Chronic exposure of developing cortical neurons to GABA down-regulates GABA/benzodiazepine receptors and GABA-gated chloride currents.

Abstract Cultures of cerebral neurons were prepared from chick embryos, 8.5 days in ovo, and maintained in vitro. Following chronic exposure of these cells to GABA, the levels of [ 3 H]flunitrazepam binding in situ and electrophysiological responsiveness to γ-aminobutyric acid (GABA) was examined. Treatment with 100 μM GABA for 7 days reduced [ 3 H]flunitrazepam binding in situ by 70 ± 8% compared to untreated controls. The binding of [ 3 H] N -methylscopolamine was unaffected by this treatment. The reduction in [ 3 H]flunitrazepam binding was prevented by concomitant exposure of developing neurones to the GABA antagonist R 5135, suggesting that GABA A receptor occupancy is required. The loss of bezodiazepine receptors was dependent on the GABA concentration in the culture medium and a half-saturation (IC 50 ) value of 11.2 ± 3.7 μM was estimated. Whole-cell patch-clamp recordings were obtained to assess the functional properties of the labile receptor pool observed in the binding studies. Neurons cultured with 100 μM GABA for 7 days showed a 60–70% reduction in the peak current amplitudes observed in response to application of 10–100 μM GABA. However, the rate of rapid desensitization, quantified by measuring changes in input conductance, was unchanged by chronic GABA exposure, yielding decay time constants of 27.1 ± 2.1 and 34.7 ± 4.7 s for control and treated cells, respectively. The results are consistent with a GABA modulation of the GABA A /benzodiazepine receptor complex by means of down-regulation.