Interbulbar axonal collateralization and morphology of anterior olfactory nucleus neurons in the rat

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.

Extrinsic neuromodulation in the rodent olfactory bulb

Evolutionarily, olfaction is one of the oldest senses and pivotal for an individual's health and survival. The olfactory bulb (OB), as the first olfactory relay station in the brain, is known to heavily process sensory information. To adapt to an animal's needs, OB activity can be influenced by many factors either from within (intrinsic neuromodulation) or outside (extrinsic neuromodulation) the OB which include neurotransmitters, neuromodulators, hormones, and neuropeptides. Extrinsic sources seem to be of special importance as the OB receives massive efferent input from numerous brain centers even outweighing the sensory input from the nose. Here, we review neuromodulatory processes in the rodent OB from such extrinsic sources. We will discuss extrinsic neuromodulation according to points of origin, receptors involved, affected circuits, and changes in behavior. In the end, we give a brief outlook on potential future directions in research on neuromodulation in the OB.

Dynamic Impairment of Olfactory Behavior and Signaling Mediated by an Olfactory Corticofugal System

Processing of olfactory information is modulated by centrifugal projections from cortical areas, yet their behavioral relevance and underlying neural mechanisms remain unclear in most cases. The anterior olfactory nucleus (AON) is part of the olfactory cortex, and its extensive connections to multiple upstream and downstream brain centers place it in a prime position to modulate early sensory information in the olfactory system. Here, we show that optogenetic activation of AON neurons in awake male and female mice was not perceived as an odorant equivalent cue. However, AON activation during odorant presentation reliably suppressed behavioral odor responses. This AON-mediated effect was fast and constant across odors and concentrations. Likewise, activation of glutamatergic AON projections to the olfactory bulb (OB) transiently inhibited the excitability of mitral/tufted cells (MTCs) that relay olfactory input to the cortex. Single-unit MTC recordings revealed that optogenetic activation of glutamatergic AON terminals in the OB transiently decreased sensory-evoked MTC spiking, regardless of the strength or polarity of the sensory response. The reduction in MTC firing during optogenetic stimulation was confirmed in recordings in awake mice. These findings suggest that glutamatergic AON projections to the OB impede early olfactory signaling by inhibiting OB output neurons, thereby dynamically gating sensory throughput to the cortex.SIGNIFICANCE STATEMENT The anterior olfactory nucleus (AON) as an olfactory information processing area sends extensive projections to multiple brain centers, but the behavioral consequences of its activation have been scarcely investigated. Using behavioral tests in combination with optogenetic manipulation, we show that, in contrast to what has been suggested previously, the AON does not seem to form odor percepts but instead suppresses behavioral odor responses across odorants and concentrations. Furthermore, this study shows that AON activation inhibits olfactory bulb output neurons in both anesthetized as well as awake mice, pointing to a potential mechanism by which the olfactory cortex can actively and dynamically gate sensory throughput to higher brain centers.

Bilateral and unilateral odor processing and odor perception

Imagine smelling a novel perfume with only one nostril and then smelling it again with the other nostril. Clearly, you can tell that it is the same perfume both times. This simple experiment demonstrates that odor information is shared across both hemispheres to enable perceptual unity. In many sensory systems, perceptual unity is believed to be mediated by inter-hemispheric connections between iso-functional cortical regions. However, in the olfactory system, the underlying neural mechanisms that enable this coordination are unclear because the two olfactory cortices are not topographically organized and do not seem to have homotypic inter-hemispheric mapping. This review presents recent advances in determining which aspects of odor information are processed unilaterally or bilaterally, and how odor information is shared across the two hemispheres. We argue that understanding the mechanisms of inter-hemispheric coordination can provide valuable insights that are hard to achieve when focusing on one hemisphere alone.

Altered brain morphology after focal radiation reveals impact of off-target effects: implications for white matter development and neurogenesis

Background

Children with brain tumors treated with cranial radiation therapy (RT) often exhibit cognitive late effects, commonly associated with reduced white matter (WM) volume and decreased neurogenesis. The impact of radiation damage in particular regions or tissues on brain development as a whole has not been elucidated.

Methods

We delivered whole-brain or focal radiation (8 Gy single dose) to infant mice. Focal treatments targeted white matter (anterior commissure), neuronal (olfactory bulbs), or neurogenic (subventricular zone) regions. High-resolution ex vivo MRI was used to assess radiation-induced volume differences. Immunohistochemistry for myelin basic protein and doublecortin was performed to assess associated cellular changes within white matter and related to neurogenesis, respectively.

