G. Robert Lynch

Influence of Ambient Temperature, Nest Availability, Huddling, and Daily Torpor on Energy Expenditure in the White-Footed Mouse Peromyscus Leucopus

Short-day (9 light:15 dark), cold-acclimated Peromyscus leucopus known to enter torpor (no. = 24) and P. leucopus never observed in torpor (no. = 12) were compared with and without a 5-g nest at 13, 7, and 1 C over 9 wk. Oxygen consumption and body temperature were monitored approximately hourly for 2-3 days under each treatment. A second group of short-day, cold-acclimated mice (13 torpid and 14 nontorpid) were monitored for oxygen consumption with and without a 5-g nest at 13 C, first individually and 10 days later as huddles of three. For individual mice, the time required to enter torpor (3.0-4.5 h), duration of torpor (4-5 h), and minimum body temperature during torpor (21-23 C) remained fairly constant at different ambient temperatures. The metabolic rate [cm³O₂/(g · h)] necessary to maintain minimum body temperature during torpor without a nest was 2.1 at 13, 3.0 at 7, and 3.7 at 1 C, while metabolic rates during the normothermic period at these temperatures were 4.8, 5.5, and 6.7, respectively. A similar trend, but with lower values, occurred with a nest. Without a nest, torpid mice had a mean daily metabolic rate which was 20% less at 13, 21% at 7, and 9% at 1 C than the corresponding value of nontorpid animals. Torpid mice with a 5-g nest reduced metabolic rate at those temperatures by 43%, 44%, and 34%, respectively, relative to nontorpid mice without a nest. Daily torpor and huddling together provided a 58% energetic saving at 13 C relative to individually housed, nontorpid mice. The addition of a nest fostered a 74% daily energy savings versus nontorpid, individual mice without a nest.

Effects of photoperiod, melatonin administration and thyroid block on spontaneous daily torpor and temperature regulation in the white-footed mouse,Peromyscus leucopus

SummaryCold exposed (13°C) mice maintained on a short day photoperiod (9L:15D) became torpid 9 times more frequently than long day photoperiod (16L:8D) animals (Table 1). Mice on a short day photoperiod also exhibited a 26% increase in nesting behavior, a 9% decrease in food consumption and a 49% increase in norepinephrine induced thermogenesis (Table 2). No change in resting metabolism was observed. Similarily, chronic melatonin administration (subcutaneously implanted beeswax pellet containing 3.5 mg melatonin) elicited a 2.5 fold increase in spontaneous daily torpor relative to shamimplanted mice (Table 3). Mice treated with melatonin exhibited a 33% increase in nesting behavior and a slight decrease in food consumption. Although thyroid block (propyl-thiouracil) increased the incidence of daily torpor only slightly, it did effect an 11% decrease in resting metabolism, a 42% decrease in norepinephrine induced thermogenesis and a 5% decrease in food consumption. Thyroid block did not alter nesting behavior.

Evidence for differences in the circadian organization of hamsters exposed to short day photoperiod

SummaryDjungarian hamsters,Phodopus sungorus, depend mainly on day length to cue seasonal adjustments in reproduction and thermoregulation. These photoperiod-induced changes are mediated by changes in the daily release of pineal melatonin. However, some hamsters fail to respond to chronic short day exposure, and these individuals lack typical short day rhythms for both daily activity and pineal melatonin content. These results indicate that nonresponding hamsters lack the circadian organization responsible for proper coding of day length. Although the nature of the disruption in circadian organization is yet not known, these results clearly demonstrate the central importance of circadian rhythms in regulating photoperiod-induced adjustments in reproduction and thermoregulation.

Melatonin induced changes in gonads, pelage and thermogenic characters in the white-footed mouse, peromyscus leucopus

Abstract 1. Pellets containing 3·1 mg melatonin and 12·4 mg beeswax were subcutaneously implanted in warm exposed mice. Controls received 15·5 mg pellets of beeswax. 2. Mice treated with melatonin exhibited regression of the reproductive system, a fall moult, hypertrophy of interscapular brown fat, and an increase in basal metabolic rate relative to controls.

In vitro electrical activity in the suprachiasmatic nucleus following splitting and masking of wheel-running behavior

Djungarian hamsters (Phodopus sungorus sungorus) were chronically exposed to constant light (60 lux) in order to generate a split rhythm of wheel-running behavior. The animals were killed and coronal hypothalamic slices prepared for extracellular recording from the left and right suprachiasmatic nuclei (SCN). Nine hamsters which exhibited a split in overt behavior also had bimodal peaks (280 cells) of SCN firing frequency (5.4 +/- 1.1 and 5.8 +/- 0.7 Hz); these peaks were about 180 degrees antiphase. Two troughs in firing frequency were also apparent (1.8 +/- 0.4 and 2.4 +/- 0.5 Hz) and coincided with the projected time of wheel-running activity. Differences in circadian pattern of electrical activity between the right and left SCN were not observed. When wheel-running activity in 5 hamsters was suppressed with high intensity (500 lux) constant light, the SCN firing profile (154 cells) failed to exhibit a daily rhythm. Firing frequency was consistently high (5.8 +/- 1.4 Hz) throughout the 24-h period. These results indicate that in vitro electrical activity of the SCN is related to overt behavior in Djungarian hamsters, in that high electrical activity occurs when locomotor activity is minimal.

