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Several lines of evidence indicate that peripheral 5-HT2A receptors are involved in the development of inflammatory and neuropathic pain. However, their localization in sensory cell bodies is not accurately known. We therefore studied 5-HT2A receptor distribution in rat lumbar dorsal root ganglia using immunocytochemistry. Forty percent of L3 lumbar dorsal root ganglion cells were immunoreactive for 5-HT2A receptor. Most were small- to medium-sized cell bodies. Double-labeled experiments revealed that they expressed various chemical phenotypes. The smaller 5-HT2AR cell bodies often bind the isolectin B4 although some 5-HT2AR cell bodies also express substance P (SP). Many 5-HT2A-positive small dorsal root ganglion cells expressed the capsaicin receptor transient receptor potential vanilloid type 1 receptor (TRPV1), confirming their nociceptive nature. In addition, a few large cell bodies were labeled for 5-HT2A, and they also expressed NF200 suggesting that they were at the origin of Adelta or Abeta fibers. A total absence of double labeling with parvalbumin showed that they were not proprioceptors. 5-HT2A immunoreactivity in dorsal root ganglia cells was found in the cytoplasm and along the plasma membrane at the interface between sensory cell and the adjacent satellite cells; this distribution was confirmed under the electron microscope, and suggested a functional role for the 5-HT2A receptor at these sites. We therefore investigated the presence of 5-HT and 5-HIAA in lumbar dorsal root ganglia by high performance liquid chromatography. There were 5.75+/-0.80 ng 5-HT and 3.19+/-0.37 ng 5-hydroxyindoleacetic acid (5-HIAA) per mg of protein with a ratio 5-HIAA/5-HT of 0.67+/-0.10, similar to values typically observed in brain tissues. These findings suggest that 5-HT, via the 5-HT2AR, may be involved in the peripheral control of sensory afferents, mainly unmyelinated nociceptors and to a lesser extent neurons with Adelta or Abeta fibers, and in the control of cellular excitability of some dorsal root cell bodies through a paracrine mechanism of action.

Peripheral glial cells consist of satellite, enteric glial, and Schwann cells. In dorsal root ganglia, besides pseudo-unipolar neurons, myelinated and nonmyelinated fibers, macrophages, and fibroblasts, satellite cells also constitute the resident components. Information on satellite cells is not abundant; however, they appear to provide mechanical and metabolic support for neurons by forming an envelope surrounding their cell bodies. Although there is a heterogeneous population of neurons in the dorsal root ganglia, satellite cells have been described to be a homogeneous group of perineuronal cells. Our objective was to characterize the ultrastructure, immunohistochemistry, and histochemistry of the satellite cells of the dorsal root ganglia of 17 adult 3-4-month-old Wistar rats of both genders. Ultrastructurally, the nuclei of some satellite cells are heterochromatic, whereas others are euchromatic, which may result from different amounts of nuclear activity. We observed positive immunoreactivity for S-100 and vimentin in the cytoplasm of satellite cells. The intensity of S-100 protein varied according to the size of the enveloped neuron. We also noted that vimentin expression assumed a ring-like pattern and was preferentially located in the cytoplasm around the areas stained for S-100. In addition, we observed nitric oxide synthase-positive small-sized neurons and negative large-sized neurons equal to that described in the literature. Satellite cells were also positive for NADPH-diaphorase, particularly those associated with small-sized neurons. We conclude that all satellite cells are not identical as previously thought because they have different patterns of glial marker expression and these differences may be correlated with the size and function of the neuron they envelope.

We studied quantitative changes in mitochondria of perineuronal satellite cell sheaths (SCSs) of rabbit spinal ganglia from young to extremely advanced age (1, 3.6, 6.7 and 8.8 years). The mitochondrial structure did not differ in the four age groups, while mitochondrial size increased progressively and significantly with age. The mean percentage of cytoplasmic volume occupied by mitochondria decreased progressively and significantly from young to old animals. This decrease was mainly due to a progressive and significant reduction in the total mitochondrial volume. Lipofuscin accumulation had a negligible influence on this reduction. These results suggest that the ability of SCSs to produce energy decreases with age and that the reduced ability of spinal ganglion neurons to respond to high energy demands in old age may be in part due to the diminished contribution of perineuronal satellite cells., (Copyright 2007 S. Karger AG, Basel.)

