Neurosecretion and the parvicellular hypothalamic system

Abstract

After the latest discoveries on the peptides identified in the vertebrate hypothalamus and on the mechanisms of neuroendocrine regulations, the classical definition should be amended of the neurosecretory cell (as a neuron producing histologically detectable secretions acting via the blood stream and unable to make true synapses).

Besides the magnocellular neurons revealed by special staining techniques, the existence has been ascertained of some other “peptidergic neurosecretory systems” of a parvicellular type. These impinge on the neuroendocrine axis by releasing substances which act locally, or are conveyed to effectors via a specialized vascular circuit (as a rule, the hypophysial portal system). These peptidergic systems can establish synapses by acting as presynaptic elements, their peptides behaving as neurohormones and/or neuromediators, and sometimes containing more than a single active principle. Furthermore, they are not confined to the hypothalamus. Information on complex neuroendocrine interactions (an example may be the mechanisms which govern prolactin secretion) indicates that the concept of the neurosecretory cell as a nerve cell engaged in a secretory activity far more intense than that of the conventional neurons is still valid, as are the speculations on the position assignable to the neurosecretory cells in the evolution of the nervous system and as part of the diffuse neuroendocrine system.