Reduced Inhibition and Sensitivity to Neurosteroids in Hippocampus of Mice Lacking the GABA_AReceptor δ Subunit

Abstract

. Reduced inhibition and sensitivity to neurosteroids in hippocampus of mice lacking the GABA A receptor ␦ subunit. J Neurophysiol 90: 903-910, 2003. First published April 17, 2003 10.1152/jn.01022.2002. The ␦ subunit of the ␥-aminobutyric acid (A) receptor (GABA A R) is expressed postnatally mostly in the cerebellum, thalamus, and dentate gyrus. Previous studies in mice with a targeted disruption of the ␦ subunit revealed a considerable attenuation of behavioral responses to neuroactive steroids but not to other neuromodulatory drugs. Here we show that ␦ subunit loss leads to a concomitant reduction in hippocampal ␣4 subunit levels. These changes were accompanied by faster decay of evoked inhibitory postsynaptic potentials (IPSPs) in dentate granule neurons of Ϫ/Ϫ mutants (decay ϭ 25 ms) compared with ϩ/ϩ controls ( ϭ 50 ms). Furthermore, the GABA A R-mediated miniature inhibitory postsynaptic currents (mIPSCs) also decayed faster in ␦-mutants ( ϭ 6.3 ms) than controls ( ϭ 7.2 ms) and had decreased frequency (controls, 10.5 Hz; mutants, 6.6 Hz). Prolongation of mIPSCs by the neuroactive steroid anesthetic, alphaxalone (1-10 M), was smaller in ␦-mutants (at 10 M, 65% increase) compared with ϩ/ϩ littermates (308% increase). In competition binding experiments, alphaxalone (0.03-1 M) modulation of [35 S]t-butylbicyclophosphorothionate binding was reduced in ␦-mutant brain homogenates, indicating that the decreased alphaxalone effects on mIPSCs were due to changes in the GABA A R protein. Faster decay of evoked IPSPs and mIPSCs in ␦-mutants suggests presence of the ␦ subunit at both synaptic and extrasynaptic GABA A Rs. Decreased synaptic and extrasynaptic inhibition likely contributes to the pro-epileptic phenotype of ␦-mutants. Reduced neurosteroid sensitivity might also contribute to seizure susceptibility. While the simplest explanation is that ␦ subunit-containing GABA A Rs represent the actual target of neurosteroids, it is possible that the behavioral and physiological sensitivity to neuroactive steroids is indirectly altered in the ␦ Ϫ/Ϫ mice.

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

“…Furthermore, the frequency of mIPSCs in  DGGCs was modestly smaller than WT DGGCs (n = 10-12 cells per group, p = 0.06), showing a small reduction in the functional number of GABAergic synapse release sites and the rate of presynaptic release in  neurons (Cherubini and Conti, 2001). Overall, these findings are similar and consistent with the results reported previously in mice (Spigelman et al, 2003).…”

“…In WT DGGCs, which have the second highest expression of the -subunit, alphaxolone strongly prolongs the decay time of mIPSCs, but not in those from  mice (Spigelman et al, 2003). KO mice were observed to have a significant reduction in the expression of the 4-subunit and an increase in the 2-subunit (Peng et al, 2002;Spigelman et al, 2003). This notion is with previous reports that 4-containing GABA-A receptors are more sensitive to neurosteroid modulation (Brown et al, 2002;Wohlfarth et al, 2002).…”

“…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.…”

“…However, this is significantly diminished in  neurons (Vicini et al, 2002). In WT DGGCs, which have the second highest expression of the -subunit, alphaxolone strongly prolongs the decay time of mIPSCs, but not in those from  mice (Spigelman et al, 2003). KO mice were observed to have a significant reduction in the expression of the 4-subunit and an increase in the 2-subunit (Peng et al, 2002;Spigelman et al, 2003).…”

A Conceptualization of Integrated Actions of Ethanol Contributing to its GABAmimetic Profile: A Commentary

“…These combined actions are proposed to be the primary contributors to the GABAmimetic profile of ethanol (Figure 4). Further, the specificity of neurosteroids on differing GABA A receptor subtypes (see Belelli et al, 2002;Bianchi and Macdonald, 2003;Lambert et al, 2001Lambert et al, , 2003Spigelman et al, 2003;Stell et al, 2003;Wallner et al, 2003) could conceivably contribute to the regional specificity of ethanol on GABA function.…”