Transformation of odor information from defined olfactory receptor units

EZEQUIEL M. ARNEODO; KRISTINA PENIKIS; NEIL RABINOWITZ; JINJI ZHANG; THOMAS BOZZA; DMITRY RINBERG

Chicago

Conferencia; Society for Neuroscience; 2015

Society for Neuroscience

Circuits within the olfactory bulb transform the odor representations of the sensory periphery. The logic behind this transformation remains elusive, because a) it has been difficult to record from olfactory bulb neurons while knowing what their inputs are, and b) recordings should be done in awake animals as network dynamics change drastically with wakefulness.In order to understand this coding transformation, we have developed a technique to optogenetically identify and then record from olfactory bulb neurons that receive input from genetically-identified inputs in awake animals. Olfactory sensory neurons (OSNs) that express the same odorant receptor gene send axons that converge to form glomeruli, where they provide excitatory input to a small number of mitral/tufted (M/T) cells. The peripheral representation of incoming odors is modified in the M/T layer via an extensive inhibitory network before odor information is relayed to cortex. Using a gene-targeted mouse line that expresses Channelrhodopsin-2 in a genetically-defined subpopulation of OSNs -- cells that express a specific receptor (M72) -- we can identify the subpopulation of M/T cells that are functionally coupled to this same glomerulus. Light stimulation of the M72 glomerulus elicits a reliable, short latency response in these ?M72? M/T cells. This technique gives us the opportunity to characterize how a specific subpopulation of ?sister? M/T cells transforms their shared input. We recorded the activity of M72 M/T cells from awake, head-fixed, freely-breathing mice while we presented a range of odorants, for which the M72 OSN responses have been characterized. We found that, for most odors, there was a surprising amount of variability in responses among sister M/T cells, despite their common excitatory input. This response diversity was comparable to that seen across the greater population of M/T cells. However, a single odor stimulus -- the strongest known ligand of M72 OSNs -- provided an exception to this response heterogeneity, as it evoked a temporal pattern of firing that was highly consistent across M72 M/T cells. This response stereotypy was not seen in the larger M/T population.These data raise the possibility that response stereotypy and diversity within specific neural subpopulations could form a substrate for sensory coding within the olfactory bulb.