Preoptic inputs and mechanisms that regulate maternal responsiveness.

The preoptic area is a well-established centre for the control of maternal behaviour. An intact medial preoptic area (mPOA) is required for maternal responsiveness because lesion of the area abolishes maternal behaviours. Although hormonal changes in the peripartum period contribute to the initiation of maternal responsiveness, inputs from pups are required for its maintenance. Neurones are activated in different parts of the mPOA in response to pup exposure. In the present review, we summarise the potential inputs to the mPOA of rodent dams from the litter that can activate mPOA neurones. The roles of potential indirect effects through increased prolactin levels, as well as neuronal inputs to the preoptic area, are described. Recent results on the pathway mediating the effects of suckling to the mPOA suggest that neurones containing the neuropeptide tuberoinfundibular peptide of 39 residues in the posterior thalamus are candidates for conveying the suckling information to the mPOA. Although the molecular mechanism through which these inputs alter mPOA neurones to support the maintenance of maternal responding is not yet known, altered gene expression is a likely candidate. Here, we summarise gene expression changes in the mPOA that have been linked to maternal behaviour and explore the idea that chromatin remodelling during mother-infant interactions mediates the long-term alterations in gene expression that sustain maternal responding.

Oestrogen-independent, experience-induced maternal behaviour in female mice.

Nulliparous female mice that have not experienced mating, pregnancy or parturition show near immediate spontaneous maternal behaviour when presented with foster pups. The fact that virgin mice display spontaneous maternal behaviour indicates that the hormonal events of pregnancy and parturition are not necessary to produce a rapid onset of maternal behaviour in mice. However, it is not known how similar maternal behaviour is between virgin and lactating mice. In the present study, we show that naturally postpartum females are faster to retrieve pups and spend more time crouching over pups than spontaneously maternal virgin females, and that these differences diminish with increased maternal experience. Moreover, 4 days of experience with pups induced pup retrieval on a novel T-maze. Furthermore, the effects of experience on subsequent maternal responsiveness are not dependent on gonadal hormones because ovariectomised females with 4 days of pup experience show pup retrieval on a novel T-maze similar to that of postpartum mice. Four days of maternal experience also induced T-maze pup retrieval in ovariectomised aromatase knockout female mice that was not significantly different from the maternal responsiveness of ovariectomised wild-type littermates. These data suggest that maternal experience can induce maternal behaviour in females that have never been exposed to oestradiol at any time in development or adulthood. Finally, ovariectomised pup-experienced females continue to retrieve pups on a novel T-maze 1 month after the initial experience, suggesting that, even in the absence of oestradiol, maternal experience produces long-lasting modifications in maternal responsiveness.