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Carbonaro TM, Nguyen V, Forster MJ, Gatch MB, Prokai L. Carisoprodol pharmacokinetics and distribution in the nucleus accumbens correlates with behavioral effects in rats independent from its metabolism to meprobamate. Neuropharmacology 2020; 174:108152. [PMID: 32479814 DOI: 10.1016/j.neuropharm.2020.108152] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 10/10/2019] [Revised: 04/29/2020] [Accepted: 05/18/2020] [Indexed: 10/24/2022]
Abstract
Carisoprodol (Soma®) is a centrally-acting skeletal-muscle relaxant frequently prescribed for treatment of acute musculoskeletal conditions. Carisoprodol's mechanism of action is unclear and is often ascribed to that of its active metabolite, meprobamate. The purpose of this study was to ascertain whether carisoprodol directly produces behavioral effects, or whether metabolism to meprobamate via cytochrome P450 (CYP450) enzymatic reaction is necessary. Rats were trained to discriminate carisoprodol (100 mg/kg) to assess time course and whether a CYP450 inhibitor (cimetidine) administered for 4 days would alter the discriminative effects of carisoprodol. Additionally, pharmacokinetics of carisoprodol and meprobamate with and without co-administration of cimetidine were assessed via in vivo microdialysis combined with liquid-chromatography-tandem mass spectrometry from blood and nucleus accumbens (NAc). The time course of the discriminative-stimulus effects of carisoprodol closely matched the time course of the levels of carisoprodol in blood and NAc, but did not match the time course of meprobamate. Administration of cimetidine increased levels of carisoprodol and decreased levels of meprobamate consistent with its interfering with metabolism of carisoprodol to meprobamate. However, cimetidine failed to alter the discriminative-stimulus effects of carisoprodol. Carisoprodol penetrated into brain tissue and directly produced behavioral effects without being metabolized to meprobamate. These findings indicate that understanding the mechanism of action of carisoprodol independently of meprobamate will be necessary to determine the validity of its clinical uses.
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Affiliation(s)
- Theresa M Carbonaro
- Department of Pharmacology and Neuroscience, University of North Texas Health Science Center, 3500 Camp Bowie Blvd, Fort Worth, TX, 76107-2699, USA
| | - Vien Nguyen
- Department of Pharmacology and Neuroscience, University of North Texas Health Science Center, 3500 Camp Bowie Blvd, Fort Worth, TX, 76107-2699, USA
| | - Michael J Forster
- Department of Pharmacology and Neuroscience, University of North Texas Health Science Center, 3500 Camp Bowie Blvd, Fort Worth, TX, 76107-2699, USA
| | - Michael B Gatch
- Department of Pharmacology and Neuroscience, University of North Texas Health Science Center, 3500 Camp Bowie Blvd, Fort Worth, TX, 76107-2699, USA
| | - Laszlo Prokai
- Department of Pharmacology and Neuroscience, University of North Texas Health Science Center, 3500 Camp Bowie Blvd, Fort Worth, TX, 76107-2699, USA.
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Kumar M, Kumar M, Freund JM, Dillon GH. A Single Amino Acid Residue at Transmembrane Domain 4 of the α Subunit Influences Carisoprodol Direct Gating Efficacy at GABA A Receptors. J Pharmacol Exp Ther 2017. [PMID: 28642232 DOI: 10.1124/jpet.117.242156] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/22/2022] Open
Abstract
The muscle relaxant carisoprodol has recently been controlled at the federal level as a Schedule IV drug due to its high abuse potential and consequences of misuse, such as withdrawal syndrome, delusions, seizures, and even death. Recent work has shown that carisoprodol can directly gate and allosterically modulate the type A GABA (GABAA) receptor. These actions are subunit-dependent; compared with other GABAA receptors, carisoprodol has nominal direct gating effects in α3β2γ2 receptors. Here, using site-directed mutagenesis and whole-cell patch-clamp electrophysiology in transiently transfected human embryonic kidney 293 cells, we examined the role of GABAA receptor α subunit transmembrane domain 4 (TM4) amino acids in direct gating and allosteric modulatory actions of carisoprodol. Mutation of α3 valine at position 440 to leucine (present in the equivalent position in the α1 subunit) significantly increased the direct gating effects of carisoprodol without affecting its allosteric modulatory effects. The corresponding reverse mutation, α1(L415V), decreased carisoprodol direct gating potency and efficacy. Analysis of a series of amino acid mutations at the 415 position demonstrated that amino acid volume correlated positively with carisoprodol efficacy, whereas polarity inversely correlated with carisoprodol efficacy. We conclude that α1(415) of TM4 is involved in the direct gating, but not allosteric modulatory, actions of carisoprodol. In addition, the orientation of alkyl or hydroxyl groups at this position influences direct gating effects. These findings support the likelihood that the direct gating and allosteric modulatory effects of carisoprodol are mediated via distinct binding sites.
