Physiology & Behavior, Vol. 23, pp. 723-732. Pergamon Press and Brain Research Publ., 1979. Printed in the U.S.A.
Observations on the Thermoregulatory Effects of Preoptic Warming in Rats I H A R R Y J. C A R L I S L E A N D M A R K L. L A U D E N S L A G E R
Department of Psychology, University of California, Santa Barbara, CA 93106 Received 17 January 1979 CARLISLE, H. J. AND M. L. LAUDENSLAGER. Observations on the thermoregulatory effects of preoptic warming in rats. PHYSIOL. BEHAV. 23(4) 723--732, 1979.--Experimental warming of the preoptic-anterior hypothalamic area was used to evaluate behavioral and autonomic thermoregulatory heat-loss responses. Hypothalamic warming was associated with reduced behavioral heat intake and decreased core and peripheral temperatures in rats working for radiant heat reward in a cold environment. Baseline rates of responding for external heat were determined by ambient temperature, but the magnitude of changes in core or peripheral temperatures during preoptic warming were not. Behavioral responses compensated for variations in ambient temperature so that the threshold hypothalamic temperature above which heat loss was activated by preoptic warming was not altered by changing ambient temperatures. Heat loss during hypothalamic warming was a function of both autonomic and behavioral thermoregulatory responses because the decrease in body heat content during preoptic warming could not be accounted for by the decreased behavioral heat intake alone. The threshold hypothalamic temperature for elicitation of tail vasodilation decreased systematically as ambient temperature increased when no behavioral option was available. In the rat, both behavioral and autonomic thermoregulatory responses cooperate to determine the magnitude of heat loss which is proportional to the magnitude of preoptic warming.
Thermoregulatory behavior
Vasodilation
Preoptictemperature
THERMOREGULATORY responses are generally divided into two classes, behavioral and autonomic, both of which are substantially influenced by thermal stimulation of the preoptic/anterior hypothalamic area (PO/AH) [7,11]. Warming the PO/AH elicits both an increase in evaporative heat loss [24] and a decrease in behavioral heat intake [9,15] while cooling the PO/AH elicits an increase in metabolic rate [12] and an increase in behavioral heat intake [15,20]. It is quite clear that both response categories involve interactions between central and peripheral temperatures. The changes in metabolic rate and evaporative heat loss produced by PO/AH thermal stimulation are significantly influenced by ambient temperature, as are behavioral responses [10]. It is not entirely clear how central and peripheral temperatures interact to determine which response category is elicited. Small displacements of PO/AH temperature are far more effective in driving autonomic responses than are equivalent displacements of peripheral temperature [11]. Conversely, the importance of peripheral temperature has been emphasized for the control of thermoregulatory behavior [6, 10, 18, 19]. In many behavioral studies, unlike autonomic studies, the animal is given some control over environmental temperature and, consequently, skin temperature. Does the opportunity to control ambient temperature preclude the appearance of autonomic thermoregulatory responses when the PO/AH is warmed or cooled? Squirrel monkeys selecting between convective air temperatures of 10°C and 50°C produce new preferred temperatures during PO/AH thermal stimulation so that core and peripheral temperatures are
Skin temperature
Tail temperature
displaced inversely and proportionately to the PO/AH stimulus [1,3]. There is little reliance on autonomic responses unless behavioral control is reduced by increasing response effort [4] or by limiting the convective alternatives [2]. A similar result has been reported for the pigeon [21,23]. Radiant heat reward provides animals in a cold environment with less direct control over ambient temperature than does convective warming or cooling. Rats working for radiant heat in a cold environment appear to use both behavioral and autonomic responses to dissipate heat during PO/AH warming [9]. The present study examines this result in more detail. The influence of PO/AH temperature on thermoregulatory behavior when ambient temperature is altered, and the influence of ambient temperature on an autonomic response, vasodilation, are examined.
EXPERIMENT 1 The PO/AH was warmed to temperatures between 39°C and 43°C for 10-min periods, and the resulting changes in core and peripheral temperatures and behavior were noted and compared to changes that occur during cold exposure with no thermal reinforcement available. METHOD
Animals Twelve adult male Sprague-Dawley rats, weighing 410 to
1This research was supported by National Institute of Mental Health Grant MH-12414 and in part by NIH Grant HD-10473. Requests for reprints should be sent to: Harry J. Carlisle, Department of Psychology, University of California, Santa Barbara, CA 93106.
