Unmasking the functions of the chromaffin cell α7 nicotinic receptor by using short pulses of acetylcholine and selective blockers

Methyllycaconitine (MLA), alpha-conotoxin ImI, and alpha-bungarotoxin inhibited the release of catecholamines triggered by brief pulses of acetylcholine (ACh) (100 mu M, 5 s) applied to fast-superfused bovine adrenal chromaffin cells, with IC(50)s of 100 nM for MLA and 300 nM for alpha-conotoxin ImI and alpha-bungarotoxin, MLA (100 nM), alpha-conotoxin ImI (1 mu M), and alpha-bungarotoxin (1 mu M) halved the entry of Ca-45(2+) stimulated by 5-s pulses of 300 mu M ACh applied to incubated cells. These supramaximal concentrations of alpha(7) nicotinic receptor blockers depressed by 30% (MLA), 25% (alpha-bungarotoxin), and 50% (alpha-conotoxin ImI) the inward current generated by 1-s pulses of 100 mu M ACh, applied to voltage-clamped chromaffin cells. In;Xenopus oocytes expressing rat brain alpha(7) neuronal nicotinic receptor for acetylcholine nAChR, the current generated by 1-s pulses of ACh was blocked by hZLA, alpha-conotoxin ImI, and alpha-bungarotoxin with IC(50)s of 0.1 nM, 100 nM, and 1.6 nM, respectively; the current through alpha(3)beta(4) nAChR was unaffected by alpha-conotoxin ImI and alpha-bungarotoxin, and weakly blocked by MLA (IC50 = 1 mu M). The functions of controlling the electrical activity, the entry of Ca2+, and the ensuing exocytotic response of chromaffin cells were until now exclusively attributed to alpha(3)beta(4) nAChR; the present results constitute the first evidence to support a prominent role of alpha(7) nAChR in controlling such functions, specially under the more physiological conditions used here to stimulate chromaffin cells with brief pulses of ACh.