Temperature regulation in the mouse, Peromyscus leucopus: effects of various photoperiods, pinealectomy and melatonin administration

Seasonal Control of Mammalian Energy Balance: Recent Advances in the Understanding of Daily Torpor and Hibernation

Endothermic mammals and birds require intensive energy turnover to sustain high body temperatures and metabolic rates. To cope with the energetic bottlenecks associated with the change of seasons, and to minimise energy expenditure, complex mechanisms and strategies are used, such as daily torpor and hibernation. During torpor, metabolic depression and low body temperatures save energy. However, these bouts of torpor, lasting for hours to weeks, are interrupted by active 'euthermic' phases with high body temperatures. These dynamic transitions require precise communication between the brain and peripheral tissues to defend rheostasis in energetics, body mass and body temperature. The hypothalamus appears to be the major control centre in the brain, coordinating energy metabolism and body temperature. The sympathetic nervous system controls body temperature by adjustments of shivering and nonshivering thermogenesis, with the latter being primarily executed by brown adipose tissue. Over the last decade, comparative physiologists have put forward integrative studies on the ecophysiology, biochemistry and molecular regulation of energy balance in response to seasonal challenges, food availability and ambient temperature. Mammals coping with such environments comprise excellent model organisms for studying the dynamic regulation of energy metabolism. Beyond the understanding of how animals survive in nature, these studies also uncover general mechanisms of mammalian energy homeostasis. This research will benefit efforts of translational medicine aiming to combat emerging human metabolic disorders. The present review focuses on recent advances in the understanding of energy balance and its neuronal and endocrine control during the most extreme metabolic fluctuations in nature: daily torpor and hibernation.

Feeding Schedule Controls Circadian Timing of Daily Torpor in SCN-Ablated Siberian Hamsters

Timing of daily torpor was assessed in suprachiasmatic nucleus-ablated (SCNx) and sham-ablated Siberian hamsters fed restricted amounts of food each day either in the light or dark phase of a 14:10 light-dark cycle. Eighty-five percent of sham-ablated and 45% of SCNx hamsters displayed a preferred hour for torpor onset. In each group, time of torpor onset was not random but occurred at a mean hour that differed significantly from chance. Time of food presentation almost completely accounted for the timing of torpor onset in SCNx animals and significantly affected timing of this behavior in intact hamsters. These results suggest that the circadian pacemaker in the SCN controls the time of torpor onset indirectly by affecting timing of food intake, rather than by, or in addition to, direct neural and humoral outputs to relevant target tissues.

Photoperiod and Thermoregulation in Vertebrates: Body Temperature Rhythms and Thermogenic Acclimation

Evidence has recently begun to accumulate that photoperiodic responses of mam mals and birds may affect the control of energy balance and thermoregulation. Exposure to short photoperiod can lower the set point for body temperature regulation in birds and mam mals, as well as the voluntarily selected body temperature in ectothermic lizards. This de crease is accompanied by a reorganization of circadian or ultradian rhythms of body temper ature, particularly an increase in periods spent at rest with minimum body temperatures. Short photoperiod is also used as an environmental cue for induction of seasonal torpor or facilita tion of hibernation. During winter, cold tolerance of small mammals is improved by an in crease of nonshivering thermogenesis in brown fat. Thermogenic capacity of brown fat (res piratory enzymes, mitochondria, uncoupling protein) is enhanced in response to short pho toperiod. This response is mediated via an increase in the activity of sympathetic innervation in brown fat. Moreover, an exposure to short photoperiod prior to low temperatures may act in preparing brown fat for facilitated thermogenesis during acclimation to cold. This shows that photoperiodic control not only affects energy balance indirectly via the control of repro duction or body mass, but may directly interact with central control of thermoregulation and may influence the process of acclimatization.

Spontaneous daily torpor and fasting-induced torpor in Djungarian hamsters are characterized by distinct patterns of metabolic rate

The Djungarian hamster is a rodent species that expresses both spontaneous daily torpor (SDT) when acclimated to winter conditions as well as fasting-induced torpor (FIT) during summer. In an earlier report we argued that these two thermoregulatory phenomena differ in several parameters. In the present study, we further complete this comparison by showing that metabolic rate patterns differ between both SDT and FIT. SDT bouts were significantly longer and deeper compared to FIT bouts. Additionally, respiratory quotient measures support the view that SDT is entered from a state of energetic balance while FIT appears to be an emergency shutdown of energy demanding thermogenesis due to a shortage of energy sources. In a second experiment, we also confirm that brief periods of food restriction during the hamsters' torpor season increase the frequency of SDT, but do not affect its depth or duration. Although winter-acclimated animals could flexibly alter torpor frequency in order to stay in energetic balance, we also found evidence for torpor expression patterns that resembled FIT, rather than SDT. Consequently, if energetic challenges cannot be compensated with increased SDT expression any longer, the hamsters seem to be driven in a negative energy balance resulting in FIT as a last resort.

