Differential expression of growth-associated protein (GAP-43) mRNA in rat primary sensory neurons after peripheral nerve lesion: a non-radioactive in situ hybridisation study

Heterogeneous responses of dorsal root ganglion neurons in neuropathies induced by peripheral nerve trauma and the antiretroviral drug stavudine

Background

Heterogeneity is increasingly recognized in clinical presentation of neuropathic pain (NP), but less often recognized in animal models. Neurochemical dysregulation in rodent dorsal root ganglia (DRG) is associated with peripheral nerve trauma, but poorly studied in non-traumatic NP conditions.

Methods

This study aimed to investigate the temporal expressions of activating transcription factor-3 (ATF-3), growth-associated protein-43 (GAP-43), neuropeptide Y (NPY) and galanin in traumatic and non-traumatic rat models of neuropathies associated with NP. Expressions of these markers were examined in the DRG at different time points following tibial nerve transection (TNT) injury and antiretroviral drug stavudine (d4T) administration using immunohistochemistry. The development of sensory gain following these insults was assessed by measuring limb withdrawal to a punctate mechanical stimulus.

Results

Both TNT-injured and d4T-treated rats developed hindpaw mechanical hypersensitivity. Robust expressions of ATF-3, GAP-43, NPY and galanin in both small- and large-sized L5 DRG neurons were observed in the DRG from TNT-injured rats. In contrast, d4T-treated rats did not exhibit any significant neurochemical changes in the DRG.

Conclusions

Taken together, the results suggest that ATF-3, GAP-43, NPY and galanin are likely indicators of nerve trauma-associated processes and not generic markers for NP. These experiments also demonstrate distinct expression patterns of neurochemical markers in the DRG and emphasize the mechanistic difference between nerve trauma and antiretroviral drug-associated NP.

Spinal subarachnoid adrenal medullary transplants reduce hind paw swelling and peripheral nerve transport following formalin injection in rats

Previous studies have demonstrated that adrenal medullary chromaffin cells transplanted into the spinal subarachnoid space significantly reduced pain-related behavior following hind paw plantar formalin injection in rats. The data suggests a centrally mediated antinociceptive mechanism. The spinal transplants may have effects on sciatic nerve function as well. To address this, the current study examined the effects of spinal adrenal transplants on hind paw edema and the anterograde transport of substance P (SP) that occur following formalin injection. Robust formalin-evoked edema, as well as hind paw flinching, was observed in striated muscle control-transplanted rats, which were not observed in adrenal-transplanted rats. To visualize transport of SP, the sciatic nerve was ligated ipsilateral to formalin injection and the nerve was processed 48 h later for immunocytochemistry. A significant formalin-induced accumulation of SP immunoreactivity (IR) was observed proximal to the ligation in control-transplanted rats. In contrast, there was significantly less SP IR observed from nerve of adrenal-transplanted rats, suggesting a diminution of anterograde axoplasmic transport by adrenal transplants. The change in SP IR may have been due to an alteration of transport due to formalin injection, thus, transport was visualized by the accumulation of growth-associated protein 43 (GAP43) at the ligation site. Formalin injection did not significantly increase proximal accumulation of GAP43 IR, indicating that formalin does not increase anterograde transport. Surprisingly, however, adrenal transplants significantly diminished GAP43 IR accumulation compared to control-transplanted rats. These data demonstrate that spinal adrenal transplants can attenuate the formalin-evoked response by modulating primary afferent responses.

