Photoperiodic regulation of hypothalamic neuropeptide messenger RNA expression: effect of pinealectomy and neuroanatomical location.

Seasonal changes in daylength (photoperiod) affect many aspects of mammalian physiology and behavior, including reproduction, metabolism, thermoregulation, and sleep. The circadian pacemaker in the hypothalamic suprachiasmatic nuclei (SCN) regulates these photoperiodic changes. Our studies of the Siberian hamster SCN have shown that two types of neuropeptide-containing neurons, vasopressin (AVP) and vasoactive intestinal peptide (VIP) neurons, respond to short photoperiod by decreasing mRNA expression. The present studies investigated whether photoperiodic inhibition of mRNA expression also occurs in somatostatin-synthesizing neurons in the SCN, depends upon the pineal gland, and occurs in neurons in other hypothalamic nuclei. Juvenile Siberian hamsters exposed to either long photoperiod (16 h light/day) or short photoperiod (10 h light/day) for 2 weeks after weaning, were used for these studies. Coronal sections throughout the SCN were prepared and processed for in situ hybridization. The results showed that photoperiod decreased the expression of AVP mRNA and VIP mRNA in the SCN, as seen previously, but not somatostatin mRNA. Furthermore, pinealectomy did not attenuate the short photoperiod inhibition of AVP mRNA and VIP mRNA expression in the SCN. Also, short photoperiod inhibition of AVP mRNA expression was found in the paraventricular and supraoptic nuclei, as well as in the SCN. These results show that short photoperiod inhibition of mRNA expression is partially selective among the neuropeptides, but is not restricted to the SCN. Furthermore, these findings suggest that photoperiodic regulation of neuropeptide mRNA expression is independent of pineal melatonin secretion and gonadal steroid secretion.