Relation of impulse conduction to electrically induced release of neurosecretory material from the urophysis of the teleost fish Tilapia mossambica

The caudal neurosecretory system of the teleost Tilapia mossambica was subjected in vivo and in vitro to intense and prolonged electrical stimulation. Repeated firing of neurosecretory units was observed in the in vivo preparations, and compound action potentials were recorded in vitro. Evidence of depletion of neurosecretory material from the urophysis was obtained on the light and electron microscope levels. Ultrastructurally two pictures were encountered in the responding axon terminals: (1) a decrease in electron density of most of the elementary neurosecretory granules in individual terminals; (2) complete or nearly complete disappearance of granules and vesicles from individual terminals. The latter picture is considered to be a more advanced stage of depletion.