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Affiliation(s)
- Manoj Kumar
- Department of Physiology and Pharmacology, Center for Neuroscience, Robert C. Byrd Health Sciences Center, West Virginia University, Morgantown, West Virginia (Mo.K., Mi.K., J.M.F., G.H.D.); and Center for Neuroscience Discovery, Institute for Healthy Aging, University of North Texas Health Science Center, Fort Worth, Texas (Mi.K., G.H.D.)
| | - Manish Kumar
- Department of Physiology and Pharmacology, Center for Neuroscience, Robert C. Byrd Health Sciences Center, West Virginia University, Morgantown, West Virginia (Mo.K., Mi.K., J.M.F., G.H.D.); and Center for Neuroscience Discovery, Institute for Healthy Aging, University of North Texas Health Science Center, Fort Worth, Texas (Mi.K., G.H.D.)
| | - John M Freund
- Department of Physiology and Pharmacology, Center for Neuroscience, Robert C. Byrd Health Sciences Center, West Virginia University, Morgantown, West Virginia (Mo.K., Mi.K., J.M.F., G.H.D.); and Center for Neuroscience Discovery, Institute for Healthy Aging, University of North Texas Health Science Center, Fort Worth, Texas (Mi.K., G.H.D.)
| | - Glenn H Dillon
- Department of Physiology and Pharmacology, Center for Neuroscience, Robert C. Byrd Health Sciences Center, West Virginia University, Morgantown, West Virginia (Mo.K., Mi.K., J.M.F., G.H.D.); and Center for Neuroscience Discovery, Institute for Healthy Aging, University of North Texas Health Science Center, Fort Worth, Texas (Mi.K., G.H.D.)
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Huang R, Chen Z, Dolan S, Schetz JA, Dillon GH. The dual modulatory effects of efavirenz on GABA A receptors are mediated via two distinct sites. Neuropharmacology 2017; 121:167-178. [PMID: 28456686 DOI: 10.1016/j.neuropharm.2017.04.038] [Citation(s) in RCA: 5] [Impact Index Per Article: 0.7] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 11/11/2016] [Revised: 03/27/2017] [Accepted: 04/24/2017] [Indexed: 11/20/2022]
Abstract
Efavirenz is a widely prescribed medicine used to treat type 1 human immunodeficiency virus (HIV-1), the most prevalent pathogenic strain of the virus responsible for the acquired immune deficiency syndrome (AIDS) pandemic. Under prescribed dosing conditions, either alone or in combination therapy, efavirenz-induced CNS disturbances are frequently reported. Efavirenz was recently reported to interact in a similar concentration range with a number of receptors, transporters and ion channels including recombinant rat α1β2γ2 GABAA receptors whose actions were potentiated (Gatch et al., 2013; Dalwadi et al., 2016). Now we report on the molecular mechanism of efavirenz on GABAA receptors as a function of concentration and subunit composition via whole-cell recordings of GABA-activated currents from HEK293 cells expressing varying subunit configurations of GABAA receptors. Efavirenz elicited dual effects on the GABA response; it allosterically potentiated currents at low concentrations, whereas it inhibited currents at higher concentrations. The allosteric potentiating action on GABAA receptors was pronounced in the α1β2γ2, α2β2γ2 and α4β2γ2 configurations, greatly diminished in the α6β2γ2 configuration, and completely absent in the α3β2γ2 or α5β2γ2 configuration. In stark contrast, the inhibitory modulation of efavirenz at higher concentrations was evident in all subunit configurations examined. Moreover, efavirenz-induced modulatory effects were dependent on GABA concentration ([GABA]), with a pronounced impact on currents activated by low [GABA] but little effect at saturating [GABA]. Mutation of a highly-conserved threonine to phenylalanine in transmembrane domain 2 of the α1 subunit abolished the inhibitory effect of efavirenz in α1β2 receptors. Finally, mutations of any of the three conserved extracellular residues in α1/2/4 subunits to the conserved residues at the corresponding positions in α3/5 subunits (i.e., R84P, M89L or I120L) completely eliminated the potentiating effect of efavirenz in α1β2γ2 configuration. These findings demonstrate that efavirenz's positive allosteric modulation of the GABAA receptor is mediated via a novel allosteric site associated with the extracellular domain of the receptor.
