Altered thermal responsiveness during regeneration of corneal cold fibers

1. To date, there has been no quantitative, systematic, study of the electrophysiology of regenerating cold receptors. This study, therefore, examines the changes in cold-receptor neural activity following a circular wound (5 mm dia, 200 microns deep) on the surface of the rabbit cornea. This is a well studied wounding model, in which neural regeneration has been anatomically quantified. 2. Extracellular recordings were obtained from a total of 90 single cold fibers, at 1, 3, 10, 20, or 30 days following wounding. The adapting temperature was 35 degrees C in all experiments. Thermal sensitivity for each fiber was determined by using a series of temperature steps, 0.2 degree C ranging from 35 to 34 degrees C, and 2 degrees C steps ranging from 35 to 24 degrees C. The rate of temperature change ranged from 0.2 to 1 degree C/s. 3. At the adapting temperature, the tonic activity of the regenerating cold-fibers was not significantly different from normals. Conduction velocities for regenerating cold-fibers were slower on day 1 postwounding compared with normal fibers, 0.59 +/- 0.04 and 0.75 +/- 0.04 (SE) M/s, respectively, however, were within the normal range by day 30 postwounding, 0.72 +/- 0.06 M/s. 4. On day 1, sprouting fibers showed decreased responsiveness to cooling (P < 0.05). At days 3 and 10 postwounding, action potential rates in response to cooling were enhanced by 180-200% of normal (P < 0.05) and returned to preinjury values by 20 to 30 days postwounding.(ABSTRACT TRUNCATED AT 250 WORDS)