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

The piriform cortex 3-layered structure pyramidal Figure 1. Schematic diagram of the location and anatomy of the piriform cortex. A, Ventrolateral aspect of the rat brain, showing the olfactory bulb (OB), lateral olfactory tract (LOT), and the locations of the anterior and posterior piriform cortex (aPC and pPC, respectively), which are approximately demarcated by the dashed line. rf, rhinal fissure. A coronal slice of the aPC, taken between the arrows, shows the laminar structure illustrated schematically in panel B. B, The three main layers of the PC. At left is a schematic representation of the density of neuronal somata in each layer, showing the high density of mainly principal cells in Layer II, and a lower density of neurons in Layers I and III. Schematic diagrams of the dendritic trees of the two main types of Layer II principal cells (SP, superficial pyramidal; SL, semilunar) and one type of Layer III principal cell (DP, deep pyramidal) are shown in grey. Schematic diagrams of the dendritic trees of four types of GABAergic interneurons are shown in black (B, bitufted; G, neurogliaform; H, horizontal; M, multipolar). The scale bar is approximate. Adapted from Neville and Haberly (2004).1 The piriform cortex is a three-layered structure on the ventral lateral surface of the cerebral hemisphere (Figure 1A). The major projection neurons of the piriform cortex, the pyramidal cells, locate their cell bodies in layers 2 and 3 and extend apical dendrites to layer 1 where they synapse with mitral cell afferents from the olfactory bulb (Figure 1B). 3-layered structure On the ventral-lateral surface of the cerebral hemisphere Synapse with mitral cell afferents in layer 1 http://aups.org.au/Proceedings/38/9-14/

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. Imaged at multiple sites in over 100 mice.

Odorant-Evoked Responses in Mouse Piriform Using In Vivo Two-Photon Calcium Imaging

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

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 in which different odorants activate unique but dispersed ensembles of cortical neurons. Neurons in the piriform cortex don’t have an apparent continuos 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 straight-forward. 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

http://www. nobelprize http://www.nobelprize.org/nobel_prizes/medicine/laureates/2004/illpres/2_olfactory.html

2-photon microscope

Practical theory of 2-photon microscopy 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. 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 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, diffraction-limited, co-localization. Imaging of UV-excited compounds with conventional optics.