Neurosecretion—Comparative and Evolutionary Aspects

Abstract

The rationale for the existence of neurosecretory phenomena and their place within the range of neurochemical mediation is inherent in their evolutionary history. The signs of neurosecretory activity—i.e., the specialization of certain neurons for the production of neurohormones—are found in all metazoans. These hormones are, thus, phylogenetically the oldest and also the only blood-borne messengers operating in the more primitive invertebrates, which lack regular glands of internal secretion. The recognition of neuronal types capable of several intermediate modes of neurochemical communication has removed the sharp dividing line between classical neurosecretory cells and conventional neurons, and the phenomenon of neurosecretion, once so difficult to assimilate, now blends into a spectrum of neurochemical activities. The conceptual model of the ancestral neuron, considered as the phylogenetic derivative of an undifferentiated and pluripotential epithelial cell, is that of a functionally versatile structure, equally endowed for the dispatch of long distance and localized chemical signals. The neurosecretory neuron has remained closer to the nerve cell precursor than has the conventional neuron with its specialization for synaptic transmission. In addition, the inherited capacity for secretory activity has been put to good use by neurosecretory neurons in various ways at every level of the evolutionary scale.