Rats prefer ambient temperatures out of phase with their body temperature circadian rhythm

Brain Research, 345 (1985) 389-393

389

Elsevier BRE 21108

Rats prefer ambient temperatures out of phase with their body temperature circadian rhythm EDUARDO BRIESE Universidad de Los Andes, MOrida 5101-A (Venezuela)

(Accepted May 29th, 1985) Key words: thermoregulation - - set-point temperature - - circadian rhythm - - thermoregulatory behavior - -

body temperature - - control theory - - rat

Rats placed in thermally graded enclosures cyclically selected ambient temperatures about 195° out of phase with the circadian variations of their hypothalamic temperature. This finding cannot be explained by the generally accepted assumption that body temperature circadian rhythm is due to a cyclic shift of the set-point temperature.

Circadian variations of e n d o t h e r m s ' b o d y t e m p e r atures are generally attributed to a cyclic shift in the set-point of the t h e r m o r e g u l a t o r y system2, 6,l°,14,1~. It is well knownS, 7,18 that a set-point t e m p e r a t u r e shift can only be d e d u c e d if a change in the b o d y core temperature is p r o d u c e d and m a i n t a i n e d by a p p r o p r i a t e thermal responses that defend the new level. F o r instance, a set-point shift to a higher t e m p e r a t u r e induces the organism to p r o d u c e and retain m o r e heat and to seek a w a r m e r e n v i r o n m e n t , and, vice versa, a lower setting will result in an increase in heat dissipation and a preference for a cooler environment. The set-point is a descriptive concept 18. It is an abstraction since it cannot be m e a s u r e d directly but can only be d e d u c e d by comparing b o d y t e m p e r a t u r e changes with the effectors' t h e r m o r e g u l a t o r y responses (autonomic and behavioral) 7. If the effectors defend the new level it can be d e d u c e d that the body t e m p e r a t u r e change was due to a set-point shift. If the effectors counteract the body t e m p e r a t u r e change, this change is not due to a set-point shift but to other causes 18. If we accept the assumption that the circadian oscillations of b o d y t e m p e r a t u r e are due to a shift in the set-point, we would then expect that during the phase of elevated b o d y t e m p e r a t u r e the animal would behave as if it had a fever, which is

the typical p a r a d i g m for a shift of the set-point to a higher level9, 22. By definition, such a shift implies that the organism abhors cold and likes and seeks warm environments (and increases heat production and conservation)3,4,10,~5, 2z. The contrary must happen in o r d e r to deduce that a decrease in the b o d y t e m p e r a t u r e is due to a lower setting of the set-point. Consequently, according to the prevalent belief, a rat allowed to choose b e t w e e n distinct t e m p e r a t u r e environments should p r e f e r a w a r m e r e n v i r o n m e n t at night, during the active period, when its central t e m p e r a t u r e is high, and a cooler e n v i r o n m e n t during the day, when its central t e m p e r a t u r e is low. However, the experiments l r e p o r t here indicate the contrary. Preferred a m b i e n t t e m p e r a t u r e cyclically oscillated about 195 ° out of phase with the normal body t e m p e r a t u r e circadian variations. A d u l t male rats of Wistar origin were allowed to choose among distinct ambient temperatures. The hypothalamic t e m p e r a t u r e (Thy) and the p r e f e r r e d ambient t e m p e r a t u r e (Ta) were continuously recorded for periods of up to 8 days. T e m p e r a t u r e s were m e a s u r e d with c o p p e r - c o n s t a n t a n t h e r m o c o u p l e s and p o t e n t i o m e t r i c recorders. The Thy was r e c o r d e d through a r e e n t r a n t stainless-steel tube~. The Ta therm o c o u p l e was fastened to the Th~ t h e r m o c o u p l e

Correspondence: E. Briese, Universidad de Los Andes, Apartado 109, M6rida 5t01-A, Venezuela.

