High-throughput segmentation of unmyelinated axons by deep learning

Axonal characterizations of connectomes in healthy and disease phenotypes are surprisingly incomplete and biased because unmyelinated axons, the most prevalent type of fibers in the nervous system, have largely been ignored as their quantitative assessment quickly becomes unmanageable as the number of axons increases. Herein, we introduce the first prototype of a high-throughput processing pipeline for automated segmentation of unmyelinated fibers. Our team has used transmission electron microscopy images of vagus and pelvic nerves in rats. All unmyelinated axons in these images are individually annotated and used as labeled data to train and validate a deep instance segmentation network. We investigate the effect of different training strategies on the overall segmentation accuracy of the network. We extensively validate the segmentation algorithm as a stand-alone segmentation tool as well as in an expert-in-the-loop hybrid segmentation setting with preliminary, albeit remarkably encouraging results. Our algorithm achieves an instance-level [Formula: see text] score of between 0.7 and 0.9 on various test images in the stand-alone mode and reduces expert annotation labor by 80% in the hybrid setting. We hope that this new high-throughput segmentation pipeline will enable quick and accurate characterization of unmyelinated fibers at scale and become instrumental in significantly advancing our understanding of connectomes in both the peripheral and the central nervous systems.

Human organ donor-derived vagus nerve biopsies allow for well-preserved ultrastructure and high-resolution mapping of myelinated and unmyelinated fibers

The vagus nerve provides motor, sensory, and autonomic innervation of multiple organs, and electrical vagus nerve stimulation (VNS) provides an adjunctive treatment option for e.g. medication-refractory epilepsy and treatment-resistant depression. The mechanisms of action for VNS are not known, and high-resolution anatomical mapping of the human vagus nerve is needed to better understand its functional organization. Electron microscopy (EM) is required for the detection of both myelinated and unmyelinated axons, but access to well-preserved human vagus nerves for ultrastructural studies is sparse. Intact human vagus nerve samples were procured intra-operatively from deceased organ donors, and tissues were immediately immersion fixed and processed for EM. Ultrastructural studies of cervical and sub-diaphragmatic vagus nerve segments showed excellent preservation of the lamellated wall of myelin sheaths, and the axolemma of myelinated and unmyelinated fibers were intact. Microtubules, neurofilaments, and mitochondria were readily identified in the axoplasm, and the ultrastructural integrity of Schwann cell nuclei, Remak bundles, and basal lamina was also well preserved. Digital segmentation of myelinated and unmyelinated axons allowed for determination of fiber size and myelination. We propose a novel source of human vagus nerve tissues for detailed ultrastructural studies and mapping to support efforts to refine neuromodulation strategies, including VNS.

[Analysis of the quantitative differences in the thickness of the retinal nerve fibre layer between time-domain and spectral-domain optical coherence tomography in patients with relapsing-remitting multiple sclerosis]

INTRODUCTION

Optical coherence tomography (OCT) is a tool that is increasingly more commonly used in the study of neuro-degenerative diseases.

AIMS

To analyse and correlate the thickness of the retinal nerve fibre layer (RNFL) by means of time-domain and spectral-domain OCT in patients with relapsing-remitting multiple sclerosis (MS), with and without a history of optic neuritis (ON).

SUBJECTS AND METHODS

We conducted a cross-sectional study of the thickness (mean and by quadrants) of the RNFL of 15 disease-free subjects, 28 with MS with no prior history of ON and 18 with a history of ON. The full ophthalmologic examination included measurement of the RNFL by means of time-domain and spectral-domain tomography.

RESULTS

Statistically significant differences are found between the two tomography scans on comparing the mean thickness of the RNFL of the control group (p = 0.000), the group with a history of ON (p = 0.000) and the group without ON (p = 0.000). We obtained a strong, statistically significant and directly proportional correlation between the mean thickness of the RNFL measured with the two types of tomography in the control group (rho = 0.842; p = 0.000), and the groups of eyes without ON (rho = 0.91; p = 0.000) and with ON (rho = 0.902; p = 0.000).

CONCLUSIONS

There is a strong correlation between the two tomography scans in the measurement of the thickness of the RNFL in patients with MS, with and without a history of ON. Time-domain OCT quantifies greater thicknesses, and therefore both types of tomography have proven to be effective in the study of MS, although the results cannot be interchanged or extrapolated.

Peri-implant bone innervation: histological findings in humans

PURPOSE

The aim of the present study was to describe nerve fibres around osseointegrated implants in humans.

MATERIALS AND METHODS

Twelve mechanically failed implants, retrieved from 10 patients were collected from three dental centres over a period of 5 years. After implant removal, decalcified semi-thin sections (0.5 μm) were stained with thionic methylene blue for light microscopic analysis. In addition, an ultrastructural analysis was performed on serial ultra-thin sections (0.06 μm) using transmission electron microscopy.

RESULTS

Both myelinated and unmyelinated nerve fibres could be identified inside the Haversian canals of the osteonal bone near the implant threads. Myelinated fibres were also located at the woven bone around the implant. However, no differentiated nerve endings could be observed around the implants.

CONCLUSIONS

This study shows the presence of nerve fibres in human peri-implant bone. Previous studies in animals showed that those fibres participate in the process of bone modelling and remodelling. Yet, the role of peri-implant bone innervation in the osseoperception phenomenon cannot be ruled out since the mechanism of mechanoreception in bone is not fully understood.

Modulation of semaphorin 3A expression by calcium concentration and histamine in human keratinocytes and fibroblasts

BACKGROUND

Both neurotrophins and chemorepellents are involved in the elongation and sprouting of itch-associated C-fibers in the skin. Nerve growth factor (NGF) and semaphorin 3A (Sema3A) are representatives of these two types of axon-guidance factors, respectively.

OBJECTIVE

We investigated the effects of calcium concentration and histamine on the expression of NGF and Sema3A in normal human epidermal keratinocytes (NHEK) and normal human fibroblasts (NHFb).

METHODS

NHEK and NHFb were cultured under different calcium concentrations (0.15-0.9 mM) with or without histamine, and the expression of mRNA for NGF and SEMA3A was assessed by real-time PCR analysis. An immunohistochemical study was performed for Sema3A using normal skin and skin cancer specimens.

RESULTS

In NHEK, SEMA3A expression was elevated by high calcium concentration and reduced by low calcium condition, while NGF expression was not dependent on calcium. Their expressions were unchanged by calcium in NHFb. Immunohistochemically, keratinocytes in the prickle layer of normal epidermis and squamous cell carcinoma cells were positive for Sema3A, sparing basal cells and suprabasal cells. The addition of histamine to NHEK at 10 μg/ml enhanced SEMA3A expression but depressed NGF expression. In NHFb, however, histamine decreased both NGF and SEMA3A levels.

CONCLUSIONS

Sema3A inhibits C-fiber elongation/sprouting in the upper layers of the epidermis, where calcium concentration is high, thereby determining the nerve endings. Histamine reduces Sema3A production by fibroblasts, allowing C-fibers to elongate in the dermis. In contrast, the histamine-augmented keratinocyte production of Sema3A might suppress C-fiber elongation and exaggerated pruritus.

Morphofunctional interactions of peripheral nerve fibers of the iris with neurons developing in the anterior chamber of the eye in rats

Electron microscopic studies were performed on intraocular transplants of embryonic septal and hippocampal tissue developing in the anterior chamber of the eye in rats for 3-4 months. The aim of the study was to seek ultrastructural identification of peripheral nerve fibers entering transplants from the iris, and to assess their ability to establish true synaptic contacts with transplanted CNS neurons. Bundles of myelinated and unmyelinated axons surrounded by Schwann cell cytoplasm were seen within the perivascular spaces of ingrowing blood vessels. Both types of peripheral fiber were also identified in the neuropil areas of transplants. At the ultrastructural level, unmyelinated axons were found to be free of glial Schwann cell sheaths and to form typical asymmetrical synapses with the dendrites and dendritic spines of transplant neurons. These results provide evidence of the high morphofunctional plasticity of both parts (central, peripheral) of the nervous system.

Microstructural and ultrastructural assessment of inferior alveolar nerve damage following nerve lateralization and implant placement: an experimental study in rabbits

PURPOSE

The present study assessed damage to the inferior alveolar nerve (IAN) following nerve lateralization and implant placement surgery through optical and transmission electron microscopy (TEM).

