Analysis of the topographical organization of olfactory epithelium projections in the rat

The degree of topographical organization of epithelium-to-bulb connections in 15-day-old rats was analyzed from the complete mapping of these projections previously described by using the HRP retrograde tracing method. We performed HRP placements in the antero-posterior (anterior, central and posterior injections) and dorso-ventral bulbar axes. Results demonstrated: lack of transposition of any plane of the epithelial sheet along the antero-posterior axis in the bulb which indicated a typical diffuse distribution of projections along this bulbar plane; a general tendency of the medial, lateral, dorsal and ventral epithelial regions to project to homologous bulbar surfaces; however, this transposition was always relative; the importance of the dorso-ventral bulbar axis in the somatotopic organization of projections since this plane was a common plane on which the different mucosal axes were represented. These results suggest that the topographical organization of projections reflects the ontogenetically determined epithelium-to-bulb connections.     

Differential projections from locus coeruleus to olfactory bulb and olfactory tubercle: an HRP study

The microiontophoretic administration of horseradish peroxidase (HRP) to the olfactory bulb (OB) or olfactory tubercle (OT) in cats and rats yielded similar results in both species. After an OB HRP-injection ipsilateral and contralateral labelled neurons were seen in the piriform cortex, polymorphic layer of OT, magnocellular preoptic region, lateral hypothalamus, ventromedial hypothalamic nucleus and locus coeruleus (LC). In both species more labelled structures were found after an OT HRP-injection than after an OB HRP-injection. The substantia nigra in rats was more abundantly labelled after an OT injection than after an OB one. In cats the dorsal and the ventral raphe were also labelled. In either species, OT HRP-injections resulted in a higher frequency of LC labelled neurons than after OB injections. These results favor the hypothesis that the OT plays an important role as a relay station for efferent inflow from the brain stem en route to the OB.     

Nucleus of the tractus solitarius projections to the olfactory tubercle: An HRP study

Labelled cells were found in the nucleus of the tractus solitarius (NTS) after horseradish peroxidase injections in the olfactory tubercle (OT) of the rat. These results suggest a direct pathway from the NTS to the OT. The importance of this pathway in a neural circuit related to autonomic functions is discussed.     

Glutathione levels in olfactory and non-olfactory neural structures of rats

Olfactory receptor neurons are a CNS entry point for a wide variety of airborne substances. Therefore, it is probable that detoxification mechanisms are present in these neurons to neutralize such agents. Glutathione (GSH) is an essential component of several detoxification schemes, and in this study we examined the distribution and levels of GSH in the olfactory epithelium, olfactory bulb, cortex, hippocampus and cerebellum in neonatal, weanling, adult and aged rats. We report that GSH is primarily localized to the olfactory receptor neurons and their oxons within the olfactory epithelium. It is also localized within the glomerular neuropil and granule cells of the olfactory bulb. Levels of GSH in the olfactory epithelium and hippocampus do not change as a function of age, although GSH levels decrease in several brain regions, including the olfactory bulb, cerebellum and cortex.     

Localization of orexins and their receptors in the rat olfactory system: possible modulation of olfactory perception by a neuropeptide synthetized centrally or locally

Orexin-A and -B, also known as hypocretins, are two neuropeptides acting on feeding and sleep. They are specific ligands for two different receptors belonging to the G-protein coupled receptors family. Orexin fibers and orexin receptor neurons have been previously described in the forebrain olfactory system. Using immunocytochemistry, we showed that both orexin-A and -B as well as their receptors were present at different levels of the olfactory system, from the nasal mucosa to nuclei of the amygdala. A punctuated staining for orexins and their receptors was detected at the apical part of the olfactory epithelium; in the lamina propria of the mucosa, the staining was localized around olfactory nerves. At the ultrastructural level, olfactory neurons and supporting cells were found immunoreactive for orexins and their receptors. The labeling was localized in dendritic knobs and cilia of neurons, in the apical part and microvilli of supporting cells. The finding of immunolabeled cisternae of reticulum strongly suggests a local synthesis of both peptides and receptors, confirmed by RT-PCR experiments. In forebrain and amygdala regions, we detected numerous orexin fibers. Orexin receptors were present in mitral-tufted cells of the bulb and in many neuronal perikarya in the anterior olfactory nuclei, piriform cortex and amygdala nuclei. Altogether, these results show that orexins and their receptors are present at all levels of the olfactory system, from cilia where odors bind to their receptors to central regions where integration of olfactory signals occurs. They suggest a possible modulation of olfactory perception by these neuropeptides.     