The volumes of the nerve cell bodies and those of the enveloping satellite cell sheaths from spinal ganglia were determined by morphometric methods applied to electron micrographs in young, adult, old and very old rabbits. The mean volume of the nerve cell bodies increased progressively with age; this is probably related to the increase with age of the body size of the rabbits studied. The mean volume of the satellite cell sheaths did not differ significantly in young, adult and old animals, but was significantly smaller in very old animals. It is extremely unlikely that this marked reduction in the volume of the satellite cell sheath is the result of a pathological process. The mean value of the volume ratio between the satellite cell sheaths and the related nerve cell bodies did not differ significantly in young and adult animals, but was significantly smaller in old and very old animals. This ratio was particularly low in very old animals. Our analysis showed that in each age group the volume of the satellite cell sheath is linearly related to the volume of the related nerve cell body. This result suggests that in rabbit spinal ganglia the quantitative relations between glial and nervous tissue are tightly controlled throughout life. It is suggested that ganglionic neurons release signals to influence and control the volume of their associated glial tissue. Since satellite cells have important support roles for the neurons they surround, it is likely that the marked reduction in the volume of perineuronal sheaths in the extremely advanced age is accompanied by a reduction of those roles, with negative consequences for neuronal activity.

Study Design: The membrane, nucleus, and cytoplasmic contents of satellite cells were evaluated using a transmission electron microscope., Objective: To delineate satellite cell morphometries., Summary of Background Data: The role of the satellite support cells associated with the neuronal cell bodies remains poorly understood. Previous research has identified one type of satellite support cells., Methods: Dorsal root ganglions were excised from 10 adult New Zealand White rabbits. Sections from L2-L5 ganglions were prepared, cut, and analyzed under a transmission electron microscope., Results: A total of 190 neurons and their associated satellite cells were selected for analysis. Three subgroups of satellite cells were identified. The two predominant subgroups consisted of previously described satellite cells. The third subgroup consisted of highly complex and unusual cells. Nineteen satellite cells (4%) did not conform to any previous description of glial cells. Cells were characterized by larger nuclei, with numerous inclusions, and by extensively convoluted reflections of the cellular membrane. These cells were "perched" or "piggy-backed" on top of a convoluted and multilayered cytoplasmic sheet., Conclusion: A new type of support cell representing a different cell line or a highly adapted cell with specific functional capacities was identified.

This study investigated quantitative aspects of the gap junctions between satellite neuroglial cells that envelope the spinal ganglion neurons in rabbits aged 1 year (young), 3.6 years (adult), 6.7 years (old), and 8.8 years (very old). Both the total number of gap junctions present in 30,000 microm2 of surface area occupied by perineuronal satellite cells, and the density of these junctions increased throughout life, including the extremely advanced age. By contrast, the mean length of individual gap junctions did not change with age. Thus, the junctional system which provides morphological support for the metabolic cooperation between satellite cells in rabbit spinal ganglia becomes more extensive as the age of the animal increases. These results support the hypothesis that the gap junctions between perineuronal satellite cells are involved in the spatial buffering of extracellular K+ and in neuroprotection.

Varicella-zoster virus (VZV) causes varicella and establishes latency in sensory ganglia. VZV reactivation results in herpes zoster. We developed a model using human dorsal root ganglion (DRG) xenografts in severe combined immunodeficient (SCID) mice to investigate VZV infection of differentiated neurons and satellite cells in vivo. DRG engrafted under the kidney capsule and contained neurons and satellite cells within a typical DRG architecture. VZV clinical isolates infected the neurons within DRG. At 14 days postinfection, VZ virions were detected by electron microscopy in neuronal cell nuclei and cytoplasm but not in satellite cells. The VZV genome copy number was 7.1 x 10(7) to 8.0 x 10(8) copies per 10(5) cells, and infectious virus was recovered. This initial phase of viral replication was followed within 4-8 weeks by a transition to VZV latency, characterized by the absence of infectious virus release, the cessation of virion assembly, and a reduction in VZV genome copies to 3.7 x 10(5) to 4.7 x 10(6) per 10(5) cells. VZV persistence in DRG was achieved without any requirement for VZV-specific adaptive immunity and was associated with continued transcription of the ORF63 regulatory gene. The live attenuated varicella vaccine virus exhibited the same pattern of short-term replication, persistence of viral DNA, and prominent ORF63 transcription as the clinical isolates. VZV-infected T cells transferred virus from the circulation into DRG, suggesting that VZV lymphotropism facilitates its neurotropism. DRG xenografts may be useful for investigating neuropathogenic mechanisms of other human viruses.

