Vol. 87, Issue 1, 170-174, July 1999
Role of AVP in mediating the altered core temperature response
to a simulated open field in pregnant rats
Patricia A.
Tang,
James E.
Fewell, and
Heather L.
Eliason
Department of Physiology and Biophysics, Health Sciences Centre,
University of Calgary, Calgary, Alberta, Canada T2N 4N1
 |
ABSTRACT |
Near the term of pregnancy, rats have an attenuated core
temperature response on exposure to a novel environment (e.g., a simulated open field) compared with that observed early in pregnancy or
in nonpregnant rats. The present experiments were carried out on 26 nonpregnant and 26 pregnant rats to test the hypothesis that arginine
vasopressin, functioning as an endogenous antipyretic substance in the
central nervous system, mediates this attenuated core temperature
response. Exposure to a simulated open field after
intracerebroventricular (ICV) vehicle produced a significant increase
in core temperature in both nonpregnant and pregnant animals, the
magnitude and duration of which were greater in the nonpregnant rats.
In nonpregnant rats, exposure to a simulated open field after ICV
vasopressin V1-receptor antagonist
altered the pattern of the core temperature response but not the core temperature index compared with that observed on exposure to a simulated open field after ICV vehicle. In pregnant animals, ICV vasopressin V1-receptor antagonist
did not alter the core temperature response to a simulated open field
compared with that observed after ICV vehicle. Thus our data do not
support the hypothesis that a pregnancy-related activation of arginine
vasopressin attenuates the core temperature response to a simulated
open field in rats near the term of pregnancy.
arginine vasopressin; endogenous antipyretic; fever; stress-induced
hyperthermia
| |
INTRODUCTION |
EXPOSURE OF A RAT to a novel environment induces a
transient increase in core temperature of ~1.5°C (3, 5). This
response is often called stress-induced hyperthermia. Several
laboratories have provided evidence that stress-induced hyperthermia
results from a regulated thermoregulatory response and shares some
common mechanisms with fever induced by bacterial pyrogens (3, 28). Considering this and the fact that pregnancy alters the
thermoregulatory responses to pyrogens such as bacterial endotoxin
(20), interleukin-1
(27) and
PGE1 (11, 29) in rats, we
previously carried out experiments to test the hypothesis that
pregnancy would alter the core temperature response to a novel
environment (i.e., a simulated open field) in rats. We
found that the core temperature index increased significantly after
exposure to a simulated open field in nonpregnant rats and
day 10 gestation rats but not in day 15 and day
20 gestation rats (13). The mechanism of this altered
core temperature response near term of pregnancy is unknown.
Recent experiments carried out in our laboratory have provided evidence
that arginine vasopressin, which functions as an endogenous antipyretic
substance in the central nervous system (16) and is elevated in a
number of hypothalamic nuclei in rats near term of pregnancy (6, 17),
attenuates the febrile response to intracerebroventricular (ICV)
administration of PGE1 near the term of pregnancy in rats (12). In these experiments, ICV injection of
PGE1 after an ICV injection of a
vasopressin V1-receptor antagonist produced a significant increase in core temperature that was similar in
magnitude and duration in both nonpregnant and pregnant animals. Considering this as well as the evidence that stress-induced
hyperthermia shares some common mechanisms with fever induced by
bacterial pyrogens (3, 28), our present experiments have been carried out to determine whether ICV administration of a vasopressin
V1 antagonist would alter the core
temperature response to a novel environment in rats near the term of pregnancy.
| |
METHODS |
Experiments were carried out on 26 nonpregnant and 26 pregnant female
Sprague-Dawley rats (8-11 wk of age) undergoing their first
pregnancy (Charles River Breeding Laboratories). The rats were housed
individually in Plexiglas cages at 22 ± 1°C in a 12:12-h light-dark cycle, with lights on from 0700 to 1900. All animals had
continuous access to Rodent Diet (PROLAB 2500, Purina Feeds) and tap
water. To familiarize the animal with the investigator, all rats were
handled three to four times before an experiment. The procedure
consisted of picking the animal up, returning the animal to its cage,
and then draping a towel over the rat for 2-3 min (i.e., the time
required to give an ICV injection as detailed in
Experimental protocol).
Surgical preparation.
