Roles of diverse glutamate receptors in brain functions elucidated by subunit-specific and region-specific gene targeting

Glutamate receptor (GluR) channels play a major role in fast excitatory synaptic transmission in vertebrate central nervous system. We revealed the molecular diversity of the GluR channel by molecular cloning and investigated their physiological roles by subunit-specific gene targeting. NMDA receptor GluRε1 KO mice showed increase in thresholds for hippocampal long-term potentiation and hippocampus-dependent contextual learning. The mutant mice performed delay eyeblink conditioning, but failed to learn trace eyeblink conditioning. GluRε1 mutant suffered less brain injury after focal cerebral ischemia. NMDA receptor GluRε2 KO mice showed impairment of the whisker-related neural pattern formation and suckling response, and died shortly after birth. Heterozygous (+/-) GluRε2 mutant mice were viable and showed enhanced startle response to acoustic stimuli. GluRδ2, a member of novel GluR channel subfamily we found by molecular cloning, is selectively expressed in the Purkinje cells of the cerebellum. GluRδ2 KO mice showed impairments of cerebellar synaptic plasticity and synapse stability. GluRδ2 KO mice exhibited impairment in delay eyeblink conditioning, but learned normally trace eyeblink conditioning. The phenotypes of NMDA receptor subunits and GluRδ2 mutant mice suggest that diverse GluR subunits play differential roles in the brain functions.