IMMUNOREACTIVE PROLACTIN FORMS COLOCALIZE WITH VASOPRESSIN IN NEURONSOF THE HYPOTHALAMIC PARAVENTRICULAR AND SUPRAOPTIC NUCLEI

system as revealed by the detection of the PRL mRNA and of PRL-like im munoreactive and biologically active proteins in hypothalamic paravent ricular (PVN) and supraoptic (SON) nuclei and in the neurohypophysis. We have investigated the distribution of cells containing PRL-like mol ecules in the PVN and SON by immunocytochemistry with a specific antis erum directed against the 16-kD N-terminal fragment of PRL. PRL-positi ve cells were found to be concentrated throughout the ventral SON and in the lateroposterior region of the PVN. The cellular distribution of PRL-immunoreactive cells resembled more closely that of vasopressin ( VP) than that of oxytocin magnocellular neurons. Moreover, double immu nofluorescence labelling, followed by confocal microscopy, indicated t he coexistence of PRL and VP-related antigens within the same neurons of the PVN and SON. Pre-embedding immunoperoxidase on the ultrastructu ral level showed a PRL-like product in granular-type particles within the neural soma and projections in the SON and PVN. These findings are consistent with the expression and secretion of PRL-like molecules by vasopressinergic neurons of the hypothalamo-neurohypophyseal system.