Factors affecting the daily rhythm of body temperature of captive mouse lemurs (Microcebus murinus)

Microcebus murinus, a small nocturnal Malagasy primate, exhibits adaptive energy-saving strategies such as daily hypothermia and gregarious patterns during diurnal rest. To determine whether ambient temperature (T(a)), food restriction and nest sharing can modify the daily body temperature (T(b)) rhythm, T(b) was recorded by telemetry during winter in six males exposed to different ambient temperatures (T(a) = 25, 20, 15 degrees C) and/or to a total food restriction for 3 days depending on social condition (isolated versus pair-grouped). At 25 degrees C, the daily rhythm of T(b) was characterized by high T(b) values during the night and lower values during the day. Exposure to cold significantly decreased minimal T(b) values and lengthened the daily hypothermia. Under food restriction, minimal T(b) values were also markedly lowered. The combination of food restriction and cold induced further increases in duration and depth of torpor bouts, minimal T(b) reaching a level just above T(a). Although it influenced daily hypothermia less than environmental factors, nest sharing modified effects of cold and food restriction previously observed by lengthening duration of torpor but without increasing its depth. In response to external conditions, mouse lemurs may thus adjust their energy expenditures through daily modifications of both the duration and the depth of torpor.

Unpredictable deprivation of water increases the probability of torpor in the Syrian hamster

The effect of unpredictable water deprivation on hibernation was investigated in the Syrian hamster under natural photoperiod and temperature. Based on our previous findings, we hypothesized that 1) deprivation of water caused testicular regression, 2) the magnitude of regression depended on unpredictability of water deprivation, 3) unpredictable deprivation of water on 50% of the days of the experimental period causes more testicular regression for animals than regular deprivation every other day, even if the total lengths were the same for the two treatments, and thus, 4) unpredictable deprivation of water induces hibernation more readily than regular deprivation or no deprivation. The results showed that the animals with the random and unpredictable deprivation a) started hibernation earlier, b) had a higher chance of being in torpor at the end of hibernation season, c) spent more time in torpor during hibernation than those animals which were deprived of water on a regular basis or the control, non-deprived animals. These findings supported our working hypothesis.

Function of melatonin in thermoregulatory processes

Accumulated information shows that, besides its role in the timing of seasonal reproduction, melatonin also plays an important role in seasonal thermoregulatory adjustments of animals including torpor and hibernation. Furthermore, melatonin has a crucial role in circadian thermoregulatory adjustments of body temperature (Tb). Melatonin appears to send signals to the preoptic area of anterior hypothalamus (PoAH) where it adjusts the set point of Tb consistent with the metabolic rate of the animal. This new function for melatonin as a transducer mediating information about energy balance has been suggested in this review. Melatonin also adjusts the activity of the biological clock in vertebrates.

Daily rhythms of metabolic rate and body temperature of two murids from extremely different habitats

Daily circadian rhythms of body temperature (Tb) and oxygen consumption (VO2) were measured in two murid species, which occupy extremely different habitats in Israel. The golden spiny mouse (Acomys russatus) is a diurnal murid distributed in arid and hot parts of the great Syrio-African Rift Valley, while the broad-toothed field mouse (Apodemus mystacinus) is a nocturnal species that inhabits the Mediterranean woodlands. In both species, the daily rhythms of Tb and VO2 are entrained by the photoperiod. Under laboratory experimental conditions (ambient temperature Ta = 33 degrees C and photoperiod regime of 12L:12D), Acomys russatus exhibits a tendency towards a nocturnal activity pattern, compared to the diurnal activity displayed by this species under natural conditions. Under the same photoperiod regime and at Ta = 28 degrees C, Apodemus mystacinus displays nocturnal activity, as observed under natural conditions. The maximal values of Tb were recorded in Acomys russatus at midnight (23:50 h), while the maximal values of VO2 were recorded at the beginning of the dark period (18:20 h). In Apodemus mystacinus, the maximal values of Tb and VO2 were recorded at 23:40 and 20:00 h, respectively. The ecophysiological significance of these results is discussed further.

