Purpose of review: A wide distribution of the eye's muscarinic receptor system has been found and several roles for the muscarinic system in the eye proposed, although functional consequences of muscarinic receptor activation are not always fully characterized. The present paper reviews current knowledge about the presence of muscarinic receptors in the ocular surface and the implication of alterations on their expression and/or functioning in eye diseases.
Recent findings: Several authors have described alterations in muscarinic receptor subtype expression in the eye under pathological conditions. In conjunctiva of vernal keratoconjunctivitis patients, the expression of muscarinic receptor subtypes is altered in both the epithelium and stroma. Under proinflammatory conditions, M2-muscarinic receptor expression is upregulated in conjunctival epithelial cells in vitro. M3-muscarinic receptor altered distribution has been found in the lacrimal gland of NOD mice.
Summary: The cholinergic muscarinic system plays diverse roles. Alteration in the expression and/or functioning of muscarinic receptors may be implicated in the etiopathogenesis of some ocular diseases. Their pharmacological regulation may therefore have therapeutic value. Knowledge of the specific receptor subtypes expressed in each tissue may help to avoid some undesired secondary side effects in some cases when muscarinic agonists or antagonists are used.