A comparative analysis of huddling in infant Norway rats and Syrian golden hamsters: does endothermy modulate behavior?

In infant rats, huddling improves surface-to-volume ratios and provides metabolic savings during cold exposure. It is unclear, however, whether endothermy is also a necessary component of huddling. In the present experiment, huddles composed of infant Norway rats (2- or 8-day-olds), which produce heat endogenously, or Syrian golden hamsters (8-day-olds), which do not produce heat endogenously, were exposed to decreases in air temperature. Behavioral and physiological responses were monitored throughout the test. Rats, especially at 8 days of age, were better able to thermoregulate using huddling than hamsters, due in part to endogenous heat production. Furthermore, 8-day-old rats exhibited behavioral responses that promote heat retention, suggesting that both physiological and behavioral mechanisms contribute to effective thermoregulation during huddling in the cold.     

Rat behavioral thermoregulation integrates with nonshivering thermogenesis during postnatal development

Rat pups are capable of behavioral thermoregulation, both in the nest and on a thermocline, as early as the 1st week of postnatal life, and these pups can also produce heat metabolically without shivering. The rat pup's primary source of nonshivering thermogenesis is the sympathetically mediated metabolism of brown adipose tissue (BAT). BAT is well formed in newborns and functions shortly after birth. While infant behavioral thermoregulation and BAT thermogenesis have been extensively studied, little is known about the extent to which thermoregulatory behavior can be influenced by BAT thermogenesis. In the present study, 2-, 7-, and 14-day-old pups were observed on a thermal gradient following pharmacological stimulation or inhibition of BAT thermogenesis, and their thermal preferences were quantified. The authors found that 7- and 14-day-old pups treated with norepinephrine (NE), which increases BAT thermogenesis, preferred cooler portions of the gradient than saline-treated controls, whereas 2-day-olds failed to show a similar NE-induced behavioral adjustment. These findings indicate that the ability to adjust thermoregulatory behavior to compensate for enhanced metabolic thermogenesis develops during the 1st week of postnatal life.     

Role of sympathetic nervous system in immobilization- and cold-induced brown adipose tissue thermogenesis in rats

The role of the sympathetic nervous system in 10-min cold (5 degrees C)- or 2-min immobilization-induced thermogenesis in brown adipose tissue (BAT) was studied in warm (25 degrees C)-acclimated rats. Both cold- and immobilization-stresses increased heat production (M), interscapular brown adipose tissue temperature ( Tbat ), and colonic temperature ( Tcol ). Resulting from both stresses, the increase in Tbat was greater than that in Tcol , the differences (delta Tbat ) becoming approximately 0.48 and 0.46 degrees C by the cold exposure and the immobilization, respectively. After sympathectomy, Tbat and delta Tbat did not change on immobilization but increased significantly on the cold exposure. Delta Tbat was 0.31 degrees C in the sympathectomized rats at the end of the cold exposure period. Immobilization-induced BAT thermogenesis may be mainly controlled by the sympathetic nervous system. On the other hand cold-induced BAT thermogenesis seems to be controlled by certain hormonal factors as well as the sympathetic nervous system.     

Huddling by rat pups: ontogeny of individual and group behavior

A full account of behavioral development in rats must include the ontogeny of both individual and group behavior. Most of our accumulated knowledge, however, pertains to individual ontogenesis. Group behavior and its development are readily seen in the huddling behavior of rat pups. A rat huddle is an entity with characteristics and capabilities distinct from those of the individuals that comprise it. The huddle is a natural context for acquiring olfactory preferences for species odors. Olfactory learning in a huddle involves thermal and tactile stimulation from the mother's body but, surprisingly, not the rewards of suckling or of milk transfer. Although there is complete developmental continuity of huddling behavior, the sensory controls of huddling change dramatically during the first 2 weeks of postnatal life. Huddling behavior is initially controlled by thermal cues ("physiological huddling") and then becomes dominated by olfactory stimuli ("filial huddling"). The complex group behavior of huddling was modeled successfully with computational methods. Group behavior emerges from individual interactions, guided entirely by rules of individual behavior (no rules for group behavior). Three simple rules of autonomous activity/inactivity can spawn the patterns of aggregon formation displayed by groups of 7-day-old pups, but not by 10-day-olds. The developmental change evident by Day 10 requires adding a rule by which each individual is affected by the activity state of adjacent pups. Group behavior responded to manipulations of central oxytocin on Day 10, but not on Day 7.     

