Short-latency single unit processing in olfactory cortex

Cortical Microcircuit Mechanisms of Mismatch Negativity and Its Underlying Subcomponents

In the neocortex, neuronal processing of sensory events is significantly influenced by context. For instance, responses in sensory cortices are suppressed to repetitive or redundant stimuli, a phenomenon termed “stimulus-specific adaptation” (SSA). However, in a context in which that same stimulus is novel, or deviates from expectations, neuronal responses are augmented. This augmentation is termed “deviance detection” (DD). This contextual modulation of neural responses is fundamental for how the brain efficiently processes the sensory world to guide immediate and future behaviors. Notably, context modulation is deficient in some neuropsychiatric disorders such as schizophrenia (SZ), as quantified by reduced “mismatch negativity” (MMN), an electroencephalography waveform reflecting a combination of SSA and DD in sensory cortex. Although the role of NMDA-receptor function and other neuromodulatory systems on MMN is established, the precise microcircuit mechanisms of MMN and its underlying components, SSA and DD, remain unknown. When coupled with animal models, the development of powerful precision neurotechnologies over the past decade carries significant promise for making new progress into understanding the neurobiology of MMN with previously unreachable spatial resolution. Currently, rodent models represent the best tool for mechanistic study due to the vast genetic tools available. While quantifying human-like MMN waveforms in rodents is not straightforward, the “oddball” paradigms used to study it in humans and its underlying subcomponents (SSA/DD) are highly translatable across species. Here we summarize efforts published so far, with a focus on cortically measured SSA and DD in animals to maintain relevance to the classically measured MMN, which has cortical origins. While mechanistic studies that measure and contrast both components are sparse, we synthesize a potential set of microcircuit mechanisms from the existing rodent, primate, and human literature. While MMN and its subcomponents likely reflect several mechanisms across multiple brain regions, understanding fundamental microcircuit mechanisms is an important step to understand MMN as a whole. We hypothesize that SSA reflects adaptations occurring at synapses along the sensory-thalamocortical pathways, while DD depends on both SSA inherited from afferent inputs and resulting disinhibition of non-adapted neurons arising from the distinct physiology and wiring properties of local interneuronal subpopulations and NMDA-receptor function.

Balanced feedforward inhibition and dominant recurrent inhibition in olfactory cortex

Throughout the brain, the recruitment of feedforward and recurrent inhibition shapes neural responses. However, disentangling the relative contributions of these often-overlapping cortical circuits is challenging. The piriform cortex provides an ideal system to address this issue because the interneurons responsible for feedforward and recurrent inhibition are anatomically segregated in layer (L) 1 and L2/3 respectively. Here we use a combination of optical and electrical activation of interneurons to profile the inhibitory input received by three classes of principal excitatory neuron in the anterior piriform cortex. In all classes, we find that L1 interneurons provide weaker inhibition than L2/3 interneurons. Nonetheless, feedforward inhibitory strength covaries with the amount of afferent excitation received by each class of principal neuron. In contrast, intracortical stimulation of L2/3 evokes strong inhibition that dominates recurrent excitation in all classes. Finally, we find that the relative contributions of feedforward and recurrent pathways differ between principal neuron classes. Specifically, L2 neurons receive more reliable afferent drive and less overall inhibition than L3 neurons. Alternatively, L3 neurons receive substantially more intracortical inhibition. These three features--balanced afferent drive, dominant recurrent inhibition, and differential recruitment by afferent vs. intracortical circuits, dependent on cell class--suggest mechanisms for olfactory processing that may extend to other sensory cortices.

Cortical Feedback Decorrelates Olfactory Bulb Output in Awake Mice

The olfactory bulb receives rich glutamatergic projections from the piriform cortex. However, the dynamics and importance of these feedback signals remain unknown. Here, we use multiphoton calcium imaging to monitor cortical feedback in the olfactory bulb of awake mice and further probe its impact on the bulb output. Responses of feedback boutons were sparse, odor specific, and often outlasted stimuli by several seconds. Odor presentation either enhanced or suppressed the activity of boutons. However, any given bouton responded with stereotypic polarity across multiple odors, preferring either enhancement or suppression. Feedback representations were locally diverse and differed in dynamics across bulb layers. Inactivation of piriform cortex increased odor responsiveness and pairwise similarity of mitral cells but had little impact on tufted cells. We propose that cortical feedback differentially impacts these two output channels of the bulb by specifically decorrelating mitral cell responses to enable odor separation.

Odor representations in olfactory cortex: distributed rate coding and decorrelated population activity

VIDEO ABSTRACT

How information encoded in neuronal spike trains is used to guide sensory decisions is a fundamental question. In olfaction, a single sniff is sufficient for fine odor discrimination but the neural representations on which olfactory decisions are based are unclear. Here, we recorded neural ensemble activity in the anterior piriform cortex (aPC) of rats performing an odor mixture categorization task. We show that odors evoke transient bursts locked to sniff onset and that odor identity can be better decoded using burst spike counts than by spike latencies or temporal patterns. Surprisingly, aPC ensembles also exhibited near-zero noise correlations during odor stimulation. Consequently, fewer than 100 aPC neurons provided sufficient information to account for behavioral speed and accuracy, suggesting that behavioral performance limits arise downstream of aPC. These findings demonstrate profound transformations in the dynamics of odor representations from the olfactory bulb to cortex and reveal likely substrates for odor-guided decisions.

Diverse patterns of odor representation by neurons in the anterior piriform cortex of awake mice

The mammalian piriform cortex receives direct synaptic input from the olfactory bulb and is likely the locus for the formation of odor percept. It remains unclear how individual cortical neurons encode olfactory information in unanesthetized animals. By single-cell recordings from head-restrained awake mice, we studied the odor response profiles of individual neurons in the anterior piriform cortex (aPCX). Neurons were juxtacellularly labeled, and their cell types were determined by their morphology and neurotransmitter phenotypes. We found a considerable level of variability in selectivity patterns among pyramidal neurons (PNs). Approximately one-quarter of PNs were broadly activated by structurally dissimilar odorants, whereas the excitations to the rest of PNs were highly selective. Broad inhibition was only observed from a subpopulation of PNs. GABAergic neurons displayed nonselective excitatory responses to test odorants and rarely exhibited inhibition. In contrast, non-GABAergic nonpyramidal neurons in the deep layer tended to be strongly inhibited by multiple different odorants. Our findings suggest that odor representation is accomplished by both broadly tuned and narrow-tuned PNs in the aPCX of awake animals. In addition, various types of interneurons may play different roles in the intracortical processing of olfactory information.

