By Dani Cooper, ABC
While most people have a childhood memory that is evoked by a certain smell, the process behind this connection has been little understood.
However, new Australian research shows long-term, odour-memory formation in the brain is linked to smell perception by affecting the expression of olfactory receptors in the nose, according to lead-author Dr Judith Reinhard, at theQueensland Brain Institute.
The study, published in the latest European Journal of Neuroscience, came out of an observation by Reinhard after moving to Brisbane from Canberra that her laboratory honeybees were not "learning" as well as they used to.
"At first I thought maybe it was a Queensland thing," admits Reinhard. "It was July and winter, but then I did the exact same experiment in November and the bees learned really well."
"The only thing that had changed was the chemical environment, the flowers and flowers on trees, outside."
Reinhard theorised from this observation that the way odours are perceived might have something to do with our scent experiences.
To test her theory, Reinhard and colleagues in New Zealand and Australia, put honeybees through a number of experiments to measure the effect of age and season.
For the age test, the team marked 200 honeybee workers from when they emerged from pupae and then collected 50 bees each at seven and 21 days of age.
To test seasonal effect, 21-day-old honeybees were collected as they returned to the hive with pollen in October, January, April and July.
The antennae of the collected bees were removed and the olfactory receptor expression levels analysed.
In the age experiment, the researchers found olfactory receptor expression levels in older honeybee workers who mostly foraged outside the hive "differed significantly" from their younger test mates.
Olfactory receptors were predominantly down-regulated, or reduced, in the older cohort, says Reinhard.
Similarly the second experiment showed differences in olfactory receptor expression levels throughout the seasons. Reinhard says the difference may be the result of flowering trees and plants that were available at different times of the year.
"This suggests that olfactory receptor expression in the honeybee antennae is not hard-wired but plastic, perhaps changing depending on the scent environment the bees are experiencing," the researchers write.
Reinhard says to test whether these changes were the result of odour memory, they trained the bees to associate a certain odour with a reward.
They did this by strapping bees in little holders from which their heads emerged. As a specific odour was blown over its antennae, the bees were given a drink of sugar water as a reward and therefore formed an associative memory between odour and reward.
A control group of honeybees received the sugar water but no odour, while a second group had the "reward" and odour offered separately so no link between them was established.
Analysis of the antennae found the bees conditioned by the reward, had down-regulated olfactory receptor expression for the specific odour.
Reinhard admits this reduction of receptor expression for the odour is counterintuitive.
"We think the reason for this is that once the bee has formed this memory of the odour, there is a very strong neural connection [so] you don't need a lot of the receptor to detect the odour and trigger recall of the memory."
However she says there are tens of thousands of odours out there and all the other odour receptors need to be up-regulated to detect new flowering scents.
The research also uncovered that drones &mrule; the male bees raised to mate with queen bees &mrule; do not have this plasticity.
Reinhard says the drones' primary role is to find and mate with a queen so their olfactory receptors are tuned to queen pheromone detection.
"It's possible the mechanism doesn't exist in drones because they don't need this plasticity. They don't need to manage changing odour environments," she says.
Reinhard says the same mechanism could be underpinning the notion of "acquired taste" in humans, where repeated sensory experience with a flavour or aroma leads to changes in preference.
She says this could be an important finding for the food and wine industry.
"If you can find out what chemicals are preferred [for a certain group of consumers or region] you can design a wine for a specific market." Read story