The gap junctions between perineuronal satellite cells were studied in the spinal ganglia of 12, 42, and 79-month-old rabbits. The mean number of gap junctions per 100 microm2 of surface of the section occupied by satellite cells was significantly greater in old rabbits than young adults. Since the mean length of individual gap junctions did not change with age, the increase in number of gap junctions cannot be due to fragmentation of pre-existing gap junctions but is very likely due to the formation of new gap junctions. The increase in number of gap junctions cannot be related to an increase in number of perineuronal satellite cells since the mean number of these cells is significantly smaller in aged rabbits than in young adults. It is suggested that the increase in number of gap junctions with age may enhance the suggested neuroprotective role of satellite cells towards ganglionic neurons. The present findings, together with previous observations, suggest that the gap junctions between perineuronal satellite cells are dynamic structures, able to adapt to varying neuronal demands and varying environmental conditions.

We studied mitochondria in the satellite cell sheaths which envelope the spinal ganglion neurons of rabbits aged 12, 42, and 79 months. While the mean cytoplasmic volume of satellite cell sheaths did not change significantly with age, the mean percentage of cytoplasmic volume occupied by mitochondria decreased with age. This decrease is mainly due to a reduction in the total mitochondrial mass and only in minor part is a consequence of lipofuscin accumulation. Mitochondrial structure did not change, while mitochondrial size increased with age. Comparison between mitochondria in nerve cell bodies and those in satellite cell sheaths showed that: (1) the mean percentage of cytoplasmic volume occupied by mitochondria was greater in nerve cell bodies than satellite cell sheaths and the ratio between these two percentages remained constant with advancing age; (2) the total mitochondrial mass was much greater in nerve cell bodies than satellite cell sheaths and the ratio between these two values increased with age; (3) the extent of increase of mitochondrial size with age was similar in nerve cell bodies and satellite cell sheaths. The results of the present study suggest that: (1) the ability of satellite cell sheaths to produce energy decreases with age; (2) the decreased ability of sensory neurons in old animals to meet high energy demands may be partly due to the diminished contribution of their associated satellite cell sheaths.

We studied the Golgi apparatus in the satellite cell sheaths enveloping spinal ganglion neurons of rabbits aged 12, 42, and 79 months. We found neither structural changes nor indications of peripheral displacement of this organelle with advancing age. The mean percentage of cytoplasmic volume occupied by the Golgi apparatus decreased significantly passing from young adult to old rabbits. This decrease was only in very minor part a consequence of lipofuscin accumulation, so that the ratio between the total volume of the Golgi apparatus and the functionally active volume of cytoplasm decreased with age. The mean cytoplasmic volume of perineuronal satellite cell sheaths did not change significantly with increasing age, whereas the total volume of the Golgi apparatus within these sheaths decreased significantly with age. This decrease strongly suggests that the activity of satellite cells diminishes in old age, further suggesting that these cells are unable to compensate for the decrease in the neuronal metabolism that a number of authors have described in old age.

This study investigated satellite cell changes in mouse L4 and L5 spinal ganglia 14 days after unilateral transection of sciatic and saphenous nerves. The ganglia were studied under the electron microscope in single and serial sections, and by dye injection. Satellite cell responses to axon injury of the neurons with which they are associated included the formation of bridges connecting previously separate perineuronal sheaths and the formation of new gap junctions, resulting in more extensive cell coupling. Some possible consequences of these satellite cell reactions are briefly discussed.

Data on the presence of NGF receptors in the satellite cells of spinal ganglia are scanty and contradictory. In the present study we used immunocytochemistry to examine the distribution of these receptors in spinal ganglia of the adult rat by light and electron microscopy. We found that (1) all satellite cells were immmunoreactive to p75 and the mean density of gold particles (mean number per microm2) was significantly greater in the satellite cell sheath than in the nerve cell body; (2) numerous satellite cells were immunoreactive for trkA with a mean density of gold particles slightly greater in the satellite cell sheath than in the nerve cell body, although the difference was not statistically significant; (3) both p75 and trkA immunoreactivity were confined to the cytoplasm. We suggest that the p75 receptor may be involved in the NGF-induced outgrowth of slender projections from the nerve cell body surface. With regard to the trkA receptor, satellite cells might be supported trophically by NGF released from the neuron with which they are associated; alternatively, satellite cells might internalize NGF to constitute a reservoir for later release to the neuron.

