Primary olfactory projections and the nervus terminalis in the African lungfish: implications for the phylogeny of cranial nerves

Primary olfactory and central projections of the nervus terminalis were investigated by injections of horseradish peroxidase into the olfactory epithelium in the African lungfish. In addition, gonadotropin-releasing hormone (GnRH) immunoreactivity of the nervus terminalis system was investigated. The primary olfactory projections are restricted to the olfactory bulb located at the rostral pole of the telencephalon; they do not extend into caudal parts of the telencephalon. A vomeronasal nerve and an accessory olfactory bulb could not be identified. The nervus terminalis courses through the dorsomedial telencephalon. Major targets include the nucleus of the anterior commissure and the nucleus praeopticus pars superior. some fibers cross to the contralateral side. A few fibers reach the diencephalon and mesencephalon. No label is present in the "posterior root of the nervus terminalis" (= "Pinkus's nerve" or "nervus praeopticus"). GnRH immunoreactivity is lacking in the "anterior root of the nervus terminalis," whereas it is abundant in nervus praeopticus (Pinkus's nerve). These findings may suggest that the nervus terminalis system originally consisted of two distinct cranial nerves, which have fused-in evolution-in most vertebrates. Theories of cranial nerve phylogeny are discussed in the light of the assumed "binerval origin" of the nervus terminalis system.     

Gonadotropin releasing hormone (GnRH) neurons project to growth hormone and somatolactin cells in the steelhead trout

Analysis of gene expression using gonadotropin-releasing hormone (GnRH) antisense oligonucleotide confirmed by immunocytochemical localization the occurrence of GnRH neurons along the nervus terminalis in the steelhead trout (Oncorhynchus mykiss). Double-label immunocytochemistry revealed the distribution of mammalian (m), salmon (s) and chicken II (cII)-type GnRHs and various pituitary hormones. Both sGnRH and mGnRH appeared to be colocalized in the same cells of the nervus terminalis. Chicken GnRH II-immunoreactivity was found only in fibers and terminals. In the younger fish [73 and 186 days after fertilization (DAF)] GnRH neurons were seen rostral to the olfactory bulb. A novel GnRH ganglion, along the nervus terminalis, was found at the cribriform bone (gCB). A few non-immunoreactive rounded cells were seen among the GnRH neurons. A second smaller ganglion was seen at the most rostrally located part of the ventromedial olfactory bulb (gROB). In the older fish (850 DAF) GnRH neurons were also observed in the basal forebrain. A small group of neurons (2–3 cells), at the caudoventromedial border of the olfactory bulb, formed the ganglion terminale. Occasionally isolated GnRH-immunoreactive cells were seen at the base of the olfactory epithelium, along the ventromedial margins of the olfactory nerve. GnRH-immunoreactive and GnRH mRNA expressing neurons were absent from midbrain regions at the ages observed. GnRH-immunoreactive fibers were present only in older fish. The pattern of distribution of fibers that were immunoreactive to all three forms of GnRH was identical. Fibers were seen along the medial side of the olfactory nerve, throughout the brain and in the pituitary, associated with growth hormone and somatolactin cells. This morphological study shows that molecular forms of GnRHs might have multiple functions.     

HRP uptake by olfactory and vomeronasal receptor neurons: use as an indicator of incomplete lesions and relevance for non-volatile chemoreception