Thermoregulatory competence and behavioral expression in the young of altricial species—Revisited

The behavioral and physiological thermoregulatory capabilities of newborn and infant mammals have been studied for over half a century. Psychobiologists have noted that the infants of altricial species (e.g., rats) have physical and physiological limitations such that heat loss overwhelms heat production, thus forcing a reliance on behavioral thermoregulation for the maintenance of body temperature. Recent evidence, however, suggests that a modification of this view is justified. Specifically, throughout a range of moderately cold air temperatures, nonshivering thermogenesis by brown adipose tissue contributes significantly to the infant rat's behavioral and physiological adaptations to cold challenge. Given the prominent use of altricial species for the study of infant behavior, increased understanding of the infant's physiological responses to cold and the effect of thermal factors on behavior is warranted. © 1998 John Wiley & Sons, Inc. Dev Psychobiol 33: 107–123, 1998     

Ultrasonic vocalizations by rat pups after adrenergic manipulations of brown fat metabolism

Rat pups emit ultrasonic vocalizations (USVs) during cold challenge. R. F. Kirby and M. S. Blumberg (1998) suggested that when brown adipose tissue (BAT) thermogenesis fails to compensate for body heat loss and heart rate declines, infant pups maintain venous return to the heart with a mechanical maneuver that is accompanied by ultrasonic emissions. Thus, manipulations that attenuate or enhance BAT thermogenesis should have inverse effects on cold-induced USVs. The authors found that hexamethonium (10 mg/kg) and propranolol (1 and 20 mg/kg) attenuated BAT metabolism while enhancing USV production, and norepinephrine (NE, 800 microg/kg) enhanced BAT metabolism while ultrasonic emissions decreased. The findings are consistent with the hypothesis that BAT metabolism influences USVs during cold challenge by affecting cardiac rate and inducing compensatory, homeostatic responses.     

Brown adipose tissue glyceroneogenesis is activated in rats exposed to cold

We have previously found that glyceroneogenesis is very active in brown adipose tissue (BAT) and increases in fasted, diabetic and high-protein-diet-fed rats, situations of reduced thermogenic activity. To understand better the role of glyceroneogenesis in BAT glycerol-3-phosphate (G3P) generation, we investigated its activity during cold exposure (10 days at 4°C), a condition in which, in contrast to the above situations, BAT thermogenesis is markedly activated. Rates of total (from all sources) BAT fatty acid (FA) synthesis and rates of incorporation of glucose carbon into BAT glyceride-FA and -glycerol in vivo were markedly increased by cold exposure. Cold exposure induced a marked increase in BAT glyceroneogenic activity, evidenced by (1) increased rates of non-glucose carbon incorporation into glyceride-glycerol in vivo and of [1-¹⁴C]-pyruvate incorporation into glyceride-glycerol in vitro, and (2) a threefold increase in phosphoenolpyruvate carboxykinase activity. Most of the glyceride-glycerol synthesized by BAT via glyceroneogenesis or from glucose was used to esterify preformed FA. This use was markedly increased by cold exposure, in parallel with a pronounced activation of BAT lipoprotein lipase activity. In conclusion, during cold exposure BAT glyceroneogenesis is markedly activated, contributing to increase the generation of G3P, which is mostly used to esterify preformed FA.     

