LACTATION AS A MODEL OF NATURALLY REVERSIBLE HYPERCORTICALISM PLASTICITY IN THE MECHANISMS GOVERNING HYPOTHALAMO-PITUITARY-ADRENOCORTICAL ACTIVITY IN RATS

Steady state levels of hypothalamic expression of the genes encoding c orticotropin-releasing hormone (CRH),proopiomelanocortin (POMC), argin ine vasopressin (AVP), and oxytocin (OT) were studied in rats to inves tigate the mechanisms underlying the transitions between hypercortical ism during lactation and normocortiralism upon weaning, During lactati on, CRH mRNA levels and blood titers of adrenocorticotropin (ACTH) wer e found to be significantly reduced, although POMC mRNA levels in the anterior pituitary were not significantly different from those found i n cycling virgin (control) rats; during all phases of lactation, an in verse relationship was observed between the blood levels of ACTH and c orticosterone (CORT), Plasma prolactin (PRL) concentrations were eleva ted similar to 30-fold during lactation, Whereas steady state levels o f OT mRNA were markedly increased throughout lactation, those of AVP m RNA were only transiently (initially) elevated, and the blood levels o f these hormones were not significantly altered in lactating as compar ed with cycling virgin and postlactating rats, CRH and POMC gene expre ssion and blood levels of ACTH, CORT, and PRL were normalized within 1 -3 d of removal of suckling pups, The temporal relationships between t he biosynthetic profiles of the various peptide hormones and the patte rns of ACTH and CORT secretion during the two physiological states sug gest that lactation-associated hypercorticalism does not merely result from increased ACTH secretion; although still not web substantiated a t this time, the evidence points to contributory roles of PRL, OT, and AVP in the hypercorticalismic state found during lactation.