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 and of stimuli derived from a litter of pups and its mother, in the acquisition of maternal motivation. Experimental design included female mice in three conditions: lactating dams, comothers (virgins housed and sharing pup care with dams) and pup-naïve virgins. Females underwent three 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 sensitisation 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 sensitisation. Prolactin responsiveness was analysed in these animals using immunohistochemical detection of pSTAT5 (prolactin-derived signalling marker). As expected, dams showed significantly higher pSTAT5 expression in most of the analysed nuclei. Moreover, comothers displayed significantly higher prolactin 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 sensitisation 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.


中文翻译：

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母亲的动机：探索催乳素和小狗刺激的作用。

做母亲需要增加对幼崽的动力，这会成为强大的强化物并指导母亲的行为。这取决于在怀孕和产后作用于女性大脑的类固醇和催乳素。由于暴露于幼崽的处女雌性小鼠几乎是自发的母体，内分泌和幼崽衍生信号在诱导母体动机中的具体作用仍不清楚。这项工作使用幼犬检索范式的一种新变体，即动机幼犬检索测试，研究了大坝和处女雌性小鼠的母性动机。我们还分析了催乳素和来自一窝幼崽及其母亲的刺激在获得母性动机中的作用。实验设计包括三种条件下的雌性小鼠：泌乳母鼠、comothers（处女被安置并与水坝分享幼犬护理）和幼犬天真的处女。雌性进行了三项有动机的幼犬检索试验，幼犬被转移到 10 厘米高的金属丝网屏障后面。取回的大坝的延迟明显低于同母或处女，这表明只有在怀孕后才会出现完全的母性动机。虽然最初的 comothers 和处女没有表现出恢复，但在整个实验过程中，comothers 显着改善，表明诱导致敏过程。延长母猪与二胎幼崽的接触时间（从测试开始时的 2 天到 5 天）对母体敏感性没有强烈影响。使用 pSTAT5（催乳素衍生信号标志物）的免疫组织化学检测在这些动物中分析催乳素反应性。正如预期的那样，大坝在大多数分析的细胞核中显示出显着更高的 pSTAT5 表达。此外，即使它们表现出相似的循环 PRL 水平，显着低于水坝，它们在视前内侧核中的催乳素反应性也显着高于幼崽的处女。鉴于该核在传递和整合幼犬衍生刺激以促进母体主动反应中的重要作用，这种 PRL 反应性的增加可能反映了这项工作中报告的母体致敏过程的潜在机制。由于大脑中母体动机和 PRL 信号的分析是在相同的动物中进行的，因此我们能够探索两组数据之间的相关性。