Sensory neuron addition in juvenile rat: time course and specificity

Scientific and Regulatory Policy Committee Technical Review: Biology and Pathology of Ganglia in Animal Species Used for Nonclinical Safety Testing

Dorsal root ganglia (DRG), trigeminal ganglia (TG), other sensory ganglia, and autonomic ganglia may be injured by some test article classes, including anti-neoplastic chemotherapeutics, adeno-associated virus-based gene therapies, antisense oligonucleotides, nerve growth factor inhibitors, and aminoglycoside antibiotics. This article reviews ganglion anatomy, cytology, and pathology (emphasizing sensory ganglia) among common nonclinical species used in assessing product safety for such test articles (TAs). Principal histopathologic findings associated with sensory ganglion injury include neuron degeneration, necrosis, and/or loss; increased satellite glial cell and/or Schwann cell numbers; and leukocyte infiltration and/or inflammation. Secondary nerve fiber degeneration and/or glial reactions may occur in nerves, dorsal spinal nerve roots, spinal cord (dorsal and occasionally lateral funiculi), and sometimes the brainstem. Ganglion findings related to TA administration may result from TA exposure and/or trauma related to direct TA delivery into the central nervous system or ganglia. In some cases, TA-related effects may need to be differentiated from a spectrum of artifactual and/or spontaneous background changes.

Developmental Changes in Pain and Spinal Immune Gene Expression after Radicular Trauma in the Rat

Neuropathic pain is chronic pain that develops after nerve injury and is less frequent in infants and children than in adults. Likewise, in animal models of neuropathic pain, allodynia and hyperalgesia are non-existent or attenuated in the infant, with a “switch” during development by which acute nerve injury transitions to chronic pain. Concomitant with the delay in neuropathic pain, there is a parallel delay in the ability of nerve injury to activate the immune system. Models of neuropathic pain in the infant have used various ligation methods and find that neuropathic pain does not occur under after postnatal days 21–28 (PN21–PN28), linked to activation of immune processes and developmental regulation of anti-inflammatory cytokines. We applied a model of neuropathic pain in the adult using a transient compression of the cervical nerve or nerve root in infant rats (injured at 10, 14, 21, or 28 days of age) to define transition periods during which injury results in no change in thermal and mechanical pain sensitivity or in short-term changes in pain. There was little to no hyperalgesia when the injury was imposed at PN10, but significant thermal hyperalgesia and mechanical allodynia 1 day after compression injury when performed at PN14, 21, or 28. Thermal withdrawal latencies returned to near baseline by 7 days postsurgery when the injuries were at PN14, and lasted up to 14 days when the injury was imposed at PN28. There was mechanical allodynia following injury at 1 day postinjury and at 14 days after injury at PN14. Measurements of mRNA from spinal cord at 1, 7, and 14 days postinjury at PN14, 21, and 28 showed that both the magnitude and duration of elevated immune markers and chemokines/cytokines were greater in the older animals, corresponding to the development of hyperalgesia. Thus, we confirm the late onset of neuropathic pain but found no evidence of emergent hyperalgesia if the injury was before PN21. This may be due to the use of a transient, and not sustained, compression ligation model.

Lamina specific postnatal development of PKCγ interneurons within the rat medullary dorsal horn

