Differences in chemo- and cytoarchitectural features within pars principalis of the rat anterior olfactory nucleus suggest functional specialization

The anterior olfactory nucleus revisited - an emerging role for neuropathological conditions?

Olfaction is an important sensory modality for many species and greatly influences animal and human behavior. Still, much about olfactory perception remains unknown. The anterior olfactory nucleus is one of the brain's central early olfactory processing areas. Located directly posterior to the olfactory bulb in the olfactory peduncle with extensive in- and output connections and unique cellular composition, it connects olfactory processing centers of the left and right hemispheres. Almost 20 years have passed since the last comprehensive review on the anterior olfactory nucleus has been published and significant advances regarding its anatomy, function, and pathophysiology have been made in the meantime. Here we briefly summarize previous knowledge on the anterior olfactory nucleus, give detailed insights into the progress that has been made in recent years, and map out its emerging importance in translational research of neurological diseases.

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.

Automated Brain Region Segmentation for Single Cell Resolution Histological Images Based on Markov Random Field

The brain consists of massive regions with different functions and the precise delineation of brain region boundaries is important for brain region identification and atlas illustration. In this paper we propose a hierarchical Markov random field (MRF) model for brain region segmentation, where a MRF is applied to the downsampled low-resolution images and the result is used to initialize another MRF for the original high-resolution images. A fractional differential feature and a gray level co-occurrence matrix are extracted as the observed vector for the MRF and a new potential energy function, which can capture the spatial characteristic of brain regions, is proposed as well. A fuzzy entropy criterion is used to fine-tune the boundary from the hierarchical MRF model. We test the model both on synthetic images and real histological mouse brain images. The result suggests that the model can accurately identify target regions and even the whole mouse brain outline as a special case. An interesting observation is that the model cannot only segment regions with different cell density but also can segment regions with similar cell density and different cell morphology texture. Thus this model shows great potential for building the high-resolution 3D brain atlas.

Colonic afferent input and dorsal horn neuron activation differs between the thoracolumbar and lumbosacral spinal cord

The distal colon is innervated by the splanchnic and pelvic nerves, which relay into the thoracolumbar and lumbosacral spinal cord, respectively. Although the peripheral properties of the colonic afferent nerves within these pathways are well studied, their input into the spinal cord remain ill defined. The use of dual retrograde tracing from the colon wall and lumen, in conjunction with in vivo colorectal distension and spinal neuronal activation labeling with phosphorylated MAPK ERK 1/2 (pERK), allowed us to identify thoracolumbar and lumbosacral spinal cord circuits processing colonic afferent input. In the thoracolumbar dorsal horn, central projections of colonic afferents were primarily labeled from the wall of the colon and localized in laminae I and V. In contrast, lumbosacral projections were identified from both lumen and wall tracing, present within various dorsal horn laminae, collateral tracts, and the dorsal gray commissure. Nonnoxious in vivo colorectal distension evoked significant neuronal activation (pERK-immunoreactivity) within the lumbosacral dorsal horn but not in thoracolumbar regions. However, noxious in vivo colorectal distension evoked significant neuronal activation in both the thoracolumbar and lumbosacral dorsal horn, with the distribution of activated neurons correlating to the pattern of traced projections. Dorsal horn neurons activated by colorectal distension were identified as possible populations of projection neurons or excitatory and inhibitory interneurons based on their neurochemistry. Our findings demonstrate how colonic afferents in splanchnic and pelvic pathways differentially relay mechanosensory information into the spinal cord and contribute to the recruitment of spinal cord pathways processing non-noxious and noxious stimuli.NEW & NOTEWORTHY In mice, retrograde tracing from the colon wall and lumen was used to identify unique populations of afferent neurons and central projections within the spinal cord dorsal horn. We show that there are pronounced differences between the spinal cord regions in the distribution pattern of colonic afferent central projections and the pattern of dorsal horn neuron activation evoked by colorectal distension. These findings demonstrate how colonic afferent input influences spinal processing of colonic mechanosensation.

