The identification of adrenergic and cholinergic nerve endings in the trabecular meshwork.

There are nerves and nerve endings in the trabecular meshwork, but their structure does not reveal whether they are adrenergic or cholinergic in function. Tranzer and Thoenen have shown that the administration of 5-hydroxydopamine, a false sympathetic transmitter, fills the synoptic vesicles of adrenergic nerve endings with an electron dense material. Cholinergic nerve endings possess synaptic vesicles which appear empty in ordinary electron micrographs. This reaction has been shown by Tranzer and Thoenen to be highly specific. After 5-hydroxydopamine was injected subconjunctivally and intraperitoneally into cynomolgus monkeys, the nerve terminals of the iris dilator and sphincter muscles and also in the trabecular meshwork were examined by electron microscopy. Some of the synaptic endings possess vesicles which appeared solid in all three tissues following this procedure. The proportion of adrenergic to cholinergic nerve terminals, determined in this manner, was three to one in the iris dilator and one-to-six in the iris sphincter muscle. Adrenergic and cholinergic nerve terminals in the trabecular meshwork are located mainly in the posterior part just anterior to the insertion of the longitudinal ciliary muscle. About one third of these nerve endings were adrenergic.