Functional role of Ca2+ currents in graded and spike-mediated synaptic transmission between leech heart interneurons

We used intracellular recording and single electrode voltage-clamp techniques to explore Ca2+ currents and their relation to graded and spike-mediated synaptic transmissions in leech heart interneurons. Low-threshold Ca2+ currents (activation begins below -50 mV) consist of a rapidly inactivating component (I-CaF) and a slowly inactivating component (I-CaS). The apparent inactivation kinetics of I-CaF appears to be influenced by Ca2+; both the substitution of Ca2+ (5 mM) with Ba2+ (5 mM) in the saline and the intracellular injection of the rapid Ca2+ chelator, bis-(o-aminophenoxy)-N,N,N',N'-tetraacetic acid (BAPTA), from the recording microelectrode, significantly increase its apparent inactivation time constant. The use of saline with a high concentration of Ba2+ (37.5 mM) permitted exploration of divalent ion currents over a broader activation range, by acting as an effective charge carrier and significantly blocking outward currents. Ramp and pulse voltage-clamp protocols both reveal a rapidly activating and inactivating Ba2+ current (I-BaF) and a less rapidly activating and slowly inactivating Ba2+ current with a broad activation range (I-BaS). Low concentrations of Cd2+ (100-150 mu M) selectively block I-BaS, without significantly diminishing I-BaF. The current that remains in Cd2+ lacks the characteristic delayed activation peak of I-BaS and inactivates with two distinct time constants. I-BaF appears to correspond to a combination of I-CaF, and I-CaS, i.e., to low-threshold Ca2+ currents, that can be described as Gamma-like. I-BaS appears to correspond to a Ca2+ current with a broad activation range, which can be described as L-like. Cd2+ (100 mu M) selectively blocks spike-mediated synaptic transmission between heart interneurons without significantly interfering with low-threshold Ca2+ currents and plateau formation in or graded synaptic transmission between heart interneurons. Blockade of spike-mediated synaptic transmission between reciprocally inhibitory heart interneurons with Cd2+ (150 mu M), in otherwise normal saline, prevents the expression of normal oscillations (during which activity in the two neurons consists of alternating bursts), so that the neurons fire tonically. We conclude that graded and spike-mediated synaptic transmission may be relatively independent processes in heart interneurons that are controlled predominantly by different Ca2+ currents. Moreover, spike-mediated synaptic inhibition appears to be required for normal oscillation in these neurons.