Quantitative analysis of axon collaterals of single superficial pyramidal cells in layer IIb of the piriform cortex of the guinea pig

Common principles for odour coding across vertebrates and invertebrates

The olfactory system is an ideal and tractable system for exploring how the brain transforms sensory inputs into behaviour. The basic tasks of any olfactory system include odour detection, discrimination and categorization. The challenge for the olfactory system is to transform the high-dimensional space of olfactory stimuli into the much smaller space of perceived objects and valence that endows odours with meaning. Our current understanding of how neural circuits address this challenge has come primarily from observations of the mechanisms of the brain for processing other sensory modalities, such as vision and hearing, in which optimized deep hierarchical circuits are used to extract sensory features that vary along continuous physical dimensions. The olfactory system, by contrast, contends with an ill-defined, high-dimensional stimulus space and discrete stimuli using a circuit architecture that is shallow and parallelized. Here, we present recent observations in vertebrate and invertebrate systems that relate the statistical structure and state-dependent modulation of olfactory codes to mechanisms of perception and odour-guided behaviour.

Cell assembly formation and structure in a piriform cortex model

The piriform cortex is rich in recurrent excitatory synaptic connections between pyramidal neurons. We asked how such connections could shape cortical responses to olfactory lateral olfactory tract (LOT) inputs. For this, we constructed a computational network model of anterior piriform cortex with 2000 multicompartment, multiconductance neurons (500 semilunar, 1000 layer 2 and 500 layer 3 pyramids; 200 superficial interneurons of two types; 500 deep interneurons of three types; 500 LOT afferents), incorporating published and unpublished data. With a given distribution of LOT firing patterns, and increasing the strength of recurrent excitation, a small number of firing patterns were observed in pyramidal cell networks: first, sparse firings; then temporally and spatially concentrated epochs of action potentials, wherein each neuron fires one or two spikes; then more synchronized events, associated with bursts of action potentials in some pyramidal neurons. We suggest that one function of anterior piriform cortex is to transform ongoing streams of input spikes into temporally focused spike patterns, called here "cell assemblies", that are salient for downstream projection areas.

Principles of odor coding in vertebrates and artificial chemosensory systems

The biological olfactory system is the sensory system responsible for the detection of the chemical composition of the environment. Several attempts to mimic biological olfactory systems have led to various artificial olfactory systems using different technical approaches. Here we provide a parallel description of biological olfactory systems and their technical counterparts. We start with a presentation of the input to the systems, the stimuli, and treat the interface between the external world and the environment where receptor neurons or artificial chemosensors reside. We then delineate the functions of receptor neurons and chemosensors as well as their overall I-O relationships. Up to this point, our account of the systems goes along similar lines. The next processing steps differ considerably: while in biology the processing step following the receptor neurons is the "integration" and "processing" of receptor neuron outputs in the olfactory bulb, this step has various realizations in electronic noses. For a long period of time, the signal processing stages beyond the olfactory bulb, i.e., the higher olfactory centers were little studied. Only recently there has been a marked growth of studies tackling the information processing in these centers. In electronic noses, a third stage of processing has virtually never been considered. In this review, we provide an up-to-date overview of the current knowledge of both fields and, for the first time, attempt to tie them together. We hope it will be a breeding ground for better information, communication, and data exchange between very related but so far little connected fields.

Distinct projection patterns of different classes of layer 2 principal neurons in the olfactory cortex

The broadly-distributed, non-topographic projections to and from the olfactory cortex may suggest a flat, non-hierarchical organization in odor information processing. Layer 2 principal neurons in the anterior piriform cortex (APC) can be divided into 2 subtypes: semilunar (SL) and superficial pyramidal (SP) cells. Although it is known that SL and SP cells receive differential inputs from the olfactory bulb (OB), little is known about their projections to other olfactory regions. Here, we examined axonal projections of SL and SP cells using a combination of mouse genetics and retrograde labeling. Retrograde tracing from the OB or posterior piriform cortex (PPC) showed that the APC projects to these brain regions mainly through layer 2b cells, and dual-labeling revealed many cells extending collaterals to both target regions. Furthermore, a transgenic mouse line specifically labeling SL cells showed that they send profuse axonal projections to olfactory cortical areas, but not to the OB. These findings support a model in which information flow from SL to SP cells and back to the OB is mediated by a hierarchical feedback circuit, whereas both SL and SP cells broadcast information to higher olfactory areas in a parallel manner.

