Enhanced GABAergic actions resulting from the coapplication of the steroid 3α-hydroxy-5α-pregnane-11,20-dione (alfaxalone) with propofol or diazepam

Abstract

Many GABAergic drugs are in clinical use as anesthetics, sedatives, or anxiolytics. We have investigated the actions of the combinations of the neuroactive steroid 3α-hydroxy-5α-pregnane-11,20-dione (alfaxalone) with the intravenous anesthetic propofol or the benzodiazepine diazepam. The goal of the study was to determine whether coapplication of alfaxalone reduces the effective doses and concentrations of propofol and diazepam. Behavioral effects of alfaxalone, propofol, diazepam, and the combinations of the drugs were evaluated during a 30-min activity test in mice. Functional effects of the individual drugs and drug combinations were tested by measuring the decay times of spontaneous inhibitory postsynaptic currents in rat hippocampal neurons, and peak current responses from heterologously expressed concatemeric α1β2γ2L GABAA receptors. Co-administration of alfaxalone increased the sedative actions of propofol and diazepam in mice. The combination of alfaxalone with propofol or diazepam increased the decay times of sIPSCs and shifted the concentration-response relationships for GABA-activated receptors to lower transmitter concentrations. We infer that alfaxalone acts as a co-agonist to enhance the GABAergic effects of propofol and diazepam. We propose that co-administration of alfaxalone, and possibly other neuroactive steroids, can be employed to reduce dosage requirements for propofol and diazepam.

Steady‐state activation and modulation of the synaptic‐type α 1 β 2 γ 2L GABA A receptor by combinations of physiological and clinical ligands

“…(Fig. ) and increases the decay time constants of sIPSCs (Cao et al ), its relative effects are similar on tonic and phasic activation of the synaptic‐type receptor and, therefore, on the ratios of relative integrated charge transfer (0.997:0.003 for tonic vs. phasic in the presence of 1 μ mol/L propofol at 0.001 Hz frequency per synapse and 0.25:0.75 at 1 Hz).…”

“…For phasic activity, it is considered that during the peak synaptic response 100% of receptors are active, that is, P Open,Peak is indistinguishable from 1 (Kitamura et al ; Takahashi et al ; McDougall et al ). The decay time constants of sIPSCs are 30 msec under control conditions, and 65 or 90 msec in the presence of 0.3 or 1 μ mol/L propofol (Cao et al ). Then, the total charge transfer for a single sIPSC is 0.03, 0.065, or 0.09 P Open × s in the presence of 0, 0.3, or 1 μ mol/L propofol, respectively.…”

Binding site location on GABAA receptors determines whether mixtures of intravenous general anaesthetics interact synergistically or additively in vivo

“…Although propofol and alphaxalone interact synergistically (Cao et al, ), propofol has only weak or no selectively between Sites 1 and 2. (Chiara et al, ).…”

“…Consequently, binding to one site will allosterically enhance the affinity of the other sites, leading to the hypothesis that combinations of general anaesthetics should act synergistically if they bind selectively to distinct sites. Pioneering studies have shown that steroids interact allosterically with etomidate and propofol both in vivo and in vitro (Cao et al, ; Chen et al, ; Drexler, Balk, & Antkowiak, ). Here, using three sets of general anaesthetics that bind with high selectivity to one of the three sites shown in Figure c, we find that the potencies of combinations of site‐selective anaesthetics that bind to the same site are additive, whereas those that bind to two or three distinct sites are increasingly synergistic.…”

Monod-Wyman-Changeux Allosteric Shift Analysis in Mutant α1β3γ2L GABAA Receptors Indicates Selectivity and Crosstalk among Intersubunit Transmembrane Anesthetic Sites

“…Thus, different mutations may divergently affect symmetry and/or crosstalk among anesthetic sites. Energy additivity in wild-type GABA A receptors is supported by studies of drug combinations (Shin et al, 2017;Cao et al, 2018) that notably used receptors formed from concatenated subunit assemblies. Concatenated receptors display less spontaneous activation than free subunit assemblies, and may also reduce heterogeneity in subunit arrangement (Baumann et al, 2001;Guitchounts et al, 2012).…”