Adult hamsters (8-week-old) were exposed to a short photoperiod (6 h light, 18 h dark) and an ambient temperature of 23°C. The testes photoregressed to the most “inactive” state at 13 weeks of treatment (D0). Then, in half of the hamsters, the ambient temperature was reduced from 23°C to 5°C to induce a hibernated state (C6). It was shown that, after 13 weeks of exposure to a short photoperiod (D0), spermatogenic activity spontaneously recrudesced after 10 to 20 weeks in both the 5°C and 23°C groups (“D10” and “C20”), albeit a 7 to 10 week delay in GDNF expression was observed in the 5°C group. (A) A line graph, including small circle graphs at each stage, shows changes in testicular weight (Y axis in the line graph; error bars indicate±SD) and the appearance ratio of GDNF-positive seminiferous tubules (small circle graph at each stage; also see Table S1) in adult testes exposed to a short photoperiod (solid lines in the line graph; D0, D6, and D10) in combination with a low ambient temperature (broken lines in the line graph; C6, C13, and C20). X axis represents weeks before and after the most “inactive” state (D0) was reached in Week 13 of treatment. (B) Anti-GDNF immunostaining patterns show no appreciable positive signals in almost any of the seminiferous tubules in “inactive” testes at D0 and C6 stages. A rapid recovery of immunoreactive GDNF-positive signals is ubiquitously observed, even at C13 stage (arrows) when the testicular weight is at a similarly low level to that in the inactive state, D0 (“D0”, “C13” in A, B). A seminiferous cycle-dependent pattern of GDNF signals recovers at D10 and C20. The insets in B indicate the constant GATA4 expression in the Sertoli cells throughout all stages (insets in B). Asterisks, non-specific signals in interstitial region. Scale bars represent 100 µm.