Inhibitory neurons in the anterior piriform cortex of the mouse: classification using molecular markers

A multifaceted architectural framework of the mouse claustrum complex

Accurate anatomical characterizations are necessary to investigate neural circuitry on a fine scale, but for the rodent claustrum complex (CLCX), this has yet to be fully accomplished. The CLCX is generally considered to comprise two major subdivisions, the claustrum (CL) and the dorsal endopiriform nucleus (DEn), but regional boundaries to these areas are debated. To address this, we conducted a multifaceted analysis of fiber- and cytoarchitecture, genetic marker expression, and connectivity using mice of both sexes, to create a comprehensive guide for identifying and delineating borders to CLCX, including an online reference atlas. Our data indicated four distinct subregions within CLCX, subdividing both CL and DEn into two. Additionally, we conducted brain-wide tracing of inputs to CLCX using a transgenic mouse line. Immunohistochemical staining against myelin basic protein (MBP), parvalbumin (PV), and calbindin (CB) revealed intricate fiber-architectural patterns enabling precise delineations of CLCX and its subregions. Myelinated fibers were abundant dorsally in CL but absent ventrally, whereas PV expressing fibers occupied the entire CL. CB staining revealed a central gap within CL, also visible anterior to the striatum. The Nr2f2, Npsr1, and Cplx3 genes expressed specifically within different subregions of the CLCX, and Rprm helped delineate the CL-insular border. Furthermore, cells in CL projecting to the retrosplenial cortex were located within the myelin sparse area. By combining own experimental data with digitally available datasets of gene expression and input connectivity, we could demonstrate that the proposed delineation scheme allows anchoring of datasets from different origins to a common reference framework.

Characterization of primary visual cortex input to specific cell types in the superior colliculus

The superior colliculus is a critical brain region involved in processing visual information. It receives visual input directly from the retina, as well as via a projection from primary visual cortex. Here we determine which cell types in the superficial superior colliculus receive visual input from primary visual cortex in mice. Neurons in the superficial layers of the superior colliculus were classified into four groups - Wide-field, narrow-field, horizontal and stellate - based on their morphological and electrophysiological properties. To determine functional connections between V1 and these four different cell types we expressed Channelrhodopsin2 in primary visual cortex and then optically stimulated these axons while recording from different neurons in the superficial superior colliculus using whole-cell patch-clamp recording in vitro. We found that all four cell types in the superficial layers of the superior colliculus received monosynaptic (direct) input from V1. Wide-field neurons were more likely than other cell types to receive primary visual cortex input. Our results provide information on the cell specificity of the primary visual cortex to superior colliculus projection, increasing our understanding of how visual information is processed in the superior colliculus at the single cell level.

Effects of stochastic coding on olfactory discrimination in flies and mice

Sparse coding can improve discrimination of sensory stimuli by reducing overlap between their representations. Two factors, however, can offset sparse coding's benefits: similar sensory stimuli have significant overlap and responses vary across trials. To elucidate the effects of these 2 factors, we analyzed odor responses in the fly and mouse olfactory regions implicated in learning and discrimination-the mushroom body (MB) and the piriform cortex (PCx). We found that neuronal responses fall along a continuum from extremely reliable across trials to extremely variable or stochastic. Computationally, we show that the observed variability arises from noise within central circuits rather than sensory noise. We propose this coding scheme to be advantageous for coarse- and fine-odor discrimination. More reliable cells enable quick discrimination between dissimilar odors. For similar odors, however, these cells overlap and do not provide distinguishing information. By contrast, more unreliable cells are decorrelated for similar odors, providing distinguishing information, though these benefits only accrue with extended training with more trials. Overall, we have uncovered a conserved, stochastic coding scheme in vertebrates and invertebrates, and we identify a candidate mechanism, based on variability in a winner-take-all (WTA) inhibitory circuit, that improves discrimination with training.

The Basolateral Amygdala Sends a Mixed (GABAergic and Glutamatergic) Projection to the Mediodorsal Thalamic Nucleus

The medial prefrontal cortex receives converging inputs from the mediodorsal thalamic nucleus (MD) and basolateral amygdala (BLA). Although many studies reported that the BLA also projects to MD, there is conflicting evidence regarding this projection, with some data suggesting that it originates from GABAergic or glutamatergic neurons. Therefore, the present study aimed to determine the neurotransmitter used by MD-projecting BLA cells in male and female rats. We first examined whether BLA cells retrogradely labeled by Fast Blue infusions in MD are immunopositive for multiple established markers of BLA interneurons. A minority of MD-projecting BLA cells expressed somatostatin (∼22%) or calretinin (∼11%) but not other interneuronal markers, suggesting that BLA neurons projecting to MD not only include glutamatergic cells, but also long-range GABAergic neurons. Second, we examined the responses of MD cells to optogenetic activation of BLA axons using whole-cell recordings in vitro Consistent with our immunohistochemical findings, among responsive MD cells, light stimuli typically elicited isolated EPSPs (73%) or IPSPs (27%) as well as coincident EPSPs and IPSPs (11%). Indicating that these IPSPs were monosynaptic, light-evoked EPSPs and IPSPs had the same latency and the IPSPs persisted in the presence of ionotropic glutamate receptor antagonists. Overall, our results indicate that the BLA sends a mixed, glutamatergic-GABAergic projection to MD, which likely influences coordination of activity between BLA, MD, and medial prefrontal cortex. An important challenge for future studies will be to examine the connections formed by MD-projecting glutamatergic and GABAergic BLA cells with each other and other populations of BLA cells.SIGNIFICANCE STATEMENT The mediodorsal thalamic nucleus (MD) and basolateral amygdala (BLA) send convergent projections to the medial prefrontal cortex. Although many studies reported that the BLA also projects to MD, there is conflicting evidence as to whether this projection is glutamatergic or GABAergic. By combining tract tracing, immunohistochemistry, optogenetics, and patch clamp recordings in vitro, we found that BLA neurons projecting to MD not only include glutamatergic cells, but also long-range GABAergic neurons. Differential recruitment of these two contingents of cells likely influences coordination of activity between the BLA, MD, and medial prefrontal cortex.

Neuronal differentiation and functional maturation of neurons from neural stem cells induced by bFGF-chitosan controlled release system

Available methods for differentiating stem cells into neurons require a large number of cytokines and neurotrophic factors, with complex steps and slow processes, and are inefficient to produce functional neurons and form synaptic contacts, which is expensive and impractical in clinical application. Here, we demonstrated a bioactive material, basic fibroblast growth factor (bFGF)-chitosan controlled release system, for facilitating neuronal differentiation from NSCs and the functional maturation of the induced neurons with high efficiency. We illustrated by immunostaining that the neurons derived from NSCs expressed mature immunomarkers of interneurons and excitatory neurons. And we found by patch-clamp that the induced neurons exhibited diverse electrophysiological properties as well as formed functional synapses. In vivo, we implanted bFGF-chitosan into lesion area in traumatic brain injury (TBI) mice and similarly observed abundance of neuroblasts in SVZ and the presence of newborn functional neurons in injury area, which integrated into synaptic networks. Taken together, our efficient and rapid tissue engineering approach may be a potential method for the generation of functional neuronal lineage cells from stem cells and a therapy of brain injury and disease.

