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

Abstract

Methyllycaconitine (MLA), α-conotoxin ImI, and α-bungarotoxin inhibited the release of catecholamines triggered by brief pulses of acetylcholine (ACh) (100 μM, 5 s) applied to fast-superfused bovine adrenal chromaffin cells, with IC₅₀s of 100 nM for MLA and 300 nM for α-conotoxin ImI and α-bungarotoxin. MLA (100 nM), α-conotoxin ImI (1 μM), and α-bungarotoxin (1 μM) halved the entry of ⁴⁵Ca²⁺ stimulated by 5-s pulses of 300 μM ACh applied to incubated cells. These supramaximal concentrations of α₇ nicotinic receptor blockers depressed by 30% (MLA), 25% (α-bungarotoxin), and 50% (α-conotoxin ImI) the inward current generated by 1-s pulses of 100 μM ACh, applied to voltage-clamped chromaffin cells. In Xenopus oocytes expressing rat brain α₇ neuronal nicotinic receptor for acetylcholine nAChR, the current generated by 1-s pulses of ACh was blocked by MLA, α-conotoxin ImI, and α-bungarotoxin with IC₅₀s of 0.1 nM, 100 nM, and 1.6 nM, respectively; the current through α₃β₄ nAChR was unaffected by α-conotoxin ImI and α-bungarotoxin, and weakly blocked by MLA (IC₅₀ = 1 μM). 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 α₃β₄ nAChR; the present results constitute the first evidence to support a prominent role of α₇ nAChR in controlling such functions, specially under the more physiological conditions used here to stimulate chromaffin cells with brief pulses of ACh.