Self-assembly of sensory neurons into ganglia-like microtissues

Unraveling intra- and inter-cellular signaling networks managing cell-fate control, coordinating complex differentiation regulatory circuits and shaping tissues and organs in living systems remain major challenges in the post-genomic era. Resting on the laurels of past-century monolayer culture technologies, the cell culture community has only recently begun to appreciate the potential of three-dimensional mammalian cell culture systems to reveal the full scope of mechanisms orchestrating the tissue-like cell quorum in space and time. Capitalizing on gravity-enforced self-assembly of monodispersed primary embryonic mouse cells in hanging drops, we designed and characterized a three-dimensional cell culture model for ganglion-like structures. Within 24h, a mixture of mouse embryonic fibroblasts (MEF) and cells, derived from the dorsal root ganglion (DRG) (sensory neurons and Schwann cells) grown in hanging drops, assembled to coherent spherical microtissues characterized by a MEF feeder core and a peripheral layer of DRG-derived cells. In a time-dependent manner, sensory neurons formed a polar ganglion-like cap structure, which coordinated guided axonal outgrowth and innervation of the distal pole of the MEF feeder spheroid. Schwann cells, present in embryonic DRG isolates, tended to align along axonal structures and myelinate them in an in vivo-like manner. Whenever cultivation exceeded 10 days, DRG:MEF-based microtissues disintegrated due to an as yet unknown mechanism. Using a transgenic MEF feeder spheroid, engineered for gaseous acetaldehyde-inducible interferon-beta (ifn-beta) production by cotransduction of retro-/ lenti-viral particles, a short 6-h ifn-beta induction was sufficient to rescue the integrity of DRG:MEF spheroids and enable long-term cultivation of these microtissues. In hanging drops, such microtissues fused to higher-order macrotissue-like structures, which may pave the way for sophisticated bottom-up tissue engineering strategies. DRG:MEF-based artificial micro- and macrotissue design demonstrated accurate key morphological aspects of ganglions and exemplified the potential of self-assembled scaffold-free multicellular micro-/macrotissues to provide new insight into organogenesis.

Distribution of GAP-43 nerve fibers in the skin of the adult human hand

Skin is an important region of somatic sensory input, and is one of the most innervated areas of the human body. In this study, we investigated in human hand skin the distribution of nervous structures immunoreactive for the growth-associated protein 43 (GAP-43) and the protein gene product 9.5 (PGP 9.5). GAP-43 is a neuronal presynaptic membrane protein that is generally considered to be a marker of neuronal plasticity. PGP 9.5 is a neuron-specific soluble protein that is widely used as general marker for the peripheral nervous system. The entire neural network of the dermis and epidermis was stained with antibody to PGP 9.5. In the dermis, there were fewer GAP-43-immunostained nerve fibers than PGP 9.5-immunostained nerve fibers, whereas in the epidermis the numbers were equal. Only some Merkel cells and Meissner corpuscles were GAP-43-immunoreactive. In conclusion, our results show that GAP-43 protein is expressed in a subset of PGP 9.5-immunoreactive nerve structures.

Expression of neuropeptides and growth-associated protein 43 (GAP-43) in cutaneous and mucosal nerve structures of the adult rat lower lip after mental nerve section

The reinnervation of the adult rat lower lip has been investigated after unilateral section of the mental nerve. Rats were sacrificed at 4, 7, 9, 14, 30, and 90 days after the operation. A further group of animals with section of the mental nerve and block of the alveolar nerve regeneration, was sacrificed at 14 days. Specimens were processed for immunocytochemistry with antibodies against PGP 9.5, GAP-43 or neuropeptides (CGRP, SP and VIP). Four days after nerve section, axonal degeneration seems evident in the mental nerve branches and inside skin and mucosa. GAP-43 immunoreactivity is intense in the mental nerve 7 days after nerve section and it reaches its maximal expression and distribution in peripheral nerve fibres at 14 days. At 30 days, the decline in its expression is associated with the increase of PGP9.5-, SP-, and CGRP immunopositivity. VIP is observed only in perivascular fibres at all times observed. Present results suggest that, after sensory denervation of the rat lip, nerve fibres in skin and mucosa remain at lower density than normal. The different time courses in the expression of neuropeptides and GAP-43 suggest a possible early involvement of GAP-43 in peripheral nerve regeneration.