Results

Both whole-brain and focal RT in infancy resulted in volume deficits in young adulthood, with whole-brain RT resulting in the largest deficits. RT of the anterior commissure, surprisingly, showed no impact on its volume or on brain development as a whole. In contrast, RT of the olfactory bulbs resulted in off-target volume reduction in the anterior commissure and decreased subventricular zone neurogenesis. RT of the subventricular zone likewise produced volume deficits in both the olfactory bulbs and the anterior commissure. Similar off-target effects were found in the corpus callosum and parietal cortex.

Conclusions

Our results demonstrate that radiation damage locally can have important off-target consequences for brain development. These data suggest that WM may be less radiosensitive than volume change alone would indicate and have implications for region-sparing radiation treatments aimed at reducing cognitive late effects.

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.

Transfer of human α-synuclein from the olfactory bulb to interconnected brain regions in mice

α-Synuclein (α-syn) is a protein prevalent in neural tissue and known to undergo axonal transport. Intracellular α-syn aggregates are a hallmark of Parkinson’s disease (PD). Braak and collaborators have suggested that in people who are destined to eventually develop PD, α-syn aggregate pathology progresses following a stereotypic pattern, starting in the olfactory bulb (OB) and the gut. α-Synuclein aggregates are postulated to spread to interconnected brain regions over several years. Thus, propagation of the pathology via neural pathways can potentially explain how α-syn aggregates spread in PD. We have now studied if α-syn can transfer from the OB to other brain structures through neural connections, by injecting different molecular species of human α-syn (monomers, oligomers, fibrils) into the OB of wild-type mice. We found that non-fibrillar human α-syn is taken up very quickly by OB neurons. Within minutes to hours, it is also found in neurons in structures connected to the OB. Conversely, when we injected bovine serum albumin used as a control protein, we found that it does not diffuse beyond the OB, is rarely taken up by OB cells, and does not transfer to other structures. Taken together, our results show that OB cells readily take up α-syn, and that monomeric and oligomeric, but not fibrillar, forms of α-syn are rapidly transferred to interconnected structures within the timeframe we explored. Our results support the idea that α-syn can transfer along neural pathways and thereby contribute to the progression of the α-syn-related pathology.

Functional properties of cortical feedback projections to the olfactory bulb

Sensory perception is not a simple feed-forward process, and higher brain areas can actively modulate information processing in "lower" areas. We used optogenetic methods to examine how cortical feedback projections affect circuits in the first olfactory processing stage, the olfactory bulb. Selective activation of back projections from the anterior olfactory nucleus/cortex (AON) revealed functional glutamatergic synaptic connections on several types of bulbar interneurons. Unexpectedly, AON axons also directly depolarized mitral cells (MCs), enough to elicit spikes reliably in a time window of a few milliseconds. MCs received strong disynaptic inhibition, a third of which arises in the glomerular layer. Activating feedback axons in vivo suppressed spontaneous as well as odor-evoked activity of MCs, sometimes preceded by a temporally precise increase in firing probability. Our study indicates that cortical feedback can shape the activity of bulbar output neurons by enabling precisely timed spikes and enforcing broad inhibition to suppress background activity.

Optophysiological analysis of associational circuits in the olfactory cortex

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

Vasopressin and social odor processing in the olfactory bulb and anterior olfactory nucleus

Central vasopressin facilitates social recognition and modulates numerous complex social behaviors in mammals, including parental behavior, aggression, affiliation, and pair-bonding. In rodents, social interactions are primarily mediated by the exchange of olfactory information, and there is evidence that vasopressin signaling is important in brain areas where olfactory information is processed. We recently discovered populations of vasopressin neurons in the main and accessory olfactory bulbs and anterior olfactory nucleus that are involved in the processing of social odor cues. In this review, we propose a model of how vasopressin release in these regions, potentially from the dendrites, may act to filter social odor information to facilitate odor-based social recognition. Finally, we discuss recent human research linked to vasopressin signaling and suggest that our model of priming-facilitated vasopressin signaling would be a rewarding target for further studies, as a failure of priming may underlie pathological changes in complex behaviors.

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.

Medullary neurons in the core white matter of the olfactory bulb: a new cell type

The structure of a new cell type, termed the medullary neuron (MN) because of its intimate association with the rostral migratory stream (RMS) in the bulbar core, is described in the adult rat olfactory bulb. The MN is a triangular or polygonal interneuron whose soma lies between the cellular clusters of the RMS or, less frequently, among the neuron progenitors therein. MNs are easily distinguished from adjacent cells by their large size and differentiated structure. Two MN subtypes have been categorized by the Golgi technique: spiny pyramidal neurons and aspiny neurons. Both MN subtypes bear a large dendritic field impinged upon by axons in the core bulbar white matter. A set of collaterals from the adjacent axons appears to terminate on the MN dendrites. The MN axon passes in close apposition to adjacent neuron progenitors in the RMS. MNs are immunoreactive with antisera raised against gamma-aminobutyric acid and glutamate decarboxylase 65/67. Electron-microscopic observations confirm that MNs correspond to fully differentiated, mature neurons. MNs seem to be highly conserved among macrosmatic species as they occur in Nissl-stained brain sections from mouse, guinea pig, and hedgehog. Although the functional role of MNs remains to be determined, we suggest that MNs represent a cellular interface between endogenous olfactory activity and the differentiation of new neurons generated during adulthood.

