Maternal Behavior in Rabbits: Regulation by Hormonal and Sensory Factors

Maternal behavior in rabbits involves building a nest of straw and body hair inside an underground burrow, before parturition, and the display of a single, brief (ca. 3 min.), nursing bout per day throughout lactation. The onset and decline of digging, strawcarrying, and hair-pulling is controlled by specific combinations of estradiol, progesterone, testosterone, and prolactin (PRL). These hormones also prime the maternal brain to respond effectively to the newborn pups: antagonizing the release of PRL that occurs before parturition (by injecting bromocryptine s.c.) abolishes maternal behavior in most rabbits. This effect is counteracted by the intracerebroventricular administration of rabbit PRL. We have investigated the brain areas involved in regulating specific aspects of maternal behavior by: a) determining the distribution of receptors for estradiol (ER-alpha) and progesterone (PR); b) implanting estradiol in the forebrain and quantifying the display of nest-building; c) establishing the expression of an immediate early gene (c-fos) in association with the display of maternal behavior. These studies have revealed that specific forebrain regions (e.g., the preoptic area and septum) play a major role in the control of nest-building and maternal responsiveness: they are rich in ER-alpha and PR; implants of estradiol benzoate stimulate digging and straw-carrying in ovariectomized rabbits treated with progesterone s.c.; an increase in the number of c-fos immunoreactive neurons is observed following nursing. In addition to hormonal stimulation, the adequate display of maternal behavior relies heavily on the perception of stimuli coming from the young. By preventing mother/young contact at parturition or during early lactation maternal responsiveness is altered or abolished. Suckling stimulation per se is not essential to maintain maternal behavior as mothers deprived of nipples (i.e., thelectomized) are responsive to the young. However, the duration of the mother/young interaction is greatly extended in thelectomized females and also in mothers whose nipples have been covered with cushions. Moreover, decreasing the amount of suckling stimulation (by providing only one pup) or providing a litter that is unable to suckle (by placing adhesive tape over the mouths) also extends the time mothers spend with the young inside the nest box. These results show a major role of suckling stimulation in timing the duration of mother/young contact. In summary, the behavior of mother rabbits is regulated by a multiplicity of hormonal and somatosensory stimuli that act in concert to determine the onset, offset and duration of specific maternal activities.