The main and the accessory olfactory systems interact in the control of mate recognition and sexual behavior

In the field of sensory perception, one noticeable fact regarding olfactory perception is the existence of several olfactory subsystems involved in the detection and processing of olfactory information. Indeed, the vomeronasal or accessory olfactory system is usually conceived as being involved in the processing of pheromones as it is closely connected to the hypothalamus, thereby controlling reproductive function. By contrast, the main olfactory system is considered as a general analyzer of volatile chemosignals, used in the context of social communication, for the identification of the status of conspecifics. The respective roles played by the main and the accessory olfactory systems in the control of mate recognition and sexual behavior are at present still controversial. We summarize in this review recent results showing that both the main and accessory olfactory systems are able to process partially overlapping sets of sexual chemosignals and that both systems support complimentary aspects in mate recognition and in the control of sexual behavior.     

嗅球、嗅束的显微解剖学研究

目的详尽了解嗅球、嗅束、嗅三角区的显微解剖学特点,为相关手术入路提供解剖学基础。方法 15例(30侧)颅骨干性标本,进行筛板区域的观察和测量。15例(30侧)头颅湿性标本,对嗅球、嗅束及嗅三角进行显微解剖学观察和拍照。结果筛孔分布在前颅底中线两侧,长(19.77±1.23)mm;中点处宽(3.15±0.45)mm的蝶形凹陷内;鼻甲侧筛孔数目明显多于鼻中隔侧(P<0.01),鸡冠两侧筛孔相对密集。嗅丝由嗅鞘包裹形成嗅神经,嗅神经于嗅窝内汇集形成嗅球。嗅球后极延续为嗅束,嗅束走形在前颅窝底直回与内侧眶回之间,并成为两者的分界,前部扁平,后部横断面呈三角形,前部相对后部较宽。嗅束于前穿质前部延续为嗅三角,以内侧嗅纹、外侧嗅纹分别进入脑实质而终。结论熟悉嗅丝、嗅球、嗅束及嗅三角的显微解剖特点,有利于提高相关手术入路中嗅觉通路的保护及嗅觉功能的保留,从而提高手术成功率,减少并发症的发生率。     

Vagus nerve stimulation modifies the electrical activity of the olfactory bulb

Evoked potential and unit activity recording techniques were used to study the effects of the vagus nerve stimulation on the olfactory bulb. A biphasic potential was evoked in the olfactory bulb by a single pulse delivered to the vagus nerve. Half of the neurons studied decreased discharge frequency after single pulse or train stimulation. The interval during which neurons ceased activity corresponded to the duration of the negative wave of the evoked potential. Responsive neurons were marked with horseradish peroxidase applied iontophoretically. Responsive neurons were located in the periglomerular layer of the olfactory bulb. These results suggest the existence of a vagus nerve-olfactory bulb pathway. The functional significance of this pathway is discussed.     

Levels of amino acid neurotransmitters during mouse olfactory bulb neurogenesis and in histotypic olfactory bulb cultures

The developmental changes in the levels of amino acid neurotransmitters were analyzed by high pressure liquid chromatography during mouse olfactory bulb neurogenesis, from embryonic day (E)13 until the young adult age, between postnatal days (P)30 and P40. During the embryonic period, high levels of glutamate, aspartate and GABA were observed, with the values of GABA about 2-fold higher than those of glutamate and aspartate. At P0, the production of these neurotransmitters experienced birth stress as shown by a significant 2-fold reduction in their levels. During the first two postnatal weeks, a progressive increase in the glutamate content was detected diminishing slightly in the adult stage. The aspartate concentrations showed a maximal value at P3 and then decreased gradually until the second postnatal week; in the young adult age, its concentration was comparable with that of glutamate. The postnatal GABA contents increased progressively from birth to maturity, showing maximal levels at P3, P11 and in the adult. Throughout the studied developmental period, the concentration of glycine remained relatively low. With regard to taurine, very low concentrations were detected during the prenatal period but after birth, the taurine content gradually increased with age, and in the adult animal, its concentration was comparable with those of GABA and glutamate. Our data demonstrate the predominance of GABA and glutamate during olfactory bulb synaptogenesis, however, in the adult animal, both glutamate and aspartate exert the same influence in the excitatory synaptic transmission; in the adult inhibitory synaptic transmission, taurine appears to play an important neuromodulatory or neurotransmitter role as that of GABA. To determine the intrinsic neurotransmitter production, primary histotypic olfactory bulb cultures were prepared from mice at P10. The comparative analysis of in vitro neurotransmitter contents with those in in situ adult animal showed higher levels of endogenously produced glutamate, glycine and GABA in the olfactory bulb than the extrinsic ones coming from olfactory nerve axons and higher olfactory brain centers. On the other hand, most of aspartate and taurine neurotransmitters apparently come from extrinsically located neurons.     