Copyright © 1979 Brain Research Publications Inc.--0031-9384/79/100723-10501.50/0
724 613 g during testing were maintained in individual cages in a colony room maintained at 23 _+ 2°C with a relative humidity of 50%. Testing occurred at approximately the same time during the light phase of an L D 12:12 cycle. Apparel tus The operant-conditioning apparatus consisted of a 24 cm diameter cage constructed of hardware cloth with Plexiglas rod flooring. A Plexiglas lever protruded 5 cm into the cage, 2 cm above the floor. Depression of the lever activated two 250 W red-bulb infrared heat lamps mounted at each side of the cage. The total power dissipated by the lamps was set for 300 W, which produced a radiant flux density, measured with an Eppley thermopile, of 180 mW/cm e in the center of the cage. Pressing the lever once activated the lamps for 3 sec; responses made while the lamps were on neither prolonged the 9ngoing reinforcement nor provided a subsequent reinforcement. Dim illumination was provided by a 7 W (1.6 lx) red-bulb incandescent lamp mounted outside the test cage. The cage and heat lamps were placed in a 17 ft 3 (0.48 m 3) freezer, maintained at - 7 - 2°C, unless noted otherwise. Programming and recording equipment were located in a room adjacent to the test room. Responses and reinforcements were recorded on electromechanical counters, a cumulative recorder, and a print-out counter which provided cumulative totals at 10-min intervals. Temperature Control and Measurement Each rat was implanted with bilateral preoptic area heating coils and a thermistor. The heating coils were constructed from 36-ga Teflon-insulated constantan wire (Thermoelectric Co.). The wire was closely wound at the tip of a size 00 stainless-steel insect pin, and the leads were connected to a miniature electrical connector (SMRE-4S, Winchester Electronics). The coils were connected in series to provide a total resistance of 4 fl. The coils were warmed by direct current, the magnitude of which was controlled by a variable resistance network. The temperature of the preoptic area (Thy) was sensed by a 0.013 in. (0.33 mm) diameter thermistor (VECO 32A7, Victory Engineering Co.) which was affixed to the outer surface of a heating coil. The thermistor formed one arm of a calibrated Wheatstone bridge, the output of which was continuously monitored on a recording potentiometer (Leeds and Northrup). Skin temperature (Ts) was measured on the dorsal surface of the thoracic region by a 0.1 in. (2.5 mm) diameter thermistor (VECO 32A231) sewn into a 2.5 cm wide elastic ribbon which fit snugly around the thorax. The skin thermistor was connected to a Wheatstone bridge and monitored with a second recording potentiometer (Texas Instruments). All thermistors were calibrated against a precision thermometer in a constant-temperature bath prior to use. Surgery and Histology The rats were anesthetized with sodium pentobarbital (Nembutal, 50 mg/kg, IP), treated with atropine sulfate (0.16 mg/kg, IP), and placed in a K o p f stereotaxic instrument. The heating coils were implanted 8.0-9.0 mm ventral to the level surface of the skull, between 1.0 mm rostral and 0,5 mm caudal to bregma, and centered 1.0--2.0 mm to each side of the midline. The implant assembly was affixed to the skull with dental acrylic cement and four stainless-steel screws in the cranium. Procaine penicillin-G (Crysticillin, 20,000 U,
CARLISLE AND LAUDENSLAGER IM) was given as prophylaxis. At the conclusion of testing, animals were given an overdose of Nembutal (100 mg/kg) and perfused via the aorta with 0.9% saline followed by 10% Formalin. The brain was removed and fixed in 10% Formalin; frozen sections were cut at 40/zm. All fourth and fifth sections were retained and stained, alternately, by the Well and cresyl violet methods. Procedure The rats were shaved the day prior to a test and also food deprived since this stabilized rate of responding for r',~liant heat. During the first training test, the heat lamps remained on as long as the lever was held down. When performance was stable, usually within a few hours, the rat was switched to a continuous reinforcement schedule in which a lever press was reinforced by 3 sec of radiant heat. Typically, 2 or 3 sessions