'Neurosecretion' by synaptic terminals and glandular discharge in the endocrine pancreas. Application of tannic acid to the teleost Xiphophorus helleri.

Nerve fibers within the large pancreatic islet of Xiphophorus helleri are apparently of a single type and form typical, well-differentiated synapses with the endocrine gland cells. A histochemical test for cholinesterases generates reaction product in association with the outer surfaces of the neuronal membranes. Two categories of presumptive secretory inclusions are present in the fiber terminals: secretory granules usually 80-120 nm in diameter have a wide distribution within the fibers, whereas electron-lucent synaptic vesicles 30-60 nm across often cluster adjacent to membrane thickenings and differentiated clefts. Infusion of tannic acid in high-K+ saline enabled sites of granule exocytosis to be visualized. These presumed sites of neuropeptide discharge were associated with morphologically undifferentiated regions of the terminal membranes. Tannic acid also facilitated the detection of exocytosis, involving granules of endocrine gland cells; discharged secretory material exposed to this agent retains immunoreactivity at a reduced level.