Synaptic depression creates a switch that controls the frequency of an oscillatory circuit

Synaptic depression is a form of short-term plasticity exhibited by many sy napses, Nonetheless, the Functional significance of synaptic depression in oscillatory networks is not well understood. We show that, in a recurrent i nhibitory network that includes an intrinsic oscillator, synaptic depressio n can give rise to two distinct modes of network operation. When the maxima l conductance of the depressing synapse is small, the oscillation period is determined by the oscillator component, Increasing the maximal conductance beyond a threshold value activates a positive-feedback mechanism that grea tly enhances the synaptic strength. In this mode, the oscillation period is determined by the strength and dynamics of the depressing synapse. Because of the regenerative nature of the feedback mechanism, the circuit can be s witched from one mode of operation to another by a very small change in the maximal conductance of the depressing synapse. Our model was inspired by e xperimental work on the pyloric network of the lobster. The pyloric network produces a simple motor rhythm generated by a pacemaker neuron that receiv es feedback inhibition from a depressing synapse. In some preparations, eli mination of the synapse had no effect on the period of the rhythm, whereas in other preparations, there was a significant decrease in the period. We p ropose that the pyloric network can operate in either of the two modes sugg ested by the model, depending on the maximal conductance of the depressing synapse.