A single injection of 17β-estradiol at the time of pup presentation promotes the onset of maternal behavior in pregnancy-terminated rats

Female rats that are hysterectomized and ovariectomized on day 15 of pregnancy (15HO) and presented with pups 48 h later show maternal behavior after 2 or 3 days of pup exposure. In contrast, if 15HO females are administered (sc) 20 μg/kg of estradiol benzoate (EB) on day 15 of pregnancy after HO, they show near immediate maternal behavior when pups are presented 48 h later. EB has typically been administered on day 15 because of the underlying assumption that EB exerts genomic effects which require a long duration before being expressed in changes in neuronal phenotype. In light of the more recent evidence that estradiol can generate rapid changes in cellular function, we examined whether injection of a water-soluble form of 17β-estradiol (E2) can facilitate maternal behavior in pregnancy-terminated females when it is administered at the time of pup presentation rather than at the time of HO. Female rats treated with 100 μg/kg of E2 showed a robust facilitation of maternal behavior, requiring a median of 1 day of pup exposure before showing maternal behavior, compared with 3 days in vehicle-treated rats.