Neuronal turnover in the Xenopus laevis olfactory epithelium during metamorphosis

Metamorphic changes in the amphibian olfactory system present many interesting questions concerning the competing possibilities of neuronal respecification versus replacement. For example, are olfactory neurons retained during this transition with their presumed sensitivity to waterborne versus airborne stimuli respecified, or are olfactory neurons completely replaced? We address this question using the African clawed frog (Xenopus laevis) as a model. The water‐sensing nose (principal cavity; PC) of larval X. laevis is respecified into an air‐sensing cavity in adults, with changes in odorant receptor gene expression, ultrastructure, and site of innervation of the receptor neurons. The vomeronasal organ (VNO) does not appear to change function, structure, or innervation during metamorphosis. We labeled PC and VNO olfactory receptor neurons with injections of retrogradely transported fluorescent microspheres into the main and accessory olfactory bulbs. Injections were performed in larvae, and animals were allowed to survive through metamorphosis. After metamorphosis, few labeled cells were observed in the PC, whereas the VNO and the olfactory bulbs remained heavily labeled. Animals that were killed before metamorphosis always had extensive label in the PC epithelium regardless of how long the beads were present. This suggests that changes in the PC olfactory epithelium that are seen during metamorphosis are due primarily to turnover of the neurons in this epithelium rather than to respecification of existing neurons. These results also are discussed in terms of natural turnover time of olfactory receptor neurons. J. Comp. Neurol. 433:124–130, 2001. © 2001 Wiley‐Liss, Inc.     

Metamorphic remodeling of the olfactory organ of the African clawed frog,Xenopus laevis

The amphibian olfactory system undergoes massive remodeling during metamorphosis. The transition from aquatic olfaction in larvae to semiaquatic or airborne olfaction in adults requires anatomical, cellular, and molecular modifications. These changes are particularly pronounced in Pipidae, whose adults have secondarily adapted to an aquatic life style. In the fully aquatic larvae of Xenopus laevis, the main olfactory epithelium specialized for sensing water‐borne odorous substances lines the principal olfactory cavity (PC), whereas a separate olfactory epithelium lies in the vomeronasal organ (VNO). During metamorphosis, the epithelium of the PC is rearranged into the adult “air nose,” whereas a new olfactory epithelium, the adult “water nose,” forms in the emerging middle cavity (MC). Here we performed a stage‐by‐stage investigation of the anatomical changes of the Xenopus olfactory organ during metamorphosis. We quantified cell death in all olfactory epithelia and found massive cell death in the PC and the VNO, suggesting that the majority of larval sensory neurons is replaced during metamorphosis in both sensory epithelia. The moderate cell death in the MC shows that during the formation of this epithelium some cells are sorted out. Our results show that during MC formation some supporting cells, but not sensory neurons, are relocated from the PC to the MC and that they are eventually eliminated during metamorphosis. Together our findings illustrate the structural and cellular changes of the Xenopus olfactory organ during metamorphosis. J. Comp. Neurol. 524:986–998, 2016. © 2015 Wiley Periodicals, Inc.     

Neuroplasticity in the olfactory system: differential effects of central and peripheral lesions of the primary olfactory pathway on the expression of B-50/GAP43 and the olfactory marker protein

The regeneration of the olfactory neuroepithelium following olfactory bulbectomy or peripheral deafferentation was studied with mRNA probes and antibodies for B-50/GAP43 and for olfactory marker protein (OMP). Two stages in the regeneration of the olfactory epithelium could be discerned with these reagents. The first stage occurs following either peripheral deafferentation of the olfactory epithelium with Triton X-100 (TX-100) or after bulbectomy and is characterized by the formation of a large population of immature olfactory receptor neurons. These newly formed neurons express B-50/GAP43, a phosphoprotein related to neuronal growth and plasticity. During the second stage of the regeneration process the newly formed olfactory neurons mature, as evidenced by a decrease in their expression of B-50/GAP43 and an increase in the expression of OMP. This stage is only manifested if the developing neurons have access to the target olfactory bulb. Formation of a full complement of OMP-expressing neurons occurs only after peripheral lesion with TX-100. In contrast, following bulbectomy the reconstituted olfactory epithelium lacks its normal target and is compromised in its ability to recover from nerve damage, as evidenced by the presence of a large number of B-50/GAP43-expressing neurons up to 3 months after the lesion and its failure to establish a full complement of OMP-expressing neurons. These results demonstrate that the olfactory epithelium is capable of replacing its sensory neurons independently of the presence of its target, the olfactory bulb. However, the differential patterns of expression of B-50/GAP43 and OMP at long times after peripheral lesion with TX-100 or bulbectomy illustrate the profound effect the olfactory bulb has on neuronal maturation in reconstituted olfactory neuroepithelium.     

