Potentiation and inhibition of neuronal nicotinic receptors by atropine: Competitive and noncompetitive effects

Atropine, the classic muscarinic receptor antagonist, inhibits ion currents mediated by neuronal nicotinic acetylcholine receptors expressed in Xenopus laevis oocytes. At the holding potential of -80 mV, 1 mu M atropine inhibits 1 mM acetylcholine-induced inward currents mediated by rat alpha 2 beta 2, alpha 2 beta 4, alpha 3 beta 2, alpha 3 beta 4, alpha 4 beta 2, alpha 4 beta 4, and alpha 7 nicotinic receptors by 12-56%. Inward currents induced with a low agonist concentration are equally inhibited (alpha 3 beta 2, alpha 3 beta 4), less inhibited (alpha 2 beta 4, alpha 7), or potentiated (alpha 4 beta 2, alpha 4 beta 4) by 1 mu M atropine. Effects on the more sensitive alpha 4 beta 4 nicotinic receptors were investigated in detail by systematic variation of acetylcholine and atropine concentrations and of membrane potential. At high agonist concentration, atropine inhibits alpha 4 beta 4 nicotinic receptor-mediated ion current in a noncompetitive, voltage-dependent way with IC50 values of 655 nM at -80 mV and of 4.5 mu M at -40 mV. At low agonist concentration, 1 mu M atropine potentiates alpha 4 beta 4 nicotinic receptor-mediated ion current. This potentiating effect is surmounted by high concentrations of acetylcholine, indicating a competitive interaction of atropine with the nicotinic receptor, and potentiation is also reversed at high atropine concentrations. Steady state effects of acetylcholine and atropine are accounted for by a model for combined receptor occupation and channel block, in which atropine acts on two distinct sites. The first site is associated with noncompetitive ion channel block. The second site is associated with competitive potentiation, which appears to occur when the agonist recognition sites of the receptor are occupied by acetylcholine and atropine. The apparent affinity of atropine for the agonist recognition sites of the alpha 4 beta 4 nicotinic acetylcholine receptor is estimated to be 29.9 mu M.