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Affiliation(s)
- Renqi Huang
- Center for Neuroscience Discovery, Institute for Healthy Aging, University of North Texas Health Science Center at Fort Worth, Fort Worth, TX 76107, United States.
| | - Zhenglan Chen
- Center for Neuroscience Discovery, Institute for Healthy Aging, University of North Texas Health Science Center at Fort Worth, Fort Worth, TX 76107, United States
| | - Sean Dolan
- Center for Neuroscience Discovery, Institute for Healthy Aging, University of North Texas Health Science Center at Fort Worth, Fort Worth, TX 76107, United States
| | - John A Schetz
- Center for Neuroscience Discovery, Institute for Healthy Aging, University of North Texas Health Science Center at Fort Worth, Fort Worth, TX 76107, United States
| | - Glenn H Dillon
- Center for Neuroscience Discovery, Institute for Healthy Aging, University of North Texas Health Science Center at Fort Worth, Fort Worth, TX 76107, United States
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Kumar M, Dillon GH. Assessment of direct gating and allosteric modulatory effects of meprobamate in recombinant GABA(A) receptors. Eur J Pharmacol 2016; 775:149-58. [PMID: 26872987 DOI: 10.1016/j.ejphar.2016.02.031] [Citation(s) in RCA: 11] [Impact Index Per Article: 1.4] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 10/25/2015] [Revised: 02/05/2016] [Accepted: 02/08/2016] [Indexed: 02/07/2023]
Abstract
Meprobamate is a schedule IV anxiolytic and the primary metabolite of the muscle relaxant carisoprodol. Meprobamate modulates GABAA (γ-aminobutyric acid Type A) receptors, and has barbiturate-like activity. To gain insight into its actions, we have conducted a series of studies using recombinant GABAA receptors. In αxβzγ2 GABAA receptors (where x=1-6 and z=1-3), the ability to enhance GABA-mediated current was evident for all α subunit isoforms, with the largest effect observed in α5-expressing receptors. Direct gating was present with all α subunits, although attenuated in α3-expressing receptors. Allosteric and direct effects were comparable in α1β1γ2 and α1β2γ2 receptors, whereas allosteric effects were enhanced in α1β2 compared to α1β2γ2 receptors. In "extrasynaptic" (α1β3δ and α4β3δ) receptors, meprobamate enhanced EC20 and saturating GABA currents, and directly activated these receptors. The barbiturate antagonist bemegride attenuated direct effects of meprobamate. Whereas pentobarbital directly gated homomeric β3 receptors, meprobamate did not, and instead blocked the spontaneously open current present in these receptors. In wild type homomeric ρ1 receptors, pentobarbital and meprobamate were ineffective in direct gating; a mutation known to confer sensitivity to pentobarbital did not confer sensitivity to meprobamate. Our results provide insight into the actions of meprobamate and parent therapeutic agents such as carisoprodol. Whereas in general actions of meprobamate were comparable to those of carisoprodol, differential effects of meprobamate at some receptor subtypes suggest potential advantages of meprobamate may be exploited. A re-assessment of previously synthesized meprobamate-related carbamate molecules for myorelaxant and other therapeutic indications is warranted.
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Affiliation(s)
- Manish Kumar
- Department of Physiology and Pharmacology and Center for Neuroscience, Robert C. Byrd Health Sciences Center, West Virginia University, Morgantown, WV, USA
| | - Glenn H Dillon
- Department of Physiology and Pharmacology and Center for Neuroscience, Robert C. Byrd Health Sciences Center, West Virginia University, Morgantown, WV, USA; Center for Neuroscience Discovery, University of North Texas Health Science Center, Fort Worth, TX 76107, USA.
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