Quantitative analysis of axon collaterals of single pyramidal cells of the anterior piriform cortex of the guinea pig

BACKGROUND

The role of the piriform cortex (PC) in olfactory information processing remains largely unknown. The anterior part of the piriform cortex (APC) has been the focus of cortical-level studies of olfactory coding, and associative processes have attracted considerable attention as an important part in odor discrimination and olfactory information processing. Associational connections of pyramidal cells in the guinea pig APC were studied by direct visualization of axons stained and quantitatively analyzed by intracellular biocytin injection in vivo.

RESULTS

The observations illustrated that axon collaterals of the individual cells were widely and spatially distributed within the PC, and sometimes also showed a long associational projection to the olfactory bulb (OB). The data showed that long associational axons were both rostrally and caudally directed throughout the PC, and the intrinsic associational fibers of pyramidal cells in the APC are omnidirectional connections in the PC. Within the PC, associational axons typically followed rather linear trajectories and irregular bouton distributions. Quantitative data of the axon collaterals of two pyramidal cells in the APC showed that the average length of axonal collaterals was 101 mm, out of which 79 mm (78% of total length) were distributed in the PC. The average number of boutons was 8926 and 7101, respectively, with 79% of the total number of boutons being distributed in the PC. The percentage of the total area of the APC and the posterior piriform cortex occupied by the average distribution region of the axon collaterals of two superficial pyramidal (SP) cells was about 18 and 5%, respectively.

CONCLUSION

Our results demonstrate that omnidirectional connection of pyramidal cells in the APC provides a substrate for recurrent processes. These findings indicate that the axon collaterals of SP cells in the PC could make synaptic contacts with all granule cells in the OB. This study provides the morphological evidence for understanding the mechanisms of information processing and associative memory in the APC.

Optophysiological analysis of associational circuits in the olfactory cortex

Primary olfactory cortical areas receive direct input from the olfactory bulb, but also have extensive associational connections that have been mainly studied with classical anatomical methods. Here, we shed light on the functional properties of associational connections in the anterior and posterior piriform cortices (aPC and pPC) using optophysiological methods. We found that the aPC receives dense functional connections from the anterior olfactory nucleus (AON), a major hub in olfactory cortical circuits. The local recurrent connectivity within the aPC, long invoked in cortical autoassociative models, is sparse and weak. By contrast, the pPC receives negligible input from the AON, but has dense connections from the aPC as well as more local recurrent connections than the aPC. Finally, there are negligible functional connections from the pPC to aPC. Our study provides a circuit basis for a more sensory role for the aPC in odor processing and an associative role for the pPC.

Pyramidal neurons are generated from oligodendroglial progenitor cells in adult piriform cortex

Previous studies have shown that oligodendroglial progenitor cells (OPCs) can give rise to neurons in vitro and in perinatal cerebral cortex in vivo. We now report that OPCs in adult murine piriform cortex express low levels of doublecortin, a marker for migratory and immature neurons. Additionally, these OPCs express Sox2, a neural stem cell marker, and Pax6, a transcription factor characteristic of progenitors for cortical glutamatergic neurons. Genetic fate-mapping by means of an inducible Cre-LoxP recombination system proved that these OPCs differentiate into pyramidal glutamatergic neurons in piriform cortex. Several lines of evidence indicated that these newly formed neurons became functionally integrated into the cortical neuronal network. Our data suggest that NG2(+)/PDGFRα(+) proteolipid protein promoter-expressing progenitors generate pyramidal glutamatergic neurons within normal adult piriform cortex.

An early critical period for long-term plasticity and structural modification of sensory synapses in olfactory cortex

Critical periods for plasticity of thalamic sensory inputs play an important role in developing neocortical circuits. During an early postnatal time window, pyramidal cells of visual, auditory, and somatosensory cortex undergo structural refinement and possess an enhanced ability for activity-dependent synaptic plasticity. In olfactory cortex, however, pyramidal cells receive direct sensory input from the olfactory bulb, and it is unclear whether the development of olfactory sensory circuits is governed by a critical period. Here, we show that NMDA receptor-dependent long-term potentiation and dendritic spine maturation occur only during a brief postnatal time window at sensory synapses of olfactory cortex pyramidal cells. In contrast, associational synapses onto the same cells retain the capacity for plasticity into adulthood.

