Altered localization of GABAA receptor subunits on dentate granule cell dendrites influences tonic and phasic inhibition in a mouse model of epilepsy

Complex changes in GABA(A) receptors (GABA(A)Rs) in animal models of temporal lobe epilepsy during the chronic period include a decrease in the delta subunit and increases in the alpha 4 and gamma 2 subunits in the dentate gyrus. We used postembedding immunogold labeling to determine whether the subcellular locations of these subunits were also altered in pilocarpine-treated epileptic mice, and related functional changes were identified electrophysiologically. The ultrastructural studies confirmed a decrease in delta subunit labeling at perisynaptic locations in the molecular layer of the dentate gyrus where these subunits are critical for tonic inhibition. Unexpectedly, tonic inhibition in dentate granule cells was maintained in the epileptic mice, suggesting compensation by other GABA(A)Rs. An insensitivity of the tonic current to the neurosteroid tetrahydrodeoxy-corticosterone was consistent with decreased expression of the delta subunit. In the pilocarpine-treated mice, alpha 4 subunit labeling remained at perisynaptic locations, but increased gamma 2 subunit labeling was also found at many perisynaptic locations on granule cell dendrites, consistent with a shift of the gamma 2 subunit from synaptic to perisynaptic locations and potential partnership of the alpha 4 and gamma 2 subunits in the epileptic animals. The decreased gamma 2 labeling near the center of synaptic contacts was paralleled by a corresponding decrease in the dendritic phasic inhibition of granule cells in the pilocarpine-treated mice. These GABA(A) R subunit changes appear to impair both tonic and phasic inhibition, particularly at granule cell dendrites, and could reduce the adaptive responses of the GABA system in temporal lobe epilepsy.