Introduction The NMDA receptor, one of the three major subtypes of ionotropic glutamate receptors, is highly permeable to Ca 2+ and plays a key role in various kinds of synaptic plasticity underlying memory and learning. On the basis of the amino acid sequence, the NMDA receptor subunits are classified into two families, the NR1 and NR2 (NR2A–NR2D), and the functional activities of NMDA receptor channels require the heteromeric assemblies of NR1 with one or more NR2 subunits. The diversity of NR2 subunit is assumed to be responsible for the functional diversity of NMDA receptor channels in the brain. 1,2 Mutant mice lacking these subunits should be useful for elucidating the roles of these diverse subtypes. Impairments in spatial learning and in the long-term potentiation (LTP) in the hippocampus have already been demonstrated in NR1- or NR2A-deficient mutant mice. 3,4 Some of the present authors reported that disruption of both NR2A and NR2C causes motor incoordination, suggesting their involvement in cerebellar function. 5 The mRNA for the NR2A subunit is widely expressed in the brain of adult mice, including cerebellar granule cells and deep nuclei, and is espe- cially enriched in the hippocampus. 3 In contrast, the mRNA for NR2C is localized only in the cere- bellar granule cells. 6 Eyeblink conditioning is a type of motor learning that depends crucially upon the cerebellum. Thompson and his colleagues demonstrated in their detailed lesion experiments that the memory trace for eyeblink conditioning, in a standard delayed paradigm, is localized in cerebellar neural circuits including the cerebellar cortex and deep nuclei. 7,8 The hippocampus was also shown to modulate this type of learning. 9–12 Although many studies using NMDA receptor antagonists suggested that activation of NMDA receptor channels are involved in the acqui- sition of eyeblink conditioning, 13–16 the subtype of NMDA receptor subunit which is critical for this acquisition has not been clarified. The present study addresses this issue by investigating the conditioned eyeblink response in mutant mice deficient in NR2A and/or NR2C. Learning and Memory 1 1 1 1 1 p © Rapid Science Publishers Vol 8 No 17 1 December 1997 3717 NMDA receptor channels, heteromeric assemblies of subunits with diverse subtypes, play critical roles in various kinds of synaptic plasticity underlying learning and memory. To elucidate the roles of subunits NR2A and NR2C in motor learning, we investigated acquisi- tion of the classically conditioned eyeblink response in a delayed-conditioning paradigm by gene knockout mice. Mutant mice lacking NR2C exhibited no significant defect; however, early acquisition of the task was impaired in mutant mice lacking NR2A or both NR2A and NR2C. Based on the distribution of these subunits in brain, these results indicate that acquisition of the conditioned response does not depend on NMDA recep- tors in the cerebellar cortex, but that its early acquisi- tion involves the hippocampus and/or cerebellar deep nuclei. Key words: Ca 2+ imaging; Cerebellar deep nuclei; Cerebellum; Classical eyeblink conditioning; Gene knockout; Granule cells; Hippocampus; Long-term poten- tiation; Mutant mice; NMDA receptor Conditioned eyeblink response is impaired in mutant mice lacking NMDA receptor subunit NR2A Yasushi Kishimoto, Shigenori Kawahara, Yutaka Kirino, CA Hiroshi Kadotani, 1 Yukihiro Nakamura, 2 Masayuki Ikeda 2 and Tohru Yoshioka 2 Laboratory of Neurobiophysics, School of Pharmaceutical Sciences, The University of Tokyo, 7-3-1 Hongo, Bunkyo-ku, Tokyo 113; 1 Department of Biological Sciences, Kyoto University Faculty of Medicine, Sakyo-ku, Kyoto 606-01; 2 Department of Molecular Neurobiology, School of Human Sciences, Waseda University, Tokorozawa 359, Japan CA Corresponding Author Website publication 24 October 1997 NeuroReport 8, 3717–3721 (1997)