Intrathecal administration of nerve growth factor delays GAP 43 expression and early phase regeneration of adult rat peripheral nerve

Whether nerve growth factor (NGF) promotes peripheral nerve regeneration in vivo, in particular in adults, is controversial. We therefore examined the effect of exogenous NGF on nerve regeneration and the expression of GAP 43 (growth-associated protein 43) in adult rats. NGF was infused intrathecally via an osmotic mini-pump, while control rats received artificial cerebrospinal fluid. Two days after the infusion was initiated, the right sciatic nerves were transected or crushed, and the animals allowed to survive for 3 to 11 days. The right DRG, the right proximal stump of the transected sciatic nerve, and the posterior horn of the spinal cord were examined by Western blotting, immunohistochemistry, and electron microscopy. GAP 43 immunoreactivity in the NGF-treated animals was significantly lower than in the aCSF-treated controls. Electron microscopy showed that the number of myelinated and unmyelinated axons decreased significantly in the NGF-treated rats as compared with the controls. These findings are indicative that exogenous NGF delayed GAP 43 induction and the early phase of peripheral nerve regeneration and supports the hypothesis that the loss of NGF supply from peripheral targets via retrograde transport caused by axotomy serves as a signal for DRG neurons to invoke regenerative responses. NGF administered intrathecally may delay the neurons' perception of the reduction of the endogenous NGF, causing a delay in conversion of DRG neurons from the normal physiological condition to regrowth state.

Increased expression of GAP-43 in small sensory neurons after stimulation by NGF indicative of neuroregeneration in capsaicin-treated rats

Intraplantar injections of human recombinant nerve growth factor (rhNGF-beta) into the hind paw of capsaicin-treated adult rats are known to lead to a recovery of depleted peptide transmitter substances, to the immunohistochemical reappearance of peptidergic innervation in the skin and in the dorsal horn of the spinal cord, as well as to a recovery of the function of capsaicin-lesioned neurons. In the present study a marker peptide for neuronal regeneration and outgrowth, growth associated protein 43 (GAP-43), was investigated in lumbar dorsal root ganglia (DRGs) and in the hindpaw skin, in order to differentiate which population of the sensory neurons responds with a neuroregenerative behaviour. In situ hybridization histochemistry (ISH) revealed that at day 8 after the capsaicin treatment GAP-43 expression was significantly increased in small DRG cells as compared to control animals, and treatment with NGF in capsaicinized rats lead to an even more pronounced increase of GAP-43 expression in the small-sized cell population. Intraepidermal labelling of GAP-43 peptide was observed in the skin of control animals, but was markedly reduced in the animals that were treated with capsaicin alone. However, intraepidermal GAP-43 immunoreactive (GAP-43-IR) fibres nearly fully recovered in the capsaicin + NGF-treated group. These results indicate that the population of small DRG cells shows spontaneous regenerative activity after a capsaicin lesion which does not lead to a successful recovery of nerve terminals in the skin. Only after an additional NGF treatment small DRG cells show an even stronger regenerative response which now also involves structural reorganization of neuron membranes and axogenesis in the periphery.

Neuropeptides, nitric oxide synthase and GAP-43 in B4-binding and RT97 immunoreactive primary sensory neurons: normal distribution pattern and changes after peripheral nerve transection and aging

We have here sought to cross-correlate the expression of immunoreactivities for several neuropeptides, nitric oxide synthase (NOS) and the growth associated protein GAP-43 in subpopulations of dorsal root ganglion (DRG) neurons tagged by the selective markers isolectin B4 and the neurofilament antibody RT97, selective for, respectively, subpopulations of small and large DRG neurons. By use of double- and triple-labeling immunohistochemistry, non-manipulated and sciatic nerve transected young adult rats as well as aged (30-months-old) rats were examined using a confocal microscope equipped with enhanced spectral separation. In young adult rats, the DRG neuron profiles could be divided into three subpopulations (B4 binding (B4+) approximately 50%; RT97-immunoreactive (RT97+) approximately 35%; B4-/RT97- approximately 15%). Calcitonin gene-related peptide (CGRP) is expressed in all three subpopulations. Galanin message-associated peptide (GMAP) colocalize with CGRP (100%) but is not expressed in RT97+ profiles. NOS is present in the RT97- subpopulations and frequently colocalize with CGRP (92%). GAP-43 is expressed in all three DRG subpopulations and colocalize with CGRP (88%), GMAP (38%) and/or NOS (22%). Only very small differences were seen among the young adult rats, implicating that the size of respective subpopulation as well as the expression pattern for neuropeptides, NOS and GAP-43 are fairly stable. Sciatic nerve transection reduced B4-binding but not RT97-like immunoreactivity. Distinct changes in the expression of neuropeptides, NOS and GAP-43 were evident in the DRG subpopulations and, furthermore, the regulatory changes were very similar among the lesioned animals. The relative size of the DRG subpopulations was unaffected by aging, while the expression of neuropeptides was altered showing similarities with the changes induced by axotomy in young adult rats.

