Stimulation of the cervical sympathetic nerves increases intraocular pressure

To test the hypothesis that a moderate electrical stimulation of the cervical sympathetic nerves in rabbits can increase intraocular pressure (IOP).

Electrical stimulations of the cervical sympathetic nerves were performed in anesthetized and conscious rabbits. Intraocular pressure, pupil size, and concentrations of aqueous humor components were monitored.

In urethane-anesthetized rabbits, stimulations of 5 V and 1 ms at 2.5 Hz for 1 hr and then at 20 Hz for 2 hr caused a short inhibition of IOP decrease and a prolonged mydriasis. Concentrations of norepinephrine (NE), neuropeptide Y (NPY), and cyclic AMP (cAMP) in aqueous humor were elevated. Aqueous humor protein concentration was not changed. In rabbits anesthetized with ketamine, chlorpromazine, and pentobarbital, electrical stimulations with the same parameters caused prolonged increases in IOP and pupil size. Aqueous humor NE and cAMP concentrations increased, while NPY and protein concentrations did not change. When the stimulations were set at 5 Hz for 3 hr under this anesthesia, the increase of IOP and mydriasis persisted. However, only the NE concentration increased. In conscious rabbits, stimulations of 5 V and 1 ms at either 5 Hz or 20 Hz were delivered from a portable stimulator for 4 hr, starting 2 hr before the onset of the dark. Stimulations at 5 Hz caused an increase in IOP in the light phase. The circadian IOP elevation in the dark phase persisted. When 20 Hz was used, a transient fall in IOP was observed, and the circadian IOP elevation was eliminated. Aqueous humor NE concentration doubled in conscious rabbits receiving electrical stimulations at 5 Hz for 1 hr.

A moderate electrical stimulation of the cervical sympathetic nerves can increase IOP in anesthetized rabbits and in conscious rabbits in the light phase.