Strychnine: a potent competitive antagonist of alpha-bungarotoxin-sensitive nicotinic acetylcholine receptors in rat hippocampal neurons.

In our study, evidence is provided that strychnine, a competitive antagonist at glycine-gated Cl- channels, is also a potent competitive antagonist at native alpha-7-containing, alpha-bungarotoxin-sensitive nicotinic acetylcholine receptor (nAChRs). To address the effects of strychnine on two types of nicotinic responses, the whole-cell mode of the patch-clamp technique was applied to rat hippocampal neurons in culture. Type IA and type II nicotinic currents evoked by acetylcholine (ACh) were inhibited by strychnine in a concentration-dependent manner with IC50S of 1.2 and 38 microM, respectively. Strychnine (2 microM) decreased the peak amplitude of the alpha-bungarotoxin-sensitive type IA current in a voltage-independent manner and prolonged the decay phase of this current. The concentration-response curve for ACh in evoking type IA current showed a parallel shift to the right in the presence of strychnine (2 microM); the EC50 for ACh was increased from 0.4 to 0.8 mM. These findings suggest that strychnine acts as a competitive antagonist of ACh at the alpha 7 nAChRs that subserve type IA current. In contrast, the inhibition by strychnine of type II current was strongly voltage dependent, and the decay phase of this current was markedly accelerated by the toxin, suggesting an open-channel blockade by strychnine of the alpha 4 beta 2 nAChRs subserving type II currents. Preexposure of the neurons to strychnine enhanced its ability to decrease the peak amplitude of type II currents, indicating that the toxin may also act on alpha 4 beta 2 nAChR channels that are not open. It is concluded that strychnine is a potent competitive antagonist of ACh at neuronal alpha 7 nAChRs and a noncompetitive antagonist at the alpha 4 beta 2 nAChR.