Peripheral axotomy induces long-term c-Jun amino-terminal kinase-1 activation and activator protein-1 binding activity by c-Jun and junD in adult rat dorsal root ganglia In vivo

One of the earliest documented molecular events after sciatic nerve injury in adult rats is the rapid, long-term upregulation of the immediate early gene transcription factor c-Jun mRNA and protein in lumbar dorsal root ganglion (DRG) neurons, suggesting that c-Jun may regulate genes that are important both in the early post-injury period and during later peripheral axonal regeneration. However, neither the mechanism through which c-Jun protein is increased nor the level of its post-injury transcriptional activity in axotomized DRGs has been characterized. To determine whether transcriptional activation of c-Jun occurs in response to nerve injury in vivo and is associated with axonal regeneration, we have assayed axotomized adult rat DRGs for evidence of jun kinase activation, c-Jun phosphorylation, and activator protein-1 (AP-1) binding. We report that sciatic nerve transection resulted in chronic activation of c-Jun amino-terminal kinase-1 (JNK) in L4/L5 DRGs concomitant with c-Jun amino-terminal phosphorylation in neurons, and lasting AP-1 binding activity, with both c-Jun and JunD participating in DNA binding complexes. The timing of JNK activation was dependent on the distance of the axotomy site from the DRGs, suggesting the requirement for a retrograde transport-mediated signal. AP-1 binding and c-Jun protein returned to basal levels in DRGs as peripheral regeneration was completed but remained elevated in the case of chronic sprouting, indicating that c-Jun may regulate target genes that are involved in axonal outgrowth.

Mechanisms of enhancement of neurite regeneration in vitro following a conditioning sciatic nerve lesion

To examine the mechanisms responsible for the more rapid nerve regeneration observed after a previous (conditioning) nerve injury, adult rats were subjected to a midthigh sciatic nerve transection by using one of three protocols designed to facilitate or restrict nerve regeneration: 1) ligation, in which transected axons were prevented from regenerating; 2) cut, in which transected axons were permitted to extend into peripheral target tissue but were separated from the denervated peripheral nerve stump; and 3) crush, in which axons could regenerate normally through the denervated distal nerve tract. The affected dorsal root ganglia (DRG) were subsequently removed, dissociated, and cultured for up to 3 days, and the timing of neurite initiation, rate of outgrowth, and arborization pattern of previously injured neurons were compared with control DRG. Our results indicate that conditioning lesions have at least four distinct and differentially regulated effects on neuronal morphogenesis: 1) conditioning lesions promote earlier neurite initiation, 2) prior nerve injury decreases the ability of neurons to extend long neurites following a second axotomy, 3) exposure to the environment of a denervated peripheral nerve stimulates greater initial rates of neurite outgrowth, and 4) conditioning lesions reduces initial neuritic branching frequency, resulting in straighter neurites whose growth cones extend further distances from their cell bodies. The primary effect of all conditioning lesions on cultured DRG neurons appeared to be to advance the timing of morphogenesis, resulting in conditioning-lesioned neurons that exhibited characteristics consistent with control neurons that had been cultured for an additional day or more. A secondary effect of conditioning lesions on neurite outgrowth rates was dependent on the local environment of the axons prior to culturing.