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.     

Transport of molecules from nose to brain: Transneuronal anterograde and retrograde labeling in the rat olfactory system by wheat germ agglutinin-horseradish peroxidase applied to the nasal epithelium

Transneuronal anterograde labeling with the conjugate wheat germ agglutinin-horseradish peroxidase (WGA-HRP) has been documented in the mammalian and immature avian visual system [6,14]. Transneuronal retrograde labeling was significant only in the chick [6]. The present study was performed to determine whether transneuronal labeling could be shown in the mammalian olfactory system, whether the phenomenon was robust in adults, and whether transneuronal retrograde transport could label several transmitter-specific centrifugal afferent projections to the olfactory bulb. In addition we wished to learn whether molecules that enter the nasal cavity can undergo transport to brain neurons. Gelfoam implants soaked with 1% WGA-HRP, surgically implanted into the nasal cavity, produced transneuronal labeling patterns that affirmed all of these questions. Transneuronal anterograde transport labeled the appropriate zones in the olfactory bulb and in all second order olfactory targets. In addition, there was transneuronal retrograde labeling of neurons in the olfactory bulb, anterior olfactory nucleus and in transmitter-specific projection neurons from the diagonal band (cholinergic), raphe (serotonergic) and locus coeruleus (noradrenergic). Transneuronal labeling was robust and consistent. The patterns of labeling indicated that transneuronal anterograde and retrograde transport occurred along known, specific circuits in the olfactory system. The present results suggest that nasal epithelial application of WGA-HRP may be a useful tool for assessing regeneration of primary olfactory neurons and the status of central circuitry following regeneration. The method should also facilitate the study of central olfactory connections after surgical or genetic lesions of the olfactory bulb. Finally, these experiments suggest the possibility that inhaled molecules including, possibly substances of abuse, may be transported to, and, possibly, influence the function of neurons in the brain, including some (diagonal band, raphe, locus coeruleus) which have extensive projections to wide areas of the CNS.     

Neurogenesis in the olfactory bulb of the frogXenopus Laevis shows unique patterns during embryonic development and metamorphosis

We determined the time of origin of neurons in the olfactory bulb of the South African clawed frog,Xenopus laevis. Tritiated thymidine injections were administered to frog embryos and tadpoles from gastrulation (stage 11/12) through metamorphosis (stage 65), paraffin sections were processed for autoradiography, and the distribution of heavily and lightly labeled cells was examined. In the ventral olfactory bulb, we observed that the mitral cells were born as early as stage 11/12 and continued to be generated through the end of metamorphosis. Interneurons (periglomerular and granule cells) were not born in the ventral bulb until stage 41, and birth of these cells also continued through metamorphosis. Labeled cells were observed in the accessory olfactory bulb, beginning at stage 41. In contrast, the cells of the dorsal olfactory bulb were not born until the onset of metamorphosis (stage 54); at this stage in the dorsal bulb, the genesis of mitral cells, interneurons, and glial cells completely overlapped. The results indicate that olfactory axon innervation is not necessary to induce early stages of neurogenesis in the ventral olfactory bulb. On the other hand, the results on the dorsal olfactory bulb are consistent with the hypothesis that innervation from new or transformed sensory neurons in the principal cavity induces neurogenesis in the dorsal bulb.     

