Functional consequences of reduction in NMDA receptor glycine affinity in mice carrying targeted point mutations in the glycine binding site

We have used site-directed mutagenesis in conjunction with homologous recom bination to generate two mouse lines carrying point mutations in the glycin e binding site of the NMDAR1 subunit (Grin1). Glycine concentration-respons e curves from acutely dissociated hippocampal neurons revealed a 5- and 86- fold reduction in receptor glycine affinity in mice carrying Grin1(D481N) a nd Grin1(K483Q) mutations, respectively, whereas receptor glutamate affinit y remained unaffected. Homozygous mutant Grin1(D481N) animals are viable an d fertile and appear to develop normally. However, homozygous mutant Grin1( K483Q) animals are significantly lighter at birth, do not feed, and die wit hin a few days. No gross abnormalities in CNS anatomy were detected in eith er Grin1(D481N) or Grin1(K483Q) mice. Interestingly, in situ hybridization and Western blot analysis revealed changes in the expression levels of NMDA receptor subunits in Grin1(D481N) mice relative to wild type that may repr esent a compensatory response to the reduction in receptor glycine affinity . Grin1(D481N) mice exhibited deficits in hippocampal theta burst-induced l ong-term potentiation (LTP) and spatial learning and also a reduction in se nsitivity to NMDA-induced seizures relative to wild-type controls, consiste nt with a reduced activation of NMDA receptors. Mutant mice exhibited norma l prepulse inhibition but showed increased startle reactivity. Preliminary analysis indicated that the mice exhibit a decreased natural aversion to an exposed environment. The lethal phenotype of Grin1(K483Q) animals confirms the critical role of NMDA receptor activation in neonatal survival. A mild er reduction in receptor glycine affinity results in an impairment of LTP a nd spatial learning and alterations in anxiety-related behavior, providing further evidence for the role of NMDA receptor activation in these processe s.