Effect of early exposure of young rats to odor on odor learning and memory

Olfaction is one of the first senses to develop in the neonatal rat pup. During the first post-natal weeks (the sensitive period), the olfactory bulbs undergo change. We examined whether exposure to a particular odor during the sensitive period influences preference for this odor at a later stage, as well as the subsequent ability to learn and retain memory of the odor. We sprayed two females twice daily on the nipple area with aniseed essence from day 1 to day 21 following birth of the litter. As control, two additional post-partum females were sprayed with water. At the age of 21 days, we tested the young rats from all litters for odor preference in a Y maze. The pups that had been exposed to aniseed showed preference for this odor (p < 0.05). Exposure to the odor was then discontinued until the age of 40 days when the maze test was repeated. No preference for the odor was found. Starting from day 41, the rats were taught to choose between aniseed and cinnamon odors in a Y maze. Rats choosing aniseed received a reward. The rats that had been exposed to aniseed in infancy demonstrated greater success in choosing the aniseed arm throughout the training period (p < 0.01). The rats were retested at 5, 6, 7, 8 and 9 months after the end of the learning experiment. We found that those rats that had had early experience with the aniseed remembered the odor better and demonstrated greater success in finding it (p < 0.01). From these results, we conclude that exposing young rats to an odor affects positively their ability to learn the odor and retain long-term memory of it.