PRESYNAPTIC CALCIUM-ACTIVATED POTASSIUM CHANNELS AND CALCIUM CHANNELS AT A CRAYFISH NEUROMUSCULAR-JUNCTION

1. We used a two-microelectrode current clamp to investigate various characteristics of the Ca2--activated K+ conductance [g(K(Ca))] and Ca2+ conductance (g(Ca)) and transmitter release in presynaptic terminals of excitatory neuromuscular junctions in the crayfish walking leg. 2. Voltage-activated Na- conductances (g(Na)) and K+ conductances [g(K(v))] were blocked with tetrodotoxin and 3,4-diaminopyridine, respectively. Under these conditions, presynaptic depolarization produced by a first (conditioning) pulse admitted Ca2- into the presynaptic terminals and activated g(K(Ca)), which modulated the amplitude of the depolarization produced by a second (rest) pulse. The relative amount of g(K(Ca)) measured at the test pulse increased with increased magnitude or duration of the conditioning pulse. 3. A brief hyperpolarization immediately after a conditioning pulse substantially reduced g(K(Ca)). 4. g(K(Ca)) activation was blocked by funnel web spider toxin (a Ca2+ channel blocker) or by injection of the presynaptic terminal region with a calcium chelator. bis-(o-aminophenoxy)-N,N,N',N'-tetraacetic acid (BAPTA). Under current-clamp conditions, g(K(Ca)) was not blocked by charybdotoxin or iberiotoxin [specific g(K(Ca)) blockers]. 5. When g(K(Ca)) as blocked or reduced, the amplitude of the depolarizing after potential of action potentials was increased. When g(K(v)) was blocked or reduced, the duration of action potentials was increased. 6. Intracellular injection of BAPTA into the presynaptic terminal region eliminated evoked neurotransmitter release before test pulse modulation was affected. suggesting that the K(Ca) channel had a greater sensitivity (greater affinity or lower stoichiometry) for Ca2+ than did the transmitter release machinery. BAPTA reduced neurotransmitter release by 66-78%, but did not affect facilitation of neurotransmitter release. 7. When g(Na), g(K(v)), and g(K(Ca)) were blocked, we detected a membrane depolarization produced by an increase in presynaptic g(Ca) that was eliminated by 2 mM Cd2- or 0 mM Ca2+.