Received for publication December 3, 2004.
Revised January 31, 2005.
Accepted for publication January 31, 2005.

Contribution of GABA_A Receptor Subtypes to the Anxiolytic-Like, Motor, and Discriminative Stimulus Effects of Benzodiazepines: Studies With the Functionally-Selective Ligand SL651498

Abstract

Benzodiazepines (BZs) are prescribed for a variety of disorders, including those involving anxiety and sleep, but have unwanted side effects that limit their use. Elucidating the GABA_A receptor mechanisms underlying the behavioral effects of BZs will help develop new drugs having both maximum clinical benefit and minimum adverse side effects. A recently developed compound is SL651498, which is a full agonist at GABA_A receptors containing 2 and 3 subunits, and a partial agonist at GABA_A receptors containing 1 and 5 subunits. We assessed the ability of SL651498 to engender anxiolytic-like, motor, and subjective effects characteristic of BZ-type drugs in non-human primates. Anxiolytic-like activity was assessed with a conflict procedure in rhesus monkeys. Motor effects were evaluated in squirrel monkeys using observational techniques, and the subjective effects of SL651498 were assessed in squirrel monkeys trained to discriminate the non-selective BZ triazolam from saline. SL651498 engendered anxiolytic-like effects similar to conventional BZs. In addition, SL651498 fully induced muscle relaxation, but unlike conventional BZs, engendered minimal ataxia. In drug discrimination studies, SL651498 partially substituted for triazolam. This effect was blocked with the 1GABA_A subtype-preferring antagonist, -carboline-3-carboxylate-t-butyl ester (-CCT), implicating 1GABA_A receptors in the subjective effects of SL651498. Together, these studies suggest that compounds such as SL651498 that have high intrinsic efficacy at 2GABA_A and/or 3GABA_A receptors may have clinical potential as anxiolytics and muscle relaxants. Moreover, a compound with reduced efficacy at 1GABA_A and/or 5GABA_A receptors may lack some of the motor and subjective effects associated with conventional BZs.

Key words: Benzodiazepines, Conflict, Drug discrimination, GABA-A receptor subtypes, Non-human primate, Observable behavior

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