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Original Article
Volume 46 - No.4:January 2002 index
 
Indian J Physiol Pharmacol 2002;46 (4);


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 (24C, 27C and 30C). 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 27C both during day and night. After the lesion of catecholaminergic terminals in the POA with 6-hydroxydopamine (6-OHDA), the animals preferred 24C on the third and seventh day and 27C 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

Animals 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.

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 24C, 27C and 30C 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 animals room having controlled room temperature (26 2C) 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 24C, 27C and 30C. 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.

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 g/0.2 l 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 -18C to -21C 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

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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, Students 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 27C (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 27C (75.89%) and spent less time at 24C (18.04%) and least at 30C (4.42%) (Fig. 3).

Fig.2

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Fig.3

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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 24C (89.7629.75 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 24C (186.4560.24 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 27C (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 24C on the third and seventh day after lesion and on the fourteenth and twentyfirst day postlesion they spent maximum time at 27C. The time spent at 30C was minimum throughout the study.

Rectal temperature

The average rectal temperature of the normal rats during the day (36.52 0.26C) was slightly lower than that at night (36.75 0.04C) (Fig. 4).

Fig.4

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It was elevated after the lesion of catecholaminergic terminals of the POA being as high as 38.5 0.39C 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 27C when allowed to choose between 24C, 27C and 30C, whereas after the lesion of the catecholaminergic terminals of the POA, it preferred 24C on the third and seventh day but spent more time at 27C 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 

1.      Satinoff E, Rutstein J. Behavioural thermoregulation in rats with anterior hypothalamic lesions. J Comp Physiol Psychol 1970; 71: 77-82.

2.      Teague RS, Ranson SW. The role of anterior hypothalamus in temperature regulation. Am J Physiol 1936; 117: 562-570.

3.      Gordon JH. Relationship between preferred ambient temperature and autonomic thermoregulatory functions in rat. Am J Physiol 1987; 252: R1130-R1137.

4.      Carlisle JH. Effect of preoptic and anterior hypothalamic lesions on behavioural thermoregulation in cold. J Comp Physiol Psychol 1969; 69: 391-402.

5.      Lipton JM. Effects of preoptic lesions on heat escape responding and colonic temperature in rat. Physiol Behav 1963; 3: 165-169.

6.      Feldberg W, Myers RD. Changes in temperature produced by microinjections of amines into the anterior hypothalamus of cats. J Physiol 1965: 177: 239-245.

7.      Ray B, Mallick H, Kumar VM. Role of the medial preoptic area in thermal preference of rats. Indian J Physiol Pharmaol 2001; 45: 445-450.

8.      De Groot J. The rat forebrain in stereotaxic coordinates. Trans R Neth Acad Sci 1959; 52: 1-40.

9.      Ramesh V and Kumar VM. Changes in sleep-wakefulness after 6-hydroxydopamine lesion of the preoptic area. Neuroscience 200; 98: 549-553.

10.  Feldberg W, Myers RD. Effects on temperature of amines injected into the cerebral ventricles. A new concept of temperature regulation. J Physiol (Lond) 1964; 173: 226-237.

11.  Kumar VM, Khan NA. Role of the preoptic neurons in thermoregulation in rats. Arch Clin Exp Med 1998;l 7: 24-27.

12.  Ray B, Mallick H, Kumar VM. Role of preoptic area in terminal preference of rats. Sleep research Online 1999; (2(suppl 1): 619.

13.  Datta S, Mohan Kumar V, Chhina GS, Singh B. Tonic activity of medial preoptic norepinephrine mechanism for body temperature maintenance in sleeping and awake rats. Brain Res Bull 1985; 15: 447-451.

14.  Ramesh V, Kumar VM. The role of alpha-2 receptors in the medial preoptic area in the regulation of sleep-wakefulness and body temperature. Neuroscience 1998; 85: 807-817.
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