A family of glutamatergic, excitatory channel types at the crayfish neuromuscular junction

Outside-out patches from membrane of muscles of crayfish (Austropotamobius torrentium) were excised, and L-glutamate (glu) was applied to these patches in pulses of different duration, performing a concentration step within about 0.2 ms. While a uniform population of cationic channels is seen in equilibrium applications of glu, four kinetically different channel types were revealed by the pulse applications of glu. All these channel types had the same single channel conductance and durations of elementary short single channel openings and closings, and they thus form a family of channels. Type I, incompletely desensitizing channels reacted to a pulse of 10 m M glu with a peak open probability of 0.7 within 0.3 ms. Thereafter open probability decayed with a time constant of desensitization of about 5 ms, reaching a plateau of about 1/20 peak probability which was maintained as long as 10 m M glu were present. The peak probabilities of channel opening were proportional to approximately power 2.5 of the glu concentration, for low concentrations. Type II, completely desensitizing channels also were activated very rapidly by glu pulses, but their time constant of desensitization was 1-2 ms, and no channel openings were observed after more than 10 ms presence of a high glu concentration. The peak probabilities of channel opening rose with about the 5th power of glu concentration (for low concentrations). Type III, non-desensitizing channels, were observed relatively rarely. They were activated much more slowly and reached much lower probabilities of opening than type I and II channels. They did not show appreciable desensitization. Type IV, short-opening channels, develop sometimes from type I channels while recording, and may revert to the type I. Type IV channels show an additional open time component of 0.08 ms average duration, and a relatively long additional closed time of on average 1.3 ms. In addition to channel measurements, distributions of amplitudes and time courses of macroscopic quantal currents were determined. It is discussed in which way the different channel types may contribute to the quantal currents.