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Behavior of rats in a thermocline during stress hyperthermia
J. therm. Biol. Vol. 18, No. 1, pp. I-6, 1993
0306-4565/93 $6.00+0.00 Copyright © 1993 Pergamon Press Ltd
Printed in Great Britain. All fights reserved
BEHAVIOR OF RATS IN A THERMOCLINE DURING STRESS HYPERTHERMIA E. BRIESE* Universidad de Los Andes, M6rida, Venezuela (Received 28 September 1991; accepted in revised form 21 November 1992)
Abstract--1. During stress hypertbermia, rats placed in a thermocline went from an ambient temperature of about 25°C to a cooler one. 2. There was an inverse relation between the central temperature variations and selected ambient temperature. 3. This suggests that the central temperature rise induced by handling and other mild stress circumstances might not be due to an upward shift of set-point temperature, as is currently believed, and that current theories may need to be revised. Key Word Index: Emotional hyperthermia; fever; behavioral temperature regulation; spreading saliva; grooming; heat loss; colonic temperature; psychological stress; handling
INTRODUCTION
Handling, environmental changes, and other mild stress conditions induce a rise of central temperature (To) of rats (Briese and de Quijada, 1970) and mice (Briese et al., 1991; Cabanac and Briese, 1992). This rise in T~, which was also described in humans, was independently called emotional hyperthermia or stress-hyperthermia. However, it is currently believed that emotional hyperthermia is due to an upward shift of the set-point temperature and has been qualified as a "true fever" (Singer et al., 1986). To arrive at this conclusion, pharmacological and physiological methods have been used. The pharmacological approach consists in reducing the emotional rise of Tc by salicylate and indomethacin under the assumption that these drugs lower the T~ only in fever but have no effect on the temperature of afebrile subjects (Rosendorff and Cranston, 1968; Rawlins et al., 1971). Then, it was reasoned, if emotional rise of Tc is diminished by antipyretic drugs it has to be a fever (Briese and Cabanac, 1980; Singer et al., 1986; Kluger et al., 1987). The physiological approach was used to investigate, during the emotional rise of To skin vasomotricity, the possible effects of ambient temperature and the effect of circadian variations of Tc (Long et al., 1990; Briese and Cabanae, 1991). No studies exist on behavioral thermoregulation during the emotional rise of T~. However, the technique of thermal preference in *Address for correspondence: Apartado 109, M6rida 5101-A, Venezuela.
humans and in dogs and mice is a powerful investigative tool (Cabanac, 1979; Satinoff, 1979). "Thermal preference is a measure of setpoint" as has been concisely stated by Satinoff (1979; p.171). Consequently, the purpose of the present study was to investigate the thermal preference of rats during stress hyperthermia.
METHODS
The experiments were done in a temperatureregulated chamber (Forma Scientific Inc.) with adult male rats of Wistar origin. The rats had food and water ad libitum. Lights were on at 06:00 h and off at 18:00 h. The animals were housed for days or weeks at a time in a thermocline (Briese, 1986). In brief, the thermocline was a plywood box 61 x 26 x 40 cm with three intercommunicable compartments. The ambient temperatures (T~) in the three compartments were maintained at about 17, 25 and 30°C by forced convection through heat exchangers. However, since the compartments intercommunicated, the animals in the thermocline could select a Ta between the extremes 17 and 30°C. Temperature measurements
The intracranial temperature To, and T, were recorded simultanously using small thermistors as sensors. The thermistors were fixed into 7ram stainless steel 23 gauge tubes closed at the tip by welding. The leads from the thermistor were soldered to Amphenol male connectors and the tube fastened
2
E. BRIESE
to the two connectors with epoxy glue. The stainless steel tube with the thermistor was implanted into the thalamus to record the temperature near the center of cranial cavity. The implant was secured to the skull with stainless steel screws and acrylic cement. The wires from the Amphenol connectors were threaded through a protecting steel spring and connected to a rotatory mercury commutator. For Ta measurements, a second thermistor was fastened on the protecting spring about 4 5 ~ 5 mm above the head of the animal and the two wires from the thermistor were threaded through the same protecting spring to the four-channel rotatory commutator. From the commutator the four wires were connected to two Wheatstone bridges and from there to a two-channel potentiometric recorder.
depth into the colon to measure the colonic temperature. The probe temperature stabilized in about 1 min, after which the rat was placed in the middle compartment of the thermocline where the Ta was about 25°C, the same as in the climatic chamber where the experiments were performed. This procedure was repeated several times every 5 or 6 min until the emotional rise of T¢ was maximal and Tc began to descend, which occurred after about 25 min.
