[show abstract][hide abstract] ABSTRACT: 1. The aim of this study was to define the biophysical properties contributed by the gamma2 subunit to native single GABAA receptors. 2. Single-channel activity was recorded from neurones of wild-type (gamma2+/+) mice and compared with that from mice which were heterozygous (gamma2+/-) or homozygous (gamma2-/-) for a targeted disruption in the gamma2 subunit gene of the GABAA receptor. Unitary currents were evoked by low concentrations of GABA (0.5-5 microM) in membrane patches from acutely isolated dorsal root ganglion (DRG) neurones (postnatal day 0) and by 1 microM GABA in patches from embryonic hippocampal neurones which were cultured for up to 3 weeks. 3. GABAA receptors from DRG and hippocampal neurones of gamma2+/+ and gamma2+/- mice displayed predominantly a conductance state of 28 pS and less frequently 18 and 12 pS states. In gamma2-/- mice, conductance states mainly of 12 pS and less frequently of 24 pS were found. 4. The mean open duration of the 28 pS state in gamma2+/+ GABAA receptors (1.5-2.6 ms) was substantially longer than for the 12 pS state of gamma2-/- GABAA receptors (0.9-1.2 ms) at all GABA concentrations. For gamma2+/+ and gamma2-/- channels, the mean open duration was increased at higher GABA concentrations. 5. Open duration frequency distributions of 28 and 12 pS receptors revealed the existence of at least three exponential components. Components with short mean durations declined and components with long mean durations increased in relative frequency at higher GABA concentration indicating at least two binding sites of GABA per 28 and 12 pS receptor. 6. Shut time frequency distributions revealed at least four exponential components of which two were identified as intraburst components in 28 pS and one in 12 pS GABAA receptors. 7. The mean burst duration and the mean number of openings per burst increased in 28 and 12 pS GABAA receptors with increasing GABA concentration. At least two burst types were identified: simple bursts consisting of single openings and complex bursts of five to six openings in 28 pS but only two to three openings in 12 pS GABAA receptors. 8. We conclude that the gamma2 subunit enhances the efficacy of GABA by determining open conformations of high conductance and long lifetime, and by prolonging the time receptors remain in the activated bursting state.
The Journal of Physiology 09/2000; 527 Pt 1:11-31. · 4.38 Impact Factor
[show abstract][hide abstract] ABSTRACT: Patients with panic disorders show a deficit of GABAA receptors in the hippocampus, parahippocampus and orbitofrontal cortex. Synaptic clustering of GABAA receptors in mice heterozygous for the gamma2 subunit was reduced, mainly in hippocampus and cerebral cortex. The gamma2 +/- mice showed enhanced behavioral inhibition toward natural aversive stimuli and heightened responsiveness in trace fear conditioning and ambiguous cue discrimination learning. Implicit and spatial memory as well as long-term potentiation in hippocampus were unchanged. Thus gamma2 +/- mice represent a model of anxiety characterized by harm avoidance behavior and an explicit memory bias for threat cues, resulting in heightened sensitivity to negative associations. This model implicates GABAA-receptor dysfunction in patients as a causal predisposition to anxiety disorders.
[show abstract][hide abstract] ABSTRACT: Most fast inhibitory neurotransmission in the brain is mediated by GABAA receptors, which are mainly postsynaptic and consist of diverse alpha and beta subunits together with the gamma 2 subunit. Although the gamma 2 subunit is not necessary for receptor assembly and translocation to the cell surface, we show here that it is required for clustering of major postsynaptic GABAA receptor subtypes. Loss of GABAA receptor clusters in mice deficient in the gamma 2 subunit, and in cultured cortical neurons from these mice, is paralleled by loss of the synaptic clustering molecule gephyrin and synaptic GABAergic function. Conversely, inhibiting gephyrin expression causes loss of GABAA receptor clusters. The gamma 2 subunit and gephyrin are thus interdependent components of the same synaptic complex that is critical for postsynaptic clustering of abundant subtypes of GABAA receptors in vivo.
[show abstract][hide abstract] ABSTRACT: Fast inhibitory neurotransmission in the brain is mediated mainly by g-aminobutyric acid (GABA) gating of heteromeric GABAA receptor chloride channels. Most types of GABAA receptor are preferentially localized in the postsynaptic membrane of GABAergic synapses and consist of diverse a and b subunits together with the g2 subunit. Although the g2 subunit is dispensable for assembly and translocation of receptors to the cell surface, we show here that it is required for clustering of major postsynaptic GABAA receptor subtypes containing either the a1 or a2 subunit. Loss of GABAA receptor clusters in g2 subunit-deficient cultured cortical neurons and brain is paralleled by loss of the synaptic clustering molecule gephyrin and loss of synaptic GABAergic function. Conversely, inhibition of gephyrin expression results in loss of GABAA receptor clusters. The g2 subunit and gephyrin are thus interdependent components of the same synaptic complex and of major importance for postsynaptic clustering of abundant subtypes of GABAA receptors in vivo .