No, I don't mean smell in the stinky sense - unless you happen to develop cravings for boiled eggs, cabbage and baked beans - but new research has revealed that part of the brain responsible for smelling grows during pregnancy, at least in rats. This discovery has exciting implications for our understanding of brain function and learning in adults, principally because instances where scientists have managed to track down newly forming brain cells (neurones) in adults are relatively rare. Until the 1980's it was thought that neurones were only created in the developing embryonic brain, and if they subsequently died in the adult then that was that. Game over, no second chances. This dogma was challenged by studies on songbirds, when researchers discovered that certain types of birds grew more brain cells at the time of year when they had to learn new songs. Presumably, it was argued, the increased number of neurones helped them cope with the learning task. But could this apply to animals? Many were sceptical, until American scientists showed that new neurones could be born in the brains of adult mice and humans. Interestingly, two sites where the new cells were found were the hippocampus (the area of the brain associated with learning and memory) and the olfactory bulb (the "smelling zone"), which is where the pregnant rodents come into the picture.
Researchers in Calgary, Canada, injected pregnant rats with a chemical that specifically labelled newly formed neurones. Intriguingly they found a burst of neurone "birth" amongst cells that contribute to the olfactory bulb, about a third of the way into the pregnancy and also just after the pups were born. The trigger for the increase in cell numbers was found to be the hormone prolactin, which increases during pregnancy.
So what is the purpose of this brain expansion? Other researchers have found that female mice that cannot use the hormone properly also fail to recognise their babies. Some have speculated that this could be because they cannot identify their offspring by smell, a common mechanism in many animal species. Perhaps the increased number of neurones in the pregnant rats helps them to learn the smell of their babies. Even more fascinating, is the observation that levels of prolactin increase in males and females immediately after sexual activity. Maybe this also stimulates the production of brain cells - although nothing has yet been proven. It's certainly worth a try as a chat-up line, though.
But how can such studies help us? One of the most keenly sought Holy Grails of modern medical science is the ability to rejuvenate the human brain and body. By studying the sort of things that can stimulate growth of new neurones, we may be able to repair or enhance our ageing brains. Other groups of researchers have shown that if you place rats in an exciting environment, with toys and physical activities, they will increase in mental power and brain size. Exposure to certain anti-depressant drugs can also lead to the birth of new neurones, while stress and depression prevent this. Sadly, the complexities of the mammalian brain make such experiments difficult to interpret, despite the exciting speculation that can be made from even the simplest results. We are a long way from understanding the signals that cause this new growth, but if we can harness the process, the potential for novel therapies could be enormous.