The transport of HRP (horseradish peroxidase) from the nasalcavity to the brain by intact olfactory receptor axons was usedto investigate the effectiveness of methods commonly used inbehavioral studies for deafferenting nasal chemoreceptor systems.The HRP experiments demonstrated that routine intranasal lavagewith zinc sulfate solution fails to destroy all olfactory receptorneurons in hamsters, in spite of the distinct behavioral deficitthat this treatment can cause in the male hamster. The intracranialdeafferentation of the accessory olfactory bulb by surgicalsection of the vomeronasal nerves was generally effective butthere was much incidental damage to main olfactory nerves thatwould probably not be detected without the HRP tracer. The distribution pattern of HRP molecules introduced into themammalian nasal cavity, as shown by the uptake of HRP by nasalchemoreceptors and its transport to the brain, was also usedto identify potential pathways for non-volatile stimulus moleculeswithin the nose. HRP reaction product was reliably detectedin the glomeruli of the main olfactory bulb after HRP was depositedat the nostril, demonstrating that nonvolatile materials, oncethey have entered the nasal cavity, can reach the main olfactoryreceptor neurons in the posterior nasal epithelium. Significantamounts of HRP reaction product were never observed in the accessoryolfactory bulbunlessa large dose of epinephrine had been givento activate the vomeronasal organ pumping mechanism, which drawssubstances into the vomeronasal organ lumen. Thus, it seemsthat stimulus access to vomeronasal receptor neurons is controlledindependently of access to main olfactory receptor neurons.     

The rodent accessory olfactory system

The accessory olfactory system contributes to the perception of chemical stimuli in the environment. This review summarizes the structure of the accessory olfactory system, the stimuli that activate it, and the responses elicited in the receptor cells and in the brain. The accessory olfactory system consists of a sensory organ, the vomeronasal organ, and its central projection areas: the accessory olfactory bulb, which is connected to the amygdala and hypothalamus, and also to the cortex. In the vomeronasal organ, several receptors—in contrast to the main olfactory receptors—are sensitive to volatile or nonvolatile molecules. In a similar manner to the main olfactory epithelium, the vomeronasal organ is sensitive to common odorants and pheromones. Each accessory olfactory bulb receives input from the ipsilateral vomeronasal organ, but its activity is modulated by centrifugal projections arising from other brain areas. The processing of vomeronasal stimuli in the amygdala involves contributions from the main olfactory system, and results in long-lasting responses that may be related to the activation of the hypothalamic–hypophyseal axis over a prolonged timeframe. Different brain areas receive inputs from both the main and the accessory olfactory systems, possibly merging the stimulation of the two sensory organs to originate a more complex and integrated chemosensory perception.     

Goα expression in the vomeronasal organ and olfactory bulb of the tammar wallaby

The vomeronasal organ (VNO) detects pheromones via 2 large families of receptors: vomeronasal receptor 1, associated with the protein Giα2, and vomeronasal receptor 2, associated with Goα. We investigated the distribution of Goα in the developing and adult VNO and adult olfactory bulb of a marsupial, the tammar wallaby. Some cells expressed Goα as early as day 5 postpartum, but by day 30, Goα expressing cells were distributed throughout the receptor epithelium of the VNO. In the adult tammar, Goα appeared to be expressed in sensory neurons whose nuclei were mostly basally located in the vomeronasal receptor epithelium. Goα expressing vomeronasal receptor cells led to all areas of the accessory olfactory bulb (AOB). The lack of regionally restricted projection of the vomeronasal receptor cell type 2 in the tammar was similar to the uniform type, with the crucial difference that the uniform type only shows expression of Giα2 and no expression of Goα. The observed Goα staining pattern suggests that the tammar may have a third accessory olfactory type that could be intermediate to the segregated and uniform types already described.     

The prenatal maturity of the accessory olfactory bulb in pigs

The morphological development of the accessory olfactory bulb of the fetal pig was studied by classical and histo-chemical methods, and the vomeronasal organ and nasal septum were studied histochemically. Specimens were obtained from an abattoir and their ages estimated from their crown-to-rump length. The accessory olfactory bulb was structurally mature in fetuses of crown-to-rump length 21-23 cm, by which time the lectin Lycopersicum esculentum agglutinin stained the same structures as in adults (in particular, the entire sensory epithelium of the vomeronasal organ, the vomeronasal nerves, and the nervous and glomerular layers of the accessory olfactory bulb). These results suggest that the vomeronasal system of the pig may, like that of vertebrates such as snakes, be functional at birth.     