The tree shrew cerebellum atlas: Systematic nomenclature, neurochemical characterization, and afferent projections

The cerebellum is involved in the control of movement, emotional responses, and reward processing. The tree shrew is the closest living relative of primates. However, little is known not only about the systematic nomenclature for the tree shrew cerebellum but also about the detailed neurochemical characterization and afferent projections. In this study, Nissl staining and acetylcholinesterase histochemistry were used to reveal anatomical features of the cerebellum of tree shrews (Tupaia belangeri chinensis). The cerebellar cortex presented a laminar structure. The morphological characteristics of the cerebellum were comprehensively described in the coronal, sagittal, and horizontal sections. Moreover, distributive maps of calbindin-immunoreactive (-ir) cells in the Purkinje cell layer of the cerebellum of tree shrews were depicted using coronal, sagittal, and horizontal schematics. In addition, 5th cerebellar lobule (5Cb)-projecting neurons were present in the pontine nuclei, reticular nucleus, spinal vestibular nucleus, ventral spinocerebellar tract, and inferior olive of the tree shrew brain. The anterior part of the paramedian lobule of the cerebellum (PMa) received mainly strong innervation from the lateral reticular nucleus, inferior olive, pontine reticular nucleus, spinal trigeminal nucleus, pontine nuclei, and reticulotegmental nucleus of the pons. The present results provide the first systematic nomenclature, detailed atlas of the whole cerebellum, and whole-brain mapping of afferent projections to the 5Cb and PMa in tree shrews. Our findings provide morphological support for tree shrews as an alternative model for studies of human cerebellar pathologies.

The olfactory bulb as the entry site for prion-like propagation in neurodegenerative diseases

Olfactory deficits are present in numerous neurodegenerative disorders and are accompanied by pathology in related brain regions. In several of these disorders, olfactory disturbances appear early and are considered as prodromal symptoms of the disease. In addition, pathological protein aggregates affect olfactory regions prior to other regions, suggesting that the olfactory system might be particularly vulnerable to neurodegenerative diseases. Exposed to the external environment, the olfactory epithelium and olfactory bulb allow pathogen and toxin penetration into the brain, a process that has been proposed to play a role in neurodegenerative diseases. Determining whether the olfactory bulb could be a starting point of pathology and of pathology spread is crucial to understanding how neurodegenerative diseases evolve. We argue that pathological changes following environmental insults contribute to the initiation of protein aggregation in the olfactory bulb, which then triggers the spread of the pathology within the brain by a templating mechanism in a prion-like manner. We review the evidence for the early involvement of olfactory structures in neurodegenerative diseases and the relationship between neuropathology and olfactory function. We discuss the vulnerability and putative underlying mechanisms by which pathology could be initiated in the olfactory bulb, from the entry of pathogens (promoted by increased permeability of the olfactory epithelium with aging or inflammation) to the sensitivity of the olfactory system to oxidative stress and inflammation. Finally, we review changes in protein expression and neural excitability triggered by pathogenic proteins that can promote pathogenesis in the olfactory bulb and beyond.

Origin and Migration of Olfactory Cajal-Retzius Cells

Early telencephalic development involves the migration of diverse cell types that can be identified by specific molecular markers. Most prominent among them are Cajal-Retzius (CR) cells that emanate mainly from the cortical hem and to a lesser extent from rostrolateral, septal and caudo-medial regions. One additional territory proposed to give rise to CR cells that migrate dorsally into the neocortex lies at the ventral pallium, although contradictory results question this notion. With the use of a cell-permeable fluorescent tracer in cultured embryos, we identified novel migratory paths of putative CR cells and other populations that originate from the rostrolateral telencephalon at its olfactory region. Moreover, extensive labeling on the lateral telencephalon along its rostro-caudal extent failed to reveal a dorsally-migrating CR cell population from the ventral pallium at the stages analyzed. Hence, this work reveals a novel olfactory CR cell migration and supports the idea that the ventral pallium, where diverse types of neurons converge, does not actually generate CR cells.

Olfactory Development, Part 1: Function, From Fetal Perception to Adult Wine-Tasting

Discrimination of odorous molecules in amniotic fluid occur after 30 weeks' gestation; fetuses exhibit differential responses to maternal diet. Olfactory reflexes enable reliable neonatal testing. Olfactory bulbs can be demonstrated reliably by MRI after 30 weeks' gestation, and their hypoplasia or aplasia also documented by late prenatal and postnatal MRI. Olfactory axons project from nasal epithelium to telencephalon before olfactory bulbs form. Fetal olfactory maturation remains incomplete at term for neuronal differentiation, synaptogenesis, myelination, and persistence of the transitory fetal ventricular recess. Immaturity does not signify nonfunction. Olfaction is the only sensory system without thalamic projection because of its own intrinsic thalamic equivalent. Diverse malformations of the olfactory bulb can be diagnosed by clinical examination, imaging, and neuropathology. Some epileptic auras might be primarily generated in the olfactory bulb. Cranial nerve 1 should be tested in all neonates and especially in patients with brain malformations, endocrinopathies, chromosomopathies, and genetic/metabolic diseases.

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.