Validation of the thermocline method The rationale for the tests to validate the thermocline method was that if the Tc of the animal was altered by physical means, and assuming that the set-point level did not change, the animal should prefer a Ta inverse to the T¢ change. If Tc temperature is made to rise (induced hyperthermia) the animal should select a cool environment and if Tc is made to fall (induced hypothermia) then the animal should move to a warmer environment. Hypotherrnia was produced by putting the animal in water of 8-1 I°C for 2 min (Spencer et al., 1990). The rat was dried off before being returned to the thermocline. Hyperthermia was produced by placing the rat in a double wall tin-plate box 24 x 12.5 x 27 cm. The box was
Experimental procedure and emotional stress paradigm Selected Ta and Tc of the animals housed in the thermocline were continuously recorded. The experiments were done in the morning. A stress situation was created by taking the animal out of the thermocline, without disconnecting the head leads, maintaining the animal on the operator's knees while inserting a lubricated thermocouple probe at 70 mm 30 28 26 24 22 20
39-
38
36
I,llll,, -90
-60
tt,tt,t,, 0
12
I ' ' 36
60
J 90
'
' i20
Time (rain)
Fig. I. Intracranial temperature and the selected ambient temperature recordings from a rat in a thermocline. Arrows indicate moments when the animal was taken out of the thermocline and its colonic temperature measured with a thermocouple probe. Speed of the recording paper was 5 cm/h during the experiment and 2.5 cm/h before it. The selected ambient temperature began to de~end from the beginning of the intracraniai temperature rise. When the intracruial temperature reached a plateau and during the descending phase the selected ambient temperature oscillated between 21 and 19°C.
Rat behavior during stress hyperthermia heated with water at 45°C poured between the two walls. T~ was monitored while the rat was in the hot environment and the rat was taken out and placed again in the middle compartment of the thermocline when T~ reached about 40°C.
3
y=-8.1228e-2 -2.1479x R2=0.849 L,
o
Analysis of the results The experiments were performed on six rats. There were 39 experiments of handling-induced rise of T~, four experiments of induced hypothermia and five experiments of induced hyperthermia. Analog data were digitized by reading from the recordings T~ and Ta values every 6 rain during the first 60 rain of each experiment. The data were than normalized by subtracting the initial values of each experiment from the following values. In this way, raw data were transformed in data representing changes of temperatures in °C, with zero taken as the initial temperature. Means and standard errors were calculated. The relationship between changes of T~ and changes of T~ was assessed by linear regression analysis. RESULTS
Selected ambient temperature during emotional rise
of T"~ During the emotional rise of To, the rats went, in most of the experiments, from the middle compartment of the thermocline, where they were placed after each colonic temperature measurement, to a cooler T¢. Figure 1 illustrates the result of a typical 0 o
=
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e~ t~ l=
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"~ ~0.5 g=
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I 6
I 12
I I J I I I I 18 24 30 36 42 48 54 Time (min)
J 60
Fig. 2. The upper curve represents mean changes in selected ambient temperature and the bottom curve mean changes in intracranial temperature produced by handling in 39 experiments. Each point represents a mean change in temperature relative to the initial temperature. Vertical bars are standard error of the means. Note that minimum selected ambient temperature is 12rain behind the maximum intracranial temperature rise.
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o~
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e~ O e~
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1.0 1.2 Change in intracranial temperature (°C) 0.4
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0.8
Fig. 3. Correlation between mean changes in intracranial temperature and mean changes in selected ambient temperature. For this analysis the delay between intracranial and ambient temperature data was suppressed by shifting the ambient temperature data 12 rain forward. Regression line equation was: y = - 0 . 0 8 1 - 2.14x, correlation coefficient r = - 0.921. experiment. The rat went gradually from about 25°C in the middle compartment of the thermocline to cooler ambient temperatures. When Tc elevation reached a plateau and during the descending phase of Tc course the selected T~ oscillated between about 21 and 19°C. Towards the end of the experiment, after about 78 min, the rat again selected a warmer T~ Figure 2 presents the mean of 39 experiments and shows the evolution in time of the rats' behavior and their body temperature. It is evident that while T¢ rose the selected ambient temperature descended and vice-versa. In addition, Fig. 2 shows that there was a 12 min gap between the lowest selected T~ and the highest rise in Tc, that is, the behavioral response occurs 12 rain after the T¢ change. The change of sign of the slope of the curve of selected T~ took place at 36 rain. For the Tc curve it took place at 24 rain. When allowance was made for the 12 min gap by shifting the Ta changes to the left the correlation between T~ changes and selected T~ was almost perfect (r = 0.913) as shown in Fig. 3.
