To clarify the anatomical organization that allows for the synergy of vasopressin and oxytocin with corticotropin-releasing factor (CRF) in promoting adrenocorticotropic hormone secretion from the anterior pituitary, immunohistochemical double staining methods were used to compare the distribution of these peptides in the hypothalamic paraventricular nucleus of normal, colchicine-treated, and adrenalectomized male rats. In untreated animals, a few CRF-stained cells were found in the parvocellular division of the paraventricular nucleus, while brightly stained oxytocin- and vasopressin-immunoreactive cells were centered in the magnocellular division. In animals treated with colchicine, and inhibitor of axonal transport, large numbers of CRF-stained cells were found in the parvocellular division of the nucleus, and 1-2% of these also stained with antivasopressin. As reported previously, a substantial number of oxytocin-stained cells, centered in a discrete anterior part of the magnocellular division, also expressed CRF immunoreactivity. In contrast, after adrenalectomy, CRF immunostaining of cells in the parvocellular division was enhanced selectively and greater than 70% of these cells also stained positively for vasopressin. The distribution of oxytocin-stained cells was not influenced by adrenalectomy. The unusual localization of vasopressin immunoreactivity in parvocellular neurosecretory neurons in the adrenalectomized rat suggests that a single population of cells can produce CRF and vasopressin, both of which are potent promoters of adrenocorticotropic hormone secretion. These findings indicate that there is a state-dependent plasticity in the expression of biologically active peptides by individual neuroendocrine neurons.
