Calcium dependent gating of thel-glutamate activated, excitatory synaptic channel on crayfish muscle

Abstract

Excitatory, glutamate-activated single channel currents were measured in outside-out patches of crayfish muscle. The open time of single channel openings, and the durations and rates of bursts were evaluated. These kinetic parameters were not appreciably affected by replacement of extracellular Na⁺ by Li⁺ or choline. Changes in extracellular Ca²⁺ concentration Ca_o also did not influence the duration of single openings. However the mean burst duration decreased for Ca_oo. At Ca_oo as the cause of disappearance of channel openings, glutamate was applied in short pulses with a liquid-filament switch. In 0 Ca_o also a glutamate pulse did not trigger channel openings. In presence of 13.5 mM Ca_o, the inorganic Ca-channel blockers La³⁺ and Cd²⁺ diminished the duration and rate of bursts of channel openings in a similar manner as low Ca_o. The effects of low Ca_o and of Cd²⁺ were tested also on quantal postsynaptic currents, EPSCs, which were recorded through a perfused macro-patch-clamp electrode. At 1.4 mM Ca_o in the perfused electrode tip, spontaneous EPSCs were reduced at least by a factor of 4, and elicited EPSCs by a factor of 16. Application of Cd²⁺ had similarly strong effects on the EPSCs. Also the decay of EPSCs was shortened substantially in 1.4 mM Ca_o or 5 mM Cd²⁺. The inhibitory Cl⁻-channel of crayfish muscle, activated by glutamate or GABA, also was studied in outside-out patches. The openings of this channel persisted in 0 Ca_o solutions; the block of channel openings in low Ca_o thus is a specific property of the excitatory channel. The action of Ca_o on the excitatory channel may be described as that of a cofactor to glutamate. A possible reaction scheme is proposed.