Architecture of odor information processing in the olfactory system

Since the discovery of the superfamily of approximately 1000 odorant receptor genes in rodents, the structural simplicity as well as the complexity of the olfactory system have been revealed. The simple aspects include the one neuron-one receptor rule and the exclusive convergence of projections from receptor neurons expressing the same receptors to one or two glomeruli in the olfactory bulb. Odor decoding in the olfactory cortex or higher cortical areas is likely to be a complicated process that depends on the sequence of signal activation and the relative signal intensities of receptors overlapping for similar but different odors. The aim of the present study was to investigate odor information processing both in receptors and in the olfactory cortex. At the receptor level, the similarity and difference in receptor codes between a pair of chiral odorants were examined using the tissue-printing method for sampling all the epithelial zones. In order to dissect odor-driven signal processing in the olfactory cortex by reducing cross-talk with the non-olfactory activities, such as cyclic respiration or other sensory inputs, an in vitro preparation of isolated whole brain with an attached nose was developed, and the methodologies and resulting hypothesis of receptor-sensitivity-dependent hierarchical odor information coding were reviewed.

Olfactory clearance: what time is needed in clinical practice?

Objective:

To determine olfactory adaptation and clearance times for healthy individuals, and to assess the effect of common variables upon these parameters.

Study design and setting:

Fourteen healthy volunteers were recruited for a series of tests. Their initial olfactory threshold levels for phenethyl alcohol were determined. After olfactory exposure to a saturated solution of phenethyl alcohol (i.e. olfactory adaptation), the time taken for subjects to return to their initial olfactory threshold was then recorded (i.e. olfactory clearance). Visual analogue scale scores for subjective variables were also recorded.

Results:

The 14 subjects performed 120 tests in total. Despite consistent linear trends within individuals, olfactory clearance times varied widely within and between individuals. The mean olfactory clearance time for phenethyl alcohol was 170 seconds (range 81–750). Univariate analysis showed a relationship between olfactory clearance times and age (p = 0.031), symptoms (p = 0.029) and mood (p = 0.048).

Conclusions:

When testing a person's sense of smell in a clinical setting, recent exposure to similar smells should be noted, and a period of 15 minutes needs to be allowed before retesting if using phenethyl alcohol. Other variables need not be controlled, but greater clearance time may be needed for older patients.

Synaptic adaptation and odor-background segmentation

Habituation is a form of non-associative memory that plays an important role in filtering stable or redundant inputs. The present study examines the contribution of habituation and cortical adaptation to odor-background segmentation. Segmentation of target odorants from background odorants is a fundamental computational requirement for the olfactory system. Recent electrophysiological data have shown that odor specific adaptation in piriform cortex neurons, mediated at least partially by synaptic adaptation between the olfactory bulb outputs and piriform cortex pyramidal cells, may provide an ideal mechanism for odor-background segmentation. This rapid synaptic adaptation acts as a filter to enhance cortical responsiveness to changing stimuli, while reducing responsiveness to static, potentially background stimuli. Using previously developed computational models of the olfactory system, we here show how synaptic adaptation at the olfactory bulb input to the piriform cortex, as demonstrated electrophysiologically, creates odor specific adaptation. We show how this known feature of olfactory cortical processing can contribute to adaptation to a background odor and to odor-background segmentation. We then show in a behavioral experiment that the odor-background segmentation is perceptually important and functions at the same time-scale as the synaptic adaptation observed between the olfactory bulb and cortex.

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.

The role of metabotropic glutamate receptors and cortical adaptation in habituation of odor-guided behavior

Decreases in behavioral investigation of novel stimuli over time may be mediated by a variety of factors including changes in attention, internal state, and motivation. Sensory cortical adaptation, a decrease in sensory cortical responsiveness over prolonged stimulation, may also play a role. In olfaction, metabotropic glutamate receptors on cortical afferent pre-synaptic terminals have been shown to underlie both cortical sensory adaptation and habituation of odor-evoked reflexes. The present experiment examined whether blockade of sensory cortical adaptation through bilateral infusion of the group III metabotropic glutamate receptor antagonist cyclopropyl-4-phosphonophenylglycine (CPPG) into the anterior piriform cortex could reduce habituation of a more complex odor-driven behavior such as investigation of a scented object or a conspecific. The results demonstrate that time spent investigating a scented jar, or a conspecific, decreases over the course of a continuous 10 minute trial. Acute infusion of CPPG bilaterally into the anterior piriform cortex significantly enhanced the time spent investigating the scented jar compared to investigation time in control rats, without affecting overall behavioral activity levels. Infusions into the brain outside of the piriform cortex were without effect. CPPG infusion into the piriform cortex also produced an enhancement of time spent investigating a conspecific, although this effect was not significant.