0006-8993/85/$03.30 © 1985 Elsevier Science Publishers B.V. (Biomedical Division)

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Time of doy (hours) Fig. 1. Preferred ambient temperature (Tog) and hypothalamic temperature (Thy) recorded in one rat during 3 circadian cycles. Numbers in each hourly area under the curves are the weights in milligrams of cut-away paper of each area representing the integral of temperature as for time. The shaded areas between the two curves are to emphasize the reciprocal course, night vs day, of the two curves. Horizontal strips from 18.00 h to 06.00 h indicate dark hours. The animal was placed in a plastic washbasin 60 cm in diameter, divided into 6 sectors by wooden partitions with a central common area 20 cm in diameter allowing the animal to pass from one sector to the other. The bottom of each sector, with one exception, was heated by a nichrome resistance wire so wound as to result in a different temperature for each. The entire enclosure was placed in a walk-in controlled-temperature chamber where the temperature was maintained at 13 °C. leads at 1 . 5 - 3 cm above the h e a d of the a n i m a l t L F o o d a n d water were p e r m a n e n t l y available. Lights were a u t o m a t i c a l l y t u r n e d o n at 06.00 h a n d off at 18.00 h. In a series of e x p e r i m e n t s o n 4 rats d u r i n g 20 nyct h e m e r a l cycles, a circular t h e r m a l l y g r a d e d e n c l o sure was used. A n e x a m p l e of the records o b t a i n e d from a rat placed in this e n c l o s u r e is given in Fig. 1. A n inverse r e l a t i o n b e t w e e n p r e f e r r e d T~ a n d Thy is a p p a r e n t . T~y was higher d u r i n g the d a r k h o u r s a n d lower d u r i n g the light hours, while the c o n t r a r y happ e n e d with the p r e f e r r e d T~. T h e a n a l o g o u s data were digitized as a time integral of t e m p e r a t u r e , that is, the area u n d e r the r e c o r d e d curves, expressed as the weight of cut-away p a p e r . First, strips of p a p e r c o r r e s p o n d i n g to the 12 hours of light a n d the 12 hours of d a r k n e s s were w e i g h e d , a n d after that they were sliced into 1-h sections. F o r every rat a n d every cycle the difference, dark phase m i n u s light phase for

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Fig, 2. Circadian variations of preferred ambient temperature (T~) and hypothalamic temperature (Tt~y)in 4 rats. Each point represents the hourly mean of 20 circadian cycles. The variations of preferred T a (0) and Tl~y(©) are given as time integral of temperature' that is, the area under the recorded curves ex' pressed as the weight of cut-away paper. Best fitting sine wave curves are also given. The difference in phase angle between best fitting curves for T a and Thy w a s 195°.

391 the 12-h strips, was positive for Thy and negative for T a. A c c o r d i n g to the sign test el, the one-tailed p r o b a bility associated with the occurrence of this distribution under null hypothesis was < 0.001. H o u r l y means of time integral of t e m p e r a t u r e expressed as weight of p a p e r are given in Fig. 2. A sine wave fit analysis indicated that the phase difference between Thy and p r e f e r r e d T~, for this series of experiments, was 195 ° . In other words, the e s t i m a t e d Thy minima occurred 1 h before the e s t i m a t e d p e a k (acrophase) of T a. A linear regression analysis of T~ VS Thy gave a correlation coefficient of r = -0.895 (t -- -9.412, df = 22, P < 0.001). In a n o t h e r series of experiments the animals could choose, at a given time, only b e t w e e n two Tas: either between a neutral and a cold one or b e t w e e n the neutral and a hot one. With this m e t h o d a direct, easy to interpret, graphic representation of the p h e n o m e n o n was obtained. F o r these experiments one of the walls of a rectangular thermally g r a d e d enclosure was m a d e of 3 heat exchangers. A blower forced air between 22.5 and 26 °C through the 3 heat exchangers. W a t e r from a thermostatically controlled t e m p e r a ture bath circulated through one of the heat exchangers, and in this way air entering part of the animal en-

dark hours the rat went to the cold portion of the enclosure, and this behavior was synchronous with the higher ultradian undulations of the nocturnal hyperthermia. During light hours it stayed in the neutralwarm environment while the fhy decreased. Note the reciprocal, mirror image of one curve with respect to the other. Similar results were obtained with the other 9 rats during 37 cycles. But to d e m o n s t r a t e a preference, behavior must shift appropriately when the situation is reversed. That was done in the following experiment. In the first part of the e x p e r i m e n t illustrated in Fig. 4, the rat could choose between a neutral, about 26 °C, and a cold environment with a minimum of 12 °C. The rat went to the cold during the nighL as did the animals in the previously described experiment. W h e r e lateL the t e m p e r a t u r e of the water circulating through the heat exchanger was changed from 10 to 34 °C, the animal stayed most of the time in an environment of 26 °C during the night, while it went frequently to the hot environment during the light phase and stayed there for longer periods. Similar results were o b t a i n e d with pigeons 20. In a cold environment deep body t e m p e r a t u r e of pigeons drops during the night. During this phase of the circadian cycle shivering was r e d u c e d while d e m a n d for warm air reenforcements and time spent in the warm air increased. In these experiments the decrease of body t e m p e r a t u r e was resisted by the behavioral responses yet facilitated by the inhibition of shivering. However, the instrumental response in these experiments was the interruption of a p h o t o g a t e by a head-