Zonal organization of the mammalian main and accessory olfactory systems

Zonal organization is one of the characteristic features observed in both main and accessory olfactory systems. In the main olfactory system, most of the odorant receptors are classified into four groups according to their zonal expression patterns in the olfactory epithelium. Each group of odorant receptors is expressed by sensory neurons distributed within one of four circumscribed zones. Olfactory sensory neurons in a given zone of the epithelium project their axons to the glomeruli in a corresponding zone of the main olfactory bulb. Glomeruli in the same zone tend to represent similar odorant receptors having similar tuning specificity to odorants. Vomeronasal receptors (or pheromone receptors) are classified into two groups in the accessory olfactory system. Each group of receptors is expressed by vomeronasal sensory neurons in either the apical or basal zone of the vomeronasal epithelium. Sensory neurons in the apical zone project their axons to the rostral zone of the accessory olfactory bulb and form synaptic connections with mitral tufted cells belonging to the rostral zone. Signals originated from basal zone sensory neurons are sent to mitral tufted cells in the caudal zone of the accessory olfactory bulb. We discuss functional implications of the zonal organization in both main and accessory olfactory systems.     

Prion Shedding from Olfactory Neurons into Nasal Secretions

This study investigated the role of prion infection of the olfactory mucosa in the shedding of prion infectivity into nasal secretions. Prion infection with the HY strain of the transmissible mink encephalopathy (TME) agent resulted in a prominent infection of the olfactory bulb and the olfactory sensory epithelium including the olfactory receptor neurons (ORNs) and vomeronasal receptor neurons (VRNs), whose axons comprise the two olfactory cranial nerves. A distinct glycoform of the disease-specific isoform of the prion protein, PrP^Sc, was found in the olfactory mucosa compared to the olfactory bulb, but the total amount of HY TME infectivity in the nasal turbinates was within 100-fold of the titer in the olfactory bulb. PrP^Sc co-localized with olfactory marker protein in the soma and dendrites of ORNs and VRNs and also with adenylyl cyclase III, which is present in the sensory cilia of ORNs that project into the lumen of the nasal airway. Nasal lavages from HY TME-infected hamsters contained prion titers as high as 10^3.9 median lethal doses per ml, which would be up to 500-fold more infectious in undiluted nasal fluids. These findings were confirmed using the rapid PrP^Sc amplification QuIC assay, indicating that nasal swabs have the potential to be used for prion diagnostics. These studies demonstrate that prion infection in the olfactory epithelium is likely due to retrograde spread from the olfactory bulb along the olfactory and vomeronasal axons to the soma, dendrites, and cilia of these peripheral neurons. Since prions can replicate to high levels in neurons, we propose that ORNs can release prion infectivity into nasal fluids. The continual turnover and replacement of mature ORNs throughout the adult lifespan may also contribute to prion shedding from the nasal passage and could play a role in transmission of natural prion diseases in domestic and free-ranging ruminants.     

A map of pheromone receptor activation in the mammalian brain

In mammals, the detection of pheromones is mediated by the vomeronasal system. We have employed gene targeting to visualize the pattern of projections of axons from vomeronasal sensory neurons in the accessory olfactory bulb. Neurons expressing a specific receptor project to multiple glomeruli that reside within spatially restricted domains. The formation of this sensory map in the accessory olfactory bulb and the survival of vomeronasal organ sensory neurons require the expression of pheromone receptors. In addition, we observe individual glomeruli in the accessory olfactory bulb that receive input from more than one type of sensory neuron. These observations indicate that the organization of the vomeronasal sensory afferents is dramatically different from that of the main olfactory system, and these differences have important implications for the logic of olfactory coding in the vomeronasal organ.     

