Abstract

In the present study, we have elucidated the functional characteristics and mechanism of action of methaqualone (2-methyl-3-o-tolyl-4(3H)-quinazolinone, Quaalude), an infamous sedative-hypnotic and recreational drug from the 1960s–1970s. Methaqualone was demonstrated to be a positive allosteric modulator at human α_1,2,3,5β_2,3γ_2S GABA_A receptors (GABA_ARs) expressed in Xenopus oocytes, whereas it displayed highly diverse functionalities at the α_4,6β_1,2,3δ GABA_AR subtypes, ranging from inactivity (α₄β₁δ), through negative (α₆β₁δ) or positive allosteric modulation (α₄β₂δ, α₆β_2,3δ), to superagonism (α₄β₃δ). Methaqualone did not interact with the benzodiazepine, barbiturate, or neurosteroid binding sites in the GABA_AR. Instead, the compound is proposed to act through the transmembrane β⁽⁺⁾/α^(–) subunit interface of the receptor, possibly targeting a site overlapping with that of the general anesthetic etomidate. The negligible activities displayed by methaqualone at numerous neurotransmitter receptors and transporters in an elaborate screening for additional putative central nervous system (CNS) targets suggest that it is a selective GABA_AR modulator. The mode of action of methaqualone was further investigated in multichannel recordings from primary frontal cortex networks, where the overall activity changes induced by the compound at 1–100 μM concentrations were quite similar to those mediated by other CNS depressants. Finally, the free methaqualone concentrations in the mouse brain arising from doses producing significant in vivo effects in assays for locomotion and anticonvulsant activity correlated fairly well with its potencies as a modulator at the recombinant GABA_ARs. Hence, we propose that the multifaceted functional properties exhibited by methaqualone at GABA_ARs give rise to its effects as a therapeutic and recreational drug.