How do male and female brains differ?

How do male and female brains differ? Investigating fruit flies has given us an insight into the neural circuits that control courtship. Naked Scientist Hannah Critchlow spoke to Johnny Cole, a postgraduate student at the Medical Research Council Laboratory of Molecular Biology in Cambridge...

Johnny -   I'm interested in differences between the male and the female brain because if you look at animals, in most species, males and females behave quite differently. And now, our big challenge is to explain at the level of neurocircuits, so at the level of how individual nerve cells in the brain wire up into circuits to explain those differences at this level.

Hannah -   And what animal are you looking at?

Johnny -   We are studying the fruit fly brain just because the human brain is a lot, lot more complex than the fly brain.  So, fly brains have about a million times fewer nerve cells, so they are a lot easier to study than the incredibly complex human brain and also in flies, we have very powerful genetic tools to monitor, to activate or inactivate signal cells in the behaving fly.

Hannah -   And you mentioned that you're looking at behaviours between males and females in the fly.  What kind of behaviours are different?

Johnny -   So, we're looking at innate behaviours. So, those are behaviours that the flies don't have to learn, so they're just being born with the knowledge of how to perform this behaviour and specifically, we're looking at one type of behaviour, courtship behaviour. So, male flies, they will try to approach the female flies, so they start following the female fly around, then they'll try to sing a love song for the female fly so they tried to serenade the female fly, then they will try to lick and touch the female fly, and eventually, try to copulate with the female fly. And during all that time, female flies respond quite subtly to those courtship attempts by either becoming docile and they let the male do its thing or if they're not impressed by the male courtship attempts, they will just kick the male in the face, literally.

Hannah -   And what's controlling these different behaviours then?

Johnny -   So, that's specifically what we're trying to find out.  So, we know that there's one type of chemical signal, a so-called pheromone, it's called CVA.  It's controlling courtship behaviour.  This chemical signal leads to completely different behaviours in males and females. So, when males smell this pheromone CVA, they increase their aggressiveness and they're in general repelled by CVA whereas females increase their receptivity when they smell this chemical signal.  That's quite interesting because they have the same physical stimulus, CVA, that leads to completely different outcomes behaviourally in males and females, and that's what we're looking at.

Hannah -   And it's usually the males that emit this pheromone, this chemical CVA.

Johnny -   Exactly.

Hannah -   And so, what brain differences have you found between the males and the females that give rise to these different responses then?

Johnny -   So, for the last 3 years, I've been recording electrical activity from single nerve cells in the fly brain when we present this pheromone to the living fly. Of the many of those recordings, I've found that there's different responses to that pheromone in brain centres that process the smell. So basically, the pheromone CVA is detected by the fly's antenna - that's the olfactory organ of the fly and there, it activates the first nerve cell. This nerve cell then talks to a second nerve cell and the second nerve cell then talks to a third nerve cell, and that's the basic circuitry for pheromone perception to fly brain. And I'm now recording activity from the third nerve cell.

Hannah - And this simple 3-nerve cell circuit is there in both male and female flies.

Johnny - Exactly, so the first two nerve cells are identical in males and females, but I found at the level of the third nerve cell, there's a difference between males and females. So, if you take the analogy of a train on rail tracks, so imagine that the pheromone signal is the train and the rail tracks are the cables with which the nerve cells in the circuit communicate. In both sexes, the train on rail tracks goes past the first station, so the first nerve cell on the antenna.  It goes past the second nerve cell deeper in the brain, but then at the level of the third nerve cell, something different happens. It can go either left or right.  So, in female flies, the train would turn left and go to one part of the fly brain.  In male flies, the train would turn right and go to a different part of the fly brain. And we've identified now that there's a circuit which at the level of the third nerve cell that tells the train to go either left in females or right in males.

Hannah - And do you know what that signal switch is? What's controlling whether it goes left or right?

Johnny - Yes, we found that surprisingly, this switch is controlled by a single gene.  It's called 'fruitless'. This gene occurs in a male and a female version, and we found that when we engineer female flies that have the male version of this gene, the switch is set in a male manner.

Hannah - And then what happens to these female flies with this different switch?

Johnny - So, these she-males as we call them, they start behaving like males.  So when they smell the pheromone, they would get on their hind legs and start boxing, so they display male typical aggressive behaviour.

Hannah - Do you think there's a similar system in humans?

Johnny - That's a long standing question. Many people are interested whether humans can actually perceive pheromones. The problem is that the human brain is very, very complex and it's hard to study those kind of questions in people. So, we know that the human brain has the anatomical potential to perceive pheromones, but as far as I'm aware, there hasn't been conclusive proof of the existence of human pheromones. So, at the moment, we just don't know, but it would be very interesting to find it out.

Hannah - And you're now coming to the end of your PhD. Do you want to stay and research, do you think?

Johnny - Absolutely. I'm planning to do a post doc and I will potentially want to investigate whether the same mechanism is in operation and more complex brains.