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

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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.

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