No less than 5 days before an experiment, each rat was anesthetized by
an intraperitoneal injection of pentobarbital sodium (50 mg/kg). A
paramedian laparotomy was done, and a free-floating battery-operated
biotelemetry device (VM-FH; Mini-Mitter) was inserted into the
peritoneal cavity for later measurement of core temperature. The
abdominal musculature and the skin were sutured to close the wound. A
topical antibiotic was applied to the wound before the skin was closed
(Gentocin, gentamicin sulfate veterinary grade, Schering Canada)
The animal's head was then placed in a stereotaxic frame, and the
skull was exposed by means of a midline scalp incision. A stainless
steel guide cannula (1.5-cm-long, 20-gauge thin-walled tubing, Small
Parts) was placed 1 mm above the left ventricle by using the
coordinates anteroposterior 
0.6 mm, lateral 2.0 mm in relation
to the bregma, and 2.0 mm below the surface of the brain (25).
Jeweler's screws and dental acrylic were used to fix the guide cannula
to the skull. The skin was then sutured to close the incision. A
25-gauge stainless steel stylet was placed into the guide cannula until
experiments were begun.
All surgical and experimental procedures were carried out in accordance
with the Guide to the Care and Use of Experimental Animals provided by the Canadian Council on Animal Care
and with the approval of the Animal Care Committee of the University of Calgary.
Conditions of observations.
Our laboratory contains two environmental chambers: a home
environmental chamber in which the animals are housed on a day-to-day basis and an experimental environmental chamber that houses a simulated
open field. The simulated open field used in our experiments consisted
of a 30-in. (wide) × 60-in. (long) × 24-in. (high) white acrylic finish box that is illuminated by two hanging fluorescent lights.
The nonpregnant and pregnant rats were randomly allocated to four
experimental groups on the basis of the combination of injectate (vehicle or vasopressin
V1-receptor antagonist) and
experimental manipulation (home cage or simulated open field). Pregnant
animals were studied on day 19,
20, or
21 of gestation (term ~22 days). All
experiments were carried out between 0800 and 1200 to avoid possible
circadian effects on the measured variables, and each animal was
studied only once.
For a home-cage experiment, each rat was left in its cage in the home
environmental chamber for the 3-h experimental period. For an
open-field experiment, each rat was carried in its cage from the home
environmental chamber to the experimental environmental chamber. The
cage was then placed on the floor, and the rat was picked up and placed
in the center of the simulated open field.
For measurement of core temperature, both the cage in the home
environmental chamber as well as the simulated open field in the
experimental environmental chamber were placed on platform antennae
(RLA1020 Receiver, Data Sciences International) that received the
output frequency (Hz) from the biotelemetry device. The received output
was then fed into a peripheral processor connected to an IBM computer
for determination of core temperature (DataQuest III, Data Sciences International).
Experimental protocol.
Core temperature was measured at 2-min intervals during an initial
10-min control period. A suitable control period was defined as one in
which five consecutive measurements of core temperature did not vary by
>0.1°C; these five values were averaged to obtain the reported
control value for core temperature. Each animal was then given an ICV
injection of a vasopressin
V1-receptor antagonist in 10.0 µl artificial cerebrospinal fluid (aCSF) or an ICV injection of an
equal volume of vehicle (aCSF). The injection procedure consisted of
draping a towel over the rat, removing the stylet, inserting the
25-gauge injection cannula into the guide cannula, and allowing the
solution to flow via gravity into the lateral ventricle over a period
of 30 s. This was followed by either a home-cage or an open-field
maneuver, during which core temperature was measured at 10-min
intervals for 3 h.
After an experiment, the rat was again anesthetized with pentobarbital
sodium. The injection cannula was reinserted into the guide cannula,
and 10 µl of black ink were injected into the ventricle via gravity
flow. The chest was then opened, and the vascular system was perfused
through the heart with saline, followed by 10% buffered Formalin to
fix the brain tissue. The brain was then removed and sectioned. The
presence of ink in the cerebroventricular system verified the correct
placement of the injection cannula.
Vasopressin V1-receptor antagonist.
