Central cholinergic stimulation affects ocular functions through sympathetic pathways

Intracerebroventricular (ICV) injection of carbachol into the third ventricle of conscious rabbits increased pupil size and intraocular pressure (IOP). A significant increase of pupil size by 0.6-1.3 mm for 60-90 min was observed with the dose of 1 microgram and an increase of IOP by 1.1-1.9 mmHg for 15-60 min was observed with doses over 10 ng. These responses were due to central cholinergic stimulation; mydriasis was eliminated by ICV injection of atropine and the elevation of IOP was significantly reduced by ICV injections of atropine and hexamethonium. Transection of the cervical sympathetic trunk eliminated the mydriasis. The efferent pathway for the mydriasis is via the cervical sympathetic nerve. The elevation of IOP was significantly reduced but not eliminated in the decentralized eye. Plasma epinephrine (Epi) and norepinephrine (NE) increased at 30 min after the ICV injection of 1 microgram carbachol. It is hypothesized that the elevation of IOP is due mainly to the stimulation of ocular sympathetic nerves and the increase of circulating catecholamines. Topical treatment of 0.01% timolol significantly reduced the IOP elevation, indicating the involvement of ocular beta-adrenergic mechanism in the elevation of IOP.