Deletion of the GABA_A receptor α1 subunit increases tonic GABA_A receptor current: A role for GABA uptake transporters

The loss of more than half the number of GABA_A receptors yet lack of pronounced phenotype in mice lacking the gene for the GABA_A α1 subunit is somewhat paradoxical. We explored the role of tonic GABA_A receptor-mediated current as a target of compensatory regulation in the α1 knock-out (-/-) mice. A 62% increase of tonic current was observed in the cerebellar granule cells (CGCs) of α1^-/- compared with wild-type (+/+) mice along with a 67% increase of baseline current variance. Examination of whole-cell currents evoked by low concentrations of GABA and 4,5,6,7-tetrahydroisoxazolo[5,4-c]pyridin-3-ol suggested no upregulation of α6 and δ subunit-containing GABA_A receptors in the α1^-/-, confirming previous biochemical studies. Single-channel current openings were on average 32% shorter in the α1^-/- neurons. Single-channel conductance and frequency of opening were not different between genotypes. Tonic current induced by application of the GABA transporter GAT-1 blocker NO711 (1- [2([(diphenylmethylene)imino]oxy)ethyl]-1,2,5,6-tetrahydro-3-pyridinecarboxylic acid hydrochloride) was significantly larger in the α1^-/-, suggesting an increase of ambient GABA concentration. Experiments done with a known concentration of extracellular GABA complemented by a series of biochemical experiments revealed a reduction of GAT activity in α1 ^-/- without an identifiable reduction of GAT-1 or GAT-3 protein. We report increased tonic GABA_A receptor-mediated current in the α1^-/- CGCs as a novel compensatory mechanism. Our data establish a role for GABA transporters as regulators of neuronal excitability in this and relevant models and examine other tonic conductance-regulating mechanisms responsible for the adaptive response of the cerebellar network to a deletion of a major synaptic GABA_A receptor subunit.