Center for aggression and sexual mounting found in male mice.

NATURE journal has a fine article by a Caltech research group about the most precise localization of the neuronal circuit for aggression (and sexual mounting) in male mice. Now, that’s important for human beings as well, because we happen to share that part of the brain with just about all other mammals. So the aggression/sexuality center in mice immediately leads to the hypothesis that we have that in the same place of our brains.

As you can see in the figure, humans and other mammals have two egg-shaped structures right at the center of the brain, called the thalamus (thalami is the plural). The thalamus is the great gateway to the cortex. Almost all of our sensory input channels — vision, audition, body touch — stop off in the thalamus before being sent to cortex.

Now below the thalamus is a beautiful structure called the “hypo-thalamus,” which simply means “below the thalamus.” (The old anatomists who labeled the brain didn’t have too much imagination.) And it’s the hypo-thalamus that’s responsible for all the trouble in the world: Aggression, sexuality, overeating, all the emotions and appetites that get us in trouble half the time. The hypothalamus has a lot of little nuclei that secrete hormones, connected with lots of neurons, so that it is a NEURO-HORMONAL structure. And right below, connected to the hypothalamus is the PITUITARY GLAND, the Master Gland of the body and brain. Together the hypothalamus and pituitary are the most powerful interface between neurons and hormones.

Emotions and appetites, like anger and sexuality, hunger and thirst, combine neuronal and hormonal features. Sexuality involves big changes to our bodies around puberty and afterwards, during pregnancy and lactation, and even when we see a sexually attractive person. Aggression does, too, and so do hunger and thirst.

Here is an picture of the hypothalamus with the pituitary suspended below.

Together these two small regions control hugely important emotional and appetitive functions.

Now – we know a lot about these two structures at the center of the brain, but we are ONLY BARELY learning about the neural circuits that run everything. The reason is that neural circuits are tiny, they are surrounded by other tissues, and they are connected to each other in ways that are really hard to trace.

Enter the new tool of optogenetics.

Optogenetics is the coolest new tool in the neuroscience toolbox. It’s amazing.

Wikipedia says, “Optogenetics (from Greek optos, meaning “visible”) uses light to control neurons which have been genetically sensitised to light. It is a neuromodulation technique employed in neuroscience that uses a combination of techniques from optics and genetics to control and monitor the activities of individual neurons in living tissue—even within freely-moving animals—and to precisely measure the effects of those manipulations in real-time.”

Here is an image of the aggression circuit in the male mouse brain, showing the light-triggered neurons glowing in response to optical input.

You have to understand that this level of control and observable detail has NEVER been available before. For decades neuroscientists have carefully stuck electrodes into the hypothalamus of animals, and tried to figure out what it does. We have learned a great deal — but never at the level of neuron-to-neuron circuits.

That’s the big news today.

Studies from the Caltech group now show that tiny neuronal circuits in the hypothalamus of male mice control aggression, so clearly that you can actually watch the neurons get active as one mouse approaches another one, ready to attack it or defend it. (Male mice often fight over access to females, when the females send out estrus pheromones to show they are ready.) Interestingly, the tiny aggression circuit in hypothalamus is connected to the sexual mounting circuit. Precisely how they interact is still not clear, but you can’t imagine that a successful male mouse can’t mixup aggression and sexual intercourse. So those two tiny circuits CANNOT be the same, but they can be related to each other. In one scenario a dominant male might chase away a competitor, and then get an opportunity to mount the female. (If she’s in the mood, needless to say).

Because dominance and sexual reproduction are SO CRUCIAL to Darwinian fitness (survival and reproduction), these hypthalamic circuits are likely to be very ancient and widespread among mammals and maybe other species. Because we now have some of the genomes, we could look for the genes that control the growth, development, and life adaptations of these tiny circuits. My bet is that we will find genetic links —- but we’ll know when we really know. Right now it’s a speculation.

Suppose we can identify the right neurons in the human brain for aggression and sexual behavior. What might be some of the ETHICAL applications??? What could be some of the possible MISUSES of those discoveries?

Like it or not, in ten years we will know much, much more…


This work by Bernard J Baars and the Society for Mind Brain Sciences is licensed under

a Creative Commons Attribution 4.0 International License

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