It is a wondrous time to be involved in olfaction. Almost 15 years ago, the field was cracked open when recent Nobel Prize winners Linda Buck and Richard Axel discovered the genes that are responsible for encoding olfactory receptors — the proteins that bind to odorants, the proteins that in many ways are responsible for unraveling the mystery of the olfactory code (see The Breakthrough for abstract). Since that time, much has been elucidated through genomic, cellular, molecular, physiological and anatomical studies of the mammalian olfactory system. These studies have broadened understanding and revealed a complexity and elegance of function difficult to predict earlier. Furthermore, it is becoming increasingly clear that these developments will have an important role in the private sector, leading to various industry applications. However, it is still unclear what exactly these new applications will be and how the transition from basic science to industry technology will occur.
How can the molecular biology and neuroscience of olfaction help shine light on the relationship between the molecular properties of an odorant and its odor quality? This question invites many others, because in order to go from stimulus to perception all the intermediate steps need to be understood. Such steps include binding mechanisms and sensitivity of olfactory receptor (OR) and odorant, transduction processes that activate the olfactory sensory neurons, mechanisms that guide the sensory neurons’ projections to the olfactory bulb (OB), local processing within the OB, and processing of odor information brain areas downstream from the OB. Understanding all of these steps is critical to unlocking the code — the good news is many of these steps already are partially understood.
In the nasal epithelium, which lines the inside of the nose, there are olfactory sensory neurons (OSNs; F-1). The neurons have hair-like cilia, and on these cilia are densely packed proteins called olfactory receptors (ORs). These proteins are located in the membrane of the OSNs. When a certain odorant molecule enters the nose, it is exposed to the nasal epithelium and binds to an OR. This interaction leads to the opening of ion channels on the surface of the cell, allowing positive ions into the OSNs, thereby changing the electrical potential between the inside and outside of the cell. This change in membrane potential can induce an action potential: an electrical spike propagating throughout the rest of the neuron, ending with the release of chemicals called neurotransmitters, which allow communication with other neurons.