Transient and persistent behavioral and molecular changes in primiparous female Wistar rats

Motherhood brings about a multitude of behavioral and physiological changes in dams and some of these persist until after weaning. We studied behavioral changes associated with reproductive experience at lactating day (LD)8, at weaning (LD21), and 28 days post-weaning (PW28) compared to nulliparous (NP) females. Furthermore, in another cohort of animals, we quantified mRNA expression of five target genes known to be associated with maternal experience: arginin-vasopressin(Avp) and its 1A receptor(Avpr1a), oxytocin(Oxt) and its receptor(Oxtr), and corticotropin-releasing hormone(Crh) in three key maternal region: the medial preoptic area (MPOA), bed nucleus of the stria terminalis (BNST) and paraventricular hypothalamic nucleus(PVN). Although dams were slightly less anxious than NP at LD8, this effect did not persist at LD21 and PW28. No differences in social preference were found between the four groups. In the maternal responsiveness test (MRT), LD8 and LD21 dams were immediately responsive to pups whereas NP largely avoided the pups throughout 12-day period. PW28 females were significantly more responsive to pups than NP females, but less than LD8 and LD21 females. The mRNA expression of Avp in the PVN, Avpr1a in the BNST and Oxtr in the MPOA and BNST was increased, whereas mRNA expression of Avpr1a was reduced in the PVN, at LD8 compared to NP. Although Oxtr in the BNST and Avp in the PVN were still somewhat (non-significantly) increased at LD21, all levels of gene expression had normalized at PW28. Our results emphasize the transient nature of these behavioral and molecular adaptations, except for a persistent up-regulation of maternal responsiveness.