Calretinin-, neurocalcin-, and parvalbumin-immunoreactive elements in the olfactory bulb of the hedgehog (Erinaceus europaeus)

The vomeronasal system of the wolf (Canis lupus signatus): The singularities of a wild canid

Wolves, akin to their fellow canids, extensively employ chemical signals for various aspects of communication, including territory maintenance, reproductive synchronisation and social hierarchy signalling. Pheromone-mediated chemical communication operates unconsciously among individuals, serving as an innate sensory modality that regulates both their physiology and behaviour. Despite its crucial role in the life of the wolf, there is a lacuna in comprehensive research on the neuroanatomical and physiological underpinnings of chemical communication within this species. This study investigates the vomeronasal system (VNS) of the Iberian wolf, simultaneously probing potential alterations brought about by dog domestication. Our findings demonstrate the presence of a fully functional VNS, vital for pheromone-mediated communication, in the Iberian wolf. While macroscopic similarities between the VNS of the wolf and the domestic dog are discernible, notable microscopic differences emerge. These distinctions include the presence of neuronal clusters associated with the sensory epithelium of the vomeronasal organ (VNO) and a heightened degree of differentiation of the accessory olfactory bulb (AOB). Immunohistochemical analyses reveal the expression of the two primary families of vomeronasal receptors (V1R and V2R) within the VNO. However, only the V1R family is expressed in the AOB. These findings not only yield profound insights into the VNS of the wolf but also hint at how domestication might have altered neural configurations that underpin species-specific behaviours. This understanding holds implications for the development of innovative strategies, such as the application of semiochemicals for wolf population management, aligning with contemporary conservation goals.

Immunohistochemical distribution of secretagogin in the mouse brain

Introduction

Calcium is essential for the correct functioning of the central nervous system, and calcium-binding proteins help to finely regulate its concentration. Whereas some calcium-binding proteins such as calmodulin are ubiquitous and are present in many cell types, others such as calbindin, calretinin, and parvalbumin are expressed in specific neuronal populations. Secretagogin belongs to this latter group and its distribution throughout the brain is only partially known. In the present work, the distribution of secretagogin-immunopositive cells was studied in the entire brain of healthy adult mice.

Methods

Adult male C57BL/DBA mice aged between 5 and 7 months were used. Their whole brain was sectioned and used for immunohistochemistry. Specific neural populations were observed in different zones and nuclei identified according to Paxinos mouse brain atlas.

Results

Labelled cells were found with a Golgi-like staining, allowing an excellent characterization of their dendritic and axonal arborizations. Many secretagogin-positive cells were observed along different encephalic regions, especially in the olfactory bulb, basal ganglia, and hypothalamus. Immunostained populations were very heterogenous in both size and distribution, as some nuclei presented labelling in their entire extension, but in others, only scattered cells were present.

Discussion

Secretagogin can provide a more complete vision of calcium-buffering mechanisms in the brain, and can be a useful neuronal marker in different brain areas for specific populations.

The vomeronasal system of the newborn capybara: a morphological and immunohistochemical study

The vomeronasal system (VNS) is responsible for the perception mainly of pheromones and kairomones. Primarily studied in laboratory rodents, it plays a crucial role in their socio-sexual behaviour. As a wild rodent, the capybara offers a more objective and representative perspective to understand the significance of the system in the Rodentia, avoiding the risk of extrapolating from laboratory rodent strains, exposed to high levels of artificial selection pressure. We have studied the main morphological and immunohistochemical features of the capybara vomeronasal organ (VNO) and accessory olfactory bulb (AOB). The study was done in newborn individuals to investigate the maturity of the system at this early stage. We used techniques such as histological stains, lectins-labelling and immunohistochemical characterization of a range of proteins, including G proteins (Gαi2, Gαo) and olfactory marking protein. As a result, we conclude that the VNS of the capybara at birth is capable of establishing the same function as that of the adult, and that it presents unique features as the high degree of differentiation of the AOB and the active cellular migration in the vomeronasal epithelium. All together makes the capybara a promising model for the study of chemical communication in the first days of life.

