Influence of intravaginal 4-vinylcyclohexene diepoxide on ovarian follicles and estrous cycle in rats

Background For the population control of stray dogs and cats, alternative methods to surgical castration are being studied and developed, such as chemosterilization. In this context, substances that lead to irreversible degeneration of the tissue administered directly to the gonads are being investigated, with most studies focusing on males due to the ease of access to the testicles. In females, ovarian degeneration is reported with the use of a chemical compound called 4-vinylcyclohexene diepoxide, a selective ovotoxic agent, used in experimental studies in rats to mimic menopause. However, it requires fifteen consecutive days of intraperitoneal application. Envisioning its use for the population control of dogs and cats, a less invasive and effective route is necessary. Thus, the aim of this study was to evaluate the effect of 4-vinylcyclohexene diepoxide on the ovaries and its chemosterilizing potential by intravaginal gel inoculation in rats. Twenty Wistar female rats were used, randomly divided into two groups (n = 10/group): control and treatment. The control group received 0.2 mL of chitosan-based gel, and the treatment group received the same gel containing 4-vinylcyclohexene diepoxide (160 mg/kg) for fifteen days. The rats were evaluated daily by vaginal cytology until euthanasia. The animals were euthanized at two time points (n = 5/group): Sixteen (M1) and thirty (M2) days after the start of treatment. The ovaries were collected, prepared, and evaluated by optical microscopy for counting of primordial and primary follicles. Results In the group treated with 4-vinylcyclohexene diepoxide, a prolongation of the diestrus phase (M1 and M2), as well as a prolongation of the estrus phase (M2), was noted after inoculations of the intravaginal gel. Even so, in the treated group, there was an increase in the number of estrous cycles compared to the control group (M1 and M2). In the follicular count, a higher median number of atretic primordial follicles was noted in the group treated in M2. Follicular evaluation in M1 and M2 revealed a higher proportion of healthy primordial follicles in the control group, as well as an increase in the proportion of atretic primordial follicles in the treated group. Conclusions It was possible to observe evidence of ovotoxicity of chitosan gel containing 4-vinylcyclohexene diepoxide, applied intravaginally, due to the increase in the population and proportion of atretic primordial follicles and interference in the estrous cycle.