J Physiol 594.3 (2016) pp 763–780
763
Effects of acute and chronic systemic methamphetamine
on respiratory, cardiovascular and metabolic function,
and cardiorespiratory reflexes
Sarah F. Hassan1, Travis A. Wearne2, Jennifer L. Cornish2 and Ann K. Goodchild1
1The Australian School of Advanced Medicine, Macquarie University, NSW 2109, Australia
2Neuropharmacology Laboratory, Department of Psychology, Macquarie University, NSW 2109, Australia
Key points
r Methamphetamine (METH) abuse is escalating worldwide, with the most common cause
of death resulting from cardiovascular failure and hyperthermia; however, the underlying
physiological mechanisms are poorly understood.
r Systemic administration of METH in anaesthetised rats reduced the effectiveness of some
protective cardiorespiratory reflexes, increased central respiratory activity independently of
metabolic function, and increased heart rate, metabolism and respiration in a pattern indicating
that non-shivering thermogenesis contributes to the well-described hyperthermia.
r In animals that showed METH-induced behavioural sensitisation following chronic METH
treatment, no changes were evident in baseline cardiovascular, respiratory and metabolic
hysiology
P
measures and the METH-evoked effects in these parameters were similar to those seen in
saline-treated or drug na¨ıve animals.
r Physiological effects evoked by METH were retained but were neither facilitated nor depressed
of
following chronic treatment with METH.
r These data highlight and identify potential mechanisms for targeted intervention in patients
vulnerable to METH overdose.
nal
Abstract
Methamphetamine (METH) is known to promote cardiovascular failure or
Jour
life-threatening hyperthermia; however, there is still limited understanding of the mechanisms
responsible for evoking the physiological changes. In this study, we systematically determined
the effects on both autonomic and respiratory outflows, as well as reflex function, following
The
acute and repeated administration of METH, which enhances behavioural responses. Arterial
pressure, heart rate, phrenic nerve discharge amplitude and frequency, lumbar and splanchnic
sympathetic nerve discharge, interscapular brown adipose tissue and core temperatures, and
expired CO2 were measured in urethane-anaesthetised male Sprague-Dawley rats. Novel findings
include potent increases in central inspiratory drive and frequency that are not dependent on
METH-evoked increases in expired CO2 levels. Increases in non-shivering thermogenesis correlate
with well-described increases in body temperature and heart rate. Unexpectedly, METH evoked
minor effects on both sympathetic outflows and mean arterial pressure. METH modified cardio-
respiratory reflex function in response to hypoxia, hypercapnia and baroreceptor unloading.
Chronically METH-treated rats failed to exhibit changes in baseline sympathetic, cardiovascular,
respiratory and metabolic parameters. The tonic and reflex cardiovascular, respiratory and
metabolic responses to METH challenge were similar to those seen in saline-treated and drug
naive animals. Overall, these findings describe independent and compound associations between
physiological systems evoked by METH and serve to highlight that a single dose of METH can
significantly impact basic homeostatic systems and protective functions. These effects of METH
persist even following chronic METH treatment.
C
2015 The Authors. The Journal of Physiology C
2015 The Physiological Society
DOI: 10.1113/JP271257
764
S. F. Hassan and others
J Physiol 594.3
(Received 9 July 2015; accepted after revision 13 November 2015; first published online 20 November 2015)
Corresponding author A. K. Goodchild: The Australian School of Advanced Medicine, Level 1, 2 Technology Place,
Macquarie University, North Ryde, New South Wales, Australia 2109.
Email: [email address]
Abbreviations
AP, arterial pressure; HR, heart rate; iBAT, interscapular brown adipose tissue; lSNA, lumbar
sympathetic nerve activity; MAP, mean arterial pressure; METH, methamphetamine; PNA, phrenic nerve activity;
PNamp, phrenic nerve amplitude; PNf, phrenic nerve frequency; SNA, sympathetic nerve activity; sSNA, splanchnic
sympathetic nerve activity.
Introduction
abstinence evokes a facilitated behavioural response in
locomotor activity and/or stereotypy when subsequently
Methamphetamine (METH) distributes rapidly throu-
challenged with METH, a process known as behavioural
ghout the brain and body to block and reverse mono-
sensitization (Pierce & Kalivas, 1997), while the locomotor
amine transporters and as a result, enhances synaptic
enhancement can further be attenuated by pretreatment
and extra-synaptic levels of dopamine, noradrenaline
with antipsychotic drugs (Abekawa
et al. 2008). The
and serotonin (Brown
et al. 2002; Sulzer
et al. 2005;
neurobiological changes associated with repeated METH
Cruickshank & Dyer, 2009). While in humans, well-
use are increasingly described, but the effects of METH
described psychogenic effects are accompanied by a
sensitisation on autonomic and respiratory function are
range of physiological changes that include tachy-
unknown.
cardia, hypertension, tachypnoea and hyperthermia
Thus, the objectives of this study were twofold.
(Schep
et al. 2010), the type and degree of change
The first aim was to define the changes in cardio-
reported are impacted by the drug dose, length of drug
vascular, sympathetic, respiratory and metabolic function,
use, and varying patient populations whose histories
as well as the effects on cardiorespiratory reflex
of METH use (binge/occasional) are complicated by
function, that are evoked by increasing concentrations
poly-drug use (Carvalho
et al. 2012). Initial attempts in
of systemically administered METH. The second aim
conscious animals to describe the physiological effects
was to determine if basal physiological and cardio-
evoked by METH indicate that METH increases arterial
respiratory reflex functions were changed in a model
pressure (AP), heart rate (HR), locomotor activity and
of behavioural sensitisation, and whether or not there
body temperature (Yoshida
et al. 1993; Sprague
et al.
is enhanced sympathetic, cardiovascular, metabolic or
2004; Rusyniak
et al. 2012); however, these earlier
respiratory sensitivity to METH challenge. Experiments
studies are limited by a narrow range of physiological
were conducted in urethane-anaesthetised rats in order
measures. As such, we do not yet understand the
to permit a comprehensive assessment of central
full range and degree of physiological effects of acute
inspiratory drive, non-shivering thermogenesis, multiple
METH administration, or their underlying mechanisms.
sympathetic nerve activities, blood pressure and heart rate,
Importantly, identifying precisely how and what extent
together with central and peripheral chemoreceptor and
these systems are altered may expose damage incurred
baroreceptor reflex function unencumbered by changes in
by chronic METH use and factors that underpin patient
locomotor activity, to investigate specific reflex pathways
mortality.
and systems altered by METH administration.
As METH administration promotes neuronal plasticity
(Guilarte
et al. 2003; Jedynak
et al. 2007) and/or neuro-
toxicity (Bowyer & Holson, 1995; Cadet & Krasnova, 2009;
Methods
Krasnova & Cadet, 2009) depending upon the dose, and
Ethical approval
overdose is typically associated with significant cardiac
effects and life threatening hyperthermia, a comprehensive
All procedures conform to the regulations detailed in
study to delineate the physiological changes evoked by
the Australian Code of Practice for the Care and Use
repeated METH use is also required. Chronic use of
of Animals for Scientific Purposes 7th edition (2004)
METH is associated with altered psychological profiles
and were approved by the Macquarie University Animal
and can evoke acute psychosis that is indistinguishable
Ethics Committee (Animal Research Authority Number
from schizophrenia (Srisurapanont
et al. 2003; Zweben
2010/045, 2010/013).
et al. 2004; Kim
et al. 2009; Glasner-Edwards
et al. 2010;
Krasnova
et al. 2010). Many of these psychological changes
Housing
induced by METH are associated with abnormalities
within the brain (Steketee, 2003) and these neural
Male Sprague-Dawley rats obtained from the Animal
changes are detectable through behavioural examination.
Resources Centre (Perth, Australia) were housed at
Repeated exposure to METH followed by a period of
Macquarie University vivarium in groups of four in
C
2015 The Authors. The Journal of Physiology C
2015 The Physiological Society
J Physiol 594.3
Physiological systems altered by acute and chronic methamphetamine
765
standard plastic cages (64 cm (L) × 40 cm (W) × 20 cm
10–20 min was allowed. Baroreceptor function (I.V. bolus
(H)) lined with wood shavings and environmental
of 10
μg sodium nitroprusside and 10
μg phenylephrine;
enrichment material and were kept in a humidity-
Sigma Aldrich) and then peripheral and central chemo-
and temperature-controlled room (21°C, 60% humidity)
reflex function (hypoxia: 14 s 100% nitrogen; hypercapnia:
maintained on a 12 h light–12 h dark cycle (lights on at
60 s of 10% carbon dioxide) were tested. METH was then
06.00 h). All experimentation was carried out during the
administered (I.P.) cumulatively at concentrations of 0.03,
light period. Food and water were available
ad libitum in
0.1, 0.3, 1 and 5 mg kg−1. The peak responses in MAP, HR,
the home cages.
splanchnic and lumbar sympathetic nerve activities (sSNA
and lSNA, respectively), phrenic nerve burst amplitude
(PNamp) and PNf, iBAT and core temperatures, and
Drugs
expired CO2 were determined at each dose with a
(±) Methamphetamine HCl (METH) was obtained
minimum period of 30 min between doses. Assessment
from the Australian Government Analytical Laboratories
of baroreceptor and then chemoreflex function was
(AGAL, Pymble, Australia). METH was dissolved in iso-
repeated at the time immediately after the peak response
tonic saline and injected into the intraperitoneal cavity
was reached following each dose, with each stimulus
(I.P.) at a volume of 1 ml kg−1. Control rats were treated
challenge presented twice and the response averaged.
with isotonic saline.
