Hypophysiotropic neurons in the hypothalamus of the catfish Clarias batrachus: a cobaltous lysine and HRP study.

Seven hypothalamic nuclei and several isolated perikarya that send projections to the pituitary gland were identified following administration of cobaltous lysine or horseradish peroxidase (HRP) to severed hypophysial stalks of previously hypophysectomized catfish, Clarias batrachus. Retrogradely labelled neurons were identified in the nucleus preopticus periventricularis, suprachiasmatic nucleus, paraventricular and supraoptic divisions of the magnocellular nucleus preopticus (NPO) and nucleus lateralis tuberis. A few neurons in the paraventricular subdivision of the NPO, however, remained unfilled; these may project to extrahypophysial sites. Three other nuclei contributing to the innervation of the pituitary gland include the paraventricular organ, nucleus recessus lateralis (NRL) and nucleus recessus posterioris (NRP), all of which contain cerebrospinal fluid-contacting aminergic neurons. These three neuronal aggregations were retrogradely labelled with cobaltous lysine but not with HRP. Isolated neurons displaying hypophysial connections were identified in the organon vasculosum laminae terminalis area, in the nucleus hypothalamicus ventromedialis, and in the vicinity of the NRL and NRP. Thus the present study reveals that hypothalamic projections to the pituitary gland of catfish have their origin in several peptidergic and aminergic nuclei and perikarya hitherto unreported.