title: The role of acetylcholine in the corneal epithelium of the rabbit
Creators: name: McKean, Catherine E.
Publisher: name: University of Glasgow
Year: 1980

description: The corneal epithelium forms the smooth anterior surface of the cornea, a structure which is a highly refractive component of the optical system. The corneal epithelium has several unusual properties; it is avascular; it is capable of rapid regeneration; it is a non-keratinised epithelium exposed to the external environment. This epithelium contains a high concentration of acetylcholine (ACh) and high levels of choline acetyltransferase (ChAc) activity, this being the enzyme which is responsible for the synthesis of ACh. There are relatively low levels of cholinesterases (ChE), the enzymes involvea in the breakdown of ACh. These components of the cholinergic system do not appear Go be entirely associated with the sensory nerves of the cornea. Several theories have been propounded for the function of this ACh, but none has received universal acceptance. This research was an attempt to elucidate possible functions for the ACh of the corneal epithelium of the rabbit. The first possibility was that the ACh played a role within the epithelium, maybe a role in the regeneration of the tissue after abrasive injury. Removal of the epithelium followed by investigation of the levels of protein and ChAc in the regenerating tissue presented some evidence to suggest that the enzyme ChAc does not play a major role in this process of regrowth. Thereafter, experiments investigated the postulate that corneal epithelial ACh may act at a site outwith the epithelium. Firstly, the possibility was explored of an anterior release of ACh into the tear film, perhaps to act on one of the structures bathed by this fluid. It was found that only drugs and stimuli which caused damage to the epithelial cells could increase the release of ACh into the tears. It seemed unlikely that this was a physiological function of epithelial ACh. Secondly, experiments were designed to investigate posterior release of ACh to act at a site most probably located in the anterior segment of the eye. Levels of cholinesterases in various tissues were measured and it was found that these levels were low in the corneal stroma and the aqueous humour, which the epithelial ACh would probably have to traverse to reach a distant site of action. A second series of experiments traced the tissues to which radio-labelled ACh diffused after it had been injected into the corneal stroma. These results indicated that detectable amounts of ACh could pass through the aqueous humour to reach the iris and ciliary body althougn only 1.5% of the injected ACh reaches the iris intact and less tnan this reaches the ciliary body. When ACh was injected into the corneal stroma and its effect on the iris observed, it was found that an injected dose of around 2nmol could produce a significant miosis. This dose of ACh is of the same order as the total amount of endogenous ACh in the corneal epithelium. A total release of the complete store of epithelial ACh to have a physiological role in the iris seemed improbable. The effect of cholinomimetics on the outflow of aqueous humour is well documented. These drugs, especially pilocarpine, are used in the treatment of glaucoma and probably act by increasing the outflow of aqueous humour from the anterior chamber. The main outflow system from the rabbit eye is situated in the iridocorneal angle of the anterior chamber. It was found that small doses of ACh (40pmol) injected into the corneal stroma could significantly increase the apparent facility of outflow of aqueous humour. The ACh appeared to act on muscarinic receptors and act through a Ca2+-mediated mechanism. It was potentiated by anticholinesterases. The mechanism of action of ACh on the outflow mechanism is discussed. The possibility that ACh from the corneal epithelium might exert an influence on the outflow of aqueous humour is put forward.