Functional consequences of prejunctional receptor activation or blockade in the iris.

The iris is innervated by nerves of the sympathetic, parasympathetic, and sensory nervous systems. The terminal nerve fibres are endowed with prejunctional receptors which modulate neurotransmitter release. Activation or blockade of prejunctional receptors by drugs may have an influence on iris smooth muscle tone. Several findings are in favour of the hypothesis that prejunctional receptors may be involved in regulation of iris smooth muscle tone and/or pathophysiological events. (i). Release of acetylcholine from parasympathetic nerves of guinea-pig iris sphincter evoked by electrical stimulation is subject to autoinhibition via prejunctional M2 muscarinic receptors, and the release can be enhanced by M2 selective antagonists such as methoctramine or gallamine. Concomitantly with the increased neurotransmitter release, the sphincter contraction is enhanced in the presence of M2 antagonists, since the postjunctional muscarinic receptors (presumably M3, or at least not M2) are not simultaneously blocked. Unlike the non-selective blocker atropine, M2 antagonists are not expected to cause mydriasis but rather miosis. (ii). Sensory nerves are involved in pathophysiological events following ocular irritation. Release of substance P and/or neurokinin A from sensory nerves of rabbit iris is followed by a non-adrenergic-non-cholinergic iris sphincter contraction (mediated by NK1 and NK3 receptors) which can be used to estimate sensory neurotransmitter release. Exocytotic release of the sensory neurotransmitters is inhibited by activation of alpha 2B-adrenoceptors and probably also via putative prejunctional imidazoline receptors. Alpha-adrenoceptors are stimulated by oxymetazoline and other imidazoline derivatives (which are agonists at imidazoline receptors) leading to a reduction of sensory neurotransmitter release, as evident from a decrease in evoked sphincter contraction. Imidazolines in eye drops may not only cause relief in ocular inflammation due to postjunctional vasoconstriction but also possibly due to a prejunctional effect, a reduction of sensory neurotransmitter release. Reinforcement of inflammation due to release of sensory neurotransmitters may thus be prevented.