The influence of multiple photoperiods and pinealectomy on gonads, pelage and body weight in male meadow voles, Microtus pennsylvanicus

1. Chronic exposure of male Microtus pennsylvanicus to photoperiods with 8, 10 and 12 hr of light per day results in gonadal regression, molt to winter pelage and significant reductions in body weight and food consumption relative to voles kept on photoperiods with 13 and 14 hr of light per day. 2. A precise critical daylength is observed in this species as exposure to photoperiods with 12 or fewer hours of light per day results in complete gonadal involution, seasonal molt and loss of body weight. 3. Pinealectomy abolishes all short-photoperiod induced responses in this species.

Prolongation of hibernation bout duration by continuous intracerebroventricular infusion of melatonin in hibernating ground squirrels

Melatonin was infused intracerebroventricularly into hibernating golden-mantled ground squirrels (Citellus lateralis) maintained at 5 degrees C in darkness. Continuous infusion at a rate of 0.5 microliter/h was accomplished using an osmotic minipump. The effect of melatonin on hibernation bout duration was determined with reference to the natural trend in bout duration for each animal. At doses of 200 and 400 ng/h, melatonin produced a dose-related increase in bout duration. No effect was observed following control infusions of artificial cerebrospinal fluid or lower doses of melatonin.

Both hydroxy- and methoxyindoles modify basal temperature in the rat

The various pineal gland tryptophan metabolites were administered to male rats intraperitoneally (100 micrograms/kg) and rectal temperatures were recorded. Of the compounds tested, hydroxytryptophan, N-acetylserotonin, hydroxytryptophol, and their corresponding methoxyindoles all caused a marked hypothermia, indicating that several indolic products may be involved in thermoregulation. Although the brain penetration of indoles is poor, a central site of action would be most likely, although peripheral actions cannot be excluded. The mechanism of induction of hypothermia may involve peptides, the pituitary-thyroid axis, the adrenal gland, or a combination of these. These results may suggest that the pineal gland integrates environmental cues to act in concert with physiological thermostats in the fine tuning of thermoregulation.

Influence of the pineal gland and melatonin on blood flow and evaporative water loss during heat stress in rats

Plasma melatonin levels of laboratory rats were elevated both during acute heat exposure (43 degrees C for 40 min) and chronic exposure (33 degrees C for 17 days) suggesting a possible correlation between melatonin and thermoregulatory mechanisms. Pinealectomy reduced the nighttime elevation in oxygen consumption and evaporative water loss. In addition, pinealectomized animals exhibited a significantly lower cutaneous evaporative water loss both at night and during the day when exposed to an acute heat exposure of 38 degrees C for 45 min. Pinealectomy elevated the blood pressure over the control group whereas melatonin infusion depressed the blood pressure without altering the cardiac output. This relationship implies an action by melatonin on the peripheral vasculature. In support of this conclusion, melatonin pretreatment tended to dampen the vasopressive effect of infused norepinephrine. These data, therefore, suggest a role of the pineal gland and melatonin in thermoregulation through an influence on the cardiovascular system and evaporative water loss.

Pineal melatonin: circadian rhythm and variations during the hibernation cycle in the ground squirrel, Spermophilus lateralis

Variations in pineal melatonin content throughout a 24-hour period and during different phases of the hibernation bout cycle were studied in the golden-mantled ground squirrel (Spermophilus lateralis). In addition to pineal melatonin, the circadian variation in the activities of pineal N-acetyltransferase (NAT) and hydroxyindole-O-methyltransferase (HIOMT) were also investigated in summer animals maintained at 22 +/- 2 degrees C, on a light:dark (L:D) schedule of 12:12 hr for 1 month (lights on at 08.00 hr). Pineal glands were collected from six animals in each group at 1200, 1600, 2000, 2400, 0200, 0400, and 0800 hr. Changes in pineal melatonin content during the hibernation bout cycle were investigated in ground squirrels housed at 4 +/- .05 degrees C in relative darkness (1.9-3.4 lux; 10:14 LD). Pineal glands were obtained between 12:00 and 18:00 hr from 30 animals during one of three phases of the cycle (deep hibernation, euthermic interbout, and entrance into hibernation). Pineal melatonin was also measured for comparison in six winter euthermic animals that were housed at 22 +/- 2 degrees C, on a L:D schedule of 10:14 hr. Melatonin was measured in individual pineal glands by radioimmunoassay. The daily melatonin rhythm in S. lateralis was characterized by a marked increase in pineal melatonin during the dark phase, in which peak nighttime values were nearly 20-fold greater than daytime basal levels. The daily rhythm for NAT activity paralleled the changes in melatonin, showing a peak activity at 0200 hr that was 45 times greater than mean daytime values.(ABSTRACT TRUNCATED AT 250 WORDS)

