Maternal aggression driven by the transient mobilisation of a dormant hormone-sensitive circuit.

Aggression is a sexually dimorphic behaviour. In some species, including the laboratory mouse, it is robustly expressed in males - while females are not aggressive in the non-puerperal state. However, during nursing, females exhibit maternal aggression, a dramatic yet transient shift in their social behaviour repertoire. This phenotypic change occurring in adulthood presents an opportunity to investigate whether sex-biased behavioural programs depend on mono- or di-morphic neural circuits. While maternal hormones are known to elicit nursing, their role in maternal aggression, particularly regarding target sites and cellular mechanisms, remains unclear. Here, we show that a molecularly defined subset of mouse ventral premammillary (PMv) neurons - with an established role in intermale aggression- transitions from quiescence to a hyperexcitable state during female lactation. The maternal hormones, prolactin and oxytocin, were found to excite these cells through pre- and post-synaptic electrophysiological actions. Gain- and loss-of-function experiments related to PMv neuron activity bidirectionally influence maternal aggression, while PMv neuron activation suppressed the expression of a competing social behaviour. This study identifies a sexually monomorphic neural substrate in mice capable of integrating hormonal cues, providing a likely mechanism that enables the transient access to a dormant behavioural program.