&agr;7-Nicotinic Acetylcholine Receptors on Cerebral Perivascular Sympathetic Nerves Mediate Choline-Induced Nitrergic Neurogenic Vasodilation

It has been suggested in isolated porcine cerebral arteries that stimulation by nicotine of &agr;7-nicotinic acetylcholine receptors (&agr;7-nAChRs) on sympathetic nerves, but not direct stimulation of parasympathetic nitrergic nerves, caused nitrergic neurogenic dilation. Direct evidence supporting this hypothesis has not been presented. The present study, which used in vitro tissue bath and confocal microscopy techniques, was designed to determine whether choline, a selective agonist for &agr;7-nAChRs, induced sympathetic-dependent nitrergic dilation of porcine basilar arterial rings. Choline and several nAChR agonists induced exclusive relaxation of basilar arterial rings without endothelium. The relaxation was blocked by tetrodotoxin, nitro-l-arginine, guanethidine, and &bgr;2-adrenoceptor antagonists. Furthermore, the relaxation was blocked by methyllycaconitine and &agr;-bungarotoxin (preferential &agr;7-nAChR antagonists) and mecamylamine but was not affected by dihydro-&bgr;-erythroidine (a preferential &agr;4-nAChR antagonist). Confocal microscopic study demonstrated that choline and nicotine induced significant calcium influx in cultured porcine superior cervical ganglionic cells but failed to affect calcium influx in cultured sphenopalatine ganglionic cells, providing direct evidence that choline and nicotine did not act directly on the parasympathetic nitrergic neurons. The increased calcium influx in superior cervical ganglionic cells was attenuated by &agr;-bungarotoxin and methyllycaconitine but not by dihydro-&bgr;-erythroidine. These results support our hypothesis that activation of &agr;7-nAChRs on cerebral perivascular sympathetic nerves causes calcium influx and the release of norepinephrine, which then act on presynaptic &bgr;2-adrenoceptors located on the neighboring nitrergic nerve terminals, resulting in NO release and vasodilation. Endogenous choline may play an important role in regulating cerebral sympathetic activity and vascular tone.