Structural, morphometric and immunohistochemical study of the rabbit accessory olfactory bulb

The accessory olfactory bulb (AOB) is the first neural integrative centre of the vomeronasal system (VNS), which is associated primarily with the detection of semiochemicals. Although the rabbit is used as a model for the study of chemocommunication, these studies are hampered by the lack of knowledge regarding the topography, lamination, and neurochemical properties of the rabbit AOB. To fill this gap, we have employed histological stainings: lectin labelling with Ulex europaeus (UEA-I), Bandeiraea simplicifolia (BSI-B4), and Lycopersicon esculentum (LEA) agglutinins, and a range of immunohistochemical markers. Anti-G proteins Gαi2/Gαo, not previously studied in the rabbit AOB, are expressed following an antero-posterior zonal pattern. This places Lagomorpha among the small groups of mammals that conserve a double-path vomeronasal reception. Antibodies against olfactory marker protein (OMP), growth-associated protein-43 (GAP-43), glutaminase (GLS), microtubule-associated protein-2 (MAP-2), glial fibrillary-acidic protein (GFAP), calbindin (CB), and calretinin (CR) characterise the strata and the principal components of the BOA, demonstrating several singular features of the rabbit AOB. This diversity is accentuated by the presence of a unique organisation: four neuronal clusters in the accessory bulbar white matter, two of them not previously characterised in any species (the γ and δ groups). Our morphometric study of the AOB has found significant differences between sexes in the numerical density of principal cells, with larger values in females, a pattern completely opposite to that found in rats. In summary, the rabbit possesses a highly developed AOB, with many specific features that highlight the significant role played by chemocommunication among this species.

Neuronal organization of the main olfactory bulb revisited

The main olfactory bulb is now one of the most interesting parts of the brain; firstly as an excellent model for understanding the neural mechanisms of sensory information processing, and secondly as one of the most prominent sites whose interneurons are generated continuously in the postnatal and adult periods. The neuronal organization of the main olfactory bulb is fundamentally important as the basis of ongoing and future studies. In this review we focus on four issues, some of which appear not to have been recognized previously: (1) axons of periglomerular cells, (2) the heterogeneity and peculiarity of dopamine-GABAergic juxtaglomerular cells, (3) neurons participating in the interglomerular connections, and (4) newly found transglomerular cells.

Secretagogin-containing neurons in the mouse main olfactory bulb

Secretagogin (SCGN) is a recently discovered calcium binding protein of the EF hand family. We studied the structural features of SCGN-positive neurons in the mouse main olfactory bulb (MOB). SCGN-positive neurons were localized throughout layers but clustered in the glomerular layer (GL), mitral cell layer (MCL) and granule cell layer (GCL). They were heterogeneous, including numerous juxtaglomerular neurons, granule cells, small to medium-sized neurons in the external plexiform layer (EPL), and a few small cells in the ependymal/subependymal layer. Calretinin and/or tyrosine hydroxylase occasionally colocalized in SCGN-positive juxtaglomerular neurons. Calretinin also frequently colocalized in SCGN-positive EPL and GCL neurons. Morphologically some of juxtaglomerular SCGN-positive neurons were classical periglomerular cells, whereas others were apparently different from those periglomerular cells, although they were further heterogeneous. Some extended one slender process into a glomerulus which passed the glomerulus and further penetrated into another nearby glomeruli, and thus their dendritic processes spanned two or three or more glomeruli. We named this type of juxtaglomerular neurons "transglomerular cells." With the stereological analysis we estimated total number of juxtaglomerular SCGN-positive neurons at about 80,000/single MOB. The present study revealed the diversity of SCGN-positive neurons in the mouse MOB and their particular structural properties hitherto unknown.

