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M Shara, KH Kulkarni, SE Ohia; Effect of Cannabinoids on Sympathetic Neurotransmission in Isolated Bovine Irides . Invest. Ophthalmol. Vis. Sci. 2002;43(13):3273.
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It is well established that cannabinoids can lower intraocular pressure (IOP) in experimental animals (Song and Slowey, J. Pharmacol. Exp. Ther. 292: 136, 2000) and humans (Porcella et al., Eur. J. Neurosci. 13: 409, 2001). The exact mechanism of ocular hypotensive action of these compounds is yet to be fully elucidated. There is evidence that cannabinoids can inhibit neurotransmitter release from several tissues and organs through specific presynaptic cannabinoid CB1-receptors (Schlicker and Kathmann, TIPS, 22: 565, 2001). Purpose: The aim of the present study was to investigate the effect of cannabinoids on sympathetic neurotransmission in bovine irides in vitro. Methods: Isolated bovine irides were incubated in oxygenated Krebs solution containing 1.6 µM [3H]NE and flurbiprofen (3 µM) for 1 hour. After incubation, tissues were prepared for studies of [3H]NE release using the superfusion method. Release of [3H]NE was elicited by 300 direct current pulses (supramaximal voltage, 2 ms pulse duration, 5 Hz) applied 80 minutes (S1) and 108 minutes (S2) after the onset of superfusion. Cannabinoid receptor agonists were added to the buffer solution 16 minutes before and during S2. Results: Both WIN 552122 (0.1 - 30 µM) and the cannabinoid CB1-receptor selective agonist, methanandamide (1 nM - 1 µM) and caused a concentration-dependent inhibition of field-stimulated [3H]NE release without affecting basal tritium efflux. Methanandamide was more potent than WIN 552122 in inhibiting evoked [3H]NE overflow. Arachidonyl-2’ -chloroethylamide (10 nM), another CB1-receptor selective agonist caused a 40% inhibition of electrically-evoked [3H]NE release. Conclusion: We conclude that cannabinoids can inhibit sympathetic neurotransmission in the isolated bovine iris through activation of cannabinoid receptors of the CB1 subtype. It is feasible that CB1-receptors localized on adrenergic neurons could be involved in the IOP lowering activity of cannabinoids.
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