Distinct signal transduction pathways for GABA-induced GABA(A) receptor down-regulation and uncoupling in neuronal culture: a role for voltage-gated calcium channels

Changes in GABA receptor (GABA(A)R) gene expression are detected in animal models of epilepsy, anxiety and in postmortem schizophrenic brain, suggesti ng a role for GABA(A)R regulation in neurological disorders. Persistent (48 h) exposure of brain neurons in culture to GABA results in downregulation of GABAAR number and uncoupling of GABA and benzodiazepine (BZD) binding si tes. Given the central role of GABA(A)Rs in fast inhibitory synaptic transm ission, GABA(A)R down-regulation and uncoupling are potentially important m echanisms of regulating neuronal excitability, yet the molecular mechanisms remain unknown. In this report we show that treatment of brain neurons in culture with tetrodotoxin, glutamate receptor antagonists, or depolarizatio n with 25 mm K+ fails to alter GABA(A)R number or coupling. Changes in neur onal activity or membrane potential are therefore not sufficient to induce either GABA(A)R down-regulation or uncoupling. Nifedipine, a voltage-gated Ca2+ channel (VGCC) blocker, inhibits both GABA-induced increases in [Ca2+] , and GABA(A)R down-regulation, suggesting that VGCC activation is required for GABA(A)R down-regulation. Depolarization with 25 mm K+ produces a sust ained increase in intracellular [Ca2+] without causing GABA(A)R down-regula tion, suggesting that activation of VGCCs is not sufficient to produce GABA AR down-regulation. In contrast to GABA(A)R down-regulation, nifedipine and 25 mm K+ fail to inhibit GABA-induced uncoupling, demonstrating that GABA- induced GABA(A)R down-regulation and uncoupling are mediated by independent molecular events. Therefore, GABA(A)R activation initiates at least two di stinct signal transduction pathways, one of which involves elevation of int racellular [Ca2+] through VGCCs.