Role of catecholaminergic terminals
in the preoptic area in behavioral thermoregulation in rats
RAKHI PAL, HRUDA NANDA MALLICK AND
VELAYUDHAN MOHAN KUMAR*
Department of Physiology,
All India Institute of Medical Science,
New Delhi – 110 029
*Corresponding Author: Telephone: 91-11-6594243; Fax: 91-11-6862663;
(Received on February 8, 2002)
This study was conducted to find out the role of the catecholaminergic
terminals in the preoptic area (POA) in selection of ambient temperature
in rats. The adult male Wistar rats (n =6) were allowed to choose
between three ambient temperatures (24°C, 27°C and 30°C). Rats
could move about freely from one ambient temperature to another,
in a specially designed environmental chamber having three interconnected
compartments, which were maintained at the above mentioned temperature.
The results show that the normal rats preferred to stay at 27°C
both during day and night. After the lesion of catecholaminergic
terminals in the POA with 6-hydroxydopamine (6-OHDA), the animals
preferred 24°C on the third and seventh day and 27°C on the fourteenth
and twenty first day after lesion. The alteration in thermal preference
was associated with an elevation of rectal temperature. The study
suggests that the catecholaminergic terminals of the POA play
an important role in integrating behavioural and non-behavioural
thermoregulatory responses, but in its absence the rest of the
brain takes over some of its functions.
words: behavioural thermoregulation rectal
have the capacity to regulate their body temperature by both
behavioural and non-behavioural mechanisms. The behavioural
responses include nestbuilding, altered activity, altered food
intake, choosing an appropriate ambient temperature, postural
adjustments and seeking shelter in burrows (1). The non-behavioural
responses are those involving vasoconstriction, vasodilatation,
sweating, panting, piloerection and shivering. All these thermoregulatory
capacities are part of a well-known integrated function of the
POA (2,3). Destruction of the POA electrolytically impairs physiological
thermoregulation and also the accuracy of behavioural thermoregulation
(1, 4). But, it has also been shown that even after destruction
of the POA enough thermoregulatory capacity is retained to prevent
death by hyperthermia (5). The thermoregulatory function of
the POA is most importantly modulated by the catecholaminergic
innervation of the POA. Microinjection of amines (norepinephrine,
epinephrine, preoptic) at the anterior hypothalamic preoptic
area alters the rectal temperature (6), but the role of catecholaminergic
terminals in the POA in behavioural thermoregulation is yet
to be understood.
study was undertaken to examine the thermal preference of the
rats (choosing an appropriate ambient temperature) by placing
them in an environmental chamber maintained at 24°C, 27°C and
30°C and to find out the role of the catecholaminergic terminals
of the POA in behavioural thermoregulation.
study was conducted on adult male Wistar rats weighing between
200 – 250 gms. The animals were kept in separate plastic cages
in an animal’s room having controlled room temperature (26 ± 2°C)
and lights on from 5: 00 hours to 19: 00 hours. Food and water
were provided ad libitum. Only those rats which showed
30% higher activity at night (selected with the help of a photoactometer)
and showed no chamber preference were used. The study was conducted
on six rats, each forming its own control.
thermal preference of the rats was examined by exposing the animals
individually into the specially designed environmental chamber
(designed by Dr. V. Mohan Kumar and fabricated by Inerterk India
Co., New Delhi). The chamber has three interconnected compartments
maintained at three different ambient temperatures in which the
animal can move around freely (7). Food and water are provided
ad libitum and lighting and humidity are maintained identical
in all the three compartments.
rat, at a time, was kept in the environmental chamber with the
compartments maintained at 24°C, 27°C and 30°C. The temperatures
of the chambers were changed randomly to rule out any chamber
preference by the animal. The temperature preference of the animals
was studied for five days before the lesion and four days (3rd,
7th, 14th and 21st days) after
the lesion between 12 : 00 – 14 :00 hours and 20 :00-22 :00 hours.
The rat was introduced into the chamber, about one hour before
the recording session, in order to habitualize it to the chamber.
The time spent in each chamber was recorded using a stopwatch.
temperature was recorded daily before each recording session using
a rectal probe (Becton-Dickson consumer products, China), which
was inserted three cm into the rectum. Recording was taken after
a steady temperature was reached which was indicated by a beep.
recording the thermal preference of the normal rats, the catecholaminergic
terminals of the POA were lesioned by microinjection of 6-OHDA
procured from Sigma Chemical Co. USA (8 µg/0.2 µl of saline containing
1 mg/ ml of asorbic acid) intracerebrally, at the stereotaxic
coordinates A 7.8, L 0.6, H –1.5 as per De Groot atlas (8). Recordings
for the thermal preference and rectal temperature were repeated
on the third, seventh, fourteenth and twenty first day and night
after the lesion.
the end of the experiment, the animals were sacrificed and the
fluorescence of the catecholaminergic terminals of the POA in
the lesioned rats were compared with the fluorescence in the normal
rat brain, using glyoxalic acid histofluorescence method. This
method is considered sufficient to detect catecholaminergic terminals
lesion (9). Brain sections were cut using a cryostat (model 2800
Frigocut-N), at a temperature of -18°C to -21°C to produce 25
mm thick sections and viewed under fluorescent
microscope (Leitz Laborlux version 513547 connected with the epifluorescence
condensor IVFI). In all the six rats of the experimental group,
there was extensive reduction in fluorescence in and around the
POA, indicating destruction of the catecholaminergic terminals
of this area (Fig. 1).