Centrifugal innervation of the mammalian olfactory bulb

Although it has been known for decades that the mammalian olfactory bulb receives a substantial number of centrifugal inputs from other regions of the brain, relatively few data have been available on the function of the centrifugal olfactory system. Knowing the role of the centrifugal projection and how it works is of critical importance to fully understanding olfaction. The centrifugal fibers can be classified into two groups, a group that release neuromodulators, such as noradrenaline, serotonin, or acetylcholine, and a group originating in the olfactory cortex. Accumulating evidence suggests that centrifugal neuromodulatory inputs are associated with acquisition of odor memory. Because the distribution of the terminals on these fibers is diffuse and widespread, the neuromodulatory inputs must affect diverse subsets of bulbar neurons at the same time. In contrast, knowledge of the role of centrifugal fibers from the olfactory cortical areas is limited. Judging from recent morphological evidence, these fibers may modify the activity of neurons located in sparse and discrete loci in the olfactory bulb. Given the modular organization of the olfactory bulb, centrifugal fibers from the olfactory cortex may help coordinate the activities of restricted subsets of neurons belonging to distinct functional modules in an odor-specific manner. Because the olfactory cortex receives inputs from limbic and neocortical areas in addition to inputs from the bulb, the centrifugal inputs from the cortex can modulate odor processing in the bulb in response to non-olfactory as well as olfactory cues.

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.

Precise circuitry links bilaterally symmetric olfactory maps

Olfactory sensory neurons expressing a common receptor gene converge onto one or a few glomeruli with stereotyped positions within the mouse main olfactory bulb (MOB), producing a map of approximately 1800 olfactory columns representing approximately 1000 odorant receptors. Here, we report that this precise olfactory map is maintained over several synapses that ultimately cross MOB hemispheres to link bilateral isofunctional olfactory columns. Focal injection of tracer into genetically identified glomeruli revealed an exquisite topography that involves a bilateral connection via the anterior olfactory nucleus pars externa (AONpE) that links isofunctional olfactory columns in the contralateral MOB. Physiological and behavioral assays revealed an important role for the AONpE in bilateral exchange of odorant-specific information. These results indicate that the interbulbar link through the AONpE integrates bilateral olfactory sensory maps and exchanges olfactory information, positioning it as a unique model system for studying interhemispheric connections.

Development and topography of the lateral olfactory tract in the mouse: imaging by genetically encoded and injected fluorescent markers

In mammals, conventional odorants are detected by OSNs located in the main olfactory epithelium of the nose. These neurons project their axons to glomeruli, which are specialized structures of neuropil in the olfactory bulb. Within glomeruli, axons synapse onto dendrites of projection neurons, the mitral and tufted (M/T) cells. Genetic approaches to visualize axons of OSNs expressing a given odorant receptor have proven very useful in elucidating the organization of these projections to the olfactory bulb. Much less is known about the development and connectivity of the lateral olfactory tract (LOT), which is formed by axons of M/T cells connecting the olfactory bulb to central neural regions. Here, we have extended our genetic approach to mark M/T cells of the main olfactory bulb and their axons in the mouse, by targeted insertion of IRES-tauGFP in the neurotensin locus. In NT-GFP mice, we find that M/T cells of the main olfactory bulb mature and project axons as early as embryonic day 11.5. Final innervation of central areas is accomplished before the end of the second postnatal week. M/T cell axons that originate from small defined areas within the main olfactory bulb, as visualized by localized injections of fluorescent tracers in wild-type mice at postnatal days 1 to 3, follow a dual trajectory: a branch of tightly packed axons along the dorsal aspect of the LOT, and a more diffuse branch along the ventral aspect. The dorsal, but not the ventral, subdivision of the LOT exhibits a topographical segregation of axons coming from the dorsal versus ventral main olfactory bulb. The NT-GFP mouse strain should prove useful in further studies of development and topography of the LOT, from E11.5 until 2 weeks after birth.

