Using Vaginal Impedance Measurement to Identify Proestrus in Rats Given Luteinizing Hormone Releasing Hormone (LHRH) Agonist

Vaginal cytology is the most common method of monitoring the estrous cycle in rats; however, this test requires specific technical training and can be subject to interpretation. Vaginal impedance offers a quicker and less technically challenging alternative and has been used successfully to identify estrus in normally cycling breeder rats. We hypothesize that vaginal impedance can also be used to stage the estrous cycle in rats that have been given luteinizing hormone releasing hormone (LHRH) for timed mating. Vaginal impedance measurements and vaginal cytology were performed in LHRH-primed female rats (n = 36) at the expected peak of proestrus and paired with proven stud males. Breeding success was determined by gross necropsy to detect embryo implantation sites in the female rats. We found that the predictive rates of vaginal cytology and impedance measurement for proestrus were similar; however, both methods resulted in high proportions of false positive and false negative determinations (28% and 31%, respectively). We further hypothesized that females respond to LHRH at variable rates, resulting in variable times of peak proestrus. To test this, vaginal impedance measurements were performed multiple times throughout the expected day of proestrus in LHRH-primed female rats (n = 36). Females were either paired with a male 24 h after reaching the proestrus threshold (n = 18) or paired according to our standard protocol at 1300 h on the day after the expected proestrus (n = 18). Sequential measurements reduced false positive and negative rates (14% and 8%, respectively). Pregnancy rates did not differ based on the time of pairing during expected estrus. Overall, we determined vaginal impedance can be more successful than vaginal cytology at identifying proestrus in the rat, but only if multiple measurements are taken.