Disruption of adult olfactory neurogenesis induces deficits in maternal behavior in sheep

Profound behavioral changes occur in the mother at parturition, a time when the maternal brain undergoes extensive remodeling of neural circuits, especially in olfactory structures. Adult neurogenesis, a form of brain plasticity, could constitute an adaptive response to motherhood. The present study hypothesized that chemical disruption of olfactory neurogenesis would impair the establishment of maternal behavior in sheep. In addition, because ewes are able to learn the olfactory signature of their offspring, we also examined whether disruption of olfactory neurogenesis altered recognition of the familiar lamb. At one month of gestation, ewes received either infusion of the antimitotic drug Ara-C or saline into the lateral ventricles via one-month-long osmotic minipumps. Ara-C infusion dramatically decreased olfactory neurogenesis but spared hippocampal neurogenesis. Mothers exhibiting more than a 70% reduction in olfactory neurogenesis emitted fewer maternal bleats during the first hours after parturition. Reduction of olfactory neurogenesis also negatively affected discrimination of the familiar lamb. Differences in ewes' aggressive behavior toward familiar and alien lambs were observed in sham mothers, but not in mothers with reduced olfactory neurogenesis. In addition, when ewes were given the choice between familiar and unfamiliar anesthetized lambs, so that only olfactory cues were available, mothers with a reduction in neurogenesis greater than 70% were not able to discriminate their own lamb from an alien lamb. These results indicate that adult-born olfactory neurons are to some extent involved in the establishment of maternal behavior in sheep by contributing to the processing of offspring odors.