Types of cholecystokinin-containing periglomerular cells in the mouse olfactory bulb

The periglomerular cells (PG) of the olfactory bulb (OB) are involved in the primary processing and the refinement of sensory information from the olfactory epithelium. The neurochemical composition of these neurons has been studied in depth in many species, and over the last decades such studies have focused mainly on the rat. The increasing use of genetic models for research into olfactory function demands a profound characterization of the mouse olfactory bulb, including the chemical composition of bulbar interneurons. Regarding both their connectivity with the olfactory nerve and their neurochemical fate, recently, two different types of PG have been identified in the mouse. In the present report, we analyze both the synaptology and the chemical composition of specific PG populations in the murine olfactory bulb, in particular, those containing the neuropeptide cholecystokinin. Our results demonstrate the existence in the mouse of non-GABAergic PG and that these establish synaptic contacts with the olfactory nerve within the glomeruli. Based on previous classifications, we propose that this population would constitute a new subtype of type 1 mouse PG. In addition, we demonstrate the partial coexistence of cholecystokinin with the calcium-binding proteins neurocalcin and parvalbumin. All these findings add further data to our knowledge of the synaptology and neurochemistry of mouse PG. The differences observed from other rodents reflect the neurochemical heterogeneity of PG in the mammalian OB.

Somatostatin interneurons delineate the inner part of the external plexiform layer in the mouse main olfactory bulb

Neuropeptides play a major role in the modulation of information processing in neural networks. Somatostatin, one of the most concentrated neuropeptides in the brain, is found in many sensory systems including the olfactory pathway. However, its cellular distribution in the mouse main olfactory bulb (MOB) is yet to be characterized. Here we show that approximately 95% of mouse bulbar somatostatin-immunoreactive (SRIF-ir) cells describe a homogeneous population of interneurons. These are restricted to the inner lamina of the external plexiform layer (iEPL) with dendritic field strictly confined to the region. iEPL SRIF-ir neurons share some morphological features of Van Gehuchten short-axon cells, and always express glutamic acid decarboxylase, calretinin, and vasoactive intestinal peptide. One-half of SRIF-ir neurons are parvalbumin-ir, revealing an atypical neurochemical profile when compared to SRIF-ir interneurons of other forebrain regions such as cortex or hippocampus. Somatostatin is also present in fibers and in a few sparse presumptive deep short-axon cells in the granule cell layer (GCL), which were previously reported in other mammalian species. The spatial distribution of somatostatin interneurons in the MOB iEPL clearly outlines the region where lateral dendrites of mitral cells interact with GCL inhibitory interneurons through dendrodendritic reciprocal synapses. Symmetrical and asymmetrical synaptic contacts occur between SRIF-ir dendrites and mitral cell dendrites. Such restricted localization of somatostatin interneurons and connectivity in the bulbar synaptic network strongly suggest that the peptide plays a functional role in the modulation of olfactory processing.

Heterogeneity of calbindin-containing neurons in the mouse main olfactory bulb: I. General description

The structural features of calbindin-positive neurons were studied in the mouse main olfactory bulb (MOB). Calbindin-positive neurons were heterogeneous, including numerous periglomerular cells, a few granule cells, small to medium-sized interneurons in the external plexiform layer, and large short-axon cells located in the external plexiform layer, internal plexiform layer, granule cell layer and ependymal/subependymal layer. These large short-axon cells were also heterogeneous; some corresponded to classically identified short-axon cells such as Blanes cells, Golgi cells, horizontal cells and vertical cells, but some others appeared to be previously unidentified. A few faintly calbindin-positive presumed tufted cells were also encountered. Near the ependymal/subependymal layer of the MOB some calbindin-positive short-axon cells extended their dendritic processes more or less parallel to the sagittal plane, presumably corresponding to medullary cells named recently. In addition we encountered a few calbindin-positive horizontal cells in the internal plexiform layer extending their axons toward the lateral olfactory tract, one of which was confirmed to extend its axon into the lateral olfactory tract, indicating that they were presumed to be one of projection neurons. The present study revealed the diversity of calbindin-positive neurons in the mouse MOB and their particular structural properties hitherto unknown.

