ROLE OF PROLACTIN AND GONADAL-STEROIDS IN REGULATION OF OXYTOCIN MESSENGER-RNA DURING LACTATION

The role of suckling, prolactin, and gonadal steroids in regulating hypothalamic oxytocin (OT) mRNA content during the 1st and 2nd wk of lactation was evaluated. On day 4 of lactation, OT mRNA content decreased in rats removed from their litter for 24 h compared with suckled controls. Either suckling (in the absence of prolactin release) or prolactin (in the absence of suckling) maintained OT mRNA content at this stage of lactation. In contrast, at day 11 of lactation, OT mRNA content remained unchanged in rats deprived of pups for 24 h. Ovariectomy did not compromise the ability of day 11 nonsuckled rats to maintain OT mRNA content; however, the increased water intake, plasma prolactin, plasma vasopressin (VP), plasma renin concentration, and hypothalamic VP mRNA content at day 11 compared with day 4 suggest a role for fluid balance in determining OT mRNA content during the 2nd wk of lactation. Thus, at day 4, suckling is a major determinant of OT mRNA content as a result of both direct activation of neuronal afferents to the OT neurons and stimulation of prolactin release. This is in contrast to day 11, when fluid balance may predominate in regulation of OT mRNA.