Variations in concentration of oxytocin and vasopressin in the paraventricular nucleus of the hypothalamus during the estrous cycle in rats.

Oxytocin (OT) and arginine-8-vasopressin (AVP) were measured by radioimmunoassay in micropunched hypothalamic neurosecretory nuclei of estrous cycling female Sprague-Dawley rats. In the paraventricular nucleus (PVN): the concentration (pg/microgram protein) of OT was significantly higher in rats in diestrus than during proestrus, estrus, or metestrus, while the concentration during metestrus was significantly greater than in proestrus and estrus; the concentration of AVP was significantly lower in animals in estrus than during the other three stages; because the paraventricular OT levels dropped before proestrus, the AVP/OT ratio was significantly greater in animals in proestrus than in diestrus, metestrus, and estrus. In the supraoptic nucleus (SON) a similar trend was noted: the concentration of OT was highest during diestrus, and AVP was lowest during estrus, though neither was significantly different from other stages. Because the OT and AVP cycles in the SON were asynchronous, the ratio of AVP to OT was significantly higher in proestrus than in metestrus or diestrus and significantly greater in estrus than during diestrus. In contrast to these two areas, peptide concentrations did not vary significantly across the estrous cycle in other sites of nonapeptide synthesis, i.e. the anterior commissural nucleus (ACN) and the suprachiasmatic nuclei (SCN).