Validation of the thermocline method Selected To during induced hyperthermia. There were five experiments of this type in five different rats. Figure 4 shows the recording of an induced hyperthermia experiment. The animal was placed in a hot box at 44°C at time 0 and T~ rose to 40°C. At the time indicated by the arrow the animal was placed in the middle compartment of the thermocline and immediately went to the coolest place. The rat
4
E. BRIESE
stayed in this cool environment until the Tc descended to 37°C. Selected To during induced hypothermia. This experiment was performed with similar results five times in four rats. Figure 5 gives an example. The animal was put in water at 11 °C, for 2 min. The T~ recorded during this time was that of the air above the head of the animal (25-27°C) and, therefore, much higher than the bath temperature. During immersion in the cold hath T~ fell from 37 to about 34.5°C and continued to fall to 34°C after the rat was replaced in the thermocline in the middle compartment at 25°C. Immediately after that the rat went to a cooler T~ (21-19°C). T~ rose gradually to reach the initial level (37°C) in about 96 min. At the same time the selected Ta rose also gradually to 30°C and started to descend when T~ was higher than 37°C.
found previously. However, the results did not confirm this hypothesis. In no instance, during the emotional rise of Tc, did the rats prefer an environment warmer than 25°C. On the contrary, the animals went to a cooler environment in all 39 experiments but two, during which the animals stayed in the middle compartment of the thermocline where they were placed. The lowest selected Ta was 12 min after the highest Tc rise. An almost perfect inverse relationship was found between changes of Tc and the changes of selected Ta set forward by 12 min. This suggests that the delay between the Tc variation and behavioral responses is built-in into the system and that the behavior could be a consequence of Tc changes. These results seem to indicate that the rise of T¢ related to psychological stress is opposed by the behavioral responses and might not be a "fever" as though previously (Briese and Cabanac, 1980, 1991; Singer et al., 1986; Kluger et al., 1987; Long et al., 1990). This is rather a disturbing conclusion because autonomic and behavioral responses are usually regarded as cooperating with or complementary to each other (Satinoff and Henderson, 1977), or, on occasion, as independent (Satinoff, 1980) but not opposing each other.
D~CUSSION This is the first study of behavioral thermoregulatory responses as they apply to the nature of stress induced rise in To. My intention was to look for further evidence that emotional rise in Tc in rats is due to an upward shift of set point level as
33
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38 37 36 HB I J=l -6O
II 0
T I
I 24
I ; I 36 8 60
I
°
I
'
,~ I 0
Time (rain)
Fig. 4. Intracraniai temperature and ambient temperature recordings during an induced hyperthermia experiment. At time zero the rat was placed in a hot box (HB) until the intracranial temperature reached 40°C; then it was replaced in the thermocline where it went immediately to the lowest temperature of the thermocline.
Rat behavior during stress hyperthermia
29
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21 19
~
34 CB T I
i
i
i
0
I
I
60
i
i
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12 24 3 6 48 120 Time (rain)
Fig. 5. Intracranial temperature and preferred ambient temperature during an induced hypothermia experiment. At the moment indicated by CB (cold bath) the rat was taken out of the thermocline and put in water at 11°C for 2 min. During the cold bath the ambient temperature recorded was that of the air above the head of the animal. Then at time zero the rat was placed in the middle compartment of the thermoeline at about 25°C. During the first 12 rain the animal went from there to a lower ambient temperature (about 20°C). Only after 48 rain when the intracranial temperature was already above 36°C did the rat go to ambient temperatures higher than 25°C. Still warmer ambient temperatures of 29-30°C were selected only 36 min before the intracranial temperature reached 37°C.