A method to estimate the cellular composition of the mouse brain from heterogeneous datasets

The mouse brain contains a rich diversity of inhibitory neuron types that have been characterized by their patterns of gene expression. However, it is still unclear how these cell types are distributed across the mouse brain. We developed a computational method to estimate the densities of different inhibitory neuron types across the mouse brain. Our method allows the unbiased integration of diverse and disparate datasets into one framework to predict inhibitory neuron densities for uncharted brain regions. We constrained our estimates based on previously computed brain-wide neuron densities, gene expression data from in situ hybridization image stacks together with a wide range of values reported in the literature. Using constrained optimization, we derived coherent estimates of cell densities for the different inhibitory neuron types. We estimate that 20.3% of all neurons in the mouse brain are inhibitory. Among all inhibitory neurons, 18% predominantly express parvalbumin (PV), 16% express somatostatin (SST), 3% express vasoactive intestinal peptide (VIP), and the remainder 63% belong to the residual GABAergic population. We find that our density estimations improve as more literature values are integrated. Our pipeline is extensible, allowing new cell types or data to be integrated as they become available. The data, algorithms, software, and results of our pipeline are publicly available and update the Blue Brain Cell Atlas. This work therefore leverages the research community to collectively converge on the numbers of each cell type in each brain region.

Long-range GABAergic projections contribute to cortical feedback control of sensory processing

In the olfactory system, the olfactory cortex sends glutamatergic projections back to the first stage of olfactory processing, the olfactory bulb (OB). Such corticofugal excitatory circuits - a canonical circuit motif described in all sensory systems- dynamically adjust early sensory processing. Here, we uncover a corticofugal inhibitory feedback to OB, originating from a subpopulation of GABAergic neurons in the anterior olfactory cortex and innervating both local and output OB neurons. In vivo imaging and network modeling showed that optogenetic activation of cortical GABAergic projections drives a net subtractive inhibition of both spontaneous and odor-evoked activity in local as well as output neurons. In output neurons, stimulation of cortical GABAergic feedback enhances separation of population odor responses in tufted cells, but not mitral cells. Targeted pharmacogenetic silencing of cortical GABAergic axon terminals impaired discrimination of similar odor mixtures. Thus, corticofugal GABAergic projections represent an additional circuit motif in cortical feedback control of sensory processing.

TNF-α Orchestrates Experience-Dependent Plasticity of Excitatory and Inhibitory Synapses in the Anterior Piriform Cortex

Homeostatic synaptic plasticity, which induces compensatory modulation of synapses, plays a critical role in maintaining neuronal circuit function in response to changing activity patterns. Activity in the anterior piriform cortex (APC) is largely driven by ipsilateral neural activity from the olfactory bulb and is a suitable system for examining the effects of sensory experience on cortical circuits. Pro-inflammatory cytokine tumor necrosis factor-α (TNF-α) can modulate excitatory and inhibitory synapses, but its role in APC is unexplored. Here we examined the role of TNF-α in adjusting synapses in the mouse APC after experience deprivation via unilateral naris occlusion. Immunofluorescent staining revealed that activity deprivation increased excitatory, and decreased inhibitory, synaptic density in wild-type mice, consistent with homeostatic regulation. Quantitative RT-PCR showed that naris occlusion increased the expression of Tnf mRNA in APC. Critically, occlusion-induced plasticity of excitatory and inhibitory synapses was completely blocked in the Tnf knockout mouse. Together, these results show that TNF-α is an important orchestrator of experience-dependent plasticity in the APC.

Fast and slow feedforward inhibitory circuits for cortical odor processing

Feedforward inhibitory circuits are key contributors to the complex interplay between excitation and inhibition in the brain. Little is known about the function of feedforward inhibition in the primary olfactory (piriform) cortex. Using in vivo two-photon-targeted patch clamping and calcium imaging in mice, we find that odors evoke strong excitation in two classes of interneurons – neurogliaform (NG) cells and horizontal (HZ) cells – that provide feedforward inhibition in layer 1 of the piriform cortex. NG cells fire much earlier than HZ cells following odor onset, a difference that can be attributed to the faster odor-driven excitatory synaptic drive that NG cells receive from the olfactory bulb. As a result, NG cells strongly but transiently inhibit odor-evoked excitation in layer 2 principal cells, whereas HZ cells provide more diffuse and prolonged feedforward inhibition. Our findings reveal unexpected complexity in the operation of inhibition in the piriform cortex.

Target-specific control of piriform cortical output via distinct inhibitory circuits

Information represented by principal neurons in anterior piriform cortex (APC) is regulated by local, recurrent excitation and inhibition, but the circuit mechanisms remain elusive. Two types of layer 2 (L2) principal neurons, semilunar (SL), and superficial pyramidal (SP) cells, are parallel output channels, and the control of their activity gates the output of APC. Here, we examined the hypothesis that recurrent inhibition differentially regulates SL and SP cells. Patterned optogenetic stimulation revealed that the strength of recurrent inhibition is target- and layer-specific: L1 > L3 for SL cells, but L3 > L1 for SP cells. This target- and layer-specific inhibition was largely attributable to the parvalbumin (PV), but not somatostatin, interneurons. Intriguingly, olfactory experience selectively modulated the PV to SP microcircuit while maintaining the overall target and laminar specificity of inhibition. Together, these results indicate the importance of target-specific inhibitory wiring for odor processing, implicating these mechanisms in gating the output of piriform cortex.

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.

Chronic loss of inhibition in piriform cortex following brief, daily optogenetic stimulation

It is well established that seizures beget seizures, yet the cellular processes that underlie progressive epileptogenesis remain unclear. Here, we use optogenetics to briefly activate targeted populations of mouse piriform cortex (PCx) principal neurons in vivo. After just 3 or 4 days of stimulation, previously subconvulsive stimuli trigger massive, generalized seizures. Highly recurrent allocortices are especially prone to “optokindling.” Optokindling upsets the balance of recurrent excitation and feedback inhibition. To understand how this balance is disrupted, we then selectively reactivate the same neurons in vitro. Surprisingly, we find no evidence of heterosynaptic potentiation; instead, we observe a marked, pathway-specific decrease in feedback inhibition. We find no loss of inhibitory interneurons; rather, decreased GABA synthesis in feedback inhibitory neurons appears to underlie weakened inhibition. Optokindling will allow precise identification of the molecular processes by which brain activity patterns can progressively and pathologically disrupt the balance of cortical excitation and inhibition.

Ryu et al. use optogenetics to briefly activate principal neurons in mouse piriform cortex. After 4 days, previously innocuous stimuli evoke massive, generalized seizures. “Optokindling” does not strengthen recurrent excitation; instead, it weakens feedback inhibition by decreasing synaptic cleft GABA concentrations and slowing vesicle refilling, consistent with decreased GABA synthesis.