Saline vehicle (1ml kg−1 I.P.) was administered as control
(
n = 4).
In order to investigate whether the physiological effects
General surgical procedures
evoked by METH were dependent or independent of the
Anaesthesia and surgical preparation was conducted
hypercapnia produced (in paralysed, ventilated animals),
as previously described by Hassan
et al. (2015).
an additional group of animals (
n = 5) received a single
Briefly, rats were anaesthetised with urethane (10% in
dose of METH (5 mg kg−1 I.P.) and the METH-evoked
saline, 1.3 g kg−1 I.P.; Sigma-Aldrich, St Louis, MO,
hypercapnia was immediately compensated by increasing
USA). Right femoral artery and veins were cannulated
the ventilation rate to maintain the expired CO2 level
for arterial pressure (AP) and drug administration,
within 0.1% of control levels.
respectively. Tracheostomy permitted artificial ventilation
and measurement of expired CO2. Colonic temperature
Experiment 2: chronic METH sensitisation. The behavi-
was recorded and maintained at 36.5–37.5°C using
oural sensitisation protocol is as described in Wearne
a thermoregulated heating blanket set to turn off at
et al. (2015). Male Sprague-Dawley rats (277 ± 7g at
37.0°C. Whole nerve recordings were made from left
the start of testing (
n = 26)) were used. Rats were
phrenic, splanchnic and lumbar sympathetic nerves. Inter-
acclimated to the vivarium for 1 week. On Day 1, all
scapular brown adipose tissue temperature was recorded.
rats were given 15 min to explore a locomotor chamber.
The left and right vagi were cut and neuromuscular
Rats then received an injection of saline (1 ml kg−1
blockade was maintained with pancuronium bromide
I.P.), were placed back into the locomotor chamber
(0.8 mg I.V. induction, 0.4mg h−1 I.V. maintenance;
and their activity measured for 1 h. Rats were then
Astra Pharmaceuticals Pty Ltd, Sydney, Australia). Rats
allocated to treatment groups (METH (
n = 13) or
were positioned prone in a stereotaxic frame. Nerve
saline (
n = 13)) based on baseline locomotor activity
recordings were amplified (CWE Inc., Ardmore, PA,
such that there was no significant difference between
USA), band pass filtered (0.1–3 kHz), sampled at 5 kHz
groups prior to the commencement of the drug schedule
(1401, CED Ltd, Cambridge, UK) and recorded on a
(
P = 0.21).
computer using Spike2 software (CED Ltd). Core and
METH-treated rats received once daily 1 mg kg−1 ml−1
iBAT temperatures and end-tidal CO2 were sampled
METH (I.P.) on Days 2 and 8 for locomotor testing, and
at 2 kHz and recorded similarly. Heart rate (HR) was
once daily injections of 5 mg kg−1 ml−1 METH (I.P.) on
derived from the AP signal. Phrenic nerve burst frequency
Days 3–7 in the home cages. Saline rats received once daily
(PNf) was derived from PNA. All parameters were
injections of physiological 1 ml kg−1 saline (I.P.) from Days
continuously recorded for the duration of experiment with
2–8. METH doses used were low to moderate and would be
additional doses of urethane administered as required. At
unlikely to cause neurotoxicity, consistent with previous
the end of each experiment, rats were killed with 3 M
methods (Iwazaki
et al. 2008; Morshedi & Meredith, 2008).
KCl I.V.
Locomotor activity was measured on Days 2 and 8 in all
rats and recorded as the number of beam breaks per 60 min
following drug injection (total time in chamber = 75 min).
Experimental protocols
At the end of the 8 day drug schedule, rats were left for
Experiment 1: acute METH. Male Sprague-Dawley rats
a withdrawal period of 14–16 days in their home cages
(350–550 g,
n = 6) were used. A stabilisation period of
prior to locomotor testing (METH
n = 6, saline
n = 6)
C
2015 The Authors. The Journal of Physiology C
2015 The Physiological Society
766
S. F. Hassan and others
J Physiol 594.3
or electrophysiological assessment (METH
n = 7, saline
To assess sympathetic baroreflex function the effects of
n = 7).
sodium nitroprusside (10
μg) and phenylephrine (10
μg)
were assessed on splanchnic SNA and MAP.
X–Y plots of
Behavioural measures. Locomotor activity in all rats was
SNA
vs. MAP were determined and the data normalised
recorded on Days 1, 2 and 8 of the drug regime. Twelve
setting 100% SNA to baseline levels of SNA. Data were
standard chambers (250 (L) × 310 (W) × 500 (H) mm)
then fitted using non-linear regression to obtain, top,
consisting of aluminum top and side panels and plexi-glass
bottom,
V50 and slope. The first derivative of the curve was
front and back panels with metal rod floor (16 rods,
calculated in order to determine the maximum baroreflex
6 mm diameter, 15 mm apart) were used. The chambers
gain (%
SNA/mmHg). To assess cardiac sympathetic
were housed in individual wooden sound attenuation
baroreflex function maximum and minimum HR values
boxes with ventilation fans providing a masking noise.
generated in response to sodium nitroprusside and
Each chamber was equipped with four infrared photo-
phenylephrine were determined. One-way ANOVA with
beam detectors (Quantum PIR motion sensor, part no.
Dunnett’s multiple comparison test was used to compare
890-087-2, NESS Security Products, Australia) positioned
doses to control responses, significance was indicated if
on the front and back panels approximately 50 mm
P < 0.05.
apart and 30 mm above the floor. Locomotor activity
For peripheral and central chemoreflex challenge, peak
was quantified as the number of photobeam inter-
values of HR, PNf and sSNA were determined following
ruptions and recorded via an attached Macintosh
METH and compared to controls.
computer equipped with Med-IV PC software (Med
Associates). Rats were placed in the test chamber 15 min
prior to drug injection to reduce environment-induced
Statistical analysis
increases in activity. The test room was maintained
Statistical calculations were performed using SPSS version
under low light conditions at an ambient temperature of
17 or GraphPad 9. For behavioural analysis statistical
21°C.
evaluations were made using two-tailed unpaired
t tests.
Following the 14–16 day withdrawal period, rats under-
One-way ANOVA (Dunnett’s multiple comparison test)
going locomotor testing only (METH
n = 6, saline
n = 6)
was used to compare dose–response to control in
were injected with 1.0 ml kg−1 saline (I.P.) to test for
physiological data. Curves were fitted using non-linear
conditioned baseline responding. The next day (Day 24),
regression where possible. Pearson’s correlation analysis
all rats were assessed for METH-induced sensitisation
examined iBAT temperatures against other autonomic
via a challenge dose of 1 mg kg−1 ml−1 METH (I.P.).
parameters, plots were then assessed for linear regression
This procedure provides a behavioural correlate to the
where possible. For chemoreflex and baroreflex analysis,
physiological changes measured in the remaining cohort
two-tailed unpaired
t tests and/or one-way ANOVA
of animals described below.
(Dunnett’s multiple comparison test) was used. Two-way
ANOVA (Bonferroni multiple comparison test) was
Electrophysiological measures. Following the 14–16 day
used to compare between treatment groups and doses.
withdrawal period the remaining animals from each
Significance was indicated if
P < 0.05.
treatment group (METH
n = 7, saline
n = 7) were assessed
for cardiovascular, respiratory and thermoregulatory
function and cardiorespiratory reflexes as described in
Results
Experiment 1. Behavioural sensitisation is maintained for
Experiment 1: acute METH
up to 1 year (Paulson
et al. 1991) and therefore these rats
were assessed on alternate days between Days 21 and 34
Effects of METH on cardiovascular, respiratory and
(1 rat day−1). Rats were collected and anaesthetised at the
metabolic outflows. To determine the effects of METH
same time on each day.
on sSNA, lSNA, MAP, HR, iBAT and core temperatures,
expired CO2 and PNamp and PNf, METH was
cumulatively administered at 0.03, 0.1, 0.3, 1 and 5 mg
Electrophysiological data analysis
kg−1 doses (Fig. 1). Very small if any response was
Nerve recordings were rectified and averaged using a 2 s
evoked in sSNA, lSNA and MAP. In contrast, clear
time constant. Baseline values were obtained by averaging
dose-related responses were generated in HR, iBAT and
5 min of data in 60 s bins prior to the injection of METH.
core temperatures, expired CO2, PNamp and PNf, with
Peak changes are expressed as peak value or percentage
thresholds consistently between 0.1 and 0.3 mg kg−1. The
(%) of baseline activity for each variable. The peak change
maximal response was typically generated between 1 and
data are expressed as means ± standard error of the mean
5 mg kg−1 for respiratory and metabolic parameters.