Protein kinase C gamma (PKCγ) interneurons, located in the superficial spinal (SDH) and medullary dorsal horns (MDH), have been shown to play a critical role in cutaneous mechanical hypersensitivity. However, a thorough characterization of their development in the MDH is lacking. Here, it is shown that the number of PKCγ-ir interneurons changes from postnatal day 3 (P3) to P60 (adult) and such developmental changes differ according to laminae. PKCγ-ir interneurons are already present at P3-5 in laminae I, IIo, and III. In lamina III, they then decrease from P11-P15 to P60. Interestingly, PKCγ-ir interneurons appear only at P6 in lamina IIi, and they conversely increase to reach adult levels at P11-15. Analysis of neurogenesis using bromodeoxyuridine (BrdU) does not detect any PKCγ-BrdU double-labeling in lamina IIi. Quantification of the neuronal marker, NeuN, reveals a sharp neuronal decline (∼50%) within all superficial MDH laminae during early development (P3-15), suggesting that developmental changes in PKCγ-ir interneurons are independent from those of other neurons. Finally, neonatal capsaicin treatment, which produces a permanent loss of most unmyelinated afferent fibers, has no effect on the development of PKCγ-ir interneurons. Together, the results show that: (i) the expression of PKCγ-ir interneurons in MDH is developmentally regulated with a critical period at P11-P15, (ii) PKCγ-ir interneurons are developmentally heterogeneous, (iii) lamina IIi PKCγ-ir interneurons appear less vulnerable to cell death, and (iv) postnatal maturation of PKCγ-ir interneurons is due to neither neurogenesis, nor neuronal migration, and is independent of C-fiber development. © 2016 Wiley Periodicals, Inc. Develop Neurobiol 77: 102-119, 2017.

Comparative gene expression study of the vestibular organ of the Igf1 deficient mouse using whole-transcript arrays

The auditory and vestibular organs form the inner ear and have a common developmental origin. Insulin like growth factor 1 (IGF-1) has a central role in the development of the cochlea and maintenance of hearing. Its deficiency causes sensorineural hearing loss in man and mice. During chicken early development, IGF-1 modulates neurogenesis of the cochleovestibular ganglion but no further studies have been conducted to explore the potential role of IGF-1 in the vestibular system. In this study we have compared the whole transcriptome of the vestibular organ from wild type and Igf1(-/-) mice at different developmental and postnatal times. RNA was prepared from E18.5, P15 and P90 vestibular organs of Igf1(-/-) and Igf1(+/+) mice and the transcriptome analysed in triplicates using Affymetrix(®) Mouse Gene 1.1 ST Array Plates. These plates are whole-transcript arrays that include probes to measure both messenger (mRNA) and long intergenic non-coding RNA transcripts (lincRNA), with a coverage of over 28 thousand coding transcripts and over 7 thousands non-coding transcripts. Given the complexity of the data we used two different methods VSN-RMA and mmBGX to analyse and compare the data. This is to better evaluate the number of false positives and to quantify uncertainty of low signals. We identified a number of differentially expressed genes that we described using functional analysis and validated using RT-qPCR. The morphology of the vestibular organ did not show differences between genotypes and no evident alterations were observed in the vestibular sensory areas of the null mice. However, well-defined cellular alterations were found in the vestibular neurons with respect their number and size. Although these mice did not show a dramatic vestibular phenotype, we conducted a functional analysis on differentially expressed genes between genotypes and across time. This was with the aim to identify new pathways that are involved in the development of the vestibular organ as well as pathways that maybe affected by the lack of IGF-1 and be associated to the morphological changes of the vestibular neurons that we observed in the Igf1(-/-) mice.

Neurogenesis in the adult peripheral nervous system

Most researchers believe that neurogenesis in mature mammals is restricted only to the subgranular zone of the dentate gyrus and the subventricular zone of the lateral ventricle in the central nervous system. In the peripheral nervous system, neurogenesis is thought to be active only during prenatal development, with the exception of the olfactory neuroepithelium. However, sensory ganglia in the adult peripheral nervous system have been reported to contain precursor cells that can proliferate in vitro and be induced to differentiate into neurons. The occurrence of insult-induced neurogenesis, which has been reported by several investigators in the brain, is limited to a few recent reports for the peripheral nervous system. These reports suggest that damage to the adult nervous system induces mechanisms similar to those that control the generation of new neurons during prenatal development. Understanding conditions under which neurogenesis can be induced in physiologically non-neurogenic regions in adults is one of the major challenges for developing therapeutic strategies to repair neurological damage. However, the induced neurogenesis in the peripheral nervous system is still largely unexplored. This review presents the history of research on adult neurogenesis in the peripheral nervous system, which dates back more than 100 years and reveals the evidence on the under estimated potential for generation of new neurons in the adult peripheral nervous system.

