MECHANISMS OF DEPOLARIZING INHIBITION AT THE CRAYFISH GIANT MOTOR SYNAPSE .1. ELECTROPHYSIOLOGY

Mechanisms of depolarizing synaptic inhibition were investigated at the crayfish giant motor synapse with the use of two-electrode current- and voltage-clamp techniques. Depolarizing inhibitory postsynaptic potentials (d-IPSPs) of between 5 and 15 mV in amplitude are produced there in the motor giant motoneuron (MoG) by motor giant inhibitor MoGI) interneurons. Three mechanisms of inhibition are activated by the d-IPSP: inactivation of a voltage-sensitive inward current (probably sodium), activation of the delayed rectifier, and reverse bias of the electrically rectifying giant motor synapse (GMS). These mechanisms supplement the inhibition produced by a α-aminobutyric acid (GABA)-mediated increase in postsynaptic conductance. The d-IPSP is produced by a fast-rising increase in postsynaptic membrane conductance that peaks at 10 μS and lasts nearly 100 ms. An 8-ms, 10-mV depolarizing prepulse inactivated 90% of the inward current evoked by a subsequent step to 33 mV above rest potential, which was -70 mV. d-IPSPs having similar amplitudes should have similar effects on the inward current evoked by an excitatory postsynaptic potential (EPSP). The input resistance of MoG decreased by >60% when the cell was depolarized to 11 mV above rest. This resistance change corresponds to delayed rectification, which should also contribute to the increase in input conductance during a d-IPSP. Depolarization of MoG by 10 mV reduced the excitatory postsynaptic current through the GMS by up to 30%. The reduction in synaptic current occurs because postsynaptic depolarization reduces the transynaptic driving force and increases the reverse bias of the electrically rectifying synapse. Greater reverse bias decreases the period of high transynaptic conductance produced by a presynaptic impulse. Two of these mechanisms, sodium inactivation and delayed rectification, may occur at d-IPSPs that are near active membrane. Unlike hyperpolarizing inhibition, they do not facilitate rapid postinhibitory excitation of the inhibited neuron. The third mechanism, reverse bias of a rectifying electrical synapse, is likely to occur at several such synapses in crayfish and hermit crab.