Projections from orbitofrontal cortex to anterior piriform cortex in the rat suggest a role in olfactory information processing

The orbitofrontal cortex (OFC) has been characterized as a higher-order, multimodal sensory cortex. Evidence from electrophysiological and behavioral studies in the rat has suggested that OFC plays a role in modulating olfactory guided behavior, and a significant projection to OFC arises from piriform cortex, the traditional primary olfactory cortex. To discern how OFC interacts with primary olfactory structures, the anterograde tracer Phaseolus vulgaris leucoagglutinin was injected into orbitofrontal cortical areas in adult male rats. Labeled fibers were found in the piriform cortex and olfactory bulb on the side ipsilateral to the injection. Notably, the projection to piriform cortex was predominantly from ventrolateral orbital cortex, and was not uniform; rostrally, the projection to the ventral portion of the anterior piriform cortex (APC) was substantial, while the dorsal APC was virtually free of labeled fibers. Labeled fibers were found in both the dorsal and ventral portions in more caudal regions of APC. Most labeled fibers were found in layer III, although a substantial number of fibers were observed in layers Ib and II. Labeled fibers in posterior piriform cortex also were seen after injection into orbitofrontal areas. Taken together with previous reports, these findings suggest that piriform cortex includes multiple subdivisions, which may perform separate, parallel functions in olfactory information processing. Further, these results suggest that the OFC, in addition to its putative role in encoding information about the significance of olfactory stimuli, may play a role in modulating odor response properties of neurons in piriform cortex.

Odor maps in the olfactory cortex

In the olfactory system, environmental chemicals are deconstructed into neural signals and then reconstructed to form odor perceptions. Much has been learned about odor coding in the olfactory epithelium and bulb, but little is known about how odors are subsequently encoded in the cortex to yield diverse perceptions. Here, we report that the representation of odors by fixed glomeruli in the olfactory bulb is transformed in the cortex into highly distributed and multiplexed odor maps. In the mouse olfactory cortex, individual odorants are represented by subsets of sparsely distributed neurons. Different odorants elicit distinct, but partially overlapping, patterns that are strikingly similar among individuals. With increases in odorant concentration, the representations expand spatially and include additional cortical neurons. Structurally related odorants have highly related representations, suggesting an underlying logic to the mapping of odor identities in the cortex.

Derivation and analysis of basic computational operations of thalamocortical circuits

Shared anatomical and physiological features of primary, secondary, tertiary, polysensory, and associational neocortical areas are used to formulate a novel extended hypothesis of thalamocortical circuit operation. A simplified anatomically based model of topographically and nontopographically projecting ("core" and "matrix") thalamic nuclei, and their differential connections with superficial, middle, and deep neocortical laminae, is described. Synapses in the model are activated and potentiated according to physiologically based rules. Features incorporated into the models include differential time courses of excitatory versus inhibitory postsynaptic potentials, differential axonal arborization of pyramidal cells versus interneurons, and different laminar afferent and projection patterns. Observation of the model's responses to static and time-varying inputs indicates that topographic "core" circuits operate to organize stored memories into natural similarity-based hierarchies, whereas diffuse "matrix" circuits give rise to efficient storage of time-varying input into retrievable sequence chains. Examination of these operations shows their relationships with well-studied algorithms for related functions, including categorization via hierarchical clustering, and sequential storage via hash- or scatter-storage. Analysis demonstrates that the derived thalamocortical algorithms exhibit desirable efficiency, scaling, and space and time cost characteristics. Implications of the hypotheses for central issues of perceptual reaction times and memory capacity are discussed. It is conjectured that the derived functions are fundamental building blocks recurrent throughout the neocortex, which, through combination, gives rise to powerful perceptual, motor, and cognitive mechanisms.

Coordinate synaptic mechanisms contributing to olfactory cortical adaptation

Anterior piriform cortex (aPCX) neurons rapidly filter repetitive odor stimuli despite relatively maintained input from mitral cells. This cortical adaptation is correlated with short-term depression of afferent synapses, in vivo. The purpose of this study was to elucidate mechanisms underlying this nonassociative neural plasticity using in vivo and in vitro preparations and to determine its role in cortical odor adaptation. Lateral olfactory tract (LOT)-evoked responses were recorded in rat aPCX coronal slices. Extracellular and intracellular potentials were recorded before and after simulated odor stimulation of the LOT. Results were compared with in vivo intracellular recordings from aPCX layer II/III neurons and field recordings in urethane-anesthetized rats stimulated with odorants. The onset, time course, and extent of LOT synaptic depression during both in vitro electrical and in vivo odorant stimulation methods were similar. Similar to the odor specificity of cortical odor adaptation in vivo, there was no evidence of heterosynaptic depression between independent inputs in vitro. In vitro evidence suggests at least two mechanisms contribute to this activity-dependent synaptic depression: a rapidly recovering presynaptic depression during the initial 10-20 sec of the post-train recovery period and a longer lasting (approximately 120 sec) depression that can be blocked by the metabotropic glutamate receptor (mGluR) II/III antagonist (RS)-alpha-cyclopropyl-4-phosphonophenylglycine (CPPG) and by the beta-adrenergic receptor agonist isoproterenol. Importantly, in line with the in vitro findings, both adaptation of odor responses in the beta (15-35 Hz) spectral range and the associated synaptic depression can also be blocked by intracortical infusion of CPPG in vivo.

Olfactory bulb mitral-tufted cell plasticity: odorant-specific tuning reflects previous odorant exposure

Olfactory system second-order neurons, mitral-tufted cells, have odorant receptive fields (ORFs) (molecular receptive ranges in odorant space for carbon chain length in organic odorant molecules). This study quantified several dimensions of these excitatory odorant receptive fields to novel odorants in rats and then examined the effects of passive odorant exposure on the shape of the ORF-tuning curve. ORFs for carbon chain length of novel ethyl esters (pure odorants that the animals had not been exposed to previously) were determined before and after a 50 sec prolonged exposure to one of the odorants. In response to novel odorants, quantitative analysis of mitral-tufted cell excitatory ORFs revealed that the median ORF width spanned 3-4 carbons, generally with a single-most excitatory odorant. Exposure to either the most excitatory odorant (ON-PEAK) or an odorant that was two carbons longer (OFF-PEAK) for 50 sec produced whole ORF suppression immediately after the end of the prolonged exposure, with the ON-PEAK exposure producing the greatest suppression. These results are consistent with a feature-detecting function for mitral-tufted cells. Redetermination of the ORF 15 and 60 min after the exposure revealed that OFF-PEAK exposure produced a reduction in responsiveness to the best odorant and an increase in responsiveness to the exposed odorant. In contrast, exposure to the ON-PEAK odorant or no odorant did not affect ORFs. Given that mitral-tufted cells receive exclusive excitatory input from olfactory receptor neurons expressing identical receptor proteins, it is hypothesized that experience-induced mitral-tufted cell ORF changes reflect modulation of lateral and centrifugal olfactory bulb circuits.

