Contribution of changes in the chloride driving force to the fading of I(GABA) in frog melanotrophs.

Chloride redistribution during type A gamma-aminobutyric acid (GABA(A)) currents (I(GABA)) has been investigated in cultured frog pituitary melanotrophs with imposed intracellular chloride concentration ([Cl(-)](i)) in the whole cell configuration or with unaltered [Cl(-)](i) using the gramicidin-perforated patch approach. Prolonged GABA exposures elicited reproducible decaying currents. The decay of I(GABA) was associated with both a transient fall of conductance (g(GABA)) and shift of current reversal potential (E(GABA)). The shift of E(GABA) appeared to be time and driving force dependent. In the gramicidin-perforated patch configuration, repeated GABA exposures induced currents that gradually vanished. The fading of I(GABA) was due to persistent shifts of E(GABA) as a result of g(GABA) recovering from one GABA application to another. In cells alternatively clamped at potentials closely flanking resting potential and submitted to a train of brief GABA pulses, a reversal of I(GABA) was observed after 150 s recording. It is demonstrated that, in intact frog melanotrophs, shifts of E(GABA) combine with genuine receptor desensitization to depress I(GABA). These findings strongly suggest that shifts of E(GABA) may act as a negative feedback, reducing the bioelectrical and secretory responses induced by an intense release of GABA in vivo.