Figure 1.

Brain regions implicated in sexually dimorphic social behaviours. A shared set of brain regions has been implicated in the control of sexually dimorphic social behaviours in females and males, raising the issue of how neurons in these areas control distinct behaviours in the two sexes. A related issue is that the molecular identity of neurons in these functionally and molecularly heterogeneous brain regions is largely unknown. Recent molecular genetic studies have now identified discrete subsets of neurons, representing 10–20% of neurons in these regions, that control one or a few sexually dimorphic social behaviours but not other behaviours regulated by that entire brain region. Thus, the control of complex social behaviours appears to be modularly organized at the level of molecularly defined neurons. Another general principle emerging from these studies is that these neurons are functionally bivalent in the sense that they control distinct behaviours in both sexes. Such sexually dimorphic function is likely to be regulated by sex differences in gene expression or connectivity. Indeed, both the MeA and VMH express genes in a sexually dimorphic manner (e.g. [10,36,37]), and a recent study showed that VMH neurons regulating social behaviours project in a sex-typical manner [37]. AOB, accessory olfactory bulb; CoA, cortical amygdala encompassing both the posterolateral and posteromedial components that receive inputs from the MOB and AOB, respectively; MOB, main olfactory bulb. *, aromatase-expressing neurons in the posterodorsal MeA (MeApd) control maternal aggression and intermale aggression; ^†, progesterone receptor (PR)-expressing neurons in the ventrolateral subdivision of VMH (VMHvl) control mating behaviour in both sexes and aggression in males. (Online version in colour.)