Ablation of the olfactory bulb up-regulates the rate of neurogenesis and induces precocious cell death in olfactory epithelium.

Young adult rats were unilaterally bulbectomized and tritiated thymidine ([3H]TdR) was injected at variable times following surgery to determine the effect of bulbectomy on the rates of cell proliferation and cell death in the olfactory epithelium. Removal of the olfactory bulb elicits a two- to fourfold increase in the proliferation rate of ipsilateral olfactory epithelial cells 7-50 days following surgery. On the contralateral side, there was a temporary twofold increase in the proliferation rate during the second week after surgery, but this returned to control values at 3 weeks. This temporary increase was in parallel with the response on the ipsilateral side so that the ratio between operated and unoperated sides remained at two. Cell death in olfactory epithelium is also up-regulated following bulbectomy. Death of cells can occur as early as 1 day following incorporation of [3H]TdR, i.e., well before the sensory neurons become mature. This means there is an over-production of sensory cells, and they die at all stages of their life cycle. The number of cells dying is greater after bulbectomy, indicating that the overproduction of olfactory cells is more pronounced after surgery.     

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 certaine 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.     

Neuronal changes in the forebrain of mice following penetration by regenerating olfactory axons

Following total, unilateral bulbectomy in neonatal mice, the olfactory sensory axons regrow from a reconstituted population of sensory neurons, cross the lamina cribrosa, and invade the spared forebrain that has leaned forward toward the anteroventral wall of the cranial cavity. The sensory axons invade several regions of the spared forebrain, at times penetrating deeply into the brain parenchyma. These axons terminate in characteristic globose structures resembling the glomeruli of the olfactory bulb. However, they can be distinguished from the latter by the absence of periglomerular cells. These ectopic glomerular structures are formed by the commingling of the olfactory axon terminals and the dendrites of brain neurons that lie in their proximity. Previously we have established that synaptic contacts occur between the sensory axon terminals and the dendrites of the brain neurons. Our present study describes large neurons, resembling mitral cells, that expand their dendrites into the intracerebral glomeruli. These neurons are recognized by virtue of their relatively large diameter, their selective stainability with silver methods, and the unorthodox arrangement of their dendrites in comparison with the neurons of the region. Their appearance is contingent upon the presence of ectopic glomeruli. The possibility is discussed that the large argyrophilic neurons may be derived from developing neuronal elements of the brain.     

Identification of a cell surface glycoprotein family of olfactory receptor neurons with a monoclonal antibody

A monoclonal antibody (Mab) has been developed which recognizes a family of cell surface glycoproteins found in high levels of rat olfactory receptor neurons. This Mab, designated 2B8, was produced by the fusion of X63-Ag8.653 myeloma cells and spleen cells of a mouse immunized with PC12 rat pheochromocytoma cells. Immunofluorescence analyses of cryostat sections of neonatal olfactory epithelium show prominent 2B8 binding to receptor neurons. Within the olfactory bulb only the glomerular and olfactory nerve layers show 2B8 binding. All other neural structures in the main olfactory bulb have background levels of reactivity. Analyses of 2B8 binding to particulate protein preparations from several central and peripheral nervous system components demonstrated highest 2B8 antigen specific activity in olfactory bulb and epithelium and detectable levels in dorsal root ganglia (DRG), whole cerebrum, cerebellum, and brainstem. However, 2B8 antigen could not be detected in non-olfactory structures by immunofluorescence. Some non-neural tissues also had the ability to bind 2B8 Mab in the particulate protein radioimmunoassay. In order to compare the 2B8-reactive molecules found in each tissue, Mab was applied to polyacrylamide gels of unlabeled membrane proteins. A family of molecules with diverse molecular weights was found. Some were unique to individual tissues whereas others were shared among tissues. Olfactory bulb and epithelium had a unique band with Mr = 215,000 and another band with Mr = 142,000. The 142,000-dalton band was also found with PC12 cells. PC12 cells also had several bands of lesser molecular weight, including 51,000 and 43,000. Testes membranes had immunoreactive bands only at Mr = 46,000 and 43,000. Bone marrow, perinatal liver, and DRG each expressed a single 2B8-reactive band with Mr = approximately 114,000. Salivary gland had four reactive bands, two common to it and only PC12 cells, the 114,000-dalton band which is similar to that found in adult rat bone marrow and DRG, and a unique band at Mr = 152,000. 2B8 immunoprecipitates of olfactory bulb and epithelium were analyzed for glycosyl groups by lectin reactivity. Wheat germ agglutinin and Ricinus communus agglutinin I bound the 2B8 antigens using two distinct assay methods. This suggests that the 2B8 antigens recognized in the olfactory system are glycoproteins having sialic acid and D-galactosyl components.(ABSTRACT TRUNCATED AT 400 WORDS)     