MATERIALS AND METHODS

IAN lateralization was performed in 16 adult female rabbits (Oryctolagus cuniculus). During the nerve lateralization procedure, one implant was placed through the mandibular canal, and the IAN was replaced in direct contact with the implant. The implant was placed in the right mandible, and the left side was used as a control (no surgical procedure). After 8 weeks, the animals were sacrificed and samples were prepared for optical and TEM analysis of IAN structural damage. Histomorphometric analysis was performed to determine the number and cross-sectional dimensions of nerve fascicles and myelin sheath thickness between experimental and control groups. The different parameters were compared by one-way analysis of variance at the 95% significance level.

RESULTS

Alterations in the perineural and endoneural regions of the IAN, with higher degrees of vascularization, were observed in the experimental group. TEM showed that the majority of the myelinated nerve fibers were not affected in the experimental samples. No significant variation in the number of fascicles was observed, significantly larger fascicle height and width were observed in the control group, and significantly thicker myelin sheaths were observed in the experimental samples.

CONCLUSION

IAN lateralization resulted in substantial degrees of tissue disorganization at the microstructural level because of the presence of edema. However, at the ultrastructural level, small amounts of fiber degeneration were observed.

[Morpho-functional interactions of the iris peripheral nervous fibers with the neurons developing in the rat anterior eye chamber]

The intraocular grafts of the septal or hippocampal embryonic tissues developing in the rat anterior eye chamber for three to four months were investigated by electron microscopy. The aim of this study was both the ultrastructural identification of the peripheral nervous fibers entering the grafts from host iris and the estimation of their capacity to establish true synaptic contacts with the central nervous system neurons of the grafts. The bundles of myelinated and unmyelinated axons, surrounded by the Schwann cell cytoplasm, were observed within the perivascular spaces of the ingrowing blood vessels. In the neuropil areas of the grafts, both types of the peripheral nervous fibers were also identified. It was demonstrated on the ultrastructural level that the unmyelinated axons lost their glial envelope of the Schwann cell and formed the typical asymmetric synapses with the dendrites and dendritic spines of the grafted neurons. The results are indicative of the high morpho-functional plasticity of both parts of the nervous system.

Hypothalamospinal oxytocinergic antinociception is mediated by GABAergic and opiate neurons that reduce A-delta and C fiber primary afferent excitation of spinal cord cells

Recent results implicate a new original mechanism involving oxytocin (OT), as a mediator via descending fibers of the paraventricular hypothalamic nucleus (PVN), in antinociception and analgesia. In rats electrical stimulation of the PVN or topical application of OT selectively inhibits A-delta and C fiber responses in superficial dorsal horn neurons, and this inhibition is reversed by a selective OT antagonist. However, little is known about the mechanisms and the spinal elements participating in this phenomenon. Here we show that topical application of bicuculline blocks the effects produced by PVN electrical stimulation or OT application. PVN electrical stimulation also activates a subpopulation of neurons in lamina II. These PVN-On cells are responsible for the amplification of local GABAergic inhibition. This result reinforces the suggestion that a supraspinal descending control of pain processing uses a specific neuronal pathway in the spinal cord in order to produce antinociception involving a GABAergic interneuron. Moreover, the topical administration of naloxone or a mu-opiate receptor antagonist beta-funaltrexamine only partially blocks the inhibitory effects produced by OT application or PVN electrical stimulation. Thus, this OT mechanism only involves opiate participation to a minor extent. The OT-specific, endogenous descending pathway represents an interesting mechanism to resolve chronic pain problems in special the neuropathic pain.

TRPV1 unlike TRPV2 is restricted to a subset of mechanically insensitive cutaneous nociceptors responding to heat

In the present study, a murine ex vivo somatosensory system preparation was used to determine the response characteristics of cutaneous sensory neurons staining positively for TRPV1 or TRPV2. TRPV1 immunostaining was found exclusively (11/11) in a specific set of mechanically insensitive unmyelinated (C) nociceptors that responded to heating of their receptive fields. No cutaneous C-fibers that responded to both mechanical and heat stimuli stained positively for TRPV1 (0/62). The relationship between TRPV2 and heat transduction characteristics was not as clear, as few unmyelinated or myelinated fibers that responded to heat contained TRPV2. TRPV2 was found most frequently in mechanically sensitive myelinated fibers, including both low threshold and high threshold mechanoreceptors (nociceptors). Although TRPV2 was found in only 1 of 6 myelinated polymodal nociceptors, it was found in a majority (10/16) of myelinated mechanical nociceptors. Thus, whereas the in vivo role of TRPV1 as a heat-sensitive channel in cutaneous sensory neurons is clearly defined, the role of TRPV2 in cutaneous neurons remains unknown. These results also suggest that TRPV1 may be essential for heat transduction in a specific subset of mechanically insensitive cutaneous nociceptors and that this subset may constitute a discrete heat input pathway for inflammation-induced thermal pain.

PERSPECTIVE

The distinct subset of murine cutaneous nociceptors containing TRPV1 has many attributes in common with mechanically insensitive C-fibers in humans that are believed to play a role in pathological pain states. Therefore, these murine fibers provide a clinically relevant animal model for further study of this group of cutaneous nociceptors.

Sciatic nerve block with resiniferatoxin: an electron microscopic study of unmyelinated fibers in the rat

BACKGROUND

Perineural administration of the naturally occurring vanilloids (capsaicin, resiniferatoxin [RTX]) produces selective nociceptive blockade. Studies using perineural vanilloids in high concentrations suggest that they can cause a degeneration of unmyelinated fibers. However, electron microscopic studies of local vanilloid toxicity produced conflicting outcomes. In the present study, we sought to determine whether RTX-induced reversible sciatic nerve block results in the degenerative changes of unmyelinated fibers.

METHODS

In rat experiments, RTX was administered percutaneously at the sciatic nerve. The effect of RTX was monitored by measuring the rat's response to noxious heat. The sciatic nerves were removed 48 h after the blockade initiation. Quantitative electron microscopic evaluation of the unmyelinated fibers was performed in three groups of animals: RTX 0.0001% (0.1 microg), RTX 0.001% (1 microg), and control (RTX vehicle, 0.1 mL).

RESULTS

Cross-sections of the sciatic nerve 48 h after the initiation of RTX-induced reversible nerve blockade appeared essentially normal. One rarely observed finding was the irregularly compacted membranous deposits in the unmyelinated axons. The frequency of this finding was approximately one per thousand fibers with both concentrations of RTX.

CONCLUSIONS

The results of the study suggest that a selective and long-lasting sciatic nerve block (up to 2 wk) can be provided by RTX without any significant damage to the unmyelinated nerve fibers.

Myelinated nerve endings in human skin

We used immunohistochemical techniques and confocal microscopy to study the morphometry of myelinated nerve endings in glabrous and hairy skin. A total of 30 healthy volunteers took part in this study designed to assess the possibility of obtaining reliable information on myelinated fibers using samples of hairy skin and to determine whether differences exist between myelinated terminations from different sites. We obtained consistent information on cutaneous myelinated terminations using hairy as well as glabrous skin samples. Myelinated endings from hairy and glabrous skin differ in density and distribution. However, from a comparison of our findings with data from nerve biopsy studies, we conclude that all cutaneous myelinated terminations are thinner terminal branches of large myelinated A beta fibers, whereas cutaneous terminations of small myelinated A delta fibers lose their myelin before entering the dermis and become indistinguishable from C-fiber terminations. The classic criteria, based on fiber size, used to distinguish myelinated fiber subgroups in sensory nerves are therefore not suitable for identifying myelinated terminations in the skin.

Potassium- and capsaicin-induced release of agmatine from spinal nerve terminals

Agmatine (decarboxylated arginine) was originally identified in the CNS as an imidazoline receptor ligand. Further studies demonstrated that agmatine antagonizes NMDA receptors and inhibits nitric oxide synthase. Intrathecally administered agmatine inhibits opioid tolerance and hyperalgesia evoked by inflammation, nerve injury, and intrathecally administered NMDA. These actions suggest an anti-glutamatergic role for agmatine in the spinal cord. We have previously reported that radiolabeled agmatine is transported into spinal synaptosomes in an energy- and temperature-dependent manner. In the present study, we demonstrate that agmatine is releasable from purified spinal nerve terminals upon depolarization. When exposed to either elevated potassium or capsaicin, tritiated agmatine (but not its precursor L-arginine or its metabolite putrescine) is released in a calcium-dependent manner. Control experiments confirmed that the observed release was specific to depolarization and not due to permeabilization of or degradation of synaptosomes. That capsaicin-evoked stimulation results in agmatine release implicates the participation of primary afferent nerve terminals. Radiolabeled agmatine also accumulates in purified spinal synaptosomal vesicles in a temperature-dependent manner, suggesting that the source of releasable agmatine may be vesicular in origin. These results support the proposal that agmatine may serve as a spinal neuromodulator involved in pain processing.