However, the idea that an emotional rise of Tc might be a hyperthermia, as suggested by the results presented here, might be supported by some arguments. F o r instance, the effect of antipyretic drugs in lowering emotional Tc rise is only indirect evidence and is not irrefutable since salicylate also lowers normal Tc (Satinoff, 1972; Green and Lomax, 1973) and does not affect the magnitude of the emotional T~ rise in mice (Briese et al., 1991; Cabanac and Briese, 1992). On the other hand, in an earlier study (Briese and Cabanac, 1991) vasomotricity was measured as changes of skin temperature and skin temperature depends on T,. The initial vasoconstriction was manifest only in a warm T, and was followed by a lengthy and strong vasodilation; in addition, vasoconstriction might be due to factors other than thermoregulation, such as those associated with stress per se, for instance, catecholamines secretion (Bfihler et aL, 1978). Furthermore, as shown in a recent work (Briese, 1992), cold increases and warm diminishes stress induced rise of Tc which again suggests that it is not a regulated rise. Gordon et aL (1991) have shown that naive rats placed in a thermal gradient prefer a cooler temperature for the first couple of hours and then slowly move to warmer temperatures. Body temperature was not measured in that study but it can be assumed that by placing the animals
in a novel environment a stress-hyperthermia was induced. In another study (Spencer et al., 1990), control, vehicle injected rats, placed in a thermocline, selected a cooler ambient temperature relative to the pre-injection level for the first 10-15 rain after injection. In the same rats there was a small hyperthermia response after vehicle injection. Lipopolysaccharide (which is supposed to produce a fever) increases ambient temperature preference of mice (Akins et al., 1991) while the stressed rats in the experiments described sought a cool environment. An alternative interpretation of the results obtained in the present work would be to challenge the postulate that "thermal preference is a measure of setpoint" (Satinoff, 1979). However, this would imply the collapse or a profound revision of the whole thermoregulatory theory. Two incidental observations in the present work deserve further comments. First, I observed but not measured, that, in numerous instances grooming, spreading saliva and extension-relaxation were preferred by the rat before seeking a cooler environment. This goes against the belief that behavioral thermoregulation, because phylogenetically older and energetically more economical, takes precedence over reflex or autonomic responses (Cabanac, 1979; Satinoff, 1980). The question arises whether or not
6
E. BRIESE
heat loss responses as grooming with spreading saliva and extension-relaxation (Roberts, 1988) should be considered autonomic or behavioral responses. The second incidental observation deserving comment was that induced hyperthermia and induced hypothermia elicited different patterns of behavior. Physically induced hyperthermia was instantly counterchecked by the animal seeking the coldest available Ta when placed in the thermocline. On the other hand, after induced hypothermia the rats placed in the thermocline did not go to the warmer environment at once but gradually. In the example given in Fig. 5, intracranial temperature rose from 34 to 36°C in 36 min while the rat selected, first, a mean Ta of 20°C and later a Ta of about 23-24°C. The rat went to about 29-30°C only after 72 min when its central temperature was already near 37°C. Thus, most of the rewarming should have been due to autonomic responses. This shows again that behavioral adjusting thermal responses are not always preferred to autonomic counteracting responses. I am tempted to speculate that behavior responses are used in preference to autonomic responses when the thermal situation requires an urgent defense of homeostasis, and autonomic or reflex responses possibly come first when thermal disturbance is not critical, overheating being more dangerous than cooling. According to this view behavioral responses would be preferred to autonomic ones because they might be more rapid in correcting the homeostatic disturbance when this was dangerous to life. In conclusion, during the emotional rise of body temperature the rats preferred a cooler Ta. An almost perfect inverse relationship was found between the Tc changes and the selected Ta. These results suggest that the emotional rise of body temperature in rats might be due to an abrupt heat load and not to an upward shift of set-point temperature. This does not exclude the possibility that both mechanisms may occur simultaneously, or follow each other at a short interval. Acknowledgements--The author gratefully acknowledges the critical comments on the manuscript of Michel Cabanac and Luis Hermlndez and the technical assistance of Richard Fern~indez. REFERENCES
Atkins C., Thiessen D. and Cocke R. (1991) Lipopolysaccharide increases ambient temperature preference in C57 BL/6J adult mice. Physiol. Behav. 50, 461-463. Briese E. (1986) Circadian body temperature rhythm and behavior of rats in thermoclines. PhysioL Behav. 37, 839-847. Briese E. (1992) Cold increases and warm diminishes stress-induced rise of colonic temperature in rats. Physiol. Behav. 51, 881-883.
