Hypophysiotropic Centers in the Brain of Amphibians and Fish

Abstract

The subject is the localization of three different hypophysiotropic centers in the brain of amphibians and fish. The thyrotropic hormone-releasing hormone (TRH) in Xenapus may originate from the dorsal magno-cellular neurons of the preoptic nucleus. This hypothesis is based on correlative changes between these cells and alterations in thyroid activity during metamorphosis. Experimental data are in support of a functional relationship between certain preoptic neurons and the thyrotropic activity of the pituitary. The MSH inhibiting activity of the hypothalamus is effected by means of an aminergic innervation of the pars intermedia in amphibians, teleosts and elasmobranchs. In amphibians the aminergic fibers originate from the caudal part of the paraventricular organ (PVO); in elasmobranchs probably from the nucleus medius hypothalamicus(NMI); in teleosts the origin still has to be investigated. Two centers producing gonadotropic hormone-releasing hormone (GRH) have been demonstrated. Lesion experiments lead to the hypothesis that GRH is produced in the caudal hypothalamus, i.e., in the nucleus infundibularis ventralis of amphibians and in the nucleus lateralis tuberis of fishes. ImmunoHuorescence studies indicate in both groups the presence of neurons, in front of the preoptic area in the telencephalon, and these neurons are immuno-reactive with anti-mammalian LH-RH.