Distribution of the SNAP25 and SNAP23 synaptosomal-associated protein isoforms in rat cerebellar cortex.

Synaptosome-associated protein of 25 kDa (SNAP25) is a component of the fusion complex that mediates synaptic vesicle exocytosis, regulates calcium dynamics and neuronal plasticity. Despite its crucial role in vesicle release, SNAP25 is not distributed homogenously within the brain. It seems to be virtually absent in mature inhibitory terminals and is observed in a subtype of excitatory neurons defined by the expression of vesicular glutamate transporter 1 (VGluT1). Since a complementary distribution of VGluT1 and VGluT2 in excitatory synapses is correlated with different probabilities of release (Pr), we evaluated whether SNAP25 localization is associated with specific synaptic properties. In the cerebellum, climbing fiber (CF) and parallel fiber (PF) inputs, which impinge onto the same Purkinje cell (PC), have very different functional properties. In the cerebellum of adult rats, using confocal and electron microscopy, we observed that VGluT2-positive CFs, characterized by a high Pr, only weakly express SNAP25, while VGluT1-positive PFs that show a low Pr abundantly express SNAP25. Moreover, SNAP25 was less profuse in the VGluT2-positive rosettes of mossy fibers (MFs) and was almost absent in inhibitory terminals. We extended our analysis to the SNAP23 homolog; this is expressed at different levels in both gamma-aminobutyric acid-containing terminals (GABAergic) and glutamatergic terminals of the cerebellar cortex. In conclusion, the preferential localization of SNAP25 in specific synaptic boutons suggests a correlation between SNAP25 and the Pr. This evidence supports the hypothesis that SNAP25 has a modulatory role in shaping synaptic responses.