Kinetic properties of the GABAA receptor main conductance state of mouse spinal cord neurones in culture.

1. The kinetic properties of the main conductance state of gamma-aminobutyric acidA (GABA) receptor channels from somata of mouse spinal cord neurones in cell culture were investigated using patch clamp techniques. 2. Whole-cell GABA receptor currents increased in a concentration-dependent manner from 0.5 to 5 microM. 3. Single-channel currents were recorded with a main conductance state of 27.2 pS and a less frequent conductance state of 15.9 pS. The main conductance state opened singly and in bursts of several openings. 4. Mean open times of GABA receptor main conductance currents were increased and open-time frequency histograms were shifted to longer times as GABA concentration was increased from 0.5 to 5 microM. Three exponential functions were required to fit the histograms at all GABA concentrations, suggesting that the channel opened into at least three open states (O1, O2 and O3). The three functions had the same time constants (1.0 +/- 0.2, 3.7 +/- 0.4 and 11.3 +/- 0.5 ms; mean +/- S.D.) at each concentration. The increase in long open times with concentration was due to a shift in relative frequency of occurrence of openings from the shortest (O1) to the two longest (O2 and O3) open states. 5. Closed-time distributions of closures between main conductance state openings were fitted with multiple exponential functions, suggesting that the channel had several closed states. The two shortest time constants (0.24 +/- 0.03 and 2.0 +/- 0.3 ms) were concentration independent (0.5 to 5 microM). Three longer time constants decreased as concentration increased. 6. Bursts were defined as groups of openings surrounded by closures greater than a critical closed time (tc = 5 ms). Mean burst durations were increased and burst duration frequency histograms were shifted to longer times as GABA concentration was increased from 0.5 to 5 microM. Burst-duration frequency histograms were best fitted with three exponential functions. The time constants were concentration independent and were 1.0 +/- 0.2, 5.5 +/- 0.2 and 29.8 +/- 1.6 ms. The increase in burst duration with concentration was due to a relative shift from short duration bursts to longer duration bursts. 7. The shortest burst time constant was similar to the shortest open time constant suggesting that there was a population of single openings of short duration. The two longest burst time constants were longer than the two longest open time constants, suggesting that the bursts from the two longest burst components were composed of two or more openings.(ABSTRACT TRUNCATED AT 400 WORDS)