Violence, Sex, and Mice

According to an article in Nature, a study shows that sex and violence are linked to one another deep within the hypothalamus of a mouse’s brain. This study showed that, when these specific neurons were shut off or silenced, these territorial animals paid no attention to intruding males they would have normally attacked. However, when these neurons were activated, the mice became overly aggressive and not only attacked other males but females and even inanimate objects. These neurons are located within the hypothalamus in an area known to be associated with aggressive and violent behavior. Similar studies to this involved shocking that area of the brain in cats and rats but these shocks stimulated an area too large to identify specific neurons involved in aggression. This study was able to narrow down the area of stimulation by using a different technique.

The first part of the study involved examining the areas of the brain that were activated when a male mouse encountered an intruding male and when he encountered a female mate. The scientists accomplished this by labeling brain cells with fluorescent markers capable of identifying recently active neurons. This experiment produced interesting results. An area known as the ventromedial hypothalamus, or VMH, was activated during both encounters. This left the researchers with more questions. To better understand what exactly was going on, they created another experiment. This experiment was aimed towards which specific cells, if any, fired during both encounters and which ones did not. This was accomplished by inserting electrodes capable of reading individual neuron firings into a male mouse’s brain. The results showed that the majority of neurons were specific to either the sex or the violence. Yet, there were some that fired during both of these seemingly different behaviors.

Now that the researchers discovered specifically which neurons fired during these actions, they designed another experiment that would allow them to control the firing of those neurons. Through a technique known as optogenetics, the researchers inserted a gene into these neurons that responds to blue-light. They then implanted an optic fiber into the brain that produced a blue light when activated. When the researchers turned on the blue light, the mice immediately became aggressive attacking males, females, and even a laboratory glove. The males always attacked the females when these neurons were switched on with one exception- while they were having sex. It did not matter how much the researchers stimulated that area, the mice were completely unresponsive to the activation. When the mice were finished, the male would attack the female again if the signal was on.

How does this research relate to humans? Scientists believe that these VMH neurons that control aggressive behavior are part of a brain circuit. This brain circuit controls their responses to male and female intruders of their territory. This can be seen through a mouse’s need to protect his territory and his need to reproduce. He is constantly protecting his territory in order to provide room for his offspring. Many scientist believe that this circuit system exists in humans as well. One study showed a link between defensive and aggressive behavior to the VMH in humans. Another revealed that VMH is also linked to aggression in monkeys. More research in this area could provide biological answers to why people commit heinous acts such as rape or murder.

Posted by Kevin McLaughlin (2)