Hormonal interaction with stimulus and situational factors in the initiation of maternal behavior in nonpregnant rats.

Previous hormonal studies have identified the hormonal and stimulus factors mediating the initiation of maternal behavior but have failed to reduce hormone-induced latencies of nonpregnant females to less than 1-2 days of continuous pup exposure. For the purpose of testing whether this delay is due to an olfactory-vomeronasal-mediated aversive reaction to pups like that found in untreated virgins, estrogen-injected hysterectomized-ovariectomized (HO-EB) nonpregnant females were subjected to olfactory-vomeronasal deafferentation. Median latencies were reduced to 0 and 1.5 hr compared with 72 hr for HO-EB nondeafferented females, which indicates that normally after HO-EB treatment, tendencies to initially avoid pup contact remain strong. Next, the hypothesis was explored that experiences during late pregnancy and/or parturition interact with hormonal priming to modify pup avoidance. Nonpregnant HO-EB females that had been exposed to pregnant-parturient females for 2 wk were tested under conditions simulating parturition. A high percentage rapidly initiated maternal behavior, but conditions during testing proved more important than prior exposure to pregnancy/parturition. The contributions of three stimulus/situation factors were analyzed: pup age--newborn versus 3-8 day; method of pup presentation--four at once versus staggered introduction; and time of day--lights versus dark phase. Hormonally treated but not sham-treated females initiated maternal behavior most rapidly when first exposed to one newborn in the nest during the light phase (76% within 1.5 hr). Prepartum caesarean-delivered females, however, responded maternally to four 3-8-day-old pups outside the nest, which indicates that additional factors operate at parturition.