Author(s): Zagon IS, Campbell AM, Sassani JW, McLaughlin PJ
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Abstract PURPOSE: To elucidate the factors in tear production, this study examined the role of endogenous opioids and opioid receptors in spontaneous episodic reduced tear volume. METHODS: A model of spontaneous episodic decreases in the quantity of tears was characterized in otherwise normal Sprague-Dawley rats using Schirmer's test. A single eye drop of 10(-5) M naltrexone (NTX), 10(-5) M [Met(5)]-enkephalin, or sterile vehicle was administered to one eye. Tear secretion, corneal sensitivity, and corneal morphology were examined in both eyes. RESULTS: At any given time period, otherwise normal rats were found to have Schirmer test scores with a bimodal distribution (6.5 mm or less, or 7.0 mm or greater). Decreased tear production was detected in male and female rats aged 4 to 24 weeks at least once per animal. The episodes of reduced tear volume ranged from 1 to 7 days. No changes in corneal sensitivity or corneal morphology were observed in any rat. One drop of NTX given to rats with a decrease in tear volume raised levels of tears to scores of 7.0 mm or greater within 1 hour, and increased tear production persisted for at least 48 hours. NTX had no effect on rats with Schirmer scores of 7.0 mm or higher. Topical application of [Met(5)]-enkephalin depressed tear secretion from baseline scores of 9.8 ± 0.6 mm to as low as 4.5 ± 0.7 mm. CONCLUSIONS: Normal rats experience fluctuations in tear production that can be modulated by opioidergic signaling pathways.
This article was published in Invest Ophthalmol Vis Sci
and referenced in Biochemistry & Analytical Biochemistry