GAMMA-AMINOBUTYRIC-ACID TYPE-A RECEPTOR ANTAGONISTS PICROTOXIN AND BICUCULLINE ALTER ACETYLCHOLINE CHANNEL KINETICS IN CULTURED EMBRYONIC RAT SKELETAL-MUSCLE

The effects of the classical gamma-aminobutyric acid type A receptor a ntagonists picrotoxin and bicuculline on nicotinic acetylcholine recep tors in cultured embryonic rat skeletal muscle were examined with whol e-cell and cell-attached single-channel recording methods. Up to 600 m u M picrotoxin had little or no effect on the amplitude of the whole-c ell current, whereas bicuculline dose-dependently blocked it, with an IC50 value of 101.2 +/- 8.9 mu M. Bicuculline reduced the maximum indu cible acetylcholine current without changing the K-d value, suggesting that bicuculline uncompetitively blocked the binding of acetylcholine to its receptor. The elementary nicotinic acetylcholine receptor curr ents recorded in the cell-attached single-channel recording configurat ion exhibited properties typical of those recorded in embryonic muscle (similar to 36 pS and similar to 6 msec). Picrotoxin dramatically tra nsformed individual channel openings into briefly interrupted bursts, so that the number of openings increased while the mean open time mark edly decreased. Bicuculline decreased mean open time to a lesser but s tatistically significant degree. The dominant component of the closed time histogram in control recordings occurred at 17 msec, whereas that recorded with picrotoxin occurred at 0.5 msec. Bicuculline prolonged the closed time, with a dominant closed time component at 52 msec. Ele mentary conductance was not altered by either agent. In conclusion, we found that the gamma-aminobutyric acid type A channel antagonists pic rotoxin and bicuculline were also blockers of embryonic nicotinic acet ylcholine receptor channels in cultured rat muscle.