The anterior olfactory nucleus and piriform cortex of the echidna and platypus

The cyto- and chemoarchitecture of the anterior olfactory nucleus and piriform cortex of the short-beaked echidna and platypus were studied to determine: (1) if these areas contain chemically distinct subdivisions, and (2) if the chemoarchitecture of those cortical olfactory regions differs from therians. Nissl and myelin staining were applied in conjunction with enzyme reactivity for NADPH diaphorase and acetylcholinesterase, and immunoreactivity for calcium-binding proteins (parvalbumin, calbindin and calretinin) and tyrosine hydroxylase. Golgi impregnations were also available for the echidna. In the echidna, the anterior olfactory nucleus is negligible in extent and merges at very rostral levels with a four-layered piriform cortex. Several rostrocaudally running subregions of the echidna piriform lobe could be identified on the basis of Nissl staining and calcium-binding protein immunoreactivity. Laminar-specific differences in calcium-binding protein immunoreactivity and NADPH-d-reactive neuron distribution were also noted. Neuron types identified in echidna piriform cortex included pyramidal neurons predominating in layers II and III and non-pyramidal neurons (e.g., multipolar profusely spiny and neurogliaform cells) in deeper layers. Horizontal cells were identified in both superficial and deep layers. By contrast, the platypus had a distinct anterior olfactory nucleus and a three-layered piriform cortex with no evidence of chemically distinct subregions within the piriform cortex. Volume of the paleocortex of the echidna was comparable to prosimians of similar body weight and, in absolute volume, exceeded that for eutherian insectivores such as T. ecaudatus and E. europaeus. The piriform cortex of the echidna shows evidence of regional differentiation, which in turn suggests highly specialized olfactory function.

A field guide to the anterior olfactory nucleus (cortex)

While portions of the mammalian olfactory system have been studied extensively, the anterior olfactory nucleus (AON) has been relatively ignored. Furthermore, the existing research is dispersed and obscured by many different nomenclatures and approaches. The present review collects and assembles the relatively sparse literature regarding the portion of the brain situated between the olfactory bulb and primary olfactory (piriform) cortex. Included is an overview of the area's organization, the functional, morphological and neurochemical characteristics of its cells and a comprehensive appraisal of its efferent and afferent fiber systems. Available evidence suggests the existence of subdivisions within the AON and demonstrates that the structure influences ongoing activity in many other olfactory areas. We conclude with a discussion of the AON's mysterious but complex role in olfactory information processing.

Postnatal development of dendritic spines on olfactory bulb granule cells in rats

Postnatal morphological changes in granule cell dendritic spines and filopodia (collectively referred to as "spines/filopodia") were examined in the rat main olfactory bulb to characterize the development of the neural circuitry for olfaction. Granule cells were labeled with a membrane dye and confocal laser scanning microscope images of labeled spines/filopodia were acquired in the following three dendritic domains: apical dendrites in the external plexiform layer, those in the granule cell layer, and basal dendrites. In all three domains the proportion of typical spines slightly increased during development, with a concomitant decrease in the proportion of "stubby" spines lacking a neck; the proportion of filopodia remained unchanged, accounting for 20-40% of all protrusions. The mean diameter and length of the spine/filopodium population were nearly constant throughout development. On the other hand, the developmental pattern of the spine/filopodium density varied markedly, depending on the domain of the dendrites. In the external plexiform layer, the density did not change remarkably during development. The density in apical dendrites in the granule cell layer increased during the initial 2 postnatal weeks, then gradually decreased. The spine/filopodium density in basal dendrites, however, continued to increase until 4 weeks of age, and then began to decrease. These results suggest that a substantial amount of input-specific synaptic remodeling occurs in granule cells during development, which proceeds from superficial dendritic domains to deeper ones, occurring most prominently in the basal dendrites.

Central olfactory connections in the microsmatic marmoset monkey (Callithrix jacchus)

The mammalian primary olfactory system consists of a set of different telencephalic structures, including paleo-, archi-, periarchi- and mesocortical components. We present the first characterisation of the normal and connectional anatomy of the primary olfactory cortex of the common marmoset, a microsmatic simian species increasingly used in primate research. The centrifugal and centripetal bulbar projections were determined by injections of the anterograde and retrograde tracer wheat germ agglutinin-conjugated horseradish peroxidase and fluorescent dyes into the ipsilateral main olfactory bulb. The efferent projections of the marmoset bulb are organised entirely ipsilaterally and are established via a rudimentary medial olfactory tract and the dominant lateral olfactory tract. Target areas are the anterior olfactory nucleus, the entire prepiriform cortex, ventral tenia tecta, periamygdaloid cortex and the rostral part of the entorhinal cortex. The bulbar axons predominantly terminate in the outer part of layer I. The anterior olfactory nucleus receives a weak additional input within layer II and III, which is not found in macrosmatic rodents. Further anterograde labelling was found in the endopiriform nucleus deep under the prepiriform cortex and within an anterolateral strip of the olfactory tubercle. However, control injections into the olfactory tubercle suggest that the marmoset olfactory tubercle receives a bisynaptic olfactory input only. Retrograde labelling after bulb injections revealed that, except for the olfactory tubercle, all primary olfactory cortices contributed to an ipsilateral bulbopetal feedback projection. Like in rodents, the only bulbopetal projection organised bilaterally in the marmoset is maintained by the anterior olfactory nucleus. With few exceptions, the projections of the marmoset olfactory brain are organised similarly to that of the macaque monkey or those of macrosmatic species.

