A daily rhythm in behavioral vasopressin sensitivity and brain vasopressin concentrations.

The concentration of Arg-vasopressin (AVP) in the suprachiasmatic nuclei of the hypothalamus (SCN), the neural generator of circadian rhythms, showed a daily rhythm, which was inversely related to the rhythm in lordosis, an aspect of sexual behavior shown by ovariectomized estradiol-17 beta-treated female rats. A threshold dose of an AVP antagonist facilitated sexual behavior most effectively if injected intracerebroventricularly when the endogenous levels of AVP in the SCN were maximal and a threshold dose of AVP inhibited the behavior most effectively if injected when these levels were minimal. The results support the suggestion that AVP may be the neuropeptide whereby the SCN generate some behavioral rhythms.