Inhibitory Postsynaptic Currents at Aplysia Cholinergic Synapses: Effects of Permeant Anions and Depressant Drugs

Inhibitory postsynaptic potentials (i.p.s.ps) and, under voltage-clamp conditions, inhibitory postsynaptic currents (i.p.s.cs) were recorded in neurons in buccal ganglia of Aplysia juliana. The decay of i.p.s.cs was exponential with a single time constant, τ , which decreased with membrane depolarization. In external solutions containing iodide or bromide ions instead of chloride ions, τ varied according to the sequence τ (I) > τ (Br) > τ (Cl), and the voltage sensitivity of τ was altered. In iodide solution, the voltage sensitivity of τ was reversed. Furthermore, 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 μ M), the decay of i.p.s.cs became biphasic. Increasing drug concentration and membrane hyperpolarization had differential effects on the rates and relative amplitudes of the two components of i.p.s.c. decay. Octanol (0.5-1 mM) reduced the amplitude of i.p.s.ps and increased the rate of decay of i.p.s.cs without changing the voltage sensitivity of τ . The effect of foreign halides and barbiturates on i.p.s.c. 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.