The brain structure that makes mothers special

We know intuitively that women are more likely than men to pursue child care. This social task division is attributed to sexual dimorphism in the brain. The hormone difference between women and men not only forges different sexual organs, but also shapes different brain. Due to the difficulties to study human behaviors, our knowledge of sexual dimorphism is mainly based on animal experiments. In a recent Nature paper, the investigators discovered a new neural circuit that control the maternal care in mice.

Dopamine is a neurotransmitter playing essential roles in controlling voluntary movements in midbrain, short of which cause Parkinson disease; it also contributes to many behavioral processes, including mother-pup interaction. In an anatomical structure called anteroventral periventricular nucleus (AVPV) in the hypothalamus, a brain area critical in coordinating sexual dimorphism, we know that there are more dopaminergic neurons in females than males. Thus the investigators of the paper raised their hypothesis — the difference of dopaminergic neurons in AVPV of males and females may cause the sex differences in parental care.

First, they confirmed the double numbers difference of dopaminergic neurons in AVPV between male and female mice, also with more AVPV dopaminergic neurons in parental females than virgin females. Then they pharmacologically destroyed these neurons or genetically overexpressed dopamine in these neurons, or selectively activated these neurons, after which they recorded the parental behavior changes of these mice, including latency to retrieve pups and parental duration in both male and female mice, also aggressive behaviors in male mice. As expected, the ablation of AVPV dopaminergic neurons increased the pup-retrieval latency, and decreased the maternal duration, while overexpressing dopamine or activating these neurons did the opposite in female but not male mice. Unexpectedly, they found the ablation of AVPV dopaminergic neurons can increase male aggressiveness to pups, while overexpressing dopamine or activating these neurons can reduce the aggressiveness.

Next, to connect the dopaminergic neuron difference with more direct functional difference in parental behavior, they tested several possible hormones involved, including oestradiol, corticosterone, prolactin, and oxytocin. Oxytocin is the only hormone that was reduced after AVPV dopaminergic neuron ablation, which let the investigators to make their second hypothesis that AVPV dopaminergic neurons control oxytocin secretion in oxytocin-secreting neurons in paraventricular nucleus (PVN) or supraoptic nucleus (SON). By using chemical tracer and electrophysiological recording, they proved that AVPV dopaminergic neurons projected to PVN oxytocin-secreting neurons, which completes the circuit for maternal care.

These experiments used classical strategy to study anatomical and functional neural circuits, with modern molecular techniques. It could be furthered if the investigators can clarify the dopaminergic control of aggressiveness in male mice during parenting, maybe through another nucleus in amygdala.