Photoperiod time measurement for activity, torpor, molt and reproduction in mice

Resonance light:dark cycles (LD 6:18, 6:30, 6:42, or 6:54) were used to establish that a circadian rhythm of light sensitivity is involved in the thermoregulatory and reproductive responses to a short day photoperiod in the mouse, Peromyscus leucopus. A fifth group was maintained on the long day photoperiod of LD 16:8. After 19 weeks animals presented with LD 6:18 or 6:42 exhibited short day photoperiod responses: gonadal regression, incidence of spontaneous daily torpor and molt to the winter pelage. In contrast animals responded to LD 6:30 and 6:54 as long day photoperiods: maintenance of gonadal system, no incidence of spontaneous daily torpor, and summer pelage. In a second study a T-experiment was conducted to determine that more than one circadian system may regulate these multiple photoperiodic effects. Mice were exposed to 1 of 8 LD cycles for 15 weeks (1:22.00, 1:22.25, 1:22.50, 1:23.00, 1:23.50, 1:23.75, 9:15, or 16:8), Entrained wheel-running activity occurred under all LD regimes. Mice on LD 1:22.50, 1:23.00, and 1:23.50, however, exhibited activity patterns similar to mice on LD 9:15, and they exhibited gonadal regression. Mice on LD 1:22.00, 1:22.25, and 1:23.75 exhibited activity patterns similar to LD 16:8 animals, and most of these animals remained reproductively competent. There was also a close association between occurrence of reproductive regression and daily torpor. In contrast, molt to the winter pelt occurred under all non-24-hr LD cycles. This dysynchrony in response suggests that at least 2 circadian systems are involved in photoperiodic time measurement in P. leucopus.

Two daily melatonin injections differentially induce nonshivering thermogenesis and gonadal regression in the mouse (Peromyscus leucopus)

Female white-footed mice (Peromyscus leucopus) were injected twice daily with 5, 10, 50, 100 micrograms melatonin (MEL) or saline. Injections were given for 7 weeks at 2 and 12 hours after lights-on under a long day (LD 16:8) photoperiod. Afternoon administration of MEL induced gonadal regression, although a dose of 50 micrograms or more was necessary to obtain a maximal response. A 5 micrograms MEL injection in the afternoon resulted in intermediate reproductive tract weights. In white-footed mice a morning MEL injection did not abolish the reproductive regression induced by an afternoon injection. Mice receiving 10, 50 or 100 micrograms MEL daily exhibited increased nonshivering thermogenesis (NST), irrespective of the timing of the injection. Daily injections of 5 micrograms MEL had little effect on NST. These observations suggest that "up and down regulation" of MEL receptors may not be important in P. leucopus. Further, the mechanism by which MEL controls reproduction is different from that for NST.

Thermoregulatory responses to photoperiod by kangaroo rats (Dipodomys ordii): influence of night lighting on nonshivering thermogenesis and resting metabolism

The influence of photoperiod on resting oxygen consumption and nonshivering thermogenesis of winter-acclimatized kangaroo rats was determined by exposing freshly captured animals to 10-week controlled photoperiod treatments. Three treatments were employed: 1) long days (LD) = 16L:8D, 2) short days (SD) = 8L:16D, and 3) short days interrupted with 15 min light at midnight (SDL). Thermoneutral oxygen consumption was uninfluenced by treatment, and posttreatment values were not different from pretreatment values. Below thermoneutrality, mean values for oxygen consumption and norepinephrine(NE)-induced thermogenesis each were significantly greater in the SD group than they were in either the LD or SDL groups. Furthermore, oxygen consumption of LD and SDL groups, and NE-induced thermogenesis of LD and SDL groups were indistinguishable. Posttreatment oxygen consumption and NE-induced thermogenesis of the SD group were not different from pretreatment values. These results indicate that thermogenic capacities of kangaroo rats are strongly influenced by photoperiod. The similarities of responses by the LD and SDL groups indicate that the length of the dark period is of importance, and indirectly support suggestions that the pineal gland, through its nocturnal secretion of melatonin, may be involved in the transduction of photic information.