Volumetric changes in the anterior olfactory nucleus of the rat after neonatal olfactory deprivation

The effect of olfactory deprivation in the postnatal development of the anterior olfactory nucleus (AON) was studied in 60-day-old rats which underwent unilateral naris closure after birth (postnatal day 1). Volumetric and morphometric analyses of the AON ipsilateral and contralateral to the closed naris were performed and data were statistically compared among them and with those of control animals. The volumes of the AONs and those of their subdivisions were calculated by the Cavalieri method and the area of the subdivisions was measured at seven established rostrocaudal levels. Whereas no statistically significant differences were detected between the ipsilateral and the contralateral AONs, comparison of these with controls revealed significant reductions in the volumes and dimensions of most AON subdivisions. The reduction was larger in the ipsilateral than in the contralateral AON and more pronounced in the rostralmost subdivisions (external and lateral) than in the caudal ones, the dorsal subdivision not being affected. These data demonstrate that the disruption of the normal afferent activity to one olfactory bulb has effects on the postnatal development of both the ipsilateral and the contralateral AONs. In addition, the most affected subdivisions were those that develop later and that receive the bulk of projections from the olfactory bulb, suggesting that the degree of maturity is an important factor in susceptibility to changes induced by reduced afferent activity. Finally, the results indicate that, contrary to the olfactory bulb, the contralateral AON cannot be used as a control structure in deprivation studies.

Calretinin immunoreactivity in the anterior olfactory nucleus of the rat

Calretinin-immunoreactive structures in the anterior olfactory nucleus of the rat were studied using a polyclonal antibody, which does not cross-react with the highly homologous calcium-binding protein calbindin-D28k, and the avidin-biotin-peroxidase technique. Calretinin-immunopositive neurons were found in all regions of the anterior olfactory nucleus, with the highest number in the medial subdivision and dorsal transition area. The immunostained neurons, although morphologically heterogeneous, demonstrated typically small size. In addition to neuronal somata, calretinin-immunopositive fibres and terminals, some of them forming basket-like arrangements surrounding immunonegative neurons, were observed. Although calretinin and calbindin-D28k colocalize in several brain regions, and both proteins showed an extensive overlap in the anterior olfactory nucleus, immunostained semithin sections demonstrated that calretinin does not co-localize with calbindin-D28k in this nucleus.

Odour-induced c-fos expression in the rat olfactory bulb: involvement of centrifugal afferents

Expression of the proto-oncogene c-fos is known to increase in granule cells of the olfactory bulb following a sustained olfactory stimulation. Most granule cells displaying high levels of Fos accumulation are located in the bulbar columns defined by the odour-induced foci of high 2-deoxyglucose glomerular uptake. The present studies were undertaken in order to assess the possible involvement of centrifugal afferents in the modulation of odour-induced patterns of either 2-deoxyglucose accumulation or c-fos expression in the olfactory bulb. A unilateral olfactory peduncle section had no effect on the odour-induced 2-deoxyglucose foci but induced a significant decrease in the number of Fos-containing neurons in odour-selective areas of both olfactory bulbs, ipsilateral and contralateral to the lesion. This suppressive effect was much more pronounced in the side ipsilateral to the peduncle section. It is concluded that c-fos expression induced by a sustained stimulation with propionic acid vapours is not only determined by the olfactory peripheral input but also by afferents of central origin. In order to estimate the contingent involvements of the cholinergic and noradrenergic afferents in this control of c-fos expression, we attempted to mimic the effects of the surgical deafferentation on odour-induced c-fos expression by using a pharmacological approach with selective cholinergic and noradrenergic antagonists. The beta-adrenergic antagonist propanolol induced a suppression of the odour-related patterns of Fos accumulation similar to the one caused by the surgical deafferentation of the olfactory bulb. The muscarinic antagonist scopolamine did not alter c-fos expression in the odour-selective area but increased significantly Fos labelling in the other bulbar aspects. Pharmacological investigations indicate that the noradrenergic and cholinergic centrifugal systems are likely involved in the central modulation of c-fos expression in the OB. The Fos protein could be expressed as an early nuclear signal triggering further long-term modifications of the neuronal phenotype, in certain conditions of sensory stimulation involving the activation of centrifugal systems.

