Cerebellar LTD vs. motor learning-lessons learned from studying GluD2.

Synaptic plasticity, such as long-term potentiation and long-term depression (LTD), is believed to underlie learning and memory processes in vivo. The cerebellum is an ideal brain region to obtain definitive proof for this hypothesis. The current belief is that the acquisition of motor learning is stored by LTD at the parallel fiber (PF)-Purkinje cell synapse in the cerebellar cortex. Recently, however, several lines of mutant mice that display normal motor learning in the absence of cerebellar LTD have been reported. A similar dichotomy between synaptic plasticity at the circuitry level and learning at the behavioral level has also been reported in the hippocampus. One possible explanation for this dichotomy is that compensatory pathways at the molecular and circuitry levels play an important role in mice that have been genetically modified for their entire lives. Mice that are genetically modified to be deficient in or to express mutant versions of the δ2 glutamate receptor (GluD2) serve as an interesting model due to the predominant expression of GluD2 at PF-Purkinje cell synapses. Furthermore, two major functions of GluD2-PF synapse formation and LTD induction-can be mechanistically dissociated so that the role of LTD in motor learning can be investigated in the absence of morphological abnormalities caused by altered synapse formation. Therefore, genetic manipulations of GluD2 will help to clarify the relationship between LTD and motor learning in the cerebellum.