Unique Antipsychotic Activities of the Selective Metabotropic Glutamate Receptor 1 Allosteric Antagonist 2-Cyclopropyl-5-[1-(2-fluoro-3-pyridinyl)-5-methyl-1H-1,2,3-triazol-4-yl]-2,3-dihydro-1H-isoindol-1-one

A newly discovered metabotropic glutamate receptor (mGluR) 1 allosteric antagonist, 2-cyclopropyl-5-[1-(2-fluoro-3-pyridinyl)-5-methyl-1 H -1,2,3-triazol-4-yl]-2,3-dihydro-1 H -isoindol-1-one (CFMTI), was tested both in vitro and in vivo for its pharmacological effects. CFMTI demonstrated potent and selective antagonistic activity on mGluR1 in vitro and in vivo after oral administration. CFMTI inhibited l-glutamate-induced intracellular Ca 2+ mobilization in Chinese hamster ovary cells expressing human and rat mGluR1a, with IC 50 values of 2.6 and 2.3 nM, respectively. The selectivity of CFMTI to mGluR1 over mGluR5 was >2000-fold, and CFMTI at 10 μM showed no agonistic or antagonistic activities toward other mGluR subtypes and other receptors. It antagonized face-washing behavior in mice induced by ( S )-3,5-dihidroxyphenylglycine at a dose range of 3 to 30 mg/kg, for which receptor occupancy was 73 to 94%. As with the classical neuroleptic haloperidol and an atypical antipsychotic, clozapine, orally administered CFMTI reduced methamphetamine-induced hyperlocomotion and disruption of prepulse inhibition (PPI) at the same dose range as required to antagonize the face-washing behavior. CFMTI and clozapine improved ketamine-induced hyperlocomotion, PPI disruption and (5 S ,10 R )-(+)-5-methyl-10,11-dihydro-5 H -dibenzo[a,d]cyclohepten-5,10-imine maleate (MK-801)-induced social withdrawal without any cataleptogenic activities, whereas haloperidol only improved ketamine-induced hyperlocomotion. CFMTI, unlike clozapine, caused neither hypolocomotion nor motor incoordination at therapeutic doses. In c-fos expression studies, CFMTI and clozapine increased the number of fos-positive neurons in the nucleus accumbens and medial prefrontal cortex but not in the dorsolateral striatum. These results suggest that the antipsychotic activities of mGluR1 antagonists are more similar to those of atypical antipsychotics than those of typical antipsychotics.