The peripartum period includes numerous neurochemical changes in the female mammalian brain that alter their behavior.  During this time, females switch from aversion to attraction to pups.  However, the neuronal and hormonal mechanisms contributing to this behavior are not entirely clear.  Serotonin pathways play a role in maternal behavior, but its molecular regulation of these behaviors is unknown.  FosB proteins, which are markers of chronic stimulation, act as a molecular switch and may regulate serotonin mechanisms involved in maternal behavior.  We are currently investigating the effects of maternal state on protein expression of FosB, and two of its isoforms, ΔFosB and Δ2Δ FosB, in dorsal raphe, medial preoptic area, bed nucleus of the stria terminalis, and nucleus accumbens.  To do so, we first examined the brains of diestrus virgins or postpartum day (PPD) 7 females.  To further investigate the effects of maternal experience, we will soon measure FosB protein expression in PPD 7 females that had pups removed on the day of parturition and PPD 7 females that remained with pups until sacrifice.  Lastly, we will quantify FosB protein expression in virgin females that were repeatedly exposed to pups in a sensitization paradigm where some females showed maternal behaviors, while others did not.  We hypothesize that differential ΔFosB protein expression may be part of the “molecular switch” from aversion to pups to attraction to pups in postpartum females with maternal experience and sensitized virgins.  Specifically, if ΔFosB is relevant for maternal behavior, it may increase early in the postpartum period compared to virgins.  Similar to what has been shown for previous work on exposure to drugs and natural rewards, FosB proteins may be also important components of the molecular mechanisms involved in the onset of maternal behavior.