Citat:
Originally Posted by negrogesic
Benzos are positive allosteric modulators of GABA-A, whereas methaqualone acts directly on GABA-A. Also, unlike benzos, methaqualone causes a direct gating of the GABA-A chloride ion channel, thus leading to an increased toxicity/overdose risk when compared to benzos.
Would you be so kind and provide a reference for the red-marked statements, plz? I'm not aware of any conclusive binding data for methaqualone (with respect to GABA-receptor subtypes resp. the detailed mechanism of action), but would be quite happy if there are any publications that I just missed.
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My own findings in the literature, in short:
It was published that methaqualone (MMQ), as well as mecloqualone (i.e. MMQ with the tolyl-group replaced by an ortho-chloro-phenyl) were able to inhibit [3H]diazepam binding in vitro (ref [1]). In the same assay, GABA-binding to the receptors was not effected. It must be stressed that the authors did not make a difference between certain GABA-receptor subtypes.
Surprisingly, piriqualone (i.e. MMQ with the 2-methyl replaced by a pyridin-2-yl-vinyl-residue) enhanced [3H]diazepam-binding. Kinetic analysis pointed towards a competitive binding mode for MMQ, with respect to diazepam.
These findings were supported shortly after by cross-tolerance studies, were MMQ was shown to exhibit cross-tolerance for diazepam, but with lesser extend for pentobarbital, barbital and ethanol (ref [2]).
Further support for a benzodiazepine-competitive binding mode was established, using Ro 15-1788 as GABA-receptor probe:
Quote:
Methaqualone (Mtq; quaaludes or 'ludes) is a controlled substance, having a molecular structure related to the imidiazobenzodiazepine series of drugs, that has gained some notoriety recently due to its history of widespread abuse on the street. Users report experiencing peripheral paresthesia and transient numbness on body parts receiving dense cutaneous innervation (lips, fingertips, etc.). Since the receptive-field (RF)-sizes of many primary somatosensory (S1) cortical neurones are controlled by local, gamma-aminobutyric acid (GABA)-mediated inhibitory processes, we tested Mtq to see whether its clinical symptoms might have a basis in an action through central GABA-mediated synaptic processes. This report supports this contention and describes a likely pharmacological mechanism involved as one being related to the Ro 15-1788-sensitive benzodiazepine (Bzd) recognition site(s) of the GABA receptor complex.
Abstract of ref [3]
See also ref [4] for a structure of Ro 15-1788; the article is available free of charge.
Summary: Except for references without specification of action on certain GABA-receptor subtypes, there is no evidence on the detailed binding mode of MMQ, at least not to my knowledge. Neither are there any publications that discuss the mechanistic mode of action in further detail than the already mentioned ones.
The one thing that we know with relative certainty is that MMQ presumably binds to the same, or partially the same site as benzodiazepines. That's it.
If you know more, please share your data with me.
Regards, Murphy