closure was h e a t e d or cooled. The results of a typical experiment are illustrated in Fig. 3. The Thy recordings were very similar to those published by other authors 11-13,19. The ultradian fluctuations superimposed on the 24-h rhythm were variable from animal to animal and within a given animal, as is evident in Fig. 3 and as has been n o t e d by others. During the

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Fig. 3. Preferred ambient temperature and hypothalamic temperature of a rat. The animal was in a rectangular plywood enclosure (61 x 26 x 40 cm). The lower part (25 cm of the total height of 40 cm) of one of the longitudinal walls of the enclosure was made of 3 heat exchangers (3 car heaters). One blower forced air from a controlled-temperature chamber through the 3 heat exchangers. Water from a thermostatically controlled-temperature bath circulated through either one of the end heat exchangers; thus, two-thirds of the rat's habitable space had a neutral-warm temperature (22.5-26 °C) and one-third was either cold or hot according to the temperature of the circulating bath. Here the animal could choose between about 15 °C and 25-26 °C. During the dark hours, the hypothalamic temperature rose, and the animal frequently went to the cold environment, During the light hours, it stayed in the neutral-warm 26 °C environment, Note that even during the light time for almost every transitory rise of Thythere is a corresponding inverse spike of T~,, that is, a short excursion of the animal into the cold.

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Time of day (hours) Fig. 4. Hypothalamic temperature (Thy) and preferred ambient temperature recorded for 72 h in one rat. The animal was in the same enclosure as described for Fig. 3. For convenience the record was cut into two parts, part B follows part A. Duringthe first 26 h, before the moment marked with the arrow, the rat could choose between an environmental temperature of approximately 26 °C and one of approximately 12 °C. During this part of the experiment, the rat spent most of the light hours at 26 °C and went to the cold during the night, In the second part of the experiment (after the arrow) the animal could choose between the neutral environment (22;5-26 °C) and a hot one (34 °C). During the night, when the Thy rises, the rat stayed most of the time in an environment of 23-25 °C. During the light hours, when the Thy decreases, the animal chose to go frequently and stayed for long periods in the hot environment.

nodding movement which could have been due to the reduction of muscular tone, characteristic of sleep. Since the defence of a reference value is essentially implicit in the concept of a set-point 2.5-t0.t4 and since behavioral thermal responses appear here to oppose the body circadian temperature variations, it can be deduced that these variations, contrary to the current theory, cannot be explained by a shift in the setpoint. Rhythmic shifts in the set-point should be reflected by the preferred temperature, and the present results do not show that. However. this deduction is valid only for behavioral thermoregulation. Circadian oscillations of body temperature persist in spite of being opposed by behavioral responses. That means that autonomic responses necessarily exceed the behavioral ones and should perform as if a cyclic change in the set-point

had taken place. The dissociation between autonomic and behavioral thermoregulation is well known 5,]6J7. This has been shown in animals with experimental lesions in the preoptic area and lateral hypothalamus, in animals in which the posterior hypothalamus was thermally stimulated and in infant mammals. But reflexive and behavioral thermoregulations are usually conceived as redundant and compensatory subsystems substituting or helping one another towards the same goalSa8,20. What may be of interest, according to the present results, is that this dissociation is now found in normal adult animals, regulating their body temperature m unstressful conditions, and that we deal here not only with a dissociation but rather with an antagonism. Although the present results appear perplexing, an

393 e x p l a n a t i o n can be s u g g e s t e d within t h e f r a m e w o r k

w e r e the case, t h e n the circadian b o d y t e m p e r a t u r e

of c o n t r o l t h e o r y , if we accept the t w o t h e r m o s t a t s o r

changes o r i g i n a t e d by the a u t o n o m i c r e s p o n s e s inte-

m u l t i p l e t h e r m o s t a t m o d e l s of S a t i n o f f 16. T h e set-

g r a t o r w o u l d increase the signal e r r o r for the b e h a v -

point of o n e t h e r m o s t a t , the o n e for a u t o n o m i c re-

ioral r e s p o n s e s i n t e g r a t o r , which w o u l d thus inter-

sponses, w o u l d h a v e a built-in cyclical n y c t o h e m e r a l

p r e t the t e m p e r a t u r e c h a n g e s as cyclical p o s i t i v e and

shift, w h e r e a s the set-point for b e h a v i o r a l r e s p o n s e s

n e g a t i v e h e a t loads and p r o c e e d to c o u n t e r a c t t h e m .