Generation of new neurons in dorsal root Ganglia in adult rats after peripheral nerve crush injury

The evidence of neurons generated ex novo in sensory ganglia of adult animals is still debated. In the present study, we investigated, using high resolution light microscopy and stereological analysis, the changes in the number of neurons in dorsal root ganglia after 30 days from a crush lesion of the rat brachial plexus terminal branches. Results showed, as expected, a relevant hypertrophy of dorsal root ganglion neurons. In addition, we reported, for the first time in the literature, that neuronal hypertrophy was accompanied by massive neuronal hyperplasia leading to a 42% increase of the number of primary sensory neurons. Moreover, ultrastructural analyses on sensory neurons showed that there was not a relevant neuronal loss as a consequence of the nerve injury. The evidence of BrdU-immunopositive neurons and neural progenitors labeled with Ki67, nanog, nestin, and sox-2 confirmed the stereological evidence of posttraumatic neurogenesis in dorsal root ganglia. Analysis of morphological changes following axonal damage in addition to immunofluorescence characterization of cell phenotype suggested that the neuronal precursors which give rise to the newly generated neurons could be represented by satellite glial cells that actively proliferate after the lesion and are able to differentiate toward the neuronal lineage.

Sox2 in the adult rat sensory nervous system

Sex-determining region Y (SRY)-box 2 (Sox2) is a member of the Sox family transcription factors. In the central nervous system, Sox2 is expressed in neural stem cells from neurogenic regions, and regulates stem cell proliferation and differentiation. In the peripheral nervous system, Sox2 is found only in the immature and dedifferentiated Schwann cells, and is involved in myelination inhibition or N-cadherin redistribution. In the present immunohistochemical study, we found that Sox2 is also expressed in other cells of the adult rat peripheral nervous system. Nuclear Sox2 was observed in all satellite glial cells, non-myelinating Schwann cells, and the majority of terminal Schwann cells that form lamellar corpuscles and longitudinal lanceolate endings. Sox2 was not found in myelinating Schwann cells and terminal Schwann cells of subepidermal free nerve endings. Satellite glial cells exhibit strong Sox2 immunoreactivity, whereas non-myelinating Schwann cells show weak immunoreactivity. RT-PCR confirmed the presence of Sox2 mRNA, indicating that the cells are likely Sox2 expressors. Our findings suggest that the role of Sox2 in the peripheral nervous system may be cell-type-dependent.

Dynamic genotype-selective "phenotypic switching" of CGRP expression contributes to differential neuropathic pain phenotype

Using a genetic model we demonstrate the role played by "phenotypic switching" of calcitonin gene related peptide (CGRP) expression in axotomized large Aβ afferents in the development of neuropathic pain behavior in rats. After nerve injury both substance P and CGRP are upregulated in Aβ afferents in the corresponding DRGs. It has been proposed that intraspinal release of these neurotransmitters upon gentle stroking of skin drives ascending pain signaling pathways resulting in tactile allodynia. We reported previously that in rat lines genetically selected for high (HA) vs. low (LA) pain phenotype, SP is upregulated equally in both strains, but that CGRP is upregulated exclusively in the pain prone HA line (Nitzan-Luques et al., 2011). This implicates CGRP as the principal driver of tactile allodynia. Here we confirm this conclusion by showing: 1) that the time of emergence of CGRP-IR in DRG Aβ neurons and their central terminals in HA rats matches that of pain behavior, 2) that following spinal nerve lesion (SNL) selective activation of low threshold afferents indeed drives postsynaptic pain-signaling neurons and induces central sensitization in HA rats, as monitored using c-Fos as a marker. These changes are much less prominent in LA rats, 3) that intrathecal (i.t.) administration of CGRP induces tactile allodynia in naïve rats and 4) that i.t. administration of the CGRP-receptor antagonist BIBN4096BS (Olcegepant) attenuates SNL-evoked tactile allodynia, without blocking baseline nociception. Together, these observations support the hypothesis that genotype-selective phenotypic switching of CGRP expression in Aβ afferents following nerve injury is a fundamental mechanism of neuropathic tactile allodynia.

