NGF receptor increase in the olfactory bulb of the rat after early odor deprivation

The Role of the Stimulus in Olfactory Plasticity

Plasticity, the term we use to describe the ability of a nervous system to change with experience, is the evolutionary adaptation that freed animal behavior from the confines of genetic determinism. This capacity, which increases with brain complexity, is nowhere more evident than in vertebrates, especially mammals. Though the scientific study of brain plasticity dates back at least to the mid-19th century, the last several decades have seen unprecedented advances in the field afforded by new technologies. Olfaction is one system that has garnered particular attention in this realm because it is the only sensory modality with a lifelong supply of new neurons, from two niches no less! Here, we review some of the classical and contemporary literature dealing with the role of the stimulus or lack thereof in olfactory plasticity. We have restricted our comments to studies in mammals that have used dual tools of the field: stimulus deprivation and stimulus enrichment. The former manipulation has been implemented most frequently by unilateral naris occlusion and, thus, we have limited our comments to research using this technique. The work reviewed on deprivation provides substantial evidence of activity-dependent processes in both developing and adult mammals at multiple levels of the system from olfactory sensory neurons through to olfactory cortical areas. However, more recent evidence on the effects of deprivation also establishes several compensatory processes with mechanisms at every level of the system, whose function seems to be the restoration of information flow in the face of an impoverished signal. The results of sensory enrichment are more tentative, not least because of the actual manipulation: What odor or odors? At what concentrations? On what schedule? All of these have frequently not been sufficiently rationalized or characterized. Perhaps it is not surprising, then, that discrepant results are common in sensory enrichment studies. Despite this problem, evidence has accumulated that even passively encountered odors can "teach" olfactory cortical areas to better detect, discriminate, and more efficiently encode them for future encounters. We discuss these and other less-established roles for the stimulus in olfactory plasticity, culminating in our recommended "aspirations" for the field going forward.

Postnatal experience modulates functional properties of mouse olfactory sensory neurons

Early experience considerably modulates the organization and function of all sensory systems. In the mammalian olfactory system, deprivation of the sensory inputs via neonatal, unilateral naris closure has been shown to induce structural, molecular and functional changes from the olfactory epithelium to the olfactory bulb and cortex. However, it remains unknown how early experience shapes the functional properties of individual olfactory sensory neurons (OSNs), the primary odor detectors in the nose. To address this question, we examined the odorant response properties of mouse OSNs in both the closed and open nostril after 4 weeks of unilateral naris closure, with age-matched untreated animals as control. Using a patch-clamp technique on genetically tagged OSNs with defined odorant receptors (ORs), we found that sensory deprivation increased the sensitivity of MOR23 neurons in the closed side, whereas overexposure caused the opposite effect in the open side. We next analyzed the response properties, including rise time, decay time, and adaptation, induced by repeated stimulation in MOR23 and M71 neurons. Even though these two types of neuron showed distinct properties with regard to dynamic range and response kinetics, sensory deprivation significantly slowed down the decay phase of odorant-induced transduction events in both types. Using western blotting and antibody staining, we confirmed the upregulation of several signaling proteins in the closed side as compared with the open side. This study suggests that early experience modulates the functional properties of OSNs, probably by modifying the signal transduction cascade.

Studies of olfactory system neural plasticity: the contribution of the unilateral naris occlusion technique

Unilateral naris occlusion has long been the method of choice for effecting stimulus deprivation in studies of olfactory plasticity. A significant body of literature speaks to the myriad consequences of this manipulation on the ipsilateral olfactory pathway. Early experiments emphasized naris occlusion's deleterious and age-critical effects. More recent studies have focused on life-long vulnerability, particularly on neurogenesis, and compensatory responses to deprivation. Despite the abundance of empirical data, a theoretical framework in which to understand the many sequelae of naris occlusion on olfaction has been elusive. This paper focuses on recent data, new theories, and underappreciated caveats related to the use of this technique in studies of olfactory plasticity.

