Ionic conductances underlying the activity of interneurons that control heartbeat in the medicinal leech

Electrical properties of interneurons that control heartbeat in the leech (HN cells) were studied using intracellular recording and stimulation in isolated ganglia bathed by salines of various ionic compositions. Substitution of Na+ ions in the bath by Tris stopped the spontaneous firing of HN cells and led to their gradual hyperpolarization by 15-20 mV. In the absence of Na+, HN neurons produced long-lasting regenerative plateau potentials with thresholds near -55 mV and peaks near -30 mV that were accompanied by an increase in membrane conductance. Elevation of Ca2+ concentration enhanced plateaus, as did replacement of Ca2+ by Ba2+. Plateaus were formed when Sr2+ replaced Ca2+, but were blocked by addition of Mg2+ or Co2+ to the bath, Co2+ being effective at lower concentrations than Mg2+. Hyperpolarization of HN neurons with injected currents revealed a time-dependent change in membrane potential, whereby initial maximum hyperpolarization was followed by a "sag" in potential towards more depolarized values. The sag showed dual voltage dependence, being diminished when HN neurons were hyperpolarized or depolarized outside the normal range of oscillation. The sag was found to depend on the presence of Na+ ions and to be blocked by Cs+ but not by Ba2+. This time-dependent change in membrane potential counters hyperpolarizations of HN neuron membrane potential and may contribute to the escape of these neurons from synaptic inhibition.