Light Microscopic Evidence for Exocytosis in Nerve Terminals

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It was very controversial over the question how the transmitters are released from the nerve terminals. Two hypotheses, the exocytosis hypothesis and the diacrine hypothesis, both had advantages and disadvantages. The purpose of this project was to determine which hypothesis is instrumental. I employed video-enhanced differential interference microscope to observe the nerve terminals enzymatically isolated from neurohypophysis of the rat and the nerve terminals of cultured chromaffin cells. Their physiological responses induced by electrical stimulation were directly observed in monitor screen at a magnification of X12, 000. Many granules of 0.2 - 0.5 ytm were seen to be fluctuating slowly in a Brownian fashion. Upon electrical stimulation, some granules abruptly changed in light intensity as if they popped one after another in both types of preparation. The same response could be induced also by application of a K-rich solution. This popping image was exactly similar to those found in salivary glands, goblet cells, anterior pituitary cells, some of chromaffin cells, and other secretory cells upon their stimulation. The popping response could be suppressed completely and reversibly by removal of Ca ions from the medium. Repetitive stimulation had facilitatory effects on the popping responses. These findings clearly indicate that nerve terminals are capable of releasing transmitters by the process of exocytosis. I am very convinced that the historical controversy has now been finally settled.