Dan D. Stettler and Richard Axel REPRESENTATIONS OF ODOR IN THE PIRIFORM CORTEX Neuron 63, p (2009)

THE OLFACTORY BULB IS QUITE ORGANIZED Cortex What about the cortex? ?

THE PIRIFORM CORTEX 3-layered structure On the ventral-lateral surface of the cerebral hemisphere Pyramidal cells synapse with mitral cell afferents in layer 1 SL = pyramidal???? pyramidal 100μm

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. 50μm Baseline fluorescence (air)

STIMULI Stimulation consisted of a 1sec??? puff of odorant at concentrations of ppm in air. 20 different odors were used

ODORANT-EVOKED RESPONSES IN MOUSE PIRIFORM USING IN VIVO TWO-PHOTON CALCIUM IMAGING Fluorescence changes were elicited in 3-15% of the piriform neurons (Fox urine) Each odorant activated a unique but dispersed ensemble of cells in layer 2. Behaviorally different odors elicited the same type of response.

50μm Responses of the 5 marked cells to 5 odorants DPG/air Adjacent cells can respond selectively to different odorants

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

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

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 ODORANT- EVOKED 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 Tuned and dispersed Investigating the activity under hyperpolarization

HETEROGENEOUS DISTRIBUTION OF PURE- TONE-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

reates/2004/illpres/2_olfactory.html

2-photon microscope

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. Practical theory of 2-photon microscopy

Advantages of 2-photon microscopy 1.Increased penetration of infrared light allows deeper imaging. 2.No out-of-focus fluorescence. 3.Photo-damage and bleaching are confined to diffraction- limited spot. 4.Multiple fluorochrome excitation allows simultaneous, diffraction- limited, co-localization. 5.Imaging of UV-excited compounds with conventional optics.