(SEM). Non-linear regression was used to curve fit the
Figure 2
A shows a representative example of the physio-
dose–response data.
logical effects evoked by METH (5mg kg−1). Decreases
C
2015 The Authors. The Journal of Physiology C
2015 The Physiological Society
J Physiol 594.3
Physiological systems altered by acute and chronic methamphetamine
767
in sSNA, lSNA and AP (only seen at this dose) were
resulted from elevated levels of expired CO2 induced
accompanied by increases in HR, PNamp, PNf, expired
by METH administration. To test this, another group of
CO2, iBAT and core temperatures. Effects were of varying
animals was administered METH (5 mg kg−1) and expired
duration, with some increases in parameters 150 min
CO2 levels in each animal were tightly preserved at control
except for PNf, which returned to baseline levels within
levels by increasing the ventilation rate; such that sub-
<20 min.
sequent changes in expired CO2 levels in the cohort were
A co-ordinated response in iBAT temperature, HR,
small (0.038 ± 0.047% expired CO2,
n = 5, compensated).
expired CO2 and core temperature was consistently
Figure 3
Aa shows representative examples during the
observed following METH administration. Figure 2
B
control period and METH-evoked increases in central
shows the correlation plots of changes in iBAT temperature
respiratory drive (PNAmp and PNf) when the expired
evoked by METH with changes in HR, PNamp,
CO2 was compensated for by increasing the ventilation
PNf, expired CO2, and core temperature; correlation
rate (right-hand side
Y axis, Fig. 3
Ab). Figure 3
B
coefficients are indicated. A strong linear correlation was
shows that METH-evoked increases in PNf and PNAmp
observed between iBAT temperature and HR (
r2 = 0.4315,
(independent of CO2 stimulus) resulted from reductions
P = 0.0057), expired CO2 (
r2 = 0.6045,
P = 0.0004), and
of the expiratory period, with little change to the
core temperature (
r2 = 0.5789,
P = 0.0006). Increases in
inspiratory period. The peak responses in all parameters
iBAT temperature were weakly associated with increases
evoked by vehicle, METH (5 mg kg−1) and ‘METH
in PNamp (
r2 = 0.3001,
P = 0.0426) but not PNf
(5 mg kg−1) + compensated’ are compared (Fig. 3
C).
(
r2 = 0.1231).
PNamp and PNf were still increased by
>3-fold following
Figure 2
A shows that the increase in PNamp appeared
METH, despite similar levels of expired CO2 during
to precede the increase in expired CO2 so we aimed to
compensation (Fig. 3
C). Compensating the increasing
determine whether increases in respiratory parameters
levels of CO2 after METH (5 mg kg−1) did not significantly
Figure 1. Effects of methamphetamine (METH) on respiratory, cardiovascular and metabolic outputs
Grouped data showing the cumulative effects of METH at 0.03, 0.1, 0.3, 1 and 5 mg kg−1 administered intra-
peritoneally on splanchnic and lumbar sympathetic nerve activity (sSNA and lSNA), mean arterial pressure (MAP),
heart rate (HR), interscapular brown adipose tissue (iBAT) and core temperatures, expired CO2, phrenic nerve (PN)
amplitude and frequency. Grouped data are presented as means ± SEM; non-linear regression curves are shown
where possible.
C
2015 The Authors. The Journal of Physiology C
2015 The Physiological Society
768
S. F. Hassan and others
J Physiol 594.3
alter the HR, PNamp, PNf and iBAT and core temperature
(
F(2,10) = 5.006,
P = 0.0312; M∗,
P < 0.05), expired
responses evoked by METH. Significant changes were
CO2 (
F(2,12) = 125.2,
P < 0.0001; M,
P < 0.0001), iBAT
evoked by both METH (5 mg kg−1) (M) alone as well
temperature (
F(2,12) = 19.90,
P = 0.0002; M,
P < 0.0001;
as ‘METH (5 mg kg−1) + compensated’ (M∗) compared
M∗,
P < 0.01) and core temperature (
F(2,12) = 8.934,
to vehicle in HR (one-way ANOVA,
F(2,12) = 21.99,
P = 0.0042; M,
P < 0.05; M∗,
P < 0.01). SSNA and lSNA
P < 0.0001; Dunnett’s multiple comparison test; M,
were unchanged following administration of 5 mg kg−1
P < 0.001; M∗,
P < 0.0001), PNamp (
F(2, 11) = 6.664,
METH alone or 5 mg kg−1 METH when expired CO2 was
P = 0.0127; M,
P < 0.05; M∗,
P < 0.05), PNf
compensated.
Figure 2. Co-ordinated respiratory,
cardiovascular and metabolic effects
evoked following METH administration
A, representative example of the effects of
systemic METH (5 mg kg−1) on lSNA, sSNA,
AP, HR, PNamp, PNf, expired CO2, iBAT and
core temperature (respectively). Grey box
indicates period in which cardiorespiratory
reflexes were tested.
B, correlation
scatterplots showing the relationship
between METH-evoked responses in HR,
PNamp, PNf, expired CO2 and core
temperature, with respect to METH-evoked
changes in iBAT temperature (
n = 15).
Strong correlations are seen between HR,
expired CO2, core temperature and iBAT
temperature.
C
2015 The Authors. The Journal of Physiology C
2015 The Physiological Society
J Physiol 594.3
Physiological systems altered by acute and chronic methamphetamine
769
Effects of METH on cardiorespiratory reflex function: peri-
67.0 ± 45.8%; 1 mg kg−1, 49.3 ± 24.7%; 5 mg kg−1,
pheral and central chemoreceptor reflexes. HR, PNf and
28.3 ± 4.9%,
P < 0.01). Sympathoexcitation in lSNA
sSNA responses to hypoxia and hypercapnia are shown in
was significantly enhanced following METH 5 mg kg−1
Fig. 4, with responses in PNamp, lSNA and MAP collected
(
F(3,18) = 3.959,
P = 0.0249; lSNA; control, 22.0 ± 7.1%;
but not shown.
METH: 0.3 mg kg−1, 18.2 ± 13.1%; 1 mg kg−1,
Hypoxia (14 s of 100% N2) evoked well-described
32.9 ± 22.6%; 5 mg kg−1, 73.9 ± 17.1%,
P < 0.05) but
responses (Sun & Reis, 1994) that included tachycardia,
unchanged in sSNA. MAP was not significantly affected by
tachypnoea and sympathoexcitation (Fig. 4
A). After
hypoxia (control, −2.9 ± 4.4 mmHg; METH: 0.3 mg kg−1,
each dose of METH, hypoxia evoked similar effects in
3.9 ± 1.0 mmHg; 1 mg kg−1, 6.9 ± 6.9 mmHg; 5 mg kg−1,
HR and in PNf (ns, one-way ANOVA) (Fig. 4
A). The
2.6 ± 6.5 mmHg, ns).
PNamp response to hypoxia was significantly reduced
Hypercapnia (60 s of 10% carbon dioxide) also
at 5 mg kg-1 METH (one-way ANOVA,
F(3,13) = 4.146,
evoked well-described responses (Makeham
et al.
P = 0.0288; control, 122.8 ± 21.9%; METH: 0.3 mg kg−1,
2004), which included bradycardia, tachypnoea and
a
b
Figure 3. Physiological effects of systemic methamphetamine (METH) before and after compensating
METH-evoked increases in expired CO2
A, representative example of expired CO2, ventilation rate (right axis, grey), integrated and raw phrenic nerve activity
during (
a) control period and (
b) after METH (5 mg kg−1)-evoked changes in expired CO2 were compensated by
increasing ventilation rate.
B, comparison of integrated phrenic nerve activity respiratory period in control (grey line)
and following METH (5 mg kg−1) with compensation of expired CO2 (continuous line).
C, group data comparing
peak responses following vehicle (open bar), METH (filled bar) and METH with CO2 compensation (shaded bar)
and. Grouped data are presented as means ± SEM; ∗
P < 0.05, ∗∗
P < 0.01, ∗∗∗
P < 0.001
vs. control.
C
2015 The Authors. The Journal of Physiology C
2015 The Physiological Society
770
S. F. Hassan and others
J Physiol 594.3
sympathoexcitation (Fig. 4
B). METH significantly
9.0 ± 2.4 mmHg; METH: 0.3 mg kg−1, 3.0 ± 4.2;
potentiated the bradycardic response (
F(3,19) = 6.188,
1 mg kg−1; 3.7 ± 1.8; 5 mg kg−1, −1.2 ± 1.6 mmHg,
P = 0.0041; METH 5 mg kg−1,
P < 0.01), whereas,
P < 0.05).
the increase in respiratory drive (PNf) was significantly
Peripheral and central chemoreflex function was also
reduced (
F(3,17) = 10.84,
P = 0.003; METH 1 mg kg−1,
assessed following METH (5 mg kg−1,
n = 5) and CO2
P < 0.01) or abolished (METH 5 mg kg−1,
P < 0.001)
compensation (Fig. 4
A and
B, open triangles). Hypo-
(Fig. 4
B). No differences were observed in the PNamp
xia elicited a slightly reduced tachycardia (two-tailed
response (control, 66.3 ± 25.6%; METH: 0.3 mg kg−1,
unpaired
t test,
P = 0.0375, Fig. 4
A) compared to
141.0 ± 118.1%; 1 mg kg−1, 36.3 ± 19.9%; 5 mg kg−1,
control and a sympathoexcitation that was similar to
22.5 ± 4.9%, ns). Similarly, sympathoexcitation was
that evoked by METH alone, which was significantly
unaffected, but the small pressor response was blunted
larger than that evoked at baseline (
P = 0.0165, Fig. 4
A).
at 5 mg kg−1 METH (
F(3,18) = 3.453,
P = 0.0385; control,
Hypercapnia following METH (5 mg kg−1) during
A
Hypoxia
acute
compensated CO2
30
50
150
40
20
100
30
Δ HR (bpm)
Δ PNf (bpm) 20
Δ sSNA (%)
10
50
eak
eak
eak
P
P 10
P
0
0
0
B
Hypercapnia
0
20
50
40
–10
10
30
–20
Δ HR (bpm)
Δ PNf (bpm)
Δ sSNA (%) 20
0
eak –30
eak
eak
P
P
P 10
–40
–10
0
0.1
1
10
0.1
1
10
0.1
1
10
Contr
C
ol
ontr
METH (mg kg–1)
Contr
C
ol
ontr
METH (mg kg–1)
Contr
C
ol
ontr
METH (mg kg–1)
C
Cardiac Baroreflex Range
40
30
20
HR (bpm)
10
0
ol
ontr
C
M 1mg M 5mg
M 0.3mg
Figure 4. Physiological effects evoked by cardiorespiratory reflex activation following acute systemic
methamphetamine (METH)
Response to hypoxia (
A) and hypercapnia (
B) before (grey boxes) and after acute METH (0.3, 1 and 5 mg kg−1) with
expired CO2 compensation (open symbol).
