Do Schwann Cells Play a Role in ‘Upstream’ Regulation of the Release Probability in Sympathetic Nerve Varicosities?

The mechanisms which control the release probability (P) in sympathetic nerve varicosities were studied by extracellular recording in some model tissues. The method permits direct recording of different components of the action potential in the nerve terminals as well as of the excitatory junction current which reflects the release of quanta of ATP. The results indicate (i) that the action potential-induced Ca influx into the release site (ICaII) is necessary but not sufficient for release to occur, (ii) that P is controlled by a mechanism which depends on the resting Ca influx (ICaI) at a distance from (upstream of) the release site, and (iii) that Schwann cells may influence ICaI by controlling the Ca and K levels in the perineuronal ‘sub-Schwann space’.