Preferential neuron loss in the rat piriform cortex following pilocarpine-induced status epilepticus

Structures within the piriform cortex (PC) including the endopiriform nucleus (DEN) and pre-endopiriform nucleus (pEn) have been implicated to be involved in seizure genesis in models of temporal lobe epilepsy. We used stereological methods to examine the specificity and extent of neuron loss in the PC of pilocarpine-treated rats. Both 7 days and 2 months post-status epilepticus rats showed significant neuron loss in the pEn and DEN, layer III of the intermediate PC, and layers II and III of the caudal PC. Total losses in the PC were 40 and 46% in 7 days and 2 months post-status epilepticus rats, respectively (p<0.01). The numbers of parvalbumin (PV)- and cholecystokinin (CCK)-immunopositive neuron profiles significantly decreased, and somatostatin (SS)-immunopositive neuron profiles tended to decrease. A large decrease in the number of PV-immunopositive neuron profiles occurred in the pEn, adjoining parts of the DEN and deep layer III of the PC, portions of the DEN bordering the claustrum and agranular insular cortex, and layer III of the caudal PC. The regions with decreased numbers of PV-, CCK-, and SS-immunopositive neuron profiles overlapped with those where many Nissl-stained neurons were lost and many degenerating cell bodies were detected. These results suggest that the decreases in the numbers of PV/SS/CCK-immunopositive neurons are related to neuron loss rather than to a low rate of synthesis of their peptides or proteins.

Neural coding by two classes of principal cells in the mouse piriform cortex

The piriform (or primary olfactory) cortex is a trilaminar structure that is the first cortical destination of olfactory information, receiving monosynaptic input from the olfactory bulb. Here, we show that the main input layer of the piriform cortex, layer II, is dominated by two classes of principal neurons, superficial pyramidal (SP) and semilunar (SL) cells, with strikingly different properties. Action potentials in SP cells are followed by a Ni2+-sensitive afterdepolarization that promotes burst firing, whereas SL cells fire nonbursting action potentials that are followed by a powerful afterhyperpolarization. Synaptic inputs from the olfactory bulb onto SP cells exhibit prominent paired-pulse facilitation, which is attributable to residual presynaptic Ca2+ and a low probability of neurotransmitter release. In contrast, the same inputs onto SL cells do not facilitate. These distinctive synaptic and firing properties cause SP and SL cells to respond differently to in vivo-like bursts of afferent stimulation: SP cells tend to fire bursts of output action potentials at a higher frequency than the input, whereas SL cells tend to fire at a lower frequency than the input. When connected together in the canonical circuit of the piriform cortex, SP and SL cells transform the pattern of synaptic inputs they receive from the olfactory bulb, dispersing the firing rate and latency of output action potentials to an extent that depends on the strength of the input. Thus, the presence of two types of principal cells in layer II of the piriform cortex may underlie coding strategies used for the representation of odors.

Neural coding by two classes of principal cells in the mouse piriform cortex

The piriform (or primary olfactory) cortex is a trilaminar structure that is the first cortical destination of olfactory information, receiving monosynaptic input from the olfactory bulb. Here, we show that the main input layer of the piriform cortex, layer II, is dominated by two classes of principal neurons, superficial pyramidal (SP) and semilunar (SL) cells, with strikingly different properties. Action potentials in SP cells are followed by a Ni2+-sensitive afterdepolarization that promotes burst firing, whereas SL cells fire nonbursting action potentials that are followed by a powerful afterhyperpolarization. Synaptic inputs from the olfactory bulb onto SP cells exhibit prominent paired-pulse facilitation, which is attributable to residual presynaptic Ca2+ and a low probability of neurotransmitter release. In contrast, the same inputs onto SL cells do not facilitate. These distinctive synaptic and firing properties cause SP and SL cells to respond differently to in vivo-like bursts of afferent stimulation: SP cells tend to fire bursts of output action potentials at a higher frequency than the input, whereas SL cells tend to fire at a lower frequency than the input. When connected together in the canonical circuit of the piriform cortex, SP and SL cells transform the pattern of synaptic inputs they receive from the olfactory bulb, dispersing the firing rate and latency of output action potentials to an extent that depends on the strength of the input. Thus, the presence of two types of principal cells in layer II of the piriform cortex may underlie coding strategies used for the representation of odors.

Cortical contributions to olfaction: Plasticity and perception

In most sensory systems, the sensory cortex is the place where sensation approaches perception. As described in this review, olfaction is no different. The olfactory system includes both primary and higher order cortical regions. These cortical structures perform computations that take highly analytical afferent input and synthesize it into configural odor objects. Cortical plasticity plays an important role in this synthesis and may underlie olfactory perceptual learning. Olfactory cortex is also involved in odor memory and association of odors with multimodal input and contexts. Finally, the olfactory cortex serves as an important sensory gate, modulating information throughput based on recent experience and behavioral state.