Olfactory perceptual learning: the critical role of memory in odor discrimination

The major problem in olfactory neuroscience is to determine how the brain discriminates one odorant from another. The traditional approach involves identifying how particular features of a chemical stimulus are represented in the olfactory system. However, this perspective is at odds with a growing body of evidence, from both neurobiology and psychology, which places primary emphasis on synthetic processing and experiential factors--perceptual learning--rather than on the structural features of the stimulus as critical for odor discrimination. In the present review of both psychological and sensory physiological data, we argue that the initial odorant feature extraction/analytical processing is not behaviorally/consciously accessible, but rather is a first necessary stage for subsequent cortical synthetic processing which in turn drives olfactory behavior. Cortical synthetic coding reflects an experience-dependent process that allows synthesis of novel co-occurring features, similar to processes used for visual object coding. Thus, we propose that experience and cortical plasticity are not only important for traditional associative olfactory memory (e.g. fear conditioning, maze learning, and delayed-match-to-sample paradigms), but also play a critical, defining role in odor discrimination.

Rapid, experience-induced enhancement in odorant discrimination by anterior piriform cortex neurons

Current views of odorant discrimination by the mammalian olfactory system suggest that the piriform cortex serves as a site of odor object synthesis. Given the enormous number of odorant feature combinations possible in nature, however, it seems unlikely that cortical synthetic receptive fields (RFs) are innate but rather require experience for their formation. The present experiment addressed two issues. First, we made a direct comparison of mitral/tufted cell and anterior piriform cortex (aPCX) neuron abilities to discriminate odorant mixtures from their components to further test whether aPCX neurons can treat collections of features different from the features themselves (synthetic coding). Second, we attempted to determine the minimum duration of experience necessary for formation of cortical synthetic RFs. Single-unit recordings were made from mitral/tufted cells and aPCX layer II/III neurons in urethan-anesthetized rats. Cross-habituation between novel binary mixtures and their novel components was used to determine odor discrimination abilities. The results suggest that after >/=50 s of experience with a binary mixture, aPCX neurons can discriminate the mixture from its components, whereas mitral/tufted cells cannot. However, when limited to 10 s of experience with the mixture, aPCX neurons appear similar to mitral/tufted cells and do not discriminate mixtures from components. These results suggest experience-dependent synthetic processing in aPCX and suggest an important role for perceptual learning in normal odor discrimination.

Piriform cortex functional heterogeneity revealed by cellular responses to odours

The role of the piriform cortex (PC) in olfactory information processing remains mainly unknown. Indeed, until recently, only a few studies have investigated the response of PC neurons to odours and these studies did not take into account the functional heterogeneity of the PC previously described using an electrical stimulation paradigm. In this experiment, extracellular activity in response to odour was recorded in urethane anaesthetized rats in the different parts of the cortex ranging from anterior to posterior. A large percentage of cortical cells were silent at rest, and this percentage increased from anterior to posterior. Analysis of odour evoked activity revealed a large percentage of nonresponsive cells that increased from anterior to posterior. Cell activity was largely synchronized with breathing and different temporal patterns were observed. The anterior PC was characterized by odour-evoked responses phase-locked with the inhalation-exhalation transition period. By contrast, activity in the posterior PC was mainly phase-locked with inhalation or exhalation. These data confirm the spatial functional heterogeneity previously reported in the PC. Functional anatomy of the PC suggests that activity in the anterior PC can be mainly driven by afferent activity coming from the OB whereas posterior cells were certainly entrained by more complex mechanisms.

Odor-evoked activity is spatially distributed in piriform cortex

Much data on the olfactory bulb (OB) indicates that structural characteristics of odorant molecules are encoded as ordered, spatially consolidated sets of active cells. New results with "genetic tracing" (Zou et al. [2001] Nature 414:173-179) suggest that spatial order is also present in projections from the OB to the olfactory cortex. For the piriform cortex (PC), results with this technique indicate that afferents conveying input derived from single olfactory receptors (ORs) are distributed to well-defined patches in the anterior PC (APC) but that these patches are much larger than in the OB. We have used c-fos induction to examine how input patterning for single ORs is translated into patterns of odor-evoked cellular activity in the PC. The laminar distribution of labeled cells and dual-immunostaining for gamma-aminobutyric acid (GABA)ergic markers indicated that activity was detected largely in pyramidal cells. In odor-stimulated rats, labeled cells were present throughout the posterior PC (PPC) but were concentrated in prominent rostrocaudal bands in APC. Analysis of responses to different odorants and concentrations revealed that locations and shapes of bands conveyed no apparent information regarding odor quality, rather, they appeared to correspond to subregions of the APC distinguished by cytoarchitecture and connectivity. Small-scale variations in labeling density were observed within APC bands and throughout the PPC that could reflect the presence of a complex topographical order, but discrete patches at consistent locations as observed by genetic tracing were absent. This finding suggests that as a result of afferent overlap and intracortical processing, odor-quality information is represented by spatially distributed sets of cells. A distributed organization may be optimal for discriminating biologically relevant odorants that activate large numbers of ORs.

Experience modifies olfactory acuity: acetylcholine-dependent learning decreases behavioral generalization between similar odorants

Perceptual learning has been demonstrated in several thalamocortical sensory systems wherein experience enhances sensory acuity for trained stimuli. This perceptual learning is believed to be dependent on changes in sensory cortical receptive fields. Sensory experience and learning also modifies receptive fields and neural response patterns in the mammalian olfactory system; however, to date there has been little reported evidence of learned changes in behavioral olfactory acuity. The present report used a bradycardial orienting response and cross-habituation paradigm that allowed assessment of behavioral discrimination of nearly novel odorants, and then used the same paradigm to examine odorant discrimination after associative olfactory conditioning with similar or dissimilar odorants. The results demonstrate that associative conditioning can enhance olfactory acuity for odors that are the same as or similar to the learned odorant, but not for odors dissimilar to the learned odorant. Furthermore, scopolamine injected before associative conditioning can block the acquisition of this learned enhancement in olfactory acuity. These results could have important implications for mechanisms of olfactory perception and memory, as well as for correlating behavioral olfactory acuity with observed spatial representations of odorant features in the olfactory system.