Characteristics of Sensory Dorsal Root Ganglia Neurons Innervating the Lumbar Vertebral Body in Rats

Characteristics of sensory dorsal root ganglia (DRG) neurons innervating the L5 vertebral body were investigated in rats by using a retrograde neurotransport method, lectin affinity- and immuno-histochemistry to further elucidate the causes of diffuse pain suffered by some elderly patients in their back, lateral trunk, and iliac crest, after lumbar osteoporotic vertebral fracture. We used calcitonin gene-related peptide (CGRP) as a marker of small peptide-containing neurons and the glycoprotein binding the isolectin from Griffonia simplicifolia (IB4) as a marker of small non-peptide-containing neurons. Neurons innervating the L5 vertebral bodies, retrogradely labeled with fluoro-gold (FG), were distributed throughout DRGs from T13 to L6. The proportion of CGRP-immunoreactive (IR) FG-labeled neurons was 32%. The proportion of IB4-binding FG-labeled neurons was significantly smaller, at 4%. Other neurons that were non-CGRP-IR and non-IB4-binding were mostly large neurons, and they may transmit proprioception from vertebral bodies. Most neurons transmitting pain are CGRP-IR peptide-containing neurons. They may have a more significant role in pain sensation in the vertebral bodies as peptidergic DRG neurons.

PERSPECTIVE

This article shows that vertebral bodies are innervated by CGRP-IR neurons. CGRP-IR neurons may play a role in pain sensation through peptidergic DRG neurons. These findings contribute to an understanding of pain associated with the vertebral body such as tumor, infection, or osteoporotic fracture.

A novel nerve bundle containing thin muscle fibers in the posterior cricoarytenoid muscle of the marmoset

We examined a novel nerve bundle in the posterior cricoarytenoid muscle of the marmoset. This intramuscular nerve bundle contained two thin muscle fibers about 10 microm in diameter, like intrafusal muscle fibers in the muscle spindle. These thin muscle fibers were individually surrounded by nerve bundles consisting of numerous nonmyelinated nerve fibers. Individual nerve axons contained clear synaptic vesicles and large granulated vesicles, being possibly cholinergic (parasympathetic) in nature. These nerve axons were often in contact with the muscle fiber with and without an interposing basal lamina. Two thin muscle fibers gradually terminated in the endoneural connective tissue around myelinated and nonmyelinated nerve fibers during their course. The innervation of thin muscle fibers in the novel nerve bundle is briefly discussed.

Anatomy of the deep fascia of the upper limb. Second part: study of innervation

Analysis of specimens taken from different areas of the deep fascia in 20 upper limbs was made in order to establish which kind of nerve fibres and endings are present in the deep muscular fascia. The flexor retinaculum and the lacertus fibrosus were also evaluated because they are anatomically hardly separable from the deep muscular fascia, although they have different functions. In particular, specimens were taken at the level of: (a) the expansion of pectoralis major onto the bicipital fascia, (b) the middle third of the brachial fascia, (c) the lacertus fibrosus, (d) the middle third of the antebrachial fascia, (e) the flexor retinaculum. This study demonstrated an abundant innervation of the fascia consisting in both free nerve endings and encapsulated receptors, in particular, Ruffini and Pacini corpuscles. However, differences in innervation were verified: the flexor retinaculum was resulted the more innervated element whilst lacertus fibrosus and the pectoralis major expansion the less innervated. These results suggest that the retinaculum has more a perceptive function whereas the tendinous expansions onto the fascia have mostly a mechanical role in the transmission of tension. The hypothesis that the fascia plays an important role in proprioception, especially dynamic proprioception, is therefore advanced. In fact, the fascia is a membrane that extends throughout the whole body and numerous muscular expansions maintain it in a basal tension. During a muscular contraction these expansions could also transmit the effect of the stretch to a specific area of the fascia, stimulating the proprioceptors in that area.

Vesicular release of glutamate from unmyelinated axons in white matter

Directed fusion of transmitter-laden vesicles enables rapid intercellular signaling in the central nervous system and occurs at synapses within gray matter. Here we show that action potentials also induce the release of glutamate from axons in the corpus callosum, a white matter region responsible for interhemispheric communication. Callosal axons release glutamate by vesicular fusion, which induces quantal AMPA receptor-mediated currents in NG2(+) glial progenitors at anatomically distinct axo-glial synaptic junctions. Glutamate release from axons was facilitated by repetitive stimulation and could be inhibited through activation of metabotropic autoreceptors. Although NG2(+) cells form associations with nodes of Ranvier in white matter, measurements of conduction velocity indicated that unmyelinated fibers are responsible for glutamatergic signaling with NG2(+) glia. This activity-dependent secretion of glutamate was prevalent in the developing and mature mouse corpus callosum, indicating that axons within white matter both conduct action potentials and engage in rapid neuron-glia communication.

Expression of hyperpolarization-activated cyclic nucleotide-gated cation channels in rat dorsal root ganglion neurons innervating urinary bladder

Afferent pathways innervating the urinary bladder consist of myelinated Adelta- and unmyelinated C-fibers, the neuronal cell bodies of which correspond to medium and small-sized cell populations of dorsal root ganglion (DRG) neurons, respectively. Since hyperpolarization-activated cyclic nucleotide-gated cation (HCN) channel currents have been identified in various peripheral sensory neurons, we examined the expression of isoforms of HCN channels in the L6-S1 spinal cord and bladder afferent neurons from L6-S1 DRG in rats. Among HCN-1, HCN-2 and HCN-4 channel subtypes, positive staining with HCN-2 antibodies was found in the superficial dorsal horn of the spinal cord and small- and medium-sized unidentified DRG neurons. In dye-labeled bladder afferent neurons, HCN-2-positive cells were found in approximately 60% of neurons, and HCN-2 was expressed in both small- and medium-sized neurons with a higher ratio (expression ratio: 61% and 50% of neurons, respectively) compared with unidentified DRG neurons, in which the HCN expression ratio was 47% and 21% of small- and medium-sized cells, respectively. These results suggest that HCN-2 is the predominant subtype of HCN channels, which can control neuronal excitability, in small-sized C-fiber and medium-sized Adelta fiber DRG neurons including bladder afferent neurons, and might modulate activity of bladder afferent pathways controlling the micturition reflex.

Ultrastructural anatomy of the renal nerves in rats

The innervation within mammalian kidneys (intrinsic innervation) has been extensively described in the literature, particularly for rats. In contrast, there is still a lack of detailed description of the morphology of the extrinsic renal nerves leading to the kidney. The aim of the present study was to describe, in detail, the morphology of the renal nerves in rats. Left renal nerves were evaluated in 6 normal adult Wistar rats. After nerve recordings, in order to ascertain that the nerves studied were the extrinsic renal nerves, rats were killed and the nerves prepared for transmission electron microscopy. Morphometry was carried out with the aid of computer software. The total numbers of myelinated and unmyelinated fibers were 22+/-6 and 1246+/-110, respectively, with a ratio of unmyelinated/myelinated fiber of 109+/-26. The diameters of myelinated fibers showed an unimodal distribution with a peak at 3.0 microm but more than 17% of the fibers showed diameters larger than 5 microm. Unmyelinated fiber distribution was unimodal, with peak between 0.5 and 0.7 microm. The present study adds new information on the morphology of renal nerves in rats and provides morphological basis for further studies involving the structural basis of altered renal responses in conditions such as hypertension, ageing, diabetes and peripheral neuropathies.

