Maternal Motivation: Exploring the Roles of Prolactin and Pup Stimuli

Motherhood entails increased motivation for pups, which become strong reinforcers and guide maternal behaviours. This depends on steroids and lactogens acting on the brain of females during pregnancy and postpartum. Since virgin female mice exposed to pups are nearly spontaneously maternal, the specific roles of endocrine and pup-derived signals in the induction of maternal motivation remain unclear. This work investigates maternal motivation in dams and virgin female mice, using a novel variant of the pup retrieval paradigm, the motivated pup retrieval test. We also analyse the role of prolactin (PRL) and of stimuli derived from a litter of pups and its mother, in the acquisition of maternal motivation. Experimental design included female mice in 3 conditions: lactating dams, comothers (virgins housed and sharing pup care with dams) and pup-naïve virgins. Females underwent 3 motivated-pup-retrieval trials, with pups displaced behind a 10-cm-high wire-mesh barrier. Dams retrieved with significantly lower latencies than comothers or virgins, indicating that full maternal motivation appears only after pregnancy. Although initially comothers and virgins showed no retrieval, comothers significantly improved throughout the experiment, suggesting an induced sensitization process. Lengthening exposure of comothers to the dyad pups-dam (from 2 to 5 days at the beginning of testing) had no strong effects on maternal sensitization. PRL responsiveness was analysed in these animals using immunohistochemical detection of phosphorylated signal transducer and activator of transcription 5 (pSTAT5, PRL-derived signalling marker). As expected, dams showed significantly higher pSTAT5 expression in most of the analysed nuclei. Moreover, comothers displayed significantly higher PRL responsiveness than pup-naïve virgins in the medial preoptic nucleus, even if they display similar circulating PRL levels, which are significantly lower than those of dams. Given the instrumental role of this nucleus in the relay and integration of pup-derived stimuli to facilitate proactive maternal responses, this increase in PRL responsiveness likely reflects the mechanism underlying the maternal sensitization process reported in this work. Since the analyses of maternal motivation and PRL signalling in the brain were performed in the same animals, we were able to explore correlation between both set of data. The results shed light on the neuroendocrine mechanisms underlying maternal motivation and other aspects of maternal behaviour.