THE EFFECTS OF SHORT PHOTOPERIOD, PINEALECTOMY, AND MELATONIN TREATMENT ON OXYTOCIN SYNTHESIS AND RELEASE IN THE MALE SYRIAN-HAMSTER

The pineal gland has been shown to affect plasma oxytocin (OT) levels, but the mechanism of this action is not apparent. In the present stud y, the ability of the photoperiod to affect plasma OT levels, neuroint ermediate lobe (NIL) OT content, and hypothalamic OT mRNA levels was s tudied in male Syrian hamsters. In addition, the ability of pinealecto my to prevent and melatonin (MEL) to mimic the short photoperiod-induc ed changes were also determined. Exposure to short days (SD) led to th e expected decrease in testes weight and plasma PRL levels, but plasma OT levels were unchanged. However, NIL OT content was increased in th e SD-exposed animals. Hypothalamic OT mRNA levels were not significant ly altered by SD exposure. Pinealectomy blocked the effects of SD on t estes weight, whereas afternoon MEL injections mimicked the effects of SD, In long day (LD)-exposed hamsters, pinealectomy induced a decreas e in NIL OT content without altering hypothalamic OT mRNA levels. In S D-exposed animals, NIL OT content was not affected by pinealectomy. Me latonin injections had no significant effect on NIL OT content or hypo thalamic OT mRNA levels. The data from the present study suggest that exposure of male Syrian hamsters to short photoperiods influences some aspects of OT synthesis and/or transport to produce its increased acc umulation in the NIL, but does not affect OT release. These changes ar e apparently not the result of SD-induced changes in MEL secretion, bu t conceivably could be related to the previously documented effects of SD on hypothalamic catecholamine turnover.