Modulation of GABAA receptor activity by alphaxalone

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Abstract
1 The modulation of the γ-aminobutyric acidA (GABAA) receptor by alphaxalone has been investigated by use of voltage-clamp recordings from enzymatically isolated bovine chromaffin cells maintained in cell culture. 2 Alphaxalone (> 30 nm) reversibly and dose-dependently potentiated the amplitude of membrane currents elicited by locally applied GABA (100 μm). The potentiation was not associated with a change in the reversal potential of GABA-evoked currents and was not influenced by the benzodiazepine receptor antagonist, Ro15-1788 (300 nm). 3 At relatively high concentrations (> 1 μm), alphaxalone directly elicited a membrane current. It is concluded that such currents result from GABAA receptor activation since they were reversibly suppressed by bicuculline (3 μm), dose-dependently enhanced by phenobarbitone (100–500 μm), and had a similar reversal potential (∼ 0 mV) to currents elicited by GABA. Additionally, on outside-out membrane patches, alphaxalone activated single channel currents with amplitudes and a reversal potential similar to those evoked by GABA. 4 Alphaxalone (30 nm-1 μm) had no effect upon the amplitude of membrane currents elicited by locally applied acetylcholine (ACh) (100 μm). However, higher concentrations of alphaxalone (10–100 μm) reversibly suppressed ACh-evoked currents, the IC50 for blockade being 20 μm. 5 The β-hydroxy isomer of alphaxalone, betaxalone (100 nm-1 μm), did not potentiate GABA-induced currents, nor did higher concentrations of the steroid (10–100 μm) directly evoke a membrane current. However, over the latter concentration range, betaxalone suppressed the amplitude of currents elicited either by GABA or ACh. 6 The relevance of the present results to the anaesthetic action of alphaxalone is discussed together with the broader implications of steroidal modulation of the GABAA receptor.