Photoperiodic exposure and time of day modulate the expression of arginine vasopressin mRNA and vasoactive intestinal peptide mRNA 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 photoperiod also alters 24-h rhythms in neuropeptide mRNA levels in the SCN of Siberian hamsters. In situ hybridization and quantitative autoradiography were used to assess messenger RNA levels for vasopressin (AVP) and vasoactive intestinal peptide (VIP) in the SCN of hamsters sacrificed at six times of day following exposure to long (16 h light/day) or short (10 h light/day) photoperiod for 2 weeks. Both AVP mRNA and VIP mRNA in the SCN were significantly affected by time of day and photoperiodic exposure. The 24-h profiles of AVP mRNA and VIP mRNA showed different relationships to the light: dark cycle, suggesting that these profiles are differentially regulated. In general, short photoperiod tended to suppress AVP mRNA and VIP mRNA in the SCN; this effect on AVP mRNA was significant at two times of day. These results complement and extend previous findings of 24-h h profiles in neuropeptide mRNA expression in the rat SCN by showing that these 24-h profiles are also characteristic of the Siberian hamster SCN and that they can be modulated by photoperiod.