Functional Role of Ca Currents in Graded and Spike-Mediated Synaptic Transmission Between Leech Heart Interneurons

Lu, Jin, John F. Dalton, IV, Darrell R. Stokes, and Ronald L. ating networks, especially those found in invertebrate ganCalabrese. Functional role of Ca currents in graded and spikeglia, graded synaptic transmission (almost invariably inhibimediated synaptic trnasmission between leech heart interneurons. tory) —release of transmitter by slow changes in membrane J. Neurophysiol. 77: 1779–1794, 1997. We used intracellular repotentials and independent of action potentials per se (Burcording and single electrode voltage-clamp techniques to explore rows 1985; Siegler 1985) —is thought to play an important, Ca currents and their relation to graded and spike-mediated synif not dominant role in network function (Büschges 1995; aptic transmissions in leech heart interneurons. Low-threshold DiCaprio 1989; Heitler and Pearson 1980; Mendelson 1971; Ca currents (activation begins below 050 mV) consist of a Paul and Mulloney 1985a,b; Pearson and Fourtner 1975; rapidly inactivating component (ICaF) and a slowly inactivating Wolf and Büschges 1995) even when the usual spike-medicomponent (ICaS) . The apparent inactivation kinetics of ICaF appears to be influenced by Ca ; both the substitution of Ca (5 mM) ated transmission is present (e.g., see Granzow et al. 1985; with Ba (5 mM) in the saline and the intracellular injection of the Graubard et al. 1983; Johnson and Harris-Warrick 1990; rapid Ca chelator, bis-(o-aminophenoxy)-N,N,N *,N *-tetraacetic Johnson et al. 1991; Mangan et al. 1994; Raper 1979). It is acid (BAPTA), from the recording microelectrode, significantly not clear whether spike-mediated and graded transmissions increase its apparent inactivation time constant. The use of saline represent separate processes or form a continuum (Hartline with a high concentration of Ba (37.5 mM) permitted exploration and Graubard 1992). Because graded and spike-mediated of divalent ion currents over a broader activation range, by acting transmissions are active in different ranges of presynaptic as an effective charge carrier and significantly blocking outward membrane potential, one possibility is that they are concurrents. Ramp and pulse voltage-clamp protocols both reveal a trolled by Ca entry through different populations of Ca rapidly activating and inactivating Ba current (IBaF) and a less channels with different activation ranges. rapidly activating and slowly inactivating Ba current with a broad