Chronic stimulation of ocular sympathetic fibers in unanesthetized rabbits

The goal of this study was to devise a technique to implant permanent electrodes in the cervical sympathetic trunk, to stimulate the ocular adrenergic fibers for periods of hours or days in awake, unrestrained rabbits. Electrodes were made of a silver wire soldered to a multistranded wire and enclosed in silicone. Two of these electrodes were wrapped around the preganglionic sympathetic nerve, their leads emerging through a hole in the back of the neck. Success of the procedure was confirmed by the mydriasis elicited by electrical stimulation of the nerve following surgery; threshold voltages for the pupillary response varied between 5-10 volts. In eight rabbits, suprathreshold sympathetic stimulation was performed on the following days by means of a portable stimulator using increasing frequencies (1, 3, 5, 8, and 10 Hz) during a 20-hr period. Dilation of the ipsilateral pupil and vasoconstriction in the ear, measured by the fall in temperature of the ear's surface, was observed as long as stimulation was maintained. Both effects were proportional to the frequency of stimulation. Maximal mydriasis was obtained at 8 Hz, whereas full vasoconstriction was elicited with 5 Hz. Intraocular pressure, measured in 10 rabbits with a Perkins tonometer at the end of a 24-hr stimulation period, did not differ from pre-stimulation values. It was concluded that chronic stimulation of the sympathetic nerve allows to maintain known levels of adrenergic activity in the eye, and may be a useful method to study the actions of the adrenergic system on various ocular functions in unanesthetized animals.