We investigated the neural mechanisms of the increases in blood flow produced by synaptic activity using the parallel fiber (PF) system of the cerebellum as a model. The midline cerebellum was exposed in anesthetized rats and the PFs were stimulated with tungsten microelectrodes. Cerebellar blood flow (BFcrb) was recorded using a laser-Doppler probe, whereas field potentials were recorded using glass micropipettes. PF stimulation produced increases in BFcrb that were related to the frequency and intensity of stimulation (+60 +/- 9%, mean +/- SE, at 100 microA and 30 Hz; n = 6). The greatest increases were confined to a band stretching along the major axis of the stimulated folium and corresponding to the beam of activated PFs. The increase in evoked by PF stimulation was associated with a corresponding increase in glucose utilization, assessed by the 2-deoxyglucose method. The increases in BFcrb and the field potentials evoked by PF stimulation were abolished by tetrodotoxin (1 microM; n = 6). Ringer solution containing 12 mM Mg2+ and 0 mM Ca2+ blocked synaptic activity in the PFs and abolished the increases in flow (P > 0.05 from baseline; n = 5). The broad-spectrum glutamate receptor antagonist kynurenate (5 mM) prevented depolarization of Purkinje cells and interneurons and abolished the increase in BFcrb evoked by PF stimulation (P > 0.05; n = 6). Treatment with tetrodotoxin, Mg2+, or kynurenate did not affect the increase in BFcrb elicited by systemic hypercapnia or by topical application of the nitric oxide donor 3-morpholino sydnonimine (P > 0.05 from Ringer solution). We conclude that the increases in flow produced by synaptic activity are linked to glutamate-induced depolarization of Purkinje cells and interneurons. These findings provide evidence that activation of glutamate receptors participates in the mechanisms of functional hyperemia, and they support the validity of the PF system as a model for study of the relationship between synaptic activity and blood flow in the CNS.
