Inhibition of nicotinic acetylcholine response by atropine in frog isolated sympathetic neurons

Abstract

The effect of atropine on nicotinic acetylcholine (ACh) response was studied in frog isolated sympathetic ganglion cells using a ‘concentration clamp’ technique which combines intra-cellular perfusion with a rapid external solution change within 3 ms. When atropine (more than 6 × 10 −7 M) was simultaneously applied to neurons with ACh (6 × 10 −6 M), the current amplitude was instantaneously reduced in a dose-dependent fashion. Further decreases in the current amplitude were observed in a time-dependent manner during about 20 min after the start of drug-application. Thereafter the current amplitude gradually restored toward the control level in the following 90–120 min in spite of the continuous presence of atropine. However, because d-tubocurarine greatly inhibited the ‘restored’ current, the last-mentioned is suggested be also mediated by nicotinic ACh receptor-ionophore complex. Therefore, the inhibitory action of atropine on the peak amplitude of ACh response was examined at 15–20 min after adding the agent. It was dose-dependent but not voltage-dependent. Respective concentrations of atropine and d-tubocurarine causing half the maximum inhibition (IC 50) of the peak current evoked by ACh (6 × 10 −6 M) were 1.8 × 10 −5 M and 1.8 × 10 −6 M. Thus, the inhibitory potency was 10 times less tha of d-tubocurarine, an antagonist of nicotinic ACh receptors. The blockade of ACh response by d-tubocaririne was competitive while that by atropine was non-competitive. The current elicited by Ach consisted of two (fast and low) exponentiald components plus a steady0state one in the control period. Atropine (3 × 10 −6 M) greatly reduced the amplitude of exponential components but it exerted a small effect on the steady-state one. Therefore, the changes in current kinetics during the application of atropine may be due to a different sensitivity of each component to atropine.