Local and propagated vascular responses evoked by focal synaptic activity in cerebellar cortex

We investigated the local and remote vascular changes evoked by activation of the cerebellar parallel fibers (PFs). The PFs were stimulated (25-150 μA. 30 Hz) in halothane-anesthetized rats equipped with a cranial window. The changes in arteriolar and venular diameter produced by PF stimulation were measured with the use of a videomicroscopy system. Cerebellar blood flow (BF(crb)) was monitored by laser Doppler flowmetry and the field potentials evoked by PF stimulation were recorded with the use of microelectrodes. PF stimulation increased the diameter of local arterioles (+26 ± 1%, mean ± SE) in the activated folium (n = 10, P < 0.05). The vasodilation was greatest in smaller arterioles (16.5 ± 0.8 μm), was graded with the intensity of stimulation, and was less marked than the vasodilation produced by hypercapnia in comparably sized vessels (+58 ± 5%, CO₂ pressure = 50-60 mmHg, n = 8). In addition, the vasodilation was greatest along the horizontal beam of activated PFs and was reduced in arterioles located away from the stimulated site in a rostrocaudal direction. The increases in vascular diameter were associated with increases in BF(crb) in the activated area (+55 ± 4%, n = 5) PF stimulation increased vascular diameter (+10 ± 0.5%, n = 10) also in larger arterioles (30-40 μm) located in the folium adjacent to that in which the PFs were stimulated. Higherorder branches of these arterioles supplied the activated area. No field potentials were evoked by PF stimulation in the area where these upstream vessels were located. The data suggest that increased synaptic activity in the PF system produces a 'local' hemodynamic response mediated by synaptic release of vasoactive agents and a 'remote' response that is propagated to upstream arterioles from vessels residing in the activated folium. These propagated vascular responses are important in the coordination of segmental vascular resistance that is required to increase flow effectively during functional brain hyperemia.