The vasopressin receptor system in the neonatal pituitary gland: evidence for reduced binding capacity and signal transmission.

The present study was aimed at evaluating the capacity of anterior pituitary cells from neonatal rats to bind arginine vasopressin (AVP) and show AVP-receptor-mediated signal transmission. We found that in cultures of pituitary cells of 10-day-old pups, in contrast to cultures of cells of adults, AVP was unable to trigger sustained adrenocorticotropin (ACTH) secretion and, in addition, was also less potent in synergizing with the effect of corticotropin-releasing factor (CRF) on both ACTH output and cyclic AMP formation. Binding studies revealed the existence of a much lower number of AVP receptor sites in membranes of neonatal pituitary gland than in those of adult tissue (32.3 +/- 9.0 and 137.6 +/- 6.2 fmol/mg protein, respectively), although the binding of agonists and the apparent molecular weight (Mr about 120,000) of the receptors were similar. Activation by phorbol ester PMA of protein kinase C, a messenger involved in AVP action, resulted in a dose-related enhancement of ACTH secretion that was 2-3 times smaller for immature corticotrophs than for mature ones. Importantly, PMA treatment allowed AVP to significantly stimulate ACTH secretion from neonatal cells, while it failed to similarly affect AVP-evoked hormone output from adult tissue. Our results indicate that pituitary corticotrophs of rat pups fail to properly transduce AVP-receptor-mediated signalling and, thereby, suggest an explanation for the postnatal 'stress nonresponsive period'.