Exocytosis by Synaptic Terminals Innervating the Adrenal Gland of the Goldfish Reveals Multiple Domains within the Plasmalemma

The adrenal chromaffin gland of the goldfish has typical synaptic terminals embedded in its surface which are homologues of the cholinergic fibres innervating the mammalian adrenal medulla. The terminals contain both lucent synaptic vesicles and larger secretory granules with dense cores, known to be storage sites for transmitters and peptides, respectively. Three domains are present within the terminal plasmalemma. Exocytosis of vesicles is thought to be associated with a ‘synaptic domain’ marked by synaptic thickenings around which the vesicles cluster. Exocytosis of granules, stimulated by high K+ and visualized with the aid of tannic acid, is almost exclusively associated with areas of the membrane adjacent to chromaffin cells, and in particular with unspecialized regions which constitute the ‘parasynaptic domain’, creating a pattern of targeted secretory discharge. Sites of release within the ‘non-synaptic domain’, which is sheathed in glial cell lamellae, are extremely rare, despite the expansive character of this domain and the close association of granules with the plasmalemma within it. The pattern of secretory release described may be correlated with the position of the terminals at the surface of the innervated organ.