In oscillatory neuronal networks that pace rhythmic behavior, Ca 2ϩ entry through voltage-gated Ca channels often supports bursting activity and mediates graded transmitter release. We monitored simultaneously membrane potential and/or ionic currents and changes of Ca fluorescence (using the fluorescence indicator Ca Orange) in spontaneously active and experimentally manipulated oscillator heart interneurons in the leech. We show that changes in Ca fluorescence in these interneurons during

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... aneous bursting and evoked activity reflect the slow wave of that activity and that these changes in Ca fluorescence are mediated by Ca 2ϩ entry primarily through lowthreshold Ca channels. Spatial and temporal maps of changes in Ca fluorescence indicate that these channels are widely distributed over the neuritic tree of these neurons. We establish a correlation between the amount of transmitter released, as estimated by the integral of the postsynaptic current, and the change in Ca fluorescence. In experiments in which we were able to record presynaptic low-threshold Ca currents, associated IPSCs, and presynaptic changes in Ca fluorescence from fine neuritic branches of heart interneurons near their region of synaptic contact with their contralateral partner, there was a close association between the rise in Ca fluorescence and the rise of the postsynaptic conductance. The changes in Ca fluorescence that we record at the end of fine neuritic branches appear to reflect changes in [Ca 2ϩ ] i that mediate graded synaptic release in leech heart interneurons. These results indicate that widely distributed low-threshold Ca currents play an important role in generating rhythmic activity and in mediating graded transmitter release. Free intracellular calcium ions (Ca 2ϩ ) play an essential role in the regulation of many cellular functions in neurons. Correspondingly, during neuronal activity, intracellular Ca 2ϩ concentration ([Ca 2ϩ ] i ) varies in a dynamic way both temporally and spatially. The spatial and temporal pattern of changes in [Ca 2ϩ ] i , monitored with different Ca 2ϩ -sensitive fluorescence dyes, is thought to reflect differences in the dynamics and cellular localization of different Ca 2ϩ channels in the plasma membrane and Ca 2ϩrelease channels of the endoplasmic reticulum (ER) (Lipscombe et al., 1988; Regehr et al.Mainen et al., 1999). Correspondingly, localized changes in [Ca 2ϩ ] i are thought to be important in neuronal function. For example, during synaptic transmission, the regulatory action of Ca 2ϩ on neurotransmitter release depends on changes in internal Ca 2ϩ concentration ([Ca 2ϩ ] i ) at specific intracellular sites (Robitaille et al.