Independence of ultrasonic vocalization and thermogenic responses in infant rats

Ultrasonic vocalizations (USV) normally accompany brown adipose tissue (BAT) thermogenesis in infant rats exposed to cold. BAT activation (measured by implanted thermistors) was pharmacologically blocked with hexamethonium (20 mg/kg ip) in 12-13-day-old pups, but they nevertheless showed normal USV responses to cold. Activation of BAT in warm pups by norepinephrine (800 micrograms/kg sc) failed to elicit USV. It is concluded that BAT activation is neither necessary nor sufficient for USV production. To evaluate how tightly the two responses may be coupled centrally, rat pups deprived of nutrients for 24 hr, in which sympathetic activation is known to be inhibited centrally (Bignall, Heggeness, & Palmer, 1975), were studied. These pups vocalized with the same latency in response to cold as normals but failed to show evidence of concurrent BAT activation. It is concluded that USV and BAT thermogenesis are normally elicited together by cold but are not tightly linked physiologically.     

Effect of immobilization stress on in vitro and in vivo thermogenesis of brown adipose tissue.

Repetitive intermittent stress such as immobilization has been shown to induce an improved cold tolerance through an enhanced capacity of nonshivering thermogenesis (NST), causing positive cross adaptation between nonthermal stress and cold. In the present study, effect of 3-h-daily immobilization stress for 4-5 weeks was investigated on in vitro and in vivo thermogenesis of interscapular brown adipose tissue (BAT). In vitro thermogenesis was measured in the minced tissue blocks incubated in Krebs-Ringer phosphate buffer with glucose and albumin at 37 degrees C, using a Clark-type oxygen electrode. The stressed rats showed less body weight gain during the experiment. The BAT weight, its protein and DNA contents were significantly greater in the stressed rats. Basal, noradrenaline- and glucagon-stimulated oxygen consumptions were significantly greater in the stressed rats. In vivo thermogenesis was assessed by the changes of temperatures in colon (Tcol), BAT (TBAT), and tail skin (Tsk) induced by noradrenaline or glucagon infusion in the anesthetized rats. Noradrenaline and glucagon increased the TBAT and the extent of increase was greater in the stressed rats. These results indicate that cross adaptation between nonthermal stress and cold may be mediated through an enhanced thermogenic activity of BAT.     

Role of the main olfactory system in recognition between individual spiny mice

Recognition between spiny mice requires a period of exposure to learn the olfactory cues of individual conspecifics that can serve as recognition signatures in subsequent discrimination. Animals received nasal irrigations of zinc sulfate to disrupt sensory input to the main olfactory system (MOS) either prior to the 9-day exposure period (Experiment 1) or immediately after the exposure period (Experiment 2). Animals rendered anosmic by zinc sulfate did not show evidence of recognition as defined by huddling preferences. In contrast, animals who received irrigations of saline were able to preferentially huddle with their cagemate who they had been housed with during the exposure period. The results suggest that the MOS is necessary in mediating behavioral discrimination between conspecifics.     

Repeated immobilization stress increases uncoupling protein 1 expression and activity in Wistar rats

Repeat immobilization-stressed rats are leaner and have improved cold tolerance due to enhancement of brown adipose tissue (BAT) thermogenesis. This process likely involves stress-induced sympathetic nervous system activation and adrenocortical hormone release, which dynamically enhances and suppresses uncoupling protein 1 (UCP1) function, respectively. To investigate whether repeated immobilization influences UCP1 thermogenic properties, we assessed UCP1 mRNA, protein expression, and activity (GDP binding) in BAT from immobilization-naive or repeatedly immobilized rats (3 h daily for 4 weeks) and sham operated or adrenalectomized (ADX) rats. UCP1 properties were assessed before (basal) and after exposure to 3 h of acute immobilization. Basal levels of GDP binding and UCP1 expression was significantly increased (140 and 140%) in the repeated immobilized group. Acute immobilization increased GDP binding in both naive (180%) and repeated immobilized groups (220%) without changing UCP1 expression. In ADX rats, basal GDP binding and UCP1 gene expression significantly increased (140 and 110%), and acute immobilization induced further increase. These data demonstrate that repeated immobilization resulted in enhanced UCP1 function, suggesting that enhanced BAT thermogenesis contributes to lower body weight gain through excess energy loss and an improved ability to maintain body temperature during cold exposure.     