Differential effects of unilateral olfactory deprivation on noradrenergic and cholinergic systems in the main olfactory bulb of the rat

The lack of environmental olfactory stimulation produced by sensory deprivation causes significant changes in the deprived olfactory bulb. Olfactory transmission in the main olfactory bulb (MOB) is strongly modulated by centrifugal systems. The present report examines the effects of unilateral deprivation on the noradrenergic and cholinergic centrifugal systems innervating the MOB. The morphology, distribution, and density of positive axons were studied in the MOBs of control and deprived rats, using dopamine-beta-hydroxylase (DBH)-immunohistochemistry and acetylcholinesterase (AChE) histochemistry in serial sections. Catecholamine content was compared among the different groups of MOBs (control, contralateral, and ipsilateral to the deprivation) using high-performance liquid chromatography analysis. Sensory deprivation revealed that the noradrenergic system developed adaptive plastic changes after olfactory deprivation, including important modifications in its fiber density and distribution, while no differences in cholinergic innervation were observed under the same conditions. The noradrenergic system underwent an important alteration in the glomerular layer, in which some glomeruli showed a dense noradrenergic innervation that was not detected in control animals. The DBH-positive glomeruli with the highest noradrenergic fiber density were compared with AChE-stained sections and it was observed that the strongly noradrenergic-innervated glomeruli were always atypical glomeruli (characterized by their strong degree of cholinergic innervation). In addition to the morphological findings, our biochemical data revealed that olfactory deprivation caused a decrease in the content of dopamine and its metabolite 3,4-dihydroxyphenylacetic acid in the ipsilateral MOB in comparison to the contralateral and control MOBs, together with an increase in noradrenaline levels in both the ipsilateral and contralateral MOBs. Our results show that regulation of the noradrenergic centrifugal system in the MOB depends on environmental olfactory stimulation and that it is highly reactive to sensory deprivation. By contrast, the cholinergic system is fairly stable and does not exhibit clear changes after the loss of sensory inputs.

Changes in glutamate receptor subunit 4 expression in the deafferented olfactory bulb of zebrafish

The distribution of ionotropic glutamate receptor subunit 4 (iGluR4) was examined in both normal and deafferented olfactory bulbs of adult zebrafish, Danio rerio. With the exception of the olfactory nerve layer, there was extensive labeling with antibodies to iGluR4 in the olfactory bulbs, specifically in juxtaglomerular cell bodies and their processes. These results are consistent with previous work, which has suggested differential distribution of glutamate receptors in the vertebrate olfactory system. Analysis of bulbs following olfactory organ removal revealed a significant loss of iGluR4 immunoreactivity by 24 h post-deafferentation. At 48 h after denervation, iGluR4 labeling had returned to normal levels and was retained through 3 weeks post-surgery. Thus, afferent input plays a role in reduced labeling of this protein immediately following injury, but return of immunoreactivity can occur even without sensory innervation.

Local neurons play key roles in the mammalian olfactory bulb

Over the past few decades, research exploring how the brain perceives, discriminates, and recognizes odorant molecules has received a growing interest. Today, olfaction is no longer considered a matter of poetry. Chemical senses entered the biological era when an increasing number of scientists started to elucidate the early stages of the olfactory pathway. A combination of genetic, biochemical, cellular, electrophysiological and behavioral methods has provided a picture of how odor information is processed in the olfactory system as it moves from the periphery to higher areas of the brain. Our group is exploring the physiology of the main olfactory bulb, the first processing relay in the mammalian brain. From different electrophysiological approaches, we are attempting to understand the cellular rules that contribute to the synaptic transmission and plasticity at this central relay. How olfactory sensory inputs, originating from the olfactory epithelium located in the nasal cavity, are encoded in the main olfactory bulb remains a crucial question for understanding odor processing. More importantly, the persistence of a high level of neurogenesis continuously supplying the adult olfactory bulb with newborn local neurons provides an attractive model to investigate how basic olfactory functions are maintained when a large proportion of local neurons are continuously renewed. For this purpose, we summarize the current ideas concerning the molecular mechanisms and organizational strategies used by the olfactory system to encode and process information in the main olfactory bulb. We discuss the degree of sensitivity of the bulbar neuronal network activity to the persistence of this high level of neurogenesis that is modulated by sensory experience. Finally, it is worth mentioning that analyzing the molecular mechanisms and organizational strategies used by the olfactory system to transduce, encode, and process odorant information in the olfactory bulb should aid in understanding the general neural mechanisms involved in both sensory perception and memory. Due to space constraints, this review focuses exclusively on the olfactory systems of vertebrates and primarily those of mammals.

