Deletion of the cannabinoid CB receptor impacts on the ultrastructure of the cerebellar parallel fiber-Purkinje cell synapses.

The cannabinoid CB receptor localizes to the glutamatergic parallel fiber (PF) terminals of the cerebellar granule cells and participates in synaptic plasticity, motor control and learning that are impaired in CB receptor knockout (CB -KO) mice. However, whether ultrastructural changes at the PF-Purkinje cell (PC) synapses occur in CB -KO remains unknown. We studied this in the vermis of the spinocerebellar lobule V and the vestibulocerebellar lobule X of CB -KO and wild-type (CB -WT) mice by electron microscopy. Lobule V, but not lobule X, of CB -KO had significantly less and longer synapses than in CB -WT. PF terminals were significantly larger in both lobules of CB -KO with no changes in PC dendritic spines. The PF terminals in lobule V of CB -KO contained less synaptic vesicles and lower vesicle density; by contrast, vesicle density in lobule X of CB -KO remained unchangeable relative to CB -WT. There were as many vesicles in lobule V of CB -KO as in CB -WT, but their distribution decreased drastically at 300 nm of the active zone. In lobule X of CB -KO, less vesicles were found within 150 nm from the presynaptic membrane; however, no vesicles were at 450-600 nm of the active zone. A significant higher amount of synaptic vesicles close to the active zone in lobule V and X of CB -KO was observed. In conclusion, the absence of CB receptors strikingly and distinctively impacts on the ultrastructural architecture of the PF-PC synapses located in cerebellar lobules that differ in vulnerability to damage and motor functions.