Genotype-selective phenotypic switch in primary afferent neurons contributes to neuropathic pain

Pain is normally mediated by nociceptive Aδ and C fibers, while Aβ fibers signal touch. However, after nerve injury, Aβ fibers may signal pain. Using a genetic model, we tested the hypothesis that phenotypic switching in neurotransmitters expressed by Aβ afferents might account for heritable differences in neuropathic pain behavior. The study examined selection-line rats in which one line, high autotomy (HA), shows higher levels of spontaneous pain in the neuroma neuropathy model, and of tactile allodynia in the spinal nerve ligation (SNL) model, than the companion low autotomy (LA) line. Changes in calcitonin gene-related peptide (CGRP) and Substance P expression were evaluated immunohistochemically in L4 and L5 dorsal root ganglia 7 days after SNL surgery. Expression of CGRP was decreased in axotomized small- and medium-diameter neurons in both rat lines. However, in HA but not in LA rats, there was a tenfold increase in CGRP immunoreactivity (CGRP-IR) in large-diameter neurons. Corresponding changes in CGRP-IR in axon terminals in the nucleus gracilis were also seen. Finally, there were indications of enhanced CGRP neurotransmission in deep laminae of the dorsal horn. Substance P immunoreactivity was also upregulated in large-diameter neurons, but this change was similar in the 2 lines. Our findings suggest that phenotypic switching contributes to the heritable difference in pain behavior in HA vs LA rats. Specifically, we propose that in HA rats, but less so in LA rats, injured, spontaneously active Aβ afferents both directly drive CGRP-sensitive central nervous system pain-signaling neurons and also trigger and maintain central sensitization, hence generating spontaneous pain and tactile allodynia.

The influence of exercise on morphological and neurochemical properties of neurons in rat nodose ganglia

Physical exercise can induce immunohistochemical changes and cell proliferation in the hippocampus. One of the main effects of prolonged exercise is resting bradycardia, most probably caused by enhanced vagal activity. To investigate whether physical exercise can cause neurochemical and morphological changes in vagal afferent neurons, we performed immunohistochemical studies of nodose neurons using isolectin B4 (IB4), 200-kDa neurofilament protein (N52) and calretinin in adult female rats. To distinguish subpopulations of neurons projecting to the left ventricle, we applied a Fast Blue patch to the epicardial surface of the left ventricle. Treadmill running for 8 weeks significantly increased the size of N52-positive cardiac projecting neurons. Furthermore, the proportion of IB4-positive neurons among all nodose ganglia neurons was significantly higher in trained animals. These data indicate that exercise leads to plastic changes in nodose ganglia neurons that may initiate changes of vagal activity caused by prolonged exercise.

