A chronic model for evaluating the itching associated with allergic conjunctivitis in rats.

The present study was performed to develop a new model for evaluating itching associated with allergic conjunctivitis in rats. Repeated topical application of antigen caused an increase in eye scratching behavior in sensitized animals, and a significant difference was observed from days 21 to 42. Almost the same findings were observed in allergic symptoms, hyperemia and edema. Instillation of histamine also resulted in an increase in eye scratching behavior. The sensitivity to histamine in eye scratching behavior was increased by topical antigen application for 42 days after sensitization. In addition, the number of conjunctival eosinophils was significantly increased by repeated topical antigen application from days 21 to 42 in sensitized rats. Some anti-allergic drugs such as olopatadine (H1 antagonist), cetiridine (H1 antagonist) and ramatroban (thromboxane A2 (TXA2) antagonist) caused an inhibition of eye scratching behavior induced by topical sensitization in a dose-related manner. However, zafirlukast (cys-LT antagonist) caused no significant inhibition even at a dose of 30 mg/kg. The findings in present model of itching in allergic conjunctivitis were mainly through histamine H1-activity, and thromboxane A2 receptors were also involved in the response.