Distribution of GluR1 is altered in the olfactory bulb following neonatal naris occlusion

The olfactory system is well suited for studies of glutamate receptor plasticity. The sensory neurons are glutamatergic, and they turn over throughout life, and the olfactory bulb neurons that process their inputs express many of the known glutamate receptor subunits. Neonatal naris occlusion alters olfactory bulb development and the expression of certain neuroactive substances and receptors, at least in part due to loss of the sensory inputs. We therefore postulated that neonatal naris occlusion might alter glutamate receptor expression during postnatal development. Single nares of newborn mice were occluded on postnatal days 1-2, and the distribution of glutamate receptor subunits was evaluated using immunoperoxidase methods. Light microscopic examination on postnatal day 6 failed to reveal adult-like staining of neuronal cell bodies in the olfactory bulbs. By day 12, cell bodies that were immunoreactive (-IR) for the GluR1 subunit were visible in the external plexiform layer (EPL) of both sides. By day 18, many of the GluR1-IR cell bodies could be identified as cell types that had previously been reported to express homomeric GluR1 receptors. Analysis of single, mid-dorsal sections from 18-25-day-old mice showed that the medial EPL of the occluded side had a significantly lower density of these cell bodies. The GluR1 staining of the adjacent mitral cell layer (MCL) was also heavier on the occluded side, but no gross differences in staining for other glutamate receptor subunits were observed. Neonatal naris occlusion therefore appears to provide a new model for studying expression of GluR1 receptors during the development of a discrete population of olfactory bulb neurons.

Neurotrophic factors in the primary olfactory pathway

The number of identified growth factors continues to increase rapidly with many being implicated in the development of the nervous system, although for most of them the autocrine and paracrine pathways of cellular regulation still remain to be elucidated. The primary olfactory pathway, consisting of the olfactory epithelium and olfactory bulb, is presented here as a very useful model for the analysis of growth factor function. Review of the available literature suggests that a large proportion of neuroactive growth factors and their receptors are present in the olfactory epithelium or olfactory bulb. Furthermore, the primary olfactory pathway is one of the most plastic in the nervous system with neurogenesis continuing to contribute new sensory neurones in the olfactory epithelium and new interneurones in the olfactory bulb throughout adult life. The rich diversity of growth factors and their receptors in the olfactory system indicates that it will be useful in elucidating how these molecules regulate the formation of the nervous system. The olfactory epithelium in particular is proving useful as a model for the actions of growth factors in directing the neuronal lineage from stem cell to mature neurone.