Briese E. and Cabanac M. (1980) Emotional fever and salicylate. In Proceedings of the 28th International Congress on Physiological Sciences Vol. 32 in Advances in Physiological Sciences (Edited by Szel6nyiZ. and Sz~kely M.), pp. 161-163. Pergamon Press, Oxford; Akademische Kiado, Budapest. Briese E. and Cabanac M. (1991) Stress hyperthermia: physiological arguments that it is a fever. Physiol. Behav. 49, 1153-1157. Briese E. and de Quljada M. G. (1970) Colonic temperature in rats during handling. Acta physiol, latinoam. 20, 97-102. Briese E., Han Hui-Wan and Parada M. A. (1991) Stress hyperthermia in mice. J. therm. Biol. 16, 333-336. Biihler H. U., Da Prada M., Haefely W. and Picotti G. B. (1978) Plasma adrenaline, noradrenaline and dopamine in man and different animal species. J. Physiol. (Lond.) 276, 311--120. Cabanac M. (1979) Le comportement thermor~gulateur. J. Physiol. (Paris)75, i15-178. Cabanac A. and Briese E. (1992) Handling elevates colonic temperature of mice. Physiol. Behav. 51, 95-98. Gordon C. J., Lee K.-L. A., Chen T.-L. A., Killough P. and Ali J. S. (1991) Dynamics of behavioral thermoregulation in the rat. Am. J. PhysioL 261, R705-RTII. Green M. D. and Lomax P. (1973) The effects of sodium salicylate on body temperature in the normothermic rat. Proc. west. Pharmac. Soc. 16, 257-261. Kluger M. J., O'Reilly B., Shope T. R. and Vander A. J. (1987) Further evidence that stress hyperthermia is a fever. PhysioL Behav. 39, 763-766. Long N. C., Vander A. J. and Kluger M. J. (1990) Stress-induced rise of body temperature in rats is the same in warm and cool environments. Physiol. Behav. 47, 773-775. Rawlins M. D., Rosendorff C. and Cranson W. I. (1971) Mechanism of action of antipyretics. In Pyrogens and Fever (Edited by Wolstrenholme G. E. W. and Birch J.), pp. 175-187. Churchill Livingstone, London. Roberts W. W. (1988) Differential thermosensor control of thermoregulatory grooming, locomotion and relaxed postural extension. Ann. N. Y. Acad. Sci. 525, 363-374. Rosendorff C. and Cranston W. I. (1968) Effects of salicylate on human temperature regulation. Clin. Sci. 35, 81-91. Satinoff E. (1972) Salicylate: action on normal body temperature in rats. Science 176, 532-533. Satinoff E. (1979) Drug and thermoregulatory behavior. In Body Temperature, Drug Effects and Therapeutic Implications (Edited by Lomax P. and Schonbaum E.), pp. 151-181. Dekker, New York. Satinoff E. (1980) Independence of behavioral and autonomic thermoregulatory responses. In Neural Mechanisms of Goal-Directed Behavior and Learning (Edited by Thompson R. F., Hicks L. and Shvyrokov V. B.), pp. 189-195. Academic Press, New York. Satinoff E. and Henderson R. (1977) Thermoregulatory behavior. In Handbook of Operant Behavior (Edited by Honig W. K. and Staddon J. E. R.), pp. 153-173. Prentice-Hall, Englewood Cliffs, N.J. Singer R., Harker C. T., Vander A. J. and Kluger M. J. (1986) Hyperthermia induced by open-field stress is blocked by salicylate. Physiol. Behav. 36, 1179-1182. Spencer R. L., Hruby V. J. and Burks T. F. (1990) Alteration of thermoregulatory setpoint with opioid agonist. J. Pharmac. exp. Ther. 252, 696-705.
0306-4565/93 $6.00+0.00 Copyright © 1993 Pergamon Press Ltd
Printed in Great Britain. All fights reserved
BEHAVIOR OF RATS IN A THERMOCLINE DURING STRESS HYPERTHERMIA E. BRIESE* Universidad de Los Andes, M6rida, Venezuela (Received 28 September 1991; accepted in revised form 21 November 1992)
Abstract--1. During stress hypertbermia, rats placed in a thermocline went from an ambient temperature of about 25°C to a cooler one. 2. There was an inverse relation between the central temperature variations and selected ambient temperature. 3. This suggests that the central temperature rise induced by handling and other mild stress circumstances might not be due to an upward shift of set-point temperature, as is currently believed, and that current theories may need to be revised. Key Word Index: Emotional hyperthermia; fever; behavioral temperature regulation; spreading saliva; grooming; heat loss; colonic temperature; psychological stress; handling
INTRODUCTION
Handling, environmental changes, and other mild stress conditions induce a rise of central temperature (To) of rats (Briese and de Quijada, 1970) and mice (Briese et al., 1991; Cabanac and Briese, 1992). This rise in T~, which was also described in humans, was independently called emotional hyperthermia or stress-hyperthermia. However, it is currently believed that emotional hyperthermia is due to an upward shift of the set-point temperature and has been qualified as a "true fever" (Singer et al., 1986). To arrive at this conclusion, pharmacological and physiological methods have been used. The pharmacological approach consists in reducing the emotional rise of Tc by salicylate and indomethacin under the assumption that these drugs lower the T~ only in fever but have no effect on the temperature of afebrile subjects (Rosendorff and Cranston, 1968; Rawlins et al., 1971). Then, it was reasoned, if emotional rise of Tc is diminished by antipyretic drugs it has to be a fever (Briese and Cabanac, 1980; Singer et al., 1986; Kluger et al., 1987). The physiological approach was used to investigate, during the emotional rise of To skin vasomotricity, the possible effects of ambient temperature and the effect of circadian variations of Tc (Long et al., 1990; Briese and Cabanae, 1991). No studies exist on behavioral thermoregulation during the emotional rise of T~. However, the technique of thermal preference in *Address for correspondence: Apartado 109, M6rida 5101-A, Venezuela.
humans and in dogs and mice is a powerful investigative tool (Cabanac, 1979; Satinoff, 1979). "Thermal preference is a measure of setpoint" as has been concisely stated by Satinoff (1979; p.171). Consequently, the purpose of the present study was to investigate the thermal preference of rats during stress hyperthermia.
