Single channels and ionic currents in peptidergic nerve terminals

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Abstract
Control of secretion, by a mechanism in which membrane depolarization leads to Ca2+ entry, has been extensively studied. The small size and inaccessibility of most nerve terminals, however, have precluded direct analysis of membrane ionic currents and their influence on secretion (with some notable exceptions). Recently, patch-clamp methods have been applied to several secretory systems for both voltage-clamp and single-channel recordings. We now report the extension of this analysis to isolated peptidergic nerve terminals. We used terminals obtained from a crustacean neurohaemal organ, the sinus gland. Analyses of currents under whole-terminal voltage clamp showed inward currents carried by Na+ and by Ca2+, and outward currents carried predominantly by K+. Furthermore, we have observed two types of single-channel currents that may be unique to nerve terminals. Both show little selectivity between Na+ and K+. The first channel is activated by intracellular Na+ and the second by intracellular Ca2+. These channels have conductances of 69 and 213 pS, respectively, in symmetrical 310 mM KCl. It should now be possible to compare electrical activity recorded intracellularly from intact nerve endings, with whole-cell and single-channel currents and with the release of peptide neurohormones from isolated neuronal terminals.