SUCKLING-INDUCED CHANGES OF VASOACTIVE-INTESTINAL-PEPTIDE CONCENTRATIONS IN HYPOTHALAMIC AREAS IMPLICATED IN THE CONTROL OF PROLACTIN-RELEASE

The present study was designed to investigate whether the vasoactive intestinal peptide (VIP) concentration in hypothalamic nuclei, dorsal raphe nucleus (DR) and pituitary lobes of lactating rats changes in physiological situations when prolactin (PRL) secretion is stimulated (suckling) or inhibited (pup separation). In addition VIP levels in blood plasma were determined in both situations. Acute suckling induced changes in VIP concentration only in the rostral part of the anterior hypothalamic (rAHN) and the paraventricular (PVN) nuclei of all the brain areas examined. VIP concentration in the rAHN increased at 5 min from 3.52 +/- 0.30 (mean +/- SEM) to 8.67 +/- 1.91 ng/mg protein (p < 0.05) but fell to baseline values after 30 min suckling (p < 0.05; 5 vs. 30 min). Although changes in VIP concentration in the suprachiasmatic nucleus (SCN) did not attain statistical significance, they followed the same trends as the changes of VIP in the rAHN. The opposite pattern of changes was observed in the PVN with a decrease in VIP concentration following 5 min suckling (p < 0.01). At 30 min the VIP values showed a trend towards 0-min values. Pup removal did not affect VIP concentrations in the rAHN, PVN, SCN, median eminence, supraoptic nucleus and DR. VIP values were not detectable in the arcuate nucleus in any of the experimental situations examined. Lactation increased VIP concentration only in the rAHN and PVN when lactating rats with their pups were compared with virgin female diestrous rast. VIP concentration in the anterior lobe of the pituitary from lactating rats did not change with pup separation or suckling. In contrast, VIP levels in the neurointermediate lobe displayed a significant increase (p < 0.01) with pup removal and fell with 5 (p < 0.05) and 30 min suckling (p < 0.01; 0 vs. 30 min). Plasma VIP levels were not altered by any of the situations examined. The present results in combination with our previous data lead us to postulate the participation of VIP in suckling-induced PRL release principally as a central neurotransmitter and secondarily as a minor PRL-releasing factor released from the posterior lobe.