click for full view
duration of stay in each compartment by all the six rats, during
five prelesion days (both during the day and night), and the mean
prelesion values along with the four postlesion values, were subjected
to two-way ANOVA, Student’s test was done between prelesion mean
and postlesion values. The data of day and night rectal temperatures
were also subject to the same analysis. As prelesion values are
compared with postlesion readings, a control group of rats in
which the vehicle of the drug was injected at the POA, is not
considered essential, at this stage.
preference in normal rats:
thermal preference of the normal rats was 27°C (75.89% of the
total time recorded) on all five days of recording, and there
was no significant variation between the animals as shown by two
way ANOVA (Fig. 2). Also, there was no diurnal variation in the
temperature preferred by the rats (Fig. 3). The trend followed
by all the animals in preferring a temperature inside the chamber
was identical i.e., all animals preferred 27°C (75.89%) and spent
less time at 24°C (18.04%) and least at 30°C (4.42%) (Fig. 3).
click for full view
click for full view
in thermal preference after the destruction of catecholaminergic
On the third day, after
destruction of the catecholaminergic terminals of the POA, the
rats spent maximum time at 24°C (89.76±29.75 min), which was significantly
higher (P < 0.001) when compared to the prelesion average value
(Fig. 2). On the seventh postlesion day, the animals spent significantly
(P<0.01) more time at 24°C (186.45±60.24 min) but it was less
than the time spent by the animals on the third day (Fig. 2).
Whereas, on the fourteenth and twenty first day the animals spent
maximum time at 27°C (Fig. 2). This was higher than the time spent
on the third and seventh day but not significantly different from
the prelesion average values.
Therefore, the rats spent
the maximum time in the chamber maintained at 24°C on the third
and seventh day after lesion and on the fourteenth and twentyfirst
day postlesion they spent maximum time at 27°C. The time spent
at 30°C was minimum throughout the study.
The average rectal temperature
of the normal rats during the day (36.52 ± 0.26°C) was slightly
lower than that at night (36.75 ± 0.04°C) (Fig. 4).
click for full view
It was elevated after the
lesion of catecholaminergic terminals of the POA being as high
as 38.5 ± 0.39°C on the
third day after the lesion. The temperature elevation was highly
significant on the third and seventh day (P<0.001) and less
significant (P<0.05) on the fourteenth and twenty-first day
after the lesion (fig.4).
The study shows that the
normal rats preferred an ambient temperature of 27°C when allowed to choose between 24°C,
27°C and 30°C, whereas after the lesion of the catecholaminergic terminals
of the POA, it preferred 24°C on the third and seventh day but spent more
time at 27°C on the fourteenth and twenty first day.
This indicates that the
destruction of the catecholaminergic terminals of the POA creates
an imbalance between the catecholaminergic and other inputs which
increase the body temperature, and those which my lead to a decrease
(8). Studies using other neurotoxins such as NMDA, which destroys
the neurons of the POA, have also reported a severe hyperthermia
during the first week and a mild hyperthermia subsequently (7,11,
12). Though the increase in rectal temperature was marked during
the initial days, it was less marked during the second and third
weeks after the lesion. This decrease cannot be attributed to
a regeneration of catecholaminergic fibres, as these fibres were
not found on postmortem histology, performed after three weeks.
Thus, the hyperthermia produced due to destruction of the catecholaminergic
terminals of the POA can be explained in the light of the previous
studies (13, 14). Microinjection of adrenergic agonists into the
mPOA a freely moving rats produced hypothermia whereas antagonists
Behavioural and non-behavioural
responses complement each other in an animal, to maintain thermal
homeostasis. This integration by the close-knit interaction amongst
the different neuronal inputs is apparently affected after destruction
of catecholaminergic fibres to the POA. After the lesion, the
behavioural thermoregulation (which can bring down the body temperature)
seems to be intact even though the non-behavioural thermoregulatory
mechanism was affected, resulting in increased rectal temperature.
In other words, the imbalance created by the destruction of catecholaminergic
fibres has elevated the body temperature. But the animal recognizes
this elevate temperature as a deviation from the normal homeostasis,
and makes behavioural alterations to bring down the temperature
by selecting a lower ambient temperature. The results of the study
thus indicate that the catecholaminergic fibres of the POA do
play a role in physiological thermoregulation, though it may not
play a direct role in behavioural thermoregulation. The higher
rectal temperature (though less significant) obtained two to three
weeks after the lesion, was not accompanied by a significant shift
in the thermal preference. Though this finding may not support
the above mentioned hypothesis, larger number of observations
may be required to state anything conclusively.
It can be concluded that
after destruction of the catecholaminergic fibres of the POA,
there was dissociation between the behavioural thermoregulation
which favours the lowering of body temperature and those non-behavioural
changes which have brought the body temperature at a higher level.
The financial assistance
from life Science Research Board is acknowledged.
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