RT Journal Article
SR Electronic
T1 A Single amino acid residue at transmembrane domain 4 of the alpha subunit influences carisoprodol direct gating efficacy at GABAA receptors
JF Journal of Pharmacology and Experimental Therapeutics
JO J Pharmacol Exp Ther
FD American Society for Pharmacology and Experimental Therapeutics
SP jpet.117.242156
DO 10.1124/jpet.117.242156
A1 Manoj Kumar
A1 Manish Kumar
A1 John Freund
A1 Glenn H Dillon
YR 2017
UL http://jpet.aspetjournals.org/content/early/2017/06/22/jpet.117.242156.abstract
AB The muscle relaxant carisoprodol (CSP, trade name Soma) 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 GABAA receptor. These actions are subunit-dependent; compared to other GABAA receptors, carisoprodol has nominal direct gating effects in α3β2γ2receptors. Here, using site-directed-mutagenesis and whole cell patch clamp electrophysiology in transiently transfected HEK293 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 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 amino acid volume correlated positively with CSP efficacy, while polarity inversely correlated with CSP efficacy. We conclude α1(415) of TM4 is involved in the direct gating, but not allosteric modulatory, actions of carisoprodol. Also, orientation of alkyl or hydroxyl groups at this position influence direct gating effects. These findings support the likelihood that direct gating and allosteric modulatory effects of carisoprodol are mediated via distinct binding sites.