Effects of neostigmine and physostigmine on the acetylcholine receptor-ionophore complex in frog isolated sympathetic neurones.

1. The effects of neostigmine and physostigmine, reversible carbamate acetylcholinesterase (AChE)-inhibitors, on nicotinic acetylcholine-induced inward currents (IACh) were investigated in enzymatically isolated single sympathetic ganglion cells from the bullfrog. The 'concentration clamp' technique which combines intracellular perfusion with a rapid external solution change under single electrode voltage-clamp conditions was used. 2. Pretreatment with neostigmine and physostigmine did not enhance IACh at any concentrations, suggesting that AChE activity had already disappeared during the enzymatic treatment of the preparation. 3. Both neostigmine and physostigmine inhibited IACh in a dose-dependent manner with IC50 values of 7.0 x 10(-4) M and 1.1 x 10(-4) M, respectively. The blockade by neostigmine was competitive, while that by physostigmine was non-competitive. 4. The inhibition of IACh by neostigmine and physostigmine showed no apparent voltage dependency. 5. Neostigmine did not cause obvious changes of the kinetics of IACh. However, physostigmine reduced both the fast and slow time constants of inactivation of IACh, thus facilitating the rate of inactivation without affecting the activation kinetics of IACh. 6. These results suggest that neostigmine and physostigmine have different direct actions on the ACh receptor-ionophore complex. Neostigmine may act on the ACh-receptor (the binding site of ACh) while physostigmine may interact with the ACh-gated cation channels.