Which GABA(A) receptor subunits are necessary for tonic inhibition in the hippocampus?

Abstract

GABA(A) receptors (GABA(A)Rs) assembled of different subunits mediate tonic and phasic inhibition in hippocampal neurons. CA1/CA3 pyramidal cells (PCs) predominantly express alpha5 subunits whereas dentate gyrus granule cells (DGGCs) and molecular layer (ML) interneurons predominantly express delta subunits. Both alpha5- and delta-containing GABA(A)Rs mediate tonic inhibition. We have shown previously that mice lacking alpha5 subunits (Gabra5-/-) have a residual tonic current in CA1/CA3 PCs because of an upregulation of delta subunits, but the basis of the residual tonic current in DGGCs and ML interneurons of mice lacking the delta subunit (Gabrd-/-) is still unknown. We now show that wild-type DGGCs have a small tonic current mediated by alpha5 subunit-containing GABA(A)Rs responsible for approximately 29% of the total tonic current. To better identify the GABA(A)Rs mediating tonic inhibition in hippocampal neurons, we generated mice lacking both alpha5 and delta subunits (Gabra5/Gabrd-/-). Recordings from CA1/CA3 PCs, DGGCs, and ML interneurons in these mice show an absence of tonic currents without compensatory changes in spontaneous IPSCs (sIPSCs), sEPSCs, and membrane resistance. The absence of tonic inhibition results in spontaneous gamma oscillations recordable in vitro in the CA3 pyramidal layer of these mice, which can be mimicked in wild-type mice by blocking alpha5 subunit-containing GABA(A)Rs with 50 nM L-655,708. In conclusion, depending on the cell type, the alpha5 and delta subunits are the principal GABA(A)R subunits responsible for mediating the lion's share of tonic inhibition in hippocampal neurons.

Figure 1.

GABA_ARs containing α5 or δ subunits mediate tonic inhibitory currents in hippocampal neurons. A, Whole-cell voltage-clamp recording of a CA1 PC (V_h = −70 mV). Top lines indicate the perfusion of L-655,708 and BMI. Right, Fit to the all-points histogram from segments of each of the three conditions. B, Whole-cell voltage-clamp recordings from Gabra 5/Gabrd^−/− CA1 and CA3 PC, DGGC and a molecular layer interneuron (V_h = −70 mV). Right, Same as in A.

Figure 2.

Hippocampal neurons from Gabra5/Gabrd^−/− mice show no tonic inhibitory currents. Mean tonic inhibitory conductance recorded from CA1/CA3 PCs, DGGCs and ML interneurons. One-way ANOVA of each cell type indicate statistical significance p < 0.05. Asterisk indicates Dunnett's post hoc test being statistically significant (p < 0.05) using WT as the control group. The number of cells recorded is indicated in Tables 1–3.

Figure 3.

Reducing tonic inhibition leads to the emergence of gamma oscillations in hippocampal CA3. Field potential recordings were obtained from the CA3 pyramidal layer of slices maintained in an interface chamber. A, The power spectral density of the field oscillation in control conditions and after cumulative application of 50 and 200 nm L-655,708 in WT (top) and Gabra5/Gabrd^−/− (bottom) mice. Right, Sample traces notch-filtered at 60 Hz. B, Mean changes in log power in the gamma frequency range (30–80 Hz). C, Mean ratio of the power recorded in the gamma and ripple (150–250 Hz) frequency ranges (WT, n = 7; Gabra5/Gabrd^−/−, n = 4).