C, comparison of heart rate range in response to sodium nitroprusside
and phenylephrine before (shaded bar) and after METH (0.3, 1 and 5 mg kg−1) (open bar). Grouped data are
presented as means ± SEM; ∗
P < 0.05, ∗∗∗
P < 0.001
vs. control (two-tailed unpaired
t tests),
†P < 0.05,
††P < 0.01,
†††P < 0.001
vs. control (one-way ANOVA, Dunnett’s multiple comparison test).
C
2015 The Authors. The Journal of Physiology C
2015 The Physiological Society
J Physiol 594.3
Physiological systems altered by acute and chronic methamphetamine
771
Table 1. Baroreflex
METH
Control
0.3 mg
1 mg
5 mg
A: upper plateau (%sSNA)
Acute
128.8 ± 14.6
174.7 ± 31.7
185.5 ± 34.3
146.0 ± 20.3
Saline treated
133.0 ± 8.0
150.7 ± 10.6
171.9 ± 12.3
168.2 ± 21.9
METH treated
126.8 ± 4.2
172.4 ± 19.3
186.6 ± 20.4
162.3 ± 25.9
B: lower plateau (%sSNA)
Acute
0.2 ± 3.3
17.9 ± 5.5
12.9 ± 15.4
17.4 ± 4.6
Saline treated
10.6 ± 4.2
12.1 ± 6.2
13.7 ± 4.3
16.9 ± 4.6
METH treated
10.6 ± 4.2
11.22 ± 3.8
9.3 ± 2.6
5.1 ± 3.6
C: gain (%sSNA/mmHg)
Acute
−2.4 ± 0.7
−2.9 ± 0.6
−2.4 ± 0.2
−2.4 ± 0.2
Saline treated
−2.3 ± 0.3
−1.9 ± 0.1
−2.6 ± 0.3
−2.8 ± 0.4
METH Treated
−1.9 ± 0.2
−3.0 ± 0.3
−2.9 ± 0.5
−2.8 ± 0.6
compensated CO2 conditions evoked a greater bradycardia
groups showed increased locomotor activity on Day 24
(
P < 0.001, Fig. 4
B) and maintained reductions in
in response to METH challenge compared to locomotor
tachypnoea (
P = 0.0005, Fig. 4
B). Compensating
activity measured on Day 8 (two-tailed unpaired
the expired CO2 levels did not alter the effects of
t test; saline treated,
P = 0.0371; METH
P = 0.0314).
METH on PNamp for either hypoxia or hypercapnia,
Sensitisation of the METH-treated group was indicated by
which were similar to control (59.7 ± 17.5% and
a greater increase in locomotor activity in METH-treated
46.2 ± 10.2%, respectively, ns). MAP was similarly
animals (
n = 6) compared to saline-treated animals
unaffected (6.2 ± 4.7 mmHg and 2.7 ± 2.1 mmHg,
(
n = 6) (
P = 0.0199). As a result the remaining animals
respectively, ns). Surprisingly sympathoexcitation was
in each cohort were assumed to demonstrate behavioural
markedly increased by hypercapnia following METH with
sensitization, so we then determined whether autonomic
CO2 compensation (
P = 0.0001).
and respiratory sensitisation to METH challenge occurred.
Sympathetic baroreceptor reflex. Cardiac sympathetic
Respiratory, cardiovascular and metabolic effects fol-
baroreflex function was assessed by determining the total
lowing chronic METH sensitisation protocol. Following
range in HR change which could be evoked by infusion
chronic METH sensitisation but prior to METH challenge,
of SNP and PE (Fig. 4
C). Increasing doses of METH
resting cardiorespiratory and metabolic measurements
significantly attenuated the HR range in response to SNP
and cardiorespiratory reflex function were determined
and PE (one-way ANOVA,
F(3,16) = 6.568,
P = 0.0042) with
Dunnett’s test showing significant effects of both 1 mg kg−1
and 5 mg kg−1 METH (
P < 0.01) (Fig. 4
C).
A
B
Sympathetic baroreflex function was also assessed
1500
Saline Treated
from the splanchnic nerve. No significant differences in
Meth Treated
sympathetic baroreflex range or gain were seen following
y
METH administration
(Table 1, acute).
ctivit
1000
or A
eaks per 60min)
Experiment 2: chronic METH sensitisation
500
Locomot
Behavioural sensitisation. Injections of METH were
(beam br
given daily for 7 days and locomotor activity recorded
0
on Days 2–8. Locomotor activity significant increased
2
8
24
(two-tailed unpaired
t test,
P = 0.0001) in the
Day
(Challenge)
METH-treated group on Day 8 (
n = 9) compared with Day
Figure 5. Behavioural sensitisation following chronic
2 (
n = 9) (Fig. 5
A). Moreover, the METH-treated animals
methamphetamine (METH)
were significantly more active than saline-treated animals
A, locomotor activity on Days 2 and 8 of the drug schedule (see text)
on Day 8 (
P < 0.0001), with no significant differences seen
in saline-treated (open bar) and METH-treated (filled bar) rats.
B,
within the saline-treated group.
locomotor activity following METH (1 mg kg−1) challenge after
14 days of abstinence in aligned cohort of rats which did not
On Day 24 (saline and METH treatment completed Day
undergo physiological assessment (grey boxed area). Grouped data
8) locomotor activity to METH challenge was measured
presented as means ± SEM; ∗
P < 0.05, ∗∗∗
P < 0.001,
in half of each cohort (Challenge, Fig. 5
B). Both treated
∗∗∗∗
P < 0.0001; METH treated
vs. saline treated.
C
2015 The Authors. The Journal of Physiology C
2015 The Physiological Society
772
S. F. Hassan and others
J Physiol 594.3
Table 2. Baseline physiological values
MAP (mmHg)
HR (bpm)
PNf (bpm)
Core (°C)
iBAT (°C)
sSNA (
μV)
Saline treated
88.3 ± 4.4
449.6 ± 4.5
31.7 ± 1.8
36.72 ± 0.19
34.15 ± 0.47
3.24 ± 0.50
METH treated
90.1 ± 3.6
438.6 ± 7.2
35.4 ± 2.8
36.80 ± 0.18
34.17 ± 0.59
4.24 ± 1.39
and compared
(Tables 1 and 2, Fig. 7
). Chronic METH
Hypercapnia at baseline evoked bradycardia, tachy-
treatment had no significant effect on baseline variables:
pnoea and sympathoexcitation, with no significant
MAP, HR, PNf, iBAT and core temperatures and sSNA.
differences between the two treatment groups observed
In
saline
(open
circles)
and
METH
(filled
except in HR (two-tailed unpaired
t tests,
P = 0.0235)
squares)-treated groups the effects of METH challenge
(Fig. 7
B). Dose-related increases in the bradycardic
(0.1, 0.3, 1 and 5 mg kg−1) on sSNA, lSNA, MAP, HR,
response with increasing doses of METH were retained in
iBAT, core temperature, expired CO2, PN amplitude
saline-treated animals (one-way ANOVA,
F(3,23) = 10.13,
and frequency were assessed (Fig. 6). The lowest
P = 0.0002) and METH (
F(3,15) = 14.61,
P < 0.0001),
doses of METH evoked little or no response in any
with significant effects at 1 mg kg−1 (
P < 0.01) and
parameter in either group. Treatment effects were
at 5 mg kg−1 (
P < 0.001) METH in saline-treated
only observed in expired CO2 (two-way ANOVA,
animals, and at 5 mg kg−1 METH (
P < 0.0001)
F(1,44) = 18.81,
P < 0.0001) with significant dose effects
in METH-treated animals. Increasing doses of METH
(
F(3,44) = 4.255,
P = 0.0101) observed at 1 mg kg−1
also blunted the tachypnoea response in both saline
(
P < 0.05) and 5 mg kg−1 METH (
P < 0.001). Dose
(
F(3,24) = 8.564,
P = 0.0005) and METH (
F(3,19) = 4.971,
related increases to METH were maintained in both
P = 0.0103)-treated animals, with significant effects at
treatment groups in HR (
F(3,43) = 37.77,
P < 0.0001),
0.3 mg kg−1 METH (
P < 0.05), 1 mg kg−1 METH
iBAT (
F(3,44) = 98.99,
P < 0.0001), core temperature
(
P < 0.01) and 5 mg kg−1 METH (
P < 0.001) in
(
F(3,44) = 23.52,
P < 0.0001), expired CO2 (
F(3,44) = 122.1,
saline-treated and 1 mg kg−1 METH (
P < 0.05) and
P < 0.0001), PNamp (
F(3,41) = 16.87,
P < 0.0001)
5 mg kg−1 METH (
P < 0.05) in METH-treated animals.
and PNf (
F(3,44) = 4.259,
P = 0.01). METH challenge
Tachypnoea responses to hypercapnia were significantly
evoked responses in sSNA, lSNA, MAP, HR, iBAT,
different between treatment groups (two-way ANOVA,
core temperature, PN amplitude and frequency in
F(1,43) = 6.88,
P = 0.0120), with greater dose-dependent
METH-treated animals (Fig. 6) were also similar to those
reductions in the METH-treated group (
F(3,43) = 12.74,
seen in treatment na¨ıve animals (Fig. 1).
