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Download Dendritic organization of sensory input to cortical neurons in vivo
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REPRESENTATIONS OF ODOR IN THE PIRIFORM CORTEX Dan D. Stettler and Richard Axel Neuron 63, p. 854-864 (2009) THE OLFACTORY BULB IS QUITE ORGANIZED ? Cortex What about the cortex? THE PIRIFORM CORTEX 100μm • 3-layered structure • On the ventral-lateral surface of the cerebral hemisphere • Pyramidal cells synapse with mitral cell afferents in layer 1 • SL = pyramidal???? http://aups.org.au/Proceedings/38/9-14/ IMAGING METHOD • For the calcium imaging the calcium sensitive fluorescent dye Oregon Green 488 BAPTA-1 AM was used. • The dye was injected into broad regions of layer 1, causing the labeling of > 90% of the pyramidal cell bodies in layers 2 and 3 across wide regions in the piriform cortex. • Imaging was done at multiple sites in more than 100 mice. Baseline fluorescence (air) 50μm STIMULI • Stimulation consisted of a 1sec??? puff of odorant at concentrations of 0.5-80ppm in air. • 20 different odors were used ODORANT-EVOKED RESPONSES IN MOUSE PIRIFORM USING IN VIVO TWO-PHOTON CALCIUM IMAGING (Fox urine) Fluorescence changes were elicited in 3-15% of the piriform neurons • Each odorant activated a unique but dispersed ensemble of cells in layer 2. • Behaviorally different odors elicited the same type of response. Adjacent cells can respond selectively to different odorants Responses of the 5 marked cells to 5 odorants 50μm DPG/air ODORANT-EVOKED CALCIUM RESPONSES IN PIRIFORM CORTEX ARE HIGHLY CONSISTENT Odor puff ODORANT-EVOKED CALCIUM RESPONSES IN PIRIFORM CORTEX ARE HIGHLY CONSISTENT Cells responsive to a given odorant within the same imaging field exhibited a range of statistically significant positive responses CONCENTRATION DEPENDENCE OF ODOR-EVOKED RESPONSES ODORANTS EVOKE RESPONSES IN UNIQUE BUT OVERLAPPING ENSEMBLES OF PIRIFORM NEURONS ODORANTS EVOKE RESPONSES IN UNIQUE BUT OVERLAPPING ENSEMBLES OF PIRIFORM NEURONS DISTRIBUTED ODORANT REPRESENTATIONS EXTEND ACROSS WIDE REGIONS OF PIRIFORM CORTEX THE RESPONSE OF PIRIFORM CELLS TO A MIX OF ODORANTS EXHIBITS STRONG SUPPRESSION AND WEAK SYNERGY THE RESPONSE OF PIRIFORM CELLS TO A MIX OF ODORANTS EXHIBITS STRONG SUPPRESSION AND WEAK SYNERGY A MODEL OF PIRIFORM RESPONSES BASED UPON RANDOM CONNECTIVITY BETWEEN THE BULB AND PIRIFORM CAN GENERATE THE OBSERVED ODORANT REPRESENTATIONS A MODEL OF PIRIFORM RESPONSES BASED UPON RANDOM CONNECTIVITY BETWEEN THE BULB AND PIRIFORM CAN GENERATE THE OBSERVED ODORANT REPRESENTATIONS CONCLUSIONS Unlike visual, auditory or somatosensory cortical sensory areas: • The piriform cortex discards the spatial segregation and chemotopy apparent in earlier stages of the olfactory system. • The piriform shows a highly distributed organization - different odorants activate unique but dispersed ensembles of cortical neurons. • Neurons in the piriform cortex don’t have an apparent continuous receptive fields (chemotopy, ….behavioral What else was checked?) → It should be remembered, though, that a relevant odors space still needs to be defined, while in other senses this step is more straightforward. Caveat – results are dependent on thresholds of imaging GLUTAMATE BLOCKADE DIMINISHES ODORANTEVOKED RESPONSES MONTE CARLO SIMULATIONS OF RESPONSIVE CELL DISTRIBUTIONS. AUTO- AND CROSS- CORRELATION ANALYSIS REVEALS NO CONSISTENT FINE-SCALE PATTERNING IN ODORANT RESPONSES. MOTIVATION Measuring the input to neurons. In electrical measurements one finds: • directional selectivity of the firing rate • but no directional selectivity under hyperpolarization – indicating a low tuning level of the inputs. POSSIBILITIES OF INPUT TUNING AND ORGANIZATION: Untuned Tuned and clustered Investigating the activity under hyperpolarization Tuned and dispersed HETEROGENEOUS DISTRIBUTION OF PURETONE-ACTIVATED SPINES ALONG DENDRITES. Xiaowei Chen, Ulrich Leischner, Nathalie L. Rochefort, Israel Nelken & Arthur Konnerth Functional mapping of single spines in cortical neurons in vivo Nature 475, 501–505 http://www.nobelprize.org/nobel_prizes/medicine/lau reates/2004/illpres/2_olfactory.html 2-photon microscope Practical theory of 2-photon microscopy 1. Near simultaneous absorption of the energy of two infrared photons results in excitation of a fluorochrome that would normally be excited by a single photon of twice the energy. 2. The probability of excitation depends on the square of the infrared intensity and decreases rapidly with distance from the focal volume. Advantages of 2-photon microscopy 1. 2. 3. 4. 5. Increased penetration of infrared light allows deeper imaging. No out-of-focus fluorescence. Photo-damage and bleaching are confined to diffraction- limited spot. Multiple fluorochrome excitation allows simultaneous, diffractionlimited, co-localization. Imaging of UV-excited compounds with conventional optics.