Olfactory neurons in bax knockout mice are protected from bulbectomy-induced apoptosis

Surgical ablation of the olfactory bulb (bulbectomy) triggers a massive wave of apoptosis in mature olfactory sensory neurons within the olfactory epithelium. The aim of the current study was to determine if this process is dependent on expression of the pro-apoptotic protein Bax. Immunohistochemical detection of caspase-3 activation and olfactory epithelial thickness was used to demonstrate and quantify neuronal apoptosis in bax knockout and wild type mice, following bulbectomy. Caspase-3 activation and epithelial thinning were both reduced in the bax knockout mouse compared to the wild type mouse, at least up to 9 days post-bulbectomy, indicating that apoptosis was inhibited not just delayed. This study demonstrates that Bax plays a major role in olfactory neuron apoptosis following surgical deafferentation.     

Mapping of an olfactory receptor population that projects to a specific region in the rat olfactory bulb

An anatomically distinct group of glomeruli, termed the modified glomerular complex (MGC), is present in the posterior dorsomedial portion of the main olfactory bulb. This region has been strongly implicated as part of the pathway that processes odor cues for suckling in neonatal rat pups. We studied the distribution pattern of olfactory receptor neurons that project to the MGC region after ionophoretic injections of WGA-HRP into the olfactory bulbs of 12-day-old rat pups. HRP label was confined to an identifiable localized region in the MGC of the main olfactory bulb. Label extended over 2-7% of the glomerular sheet of the main olfactory bulb, including the MGC. Olfactory receptor neurons within the olfactory epithelium of the nasal cavity were labeled with HRP ipsilateral to the injected side. Maps constructed of the olfactory epithelium revealed that the labeled neurons occurred within topographically defined regions. Anteriorly, labeled olfactory neurons were confined to a narrow strip medial to the dorsal recess, and, more posteriorly, this strip widened medially along the septal wall and laterally onto a limited area on the nasal turbinates. Only a portion of the receptor population within a region was labeled. The boundaries between labeled and unlabeled regions were sharp. These findings support the concept that the olfactory epithelium is an anatomical mosaic in which receptors with different glomerular projections sites are intermingled. In conjunction with previous evidence on the functional specificity of the MGC, and staining of receptor neuron subgroups with monoclonal antibodies, these findings further suggest that olfactory receptor neurons form a functional mosaic within the olfactory epithelium.     

Monoclonal antibodies reveal novel aspects of the biochemistry and organization of olfactory neurons following unilateral olfactory bulbectomy

