Kinetic properties of the glycine receptor main- and sub-conductance states of mouse spinal cord neurones in culture

Abstract

1. The kinetic properties of the two most frequent conductance states of glycine receptor channels from somata of mouse spinal cord neurones in cell culture were investigated using the outside-out patch clamp technique. At low concentrations of glycine (0.5, 1 and 2 microM), single-channel currents were recorded with two predominant amplitudes corresponding to a dominant or main-conductance state of about 42 pS and a sub-conductance state of about 27 pS. Both conductance states opened singly and in bursts of several openings. Total current evoked and single-channel opening frequency increased as glycine concentration was increased from 0.5 to 2 microM. 2. For both conductance states mean open times were increased and open time frequency histograms were shifted to longer times as glycine concentration was increased from 0.5 to 2 microM. For both conductance states, three exponential components were required to fit best open time frequency distribution histograms at all glycine concentrations (0.5, 1 and 2 microM). The time constants of the exponential components for each conductance state were not significantly different across concentration, suggesting that the main- and sub-conductance states of the channel each opened into at least three open states. For the main-conductance state, the time constants were 1.09 +/- 0.09, 4.06 +/- 0.26 and 9.79 +/- 0.30 ms. For the sub-conductance state, the time constants were 0.55 +/- 0.04, 2.64 +/- 0.35 and 8.57 +/- 1.08 ms. The increase in long open times with concentration was due primarily to a shift in relative frequency of occurrence of openings from the shortest to the two longest open states. 3. Closed time frequency distributions of closures between main-conductance state openings, closures between sub-conductance state openings and closures between both main- and sub-conductance state openings were fitted with multiple exponential components, suggesting that the channel had several closed states. The two shortest time constants (0.16 +/- 0.01 and 1.26 +/- 0.13 ms) did not vary significantly with concentration (0.5-2 microM) or method of analysis. The longer time constant varied with concentration. 4. Bursts were defined as groups of openings surrounded by closures greater than a critical closed time. For both conductances states, mean burst durations were increased and burst duration frequency histograms were shifted to longer times as glycine concentration was increased from 0.5 to 2 miroM. Burst duration frequency histograms contained four exponential components for the main-conductance state and three exponential components for the sub-conductance state.(ABSTRACT TRUNCATED AT 400 WORDS)