of 6-hr duration were sufficient to produce stable responding for 3-sec radiant heat rewards. Surgery was then performed, and one week allowed for recuperation. Sensitivity to thermal stimulation of the PO/AH was determined during the first postoperative test, and the most responsive animals were selected for further study (see Results). At the beginning of a test session, the rat was connected to the temperature measuring equipment and placed in a circular cage on wood shavings at a neutral (23°C) ambient temperature for 30-60 rain prior to entering the cold; another 30-60 min were permitted for behavioral response rates and body temperature to stabilize after cold exposure. The PO/AH was then warmed for 10 min, and changes in response and reinforcement rates, skin temperature during hypothalamic warming, and the range in PO/AH temperature at the termination of warming were noted. The PO/AH was warmed to temperatures between 39°C and 42°C in i°C increments; warming to 43°C was used occasionally. A typical session lasted about 7 hr and consisted of an ascending series of trials in which the PO/AH was warmed from 39°C to 42°C. A recovery period of 30-50 min was interposed after each warming trial to allow response rates and body temperatures to stabilize. A 10-rain extinction period, during which time the heat lamps were disconnected, was given prior to removal from the cold. Results and Discussion Warming the PO/AH was associated with decreases in core and peripheral temperatures. The magnitude o f the fall in core temperature was determined by taking the difference between Thy prior to and at the termination of warming. The hypothalamic thermistor thus measured the effect of warming the PO/AH heating coils during a warming trial and tissue temperature at all other times. It is assumed that the change in Thy (AThy) during warming is an appropriate index of the change in core temperature. That this is the case can be seen in Experiment 3. The rats were first tested for sensitivity to warming of the PO/AH by measuring the magnitude of the fall in Thy (ATby) after a series of warming trials between 39 ° and 42°C while the animals worked for radiant heat in the cold. All rats showed some sensitivity to increases in Thy although there were large differences among animals. Six rats, which fell into a high-sensitivity group, showed reductions in Thy greater than 0.7°C for each I°C increment in Thy above 39°C. These rats were selected for further study; the lowsensitivity animals (A Thy
Observations on the Thermoregulatory Effects of Preoptic Warming in Rats I H A R R Y J. C A R L I S L E A N D M A R K L. L A U D E N S L A G E R
Department of Psychology, University of California, Santa Barbara, CA 93106 Received 17 January 1979 CARLISLE, H. J. AND M. L. LAUDENSLAGER. Observations on the thermoregulatory effects of preoptic warming in rats. PHYSIOL. BEHAV. 23(4) 723--732, 1979.--Experimental warming of the preoptic-anterior hypothalamic area was used to evaluate behavioral and autonomic thermoregulatory heat-loss responses. Hypothalamic warming was associated with reduced behavioral heat intake and decreased core and peripheral temperatures in rats working for radiant heat reward in a cold environment. Baseline rates of responding for external heat were determined by ambient temperature, but the magnitude of changes in core or peripheral temperatures during preoptic warming were not. Behavioral responses compensated for variations in ambient temperature so that the threshold hypothalamic temperature above which heat loss was activated by preoptic warming was not altered by changing ambient temperatures. Heat loss during hypothalamic warming was a function of both autonomic and behavioral thermoregulatory responses because the decrease in body heat content during preoptic warming could not be accounted for by the decreased behavioral heat intake alone. The threshold hypothalamic temperature for elicitation of tail vasodilation decreased systematically as ambient temperature increased when no behavioral option was available. In the rat, both behavioral and autonomic thermoregulatory responses cooperate to determine the magnitude of heat loss which is proportional to the magnitude of preoptic warming.
