The mechanism of steroid anaesthetic (alphaxalone) block of acetylcholine‐induced ionic currents

Gillo, Boaz; Lass, Yoram

doi:10.1111/j.1476-5381.1984.tb16474.x

Cited by 27 publications

(13 citation statements)

References 30 publications

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“…It has been observed that the acetylcholineinduced ionic channels of voltage-clamped myoballs in culture are blocked by high micromolar concentrations of alphaxalone in the open state (Gillo & Lass, 1984). This finding substains the hypothesis of direct interactions between steroid molecules and membrane proteins, i.e., acetylcholine receptors.…”

Section: Introductionsupporting

confidence: 48%

Untitled

1988

British J Pharmacology

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1 The effects of the anaesthetic alphaxalone (0.05 to 1 mM) on the node of Ranvier of isolated myelinated nerve fibres of the frog were studied under voltage-clamp conditions. 2 When added to the solution bathing voltage-clamped nodes, alphaxalone modified neither linear leakage nor capacitative currents but rapidly and reversibly blocked K and Na currents. The blocking effects of the anaesthetic on both types of current were not dependent on the frequency of stimulation of the nerve fibres between 0.7 and 10 Hz. 3 The kinetics of the Na current were not modified by alphaxalone but, in the presence of the drug, the K current showed an apparent fast inactivation. 4 Alphaxalone rapidly and reversibly shifted towards negative voltages both the steady-state K conductance-voltage and the peak Na steady-state inactivation-voltage relationships, without noticeable modification of their shape. In contrast, the anaesthetic reversibly decreased the slope of the peak Na conductance-voltage curve. 5 The reduction of the K current induced by alphaxalone was voltage-dependent with an apparent dissociation constant first decreasing from about 0.25 to 0.08 mm between -20mV and +20 mV and then remaining constant above + 20 mV. In contrast, the apparent dissociation constant for the Na current was almost constant with increasing voltages and equalled about 0.30 mM.Hill coefficient values for both K and Na currents were noticeably less than one. 6 It is concluded that, at higher concentrations than those attainable in the brain or in the plasma during surgical anaesthesia in man, alphaxalone has a 'local anaesthetic-like' action on the peripheral nervous system in that it specifically and differentially interacts with K and Na channel gating systems: it is suggested that the anaesthetic would preferentially modify open K and inactivated Na channels.

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Section: Introductionsupporting

confidence: 48%

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1988

British J Pharmacology

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“…A somewhat lower IC50 for alphaxalone of 6;tM has been reported for the antagonism of ACh-evoked currents in cultured rat myoballs (Gillo & Lass, 1984).…”

Section: Suppression Ofacetylcholine-evoked Currents By Alphaxalonementioning

confidence: 75%

Modulation of GABA_A receptor activity by alphaxalone

Cottrell

Lambert

Peters

1987

British J Pharmacology

199

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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.

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“…Another pharmacological action of alphaxalone which occurs at higher concentrations (3-100 ,tM) is the depression of excitatory responses to acetylcholine acting on nicotinic receptors (Gillo & Lass, 1984;Cottrell et al 1985).…”

Section: Discussionmentioning

confidence: 99%

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

Barker

Harrison

Lange

et al. 1987

The Journal of Physiology

165

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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.

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The mechanism of steroid anaesthetic (alphaxalone) block of acetylcholine‐induced ionic currents

Abstract: 1 The effects of the steroid anaesthetic alphaxalone on acetylcholine (ACh)-induced ionic channels were studied in voltage clamped 'myoballs' in culture. 2 Alphaxalone produced a reversible blockade of the ACh-evoked inward current, ED50 = 6.0 MM.

Cited by 27 publications

References 30 publications

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Modulation of GABA_A receptor activity by alphaxalone

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

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The mechanism of steroid anaesthetic (alphaxalone) block of acetylcholine‐induced ionic currents

Abstract: 1 The effects of the steroid anaesthetic alphaxalone on acetylcholine (ACh)-induced ionic channels were studied in voltage clamped 'myoballs' in culture. 2 Alphaxalone produced a reversible blockade of the ACh-evoked inward current, ED50 = 6.0 MM.

Cited by 27 publications

References 30 publications

Untitled

Untitled

Modulation of GABAA receptor activity by alphaxalone

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

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Modulation of GABA_A receptor activity by alphaxalone