Fine Structural Localization of Acetylcholinesterase at a Cholinergic Vasodilator Nerve-Arterial Smooth Muscle Synapse

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Abstract
An electron-microscope study of the localization of acetylcholinesterase (AChE) has been made in the main uterine artery of the guinea pig, which is supplied by cholinergic dilator and adrenergic constrictor nerves. Large bundles of nonterminal axons were present in the outer adventitia, and terminal axons, containing numerous vesicles, were scattered through the inner adventitia up to 1µ away from the outer layer of smooth muscle cells. About 50% of these axons were within 0.2µ of a muscle cell; the minimum neuromuscular gap found was 0.05µ. No axons were within the media. The outer membranes of axons stained either heavily or lightly for AChE. These two distinct classes appear to represent cholinergic and noncholinergic axons, respectively. In late pregnancy, all cholinergic terminal axons which contained vesicles and were not separated from the muscle by Schwann or elastic tissue were associated with heavy postsynaptic staining which was restricted to that region of the muscle membrane adjacent to the axon. In tissue from virgin animals, most cholinergic terminal axons were associated with only scanty postsynaptic staining. The increase in postsynaptic AChE during pregnancy can be correlated with the increase in sensitivity of the arterial muscle to acetylcholine over this period. Assuming that the association of heavy AChE staining both pre- and postsynaptically is indicative of a functional cholinergic synapse, acetylcholine released up to 1µ away from the muscle may participate in transmission from vasodilator axons in this vessel. Since postsynaptic AChE staining is restricted to discrete areas on the outer muscle cells, probably only these cells are directly affected by transmitter released from nerves.