Fos protein immunoreactivity in the developing olfactory bulbs of normal and naris-occluded rats

Immediate early genes such as c-fos may be a route through which extracellular events affect genomic expression. Expression of immediate early genes is important in the transcriptional regulation necessary for the normal development of the nervous system. Developmental patterns of Fos protein (the product of c-fos immediate early gene expression) were studied in the main olfactory bulb of the rat using immunocytochemistry. Embryonic Day 21 (E21, the last prenatal day), as well as Postnatal Day 0 (P0), P1, P5, P10, P15, P20 and P30 subjects were examined. Although staining was absent in the E21 bulb, there was a rapid onset of Fos synthesis within hours after birth. Distribution of Fos-immunoreactive (Fos-ir) nuclei corresponded to the sequence of bulb maturation: numerous mitral/tufted and granule cells were labeled on P0, followed by the appearance of Fos-ir in the nuclei of periglomerular cells and an increase in the number of stained granule cells with development. Surgical closure of an external naris on P1 resulted in a 70% reduction in the number of Fos-ir granule cell nuclei as early as 2 h after the manipulation. During the next 30 days, levels of Fos staining further diminished in experimental bulbs when compared to their contralateral controls. Nevertheless, electrical stimulation of the contralateral bulb in P20 pups resulted in a robust increase of Fos labeling in most main and accessory olfactory bulb mitral cells and in many granule and periglomerular neurons, suggesting that the experimental bulbs remain competent to express Fos protein.

The CVS strain of rabies virus as transneuronal tracer in the olfactory system of mice

The sequential distribution of transneuronally infected neurons was studied in the olfactory pathway of mice after unilateral inoculation of the challenge virus standard (CVS) strain in the nasal cavity. A first cycle of viral multiplication was observed in a subpopulation of receptor cells scattered in the main olfactory epithelium and in the septal organ. No viral spread from cell body to cell body was reported even in later stages of infection. The second round of viral replication which took place in the ipsilateral main olfactory bulb at 2 and 2.5 days post-inoculation (p.i.), involved second order neurons and periglomerular cells, known to be directly connected with the axon terminals of receptor cells. Also reported as a result of a second cycle of viral replication, was surprisingly the spread of CVS at 2 and 2.5 days p.i. in bulbar interneurons located in the internal plexiform layer and in the superficial granule cell layer, as well as that of 2 ipsilateral cerebral nuclei, the anterior olfactory nucleus and the horizontal limb of the diagonal band. From day 3, a rapid spread of CVS was suggested by detection of virus in all ipsilateral direct terminal regions of the second order neurons and in most tertiary olfactory projections. The locus coeruleus, a noradrenergic nucleus which sends direct afferents to the olfactory bulb, never appeared immunoreactive. In spite of a certain inability of CVS to infect some neuron types, the virus appears relevant to provide new information regarding the complex network of olfactory-related neurons into the CNS.

Distribution of calbindin D-28K and parvalbumin immunoreactivities in the nucleus olfactorius anterior of the rat

The distributions of calbindin D-28K (CaBP) and parvalbumin (PV) in the rat nucleus olfactorius anterior (NOA) were described using monoclonal antibodies and the avidin-biotin-peroxidase method. The NOA showed a high immunoreactivity for CaBP, with a rostrocaudal increase in the positive neurons and fibres. Pars externa (NOAe) was the only subdivision which showed a low CaBP immunostaining. PV-positive elements were less abundant than those CaBP immunostained. The main difference in the distributions for both proteins was observed in the pars medialis which was practically PV negative. PV- and CaBP-stained neurons showed similar morphologies in the subdivisions where they were present. In NOAe, we observed a characteristic PV- and CaBP-positive neuronal type, with an oriented dendritic pattern. Transition areas were clearly observable in both CaBP- and PV-labelled sections.

Localization of corticotropin-releasing factor-like immunoreactivity in monkey olfactory bulb and secondary olfactory areas

Electrophysiological and anatomical observations suggest that terminals of olfactory bulb mitral cells ending in rat primary olfactory cortex exert certain postsynaptic effects via an excitatory amino acid neurotransmitter. Recent anatomical studies have shown that several peptides, most notably corticotropin-releasing factor (CRF) (Imaki et al., '89) Brain Res., 496: 35-44), are also localized within rat olfactory bulb projection neurons, thus raising the possibility that there is a peptide cotransmitter in this system. In contrast to the availability of data for rodents, very little is known about the distribution of peptides and other putative transmitters in the olfactory systems of primate species. In the present study, sections through the olfactory bulb and its target areas were obtained from two monkey species (Saimiri sciureus and Macaca fascicularis) and processed for immunohistochemistry with a well-characterized polyclonal antiserum directed against the human form of CRF. Virtually identical results were obtained in the two species. Within the olfactory bulb, nearly all mitral and many tufted cells contained CRF-like immunoreactivity. CRF-positive fibers were seen within the olfactory tract and olfactory stria, which contain the axons of mitral and tufted cells. Within the anterior olfactory nucleus and layer Ia of the olfactory tubercle and piriform cortex, immunoreactivity was seen within fine processes, as well as in coarse, varicose fibers and isolated puncta. CRF-positive cells were seen within layer III of the olfactory tubercle and piriform cortex. Immunoreactive fibers and varicosities were also seen within olfactory-recipient regions of the amygdala and entorhinal cortex. These observations suggest that CRF may act as a transmitter and/or neuromodulator in primate olfactory system.

