1. Seasonal adjustments in wild-caughtPeromyscus leucopus include autumn increases in basal metabolic, nonshivering thermogenesis, and interscapular brown fat and decreases in weights of gonads, liver, adrenal glands, and total lipid. Body weight and nonextractable dry weight do not change. 2. Basal metabolic rate, nonshivering thermogenesis, and interscapular brown fat increase following initial cold exposure in mid-September and are maintained at similar levels through January. 3. There is a positive correlation between nonshivering thermogenesis and interscapular brown fat, and negative correlations for both nonshivering thermogenesis and interscapular brown fat with body weight. 4. These seasonal changes in wild-caughtP. leucopus are compared with laboratory studies on the effects of chronic exposure to differences in temperature and photoperiod on these characters. It is concluded that disparities between the effects of cold acclimatization and cold acclimation could result from the influence of additional environmental cues, such as photoperiod, in cold acclimatized mice.

Bidirectional artificial selection for (High Line) and against (Low Line) photoresponsiveness altered the percent of photoresponsive hamsters within lines and affected circadian function of hamsters identical in photoresponsiveness. For example, free-running period was shorter in responsive relative to nonresponsive hamsters. Between-line differences for responders and nonresponders were also found: hamsters from the High Line had a shorter free-running period relative to Low Line hamsters. However, phase angle of entrainment to long and short days was not affected. In general, expression of circadian rhythmicity was extraordinarily inflexible in photononresponsive hamsters from both lines: 1) phase angle of entrainment to lights on was similar under short and long day; 2) activity duration was similar under long and short days, although some decompression occurred in constant dark; 3) aftereffects on the free-running period were absent; and 4) amplitude of the phase-response curve was small (+/- 1 h) and present only at circadian times 10-24. We propose that selection for or against photoresponsiveness may have affected the interaction of component oscillators underlying circadian rhythmicity.

No abstract

Long-term implants releasing a small quantity of melatonin (45 nanograms per day) were used to determine the brain sites of the hormone's antigonadal action in a photoperiodic species, the white-footed mouse (Peromyscus leucopus). Implants in the medial preoptic and supra- and retrochiasmatic areas elicited completed gonadal regression after 7 weeks. Implants in other brain regions had little effect on the animals' reproductive state.

Djungarian hamsters (Phodopus sungorus sungorus) depend mainly on day length to cue seasonal adjustments. However, not all individuals respond to short day conditions. A previous study from this laboratory proposed that nonresponsiveness to short day conditions rests with a defect in the circadian organization of these hamsters. In this study we found pronounced differences between responsive and nonresponsive hamsters in the expression of circadian rhythmicity under constant darkness and under constant illumination. While responsive hamsters showed a free-running activity pattern with a period of 23.86 +/- 0.04 h and responded to brief light pulses with the expected phase delays and phase advances, nonresponsive hamsters exhibited a period of 24.04 +/- 0.05 h and responded to light pulses with phase advances. Furthermore, 9 out of 15 responsive hamsters showed a clear split in the activity pattern within 8 weeks under constant light (80-100 lux), while only 1 of the 7 nonresponsive hamsters exhibited a split activity pattern. As a result of these differences in circadian function, nonresponsive Djungarian hamsters are incapable of proper photoperiod time measurement and photoperiod-induced seasonality.