Synaptic plasticity at the cerebellum input stage: mechanisms and functional implications

The mf-GrC relay provides the case of a synapse at which elementary neurotransmission mechanisms are particularly well understood allowing a precise investigation of synaptic plasticity. An interesting consequence is that a presynaptic mechanism of LTP could be precisely documented on the basis of quantal analysis. By being presynaptically expressed, LTP becomes instrumental to regulation of short-term synaptic dynamics thereby controlling time-dependent transformations of the incoming mossy fiber input. It is unknown to what extent these considerations could be generalized, but early observations were provided for comparable concepts and mechanisms in neocortical synapses (Tsodyks and Markram, 1997). Although several aspects remain to be investigated, mf-GrC LTP provides a wide substrate for information storage in the cerebellum. In the rat cerebellum, there are 10(11) GrCs and 4 times as many mf-GrC synapses. Mathematical models have suggested that mf-GrC LTP improves mutual information transfer, and that the combination of synaptic and non-synaptic changes improves sparse representation of the mf input (Schweighofer et al., 2000; Philipona et al., 2003). Moreover, mf-GrC LTP could play a key role in regulating neurotransmission dynamics, implementing adaptability in delay lines early envisioned by Breitenberg (1967) and then revisited by Medina and Mauk (2002). These observations challenge the simple view of spatial pattern separation proposed by Marr (1969). The potential consequences of mf-GrC LTP need to be further investigated and confronted with computational models of the cerebellar network.