Deafferentation-induced changes in the olfactory bulb of adult zebrafish

The influence of the olfactory organ on maintenance of olfactory bulb structure was examined in zebrafish, using peripheral deafferentation. This fish provides a model in which the olfactory organ is easily accessible for removal, the animals easily survive the surgery, and the olfactory bulbs are small enough to allow rigorous analysis of the resulting effects. Unilateral olfactory organ ablations were performed on anesthetized adult zebrafish using a small-vessel cautery iron. Fish were allowed to survive for 1, 3, or 6 weeks following the procedure. Analysis of deafferented animals revealed that most, if not all, of the olfactory organ was missing on the ablated side, and the structure did not regenerate. The morphology of the olfactory bulb was affected notably by the removal of its primary afferent innervation. The olfactory nerve layer was diminished at 1 week and absent by 3 weeks post-deafferentation. At all of the survival times the deafferented bulb appeared significantly smaller at the gross level, and there was a statistically significant effect on bulb size and cell number after 6 weeks. Tyrosine hydroxylase expression, as revealed by immunohistochemistry, was decreased noticeably on the ablated side. In conclusion, the olfactory organ is important in the preservation of normal olfactory bulb anatomy and neurochemistry in adult zebrafish. Thus, the influence of the periphery does not end with the formation of the mature olfactory bulb.

Activity-dependent regulation of interleukin-1 beta immunoreactivity in the developing rat olfactory bulb

Interleukin-1beta is a relatively small and abundant polypeptide that plays diverse roles in the central nervous system. In the present study, patterns of interleukin-1beta expression were observed in the olfactory bulbs of rats that had either undergone unilateral closure of the external naris or sham surgery on postnatal day 1 and then survived until postnatal day 30. Interleukin-1beta-immunoreactive fibers occupied distinct layers of the olfactory bulb. Dense immunostaining was found in the periglomerular and granule cell layers. Odor deprivation resulted in a noticeable reduction in interleukin-1beta immunoreactivity only in the periglomerular layer. The data demonstrate that interleukin-1beta is present abundantly in the bulbs, and that it can be regulated in an activity-dependent manner.

Visual input regulates the expression of basic fibroblast growth factor and its receptor

Emerging evidence indicates that the expression of trophic factors in the brain is regulated in an activity-dependent manner, which suggests an involvement of trophic factors in events controlled by input activity. We have investigated the possibility that visual sensory input impacts the expression of basic fibroblast growth factor and its receptor in the brain. Rats were maintained for seven days in darkness and then re-exposed to normal illumination for 0, 1, 3 or 6 h. We assessed relative levels of basic fibroblast growth factor and fibroblast growth factor receptor messenger RNAs using nuclease protection assays, and examined possible changes in the phenotypic expression of basic fibroblast growth factor and its receptor using immunohistochemistry. There was a significant decrease in levels of basic fibroblast growth factor and fibroblast growth factor receptor messenger RNAs as a result of dark rearing, and levels of messenger RNAs increased progressively with light re-exposure. Changes in messenger RNAs were observed primarily in the cerebral cortex (caudal portion) and were accompanied by alterations in the staining intensity and density of cells exhibiting basic fibroblast growth factor and fibroblast growth factor receptor phenotypes. Regulation of the basic fibroblast growth factor system by sensory input suggests that basic fibroblast growth factor, and perhaps other trophic factors, are mediators of the effects of experience on the structure and function of the CNS.

Calcium-binding proteins: differential expression in the rat olfactory cortex after neonatal olfactory bulbectomy

Calbindin, parvalbumin, and calretinin, members of EF-hand calcium-binding proteins, play important roles in buffering intracellular calcium ions. These proteins are localized in distinct populations of cells in the olfactory bulb (the primary sensory relay in the olfactory system) and its major synaptic target, the primary olfactory cortex (POC). In the present study, the postnatal expression of these calcium-binding proteins in layer III of POC was quantitatively examined 30 days after neonatal bulbectomy, a manipulation known to cause cell death and neurotransmitter changes. The numbers of both calbindin and parvalbumin-immunoreactive profiles showed significant increases (68% and 163%, respectively), while calretinin-immunoreactive profiles exhibited a 46% reduction. The data demonstrate that the expression of these calcium-binding proteins is regulated in part by the afferent input from the olfactory bulb. Furthermore, the resultant increase in calbindin and parvalbumin expression may provide neuroprotective support necessitated by possible alterations in intracellular calcium ions and other neurochemical factors that accompany neonatal bulb removal.

