Topical application of serotonin or the 5-HT1-agonist 5-CT intraocular pressure in rabbits

The effect of serotonin (5-hydroxytryptamine: 5-HT) and other agonists on rabbit intraocular pressure (IOP), pupil size, and the breakdown of blood-aqueous barrier were evaluated. Serotonin and various other agonists were applied topically to the rabbit eye, and intraocular pressure was followed over the next 3 hours using a Digilab 30D pneumatometer. It was demonstrated immunohistochemically that topical 5-HT reached the anterior chamber within 1 hour. Serotonin raised the IOP in a dose-dependent manner over a period of up to 4 hours, with a maximum reached between 30 minutes and 1 hour. A similar effect was observed with the 5-HT1-agonist 5-carboxamidotryptamine (5-CT). Neither tryptamine, 5-hydroxytryptophan, melatonin, gepirone, nor 5,6/5,7-dihydroxytryptamine caused any changes in IOP. Serotonin did not cause a change in pupil size or a breakdown of the blood-aqueous barrier, nor did the aqueous cAMP change significantly after topical 5-HT administration. The data presented suggest a role for 5-HT in the control of IOP. Previously demonstrated receptors on the iris-ciliary body and the effect of the 5-HT1-agonist 5-CT suggest that the rise in IOP may be caused partly or entirely by an increase in aqueous secretion mediated by 5-HT1-like receptors. Whether or not 5-HT has a role in altering aqueous outflow resistance remains to be seen. An effect of serotonin on other aspects of aqueous dynamics or on the extraocular muscles to cause a change in IOP cannot be excluded.