M 2-type Muscarinic Receptors Mediate Prejunctional Inhibition of Norepinephrine Release in the Human Iris-Ciliary Body

Abstract

Human muscarinic receptors comprise a family of five separate gene products, three of which (designated as M 1, M 2 and M 3 subtypes) can be distinguished pharmacologically. Previous work indicates that sympathetic nerve terminals in the anterior uvea contain prejunctional muscarinic receptors that, upon activation by agonists, inhibit the neural release of norepinephrine. The aim of this study was to characterize the prejunctional effects of muscarinic agents on the electrically-evoked secretion of 3H-norepinephrine in isolated, superfused human iris-ciliary body tissue segments. Stimulation-evoked 3H-norepinephrine release was inhibited > 80% by carbachol and muscarine, but was unaffected by the M 1-selective agonist McN A-343. Pilocarpine behaved as a partial agonist in this system, producing less than 40% of the maximum inhibition. The rank order of potency of selective antagonists at prejunctional muscarinic receptors was methoctramine (M 2) > AF-DX 116 (M 2) > pirenzepine (M 1) ≥ para-fluoro hexahydro-sila-definidol (M 3). These data suggest that prejunctional muscarinic receptors in the human iris-ciliary body correspond to the M 2 subtype. No evidence for age-related differences in prejunctional muscarinic receptor activity was seen in tissues obtained from 13 human donors, aged 10-83 years. Prejunctional muscarinic receptors may play a role in mediating the inhibitory effects of parasympathetic nerve stimulation or cholinomimetic drugs on ocular sympathetic neurotransmission in vivo.