Sex and aggression linked in the brain

LONDON - Scientists in US have found a link between aggressive behaviour and sexual arousal.

Violent behaviour in mice has been tracked to neurons within a part of the brain associated with sex.

Researchers discovered that sex and violence are intertwined in mice. A tiny patch of cells buried deep within a male’s brain determines whether it fights or mates, and there is good reason to believe humans possess a similar circuit.

The study shows that when these neurons are quieted, mice ignore intruding males they would otherwise attack.

Yet when the cells are activated, mice assault inanimate objects, and even females they ought to court.

“We really don’t know which part of the brain went wrong in those mice. Consequently it’s tough to make sense of that behaviour,” says Dayu Lin, a neuroscientist now at New York University and an author of the study, who began searching for the seat of aggression in mice while working with David Anderson at the California Institute of Technology in Pasadena.

As part of the study, the researchers 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.

The researchers next infected neurons in this region with a virus that inserts a gene that renders them responsive to blue light - a technique called optogenetics. With an optic fibre implanted into the brains of these mice, Lin and Anderson could fire these neurons on command.

When they did so, male mice wasted little time attacking other intruding males. Activating neurons in the aggression centre also provoked assaults on castrated males, whom males would usually ignore, as well as anesthetized animals and even an inflated laboratory glove.

Switching on these neurons also drove males to attack females - 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, at though their sexual appetites remained.

Lin and Anderson hypothesize that the entanglement of brain circuits involved in sex and violence could help mice to respond appropriately to intruders, whether male or female.

They suggest that the neurons activated by sex suppress the urge to lash out against an unknown female.

The study has been published in Nature today1. (ANI)