Molecular diversity of deep short-axon cells of the rat main olfactory bulb

Local circuit GABAergic interneurons comprise the most diverse cell populations of neuronal networks. Interneurons have been characterized and categorized based on their axo-somato-dendritic morphologies, neurochemical content, intrinsic electrical properties and their firing in relation to in-vivo population activity. Great advances in our understanding of their roles have been facilitated by their selective identification. Recently, we have described three major subtypes of deep short-axon cells (dSACs) of the main olfactory bulb (MOB) based on their axo-dendritic distributions and synaptic connectivity. Here, we investigated whether dSACs also display pronounced molecular diversity and whether distinct dSAC subtypes selectively express certain molecules. Multiple immunofluorescent labeling revealed that the most commonly used molecular markers of dSACs (e.g. vasoactive intestinal polypeptide, calbindin and nitric oxide synthase) label only very small subpopulations (< 7%). In contrast, voltage-gated potassium channel subunits Kv2.1, Kv3.1b, Kv4.3 and the GABA(A) receptor alpha1 subunit are present in 70-95% of dSACs without showing any dSAC subtype-selective expression. However, metabotropic glutamate receptor type 1alpha mainly labels dSACs that project to the glomerular layer (GL-dSAC subtype) and comprise approximately 20% of the total dSAC population. Analysing these molecular markers with stereological methods, we estimated the total number of dSACs in the entire MOB to be approximately 13,500, which is around a quarter of the number of mitral cells. Our results demonstrate a large molecular heterogeneity of dSACs and reveal a unique neurochemical marker for one dSAC subtype. Based on our results, dSAC subtype-specific genetic modifications will allow us to decipher the role of GL-dSACs in shaping the dynamic activity of the MOB network.

Age-related changes in calbindin-D28k, parvalbumin, and calretinin immunoreactivity in the dog main olfactory bulb

Expression and age-related changes of calbindin-D28k (CB), parvalbumin (PV), and calretinin (CR) in the main olfactory bulb of the dog were investigated by immunohistochemistry and western blot analysis. Neurons that expressed these calcium-binding proteins showed a characteristic laminar distribution. Most of CB-immunoreactive neurons were observed in the glomerular layer (GL) and the inner sublayer of the external plexiform layer (EPL). Most of PV-immunoreactive neurons were observed in the outer sublayer of the EPL. CR-immunoreactive neurons were mainly distributed in the GL and the granule cell layer. With regard to age-related changes, CB-immunoreactive neurons in the GL were stable among all age groups; however, in the EPL they decreased with age. PV-immunoreactive neurons decreased in middle-aged and aged groups. However, CR-immunoreactive neurons were not decreased in middle-aged and aged groups. These results suggest that CB-immunoreactive neurons in the EPL were most sensitive to aging, and that their reduction may be related to aging in the dog.

Distribution of calretinin during development of the olfactory system in the brown trout, Salmo trutta fario: Comparison with other immunohistochemical markers

Immunocytochemical techniques were used to investigate the appearance and distribution of calretinin in the olfactory system of developing and adult brown trout (Salmo trutta fario L.). The earliest calretinin-immunoreactive (CR-ir) cells were detected in the olfactory placode of 5-mm embryos. In 8-mm embryos, a CR-ir olfactory nerve was observed. The number of CR-ir olfactory receptor cells increased rapidly, and in fry and adults they were characterized by light and electron microscopy as pertaining to three morphological types of receptor cell, called microvillous, ciliated and rod-like cells or crypt cells. Comparisons of the cells labeled with CR and with more general olfactory markers (acetylated tubulin and keyhole limpet haemocyanin) in alevins and fry revealed that CR-ir cells represent only a subpopulation of olfactory receptor cells. Large cells located in the primordial mitral cell layer were the first CR-ir neuronal population of the olfactory bulbs and were observed in 7-mm embryos. These cells express high HuC/D immunoreactivity and were negative for glutamic acid decarboxylase and tyrosine hydroxylase. CR immunoreactivity diminished with development and most large cells of the mitral cell layer were CR-negative in fry. In later embryos and in alevins, CR-ir granule-like cells were observed in the olfactory bulbs. Comparisons of the terminal fields of primary olfactory fibers labeled with CR and with a more general olfactory marker in the olfactory bulbs of fry and adults revealed significant differences, with most glomeruli of the dorsomedial field receiving CR-negative olfactory fibers. These results suggest new criteria for understanding the organization of the olfactory system of the trout, and hence of teleosts. Our results also suggest that CR is involved in specific functions in the olfactory system during development.

