Pressure reversal of alphaxalone/alphadolone and methohexitone in tadpoles: evidence for different molecular sites for general anaesthesia

Halsey, Michael J.; Wardley‐Smith, Bridget; Wood, Steven

doi:10.1111/j.1476-5381.1986.tb10260.x

Cited by 14 publications

(6 citation statements)

References 19 publications

Supporting

Mentioning

Contrasting

Order By: Relevance

“…First, we examined the tadpole LRR in the presence of etomidate and various concentrations of 3α5βP. Loss of righting can be considered a surrogate for sedation in animal models (Halsey et al ., ; Franks, ). Our data show that the etomidate EC 50 for LRR was reduced from 1.6 to 0.5 or 0.1 μM in the presence of 50 or 100 nM 3α5βP respectively (Figure ).…”

Section: Resultsmentioning

confidence: 99%

The neurosteroid 5β‐pregnan‐3α‐ol‐20‐one enhances actions of etomidate as a positive allosteric modulator of α1β2γ2L GABA_A receptors

Bracamontes

Manion

et al. 2014

British J Pharmacology

View full text Add to dashboard Cite

BACKGROUND AND PURPOSENeurosteroids potentiate responses of the GABAA receptor to the endogenous agonist GABA. Here, we examined the ability of neurosteroids to potentiate responses to the allosteric activators etomidate, pentobarbital and propofol. EXPERIMENTAL APPROACHElectrophysiological assays were conducted on rat α1β2γ2L GABAA receptors expressed in HEK 293 cells. The sedative activity of etomidate was studied in Xenopus tadpoles and mice. Effects of neurosteroids on etomidate-elicited inhibition of cortisol synthesis were determined in human adrenocortical cells. KEY RESULTSThe neurosteroid 5β-pregnan-3α-ol-20-one (3α5βP) potentiated activation of GABAA receptors by GABA and allosteric activators. Co-application of 1 μM 3α5βP induced a leftward shift (almost 100-fold) of the whole-cell macroscopic concentration-response relationship for gating by etomidate. Co-application of 100 nM 3α5βP reduced the EC50 for potentiation by etomidate of currents elicited by 0.5 μM GABA by about three-fold. In vivo, 3α5βP (1mg kg -1 ) reduced the dose of etomidate required to produce loss of righting in mice (ED50) by almost 10-fold. In tadpoles, the presence of 50 or 100 nM 3α5βP shifted the EC50 for loss of righting about three-or ten-fold respectively. Exposure to 3α5βP did not influence inhibition of cortisol synthesis by etomidate. CONCLUSIONS AND IMPLICATIONSPotentiating neurosteroids act similarly on orthosterically and allosterically activated GABAA receptors. Co-application of neurosteroids with etomidate can significantly reduce dosage requirements for the anaesthetic, and is a potentially beneficial combination to reduce undesired side effects. Abbreviations3α5αP, 5α-pregnan-3α-ol-20-one; 3α5βP, 5β-pregnan-3α-ol-20-one (pregnanolone); LRR, loss of righting reflex BJP British Journal of Pharmacology

show abstract

Section: Resultsmentioning

confidence: 99%

The neurosteroid 5β‐pregnan‐3α‐ol‐20‐one enhances actions of etomidate as a positive allosteric modulator of α1β2γ2L GABA_A receptors

Bracamontes

Manion

et al. 2014

British J Pharmacology

View full text Add to dashboard Cite

show abstract

“…Furthermore, in electrophysiological studies the potentiation of GABA-evoked responses produced by either barbiturates or steroids is insensitive to the benzodiazepine receptor antagonist Rol5-1788 (Simmonds, 1985;Callachan et al, 1987a; and appears to be due to a prolongation of GABA channel burst duration (Barker et al, 1987;Callachan et al, 1987a;Lambert et al, 1987). However, despite these similarities, studies evaluating the anaesthetic effect of binary mixtures of the synthetic steroid alphaxalone and depressant barbi-turates suggest that they do not act through a common site or mechanism (Richards & White, 1981;Halsey et al, 1986).…”

Section: Introductionmentioning

confidence: 99%

Modulation of the GABA_A receptor by depressant barbiturates and pregnane steroids

