n-n--dicyclopentyl-2-methylsulfanyl-5-nitro-pyrimidine-4-6-diamine and Epilepsy--Reflex

The most common cause of inherited mental retardation, fragile X syndrome, results from a triplet repeat expansion in the FMR1 gene and loss of the mRNA binding protein, fragile X mental retardation protein (FMRP). In the absence of FMRP, signaling through group I metabotropic glutamate receptors (mGluRs) is enhanced. We previously proposed a mechanism whereby the audiogenic seizures exhibited by FMR1 null mice result from an imbalance in excitatory mGluR and inhibitory GABA(B) receptor (GABA(B)R) signaling (Mol Pharmacol 76:18-24, 2009). Here, we tested the mGluR5-positive allosteric modulator 3-cyano-N-(1,3-diphenyl-1H-pyrazol-5-yl)benzamide (CDPPB), the mGluR5 inverse agonist 2-methyl-6-(phenylethynyl)pyridine (MPEP), and GABA(B) receptor agonists, alone and in combination on receptor protein expression and audiogenic seizures in FMR1 mice. Single doses of MPEP (30 mg/kg), the GABA(B)R orthosteric agonist R-baclofen (1 mg/kg), or the GABA(B)R-positive allosteric modulator N,N'-dicyclopentyl-2-(methylthio)-5-nitro-4,6-pyrimidine diamine (GS-39783) (30 mg/kg), reduced the incidence of seizures. However, when administered subchronically (daily injections for 6 days), MPEP retained its anticonvulsant activity, whereas R-baclofen and GS-39783 did not. When administered at lower doses that had no effect when given alone, a single injection of MPEP plus R-baclofen also reduced seizures, but the effect was lost after subchronic administration. We were surprised to find that subchronic treatment with R-baclofen also induced tolerance to a single high dose of MPEP. These data demonstrate that tolerance develops rapidly to the antiseizure properties of R-baclofen alone and R-baclofen coadministered with MPEP, but not with MPEP alone. Our findings suggest that cross-talk between the G-protein signaling pathways of these receptors affects drug efficacy after repeated treatment.

Topics: Animals; Anticonvulsants; Baclofen; Benzamides; Blotting, Western; Cyclopentanes; Drug Interactions; Drug Tolerance; Epilepsy, Reflex; Excitatory Amino Acid Antagonists; Fragile X Mental Retardation Protein; Fragile X Syndrome; GABA Agonists; GABA Modulators; Mice; Mice, Inbred C57BL; Mice, Knockout; Pyrazoles; Pyridines; Pyrimidines; Receptors, GABA-B; Receptors, Kainic Acid; Receptors, Metabotropic Glutamate