Attenuated sensitivity to neuroactive steroids in GABAA receptor δ-subunit knockout mice

University of Turku, Turku, Southwest Finland, Finland
Proceedings of the National Academy of Sciences (Impact Factor: 9.67). 11/1999; 96(22):12905-10. DOI: 10.1073/pnas.96.22.12905
Source: PubMed


gamma-Aminobutyric acid (GABA) type A receptors mediate fast inhibitory synaptic transmission and have been implicated in responses to sedative/hypnotic agents (including neuroactive steroids), anxiety, and learning and memory. Using gene targeting technology, we generated a strain of mice deficient in the delta subunit of the GABA type A receptors. In vivo testing of various behavioral responses revealed a strikingly selective attenuation of responses to neuroactive steroids, but not to other modulatory drugs. Electrophysiological recordings from hippocampal slices revealed a significantly faster miniature inhibitory postsynaptic current decay time in null mice, with no change in miniature inhibitory postsynaptic current amplitude or frequency. Learning and memory assessed with fear conditioning were normal. These results begin to illuminate the novel contributions of the delta subunit to GABA pharmacology and sedative/hypnotic responses and behavior and provide insights into the physiology of neurosteroids.

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    • "subunit knockout mice (Mihalek et al., 1999). These results thus suggest that ganaxolone may be useful in clinical practice for a subpopulation of patients in whom anxiety or PTSD symptoms are related to deficient ALLO biosynthesis. "
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    ABSTRACT: Allopregnanolone and its equipotent stereoisomer, pregnanolone (together termed ALLO), are neuroactive steroids that positively and allosterically modulate the action of gamma-amino-butyric acid (GABA) at GABAA receptors. Levels of ALLO are reduced in the cerebrospinal fluid of female premenopausal patients with posttraumatic stress disorder (PTSD), a severe, neuropsychiatric condition that affects millions, yet is without a consistently effective therapy. This suggests that restoring downregulated brain ALLO levels in PTSD may be beneficial. ALLO biosynthesis is also decreased in association with the emergence of PTSD-like behaviors in socially isolated (SI) mice. Similar to PTSD patients, SI mice also exhibit changes in the frontocortical and hippocampal expression of GABAA receptor subunits, resulting in resistance to benzodiazepine-mediated sedation and anxiolysis. ALLO acts at a larger spectrum of GABAA receptor subunits than benzodiazepines, and increasing corticolimbic ALLO levels in SI mice by injecting ALLO or stimulating ALLO biosynthesis with a selective brain steroidogenic stimulant, such as S-norfluoxetine, at doses far below those that block serotonin reuptake, reduces PTSD-like behavior in these mice. This suggests that synthetic analogs of ALLO, such as ganaxolone, may also improve anxiety, aggression, and other PTSD-like behaviors in the SI mouse model. Consistent with this hypothesis, ganaxolone (3.75-30 mg/kg, s.c.) injected 60 minutes before testing of SI mice, induced a dose-dependent reduction in aggression toward a same-sex intrude and anxiety-like behavior in an elevated plus maze. The EC50 dose of ganaxolone used in these tests also normalized exaggerated contextual fear conditioning and, remarkably, enhanced fear extinction retention in SI mice. At these doses, ganaxolone failed to change locomotion in an open field test. Therefore, unlike benzodiazepines, ganaxolone at non-sedating concentrations appears to improve dysfunctional emotional
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    • "Our in vitro data, together with a computational study (Talavera et al., 2009), suggest that the etomidate-induced inhibition of TC output is dependent on synaptic and extrasynaptic GABA A Rs. Thus, it is surprising that deletion of a1, a4 or d subunits exerts little or no influence on the sedative or hypnotic actions of etomidate (Mihalek et al., 1999; Kralic et al., 2003; Iyer et al., 2013). However , the interpretation of behavioural data obtained from mice harbouring global GABA A R subunit deletions is complicated by the non-uniform up-regulation of alternative GABA A R isoforms following subunit loss (Sur et al., 2001; Kralic et al., 2002a,b; Peng et al., 2002). "
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    ABSTRACT: Modulation of thalamocortical (TC) relay neuron function has been implicated in the sedative and hypnotic effects of general anaesthetics. Inhibition of TC neurons is mediated predominantly by a combination of phasic and tonic inhibition, together with a recently described ‘spillover’ mode of inhibition, generated by the dynamic recruitment of extrasynaptic γ-aminobutyric acid (GABA)A receptors (GABAARs). Previous studies demonstrated that the intravenous anaesthetic etomidate enhances tonic and phasic inhibition in TC relay neurons, but it is not known how etomidate may influence spillover inhibition. Moreover, it is unclear how etomidate influences the excitability of TC neurons. Thus, to investigate the relative contribution of synaptic (α1β2γ2) and extrasynaptic (α4β2δ) GABAARs to the thalamic effects of etomidate, we performed whole-cell recordings from mouse TC neurons lacking synaptic (α10/0) or extrasynaptic (δ0/0) GABAARs. Etomidate (3 μm) significantly inhibited action-potential discharge in a manner that was dependent on facilitation of both synaptic and extrasynaptic GABAARs, although enhanced tonic inhibition was dominant in this respect. Additionally, phasic inhibition evoked by stimulation of the nucleus reticularis exhibited a spillover component mediated by δ-GABAARs, which was significantly prolonged in the presence of etomidate. Thus, etomidate greatly enhanced the transient suppression of TC spike trains by evoked inhibitory postsynaptic potentials. Collectively, these results suggest that the deactivation of thalamus observed during etomidate-induced anaesthesia involves potentiation of tonic and phasic inhibition, and implicate amplification of spillover inhibition as a novel mechanism to regulate the gating of sensory information through the thalamus during anaesthetic states.
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    • "Unlike benzodiazepines, which have a selective pharmacological profile and fail to activate GABA A receptors containing α4 and α6 subunits (Brown et al., 2002), allopregnanolone modulation of GABA A receptors exhibits a broad pharmacological profile. Although allopregnanolone acts preferentially on δ subunit-containing GABA A receptors, which confers neurosteroid sensitivity, it also exerts effects on other GABA A receptor subtypes at higher concentrations (Mihalek et al., 1999; Stell et al., 2003). Thus, increasing corticolimbic allopregnanolone levels with allopregnanolone injections or stimulating allopregnanolone biosynthesis with S-norfluoxetine, or directly activation of GABA A receptors with ganaxolone likely improved anxiety because allopregnanolone/ganaxolone acts on a larger spectrum of GABA A receptor subunits. "

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