Inhibition of the Pial Artery Constriction Induced by Sympathetic Stimulation by Local Microapplication of a Cholinomimetic Agent

We studied the effects of microapplications of carbachol plus atropine on cat pial artery diameter in vivo during resting conditions and during stimulation of the cervical sympathetic nerve. The cats were anesthetized with α-chloralose and artificially ventilated. The pial surface was exposed by trepanation and protected by a 1–2-cm layer of oil. Calibrated applications of solutions were made by micropipette into the subarachnoid space, while the pial artery diameter was measured by the television image-splitting method. Sympathetic stimulation during 100 s induced a constrictive response of about 10%, which was constant from 60 to 100 s and which remained so during application of inert mock spinal fluid from 65 to 100 s. Application of 10 −5 M carbachol plus 10 −7 M atropine (solution A) or 10 −4 M carbachol plus 10 −6 M atropine (solution B) did not produce any significant changes in diameter during resting conditions. During sympathetic stimulation, application of solution A from 65 to 100 s induced a small nonsignificant reduction of the constriction, whereas application of solution B induced a highly significant reduction of the constriction from 9.63 ± 1.09% at 60 s to 1.20 ± 2.40% at 100 s. These results are discussed in terms of the hypothesis that carbachol may act on the sympathetic fibers on the pial arteries by a nonmuscarinic mechanism to reduce the liberation of the transmitter.