Trends in Neurosciences
Volume 32, Issue 10, October 2009, Pages 525-531
Journal home page for Trends in Neurosciences

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From the top down: flexible reading of a fragmented odor map

https://doi.org/10.1016/j.tins.2009.06.001Get rights and content

Animals that depend on smell for communication and survival extract multiple pieces of information from a single complex odor. Mice can collect information on sex, genotype, health and dietary status from urine scent marks, a stimulus made up of hundreds of molecules. This ability is all the more remarkable considering that natural odors are encountered against varying olfactory backgrounds; the olfactory system must therefore provide some mechanism for extracting the most relevant information. Here we discuss recent data indicating that the readout of olfactory input by mitral cells in the olfactory bulb can be modified by behavioral context. We speculate that the olfactory cortex plays a key role in tuning the readout of olfactory information from the olfactory bulb.

Section snippets

Lateral interactions could allow flexible readout of the fragmented chemotopic odor map

Sensory systems must optimize the processing of input to permit timely and efficient extraction of information. One elegant solution to this challenge is to organize information into a spatial map. In vision, for instance, the cornea focuses a spatial representation of an image onto the retinal surface in the eye, while in hearing the representation of sound is organized as a frequency map in the cochlea. Odor maps also appear to have a gross chemotopic arrangement. For example, carboxylic

Mitral cell odor responses are influenced by learning, behavior, and context

Evidence for top-down regulation of processing in the olfactory bulb was first provided by Kerr and Hagbarth [15] who showed that excitation of centrifugal fibers enhances the local field potential (LFP) activity of the olfactory bulb. The LFP, first described by Adrian, is a field potential recorded extracellularly in the olfactory bulb that reflects the oscillatory synchronous activity of neurons aligned on the average in the same direction 16, 17. Since 1955 other groups have shown that

Local processing in the olfactory bulb is intrinsically dynamic

As discussed in Box 1, the spatio–temporal information contained in odor maps is processed by the interplay between the principal neurons of the bulb (tufted, T, and mitral, MT, cells) and the interneurons in the glomerular and external plexiform layers (EPL). This interplay gives rise to lateral inhibition 12, 14, a potential mechanism for synchronizing MT cell firing. Synchronization of MT cells through reciprocal connections to granule cells has been demonstrated in OB slices [32], and

Mechanisms for top-down regulation of mitral cell responsiveness

Modulation of MT cell responsiveness by adrenergic, cholinergic and serotonergic fibers is fairly well established [40]. In a recent study, Shea and co-workers found that odor-evoked increases in MT cell firing are suppressed in anesthetized mice when odor stimulation is paired with activation of the locus coeruleus (LC), the brainstem nucleus that houses the adrenergic neurons that innervate the olfactory bulb [41]. In another study, bulbar acetylcholine enhanced learning to discriminate

Acknowledgements

We would like to thank Drs. Peter Brunjes, Tom Finger, Kurt Illig, Laura López-Mascaraque, Nathan Schoppa, Michael Shipley, John Scott and Ben Strowbridge and Mr. David Gire for enlightening discussions. We would like to acknowledge funding by NIH grants DC00566 and HD041697 (D. Restrepo), DC008066 (W. Doucette), DC009369 (T. McTavish) and DC008679 (Salcedo).

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      Olfaction is crucial for animals to survive in early life, to find food and mates, to recognize prey and predators, and to communicate with con-species (Xu, 2001). To achieve these tasks, the olfactory system must possess the capacity to discriminate among the thousands of odors that occur in nature via accurate representation in neural systems (Doucette and Restrepo, 2008; Restrepo et al., 2009; Doucette et al., 2011). The olfactory bulb (OB) is the first relay station in the olfactory system and is essential for coding, processing and transmitting odor information (Shipley and Ennis, 1996; Mori et al., 1999; Schoppa and Urban, 2003).

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