A selective vasopressin
V1-receptor antagonist
(Pmp1,
O-Me-Tyr2-[Arg8]vasopressin)
was purchased as powder from Peninsula Laboratories. The
powder was dissolved in aCSF [128 mM
Na+, 2.5 mM
K+, 1.3 mM
Ca2+, 1.0 mM
Mg2+, 135 mM
Cl
(17)] to make a
working solution of 0.2 nmol/µl. This solution was divided into
0.25-ml aliquots and stored in sterile plastic vials at
70°C. At the time of injection, the desired solution was
removed from the freezer, and the injection cannula was filled with the
appropriate volume of V1-receptor
antagonist and vehicle to make a total injected volume of 10 µl. A
dose of 1.0 nmol vasopressin V1-receptor antagonist was
selected because we have previously shown that this dose restores the
febrile response to an ICV injection of
PGE1 in near-term pregnant rats
(12). Vehicle was aCSF.
Statistical analysis.
Statistical analysis was carried out by using a four-factor ANOVA for
repeated measures followed by Newman-Keul's multiple-comparison test
to determine whether time, experiment (home cage or open field),
gestation (nonpregnant or pregnant), or injectate (vehicle or
vasopressin V1-receptor
antagonist) affected core temperature (32). In addition, a three-factor
ANOVA followed by Newman-Keul's multiple-comparison test was used to
determine whether experiment, gestation, or injectate affected the core
temperature index, expressed as area under the core temperature time curve (in °C × h) (9). All results are presented as
means ± SD; P < 0.05 was considered to be of statistical significance.
| |
RESULTS |
Exposure to a simulated open field produced significant increases in
core temperature in nonpregnant and pregnant animals after ICV
administration of aCSF (Figs. 1 and
2). The core temperature response, however,
was greater in the nonpregnant than in the pregnant animals as
evidenced by the magnitude and duration of the core temperature
responses as well as the core temperature indexes (Fig.
3).
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Fig. 1.
Core temperatures before (C) and after intracerebroventricular
administration of vehicle [artificial cerebrospinal fluid
(aCSF)] or vasopressin
V1-receptor antagonist
(V1) followed by either a
home-cage or open-field maneuver in nonpregnant rats.
A: home cage-aCSF
(n = 5).
B: home
cage-V1
(n = 6).
C: open field-aCSF
(n = 8).
D: open
field-V1
(n = 7). Values are means ± SD.
* P < 0.05 vs. C.
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Fig. 2.
Core temperatures before (C) and after intracerebroventricular
administration of vehicle or vasopressin
V1-receptor antagonist followed by
either a home-cage or open-field maneuver in pregnant rats.
A: home cage-aCSF
(n = 5).
B: home
cage-V1
(n = 5).
C: open field-aCSF
(n = 8).
D: open
field-V1
(n = 8). Values are as means ± SD.
* P < 0.05 vs. C.
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Fig. 3.
Core temperature indexes in nonpregnant
(A) and pregnant
(B) rats after
intracerebroventricular administration of vehicle or vasopressin
V1-receptor antagonist followed by
either a home-cage or open-field maneuver. Values are means ± SD.
# P < 0.05 vs. home cage in nonpregnant rats.
$ P < 0.05 vs. open field in nonpregnant rats.
|
|
In nonpregnant animals, ICV administration of a vasopressin
V1-receptor antagonist altered the
pattern of the core temperature response to both home-cage and
open-field maneuvers compared with that observed after ICV
administration of aCSF; however, it did not significantly affect
overall heat gain as indicated by the core temperature indexes. In
pregnant animals, ICV administration of a vasopressin
V1-receptor antagonist did not
significantly affect the core temperature responses to either home-cage
or open-field maneuvers compared with that observed after the ICV
administration of aCSF.
| |
DISCUSSION |
Our experiments provide new information about pregnancy and
stress-induced hyperthermia in rats. Novel findings were the following. 1) Exposure to a simulated
open-field after ICV injection of vehicle produced a significant
increase in core temperature in both nonpregnant and pregnant animals.
The magnitude and duration of this increase, however, were
significantly greater in nonpregnant compared with pregnant rats.
2) In nonpregnant animals, exposure
to a simulated open field after an ICV injection of a vasopressin
V1-receptor antagonist produced an
increase in core temperature that was shorter in duration than that
observed after an ICV injection of vehicle. Overall heat gain, however,
was not significantly different as evidenced by similar core
temperature indexes. 3) In pregnant animals, ICV injection of a vasopressin
V1-receptor antagonist did not
alter the core temperature response to a simulated open field compared
with that observed after ICV injection of vehicle. Thus our data do not
support the hypothesis that a pregnancy-related activation of arginine
vasopressin as an endogenous antipyretic substance in the central
nervous system attenuates the core temperature response to a simulated
open field near the term of pregnancy in rats.
