EXPRESSION OF CIRCADIAN RHYTHMICITY CORRELATES WITH THE NUMBER OF ARGININE-VASOPRESSIN-IMMUNOREACTIVE CELLS IN THE SUPRACHIASMATIC NUCLEUS OF COMMON VOLES, MICROTUS-ARVALIS

We have studied the number of arginine-vasopressin (AVP)-immunoreactive (IR) cells in the suprachiasmatic nuclei (SCN) in common voles, Microtus arvalis, with a strong (n = 18), weak (n = 10) or absent (n = 9) expression of circadian rhythmicity. Spontaneous expression of rhythmicity was assessed from records of wheel running activity and general activity, measured with passive infra-red detection during 4 weeks of continuous low light (LL) conditions. Subsequently, 20 voles were perfused in LL. After additional exposure to a 12:12 h light-dark (LD) cycle during 14 days, the other 17 voles were perfused in the early morning. AVP-positive neurons were visualized immunocytochemically with a polyclonal IgG antibody. AVP-stained cell somata were present predominantly in the dorsomedial SCN, whereas terminals, contacting both AVP positive and negative neurons, were distributed throughout the SCN. In LL conditions, AVP-staining intensity of neurons was low in all 20 voles. Also the number of AVP-IR cells in the SCN in brain sections obtained in LL conditions was low and not different for the three categories of rhythmicity. In LD conditions, the staining intensity of AVP positive SCN neurons was low in rhythmic animals, high in non-rhythmic animals not expressing, and intermediate in animals weakly expressing circadian rhythmicity. The number of AVP positive cells in LD conditions showed a negative correlation with the propensity of animals to lose circadian rhythmicity under LL conditions. Differences in AVP-IR were found solely in the dorsomedial SCN, and neither in the rostral and caudal aspects of the SCN nor in the area adjacent to the SCN. These results suggest that the AVP-neurons of the SCN form part of a discrete neuronal substrate which functionally correlates with the expression of circadian rhythmicity.