Sexual dimorphism of the vomeronasal organ and the accessory olfactory bulb of the mandarin vole<Emphasis Type="Italic">Microtus mandarinus</Emphasis> and the reed vole<Emphasis Type="Italic">M. fortis</Emphasis>

Sexual dimorphisms of the vomeronasal organ (VNO) and the accessory olfactory bulb (AOB) of the mandarin voleMicrotus mandarinus Milne-Edwards, 1871 and reed voleM. fortis Büchner, 1889 are reported for the first time in the present work. The thickness and length of the vomeronasal epithelium (VE) and the nuclear size of the receptor cells, the width and length of the granule cell zone, the width and length of the mitral cell zone, and the density of the mitral cells were surveyed. The thickness and length of the vomeronasal epithelium (VE), the length of the granule cell zone and the mitral cell zone, and the densities of mitral cells were significantly different between male and female reed voles. Male and female mandarin voles had no significant differences in any of these parameters. Polygamous reed voles had a greater degree of sexual dimorphism in VNO and AOB than did monogamous mandarin voles. The present results provide evidence to the hypothesis that the degree of sexual dimorphism may be related to the mating system.     

Neuropilin-2 mediates axonal fasciculation, zonal segregation, but not axonal convergence, of primary accessory olfactory neurons

The mechanisms that underlie axonal pathfinding of vomeronasal neurons from the vomeronasal organ (VNO) in the periphery to select glomeruli in the accessory olfactory bulb (AOB) are not well understood. Neuropilin-2, a receptor for secreted semaphorins, is expressed in V1R- and V3R-expressing, but not V2R-expressing, postnatal vomeronasal neurons. Analysis of the vomeronasal nerve in neuropilin-2 (npn-2) mutant mice reveals pathfinding defects at multiple choice points. Vomeronasal sensory axons are severely defasciculated and a subset innervates the main olfactory bulb (MOB). While most axons of V1R-expressing neurons reach the AOB and converge into distinct glomeruli in stereotypic locations, they are no longer restricted to their normal anterior AOB target zone. Thus, Npn-2 and candidate pheromone receptors play distinct and complementary roles in promoting the wiring and patterning of sensory neurons in the accessory olfactory system.     

Olfactory and vomeronasal projections and the pathway of the nervus terminalis in ten species of salamanders

Summary Primary olfactory and vomeronasal projections as well as the pathway of the nervus terminalis were studied in 10 representative species of salamandrid and plethodontid salamanders by means of injections of horseradish peroxidase and examination of whole-mount preparations. Olfactory projections are very similar in the different urodeles, but vomeronasal projections differ in shape and number of termination fields. Whereas the direct-developing Plethodontini and Bolitoglossini reveal only one or two fields, the salamandrid species and the members of the plethodontid tribes Desmognathinae and Hemidactyliini, all possessing an aquatic larval stage, exhibit several vomeronasal projection fields. In all species examined centrifugal axons of the nervus terminalis leave the olfactory projection area ventrocaudally and terminate in the preoptic region and the hypothalamus.Abbreviations COM. ANT commissura anterior - DGL displaced glomeruli - HY hypophysis - HYTH hypothalamus - LF lateral fibers of the nervus terminalis - ME medulla oblongata - MF medial fibers of the nervus terminalis - Nt nervus terminalis - Npo nucleus praeopticus     

Localization of choline acetyltransferase and vasoactive intestinal polypeptide-like immunoreactivity in the nervus terminalis of the fetal and neonatal rat

Ganglia of the nervus terminalis have been shown to contain luteinizing hormone-releasing hormone (LHRH) immunoreactive cells in several mammalian species. These cells are always accompanied by clusters of cells non-immunoreactive to antiserum to LHRH. Using immunocytochemical procedures, we found choline acetyltransferase (ChAT) and vasoactive intestinal polypeptide (VIP) present in cell bodies and in nerve processes throughout the peripheral, intracranial and central projections of the nervus terminalis. In addition, a dense plexus of substance P (SP) immunoreactive fibers was seen in the nasal mucosa surrounding the nasal glandular acini and blood vessels. A number of SP reactive fibers were traced with the olfactory nerves through the cribriform plate of the ethmoid bone and appeared to enter the brain in the area of the central roots of the nervus terminalis.     