w o u l d not shift with the circadian r h y t h m . If that 1 Abrams, R. and Hammel, H. T., Cyclic variations in hypothalamic temperature in unanesthetized rats, Am. J. Physiol., 208 (1965) 698-702. 2 Aschoff, J., Circadian rhythm of activity and body temperature. In J. D. Hardy, A. P. Gagge and J. A. J. Stolwijk (Eds.), Physiological and Behavioral Temperature Regulation, C. C. Thomas, Springfield, IL, 1970, pp. 905-919. 3 Cabanac, M., Plaisir ou ddplaisir de la sensation thermique et hom6othermie, Physiol. Behav., 4 (1969) 359-364. 4 Cabanac, M., Duclaux, R. and Gillet, A., Thermordgulation comportamentale chez le chien: effet de la fi~vre et de la thyrosine, Physiol. Behav., 5 (1970) 697-704. 5 Cabanac, M., Temperature regulation, Ann. Rev. Physiol., 37 (1975) 415-439. 6 Cabanac, M., Le comportement thermor6gulateur, J. Physiol. (Paris), 75 (1979) 115-178. 7 Cabanac, M., Hildebrandt, G., Massonnet, B. and Strempel, H., A study of the nyctohemeral cycle of behavioral temperature regulation in man, J. Physiol. (Lond.), 257 (1976) 275-291. 8 Clark, W. G. and Lipton, J. M., Complementary lowering of behavioural and physiological thermoregulatory setpoint by tetrodotoxin and saxitoxin in the cat, J. Physiol. (Lond.), 238 (1974) 181-191. 9 Gordon, C. J., A review of terms for regulated vs forced, neurochemical-induced changes in body temperature, Life Sci., 32 (1983) 1285-1295. 10 Hensel, H., Neural processes in thermoregulation, Physiol. Rev., 53 (1973) 948-1017. 11 Heusner, A., Variation nycthdmerale de la temperature centrale chez le rat adapt6 ~ la neutralit6 thermique, C. R. Soc. Biol. (Paris), 153 (1959) 1258-1260. 12 Honma, K. and Hiroshige, T., Simultaneous determination

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of circadian rhythms of locomotor activity and body temperature in the rat, Jap. J. Physiol., 28 (1978) 159-169. Miles, G. H., Telemetering techniques for periodicity studies, Ann. N. Y. Acad. Sci., 98 (1962) 858-865. Mills, J. N., Human circadian rhythms, Physiol. Rev., 46 (1966) 128-171. Myhre, K., Cabanac, M. and Myhre, G., Fever and behavioral temperature regulation in the frog Rana esculenta, Acta Physiol. Scand., 101 (1977) 219-229. Satinoff, E., Neural organization and evolution of thermal regulation in mammals, Science, 201 (1978) 16-22. Satinoff, E., Independence of behavioral and autonomic thermoregulatory responses. In R. F. Thompson, L. H. Hicks and V. B. Shvyrkov (Eds.), Neural Mechanisms of Goal-Directed Behavior and Learning, Academic Press, New York, 1980, pp. 189-196. Satinoff, E. and Hendersen, R., Thermoregulatory behavior. In W. K. Honig and E. R. Staddon (Eds.), Handbook of Operant Behavior, Prentice-Hall, Englewood Cliffs, N J, 1977, pp. 153-173. Satinoff, E., Liram J. and Clapman, R., Aberrations of circadian body temperature rhythms in rats with medial preoptic lesions, Am. J. Physiol., 242 (1982) R352-R357. Schmidt, I., Graf, R. and Rautenberg, W., Diurnal variations in the cooperation of shivering and instrumental behavior for cold defense in the pigeon. In Y. Houdas and J. D. Guien (Eds.), Thermal Regulation, Masson, Paris, 1978, pp. 135-138. Siegel, S., Nonparametric Statistics for the Behavioral Sciences, McGraw Hill, New York, 1956, pp. 68-75. Stitt, J. T., Fever versus hyperthermia, Fed. Proc., 38 (1979) 39-43.