Primary Axons Research Articles

Expression of the amyloid protein precursor of Alzheimer's disease in the developing rat olfactory system

The expression of the amyloid protein precursor (APP) of Alzheimer's disease (AD) was examined in the olfactory system of the developing rat. Two monoclonal antibodies were used to detect APP: Alz-90, which specifically recognizes APP, and 22C11 which recognizes both APP and the structurally related protein APLP-2. Very similar patterns of immunoreactivity were observed with both antibodies. APP immunoreactivity was first detected in a subpopulation of olfactory epithelial cells at embryonic day 16 (E16), at a time when primary sensory olfactory axons are first beginning to pierce the glia limitans of the olfactory bulb. At E16, there were more olfactory receptor neurons which expressed APP than the olfactory marker protein (OMP), indicating that some APP-containing neurons were not fully mature. Between E16 and postnatal day 8 (P8), there was a marked increase in the number of primary sensory olfactory neurons expressing APP. In the olfactory bulb, APP was first detected in the mitral cell layer at E18, at a time when synapses are first beginning to form between the dendrites of these cells and primary sensory axons. The level of APP detected within mitral cell perikarya decreased after birth and could no longer be detected between P3 and P8. This indicated that once synaptic connections had been initiated within olfactory glomeruli, the expression of APP within the mitral cells was down-regulated. High levels of APP were, however, detected within the olfactory nerve fiber layer and glomeruli between P3 and P8. The results demonstrate that APP expression in the olfactory system is coordinately regulated with the major periods of synaptogenesis. Since APP is a known neurite outgrowth promoting molecule, we suspect that it may play an important regulatory role in terminal sprouting during synaptogenesis in the olfactory bulb.

Identification of cells expressing galectin-1, a galactose-binding receptor, in the rat olfactory system.

Interactions between carbohydrate ligands and their receptors play an important role in cell adhesion and migration in many tissues. Cell-surface carbohydrates that contain terminal galactose have previously been implicated in primary sensory axon growth in the rodent olfactory system. The aim of the present study was to determine whether galectin-1, a galactose-binding receptor, was expressed within the rat primary olfactory pathway. Immunohistochemical and in situ hybridisation analyses revealed expression of galectin-1 by primary sensory olfactory neurons during the major embryonic period of axonogenesis as well as in maturity. In the adult olfactory bulb, galectin-1 was expressed by both second-order projection neurons and interneurons and was selectively localised to the synaptic neuropil layers. Mitral cells, the principal postsynaptic target of primary olfactory axons, began expressing this lectin soon after genesis and maintained high levels into adulthood. The expression of galectin-1 in the primary olfactory pathway and olfactory bulb neuropil suggests a role for this lectin both in the initial formation and in the subsequent maintenance of neuronal connections between the peripheral and the central olfactory neurons as well as between neurons within the bulb.

Host primary olfactory axons make synaptic contacts in a transplanted olfactory bulb

Previous light microscopic studies have shown that host olfactory neurons are able to grow into a transplanted fetal olfactory bulb, and behavioral studies have shown that animals with transplanted olfactory bulbs recover functional olfactory abilities. We examined the olfactory bulb transplant at the ultrastructural level to determine whether synaptic contacts are reestablished between host olfactory neurons and donor olfactory bulb. Mature rats that, as neonates, had received embryonic olfactory bulb transplants following olfactory bulb removal were studied. An antibody specific for olfactory marker protein was used to identify the primary olfactory neurons; it was bound by a gold-conjugated secondary antibody for visualization. To preserve the antigenicity of the olfactory marker protein for immunolabeling, Lowicryl K4M hydrophilic resin was used. Synaptic contacts were unmistakable between labeled axons of host olfactory neurons and unlabeled processes within glomerulus-like areas of the transplanted olfactory bulb. The surrounding neuropil contained other elements similar to those found in normal tissue, including synaptic contacts between unlabeled profiles. We clearly show that the transplanted olfactory bulb exhibits sufficient plasticity to form an array of normal synaptic contacts, including the contacts from host primary olfactory neurons.