Seasonal Study of Spontaneous Daily Torpor in the White-Footed Mouse, Peromyscus leucopus

Thirty-five wild-caught Peromyscus leucopus were individually caged in outdoor enclosures throughout the year, and daily change in body temperature was monitored with telemetry. Spontaneous daily torpor (in the presence of food) was observed during most of the year, except during late summer, and was most frequent during winter. At this time mice became torpid on about 50% of the days that they were monitored. In order to study daily torpor in free-ranging populations, two 1.7-hectare study plots, consisting of either 72 or 60 nest boxes, were established in a Connecticut oakhickory forest. Daily torpor was observed in both mouse populations from mid-December through mid-February. The highest incidence of torpor occurred during early January when 20 out of 36 mice were found torpid. Torpor in free-ranging mice was never recorded on days when ambient temperature was above 3 C. Although abundant food was provided on one study plot, no decrease in daily torpor was observed. Free-ranging mice also exhibited considerable huddling behavior from mid-December through mid-February, and 96% of the animals in torpor were found in groups of two to six mice. In such cases, all mice in the nest box were usually torpid.

Genetic analyses of photoresponsiveness in the Djungarian hamster,Phodopus sungorus

SummaryEndotherms living at temperate and arctic latitudes must adjust their physiology and behavior in order to survive seasonal change. The Djungarian hamster uses photoperiod to cue annual cycles of reproduction and thermoregulation, and its responses to short photoperiod include loss of body weight and change in pelage color. Some individuals do not exhibit these responses when exposed to short days.In this study individual variation in photoresponsiveness is quantified, and four lines of evidence for a genetic component to that variation are provided. First, two separate breeding stocks differed in both the percent of animals responding to a short-day lighting regimen (SD) and in the degree and timing of their response. Second, analysis of variance within and between families of full sibs for a photoresponsive index, PI (body weightloss +2 (molt index −1)) following 12 weeks in SD demonstrated a significant family resemblance (intraclass correlation of 0.36±0.03). Third, heritability estimates from regression of offspring scores on parent scores for body weight loss, molt index and PI after 12 weeks in SD were 0.34∓0.13, 0.36±0.10 and 0.37±0.12, respectively, indicating a strong additive genetic component for the three characters. Finally, a significant response occurred after one generation of artificial selection for and against photoresponsiveness.

Evidence for Genetic Variation in the Occurrence of the Photoresponse of the Djungarian Hamster, Phodopus sungorus

The Djungarian hamster generally responds to a short-day photoperiod with a complex syndrome of physiological and behavioral changes; however, not all hamsters are photoresponsive. The phenotypic difference is, in part, genetically determined. Parent-offspring regression on a number of continuous and discontinuous measures indicated significant heritability for photoresponsiveness. Four generations of replicated bidirectional selection on a photoresponse index (PI) resulted in significant shifts in the percentage of responsive hamsters, although the average PI of responsive individuals was not significantly changed. Eight estimates of heritability ranged from 0.20 to 0.52. We hypothesize that the circadian system is responsible for the occurrence of the photoresponse, but that the extent of photoresponse is controlled by a separate functional system.

Circadian differences in neuronal activity of the suprachiasmatic nucleus in brain slices prepared from photo-responsive and photo-non-responsive Djungarian hamsters

The suprachiasmatic nucleus (SCN) of the hypothalamus, as the putative generator of circadian rhythmicity, plays an important role in mammalian photoinduction. To determine if SCN function differs in photo-non-responsive Djungarian hamsters, we defined the pattern of spontaneous neuronal discharge of single cells from SCN slices in vitro of photo-responsive and photo-non-responsive phenotypes. Responsive hamsters exhibited a peak neuronal discharge rate (4.8 +/- 0.5 Hz) during the mid day which gradually attenuated through the late day and early night. In non-responsive hamsters, a similar discharge rate (5.1 +/- 0.5) was maintained through the late day and early night. The delayed decline in spontaneous firing rate of non-responders correlates with their delayed activity onset and delayed nocturnal pineal melatonin pulse. These data support the argument that the absence of photoperiodic adjustments in Phodopus sungorus rests with differences in SCN function.

Intraspecific differences for melatonin-induced reproductive regression and the seasonal molt in Peromyscus leucopus

Laboratory-reared female Peromyscus leucopus from Connecticut (CT) and Georgia (GA) stocks were injected with 50 μg melatonin or saline for either 12 or 18 weeks at 12 hr after lights on (16L:8D). After 12 weeks 93% of the melatonin-treated CT mice exhibited an imperforate vagina and a reduced reproductive tract weight (31 ± 11, P < 0.01) and follicular index (6.0 ± 0.2, P < 0.001) relative to saline-injected mice (18%, 81 ± 11, 7.4 ± 0.2, respectively). Six of the 14 melatonin-treated CT mice molted to a winter pelt. Spontaneous gonadal recrudescence was occurring by Week 18 in CT mice injected with melatonin. In contrast, melatonin had little effect on these parameters in GA mice. Though a few GA animals exhibited a reduced reproductive tract weight and an imperforate vagina, follicular index was unaffected. Such differential sensitivity to melatonin injections is analagous to a previous report for differences in photoperiodic sensitivity between these two populations.