Heterogeneity of parvalbumin-containing neurons in the mouse main olfactory bulb, with special reference to short-axon cells and betaIV-spectrin positive dendritic segments

The structural features of parvalbumin-positive neurons were studied in the mouse main olfactory bulb (MOB). Parvalbumin-positive neurons were heterogeneous, including numerous medium-sized interneurons in the external plexiform layer (EPL), some few large short-axon cells and a few periglomerular cells. Their overall distribution pattern and structural features resembled those of the rat MOB. However, large short-axon cells were frequently encountered in the internal plexiform and granule cell layers, which were rare in the rat MOB. In addition a few large short-axon cells were also encountered throughout the EPL. These short-axon cells extended their axons mainly in the EPL, usually making columnar axonal fields. Most parvalbumin-positive cells except periglomerular cells were confirmed to be glutamic acid decarboxylase positive. We examined the immuno-localization of the markers for the axon initial segments (AISs), betaIV-spectrin and sodium channels, to determine whether or not heterogeneous parvalbumin-positive neurons have axons. We confirmed their localization on the AISs of the large short-axon cells and periglomerular cells. However, these markers were encountered on some patch-like segments on the dendritic processes instead of the thin axon-like processes of the medium-sized EPL interneurons. The present study revealed the diversity of parvalbumin-positive neurons in the mouse MOB and their particular structural properties hitherto unknown.

Calretinin immunoreactivity in the brain of the zebrafish, Danio rerio: distribution and comparison with some neuropeptides and neurotransmitter-synthesizing enzymes. I. Olfactory organ and forebrain

The distribution of calretinin (CR) in the forebrain and the olfactory system of the adult zebrafish was studied by using immunocytochemical techniques. Previous studies in trout forebrain have indicated that CR-immunoreactive neurons acquire this phenotype rather early in development (Castro et al., J. Comp. Neurol. 467:254-269, 2003). Thus, precise knowledge of CR-expressing neuronal populations in adult zebrafish may help to decipher late stages of forebrain morphogenesis. For analysis of some forebrain nuclei and regions, CR distribution was compared with that of various ancillary markers: choline acetyltransferase, glutamic acid decarboxylase, tyrosine hydroxylase, neuropeptide Y, thyrotropin-releasing hormone, and galanin. The results reveal that calretinin is a specific marker of olfactory receptor neurons and of various neuronal populations distributed throughout the telencephalon and diencephalon. In addition, CR immunocytochemistry revealed characteristic patterns of fibers and neuropil in several telencephalic and diencephalic regions, indicating that it is a useful marker for characterizing a number of neural centers, pathways, and neuronal subpopulations in the zebrafish forebrain. Some ancillary markers also showed a distinctive distribution in pallial and subpallial regions, revealing additional aspects of forebrain organization. Comparison of the distribution of CR observed in the forebrain of zebrafish with that reported in other teleosts revealed a number of similarities and also some interesting differences. This indicates that various neuronal populations have maintained the CR phenotype in widely divergent teleost lines and suggests that CR studies may prove very useful for comparative analysis.