Metabolic and thermoregulatory effects of photoperiod and melatonin on Peromyscus maniculatus acclimatization

A photoperiod-related seasonal rhythm in active period (scotophase), metabolic rate and core temperature was documented for animals held at 21.0 +/- 0.1 degrees C ambient; animals that were habituated to long nights (10:14LD) had a greater metabolic reserve than those held in summer photoperiods (14:10LD). While relative weights of gonads and sex accessory tissues of mice show typical "winter" regression, interscapular brown adipose tissue mass was unaffected by photoperiod; moreover, IBAT beta adrenergic responses under "winter" photoperiods did not differ from "summer" photoperiods in the absence of cold stimulus. Thermogenic efficiency, measured as the increment of active temperature level achieved per increment of active period metabolic effort, was highest for animals exposed to short photoperiods. Thermal conductance was reduced in animals exposed to short (10:14LD) photoperiods. Heat conservation and thermogenic response capacity was enhanced by melatonin treatment and short photoperiod.

Decreases in pineal melatonin content during the hibernation bout in the golden-mantled group squirrel, Spermophilus lateralis

The role of the pineal gland in modulating the rhythmic bouts of hibernation in the golden-mantled ground squirrel (S. lateralis) was explored by comparing pineal melatonin content in hibernating animals with that of euthermic animals at the same time of year. Significant decreases in pineal melatonin content were found in hibernating versus euthermic animals. In addition, significantly lower values for pineal melatonin were observed in hibernating animals that were sacrificed in the late bout period, just prior to expected spontaneous arousal, as compared to hibernating animals that were sacrificed on the first day of their respective bouts. A strong correlation was evident between pineal melatonin content and the duration of the individual hibernation bout. These data suggest that pineal melatonin may be important in determining the duration of individual bouts of hibernation in this species.

Seasonal changes in glycogen level and size of pinealocytes of the white-footed mouse, Peromyscus leucopus: a semiquantitative histochemical study

Glycogen level in and size of pinealocytes of the feral, white-footed mouse Peromyscus leucopus, were studied by a semiquantitative histochemical method to determine whether seasonal changes exist in them under natural conditions, what temporal pattern they exhibit, and whether 24-hour changes in these parameters exist in different seasons, as shown in the laboratory dd-mice. Marked seasonal changes were seen in both glycogen levels and nuclear densities (ANOVA p less than 0.005). The size of pinealocytes at 09:00 to 10:00 showed one peak (and nadir) seasonal change, with the smallest size in winter (December and February) and a larger size in warmer seasons, with the maximum value in July. Glycogen level in pinealocytes at 09:00 to 10:00 showed bimodal seasonal changes, with lower levels in fall and spring and higher levels in winter and summer. In fall, a circadian trend in glycogen level in pinealocytes was seen, with a higher level at the end of the light period. In winter, the glycogen levels were very high at 09:00, 13:00, 17:00, and 21:00 examined and showed dampening of time-of-day differences. On the other hand, the size of pinealocytes followed a time-of-day change (P less than 0.005), being largest at 13:00 and smallest at 21:00. Thus, marked changes in quantitative structure and chemical activities, suggesting changes in functional activity, in pinealocytes were noted especially in severe, cold winter.

Effects of 18 weeks of daily melatonin injection on reproduction and temperature regulation in the mouse, Peromyscus leucopus

Mice injected daily with 50 microgram of melatonin, 12 hr after lights on, for 18 weeks, underwent gonadal regression after 4-7 weeks and reproductive recrudescence after 15 weeks. Most treated animals molted to the winter pelt after 9-11 weeks of injections, and 2 of the 10 mice experienced bouts of daily torpor after 14 weeks.

Pineal atrophy and other neuroendocrine and circumventricular features of the naked mole-rat, Heterocephalus glaber (Rüppell), a fossorial, equatorial rodent

Quantitative aspects of the microanatomy of the pineal gland and other neuroendocrine and circumventricular structures were studied in a small, reproductively suppressed, female Naked Mole-rat from central Kenya, Africa. The atrophic pineal is the smallest in absolute size (0.002135 mm3) of any so far described in a species of rodent, and in size relative to body weight is second only to that of another tropical species. The subcommissural organ and posterior collicular recess are also relatively small and less well differentiated than those in most other examined rodent species. In contrast, the subfornical organ, OVLT and median eminence are large and well vascularized. It is concluded that the pineal in this species follows the previously described trend among rodents of relatively smaller size in species whose centers of distribution are in lower latitudes. Although the pineal is atrophic, the Naked Mole-rat still exhibits 24-hour and seasonally timed patterns of behavior and seasonal reproduction. However, in this species these events are probably cued by moisture, temperature and social factors rather than by photic information.