Exposure of a rat to a novel stimulus, whether it be restraint,
handling, a loud noise, or a new environment, causes a rise in core
temperature (for recent review, see Ref. 22). The increase in core
temperature that occurs after exposure to a novel environment, which is
often called "stress-induced hyperthermia," is thought to result
from a regulated thermoregulatory response because it occurs when the
animals are studied in a cold environment as well as when they are
studied in a warm environment (2, 4, 19), and it is accompanied by
activation of heat-producing (26) and heat-conserving mechanisms (3,
4). Although the mechanisms that initiate stress-induced hyperthermia
are not clear, prostaglandins (3, 28) and endogenous opioids (1, 24)
appear to play important roles in mediating the core temperature
response, and glucocorticoids appear to play an important role in
modulating (21, 23) the core temperature response. Circulating
interleukin-1 (IL-1) does not appear to be involved in mediating
stress- induced hyperthermia because Long et al. (18) and Watkins et
al. (31), respectively, have shown that neither an intraperitoneal
injection of antiserum against IL-1
nor a subcutaneous injection of
recombinant human IL-1-receptor antagonist alters the core
temperature response of rats after exposure to a novel environment.
Furthermore, Hunter (15) has shown that ablation of the organum
vasculosum laminae terminalis does not alter the core temperature
response to a novel stimulus in rats. Interestingly, Watkins et al.
(31) have recently shown that subdiaphragmatic vagotomy blocks stress
induced hyperthermia in rats after their exposure to a novel environment.
Terlouw et al. (30) have recently shown that ICV administration of
arginine vasopressin attenuates the core temperature response to
restraint. The core temperature response to restraint, however, was not
significantly altered by prior ICV administration of a vasopressin
V1-receptor antagonist, suggesting
that, under normal physiological conditions, arginine vasopressin does
not play a role in limiting the core temperature response to restraint in male rats. Our present experiments provide additional information that arginine vasopressin, which is elevated in a number of
hypothalamic nuclei (6, 17), does not play a significant role in
attenuating the core temperature response to a novel environment in
rats near the term of pregnancy.
There are other possibilities for the mechanism of the attenuated core
temperature response to a novel environment that is observed in rats
near the term of pregnancy. For example, corticosterone, which appears
to modulate stress-induced hyperthermia after exposure to a novel
environment (21, 23), is elevated from day
18 of gestation through to parturition in the rat (10).
Glucocorticoids (e.g., corticosterone) are antipyretic (7, 8) and are
known to stimulate the production of lipocortin-1, a calcium-dependent phospholipid-binding protein that inhibits phospholipase
A2, a key enzyme involved in the
synthesis of prostaglandins (14). Thus it is possible that
corticosterone mediates the attenuated thermoregulatory response via a
mechanism that is upstream to the synthesis and release of
prostaglandins. This would reconcile the apparent quandary raised by
our present results and our previous findings that arginine vasopressin
mediates the attenuated febrile response to ICV injection of
PGE1 in rats near the term of
pregnancy (12). This possible mechanism require further investigation.
| |
ACKNOWLEDGEMENTS |
This study was supported by the Medical Research Council of Canada.
This work was done during J. E. Fewell's tenure as a Senior Medical
Scholar of the Alberta Heritage Foundation for Medical Research. P. A. Tang was supported by a Summer Research Studentship from the Alberta
Heritage Foundation for Medical Research.
| |
FOOTNOTES |
The costs of publication of this
article were defrayed in part by the
payment of page charges. The article
must therefore be hereby marked
"advertisement"
in accordance with 18 U.S.C. §1734 solely to indicate this fact.
Address for reprint requests and other correspondence: J. E. Fewell,
Heritage Medical Research Bldg., Univ. of Calgary, 3330 Hospital Dr.,
N.W., Calgary, AB, Canada T2N 4N1 (E-mail:
fewell{at}acs.ucalgary.ca).
Received 17 December 1998; accepted in final form 15 March 1999.
| |
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ABSTRACT
INTRODUCTION
