Carisoprodol-Mediated Modulation of GABA_A Receptors: In Vitro and in Vivo Studies

Abstract

Carisoprodol is a frequently prescribed muscle relaxant. In recent years, this drug has been increasingly abused. The effects of carisoprodol have been attributed to its metabolite, meprobamate, a controlled substance that produces sedation via GABA A receptors (GABA A Rs). Given the structural similarities between carisoprodol and meprobamate, we used electrophysiological and behavioral approaches to investigate whether carisoprodol directly affects GABA A R function. In whole-cell patch-clamp studies, carisoprodol allosterically modulated and directly activated human ␣1␤2␥2 GABA A R function in a barbiturate-like manner. At millimolar concentrations, inhibitory effects were apparent. Similar allosteric effects were not observed for homomeric 1 GABA or glycine ␣1 receptors. In the absence of GABA, carisoprodol produced picrotoxin-sensitive, inward currents that were significantly larger than those produced by meprobamate, suggesting carisoprodol may directly produce GABAergic effects in vivo. When administered to mice via intraperitoneal or oral routes, carisoprodol elicited locomotor depression within 8 to 12 min after injection. Intraperitoneal administration of meprobamate depressed locomotor activity in the same time frame. In drug discrimination studies with carisoprodol-trained rats, the GABAergic ligands pentobarbital, chlordiazepoxide, and meprobamate each substituted for carisoprodol in a dose-dependent manner. In accordance with findings in vitro, the discriminative stimulus effects of carisoprodol were antagonized by a barbiturate antagonist, bemegride, but not by the benzodiazepine site antagonist, flumazenil. The results of our studies in vivo and in vitro collectively suggest the barbiturate-like effects of carisoprodol may not be due solely to its metabolite, meprobamate. Furthermore, the functional traits we have identified probably contribute to the abuse potential of carisoprodol.

Assessment of subunit-dependent direct gating and allosteric modulatory effects of carisoprodol at GABAA receptors

“…The patterns of potentiation and inhibition by carisoprodol at α1β2 GABA A Rs were similar to those observed at α1β2γ2 GABA A Rs, shown here (Fig. 1A) and previously using a stable α1β2γ2 cell line (Gonzalez et al, 2009b). The estimated EC 50 for carisoprodol at α1β2 GABA A Rs was 87.4 ± 16.4 µM compared to 88.2 ± 20 µM for receptors containing the γ2 subunit, with direct gating by carisoprodol likely contributing to maximal current amplitude elicited by higher concentrations of the drug.…”

“…However, carisoprodol direct gating potency and efficacy was significantly less at α3-containing receptors compared to other α isoform receptors (Table 2). As originally reported for α1-containing receptors (Gonzalez et al, 2009b), attenuation of current amplitude at high concentrations, followed by rebound current, was consistently observed regardless of the α subunit.…”

“…As noted, the metabolite of carisoprodol has barbiturate-like effects at GABA A Rs (Rho et al, 1997). We have demonstrated carisoprodol itself allosterically modulates and directly activates human α1β2γ2 GABA A Rs, and its actions are not mediated via reported sites of action for benzodiazepines or barbiturates (Gonzalez et al, 2009b). Although receptors of α1β2γ2 subunit composition are the prevalent configuration in the brain, a vast array of GABA A R configurations have been shown to exist throughout the CNS, with each configuration contributing to specific physiological and pharmacological responses (Olsen and Sieghart, 2008).…”

Pharmacologically Active Drug Metabolites: Impact on Drug Discovery and Pharmacotherapy

“…65). The mechanism of action is not entirely clear, although some activity may be mediated through the GABA A receptor (Gonzalez et al, 2009). The conversion of carisprodol to meprobmate is partially catalyzed by CYP2C19 (Dalen et al, 1996), an enzyme subject to genetic polymorphism, and thus concentrations of parent drug and metabolite differ between extensive and poor metabolizers (Olsen et al, 1994).…”

The HIV Antiretroviral Drug Efavirenz has LSD-Like Properties

“…Whole-cell currents were obtained and analyzed from HEK293 cells stably expressing (a 1 b 2 g 2 (long isoform of the g 2 -subunit) or transiently expressed NMDA receptors (NR 1a with either NR 2A or NR 2B subunits), as described previously (Gonzalez et al, 2009;Huang et al, 2010). A minimum of three individual experiments was conducted for each paradigm.…”