Prejunctional 5-HT Receptors Enhance Cholinergic Neurosecretion in the Rabbit and Human Iris-ciliary Body

Purpose: To characterize prejunctional 5-HT heteroreceptors which modulate nacetylcholine (3 H-ACh release) in isolated rabbit and human iris-ciliary bodies (ICBS) . Methods: ICB tissue segments were incubated with H-choline, superfused and electrically stimulated four times (S1,S2,S3,S4) at 3 - 10 Hz for 1 min to elicit 3H-ACh. secretion. Test agents (5-HT agonists and antagonists) were added before S2,S3 and S4 and their effects determined by the stimulation ratio (Sx/S1) of evoked 3H-ACh. release. 3H-ACh in super-fusate fractions was fractionated and quantified by ion exchange chromatography. Results: In rabbit ICBs, evoked 3H-ACh. release was enhanced in a concentration-dependent manner by 5-HT (10- 9 - 10-5 M, EC50 = 5.8× 10-8 M). The maximum effect of 5-HT (10-6M) corresponded to a 45. 14 ± 7.40%) (n = 6) increase in 3H-ACh release. Higher concentrations of 5-HT ( > 10- M) induced desensitization. The response to 5-HT ( 10-6 M) in the presence of the 5-HT3/4 antagonist tropisetron (10-9 M) ,

Purpose: To characterize prejunctional 5-HT heteroreceptors which modulate nacetylcholine (3 H-ACh release) in isolated rabbit and human iris-ciliary bodies (ICBS) . Methods: ICB tissue segments were incubated with H-choline, superfused and electrically stimulated four times (S1,S2,S3,S4) at 3 - 10 Hz for 1 min to elicit 3H-ACh. secretion. Test agents (5-HT agonists and antagonists) were added before S2,S3 and S4 and their effects determined by the stimulation ratio (Sx/S1) of evoked 3H-ACh. release. 3H-ACh in super-fusate fractions was fractionated and quantified by ion exchange chromatography. Results: In rabbit ICBs, evoked 3H-ACh. release was enhanced in a concentration-dependent manner by 5-HT (10- 9 - 10-5 M, EC50 = 5.8× 10-8 M). The maximum effect of 5-HT (10-6M) corresponded to a 45. 14 ± 7.40%) (n = 6) increase in 3H-ACh release. Higher concentrations of 5-HT ( > 10- M) induced desensitization. The response to 5-HT ( 10-6 M) in the presence of the 5-HT3/4 antagonist tropisetron (10-9 M) , 5-HT1 antagonist methiothepin ((10-7 M), equated to 89.96% and 88.78% respectively of control value; in the presence of non-selective 5-HT antagonist mianserin, 5-HT4 antagonist SDZ-205557 (10-7 M) was - 15.20% and 32.84% of control. The 5-HT response was mimicked by the selective 5-HT4 agonist 5-methoxytryptamine ( 10-9 - 10-4 M, EC50 = 7 × 10-8 M), whereas the 5-HT3 agonist M-CPBG [ ( 1 - m - chlorophenyl) - biguanide ] and phenyl-biguanide, the 5-HT1 agonist 5-carboxamidotryptamine and the 5-HT2A agonist a-methyl-5-HT were ineffective. The selective 5-HT1A agonist ( + )-8-OH-DPAT (10-8-10-5M) had no significant effect, but at concentration of 10-4M, inhibited evoked 3H-Ach. release. Similar results were obtained in the human ICB. The evoked H-Ach. release was enhanced in a concentration-dependent manner by 5-HT (10-9-10-5M, EC50 = 3. 36 × 10-8M) or 5-MOT (10-8-10-5M, EC50 = 6.59×10-7M), The selective 5-HT4antagonist, GR113808A ( 10-8M) inhibited the 5-HT-inducing increase of the cholinergic response, producing parallel right shits of the concentration-response curves to 5-HT. The selective 5-HT3 antagonist ondersetron (5 × 10-7M) and tropisetron (10-9M) did not affect 5-HT inducing 3H-Ach release. Conclusions: Our results indicate that cholinergic terminals in the rabbit and human ICB contain facilitatory 5-HT heteroreceptors that belong to the 5-HT4 subtype. Their role in modulation of intraocular pressure remains to be elucidated. Eye Science 1998; 14 : 149 -155.