Functional neural circuits are formed by eliminating early-formed redundant synapses and strengthening necessary connections during postnatal development. In adult cerebellum, a single strong climbing fiber (CF) innervates the dendrites of each Purkinje cell (PC)(D). In contrast, multiple CFs with almost similar synaptic strengths innervate the soma of each PC in newborn cerebellum (A). By postnatal day 7 (P7), a single CF is selectively strengthened as a result of competition among multiple CFs. We find that this competition occurs on the soma before CFs form synapses along dendrites (B). Notably, in most PCs, the single CF that has been functionally strengthened (the "winner" CF) undergoes translocation to dendrites while keeping its synapses on the soma after P9 (C). Synapses of the weaker CFs (the "loser" CFs) remain around the soma and form "pericellular nests" with synapses of the winner CFs (C). Then, most perisomatic CF synapses are eliminated nonselectively by P15, which results in selective elimination of surplus CFs (D). Thus, our results suggest that the selective translocation of the winner CF to dendrites in each PC determines the single CF that survives subsequent synapse elimination and persistently innervates the PC.

Program member
Masanobu Kano (Department of Neurophysiology, Graduate School of Medicine)

Multiple CFs with almost similar synaptic strengths innervate the PC soma at P2-P3 (A). Subsequently, a single CF is selectively strengthened as a result of competition on the PC soma by P7-P8 (B). After P9, the strengthened CF undergoes translocation to PC dendrites, but the other weaker CFs remain around the PC soma (C). By P15, perisomatic synapses are eliminated non-selectively (D).