Overnight circuit remodelling drives juvenile alloparental care

Parental care is critical for the survival of altricial young and is mediated by neural circuits that are well characterised in adult rodents. Although adults can exhibit caregiving even before becoming parents, the developmental origins of this so-called alloparental behaviour remain unclear. Here, we show that alloparental behaviour in mice emerges abruptly between postnatal day (P)14 and 15, independently of prior social experience. This behavioural transition coincides with the onset of pup-specific activity in galanin-expressing medial preoptic area (MPOAGal) neurons, which are essential for parental behaviour in adulthood. Chemogenetic silencing of MPOAGal neurons abolishes caregiving in juveniles, suggesting that similar circuits control parenting across life stages. Viral trans-synaptic tracing and whole-cell recordings reveal extensive input remodelling of MPOAGal neurons between P14 and 15, marking a rapid transition from a highly connected, immature network to a sparser, adult-like circuit configuration. We identify microglia as key mediators of this process, as their ablation prevents both synaptic reorganisation and the emergence of alloparenting. Together, these findings uncover a previously unrecognised, microglia-dependent developmental switch that enables caregiving in juveniles through rapid circuit reconfiguration.