Neuropsychopharmacology (2007) 32, 745–756 & 2007 Nature Publishing Group All rights reserved 0893-133X/07 $30.00 www.neuropsychopharmacology.org The Ampakine CX546 Restores the Prepulse Inhibition and Latent Inhibition Deficits in mGluR5-Deficient Mice ,1 1 1,2 Tatiana Lipina* , Karin Weiss and John Roder 1Samuel Lunenfeld Research Institute, Mount Sinai Hospital, Toronto, ON, Canada; 2Program in Neuroscience, University of Toronto, Toronto, ON, Canada In order to test the possible role of mGluR5 signaling in the behavioral endophenotypes of schizophrenia and other psychiatric disorders, +/+ À/À we used genetic engineering to create mice carrying null mutations in this gene. Compared to their mGluR5 littermates, mGluR5 mice have disrupted latent inhibition (LI) as measured in a thirst-motivated conditioned emotional response procedure. Administration of the positive modulator of a-amino-3-hydroxy-5-methyl-4-isoxazole-propionic acid receptors (AMPAR), CX546, during the conditioning phase only, improved the disrupted LI in mGluR5 knockout mice and facilitated LI in control C57BL/6J mice, given extended number of À/À conditioning trails (four conditioning stimulus–unconditioned stimulus). Prepulse inhibition (PPI) was impaired in mGluR5 mice to a F À/À level that could not be disrupted further by the antagonist of N-methyl-D-aspartate receptors MK-801. PPI deficit of mGluR5 mice was effectively reversed by CX546, whereas aniracetam had a less pronounced effect. These data provide evidence that a potent positive AMPAR modulator can elicit antipsychotic action and represents a new approach for treatment of schizophrenia. Neuropsychopharmacology (2007) 32, 745–756. doi:10.1038/sj.npp.1301191; published online 23 August 2006 Keywords: mGluR5 knockout mice; PPI; latent inhibition; MK-801; CX546; aniracetam INTRODUCTION schizophrenics. Compounds that enhance the activity of the NMDAR, including glycine, D-serine, the partial coagonist In recent years, theories focusing on glutamatergic dysfunc- D-cycloserine, and glycine transporter type 1 inhibitors, tion in the etiology of schizophrenia have received much reverse NMDAR antagonist-induced behavioral effects in support (Olney et al, 1999; Goff and Coyle, 2001). humans and rodents (Javitt et al, 1999; Lipina et al, 2005). Glutamatergic mechanisms underlying schizophrenia are Metabotropic glutamate receptors (mGluR) add to the of special interest given their critical role in the modulation complexity of glutamatergic neurotransmission in the brain. of cognition, mood, and motor function, which are In the context of NMDAR-mediated neurotransmission, the impaired in schizophrenics (Paul and Skolnick, 2003). mGluR5 subtype of mGluRs is of special interest. The Historically, it was found that pharmacological blockade mGluR5 binds through G-dependent or independent path- of N-methyl-D-aspartate receptor (NMDAR) induced psy- ways (Heuss et al, 1999) with predominantly couple via Gq chotic states in healthy human volunteers and exacerbated to phospholipase C. This yields diacylglycerol, which existing symptomatology in schizophrenics (Javitt and activates protein kinase C (PKC), and inositol-1,4,5-tripho- Zukin, 1991). This observation has led to the hypothesis 2+ sphate, which releases intracellular Ca (Conn and Pin, that NMDAR hypofunction may be critically involved in the 1997), and this in turn can increase NMDAR-evoked etiology and/or underlying symptoms of schizophrenia responses in a variety of neuronal tissues (Doherty et al, (Olney et al, 1999). Hence, facilitation of the NMDAR may 1997; Awad et al, 2000), indicating one potential functional represent an effective therapeutic strategy for normalization link between mGluR5 and NMDAR. Indeed, administration of NMDAR function in schizophrenia. Indeed, it was shown of mGluR5 antagonist 2-methyl-6-(phenylethynyl)pyradine that coagonists of the NMDAR acting at the glycine-binding (MPEP) facilitated in vivo effects of NMDAR antagonists in site have antipsychotic efficacy in animal models and in rodents, producing anxiolytic (Spooren et al, 2000), neuroprotective (Bruno et al, 2000), and anticonvulsant *Correspondence: Dr T Lipina, Samuel Lunenfeld Research Institute, (Chapman et al, 2000) effects, disrupting prepulse inhibi- Mount Sinai Hospital, 600 University Avenue, Toronto, Ontario, Canada M5G 1X5, Tel: + 1 416 586 8242, Fax: + 1 416 586 8588, tion (PPI) (Kinney et al, 2003) and enhancing the E-mail: [email protected] detrimental effects of MK-801 on cognition and stereotypy Received 2 June 2006; revised 7 June 2006; accepted 7 July 2006 (Homayoun et al, 2004). This demonstrates that mGluR5 Online publication: 20 July 2006 at http://www.acnp.org/citations/ play a major role in regulating NMDAR-dependent func- Npp072006060079/default.pdf