Some Questions on the Nature and Function of Cranial and Caudal Neurosecretory Systems in Non-Mammalian Vertebrates

Abstract

There are two separable cranial (hypothalamic) neurosecretory systems in vertebrates: one originating in the preoptic (and homologous) nuclei and the other in the lateral tuberal nuclei. However, in teleosts, the median eminence is at most residual and there is a specialized system of neurosecretory innervation of adenohypophysial cells. Innervation of the adenohypophysis, especially of the pars intermedia, occurs in other vertebrates as well. Some hypophysiotropic control systems differ among the several vertebrate groups. Nothing definitive is known about the chemistry of nonmammalian hypophysiotropins. A caudal neurosecretory system is probably present in all fish groups except cyclostomes and lungfishes, although the urophysis is characteristic only of teleosts. The function of this system is still unknown, but principles with smooth muscle contracting, hypotensive, and hydrosmotic effects are located in and released from the urophysis. The hydrosmotic effect is probably due to arginine vasotocin; the other factors, although peptidic, are not related to neurohypophysial octapeptides. Proteins unique to the urophysis can be demonstrated and undergo changes in response to certain stimuli. An increasing amount of pharmacological information on urophysial principles is available, but the physiology of the system remains essentially unknown.