First-of-its-kind map of the mouse nose reveals surprises about the sense of smell
Scientists have created a comprehensive map of olfactory receptors in the mouse nose, revealing some surprises about this basic sense.
Olfactory receptors, or olfactory receptors, were previously thought to be randomly distributed throughout the lining of the nasal cavity. But today the first map of its kind shows them to be highly organized, with different types separated into narrow bands.
Odors are detected by olfactory sensory neurons in the nasal cavity. Each neuron expresses one of 1,172 different receptors encoded in the mouse’s DNA, with each receptor detecting a different type of odor.
Other senses, such as touch, vision and hearing, use sensory maps. For example, to hearDifferent frequencies are encoded at different positions in the cochlea of the inner ear, and from there this information is relayed to the brain. The sense of smell was not thought to use such mapping, but over the past six or seven years, new techniques have allowed scientists to examine about 5.5 million neurons in more than 300 individual mice and better understand which genes are active in different cells of the nose.
One such technique is called single-cell sequencing, the study’s lead author said. Dr Sandeep Dattaneurobiologist at Harvard Medical School. This allowed the researchers to examine each mature olfactory sensory neuron “one at a time, to identify which receptor is expressed,” he explained. Then, a technique called spatial transcriptomics helped researchers locate these receptors.
Using this data, the team created a “beautiful map” of more than 1,100 olfactory receptors present in the mouse’s nose. The map shows “a thousand distinct bands of odor receptor expression that overlap but are highly organized,” Datta said.
Neurons that express the same receptor in the nose target the same location in the olfactory bulb, the brain’s main smell-processing center, the team found. “The map in the nose is precisely aligned with the map in the brain,” Datta said.
The degree of complexity of the nasal mucosa is remarkable, he added. “Mice, for example, have about 20 million olfactory neurons that express more than a thousand types of olfactory receptors, compared to just three main types of visual receptors for color vision,” he said.
Interestingly, the positions of the approximately 1,100 types of receptors were essentially the same in all of the laboratory mice examined by the researchers. The work also identified a molecule called retinoic acid (RA) that likely guides each neuron to express the correct receptor based on its location. Adding or removing PR resulted in an up or down shift in the receptor map, suggesting that the molecule may help control the position and influence of neurons.
Researchers are now studying why the stripes are arranged in this specific order. “[Another] The question we ask is: To what extent are human noses organized in this way?” Datta said.
“The human olfactory system is, in many ways, similar to the mouse olfactory system. [though we] have fewer odor receptors,” he noted. “But we don’t know much about whether these basic principles we’re learning in mice apply to humans.” Understanding this could help develop treatments for loss of smell and its consequences, including a increased risk of depression.
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