What the right subpopulation tells about the right odor

EZEQUIEL M. ARNEODO

Sao Bernardo Do Campo

Encuentro; Brazilian Meeting on Brain and Cognition; 2015

Universidade Federal do ABC

Sensory systems extract information from stimuli despite their variability: perceptual features can be identified over a wide range of stimulus properties (concentration, loudness, intensity). Understanding the neural coding mechanisms behind this task is a fundamental question that remains mostly unanswered. Olfaction is ideal to explore such mechanisms for two reasons: the primary representation of an odorant is segregated in channels, determined by the activities evoked on genetically identified and tractable olfactory receptors (ORs); and the compactness of the first layer of processing, since the second order cells already project to higher brain.In the mammalian olfactory bulb (OB) axons of each different OR type converge into glomeruli and form synapses with second order mitral/tufted (M/T) cells. These cells receive excitatory input from only one glomerulus and readily relay to higher brain areas, yet they do not faithfully reflect their glomerular excitatory input, since they are modulated by the inhibition delivered by a vast network of inhibitory interneurons, whose activities depend on wakefulness state. What odor information they carry is unknown so far, mostly because it has not been possible to know the precise input of a particular subpopulation of M/T cells. We used optogenetics to develop a technique to identify and record activities of M/T cells connected to one particular glomerulus in awake mice, and study their responses to odorants for which the corresponding OR type sensitivity is well characterized.This specific subpopulation responds to odors for which the receptors are maximally sensitive as a significantly more homogeneous ensemble than to other stimuli. This supports a model for identity coding of an odor, which relies primarily on the set of receptors that are most sensitive to it.