Numerous studies have been done on the effect of nerve injury on neurons of sensory ganglia but little is known about the contribution of satellite glial cells (SCs) in these ganglia to post-injury events. We investigated cell-to-cell coupling and ultrastructure of SCs in mouse dorsal root ganglia after nerve injury (axotomy). Under control conditions SCs were mutually coupled, but mainly to other SCs around a given neuron. After axotomy SCs became extensively coupled to SCs that enveloped other neurons, apparently by gap junctions. Serial section electron microscopy showed that after axotomy SC sheaths enveloping neighboring neurons formed connections with each other. Such connections were absent in control ganglia. The number of gap junctions between SCs increased 6.5-fold after axotomy. We propose that axotomy induces growth of perineuronal SC sheaths, leading to contacts between SCs enveloping adjacent neurons and to formation of new gap junctions between SCs. These changes may be an important mode of glial plasticity and can contribute to neuropathic pain.

Patients with a peripheral nerve injury often suffer from persistent chronic pain, but the underlying mechanism remains largely unknown. The persistent nature of the pain suggests injury-induced profound structural changes along the sensory pathways. In the present study, using the plant Griffonia simplicifolia I isolectin B4 (IB4) as a marker for nonpeptidergic small sensory neurons, we sought to examine whether these neurons sprout in the dorsal root ganglia (DRG) in response to peripheral nerve injury. The lumbar 5 (L5) spinal nerve was transected, and rats were allowed to survive for varying lengths of time before IB4 histology was performed. We found that a subpopulation of IB4-positive sensory neurons sprouted robustly after spinal nerve injury. Twelve weeks after spinal nerve injury, the IB4-positive ring structures became dramatic and encircled both large and small neurons in the DRG. The aberrant sprouting of small sensory neurons was also demonstrated by retrograde labeling. The processes of satellite cells surrounding large sensory neurons also became IB4 positive, and 87.8% of perineuronal IB4-positive ring structures intermingled and/or coexpressed with glial fibrillary acidic protein-positive satellite cells. Thus, the sprouting axons of IB4-positive neurons were intermingled with IB4-positive satellite cells, forming perineuronal ring structures surrounding large-diameter neurons. Ultrastructural examinations further confirmed that IB4-positive nerve terminals were entangled with satellite cells and IB4-negative unmyelinated sprouting fibers around sensory neurons. These studies have provided the first evidence that a subpopulation of IB4-binding small sensory neurons sprouts and forms perineuronal ring structures together with IB4-positive satellite cells in response to nerve injury. The significance of the sprouting of IB4-positive neurons remains to be determined., (Copyright 2001 Wiley-Liss, Inc.)

The mechanisms governing the regeneration of denervated peripheral mechanoreceptors are similar to those of peripheral nerves. The ability to regenerate depends partly on changes of the Schwann cell phenotype. The transforming growth factor beta (TGF-beta) family have been implicated in induction of Schwann cell proliferation, production of extracellular matrix and neurotrophin synthesis as well as synthesis or repression of cell adhesion molecules. Hence, they may prove to be of importance for regenerative mechanisms in peripheral mechanoreceptors. The distribution of TGF-beta, the receptors I and II and intra-cellular second messengers, Smad 2/3 and 4 was assessed in sensory neurones, peripheral nerves and mechanoreceptors by immuno-histochemistry, immuno-electron microscopy and in situ hybridisation. TGF-beta2 mRNA and TGF-beta2-like immunoreactivity (IR) were expressed in injured small and medium sized rat sensory neurones of dorsal root ganglia. TGF-beta and receptor II mRNA and immunoreactivities (IR) were present in satellite cells. Intact and injured sensory neurones expressed receptor I mRNA and Smad 2 mRNA. TGF-beta2 mRNA was found in transected nerve stumps and in sensory mechanoreceptors. TGF-beta1, 2 and Smad 4 were also observed in inner core lamellar cells of intact and denervated cat Pacinian corpuscles. Lamellar cells of intact and denervated Meissner corpuscles were TGF-beta immunoreactive. Merkel cells were receptors I and II immunoreactive. In conclusion, cutaneous and subcutaneous mechanoreceptors differ with regard to the expression of TGF-beta isoforms and receptors.