Djungarian 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.

A brain site of melatonin action has been determined for the white-footed mouse (Peromyscus leucopus). Melatonin-beeswax implants releasing small quantities of melatonin (<100 ng/day) caused a 50% reduction in reproductive tract weight relative to controls (p < 0.025) with 83% of these animals having an imperforate vagina, when implanted in the anterior hypothalamic nuclei (AH) and suprachiasmatic nuclei (SCN). Subcutaneous implants had little effect. Implants in the AH and SCN also had a pronounced effect on both lipid-free interscapular brown fat and nesting behavior. Mice implanted in these regions exhibited a 59% increase in interscapular brown fat and 65% more nesting than controls (both p < 0.01). These results suggest that melatonin acts at a region in the anterior hypothalamus which controls photoperiodic adjustments.

Endotherms 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 weight loss +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.

Djungarian hamsters (Phodopus sungorus) kept under a long-day photoperiod (16 h light :8 h dark) were injected with melatonin each day. Hamsters which responded physiologically to this treatment (gonadal regression, molt, body weight loss) phase-advanced onset and extended duration of activity. Hamsters which were physiologically insensitive to melatonin injections did not exhibit such changes in activity pattern and often failed to entrain to the light :dark cycle. Hamsters given saline injections did not alter activity or exhibit gonadal regression, weight loss and molt to the winter pelt. Melatonin-sensitive hamsters compressed duration of activity when they became physiologically refractory to the melatonin treatment (weeks 27–29). At the same time, melatonin-insensitive hamsters became entrained to the light :dark cycle. Thus, daily melatonin injections induce short-day-like adjustments in activity under a long-day photoperiod. These changes in activity are correlated with melatonin-induced gonadal regression, weight loss and molt.

A pronounced diurnal change in responsiveness to intrahypothalamic melatonin injections was observed in the white-footed mouse, Peromyscus leucopus. 7 weeks of chronic daily afternoon (12 h after lights-on) injections of 500 ng melatonin into the vicinity of the suprachiasmatic nuclei (SCN) caused a 66% reduction in female reproductive tract weight relative to saline-injected controls (p < 0.01), with 83% of these animals having an imperforate vagina, and no animals having mature follicles. Subcutaneous injections of 500 ng melatonin had little effect. Chronic morning (2 h after lights-on) injections of melatonin into the vicinity of the SCN had little effect on the maintenance of normal reproductive tract weight, with 18% of these animals having an imperforate vagina, and 71% having mature follicles. These results indicate that the daily rhythm of melatonin antigonadal action is due to change in responsiveness to melatonin of target neurons in the region of the SCN.