Retentive multipotency of adult dorsal root ganglia stem cells

Preservation of neural stem cells (NSCs) in the adult peripheral nervous system (PNS) has recently been confirmed. However, it is not clear whether peripheral NSCs possess predestined, bona fide phenotypes or a response to innate developmental cues. In this study, we first demonstrated the longevity, multipotency, and high fidelity of sensory features of postmigrating adult dorsal root ganglia (aDRG) stem cells. Derived from aDRG and after 4-5 years in culture without dissociating, the aDRG NSCs were found capable of proliferation, expressing neuroepithelial, neuronal, and glial markers. Remarkably, these aDRG NSCs expressed sensory neuronal markers vesicular glutamate transporter 2 (VGluT2--glutamate terminals), transient receptor potential vanilloid 1 (TrpV1--capsaicin sensitive), phosphorylated 200 kDa neurofilaments (pNF200--capsaicin insensitive, myelinated), and the serotonin transporter (5-HTT), which normally is transiently expressed in developing DRG. Furthermore, in response to neurotrophins, the aDRG NSCs enhanced TrpV1 expression upon exposure to nerve growth factor (NGF), but not to brain-derived neurotrophic factor (BDNF). On the contrary, BDNF increased the expression of NeuN. Third, the characterization of aDRG NSCs was demonstrated by transplantation of red fluorescent-expressing aDRG NSCs into injured spinal cord. These cells expressed nestin, Hu, and beta-III-tubulin (immature neuronal markers), GFAP (astrocyte marker) as well as sensory neural marker TrpV1 (capsaicin sensitive) and pNF200 (mature, capsaicin insensitive, myelinated). Our results demonstrated that the postmigrating neural crest adult DRG stem cells not only preserved their multipotency but also were retentive in sensory potency despite the age and long-term ex vivo status.

Proliferation and differentiation of glial and neuronal progenitors in the development of human spinal ganglia

Development and differentiation of the spinal ganglia were investigated in 10 human embryos and foetuses, ranging in age between 5th and 10th developmental weeks. The aim of the study was to estimate the spatial and temporal appearance, percentage and duration of proliferation process among neural crest cells and differentiating glial cells and neurons. The process of proliferation and differentiation of cell lineages from neural crest to neurons or glial cell was analysed using immunohistochemical and immunofluorescence methods in paraffin sections. Quantification of reacting cells was performed by counting the ratio of cells stained or double-stained to specific antibodies in the number of total cell population. Data were expressed as mean+/-SD, while the difference between dorsal and ventral parts of the spinal ganglia were analysed by the Mann-Whitney test. The Ki-67 proliferation marker had the strongest expression in the 5th and 6th developmental weeks (42% of positive cells), showing also significantly higher proliferation rate in the dorsal parts of the spinal ganglia than in the ventral parts (Mann-Whitney, p=0.003). During further development, the number of proliferating cells subsequently decreased to 32% in the foetal period. A majority of the proliferating cells expressed neural crest marker nestin (71.5%) or glial cell marker S100 protein (17%). Neurons (stained with PGP9.5 marker) showed no signs of proliferation. Some cells co-expressed both neural crest cells and glial cell markers. Our results indicate the highest proliferation activity of the progenitor neural crest cells, which slightly decreased with progression of spinal ganglia differentiation. On the contrary, glial cells displayed increasing proliferation activity at later developmental stages, thus conforming significance of gliogenesis during human spinal ganglia development. Although neurogenesis was not found during the investigated period, we could not exclude the possibility of neuronal differentiation from neural crest cells, or even immature glial cells.

Primary sensory neuron addition in the adult rat trigeminal ganglion: evidence for neural crest glio-neuronal precursor maturation

It is debated whether primary sensory neurons of the dorsal root ganglia increase the number in adult animals and, if so, whether the increase is attributable to postnatal neurogenesis or maturation of dormant, postmitotic precursors. Similar studies are lacking in the trigeminal ganglion (TG). Here we demonstrate by stereological methods that the number of neurons in the TG of adult male rats nearly doubles between the third and eighth months of age. The increase is mainly attributable to the addition of small, B-type neurons, with a smaller contribution of large, A-neurons. We looked for possible proliferative or maturation mechanisms that could explain this dramatic postnatal expansion in neuron number, using bromodeoxyuridine (BrdU) labeling, immunocytochemistry for neural precursor cell antigens, retrograde tracing identification of peripherally projecting neurons, and in vitro isolation of precursor cells from adult TG explant cultures. Cell proliferation identified months after an extended BrdU administration was sparse and essentially corresponded to glial cells. No BrdU-labeled cell took up the peripherally injected tracer, and only a negligible number coexpressed BrdU and the pan-neuronal tracer neuron-specific enolase. In contrast, a population of cells not recognizable as mature neurons in the TG and neighboring nerve expressed neuronal precursor antigens, and neural crest glioneuronal precursor cells were successfully isolated from adult TG explants. Our data suggest that a protracted maturation process persists in the TG that can be responsible for the neuronal addition found in the adult rat.

