INFLUENCE OF PARALLEL FIBER-PURKINJE CELL SYNAPSE FORMATION ON POSTNATAL DEVELOPMENT OF CLIMBING FIBER-PURKINJE CELL SYNAPSES IN THE CEREBELLUM

The climbing fiber (CF) to Purkinje cell (PC) synapse in the cerebellum provides an ideal model for the study of developmental rearrangements of neural circuits. At birth, each PC is innervated by multiple CFs. These surplus CFs are eliminated during postnatal development, and mono innervation is attained by postnatal day 20 (P20) in mice. Earlier studies on spontaneous mutant mice and animals with hypogranular cerebella indicate that regression of surplus CFs requires normal generation of granule cells and their axons, parallel fibers (PFs), and normal formation of PF-PC synapses. Our understanding of how PF-PC synapse formation affects development of CF-PC synapse has been greatly advanced by analyses of mutant mice deficient in glutamate receptor 82 subunit (GluR delta 2), an orphan receptor expressed selectively in PCs. Deletion of GluR delta 2 results in impairment of PF-PC synapse formation, which leads to defects in development of CF-PC synapses. In this article, we review how impaired PF-PC synapse formation affects wiring of CFs to PCs based mostly on our data on GluR delta 2 knockout mice. We propose a new scheme that CF-PC synapses are shaped by the three consecutive events, namely functional differentiation of multiple CFs into one strong and a few weak inputs from P3 to P7, early phase of CF synapse elimination from P7 to around P11, and late phase of CF synapse elimination from around P12. Normal PF-PC synapse formation is required for the late phase of CF synapse elimination. (C) 2009 IBRO. Published by Elsevier Ltd. All rights reserved.