Sex and violence linked in the brain
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By Ewen Callaway
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, published February 9 in Nature, 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.
The cells lie within an area of the hypothalamus with known links to violent behavior. An electrical jolt to this vicinity causes cats and rats to turn violent, but neurophysiological experiments conducted decades ago stimulated too big an area to identify the specific brain circuits, let alone the individual neurons, involved in aggression.
More recently, scientists studying mice engineered to lack specific genes have found that some of them act more aggressively than normal mice. "We really don't know which part of the brain went wrong in those mice. Consequently it's tough to make sense of that behavior," 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.
Snapping into action
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 behavior, 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 labeling 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 behaviors.
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 fiber 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 center also stopped mice from acting on violent urges. Animals expressing a gene in these cells that silences them didn't attack intruding males, although their sexual appetites remained.
Entangled circuits
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. The neurons activated by sex, they suggest, suppress the urge to lash out against an unknown female.
"There is a need to protect their own territories against a male invader and a need to have sex with female invaders, and this is sort of built into the circuitry of the brain," says Clifford Saper, a neuroscientist at Harvard Medical School. "It's the way animals protect their territory so they have enough room to feed their progeny."
The same circuits probably exist in human, too, says Newton Canteras, a neuroscientist at the University of São Paulo in Brazil. Deep-brain electrical stimulation has linked the VMH to defensive behavior such as panic attacks, and the region is likely to be involved in aggression as well, he says.
"I think there's every reason to think that this would be true in humans," says Anderson. The hypothalamus is one of the brain's oldest structures, and the region is also linked to aggression in monkeys.
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."
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8 Comments
Add CommentJanes Addiction told us this years ago.
Reply | Report Abuse | Link to thisThis study seems to come to an erroneous conclusion. The cells for sex and violence are not the same. If the cells responsible for these violent outbursts were inhibited, it should induce mating. Instead, the only thing this study shows is that mice who have these cell activated during sex do not respond violently. This study seems to indicate that there are other sex cells that send chemical or electrical signals to override these violent cells, just as they probably override other needs.
Reply | Report Abuse | Link to thisFrom the article:
Reply | Report Abuse | Link to this"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 behaviors."
What the author is saying here is that some of the cells fire during sex and some of them (different) fire during fighting. A few others fire for both sex and fighting.
I don't see a problem here.
This is no surprise to me, since I never could tell whether the cats outside my window were having sex, fighting, or both...
Reply | Report Abuse | Link to thisKonrad Lorentz was one of the first to recognize the intimate relationship between sex and aggression. His research illuminated the processes that an organism must adopt to balance the need for self preservation and the need to procreate. Similarly Russian breeding experiments with foxes have shown that the domestication process of reducing aggression tends to emphasize infantile (non-breeding) traits.
Reply | Report Abuse | Link to thisFreud!
Reply | Report Abuse | Link to thisThe inference that such circuits exist and function in the same way in humans is not necessarily correct. I am not denying the connection between sex and aggression but there has been plenty of time for humans to have evolved more sophisticated versions of these type of circuits.
Reply | Report Abuse | Link to thisThe response could then be mediated by a variety of other stimuli producing a wider range of behaviours. I think this is closer to what we actually observe.
Blindboy, why do you think that "there has been plenty of time for humans to have evolved more sophisticated versions of these type of circuits" but not time enough for mice to have done the same?
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