Schwann cells modulate sodium channel expression in spinal sensory neurons in vitro

In order to study the factors that govern the expression of sodium channel alpha-, beta1- and beta2-subunits, the influence that Schwann cells (SC) exert in the expression of sodium channels in DRG neurons was examined with in situ hybridization, immunocytochemistry, and patch clamp recording. The expression of sodium channel alpha-, beta1-, and beta2-subunit mRNAs in DRG neurons isolated from E15 rats cultured in defined medium in the absence (control) or presence of SC, or in SC-conditioned medium, was examined with isoform-specific riboprobes for sodium channel alpha-subunits I, II, III, NaG, Na6, hNE/PN1, SNS, and beta1- and beta2-subunits. DRG neurons cultured in the presence of SC displayed a significant (P < 0.05) increase in the hybridization signal for NaG, Na6, SNS, and Na beta2 mRNAs in comparison to control DRG neurons. In contrast, in SC-conditioned medium, only the hybridization signal for SNS mRNA was significantly increased. The upregulation of sodium channel mRNAs in DRG neurons co-cultured with SC was paralleled by an increase in sodium channel immunoreactivity of these cells. An increase in the mean sodium current density in DRG neurons in the presence of SC was also observed. These results demonstrate that a SC-derived factor selectively upregulates sodium channel alpha- and beta-subunit mRNAs in DRG neurons isolated from E15 rats that is reflected in an increase in functional sodium channels in these cells. This culture system may allow elucidation of the SC factor(s) that modulate the expression of sodium channels in DRG neurons.

B-50, the growth associated protein-43: modulation of cell morphology and communication in the nervous system

The growth-associated protein B-50 (GAP-43) is a presynaptic protein. Its expression is largely restricted to the nervous system. B-50 is frequently used as a marker for sprouting, because it is located in growth cones, maximally expressed during nervous system development and re-induced in injured and regenerating neural tissues. The B-50 gene is highly conserved during evolution. The B-50 gene contains two promoters and three exons which specify functional domains of the protein. The first exon encoding the 1-10 sequence, harbors the palmitoylation site for attachment to the axolemma and the minimal domain for interaction with G0 protein. The second exon contains the "GAP module", including the calmodulin binding and the protein kinase C phosphorylation domain which is shared by the family of IQ proteins. Downstream sequences of the second and non-coding sequences in the third exon encode species variability. The third exon also contains a conserved domain for phosphorylation by casein kinase II. Functional interference experiments using antisense oligonucleotides or antibodies, have shown inhibition of neurite outgrowth and neurotransmitter release. Overexpression of B-50 in cells or transgenic mice results in excessive sprouting. The various interactions, specified by the structural domains, are thought to underlie the role of B-50 in synaptic plasticity, participating in membrane extension during neuritogenesis, in neurotransmitter release and long-term potentiation. Apparently, B-50 null-mutant mice do not display gross phenotypic changes of the nervous system, although the B-50 deletion affects neuronal pathfinding and reduces postnatal survival. The experimental evidence suggests that neuronal morphology and communication are critically modulated by, but not absolutely dependent on, (enhanced) B-50 presence.

A transcription-dependent switch controls competence of adult neurons for distinct modes of axon growth

Although maturing neurons undergo a precipitous decline in the expression of genes associated with developmental axon growth, structural changes in axon arbors occur in the adult nervous system under both normal and pathological conditions. Furthermore, some neurons support extensive regrowth of long axons after nerve injury. Analysis of adult dorsal root ganglion (DRG) neurons in culture now shows that competence for distinct types of axon growth depends on different patterns of gene expression. In the absence of ongoing transcription, newly isolated neurons can extend compact, highly branched arbors during the first day in culture. Neurons subjected to peripheral axon injury 2-7 d before plating support a distinct mode of growth characterized by rapid extension of long, sparsely branched axons. A transition from "arborizing" to "elongating" growth occurs in naive adult neurons after approximately 24 hr in culture but requires a discrete period of new transcription after removal of the ganglia from the intact animal. Thus, peripheral axotomy-by nerve crush or during removal of DRGs--induces a transcription-dependent change that alters the type of axon growth that can be executed by these adult neurons. This transition appears to be triggered, in large part, by interruption of retrogradely transported signals, because blocking axonal transport in vivo can elicit competence for elongating growth in many DRG neurons. In contrast to peripheral axotomy, interruption of the centrally projecting axons of DRG neurons in vivo leads to subsequent growth in vitro that is intermediate between "arborizing" and "elongating" growth. This suggests that the transition between these two modes of growth is a multistep process and that individual steps may be regulated separately. These observations together suggest that structural remodeling in the adult nervous system need not involve the same molecular apparatus as long axon growth during development and regeneration.