Effects of orochemical stimulation on postnatal development of gustatory recipient zones within the nucleus of the solitary tract

Previous receptor damage studies and artificial rearing (AR) studies in rat have demonstrated that orochemical stimulation between the postnatal ages of P2 and P14 is necessary for development of primary gustatory axons and terminal endings in the rostral nucleus of the solitary tract (NST). Objectives of the present experiment were to evaluate the qualitative nature of orochemical stimulation and amount of orochemical stimulation that is necessary to produce normal axonal and terminal development in gustatory recipient zones of the rostral and intermediate NST. To this end, ultra-pure water, 30,150, and 500 mM NaCl, 80 and 340 mM lactose, whole rat milk, and rat milk that had been subject to extensive dialysis (12–14 kD MWCO) was delivered to independent groups of rat pups during AR procedures. Unstimulated AR animals and matched mother-reared (MR) animals were used as controls. AR animals received experimental orochemical stimulation between the ages of P4 and P10, and were there-after returned to lactating dams until the time of weaning; MR animals received experimental orochemical stimulation during the course of normal nursing. Following maturation, anterograde fluorescent dual-labeling experiments were conducted to map the course and distribution of primary gustatory axons within the NST. Results show that experimental stimulation with water during AR procedures is not sufficient to produce normal development of primary gustatory axons and terminal endings in the gustatory NST. Stimulation with 30,150, and 500 mM NaCl produced normal axonal development in the NST, as did 80 and 340 mM lactose, whole rat milk, and hdialyzed rat milk. These results indicate that orochemical stimulation with qualitatively different tastants during critical periods of postnatal development is sufficient to produce normal development of primary gustatory axons within the rostral NST. In a second experiment, rat pups received either 1 day or 3 days of stimulation with 150 mM NaCl to determine the time-course of stimulation necessary to produce normal development of terminal fields in the rostral NST. Results indicated that 1 day of stimulation with NaCl was not sufficient to produce normal development of rostral NST axons, whereas 3 days stimulation with NaCl was effective in producing normal axonal development. Therefore, like other sensory systems, the gustatory system is susceptible to anatomical modification by taste stimulation during critical or sensitive periods of postnatal development.

Axogenesis in the embryonic brain of the grasshopper Schistocerca gregaria: an identified cell analysis of early brain development.

Axogenesis in the embryonic brain was studied at the single cell level in the grasshopper Schistocerca gregaria. A small set of individually identifiable pioneer neurons establishes a primary axon scaffold during early embryogenesis. At the beginning of scaffold formation, pioneering axons navigate along and between glial borders that surround clusters of proliferating neuroblasts. In each brain hemisphere, an axonal outgrowth cascade involving a series of pioneer neurons establishes a pathway from the optic ganglia to the brain midline. At the midline the primary preoral commissural interconnection in the embryonic brain is pioneered by a pair of midline-derived pioneer neurons. A second preoral commissural connection is pioneered by two pairs of pars intercerebralis pioneer neurons. Descending tracts are pioneered by the progeny of identified neuroblasts in the pars intercerebralis, deutocerebrum and tritocerebrum; the postoral tritocerebral commissure is pioneered by a pair of tritocerebral neurons. All of the pioneering brain neurons express the cell adhesion molecule fasciclin I during initial axon outgrowth and fasciculation. Once established, the primary axon scaffold of the brain is used for fasciculation by subsequently differentiating neurons and, by the 40% stage of embryogenesis, axonal projections that characterize the mature brain become evident. The single cell analysis of grasshopper brain development presented here sets the stage for manipulative cell biological experiments and provides the basis for comparative molecular genetic studies of embryonic brain development in Drosophila.

Selective association of nerve fibres immunoreactive for substance P or bombesin with putative cholinergic neurons of the male rat major pelvic ganglion.