Compensatory responses to early olfactory restriction

The emphasis on the effects of early olfactory restriction has been on the deleterious nature of the consequences of this abnormal early experience. While these effects are certainly present and dramatic, it is also the case that the olfactory system is left fairly intact by this experimental manipulation. If one asks what mechanisms may account for the preservation of the developing olfactory system in the face of diminished activity, there are a number of neural responses that could help the system compensate for the damage inflicted upon the system by the loss of early sensory stimulation. I describe what may be compensatory responses to the decrease in dopamine, the increase in norepinephrine, the structural changes induced in the glomeruli and the increase in cell death within the olfactory bulb that follow neonatal unilateral naris occlusion.

Expression of the low affinity neurotrophin receptor, P75NGFR, in the rat forebrain, following unilateral bulbectomy

It has been hypothesized that the main olfactory bulb, with its relatively rich source of neurotrophins, may provide trophic support for neurons that project to the bulb. We monitored expression of the common, low affinity receptor for neurotrophins, p75NGFR, in the olfactory bulb and basal forebrain of unilaterally bulbectomized and sham-treated rats, 1-16 weeks post-surgery, using the monoclonal antibody MAb192. An induction of p75NGFR-immunoreactivity was observed in both the glomerular and olfactory nerve layers of the right, contralateral main olfactory bulb of lesioned animals. The naturally occurring regeneration taking place in the olfactory neuroepithelium is known to be altered by olfactory bulbectomy, with subsequent changes in the sensory input to the remaining bulb. These changes in expression of p75NGFR in the olfactory bulb support the hypothesis we have developed in previous papers, that changes in the extent of the peripheral input from the olfactory neuroepithelium to the main olfactory bulb regulate p75NGFR expression in both the glomerular and the olfactory nerve layers. Expression of p75NGFR in the basal forebrain of bulbectomized animals was found to be no different than sham-treated controls and does not support the hypothesis that the olfactory bulb provides trophic support to this region of the central nervous system.

The effects of variable periods of functional deprivation on olfactory bulb development in rats

Dramatic alterations occur in the developing olfactory bulb when air flow is reduced through one-half of the nasal cavity. Naris closure on the day after the day of birth (P1) in rats, for example, results in reduced cell survival in the ipsilateral bulb by P20 and a substantial (25%) decrease in bulb size by P30. Almost immediate changes in protein synthesis and cell metabolism are also observed, and one prevalent theory suggests that these changes may be important in specifying which cells are subsequently eliminated. In the present study we used a reversible technique for unilateral naris closure to examine the sensitive period for the effects of olfactory deprivation on bulb size and cell survival. This technique involves the insertion of removable plugs into a rat pup's external naris. We occluded the naris for increasing periods of time (P1-P10, P1-P15, or P1-P20), reared all animals to P30, and measured volumes of bulb laminae. In addition, we examined the duration of naris closure needed to affect cell survival by injecting animals with the thymidine analogue bromodeoxyuridine to label cells born soon after the onset of olfactory deprivation. Results indicate that relatively long periods of naris occlusion (P1-P15 or longer) are required to produce a substantial reduction in experimental bulb size. Cell survival was decreased following olfactory deprivation from P1 to P10, but not after deprivation from P1 to P3. These data support the hypothesis that changes that occur within 48 h of naris closure are not sufficient to affect cell survival.