Thermoregulatory behavior
Vasodilation
Preoptictemperature
THERMOREGULATORY responses are generally divided into two classes, behavioral and autonomic, both of which are substantially influenced by thermal stimulation of the preoptic/anterior hypothalamic area (PO/AH) [7,11]. Warming the PO/AH elicits both an increase in evaporative heat loss [24] and a decrease in behavioral heat intake [9,15] while cooling the PO/AH elicits an increase in metabolic rate [12] and an increase in behavioral heat intake [15,20]. It is quite clear that both response categories involve interactions between central and peripheral temperatures. The changes in metabolic rate and evaporative heat loss produced by PO/AH thermal stimulation are significantly influenced by ambient temperature, as are behavioral responses [10]. It is not entirely clear how central and peripheral temperatures interact to determine which response category is elicited. Small displacements of PO/AH temperature are far more effective in driving autonomic responses than are equivalent displacements of peripheral temperature [11]. Conversely, the importance of peripheral temperature has been emphasized for the control of thermoregulatory behavior [6, 10, 18, 19]. In many behavioral studies, unlike autonomic studies, the animal is given some control over environmental temperature and, consequently, skin temperature. Does the opportunity to control ambient temperature preclude the appearance of autonomic thermoregulatory responses when the PO/AH is warmed or cooled? Squirrel monkeys selecting between convective air temperatures of 10°C and 50°C produce new preferred temperatures during PO/AH thermal stimulation so that core and peripheral temperatures are
Skin temperature
Tail temperature
displaced inversely and proportionately to the PO/AH stimulus [1,3]. There is little reliance on autonomic responses unless behavioral control is reduced by increasing response effort [4] or by limiting the convective alternatives [2]. A similar result has been reported for the pigeon [21,23]. Radiant heat reward provides animals in a cold environment with less direct control over ambient temperature than does convective warming or cooling. Rats working for radiant heat in a cold environment appear to use both behavioral and autonomic responses to dissipate heat during PO/AH warming [9]. The present study examines this result in more detail. The influence of PO/AH temperature on thermoregulatory behavior when ambient temperature is altered, and the influence of ambient temperature on an autonomic response, vasodilation, are examined.
EXPERIMENT 1 The PO/AH was warmed to temperatures between 39°C and 43°C for 10-min periods, and the resulting changes in core and peripheral temperatures and behavior were noted and compared to changes that occur during cold exposure with no thermal reinforcement available. METHOD
Animals Twelve adult male Sprague-Dawley rats, weighing 410 to
1This research was supported by National Institute of Mental Health Grant MH-12414 and in part by NIH Grant HD-10473. Requests for reprints should be sent to: Harry J. Carlisle, Department of Psychology, University of California, Santa Barbara, CA 93106.
Copyright © 1979 Brain Research Publications Inc.--0031-9384/79/100723-10501.50/0
724 613 g during testing were maintained in individual cages in a colony room maintained at 23 _+ 2°C with a relative humidity of 50%. Testing occurred at approximately the same time during the light phase of an L D 12:12 cycle. Apparel tus The operant-conditioning apparatus consisted of a 24 cm diameter cage constructed of hardware cloth with Plexiglas rod flooring. A Plexiglas lever protruded 5 cm into the cage, 2 cm above the floor. Depression of the lever activated two 250 W red-bulb infrared heat lamps mounted at each side of the cage. The total power dissipated by the lamps was set for 300 W, which produced a radiant flux density, measured with an Eppley thermopile, of 180 mW/cm e in the center of the cage. Pressing the lever once activated the lamps for 3 sec; responses made while the lamps were on neither prolonged the 9ngoing reinforcement nor provided a subsequent reinforcement. Dim illumination was provided by a 7 W (1.6 lx) red-bulb incandescent lamp mounted outside the test cage. The cage and heat lamps were placed in a 17 ft 3 (0.48 m 3) freezer, maintained at - 7 - 2°C, unless noted otherwise. Programming and recording equipment were located in a room adjacent to the test room. Responses and reinforcements were recorded on electromechanical counters, a cumulative recorder, and a print-out counter which provided cumulative totals at 10-min intervals. Temperature Control and Measurement Each rat was implanted with bilateral preoptic area heating coils and a thermistor. The heating coils were constructed from 36-ga Teflon-insulated constantan wire (Thermoelectric Co.). The wire was closely wound at the tip of a size 00 stainless-steel insect pin, and the leads were