PRESYNAPTIC CALCIUM CURRENTS AT VOLTAGE-CLAMPED EXCITER AND INHIBITORNERVE-TERMINALS OF CRAYFISH

1. A two-electrode voltage clamp was used to record calcium currents f rom the excitatory and inhibitory nerve terminals that innervate the c rayfish (Procambarus spp.) opener muscle. Other voltage-dependent curr ents were blocked with tetrodotoxin, 3,4-diaminopyridine, 4-aminopyrid ine and tetraethylammonium. 2. The presynaptic calcium current at both excitatory and inhibitory synapses was blocked by cadmium and omega-a gatoxin IVA but was not affected by omega-conotoxin GVIA, omega-conoto xin MVIIC or nifedipine, suggesting that the calcium currents flow thr ough P-type calcium channels. 3. Current-voltage (I-V) relations at bo th excitatory and inhibitory synapses were similar, with current activ ation near -40 mV, peak current near -10 mV and current reversal at me mbrane potentials greater than +25 mV. I-V relations were scaled along the current axis by partial calcium current blockade with cobalt, sug gesting that series resistance and space-clamp errors were small. 4. A subset of terminals on one muscle fibre was locally superfused with a physiological saline containing barium; the rest of the preparation w as superfused with a physiological saline containing calcium channel a ntagonists. Under such conditions the characteristics of the I-V relat ion were very similar to the I-V relations recorded when the entire pr eparation was bathed in physiological levels of calcium, suggesting th at the space clamp was adequate. 5. Calcium channel activation, as det ermined from tail current analyses, was similar when the entire prepar ation was bathed in physiological levels of calcium or if terminals on one muscle fibre were locally superfused with barium. 6. During a 30 ms depolarization, calcium currents inactivated to a greater extent in inhibitory than in excitatory terminals. The inactivation was of smal l magnitude (<20%) and was eliminated by intracellular injection of th e calcium chelator BAPTA, suggesting that the inactivation was calcium dependent. 7. These data show that biophysical and pharmacological pr operties of calcium currents at crayfish neuromuscular junctions resem ble those found at stellate synapses in squid.