Projections from the paraventricular nucleus to the zona externa of the median eminence of the rhesus monkey: an immunohistochemical study.

Immunoperoxidase techniques and light microscopy were used to investigate the origin of the fibers containing vasopressin, oxytocin and related neurophysins (neurohypophyseal peptides), in the zona externa of the median emience of the rhesus monkey. Unilateral lesions were produced in the paraventricular nucleus using a modified Halasz-type knife. Complete destruction of the paraventricular nucleus resulted in a total depletion of all four neurohypophyseal peptides in the ipsilateral zona externa of the median emience. Partial lesions of different segments of the paraventricular nucleus resulted in depletion in different parts of the zona externa. These observations indicate that the paraventricular nucleus is the source of vasopressin, oxytocin and neurophysin in axons which end in the zona externa of the median eminence. These projections are strictly ipsilateral and arranged topographically, so that the more caudal paraventricular neurons project to the dorsal part of th zona externa and the rostral ones to the ventral segment. The supraoptic nucleus does not appear to project to the zona externa, and sends the great majority of its fibers to the neurohypophysis.