Behavior and Physiology of Mice Lacking the GABA_A‐Receptor δ Subunit

Abstract

Summary:Purpose: Mice with a targeted disruption of the ␥-aminobutyric acid (A) receptor (GABA A R) ␦ subunit exhibited spontaneous seizures and a strikingly selective attenuation of responses to neuroactive steroids, but not to other neuromodulators. This study further characterized the behavior and physiology of the ␦ mutants.Methods: Seizure susceptibility to pentylenetetrazol (PTZ) injection, intracellular recordings, and whole-cell patch recordings in dentate granule neurons was determined.Results: Susceptibility to PTZ-induced convulsive seizures was significantly higher in −/− versus +/+ mice. In intracellular recordings, the evoked inhibitory postsynaptic potentials (IPSPs) had much faster decay in −/− mutants (25 ± 3.5 ms) compared with +/+ controls (51 ± 5.2 ms). In whole-cell patch recordings, the decay of pharmacologically isolated miniature spontaneous GABA A R-mediated synaptic currents (mIPSCs) from −/− mice was only slightly, but significantly faster (5.7 ± 0.13 ms; n ‫ס‬ 18) than that of +/+ controls (6.8 ± 0.32 ms; n ‫ס‬ 20). Furthermore, mIPSC prolongation by bath application of alphaxalone (10 M), was much smaller in −/− mice (52 ± 12%; n ‫ס‬ 3) compared with +/+ littermates (192 ± 14%; n ‫ס‬ 3).Conclusions: Faster decay of evoked IPSPs and mIPSCs is consistent with altered GABA A R subunit composition in the ␦ mutants, but suggests predominant extrasynaptic ␦ subunit location in the dentate gyrus of normal mice. Faster-decaying IPSPs likely contribute to the proepileptic phenotype of the ␦ mutants. Reduced neurosteroid sensitivity might also contribute to seizure susceptibility. It remains to be determined whether ␦ subunit-containing GABA A Rs represent the actual target of neurosteroids or whether the behavioral and physiologic sensitivity to neurosteroids is indirectly altered in the ␦ −/− mice.

δ Subunit Susceptibility Variants E177A and R220H Associated with Complex Epilepsy Alter Channel Gating and Surface Expression of α4β2δ GABAAReceptors

“…␦ subunits preferentially coassemble with ␣4 subunits in the thalamus, a structure critical for pathogenesis of IGEs (Wisden et al, 1992;Sur et al, 1999), and GABRD knock-out animals exhibit spontaneous seizures (Spigelman et al, 2002), reinforcing our finding that impaired ␣4␤␦ receptor function is associated with complex epilepsy in humans (Dibbens et al, 2004). Also, ␣4 and/or ␦ subunit expression was changed in experimental seizures (Clark et al, 1994;Schwarzer et al, 1997;Brooks-Kayal et al, 1998;Peng et al, 2004), and reduction of ␦ subunits may contribute to epileptogenesis in experimental epilepsy (Peng et al, 2004).…”

3β-Methyl-Neurosteroid Analogs Are Preferential Positive Allosteric Modulators and Direct Activators of Extrasynapticδ-Subunitγ-Aminobutyric Acid Type A Receptors in the Hippocampus Dentate Gyrus Subfield

“…The -subunit and its expressional plasticity remain key factors in understanding neurosteroid sensitivity, network excitability, and seizure susceptibility (Mihalek et al, 1999;Carver and Reddy, 2013;Whissell et al, 2015;Reddy et al, 2017;Chuang and Reddy, 2018). Mice lacking the -subunit display significantly decreased sleep time in response to neurosteroid alphaxolone and reduced sensitivity to GX-exerted anxiolytic response (Mihalek et al, 1999;Spigelman et al, 2002;Porcello et al, 2003;Spigelman et al, 2003). The -selective effect for neurosteroid modulation is also established by electrophysiological studies.…”

Molecular Targets for Antiepileptic Drug Development

“…10 Mice lacking ␦ subunits exhibit spontaneous seizures and greater sensitivity to the convulsant PTZ, demonstrating a role of the ␦ subunit containing GABA A receptors in regulating seizure susceptibility. 188 Such GABA A receptors are particularly sensitive to modulation by neurosteroids, and this effect of neurosteroids may be lost in ␦ knockout animals. 189,190 [4,5,6,7-tetrahydroisoxazolo(5,4-c)pyridin-3-ol]), act as GABA recognition site agonists.…”