P < 0.0001). In contrast, sympathoexcitation was similar
in both treatment groups.
Peripheral and central chemoreceptor reflex function
following METH sensitisation protocol. In both the saline
and METH-treated groups responses to hypoxia and
Sympathetic baroreceptor reflexes following METH
hypercapnia were obtained at baseline prior to METH
sensitisation protocol. Sympathetic cardiac baroreflex
administration and at the peak of METH-evoked effects
range was reduced in the METH-treated group at base-
(Fig. 7). Hypoxia at baseline evoked characteristic tachy-
line control (two-tailed unpaired
t tests,
P = 0.0387,
cardia, tachypnoea and sympathoexcitation, with no
Fig. 7
C). HR range following METH challenge was
significant differences between the groups (two-tailed
significantly different to baseline in both saline (one-way
unpaired
t tests) (Fig. 7
A). Tachycardia and tachy-
ANOVA,
F (3,24) = 12.66,
P < 0.0001) and METH groups
pnoea to hypoxia were unaffected throughout the
(
F(3,21) = 9.828,
P = 0.0003), with significant reductions
dose range of METH challenge within or between
in HR following METH 0.3 mg kg−1 (saline treated;
treatment groups. METH-evoked sympathoexcitation
P < 0.01), 1 mg kg−1 (saline treated,
P < 0.01; METH
was significantly increased in both saline (one-way
treated,
P < 0.05) and METH 5 mg kg−1 (saline treated,
ANOVA,
F(3,24) = 20.61,
P < 0.0001) and METH
P < 0.0001; METH treated,
P < 0.001). However, there
(
F(3,24) = 5.007,
P = 0.0078)-treated animals, with
was no significant difference between treatment groups
significant increases at 5 mg kg−1 METH in both
(two-way ANOVA, Fig. 7
C).
saline-treated (
P < 0.0001) and METH-treated (
P < 0.01)
Upper and lower plateau values and sympathetic
animals compared to their respective baseline responses.
baroreflex gains following 0.3, 1 and 5 mg kg−1 METH
Surprisingly, saline-treated animals showed exaggerated
compared to control are shown in (Table 1
A–C). No
sympathoexcitation (two-way ANOVA,
F(3,104) = 4.77,
differences in sympathetic baroreflex function were
P = 0.0037) in response to hypoxia with increasing METH
observed within or between treatment groups following
doses (
F(3,104) = 53.57,
P < 0.0001).
METH challenge (one-way ANOVA/two-way ANOVA).
C
2015 The Authors. The Journal of Physiology C
2015 The Physiological Society
J Physiol 594.3
Physiological systems altered by acute and chronic methamphetamine
773
Discussion
The patterned physiological response of increased
HR and expired CO2 which was correlated with iBAT
The principal findings of this study are that systemic
temperature and accompanied by increased respiratory
METH administration (a) increased HR, iBAT tempe-
drive,
is
typical
of
non-shivering
thermogenesis
rature, expired CO2 and inspiratory drive and frequency
(Morrison, 2011). This suggests that METH triggers
in a dose-dependent manner, with little effect on MAP
non-shivering thermogenesis which contributes to
or the sympathetic outflows to the viscera or hind-
the well-described METH-evoked hyperthermia. The
limb, (b) increased centrally mediated ventilatory drive
simplest explanation is that METH acts to release
independently of changes in CO2, (c) modified protective
noradrenaline from BAT sympathetic postganglionic
cardiorespiratory reflexes, and (d) was not associated with
terminals, but other effectors may be possible as centrally
facilitated sympathetic, respiratory, metabolic, cardio-
mediated release at other sympathetic outflows was not
vascular responses or cardiorespiratory reflex function
evident. In addition, METH drives centrally evoked
following a behaviourally validated METH sensitisation
increases in ventilatory drive independent of metabolically
protocol.
induced changes in CO2 which underlies the tachypnoea
100
sSNA
100
ISNA
20
MAP
saline treated
METH treated
Δ (%)
0
Δ (%)
0
0
Δ (mmHg)
eak
eak
P
P
eak
P
–100
–100
–20
0.1
1
10
0.1
1
10
0.1
1
10
150
6
2.0
HR
iBAT Temp
Core Temp
4
1.0
Δ (bpm)
Δ (°C)
Δ (°C)
eak
eak
eak
P
P
2
P
0.0
0
0
–0.5
0.1
1
10
0.1
1
10
0.1
1
10
4
Expired CO
200
15
2
PN Amplitude
PN Frequency
10
2
100
Δ (%)
Δ (%)
Δ (bpm)
eak
eak
0
P
P
eak
P
0
0
–1
–50
–10
0.1
1
10
0.1
1
10
0.1
1
10
Meth (mg kg–1)
Meth (mg kg–1)
Meth (mg kg–1)
Figure 6. Effects of methamphetamine (METH) challenge on respiratory, cardiovascular and metabolic
outputs following chronic treatment with METH or saline
Dose–response curves to METH (0.1, 0.3, 1 and 5 mg kg−1) in saline (open symbols) and METH (filled
symbols)-treated animals showing effects on splanchnic and lumbar sympathetic nerve activity (sSNA and lSNA),
mean arterial pressure (MAP), heart rate (HR), interscapular brown adipose tissue (iBAT) and core temperatures,
expired CO2, phrenic (PN) amplitude and frequency. Grouped data are presented as means ± SEM; non-linear
regression curves are shown where possible.
C
2015 The Authors. The Journal of Physiology C
2015 The Physiological Society
774
S. F. Hassan and others
J Physiol 594.3
described in the clinic. The data further indicate that
Methodological considerations
METH-evoked behavioural sensitisation is mediated
Several studies have reported the effects of METH on
independently of mechanisms which evoke sympathetic,
a limited range of physiological variables able to be
cardiovascular, respiratory and metabolic changes. Finally,
measured in conscious rodents (MAP, HR and body
acutely administered METH alters tonic and reflex control
temperature; Yoshida
et al. 1993; Sprague
et al. 2004;
of cardiovascular, respiratory and metabolic systems in
Myles
et al. 2008; Rusyniak
et al. 2012; Varner
et al.
such a way that does not provide an optimal environment
2013). An anaesthetised model was chosen for the current
to withstand cardiorespiratory challenge.
study, permitting the concurrent measurement of multiple
A
saline treated
Hypoxia
METH treated
30
50
150
40
20
100
30
HR (bpm)
PNf (bpm)
Δ
Δ 20
Δ sSNA (%)
10
50
eak
eak
eak
P
P 10
P
0
0
0
B
Hypercapnia
0
20
50
40
–10
10
30
Δ PNf (bpm)
Δ sSNA (%) 20
Δ HR (bpm) –20
0
eak
eak
eak
P
10
P
P
–30
–10
0
0.1
1
10
0.1
1
10
0.1
1
10
Control
Meth (mg/kg–1)
Control
Meth (mg/kg–1)
Control
Meth (mg/kg–1)
C
Cardiac Baroreflex Range
40
30
20
HR (bpm)
10
0
ol
ol
ontr
ontr
C
M 1mg M 5mg C
M 1mg M 5mg
M 0.3mg
M 0.3mg
Saline treated
Meth treated
Figure 7. Physiological effects evoked by cardiorespiratory reflex activation following chronic
administration of methamphetamine (METH) or saline
Response to hypoxia (
A) and hypercapnia (
B) before (grey boxes) and after different doses of METH (0.3, 1 and
5 mg kg−1) in saline (open symbols) and METH (filled symbol)-treated animals.
C, comparison of heart rate range
in response to sodium nitroprusside and phenylephrine following METH challenge (0.3, 1 and 5 mg kg−1) in saline
(open bars) and METH (dark shaded bar)-treated animals, compared to control. Grouped data are presented as
means ± SEM; ∗
P < 0.05 (two-tailed unpaired
t tests). ♦, METH treated;
†, saline treated; ♦/
†P < 0.05, ♦♦/
††P < 0.01,
♦♦♦/
†††P < 0.001,
††††P < 0.0001
vs. control (one-way ANOVA, Dunnett’s multiple comparison test). #
P < 0.05,
##
P < 0.01 (two-way ANOVA).
C
2015 The Authors. The Journal of Physiology C
2015 The Physiological Society
J Physiol 594.3
Physiological systems altered by acute and chronic methamphetamine
775
variables, thus providing a greater understanding of the
The
METH-evoked
temperature
changes
were
physiological systems altered by METH administration.
independent of physical exertion, which may contribute
However, the effects of urethane anaesthesia on neuro-
to hyperthermia measured under conscious conditions
transmitter function in the brain (Hara & Harris, 2002)
(Yoshida
et al. 1993; Sprague
et al. 2004; Rusyniak
and specifically the inhibitory effects when urethane is
et al. 2012). The poor temporal relationship between
directly applied to sympathetic premotor centres in the
temperature and locomotor-induced activity has pre-
medulla (Sun & Reis, 1995) may mask METH-induced
viously suggested that factors other than locomotion
physiological effects seen in conscious but not in
contribute to the hyperthermia (Phelps
et al. 2010;
anaesthetised rats. The lack of effect on the sympathetic
Rusyniak
et al. 2012). Data described here provide the
outflows measured was also surprising considering METH
first measurement of iBAT temperature indicating that
concentrates in brain regions containing aminergic
it contributes to METH-evoked hyperthermia, adding
presympathetic populations, potentially driving these
to previous reports that also implicate glucocorticoids
outflows at least at very high doses of the drug (Li
(Makisumi
et al. 1998), the thyroid gland and hyper-
et al. 2012
b). The threshold dose of METH for variables
metabolism in skeletal muscle (Makisumi
et al. 1998;
measured in this study was between 0.1 and 0.3 mg kg−1,
Sprague
et al. 2004).
in keeping with studies in conscious animals (Yoshida
et al.