METHODS
The experiments were done in a temperatureregulated chamber (Forma Scientific Inc.) with adult male rats of Wistar origin. The rats had food and water ad libitum. Lights were on at 06:00 h and off at 18:00 h. The animals were housed for days or weeks at a time in a thermocline (Briese, 1986). In brief, the thermocline was a plywood box 61 x 26 x 40 cm with three intercommunicable compartments. The ambient temperatures (T~) in the three compartments were maintained at about 17, 25 and 30°C by forced convection through heat exchangers. However, since the compartments intercommunicated, the animals in the thermocline could select a Ta between the extremes 17 and 30°C. Temperature measurements
The intracranial temperature To, and T, were recorded simultanously using small thermistors as sensors. The thermistors were fixed into 7ram stainless steel 23 gauge tubes closed at the tip by welding. The leads from the thermistor were soldered to Amphenol male connectors and the tube fastened
2
E. BRIESE
to the two connectors with epoxy glue. The stainless steel tube with the thermistor was implanted into the thalamus to record the temperature near the center of cranial cavity. The implant was secured to the skull with stainless steel screws and acrylic cement. The wires from the Amphenol connectors were threaded through a protecting steel spring and connected to a rotatory mercury commutator. For Ta measurements, a second thermistor was fastened on the protecting spring about 4 5 ~ 5 mm above the head of the animal and the two wires from the thermistor were threaded through the same protecting spring to the four-channel rotatory commutator. From the commutator the four wires were connected to two Wheatstone bridges and from there to a two-channel potentiometric recorder.
depth into the colon to measure the colonic temperature. The probe temperature stabilized in about 1 min, after which the rat was placed in the middle compartment of the thermocline where the Ta was about 25°C, the same as in the climatic chamber where the experiments were performed. This procedure was repeated several times every 5 or 6 min until the emotional rise of T¢ was maximal and Tc began to descend, which occurred after about 25 min.
Validation of the thermocline method The rationale for the tests to validate the thermocline method was that if the Tc of the animal was altered by physical means, and assuming that the set-point level did not change, the animal should prefer a Ta inverse to the T¢ change. If Tc temperature is made to rise (induced hyperthermia) the animal should select a cool environment and if Tc is made to fall (induced hypothermia) then the animal should move to a warmer environment. Hypotherrnia was produced by putting the animal in water of 8-1 I°C for 2 min (Spencer et al., 1990). The rat was dried off before being returned to the thermocline. Hyperthermia was produced by placing the rat in a double wall tin-plate box 24 x 12.5 x 27 cm. The box was
Experimental procedure and emotional stress paradigm Selected Ta and Tc of the animals housed in the thermocline were continuously recorded. The experiments were done in the morning. A stress situation was created by taking the animal out of the thermocline, without disconnecting the head leads, maintaining the animal on the operator's knees while inserting a lubricated thermocouple probe at 70 mm 30 28 26 24 22 20
39-
38
36
I,llll,, -90
-60
tt,tt,t,, 0
12
I ' ' 36
60
J 90
'
' i20
Time (rain)
Fig. I. Intracranial temperature and the selected ambient temperature recordings from a rat in a thermocline. Arrows indicate moments when the animal was taken out of the thermocline and its colonic temperature measured with a thermocouple probe. Speed of the recording paper was 5 cm/h during the experiment and 2.5 cm/h before it. The selected ambient temperature began to de~end from the beginning of the intracraniai temperature rise. When the intracruial temperature reached a plateau and during the descending phase the selected ambient temperature oscillated between 21 and 19°C.
Rat behavior during stress hyperthermia heated with water at 45°C poured between the two walls. T~ was monitored while the rat was in the hot environment and the rat was taken out and placed again in the middle compartment of the thermocline when T~ reached about 40°C.
3
y=-8.1228e-2 -2.1479x R2=0.849 L,
o
Analysis of the results The experiments were performed on six rats. There were 39 experiments of handling-induced rise of T~, four experiments of induced hypothermia and five experiments of induced hyperthermia. Analog data were digitized by reading from the recordings T~ and Ta values every 6 rain during the first 60 rain of each experiment. The data were than normalized by subtracting the initial values of each experiment from the following values. In this way, raw data were transformed in data representing changes of temperatures in °C, with zero taken as the initial temperature. Means and standard errors were calculated. The relationship between changes of T~ and changes of T~ was assessed by linear regression analysis. RESULTS
Selected ambient temperature during emotional rise
of T"~ During the emotional rise of To, the rats went, in most of the experiments, from the middle compartment of the thermocline, where they were placed after each colonic temperature measurement, to a cooler T¢. Figure 1 illustrates the result of a typical 0 o
=
-2
t.
