Excitatory postsynaptic channels operated by quisqualate in crayfish muscle

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Abstract
Small crayfish muscle fibers were voltage clamped and membrane current noise elicited by bath application of quisqualate, a compound structurally related to the excitatory transmitter glutamate, was measured. It was found that quisqualate activates the same excitatory postsynaptic receptors as glutamate but its affinity for these receptors is about one hundred times larger. Noise analysis revealed that the high potency of quisqualate was attributable in part to an increased apparent mean channel open time, τnoise (quisqualate), which was about ten times larger than on activation by glutamate. The channel conductance γ(quisqualate), however, was about three times smaller than γ(glutamate). AtT=8°C andE=−60 mV, τnoise (quisqualate)=9.3±1.8 ms and γ(quisqualate)=9.7±1.1 pS resulted. τnoise (quisqualate) decreased with hyperpolarization but it was much less voltage dependent than τnoise (glutamate): τnoise (quisqualate) =6.0 ms·exp (E/362 mV). Both γ(quisqualate) and α = τ noise -1 (quisqualate) increased with temperature (Q10∼1.6). This temperature dependence was characterized by the temperature independent activation energiesE γ=29.1±1.7 kJ/mol andE α=33.9±1.3 kJ/mol. Concanavalin A which blocks desensitization of the quisqualate/glutamate receptors did not influence τnoise (quisqualate) significantly.