Modulation of NMDA receptors in the cerebellum. 1. Properties of the NMDA receptor that modulate its function

NMDA receptors modulate important cerebral processes such as synaptic plasticity, long-term potentiation, learning and memory, etc. NMDA receptors in cerebellum have specific characteristics that make their function and modulation different from those of NMDA receptors in other brain areas. In this and the accompanying review we summarize the information available on the modulation of NMDA receptors in cerebellum. We review the properties of the NMDA receptor that modulate its function: subunit composition, post-translational modifications and synaptic localization. NMDA receptors are heteromeric ligand-gated ion channels assembled from two families of subunits, NR1 and NR2. There are at least eight splicing variant isoforms of the NR1 subunit and four types of NR2 subunits: NR2A, NR2B, NR2C and NR2D. NMDA receptors with different subunit composition or different splice variants of NR1 subunit have different properties. The expression of the different subunits and splicing variants varies during development. Two special characteristics of NMDA receptors in cerebellum that do not occur in other brain areas are the enrichment in the NR2C subunit and in the splice variant NR1b. As a consequence of these and other factors the pharmacology of NMDA receptors is also different in cerebellum than in other brain areas. The function and localization of NMDA receptors is also modulated by postranslational modifications including phosphorylation, glycosylation and nytrosylation. NMDA receptors are phosphorylated in serines of both NR1 and NR2 subunits and in tyrosines of NR2 subunits. Another factor modulating NMDA receptors function is the synaptic localization. The trafficking and clustering of NMDA receptors is modulated by phosphorylation and by interaction with other proteins. The signaling pathways and physiological modulators regulating NMDA receptor function as well as the role of these receptors in motor learning and coordination are reviewed in an accompanying article.