Abstract
N-methyl-D-aspartate (NMDA) receptors play an important role in excitatory neurotransmission and mediate synaptic plasticity associated with learning and memory and are postulated to play an important role in numerous neurological diseases such as stroke, traumatic brain injury, alzheimers, dementia and schizophrenia. NMDA receptors are composed to two GluN1 and two GluN2 subunits and the identity of the GluN2 subunit (GluN2A-D) confers unique electrophysiologic and pharmacologic properties to the receptor. The precise role of GluN2C-containing receptors in vivo and in neuropsychiatric disorders such as schizophrenia is poorly understood. The purpose of this study was to determine the global and local in vivo role of GluN2C-containing NMDA receptors with the hypothesis that GluN2C-containing receptors are uniquely involved in the schizophrenia-like phenotypes in rodents and that pharmacologic potentiation of GluN2C-containing receptors is a viable therapeutic strategy in the treatment of schizophrenia. |The first aim of this study was to determine the result of genetic deletion of GluN2C from mice by performing a series of behavioral tests on GluN2C wildtype and knockout mice. Our results show that GluN2C knockout mice behave similar to wildtype in locomotor activity, anxiety, forced-swim induced despair, novel object recognition, pain sensitivity and reference memory. However, GluN2C knockout mice were found to exhibit deficits in fear acquisition and working memory which are two behaviors associated with the schizophrenia-like phenotype in rodents that correlate to the negative symptoms and cognitive deficits in schizophrenia. |The second aim of this study was to then determine the effect of partial or complete deletion of GluN2C subunit on schizophrenia-like behaviors in mice. We found again that GluN2C heterozygous and wildtype mice have normal spontaneous locomotor activity but exhibit hypersensitivity to NMDA channel blocker-induced hyperlocomotion and deficit in working memory versus wildtype counterpart. Additionally, GluN2C deficient mice were found to be more sensitive to social isolation-induced schizophrenia-like behaviors versus wildtype suggesting gene-environment interaction in the GluN2C knockout mouse. Furthermore, we show that systemic potentiation of GluN2C/D-containing receptors with CIQ restores naïve fear acquisition deficit and blocks NMDA antagonist-induced hyperlocomotion and deficit in working memory in GluN2C heterozygous mice. The results of this study show that deficit in function of GluN2C-containing NMDA receptors leads to schizophrenia-like behaviors and that pharmacologic enhancement of GluN2C-containing receptors may serve as a viable therapeutic strategy in the reversal of behavioral and cognitive deficits in schizophrenia. |The third aim of this study builds on the previous result of potentiation of GluN2C-containing receptors by delivering a selective potentiator of GluN2C-containing receptors into the prefrontal cortex (PFC) of wildtype and knockout mice after systemic injection of noncompetitive NMDA receptor antagonist and observing the schizophrenia-like behaviors of locomotor behavior, spatial working memory, and preference for social interaction. Our results show that potentiation of GluN2C-containing NMDA receptors in the prefrontal cortex reverses NMDA antagonist-induced hyperlocomotion, stereotypic events and deficit in working memory in wildtype but not knockout mice. This study addresses the primary concerns from specific aim 2 that CIQ may have an effect on GluN2D-containing receptors or that systemic administration of CIQ may be working indirectly in the behavioral assays. We address these concerns by using a GluN2C-specific potentiator and a local administration technique into the PFC to show that potentiation of GluN2C-containing NMDA receptors reverses the effect of NMDA antagonist-induced behaviors. |This research design progresses from the knockout mouse model to in vivo pharmacologic manipulation of GluN2C-containing NMDA receptors with GluN2C-specific potentiators. The results presented herein highlight a novel role of GluN2C-containing NMDA receptors in schizophrenia-like behaviors and fear learning. Furthermore, these results demonstrate that GluN2C-containing NMDARs may be a novel therapeutic target in the alleviation of schizophrenia that is consistent with the glutamate hypofunction hypothesis.