Chemoarchitecture of the monotreme olfactory bulb

The cyto- and chemoarchitecture of the olfactory bulb of two monotremes (shortbeaked echidna and platypus) was studied to determine if there are any chemoarchitectural differences from therian mammals. Nissl staining in conjunction with enzyme reactivity for NADPH diaphorase, and immunoreactivity for calcium binding proteins (parvalbumin, calbindin and calretinin), neuropeptide Y, tyrosine hydroxylase and non-phosphorylated neurofilament protein (SMI-32 antibody) were applied to the echidna. Material from platypus bulb was Nissl stained, immunoreacted for calretinin, or stained for NADPH diaphorase. In contrast to eutherians, no immunoreactivity for either the SMI-32 antibody or calretinin was found in the mitral or dispersed tufted cells of the monotremes and very few parvalbumin or calbindin immunoreactive neurons were found in the bulb of the echidna. On the other hand, immunoreactivity for tyrosine hydroxylase in the echidna was similar in distribution to that seen in therians, and periglomerular and granule cells showed similar patterns of calretinin immunoreactivity to eutherians. Multipolar neuropeptide Y immunoreactive neurons were confined to the deep granule cell layer and underlying white matter of the echidna bulb and NADPH diaphorase reactivity was found in occasional granule cells, fusiform and multipolar cells of the inner plexiform and granule cell layers, as well as underlying white matter. Unlike eutherians, no NPY immunoreactive or NADPH diaphorase reactive neurons were seen in the glomerular layer. The bulb of the echidna was comparable in volume to prosimians of similar body weight, and its constituent layers were highly folded. In conclusion, the monotreme olfactory bulb does not show any significant chemoarchitectural dissimilarities from eutheria, despite differences in mitral/tufted cell distribution.

Calbindin D28k, parvalbumin, and calretinin immunoreactivity in the main and accessory olfactory bulbs of the gray short-tailed opossum,Monodelphis domestica

The vertebrate main and accessory olfactory bulbs (MOB and AOB) are the first synaptic sites in the olfactory pathways. The MOB is a cortical structure phylogenetically well conserved in its laminar structure and overall synaptic organization, while the AOB has significant species variation in size. In order to better understand signal processing in the two olfactory systems and the species differences, immunocytochemical staining and analysis were done of the neuronal expression patterns of the calcium-binding proteins calbindin D28k (CB), parvalbumin (PV), and calretinin (CR) in the MOB and AOB in a marsupial species, the gray short-tailed opossum, Monodelphis domestica. In the MOB, antibody to CB labeled periglomerular cells, superficial short axon cells / Van Gehuchten cells; antibody to PV labeled Van Gehuchten cells; and antibody to CR immunostained periglomerular cells, superficial short axon cells / Van Gehuchten cells, and granule cells. In the AOB, CB immunoreactivity was detected in periglomerular cells and a subpopulation of granule cells; antibody to PV labeled the superficial short axon cells / Van Gehuchten cells and granule cells; and antibody to CR labeled a small number of periglomerular cells, superficial short axon cells / Van Gehuchten cells, and granule cells. These results showed that the patterns of CB, PV, and CR expression differ in the opossum main and accessory olfactory bulbs and differ from that in other animal species. These varying patterns of neuronal immunostaining may be related to the different functions of the main and accessory olfactory bulbs and to the differing signal processing features.

Organization of the main olfactory bulbs of some mammals: Musk shrews, moles, hedgehogs, tree shrews, bats, mice, and rats

We immunohistochemically examined the organization of the main olfactory bulbs (MOBs) in seven mammalian species, including moles, hedgehogs, tree shrews, bats, and mice as well as laboratory musk shrews and rats. We focused our investigation on two points: 1) whether nidi, particular spheroidal synaptic regions subjacent to glomeruli, which we previously reported for the laboratory musk shrew MOBs, are also present in other animals and 2) whether the compartmental organization of glomeruli and two types of periglomerular cells we proposed for the rat MOBs are general in other animals. The general laminar pattern was similar among these seven species, but discrete nidi and the nidal layer were recognized only in two insectivores, namely, the mole and laboratory musk shrew. Olfactory marker protein-immunoreactive (OMP-IR) axons extended beyond the limits of the glomerular layer (GL) into the superficial region of the external plexiform layer (EPL) or the nidal layer in the laboratory musk shrew, mole, hedgehog, and tree shrew but not in bat, mouse, and rat. We observed, in nidi and the nidal layer in the mole and laboratory musk shrew MOBs, only a few OMP-IR axons. In the hedgehog, another insectivore, OMP-IR processes extending from the glomeruli were scattered and intermingled with calbindin D28k-IR cells at the border between the GL and the EPL. In the superficial region of the EPL of the tree shrew MOBs, there were a small number of tiny glomerulus-like spheroidal structures where OMP-IR axons protruding from glomeruli were intermingled with dendritic branches of surrounding calbindin D28k-IR cells. Furthermore, we recognized the compartmental organization of glomeruli and two types of periglomerular cells in the MOBs of all of the mammals we examined. These structural features are therefore considered to be common and important organizational principles of the MOBs.

