Author: Marley Deena Kass

Publisher:

Published: 2017

Total Pages: 318

ISBN-13:

Conventional wisdom suggests that the body's sensory systems should be consistent, so that a given sensory stimulus always produces more-or-less the same signal to the brain, which can then retrieve related memories or information. However, using optical neurophysiological tools to observe the earliest parts of the mouse olfactory system, we have found that actually these signals are highly flexible, such that different sensory experiences and previously learned information radically affect the way sensory stimuli are processed in the brain. The first stage of sensory processing in the olfactory system takes place in the olfactory bulb, where axons from olfactory sensory neurons (OSNs) in the nose segregate by receptor type and converge into one or two glomeruli on the surface of the bulb. The brain's initial (primary) neural code for the identity of an odor in the nose is thus the spatiotemporal pattern of olfactory bulb glomeruli receiving synaptic input from OSNs, which can be modulated by local circuits in the glomerular layer of the bulb. Here, we demonstrate that these primary odor representations are changed in vivo through simple environmental manipulations, such as olfactory sensory deprivation or odor exposure. Subsequent experiments show that passive odor exposure leads to changes in temporal patterns of OSN synaptic output that are correlated with perceptual changes in odor quality. We move on from simple environmental manipulations to explore how emotional learning can influence early sensory processing, and surprisingly find that discriminative olfactory fear conditioning can selectively enhance the synaptic output of OSNs during the presentation of threat-predictive odorants. By contrast, when conditioned fear generalizes across olfactory stimuli that are quite different from a threat-predictive odor, there is a corresponding facilitation of odor-evoked activity in inhibitory interneurons in the olfactory bulb that generalizes across threatening and non-threatening odors. These experience-dependent effects may be further modulated by individual differences in endogenous factors such as the expression of certain transduction proteins or circulating levels of sex hormones that can independently shape primary sensory odor representations. Collectively, the results from these experiments demonstrate that early neural representations of odors are highly malleable on the basis of prior sensory experience and learning, even as early as the primary sensory input to the brain. Such plasticity presumably maximizes the detection and discrimination of meaningful sensory stimuli in a constantly changing olfactory environment, and is of broad importance for downstream brain regions that receive input from the bulb.