Modulation of GABA_A receptor activity by alphaxalone

The modulation of the γ‐aminobutyric acid_A (GABA_A) receptor by alphaxalone has been investigated by use of voltage‐clamp recordings from enzymatically isolated bovine chromaffin cells maintained in cell culture. 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). At relatively high concentrations (> 1 μm), alphaxalone directly elicited a membrane current. It is concluded that such currents result from GABA_A 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. 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. 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. The relevance of the present results to the anaesthetic action of alphaxalone is discussed together with the broader implications of steroidal modulation of the GABA_A receptor. 1987 British Pharmacological Society