Absence of mating behaviors in the female dogs neonatally treated with estrogen and progesterone

This study aimed to develop a non-surgical method to neutralize reproduction in female dogs. Female Beagle puppies, aged 6 days, were treated with pellets designed to release estradiol benzoate (EB; 1.0 mg) and progesterone (P4; 5.0 mg) over approximately 3 weeks. Their estrous cycles were monitored from 6 to 34 months of age by examining their vulvas daily and measuring their serum P4 levels once a month. Vulvar edema and discharge, followed by a serum P4 level above 5 ng/ml, indicated the potential estrus. Each time a dog showed these signs, breeding was attempted by housing with a proven male Beagle. All the treated dogs displayed cyclic progesterone surges with 5 to 6-month-long anestrous intervals. Surprisingly, none exhibited sexual behaviors, and no mating occurred (i.e., no intromission and copulatory tie), resulting in no pups being born. This phenomenon was further explored in laboratory animals. Neonatal female rats were treated with microspheres containing smaller doses of the same steroids (0.3 mg EB + 3.0 mg P4) at 1 or 2 days old. At 3 months old, the rats were ovariectomized, chemically stimulated to exhibit estrus behaviors using a standard protocol and tested for receptivity to proven male rats. Untreated control rats showed normal receptivity (i.e., lordosis) and allowed males to mate. However, rats treated with EB+P4 did not exhibit lordosis or allow mating. These results indicate that the combined use of estrogen and progesterone could be an effective non-surgical method for inhibiting mating behavior and, consequently, neutralizing female dog reproduction.