Effect on cortical blood flow of electrical stimulation of trigeminal cerebrovascular nerve fibres in the rat

It has recently been demonstrated in the rat that the majority of cerebrovascular pain fibres containing immunoreactive substance P and calcitonin gene-related peptide reach the vessels via the nasociliary nerve, a branch of the ophthalmic trigeminal division. In order to elucidate the effect of these nerves on blood flow in vivo, the relative changes in cortical microvascular flow were continuously monitored by a laser-Doppler flowmeter system during electrical nerve stimulation, with the central nerve connection cut and after removal of neighbouring dilatory parasympathetic nerves. The nasociliary nerve on one side was stimulated proximal to the ethmoidal foramen by a bipolar platinum electrode. Activation at different frequencies, continuously, or as bursts with a constant voltage, impulse duration and total stimulus length, revealed that a maximum increase in blood flow amounting to 16.7% after 36 s was obtained with continuous stimulation at 10 Hz. Flow markedly declined during the following 1-min stimulation period. No changes in contralateral cortical blood flow, mean arterial blood pressure or blood gases were observed during or after stimulation. The present study demonstrates for the first time that direct and selective electrical activation of trigeminal cerebrovascular nerves induces an increase, albeit small and transient, in blood flow within the brain.