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

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 (IC₅₀) 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.