Isolation and Characterization of Neural Crest Progenitors from Adult Dorsal Root Ganglia

After peripheral nerve injury, the number of sensory neurons in the adult dorsal root ganglia (DRG) is initially reduced but recovers to a normal level several months later. The mechanisms underlying the neuronal recovery after injury are not clear. Here, we showed that in the DRG explant culture, a subpopulation of cells that emigrated out from adult rat DRG expressed nestin and p75 neurotrophin receptor and formed clusters and spheres. They differentiated into neurons, glia, and smooth muscle cells in the presence or absence of serum and formed secondary and tertiary neurospheres in cloning assays. Molecular expression analysis demonstrated the characteristics of neural crest progenitors and their potential for neuronal differentiation by expressing a set of well-defined genes related to adult stem cells niches and neuronal fate decision. Under the influence of neurotrophic factors, some of these progenitors gave rise to neuropeptide-expressing cells and protein zero-expressing Schwann cells. In a 5-bromo-2'-deoxyuridine chasing study, we showed that these progenitors likely originate from satellite glial cells. Our study suggests that a subpopulation of glia in adult DRG is likely to be progenitors for neurons and glia and may play a role in neurogenesis after nerve injury. Disclosure of potential conflicts of interest is found at the end of this article.

Evidence of Postnatal Neurogenesis in Dorsal Root Ganglion: Role of Nitric Oxide and Neuronal Restrictive Silencer Transcription Factor

The various mechanisms underlying postnatal neurogenesis from discrete CNS regions have emerged recently. However, little is known about postnatal neurogenesis in dorsal root ganglion (DRG). BrdU incorporation and subsequent immunostaining for BrdU, neural stem cell marker, nestin and neuronal marker, PGP 9.5 have provided evidence for postnatal neurogenesis in DRG. We further demonstrate, in vivo and in vitro, that nitric oxide (NO) regulates neural stem cells (nestin+) proliferation and, possibly, differentiation into neurons. Surprisingly, nerve growth factor (NGF) had no effect on nestin+ cells proliferation. Axotomy or NGF-deprivation of DRG neurons-satellite glia co-culture increases NO production by neurons and treating with a NO synthase (NOS) inhibitor, N G-nitro-L-arginine methylester (L-NAME) in vitro or 7-nitroindazole (7NI) in vivo, causes a significant increase in nestin+ cell numbers. However, a soluble guanylyl cyclase (sGC) blocker, 1H-[1, 2, 4] oxadiazolo [4, 3-a] quinoxalin-1-one (ODQ) treatment of NGF-deprived DRG neurons-satellite glia co-culture had no significant effect on nestin+ cell numbers. This implies NO regulates nestin+ cell proliferation independent of cGMP. We hypothesised that the neuronal-restrictive silencer transcription factor (NRSF, also termed REST), a master regulator of neuronal genes in non-neuronal cells, may be modulated by NO in satellite glia cultures. A NO donor, dimethyl-triamino-benzidine (DETA)-NO treatment of satellite glia cell cultures results in a significant increase in the NRSF/REST mRNA expression. The majority of cultured satellite glia cells express nestin, and also show increased levels of NOS, thus L-NAME treatment of these cultures causes a dramatic reduction in NRSF/REST mRNA. Overall these results suggest that NO inhibits neurogenesis in DRG and this is correlated with modulation of NRSF, a known modulator of differentiation.

