Immunohistochemical Localization of Chromogranins in Sheep Sympathetic Neurones and their Release by Nerve Impulses

The theory that chemical transmission of nerve impulses at synapses occurs by the release of a number of discrete packages or “quanta” of transmitter, is based primarily on electrophysiological evidence. Since the discovery of synaptic vesicles 15 years ago it has often been suggested that they constitute the morphological basis of the quanta but direct evidence is still lacking that they contain the required amount of transmitter or indeed that they release it during transmission. In the adrenal medulla, however, considerable progress on the mechanism of secretion of catecholamines has been made possible by the recognition that the catecholamine storage vesicles also contain a large proportion of specific soluble proteins called chromogranins, and that these proteins are secreted together with the catecholamines and the other major constituent of the vesicles, adenine nucleotides (Smith, 1968). The vesicles appear to release their entire content and it has been calculated that each nerve impulse to the adrenal medulla releases the equivalent of the contents of one vesicle per cell (Viveros et al., 1969).