Receptors with low affinity for neurosteroids and GABA contribute to tonic inhibition of granule cells in epileptic animals

Neurosteroid sensitivity of GABAA receptor mediated inhibition of the hippocampal dentate granule cells (DGCs) is reduced in animal models of temporal lobe epilepsy. However, the properties and subunit composition of GABAA receptors mediating tonic inhibition in DGCs of epileptic animals have not been described. In the DGCs of epileptic animals, allopregnanolone and L-655708 sensitivity of holding current was diminished and δ subunit was retained in the endoplasmic reticulum and its surface expression was decreased the in the hippocampus. Ro15-4513 and lanthanum had distinct effects on holding current recorded from DGCs of control and epileptic animals suggesting that the pharmacological properties of GABAA receptors maintaining tonic inhibition in DGCs of epileptic animals were similar to those containing the α4βxγ2 subunits. Furthermore, surface expression of the α4 subunit increased and a larger fraction of the subunit co-immunoprecipitated with theγ2 subunit in hippocampi of epileptic animals. Together, these studies revealed that functional α4βxδ and α5βxγ2 receptors were reduced in the hippocampi of epileptic animals and that novel α4bxγ2 receptors contributed to the maintenance of tonic inhibition. The presence of α4βxγ2 receptors resulted in low GABA affinity and neurosteroid sensitivity of tonic currents in the DGCs of epileptic animals that could potentially increase seizure vulnerability. These receptors may represent a novel therapeutic target for anticonvulsant drugs without sedative actions.

Research Highlights
► GABAA receptor-mediated tonic currents recorded from hippocampal granule cells are enhanced by neurosteroids in health.
► Neurosteroid sensitivity of tonic currents recorded from granule cells of experimental animals with temporal lobe epilepsy is diminished.
► Diminished surface expression of the δ subunit-containing GABAA receptors and the assembly of novel α4γ2 subunit-containing receptors could explain diminished neurosteroid sensitivity observed in epilepsy.