Neurosteroids Shift Partial Agonist Activation of GABAAReceptor Channels from Low- to High-Efficacy Gating Patterns

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Abstract
Although GABA activates synaptic (αβγ) GABAAreceptors with high efficacy, partial agonist activation of αβγ isoforms and GABA activation of the primary extrasynaptic (αβδ) GABAAreceptors are limited to low-efficacy activity, characterized by minimal desensitization and brief openings. The unusual sensitivity of αβδ receptor channels to neurosteroid modulation prompted investigation of whether this high sensitivity was dependent on the δ subunit or the low-efficacy channel function that it confers. We show that the isoform specificity (αβδ > αβγ) of neurosteroid modulation could be reversed by conditions that reversed isoform-specific activity modes, including the use of β-alanine to achieve increased efficacy with αβδ receptors and taurine to render αβγ receptors low efficacy. We suggest that neurosteroids preferentially enhance low-efficacy GABAAreceptor activity independent of subunit composition. Allosteric conversion of partial to full agonism may be a general mechanism for reversibly scaling the efficacy of GABAAreceptors to endogenous partial agonists.