Effects of photoperiod and androgen on proopiomelanocortin gene expressionin the arcuate nucleus of golden hamsters

In golden hamsters, seasonal changes in day length act via a pineal-depende nt mechanism to regulate feedback and behavioral effects of androgen. Endog enous opiates participate in photoperiodically regulated neuroendocrine fun ctions, but the effects of androgen on expression of the gene encoding POMC , the precursor of beta-endorphin, have been controversial. We used quantit ative in, situ hybridization to examine regulation of POMC messenger RNA (m RNA) by testosterone and to test the hypothesis that short day lengths act through the pineal gland to amplify POMC mRNA expression. We studied intact hamsters and castrates with or without androgen treatment held in long (14 h of light, 10 h of darkness) or short (5 h of light, 19 h of darkness) da ys for 10 weeks. POMC gene expression differed with rostral-caudal plane, photoperiod, and s urgical treatment (castration and testosterone administration). Testosteron e increased the number of silver grains in labeled cells throughout the arc uate nucleus, and short day castrates given androgen consistently had more silver grains per labeled cell than did their long day counterparts. Testos terone exerted an inhibitory effect, however, on the number of POMC mRNA-po sitive cells, and more POMC mRNA-labeled cells were found in the arcuate nu cleus of long than short day castrates treated with testosterone. Photoperi od had no significant influence in castrates not receiving androgen. Testos terone treatment had generally similar effects whether it was begun at the time of castration or 5 weeks later. Pinealectomy blocked the influence of photoperiod on both the mean number of silver grains per labeled cell and t he number of labeled cells. The results indicate that day length regulates POMC gene expression when an drogen levels are held constant, but that androgen is necessary for photope riod effects to be expressed.