1993; Varner
et al. 2013).
Respiration. We demonstrate for the first time that
METH evokes centrally mediated increases in inspiratory
drive and frequency, which were independent of
METH-driven metabolic increases in plasma CO2 levels.
Physiological effects of acute METH
The increased inspiratory drive and reduced expiratory
Thermoregulation. METH increased core temperature
period underlie the tachypnoea often reported in clinical
as described previously (Bowyer
et al. 1992; Callaway
case and animal studies following METH use (Lin
&
Clark,
1994;
Makisumi
et
al.
1998;
Arora
et al. 1980; Mendelson
et al. 2006; Cruickshank &
et al. 2001; Rusyniak
et al. 2012), with our data
Dyer, 2009). These changes may be mediated via
indicating that non-shivering thermogenesis contributes
activation of central adrenergic receptors, as central
to METH-evoked hyperthermia. METH evoked an
β-adrenoreceptor blockade abolishes the tachypnoea seen
increase in iBAT temperature of up to 4°C, which was
following amphetamine administration (Mediavilla
et al.
strongly correlated with increases in core temperature,
1979; Mora
et al. 1983). Whether central respiratory
despite the use of a thermoregulated heating blanket.
centres are directly affected is yet to be determined, but
METH also evoked tachycardia and an increase in expired
it is of interest that METH when administered at lethal
CO
concentrations has been reported to concentrate in brain-
2, which were positively correlated both temporally and
with the magnitude of the iBAT response. Non-shivering
stem regions (Li
et al. 2012
b) where major respiratory
thermogenesis is consistently accompanied by increased
centres are located (Richter & Smith, 2014).
HR, probably increasing BAT blood flow in order to
distribute heat, and increased expired CO2 resulting from
Cardiovascular. In the present study, METH consistently
increased BAT metabolism (Morrison, 2004; Morrison
evoked tachycardia but had no effect on blood pressure
et al. 2012). Our findings are supported by a recent
except at 5 mg kg−1 where depressor responses
study showing that iBAT ablation reduced METH-induced
were evoked. METH-evoked tachycardia is consistently
hyperthermia in conscious mice (Sanchez-Alavez
et al.
reported in conscious and anaesthetised animal studies
2013) and is in keeping with a previous report in which
(Yoshida
et al. 1993; Arora
et al. 2001) and in clinical
systemic amphetamine in female rats evoked an increase
reports (Chan
et al. 1994; West
et al. 2010). This is probably
in iBAT temperature (Wellman, 1983). However, no other
mediated via
β1 adrenoreceptors (Schindler
et al. 1992) by
parameters were measured in this study so the effects
direct actions of METH at the heart and/or via activation of
could not be reliably attributed to increased thermogenesis
the cardiac sympathetic outflow (as the animals reported
and changes in blood flow could strongly influence iBAT
here were vagotomised), or as a consequence of the
temperature. Non-shivering thermogenesis is likely to be
increased thermogenesis as indicated above.
evoked by METH increasing the availability of biogenic
The depressor response evoked at high doses of
amines directly at brown adipocytes by noradrenaline
METH in anaesthetised rats could be indicative of
release from BAT sympathetic postganglionic terminals
approaching thresholds of METH toxicity, which at
evoked either at the level of the synapse or centrally as
12 mg kg−1 I.V. can lead to cardiovascular collapse and
iBAT denervation reduces METH-induced mitochondrial
death in conscious animals (Chan
et al. 1994; Kaye
dysfunction in iBAT (Sanchez-Alavez
et al. 2013), or
et al. 2007, 2008; Li
et al. 2012
a,
b). The lack of effect
possibly via indirect mechanisms.
of lower doses of METH on MAP contrasts strongly
C
2015 The Authors. The Journal of Physiology C
2015 The Physiological Society
776
S. F. Hassan and others
J Physiol 594.3
to the pressor responses evoked in conscious animals
Despite this significant behavioural sensitisation we
(Yoshida
et al. 1993) and thus may result from the
found no evidence either of changes in baseline sympa-
anaesthesia (see Methodological considerations above). It
thetic, cardiovascular, respiratory or metabolic variables
is possible that in non-vagotomised conscious animals
or cardiorespiratory reflex function. Furthermore, exag-
the METH-evoked tachycardia, which is significantly
gerated respiratory or autonomic responses were not
greater than in vagotomised anaesthetised rats used
seen at any dose of METH administered in animals
here, contributed to the relatively modest pressor effect
chronically treated with METH. Yoshida and colleagues
(17 mmHg at 1 mg kg−1 METH) seen in conscious
(1993) showed that repeated administration of METH
rats or, alternatively, that MAP changes seen in conscious
in telemetered animals evoked an enhanced response
animals simply reflect those required to accommodate the
in MAP, body temperature, as well as activity (but
increased locomotor activity and altered behaviour elicited
not HR) following I.P. METH challenge. However, they
by METH (Shoblock
et al. 2003; Hall
et al. 2008).
showed that only activity was sensitised following intra-
cerebroventricular METH challenge, so it is possible that
the enhanced locomotor activity could have contributed
Cardiorespiratory reflexes. We addressed the effects of
to these larger changes in MAP and body temperature.
METH on chemo- and baroreceptor reflexes which act
Also, rats receiving an escalating dose of METH or
to maintain blood pressure, appropriate tissue perfusion
saline for 12 days and then challenged with METH
and gas exchange. METH had little effect on the response
(10 mg kg−1) on Day 13 showed no change in temperature
to hypoxia except for a reduction in respiratory drive,
within 120 min in the METH-treated group, with a large
likely to be due to a ceiling effect resulting from
rise in core temperature exhibited in the saline-treated
METH-evoked CO2 load. In contrast, METH altered the
group (Myles & Sabol, 2008). However, we cannot
response to hypercapnia, facilitating the bradycardia and
rule out that significant differences due to METH
the sympathoexcitation (under CO2 compensation), but
administration regimes (both time course and route of
abolishing the tachypnoea.
administration) or the contribution of anaesthesia could
The sympathetic baroreflex was unaffected by METH,
account for the differences between all these studies.
but METH reduced the range of the sympathetic cardiac
Nevertheless, using a protocol which demonstrated
baroreflex. This effect could be mediated centrally or
behavioural sensitisation following a period of abstinence
directly at the heart, although the effect may simply result
in our conscious animal cohort, there was no facilitation of
from a ceiling effect, as METH evoked a large increase
METH-evoked autonomic responses in the anaesthetised
in HR which may not be further increased under the
animals.
anaesthetised and paralysed conditions here. Nevertheless,
The sensitised locomotor response is likely to be
it is interesting to note that reduced HR variability
mediated by the mesolimbic dopamine system (Pierce &
indicative of baroreceptor dysfunction is common in those
Kalivas, 1997; Kelly
et al. 2008). Although little evidence
suffering METH addiction (Henry
et al. 2012) and in
is available to describe the neurotransmitters involved in
a model of METH overdose using very high levels of
mediating central respiratory and autonomic effects of
METH, baroreceptor function is severely attenuated in
METH, we have shown, at least in the prefrontal cortex,
those animals that succumbed to fatal doses of METH (Li
that noradrenaline rather than dopamine best mimics the
et al. 2012
b).
physiological effects of systemic METH administration
These data indicate that respiratory and blood pressure
(Hassan
et al. 2015). It is possible that sensitisation
challenges assessed here following METH administration
to repeated METH administration does not occur in
fail to elicit appropriate protective reflex responses.
all systems. However, we have also demonstrated that
METH sensitisation alters proteins associated with the
Physiological effects of acute METH following
GABAergic inhibitory network (Wearne
et al. 2015),
one of many neurotransmitter systems targeted by the
METH-induced behavioural sensitisation
general anaesthetic used here (Hara & Harris, 2002). It is
In line with others, rats that received repeated METH
conceivable that such an interaction could limit measures
showed increased locomotor activity compared to those
of physiological facilitation induced by chronic METH
that received saline (Stewart & Badiani, 1993; McDaid
et al.
exposure.
2006). Following 14 days of abstinence, locomotor activity
In response to peripheral and central chemoreceptor
was further exaggerated as previously described (Paulson
stimulation, METH-pretreated animals responded very
et al. 1991; McDaid
et al. 2006). Evidence suggests this
similarly to both saline-pretreated and na¨ıve animals,
is mediated by long-term neuroadaptative changes in the
with only small differences in a few parameters between
brain (Cornish & Kalivas, 2001), particularly associated
treatment groups. Curiously, the sympathoexcitation
with the mesolimbic dopamine system (Robinson &
evoked by hypoxia following METH challenge was seen
Becker, 1986).
in both treatment groups and in naive animals. Chronic
C
2015 The Authors. The Journal of Physiology C
2015 The Physiological Society
J Physiol 594.3
Physiological systems altered by acute and chronic methamphetamine
777
METH treatment does not alter the responses to hypoxia
References
or hypercapnia at baseline but was similarly deleterious in
Abekawa T, Ito K, Nakagawa S, Nakato Y & Koyama T (2008).
both METH-pretreated and drug na¨ıve animals during
Olanzapine and risperidone block a high dose of
METH challenge. Likewise, the cardiac sympathetic
methamphetamine-induced schizophrenia-like behavioral
baroreflex range was reduced in the METH-pretreated
abnormalities and accompanied apoptosis in the medial
group, which mimicked the effect in drug na¨ıve animals.
prefrontal cortex.