Implantation of a scaffold following bulbectomy induces laminar organization of regenerating olfactory axons

Primary olfactory axons expressing different odorant receptors are interspersed within the olfactory nerve. However, upon reaching the outer nerve fiber layer of the olfactory bulb they defasciculate, sort out, and refasciculate prior to targeting glomeruli in fixed topographic positions. While odorant receptors are crucial for the final targeting of axons to glomeruli, it is unclear what directs the formation of the nerve fiber and glomerular layers of the olfactory bulb. While the olfactory bulb itself may provide instructive cues for the development of these layers, it is also possible that the incoming axons may simply require the presence of a physical scaffold to establish the outer laminar cytoarchitecture. In order to begin to understand the underlying role of the olfactory bulb in development of the outer layers of the olfactory bulb, we physically ablated the olfactory bulbs in OMP-IRES-LacZ and P2-IRES-tau-LacZ neonatal mice and replaced them with artificial biological scaffolds molded into the shape of an olfactory bulb. Regenerating axons projected around the edge of the cranial cavity at the periphery of the artificial scaffold and were able to form an olfactory nerve fiber layer and, to some extent, a glomerular layer. Our results reveal that olfactory axons are able to form rudimentary cytoarchitectonic layers if they are provided with an appropriately shaped biological scaffold. Thus, the olfactory bulb does not appear to provide any tropic substance that either attracts regenerating olfactory axons into the cranial cavity or induces these axons to form a plexus around its outer surface.     

Dopaminergic modulation at the olfactory nerve synapse

Dopamine can change the membrane potential, regulate cyclic nucleotides, and modulate transmitter release in central neurons. In the olfactory bulb (OB), the dopamine synthetic enzyme, tyrosine hydroxylase, is largely confined to neurons in the glomerular layer. After demonstrating dopamine D2 receptors in the glomerular and olfactory nerve (ON) layers, Nickell et al. [W.T. Nickell, A.B. Norman, L.M. Wyatt, M.T. Shipley, Olfactory bulb DA receptors may be located on terminals of the olfactory nerve, NeuroReport, 2 (1991) 9-12.] proposed that these receptors may reduce transmitter release due to their localization to ON presynaptic boutons. We have previously demonstrated that olfactory receptor neurons use glutamate to excite OB neurons through activation of glutamate receptors subtypes, NMDA and AMPA/kainate [D.A. Berkowicz, P.Q. Trombley, G.M. Shepherd, Evidence for glutamate as the olfactory receptor cell neurotransmitter. J. Neurophysiol., 71 (1994) 2557-2561]. Here, we used a hemisected turtle OB preparation and patch-clamp recording techniques to assess dopamine modulation of the ON/OB neuron synapse. We found that dopamine (10-300 microM) reversibly decreased the excitatory postsynaptic response to ON stimulation. This effect could be overcome by recruiting additional nerve fibers by increasing the intensity of ON stimulation. Quinpirole (10 microM), a D2 agonist, mimicked the effects of dopamine. Conversely, sulpiride (300 microM), a D2 antagonist, prevented the inhibitory effects of dopamine on synaptic transmission. Whereas dopamine appeared to equally affect the NMDA and AMPA/kainate receptor-mediated components of the synaptically evoked response, it had no direct effect on membrane currents evoked by exogenous glutamate, kainate or NMDA applied to cultured OB neurons. Our data, therefore, support the notion that dopamine modulates synaptic transmission between olfactory receptor neurons and OB neurons via a presynaptic mechanism involving D2 receptor activation. Our abstract (Berkowicz et al. (1994) Neuroscience Abs. 20:328) is the first report of these results.     