Pyramidal Cells in Olfactory Cortex

The neocortex and olfactory cortices share many features including their laminar organization, developmental sequences, and cell types. Previous work indicates that neocortical pyramidal cells exhibit a gradient of dendritic size: cells involved in the initial processing of information are less complex than those in subsequent, higher processing areas. Results presented here confirm that the same is true for the olfactory cortex: pyramidal cells in the region closest to the olfactory bulb, the anterior olfactory nucleus, have smaller total dendritic length and occupy less neural space than those in the posterior piriform cortex. These findings add to the evidence for general rules of development, organization, and function across forebrain cortices.

Synaptic Organization of Anterior Olfactory Nucleus Inputs to Piriform Cortex

Odors activate distributed ensembles of neurons within the piriform cortex, forming cortical representations of odor thought to be essential to olfactory learning and behaviors. This odor response is driven by direct input from the olfactory bulb, but is also shaped by a dense network of associative or intracortical inputs to piriform, which may enhance or constrain the cortical odor representation. With optogenetic techniques, it is possible to functionally isolate defined inputs to piriform cortex and assess their potential to activate or inhibit piriform pyramidal neurons. The anterior olfactory nucleus (AON) receives direct input from the olfactory bulb and sends an associative projection to piriform cortex that has potential roles in the state-dependent processing of olfactory behaviors. Here, we provide a detailed functional assessment of the AON afferents to piriform in male and female C57Bl/6J mice. We confirm that the AON forms glutamatergic excitatory synapses onto piriform pyramidal neurons; and while these inputs are not as strong as piriform recurrent collaterals, they are less constrained by disynaptic inhibition. Moreover, AON-to-piriform synapses contain a substantial NMDAR-mediated current that prolongs the synaptic response at depolarized potentials. These properties of limited inhibition and slow NMDAR-mediated currents result in strong temporal summation of AON inputs within piriform pyramidal neurons, and suggest that the AON could powerfully enhance activation of piriform neurons in response to odor.SIGNIFICANCE STATEMENT Odor information is transmitted from olfactory receptors to olfactory bulb, and then to piriform cortex, where ensembles of activated neurons form neural representations of the odor. While these ensembles are driven by primary bulbar afferents, and shaped by intracortical recurrent connections, the potential for another early olfactory area, the anterior olfactory nucleus (AON), to contribute to piriform activity is not known. Here, we use optogenetic circuit-mapping methods to demonstrate that AON inputs can significantly activate piriform neurons, as they are coupled to NMDAR currents and to relatively modest disynaptic inhibition. The AON may enhance the piriform odor response, encouraging further study to determine the states or behaviors through which AON potentiates the cortical response to odor.

Circuit-Specific Dendritic Development in the Piriform Cortex

Dendritic geometry is largely determined during postnatal development and has a substantial impact on neural function. In sensory processing, postnatal development of the dendritic tree is affected by two dominant circuit motifs, ascending sensory feedforward inputs and descending and local recurrent connections. In the three-layered anterior piriform cortex (aPCx), neurons in the sublayers 2a and 2b display vertical segregation of these two circuit motifs. Here, we combined electrophysiology, detailed morphometry, and Ca²⁺ imaging in acute mouse brain slices and modeling to study circuit-specific aspects of dendritic development. We observed that determination of branching complexity, dendritic length increases, and pruning occurred in distinct developmental phases. Layer 2a and layer 2b neurons displayed developmental phase-specific differences between their apical and basal dendritic trees related to differences in circuit incorporation. We further identified functional candidate mechanisms for circuit-specific differences in postnatal dendritic growth in sublayers 2a and 2b at the mesoscale and microscale levels. Already in the first postnatal week, functional connectivity of layer 2a and layer 2b neurons during early spontaneous network activity scales with differences in basal dendritic growth. During the early critical period of sensory plasticity in the piriform cortex, our data are consistent with a model that proposes a role for dendritic NMDA-spikes in selecting branches for survival during developmental pruning in apical dendrites. The different stages of the morphologic and functional developmental pattern differences between layer 2a and layer 2b neurons demonstrate the complex interplay between dendritic development and circuit specificity.

Somatostatin, Olfaction, and Neurodegeneration

Alzheimer's and Parkinson's diseases are the most prevalent neurodegenerative disorders in aging. Hyposmia has been described as an early symptom that can precede cognitive and motor deficits by decades. Certain regions within the olfactory system, such as the anterior olfactory nucleus, display the neuropathological markers tau and amyloid-β or α-synuclein from the earliest stages of disease progression in a preferential manner. Specific neuronal subpopulations, namely those expressing somatostatin (SST), are preferentially affected throughout the olfactory and limbic systems. SST is a neuropeptide present in a subpopulation of GABAergic interneurons throughout the brain and its main function is to inhibit principal neurons and/or other interneurons. It has been reported that SST expression is reduced by 50% in Alzheimer's disease and that it is related to the formation of Aβ oligomers. The mechanisms underlying the preferential vulnerability of SST-expressing neurons in Alzheimer's disease (and, to a minor extent, in Parkinson's disease) are not known but analysis of the available data could shed light on their etiology. This short review aims to update the knowledge of functional features of somatostatin within the olfactory system and its role in olfactory deficits during neurodegeneration.

Temporal Differences in Interneuron Invasion of Neocortex and Piriform Cortex during Mouse Cortical Development

Establishing a balance between excitation and inhibition is critical for brain functions. However, how inhibitory interneurons (INs) generated in the ventral telencephalon integrate with the excitatory neurons generated in the dorsal telencephalon remains elusive. Previous studies showed that INs migrating tangentially to enter the neocortex (NCx), remain in the migratory stream for days before invading the cortical plate during late corticogenesis. Here we show that in developing mouse cortices, INs in the piriform cortex (PCx; the major olfactory cortex) distribute differently from those in the NCx. We provide evidence that during development INs invade and mature earlier in PCx than in NCx, likely owing to the lack of CXCR4 expression in INs from PCx compared to those in NCx. We analyzed IN distribution patterns in Lhx2 cKO mice, where projection neurons in the lateral NCx are re-fated to generate an ectopic PCx (ePCx). The PCx-specific IN distribution patterns found in ePCx suggest that properties of PCx projection neurons regulate IN distribution. Collectively, our results show that the timing of IN invasion in the developing PCx fundamentally differs from what is known in the NCx. Further, our results suggest that projection neurons instruct the PCx-specific pattern of IN distribution.

CB1 Receptors in the Anterior Piriform Cortex Control Odor Preference Memory

The retrieval of odor-related memories shapes animal behavior. The anterior piriform cortex (aPC) is the largest part of the olfactory cortex, and it plays important roles in olfactory processing and memory. However, it is still unclear whether specific cellular mechanisms in the aPC control olfactory memory, depending on the appetitive or aversive nature of the stimuli involved. Cannabinoid-type 1 (CB1) receptors are present in the aPC (aPC-CB1), but their potential impact on olfactory memory was never explored. Here, we used a combination of behavioral, genetic, anatomical, and electrophysiological approaches to characterize the functions of aPC-CB1 receptors in the regulation of appetitive and aversive olfactory memory. Pharmacological blockade or genetic deletion of aPC-CB1 receptors specifically impaired the retrieval of conditioned odor preference (COP). Interestingly, expression of conditioned odor aversion (COA) was unaffected by local CB1 receptor blockade, indicating that the role of aPC endocannabinoid signaling is selective for retrieval of appetitive memory. Anatomical investigations revealed that CB1 receptors are highly expressed on aPC GABAergic interneurons, and ex vivo electrophysiological recordings showed that their pharmacological activation reduces miniature inhibitory post-synaptic currents (mIPSCs) onto aPC semilunar (SL), but not pyramidal principal neurons. COP retrieval, but not COA, was associated with a specific CB1-receptor-dependent decrease of mIPSCs in SL cells. Altogether, these data indicate that aPC-CB1 receptor-dependent mechanisms physiologically control the retrieval of olfactory memory, depending on odor valence and engaging modulation of local inhibitory transmission.

