Nonsensory target-dependent organization of piriform cortex

Abstract

The piriform cortex (PCX) is the largest component of the olfactory cortex and is hypothesized to be the locus of odor object formation. The distributed odorant representation found in PCX contrasts sharply with the topographical representation seen in other primary sensory cortices, making it difficult to test this view. Recent work in PCX has focused on functional characteristics of these distributed afferent and association fiber systems. However, information regarding the efferent projections of PCX and how those may be involved in odor representation and object recognition has been largely ignored. To investigate this aspect of PCX, we have used the efferent pathway from mouse PCX to the orbitofrontal cortex (OFC). Using double fluorescent retrograde tracing, we identified the output neurons (OPNs) of the PCX that project to two subdivisions of the OFC, the agranular insula and the lateral orbitofrontal cortex (AI-OPNs and LO-OPNs, respectively). We found that both AI-OPNs and LO-OPNs showed a distinct spatial topography within the PCX and fewer than 10% projected to both the AI and the LO as judged by double-labeling. These data revealed that the efferent component of the PCX may be topographically organized. Further, these data suggest a model for functional organization of the PCX in which the OPNs are grouped into parallel output circuits that provide olfactory information to different higher centers. The distributed afferent input from the olfactory bulb and the local PCX association circuits would then ensure a complete olfactory representation, pattern recognition capability, and neuroplasticity in each efferent circuit.olfaction | piriform cortex | olfactory perception T he olfactory system creates perceptual odor objects from often complex mixtures of diverse airborne chemicals (1, 2). This formidable job is mainly accomplished by a surprisingly "shallow" three-level pathway, comprising the olfactory epithelium, olfactory bulb, and olfactory cortex (3). The olfactory epithelium accommodates millions of olfactory sensory neurons (OSNs), each of which can be defined by the particular receptor protein selected for expression from the ∼1,000 odor receptor genes in the typical mammalian genome (4, 5). Axons from all OSNs expressing the same odor receptor coalesce into a few glomeruli on the surface of the olfactory bulb (6-8). Each glomerulus is therefore dedicated to a particular receptor. The position of each glomerulus appears to vary only slightly from animal to animal, giving rise to speculation that the glomeruli form a spatial map of odor sensitivities.Within the glomeruli, the incoming OSN axons form synapses with the apical dendrites of second-order neurons and the mitral and tufted cells, providing what would seem to be an anatomical basis for topographical odorant representation (9-11). Each of about a dozen mitral or tufted cells innervating only a single glomerulus send their axons to targets in a number of ventral forebrain areas, collectively termed the olfactory cortex (12).However, this...

Complementary codes for odor identity and intensity in olfactory cortex

“…Different subtypes can be distinguished genetically (Diodato et al, 2016) or by their projection targets (Chen et al, 2014). We now identify multiple functionally distinct cell types.…”

Respiration competes with theta for modulating parietal cortex neurons

“…Our results suggest that the integration of the multimodal information arriving in this region may be influenced by respiration-coupled inputs mediated by the ACC. In turn, respiration inputs are likely to arrive in frontal cortical regions via connections from the piriform and insular cortices [44,54].…”

What is so special about smell? Olfaction as a model system in neurobiology

“…Studies of the olfactory bulb (a multilayered neural structure situated at the frontal lobe of the brain) have successfully established a clear activation pattern 25 26. Contrary to expectations, however, this topographic pattern of the olfactory bulb is not maintained through further projections to the olfactory cortex 24 27 56 57. The implications following from such apparent idiosyncrasies of the olfactory cortex for our understanding of higher brain processing and olfaction need to be seen.…”

“…In the following sections, developments in the first of these two research questions will be examined in more detail. Concerning studies on higher-level brain processing in olfaction, which exceeds the scope of this review, see refs 10 24…”

“…OSNs expressing one receptor gene join in a spherical neural structure (glomerulus) in the olfactory bulb at the frontal lobe, forming topographic odour activation patterns 25 26. Signals from the bulb are projected to the olfactory cortex, whose organisation principles are still a matter of considerable debate 24 27…”