NEUROVASCULAR COUPLING IN THE CEREBELLAR GRANULAR LAYER

The tight coupling between neuronal activity and cerebral blood flow (CBF) is called neurovascular coupling (NVC). This phenomenon controls blood vessel diameter to ensure the proper supply of oxygen and nutrients to the brain and contributes to generate the BOLD (blood-oxygenation-level-dependent) signals in functional magnetic resonance imaging (fMRI).The NVC has been investigated in several brain regions, but its neuronal drive and biochemical pathways in the cerebellum are still unclear. In particular, attention has been mostly given to the cerebellar molecular layer components, as parallel fibers (Bouvier, 2016), local interneurons (Akgoren, 1994), and Purkinje cells (where NVC was found dissociated from spiking activity, Thomsen, 2004). This may be due to the inability of these cells to release nitric oxide (NO), a well known vasoactive agent. Surprisingly enough, there is no information about the role of the granular layer in this phenomenon, even though granule cells (GrCs): i) are the most abundant brain neurons and the most energy consuming elements in the cerebellum (Howarth, 2014), ii) show a high expression of NMDA receptors (NMDARs) (Monaghan and Anderson, 1991) and of the neural isoform of nitric oxide synthase (nNOS) (Southam, 1992), and iii) produce and release NO following high frequency mossy fibers (MFs) stimulation (Maffei, 2003). NO is also implicated in long-term synaptic plasticity at the MF-GrC connection in the cerebellar granular layer (D'Angelo, 2014). Therefore, the granular layer is particularly suitable for the study of NVC mechanisms and it was then compelling to investigate its role in cerebellar NVC. At first, we described the vascular organization of the rat cerebellar cortex in immunostained slices. Secondly, we investigated whether and how synaptic activity was coupled to vascular motility in the granular layer, by combining bright-field microscopy an NO-related imaging techniques. We focused our attention on MF-GrC synapses (the cerebellar input stage) and on capillaries, since these vessels are able to change their lumen diameter earlier than upstream arterioles, in response to neuronal activity and following pericytes activation (Hall, 2014).