Time course of substance P expression in dorsal root ganglia following complete spinal nerve transection

Recent evidence suggests that substance P (SP) is up-regulated in primary sensory neurons following axotomy and that this change occurs in larger neurons that do not usually produce SP. If this is so, then the up-regulation may allow normally neighboring, uninjured, and nonnociceptive dorsal root ganglion (DRG) neurons to become effective in activating pain pathways. By using immunohistochemistry, we performed a unilateral L5 spinal nerve transection on male Wistar rats and measured SP expression in ipsilateral L4 and L5 DRGs and contralateral L5 DRGs at 1-14 days postoperatively (dpo) and in control and sham-operated rats. In normal and sham-operated DRGs, SP was detectable almost exclusively in small neurons (< or =800 microm2). After surgery, the mean size of SP-positive neurons from the axotomized L5 ganglia was greater at 2, 4, 7, and 14 dpo. Among large neurons (>800 microm2) from the axotomized L5, the percentage of SP-positive neurons increased at 2, 4, 7, and 14 dpo. Among small neurons from the axotomized L5, the percentage of SP-positive neurons was increased at 1 and 3 dpo but was decreased at 7 and 14 dpo. Thus, SP expression is affected by axonal damage, and the time course of the expression is different between large and small DRG neurons. These data support a role for SP-producing, large DRG neurons in persistent sensory changes resulting from nerve injury.

The Plasticity of the DRG Neurons Belonging to Different Subpopulations After Dorsal Rhizotomy

1. The plasticity of sensory neurons following the injury to their axons is very important for prognosis of recovery of afferent fibers with different modality. It is evident that the response of dorsal root ganglion (DRG) neurons after peripheral axotomy is different depending on the deficiency in neurotrophic factors from peripheral region. The loss of cells appears earlier and is more severe in B-cells (small, dark cells with unmyelinated axons) than in A-cells (large, light cells with myelinated axons). 2. We studied using immunohistochemical methods the response of DRG neurons to dorsal rhizotomy and combined injury of central and peripheral neuronal processes. A quantitative analysis of DRG neurons tagged by the selective markers isolectin B4 (IB4) and the heavy molecular component of the neurofilament triplet (NF200) antibody, selective for subpopulations of small and large/medium DRG neurons, respectively, was performed after dorsal rhizotomy, peripheral axotomy, and their combination. 3. The number of NF200(+)-neurons is reduced substantially after both dorsal rhizotomy and peripheral axotomy, while the decrease of IB4(+)-neurons is observed only in combined injury, i.e., dorsal rhizotomy accompanied with sciatic nerve injury. 4. Our results show that distinct subpopulations of DRG neurons respond differently to the injury of their central processes. The number of NF200(+)-neurons decreases to greater degree following dorsal rhizotomy in comparison to IB4(+)-neurons.

Effects of systemically administered NT-3 on sensory neuron loss and nestin expression following axotomy

Previous work has shown that administration of the neurotrophin NT-3 intrathecally or to the proximal stump can prevent axotomy-induced sensory neuron loss and that NT-3 can stimulate sensory neuron differentiation in vitro. We have examined the effect of axotomy and systemic NT-3 administration on neuronal loss, apoptosis (defined by morphology and activated caspase-3 immunoreactivity), and nestin expression (a protein expressed by neuronal precursor cells) in dorsal root ganglia (DRG) following axotomy of the adult rat sciatic nerve. Systemic administration of 1.25 or 5 mg of NT-3 over 1 month had no effect on the incidence of apoptotic neurons but prevented the overall loss of neurons seen at 4 weeks in vehicle-treated animals. Nestin-immunoreactive neurons began to appear 2 weeks after sciatic transection in untreated animals and steadily increased in incidence over the next 6 weeks. NT-3 administration increased the number of nestin-immunoreactive neurons at 1 month by two- to threefold. Nestin-IR neurons had a mean diameter of 20.78 +/- 2.5 microm and expressed the neuronal markers neurofilament 200, betaIII-tubulin, protein gene product 9.5, growth associated protein 43, trkA, and calcitonin gene-related peptide. Our results suggest that the presence of nestin in DRG neurons after nerve injury is due to recent differentiation and that exogenous NT-3 may prevent neuron loss by stimulating this process, rather than preventing neuron death.

