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;
Email: mohankum@medinst.ernet..in
(Received on February 8, 2002)

Abstract: 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.

Key words: behavioural thermoregulation                     rectal temperature

                     Preoptic area                                           catecholaminergic terminals

Introduction
Methods
Results
Discussion
References

INTRODUCTION

This 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.
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METHODS

The 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.

The 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.

One 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 (3^rd, 7^th, 14^th and 21^st 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.

Rectal 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.

After 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.

At 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).

Fig.1 click for full view

The 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.
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RESULTS

Thermal preference in normal rats:

The 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).

Fig.2 click for full view

Fig.3 click for full view

Changes in thermal preference after the destruction of catecholaminergic terminals.

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.

Rectal temperature

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).

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).
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DISCUSSION

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 produced hyperthermia.

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.

 

ACKNOWLEDGEMENT

The financial assistance from life Science Research Board is acknowledged.



REFERENCES 

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