Mucous contribution to rat tear-film thickness measured with a microelectrode technique

Effects of Topical Mucolytic Agents on the Tears and Ocular Surface: A Plausible Animal Model of Mucin-Deficient Dry Eye

Purpose

A topical mucolytic agent, N-acetylcysteine (NAC), has been used to create an animal model without the intestinal mucus layer. In this study, we investigated the effects of topical NAC on the tears and ocular surface.

Methods

NAC-treated models were established by topically administering 10% NAC four times daily for 5 days in male Sprague-Dawley rats. Clinical parameters and the expression of mucin proteins and genes were evaluated. Alterations in the conjunctival epithelium and goblet cells were observed.

Results

The NAC group showed significant decreases in tear secretion, corneal wetting ability, tear MUC5AC concentration, and conjunctival goblet cell numbers as compared with the control group (all P < 0.01). In addition, significant increases in corneal fluorescein score and rose bengal scores were observed in the NAC group versus in the control group (P < 0.05 and P < 0.01, respectively). Hematoxylin and eosin (H&E) staining and scanning electron microscopy clearly showed damage in the epithelial cell layer and microvilli of the NAC group. Although there was no significant difference in MUC16 gene expression, the MUC16 concentration of the tear film and ocular surface tissue was significantly increased in the NAC group versus in the control group (P < 0.01 and P < 0.05, respectively). Five-day treatment with 3% diquafosol had minimal therapeutic effect in NAC-treated rat eyes.

Conclusions

Topical administration of 10% NAC induced ocular surface damage and tear film instability by prompting MUC16 disruption and release from the ocular surface. This animal model could be used to study dry eye disease, especially the mucin-deficiency subtype.

The thickness of the tear film

Measurements of the thickness of the pre-corneal tear film, pre-lens tear film, post-lens tear film, and the lipid layer on the surface of the tear film are summarized. Spatial and temporal variations in tear film thickness are described. Theoretical predictions of tear film thickness are discussed. Mechanisms involved in the upward drift of the tear film after a blink, and in the formation of dry spots, are considered.

The effect of N-acetylcysteine on the microscopic fluid dynamics of rat tears

BACKGROUND: The classical view of the tear film is of a 10-micron film of aqueous tears, sandwiched between thin layers of lipid and mucus. This has been challenged recently by the revelation that the tear film may be considerably thicker than 10 microns and that dissolved mucus and protein may play a much more important role than simply promoting tear adherence. In particular, the primary role of mucus may be to form a structured aqueous gel that adheres closely to the corneal surface and evens out its irregularities, thus providing a high-quality optical surface. METHODS: We have used the robust tear film of the rat eye as an animal model to investigate the contribution of mucus and low-molecular-weight (LMW) proteins to tear film structure. The ocular surface was first exposed to saline, which washed away the tear film. Single drops of a tear/saline mixture, treated with various concentrations of the thiol-reducing agent N-acetylcysteine (NAC), were placed on the ocular surface and the resulting fluid behaviour was recorded with video-microscopy. RESULTS: At five per cent concentration, NAC appeared to degrade the gap-filling and anti-evaporative qualities of the tears, features that give the rat tear film its robust characteristics. Lower concentrations had no significant effect. DISCUSSION: In a previous publication, we showed that five per cent NAC alters the profile of LMW proteins in rat tears. The present observations suggest that the robust wetting properties of rat tears depend critically on their mucus and/or LMW protein content and possibly are related to the formation of an aqueous/mucous gel.