In order to clarify the functional role of the cerebrovascular parasympathetic and sensory nerves on the vessel tone in vivo, and the transmitters responsible for such an effect, brain cortical microvascular blood flow was measured in rats during electrical stimulation of these particular postganglionic parasympathetic and sensory fibers. Their respective central connections were cut prior to stimulation, to avoid cerebral flow changes sencondary to activation of central structures. Relative

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... nges in microvascular blood flow was continuously recorded by a laser-Doppler flowmeter system under cc-chloralose or ketamine anesthesia. Stimulation of the postganglionic parasympathetic fibers induced a marked increase in the cortical blood flow on the ipsilateral side, whereas no change was observed on the contralateral side. Atropine and scopolamine, applied locally or systemically in effective concentrations, failed to alter the flow increase upon parasympathetic stimulation. The strong vasodilator vasoactive intestinal polypeptide found in these nerves remains as a possible transmitter responsible for the flow increase, although a proposed antagonist tested did not attenuate the response. The modest flow increase seen ipsilaterally upon stimulation of the sensory nerve fibers is likely to be caused by antidromic release of transmitter in pain fibers. Antagonists to substance P did not affect the response. The strong vasodilator in these nerves, calcitonin generelated peptide, is a possible candidate responsible for the flow increase. Cerebral blood vessels are supplied with sympathetic, parasympathetic and sensory nerve fibers (29). The origins and pathways of para-