Rapid down-regulation of tyrosine hydroxylase expression in the olfactory bulb of naris-occluded adult rats

In most sensory systems, afferent innervation regulates morphological and biochemical characteristics of target cells for a limited time during development. Sensory deprivation experiments in adult rats also have suggested a critical period for afferent influences on olfactory bulb structure and function. Previous odorant deprivation studies that employed unilateral naris closure in neonatal rats demonstrated down-regulation of the catecholamine biosynthetic enzyme tyrosine hydroxylase (TH) in dopamine neurons intrinsic to the olfactory bulb. Accompanying the altered biochemical parameters was a decrease in bulb size. To distinguish between deprivation-induced alterations in TH expression secondary to developmental sequelae and those occurring in mature neurons, the consequences of unilateral naris closure were assessed in young adult rats. In agreement with previous studies significant postnatal increases occurred in TH expression and total protein, an indication of bulb size. At 30 days post-closure, total protein was unaltered in the ipsilateral olfactory bulb but showed a small (12.9%), significant decline at 60 days. In contrast to the limited morphological consequences of odor deprivation, profound reductions occurred in TH expression. TH activity ipsilateral to the closure decreased significantly by 14 days post-closure and remained depressed for up to 6 months. In parallel with enzyme activity, TH immunoreactivity did not decline in the first few days post-closure. In situ hybridization revealed that TH mRNA levels decreased rapidly, i.e., by 2 days post-closure, reached a nadir at 1 month, and remained depressed for at least 6 months. The capacity of odor deprivation in the adult rat olfactory system to down-regulate TH expression suggests that this phenotypic alteration occurs independently of a presumed critical period.

Changes in expression of the low affinity receptor for neurotrophins, p75NGFR, in the regenerating olfactory system

We have disrupted the integrity of the rat olfactory neuroepithelium using intranasally applied TX-100, a procedure known to reversibly eliminate the sensory neuron input from the neuroeithelium to the olfactory bulb [Margolis et al. (1974) Denervation in the primary olfactory pathway of mice: biochemical and morphological effects. Brain Res. 81, 469-483]. One week after TX-100 exposure, we observed a disruption of the pseudo-stratified organization of the neuroepithelium which was accompanied by a 60% reduction in neuroepithelial width, compared to saline-treated controls. Full recovery of the neuroepithelium was not observed until 16 weeks post-lesion. During this post-lesion period, we monitored the expression of the low affinity receptor for neurotrophins, p75NGFR, in the olfactory bulb of saline- and TX-100-treated animals, using the monoclonal antibody, MAb192. In saline-treated animals, p75NGFR-immunoreactivity (p75NGFR-ir) was localized to individual glomeruli in the olfactory bulb, with little or undetectable p75NGFR-ir in the olfactory nerve layer. We have previously reported that pre-lesioned levels of p75NGFR-ir in the glomerular layer were dramatically reduced while an induction of p75NGFR-ir was observed in the olfactory nerve layer, one and two weeks after intranasal exposure to TX-100 [Turner & Perez-Polo (1992) Regulation of the low affinity receptor for nerve growth factor, p75NGFR, in the olfactory system of neonatal and adult rat. Int. J. Devl Neurosci. 10, 343-359]. In this paper, we demonstrate that this previously reported reduction in glomerular p75NGFR-ir took 16 weeks to fully recover and was, thus, coincident with the post-lesion recovery of the neuroepithelium. In the olfactory nerve layer, the return of p75NGFR-ir to pre-lesioned levels took only four weeks. No changes in neuroepithelial width and integrity or alterations in p75NGFR-ir in the olfactory bulb were observed in saline-treated animals. Thus, the TX-100-induced removal of the peripheral input to the olfactory bulb resulted in a reversible change in expression of p75NGFR-ir in the bulb. We believe that these changes are a reflection of the regenerative capacity of the olfactory system.

Unilateral naris closure and olfactory system development

In most animals there is bilateral access of odorants to the olfactory sensory epithelium. Air enters the nose through two external nares and passes back through the nasal cavity, which is divided down the midline by a cartilaginous nasal septum. The olfactory mucosa, a sheet of ciliated bipolar receptor cells, is found in the caudal two thirds of the nasal cavity. Axons from the sensory cells project to an ipsilateral extension of the telencephalon known as the olfactory bulb. If a single external naris of a rat pup is surgically closed (usually via brief cauterization) on the day after the day of birth (P1) and the subject is examined on P30, the size of the ipsilateral olfactory bulb is reduced by approximately 25%. The large reduction in size, coupled with the clear lamination and other features of the olfactory system, indicates that the manipulation is an ideal preparation for examining the regulation of early growth. We know that both olfactory bulbs are of equal size at the time of occlusion, but that 30 days later there is a large discrepancy. What series of events produces the changes? The present paper outlines what is known about the anatomical, biochemical and physiological changes introduced by naris occlusion in order to lay a framework for further work.