Spike coding in pyramidal cells of the piriform cortex of rat

The study of cortical oscillations has undergone a renaissance in recent years because of their presumed role in cognitive function. Of particular interest are frequencies in the gamma (30-100 Hz) and theta (3-12 Hz) ranges. In this paper, we use spike coding techniques and in vitro whole cell recording to assess the ability of individual pyramidal cells of the piriform cortex to code inputs occurring in these frequencies. The results suggest that the spike trains of individual neurons are much better at representing frequencies in the theta range than those in the gamma range.

A new subdivision of anterior piriform cortex and associated deep nucleus with novel features of interest for olfaction and epilepsy

The anterior part of the piriform cortex (the APC) has been the focus of cortical-level studies of olfactory coding and associative processes and has attracted considerable attention as a result of a unique capacity to initiate generalized tonic-clonic seizures. Based on analysis of cytoarchitecture, connections, and immunocytochemical markers, a new subdivision of the APC and an associated deep nucleus are distinguished in the rat. As a result of its ventrorostral location in the APC, the new subdivision is termed the APC(VR). The deep nucleus is termed the pre-endopiriform nucleus (pEn) based on location and certain parallels to the endopiriform nucleus. The APC(VR) has unique features of interest for normal function: immunostaining suggests that it receives input from tufted cells in the olfactory bulb in addition to mitral cells, and it provides a heavy, rather selective projection from the piriform cortex to the ventrolateral orbital cortex (VLO), a prefrontal area where chemosensory, visual, and spatial information converges. The APC(VR) also has di- and tri-synaptic projections to the VLO via the pEn and the submedial thalamic nucleus. The pEn is of particular interest from a pathological standpoint because it corresponds in location to the physiologically defined "deep piriform cortex" ("area tempestas") from which convulsants initiate temporal lobe seizures, and blockade reduces ischemic damage to the hippocampus. Immunostaining revealed novel features of the pEn and APC(VR) that could alter excitability, including a near-absence of gamma-aminobutyric acid (GABA)ergic "cartridge" endings on axon initial segments, few cholecystokinin (CCK)-positive basket cells, and very low gamma-aminobutyric acid transporter-1 (GAT1)-like immunoreactivity. Normal functions of the APC(VR)-pEn may require a shaping of neuronal activity by inhibitory processes in a fashion that renders this region susceptible to pathological behavior.

Comparison of odor receptive field plasticity in the rat olfactory bulb and anterior piriform cortex

Recent work in the anterior piriform cortex (aPCX) has demonstrated that cortical odor receptive fields are highly dynamic, showing rapid changes of both firing rate and temporal patterning within relatively few inhalations of an odor, despite relatively maintained, patterned input from olfactory bulb mitral/tufted cells. The present experiment examined the precision (odor-specificity) of this receptive field plasticity and compared it with the primary cortical afferent, olfactory bulb mitral/tufted cells. Adult Long-Evans hooded rats, urethan anesthetized and freely breathing, were used for single-unit recording from mitral/tufted and aPCX layer II/III neurons. Partial mapping of receptive fields to alkane odors (pentane, heptane, and nonane) was performed before and immediately after habituation (50-s exposure) to one of the alkanes. The results demonstrated that odor habituation of aPCX responses was odor specific, with minimal cross-habituation between alkanes differing by as few as two carbons. Mitral/tufted cells, however, showed strong cross-habituation within the odor set with the most profound cross effects to carbon chains shorter than the habituating stimulus. The results suggest that although mitral/tufted cells and aPCX neurons have roughly similar odor receptive fields, aPCX neurons have significantly better odor discrimination within their receptive field. The results have important implications for understanding the underlying bases of receptive fields in olfactory system neurons and the mechanisms of odor discrimination and memory.

Physiological characterization of layer III non-pyramidal neurons in piriform (olfactory) cortex of rat

We performed whole-cell recordings of layer III non-pyramidal neurons in the piriform cortex of Sprague-Dawley rats. For comparison purposes, recordings were made from deep pyramidal cells, which are also present in layer III. These two cell types could be distinguished both anatomically and physiologically. Anatomically, the layer III non-pyramidal neuron displayed smooth beady dendrites, while deep pyramidal cells showed thicker dendrites with spines. The dendrites of the layer III non-pyramidal neuron also tended to be restricted to layer III while deep pyramidal cells had long apical dendrites that spanned layers I and II. Although the resting membrane potentials of both cell types were very similar, significant differences were noted in other physiological measures. Layer III non-pyramidal neurons typically displayed higher input resistances, faster time constants, smaller spike amplitudes, shorter spike widths, and higher spike thresholds. In addition, layer III non-pyramidal neurons were able to spike at much higher rates when stimulated with the same level of threshold normalized current injection. The most dramatic differences in physiology were seen in the pattern of spiking in response to increasing levels of positive constant current pulses. Layer III non-pyramidal neurons showed qualitatively different responses at low and high levels of stimulation. At low levels, spikes occurred with long latency and the firing frequency increased throughout the duration of the current pulse. At high levels, non-pyramidal neurons started spiking with short latency, followed by a decrease in firing frequency, which in turn was followed by an increase in firing frequency. Deep pyramidal neurons differed dramatically from this pattern, displaying a qualitatively similar response at all levels of current injection. This response was characterized by short latency spikes and spike adaptation for the duration of the current pulse.