Experimental studies on the olfactory marker protein. V. Olfactory marker protein in the olfactory neurons transplanted within the olfactory bulb

The olfactory mucosa of neonatal rats was transplanted within the olfactory bulb of littermates to investigate whether the olfactory bulb would have played a role in the differentiation of the olfactory neurons and whether the olfactory axons, growing out from the transplant, would have interacted with the olfactory glomeruli of the host. The observations were conducted on sections stained with Gill's hematoxylin, Loots' silver method, and the immunohistochemical technique for the demonstration of the olfactory marker protein (OMP). The olfactory neurons of the transplant (those localized in the neuroepithelium and those migrating from it into the bulbar parenchyma) could become fully differentiated but only few of them were OMP positive. Numerous sensory axons originated from the transplanted olfactory mucosa, however, they did not form ectopic glomeruli nor did they interact with the glomeruli of the host. These results indicate that the olfactory bulb, in vivo, does not affect the number of olfactory neurons expressing OMP and that the ectopically located neurons lack the cues to recognize the host glomeruli.     

Efferents and centrifugal afferents of the main and accessory olfactory bulbs in the hamster

The efferents and centrifugal afferents of the hamster olfactory bulbs were studied using orthograde and retrograde tracing techniques. Following injections of tritiated amino acids which were restricted to the main olfactory bulb (MOB), autoradiographic grains were observed ipsilaterally over layer IA of the entire anterior olfactory nucleus (AON), the ventral portion of the hippocampal rudiment (HR), the entire prepyriform cortex and olfactory tubercle, the anterior and posterolateral cortical amygdaloid nuclei and the lateral entorhinal cortex. An ipsilateral projection to the nucleus of the lateral olfactory tract (nLOT) was also indicated. No subcortical or contralateral projections were observed. Amino acid injections into the accessory olfactory bulb (AOB) revealed ipsilateral projections to the superficial plexiform layer of the medial and posteromedial cortical amygdaloid nuclei and to the bed nucleus of the accessory olfactory tract (nAOT) and the bed nucleus of the stria terminalis (nST). Following injections of HRP which were restricted to the MOB, contralateral HRP-positive neurons were found predominantly in pars externa and to a lesser extent in the other subdivisions of the AON. Centrifugal projections to the MOB were identified ipsilaterally from the entire AON, the ventral portion of the HR, the anterior portion of the prepyriform cortex, and the nLOT. No labelled neurons were found in the olfactory tubercle, the anterior and posterolateral cortical amygdaloid nuclei or the entorhinal cortex. Centrifugal projections to the MOB were also identified from subcortical structures of the ipsilateral basal forebrain and from midline structures of the midbrain. Labelling occurred in the fusiform neurons of the diagonal band near the medial base of the forebrain at the level of caudal olfactory tubercle. Heavy labelling was seen in a distinct group of large, predominantly multipolar neurons (magnocellular preoptic area) that continued from the level of caudal olfactory tubercle to the level of the nLOT. This band of HRP-positive neurons could be followed more caudally to a position dorsal and medial to the nLOT near the lateral margin of the lateral anterior hypothalamic area. The midbrain projections to the MOB originated in the dorsal and median raphe nuclei. After injections of HRP into the AOB, centrifugal projections were identified from the nAOT and the posteromedial cortical amygdaloid nucleus. In addition, isolated neurons were labelled in the medial cortical amygdaloid nucleus but no labelled neurons were found in the nST. These results support the notion of two anatomically distinct olfactory systems and demonstrate two previously unreported pathways through which the limbic system may modulate sensory processing in the olfactory bulb.     