Calbindin D28K immunoreactive neurons in vomeronasal organ and their projections to the accessory olfactory bulb in the rat

The vomeronasal system is a nasal chemosensory system involved in pheromone detection. The chemosensory receptor neurons are located in the sensory epithelium of the vomeronasal organ (VNO). Their axons terminate in the glomeruli of the accessory olfactory bulb (AOB). In this study, we examined the expression of calbindin D28k (CB) in the rat VNO and AOB. In the VNO, a subpopulation of receptor neurons in the middle layer of the sensory epithelium was immunostained with antibodies to CB. Their axons could be traced to terminate in a group of glomeruli in the anterior half of the AOB glomerular layer. This group of CB-immunostained glomeruli in the anterior half of the AOB included a few large glomeruli close to the boundary between the anterior and posterior halves of the AOB, and several small glomeruli scattered in the anterior region of the AOB glomerular layer. The positions of the CB-immunostained glomeruli in the AOB, especially those close to the anterior-posterior boundary, were similar in the two bulbs and in different rats. No sex difference was found. A developmental study showed that the CB-immunoreactive receptor neurons in the middle layer of the VNO sensory epithelium and CB-immunoreactive glomeruli in the anterior AOB were present on the 14th postnatal day and older. The distribution pattern of the CB-immunostained receptor neurons and their localized projection suggest the possibility that these neurons may express the same or functionally related pheromone receptor genes.

Vasoactive intestinal polypeptide-containing elements in the olfactory bulb of the hedgehog (Erinaceus europaeus)

The distribution of vasoactive intestinal polypeptide (VIP)-immunopositive elements was analyzed in the olfactory bulb (OB) of the Western European hedgehog (Erinaceus europaeus) under light and electron microscopy. The immunoreactivity appeared in an abundant population of periglomerular cells of the glomerular layer, in interneurons of the external plexiform layer, and in a restricted group of deep short-axon cells of the internal plexiform layer, the granule cell layer and the white matter. In the glomerular layer, VIP-containing periglomerular cells constituted a population of non-GABAergic neurons and did not receive synapses from olfactory axons. In the EPL, VIP-immunoreactivity appeared in a morphologically heterogeneous population of GABAergic interneurons, most of them identified as satellite cells and Van Gehuchten cells. These interneurons exerted an abundant and selective innervation of the somata, primary and secondary dendrites of the principal mitral and tufted cells, but did not contact granule cells. Perisomatic innervation of the principal cells followed two different patterns. The first included 'normal' basket-like arrangements of VIP-containing varicosities surrounding the somata of mitral and tufted cells. In the second, a set of satellite cells gave rise to short dendritic shafts that embraced the somata of principal cells in an 'exuberant' basket-like arrangement. These two morphological patterns of perisomatic innervation of principal cells were correlated with a neurochemical specificity of the target. In this sense, the 'exuberant' basket-like structures were always found surrounding a subpopulation of principal cells that did not contain the calcium-binding protein parvalbumin (PV). By contrast, they were never found surrounding the subpopulation of PV-containing principal cells, which only showed 'normal' basket-like structures. This study provides new data on the connectivity and neurochemical features of the hedgehog olfactory bulb and suggests that the olfactory circuits in this species are more complex than those described in other mammals.