The distributed circuit within the piriform cortex makes odor discrimination robust

Distributed circuits wherein connections between subcircuit components seem randomly distributed are common to the olfactory circuit, hippocampus, and cerebellum. In such circuits, activation patterns seem random too, showing no detectable spatial preference, and contrast with regions that have topographic connections between subcircuits and topographic activation patterns. Quantitative studies of topographic circuits in the neocortex have yielded common principles of organization. Whether distributed circuits share similar principles of organization is unknown because similar quantitative information is missing and understanding the way they encode information remains a challenge. We addressed these needs by providing a quantitative description of the mouse piriform cortex, a paleocortical distributed circuit that subserves olfaction. The quantitative information provided two insights. First, with a nearly parameter-free model of the olfactory circuit, we show that the piriform cortex robustly maintains odor information and discrimination ability present in the olfactory bulb. Second, the paleocortex is quantitatively different from the neocortex: it has a lower surface area density, which decreases from the anterior to posterior paleocortex contrasting with the uniform neuronal density of the neocortex. These insights might also apply to other distributed circuits.

Modulation of olfactory-driven behavior by metabolic signals: role of the piriform cortex

Olfaction is one of the major sensory modalities that regulates food consumption and is in turn regulated by the feeding state. Given that the olfactory bulb has been shown to be a metabolic sensor, we explored whether the anterior piriform cortex (aPCtx)-a higher olfactory cortical processing area-had the same capacity. Using immunocytochemical approaches, we report the localization of Kv1.3 channel, glucose transporter type 4, and the insulin receptor in the lateral olfactory tract and Layers II and III of the aPCtx. In current-clamped superficial pyramidal (SP) cells, we report the presence of two populations of SP cells: glucose responsive and non-glucose responsive. Using varied glucose concentrations and a glycolysis inhibitor, we found that insulin modulation of the instantaneous and spike firing frequency are both glucose dependent and require glucose metabolism. Using a plethysmograph to record sniffing frequency, rats microinjected with insulin failed to discriminate ratiometric enantiomers; considered a difficult task. Microinjection of glucose prevented discrimination of odorants of different chain-lengths, whereas injection of margatoxin increased the rate of habituation to repeated odor stimulation and enhanced discrimination. These data suggest that metabolic signaling pathways that are present in the aPCtx are capable of neuronal modulation and changing complex olfactory behaviors in higher olfactory centers.

Differential inhibition of pyramidal cells and inhibitory interneurons along the rostrocaudal axis of anterior piriform cortex

The spatial representation of stimuli in sensory neocortices provides a scaffold for elucidating circuit mechanisms underlying sensory processing. However, the anterior piriform cortex (APC) lacks topology for odor identity as well as afferent and intracortical excitation. Consequently, olfactory processing is considered homogenous along the APC rostral-caudal (RC) axis. We recorded excitatory and inhibitory neurons in APC while optogenetically activating GABAergic interneurons along the RC axis. In contrast to excitation, we find opposing, spatially asymmetric inhibition onto pyramidal cells (PCs) and interneurons. PCs are strongly inhibited by caudal stimulation sites, whereas interneurons are strongly inhibited by rostral sites. At least two mechanisms underlie spatial asymmetries. Enhanced caudal inhibition of PCs is due to increased synaptic strength, whereas rostrally biased inhibition of interneurons is mediated by increased somatostatin-interneuron density. Altogether, we show differences in rostral and caudal inhibitory circuits in APC that may underlie spatial variation in odor processing along the RC axis.

Response Patterns of GABAergic Neurons in the Anterior Piriform Cortex of Awake Mice

Local inhibition by γ-amino butyric acid (GABA)-containing neurons is of vital importance for the operation of sensory cortices. However, the physiological response patterns of cortical GABAergic neurons are poorly understood, especially in the awake condition. Here, we utilized the recently developed optical tagging technique to specifically record GABAergic neurons in the anterior piriform cortex (aPC) in awake mice. The identified aPC GABAergic neurons were stimulated with robotic delivery of 32 distinct odorants, which covered a broad range of functional groups. We found that aPC GABAergic neurons could be divided into 4 types based on their response patterns. Type I, type II, and type III neurons displayed broad excitatory responses to test odorants with different dynamics. Type I neurons were constantly activated during odorant stimulation, whereas type II neurons were only transiently activated at the onset of odorant delivery. In addition, type III neurons displayed transient excitatory responses both at the onset and termination of odorant presentation. Interestingly, type IV neurons were broadly inhibited by most of the odorants. Taken together, aPC GABAergic neurons adopt different strategies to affect the cortical circuitry. Our results will allow for better understanding of the role of cortical GABAergic interneurons in sensory information processing.

Pre- and Postsynaptic Activation of GABAB Receptors Modulates Principal Cell Excitation in the Piriform Cortex

The piriform cortex (PC), like other cortical regions, normally operates in a state of dynamic equilibrium between excitation and inhibition. Here we examined the roles played by pre- and postsynaptic GABA_B receptors in maintaining this equilibrium in the PC. Using whole-cell recordings in brain slices from the anterior PC of mice, we found that synaptic activation of postsynaptic GABA_B receptors hyperpolarized the two major classes of layer 2 principal neurons and reduced the intrinsic electrical excitability of these neurons. Presynaptic GABA_B receptors are expressed on the terminals of associational (intracortical) glutamatergic axons in the PC. Heterosynaptic activation of these receptors reduced excitatory associational inputs onto principal cells. Presynaptic GABA_B receptors are also expressed on the axons of GABAergic interneurons in the PC, and blockade of these autoreceptors enhanced inhibitory inputs onto principal cells. Hence, presynaptic GABA_B autoreceptors produce disinhibition of principal cells. To study the functional consequences of GABA_B activation in vivo, we used 2-photon calcium imaging to simultaneously monitor the activity of ~200 layer 2 neurons. Superfusion of the GABA_B agonist baclofen reduced spontaneous random firing but also promoted synchronous epileptiform activity. These findings suggest that, while GABA_B activation can dampen excitability by engaging pre- and postsynaptic GABA_B heteroreceptors on glutamatergic neurons, it can also promote excitability by disinhibiting principal cells by activating presynaptic GABA_B autoreceptors on interneurons. Thus, depending on the dynamic balance of hetero- and autoinhibition, GABA_B receptors can function as variable modulators of circuit excitability in the PC.