The male rat major pelvic ganglion contains both sympathetic and parasympathetic neurons that supply the lower urinary and digestive tracts, and the reproductive organs. The aim of this study was to describe the distribution and identify potential targets of sensory and intestinofugal axons in this ganglion. Two putative markers of these projections were chosen, substance P for primary sensory axons and bombesin for myenteric intestinofugal projections. Varicose substance P-immunoreactive axons were associated only with non-noradrenergic (putative cholinergic) somata, and most commonly with those that contained vasoactive intestinal peptide. Immunoreactivity for substance P was also present in a small group of non-noradrenergic somata, many of which were immunoreactive for enkephalins, neuropeptide Y or vasoactive intestinal peptide. Bombesin immunoreactivity was found only in preterminal and terminal (varicose) axons, the latter of which were exclusively associated with non-noradrenergic somata that contain neuropeptide Y-immunoreactivity. Some varicose axons containing either substance P- or bombesin-immunoreactivity were intermingled with clumps of small, intensely fluorescent cells. These studies indicate that substance P- and bombesin-immunoreactive axons are likely to connect with numerically small, but discrete, populations of pelvic neurons.

Dorsal root ganglion cell death and surviving cell numbers in relation to the development of sensory innervation in the rat hindlimb

This study correlates the numbers of dying, surviving and proliferating L 4 primary afferent neurons with the development of peripheral hindlimb sensory innervation in the rat. Cell death occurs from embryonic day 15 (E15) to just after birth and peaks at E17–E19. Despite this, surviving cell numbers rise steadily to birth indicating that cell death is more than balanced by cell proliferation over this period. GAP-43 immunostaining indicates that the peripheral sensory axons are only in central parts of the hindlimb by E15 and do not finish arriving at their distal peripheral targets until birth so prenatal cell death in the L4 ganglion is not well correlated with the development of the peripheral innervation by these primary sensory axons. Prenatal cell death does, however, correlate well with the innervation of the cord by central sensory axons. In contrast to the steady rise of surviving cell numbers from E15 to birth, cell numbers go down 16% in the period from birth to postnatal day 5. This loss is correlated with the development of the peripheral innervation. We conclude that the bulk of cell death in the rat L4 dorsal root ganglion, which is prenatal, is controlled by local or central factors whereas peripheral target factors may exert their influence postnatally to determine the final numbers of mammalian sensory neurons. The data also suggest that there may be two phases of cell death, an early phase involving large light cells and a late phase involving small dark cells.

Characterization of basic fibroblast growth factor-mediated acceleration of axonal branching in cultured rat hippocampal neurons

We analyzed in more detail the effect of basic fibroblast growth factor (bFGF) on morphogenesis of rat hippocampal neurons in dissociated cell culture. As a result, we found that bFGF selectively promoted the bifurcation and growth of axonal branches without affecting the elongation rate of primary axons. The dendritic outgrowth was rather inhibited by bFGF. These effects of bFGF resulted in increased complexity of axonal trees. The effect of bFGF was concentration dependent (0.1–10 ng/ml) and was abolished by the presence of anti-bFGF neutralizing antibody. The accelerated axonal branch formation in the presence of bFGF was restored to the basal rate following removal of bFGF, suggesting that the action of bFGF is reversible and that the continuous presence is required for bFGF to accelerate the branch formation. bFGF probably works as a progression signal rather than as a triggering signal. The bFGF-mediated acceleration of axonal branch formation was blocked by treatment with heparitinase and by tyrosine kinase inhibitors, herbimycin A and lavendustin A, indicating the importance of heparan sulfate and tyrosine kinase in bFGF signal transduction. Treatment with a protein kinase C activator phorbol-12-myristate-13-acetate did not significantly affect the neurite branching, and the action of bFGF was not blocked by a protein kinase C inhibitor staurosporine. Protein kinase C is unlikely to play a role in branch formation. The novel action of bFGF as a regulator of axonal branching must be a particularly useful model for the study of neuritogenesis and synaptogenesis of brain neurons.

Early morphological changes of primary afferent neurons and their processes in newborn mice after treatment with capsaicin.

