Final Report Summary - RAPIDODORADAPTATION (Rapid adaptation to varying odor concentration)
Navigation using olfactory cues entails processing of three distinct types of information: odor identity, odor concentration, and time of cue sampling. The last two components govern the odor source localization process. Localization of the odor source relies on sampling and comparing odor concentration. This may be done in the spatial domain, by bilateral sampling and comparing between two nostrils, or temporal domain, by sequential sampling and comparison of concentrations sampled at consecutive times. While the role of the spatial strategy in olfactory navigation has been studied in various species, mechanism of temporal sampling and its contribution to odor localization remain obscure. Scientific results in order to adequately address the question of temporal sampling in olfactory bulb, we developed an odor delivery setup that allows switching and stabilization of different odor concentrations between consecutive sniff cycles. To study the neural basis of temporal sampling in mouse olfactory bulb, we monitored activity of mitral/tufted (M/T) cells in response to an olfactory stimulus, which flickered between four odor concentrations, in both anesthetized and awake animals. M/T responses to the flickering task revealed two distinct M/T populations: a) concentration tracking cells – providing information on actual odor concentration, and b) gradient detection cells – providing information on the concentration change. What is the mechanism underlying the ability of M/T cells to detect the change of concentration, and not the actual concentration? Odors activate numerous glomeruli, and consequently a large fraction of relevant M/T cells. Therefore, gradient-type responses may be due to lateral/reciprocal connections. To examine the necessity of lateral/reciprocal connections, we minimized their contribution by optogenetic activation of a subset of M/T cells selectively belonging to the dorsal M72 glomerulus. We measured the difference of M/T cell responses to light pulses of different intensity, given in ascending or descending order. Light stimulation of this specific glomerulus revealed that the associated M/T cells’ responses to identical light stimuli depend on previous stimuli intensity. These responses resembled those seen in gradient detection cells, suggesting that the underlying mechanism might be intrinsic. In summary, our results indicate that M/T cells respond to specific concentration or to change of odor concentration. The coexistence of this two different M/T responses may reveal two different streams of information in the olfactory system, resembling “what” and “where” streams of the visual system.