Abstract
γ-Hydroxybutyrate (GHB) is a psychotropic compound endogenous to the brain. Despite its potentially great physiological significance, its exact molecular mechanism of action is unknown. GHB is a weak agonist at GABAB receptors, but there is also evidence of specific GHB receptor sites, the molecular cloning of which remains a challenge. Ligands with high affinity and specificity for the reported GHB binding site are needed for pharmacological dissection of the GHB and GABAB effects and for mapping the structural requirements of the GHB receptor-ligand interactions. For this purpose, we have synthesized and assayed three conformationally restricted GHB analogs for binding against the GHB-specific ligand [3H]NCS-382 [(E,RS)-(6,7,8,9-tetrahydro-5-hydroxy-5H-benzocyclohept-6-ylidene-)acetic acid] in rat brain homogenate. The cyclohexene and cyclopentene analogs, 3-hydroxycyclohex-1-enecarboxylic acid [(RS)-HOCHCA] and 3-hydroxycyclopent-1-enecarboxylic acid [(RS)-HOCPCA], were found to be high-affinity GHB ligands, with IC50 values in the nanomolar range, and had 9 and 27 times, respectively, higher affinity than GHB. The stereo-selectively synthesized R,R-isomer of the trans-cyclopropyl GHB analog, HOCPrCA, proved to have 10-fold higher affinity than its enantiomer. Likewise, the R-enantiomers of HOCHCA and HOCPCA selectively inhibited [3H]NCS-382 binding. The best inhibitor of these, (R)-HOCPCA, has an affinity 39 times higher than GHB and is thus among the best GHB ligands reported to date. Neither of the cycloalkenes showed any affinity (IC50 > 1 mM) for GABAA or GABAB receptors. These compounds show excellent potential as lead structures and novel tools for studying specific GHB receptor-mediated pharmacology.
Footnotes
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This work was supported by the Danish Medical Research Council, Apotekerfonden of 1991, European Union Grant HPAW-CT-2002-80057 (to P.W.), the Lundbeck Foundation, the computer resources of the Danish Center for Supercomputing, and the Australian Centre for Advanced Computing and Communications.
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Article, publication date, and citation information can be found at http://jpet.aspetjournals.org.
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doi:10.1124/jpet.105.090472.
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ABBREVIATIONS: GHB, γ-hydroxybutyrate; [3H]NCS-382, (E,RS)-(6,7,8,9-tetrahydro-5-hydroxy-5H-benzocyclohept-6-ylidene)acetic acid; HOCPCA, 3-hydroxycyclopent-1-enecarboxylic acid; T-HCA, trans-4-hydroxycrotonic acid; HOCHCA, 3-hydroxycyclohex-1-enecarboxylic acid; HOCPrCA, trans-2-(hydroxymethyl)cyclopropane carboxylic acid; OxCPCA, 3-oxocyclopent-1-enecarboxylic acid; ee, enantiomeric excess.
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↵ The online version of this article (available at http://jpet.aspetjournals.org) contains supplemental material.
- Received June 3, 2005.
- Accepted July 7, 2005.
- The American Society for Pharmacology and Experimental Therapeutics
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