Durkin, EJ; (2012) Physiological roles of endogenous neurosteroids at α2 subunit-containing GABAA receptors. Doctoral thesis , UCL (University College London).

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Abstract

Neurosteroids are important endogenous modulators of the major inhibitory neurotransmitter receptor in the brain, the γ-amino-butyric acid type A (GABAA) receptor. They are involved in numerous physiological processes, and are linked to several central nervous system disorders, including depression and anxiety. The neurosteroids allopregnanolone and allo-tetrahydro-deoxy-corticosterone (THDOC) have many effects in animal models (anxiolysis, analgesia, sedation, anticonvulsion, antidepressive), suggesting they could be useful therapeutic agents, for example in anxiety, stress and mood disorders. Neurosteroids potentiate GABA-activated currents by binding to a conserved site within α subunits. Potentiation can be eliminated by hydrophobic substitution of the α1Q241 residue (or equivalent in other α isoforms). Previous studies suggest that α2 subunits are key components in neural circuits affecting anxiety and depression, and that neurosteroids are endogenous anxiolytics. It is therefore possible that this anxiolysis occurs via potentiation at α2 subunit-containing receptors. To examine this hypothesis, α2Q241M knock-in mice were generated, and used to define the roles of α2 subunits in mediating effects of endogenous and injected neurosteroids. Biochemical and imaging analyses indicated that relative expression levels and localization of GABAA receptor α1-α5 subunits were unaffected, suggesting the knock-in had not caused any compensatory effects. Electrophysiological characterization of cells in hippocampal and nucleus accumbens brain slices revealed faster-decaying inhibitory synaptic transmission in α2Q241M mice. Furthermore, the response to applied THDOC was markedly reduced compared to wild-type cells. α2 subunits therefore formed a major component of synaptic GABAA receptors in these areas. The α2Q241M knock-ins showed greater anxiety levels in two classical rodent anxiety paradigms (light-dark box and elevated plus maze), consistent with endogenous neurosteroids mediating anxiolysis via α2-type GABAA receptors. In addition, the anxiolytic response to injected THDOC is impaired by the α2Q241M mutation, which would identify the α2 isoform as an appropriate target for generating receptor subtype-selective neurosteroid therapeutics for anxiety disorders.

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