Comparison of excitatory currents activated by different transmitters on crustacean muscle: I. Acetylcholine-activated channels

The properties of acetylcholine-activated excitatory currents on the gml muscle of three marine decapod crustaceans, the spiny lobsters Panuhrus argus and interruptus, and the crab Cancer borealis, were examined using either noise analysis, analysis of synaptic current decays, or analysis of the voltage dependence of ionophoretically activated cholinergic conductance increases. The apparent mean channel open time (τ_n) obtained from noise analysis at -80 mV and 12°C was ~13 ms; Tn was prolonged e-fold for about every 100-mV hyperpolarization in membrane potential; Tn was prolonged e-fold for every 10°C decrease in temperature. γ, the single-channel conductance, at 12°C was ~18 pS and was not affected by voltage; γ was increased ~2.5-fold for every 10°C increase in temperature. Synaptic currents decayed with a single exponential time course, and at -80 mV and 12°C, the time constant of decay of synaptic currents, τ_ejc, was ~14-15 ms and was prolonged e-fold about every 140-mV hyperpolarization; τ_ejc, was prolonged about e-fold for every 10°C decrease in temperature. The voltage dependence of the amplitude of steadystate cholinergic currents suggests that the total conductance increase produced by cholinergic agonists is increased with hyperpolarization. Compared with glutamate channels found on similar decapod muscles (see the following article), the acetylcholine channels stay open longer, conduct ions more slowly, and are more sensitive to changes in the membrane potential.