Calcium currents, transmitter release and facilitation of release at voltage-clamped crayfish nerve terminals

1. The presynaptic terminals at crayfish (Procambarus spp.) opener neuromuscular junctions were voltage clamped. Calcium currents were measured during (I-Ca) and following (tail I-Ca) presynaptic depolarizations; EPSPs or IPSPs were simultaneously recorded from the (postsynaptic) muscle fibre directly beneath the presynaptic impalement. 2. For short (less than or equal to 6 ms) presynaptic depolarizations, most of the transmitter release occurred during the tail I-Ca. EPSP or IPSP amplitudes at the end of the 6 ms pulse (end EPSP or end IPSP) increased monotonically with the integral of the I-Ca (integral I-Ca). The suppression potential for transmitter release was near the apparent reversal potential for I-Ca. 3. When the end EPSP or end IPSP amplitude was plotted against the peak I-Ca elicited during a presynaptic pulse (peak I-Ca), large and small depolarizations which evoked the same peak I-Ca evoked different amounts of transmitter release. These differences in transmitter release were eliminated when end EPSP amplitude was plotted against integral I-Ca, suggesting that transmitter release during a depolarization depends only upon calcium current and not upon a subsequent voltage-dependent step. 4. The synaptic transfer function of various measurements of EPSP or IPSP amplitude vs. integral I-Ca evoked during a presynaptic depolarization was a power function having an exponent of about 3. Similar measurements of EPSP amplitude vs. integral tail I-Ca evoked following a presynaptic depolarization had an exponent of about 2. 5. Facilitation of an EPSP or IPSP was not due to increases in calcium current at the test depolarization. 6. When the conditioning depolarization was increased and the test depolarization remained constant, EPSP amplitude at the test depolarization and facilitation increased. When the conditioning depolarization remained constant and the test depolarization was increased, EPSP amplitude at the test depolarization increased, while facilitation decreased. 7. Our data suggest that transmitter release at crayfish neuromuscular junctions is a non-linear function of calcium influx, and that facilitated release utilizes intracellular calcium differently from non-facilitated release. These data contradict simple models of facilitation which combine the residual calcium hypothesis with the calcium co-operativity hypothesis non-facilitated release.