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A devastating psychiatric disorder, schizophrenia is characterized by three major symptoms, positive and negative symptoms and cognitive deficit. Almost all current therapeutic drugs for schizophrenia have efficacy for positive symptoms, and weak efficacy for negative and cognitive deficit. In particular, social withdrawal, diminished motivation, and anhedonia as the depressive aspects of negative symptoms are resistant to the treatment of antipsychotic drugs. Therefore, there is a need for development of new therapeutic drugs for negative symptoms of schizophrenia, and it is necessary to have comprehensive animal models to understand the neurobiological foundations of their symptoms. In this review, we represent the behavioral phenotypes in available animal models of schizophrenia for drug discovery, focusing on the depressive aspects of negative symptoms. We mention here animal models based on the pathology and epidemiology of schizophrenia, e.g., the pharmacological, neurodevelopmental, genetic, and gene-environment combination models. The animal models of schizophrenia are developed by various approaches and are assessed, but there are few models demonstrating negative symptoms with sensitivities to available therapeutic drugs. The development of comprehensive animal model reflecting negative symptoms and of novel compounds that can remedy them provide certain insight into the neurobiological process of schizophrenia and also point the way to a new therapeutic strategy. Keywords:: schizophrenia, social withdrawal, diminished motivation, anhedonia, animal models

The NMDA subtype of glutamate receptor (GluR) plays an important role in excitatory neurotransmission, synaptic plasticity, brain development, and neurodegeneration. NMDA receptors are inherent high Ca(2+)-permeable channels, which are formed by heteromeric assembly of the GluR zeta 1 subunit (NR1) and any one of four GluR epsilon subunits (GluR epsilon 1-4; NR2A-D). Mice lacking the GluR epsilon 1 subunit exhibited a malfunction of NMDA receptors, as evidenced by reduction of NMDA receptor channel current, hippocampal long-term potentiation, [3H]MK-801 binding, and NMDA-stimulated 45Ca2+ uptake. A biochemical analysis revealed a hyperfunction of dopaminergic and serotonergic neuronal systems in the frontal cortex and striatum of GluR epsilon 1 mutant mice. The enhancement of dopaminergic neuronal activity in the striatum, at least, due to the disinhibition of inhibitory GABAergic neuronal input. GluR epsilon 1 mutant mice showed an increase of locomotor activity in a novel environment attributed to the hyperfunction of the dopaminergic neuronal system, and an impairment of spatial, contextual, and latent learning. These findings provide evidence that NMDA receptors regulate behavior through the modulation of not only glutamatergic but also GABAergic and dopaminergic neuronal systems. Moreover, it is suggested that GluR epsilon 1 mutant mice are useful as an animal model, which is associated with the malfunction of NMDA receptors and hyperfunction of the dopaminergic neuronal system.