FUNCTIONAL-ROLE OF CA2-MEDIATED SYNAPTIC TRANSMISSION BETWEEN LEECH HEART INTERNEURONS( CURRENTS IN GRADED AND SPIKE)

We used intracellular recording and single electrode voltage-clamp tec hniques to explore Ca2+ currents and their relation to graded and spik e-mediated synaptic transmissions in leech heart interneurons. Low-thr eshold Ca2+ currents (activation begins below -50 mV) consist of a rap idly inactivating component (I-CaF) and a slowly inactivating componen t (I-CaS). The apparent inactivation kinetics of I-CaF appears to be i nfluenced by Ca2+; both the substitution of Ca2+ (5 mM) with Ba2+ (5 m M) in the saline and the intracellular injection of the rapid Ca2+ che lator, bis-(o-aminophenoxy)-N,N,N',N'-tetraacetic acid (BAPTA), from t he recording microelectrode, significantly increase its apparent inact ivation time constant. The use of saline with a high concentration of Ba2+ (37.5 mM) permitted exploration of divalent ion currents over a b roader 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+ curre nt (I-BaF) and a less rapidly activating and slowly inactivating Ba2current with a broad activation range (I-BaS). Low concentrations of C d2+ (100-150 mu M) selectively block I-BaS, without significantly dimi nishing I-BaF. The current that remains in Cd2+ lacks the characterist ic 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 describe d 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 he art 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 trans mission between reciprocally inhibitory heart interneurons with Cd2+ ( 150 mu M), in otherwise normal saline, prevents the expression of norm al oscillations (during which activity in the two neurons consists of alternating bursts), so that the neurons fire tonically. We conclude t hat graded and spike-mediated synaptic transmission may be relatively independent processes in heart interneurons that are controlled predom inantly by different Ca2+ currents. Moreover, spike-mediated synaptic inhibition appears to be required for normal oscillation in these neur ons.