The Endocytotic Role of Vesicles at the Cholinergic Synapse

Abstract

The dominant role of vesicles in the cholinergic synapse for storing ACh is well known (5). Furthermore, there is no doubt about their migration to the presynaptic membrane and opening into the synaptic cleft. This indirect delivery of neurotransmitter is, in the opinion of several research groups, the main object of this mechanism (11, 12). However the possibility is not excluded that ACh is leaking out from the nerve terminal’s axoplasm, partly originating from the vesicles and directly stimulating the postsynaptic membrane. There is even some good experimental evidence that this may be a major source of released neurotransmitter (1). Electrical stimulation of intact organs, either with single trains or repeated impulses, did not change the number of vesicles per area in the nerve terminal. Only their diameter was diminished (6, 7). In order to investigate a possible endocytotic role of vesicles, as proposed by Heuser and Reese (3), we applied horseradish peroxidase (HRP) to in vitro preparations of electric organs of Torpedo. They were put under the influence of different drugs and electrically stimulated. Different electron microscope techniques for quantitative evaluation of thin sections or freeze etching were used. In this short communication only part of the results, particularly those concerning the endocytotic role of vesicles, will be discussed.