Short-Term Dynamics of a Mixed Chemical and Electrical Synapse in a Rhythmic Network

In the rhythmically active pyloric circuit of the spiny lobster, the synapse between the lateral pyloric (LP) neuron and pyloric constrictor (PY) neuron has an inhibitory depressing chemical and an electrical component. To understand how the dynamics of the LP→PY synapse affect the relative firing times between these two neurons in an ongoing rhythm, we characterized the dynamics of the LP→PY synapse after a pharmacological block of ongoing activity. When a train of voltage pulses was applied to the voltage-clamped LP neuron, the inhibitory chemical component of the postsynaptic potential (PSP) in the PY neuron rapidly depressed. Thus, after the first few pulses, the PSP was either hyperpolarizing or depolarizing, depending on the interpulse duration, with shorter interpulse durations producing depolarizing PSPs. To characterize the synaptic response during rhythmic activity, we played back prerecorded realistic waveforms in the voltage-clamped LP neuron. After an initial transient, the resulting PSP in PY was always depolarizing, suggesting that in an ongoing rhythm, the electrical component of the synapse is dominant. However, our results indicate that the chemical component of the synapse acts to delay the peak time of the PSP and to reduce its amplitude, and that these effects become more important at slower cycle periods.