Diverse Representations of Olfactory Information in Centrifugal Feedback Projections

Although feedback or centrifugal projections from higher processing centers of the brain to peripheral regions have long been known to play essential functional roles, the anatomical organization of these connections remains largely unknown. Using a virus-based retrograde labeling strategy and 3D whole-brain reconstruction methods, we mapped the spatial organization of centrifugal projections from two olfactory cortical areas, the anterior olfactory nucleus (AON) and the piriform cortex, to the granule cell layer of the main olfactory bulb in the mouse. Both regions are major recipients of information from the bulb and are the largest sources of feedback to the bulb, collectively constituting circuits essential for olfactory coding and olfactory behavior. We found that, although ipsilateral inputs from the AON were uniformly distributed, feedback from the contralateral AON had a strong ventral bias. In addition, we observed that centrifugally projecting neurons were spatially clustered in the piriform cortex, in contrast to the distributed feedforward axonal inputs that these cells receive from the principal neurons of the bulb. Therefore, information carried from the bulb to higher processing structures by anatomically stereotypic projections is likely relayed back to the bulb by organizationally distinct feedback projections that may reflect different coding strategies and therefore different functional roles.

SIGNIFICANCE STATEMENT

Principles of anatomical organization, sometimes instantiated as "maps" in the mammalian brain, have provided key insights into the structure and function of circuits in sensory systems. Generally, these characterizations focus on projections from early sensory processing areas to higher processing structures despite considerable evidence that feedback or centrifugal projections often constitute major conduits of information flow. Our results identify structure in the organization of centrifugal feedback projections to the olfactory bulb that is fundamentally different from the organization of feedforward circuits. Our study suggests that understanding computations performed in the olfactory bulb, and more generally in the olfactory system, requires understanding interactions between feedforward and feedback "maps" both structurally and functionally.

Complementary codes for odor identity and intensity in olfactory cortex

The ability to represent both stimulus identity and intensity is fundamental for perception. Using large-scale population recordings in awake mice, we find distinct coding strategies facilitate non-interfering representations of odor identity and intensity in piriform cortex. Simply knowing which neurons were activated is sufficient to accurately represent odor identity, with no additional information about identity provided by spike time or spike count. Decoding analyses indicate that cortical odor representations are not sparse. Odorant concentration had no systematic effect on spike counts, indicating that rate cannot encode intensity. Instead, odor intensity can be encoded by temporal features of the population response. We found a subpopulation of rapid, largely concentration-invariant responses was followed by another population of responses whose latencies systematically decreased at higher concentrations. Cortical inhibition transforms olfactory bulb output to sharpen these dynamics. Our data therefore reveal complementary coding strategies that can selectively represent distinct features of a stimulus.

DOI:http://dx.doi.org/10.7554/eLife.22630.001

Spontaneous activity in the piriform cortex extends the dynamic range of cortical odor coding

Neurons in the neocortex exhibit spontaneous spiking activity in the absence of external stimuli, but the origin and functions of this activity remain uncertain. Here, we show that spontaneous spiking is also prominent in a sensory paleocortex, the primary olfactory (piriform) cortex of mice. In the absence of applied odors, piriform neurons exhibit spontaneous firing at mean rates that vary systematically among neuronal classes. This activity requires the participation of NMDA receptors and is entirely driven by bottom-up spontaneous input from the olfactory bulb. Odor stimulation produces two types of spatially dispersed, odor-distinctive patterns of responses in piriform cortex layer 2 principal cells: Approximately 15% of cells are excited by odor, and another approximately 15% have their spontaneous activity suppressed. Our results show that, by allowing odor-evoked suppression as well as excitation, the responsiveness of piriform neurons is at least twofold less sparse than currently believed. Hence, by enabling bidirectional changes in spiking around an elevated baseline, spontaneous activity in the piriform cortex extends the dynamic range of odor representation and enriches the coding space for the representation of complex olfactory stimuli.

Development and Organization of the Evolutionarily Conserved Three-Layered Olfactory Cortex

The olfactory cortex is part of the mammalian cerebral cortex together with the neocortex and the hippocampus. It receives direct input from the olfactory bulbs and participates in odor discrimination, association, and learning (Bekkers and Suzuki, 2013). It is thought to be an evolutionarily conserved paleocortex, which shares common characteristics with the three-layered general cortex of reptiles (Aboitiz et al., 2002). The olfactory cortex has been studied as a “simple model” to address sensory processing, though little is known about its precise cell origin, diversity, and identity. While the development and the cellular diversity of the six-layered neocortex are increasingly understood, the olfactory cortex remains poorly documented in these aspects. Here is a review of current knowledge of the development and organization of the olfactory cortex, keeping the analogy with those of the neocortex. The comparison of olfactory cortex and neocortex will allow the opening of evolutionary perspectives on cortical development.

Optogenetic Mapping of Intracortical Circuits Originating from Semilunar Cells in the Piriform Cortex

Despite its comparatively simple trilaminar architecture, the primary olfactory (piriform) cortex of mammals is capable of performing sophisticated sensory processing, an ability that is thought to depend critically on its extensive associational (intracortical) excitatory circuits. Here, we used a novel transgenic mouse model and optogenetics to measure the connectivity of associational circuits that originate in semilunar (SL) cells in layer 2a of the anterior piriform cortex (aPC). We generated a mouse line (48L) in which channelrhodopsin-2 (ChR) could be selectively expressed in a subset of SL cells. Light-evoked excitatory postsynaptic currents (EPSCs) could be evoked in superficial pyramidal cells (17.4% of n = 86 neurons) and deep pyramidal cells (33.3%, n = 9) in the aPC, but never in ChR- SL cells (0%, n = 34). Thus, SL cells monosynaptically excite pyramidal cells, but not other SL cells. Light-evoked EPSCs were also selectively elicited in 3 classes of GABAergic interneurons in layer 3 of the aPC. Our results show that SL cells are specialized for providing feedforward excitation of specific classes of neurons in the aPC, confirming that SL cells comprise a functionally distinctive input layer.

Whole-Brain Monosynaptic Afferent Inputs to Basal Forebrain Cholinergic System

The basal forebrain cholinergic system (BFCS) robustly modulates many important behaviors, such as arousal, attention, learning and memory, through heavy projections to cortex and hippocampus. However, the presynaptic partners governing BFCS activity still remain poorly understood. Here, we utilized a recently developed rabies virus-based cell-type-specific retrograde tracing system to map the whole-brain afferent inputs of the BFCS. We found that the BFCS receives inputs from multiple cortical areas, such as orbital frontal cortex, motor cortex, and insular cortex, and that the BFCS also receives dense inputs from several subcortical nuclei related to motivation and stress, including lateral septum, central amygdala, paraventricular nucleus of hypothalamus, dorsal raphe, and parabrachial nucleus. Interestingly, we found that the BFCS receives inputs from the olfactory areas and the entorhinal–hippocampal system. These results greatly expand our knowledge about the connectivity of the mouse BFCS and provided important preliminary indications for future exploration of circuit function.