Schizophr Res 101, 84–94.
The effect of all doses of METH on the physio-
Arora H, Owens SM & Gentry WB (2001). Intravenous
logical measures here were equivalent across all treated
(+)-methamphetamine causes complex dose-dependent
groups.
physiologic changes in awake rats.
Eur J Pharmacol 426,
This lack of sensitisation of cardiovascular, respiratory
81–87.
and metabolic effects following chronic METH is in
Bowyer J & Holson R (1995). Methamphetamine and
contrast to that seen following other chronic stimuli.
amphetamine neurotoxicity.
Neurological Disease And
Sustained hypoxia and chronic intermittent hypoxia
Therapy 36, 845–845.
Bowyer JF, Tank AW, Newport GD, Slikker W, Jr., Ali SF &
(CIH) evoke increased baseline ventilation (Reeves
et al.
Holson RR (1992). The influence of environmental
2003) and CIH causes increased sympathetic vaso-
temperature on the transient effects of methamphetamine
motor tone, MAP, and haematocrit, with enhanced
on dopamine levels and dopamine release in rat striatum.
peripheral and central chemoreflex sympathetic and
J Pharmacol ExP Ther 260, 817–824.
respiratory responses as well as augmentation of other
Brown JM, Riddle EL, Sandoval V, Weston RK, Hanson JE,
sympathoexcitatory reflexes (Greenberg
et al. 1999; Zoccal
Crosby MJ, Ugarte YV, Gibb JW, Hanson GR & Fleckenstein
et al. 2009; Silva & Schreihofer, 2011). Furthermore,
AE (2002). A single methamphetamine administration
these changes were seen in anaesthetised preparations.
rapidly decreases vesicular dopamine uptake.
J Pharmacol
Physiological adjustments to chronic exposure to low
ExP Ther 302, 497–501.
environmental temperatures include maintenance of
Cadena V & Tattersall GJ (2014). Body temperature regulation
lower levels of body temperature and a shift from shivering
during acclimation to cold and hypoxia in rats.
J Therm Biol
46, 56–64.
to non-shivering thermogenesis (Cadena & Tattersall,
Cadet JL & Krasnova IN (2009). Molecular bases of
2014). Following cessation of all these chronic insults the
methamphetamine-induced neurodegeneration.
Int Rev
physiological changes do recover over time, so that it is
Neurobiol 88, 101–119.
possible that immediately following a chronic exposure
Callaway CW & Clark RF (1994). Hyperthermia in
to METH physiological effects may be detected, but this
psychostimulant overdose.
Ann Emerg Med 24, 68–76.
remains to be determined. Alternatively, the mechanisms
Carvalho M, Carmo H, Costa VM, Capela JP, Pontes H,
affected by METH to evoke physiological change may not
Remiao F, Carvalho F & Bastos Mde L (2012). Toxicity of
be amenable to such plasticity.
amphetamines: an update.
Arch Toxicol 86, 1167–1231.
Chan P, Chen JH, Lee MH & Deng JF (1994). Fatal and
nonfatal methamphetamine intoxication in the intensive
care unit.
J Toxicol Clin Toxicol 32, 147–155.
Conclusions
Cornish JL & Kalivas PW (2001). Cocaine sensitization and
In summary, acute METH administered in the anaes-
craving: differing roles for dopamine and glutamate in the
nucleus accumbens.
J Addict Dis 20, 43–54.
thetised animal increases cardiac, metabolic and respi-
Cruickshank CC & Dyer KR (2009). A review of the clinical
ratory function in a pattern that mimics activation of
pharmacology of methamphetamine.
Addiction 104,
non-shivering thermogenesis, indicating that thermo-
1085–1099.
genesis probably contributes to METH-evoked hyper-
Glasner-Edwards S, Mooney LJ, Marinelli-Casey P, Hillhouse
thermia. METH increases ventilatory drive independently
M, Ang A, Rawson RA & Methamphetamine Treatment
of changes in CO2. Acute METH administration also
Project Corporate Authors (2010). Psychopathology in
reduces the effectiveness of some components of protective
methamphetamine-dependent adults 3 years after treatment.
cardiorespiratory reflexes, probably due to heightened
Drug Alcohol Rev 29, 12–20.
METH-evoked respiratory and cardiac functions. In
Greenberg HE, Sica A, Batson D & Scharf SM (1999). Chronic
contrast, chronic treatment with METH that facilitated
intermittent hypoxia increases sympathetic responsiveness
METH-evoked locomotor activity failed to modify basal
to hypoxia and hypercapnia.
J Appl Physiol 85, 298–305.
Guilarte TR, Nihei MK, McGlothan JL & Howard AS (2003).
physiological parameters or sympathetic, cardiovascular,
Methamphetamine-induced deficits of brain
respiratory and metabolic responses evoked by acute
monoaminergic neuronal markers: distal axotomy or
METH challenge. These findings highlight the potent
neuronal plasticity.
Neuroscience 122, 499–513.
and dangerous physiological effects of METH use and
Hall DA, Stanis JJ, Marquez Avila H & Gulley JM (2008). A
indicate some underlying mechanisms responsible for the
comparison of amphetamine- and methamphetamine-
increased cardiorespiratory vulnerability and lethal hyper-
induced locomotor activity in rats: evidence for qualitative
thermia seen in those that abuse METH.
differences in behavior.
Psychopharmacology 195, 469–478.
C
2015 The Authors. The Journal of Physiology C
2015 The Physiological Society
778
S. F. Hassan and others
J Physiol 594.3
Hara K & Harris RA (2002). The anesthetic mechanism of
Makisumi T, Yoshida K, Watanabe T, Tan N, Murakami N &
urethane: the effects on neurotransmitter-gated ion
Morimoto A (1998). Sympatho-adrenal involvement in
channels.
Anesth Analg 94, 313–318.
methamphetamine-induced hyperthermia through skeletal
Hassan SF, Zumut S, Burke PG, McMullan S, Cornish JL &
muscle hypermetabolism.
Eur J Pharmacol 363, 107–112.
Goodchild AK (2015). Comparison of noradrenaline,
Mediavilla A, Feria M, Fernandez JF, Cagigas P, Pazos A &
dopamine and serotonin in mediating the tachycardic and
Florez J (1979). The stimulatory action of d-amphetamine
thermogenic effects of methamphetamine in the ventral
on the respiratory centre, and its mediation by a central
medial prefrontal cortex.
Neuroscience 295, 209–220.
alpha-adrenergic mechanism.
Neuropharmacology 18,
Henry BL, Minassian A & Perry W (2012). Effect of
133–142.
methamphetamine dependence on heart rate variability.
Mendelson J, Uemura N, Harris D, Nath RP, Fernandez E,
Addict Biol 17, 648–658.
Jacob P 3rd, Everhart ET & Jones RT (2006). Human
Iwazaki T, McGregor IS & Matsumoto I (2008). Protein
pharmacology of the methamphetamine stereoisomers.
Clin
expression profile in the amygdala of rats with
Pharmacol Ther 80, 403–420.
methamphetamine-induced behavioral sensitization.
Mora F, Lee TF & Myers RD (1983). Involvement of alpha- and
Neurosci Lett 435, 113–119.
beta-adrenoreceptors in the central action of norepinephrine
Jedynak JP, Uslaner JM, Esteban JA & Robinson TE (2007).
on temperature, metabolism, heart and respiratory rates of
Methamphetamine-induced structural plasticity in the
the conscious primate.
Brain Res Bull 11, 613–616.
dorsal striatum.
Eur J Neurosci 25, 847–853.
Morrison SF (2004). Central pathways controlling brown
Kaye S, Darke S, Duflou J & McKetin R (2008).
adipose tissue thermogenesis.
News Physiol Sci 19, 67–74.
Methamphetamine-related fatalities in Australia:
Morrison SF (2011). 2010 Carl Ludwig Distinguished
demographics, circumstances, toxicology and major organ
Lectureship of the APS Neural Control and Autonomic
pathology.
Addiction 103, 1353–1360.
Regulation Section: Central neural pathways for
Kaye S, McKetin R, Duflou J & Darke S (2007).
thermoregulatory cold defense.
J Appl Physiol 110,
Methamphetamine and cardiovascular pathology: a review
1137–1149.
of the evidence.
Addiction 102, 1204–1211.
Morrison SF, Madden CJ & Tupone D (2012). Central control
Kelly MA, Low MJ, Rubinstein M & Phillips TJ (2008). Role of
of brown adipose tissue thermogenesis.
Front Endocrinol 3
dopamine D1-like receptors in methamphetamine
pii: 00005.
locomotor responses of D2 receptor knockout mice.
Gene
Morshedi MM & Meredith GE (2008). Repeated amphetamine
Brain Behav 7, 568–577.
administration induces Fos in prefrontal cortical neurons
Kim YT, Lee SW, Kwon DH, Seo JH, Ahn BC & Lee J (2009).
that project to the lateral hypothalamus but not the nucleus
Dose-dependent frontal hypometabolism on FDG-PET in
accumbens or basolateral amygdala.
Psychopharmacology
methamphetamine abusers.
J Psychiatr Res 43, 1166–1170.
197, 179–189.