Effects of cold and immobilization stress on noradrenaline turnover in brown adipose tissue of rat

Noradrenaline (NA) turnover of the interscapular brown adipose tissue (BAT) was determined in order to evaluate a role of sympathetic NA of this tissue in an enhanced nonshivering thermogenesis which had been previously evidenced in the repetitively stressed rats by immobilization (daily 3-h immobilization for 4 weeks) and the cold-acclimated ones (5 degrees C, 4 weeks). The disappearance rate of NA from the BAT following blockade of NA synthesis with alpha-methyl-p-tyrosine was adopted for estimation of NA turnover of the tissue. Cold acclimation increased both fractional turnover rate (%/h) (k) and turnover rate (ng/(g BAT.h)). Repetitive immobilization stress also elevated turnover rate, but not k. In the warm non-stressed controls acute cold exposure to -5 degrees C and acute immobilization stress elevated the turnover rate. The effect of cold exposure was significantly greater than that of immobilization stress for both indices of NA turnover. In the cold-acclimated rats acute cold exposure increased k as well as turnover rate, but not acute immobilization stress. In the repetitively immobilized rats both acute cold exposure and acute immobilization stress elevated k and turnover rate. These results indicate that immobilization enhances sympathetic activity of thermogenic tissue, BAT. The results also suggest that the extent of sympathetic participation is not necessarily the same between the cold-acclimated and the stressed rats.     

Brown adipose tissue and thermogenesis in hypophysectomized rats in relation to temperature acclimation

The aim of this work was to test the role of pituitary dependent hormones in cold-induced non-shivering thermogenesis. In the 28°C-acclimated rat, hypophysectomy inhibited body growth and led to an atrophy of thyroid and adrenals. In brown adipose tissue (BAT) some alterations were induced which are usually observed after cold acclimation of the animal: increase in relative weight, decreases in the relative amount of lipids, increases in the amounts of protein and DNA and modification of the proportions of several phospholipid fatty acids; moreover, basal lipolysis, in vitro, was enhanced to the same extent as that following cold acclimation of the normal rat. The in vivo stimulation by norepinephrine (NE) of O₂ consumption (test for nonshivering thermogenesis) and of fatty acid release into blood were suppressed.Progressive cold acclimation of the hypophysectomized rats at 15°C led to a hypertrophy of BAT to the same extent as in the sham-operated animals. The in vivo sensitivity to NE was partially restored. The results suggest that hypophysectomy does not suppress the ability to acclimate to moderate cold by means of BAT dependent non-shivering thermogenesis. However, the low ability to produce heat seems to indicate that pituitary or pituitary-dependent hormones are necessary to optimize the cold stimulation of brown fat thermogenesis.     

Pituitary and autonomic responses to cold exposures in man

This review presents hormonal responses to various cold exposures and their calorigenic effects in man and some animals. Previous studies in rats have shown that cold exposures activate the hypothalamic-pituitary-thyroid axis. Increased thyroid hormone concentrations lead to heat production via general stimulation of metabolism (obligatory thermogenesis) and possibly via activation of thyroid hormone receptors and uncoupling protein 1 (UCP 1) and deiodinase enzyme genes in the brown adipose tissue (BAT). In human subjects long-term cold exposures do not seem to activate the pituitary-thyroid axis, but rather accelerate the elimination of triiodothyronine (T3), leading to low serum concentrations of free T3 hormone. In corollary to this a hypothyreotic condition with increased serum thyroid-stimulating hormone and impaired mood and cognitive performance can be observed after long-term cold exposures such as wintering. During cold exposures the sympathetic nerve system is activated and noradrenaline is released to blood circulation and to BAT, where it leads to production of cAMP, lipolysis and free fatty acids. Free fatty acids open the mitochondrial proton channel protein in BAT. Protons enter the mitochondria and inhibit ATP synthesis (uncoupling). By this way energy is transformed into heat (facultatory or adaptive thermogenesis). In adult human subjects the amount of BAT is small and adaptive thermogenesis (non-shivering thermogenesis) has a smaller role. UCP 1 with other uncoupling proteins may have other functions in the control of body weight, sugar balance and formation of reactive oxygen species.     

