MECHANISMS OF OSCILLATION IN DYNAMIC CLAMP CONSTRUCTED 2-CELL HALF-CENTER CIRCUITS

1. The dynamic clamp was used to create reciprocally inhibitory two-ce ll circuits from pairs of pharmacologically isolated gastric mill neur ons of the stomatogastric ganglion of the crab, Cancer borealis. 2. We used this system to study how systematic alterations in intrinsic and synaptic parameters affected the network behavior. This has previousl y only been possible in purely computational systems. 3. In the absenc e of additional hyperpolarization-activated inward current (I-H), stab le half-center oscillatory behavior was not observed. In the presence of additional I-H, a variety of circuit dynamics, including stable hal f-center oscillatory activity, was produced. 4. Stable half-center beh avior requires that the synaptic threshold lie within the voltage enve lope of the slow wave oscillation. 5. Changes in the synaptic threshol d produce dramatic changes in half-center period. As predicted by prev ious theoretical work, when the synaptic threshold is depolarized, the period first increases and then decreases in a characteristic inverte d U-shaped relationship. Analysis of the currents responsible for the transition between the active and inhibited neurons shows that the mec hanism of oscillation changes as the synaptic threshold is varied. 6. Increasing the time constant and the conductance of the inhibitory syn aptic current increased the period of the half-center oscillater. 7. I ncreasing the conductance of I-H or changing the voltage dependence of I-H can either increase or decrease network period, depending on the initial mode of network oscillation. A depolarization of the activatio n curve causes the network to respond in a similar fashion as increasi ng the conductance of I-H. 8. Many neuromodulatory substances are know n to alter synaptic strength and the conductance and voltage dependenc e of I-H, parameters we studied with the dynamic clamp. To understand the response of the network to modulation of a single parameter, it is necessary to understand the nature of the altered conductance and how it interacts with the other conductances in the system.