Modulation of GABA_A receptor activity by alphaxalone

Abstract

1 The modulation of the y-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 ptM). 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, Rol 5-1788 (300 nM). 3 At relatively high concentrations (>1 JM), 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 AM), dose-dependently enhanced by phenobarbitone (100-500 !LM), 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 tiM) had no effect upon the amplitude of membrane currents elicited by locally applied acetylcholine (ACh) (100 pM). However, higher concentrations of alphaxalone (10-100 JM) reversibly suppressed ACh-evoked currents, the IC50 for blockade being 20 JAM.5 The P-hydroxy isomer of alphaxalone, betaxalone (100 nM-l pM), did not potentiate GABAinduced currents, nor did higher concentrations of the steroid (10-100 pM) 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.

A Conceptualization of Integrated Actions of Ethanol Contributing to its GABAmimetic Profile: A Commentary

“…While reports suggested a direct interaction between ethanol and neurosteroids on GABA A receptor function (Akk and Steinbach, 2003;Criswell et al, 1999), recent data from our laboratory questions this observation (Criswell et al, 2003). Further, it should be recognized that the concentration of neurosteroid following ethanol (o100 nM) is likely insufficient to have a direct effect on GABA A receptors to gate current in the absence of GABA (Callachan et al, 1987;Cottrell et al, 1987;Puia et al, 1990). This circumstance minimizes the possibility that a direct activation of GABA A receptor function by neurosteroids is responsible for the neurosteroid contribution to the GABAmimetic profile of ethanol.…”

The actions of propofol on inhibitory amino acid receptors of bovine adrenomedullary chromaffin cells and rodent central neurones

“…Visual inspection of the figure suggests that propofol (1.7,UM) has little effect on the GABA single channel amplitudes (see Table 1 The mean chord conductances (pS) of GABA channels calculated assuming a reversal potential of 0 mV (Bormann et al, 1987 Figure 4c). Electrophysiological (Callachan et al, 1987;Cottrell et al, 1987;Lambert et al, 1987) and radioligand binding experiments (Gee et al, 1988;Peters et al, 1988;Kirkness, 1989;Turner et al, 1989) suggest that anaesthetic steroids and barbiturates bind to different sites on the GABAA receptors to potentiate the action of GABA. In confirmation of previous results (Peters et al, 1988) the anaesthetic steroid 5fi-pregnan3a-ol-20-one (0.5 pM) produced a marked potentiation of both GABA currents (856 + 194% of control, n = 10) and pentobarbitone currents (906 + 121% of control, n = 16).…”

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

“…Similar steroid interactions have also been observed at the barbiturate site of GABAA-receptor complexes (Majewska et al, 1986). Whilst aspects of these interactions resemble those of barbiturates, as do the actions of alfaxalone on chloride conductances in isolated neurones (Harrison et al, 1987b), the direct actions of alfaxalone in activating chloride channel conductance can themselves be potentiated by benzodiazepines and barbiturates Cottrell et al, 1987), suggesting that alfaxalone modulates GABA-induced responses at some site other than that affected by barbiturates. The present results confirm that alfaxalone exerts two effects at GABAA-receptor-ionophore complexes, low concentrations potentiating GABA-induced responses, whilst high concentrations elicit direct GABAmimetic responses.…”

“…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. In other preparations, such potentiation of GABA-induced responses is in some way related to modulation by alfaxalone of GABAA-receptor-ionophore-complexes; alfaxalone increases the affinity of [3H]-muscimol at low affinity binding sites , and interacts with GABA in displacing [35S]-t-butylcyclophosphorothionate from the convulsant site at the ionophore-complexes in a bicuculline-sensitive manner (Gee et al, 1987).…”

“…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).…”