Absence of large-diameter sensory fibres in a nerve to the cat humerus

A fine branch of the median nerve innervates the periosteum and medullary cavity of the cat humerus. After branching to innervate the periosteum on the medial surface of the humerus, the nerve enters and supplies the medullary cavity via a nutrient foramen, accompanied by a small artery and vein. The composition of the fibres in the nerve was examined using electron microscopy. Myelinated fibres with diameters of 0.8-6.6 microm and unmyelinated fibres with diameters of 0.1-1.4 microm were observed. These diameters indicate that afferent fibres of this nerve are confined within the Group III and IV categories, and may therefore be nociceptive or mechanoreceptive in function. In addition, autonomic efferent fibres may also be present in these fibre groups. As no fibre diameters greater than 7 microm were noted, it appears that Group I and II fibres are absent in this nerve. The fibre distribution suggests that the principal role of this nerve is to relay bone-related nociceptive or mechanoreceptive information to the central nervous system and to provide autonomic regulatory influences on the bone.

Characterization of sensory neuron subpopulations selectively expressing green fluorescent protein in phosphodiesterase 1C BAC transgenic mice

BACKGROUND

The complex neuronal circuitry of the dorsal horn of the spinal cord is as yet poorly understood. However, defining the circuits underlying the transmission of information from primary afferents to higher levels is critical to our understanding of sensory processing. In this study, we have examined phosphodiesterase 1C (Pde1c) BAC transgenic mice in which a green fluorescent protein (GFP) reporter gene reflects Pde1c expression in sensory neuron subpopulations in the dorsal root ganglia and spinal cord.

RESULTS

Using double labeling immunofluorescence, we demonstrate GFP expression in specific subpopulations of primary sensory neurons and a distinct neuronal expression pattern within the spinal cord dorsal horn. In the dorsal root ganglia, their distribution is restricted to those subpopulations of primary sensory neurons that give rise to unmyelinated C fibers (neurofilament 200 negative). A small proportion of both non-peptidergic (IB4-binding) and peptidergic (CGRP immunoreactive) subclasses expressed GFP. However, GFP expression was more common in the non-peptidergic than the peptidergic subclass. GFP was also expressed in a subpopulation of the primary sensory neurons immunoreactive for the vanilloid receptor TRPV1 and the ATP-gated ion channel P2X3. In the spinal cord dorsal horn, GFP positive neurons were largely restricted to lamina I and to a lesser extent lamina II, but surprisingly did not coexpress markers for key neuronal populations present in the superficial dorsal horn.

CONCLUSION

The expression of GFP in subclasses of nociceptors and also in dorsal horn regions densely innervated by nociceptors suggests that Pde1c marks a unique subpopulation of nociceptive sensory neurons.

Disrupted Schwann cell-axon interactions in peripheral nerves of mice with altered L1-integrin interactions

The cell adhesion molecule L1 is important for peripheral nerve development. Mice lacking the 6th Ig domain of L1 (L1-6D mice) lose L1 homophilic binding and RGD dependent LI-integrin binding [Itoh,K., Cheng, L., Kamei, Y., Fushiki, S., Kamiguchi, H., Gutwein, P.,Stoeck, A., Arnold, B., Altevogt, P., Lemmon, V., 2004. Brain development in mice lacking Li-L homophilic adhesion. J. Cell Biol.165, 145-154]. We examined the ultrastructure of sciatic nerves from L1-6D at postnatal day 7 and 8 weeks. Unmyelinated axons frequently detached at the edge of Schwann cells, and naked axons were observed. Myelin was thinner in L1-6D and abnormal, multiple axons wrapped in a single myelin sheath were routinely observed. Previous work has shown that L1 on axons interacts with a heterophilic binding partner on Schwann cells to facilitate normal peripheral nerve formation. Taken together, it is likely that L1 on axons binds integrins on Schwann cells, resulting in interactions between axons and Schwann cells that are essential for ensheathment and myelination.

Glial cell-line-derived neurotrophic factor expression in skin alters the mechanical sensitivity of cutaneous nociceptors

Neurons classified as nociceptors are dependent on nerve growth factor (NGF) during embryonic development, but a large subpopulation lose this dependence during embryonic and postnatal times and become responsive to the transforming growth factor beta family member, glial cell line-derived growth factor (GDNF). To elucidate the functional properties of GDNF-dependent nociceptors and distinguish them from nociceptors that retain NGF dependence, the cellular and physiologic properties of sensory neurons of wild-type and transgenic mice that overexpress GDNF in the skin (GDNF-OE) were analyzed using a skin, nerve, dorsal root ganglion, and spinal cord preparation, immunolabeling, and reverse transcriptase-PCR assays. Although an increase in peripheral conduction velocity of C-fibers in GDNF-OE mice was measured, other electrophysiological properties, including resting membrane potential and somal action potentials, were unchanged. We also show that isolectin B4 (IB4)-positive neurons, many of which are responsive to GDNF, exhibited significantly lower thresholds to mechanical stimulation relative to wild-type neurons. However, no change was observed in heat thresholds for the same population of cells. The increase in mechanical sensitivity was found to correlate with significant increases in acid-sensing ion channels 2A and 2B and transient receptor potential channel A1, which are thought to contribute to detection of mechanical stimuli. These data indicate that enhanced expression of GDNF in the skin can change mechanical sensitivity of IB4-positive nociceptive afferents and that this may occur through enhanced expression of specific types of channel proteins.

Development of the corticospinal tract in the mouse spinal cord: A quantitative ultrastructural analysis

The growth of corticospinal tract (CST) axons was studied quantitatively at the 7th cervical (C7) and the 4th lumbar (L4) spinal segments in the balb/cByJ mice at the ages of postnatal day (P) 0, 2, 4, 6, 8, 10, 14, and 28. The cross-sectional area of the CST increased progressively with time. Unmyelinated axons, the most prominent CST element during early development, reached maximum at C7 and L4 on P14. Two phases of increase in the number of unmyelinated axons were observed at C7, while only one surge of axonal outgrowth was found at the L4 level. Pro-myelinated axons, defined as axons surrounded by only one layer of oligodendrocytic process, were first seen at P2 and P4 in the C7 and the L4 level, respectively, followed by a dramatic increase in the number of myelinated axons from P14 onwards at both spinal levels. Myelination of the CST axons occurred topographically in a dorsal-to-ventral pattern. The number of growth cones increased rapidly at the C7 level to reach its maximum at P4, while those at L4 increased steadily to the peak at P10. Growth cones with synapse-like junctions were occasionally observed in the growing CST. Degenerating axons and growth cones partly accounted for the massive axon loss at both spinal segments during CST development. Overall, the mouse CST elements changed dynamically in numbers during postnatal development, suggesting a vigorous growing and pruning activity in the tract. The mouse CST also showed a similar growth pattern to that of the rat CST.

Studies of peripheral sensory nerves in paclitaxel-induced painful peripheral neuropathy: evidence for mitochondrial dysfunction

Paclitaxel chemotherapy frequently induces neuropathic pain during and often persisting after therapy. The mechanisms responsible for this pain are unknown. Using a rat model of paclitaxel-induced painful peripheral neuropathy, we have performed studies to search for peripheral nerve pathology. Paclitaxel-induced mechano-allodynia and mechano-hyperalgesia were evident after a short delay, peaked at day 27 and finally resolved on day 155. Paclitaxel- and vehicle-treated rats were perfused on days 7, 27 and 160. Portions of saphenous nerves were processed for electron microscopy. There was no evidence of paclitaxel-induced degeneration or regeneration as myelin structure was normal and the number/density of myelinated axons and C-fibres was unaltered by paclitaxel treatment at any time point. In addition, the prevalence of ATF3-positive dorsal root ganglia cells was normal in paclitaxel-treated animals. With one exception, at day 160 in myelinated axons, total microtubule densities were also unaffected by paclitaxel both in C-fibres and myelinated axons. C-fibres were significantly swollen following paclitaxel at days 7 and 27 compared to vehicle. The most striking finding was significant increases in the prevalence of atypical (swollen and vacuolated) mitochondria in both C-fibres (1.6- to 2.3-fold) and myelinated axons (2.4- to 2.6-fold) of paclitaxel-treated nerves at days 7 and 27. Comparable to the pain behaviour, these mitochondrial changes had resolved by day 160. Our data do not support a causal role for axonal degeneration or dysfunction of axonal microtubules in paclitaxel-induced pain. Instead, our data suggest that a paclitaxel-induced abnormality in axonal mitochondria of sensory nerves contributes to paclitaxel-induced pain.

