MODELING THE LEECH HEARTBEAT ELEMENTAL OSCILLATOR .1. INTERACTIONS OFINTRINSIC AND SYNAPTIC CURRENTS

We have developed a biophysical model of a pair of reciprocally inhibi tory interneurons comprising an elemental heartbeat oscillator of the leech. We incorporate various intrinsic and synaptic ionic currents ba sed on voltage-clamp data. Synaptic transmission between the interneur ons consists of both a graded and a spike-mediated component. By using maximal conductances as parameters, we have constructed a canonical m odel whose activity appears close to the real neurons. Oscillations in the model arise from interactions between synaptic and intrinsic curr ents. The inhibitory synaptic currents hyperpolarize the cell, resulti ng in activation of a hyperpolarization-activated inward current I-h a nd the removal of inactivation from regenerative inward currents, Thes e inward currents depolarize the cell to produce spiking and inhibit t he opposite cell. Spike-mediated IPSPs in the inhibited neuron cause i nactivation of low-threshold Ca++ currents that are responsible for ge nerating the graded synaptic inhibition in the opposite cell, Thus, al though the model cells can potentially generate large graded IPSPs, sy naptic inhibition during canonical oscillations is dominated by the sp ike-mediated component.