Termination of neuroendocrine refractoriness to melatonin in Siberian hamsters (Phodopus sungorus).

Siberian hamsters maintained from birth in a short day length (DL), unlike their long-day counterparts, fail to undergo reproductive development by 5 weeks of age. Instead, reproductive maturation of short-day males is delayed for approximately 20 weeks, at which point neuroendocrine refractoriness to the inhibitory effects of short DLs develops, resulting in growth of the gonads. To terminate refractoriness and re-establish responsiveness to short photoperiods, 10-15 weeks of long-day exposure is required. We assessed whether continuous exposure to long days is necessary to terminate refractoriness or whether the first few weeks of long days initiate a process that culminates several months later in the breaking of refractoriness. Male hamsters refractory to short DLs were transferred to a long-day photoperiod, pinealectomized (PINx) after 0, 3, 6 or 15 weeks, and subsequently infused for 6 weeks with a short-day melatonin signal. This melatonin treatment induces gonadal regression in photosensitive but not in photorefractory hamsters. Six percent of males PINx at week 0 and 88% of those PINx at week 15 underwent gonadal atrophy by the end of the melatonin infusion treatment initiated on week 15. Among hamsters PINx on week 6, 17% versus 76% underwent testicular involution in response to melatonin infusions initiated on week 6 and week 15, respectively. This finding indicates that a fraction of the long days that hamsters experience during spring and summer are sufficient to trigger the processes that restore responsiveness to short DLs. Additional groups of pineal-intact photorefractory animals were given 3, 6 or 15 weeks of long-day exposure and then returned to a short DL for several months; only those treated for 15 weeks terminated refractoriness. The breaking of refractoriness, once triggered by long-day melatonin signals, proceeds to completion only in the absence of short-day melatonin signals.