Krasnova IN & Cadet JL (2009). Methamphetamine toxicity
Myles BJ, Jarrett LA, Broom SL, Speaker HA & Sabol KE
and messengers of death.
Brain Res Rev 60, 379–407.
(2008). The effects of methamphetamine on core body
Krasnova IN, Justinova Z, Ladenheim B, Jayanthi S, McCoy
temperature in the rat – part 1: chronic treatment
MT, Barnes C, Warner JE, Goldberg SR & Cadet JL (2010).
and ambient temperature.
Psychopharmacology 198,
Methamphetamine self-administration is associated with
301–311.
persistent biochemical alterations in striatal and cortical
Paulson PE, Camp DM & Robinson TE (1991). Time course of
dopaminergic terminals in the rat.
PloS One 5, e8790.
transient behavioral depression and persistent behavioral
Li FC, Yen JC, Chan SH & Chang AY (2012
a). Bioenergetics
sensitization in relation to regional brain monoamine
failure and oxidative stress in brain stem mediates
concentrations during amphetamine withdrawal in rats.
cardiovascular collapse associated with fatal
Psychopharmacology 103, 480–492.
methamphetamine intoxication.
PloS One 7, e30589.
Phelps G, Speaker HA & Sabol KE (2010). Relationship
Li FC, Yen JC, Chan SH & Chang AY (2012
b). Defunct brain
between methamphetamine-induced behavioral activation
stem cardiovascular regulation underlies cardiovascular
and hyperthermia.
Brain Res 1357, 41–52.
collapse associated with methamphetamine intoxication.
Pierce RC & Kalivas PW (1997). A circuitry model of the
J Biomed Sci 19, 16.
expression of behavioral sensitization to amphetamine-
Lin MT, Chandra A, Chern YF & Tsay BL (1980). Effects of
like psychostimulants.
Brain Res Brain Res Rev 25,
intracerebroventricular injection of d-amphetamine on
192–216.
metabolic, respiratory, and vasomotor activities and body
Reeves SR, Gozal E, Guo SZ, Sachleben LR Jr, Brittian KR,
temperatures in the rat.
Can J Physiol Pharmacol 58,
Lipton AJ & Gozal D (2003). Effect of long-term intermittent
903–908.
and sustained hypoxia on hypoxic ventilatory and
McDaid J, Graham MP & Napier TC (2006).
metabolic responses in the adult rat.
J Appl Physiol 95,
Methamphetamine-induced sensitization differentially alters
1767–1774.
pCREB and DeltaFosB throughout the limbic circuit of the
Richter DW & Smith JC (2014). Respiratory rhythm generation
mammalian brain.
Mol Pharmacol 70, 2064–2074.
in vivo.
Physiology 29, 58–71.
Makeham JM, Goodchild AK, Costin NS & Pilowsky PM
Robinson TE & Becker JB (1986). Enduring changes in brain
(2004). Hypercapnia selectively attenuates the
and behavior produced by chronic amphetamine
somato-sympathetic reflex.
ResP Physiol Neurobiol 140,
administration: a review and evaluation of animal models of
133–143.
amphetamine psychosis.
Brain Res 396, 157–198.
C
2015 The Authors. The Journal of Physiology C
2015 The Physiological Society
J Physiol 594.3
Physiological systems altered by acute and chronic methamphetamine
779
Rusyniak DE, Zaretsky DV, Zaretskaia MV, Durant PJ &
Wearne TA, Mirzaei M, Franklin JL, Goodchild AK, Haynes PA
Dimicco JA (2012). The orexin-1 receptor antagonist
& Cornish JL (2015). Methamphetamine-induced
SB-334867 decreases sympathetic responses to a moderate
sensitization is associated with alterations to the proteome of
dose of methamphetamine and stress.
Physiol Behav 107,
the prefrontal cortex: implications for the maintenance of
743–750.
psychotic disorders.
J Proteome Res 14, 397–410.
Sanchez-Alavez M, Conti B, Wood MR, Bortell N, Bustamante
Wellman PJ (1983). Influence of amphetamine on brown
E, Saez E, Fox HS & Marcondes MC (2013). ROS and
adipose thermogenesis.
Res Commun Chem Pathol
sympathetically mediated mitochondria activation in brown
Pharmacol 41, 173–176.
adipose tissue contribute to methamphetamine-induced
West PL, McKeown NJ & Hendrickson RG (2010).
hyperthermia.
Fron Endocrinol 4, 44.
Methamphetamine body stuffers: an observational case
Schep LJ, Slaughter RJ & Beasley DM (2010). The clinical
series.
Ann Emerg Med 55, 190–197.
toxicology of metamfetamine.
Clin Toxicol 48, 675–694.
Yoshida K, Morimoto A, Makisumi T & Murakami N (1993).
Schindler CW, Zheng JW, Tella SR & Goldberg SR (1992).
Cardiovascular, thermal and behavioral sensitization to
Pharmacological mechanisms in the cardiovascular
methamphetamine in freely moving rats.
J Pharmacol
effects of methamphetamine in conscious squirrel
ExP Ther 267, 1538–1543.
monkeys.
Pharmacol Biochem Behav 42,
Zoccal DB, Bonagamba LG, Paton JF & Machado BH (2009).
791–796.
Sympathetic-mediated hypertension of awake juvenile rats
Shoblock JR, Sullivan EB, Maisonneuve IM & Glick SD (2003).
submitted to chronic intermittent hypoxia is not linked to
Neurochemical and behavioral differences between
baroreflex dysfunction.
ExP Physiol 94, 972–983.
d-methamphetamine and d-amphetamine in rats.
Zweben JE, Cohen JB, Christian D, Galloway GP, Salinardi M,
Psychopharmacology 165, 359–369.
Parent D, Iguchi M & Methamphetamine Treatment P
Silva AQ & Schreihofer AM (2011). Altered sympathetic
(2004). Psychiatric symptoms in methamphetamine users.
reflexes and vascular reactivity in rats after exposure to
Am J Addict 13, 181–190.
chronic intermittent hypoxia.
J Physiol 589, 1463–1476.
Sprague JE, Mallett NM, Rusyniak DE & Mills E (2004). UCP3
and thyroid hormone involvement in
Additional information
methamphetamine-induced hyperthermia.
Biochem
Pharmacol 68, 1339–1343.
Competing interests
Srisurapanont M, Ali R, Marsden J, Sunga A, Wada K &
None declared.
Monteiro M (2003). Psychotic symptoms in
methamphetamine psychotic in-patients.
Int J
Neuropsychopharmacol 6, 347–352.
Author contributions
Steketee JD (2003). Neurotransmitter systems of the medial
S.F.H., J.L.C. and A.K.G. designed the experiment. The
prefrontal cortex: potential role in sensitization to
experiments and analysis were conducted by S.F.H. and T.W.
psychostimulants.
Brain Res Brain Res Rev 41,
S.F.H. and A.K.G. interpreted the data. S.F.H. and A.K.G.
203–228.
wrote the manuscript. All authors critically reviewed content
Stewart J & Badiani A (1993). Tolerance and sensitization
and approved the final version for publication. All agree to be
to the behavioral effects of drugs.
Beha Pharmacol 4,
accountable for all aspects of the work. All persons designated
289–312.
Sulzer D, Sonders MS, Poulsen NW & Galli A (2005).
as authors qualify for authorship, and all those who qualify
Mechanisms of neurotransmitter release by amphetamines: a
for authorship are listed.
review.
Prog Neurobiol 75, 406–433.
Sun MK & Reis DJ (1994). Hypoxia selectively excites
Funding
vasomotor neurons of rostral ventrolateral medulla in rats.
Am J Physiol 266, R245–256.
The following grants supported the research reported
Varner KJ, Daigle K, Weed PF, Lewis PB, Mahne SE,
in this manuscript: The Hillcrest Foundation (Perpetual
Sankaranarayanan A & Winsauer PJ (2013). Comparison of
Trustees FR2013/1308, FR2014/0781), National Health and
the behavioral and cardiovascular effects of mephedrone
Medical Research Council (APP1028183, APP1030301) and
with other drugs of abuse in rats.
Psychopharmacology 225,
the Australian Research Council (DP120100920). S.F.H. was in
675–685.
receipt of an Australian Postgraduate Award.
C
2015 The Authors. The Journal of Physiology C
2015 The Physiological Society
780
S. F. Hassan and others
J Physiol 594.3
Translational perspective
In the emergency room, the behavioural and physiological consequences of methamphetamine
(METH) intoxication are commonly seen. The use of METH is increasing worldwide and the ease
with which it can be manufactured is fuelling this. Death from an overdose of METH is due to
the consequences of extreme temperatures generated in the body or from heart-related events.
METH evokes widespread physiological changes but the underlying mechanisms responsible are
largely unknown. The data presented in this study provide the first comprehensive analysis of cardio-
vascular, respiratory and metabolic function altered by METH in the anaesthetised rat. The effects
are described over a range of doses and following chronic administration. We demonstrate that
non-shivering thermogenesis due to the increased activity of brown adipose tissue, contributes to the
METH-induced increase in body temperature, potentially providing a site of action that could be
targeted to lower the body temperature of patients who abuse METH. Furthermore, the data show
that METH evokes centrally mediated increases in respiration which are independent of the metabolic
consequences of the drug, and, further demonstrate a reduced capability of the heart and respiratory
system to generate appropriate cardiorespiratory reflex functions which are normally protective. Such
studies could ultimately pave the way for therapeutic intervention in patients suffering from METH
overdose.
C
2015 The Authors. The Journal of Physiology C
2015 The Physiological Society