Kappa opioid receptors in the rostral ventromedial medulla of male and female rats

Kappa opioid receptor (KOR) ligands alter nociceptive responses when applied to the rostral ventromedial medulla (RVM). However, the effects of kappa opioid receptor ligands are distinct in males and females. The present study examined the distribution of kappa opioid receptor immunoreactivity in the RVM of male and female rats. KOR immunoreactivity was found at pre- and postsynaptic sites within the RVM of both sexes. The most common KOR-immunoreactive (KOR-ir) neuronal structures were unmyelinated axons, followed by axon terminals, dendrites, and somata. Different proportions of KOR-ir axon terminals and dendrites were found in females at different estrous stages. Specifically, dendrites containing KOR immunoreactivity were less abundant in proestrus females compared with estrus females and showed a trend toward being less abundant in males, suggesting that KOR ligands applied to the RVM may be less potent in proestrus females. These findings suggest that the distribution of KORs in the RVM may be influenced by reproductive hormone levels. We also found KOR immunoreactivity in many spinally projecting neurons within the RVM of female rats. These findings are consistent with the hypothesis that KOR ligands influence nociceptive behaviors by altering the activity of specific populations of neurons within the RVM. The abundance of KOR in axons and axon terminals in RVM indicates a substantial role for presynaptic effects of KOR ligands through pathways that have not been clearly delineated. Altering the balance between pre- and postsynaptic receptive sites may underlie differences in the effects of KOR agonists on nociceptive responses in males and females.

Disrupted Schwann cell-axon interactions in peripheral nerves of mice with altered L1-integrin interactions

The cell adhesion molecule L1 is important for peripheral nerve development. Mice lacking the 6th Ig domain of L1 (L1-6D mice) lose L1 homophilic binding and RGD dependent LI-integrin binding [Itoh,K., Cheng, L., Kamei, Y., Fushiki, S., Kamiguchi, H., Gutwein, P.,Stoeck, A., Arnold, B., Altevogt, P., Lemmon, V., 2004. Brain development in mice lacking Li-L homophilic adhesion. J. Cell Biol.165, 145-154]. We examined the ultrastructure of sciatic nerves from L1-6D at postnatal day 7 and 8 weeks. Unmyelinated axons frequently detached at the edge of Schwann cells, and naked axons were observed. Myelin was thinner in L1-6D and abnormal, multiple axons wrapped in a single myelin sheath were routinely observed. Previous work has shown that L1 on axons interacts with a heterophilic binding partner on Schwann cells to facilitate normal peripheral nerve formation. Taken together, it is likely that L1 on axons binds integrins on Schwann cells, resulting in interactions between axons and Schwann cells that are essential for ensheathment and myelination.

Myelinated and unmyelinated axons of the corpus callosum differ in vulnerability and functional recovery following traumatic brain injury

Traumatic axonal injury (TAI), a common feature of traumatic brain injury, is associated with postinjury morbidity and mortality. However, TAI is not uniformly expressed in all axonal populations, with fiber caliber and anatomical location influencing specific TAI pathology. To study differential axonal vulnerability to brain injury, axonal excitability and integrity were assessed in the corpus callosum following fluid percussion injury in the rat. In brain slice electrophysiological recordings, compound action potentials (CAPs) were evoked in the corpus callosum, and injury effects were quantified separately for CAP waveform components generated by myelinated axons (N1 wave) and by unmyelinated axons (N2 wave). Ultrastructural analyses were also conducted of TAI-induced morphological changes in these axonal populations. The two populations of axons differed in response to brain injury, and in their functional recovery, during the first week postinjury. Amplitudes of N1 and N2 were significantly depressed at 3 h, 1 day, and 3 days survival. N1 amplitudes exhibited a recovery to control levels by 7 days postinjury. In contrast, N2 amplitudes were persistently suppressed through 7 days postinjury. Strength-duration properties of evoked CAPs further differentiated the effects of injury in these axonal populations, with N2 exhibiting an elevated strength-duration time constant postinjury. Ultrastructural observations revealed degeneration of myelinated axons consistent with diffuse injury sequelae, as well as previously undocumented pathology within the unmyelinated fiber population. Collectively, these findings demonstrate differential vulnerabilities of axons to brain injury and suggest that damage to unmyelinated fibers may play a significant role in morbidity associated with brain injury.

Unmyelinated fiber sensory neuropathy differs in type 1 and type 2 diabetes

BACKGROUND

Neuropathic pain is common in diabetic patients. Degeneration of sensory C-fibers in peripheral nerve plays a prominent role in the generation of neuropathic pain. We examined degenerative changes of C-fibers in two rat models with type 1 and type 2 diabetes.

METHODS

Type 1 insulinopenic BB/Wor and type 2 hyperinsulinemic diabetic BBZDR/Wor-rats of 8 months duration with equal exposure to hyperglycemia were examined. Thermal hyperalgesia was monitored using an infrared thermal probe. C-fiber size, number, frequencies of denervated Schwann cells, regenerating C-fibers, type 2 axon/Schwann cell relationship and collagen pockets in the sural nerve were examined morphometrically. Neurotrophic receptor expression was examined by Western blotting. Neurotrophins and neuropeptides were examined by ELISA.

RESULTS

Type 1 rats showed increased thermal hyperalgesia followed by a decrease. Hyperalgesia in type 2 rats showed a slower progression. These findings were associated with a 50% (p < 0.001) loss of C-fibers, increased frequencies of denervated Schwann cells (p < 0.001), regenerating fibers (p < 0.001), collagen pockets (p < 0.001) and type 2 axon/Schwann cell relationship (p < 0.001) in type 1, but not in type 2 rats. Expression of insulin receptor, IGF-1R, TrkA and C was decreased in BB/Wor rats, whereas BBZDR/Wor rats showed milder or no deficits. NGF and NT-3 in sciatic nerve and substance P and calcitonin gene-related peptide in dorsal root ganglia were decreased in type 1, but not in type 2 rats.

CONCLUSION

The more severe molecular, functional and morphometric abnormalities of nociceptive C-fibers in type 1 insulinopenic rats compared to type 2 hyperinsulinemic rats suggest that impaired insulin action may play a more important pathogenetic role than hyperglycemia per se.

Ablation of primary afferent terminals reduces nicotinic receptor expression and the nociceptive responses to nicotinic agonists in the spinal cord

A variety of studies indicate that spinal nicotinic acetylcholine receptors modulate the behavioral and autonomic responses elicited by afferent stimuli. To examine the location of and role played by particular subtypes of nicotinic receptors in mediating cardiovascular and nociceptive responses, we treated neonatal and adult rats with capsaicin to destroy C-fibers in primary afferent terminals. Reduction of C-fiber terminals was ascertained by the loss of isolectin B4, CGRP and vanilloid receptors as monitored by immunofluorescence. Receptor autoradiography shows a reduction in number of epibatidine binding sites following capsaicin treatment. The reduction is particularly marked in the dorsal horn and primarily affects the class of high affinity epibatidine binding sites thought to modulate nociceptive responses. Accompanying the loss of terminals and nicotinic binding sites were significant reductions in the expression of alpha 3, alpha 4, alpha 5, beta 2 and beta 4 nicotinic receptor subunits in the superficial layers of the spinal cord as determined by antibody staining and confocal microscopy. The loss of nicotinic receptors that follows capsaicin treatment results in attenuation of the nociceptive responses to both spinal cytisine and epibatidine. Capsaicin treatment also diminishes the capacity of cytisine to desensitize nicotinic receptors mediating nociception, but it shows little effect on intrathecal nicotinic agonist elicited pressor and heart rate responses. Hence, our data suggest that alpha 3, alpha 4, alpha 5, beta 2 and beta 4 subunits of nicotinic receptors are localized in the spinal cord on primary afferent terminals that mediate nociceptive input. A variety of convergent data based on functional studies and subunit expression suggest that alpha 3 and alpha 4, in combination with beta 2 and alpha 5 subunits, form the majority of functional nicotinic receptors on C-fiber primary afferent terminals. Conversely, spinal nicotinic receptors not located on C-fibers play a primary role in the spinal pathways evoking spinally coordinated autonomic cardiovascular responses.

