MECHANISMS OF DEPOLARIZING INHIBITION AT THE CRAYFISH GIANT MOTOR SYNAPSE .2. QUANTITATIVE RECONSTRUCTION

The relative strengths of four mechanisms of depolarizing synaptic inhibition described in the previous paper were evaluated with an electrical model of the giant motor synapse (GMS) and postsynaptic region of the motor giant motoneuron (MoG). The model consists of one compartment that represents the presynaptic region of the medial giant (MG) interneuron and three compartments that represent the postsynaptic region and proximal axon of the MoG. The presynaptic MG compartment is linked to a postsynaptic MoG compartment by a rectifying conductance that represents the GMS. Each compartment consists of parallel paths to ground for active and/or passive membrane currents. Parameter values of the model were set so the MG compartment would replicate an MG impulse and the MoG compartments would replicate the current-clamp, voltage-clamp, and synaptic responses of a single MoG neuron described in the previous paper. The Hodgkin-Huxley equations described voltage-sensitive sodium and potassium currents. Comparison of the MoG compartment currents that mediate an inhibited excitatory postsynaptic potential (EPSP) [triggered during a depolarizing inhibitory postsynaptic potential (d-IPSP)] with those of an uninhibited EPSP indicate that all four mechanisms have significant inhibitory effects. Reverse bias of the GMS by the d-IPSP reduced the GMS current by 65 nA (12%). The remaining inward current was further reduced by a 243-nA outward current through the inhibitory postsynaptic conductance. The d-IPSP inactivated sodium conductance so the inward sodium current evoked by the EPSP was reduced by 319 nA (-68%). The d-IPSP reduced the latency for potassium activation by the EPSP so that the outward potassium current coincided with the inward sodium current and reduced the net inward current by 100 nA. Together, these mechanisms reduced the EPSP amplitude by 69%. The resting potential of MoG is normally 15 mV more positive than MG rest potential, but in some preparations this difference may be as much as 25 mV or as little as 0 mV. Corresponding differences in the rest potentials of the MoG and MG models have little effect on the amplitude of the model MoG EPSP because changes in the inward synaptic and sodium currents are balanced by corresponding changes in the outward potassium current.