Following unilateral olfactory bulbectomy in rats the ipsilateral olfactory neuroepithelium undergoes degeneration. Subsequently, the receptor neuron complement of the tissue is restored by the proliferation and differentiation of immature neuroblasts. However, as noted by other workers, in the absence of a target organ the dynamics of neuron regeneration is altered such that there is an overall reduction in the number of cells positive for the olfactory marker protein when cellular equilibrium is re-established. Immunocytochemical staining of the olfactory epithelium of unilaterally bulbectomized rats with a series of anti-neuronal monoclonal antibodies reveals an attenuation of binding of some antibodies to the neurons of the ipsilateral epithelium. In contrast, other anti-neuronal monoclonal antibodies show no difference in staining intensity when ipsilateral and control contralateral epithelia are compared. These data suggest that the expression of some neuronal antigens is subject to control by the target olfactory bulb, whereas others are independent of such putative regulation. Besides altering the expression of some antigenic determinants, olfactory bulbectomy also results in certain organizational changes in epithelium. First, bulbectomy produces an increase in the incidence of a cell type that appears to span the neuroepithelium. Although the morphology of these cells is more akin to a sustentacular cell than to a receptor neuron, they are not immunoreactive with antibodies to sustentacular cells. The cells are stained, however, by an anti-neuronal antibody, NEU-9. The second aspect of altered organization is the appearance of novel olfactory marker protein-positive structures in the olfactory mucosa.(ABSTRACT TRUNCATED AT 250 WORDS)     

Bulbar projecting subcortical GABAergic neurons send collateral branches extensively and selectively to primary olfactory cortical regions

Circuit operations of the olfactory bulb are modulated by higher order projections from multiple regions, many of which are themselves targets of bulbar output. Multiple glutamatergic regions project to the olfactory bulb, including the anterior olfactory nucleus (AON), prefrontal cortex (PFC), piriform cortex (PC), entorhinal cortex (EC), and tenia tecta (TT). In contrast, only one region provides GABAergic projections to the bulb. These GABA neurons are located in the horizontal limb of the diagonal band of Broca extending posteriorly through the magnocellular preoptic nucleus to the nucleus of the lateral olfactory bulb. However, it was unclear whether bulbar projecting GABAergic neurons collaterallize projecting to other brain regions. To address this, we mapped collateral projections from bulbar projecting GABAergic neurons using intersectional strategies of viral and traditional tract tracers. This approach revealed bulbar projecting GABAergic neurons show remarkable specificity targeting other primary olfactory cortical regions exhibiting abundant collateral projections into the accessory olfactory bulb, AON, PFC, PC, and TT. The only "nonolfactory" region receiving collateral projections was sparse connectivity to the medial prefrontal orbital cortex. This suggests that basal forebrain inhibitory feedback also modulates glutamatergic feedback areas that are themselves prominent bulbar projection regions. Thus, inhibitory feedback may be simultaneously modulating both synaptic processing of olfactory information in the bulb and associational processing of olfactory information from primary olfactory cortex. We hypothesize that these olfactory GABAergic feedback neurons are a regulator of the entire olfactory system.     

Distinct subsets of sensory olfactory neurons in mouse: Possible role in the formation of the mosaic olfactory projection

The axons of the primary sensory olfactory neurons project from the olfactory neuroepithelium lining the nasal cavity, onto glomeruli covering the surface of the olfactory bulb. Neuroanatomical studies have shown previously that individual olfactory glomeruli are innervated by neurons that are dispersed widely within the nasal cavity. The aim of the present study was to test the hypothesis that phenotypically unique subsets of primary sensory olfactory neurons, scattered throughout the nasal cavity, project to a subset of glomeruli in specific olfactory bulb loci. Immunochemical and histochemical analyses in neonatal mice revealed that the plant lectin, Dolichos biflorus agglutinin, bound to a subset of mature primary sensory olfactory neurons which express the olfactory marker protein. This subset of neurons was principally located in the rostromedial and dorsal portions of the nasal cavity and projected specifically to a subset of glomeruli in the rostromedial and caudodorsal portions of the olfactory bulb. Analysis of Dolichos biflorus-reactive axons revealed that these axons coursed randomly, with no evidence of their selective fasciculation, within the olfactory nerve. It was only at the level of the rostral olfactory bulb that a significant reorganisation of their trajectory was observed. Within the outer fibre layer of the bulb, discrete bundles of lectin-reactive axons began to coalesce selectively into fascicles which preferentially oriented toward the medial side of the olfactory bulb. These data demonstrated that a phenotypically distinct subset of primary sensory olfactory neurons exhibits a topographical projection from the olfactory epithelium to the olfactory bulb, and suggests that these, and other subsets, may form the basis of the mosaic nature of this pathway. Moreover, it appears that the outer nerve fibre layer in the rostral olfactory bulb plays an important instructive role in the guidance and fasciculation of olfactory sensory axons. © 1993 Wiley-Liss, Inc.     