Ultrastructural Identification of a Sympathetic Component in the Hypoglossal Nerve of Hamsters Using Experimental Degeneration and Horseradish Peroxidase Methods

We employed experimental degeneration, tract-tracing with wheatgerm agglutinin conjugated with horseradish peroxidase (WGA-HRP) and electron microscopy to explore the postganglionic sympathetic fibers in the hypoglossal nerve of hamsters. Quantitative results of normal untreated animals at the electron microscopic level showed the existence of unmyelinated fibers, which made up about 20% of the total fibers in the nerve, being more numerous on the left side. The nerve fibers were preferentially distributed at the periphery of the nerve. Following superior cervical ganglionectomy, most of the unmyelinated fibers underwent degenerative changes. Tract-tracing studies showed that some of the unmyelinated fibers were labeled by WGA-HRP injected into the superior cervical ganglion (SCG). It is suggested that the unmyelinated fibers represent the postganglionic sympathetic fibers originated from the SCG.

Nerve cells and synapses in the human foetal hypogastric nerves

The hypogastric nerves of a human foetus of 220 mm C-R length (23(rd) week) were investigated with an electron microscope. These nerves were composed mainly of bundles of unmyelinated fibres and single myelinated fibres. Small ganglia and single ganglion cells were observed in the hypogastric nerves. Light and dark cells were found among the nerve cells. The two types of cell differed in the number of ribosomes and the amount rough endoplasmic reticulum. In the period of development investigated protosynapses and mature synapses were observed in the hypogastric nerves.

Ionotropic glutamate receptors are expressed in GABAergic terminals in the rat superficial dorsal horn

Ionotropic glutamate receptors (IGR), including NMDA, AMPA, and kainate receptors, are expressed in terminals with varied morphology in the superficial laminae (I-III) of the dorsal horn of the spinal cord. Some of these terminals can be identified as endings of primary afferents, whereas others establish symmetric synapses, suggesting that they may be gamma-aminobutyric acid (GABA)-ergic. In the present study, we used confocal and electron microscopy of double immunostaining for GAD65, a marker for GABAergic terminals, and for subunits of IGRs to test directly whether IGRs are expressed in GABAergic terminals in laminae I-III of the dorsal horn. Although colocalization is hard to detect with confocal microscopy, electron microscopy reveals a substantial number of terminals immunoreactive for GAD65 also stained for IGRs. Among all GAD65-immunoreactive terminals counted, 37% express the NMDA receptor subunit NR1; 28% are immunopositive using an antibody for the GluR2/4 subunits of the AMPA receptor; and 20-35% are immunopositive using antibodies for the kainate receptor subunits GluR5, GluR6/7, KA1, or KA2. Terminals immunoreactive for IGR subunits and GAD65 establish symmetric synapses onto dendrites and perikarya and can be presynaptic to primary afferent terminals within both type 1 and type 2 synaptic glomeruli. Activation of presynaptic IGR may reduce neurotransmitter release. As autoreceptors in terminals of Adelta and C afferent fibers in laminae I-III, presynaptic IGRs may play a role in inhibiting nociception. As heteroreceptors in GABAergic terminals in the same laminae, on the other hand, presynaptic IGRs may have an opposite role and even contribute to central sensitization and hyperalgesia.

Speed limits in the cerebellum: constraints from myelinated and unmyelinated parallel fibers

Cerebellar parallel fibers are among the thinnest known vertebrate axons and represent an extreme anatomical adaptation. Until now a systematic examination of their properties across species has not been carried out. We used transmission electron microscopy and light microscopy to compare parallel fibers in mammals of different brain sizes. From mouse to macaque, the average unmyelinated parallel fiber diameter was 0.2-0.3 microm, consistent with the idea that they are evolutionarily selected for compactness. Average unmyelinated parallel fiber diameter scaled up slightly with brain size, and across species the estimated total conduction time is 5-10 ms. However, these conduction times can vary by milliseconds, and unmyelinated PFs consume large amounts of energy per action potential. These functional disadvantages are overcome in myelinated parallel fibers, which we found in the deep regions nearest the Purkinje cell layer in marmoset, cat and macaque. These axons were 0.4-1.1 microm wide, have expected conduction times of 0.5-1.0 ms, and may convey fast feedforward inhibition via basket cells to Purkinje cells.

Innervation of the Female Human Urethral Sphincter: 3D Reconstruction of Immunohistochemical Studies in the Fetus

OBJECTIVES

The precise location, origin and nature of nerve fibers innervating the urethral sphincter have not been clearly established. Classical anatomical studies based on cadaver dissections have provided conflicting results concerning the location of pudendal and autonomic nerve fibers. This study was designed to identify nerve fibers innervating the urethral sphincter and to provide a three-dimensional representation of their tissue relations in the female human fetus.

MATERIALS AND METHODS

Histology and immunohistochemistry (Masson's Trichromic, Luxol Fast Blue, Protein S 100 immunostaining and smooth fiber actin immunostaining) were performed on the external urethral sphincter of ten female fetuses with a crown-rump length of 112 to 340mm. Three-dimensional reconstructions of the urethral structure and innervation were obtained from serial sections using Surf Driver 3.5.3 software (David Moody and Scott Lozanoff).

RESULTS

Three-dimensional reconstructions of the same sections with different stains demonstrated the precise structure of the muscle layers (smooth and striated muscle fibers) and nerve fibers (myelinated and unmyelinated) and their relations with the urethra and vaginal wall. The proximal third consisted of a circular smooth muscle sphincter, the middle third consisted of two circular layers of smooth and striated muscle fibers and the distal third consisted of a circular layer of smooth muscle fibers surrounded by an omega-shaped layer of striated muscle fibers. In the proximal third of the urethral sphincter, myelinated fibers were identified running with unmyelinated fibers from the pelvic plexus. These fibers were closely related to the lateral and anterior aspects of the vagina. Unmyelinated fibers entered the smooth muscle part of the sphincter at 4 o'clock and at 8 o'clock. Most myelinated fibers entered the sphincter at 3 o'clock and at 9 o'clock.

CONCLUSION

Histological and immunohistochemical three-dimensional reconstruction of the anatomical structures of the urethral sphincter provides a better understanding of the origin and nature of the Innervation participating in urinary continence. It provides a very informative view of the three-dimensional arrangement of sphincter muscle layers.

Spaceflight alters the fiber composition of the aortic nerve in the developing rat

Hydrostatic pressure gradients due to the gravitational force in blood vessels disappear under conditions of microgravity during spaceflight, and the ability of the baroreceptor reflex to control arterial pressure and blood distribution may be altered. We hypothesized, on the basis of the results obtained in our previous experiments using the head-down tilt method in rats and rabbits, that the range of increase in arterial pressure caused by animal behavior narrows under conditions of microgravity, affecting the development of high-threshold unmyelinated fibers in the rat aortic nerve which sends signals from baroreceptors located in the aortic wall to the reflex center. We verified this hypothesis using 9-day-old rat neonates housed with their dams for 16 days on the space shuttle Columbia in outer space (STS-90, Neurolab Mission). Age-matched neonatal rats with the dams remained on the ground as controls. After breeding was carried out in the three experimental groups (FLT, spaceflight; AGC, asynchronous ground control; VIV, vivarium ground control), specimens of the 25-day-old rats were excised and five left aortic nerves in each group were examined by electron microscopy. The number of aortic unmyelinated fibers was significantly less in the FLT group than in each ground control (mean+/-S.D.; 139+/-37 in the FLT, 207+/-36 in the AGC, 283+/-121 in the VIV; P<0.05), which may be related to the weakness of the baroreceptor reflex under conditions of microgravity in space. This result may contribute to understanding of the several cardiovascular issues which occur under microgravity and after reexposure to gravity in human.

C-fiber (Remak) bundles contain both isolectin B4-binding and calcitonin gene-related peptide-positive axons

Unmyelinated nerve fibers (Remak bundles) in the rodent sciatic nerve typically contain multiple axons. This study asked whether C-fiber bundles contain axons arising from more than one type of neuron. Most small neurons of the lumbar dorsal root ganglion (DRG) are either glial cell line-derived neurotrophic factor dependent or nerve growth factor dependent, binding either isolectin B4 (IB4) or antibodies to calcitonin gene-related peptide (CGRP), respectively. Injection of IB4-conjugated horseradish peroxidase into a lumbar DRG resulted in intense labeling of IB4 axons, with very low background. Visualized by confocal fluorescence, IB4-binding and CGRP-positive nerve fibers originating from different DRG neurons came together and remained closely parallel over long distances, suggesting that these two types of axon occupy the same Remak bundle. With double-labeling immunogold electron microscopy (EM), we confirmed that IB4 and CGRP axons were distinct and were found together in single Remak bundles. Previous studies indicate that some DRG neurons express both CGRP and IB4 binding. To ensure that our immunogold results were not a consequence of coexpression, we studied large populations of unmyelinated axons by using quantitative single-label EM. Tetramethylbenzidine, a chromogen with strong intrinsic signal amplification of IB4-horseradish peroxidase, labeled as many as 52% of unmyelinated axons in the dorsal root. Concomitantly, 97% of the Remak bundles with more than one axon contained at least one IB4-labeled axon. Probabilistic modeling using binomial distribution functions rejected the hypothesis that IB4 axons segregate into IB4-specific bundles (P < 0.00001). We conclude that most Remak bundle Schwann cells simultaneously support diverse axon types with different growth factor dependences.