Top-down modulation of olfactory-guided behaviours by the anterior olfactory nucleus pars medialis and ventral hippocampus

Olfactory processing is thought to be actively modulated by the top-down input from cortical regions, but the behavioural function of these signals remains unclear. Here we find that cortical feedback from the anterior olfactory nucleus pars medialis (mAON) bidirectionally modulates olfactory sensitivity and olfaction-dependent behaviours. To identify a limbic input that tunes this mAON switch, we further demonstrate that optogenetic stimulation of ventral hippocampal inputs to the mAON is sufficient to alter olfaction-dependent behaviours.

An Automatic Method for Nucleus Boundary Segmentation Based on a Closed Cubic Spline

The recognition of brain nuclei is the basis for localizing brain functions. Traditional histological research, represented by atlas illustration, achieves the goal of nucleus boundary recognition by manual delineation, but it has become increasingly difficult to extend this handmade method to delineating brain regions and nuclei from large datasets acquired by the recently developed single-cell-resolution imaging techniques for the whole brain. Here, we propose a method based on a closed cubic spline (CCS), which can automatically segment the boundaries of nuclei that differ to a relatively high degree in cell density from the surrounding areas and has been validated on model images and Nissl-stained microimages of mouse brain. It may even be extended to the segmentation of target outlines on MRI or CT images. The proposed method for the automatic extraction of nucleus boundaries would greatly accelerate the illustration of high-resolution brain atlases.

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.

Functional imaging of cortical feedback projections to the olfactory bulb

Processing of sensory information is substantially shaped by centrifugal, or feedback, projections from higher cortical areas, yet the functional properties of these projections are poorly characterized. Here, we used genetically-encoded calcium sensors (GCaMPs) to functionally image activation of centrifugal projections targeting the olfactory bulb (OB). The OB receives massive centrifugal input from cortical areas but there has been as yet no characterization of their activity in vivo. We focused on projections to the OB from the anterior olfactory nucleus (AON), a major source of cortical feedback to the OB. We expressed GCaMP selectively in AON projection neurons using a mouse line expressing Cre recombinase (Cre) in these neurons and Cre-dependent viral vectors injected into AON, allowing us to image GCaMP fluorescence signals from their axon terminals in the OB. Electrical stimulation of AON evoked large fluorescence signals that could be imaged from the dorsal OB surface in vivo. Surprisingly, odorants also evoked large signals that were transient and coupled to odorant inhalation both in the anesthetized and awake mouse, suggesting that feedback from AON to the OB is rapid and robust across different brain states. The strength of AON feedback signals increased during wakefulness, suggesting a state-dependent modulation of cortical feedback to the OB. Two-photon GCaMP imaging revealed that different odorants activated different subsets of centrifugal AON axons and could elicit both excitation and suppression in different axons, indicating a surprising richness in the representation of odor information by cortical feedback to the OB. Finally, we found that activating neuromodulatory centers such as basal forebrain drove AON inputs to the OB independent of odorant stimulation. Our results point to the AON as a multifunctional cortical area that provides ongoing feedback to the OB and also serves as a descending relay for other neuromodulatory systems.

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.

Adolescent changes in dopamine D1 receptor expression in orbitofrontal cortex and piriform cortex accompany an associative learning deficit

The orbitofrontal cortex (OFC) and piriform cortex are involved in encoding the predictive value of olfactory stimuli in rats, and neural responses to olfactory stimuli in these areas change as associations are learned. This experience-dependent plasticity mirrors task-related changes previously observed in mesocortical dopamine neurons, which have been implicated in learning the predictive value of cues. Although forms of associative learning can be found at all ages, cortical dopamine projections do not mature until after postnatal day 35 in the rat. We hypothesized that these changes in dopamine circuitry during the juvenile and adolescent periods would result in age-dependent differences in learning the predictive value of environmental cues. Using an odor-guided associative learning task, we found that adolescent rats learn the association between an odor and a palatable reward significantly more slowly than either juvenile or adult rats. Further, adolescent rats displayed greater distractibility during the task than either juvenile or adult rats. Using real-time quantitative PCR and immunohistochemical methods, we observed that the behavioral deficit in adolescence coincides with a significant increase in D1 dopamine receptor expression compared to juvenile rats in both the OFC and piriform cortex. Further, we found that both the slower learning and increased distractibility exhibited in adolescence could be alleviated by experience with the association task as a juvenile, or by an acute administration of a low dose of either the dopamine D1 receptor agonist SKF-38393 or the D2 receptor antagonist eticlopride. These results suggest that dopaminergic modulation of cortical function may be important for learning the predictive value of environmental stimuli, and that developmental changes in cortical dopaminergic circuitry may underlie age-related differences in associative learning.