Development of the anterior olfactory nucleus in normal and unilaterally odor deprived rats

The development of a second order structure in the olfactory pathway, the anterior olfactory nucleus, was examined in both normal rat pups and in subjects which underwent unilateral naris closure on postnatal day 1 (P1). Naris occlusion in neonatal rats produces a constellation of changes within the first relay in the pathway, the olfactory bulb, including a 25% reduction in total volume. Such large changes suggest that higher order structures might also be affected. Anterior olfactory nucleus development was quantified in several ways. Laminar volumes were computed by using serial section planimetry. In control animals differential development was observed, with regions extending most rostrally (e.g., pars externa and pars lateralis) exhibiting the least growth. The anterior olfactory nucleus on the "deprived" side of subjects with a single naris occluded was identical in size to that observed in controls, development within the pars lateralis was examined in control animals at P10, P20, P30, and adults. Developmental increases in numbers of both branches per cell and spines were noted, but mean branch length remained relatively constant. Finally, the effects of naris occlusion on histological patterns of succinate dehydrogenase (SDH) staining and 2-deoxyglucose uptake within pars lateralis were examined at P20 to test for more subtle effects of naris occlusion. SDH staining was quite similar in deprived and control rats at P20. However, 3H-2-DG uptake was decreased in rostral areas of the anterior olfactory nucleus ipsilateral to the deprived olfactory bulb, suggesting that naris closure does affect the structure.

Region-specific consequences of PCD gene expression in the olfactory system

These studies investigated the response of olfactory bulb juxtaglomerular dopamine neurons to the loss of mitral cells in 6-7-month-old Purkinje cell degeneration (PCD) mice. Previous studies in normal mice, with tyrosine hydroxylase (TH) enzyme as a marker, demonstrated that following peripheral olfactory afferent denervation the juxtaglomerular dopamine neurons exhibited a large reduction in TH activity and immunoreactivity. These intrinsic dopamine neurons also receive afferent input via dendrodendritic contacts with mitral cells. In contrast to the deficits produced by peripheral denervation, following mitral cell degeneration in homozygous recessive PCD mice, TH activity and immunoreactivity were unaltered as compared to normal heterozygous littermates. Moreover, TH activity in the substantia nigra also was unchanged, thus suggesting that the dopamine phenotype is resistant to the influences of the pcd gene. Despite the absence of a well-defined effect of the pcd gene on neurons bearing the TH phenotype, the expression of this mutation within the olfactory system is not limited to mitral cell degeneration. The current studies also demonstrate the absence of the anterior commissure, especially pars anterior, in homozygous recessive PCD mice at 6-7 months postnatal. Whether or not the loss of the anterior commissure is a primary effect or one that is secondary to mitral cell degeneration, this structural alteration provides evidence that the pcd gene exerts more widespread effects within the olfactory system that previously appreciated. The neuronal specificity of those effects remains apparent as indicated by the lack of change in TH expression.

Morphology of neuropeptide Y-immunoreactive neurons in the cat olfactory bulb and olfactory peduncle: postnatal development and species comparison

The distribution and morphology of Neuropeptide Y-immunoreactive (NPY-ir) neurons in the olfactory bulb and the olfactory peduncle was studied in the adult cat and rat, and the common marmoset Callithrix jacchus. Significant species differences were not observed. In all three species, the population of NPY-ir neurons is localized in the white matter extending from the main olfactory bulb to the border of the striatum. The neurons are characterized by a conspicuously looping axonal ramification pattern with some major collaterals running toward the olfactory bulb and others running toward the internal olfactory tract. The former, ipsilateral projection terminates in the granule cell layer of the main and accessory olfactory bulb and in layer II/III of the anterior olfactory nucleus. Reconstruction of the latter projection has revealed that the fibers are continuous with the olfactory limb of the anterior commissure and the anterior commissure proper suggesting a commissural contralateral projection. The analysis of the postnatal development of the cat NPY neuron system supports this assumption in a very clear-cut way. In young animals growing fibers are observed to cross the brachium of the commissure. The NPY neuron system develops postnatally. The maximum cell number is reached during the third postnatal week. The appearance of more and more NPY-ir neurons slightly precedes the formation of the terminal fields and of the fiber projection in the internal olfactory tract. The density of this early fiber projection by far exceeds the fiber density observed in the adult. Later in development the fiber density in the olfactory limb and the anterior commissure becomes considerably reduced. In contrast, the plexus density in the anterior olfactory nucleus and the granule cell layer of the main and accessory olfactory bulb undergoes only a slight reduction, and the NPY-ir cell number remains roughly constant. These observations suggest that the ipsilateral NPY-ir projection remains largely unchanged, in contrast to the contralateral projection, which exists to a large extent only for the first four postnatal months. The observation that the NPY neuron system gives rise to a contralateral projection does not support a classification of NPY neurons as short axon cells.