The astrocytic barrier to axonal regeneration at the dorsal root entry zone is induced by rhizotomy

After dorsal rhizotomy, sensory axons fail to regenerate beyond the astrocytic glia limitans at the dorsal root entry zone (DREZ) but this inhibition can be overcome with the delivery of exogenous neurotrophin-3. We investigated whether axonal inhibition at the DREZ is constitutive or induced after dorsal rhizotomy. Primary afferent neurones from enhanced green fluorescent protein-expressing mice were transplanted into adult rat dorsal root ganglia in the presence or absence of dorsal rhizotomy. In the absence of dorsal rhizotomy mouse axons freely extended into the rat central nervous system. After host dorsal rhizotomy, mouse axons were unable to cross the DREZ. However, in rats that received a dorsal rhizotomy concomitant with intrathecal neurotrophin-3, the mouse axons were able to cross the DREZ. These results indicate that, under normal circumstances, the adult DREZ is permissive to the regeneration of adult sensory axons and that it only becomes inhibitory once dorsal root axons have been injured and astrocytes at the DREZ have become reactive.

Age dependence of organophosphate and carbamate neurotoxicity in the postnatal rat: extrapolation to the human

One important aspect of risk assessment for the organophosphate and carbamate pesticides is to determine whether their neurotoxicity occurs at lower dose levels in human infants compared to adults. Because these compounds probably exert their neurotoxic effects through the inhibition of acetylcholinesterase (AChE), the above question can be narrowed to whether the cholinesterase inhibition and neurotoxicity they produce is age-dependent, both in terms of the effects produced and potency. The rat is the animal model system most commonly used to address these issues. This paper first discusses the adequacy of the postnatal rat to serve as a model for neurodevelopment in the postnatal human, concluding that the two species share numerous pathways of postnatal neurodevelopment, and that the rat in the third postnatal week is the neurodevelopmental equivalent of the newborn human. Then, studies are discussed in which young and adult rats were dosed by identical routes with organophosphates or carbamates. Four pesticides were tested in rat pups in their third postnatal week: aldicarb, chlorpyrifos, malathion, and methamidophos. The first three, but not methamidophos, caused neurotoxicity at dose levels that ranged from 1.8- to 5.1-fold lower (mean 2.6-fold lower) in the 2- to 3-week-old rat compared to the adult. This estimate in the rat, based on a limited data set of three organophosphates and a single carbamate, probably represents the minimum difference in the neurotoxicity of an untested cholinesterase-inhibiting pesticide that should be expected between the human neonate and adult. For the organophosphates, the greater sensitivity of postnatal rats, and, by analogy, that expected for human neonates, is correlated with generally lower levels of the enzymes involved in organophosphate deactivation.

Late differentiation contributes to the apparent increase in sensory neuron number in juvenile rat

Using both profile counts and unbiased stereological methods, estimates of neuron number in the lumbar dorsal root ganglia of the rat have been shown to increases approximately 35% during postnatal life [J. Comp. Neurol. 386 (1997) 8-15; J. Comp. Neurol. 449 (2002) 158-165]. The mechanism underlying this addition of neurons was investigated. No evidence of incorporation of (BrdU), a mitotic marker, was found. Similarly, counts of myelinated and unmyelinated axons in the sural nerve were the same in neonates and adults. These results are not consistent with the possibility that neurogenesis accounts for neuron addition. A population of neurons that stains with TuJ1, an antibody against neuronal class III beta tubulin, but not with an antibody against the phosphorylated and non-phosphorylated forms of heavy chain neurofilament protein (NF-H) was found in neonates, but not adults. These less-differentiated (type-L) neurons are not detected by either profile counts or unbiased stereology and do not transport HRP retrogradely. Maturation of this pool of incompletely differentiated neurons appears to be one mechanism whereby neuron number is augmented during postnatal life.