The mouse olfactory peduncle. 2.The anterior limb of the anterior commissure

The central core of the olfactory peduncle [the tissue connecting the olfactory bulb (OB) to the forebrain] includes a white matter tract that extends caudally to the anterior commissure (AC). The purpose of the present study was to examine this “anterior limb of the anterior commissure” (ALAC) to determine if the axons that progress through it segregate on the basis of their point of origin, neurotransmitter type, size, or shape. While local differences in axon density were observed in the ALAC, they were not consistent between samples of the anterior and posterior peduncle, and no other compartmentalization within the tract was observed. The innervation of the caudal olfactory peduncle by neuromodulatory fibers was examined to determine if they enter the region via the ALAC. Cholinergic fibers (CHAT) densely filled the peduncle, followed in order by serotonergic, noradrenergic, histaminergic, and orexinergic processes. Differences in the distribution of the fibers were noted for each system. While each axon type could be observed in the ALAC, it is probable that they enter the peduncle though several routes. Data for axon caliber in the ALAC was compared to information previously collected on the peduncle's other white matter region, the lateral olfactory tract (LOT). Axons in the ALAC were smaller, suggesting that the olfactory system is organized with a fast system for distributing incoming sensory information and a more economical, distributed system for subsequent processing.

Sprouting of colonic afferent central terminals and increased spinal mitogen-activated protein kinase expression in a mouse model of chronic visceral hypersensitivity

Visceral pain following infection or inflammation is a major clinical problem. Although we have knowledge of how peripheral endings of colonic afferents change in disease, their central projections have been overlooked. With neuroanatomical tracing and colorectal distension (CRD), we sought to identify colonic afferent central terminals (CACTs), the dorsal horn (DH) neurons activated by colonic stimuli in the thoracolumbar (T10-L1) DH, and determine how they are altered by postinflammatory chronic colonic mechanical hypersensitivity. Retrograde tracing from the colon identified CACTs in the DH, whereas immunohistochemistry for phosphorylated MAP kinase ERK 1/2 (pERK) identified DH neurons activated by CRD (80 mmHg). In healthy mice, CACTs were located primarily in DH laminae I (LI) and V (LV) and projected down middle and lateral DH collateral pathways. CRD evoked pERK immunoreactivity in DH neurons, the majority of which were located in LI and LV, the same regions as CACTs. In postinflammatory mice, CACTs were significantly increased in T12-L1 compared with healthy mice. Although CACTs remained abundant in LI, they were more widespread and were now present in deeper laminae. After CRD, significantly more DH neurons were pERK-IR postinflammation (T12-L1), with abundant expression in LI and deeper laminae. In both healthy and postinflammatory mice, many pERK neurons were in close apposition to CACTs, suggesting that colonic afferents can stimulate specific DH neurons in response to noxious CRD. Overall, we demonstrate that CACT density and the number of responsive DH neurons in the spinal cord increase postinflammation, which may facilitate aberrant central representation of colonic nociceptive signaling following chronic peripheral hypersensitivity.

The mouse olfactory peduncle

The olfactory peduncle, the region connecting the olfactory bulb with the basal forebrain, contains several neural areas that have received relatively little attention. The present work includes studies that provide an overview of the region in the mouse. An analysis of cell soma size in pars principalis (pP) of the anterior olfactory nucleus (AON) revealed considerable differences in tissue organization between mice and rats. An unbiased stereological study of neuron number in the cell-dense regions of pars externa (pE) and pP of the AON of 3-, 12-, and 24-month-old mice indicated that pE has about 16,500 cells in 0.043 mm(3) and pP about 58,300 cells in 0.307 mm(3) . Quantitative Golgi studies of pyramidal neurons in pP suggested that mouse neurons are similar to although smaller than those of the rat. An immunohistochemical analysis demonstrated that all peduncular regions (pE, pP, the dorsal peduncular cortex, ventral tenia tecta, and anterior olfactory tubercle and piriform cortex) have cells that express either calbindin, calretinin, parvalbumin, somatostatin, vasoactive intestinal polypeptide, neuropeptide Y, or cholecystokinin (antigens commonly co-expressed by subspecies of γ-aminobutyric acid [GABA]ergic neurons), although the relative numbers of each cell type differ between zones. Finally, an electron microscopic comparison of the organization of myelinated fibers in lateral olfactory tract in the anterior and posterior peduncle indicated that the region is less orderly in mice than in rats. The results provide a caveat for investigators who generalize data between species, as both similarities and differences between the laboratory mouse and rat were observed.

