Sensory maps in the olfactory cortex defined by long-range viral tracing of single neurons

• Published in: Nature (London) 2011-04, Vol.472 (7342), p.217-220
• Main Authors: Ghosh, Sulagna, Larson, Stephen D., Hefzi, Hooman, Marnoy, Zachary, Cutforth, Tyler, Dokka, Kartheek, Baldwin, Kristin K.
• Format: Article
• Language: English
• Subjects: 631/1647/245/2221; 631/1647/245/2227; 631/378/2624/1704; Animals; Biological and medical sciences; Brain Mapping; Female; Fundamental and applied biological sciences. Psychology; Genetic aspects; Humanities and Social Sciences; letter; Male; Methods; Mice; multidisciplinary; Neuroanatomical Tract-Tracing Techniques; Neurons; Odorants - analysis; Olfaction; Olfactory Bulb - anatomy & histology; Olfactory Bulb - cytology; Olfactory Bulb - physiology; Olfactory cortex; Olfactory Pathways - anatomy & histology; Olfactory Pathways - cytology; Olfactory Pathways - physiology; Olfactory Perception - physiology; Olfactory Receptor Neurons - cytology; Olfactory Receptor Neurons - physiology; Olfactory system and olfaction. Gustatory system and gustation; Physiological aspects; Science; Science (multidisciplinary); Sindbis Virus - genetics; Sindbis Virus - physiology; Smell - physiology; Vertebrates: nervous system and sense organs
• ISSN: 0028-0836; 1476-4687
• DOI: 10.1038/nature09945

Scent tracking In the mouse, glomeruli in the olfactory bulb receive projections from single classes of olfactory neurons, thereby forming an odour map. Information from the glomeruli is then relayed to the cortex but the projection patterns from individual glomeruli are not known. Three papers now examine the details of this projection. Luo and colleagues use a combination of genetics and retrograde mono-trans-synaptic rabies virus labelling. They trace the presynaptic connections of individual cortical neurons and find no evidence of connections supporting a stereotyped odour map in the cortex, but see systematic topographical differences in amygdala connectivity. The lack of stereotypical cortical projection is corroborated, both at the level of bulk axonal patterning and in projections of individually labelled neurons, by two papers — one from the Axel laboratory, and one from the Baldwin laboratory — that examine the anterograde projections from individual glomeruli. Together, these findings provide anatomical evidence for combinatorial processing of information from diverse glomeruli by cortical neurons and may also reflect different functions of various areas in mediating innate or learned odour preferences. 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 1 , 2 . Activation of each glomerulus is relayed to higher cortical processing centres by a set of ∼20–50 ‘homotypic’ mitral and tufted (MT) neurons 3 . In the cortex, target neurons integrate information from multiple glomeruli to detect distinct features of chemically diverse odours 4 , 5 , 6 . 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 stereo