Inhibition by Somatostatin Interneurons in Olfactory Cortex

Inhibitory circuitry plays an integral role in cortical network activity. The development of transgenic mouse lines targeting unique interneuron classes has significantly advanced our understanding of the functional roles of specific inhibitory circuits in neocortical sensory processing. In contrast, considerably less is known about the circuitry and function of interneuron classes in piriform cortex, a paleocortex responsible for olfactory processing. In this study, we sought to utilize transgenic technology to investigate inhibition mediated by somatostatin (SST) interneurons onto pyramidal cells (PCs), parvalbumin (PV) interneurons, and other interneuron classes. As a first step, we characterized the anatomical distributions and intrinsic properties of SST and PV interneurons in four transgenic lines (SST-cre, GIN, PV-cre, and G42) that are commonly interbred to investigate inhibitory connectivity. Surprisingly, the distributions SST and PV cell subtypes targeted in the GIN and G42 lines were sparse in piriform cortex compared to neocortex. Moreover, two-thirds of interneurons recorded in the SST-cre line had electrophysiological properties similar to fast spiking (FS) interneurons rather than regular (RS) or low threshold spiking (LTS) phenotypes. Nonetheless, like neocortex, we find that SST-cells broadly inhibit a number of unidentified interneuron classes including putatively identified PV cells and surprisingly, other SST cells. We also confirm that SST-cells inhibit pyramidal cell dendrites and thus, influence dendritic integration of afferent and recurrent inputs to the piriform cortex. Altogether, our findings suggest that SST interneurons play an important role in regulating both excitation and the global inhibitory network during olfactory processing.

Functional neuroanatomy of amygdalohippocampal interconnections and their role in learning and memory

The amygdalar nuclear complex and hippocampal/parahippocampal region are key components of the limbic system that play a critical role in emotional learning and memory. This Review discusses what is currently known about the neuroanatomy and neurotransmitters involved in amygdalo-hippocampal interconnections, their functional roles in learning and memory, and their involvement in mnemonic dysfunctions associated with neuropsychiatric and neurological diseases. Tract tracing studies have shown that the interconnections between discrete amygdalar nuclei and distinct layers of individual hippocampal/parahippocampal regions are robust and complex. Although it is well established that glutamatergic pyramidal cells in the amygdala and hippocampal region are the major players mediating interconnections between these regions, recent studies suggest that long-range GABAergic projection neurons are also involved. Whereas neuroanatomical studies indicate that the amygdala only has direct interconnections with the ventral hippocampal region, electrophysiological studies and behavioral studies investigating fear conditioning and extinction, as well as amygdalar modulation of hippocampal-dependent mnemonic functions, suggest that the amygdala interacts with dorsal hippocampal regions via relays in the parahippocampal cortices. Possible pathways for these indirect interconnections, based on evidence from previous tract tracing studies, are discussed in this Review. Finally, memory disorders associated with dysfunction or damage to the amygdala, hippocampal region, and/or their interconnections are discussed in relation to Alzheimer's disease, posttraumatic stress disorder (PTSD), and temporal lobe epilepsy. © 2016 Wiley Periodicals, Inc.

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.

Inhibitory dysfunction in amyotrophic lateral sclerosis: future therapeutic opportunities

In amyotrophic lateral sclerosis, motor neuron hyperexcitability and inhibitory dysfunction is emerging as a potential causative link in the dysfunction and degeneration of the motoneuronal circuitry that characterizes the disease. Interneurons, as key regulators of excitability, may mediate much of this imbalance, yet we know little about the way in which inhibitory deficits perturb excitability. In this review, we explore inhibitory control of excitability and the potential contribution of altered inhibition to amyotrophic lateral sclerosis disease processes and vulnerabilities, identifying important windows of therapeutic opportunity and potential interventions, specifically targeting inhibitory control at key disease stages.

A direct anterior cingulate pathway to the primate primary olfactory cortex may control attention to olfaction

Behavioral and functional studies in humans suggest that attention plays a key role in activating the primary olfactory cortex through an unknown circuit mechanism. We report that a novel pathway from the anterior cingulate cortex, an area which has a key role in attention, projects directly to the primary olfactory cortex in rhesus monkeys, innervating mostly the anterior olfactory nucleus. Axons from the anterior cingulate cortex formed synapses mostly with spines of putative excitatory pyramidal neurons and with a small proportion of a neurochemical class of inhibitory neurons that are thought to have disinhibitory effect on excitatory neurons. This novel pathway from the anterior cingulate is poised to exert a powerful excitatory effect on the anterior olfactory nucleus, which is a critical hub for odorant processing via extensive bilateral connections with primary olfactory cortices and the olfactory bulb. Acting on the anterior olfactory nucleus, the anterior cingulate may activate the entire primary olfactory cortex to mediate the process of rapid attention to olfactory stimuli.

Extrinsic origins of the somatostatin and neuropeptide Y innervation of the rat basolateral amygdala

The amygdalar basolateral nuclear complex (BLC) is a cortex-like structure that receives inputs from many cortical areas. It has long been assumed that cortico-amygdalar projections, as well as inter-areal intracortical connections, arise from cortical pyramidal cells. However, recent studies have shown that GABAergic long-range nonpyramidal neurons (LRNP neurons) in the cortex also contribute to inter-areal connections. The present study combined Fluorogold (FG) retrograde tract tracing with immunohistochemistry for cortical nonpyramidal neuronal markers to determine if cortical LRNP neurons project to the BLC in the rat. Injections of FG into the BLC produced widespread retrograde labeling in the cerebral hemispheres and diencephalon. Triple-labeling for FG, somatostatin (SOM), and neuropeptide Y (NPY) revealed a small number of FG+/SOM+/NPY+ neurons and FG+/SOM+/NPY- neurons in the lateral entorhinal area, amygdalopiriform transition area, and piriform cortex, but not in the prefrontal and insular cortices, or in the diencephalon. In addition, FG+/SOM+/NPY+ neurons were observed in the amygdalostriatal transition area and in a zone surrounding the intercalated nuclei. About half of the SOM+ neurons in the lateral entorhinal area labeled by FG were GABA+. FG+ neurons containing parvalbumin were only seen in the basal forebrain, and no FG+ neurons containing vasoactive intestinal peptide were observed in any brain region. Since LRNP neurons involved in corticocortical connections are critical for synchronous oscillations that allow temporal coordination between distant cortical regions, the LRNP neurons identified in this study may play a role in the synchronous oscillations of the BLC and hippocampal region that are involved in the retrieval of fear memories.

Somatostatin cells regulate sensory response fidelity via subtractive inhibition in olfactory cortex

Diverse types of local GABAergic interneurons shape the cortical representation of sensory information. Here we show how somatostatin-expressing interneurons (SOM cells) contribute to odor coding in mouse olfactory cortex. We find that odor-tuned SOM cells regulate principal cells through a purely subtractive operation that is independent of odor identity or intensity. This operation enhances the salience of odor-evoked activity without changing cortical odor tuning. SOM cells inhibit both principal cells and fast-spiking interneurons, indicating that subtractive inhibition reflects the interplay of multiple classes of interneurons.