Spatial distribution of neural activity in the anterior olfactory nucleus evoked by odor and electrical stimulation

Several lines of evidence indicate that complex odorant stimuli are parsed into separate data streams in the glomeruli of the olfactory bulb, yielding a combinatorial "odotopic map." However, this pattern does not appear to be maintained in the piriform cortex, where stimuli appear to be coded in a distributed fashion. The anterior olfactory nucleus (AON) is intermediate and reciprocally interconnected between these two structures, and also provides a route for the interhemispheric transfer of olfactory information. The present study examined potential coding strategies used by the AON. Rats were exposed to either caproic acid, butyric acid, limonene, or purified air and the spatial distribution of Fos-immunolabeled cells was quantified. The two major subregions of the AON exhibited different results. Distinct odor-specific spatial patterns of activity were observed in pars externa, suggesting that it employs a topographic strategy for odor representation similar to the olfactory bulb. A spatially distributed pattern that did not appear to depend on odor identity was observed in pars principalis, suggesting that it employs a distributed representation of odors more similar to that seen in the piriform cortex.

Membrane and synaptic properties of pyramidal neurons in the anterior olfactory nucleus

The anterior olfactory nucleus (AON) is positioned to coordinate activity between the piriform cortex and olfactory bulbs, yet the physiology of AON principal neurons has been little explored. Here, we examined the membrane properties and excitatory synapses of AON principal neurons in brain slices of PND22-28 mice and compared their properties to principal cells in other olfactory cortical areas. AON principal neurons had firing rates, spike rate adaptation, spike widths, and I-V relationships that were generally similar to pyramidal neurons in piriform cortex, and typical of cerebral cortex, consistent with a role for AON in cortical processing. Principal neurons in AON had more hyperpolarized action potential thresholds, smaller afterhyperpolarizations, and tended to fire doublets of action potentials on depolarization compared with ventral anterior piriform cortex and the adjacent epileptogenic region preendopiriform nucleus (pEN). Thus, AON pyramidal neurons have enhanced membrane excitability compared with surrounding subregions. Interestingly, principal neurons in pEN were the least excitable, as measured by a larger input conductance, lower firing rates, and more inward rectification. Afferent and recurrent excitatory synapses onto AON pyramidal neurons had small amplitudes, paired pulse facilitation at afferent synapses, and GABA(B) modulation at recurrent synapses, a pattern similar to piriform cortex. The enhanced membrane excitability and recurrent synaptic excitation within the AON, together with its widespread outputs, suggest that the AON can boost and distribute activity in feedforward and feedback circuits throughout the olfactory system.

The anterior olfactory nucleus: quantitative study of dendritic morphology

The anterior olfactory nucleus (AON) occupies a crucial position within the olfactory circuit, as it is able to influence function in nearly every major synaptic processing stage of both the ipsilateral and the contralateral pathways. Nevertheless, very little is known about the region's internal organization and circuitry. The present study provides basic quantitative and qualitative data on the morphology of several cell types within the two major regions of the AON, pars externa and pars principalis. In pars externa two types of cells are analyzed, the "classical" cell (type I), containing only apically directed dendrites with large spines, and a previously unreported cell with basilar dendrites and complex, spiny apical processes (type II). In pars principalis the characteristic pyramidal cell is described both on the basis of the depth of the cell bodies in the cell layer comprising the structure and on the basis of their radial location. Several other nonpyramidal neurons are also described. The findings provide useful basic information necessary for understanding and modeling the circuitry of the AON.

Contralateral projections of the rat anterior olfactory nucleus

The anterior olfactory nucleus (AON) is a central olfactory cortical structure that has heavy reciprocal connections with both the olfactory bulb (OB) and piriform cortex. While it has been firmly established that the AON is a primary source of bilateral projections in the olfactory system through extensive connections with both the ipsilateral and contralateral OB, AON, and piriform cortex, few studies have examined this circuitry in detail. In the present study we used small injections of the anterograde tracer Phaseolus vulgaris leucoagglutinin (PHA-L) and the retrograde tracer FluoroGold in specific subregions of the AON to explore the topography of the interconnections between the left and right AONs. Labeled fibers were found in the contralateral AON following injections in all areas. However, detailed quantitative analyses revealed that different regions of the AON have distinct patterns of interhemispheric innervation; contralateral fibers were most heavily targeted to dorsal and lateral AON subregions, while the medial and ventral areas received relatively light projections. These results demonstrate important features of the interhemispheric circuitry of the AON and suggest separate functional roles for subregions of the AON in olfactory information processing.