Degenerating figures of dorsal root ganglion (DRG) neurons and their central and peripheral processes (dorsal root and saphenous nerve) and terminals (central terminals in the superficial dorsal horn and cutaneous nerve of the hind paw dorsal skin) of neonatal mice were examined 30 min, 1, 2 and 5 h, and 2, 3, 5, and 10 days after subcutaneous injection of capsaicin on post-natal day 2. Many small DRG neurons showed degeneration 1 h after treatment. Scarcely any features of degeneration were seen in the DRG and dorsal root 10 days after treatment. The degenerating aspects of terminal axons in the marginal layer of the superficial dorsal horn were characterized by enlarged round axons with closely packed osmiophilic materials, lamellar bodies, and loss of axoplasmic organelles. Two types of central terminals (C-terminals) showed degeneration in the substantia gelatinosa from 30 min after treatment onward. One type consisted of small, round, sinuous or slender dark terminals (CI-terminals), and the other of large, pale, round or angular terminals (CII-terminals). Those that degenerated markedly had homogeneously electron-dense axoplasm with dilated synaptic vesicles and inclusion bodies. Extensive degeneration of terminal axons in the marginal layer occurred 5 h after treatment, whereas conspicuous degeneration of C-terminals occurred from 30 min to 10 days after treatment in the substantia gelatinosa. CI-terminals showed marked degeneration during the first 3 days, whereas marked degeneration of CII-terminals occurred between 5 and 10 days after treatment. This time difference between the peaks of degeneration of CI- and CII-terminals indicates an important difference in the origins of these two types of capsaicin-sensitive, nociceptive fibers in the superficial dorsal horn; CI-terminals are derived from small DRG cells, whereas CII-terminals are derived from larger DRG cells. Unmyelinated axons in the dorsal root, saphenous nerve, and dorsal skin of the hind paw showed similar degeneration patterns 2 h after treatment to those of terminal axons in the marginal layer. Thus, the degenerating profiles in the marginal layer suggest that these axons arose from collaterals of unmyelinated primary axons descending or ascending within the marginal layer. Numerous enlarged degenerating axons showing vacuolation were conspicuous in the dorsal skin 3 days after treatment. The synchronous degeneration of the smaller DRG neurons, their central and peripheral processes, and their CI-terminals in the substantia gelatinosa supports the idea that the smaller DRG neurons are directly influenced by capsaicin, and that their degeneration is followed by centrifugal degeneration.

Topographic specificity of corticospinal connections formed in explant coculture

The corticospinal pathway connects layer V pyramidal neurons in discrete regions of the sensorimotor cortex to topographically matching targets in the spinal cord. In rodents initial pathway errors occur transiently during early postnatal development, such that visual cortical axons project inappropriately into the corticospinal tract. Nevertheless, only sensorimotor axons form corticospinal connections, which are topographically ordered in hamsters from the earliest stages of innervation. Previous work in vivo suggests that pathfinding is carried out by primary cortical axons whereas target innervation occurs by extension of axon collaterals at appropriate locations. In vitro studies have provided evidence that chemotropic factors may selectively attract extension of neurites into specific targets. To investigate the basis for corticospinal target selection during development, we have used an in vitro explant coculture system. Sensorimotor and visual cortical explants from newborn hamsters were presented with inappropriate targets from olfactory bulb and cerebellum and targets from the cervical (forelimb) and lumbar (hindlimb) enlargements of the early postnatal spinal cord. Under in vitro conditions, corticospinal target selection was highly specific and remarkably similar to corticospinal connectivity in vivo. Visual and sensorimotor cortical neurites extended nonselectively into the white matter of the spinal cord. However, only neurites from the sensorimotor cortex were able to extend into and arborize within the spinal gray. In the majority of cases, these connections were topographically appropriate, matching forelimb cortex to cervical cord and hindlimb cortex to lumbar cord. However, we found no evidence that chemotropic attraction was responsible for selection of appropriate targets by cortical neurites or that spinal target tissue promoted extension of cortical axon collaterals within the collagen matrix. These results suggest that the ability of cortical neurites to recognize correct spinal targets and form terminal arbors may require direct axon target interaction.