Mating Increases Neuronal Tyrosine Hydroxylase Expression and Selectively Gates Transmission of Male Chemosensory Information in Female Mice

Exposure to chemosensory signals from unfamiliar males can terminate pregnancy in recently mated female mice. The number of tyrosine hydroxylase-positive neurons in the main olfactory bulb has been found to increase following mating and has been implicated in preventing male-induced pregnancy block during the post-implantation period. In contrast, pre-implantation pregnancy block is mediated by the vomeronasal system, and is thought to be prevented by selective inhibition of the mate’s pregnancy blocking chemosignals, at the level of the accessory olfactory bulb. The objectives of this study were firstly to identify the level of the vomeronasal pathway at which selective inhibition of the mate’s pregnancy blocking chemosignals occurs. Secondly, to determine whether a post-mating increase in tyrosine hydroxylase-positive neurons is observed in the vomeronasal system, which could play a role in preventing pre-implantation pregnancy block. Immunohistochemical staining revealed that mating induced an increase in tyrosine-hydroxylase positive neurons in the arcuate hypothalamus of BALB/c females, and suppressed c-Fos expression in these neurons in response to mating male chemosignals. This selective suppression of c-Fos response to mating male chemosignals was not apparent at earlier levels of the pregnancy-blocking neural pathway in the accessory olfactory bulb or corticomedial amygdala. Immunohistochemical staining revealed an increase in the number of tyrosine hydroxylase-positive neurons in the accessory olfactory bulb of BALB/c female mice following mating. However, increased dopamine-mediated inhibition in the accessory olfactory bulb is unlikely to account for the prevention of pregnancy block to the mating male, as tyrosine hydroxylase expression did not increase in females of the C57BL/6 strain, which show normal mate recognition. These findings reveal an association of mating with increased dopaminergic modulation in the pregnancy block pathway and support the hypothesis that mate recognition prevents pregnancy block by suppressing the activation of arcuate dopamine release.     

Histochemical localization of NADPH-diaphorase in the rat accessory olfactory bulb

The distribution of NADPH-diaphorase activity was examined inthe accessory olfactory bulb of the rat using a direct histochemicaltechnique. Labeled fibers and somata were found in all layersof the accessory olfactory bulb. The entire vomeronasal nerveand all vomeronasal glomeruli were strongly labeled, contraryto the main olfactory bulb, where only dorsomedial olfactoryglomeruli displayed NADPH-diaphorase activity. NADPH-diapborasepositive neurons were identified as periglomerular cells inthe glomerular layer and external plexiform layer, horizontalcells in the internal plexiform layer, and granule cells anddeep short-axon cells in the granule cell layer. The labeleddendrites of the granule cells formed a dense neuropile in thegranule cell layer, internal plexiform layer and external plexiformlayer. The staining pattern in the accessory olfactory bulbwas more complex than what has been previously reported, anddemonstrated both similarities and differences with the distributionof NADPH-diaphorase in the main olfactory bulb.     

The nose knows who's who: chemosensory individuality and mate recognition in mice

Individual recognition is an important component of behaviors, such as mate choice and maternal bonding that are vital for reproductive success. This article highlights recent developments in our understanding of the chemosensory cues and the neural pathways involved in individuality discrimination in rodents. There appear to be several types of chemosensory signal of individuality that are influenced by the highly polymorphic families of major histocompatibility complex (MHC) proteins or major urinary proteins (MUPs). Both have the capability of binding small molecules and may influence the individual profile of these chemosignals in biological fluids such as urine, skin secretions, or saliva. Moreover, these proteins, or peptides associated with them, can be taken up into the vomeronasal organ (VNO) where they can potentially interact directly with the vomeronasal receptors. This is particularly interesting given the expression of major histocompatibility complex Ib proteins by the V2R class of vomeronasal receptor and the highly selective responses of accessory olfactory bulb (AOB) mitral cells to strain identity. These findings are consistent with the role of the vomeronasal system in mediating individual discrimination that allows mate recognition in the context of the pregnancy block effect. This is hypothesized to involve a selective increase in the inhibitory control of mitral cells in the accessory olfactory bulb at the first level of processing of the vomeronasal stimulus.     