Peters

Kirkness

Callachan

et al. 1988

British J Pharmacology

280

136

View full text Add to dashboard Cite

University of Leeds, Leeds LS2 9JT1 The modulation of the y-aminobutyric acidA (GABAAJ receptor by reduced metabolites of progesterone and deoxycorticosterone has been compared with that produced by depressant barbiturates in: (a) voltage-clamp recordings from bovine enzymatically isolated chromaffin cells in cell culture, and (b) an assay of the specific binding of [3H]-muscimol to a preparation of porcine brain membranes. 2 The progesterone metabolites 5a-and 5f-pregnan-3a-ol-20-one (> 30 nM) reversibly and dosedependently enhanced the amplitude of membrane currents elicited by locally applied GABA (100 gM), and over the concentration range 30nM-tOO pM stimulated the binding of [3H]-muscimol. In contrast, 5a-and 5#-pregnan-3#-ol-20-one (30 nM-IOO OM) had little effect in either assay, indicating a marked stereoselectivity of steroid action. 3 Scatchard analysis of the ligand binding data suggested an apparent increase in the number, rather than the affinity, of detectable [3H]-muscimol binding sites as the principle action of the active steroid isomers.4 GABA-evoked currents were also potentiated by androsterone (1 yM) and the deoxycorticosterone metabolite 5a-pregnane-3a,21-diol-20-one (100nM).5 Secobarbitone (1O-1OOjiM), pentobarbitone (10-3001pM) and phenobarbitone (100-500 pM) reversibly and dose-dependently potentiated the amplitude of GABA-evoked currents in the absence of any change in their reversal potential. 6 At relatively high concentrations (> 30pM) secobarbitone and pentobarbitone directly elicited a membrane current. It is concluded that such currents result from GABAA receptor-channel activation since they share a common reversal potential with GABA-evoked responses (-OmV), are reversibly antagonized by bicuculline (3pM), and potentiated by either diazepam (1 pM) or 5#-pregnan-3a-ol-20-one (500 nM). 7 Secobarbitone (1 pM-1 mM) dose-dependently enhanced the binding of [3H]-muscimol. In common with the active steroids, an increase in the apparent number of binding sites was responsible for this effect. 8 A saturating concentration (1 mM) of secobarbitone in the ligand binding assay did not suppress the degree of enhancement of control binding produced by 5#-pregnan-3a-ol-20-one (30nM-1001pM). Similarly the steroid, at a concentration of 100pM, did not influence the enhancement of [3H]-muscimol binding by secobarbitone (1 yM-1 mM). In all combinations of concentrations tested, the effects of secobarbitone and 5#-pregnan-3a-ol-20-one on [3H]-muscimol binding were additive.9 In conjunction with previously published observations, the present data indicate close similarities in the GABA-mimetic and potentiating actions of barbiturates and steroids. However, the results obtained with combinations of steroids and barbiturates in the ligand binding assay appear inconsistent with the two classes of compound interacting with a common site to modulate the GABAA receptor activity.

show abstract

“…Where biological end‐points were used, they varied greatly, from single cell functions (e.g., axonal height or conductance, or synaptic function 36 ) to various end‐points in terrestrial or aquatic animals (e.g., movement or loss of righting reflex 37‐40 ). Clearly, these are all at several removes from ‘general anaesthesia’, a constraint also being to administer suitable painful stimuli whilst also maintaining the closed high‐pressure environment for experiment.…”

Section: Resultsmentioning

confidence: 99%

General anaesthetics as ‘awakening agents’? Re‐appraising the evidence for suggested ‘pressure reversal’ of anaesthesia

George

Pandit

2021

Clin Exp Pharma Physio

View full text Add to dashboard Cite

Increasing ambient pressure has been suggested to reverse general anaesthesia and provides support for the ‘lipid theory’. Anaesthetic dissolution into cell membranes is said to cause their expansion to a critical volume. This triggers a sequence of events as basis of a unitary theory of anaesthestic mechanism. Pressure is argued to restore membrane volume to below critical level, reversing this process. We wished to review the original literature to assess internal consistency within and across papers, and to consider if alternative interpretations were possible. A literature search yielded 31 relevant ‘pressure reversal’ papers for narrative review, and 8 papers that allowed us to re‐plot original data more consistently as ‘dose‐response’ curves for the anaesthetics examined. Original studies were heterogenous for end‐points, pressure ranges, species, and agents. Pressure effects were inconsistent, with narcosis at certain pressures and excitation at others, influenced by carrier gas (e.g., nitrogen vs helium). Pressure reversal (a right‐ or downward‐shift on the re‐plotted dose‐response curves) was evident, but only in some species and at certain pressures and anaesthetic concentrations. However, even more striking was a novel ‘awakening’ effect of anaesthetics: i.e., anaesthetics reversed the narcotic effect of pressure, but this was limited to certain pressures at generally low anaesthetic concentrations. Contrary to the established view, ‘pressure reversal’ is not a universal phenomenon. The awakening effect of anaesthetics – described here for the first time ‐ has equal evidence to support it, within the same literature, and is something that cannot be fully explained. Pressure cannot meaningfully be used to gain insight into anaesthetic mechanisms because of its heterogenous, non‐specific and unpredictable effects on biological systems.

show abstract

Pressure reversal of alphaxalone/alphadolone and methohexitone in tadpoles: evidence for different molecular sites for general anaesthesia

Cited by 14 publications

References 19 publications

The neurosteroid 5β‐pregnan‐3α‐ol‐20‐one enhances actions of etomidate as a positive allosteric modulator of α1β2γ2L GABA_A receptors

The neurosteroid 5β‐pregnan‐3α‐ol‐20‐one enhances actions of etomidate as a positive allosteric modulator of α1β2γ2L GABA_A receptors

Modulation of the GABA_A receptor by depressant barbiturates and pregnane steroids

General anaesthetics as ‘awakening agents’? Re‐appraising the evidence for suggested ‘pressure reversal’ of anaesthesia

Contact Info

Product

Resources

About

Pressure reversal of alphaxalone/alphadolone and methohexitone in tadpoles: evidence for different molecular sites for general anaesthesia

Cited by 14 publications

References 19 publications

The neurosteroid 5β‐pregnan‐3α‐ol‐20‐one enhances actions of etomidate as a positive allosteric modulator of α1β2γ2L GABAA receptors

The neurosteroid 5β‐pregnan‐3α‐ol‐20‐one enhances actions of etomidate as a positive allosteric modulator of α1β2γ2L GABAA receptors

Modulation of the GABAA receptor by depressant barbiturates and pregnane steroids

General anaesthetics as ‘awakening agents’? Re‐appraising the evidence for suggested ‘pressure reversal’ of anaesthesia

Contact Info

Product

Resources

About

The neurosteroid 5β‐pregnan‐3α‐ol‐20‐one enhances actions of etomidate as a positive allosteric modulator of α1β2γ2L GABA_A receptors

The neurosteroid 5β‐pregnan‐3α‐ol‐20‐one enhances actions of etomidate as a positive allosteric modulator of α1β2γ2L GABA_A receptors

Modulation of the GABA_A receptor by depressant barbiturates and pregnane steroids