Connections of the olfactory bulb and nucleus olfactorius anterior in the hedgehog (Erinaceus europaeus): fluorescent tracers and HRP study

The projections of the main olfactory bulbs (MOBs) and the dorsal part of the anterior olfactory nucleus (NOA) in the hedgehog (Erinaceus europaeus) have been studied by fluorescent tracers and the horseradish peroxidase method (HRP), respectively, to reveal the pattern of labeling from these structures. After different dye injections in both MOBs, labeled cells were present in the following structures: tenia tecta, vertical limb of the diagonal band of Broca, and medial septal nucleus in the ipsilateral injection site; and the NOA, piriform cortex, nucleus of the lateral olfactory tract, horizontal limb of the diagonal band of Broca, posterolateral cortical amygdaloid nucleus, anterior amygdaloid area, and dorsal raphe nucleus in both hemispheres. Structures showing double-labeled cells were the NOA, horizontal limb of the diagonal band of Broca, nucleus of the lateral olfactory tract, anterior amygdaloid area, and posterolateral cortical amygdaloid nucleus. After HRP injections in the dorsal part of the NOA, labeled cells were distributed in the NOA, nucleus of the lateral olfactory tract, posterolateral cortical amygdaloid nucleus, piriform cortex, horizontal and vertical limbs of the diagonal band of Broca, mitral cell layer of the MOB, tenia tecta, anterior amygdaloid area, and the contralateral NOA. We suggest that the contralateral projection nuclei to the MOB of the hedgehog, unusual in other mammals, and the large number of cells with axonal collaterals projecting to both hemispheres, may be a strategy in these animals to bilaterally integrate brain functions at the expense of its reduced corpus callosum.

Structure of the nucleus olfactorius anterior of the hedgehog (Erinaceus europaeus)

The cytoarchitecture, topography, and cellular structure of the nucleus olfactorius anterior (NOA) in the hedgehog have been studied in Nissl-stained and Golgi preparations. The NOA is an important receptive allocortical formation for olfactory fibers and the major source of association fibers relating the main olfactory bulb with the rest of the olfactory brain. It was divided into a bulbar part; four subdivisions named lateral, dorsal, medial, and ventral; an external part; and a posterior part. Except for the external and posterior subdivisions, the NOA is relatively homogeneous and, in spite of the apparent lack of sublamination in Niss-stained material, four clearly defined cellular laminae were distinguished by the Golgi method. These layers were found to be strikingly similar to those in the piriform cortex. Layer I contains the terminal ramifications of apical dendrites of pyramidal cells and the collaterals of the lateral olfactory tract. The superficial part of layer II contains extraverted pyramidal cells with two or three apical dendrites ramifying in layer I. Most pyramidal cells in the deep part of layer II and layer III are typical pyramidal cells with axons entering the commissura anterior. Some pyramidal cell axons bifurcate into two branches running in opposite directions in the commissura anterior. The interstitial zone below layer III contains deep pyramidal cells and polymorphic cells with ascending branches. Cells with intrinsic axons were classified into four main categories according to the distribution of their axonal ramifications: 1) cells with very restricted axons, 2) cells with axons oriented tangentially in the superficial part of layer II, 3) cells with ascending axons located in the deep part, and 4) chandelierlike cells. Finally, some functional considerations are discussed.

Persistence of the pars externa system of the anterior olfactory nucleus in a microsmatic primate, Callithrix jacchus

The pars externa (PE) system of the anterior olfactory nucleus (AON) in a primate, Callithrix jacchus, was defined by its architecture and by its connection patterns with the main olfactory bulb (MOB) as revealed by tracing techniques. Focal, unilateral injections of wheat germ agglutinin-conjugated horseradish peroxidase into the MOB yielded ipsilaterally labelled afferent neurons in all subdivisions of the AON, with the exception of a clearly circumscribed area in the ventrolateral retrobulbar field of the basocaudal frontal lobe; in the contralateral hemisphere, this same area contained intensely stained neurons forming a horizontal flat plate of small neurons. This unique commissural connection pattern parallels the organization of the PE to MOB connection in sub-primates (Schoenfeld and Macrides, 1984, J. Comp. Neurol., 227: 121-135). Thus, despite earlier controversy (Crosby and Humphrey, 1939, J. Comp. Neurol., 71: 121-213), there appears to be a PE system in a microsmatic primate whose organization is quite similar to that in sub-primates.