Physiological properties of supragranular cortical inhibitory interneurons expressing retrograde persistent firing

Neurons are polarized functional units. The somatodendritic compartment receives and integrates synaptic inputs while the axon relays relevant synaptic information in form of action potentials (APs) across long distance. Despite this well accepted notion, recent research has shown that, under certain circumstances, the axon can also generate APs independent of synaptic inputs at axonal sites distal from the soma. These ectopic APs travel both toward synaptic terminals and antidromically toward the soma. This unusual form of neuronal communication seems to preferentially occur in cortical inhibitory interneurons following a period of intense neuronal activity and might have profound implications for neuronal information processing. Here we show that trains of ectopically generated APs can be induced in a large portion of neocortical layer 2/3 GABAergic interneurons following a somatic depolarization inducing hundreds of APs. Sparsely occurring ectopic spikes were also observed in a large portion of layer 1 interneurons even in absence of prior somatic depolarization. Remarkably, we found that interneurons which produce ectopic APs display specific membrane and morphological properties significantly different from the remaining GABAergic cells and may therefore represent a functionally unique interneuronal subpopulation.

The piriform cortex and human focal epilepsy

It is surprising that the piriform cortex, when compared to the hippocampus, has been given relatively little significance in human epilepsy. Like the hippocampus, it has a phylogenetically preserved three-layered cortex that is vulnerable to excitotoxic injury, has broad connections to both limbic and cortical areas, and is highly epileptogenic – being critical to the kindling process. The well-known phenomenon of early olfactory auras in temporal lobe epilepsy highlights its clinical relevance in human beings. Perhaps because it is anatomically indistinct and difficult to approach surgically, as it clasps the middle cerebral artery, it has, until now, been understandably neglected. In this review, we emphasize how its unique anatomical and functional properties, as primary olfactory cortex, predispose it to involvement in focal epilepsy. From recent convergent findings in human neuroimaging, clinical epileptology, and experimental animal models, we make the case that the piriform cortex is likely to play a facilitating and amplifying role in human focal epileptogenesis, and may influence progression to epileptic intractability.

Preprodynorphin-expressing neurons constitute a large subgroup of somatostatin-expressing GABAergic interneurons in the mouse neocortex

Dynorphins, leumorphin, and neoendorphins are preprodynorphin (PPD)-derived peptides and ligands for κ-opioid receptors. Using an antibody to PPD C-terminal, we investigated the chemical and molecular characteristics of PPD-expressing neurons in mouse neocortex. PPD-immunopositive neuronal somata were distributed most frequently in layer 5 and less frequently in layers 2-4 and 6 throughout neocortical regions. Combined labeling of immunofluorescence and fluorescent mRNA signals revealed that almost all PPD-immunopositive neurons expressed glutamic acid decarboxylase but not vesicular glutamate transporter, indicating their γ-aminobutyric acid (GABA)ergic characteristics, and that PPD-immunopositive neurons accounted for 15% of GABAergic interneurons in the primary somatosensory area. As GABAergic interneurons were divided into several groups by specific markers, we further examined the chemical characteristics of PPD-expressing neurons by the double immunofluorescence labeling method. More than 95% of PPD-immunopositive neurons were also somatostatin (SOM)-immunopositive in the primary somatosensory, primary motor, orbitofrontal, and primary visual areas, but only 24% were SOM-immunopositive in the medial prefrontal cortex. In the primary somatosensory area, PPD-immunopositive neurons constituted 50%, 79%, 55%, and 17% of SOM-immunopositive neurons in layers 2-3, 4, 5, and 6, respectively. Although SOM-expressing neurons contained calretinin-, neuropeptide Y-, nitric oxide synthase-, and reelin-expressing neurons as subgroups, only reelin immunoreactivity was detected in many PPD-immunopositive neurons. These results indicate that PPD-expressing neurons constitute a large subgroup of SOM-expressing cortical interneurons, and the PPD/SOM-expressing GABAergic neurons might serve not only as inhibitory elements in the local cortical circuit, but also as modulators for cortical neurons expressing κ-opioid and/or SOM receptors.

Aging, but not tau pathology, impacts olfactory performances and somatostatin systems in THY-Tau22 mice

Somatostatin (SOM) cortical levels decline in Alzheimer's disease (AD) in correlation with cognitive impairment severity, the latter being closely related to the presence of neurofibrillary tangles. Impaired olfaction is another hallmark of AD tightly related to tau pathology in the olfactory pathways. Recent studies showed that SOM modulates olfactory processing, suggesting that alterations in SOM levels participate to olfactory deficits in AD. Herein, we first observed that human olfactory peduncle and cortex are enriched in SOM cells and fibers, in aged postmortem brains. Then, the possible link between SOM alterations and olfactory deficits was evaluated by exploring the impact of age and tau hyperphosphorylation on olfactory SOM networks and behavioral performances in THY-Tau22 mice, a tauopathy transgenic model. Distinct molecular repertoires of SOM peptide and receptors were associated to sensory or cortical olfactory processing structures. Aging mainly affected SOM neurotransmission in piriform and entorhinal cortex in wild-type mice, although olfactory performances decreased. However, no further olfactory impairment was evidenced in THY-Tau22 mice until 12 months although tau pathology early affected olfactory cortical structures. Thus, tau hyperphosphorylation per se has a limited impact on olfactory performances in THY-Tau22 mice.

Functional properties and projections of neurons in the medial amygdala

The medial nucleus of the amygdala (MeA) plays a key role in innate emotional behaviors by relaying olfactory information to hypothalamic nuclei involved in reproduction and defense. However, little is known about the neuronal components of this region or their role in the olfactory-processing circuitry of the amygdala. Here, we have characterized neurons in the posteroventral division of the medial amygdala (MePV) using the GAD67-GFP mouse. Based on their electrophysiological properties and GABA expression, unsupervised cluster analysis divided MePV neurons into three types of GABAergic (Types 1-3) and two non-GABAergic cells (Types I and II). All cell types received olfactory synaptic input from the accessory olfactory bulb and, with the exception of Type 2 GABAergic neurons, sent projections to both reproductive and defensive hypothalamic nuclei. Type 2 GABAergic cells formed a chemically and electrically interconnected network of local circuit inhibitory interneurons that resembled neurogliaform cells of the piriform cortex and provided feedforward inhibition of the olfactory-processing circuitry of the MeA. These findings provide a description of the cellular organization and connectivity of the MePV and further our understanding of amygdala circuits involved in olfactory processing and innate emotions.