Comparative effects of FK-506, rapamycin and cyclosporin A, on the in vitro differentiation of dorsal root ganglia explants and septal cholinergic neurons

There is increasing evidence that immunophilins play a role in neural development and differentiation. We have studied the neurotrophic effects of FK-506, rapamycin and cyclosporin A (CsA) on dorsal root ganglia (DRG) taken from different segmental levels (cervical, thoracic and lumbar/sacral), and on rat embryonic septal cholinergic neurons in culture. At a low concentration (1 nM), FK-506 significantly increased (+83%) the number of neurites of thoracic DRG explants. At a higher concentration (100 nM), it also enhanced the neuritogenesis of thoracic (+100%) and lumbar/sacral (+57%) DRG, but not cervical DRG explants. Rapamycin displayed a converse effect, reducing the development of DRG explants from cervical and thoracic segments (-78% at 1 nM in thoracic DRG). CsA (from 1 to 100 nM) was without effect on DRG neuritogenesis. In contrast to nerve growth factor (NGF), which increased neurite length (+116% at 3 ng/ml), neither FK-506 nor rapamycin affected this parameter.

Sympathectomy induces c-Jun in adult trigeminal neurons: an immunohistochemical and tract-tracing study

The induction of the immediate early gene product c-Jun was investigated in trigeminal ganglia following surgical removal of the superior cervical ganglion (SCG). A 5-fold increase in c-Jun-immunoreactive neurons was detected by 48 h post-surgery in ipsilateral trigeminal ganglia of sympathectomized rats. This increase persisted for 6 days. When examined 4 months after sympathectomy, c-Jun expression had returned to basal levels. The possibility that trigeminal neurons project to the SCG, and therefore induce c-Jun by being axotomized by sympathectomy, was also examined using retrograde fluorescent tracing. A very limited number of trigeminal neurons were retrogradely labeled from the SCG. These data indicate that c-Jun induction occurs in sensory neurons following perturbations to sympathetic ganglia, and that trigeminal neurons may project to the SCG, but that c-Jun induction cannot be ascribed to axotomy following surgical sympathectomy. Thus, these findings support anatomical and functional interconnections between the sensory and autonomic nervous systems. The relationship of c-Jun induction with plasticity phenomena is discussed.

Regenerating sensory neurones of diabetic rats express reduced levels of mRNA for GAP-43, gamma-preprotachykinin and the nerve growth factor receptors, trkA and p75NGFR

Nerve growth factor (NGF) is considered to play a role in neurite outgrowth of small nerve fibres which express its high-affinity receptor, trkA. Nerve regeneration is delayed in diabetes mellitus following an experimental crush injury. In steady-state (i.e., in the absence of axotomy) diabetic rats also show reduced expression of NGF in certain target tissues. This study was designed to measure expression of messenger RNA (mRNA) coding for NGF and its receptors, trkA and p75NGFR, during nerve regeneration and degeneration in rats with streptozotocin-induced diabetes; mRNA coding for preprotachykinin A (the substance P precursor), whose expression is stimulated by NGF, and mRNA for growth-associated protein-43 (GAP-43) were also measured in blots from L4 + L5 (pooled unilaterally) dorsal root ganglia. Unexpectedly, distal stumps of diabetic injured sciatic nerve contained higher levels of NGF mRNA than those of control rats. In ipsilateral dorsal root ganglia of control animals, mRNA for trkA and preprotachykinin A were decreased and GAP-43 mRNA increased after nerve injury; mRNA for p75NGFR was decreased only 3 weeks after nerve transection. In diabetic rats, the levels of all of these mRNA, both in intact and lesioned dorsal root ganglia, were lower than those from control rats. These results suggest that regenerating sensory neurones of diabetic rats receive less NGF support in spite of enhanced NGF mRNA levels in distal stumps compared to non-diabetic rats. Reduced expression of its high-affinity receptor, trkA, in ganglia of diabetic rats might explain this discrepancy.

