Activation of scleral cold thermoreceptors by temperature and blood flow changes.

PURPOSE: To study the response of scleral cold receptors located in areas of the eye unexposed to temperature and blood flow changes.
METHODS: In anesthetized cats, the neural activity was recorded from single, cold-sensory fibers of the ciliary nerves innervating the sclera and limbus. Controlled temperature changes of the receptive field were performed with a contact thermode. Ocular blood flow reductions were obtained by occluding the ipsilateral common carotid artery for 30 to 60 seconds with a compressor placed around the artery. Local blood flow was measured with a laser Doppler flowmeter. Temperature was measured with a microprobe introduced in the subscleral space. Ocular sympathetic stimulation was performed with a pair of silver electrodes placed on the preganglionic cervical sympathetic trunk. To induce local hypoxia, N(2) was applied on the scleral surface with a specially designed chamber. For systemic hypoxia the breathing air was replaced with a gas mixture containing 10% O(2) in N(2).
RESULTS: Sensory nerve fibers identified as cold receptors exhibited ongoing nerve activity in bursts at 35 degrees C and responded to cooling pulses applied to their receptive fields with an increase in the impulse discharge that reached a peak and decayed gradually to a lower level. When temperature was reduced from 35 degrees C to 34 degrees C, frequency increased monotonically with decreasing temperature of the sclera. Between 35 degrees C and 30 degrees C, peak and mean frequencies were roughly proportional to temperature of the sclera. The characteristics of burst discharges also depended on scleral temperature. Electrical stimulation of the cervical sympathetic trunk induced a decrease in blood flow and temperature and evoked an increase in the firing frequency of cold-sensory fibers that was proportional to the frequency of stimulating pulses. Carotid occlusion also elicited an increase of the discharge of cold thermoreceptor fibers that occurred in parallel with a decrease in blood flow and temperature in the receptive field area. Local or systemic hypoxia did not modify appreciably the spontaneous firing frequency of scleral cold-sensory fibers.
CONCLUSIONS: Scleral and episcleral cold-sensory fibers encoded as a change in their impulse frequency and firing pattern temperature reductions of less than 1 degrees C in scleral tissues. Activation of scleral and episcleral cold-sensory fibers by sympathetic vasoconstriction and acute arterial pressure reductions appear to be secondary to the temperature decrease that accompanies the reduction in ocular blood flow caused by these maneuvers. Scleral thermosensory fibers are located in ocular territories not directly exposed to external temperature changes. Thus, the sensory information on local blood flow variations provided by these receptors may be involved in a reflex regulation of choroidal blood flow that functions to maintain a constant temperature and blood supply to the retina.