2
1.0
e~ t~ l=
m
"~ ~0.5 g=
0.0
I 6
I 12
I I J I I I I 18 24 30 36 42 48 54 Time (min)
J 60
Fig. 2. The upper curve represents mean changes in selected ambient temperature and the bottom curve mean changes in intracranial temperature produced by handling in 39 experiments. Each point represents a mean change in temperature relative to the initial temperature. Vertical bars are standard error of the means. Note that minimum selected ambient temperature is 12rain behind the maximum intracranial temperature rise.
o .o
"O eJ
-2
o~
o
e~ O e~
I
I
I
I
1.0 1.2 Change in intracranial temperature (°C) 0.4
0.6
0.8
Fig. 3. Correlation between mean changes in intracranial temperature and mean changes in selected ambient temperature. For this analysis the delay between intracranial and ambient temperature data was suppressed by shifting the ambient temperature data 12 rain forward. Regression line equation was: y = - 0 . 0 8 1 - 2.14x, correlation coefficient r = - 0.921. experiment. The rat went gradually from about 25°C in the middle compartment of the thermocline to cooler ambient temperatures. When Tc elevation reached a plateau and during the descending phase of Tc course the selected T~ oscillated between about 21 and 19°C. Towards the end of the experiment, after about 78 min, the rat again selected a warmer T~ Figure 2 presents the mean of 39 experiments and shows the evolution in time of the rats' behavior and their body temperature. It is evident that while T¢ rose the selected ambient temperature descended and vice-versa. In addition, Fig. 2 shows that there was a 12 min gap between the lowest selected T~ and the highest rise in Tc, that is, the behavioral response occurs 12 rain after the T¢ change. The change of sign of the slope of the curve of selected T~ took place at 36 rain. For the Tc curve it took place at 24 rain. When allowance was made for the 12 min gap by shifting the Ta changes to the left the correlation between T~ changes and selected T~ was almost perfect (r = 0.913) as shown in Fig. 3.
Validation of the thermocline method Selected To during induced hyperthermia. There were five experiments of this type in five different rats. Figure 4 shows the recording of an induced hyperthermia experiment. The animal was placed in a hot box at 44°C at time 0 and T~ rose to 40°C. At the time indicated by the arrow the animal was placed in the middle compartment of the thermocline and immediately went to the coolest place. The rat
4
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stayed in this cool environment until the Tc descended to 37°C. Selected To during induced hypothermia. This experiment was performed with similar results five times in four rats. Figure 5 gives an example. The animal was put in water at 11 °C, for 2 min. The T~ recorded during this time was that of the air above the head of the animal (25-27°C) and, therefore, much higher than the bath temperature. During immersion in the cold hath T~ fell from 37 to about 34.5°C and continued to fall to 34°C after the rat was replaced in the thermocline in the middle compartment at 25°C. Immediately after that the rat went to a cooler T~ (21-19°C). T~ rose gradually to reach the initial level (37°C) in about 96 min. At the same time the selected Ta rose also gradually to 30°C and started to descend when T~ was higher than 37°C.
found previously. However, the results did not confirm this hypothesis. In no instance, during the emotional rise of Tc, did the rats prefer an environment warmer than 25°C. On the contrary, the animals went to a cooler environment in all 39 experiments but two, during which the animals stayed in the middle compartment of the thermocline where they were placed. The lowest selected Ta was 12 min after the highest Tc rise. An almost perfect inverse relationship was found between changes of Tc and the changes of selected Ta set forward by 12 min. This suggests that the delay between the Tc variation and behavioral responses is built-in into the system and that the behavior could be a consequence of Tc changes. These results seem to indicate that the rise of T¢ related to psychological stress is opposed by the behavioral responses and might not be a "fever" as though previously (Briese and Cabanac, 1980, 1991; Singer et al., 1986; Kluger et al., 1987; Long et al., 1990). This is rather a disturbing conclusion because autonomic and behavioral responses are usually regarded as cooperating with or complementary to each other (Satinoff and Henderson, 1977), or, on occasion, as independent (Satinoff, 1980) but not opposing each other.
D~CUSSION This is the first study of behavioral thermoregulatory responses as they apply to the nature of stress induced rise in To. My intention was to look for further evidence that emotional rise in Tc in rats is due to an upward shift of set point level as
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Fig. 4. Intracraniai temperature and ambient temperature recordings during an induced hyperthermia experiment. At time zero the rat was placed in a hot box (HB) until the intracranial temperature reached 40°C; then it was replaced in the thermocline where it went immediately to the lowest temperature of the thermocline.