Close apposition and exposure of non-myelinated axons in traumatic neuromas of the human lingual nerve

Peripheral nerve injury is sometimes followed by the development of persistent painful sensory disorders, such as dysaesthesia. The aetiology of these disorders is not clear, but abnormal behaviour of damaged axons at the injury site is likely to be involved. In this study, we quantified some ultrastructural characteristics that may be related to the development of abnormal spontaneous activity, sympathetic interactions, and fibre-to-fibre crosstalk. Using electron microscopy, we have determined the frequency and extent of axonal exposure and close apposition among non-myelinated axons from 34 traumatic neuromas of the human lingual nerve. These specimens were removed at the time of microsurgical nerve repair, and the presence or absence of symptoms of dysaesthesia was determined pre-operatively. Comparisons were also made with eight normal control lingual nerve specimens obtained from patients undergoing organ donor retrieval. More non-myelinated axons showed signs of axonal exposure in traumatic neuromas (26%) than in controls (5%), and exposure was higher in nerve-end neuromas (31%) than in neuromas-in-continuity (22%). In addition, the proportion of the non-myelinated axolemma that was exposed was significantly higher in neuromas (32%) than in controls (21%). The frequency of close apposition between neighbouring non-myelinated axons was also higher in neuromas (11%) than in controls (0.35%). The majority of axons showing signs of exposure or close apposition had diameters <1 microm. These ultrastructural changes may account for some of the altered electrophysiological properties of axons within neuromas. However, no significant correlations were found between these ultrastructural characteristics and the patients' reported symptoms of dysaesthesia.

Morphology of the long and short uveal nerves in the human eye

The aim of this study was to examine the architecture of the uveal nerves in the sclera and suprachoroid of human eyes. Eyes from 17 adult human donors were investigated. The uveal nerves in different regions (retrobulbar, intrascleral, suprachoroidal, pars plana) were prepared and studied by light and electron microscopy. In addition, immunohistochemistry was performed for various neuronal markers. The long uveal nerves showed a characteristic suprachoroidal location with no branches supplying the choroid. It was found that typically they are composed of myelinated (75%) and non-myelinated (25%) nerve fibres. They mainly contain aminergic and sensory nerve fibres. A separate set of cholinergic non-myelinated nerve fibre bundles runs parallel with these long uveal nerves. The short uveal nerves supply the suprachoroidal nerve plexus with approximately 13% of their nerve fibres. The nerves and the branches supplying the choroid appear as mixed nerves containing sympathetic, parasympathetic and sensory axons. This study therefore provides new information about the quantity, type and distribution of myelinated and non-myelinated nerve fibres in the posterior uvea of the human eye.

Regeneration of unmyelinated and myelinated sensory nerve fibres studied by a retrograde tracer method

Regeneration of myelinated and unmyelinated sensory nerve fibres after a crush lesion of the rat sciatic nerve was investigated by means of retrograde labelling. The advantage of this method is that the degree of regeneration is estimated on the basis of sensory somata rather than the number of axons. Axonal counts do not reflect the number of regenerated neurons because of axonal branching and because myelinated axons form unmyelinated sprouts. Two days to 10 weeks after crushing, the distal sural or peroneal nerves were cut and exposed to fluoro-dextran. Large and small dorsal root ganglion cells that had been labelled, i.e., that had regenerated axons towards or beyond the injection site, were counted in serial sections. Large and small neurons with presumably myelinated and unmyelinated axons, respectively, were classified by immunostaining for neurofilaments. The axonal growth rate was 3.7 mm/day with no obvious differences between myelinated and unmyelinated axons. This contrasted with previous claims of two to three times faster regeneration rates of unmyelinated as compared to myelinated fibres. The initial delay was 0.55 days. Fewer small neurons were labelled relative to large neurons after crush and regeneration than in controls, indicating that regeneration of small neurons was less complete than that of large ones. This contrasted with the fact that unmyelinated axons in the regenerated sural nerve after 74 days were only slightly reduced.

Ultrastructural analysis of the central terminals of primary sensory neurones labelled by transganglionic transport of bandeiraea simplicifolia I-isolectin B4

In this study the ultrastructural appearance of primary sensory neurones labelled by the injection of the plant lectin Bandeiraea simplicifolia I-isolectin B(4) (BSI-B(4)) into a peripheral nerve has been examined in the rat. Electron microscopy of the somata of retrogradely labelled neurones showed the lectin to be associated with the inner surface of cytoplasmic vesicles, supporting the premise that the uptake of BSI-B(4) into sensory neurones is by the process of receptor-mediated endocytosis. Light and electron microscopic analysis of the spinal cord revealed transganglionically transported lectin in unmyelinated axons in the dorsolateral funiculus and axon terminals concentrated mainly within lamina II of the dorsal horn. Detailed analysis of 1377 of these axon terminals revealed that the majority were glomerular in shape and surrounded by up to 14 other unlabelled profiles. These findings suggest that primary sensory neurones which transganglionically transport BSI-B(4) have a synaptic ultrastructure similar to that which has been previously reported for unmyelinated primary sensory neurones. Moreover, it appears that the axon terminals of these neurones are subjected to extensive modulation. Examination of the vesicle content of lectin labelled axon terminals revealed that the majority contained small agranular vesicles while large granular vesicles were observed only occasionally. These findings support the suggestion that the populations of neurones expressing binding sites for BSI-B(4) are fairly distinct from those containing neuroactive peptides. In conclusion, the results of the current study suggest that the lectin BSI-B(4) can be used as a histological marker for a subpopulation of small diameter primary sensory neurones and provide further evidence for the potential of this lectin as a useful tool in the study of pain.

A novel primary culture technique for adult retina allows for evaluation of CNS axon regeneration in rodents

Unraveling the causes of regeneration failure in the adult injured CNS has remained a challenge in neurobiology. The notion that CNS neurons lose their regenerative potential during development has been challenged by the identification of several promoters of axon growth. Novel methods are required that allow to study and quantify interactions of molecular determinants, and to envisage future treatment applications. Here we report a novel, highly reproducible method for monitoring axonal regeneration of mature retinal ganglion cells (RGCs) in vitro. In contrast to earlier explantation methods, primary cultures derived from adult rodent retina are kept viable without growth factor supplements. Further, since intraretinal RGC axons remain unmyelinated, regeneration can be followed independently of non-permissive white matter compounds. Applying tracing techniques prior to retinal explantation, cell survival can be correlated to outgrowth activity on the single cell level. Following intervention with pharmacological, growth factor, or gene transfer treatments, retinal explants, and partially RGC neurites, can be processed for protein and gene expression analysis. This novel procedure will prove useful to get insight into complex cell survival and regeneration promoting cascades, and will complement in vivo strategies such as transgenic and knock out mouse models.

Protein 4.1B localizes on unmyelinated axonal membranes in the mouse enteric nervous system

Recent molecular studies on anchoring structures between myelin sheaths by glial cells (oligodendrocytes and Schwann cells (Sc) in the central (CNS) and peripheral nervous system (PNS), respectively) and axons indicated protein-protein interaction for the polarization of paranodes in the axons. The protein 4.1 (4.1) family was originally found in erythrocytes as a component of membrane skeletons, and genetic approaches revealed the precise family members. One of them, 4.1B, has been reported to be localized in paranodes and juxtaparanodes of myelinated axons. In this study, in addition to the myelinated axons, we also present the localization of 4.1B in nerve fibers in the adult mouse enteric nervous system, a subpopulation of mature unmyelinated nerve fibers in PNS. Ultrastructurally, 4.1B localized along the membranes of unmyelinated axons. Such unmyelinated axons were surrounded only by Sc, suggesting that the 4.1B may also have a role in direct Sc-axon interactions and maturation of the axons, as well as myelinating glial cell-axon interactions.

