By Shweta Ramdas
A month or so ago, I casually mentioned to my labmates that I can’t get enough of the smell of gasoline, and that I had stolen a whiteboard marker from our lab to sniff when I was particularly frustrated with research. This had two results: my labmates now tease me mercilessly, and I realize that not everybody is as enamored by these smells as I am.
The latter was quite an epiphany: I had imagined that everyone finds the smell of gasoline ambrosial. So why isn’t it true? Being a geneticist, of course my first thought was that it must be all in the genes.
The genetics of olfaction
Humans have around 400 genes (400!) that encode olfactory receptors (let’s call them ORs) responsible for smell perception (to make things more complicated, we have 600 “pseudo-genes”, or non-functional genes, that resemble these 400 functional ones). This is among the most diverse sets of genes in humans: your ORs could have as many as 3 out of 10 functional differences (or 30%) from the ORs of another human. This has led scientists to quip that each of us has our own “unique nose” and an olfactory fingerprint nearly unique to each person!
Our noses are lined with nerves containing these OR proteins, which bind to the molecules in odors called odorant molecules. Each of these proteins recognizes and binds to a specific molecule (or a distinct set of molecules); and vice-versa, each odorant molecule type can bind to multiple ORs. Since each odor is a combination of odorant molecules, and each receptor in turn binds to different subsets of these, the odor we perceive is a combination of responses by different ORs. For instance, the scent from the food on your plate enters your nose as a mix of many, many molecules. Each of these binds to different ORs lining your nose, with some molecule types being recognized by multiple different ORs. Each bound receptor now transmits a signal to the brain, which then associates the combined messages to a particular scent. This cascading process is a beautiful symphony that leads to our perception of smell, something we take for granted.
Since each of us have different ORs, we likely possess the ability to discern some smells that go unnoticed by others. There have been numerous genetic studies on differences in responses to food-related odors (which in turn affect how to perceive taste), ‘grassiness’, male sweat, and perfumes: most of these genetic studies link differences in perception to the aforementioned OR genes. Going back to the nagging question: what is it about the way I smell gasoline? There have not been any major studies connecting the prolific ORs to the perception of compounds in gasoline (or whiteboard markers), which might mean that there is more work to be done in this area, or that there are other mechanisms determining our sensitivity to them. In fact, there are other genes that could influence your response—those responsible for the production of dopamine receptors!
Neurotransmitters and olfaction
Dopamine is a neurotransmitter: a protein that transmits signals in the brain. In particular, it transmits signals of reward. Think of how you feel after eating chocolate or getting a high score on a tough game. If some things we smell lead to an activation of the reward center (like some compounds in gasoline or chocolate), then perhaps our sense of smell will also seem enhanced or suppressed depending on how rewarding we perceive it to be. Some researchers have found that reducing the response of one particular dopamine receptor (the proteins responding to the dopamine signal in the brain) decreased the ability to smell an odor, similar to what would happen if you walked away from the odor. This suggests that the more ‘rewarding’ a smell is, the more likely we are to perceive it. The above theory has intriguing implications for connections between odor perception and other manifestations of the reward system: mood disorders and addiction.
Our associations with memories influence our noses
Another popular hypothesis is the associative one: how we respond to smells often depends on what we associate with them. For instance, if you swam a lot in your childhood, you are very likely to associate the smell of chlorine with fond memories of swimming. Every time you catch a whiff of chlorine, the neurons recognizing this smell fire up the neurons encoding swimming, to which they are strongly connected. The pleasant memory associated with the smell becomes a reward that makes the original stimulus more attractive.
So, the short answer to my initial question (arrived at after 600 words) is: we don’t know. The perception of smell is a complex process involving many biological sub-processes that work in concert. An odorant molecule passes through a maze of receptors in my nose largely determined by my genetics, causes changes in my brain according to my neurotransmitters, and is imbued with a subjective sprinkling of memories that are uniquely mine. All of us have our own unique noses, and some of these just happen to love the intoxicating scents of whiteboard markers and gasoline.
About the author
Shweta is a graduate student in Bioinformatics at the University of Michigan. Her research involves studying computational methods to understand the genetic basic of psychiatric disease. Her undergraduate degree is from the National University of Singapore where she studied computational biology. Outside of research, Shweta enjoys reading, yoga, and figuring out the genetic basis for being a muggle. Follow Shweta on twitter @shramdas.
Read all posts by Shweta here.