Rat behavior during stress hyperthermia
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Fig. 5. Intracranial temperature and preferred ambient temperature during an induced hypothermia experiment. At the moment indicated by CB (cold bath) the rat was taken out of the thermocline and put in water at 11°C for 2 min. During the cold bath the ambient temperature recorded was that of the air above the head of the animal. Then at time zero the rat was placed in the middle compartment of the thermoeline at about 25°C. During the first 12 rain the animal went from there to a lower ambient temperature (about 20°C). Only after 48 rain when the intracranial temperature was already above 36°C did the rat go to ambient temperatures higher than 25°C. Still warmer ambient temperatures of 29-30°C were selected only 36 min before the intracranial temperature reached 37°C.
However, the idea that an emotional rise of Tc might be a hyperthermia, as suggested by the results presented here, might be supported by some arguments. F o r instance, the effect of antipyretic drugs in lowering emotional Tc rise is only indirect evidence and is not irrefutable since salicylate also lowers normal Tc (Satinoff, 1972; Green and Lomax, 1973) and does not affect the magnitude of the emotional T~ rise in mice (Briese et al., 1991; Cabanac and Briese, 1992). On the other hand, in an earlier study (Briese and Cabanac, 1991) vasomotricity was measured as changes of skin temperature and skin temperature depends on T,. The initial vasoconstriction was manifest only in a warm T, and was followed by a lengthy and strong vasodilation; in addition, vasoconstriction might be due to factors other than thermoregulation, such as those associated with stress per se, for instance, catecholamines secretion (Bfihler et aL, 1978). Furthermore, as shown in a recent work (Briese, 1992), cold increases and warm diminishes stress induced rise of Tc which again suggests that it is not a regulated rise. Gordon et aL (1991) have shown that naive rats placed in a thermal gradient prefer a cooler temperature for the first couple of hours and then slowly move to warmer temperatures. Body temperature was not measured in that study but it can be assumed that by placing the animals
in a novel environment a stress-hyperthermia was induced. In another study (Spencer et al., 1990), control, vehicle injected rats, placed in a thermocline, selected a cooler ambient temperature relative to the pre-injection level for the first 10-15 rain after injection. In the same rats there was a small hyperthermia response after vehicle injection. Lipopolysaccharide (which is supposed to produce a fever) increases ambient temperature preference of mice (Akins et al., 1991) while the stressed rats in the experiments described sought a cool environment. An alternative interpretation of the results obtained in the present work would be to challenge the postulate that "thermal preference is a measure of setpoint" (Satinoff, 1979). However, this would imply the collapse or a profound revision of the whole thermoregulatory theory. Two incidental observations in the present work deserve further comments. First, I observed but not measured, that, in numerous instances grooming, spreading saliva and extension-relaxation were preferred by the rat before seeking a cooler environment. This goes against the belief that behavioral thermoregulation, because phylogenetically older and energetically more economical, takes precedence over reflex or autonomic responses (Cabanac, 1979; Satinoff, 1980). The question arises whether or not
6
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heat loss responses as grooming with spreading saliva and extension-relaxation (Roberts, 1988) should be considered autonomic or behavioral responses. The second incidental observation deserving comment was that induced hyperthermia and induced hypothermia elicited different patterns of behavior. Physically induced hyperthermia was instantly counterchecked by the animal seeking the coldest available Ta when placed in the thermocline. On the other hand, after induced hypothermia the rats placed in the thermocline did not go to the warmer environment at once but gradually. In the example given in Fig. 5, intracranial temperature rose from 34 to 36°C in 36 min while the rat selected, first, a mean Ta of 20°C and later a Ta of about 23-24°C. The rat went to about 29-30°C only after 72 min when its central temperature was already near 37°C. Thus, most of the rewarming should have been due to autonomic responses. This shows again that behavioral adjusting thermal responses are not always preferred to autonomic counteracting responses. I am tempted to speculate that behavior responses are used in preference to autonomic responses when the thermal situation requires an urgent defense of homeostasis, and autonomic or reflex responses possibly come first when thermal disturbance is not critical, overheating being more dangerous than cooling. According to this view behavioral responses would be preferred to autonomic ones because they might be more rapid in correcting the homeostatic disturbance when this was dangerous to life. In conclusion, during the emotional rise of body temperature the rats preferred a cooler Ta. An almost perfect inverse relationship was found between the Tc changes and the selected Ta. These results suggest that the emotional rise of body temperature in rats might be due to an abrupt heat load and not to an upward shift of set-point temperature. This does not exclude the possibility that both mechanisms may occur simultaneously, or follow each other at a short interval. Acknowledgements--The author gratefully acknowledges the critical comments on the manuscript of Michel Cabanac and Luis Hermlndez and the technical assistance of Richard Fern~indez. REFERENCES
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