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.