Release of Transmitter From Degenerating Locust Motorneurones

Abstract

ABSTRACT When the motorneurones to the femoral part of the retractor unguis muscle of the locust Schistocerca gregaria are severed, the changes in impulse-linked release and spontaneous release of transmitter which take place at the synapses between the motorneurones and the retractor unguis muscle fibres can be related to known changes in the numbers and distribution of synaptic vesicles at these synapses. Impulse-linked transmitter release fails when the synaptic vesicles aggregate to form clumps of vesicles, and at this time the miniature discharge recorded from the muscle fibres become characterized, initially, by bursts of miniature excitatory post-synaptic potentials (min. EPSPs) and, eventually, by ‘giant’ intracellular min. EPSPs up to 15 mV in amplitude. The appearance of ‘giant’ potentials is accompanied by an overall fall in the frequency of the miniature discharge, which continues to decline thereafter until spontaneous transmitter release fails. The miniature discharge ceases at a time when the axon terminals are engulfed by glial tissue or become devoid of synaptic vesicles. The parallel changes in temporal arrangement of min. EPSPs and spatial arrangement of synaptic vesicles seen during degeneration of locust nerve-muscle synapses provide further support for the ‘vesicle hypothesis’.