Molecular and cellular properties of mammalian primary olfactory axons

How are the axonal projections of olfactory and vomeronasal receptor neurons to the olfactory bulb formed during development? How are the primary olfactory axonal connections functionally organized? With progress in molecular biological techniques and histochemical methods, it became possible to study cellular strategies and molecular mechanisms which guide the primary olfactory axons of the main and accessory olfactory systems to the target glomeruli in the bulb. In addition, new methodologies have begun to elucidate various subsets of the primary olfactory axons with distinctive central connections. The aim of the present paper is to review (1) the characteristic organization of the projection of the primary olfactory axons, (2) projection patterns of histochemically defined subsets of primary olfactory axons, and (3) information on molecules expressed by the surface membrane of the primary olfactory axons. This knowledge gives insight into the functional organization of the primary olfactory axon projection, which is indispensable for understanding signal processing in the olfactory system. This knowledge also underscores the notion that the primary olfactory axon projection provides an excellent model system in which to study axonal guidance and the formation of specific synaptic connections.

Regulation of the low affinity receptor for nerve growth factor, p75NGFR, in the olfactory system of neonatal and adult rat

Using MAb192, a monoclonal antibody to the rat low affinity receptor for nerve growth factor (p75NGFR), we determined the expression of p75NGFR in rat neonatal and adult olfactory system. In neonates and adults, we observed discrete p75NGFR-immunoreactivity (p75NGFR-ir) in the glomerular layer of the main olfactory bulb. The intensity and organization of glomerular p75NGFR-ir increased with age. This was in keeping with the general ontogeny of the main olfactory bulb. Generally, granule cells, mitral cells and periglomerular cells of the main olfactory bulb were not specifically stained. However, in early neonates, granule cells close to the lateral olfactory tract exhibited p75NGFR-ir. Additional specific staining was found in the olfactory receptor neurons of neonatal and adult olfactory neuroepithelium, the olfactory fascicles and in the glomeruli of the accessory olfactory bulb. The intensity, but not the organization, of specific staining in the accessory olfactory bulb increased as the animal matured. We believe that p75NGFR-ir in the olfactory system is associated with its unique capacity to regenerate its peripheral input to the main olfactory bulb. The presence of p75NGFR-ir in the accessory olfactory bulb would suggest a broader role for this protein. Here we discuss the implications of these findings with regards to nerve growth factor, other trophic molecules, and their receptors. The data presented provide a foundation for studies involving manipulation of regenerative phenomena while monitoring the expression of neurotrophic factors and their receptors.

Monoclonal antibody immunohistochemistry of a temporal relationship between axonal elimination of aberrant olfactory nerves and synaptogenesis in the rabbit olfactory bulb during middle embryonic periods

Immunostaining using olfactory nerve- and synaptic vesicle protein-specific monoclonal antibodies revealed their characteristic appearance in the rabbit olfactory bulb during prenatal development. Prior to formation of glomeruli, olfactory nerve fibers extended beyond their target region deep into the bulb zones. Subsequently the aberrant axons decreased in number, and correspondingly, synaptic vesicle protein occurred in the innermost region of the olfactory nerve layer. It is concluded that the lack of synaptogenesis causes axonal elimination of aberrant olfactory nerves. Present results support the hypothesis that supernumerary axons degenerate unless synaptic contacts are secured; the olfactory nerves must arrive at the appropriate terminal zone in the glomeruli.