Calcium-dependent regulation of climbing fibre synapse elimination during postnatal cerebellar development.

Functional neural circuit formation during postnatal development involves massive elimination of early-formed redundant synapses and strengthening of necessary synaptic connections. In the cerebellum, one-to-one connection from a climbing fibre (CF) to a Purkinje cell (PC) is established through four distinct phases: (1) strengthening of a single CF among multiple CFs in each PC at postnatal age P3-P7 days, (2) translocation of a single strengthened CF to PC dendrites from around P9, (3) early-phase (P7 to around P11) and (4) late-phase (around P12-P17) elimination of weak CF synapses from PC somata. Mice with PC-selective deletion of the P/Q-type voltage-dependent Ca(2+) channel (VDCC) exhibit severe defects in strengthening of single CFs, dendritic translocation of single CFs and CF elimination from P7. In contrast, mice with a mutation of a single allele for the GABA synthesizing enzyme GAD67 show selective impairment of CF elimination from P10. Electrophysiological and Ca(2+) imaging data suggest that GABAA receptor-mediated inhibition onto PC somata from putative basket cells influences CF-induced Ca(2+) transients and regulates elimination of redundant CF synapses from PC somata at P10-P16. Thus, regulation of Ca(2+) influx to PCs through VDCCs is crucial for the four phases of CF synapse elimination during postnatal development.