Actions of gamma-aminobutyric acid on neurones of guinea-pig myenteric plexus

Abstract

The effects of gamma-aminobutyric acid (GABA) applied by ionophoresis, pressure ejection and superfusion to myenteric neurones of the guinea-pig ileum were investigated by intracellular recording techniques. Ionophoretic or pressure application of GABA (10 pC-30 nC) caused membrane depolarizations of AH neurones but not S neurones. This depolarization was associated with a conductance increase. It reversed polarity at a membrane potential of -18 mV when intracellular electrodes contained KCl, and -39 mV when electrodes contained K acetate, citrate or sulphate. The ionophoretic depolarization was antagonized by bicuculline (1-30 microM) in an apparently competitive manner. During prolonged or repeated ionophoretic application of GABA, both the depolarization and conductance increase desensitized. Superfusion of GABA (1-100 microM) caused a membrane depolarization in AH neurones, associated with an increase in membrane conductance. The increase in conductance was always smaller than that evoked by ionophoresis of GABA. Bicuculline only partially depressed the depolarization induced by superfusion of GABA, particularly slowing its rising phase. beta-p-Chlorophenyl GABA (baclofen) (10 microM) caused a depolarization similar to that observed with GABA in the presence of bicuculline. The depolarization induced by baclofen and GABA (in presence of bicuculline) superfusion did not decline during prolonged applications; superfusion of GABA but not baclofen reversibly reduced or eliminated the effects of GABA ionophoresis. It is concluded that GABA has two effects on the membrane of myenteric neurones. The first is a bicuculline-sensitive, rapidly desensitizing chloride activation: the second is a bicuculline-insensitive, non-desensitizing depolarization.