Learning-induced multiple synapse formation in rat cerebellar cortex.

Strengthening of synaptic connections has been proposed to underlie information storage in the brain, and experience-dependent increases in synapse number have been observed. However, the effect of these new synapses on the specific connectivity, and thus function, of a given brain area remains largely unknown. We report here that motor learning specifically induces the formation of multiple synapses--two post-synaptic contacts at a single pre-synaptic varicosity--in the cerebellum. Rats undergoing motor learning had more multiple synapses (two Purkinje cell spines contacting a given parallel fiber varicosity) per Purkinje cell than did active or inactive controls. The formation of multiple synapses provides an additional connection between a given parallel fiber and Purkinje cell, thereby enhancing particular pathways, and may constitute a fundamental mechanism of neural encoding.