AGE-RELATED CHANGES IN MORPHOLOGICAL AND FUNCTIONAL CHARACTERISTICS OF CORNEAL NERVES AND WOUND HEALING RATE OF GUINEA PIGS.

Abstract

Purpose: To study the corneal nerve morphology, electrical nerve activity and epithelial wound healing in young and adult guinea pigs.

Methods: Young (1-3 months) and adult (7-9 months) guinea pigs of both sexes were used in compliance with the ARVO Statement on the Use of Animals. Nerve morphology: Dissected eyes were fixed and cryoprotected. Whole-mount corneas were stained with neuronal class III β-tubulin antibody. Nerve density was calculated from camera lucida drawings and confocal microscopy images. Wound healing: 2mm-diameter corneal epithelium debridations were performed with n-heptanol. Fluorescein-stained lesions were photographed regularly until complete closure. Images were analyzed with image processing software, and epithelial migration rate (EMR) and estimated time of healing (ETH) were calculated. Nerve recording: Whole eyes or isolated corneas were superfused with physiological saline at 34°C. Electrical activity was recorded using conventional electrophysiological equipment. The response to thermal (changing the temperature of the solution down to 20°C or up to 50°C), mechanical (calibrated von Frey hairs) and chemical stimulation (30s gas jets of CO2 applied to the corneal surface) was explored. The characteristics of the spontaneous and stimulus-evoked activity were analyzed. Basal tear rate was also measured in young and adult guinea pigs using phenol red threads.

Results: Density of subbasal nerves was significantly lower in adult animals than in young animals, while their length increased significantly. Also subbasal nerves leashes appeared less branched and the number of epithelial nerve terminals was lower in adult guinea pigs. EMR was slower and ETH was significantly increased in adult animals compared to young. However, no significant differences were observed between spontaneous or stimulus-evoked activity of the different types of corneal sensory receptors recorded from young and adult animals. Basal tear secretion was similar in young and adult animals.

Conclusions: The corneal nerve architecture changes with time in the guinea pig, exhibiting a reduction in the subbasal and epithelial nerve density, which might lead to a neurotrophic slowdown of epithelial wound healing in adult animals. In spite of that, the characteristics of the spontaneous and stimulus-evoked activity of corneal nerve activity are similar in young and adult animals.

Keywords: 565 innervation: sensation • 486 cornea: tears/tear film/dry eye • 413 aging