connected to a miniature electrical connector (SMRE-4S, Winchester Electronics). The coils were connected in series to provide a total resistance of 4 fl. The coils were warmed by direct current, the magnitude of which was controlled by a variable resistance network. The temperature of the preoptic area (Thy) was sensed by a 0.013 in. (0.33 mm) diameter thermistor (VECO 32A7, Victory Engineering Co.) which was affixed to the outer surface of a heating coil. The thermistor formed one arm of a calibrated Wheatstone bridge, the output of which was continuously monitored on a recording potentiometer (Leeds and Northrup). Skin temperature (Ts) was measured on the dorsal surface of the thoracic region by a 0.1 in. (2.5 mm) diameter thermistor (VECO 32A231) sewn into a 2.5 cm wide elastic ribbon which fit snugly around the thorax. The skin thermistor was connected to a Wheatstone bridge and monitored with a second recording potentiometer (Texas Instruments). All thermistors were calibrated against a precision thermometer in a constant-temperature bath prior to use. Surgery and Histology The rats were anesthetized with sodium pentobarbital (Nembutal, 50 mg/kg, IP), treated with atropine sulfate (0.16 mg/kg, IP), and placed in a K o p f stereotaxic instrument. The heating coils were implanted 8.0-9.0 mm ventral to the level surface of the skull, between 1.0 mm rostral and 0,5 mm caudal to bregma, and centered 1.0--2.0 mm to each side of the midline. The implant assembly was affixed to the skull with dental acrylic cement and four stainless-steel screws in the cranium. Procaine penicillin-G (Crysticillin, 20,000 U,
CARLISLE AND LAUDENSLAGER IM) was given as prophylaxis. At the conclusion of testing, animals were given an overdose of Nembutal (100 mg/kg) and perfused via the aorta with 0.9% saline followed by 10% Formalin. The brain was removed and fixed in 10% Formalin; frozen sections were cut at 40/zm. All fourth and fifth sections were retained and stained, alternately, by the Well and cresyl violet methods. Procedure The rats were shaved the day prior to a test and also food deprived since this stabilized rate of responding for r',~liant heat. During the first training test, the heat lamps remained on as long as the lever was held down. When performance was stable, usually within a few hours, the rat was switched to a continuous reinforcement schedule in which a lever press was reinforced by 3 sec of radiant heat. Typically, 2 or 3 sessions of 6-hr duration were sufficient to produce stable responding for 3-sec radiant heat rewards. Surgery was then performed, and one week allowed for recuperation. Sensitivity to thermal stimulation of the PO/AH was determined during the first postoperative test, and the most responsive animals were selected for further study (see Results). At the beginning of a test session, the rat was connected to the temperature measuring equipment and placed in a circular cage on wood shavings at a neutral (23°C) ambient temperature for 30-60 rain prior to entering the cold; another 30-60 min were permitted for behavioral response rates and body temperature to stabilize after cold exposure. The PO/AH was then warmed for 10 min, and changes in response and reinforcement rates, skin temperature during hypothalamic warming, and the range in PO/AH temperature at the termination of warming were noted. The PO/AH was warmed to temperatures between 39°C and 42°C in i°C increments; warming to 43°C was used occasionally. A typical session lasted about 7 hr and consisted of an ascending series of trials in which the PO/AH was warmed from 39°C to 42°C. A recovery period of 30-50 min was interposed after each warming trial to allow response rates and body temperatures to stabilize. A 10-rain extinction period, during which time the heat lamps were disconnected, was given prior to removal from the cold. Results and Discussion Warming the PO/AH was associated with decreases in core and peripheral temperatures. The magnitude o f the fall in core temperature was determined by taking the difference between Thy prior to and at the termination of warming. The hypothalamic thermistor thus measured the effect of warming the PO/AH heating coils during a warming trial and tissue temperature at all other times. It is assumed that the change in Thy (AThy) during warming is an appropriate index of the change in core temperature. That this is the case can be seen in Experiment 3. The rats were first tested for sensitivity to warming of the PO/AH by measuring the magnitude of the fall in Thy (ATby) after a series of warming trials between 39 ° and 42°C while the animals worked for radiant heat in the cold. All rats showed some sensitivity to increases in Thy although there were large differences among animals. Six rats, which fell into a high-sensitivity group, showed reductions in Thy greater than 0.7°C for each I°C increment in Thy above 39°C. These rats were selected for further study; the lowsensitivity animals (A Thy