Prion infection of oral and nasal mucosa

Centrifugal spread of the prion agent to peripheral tissues is postulated to occur by axonal transport along nerve fibers. This study investigated the distribution of the pathological isoform of the protein (PrP(Sc)) in the tongues and nasal cavities of hamsters following intracerebral inoculation of the HY strain of the transmissible mink encephalopathy (TME) agent. We report that PrP(Sc) deposition was found in the lamina propria, taste buds, and stratified squamous epithelium of fungiform papillae in the tongue, as well as in skeletal muscle cells. Using laser scanning confocal microscopy, PrP(Sc) was localized to nerve fibers in each of these structures in the tongue, neuroepithelial taste cells of the taste bud, and, possibly, epithelial cells. This PrP(Sc) distribution was consistent with a spread of HY TME agent along both somatosensory and gustatory cranial nerves to the tongue and suggests subsequent synaptic spread to taste cells and epithelial cells via peripheral synapses. In the nasal cavity, PrP(Sc) accumulation was found in the olfactory and vomeronasal epithelium, where its location was consistent with a distribution in cell bodies and apical dendrites of the sensory neurons. Prion spread to these sites is consistent with transport via the olfactory nerve fibers that descend from the olfactory bulb. Our data suggest that epithelial cells, neuroepithelial taste cells, or olfactory sensory neurons at chemosensory mucosal surfaces, which undergo normal turnover, infected with the prion agent could be shed and play a role in the horizontal transmission of animal prion diseases.     

A divergent pattern of sensory axonal projections is rendered convergent by second-order neurons in the accessory olfactory bulb

The mammalian vomeronasal system is specialized in pheromone detection. The neural circuitry of the accessory olfactory bulb (AOB) provides an anatomical substrate for the coding of pheromone information. Here, we describe the axonal projection pattern of vomeronasal sensory neurons to the AOB and the dendritic connectivity pattern of second-order neurons. Genetically traced sensory neurons expressing a given gene of the V2R class of vomeronasal receptors project their axons to six to ten glomeruli distributed in globally conserved areas of the AOB, a theme similar to V1R-expressing neurons. Surprisingly, second-order neurons tend to project their dendrites to glomeruli innervated by axons of sensory neurons expressing the same V1R or the same V2R gene. Convergence of receptor type information in the olfactory bulb may represent a common design in olfactory systems.     

Mating behavior induces changes of expression of Fos protein,plasma testosterone and androgen receptors in the accessory olfactory bulb (AOB) of the male mandarin vole Microtus mandarinus

In order to investigate the neuroendocrine mechanism of the mating behavior in the adult male mandarin voles Microtus mandarinus,the radioimmunoassay(RIA)and immunohistochemistry methods were used to investigate the differences in plasma testosterone(T)concentrations and distribution of T immunoreactive neurons(T-IRs),androgen receptor immunoreactive neurons(AR-IRs)and Fos protein immunoreactive neurons(Fos-IRs)in the accessory olfactory bulb(AOB)and the main olfactory bulb(MOB)following exposure to clean h...     

Human Neural Cells Transiently Express Reelin during Olfactory Placode Development

Reelin, an extracellular glycoprotein is essential for migration and correct positioning of neurons during development. Since the olfactory system is known as a source of various migrating neuronal cells, we studied Reelin expression in the two chemosensory olfactory systems, main and accessory, during early developmental stages of human foetuses/embryos from Carnegie Stage (CS) 15 to gestational week (GW) 14. From CS 15 to CS 18, but not at later stages, a transient expression of Reelin was detected first in the presumptive olfactory and then in the presumptive vomeronasal epithelium. During the same period, Reelin-positive cells detach from the olfactory/vomeronasal epithelium and migrate through the mesenchyme beneath the telencephalon. Dab 1, an adaptor protein of the Reelin pathway, was simultaneously expressed in the migratory mass from CS16 to CS17 and, at later stages, in the presumptive olfactory ensheathing cells. Possible involvements of Reelin and Dab 1 in the peripheral migrating stream are discussed.