Sensory maps in the olfactory cortex defined by long-range viral tracing of single neurons.
ABSTRACT Sensory information may be represented in the brain by stereotyped mapping of axonal inputs or by patterning that varies between individuals. In olfaction, a stereotyped map is evident in the first sensory processing centre, the olfactory bulb (OB), where different odours elicit activity in unique combinatorial patterns of spatially invariant glomeruli. Activation of each glomerulus is relayed to higher cortical processing centres by a set of ∼20-50 'homotypic' mitral and tufted (MT) neurons. In the cortex, target neurons integrate information from multiple glomeruli to detect distinct features of chemically diverse odours. How this is accomplished remains unclear, perhaps because the cortical mapping of glomerular information by individual MT neurons has not been described. Here we use new viral tracing and three-dimensional brain reconstruction methods to compare the cortical projections of defined sets of MT neurons. We show that the gross-scale organization of the OB is preserved in the patterns of axonal projections to one processing centre yet reordered in another, suggesting that distinct coding strategies may operate in different targets. However, at the level of individual neurons neither glomerular order nor stereotypy is preserved in either region. Rather, homotypic MT neurons from the same glomerulus innervate broad regions that differ between individuals. Strikingly, even in the same animal, MT neurons exhibit extensive diversity in wiring; axons of homotypic MT pairs diverge from each other, emit primary branches at distinct locations and 70-90% of branches of homotypic and heterotypic pairs are non-overlapping. This pronounced reorganization of sensory maps in the cortex offers an anatomic substrate for expanded combinatorial integration of information from spatially distinct glomeruli and predicts an unanticipated role for diversification of otherwise similar output neurons.
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ABSTRACT: Sensory cues that predict reward or punishment are fundamental drivers of animal behavior. For example, attractive odors of palatable food or a potential mate predict reward, while aversive odors of pathogen-laced food or a predator predict punishment. Aversive and attractive odors can be detected by intermingled sensory neurons that express highly related olfactory receptors and display similar central projections. These findings raise basic questions of how innate odor valence is extracted from olfactory circuits, how such circuits are developmentally endowed and modulated by state, and how innate and learned odor responses are related. Here, we review odors, receptors and neural circuits associated with stimulus valence, discussing salient principles derived from studies on nematodes, insects and vertebrates. Understanding the organization of neural circuitry that mediates odor aversion and attraction will provide key insights into how the brain functions. Copyright © 2015 Elsevier Ltd. All rights reserved.
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ABSTRACT: Central to the understanding of brain functions is insight into the distribution of neuronal activity that drives behavior. Local measurements of brain activity in behaving mice can be made with electrodes and fluorescent calcium indicators (Buzsáki, 2004 and Grewe and Helmchen, 2009), but such approaches provide information regarding only a very small fraction of the ∼70 million neurons that comprise the mouse brain. The detection of elevated levels of the immediate-early genes (IEGs) linked to recent neuronal activity (Clayton, 2000 and Guzowski et al., 2005) is a more spatially comprehensive technique. While it lacks the time resolution of electrophysiological recordings or calcium imaging, it does have the potential of providing a complete view of recent whole-brain activity. Once determined, the whole-brain IEG-based map can be used to generate structure-function hypotheses to be probed by high-resolution recordings as well as optogenetic and chemogenetic methods (Fenno et al., 2011 and Lee et al., 2014).Cell Reports 12/2014; 71(2). DOI:10.1016/j.celrep.2014.12.014 · 7.21 Impact Factor
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ABSTRACT: It is surprising that the piriform cortex, when compared to the hippocampus, has been given relatively little significance in human epilepsy. Like the hippocampus, it has a phylogenetically preserved three-layered cortex that is vulnerable to excitotoxic injury, has broad connections to both limbic and cortical areas, and is highly epileptogenic - being critical to the kindling process. The well-known phenomenon of early olfactory auras in temporal lobe epilepsy highlights its clinical relevance in human beings. Perhaps because it is anatomically indistinct and difficult to approach surgically, as it clasps the middle cerebral artery, it has, until now, been understandably neglected. In this review, we emphasize how its unique anatomical and functional properties, as primary olfactory cortex, predispose it to involvement in focal epilepsy. From recent convergent findings in human neuroimaging, clinical epileptology, and experimental animal models, we make the case that the piriform cortex is likely to play a facilitating and amplifying role in human focal epileptogenesis, and may influence progression to epileptic intractability.Frontiers in Neurology 12/2014; 5:259. DOI:10.3389/fneur.2014.00259