...Unlike with cats and rats, zapping a male mouse's hypothalamus with electricity fails to make it more bellicose. To understand which other areas might be implicated in violent behaviour, Lin and Anderson's team exposed male mice to consecutive encounters with other intruding male and female mice. They then examined the brain areas activated by the encounters by labelling brain cells with a fluorescent tag that can distinguish recently active neurons. Surprisingly, neurons within a region called the ventromedial hypothalamus (VMH) snapped into action during fights — but also during sex.
Perplexed, the team implanted male mice with electrodes capable of measuring single cells in this area of the brain and watched what happened when mice fought or mated. Most of the neurons fired specifically during sex or bouts of violence, but a handful fired during both of these seemingly opposing behaviours.
...Switching on these neurons also drove males to attack females (see video) — but only up to a point. When males first encountered a female, activating the neurons sent them into attack mode. However, if sex had already ensued, the researchers could not elicit the mice to attack. "It's kind of in its own world. It doesn't listen to anything else," Lin says. However, activating the aggression circuit post-coitus provoked a swift attack on the female.
Quieting the aggression centre also stopped mice from acting on violent urges. Animals expressing a gene in these cells that silences them didn't attack intruding males, though their sexual appetites remained.
...Perhaps, Anderson says, the brain pathway his team identified could malfunction in some violent sex offenders. "Maybe in those individuals there's some sort of miswiring in these circuits in the brain, so the violent impulses and sexual impulses are not properly segregated from each other.
Nature News
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