Potentiation of gamma-aminobutyric-acid-activated chloride conductance by a steroid anaesthetic in cultured rat spinal neurones.

1. Intracellular recordings from cultured rat spinal cord neurones demonstrated that Cl(-)-dependent responses to GABA (gamma-aminobutyric acid) (but not glycine) were increased in amplitude and duration by the steroid anaesthetic alphaxalone (3 alpha-hydroxy-5 alpha-pregnane-11,20-dione) at submicromolar concentrations that produced little or no effect on passive electrical properties. The non-anaesthetic 3 beta-hydroxy analogue was without effect on GABA-evoked responses. 2. Under voltage clamp, membrane currents evoked by GABA were potentiated by alphaxalone without change in the reversal potential for the GABA-evoked response. Fluctuation analysis of GABA-evoked currents suggested that the mean open-time of GABA-activated channels was prolonged from 30 to 74 ms in the presence of the anaesthetic. 3. Higher concentrations of alphaxalone, similar to those reported during surgical anaesthesia, increased membrane conductance in the absence of exogenously applied GABA. Under voltage clamp, current responses to alphaxalone reversed at the same potential as did responses to GABA, suggesting that they result from increased Cl- conductance. 4. Alphaxalone responses were reduced by the GABA antagonist bicuculline. Fluctuation analysis of current responses to the anaesthetic suggest that they result from the activation of ion channels of long (100 ms) open-time and elementary conductance indistinguishable from that of channels activated by GABA (20 pS). Taken together, these findings indicate that the steroid anaesthetic is able to directly activate Cl- conductance normally activated by GABA in spinal neurones. 5. The actions of the steroid at GABA-receptor-Cl(-)-channel complexes are similar to those produced by the anaesthetic barbiturates (e.g. pentobarbitone), although obtained at 50-100-fold lower concentrations. These effects on the inhibitory Cl(-)-conductance mechanism may be partly responsible for the depressant actions of alphaxalone on the mammalian central nervous system.