Development of specificity in corticospinal connections by axon collaterals branching selectively into appropriate spinal targets

Corticospinal projections in adult rodents arise exclusively from layer V neurons in the sensorimotor cortex. These neurons are topographically organized in their connections to spinal cord targets. Previous studies in rodents have shown that the mature distribution pattern of corticospinal neurons develops during the first 2 weeks postnatal from an initial widespread pattern that includes the visual cortex to a distribution restricted to the sensorimotor cortex. To determine whether specificity in corticospinal connections also emerges from an initially diffuse set of projections, we have studied the outgrowth of corticospinal axons and the formation of terminal arbors in developing hamsters. The sensitive fluorescent tracer 1,1',dioctadecyl-3,3,3',3'-tetramethylindocarbocyanine perchlorate (DiI) was used to label corticospinal axons from the visual cortex or from small regions of the forelimb or hindlimb sensorimotor cortex in living animals at 4-17 days postnatal. Initially axon outgrowth was imprecise. Some visual cortical axons extended transiently beyond their permanent targets in the pontine nuclei, by growing through the pyramidal decussation and in some cases extending as far caudally as the lumbar enlargement. Forelimb sensorimotor axons also extended past their targets in the cervical enlargement, in many cases growing in the corticospinal tract to lumbar levels of the cord. By about 17 days postnatal these misdirected axons or axon segments were withdrawn from the tract. Despite these errors in axon trajectories within the corticospinal tract, terminal arbors branching into targets in the spinal gray matter were topographically appropriate from the earliest stages of innervation. Thus visual cortical axons never formed connections in the spinal cord, forelimb sensorimotor axons arborized only in the cervical enlargement, and hindlimb cortical axons terminated only in the lumbar cord at all stages of development examined. Corticospinal arbors formed from collaterals that extended at right angles from the shafts of primary axons, most likely by the process of interstitial branching after the primary growth cone had extended past the target. Once collaterals extended into the spinal gray matter, highly branched terminal arbors formed within 2-4 days, beginning at about 4 and 8 days postnatal for the cervical and lumbar enlargements, respectively. These results show that specificity in corticospinal connectivity is achieved by selective growth of axon collaterals into appropriate spinal targets from the beginning and not by the later remodeling of initially diffuse connections. In contrast, errors occur in the initial outgrowth of axons in the corticospinal tract, which are subsequently corrected.

Olfactory ensheathing glia and platelet-derived growth factor B-chain reactivity in the transplanted rat olfactory bulb

Using a monoclonal antibody against the B-chain of platelet-derived growth factor as a marker, we have examined the behavior of olfactory ensheathing glia in the normal and transplanted rat olfactory bulb. In the normal postnatal olfactory bulb, these glia are found to ensheath the bundles of incoming primary olfactory nerve fibers as well as those in the olfactory nerve layer. Olfactory marker protein antibody was used to identify the olfactory nerve proper. Within the transplant, the same glia: 1.(1) ensheath bundles of both primary olfactory and non-primary olfactory axons,2.(2) ensheath axonal bundles deep within the donor tissue, and3.(3) eventually permit radiation of individual axons from bundles to surrounding neuropil.We believe that ensheathing glia (being rich in growth-related factors and extracellular matrix molecules) may be useful in providing trophic support and guidance for the reconstruction of developmentally or traumatically damaged neuronal pathways not directly related to the olfactory system. The evidence presented here indicates that ensheathing glia are capable of existing in deep brain areas and ensheathing other than primary olfactory axons. The special molecular characteristics of these glia along with the morphological findings presented here provide a foundation for further studies of these unique glia and their potential utility in the restoration of damaged neural pathways.

Ulex europaeus I and glycine max bind to the human olfactory bulb

The distribution of binding sites for the fucose-selective lectin Ulex europaeus I and the terminal N-acetylgalactosamine-selective lectin glycine max in the human olfactory bulb were studied. These lectins bound to primary olfactory axons in the olfactory nerve layer and the glomerular layer. They also bound to fibers located in the deeper layers such as the external plexiform layer and the granular layer. Furthermore they projected to the olfactory stalk but not in the cerebrum. The deeper projections of the lectin binding fibers may affect the function of the olfactory bulb in humans.