Thorough GABAergic innervation of the entire axon initial segment revealed by an optogenetic 'laserspritzer'

GABAergic terminals of chandelier cells exclusively innervate the axon initial segment (AIS) of excitatory neurons. Although the anatomy of these synapses has been well-studied in several brain areas, relatively little is known about their physiological properties. Using vesicular γ-aminobutyric acid transporter-channelrhodopsin 2-enhanced yellow fluorescence protein (VGAT-ChR2-YFP)-expressing mice and a novel fibreoptic 'laserspritzer' approach that we developed, we investigated the physiological properties of axo-axonic synapses (AASs) in brain slices from the piriform cortex (PC) of mice. AASs were in close proximity to voltage-gated Na(+) (NaV) channels located at the AIS. AASs were selectively activated by a 5 μm laserspritzer placed in close proximity to the AIS. Under a minimal laser stimulation condition and using whole-cell somatic voltage-clamp recordings, the amplitudes and kinetics of IPSCs mediated by AASs were similar to those mediated by perisomatic inhibitions. Results were further validated with channelrhodopsin 2-assisted circuit mapping (CRACM) of the entire inhibitory inputs map. For the first time, we revealed that the laserspritzer-induced AAS-IPSCs persisted in the presence of TTX and TEA but not 4-AP. Next, using gramicidin-based perforated patch recordings, we found that the GABA reversal potential (EGABA) was -73.6 ± 1.2 mV when induced at the AIS and -72.8 ± 1.1 mV when induced at the perisomatic site. Our anatomical and physiological results lead to the novel conclusions that: (1) AASs innervate the entire length of the AIS, as opposed to forming a highly concentrated cartridge, (2) AAS inhibition suppresses action potentials and epileptiform activity more robustly than perisomatic inhibitions, and (3) AAS activation alone can be sufficient to inhibit action potential generation and epileptiform activities in vitro.

Matching of feedback inhibition with excitation ensures fidelity of information flow in the anterior piriform cortex

Odor-evoked responses in mitral cells of the olfactory bulb are characterized by prolonged patterns of action potential (spike) activity. If downstream neurons are to respond to each spike in these patterns, the duration of the excitatory response to one spike should be limited, enabling cells to respond to subsequent spikes. To test for such mechanisms, we performed patch-clamp recordings in slices of the mouse anterior piriform cortex. Mitral cell axons in the lateral olfactory tract (LOT) were stimulated electrically at different intensities and with various frequency patterns to mimic changing input conditions that the piriform cortex likely encounters in vivo. We found with cell-attached measurements that superficial pyramidal (SP) cells in layer 2 consistently responded to LOT stimulation across conditions with a limited number (1-2) of spikes per stimulus pulse. The key synaptic feature accounting for the limited spike number appeared to be somatic inhibition derived from layer 3 fast-spiking cells. This inhibition tracked the timing of the first spike in SP cells across conditions, which naturally limited the spike number to 1-2. These response features to LOT stimulation were, moreover, not unique to SP cells, also occurring in a population of fluorescently labeled interneurons in glutamic acid decarboxylase 65-eGFP mice. That these different cortical cells respond to incoming inputs with 1-2 spikes per stimulus may be especially critical for relaying bulbar information contained in synchronized oscillations at beta (15-30Hz) or gamma (30-80Hz) frequencies.

Learning-induced modulation of the GABAB-mediated inhibitory synaptic transmission: mechanisms and functional significance

Complex olfactory-discrimination (OD) learning results in a series of intrinsic and excitatory synaptic modifications in piriform cortex pyramidal neurons that enhance the circuit excitability. Such overexcitation must be balanced to prevent runway activity while maintaining the efficient ability to store memories. We showed previously that OD learning is accompanied by enhancement of the GABAA-mediated inhibition. Here we show that GABAB-mediated inhibition is also enhanced after learning and study the mechanism underlying such enhancement and explore its functional role. We show that presynaptic, GABAB-mediated synaptic inhibition is enhanced after learning. In contrast, the population-average postsynaptic GABAB-mediated synaptic inhibition is unchanged, but its standard deviation is enhanced. Learning-induced reduction in paired pulse facilitation in the glutamatergic synapses interconnecting pyramidal neurons was abolished by application of the GABAB antagonist CGP55845 but not by blocking G protein-gated inwardly rectifying potassium channels only, indicating enhanced suppression of excitatory synaptic release via presynaptic GABAB-receptor activation. In addition, the correlation between the strengths of the early (GABAA-mediated) and late (GABAB-mediated) synaptic inhibition was much stronger for each particular neuron after learning. Consequently, GABAB-mediated inhibition was also more efficient in controlling epileptic-like activity induced by blocking GABAA receptors. We suggest that complex OD learning is accompanied by enhancement of the GABAB-mediated inhibition that enables the cortical network to store memories, while preventing uncontrolled activity.

Diversity among principal and GABAergic neurons of the anterior olfactory nucleus

Understanding the cellular components of neural circuits is an essential step in discerning regional function. The anterior olfactory nucleus (AON) is reciprocally connected to both the ipsi- and contralateral olfactory bulb (OB) and piriform cortex (PC), and, as a result, can broadly influence the central processing of odor information. While both the AON and PC are simple cortical structures, the regions differ in many ways including their general organization, internal wiring and synaptic connections with other brain areas. The present work used targeted whole-cell patch clamping to investigate the morphological and electrophysiological properties of the AON's two main neuronal populations: excitatory projection neurons and inhibitory interneurons. Retrograde fluorescent tracers placed into either the OB or PC identified projection neurons. Two classes were observed with different physiological signatures and locations (superficial and deep pyramidal neurons), suggesting the AON contains independent efferent channels. Transgenic mice in which GABA-containing cells expressed green fluorescent protein were used to assess inhibitory neurons. These cells were further identified as containing one or more of seven molecular markers including three calcium-binding proteins (calbindin, calretinin, parvalbumin) or four neuropeptides (somatostatin, vasoactive intestinal peptide, neuropeptide Y, cholecystokinin). The proportion of GABAergic cells containing these markers varied across subregions reinforcing notions that the AON has local functional subunits. At least five classes of inhibitory cells were observed: fast-spiking multipolar, regular-spiking multipolar, superficial neurogliaform, deep neurogliaform, and horizontal neurons. While some of these cell types are similar to those reported in the PC and other cortical regions, the AON also has unique populations. These studies provide the first examination of the cellular components of this simple cortical system.

Interneurons, tau and amyloid-β in the piriform cortex in Alzheimer's disease

Impaired olfaction has been described as an early symptom of Alzheimer's disease. Neuroanatomical changes underlying this deficit in the olfactory system are largely unknown. Interestingly, neuropathology begins in the transentorhinal cortex and extends to the neighboring limbic system and basal telencephalic structures that mediate olfactory processing, including the anterior olfactory nucleus and olfactory bulb. The human piriform cortex has been described as a crucial area in odor quality coding; disruption of this region mediates early olfactory deficits in Alzheimer's disease. Most neuropathological investigations have focused on the entorhinal cortex and hippocampus, whereas the piriform cortex has largely been neglected. This work aims to characterize the expression of the neuropathological amyloid-β peptide, tau protein and interneuron population markers (calretinin, parvalbumin and somatostatin) in the piriform cortex of ten Alzheimer-diagnosed (80.4 ± 8.3 years old) and five control (69.6 ± 11.1) cases. Here, we examined the distribution of different interneuronal markers as well as co-localization of interneurons and pathological markers. Results indicated preferential vulnerability of somatostatin- (p = 0.0001 < α = 0.05) and calretinin-positive (p = 0.013 < α = 0.05) cells that colocalized with amyloid-β peptide, while the prevalence of parvalbumin-positive cells was increased (p = 0.045 < α = 0.05) in the Alzheimer's cases. These data may help to reveal the neural basis of olfactory deficits linked to Alzheimer's disease as well as to characterize neuronal populations preferentially vulnerable to neuropathology in regions critically involved in early stages of the disease.