Maternal care can rapidly induce an odor-guided huddling preference in rat pups

Olfactory-guided huddling is learned and expressed by postnatal day (PND) 15, when rat pups huddle preferentially with conspecifics or with targets bearing an odor previously associated with maternal care. Experiment 1 replicated this induction of an odor-guided huddling preference with a truncated regime of conditioning with a scented foster dam. Pups exposed to an odor in association with foster maternal care during five daily 2-hr sessions on PNDs 1-5, 5-9, or 10-14, but not pups merely exposed to the odor, displayed a huddling preference for the conditioned odor, but only when conditioning commenced after PND5. Experiment 2 demonstrated that a single, 2-hr exposure to a scented foster dam can induce a huddling preference in pups. Analysis of maternal behavior during the 2-hr conditioning sessions on PND14 revealed that frequency of maternal hovering over pups, but not licking/grooming or duration of contact, was associated with induction of the odor preference.     

Cross adaption between stress and cold in rats

Three-hour immobilization stress was imposed on male adult rats of Wistar strain by restraining them on a board 6 days a week for 1–8 weeks. The stressed rats showed less body weight gain during the experiment compared to the controls. These stressed animals manifested an improved cold tolerance as shown by no significant fall in colonic temperature in the cold at –5° C for 300 min during the experimental period, while the colonic temperature of the controls fell progressively. Nonshivering thermogenesis as assessed by noradrenaline-induced increase in oxygen consumption was significantly potentiated in the stressed rats. The weight and protein content of the intercapsular brown adipose tissue (BAT) increased and BAT mitochondria were more packed in the stressed rats. Plasma insulin, insulin/glucagon molar ratio and thyroxine levels were lowered in the stressed rats, while the plasma triiodothyronine level remained unchanged. Removal of interscapular BAT led to a loss of improved cold tolerance and a significant reduction of nonshivering thermogenesis in the stressed rats.These results indicate that repetitive stress may induce cross adaptation between stress and cold through an enhanced capacity of nonshivering thermogenesis mediated, at least in part, via stimulation of BAT function.     

Metabolic cold acclimation after repetitive intermittent cold exposure in rat

Repetitive intermittent cold exposure (5 degrees C, 6 h/day, 4 weeks) (ICE) resulted in the same cold adaptability as assessed by an enhanced cold tolerance (less drop of colonic temperature at -5 degrees C) and nonshivering thermogenesis (NST) (greater noradrenaline-induced heat production) as that elicited by continuous cold exposure (5 degrees C, 4 weeks) (CA) in rats. Although shorter intermittent (5 degrees C, 2 h/day, 4 weeks) (ICE-2 hr) as well as shorter continuous (5 degrees C, 1 week) (CA-1 wk) cold exposure effected an improved cold adaptability, the magnitude of cold tolerance and NST was smaller as compared with that in CA and ICE. The cold deacclimation process as reflected on the decreased NST did not differ between CA and ICE. Food intake was less in ICE than CA, while increase in body weight during the acclimation period was greater in the former. Increase in adrenal weight was greater in CA than ICE, but plasma corticosterone level did not differ among warm controls (WC), CA, and ICE in resting state (after 18-20 h at warm control temperature of 25 degrees C). Weights of interscapular and dorsocervical brown adipose tissue (BAT) increased to the same degree in CA and ICE. Plasma glucagon level in resting state did not differ among groups, while BAT glucagon levels significantly increased in CA and ICE, but they were higher in dorsocervical site than interscapular site in all acclimated states. Acute cold exposure (-5 degrees C, 15 min) caused increases in plasma corticosterone, glucagon levels, and in BAT glucagon levels in all acclimated groups. The extent of increase was significantly less for plasma glucagon in CA, while plasma corticosterone increased similarly in all groups. These results indicate that repetitive short-term cold exposure could elicit the same cold adaptability as that induced by continuous exposure, but requiring only one-fourth of the time of continuous cold exposure. Moreover, it is suggested that glucagon is involved in both CA and ICE, but the same extent of cold adaptability can be obtained in the less energy-requiring and less stressful state in ICE.     

