Delta Subunit-containing GABA Research Articles

DS1, a delta subunit-containing GABA(A) receptor agonist, increases gonadotropin subunit gene expression in mouse pituitary gonadotrophs.

4-Chloro-N-[6,8-dibromo-2-(2-thienyl)imidazo[1,2-alpyridine-3-yl] (DS1) is a GABA(A) receptor agonist that selectively binds to delta subunit-containing GABA(A) alpha4beta3delta receptors. In the present study, we examined the effect of DS1 on pituitary gonadotropin subunit gene expression using the mouse pituitary gonadotroph cell line LbetaT2. DS1 increased the promoter activity of the gonadotropin subunits luteinizing hormone beta (LHbeta), follicle-stimulating hormone beta (FSHbeta), and alpha. Gonadotropin-releasing hormone (GnRH) receptor promoters were also activated by DS1. The effects of DS1 on gonadotropin subunit promoters were obvious, but they were less than those induced by stimulation with GnRH. GnRH-stimulated gonadotropin subunit promoters were enhanced in the presence of DS1. A prototypic specific agonist for GABAA receptors, muscimol, failed to increase LHbeta and FSHbeta subunit promoter activity and had no effect on GnRH-increased LHbeta and FSHbeta promoter activity. In addition, SKF97541, a specific agonist for GABAB receptors, did not modulate basal or GnRH-induced LHbeta and FSHbeta promoter activity. A natural GABA compound failed to increase gonadotropin promoter activity and potentiated the effect of GnRH on the FSHbeta promoter. DS1 increased the activity of serum response element (SRE) and cAMP response element (CRE) promoters, which reflect the activity of the extracellular signal-regulated kinase and cAMP/protein kinase A (PKA) pathways, and GnRH-increased SRE and CRE promoter activity was enhanced in the presence of DS1. A specific inhibitor of the ERK signaling pathway, U0126, prevented DS1-induced LHbeta and FSHbeta promoter activity almost completely; however, H89, a PKA inhibitor, did not modulate the effect of DS1. Our current observations demonstrate that the GABAA alpha4beta3delta receptor agonist DS1 can stimulate gonadotropin subunit gene expression in association with the ERK signaling pathway.

Control of hippocampal gamma oscillation frequency by tonic inhibition and excitation of interneurons

Gamma-frequency oscillations depend on phasic synaptic GABAA receptor-mediated inhibition to synchronize spike timing. The spillover of synaptically-released GABA can also activate extrasynaptic GABAA receptors, and such tonic inhibition may also contribute to modulating network dynamics. In many neuronal cell types, tonic inhibition is mediated by δ subunit-containing GABAA receptors. We show that the frequency of in vitro cholinergically-induced gamma oscillations in the mouse hippocampal CA3 region is increased by the activation of NMDA receptors (NMDAR) on interneurons. The NMDAR-dependent increase of gamma oscillation frequency is counteracted by the tonic inhibition of the interneurons mediated by δ subunit-containing GABAA receptors. Recordings of synaptic currents during gamma activity show that NMDAR-mediated increases in oscillation frequency correlate with a progressive synchronization of phasic excitation and inhibition in the network. Thus, the balance between tonic excitation and tonic inhibition of interneurons may modulate gamma frequency by shaping interneuronal synchronization.

Ovarian cycle–linked changes in GABAA receptors mediating tonic inhibition alter seizure susceptibility and anxiety

Disturbances of neuronal excitability changes during the ovarian cycle may elevate seizure frequency in women with catamenial epilepsy and enhance anxiety in premenstrual dysphoric disorder (PMDD). The mechanisms underlying these changes are unknown, but they could result from the effects of fluctuations in progesterone-derived neurosteroids on the brain. Neurosteroids and some anxiolytics share an important site of action: tonic inhibition mediated by delta subunit-containing GABA(A) receptors (deltaGABA(A)Rs). Here we demonstrate periodic alterations in specific GABA(A)R subunits during the estrous cycle in mice, causing cyclic changes of tonic inhibition in hippocampal neurons. In late diestrus (high-progesterone phase), enhanced expression of deltaGABA(A)Rs increases tonic inhibition, and a reduced neuronal excitability is reflected by diminished seizure susceptibility and anxiety. Eliminating cycling of deltaGABA(A)Rs by antisense RNA treatment or gene knockout prevents the lowering of excitability during diestrus. Our findings are consistent with possible deficiencies in regulatory mechanisms controlling normal cycling of deltaGABA(A)Rs in individuals with catamenial epilepsy or PMDD.

