Abstract
Siberian hamsters transferred from a long (16 h light/day [16 L]) to an intermediate (13.5 L) day length (DL) undergo testicular regression within 2 months followed 2 months later by “spontaneous” testicular recrudescence. Recovery of gonadal function after prolonged exposure to intermediate DLs is thought to reflect development of neuroendocrine refractoriness to intermediate- duration melatonin signals. The authors tested the alternative hypothesis that testicular recrudescence in 13.5 L occurs when the “memory” for the 16-L photoperiod fades and hamsters can no longer compare the 13.5-Lto the prior16- L day length. Adult hamsters transferred from 16 L to 13.5 L that underwent testicular involution were either maintained continuously in 13.5 L for 41 weeks or given a supplementary 2-week treatment of 16 L before being returned to 13.5 L. The supplementary treatment was administered either after hamsters had been in 13.5 L for 10 weeks and had involuted testes, or after 24weeks, when the gonads had undergone recrudescence. The authors found that 16 L treatment administered at week 10 delayed final gonadal recrudescence by 12 weeks; similar 16-Ltreatment at week 24 induced a second gonadal regression when animals were returned to 13.5 L. The most parsimonious hypothesis to account for these findings is that gonadal recrudescence in intermediateDLs reflects fading of the “memory” for prior long DLs rather thaninduction of refractoriness to melatonin signals generated in intermediate DLs.