Orbital cortex neuronal responses during an odor-based conditioned associative task in rats

Neuronal activity in the rat orbital cortex during discrimination of various odors [five volatile organic compounds (acetophenone, isoamyl acetate, cyclohexanone, p-cymene and 1,8-cineole), and food- and cosmetic-related odorants (black pepper, cheese, rose and perfume)] and other conditioned sensory stimuli (tones, light and air puff) was recorded and compared with behavioral responses to the same odors (black pepper, cheese, rose and perfume). In a neurophysiological study, the rats were trained to lick a spout that protruded close to its mouth to obtain sucrose or intracranial self-stimulation reward after presentation of conditioned stimuli. Of 150 orbital cortex neurons recorded during the task, 65 responded to one or more types of sensory stimuli. Of these, 73.8% (48/65) responded during presentation of an odor. Although the mean breadth of responsiveness (entropy) of the olfactory neurons based on the responses to five volatile organic compounds and air (control) was rather high (0.795), these stimuli were well discriminated in an odor space resulting from multidimensional scaling using Pearson's correlation coefficients between the stimuli. In a behavioral study, a rat was housed in an equilateral octagonal cage, with free access to food and choice among eight levers, four of which elicited only water (no odor, controls), and four of which elicited both water and one of four odors (black pepper, cheese, rose or perfume). Lever presses for each odor and control were counted. Distributions of these five stimuli (four odors and air) in an odor space derived from the multidimensional scaling using Pearson's correlation coefficients based on behavioral responses were very similar to those based on neuronal responses to the same five stimuli. Furthermore, Pearson's correlation coefficients between the same five stimuli based on the neuronal responses and those based on behavioral responses were significantly correlated. The results demonstrated a pivotal role of the rat orbital cortex in olfactory sensory processing and suggest that the orbital cortex is important in the manifestation of various motivated behaviors of the animals, including odor-guided motivational behaviors (odor preference).

Time course of odorant-induced activation in the human primary olfactory cortex

Paradoxically, attempts to visualize odorant-induced functional magnetic resonance imaging (fMRI) activation in the human have yielded activations in secondary olfactory regions but not in the primary olfactory cortex-piriform cortex. We show that odorant-induced activation in primary olfactory cortex was not previously made evident with fMRI because of the unique time course of activity in this region: in primary olfactory cortex, odorants induced a strong early transient increase in signal amplitude that then habituated within 30-40 s of odorant presence. This time course of activation seen here in the primary olfactory cortex of the human is almost identical to that recorded electrophysiologically in the piriform cortex of the rat. Mapping activation with analyses that are sensitive to both this transient increase in signal amplitude, and temporal-variance, enabled us to use fMRI to consistently visualize odorant-induced activation in the human primary olfactory cortex. The combination of continued accurate odorant detection at the behavioral level despite primary olfactory cortex habituation at the physiological level suggests that the functional neuroanatomy of the olfactory response may change throughout prolonged olfactory stimulation.

Odor specificity of habituation in the rat anterior piriform cortex

Exposure to odorants results in a rapid (<10 s) reduction in odor-evoked activity in the rat piriform cortex despite relatively maintained afferent input from olfactory bulb mitral cells. To further understand this form of cortical plasticity, a detailed analysis of its odor specificity was performed. Habituation of odor responses in anterior piriform cortex single units was examined in anesthetized, freely breathing rats. The magnitude of single-unit responses of layer II/III neurons to 2-s odor pulses were examined before and after a 50-s habituating stimulus of either the same or different odor. The results demonstrated that odor habituation was odor specific, with no significant cross-habituation between either markedly different single odors or between odors within a series of straight chain alkanes. Furthermore, habituation to binary 1:1 mixtures produced minimal cross-habituation to the components of that mixture. These latter results may suggest synthetic odor processing in the olfactory system, with novel odor mixtures processed as unique stimuli. Potential mechanisms of odor habituation in the piriform cortex must be able to account for the high degree of specificity of this effect.

Habituation of odor responses in the rat anterior piriform cortex

Simultaneous recordings of main olfactory bulb (MOB) and anterior piriform cortex (aPCX) neuron responses to repeated and prolonged odor pulses were examined in freely breathing, urethan-anesthetized rats. Comparisons of odor responses were made between multi-unit recordings of MOB activity and single-unit extracellular and intracellular recordings of Layer II/III aPCX neurons. Odor stimuli consisted of either 2-s pulses repeated at 30-s intervals or a single, prolonged 50-s stimulus. Respiration rate was monitored throughout. MOB and aPCX neuron responses to odor were quantified both through firing frequency and through the temporal patterning of firing over the respiratory cycle. The results demonstrate that aPCX neurons habituate significantly more (faster) than MOB neurons with both repeated and prolonged stimulation paradigms. This enhanced habituation is expressed as both a decrease in aPCX firing despite maintained odor-evoked MOB input and as a decrease in aPCX respiratory cycle entrainment despite maintained MOB cyclic input. Intracellular aPCX recordings suggest that several mechanisms may be involved in this experience-induced change in aPCX function, including 1) decreased excitatory driveof aPCX neurons, 2) decreased excitability of aPCX neurons,and/or 3) enhancement in odor-evoked inhibition of aPCX neurons. These studies provide the initial basis for understanding the mechanisms of nonassociative plasticity in olfactory cortex.

Optical recording of the rat piriform cortex activity

The piriform cortex (PCx) is a phylogenetically old brain structure which presents characteristics of a content-addressable memory. Taking into account its particular anatomo-functional organization, we hypothesized that this cortex could behave rather as an assembly of different functional units than as a functionally homogeneous structure. This hypothesis was tested by using both anatomical and functional approaches. Immunohistological and tracing experiments demonstrated that both the connections of the PCx with the higher nervous centres, and its monoaminergic and cholinergic modulatory afferents exhibited a heterogeneous distribution. Then, optical monitoring of its neuronal activity with a voltage-sensitive dye pointed out that the PCx is a functionally heterogeneous structure. Electrical stimulations of the olfactory bulb showed that the inhibitory processes which control the cortical responsiveness were not identical in all the PCx area. Two different functional areas at least could be distinguished: in the ventromedial PCx, the afferent activity is privileged since the level of inhibition of disynaptic activation remained large during repetitive stimuli. Contrarily, in the posterior PCx, the disynaptic activity remained unchanged in response to successive stimulations and the responses of neighbouring sites were statistically more synchronized than in its anterior part. Moreover, a late depolarization wave was significantly larger in the posterior PCx. These data are in good agreement with the results provided by computational models of the PCx. In the future, theoretical and experimental investigations of this cortex will be useful for understanding olfactory information processing and as a model of brain functioning at the neocortical level as well.

