Electron microscopic, autoradiographic and histochemical techniques were used to determine the distribution of adrenergic and cholinergic nerve fibers within the walls of arterioles having a width of 35 to 60 P in the hamster cheek pouch. Nerves were confined to the adventitia and their varicose regions were distributed so that 86% were located within 1 P of the media. Arterioles exposed to 3H-norepinephrine and examined by light microscopic autoradiography exhibited several sites of silver grain accumulation, as a result of the uptake of 3H-norepinephrine by adrenergic nerve fibers. In electron autoradiographs, approximately 70% of all adventitial nerve fibers exhibited silver grains and are therefore considered to be noradrenergic. The majority of all nerve-associated silver grains were located near the adventitial-medial junction. Smooth muscle cells sometimes possessed concentrations of silver grains above background levels, perhaps reflecting the capacity of these cells to take up norepinephrine. Acetylcholinesterase activity was demonstrated in 16 to 44% of the adventitial nerve fibers and they are presumed to be cholinergic. Acetylcholinesterase-stained fibers were distributed throughout the adventitia with 43% being located closer than 1 P to the media. The morphological identification of a dual adrenergic and cholinergic innervation of arterioles of the hamster cheek pouch indicates the probable presence of an active as opposed to a passive vasodilatory mechanism.Arterioles of the hamster cheek pouch are innervated by an extensive perivascuIar nerve plexus (Fulton et al., '60; Berman et al., '68) located in the vascular adventitia. Physiological evidence that this plexus might contain cholinergic vasodilator nerve fibers as well as adrenergic vasoconstrictor fibers was obtained by Fulton et al. ('60) who observed vasodilation of arterioles and precapillary sphincters in response to topical application of rubefacients. Since vasodilation occurred simultaneously along a greater length of the blood vessel wall than was exposed to each substance, vasodilation was thought to be mediated by conduction of an impulse along nerve fibers. Electrical stimulation of branches of the facial nerve produced occasional vasodilation of small arterioles but not constriction and these investigators concluded that the vasomotor AM. J. ANAT., 137: 31-46.
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