Temperature dependence of gonadal regression in Syrian hamsters exposed to short day lengths

We sought to determine whether ambient temperature (T-a) affects gonadal function by altering the rate at which circadian rhythms entrain to short day lengths. Syrian hamsters were housed in cages where they received 14 h of light per day (long days, 14L) at 22degreesC. Hamsters were then transferred to cages to receive 10 h of light per day (short days, 10L) and kept at 5, 22, or 28degreesC or were maintained in 14L at 22degreesC. Body mass and estimated testis volume as well as duration of nocturnal locomotor activity (alpha), previously established as a reliable indicator of the duration of nocturnal melatonin secretion, were determined over the course of 24 wk. Testicular regression in short days was accelerated by 4 wk at 5degreesC and delayed by 3 wk at 28degreesC relative to 22degreesC. The interval between alpha-expansion and initiation of testicular regression was markedly affected by T-a with delays of 0, 3, and 6 wk at 5, 22, and 28degreesC, respectively. All hamsters held at 5 and 22degreesC underwent testicular regression, but 25% of those maintained at 28degreesC failed to do so. We suggest that T-a modulates testicular regression primarily by affecting responsiveness of neuroendocrine target tissues to long melatonin signals.