Binaral interactions in the rat piriform cortex

Single-unit recordings were made from layer II/III anterior piriform cortex (aPCX) neurons in adult Wistar rats to examine odor response patterns to unilaterally and bilaterally delivered stimuli. Isoamyl acetate odor stimulation was presented either unilaterally through tubes inserted into the external nares, or bilaterally during unilateral olfactory bulb lidocaine infusions. Olfactory bulb multiunit or slow-wave activity was recorded simultaneously bilaterally to monitor selectivity of unilateral odor stimulation. The results demonstrate that 1) commissural input to aPCX neurons is sufficient to drive odor responses, and 2) aPCX neurons can be classified on the basis of spatial receptive field type. These receptive fields include cells that respond 1) selectively to ipsilateral stimulation, 2) selectively to contralateral stimulation, 3) to either ipsilateral or contralateral stimulation, and 4) selectively to bilateral stimulation. The potential functions of binaral convergence in the piriform cortex are discussed, and may include enhancement of perceived odor intensity and bilateral access to olfactory memory.

Odor coding properties of frog olfactory cortical neurons

Until now, in amphibians, response odor properties of primary cortical neurons had never been investigated. Furthermore, very few data on this subject are available in other species. This prompted us to explore the functional properties of olfactory cortical neurons at rest and in response to odors. To achieve this, our experience with odor coding in the first two stages of the frog olfactory system, the olfactory mucosa and the olfactory bulb, led us to use odor stimuli which were chemical compounds with known stimulating properties, delivered to the mucosa in controlled conditions over a wide concentration range. Most of the cortical neurons were found to be very silent at rest, their average spontaneous activity being significantly lower than that of bulb neurons recorded previously in the same conditions. Cortical cells displayed, with all odors combined, 35% excitatory responses and 8% inhibitory responses. The excitatory response rate was similar to that of the bulb, while the inhibitory response rate was about 4.5-fold lower. Interestingly, two functional groups of cortical cells emerged based both on differences in response temporal patterning to odors delivered at increasing concentrations and in qualitative discrimination power. Regarding intensity coding, group 1 cells (53%) displayed "classical" temporal pattern evolution, increase of discharge frequencies and decrease of latency and burst duration, over the concentration range. The responses of group 2 cells (47%) were clearly original, since they consisted of a single spike (or more rarely two spikes) occurring with a strictly reproducible latency at a given concentration and a decreased latency as a function of increasing concentration. The dynamics of cell recruitment in the cortex showed that group 1 cell recruitment mimicked that of mitral cells, group 2 cells being recruited at higher concentrations. The analysis of qualitative discrimination properties of cortical cells regarding the eight-odor set revealed that the discrimination power of group 2 cells was similar to that of mitral cells. By contrast, the qualitative discrimination power of group 1 cells was found to be similar to that of neuroreceptor cells. In conclusion, this pioneer approach leads us to report that olfactory cortical neurons of the frog are responsive to odors and can be clearly divided into two groups based on functional criteria. Group 1 cells, which were relatively selective, poorly discriminating but sensitive, may be mainly devoted to intensity coding. By contrast, group 2 cells, which were not very sensitive but were selective and discriminating, were hypothesized to provide minimal intensity coding and thus to be mainly devoted to qualitative discrimination tasks.

Distinct memory circuits composing the hippocampal region

The very different anatomical designs of the adjacent circuitries of the cortico-hippocampal pathway, along with their somewhat different synaptic plasticity mechanisms, suggest a nearly serial pathway of distinct memory circuits each contributing its own specialized processing operation to overall hippocampal function. Modeling and formal theoretical analysis of the prominent anatomical design features of particular circuits (piriform/entorhinal cortex; hippocampal field CA3; hippocampal field CA1) are found to identify potential emergent function not readily arrived at in the absence of these formal models, and yet which once derived can be seen potentially to confer unique capabilities to an integrated hippocampal mechanism for processing memories during behavior.

Further characteristics of long-term potentiation in piriform cortex

Mechanisms for the induction and expression of long-term potentiation (LTP) were studied in slices of piriform cortex. Cooperativity among afferent inputs as a controlling factor for induction of LTP was tested by pairing stimulation of one input that normally does not induce LTP with stimulation of another input. Combined stimulation, given either to two weak inputs with simultaneous bursts or by pairing single pulses with bursts, did effectively induce LTP. Tests for expression of LTP by NMDA vs. non-NMDA receptors indicated that non-NMDA receptor-mediated responses expressed much greater LTP than NMDA receptor-mediated responses. Ratios for paired-pulse facilitation and depression were not altered after induction of LTP. These characteristics are comparable to those exhibited by synapses in the CA1 field of hippocampus.

