Voltage dependence of amplitude and time course of inhibitory synaptic current in crayfish muscle

Abstract

The membrane of small crayfish muscle fibers was clamped to potentials between −150 and −20 mV and amplitude and time course of inhibitory postsynaptic currents (IPSCs) were studied. The IPSCs were recorded extracellularly by means of a focal microelectrode and also as total clamp current. The IPSCs lasted about 40 ms and were slowed by depolarization. The rate constant α of decay of the IPSC depended on membrane potential E according to the relation α=44 s⁻¹ · e^{−8.7V E} at 13.5°C. α increased with temperature with aQ ₁₀ of 1.9 to 2.5. The amplitudei _I of the IPSC depended nonlinearly on E and decreased with time after a potential shift. This was partly due to movement of Cl⁻-ions, the difference (E-E_I) between clamp potential and reversal potential for the IPSC decreasing to a few mV within several minutes after a shift in E. The inhibitory conductanceg _I increased up to 30-fold for 100 mV depolarization and also changed with time. However, the inhibitory permeability P_I proved to be independent of membrane potential and time. The potential dependence ofg _I is thus largely due to changes in the internal Cl⁻-concentration.