This research focuses on the prejunctional regulation of cholinergic neurotransmission in the isolated rabbit iris-ciliary body, a well established model for studies of ocular autonomic physiology and pharmacology. Using a method of radiolabeling and ion exchange chromatography, we developed a sensitive in vitro assay to measure electrically-evoked release of 3H-acetylcholine (3H-ACh) in this tissue. Using this assay, we evaluated the prejunctional effects of various substances (i.e., neurotransmitters, peptides, autacoids, etc.) reported to modulate acetylcholine release in other systems. Agents found to inhibit 3H-ACh release in the rabbit iris-ciliary body included muscarinic agonists, alpha2 -adrenergic agonists, A1 purinergic agonists, histamine, delta-opiate agonists, substance P and somatostatin. Agents causing enhancement of 3H-ACh release included serotonin, A2 purinergic agonists, and alpha1-adrenergic agonists. Other putative neuromodulators including prostaglandins, beta-adrenergic agonists, mu- and kappa-opiates, vasoactive intestinal peptide, neuropeptide Y, calcitonin gene related peptide and endothelin had no significant effects in this system.; Further investigations of purinergic effects revealed the presence of both inhibitory A1 and facilitatory A2 purinoceptors on ocular cholinergic nerves as well as postjunctional A1 receptors that potentiate cholinergic contractions of iris sphincter muscle. Studies of purine secretion indicated that ATP is released from anterior uveal tissues in response to activation of postjunctional muscarinic, alpha1-adrenergic and substance P receptors, suggesting that endogenously secreted purines act as both autocrine and paracrine (transsynaptic) modulators of cholinergic neuromuscular transmission. These studies may suggest new pharmacological targets for manipulation of ocular cholinergic neurotransmission in glaucoma therapy.
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