The number, size, and type of axons in rat subcortical white matter on left and right sides: a stereological, ultrastructural study

Abundant evidence indicates important functional differences between the two cerebral hemispheres of humans, although the cellular basis of these differences is unknown. A recent hypothesis proposes that these functional differences depend on differences between sides in the "repertoire" of axonal conduction delays for cortico-cortical axons. In morphological terms this corresponds to differences in caliber, or proportion, of myelinated versus unmyelinated axons. Several behavioural studies have indicated that cerebral asymmetry occurs in rodents, in which rigorous morphological analysis is possible. The hypothesis was therefore tested for the first time in adult male Wistar rats, using transmission electron microscopy and stereological methods. Subcortical white matter was compared between left and right sides in three regions (frontal, parietal, and occipital). The average caliber and numerical density of unmyelinated and myelinated axons was compared between sides and between regions. All data were corrected for shrinkage. No significant differences between sides were found in the average caliber of either type of axon in any region. The numerical density of either type of axon also yielded no significant differences between sides in any region. Significant differences were evident between regions in both caliber and numerical density of the two axonal types, and these quantitative data are reported. The proportion of unmyelinated axons in the lateral white matter was also higher than in previous studies of hemispheric white matter that studied the corpus callosum. The present study provides no evidence supporting the hypothesis that functional hemispheric specialization is due to differences in axonal number, caliber or type.

Morphometric changes in vagal nerves of fourth generation mice passage-bred in a 2-G environment

INTRODUCTION

Previous studies have shown that microgravity induces both functional and structural adaptations in the autonomic nerves. Functional adaptation to hypergravity has also been reported, but structural change has not yet been isolated. The purpose of this study was to evaluate structural adaptation to hypergravity in the parasympathetic nerve.

METHOD

We selected fourth generation mice which were passage-bred in a 2-G environment by cycles of coupling, delivery, and growth. Complete left cervical vagal nerves of these mice were studied in transverse sections by electron microscopy. The number of small (diameter < 5 microm, thin and light-stained myelin sheath) and large (diameter > 5 microm, thick and dark-stained myelin sheath) myelinated fibers was counted.

RESULTS

The total number of all myelinated fibers (2 G: 795 +/- 103, 1 G: 644 +/- 60) and the number of small myelinated fibers (2 G: 657 +/- 95, 1 G: 522 +/- 66) were significantly greater in the 2-G mice than those in the 1-G mice (p < 0.05). The number of large myelinated fibers in the 2-G mice was greater than that in the 1-G mice, although it was not statistically significant (2 G: 138 +/- 15, 1-G: 122 +/- 16; p = 0.091).

DISCUSSION

The results show that the autonomic nerves can adapt structurally to hypergravity. We contend that the present results are due to the fact that the mice were passage-bred. As far as we know, this is the first report to show an increase in myelinated fibers in autonomic nerves under prolonged exposure to an increased G environment.

Differential expression of tetrodotoxin-resistant sodium channels Nav1.8 and Nav1.9 in normal and inflamed rats

In an attempt to understand mechanisms underlying peripheral sensitization of primary afferent fibers, we investigated the presence of the tetrodotoxin-resistant Na+ channel subunits Nav1.8 (SNS) and Nav1.9 (SNS2) on axons in digital nerves of normal and inflamed rat hindpaws. In normal animals, 14.3% of the unmyelinated and 10.7% of the myelinated axons labeled for the Nav1.8 subunit. These percentages significantly increased in 48 h inflamed animals to 22.0% (1.5-fold increase) and 57.5% (6-fold increase) for unmyelinated and myelinated axons, respectively. In normal animals, Nav1.9 labeled 9.9% of the unmyelinated and 2.1% of the myelinated axons and following inflammation, the proportion of Nav1.9-labeled unmyelinated axons significantly decreased to 3.0% with no change in the proportion of labeled myelinated axons. These data indicate that Nav1.8 and Nav1.9 subunits are transported to the periphery in normal animals and are differentially regulated during inflammation. The massive increase in Nav1.8 expression in myelinated axons suggests that these may contribute to peripheral sensitization and inflammatory hyperalgesia.

Subpopulations of rat dorsal root ganglion neurons express active vesicular acetylcholine transporter

The vesicular acetylcholine transporter (VAChT) is a transmembrane protein required, in cholinergic neurons, for selective storage of acetylcholine into synaptic vesicles. Although dorsal root ganglion (DRG) neurons utilize neuropeptides and amino acids for neurotransmission, we have previously demonstrated the presence of a cholinergic system. To investigate whether, in sensory neurons, the vesicular accumulation of acetylcholine relies on the same mechanisms active in classical cholinergic neurons, we investigated VAChT presence, subcellular distribution, and activity. RT-PCR and Western blot analysis demonstrated the presence of VAChT mRNA and protein product in DRG neurons and in the striatum and cortex, used as positive controls. Moreover, in situ hybridization and immunocytochemistry showed VAChT staining located mainly in the medium/large-sized subpopulation of the sensory neurons. A few small neurons were also faintly labeled by immunocytochemistry. In the electron microscope, immunolabeling was associated with vesicle-like elements distributed in the neuronal cytoplasm and in both myelinated and unmyelinated intraganglionic nerve fibers. Finally, [(3)H]acetylcholine active transport, evaluated either in the presence or in the absence of ATP, also demonstrated that, as previously reported, the uptake of acetylcholine by VAChT is ATP dependent. This study suggests that DRG neurons not only are able to synthesize and degrade ACh and to convey cholinergic stimuli but also are capable of accumulating and, possibly, releasing acetylcholine by the same mechanism used by the better known cholinergic neurons.

C-Fiber Structure Varies with Location in Peripheral Nerve

Recent advances in regeneration and pain research have revealed gaps in the understanding of normal C-fiber anatomy. In the rat PNS, C-fiber axons assemble into Remak bundles, but beyond this, features of C-fiber organization are not defined. Systematic sampling and quantitation reveals that Remak bundles exiting from the L5 dorsal root ganglion (DRG) contain large numbers of axons, for example, 56% of unmyelinated axons were in bundles of >20 axons. This is different from distal nerve segments such as the hindpaw plantar nerve where the median number of axons per bundle is 3. The cross-sectional area of unmyelinated axons in dorsal root was homogeneous near the DRG but variability in axonal area increased near the spinal cord (p = 0.00001) and the mean axonal area was unchanged. Unmyelinated axons in peripheral nerve were almost always isolated from one another by Schwann cell processes; however, in dorsal root 7% to 9% of unmyelinated axons were immediately adjacent within pockets containing 2 or more axons. Remak bundles in the distal peripheral nerve clustered with other Remak bundles. We observe that multiple unmyelinated axons are juxtaposed within the C-fiber/Remak bundle and that the close association of afferent axons may have important functional implications.

Distribution of antinociceptive adenosine A1 receptors in the spinal cord dorsal horn, and relationship to primary afferents and neuronal subpopulations

Adenosine can reduce pain and allodynia in animals and man, probably via spinal adenosine A1 receptors. In the present study, we investigate the distribution of the adenosine A1 receptor in the rat spinal cord dorsal horn using immunohistochemistry, in situ hybridization, radioligand binding, and confocal microscopy. In the lumbar cord dorsal horn, dense immunoreactivity was seen in the inner part of lamina II. This was unaltered by dorsal root section or thoracic cord hemisection. Confocal microscopy of the dorsal horn revealed close anatomical relationships but no or only minor overlap between A1 receptors and immunoreactivity for markers associated with primary afferent central endings: calcitonin gene-related peptide, or isolectin B4, or with neuronal subpopulations: mu-opioid receptor, neuronal nitric oxide synthase, met-enkephalin, parvalbumin, or protein kinase Cgamma, or with glial cells: glial fibrillary acidic protein. A few adenosine A1 receptor positive structures were double-labeled with alpha-amino-3-hydroxy-5-methyl-4-isoaxolepropionic acid glutamate receptor subunits 1 and 2/3. The results indicate that most of the adenosine A1 receptors in the dorsal horn are located in inner lamina II postsynaptic neuronal cell bodies and processes whose functional and neurochemical identity is so far unknown. Many adenosine A1 receptor positive structures are in close contact with isolectin B4 positive C-fiber primary afferents and/or postsynaptic structures containing components of importance for the modulation of nociceptive information.