Prejunctional 5-HT receptors enhance cholinergic neurosecretion in the rabbit and human iris-ciliary body.

PURPOSE

To characterize prejunctional 5-HT heteroreceptors which modulate 3Hacetylcholine (3H-ACh release) in isolated rabbit and human iris-ciliary bodies (ICB3).

METHODS

ICB tissue segments were incubated with 3H-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 superfusate 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 x 10(-8) M). The maximum effect of 5-HT (10(-6) M) corresponded to a 45.14 +/- 7.40%) (n = 6) increase in 3H-ACh release. Higher concentrations of 5-HT (> 10(-4) 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 x 10(-8) M), whereas the 5-HT3 agonist M-CPBG[(1-m-chlorophenyl)-biguanide] and phenylbiguanide, the 5-HT1 agonist 5-carboxamidotryptamine and the 5-HT2A agonist alpha-methyl-5-HT were ineffective. The selective 5-HT1A agonist (+)-8-OH-DPAT (10(-8)-10(-5) M) had no significant effect, but at concentration of 10(-4) M, inhibited evoked 3H-Ach. release. Similar results were obtained in the human ICB. The evoked 3H-Ach. release was enhanced in a concentration-dependent manner by 5-HT (10(-9)-10(-5) M, EC50 = 3.36 x 10(-8) M) or 5-MOT (10(-8)-10(-5) M, EC50 = 6.59 x 10(-7) M), The selective 5-HT4 antagonist, GR113808A (10(-8) M) 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 ondersetrone (5 x 10(-7) M) and tropisetron (10(-9) M) 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.