Abstract
Prolactin activity in the medial preoptic area (MPOA) of the hypothalamus has been shown to be essential for the display of maternal behaviours in postpartum female mice. Neuronal outputs from the MPOA that express the prolactin receptor (Prlr) have also been observed to project to a number of prolactin-responsive brain regions, suggestive of a complex, prolactin-sensitive neural circuit responsible for controlling maternal behaviour. However, the exact mechanism by which prolactin acts on this maternal neural circuit remains unclear. Here, we hypothesised that prolactin stimulates maternal behaviour by causing neurons in the MPOA to become acutely activated in response to pup interactions. This was evaluated by use of virgin and lactating female mice that expressed the red fluorescent marker, td tomato, in all cells that express the Prlr. We characterized the expression of cFos, a marker of neuronal activation, colocalised with td tomato two hours after exposure to pups. We found that in the virgin mice, pup exposure lead to significantly greater cFos expression in the MPOA in comparison to the unexposed controls. Of these activated cells, only ~15% were Prlr-positive. In the lactating mice, levels of cFos in the MPOA were high regardless of whether they had been exposed to pups or not. Similarly to our observations in virgin mice, only ~25% of cFos-expressing cells were also Prlr-expressing in the lactating mice. These data suggested that prolactin's role in the MPOA to modulate maternal behaviour does not appear to be caused by acute changes in cellular activity. cFos expression was visually assessed in the rostral preoptic area (rPOA) and arcuate nucleus (ARN), areas that are known contribute to the prolactin-sensitive maternal neural circuit. We observed that in the rPOA of lactating mice, cFos levels did not seem to differ between pup-exposed and non-exposed mice. However, an increase in cFos expression was observed in the ARN, as well as the lateral hypothalamic area (LHA), suggesting that these brain regions are regulated by acute exposure to pups and therefore may be involved in maternal behaviour.
Previous findings from the Grattan lab have shown that a population of prolactin-responsive neurons in the MPOA project to the ventral tegmental area (VTA) of the midbrain, a region with a well-established role in reward and motivational behaviours. This finding was suggestive of a potential mechanism by which prolactin could stimulate maternal behaviour, perhaps acting through the VTA to enhance the rewarding nature of pup exposure, thus reinforcing the mother's motivation to care for her young. Hence, we proposed our second hypothesis that prolactin action in the MPOA drives maternal behaviour by acting through the VTA and subsequent reward neural circuitry. To test this hypothesis, we aimed to establish behavioural tests that could be used to assess reward and motivational behaviours in female mice in response to pups. The conditioned place preference (CPP) test was used to examine whether pregnant and virgin mice prefer a chamber associated with foster pups. A barrier testing procedure was also used to investigate the motivation of lactating and non-pregnant mice to interact with pups. No significant differences were observed between maternal and control animals in either of these behavioural tests, indicating that further refinement of these tests is required to effectively investigate prolactin's role in reward aspects of maternal behaviour.