Effects of glycine on the crayfish neuromuscular junction. I. Glycine-operated inhibitory postsynaptic channels and a glycine-effected decrease in membrane conductance

Abstract

Inhibitory postsynaptic membrane channels which are activated by glycine were investigated by means of the noise analysis technique. Dose-response curves were obtained for gamma-aminobutyric acid (GABA) in the presence and in the absence of glycine, and it was concluded that GABA and glycine are likely to activate the same receptors. However, glycine proved to have a very low affinity for the inhibitory postsynaptic receptors; this affinity was smaller than that of GABA by a factor of 1 . 10(3)-2 . 10(3). The mean open time tau of the postsynaptic Cl- channels activated by glycine at E = -100 mV and E = -60 mV membrane potentials were tau = 6.1 ms +/- 1.5 ms and tau = 17.7 ms +/- 2.2 ms, respectively. These values are in agreement with the tau obtained by activation with GABA (Dudel et al. 1980); however, on activation by glycine the potential dependence of tau was larger by a factor of 1.35. At E = -100 mV the conductance gamma of glycine-operated channels was about 3 pS which is a third of the respective conductance elicited by GABA. In the presence of high concentrations of glycine (0.1-0.5 mol/l) spontaneous inhibitory postsynaptic currents (sIPSCs) and 'giant' spontaneous inhibitory postsynaptic currents (gsIPSCs) were observed. Furthermore at high concentrations of glycine an additional glycine-induced noise component was found in the power spectra of current fluctuations at higher frequencies. It was concluded that this spectral component resulted from the closing of otherwise open K+ channels, as has been observed already on application of GABA (Dudel and Finger 1980). The mean duration of the low conductance state was tau- = 2.2 ms +/- 0.9 ms and the conductance decrease gamma- coupled to this process was estimated to be about 3 pS. In Na+ free- and Ca2+-enriched bathing solutions the glycine-induced conductances gamma and gamma- were reduced by a factor of about 1.7 while tau and tau- remained unchanged. The decrease in gamma and gamma- was most likely effected by the increase in concentration of divalent cations.