Summary

The rhythmically active heart interneuron HN(5) in the medicinal leech exhibits two distinct activity states, which have been associated with different coordination states of the two hearts. During the active state, it discharges high-frequency bursts of action potentials interrupted by rhythmic inhibitory input from other interneurons. In the inactive state, the same cell receives rhythmic inhibition but the membrane potential remains subthreshold between these volleys, producing few or no action potentials. We investigated differences in the membrane properties of the cell during the active and inactive states. The membrane potential in the active state oscillates on average between about −56 +/− 6 mV (S.D.) and −45 +/− 7 mV; the mean oscillation amplitude is 11 +/− 4 mV. In the inactive state, the membrane potential oscillates on average between −58 +/− 6 mV and −55 +/− 6 mV with a mean amplitude of 3 +/− 1 mV. The overall conductance of an HN(5) interneuron during the active state is approximately 10 nS lower than that during the inactive state, indicating that an outward current is turned off during the active state or turned on during the inactive state. This outward current is not voltage-dependent in the range −80 mV to −10 mV, as shown in voltage-clamp experiments by a linear current-voltage relationship. The reversal potential of this current is approximately −60 mV, indicating that chloride or potassium ions underlie the current. Using dynamic-clamp, we show that by adding an artificial current with a linear voltage-dependence (leak conductance) to an HN(5) interneuron (conductance 15 nS, reversal potential −60 mV), the cell can be transferred from its active to its inactive state.