Inhibitory postsynaptic currents atAplysiacholinergic synapses : effects of permeant anions and depressant drugs

Abstract

Inhibitory postsynaptic potentials (IPSP) and, under vultage-clamp conditions, inhibitory postsynaptic currents IPSC were recorded in neurons in buccal ganglia of A. juliana. The decay of IPSC was exponential with a single time constant, .tau., which decreased with membrane depolarization. In external solutions containing I- or Br- instead of Cl-, .tau., varied according to the sequence .tau. (I) > .tau.(Br) > .tau.(Cl), and the voltage sensitivity of .tau. was altered. In iodide solution, the voltage sensitivity of .tau. was reversed. The foreign halides depressed the peak current amplitude and shifted the reversal (zero-current) potential to more positive membrane potentials. In low concentrations of sodium pentobarbitone (100-200 .mu.M), the decay of IPSC became biphasic. Increasing drug concentration and membrane hyperpolarization had differential effects on the rates and relative amplitudes of the 2 components of IPSC decay. Octanol (0.5-1 mM) reduced the amplitude of IPSP and increased the rate of decay of IPSC without changing the voltage sensitivity of .tau.. The effect of foreign halides and barbiturates on IPSC decay were interpreted in terms of a reaction between the anion and an ion-binding site(s) associated with the anion-selective channel, which affects the probability of anions entering the channel and normal channel closure.