Visualization of neurogenesis in the central nervous system using nestin promoter-GFP transgenic mice

Neurons are generated from neural progenitor cells not only during development but also in the mature brain. To develop an in vivo system for analyzing neurogenesis, we generated transgenic mice expressing green fluorescent protein (GFP) under the control of regulatory regions of the nestin gene. GFP fluorescence was observed in areas and during periods connected with neurogenesis, including embryonic neuroepithelium, neonatal cerebellum, and hippocampal dentate gyrus and rostral migratory pathway from the subventricular zone to the olfactory bulb in the adult. GFP-positive cells in the adult brain included immature neuronal cells expressing polysialylated NCAM. BrdU labeling experiments revealed that newly generated interneurons which migrated rostrally from the subventricular zone expressed GFP until they reached the olfactory bulb. These results indicate that nestin promoter-GFP transgenic mice can be utilized to visualize the regions of neurogenesis throughout the life of the animals and to follow the migration and differentiation of newly generated neurons.     

B-50/GAP43 Gene Expression in the Rat Olfactory System During Postnatal Development and Aging

The olfactory neuroepithelium exhibits neurogenesis throughout adult life, and in response to lesions, a phenomenon that distinguishes this neural tissue from the rest of the mammalian brain. The newly formed primary olfactory neurons elaborate axons into the olfactory bulb. Thus, denervation and subsequent re-innervation of olfactory bulb neurons may occur throughout life. In this study the authors demonstrate the distribution of the growth-associated phosphoprotein B-50/GAP43 and its mRNA in the olfactory neuroepithelium and olfactory bulb during development and aging. In neonatal rats B-50/GAP43 mRNA was expressed in primary olfactory neurons throughout the olfactory epithelium and in their target neurons in the olfactory bulb, the mitral, juxtaglomerular and tufted cells. In contrast, in adult (7.5 weeks) and aging animals (6 - 18 months of age) B-50/GAP43 mRNA expression was progressively restricted to neurons in the basal region of the neuroepithelium and to some of their target mitral and juxtaglomerular cells in the olfactory bulb. The continuing expression of B-50/GAP43 mRNA in mitral- and juxtaglomerular cells in mature animals is thought to be related to their capacity to respond to continuously changing input from the primary olfactory neurons present in the olfactory neuroepithelium.     

Serotonergic nerve fibers in the primary olfactory pathway of the larval sea lamprey, Petromyzon marinus

In this study, serotonin (5-hydroxytryptamine; 5HT)-immunoreactive (5HT-IR) neuronal fibers were identified in the primary olfactory pathway of the sea lamprey. These neurons are likely part of a nonolfactory neural system that innervates the olfactory sac. Cell bodies with 5HT immunoreactivity predominated in the lamina propria of the rostral portion of the nasal cavity and were less prevalent adjacent to the olfactory epithelium. The 5HT-IR fibers were parallel to axons of the olfactory receptor neurons in the lamina propria of the olfactory mucosa and in the olfactory nerve. Serotonergic fibers crossed from the olfactory nerve into the olfactory bulb or branched in the caudal portion of the olfactory nerve and terminated at the junction of the olfactory nerve with the olfactory bulb. In the dorsal olfactory bulb, 5HT-IR fibers coursed along the layer of olfactory fibers. Throughout the layer with glomeruli and mitral cells, 5HT-IR fibers were seen along the border of glomerular units. Experimental lesion of the olfactory nerve was used to determine the origin of 5HT-IR fibers rostral to the olfactory bulb. The loss of these fibers and their reappearance during outgrowth of olfactory receptor neurons inferred that they emanate from the cell bodies in the olfactory sac. The results from this study suggest that axons of olfactory receptor neurons in larval lampreys receive modulation by 5HT from these neuronal fibers.