不同组织来源人嗅神经鞘细胞生物学与免疫学特性的对比研究

目的 比较成人嗅球与嗅黏膜来源的嗅神经鞘细胞(OECs)的生物学与免疫学特性,探讨后者取代前者行细胞或组织移植治疗的可行性. 方法 分别分离、培养、纯化两种来源的OECs,应用MTT法测定细胞活力,结合对比骨髓间充质干细胞(BMSCs)的免疫特性,应用免疫细胞化学染色、定量技术、流式细胞术、双向混合淋巴细胞反应等方法来比较分析不同组织来源的人OECs的生物学与免疫学特性.结果 两种OECs开始分化时间相同,细胞形态基本一致:生长曲线与活力曲线均走行一致,曲线大部分重合;免疫细胞化学p75~(NTR)染色两者均呈阳性;流式分析两种OECs均不表达CD34与B7-1(CD80)、B7-2(CD86)、HLA-DR、CD40、CD40L等移植免疫标志:嗅球源性嗅神经鞘细胞组双向混合淋巴细胞反应抑制率(51.18%±6.01%)与嗅黏膜源性嗅神经鞘细胞组(51.00%±5.87%)、BMSCs组(52.79%±3.12%)比较,差异均无统计学意义(P<0.05).结论 成人嗅球与嗅黏膜来源的OECs在生物学与免疫学特性方面基本一致,可考虑使用来源更为便捷的成人嗅黏膜源性OECs代替嗅球源性OECs行细胞或组织移植治疗.     

The peripheral olfactory system of the domestic chicken: physiology and development

Olfaction is a ubiquitous sensory system found in all terrestrial vertebrates. Birds use olfaction for several important activities such as feeding and mating; thus, understanding bird biology would also require the systematic study olfaction. In addition, the olfactory system has several unique features that are useful for the study of nervous system function and development, including a large multigene family for olfactory receptor expression, peripheral neurons that regenerate, and a complex system for sensory innervation of the olfactory bulb. We focused on physiological, anatomical and behavioral approaches to study the chick olfactory neurons and the olfactory bulb. Chick olfactory neurons displayed some properties similar to those found in mature neurons of other vertebrate species, and other properties that were unique. Since information from these neurons is initially processed in the olfactory bulb, we also conducted preliminary studies on the developmental timeline of this structure and showed that glomerular structures are organized in ovo during a critical time period, during which embryonic chicks can form behavioral associations with odorants introduced in ovo. Lastly, we have shown that chick olfactory neurons can grow and mature in vitro, allowing their use in cell culture studies. These results collectively demonstrate some of the features of the olfactory system that are common to all vertebrates, and some that are unique to birds. These highlight the potential for the use of the physiology and development of the olfactory system as a model system for avian brain neurobiology.     

Evidence of lateral synaptic interactions in olfactory bulb output cell responses to odors

Olfactory bulb responses to paired-pulse stimulation of the lateral olfactory tract were examined in urethane-anesthetized rats, aged 5 days to adult. Brief inter-pulse intervals resulted in a depression of test responses at all ages. The magnitude of this depression decreased dramatically between postnatal days 19 and 20 to approach adult levels. Longer inter-pulse intervals resulted in a facilitation of test response amplitude in adult animals. This facilitation was evident at adult levels by postnatal day 10. These results suggest that both inhibitory and facilitatory synaptic mechanisms appear early in the course of rat olfactory bulb development. Furthermore, presumed granule cell-mediated inhibition is present at unusually high levels in the developing bulb, decreasing sharply between days 19 and 20.     

Anatomical evidence for convergence of olfactory, gustatory, and visceral afferent pathways in mouse cerebral cortex

Flavor perception requires the neural integration of olfactory, gustatory and, possibly, visceral afferent information. Presently, it is not known where, or how this integration takes place in the brain. Neuroanatomical data presented here suggest that pathways subserving these sensory modalities converge in mouse insular cortex after surprisingly few synaptic relays. Orthograde transport of wheat germ agglutinin conjugated to horseradish peroxidase (WGA-HRP) was used to label main olfactory bulb (MOB) efferents. A projection into layer I of insular cortex was present in every case. Bulb transections were made to provoke anterograde degeneration and EM analysis confirmed that the olfactory projection to insular cortex was a terminal pathway. WGA-HRP injections in the MOB-recipient zone of insular cortex resulted in ortho and retrograde labeling of ascending and descending gustatory-visceral afferent pathways. It is concluded that in the mouse, there is a remarkably direct convergence of olfactory and gustatory-visceral sensory pathways in insular cortex. Together with the descending connections from insular cortex to the amygdala and to brainstem autonomic structures, it is possible that the cortical integration of olfactory and gustatory-visceral information could modulate mechanisms involved in food selection and autonomic reactions relating to the chemical senses. Basic mechanisms subserving flavor perception might be usefully modelled in mouse insular cortex.