Cold-induced changes in glucagon of brown adipose tissue

In order to evaluate the role of glucagon in brown adipose tissue (BAT) function under cold or stress, the changes in immunoreactive glucagon of BAT and plasma as well as uptake and metabolism of radioactive glucagon (125I-G) in this tissue were studied in rats. Glucagon per g fresh tissue was higher in the dorso-cervical BAT than in the interscapular BAT. In warm controls (WC), acute cold exposure (-5 degrees C, 15 min) (CE) or stress (immobilization, 30 min) (AS) elevated glucagon of both sites of BAT as well as plasma. In cold-acclimated animals (CA), the resting levels of BAT glucagon, but not plasma glucagon, were higher than WC. CE caused elevation of plasma glucagon, but not BAT glucagon in CA. AS did not affect glucagon levels in both plasma and BAT in CA. Cold acclimation did not influence 125I-G uptake by BAT, but resulted in a rather lower 125I-G level in plasma and liver. The present results suggest that BAT is a target tissue for glucagon to cause nonshivering thermogenesis in response to cold or stress and that turnover of glucagon is enhanced by cold acclimation.     

Lasting consistency of cold adaptability in rats reared in cold for many generations

Wistar rats were successively reared in cold at 5 degrees C from 1969 to 1984. The historical changes observed in these rats were reported. The cold-adapted rats reared in cold for 8 to 11 successive generations (C8-11G) were examined on their cold tolerance and non-shivering thermogenesis. C8-11G rats showed greater nonshivering thermogenesis than that of the warm-adapted control group (W), and rats exposed to cold for periods of 2 to 8 weeks (C). The nonshivering thermogenesis of C8-11G rats was diminished to a similar level to that of W and C rats by administration of a ganglion blocker, of reserpine, or of beta-adrenoceptor blocker. The de-adapted rats reared in warm at 25 degrees C for 3 generations after being reared for many generations in cold (DA-3G) showed much more nonshivering thermogenesis as compared to W rats. Cold tolerance of DA-3G rats was at a level of intermediate between that of W and C rats. Brown adipose tissue (BAT) weight of DA-3G rats was similar to that of C rats, while chemical composition of BAT in DA-3G rats differed from that of C and W rats.     

Thermogenic and vocalization responses to cold in the chicken hatchling during normoxia and hypoxia

We investigated the vocalization and the thermogenic responses to cold during hypoxia in chicken hatchlings during the first postnatal day. Calls were quantified in number and sound characteristics (amplitude and frequency); the change in oxygen (O2) consumption, measured by an open-flow methodology, represented thermogenesis. The cold challenge consisted of a decrease in ambient temperature (Ta) from ~39 to 28 °C, in steps of 2 °C, or an acute exposure to ~28 °C, either in normoxia or hypoxia (10% O2). Hypoxia lowered thermogenesis and the critical Ta, suggesting a decrease in the set point for thermoregulation. The vocalization response to cold was rapid; did not progress with the duration or intensity of the cold stimulus; was similar in very young (<8 hr old) and older (12-24 hr) hatchlings despite their differences in thermogenic capacity; and was essentially unaffected by hypoxia. We conclude that the hatchling's vocalization in the cold follows a stereotyped pattern not related to the thermogenic regulation of body temperature. The dissociation between vocalization and thermogenesis might carry some advantage in conditions of cold and hypoxia.