Influence of the estrus cycle on the evaluation of a vaginal irritation study in intact and ovariectomized rats

When conducting vaginal irritation studies, ovariectomized rats or rabbits are typically used according to practical reports. In the present study, we evaluated the influence of the estrus cycle in a vaginal irritation study using intact rats and ovariectomized rats, which exhibit a late diestrus-like condition, to determine whether intact rats can be useful for evaluating vaginal irritancy. Rats were divided into 4 groups: proestrus, estrus, and metestrus or diestrus in intact rats and ovariectomized rats. All the rats in each group were treated with a vehicle or sodium dodecyl sulfate, as the irritant, in single-dose and 4-day repeat-dose vaginal irritation studies. Each rat’s vagina was examined histopathologically, and the irritation score was calculated using a semiquantitative scoring system. In the single-dose study, the irritation scores for the proestrus or ovariectomized groups treated with sodium dodecyl sulfate were higher than those of the estrus group or metestrus or diestrus group. In the 4-day repeat-dose study, a significant histopathological difference was not found among the intact rats (proestrus, estrus, and metestrus or diestrus groups), and the irritation score range of the intact rats was similar to that of the ovariectomized rats, though the mean score of the intact rats was slightly lower than that of the ovariectomized rats. These results suggest that intact rats might be well suited for 4-day vaginal irritation studies and useful for evaluating vaginal irritancy using not only the mean score, but also individual irritation score ranges, whereas the estrus cycle would need to be identified in single-dose vaginal irritation studies.