Central olfactory connections in the macaque monkey

The connections between the olfactory bulb, primary olfactory cortex, and olfactory related areas of the orbital cortex were defined in macaque monkeys with a combination of anterograde and retrograde axonal tracers and electrophysiological recording. Anterograde tracers placed into the olfactory bulb labeled axons in eight primary olfactory cortical areas: the anterior olfactory nucleus, piriform cortex, ventral tenia tecta, olfactory tubercle, anterior cortical nucleus of the amygdala, periamygdaloid cortex, and olfactory division of the entorhinal cortex. The bulbar axons terminate in the outer part of layer I throughout these areas and are most dense in areas that are close to the lateral olfactory tract. Labeled axons also were found in the superficial part of nucleus of the horizontal diagonal band. Retrograde tracers injected into the olfactory bulb labeled cells in the nucleus of the diagonal band and in all of the primary olfactory cortical areas except the olfactory tubercle. Electrical stimulation of the olfactory bulb evoked short-latency unit responses and a characteristic field wave in the primary olfactory cortex. Multiunit activity in layer II tended to be of shorter latency than that in layer III and the endopiriform nucleus. Associational connections within the primary olfactory cortex were demonstrated with anterograde tracer injections into the piriform cortex and the entorhinal cortex. Injections into the piriform cortex near the lateral olfactory tract labeled axons in the deep part of layer I of many primary olfactory areas, but especially in areas near the tract. An injection into the rostral entorhinal cortex, distant to the lateral olfactory tract, labeled a complementary distribution of axons in deep layer I of olfactory areas medial and caudoventral to the tract. This organization resembles that reported in the primary olfactory cortex of the rat [Luskin and Price (1983) J. Comp. Neurol. 216:264-291]. The anterograde tracer injections into the piriform cortex and retrograde tracer injections into the orbital and medial prefrontal cortex and rostral insula label connections from the primary olfactory cortex to nine areas in the caudal orbital cortex, including the agranular insula areas Iam, Iai, Ial, Iapm, and Iapl and areas 14c, 25, 13a, and 13m. The piriform cortex projects most heavily to layer I of these areas. Only Iam, Iapm, and 13a receive a substantial projection to the deeper layers. Areas Iam, Iapm, and 13a were also the only areas that responded with multiunit action potentials to olfactory bulb stimulation in anesthetized animals.(ABSTRACT TRUNCATED AT 400 WORDS)

Serotonin2 receptor-mediated excitation of interneurons in piriform cortex: antagonism by atypical antipsychotic drugs

Rat piriform cortex contains a subpopulation of presumed GABAergic interneurons located near the border of layers 2 and 3 that express excitatory serotonin2 receptors. These serotonin2-responsive interneurons send axons to layer 2 pyramidal cells. Using an in vitro brain slice preparation, serotonin2 receptor-mediated excitation can be assessed either by directly recording from the interneurons or by recording the increase in inhibitory postsynaptic potentials in the pyramidal cells. Intracellular recordings from the interneurons demonstrated that compared to pyramidal cells they had a more depolarized resting membrane potential, a higher input resistance and shorter action potential duration. The serotonin2 receptor-mediated excitation was associated with a strong depolarization (range 3-22 mV). We found that the atypical antipsychotic drugs, risperidone and clozapine, which have relatively high affinity for serotonin2 receptors, each dose-dependently inhibited the serotonin2-mediated excitation of the interneurons with IC50 values of 7 nM and 1.4 microM, respectively. This antagonism was specific to the extent that excitation mediated by agonists at excitatory amino acid receptors were not blocked at concentrations of risperidone and clozapine that completely antagonized the serotonin2 receptor-mediated excitation. The typical antipsychotic drug, chlorpromazine, inhibited the serotonin2-mediated excitation of the interneurons with an IC50 of 14 microM. Haloperidol, another typical antipsychotic drug, decreased the serotonin2 response to about half of baseline at a concentration of 10 microM (the exact IC50 could not be calculated because higher concentrations produced non-specific effects on cells). Both risperidone and clozapine blocked the serotonin-elicited inhibitory postsynaptic potentials in layer 2 pyramidal cells at concentrations that approximated the IC50 for antagonizing the serotonin2-mediated excitation of the interneurons. Chlorpromazine and haloperidol, in the concentration range that blocked serotonin2 receptor-mediated excitation of interneurons, also blocked the serotonin-elicited inhibitory postsynaptic potentials in the pyramidal cells. The IC50 values for risperidone and clozapine, but not for chlorpromazine or haloperidol, for blocking serotonin2 receptor-mediated actions in rodent piriform cortical slice are in the range of the plasma concentrations of the drug that are clinically efficacious. Our data suggest that a potential site of action of the atypical antipsychotic drugs risperidone and clozapine could be antagonism of serotonin acting through serotonin2 receptors on GABAergic interneurons in cerebral cortex.

Pyramidal cells in piriform cortex receive a convergence of inputs from monoamine activated GABAergic interneurons

Previously, serotonin (5-HT) was shown to increase inhibitory post-synaptic potentials (IPSPs) in layer II pyramidal cells, and excite a subpopulation of interneurons located on the layer II/III border of piriform cortex in rat in vitro brain slices. In the present study, the effects of norepinephrine (NE) and dopamine (DA) on these two populations of neurons were examined in brain slices using intracellular and extracellular recordings. All three monoamines increased GABAergic IPSPs in many pyramidal cells; overall, 5-HT was most effective in eliciting IPSPs (58% of cells), followed by NE (45%), then DA (24%). Commonly, pyramidal cells responded with an increase in IPSPs to more than one of the monoamines. The increase in IPSPs was found to include an increase in the frequency of IPSPs present at baseline, as well as recruitment of additional IPSPs of different amplitudes. In interneurons the effects of the monoamines paralleled that which was found for the pyramidal cells. Thus, all three monoamines increased the firing rate of many interneurons; again 5-HT was most effective (56%), followed by NE (51%), then DA (42%). In about 10% of the interneurons the monoamines inhibited cell firing. Interneurons frequently had responses to more than one of the monoamines. The excitatory amino acid (EAA) antagonist, kynurenic acid (200-400 microM), spared most 5-HT and NE responses on interneurons, suggesting that these effects were directly mediated. We conclude that IPSPs elicited by monoamines in pyramidal cells result from a convergence of inputs from populations of layer II/III interneurons that are activated by one, two or all three of the monoamines.

Higher olfactory processes: perceptual learning and memory

The past year has seen several important findings emerge from studies of higher olfactory processes. The identification of synaptic long-term potentiation in the olfactory cortex, induced via repetitive burst stimulation at the theta rhythm, and physiological activity patterns associated with learning, some of which mimic long-term potentiation induction patterns, have suggested relationships between rhythmic activity, behavioral learning and synaptic plasticity. In addition, the construction of computational models of the olfactory bulb and cortex have generated testable behavioral and physiological predictions which have been supported by experimental evidence.