LOCAL AND PROPAGATED VASCULAR-RESPONSES EVOKED BY FOCAL SYNAPTIC ACTIVITY IN CEREBELLAR CORTEX

We investigated the local and remote vascular changes evoked by activa tion of the cerebellar parallel fibers (PFs). The PFs were stimulated (25-150 mu A, 30 Hz) in halothane-anesthetized rats equipped with a cr anial window. The changes in arteriolar and venular diameter produced by PF stimulation were measured with the use of a videomicroscopy syst em. Cerebellar blood flow (BFcrb) was monitored by laser Doppler flowm etry and the field potentials evoked by PF stimulation were recorded w ith 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 arteriole s (16.5 +/- 0.8 mu m), was graded with the intensity of stimulation, a nd was less marked than the vasodilation produced by hypercapnia in co mparably 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 s timulated site in a rostrocaudal direction. The increases in vascular diameter were associated with increases in BFcrb in the activated area (+55 +/- 4%, n = 5). PF stimulation increased vascular diameter (+10 +/- 0.5%, n = 10) also in larger arterioles (30-40 mu m) located in th e folium adjacent to that in which the PFs were stimulated. Higher-ord er branches of these arterioles supplied the activated area. No field potentials were evoked by PF stimulation in the area where these upstr eam vessels were located. The data suggest that increased synaptic act ivity in the PF system produces a ''local'' hemodynamic response media ted by synaptic release of vasoactive agents and a ''remote'' response that is propagated to upstream arterioles from vessels residing in th e 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.