PHOTOPERIODIC EXPOSURE AND TIME OF DAY MODULATE THE EXPRESSION OF ARGININE-VASOPRESSIN MESSENGER-RNA AND VASOACTIVE-INTESTINAL-PEPTIDE MESSENGER-RNA IN THE SUPRACHIASMATIC NUCLEI OF SIBERIAN HAMSTERS

In hamsters, changes in ambient photoperiod lead to alterations in the circadian rhythm of pineal melatonin secretion and subsequent changes in reproductive function. The present study examined whether photoper iod also alters 24-h rhythms in neuropeptide mRNA levels in the SCN of Siberian hamsters. In situ hybridization and quantitative autoradiogr aphy were used to assess messenger RNA levels for vasopressin (AVP) an d vasoactive intestinal peptide (VIP) in the SCN of hamsters sacrifice d at six times of day following exposure to long (16 h light/day) or s hort (10 h light/day) photoperiod for 2 weeks. Both AVP mRNA and VIP m RNA in the SCN were significantly affected by time of day and photoper iodic exposure. The 24-h profiles of AVP mRNA and VIP mRNA showed diff erent relationships to the light:dark cycle, suggesting that these pro files are differentially regulated. In general, short photoperiod tend ed to suppress AVP mRNA and VIP mRNA in the SCN; this effect on AVP mR NA was significant at two times of day. These results complement and e xtend previous findings of 24-h h profiles in neuropeptide mRNA expres sion in the rat SCN by showing that these 24-h profiles are also chara cteristic of the Siberian hamster SCN and that they can be modulated b y photoperiod.