I really should know better than to be surprised at the findings - and techniques – of science, but this is pretty cool. The 17 October edition of Nature Neuroscience published a paper detailing experiments in optogenetics. The researchers incorporated light-reactive proteins into the olfactory systems of mice, so that the systems are triggered by light rather than smell.

Basically they have genetically engineered the mice so their noses act like retina.

Now, the idea of mice being able to smell light is interesting (although perhaps not as fantastic as mice that can see odours), but of course that’s not the point of the research.

As you might imagine, administering an odour in an experimental setting can be a fairly imprecise act. The main point of this research is to improve the efficiency of neuroscientific experimentation, by replacing odour with light.

With their genetically engineered mice, the researchers (from Cold Spring Harbor Laboratory, the National Centre for Biological Sciences in Bangalore and Harvard University) were able to conduct experiments targeting specific neurons in the nose and observe the neuronal activity that was triggered in the odour scenting section of the brain (the olfactory bulb).

Now, don’t panic, but here’s just a little bit of neuro-morphology which help make the value (and the point) of this research a bit clearer.

The main olfactory bulb of an adult mouse. Image taken with a confocal microscope. False colour (applied with Photoshop) indicates the three main layers: blue - Glomerular layer; red - External Plexiform and Mitral cell layer; green - Internal Plexiform and granule cell layer. Image: Matt Valley

When a smell enters the nose, it is detected by the olfactory epithelium (in the nasal cavity). Millions of neurons then transfer this information to the olfactory bulb to be “sorted”. Each of these neurons is specialised to express a single molecular odour receptor type – one of 1500 that a mouse can detect. The axons of these neurons cluster in structures known as glomeruli (singular glomerulus). These form the outer section of the olfactory bulb, known (unsurprisingly) as the glomerular layer (coloured blue in the image on the right). The section coloured green is the plexiform layer (supporting nerve tissue), but it’s the red section that was of most interest to this study. This section is the mitral cell layer. Cells in the mitral layer transfer information - that has first been processed in the glomerular layer - to the olfactory cortex. In the olfactory cortex “decisions” are made as to what the smells “mean” and what to do about them.

This research has already revealed a great deal about the structure and layout of the olfactory systems. For a start, it seems that there are more information channels leaving the olfactory bulb than there information types entering it. It also seems that the timing of neuronal stimulation plays a part in the perception of smell. Each glomerulus gathers information about one specific type of odour, but is connected to about 15,000 mitral cells. Even though the information coming to these cells from the glomeruli come from neurons detecting the same odour molecules at about the same time, it turns out that this information is not simply transferred to the olfactory cortex. The mitral cells fire at different rates and asynchronously. It is early days, but this might provide more detailed or nuanced information to the olfactory cortex.

Apart from developing a system that uses light to trigger a response in cells that are designed to detect smells (which I still think is really cool) the techniques used in the research have great potential to help elucidate the complex processes the underpin neural “wiring” and the processes of the brain in general. Dare I say it – this could be a sniff of things to come.

The paper by Ashesh K. Dhawale, Akari Hagiwara, Upinder S. Bhalla, Venkatesh Murthy and Dinu F. Albeanu can be found at http://www.nature.com/neuro/index.html