PHOTOPERIODIC REGULATION OF HYPOTHALAMIC NEUROPEPTIDE MESSENGER-RNA EXPRESSION - EFFECT OF PINEALECTOMY AND NEUROANATOMICAL LOCATION

Seasonal changes in daylength (photoperiod) affect many aspects of mam malian physiology and behavior, including reproduction, metabolism, th ermoregulation, and sleep. The circadian pacemaker in the hypothalamic suprachiasmatic nuclei (SCN) regulates these photoperiodic changes. O ur studies of the Siberian hamster SCN have shown that two types of ne uropeptide-containing neurons, vasopressin (AVP) and vasoactive intest inal peptide (VIP) neurons, respond to short photoperiod by decreasing mRNA expression. The present studies investigated whether photoperiod ic inhibition of mRNA expression also occurs in somatostatin-synthesiz ing neurons in the SCN, depends upon the pineal gland, and occurs in n eurons in other hypothalamic nuclei. Juvenile Siberian hamsters expose d 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 Mat photoperiod decreased the expression of AVP mRNA and VIP mRNA in the SCN, as seen previously, bu t not somatostatin mRNA. Furthermore, pinealectomy did not attenuate t he short photoperiod inhibition of AVP mRNA and VIP mRNA expression in the SCN. Also, short photoperiod inhibition of AVP mRNA expression wa s found in the paraventricular and supraoptic nuclei, as well as in th e SCN. These results show that short photoperiod inhibition of mRNA ex pression is partially selective among the neuropeptides, but is not re stricted to the SCN. Furthermore, these findings suggest that photoper iodic regulation of neuropeptide mRNA expression is independent of pin eal melatonin secretion and gonadal steroid secretion. (C) 1998 Elsevi er Science B.V. All rights reserved.