On the mechanism of the .gamma.-aminobutyric acid receptor in the mammalian (mouse) cerebral cortex. Chemical kinetic investigations with a 10-ms time resolution adapted to measurements of neuronal receptor function in single cells

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Abstract
The gamma-aminobutyric acidA (GABA) receptor belongs to a superfamily of proteins involved in chemical reactions that regulate signal transmission between cells of the nervous system and is the target of some of the agents most frequently used in medicine to control disorders of the central nervous system. In contrast to the nicotinic acetylcholine receptor, which initiates signal transmission and is the best characterized member of the superfamily, the GABA receptor forms anion-specific transmembrane channels and inhibits signal transmission. The chemical kinetic experiments described here, in which fast chemical reaction techniques were used, indicate that both receptor proteins may operate by the same mechanism. Also described is the use of a chemical kinetic technique with a 10-ms time resolution that we have developed for making measurements with single cells isolated from specific areas of the nervous system, in this case the cerebral cortex of embryonic mice. A flow device was used to equilibrate receptors on the cell surface with GABA, and the concentration of open transmembrane channels in the cells was then measured by recording the whole-cell currents at pH 7.2, 21-23 degrees C, and a transmembrane voltage of -70 mV. Two different receptor forms, A alpha and A beta, were detected in cerebral cortical cells. Although the ratio of A alpha to A beta varied from cell to cell, on average 35% and 65% of the receptor-controlled current was associated with receptor forms A alpha and A beta, respectively. At saturating concentrations of GABA, the rate coefficients of desensitization, alpha and beta, associated with these two forms have maximal values of 4.4 and 0.7 s-1, respectively. The constants of a mechanism that accounts for the open transmembrane channels of both receptor forms were evaluated over a 50-fold range of GABA concentration. The dissociation constant of the site controlling channel opening was 40 microM for A alpha and 320 microM for A beta. The channel-opening equilibrium constant, phi-1, was 3.5 for A alpha and 20 for A beta. The evaluated constants allow one to calculate Po, the conditional probability that at a given concentration of GABA the receptor-channel is open. Po could also be determined in the presence of 100 microM GABA by an independent method in which different assumptions are made in the interpretation of the experimental results, the single-channel current-recording technique. The value of Po obtained (0.56) was in good agreement with the Po value (0.61) calculated for receptor form A alpha from chemical kinetic measurements at 100 microM GABA.(ABSTRACT TRUNCATED AT 400 WORDS)