Muscle afferents innervating skin form somatotopically appropriate connections in the adult rat dorsal horn.

We have studied the somatotopic reorganization in dorsal horn neurons after a disruption in the normal spatial arrangement of primary sensory axons in adult rats. Muscle afferents were redirected to skin by cutting and cross-anastomosing the hindlimb gastrocnemius nerve (GN) and sural nerve (SN). It has previously been shown that after 10-12 weeks GN afferents innervate the hairy skin of the lateral ankle and calf (previously innervated by SN afferents) and become potentially capable of relaying information on the location and intensity of stimuli applied to the skin. We determined the receptive field and response properties of dorsal horn neurons in the lumbar spinal cord, in regions where the lower hindlimb is normally represented. In control animals (with intact or self-anastomosed sural nerves) very few neurons (< 8%) received any synaptic input from the GN as assessed by electrical stimulation of the nerve. In contrast, when this nerve innervated skin, many cells responded to GN stimulation, and these nearly all had receptive field components in the former SN territory. Moreover, in animals with cross-anastomosed nerves, cells without GN inputs all had receptive fields outside the former SN skin territory. We have shown that in all likelihood GN afferents substituted for SN afferents in subserving the low and high threshold receptive fields of dorsal horn neurons. Furthermore, for many neurons, receptive fields were formed from inappropriately regrown GN afferents and adjacent intact cutaneous afferents (in the tibial or common peroneal nerves). Therefore, when GN afferents innervate skin in adult animals, they alter their central connectivity in an appropriate manner for their new peripheral terminations, so that an orderly somatotopic representation of the hind limb skin is maintained. We suggest that this plasticity of dorsal horn somatotopy is driven in part by activity-dependent mechanisms.

Odor stimulation causes disappearance of R4B12 epitope on axonal surface molecule of olfactory sensory neurons

Monoclonal antibodies R4B12 and R1D1 label the same subsets of rabbit primary olfactory axons. In the present study, we characterized the R4B12 antigens using immunohistochemical, immunoelectron-microscopic, and biochemical techniques. The R4B12 antigens are expressed on the surface membrane of a subset of primary olfactory axons. Western blot analysis revealed the existence of two forms (115,000 and 90,000 mol. wt) of the R4B12 antigens with different membrane-anchoring structures. Of the two forms, the smaller antigen (90,000 mol. wt) is anchored to the plasma membrane via a phosphatidylinositol linkage and expressed exclusively by the olfactory system. When the rabbit olfactory epithelium was stimulated by odors for 2–8 h in situ, the R4B12 immunoreactivity disappeared from the primary olfactory axons in the glomeruli of the olfactory bulb. These results suggest that the cell surface antigens R4B12 expressed by subsets of primary olfactory axons undergo stimulus-dependent changes by odor stimulation and may be involved in plasticity of olfactory sensory neurons.

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.

Polyneuropathy associated with IgA monoclonal gammopathy of undetermined significance

Although polyneuropathies associated with IgM and IgG monoclonal gammopathies have been well described, polyneuropathy with IgA monoclonal gammopathy of undetermined significance (MGUS) is less commonly seen and has not been well studied. We reviewed the clinical and electrodiagnostic features of 5 such patients, and the sural nerve biopsy findings in 4 of them. One patient was diabetic, while 4 were free of other diagnoses commonly associated with neuropathy. Clinical presentations were varied. Electrodiagnostic and histological studies ranged from primary demyelination to primary axon loss to a mixed axonal/demyelinating picture. Three patients who were treated appeared to respond to prednisone or intravenous gamma globulin, despite clear clinical, electrodiagnostic, and histological differences. We conclude that the polyneuropathy associated with IgA MGUS is heterogeneous, similar to that in IgM and IgG MGUS. A trial of immunomodulating therapy appears to be warranted in such patients if the neuropathy is sufficiently severe.

