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

Abstract

SUMMARY1. Intracellular recordings from cultured rat spinal cord neurones demonstrated that Cl--dependent responses to GABA (y-aminobutyric acid) (but not glycine) were increased in amplitude and duration by the steroid anaesthetic alphaxalone (3a-hydroxy-5a-pregnane-I 1,20-dione) at submicromolar concentrations that produced little or no effect on passive electrical properties. The non-anaesthetic 3,1-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 Clconductance.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.

Potentiation by sevoflurane of the γ‐aminobutyric acid‐induced chloride current in acutely dissociated CA1 pyramidal neurones from rat hippocampus

“…These results strongly indicate that Sev acts on the GABAA-receptor-Clchannel complex but not on the glycine receptor. Also these data suggest that the action of Sev in the CNS is rather specific, which supports the hypothesis of a more specific action on receptor proteins or ion channels of the volatile anaesthetics (Richards 1976;Halsey, 1984;Franks & Lieb, 1994;Maclver et al, 1989 (Barker & Ransome 1978;Hattori et al, 1986;Barker et al, 1987;Yakushiji et al, 1989a,b,c;Akaike et al, 1990 (Mathers, 1985;1987). This idea confirms the biochemical findings that barbiturates enhance the binding of GABA to its receptor and decrease the dissociation rate (Willow & Johnston, 1980;Olsen, 1982).…”

“…More and more evidence supports the hypothesis that anaesthetics might act by potentiating inhibitory synaptic transmission, and the GABAA receptor channel complex has been established as a prime anaesthetic target by recent electrophysiological studies (Franks & Leib, 1994 barbiturates, benzodiazepines, steroids and propofol bind to the GABAA receptor-channel complex and consequently augment the inhibitory GABAA receptor-mediated responses via allosteric modulation of receptor function (Nicoll & Wojtowicz, 1980;Study & Barker, 1981;Barker et al, 1987;Itabashi et al, 1992;Hara et al, 1994). A similar picture is also emerging for volatile anaesthetics.…”

“…(Richards & Smaje, 1976;Halsey, 1984;Franks & Lieb, 1994;Maclver et al, 1989). For example, several classes of general anaesthetics, including the barbiturates, steroids, and etomidate, share the property of potentiating GABAA receptor operated Cl-currents (ICI) at low concentrations; at higher concentrations, these compounds show GABAAmimetic properties (Barker & Ransome, 1978;Evans & Hill, 1978;Nicoll & Wojtowicz, 1980;Barker et al, 1987). The enhancement of GABA-induced IcI was produced by the volatile anaesthetics as well (Gage & Robertson, 1985a;Wakamori et al, 1991;Jones et al, 1992;Yang et al, 1992).…”

Alfaxalone potentiates and mimics GABA‐induced contractile responses in the guinea‐pig isolated ileum

“…Although the anaesthetic actions of alfaxalone may be substantially dependent on the GABAmimetic properties of alfaxalone which appear at higher concentrations (> 1 uM), close to those found in the brain during anaesthesia (Sear & PrysRoberts, 1979;Barker et al, 1987), this steroid also modulates GABA-induced actions when used at lower concentrations . The threshold for this potentiating effect in chromaffin cells was close to 30nm (Cottrell et al, 1987), whereas substantial potentiation of GABA-induced ileal contractions was already found here at 1 nm, the threshold lying between 0.1 and 1 nM.…”

“…These actions evidently involve altered chloride channel conductance at GABAA-receptor-ionophore complexes (Cottrell et al, 1987), with prolongation of the mean open-time of GABA-activated channels Harrison et al, 1987a,b) in a manner resembling the effect of anaesthetic barbiturates (Schultz & Macdonald, 1981;Study & Barker, 1981). This suggests that alfaxalone modulates the GABA-induced acti-vation of chloride channels by acting at an allosteric site associated with GABAA-receptors; in addition, higher concentrations of alfaxalone directly activate chloride channel opening in central neurones without the involvement of GABA Cottrell et al, 1987;Barker et al, 1987).…”