EVIDENCE FOR A DOUBLY INNERVATED SECRETORY UNIT IN ANURAN PARS INTERMEDIA .I. ELECTROPHYSIOLOGIC STUDIES

Two types of spontaneously active electrical units were found in the pars intermedia of frogs. Lesion experiments show that these units are neurons that come to the pars intermedia separately grouped in the infundibular floor. One neuron type is inhibited by light; the other is indifferent to changes in illumination. For inhibition of the light-sensitive unit, the eye is not the photoreceptor but the pineal organ appears to be. Reconciling these data with the published literature suggests that the basic secretory unit in the anuran pars intermedia is a doubly innervated cell. The light-indifferent neuron is probably adrenergic and tonically inhibitory. The light-inhibitable neuron probably stimulates MSH release when its electrical activity is maximal (in environments of low light value); in the light, when spontaneous electrical activity of the second neuronal type is minimal, the tonically inhibitory influence supervenes, presumably reducing MSH release. © 1969.