A study of the cholinolytic actions of strychnine using the technique of concentration jump relaxation analysis

The blocking actions of strychnine on excitatory acetylcholine (ACh) responses in isolated, voltage clamped Aplysia neuronal cell bodies has been studied using a rapid drug application technique. Rapid microperfusion of strychnine (10-50 microM) produced a reduction of the steady-state ACh-induced inward current in Aplysia neurons which decayed exponentially with a highly dose-dependent time constant. At the cessation of strychnine perfusion the ACh-induced current recovered to its original value with an exponential time course which was not sensitive to the dose of strychnine previously applied. The calculated association (k1) and dissociation (k-1) constants for a pseudo-first-order reaction between strychnine and its binding site were k1 = 1.2 X 10(4) M-1. sec-1 and k-1 = 0.12 sec-1 (KD = 1 X 10(-5) M-1). These results demonstrate that concentration jump relaxation experiments can be performed on isolated neurons for the study of voltage-independent antagonists by the use of rapid microperfusion systems and provide the first direct estimates to date of the rate constants of the cholinolytic effect of strychnine.