tion. Therefore, modulation of mGluR5 may be an effective CX546 recovers schizophrenic phenotypes T Lipina et al 746 therapeutic strategy for manipulation of NMDAR-mediated correlate with the level of negative symptoms in schizo- neurotransmission in schizophrenia (Goff and Coyle, 2001). phrenics (Cohen et al, 2004) and was found in NMDAR There is also evidence, indicating a role for mGluR5 itself in antagonists-treated rodents (Gaisler-Salomon and Weiner, the psychopathology of schizophrenia. For example, the 2003; Lipina et al, 2005). The administration of mGluR5 increased neuronal mGluR5 mRNA has been found in antagonistFMPEP to rats disrupted LI based on condi- cortical pyramidal cell layers of schizophrenics (Ohnuma tioned taste aversion (Bills et al, 2005). In addition, et al, 1998). More importantly, linkage analysis revealed cognitive deficits relevant to schizophrenia were observed that mGluR5 has been mapped to 11q14, closely to a after MPEP treatment of rats in the spontaneous alteration translocation that segregates with schizophrenia and related test of working memory (Homayoun et al, 2004), which is psychosis in a large Scottish family (Devon et al, 2001). The among the most commonly observed cognitive impairments creation of the mGluR5 null mutant (mGluR5À/À) (Jia et al, in schizophrenia patients (Goldman-Rakic, 1994). 1998) in our lab, offered a unique possibility to investigate If mGluR5 modulates the functional state of the NMDAR, how the absence of mGluR5 can modulate NMDAR- then the absence of mGluR5 in null mutant mice could dependent schizophrenia-related processes in vivo, with cause dysfunction of NMDAR. Indeed, using electrophysio- potential discovery of new approaches for schizophrenia logical measurements, we found that CA1 neurons from treatments in human. mGluR5À/À mice showed a complete loss of the NMDAR- Animal models of schizophrenia represent an important mediated component of long-term potentiation (LTPNMDA), preclinical tool for testing novel pharmacological com- but an unchanged LTP of AMPAR-mediated component pounds in the treatment of schizophrenia. PPI and latent (LTPAMPA) (Jia et al, 1998). We hypothesized here that inhibition (LI) are two important behavioral models in membrane depolarization via AMPAR facilitation could neuropharmacological research of schizophrenia with face, activate NMDAR complex as there is a link between these predictive, and construct validity (Geyer and Ellenbroek, two receptors (Hanley and Henley, 2005) and overcome 2003). PPI is a measure of sensorimotor gating and refers to behavioral deficits of mGluR5À/À animals. AMPAR are the attenuation of the startle response by a weak stimulus heteromeric complexes composed of four subunits (GluR1, (prepulse) appearing a short time before the startle GluR2, GluR3, and GluR4), each of which has a binding stimulus. Deficits in PPI have been reported in schizo- pocket for the transmitter glutamate and can be expressed phrenia (Swerdlow et al, 1994; Braff et al, 2001), in patients as flip or flop splice variant, which is regulated regionally with Huntington’s disease (Swerdlow et al, 1995), Tourett’s (Black, 2005). Several distinct classes of AMPAR modulators syndrome (Castellanos et al, 1996), and obsessive-compul- have been reported (Kew and Kemp, 2005) such as sive disorder (Swerdlow and Geyer, 1993). The mGluR5À/À aniracetam (1-(4-methoxybenzoyl)-2-pyrrolidinone com- mice exhibited robust PPI deficits found in different pound); benzamide compoundsF‘ampakines’ (CX516, laboratories (Henry et al, 1999; Brody et al, 2004a, b; CX546, and CX614); benzothiadiazides and related IDRA Kinney et al, 2003) using mice on three different back- and PEPA compounds, and group of biarypropylsulfona- ground strains, based on procedures with different stimuli mides (LY392098, LY404187, LY395153, LY503430). Positive modalities and startle magnitudes. However, the observed AMPAR modulators do not bind to the glutamate binding PPI deficit in mGluR5À/À mice was resistant to the site, but they interact with the receptor at an allosteric site antipsychotics (Brody et al, 2004b), suggesting that these and augment function by decreasing desensitization and/or mice cannot serve as a model with predictive validity to test deactivation. Such compounds have been shown to facilitate for typical/atypical antipsychotic drug effects and may AMPAR-mediated synaptic activity both in vitro and in vivo model some other psychiatric disorder (Brody et al, 2004b). and to stimulate memory-dependent processes in animals Here, we investigated LI as another distinctive assay for (Black, 2005) and are currently under development for schizophrenic-like endophenotypes in mGluR5À/À mice. LI treatment of cognitive dysfunction in various neuropsy-