Activity of chlormethiazole at human recombinant GABA(A) and NMDA receptors

1. Investigation into the modulatory effects of chlormethiazole at human recombinant gamma-aminobutyric acid A receptor (GABAA) and N-methyl-d-aspartate (NMDA) receptors was undertaken to gain insight into its mechanism of action and determine if the drug exhibited any subtype-selective activity. 2. Despite a structural similarity to the beta-subunit-selective compound loreclezole, chlormethiazole did not show any difference in maximum efficacy and only a slight difference in EC50 in its potentiating action at alpha1beta1gamma2 and alpha1beta2gamma2 GABAA receptor subtypes with preference for alpha1beta1gamma2. 3. Similar to the previously reported subtype-dependent activity of pentobarbital, chlormethiazole elicited a significantly greater degree of maximum potentiation on receptors lacking a gamma2 subunit, and also those receptors containing an alpha4 or alpha6 subunit. This also demonstrates that chlormethiazole does not act via the benzodiazepine binding site. 4. Unlike pentobarbital and propofol, chlormethiazole elicited only a slight direct GABAA receptor activation at concentrations up to 1 mm. In addition, the drug did not potentiate anaesthetic-mediated currents elicited by pentobarbital or propofol, suggesting that chlormethiazole may be acting via an anaesthetic binding site. 5. Chlormethiazole produced weak nonselective inhibition of human NMDA NR1a+NR2A and NR1a+NR2B receptors. IC50's were approximately 500 microm that likely exceed the therapeutic dose range for chlormethiazole, indicating that the primary mechanism of the compounds in vivo activity is via GABAA receptors.