Modulation of cochlear nerve spike rate by cardiac activity in the gerbil

Among primary auditory axons with characteristic frequencies (CFs) below 2500 Hz, a substantial subpopulation was found in which spike activity was driven by cardiac events. The presence of cardiac-driven activity was inferred from cycle histograms triggered on the peak of the electrocardiogram (ECG). This driven activity was either like a simple onset response (often followed by a reduction of spike activity to below background level), or as a longer lasting series of peaks and troughs. In two axons with high CFs (7 kHz and 12.5 kHz), cardiac-driven suppression was observed. Recordings made by a probe microphone revealed the presence of heart-related sound in the external ear canal. The onset of that sound coincided with the onset of cardiac-driven spike activity (and suppression).

Immunocytochemical identification of primary olfactory afferents in rainbow trout.

We have used a combination of techniques to analyze the primary olfactory projection in trout: anterograde tract tracing with horseradish peroxidase (HRP) and immunocytochemistry with antisera to olfactory marker protein (OMP) and to keyhole limpet hemocyanin (KLH). HRP labeling and the OMP antiserum revealed a subset of ciliated receptor neurons with a wide dendrite that lacked the protruding knob found on other receptor neurons. The organization of the primary olfactory axons was clearly revealed by antisera to KLH, which reacted with no other neurons. When visualized with anti-KLH, fascicles of olfactory axons penetrated the basal lamina of the olfactory rosette at scattered sites and converged to form the olfactory nerve. Fascicles within the olfactory nerve traveled parallel to the long axis of the nerve until resorted by extensive intermixing as they entered the olfactory bulb. Within the olfactory bulb, most axons terminated in nine discrete terminal fields in the glomerular layer; however, a few olfactory nerve axons projected into the ventral medial telencephalon. Fascicles supplying each terminal field in the glomerular layer followed distinctive trajectories within the olfactory nerve layer. Axons ending in two terminal fields made brush-like terminations rather than the glomerular terminations characteristic of the remaining seven fields. After unilateral olfactory nerve transection, returning olfactory axons reestablished the normal pattern of terminal fields within 14 weeks. It is likely that the organization of afferents in the trout olfactory bulb is similarly well regulated during normal receptor cell replacement.

Developmental pattern and distribution of nerve growth factor low-affinity receptor immunoreactivity in human spinal cord and dorsal root ganglia: Comparison with synaptophysin, neurofilament and neuropeptide immunoreactivities

Immunocytochemical expression of the low-affinity nerve growth factor receptor was studied in human fetal and adult tissues using the monoclonal antibody ME20.4. In dorsal root ganglia, a few immunoreactive neurons were first detected in nine-week-old fetuses and many more were found in the following weeks of gestation. However, none was present in adult ganglia. The ME20.4-positive cells were larger than neurons immunostained by substance P, calcitonin gene-related peptide or galanin antibodies. In the spinal cord, fibres immunostained by ME20.4 appeared in a characteristic pattern that differed from the spatial and temporal distributions of synaptophysin- and neurofilament-immunoreactive fibres. Those expressing the low-affinity nerve growth factor receptor were only detected in regions containing collaterals of primary sensory axons: (i) in the dorsal funiculus between seven and 18 weeks of gestation; (ii) in a ventrodorsal bundle reaching the ventral horn from weeks 12–14; (iii) in the medial region of the dorsal horn between weeks 12 and 20; (iv) in the superficial layers and lateral portion of the dorsal horn after the 14th week of gestation and also in adult spinal cord. During the fetal period, ME20.4 immunoreactivity was also found in motoneurons and peripheral nerve fibres in the skin, myotomes and gut. Sheaths of peripheral nerves and the adventitia of blood vessels were stained both in fetal and adult tissues. Thus, the low-affinity nerve growth factor receptor is: (i) strongly expressed in the developing human nervous system; (ii) transiently associated with a subset of large primary sensory neurons and with motoneurons; (iii) transiently and sequentially expressed by various groups of sensory afferents to the spinal cord; (iv) permanently expressed by fibres in the superficial layers of the dorsal horn, Clarke's column, nerve sheaths and the adventitia of blood vessels.