'Neurosecretion' by synaptic terminals in the locust corpus cardiacum: is non-synaptic exocytosis part of the regulated or the constitutive pathway?

Abstract

Nerve fibres form conventional synaptic junctions with gland cells in the corpus cardiacum of the locust, Schistocerca gregaria. They contain synaptic vesicles whose contents are normally electron-lucent, but which react positively to cytochemical tests for amines (eg, incubation in the false transmitter 5-OHDA). Secretory granules are also present in the terminals and such inclusions are known to contain neuropeptides. The granules undergo non-synaptic exocytosis and this process has been visualized by the application of tannic acid. Granule exocytosis gives clear signs of being part of a regulated secretory pathway: it is elevated in vivo by flight--a natural stimulus known to activate the gland (this effect is blocked by prior injection of trehalose); its incidence is closely correlated with a postsynaptic response, suggesting a role for the materials discharged in short-term signalling; and when induced in vitro by high K+, it is Ca(2+)-dependent. However, a low level of exocytosis was encountered under all the conditions employed, suggesting the existence of a constitutive component. It is postulated that the regulated and constitutive patterns of discharge of neuropeptides are related to the roles of these materials as neurotransmitters/modulators and neurotrophic substances, respectively.