Altered expression of the delta subunit of the GABAA receptor in a mouse model of temporal lobe epilepsy.

delta Subunit-containing GABA(A) receptors are located predominantly at nonsynaptic sites in the dentate gyrus where they may play important roles in controlling neuronal excitability through tonic inhibition and responses to GABA spillover. Immunohistochemical methods were used to determine whether delta subunit expression was altered after pilocarpine-induced status epilepticus in C57BL/6 mice in ways that could increase excitability of the dentate gyrus. In pilocarpine-treated animals, the normal diffuse labeling of the delta subunit in the dentate molecular layer was decreased by 4 d after status epilepticus (latent period) and remained low throughout the period of chronic seizures. In contrast, diffuse labeling of alpha4 and gamma2 subunits, potentially interrelated GABA(A) receptor subunits, was increased during the chronic period. Interestingly, delta subunit labeling of many interneurons progressively increased after pilocarpine treatment. Consistent with the observed changes in delta subunit labeling, physiological studies revealed increased excitability in the dentate gyrus of slices obtained from the pilocarpine-treated mice and demonstrated that physiological concentrations of the neurosteroid tetrahydrodeoxycorticosterone were less effective in reducing excitability in the pilocarpine-treated animals than in controls. The findings support the idea that alterations in nonsynaptic delta subunit-containing GABA(A) receptors in both principal cells and interneurons could contribute to increased seizure susceptibility in the hippocampal formation in a temporal lobe epilepsy model.

Reduced inhibition and sensitivity to neurosteroids in hippocampus of mice lacking the GABA(A) receptor delta subunit.

The delta subunit of the gamma-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 delta subunit revealed a considerable attenuation of behavioral responses to neuroactive steroids but not to other neuromodulatory drugs. Here we show that delta subunit loss leads to a concomitant reduction in hippocampal alpha4 subunit levels. These changes were accompanied by faster decay of evoked inhibitory postsynaptic potentials (IPSPs) in dentate granule neurons of -/- mutants (decay tau = 25 ms) compared with +/+ controls (tau = 50 ms). Furthermore, the GABA(A)R-mediated miniature inhibitory postsynaptic currents (mIPSCs) also decayed faster in delta-mutants (tau = 6.3 ms) than controls (tau = 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 microM), was smaller in delta-mutants (at 10 microM, 65% increase) compared with +/+ littermates (308% increase). In competition binding experiments, alphaxalone (0.03-1 microM) modulation of [35S]t-butylbicyclophosphorothionate binding was reduced in delta-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 delta-mutants suggests presence of the delta subunit at both synaptic and extrasynaptic GABA(A)Rs. Decreased synaptic and extrasynaptic inhibition likely contributes to the pro-epileptic phenotype of delta-mutants. Reduced neurosteroid sensitivity might also contribute to seizure susceptibility. While the simplest explanation is that delta 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 delta -/- mice.

Behavior and physiology of mice lacking the GABAA-receptor delta subunit.

Mice with a targeted disruption of the gamma-aminobutyric acid (A) receptor (GABA(A)R) delta 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 delta mutants. Seizure susceptibility to pentylenetetrazol (PTZ) injection, intracellular recordings, and whole-cell patch recordings in dentate granule neurons was determined. 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 tau in -/- mutants (25 +/- 3.5 ms) compared with +/+ controls (51 +/- 5.2 ms). In whole-cell patch recordings, the decay tau 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 = 18) than that of +/+ controls (6.8 +/- 0.32 ms; n = 20). Furthermore, mIPSC prolongation by bath application of alphaxalone (10 microM), was much smaller in -/- mice (52 +/- 12%; n = 3) compared with +/+ littermates (192 +/- 14%; n = 3). Faster decay of evoked IPSPs and mIPSCs is consistent with altered GABA(A)R subunit composition in the delta mutants, but suggests predominant extrasynaptic delta subunit location in the dentate gyrus of normal mice. Faster-decaying IPSPs likely contribute to the proepileptic phenotype of the delta mutants. Reduced neurosteroid sensitivity might also contribute to seizure susceptibility. It remains to be determined whether delta 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 delta-/- mice.