Fears and phobias can be passed from mother to offspring through smell, scientists have discovered.
University of New York researcher Jacek Dubiec began by making female rats fearful of the smell of peppermint, which was achieved by administering a small electric shock to the animals at the same time that the odour was presented.
This led to the animals associating the presence of the smell with a forthcoming electric shock so that, subsequently, whenever the peppermint odour was presented, the animals momentarily froze, a characteristic rodent fear response.
These animals were then mated and, after they gave birth to their pups, the smell was again presented, leading them to become temporarily fearful in front of the baby rats. When these pups were tested later, they also showed an aversion to the smell of peppermint.
The animals weren't born this way though, because pups whose mothers were not frightened in their presence did not show any signs subsequently of having caught their mother's fear.
Dubiec speculates that the fear signal is a smell, because pups placed in a separate enclosure - but had air piped in from their frightened mothers' cages - also developed an aversion to peppermint smells. Tests on the animals suggest that a pheromone of some kind is produced by the mother and is detected by the juveniles' olfactory system using a specialised nerve network called the Grueneberg ganglion. This triggers the production of cortisol, the body's stress hormone, which in turn leads to the formation of fear memories associated with the peppermint smell in the brains of the young rats.
The findings are important because they shed light on the transmission of phobias and irrational fear responses between human adults and their children. According to Dubiek, "understanding of the neural and molecular mechanisms controlling intergenerational transmission of fear will help to develop better preventative and therapeutic methods." The work was published this week in the journal PNAS.