Neurite outgrowth and GAP-43 mRNA expression in cultured adult rat dorsal root ganglion neurons: effects of NGF or prior peripheral axotomy

Adult dorsal root ganglion (DRG) cells are capable of neurite outgrowth in vivo and in vitro after axotomy. We have investigated, in cultured adult rat DRG cells, the relative influence of nerve growth factor (NGF) or a prior peripheral nerve lesion on the capacity of these neurons to produce neurites. Since there is evidence suggesting that the growth-associated protein GAP-43 may play a crucial role in axon elongation during development and regeneration, we have also compared the effect of these treatments on GAP-43 mRNA expression. NGF increased the early neurite outgrowth in a subpopulation of DRG cells. This effect was substantially less, however, than that resulting from preaxotomy, which initiated an early and profuse neurite outgrowth in almost all cells. No difference in the expression of GAP-43 mRNA was found between neurons grown in the presence or absence of NGF over 1 week of culture, in spite of the increased growth produced by NGF. In contrast, cultures of neurons that had been preaxotomized showed substantial increases in GAP-43 mRNA and NGF had, as expected, a significant effect on substance P mRNA levels. Two forms of growth may be present in adult DRG neurons: an NGF-independent, peripheral nerve injury-provoked growth associated with substantial GAP-43 upregulation, and an NGF-dependent growth that may underlie branching or sprouting of NGF-sensitive neurons, but which is not associated with increased levels of GAP-43 mRNA.

Nitric oxide synthase-like immunoreactivity in lumbar dorsal root ganglia and spinal cord of rat and monkey and effect of peripheral axotomy

With the immunofluorescence technique, nitric oxide synthase (NOS)-like immunoreactivity (LI) was found in a few medium-sized and small sensory neurons in lumbar (L) 4 and L5 dorsal root ganglia (DRG) of normal rat, and in most of these neurons, NOS-LI coexisted with calcitonin gene-related peptide and sometimes with substance P and galanin. NOS-immunoreactive nerve fibers, terminals and small neurons were also located in the dorsal horn of the segments 4 and 5 of the rat lumbar spinal cord with the highest density in inner lamina II. Many NOS-positive neurons and fibers were seen in the area around the central canal. A sparse network of NOS-immunoreactive nerve fibers was found in the ventral horn. After unilateral sciatic nerve cut in the rat, the number of NOS-positive neurons increased in the ipsilateral L4 and L5 DRGs, mainly in medium and small neurons, but also in some large neurons and very small neurons. NOS-LI could now also be seen in the ipsilateral dorsal roots, and in an increased number of fibers and terminals in both outer and inner lamina II of the ipsilateral dorsal horn. The number of NOS-immunoreactive neurons in lamina II of the ipsilateral dorsal horn was reduced. In the monkey L4 and L5 DRGs, many small neurons were NOS-immunoreactive, but only a few weakly stained nerve fibers and terminals were found in laminae I-IV of the dorsal horn at L4 and L5 lumbar levels. A few NOS-positive neurons were present in lamina X. The number of NOS-immunoreactive neurons was somewhat reduced in DRGs 14 days after peripheral axotomy, but no certain effect was seen in the dorsal horn. These results, together with earlier in situ hybridization studies, demonstrate that axotomy in rat induces a marked upregulation of NOS synthesis in primary sensory neurons, thus suggesting a role for NO in lesioned sensory neurons. In contrast, no such effect was recorded in monkey, perhaps indicating distinct species differences.