Marked increase in nitric oxide synthase mRNA in rat dorsal root ganglia after peripheral axotomy: in situ hybridization and functional studies

Vagus nerve stimulation and Neurotrophins: a biological psychiatric perspective

Since 2004, vagus nerve stimulation (VNS) has been used in treatment-resistant or treatment-intolerant depressive episodes. Today, VNS is suggested as possible therapy for a larger spectrum of psychiatric disorders, including schizophrenia, obsessive compulsive disorders, and panic disorders. Despite a large body of literature supports the application of VNS in patients' treatment, the exact mechanism of action of VNS remains not fully understood. In the present study, the major knowledges on the brain areas and neuronal pathways regulating neuroimmune and autonomic response subserving VNS effects are reviewed. Furthermore, the involvement of the neurotrophins (NTs) Nerve Growth Factor (NGF) and Brain Derived Neurotrophic Factor (BDNF) in vagus nerve (VN) physiology and stimulation is revised. The data on brain NGF/BDNF synthesis and in turn on the activity-dependent plasticity, connectivity rearrangement and neurogenesis, are presented and discussed as potential biomarkers for optimizing stimulatory parameters for VNS. A vagus nerve-neurotrophin interaction model in the brain is finally proposed as a working hypothesis for future studies addressed to understand pathophysiology of psychiatric disturbance.

Nitric oxide as a messenger between neurons and satellite glial cells in dorsal root ganglia

Abnormal neuronal activity in sensory ganglia contributes to chronic pain. There is evidence that signals can spread between cells in these ganglia, which may contribute to this activity. Satellite glial cells (SGCs) in sensory ganglia undergo activation following peripheral injury and participate in cellular communication via gap junctions and chemical signaling. Nitric oxide (NO) is released from neurons in dorsal root ganglia (DRG) and induces cyclic GMP (cGMP) production in SCGs, but its role in SGC activation and neuronal excitability has not been explored. It was previously reported that induction of intestinal inflammation with dinitrobenzoate sulfonate (DNBS) increased gap junctional communications among SGCs, which contributed to neuronal excitability and pain. Here we show that DNBS induced SGC activation in mouse DRG, as assayed by glial fibrillary acidic protein upregulation. DNBS also upregulated cGMP level in SGCs, consistent with NO production. In vitro studies on intact ganglia from DNBS-treated mice showed that blocking NO synthesis inhibited both SGCs activation and cGMP upregulation, indicating an ongoing NO production. Application of NO donor in vitro induced SGC activation, augmented gap junctional communications, and raised neuronal excitability, as assessed by electrical recordings. The cGMP analog 8-Br-cGMP mimicked these actions, confirming the role of the NO-cGMP pathway in intraganglionic communications. NO also augmented Ca²⁺ waves propagation in DRG cultures. It is proposed that NO synthesis in DRG neurons increases after peripheral inflammation and that NO induces SGC activation, which in turn contributes to neuronal hyperexcitability. Thus, NO plays a major role in neuron-SGC communication.

Nitric Oxide Synthase (NOS) Isoform Expression after Peripheral Nerve Transection in Mice

Localization of the nitric oxide (NO)-producing enzyme, nitric oxide synthase (NOS), and its functions are currently being investigated in several tissues and organs. It has been suggested that NO is involved in nerve cell death and the development of neurodegenerative disease. The purpose of this study was to immunohistochemically investigate expression of NOS to clarify its function in the degeneration and regeneration of transected mouse sciatic nerve. Scattered neuronal NOS (nNOS)-positive Schwann cells observed on the central side of the stump on day 1 after transection showed an increase in number on day 7. None were observed at the stump on day 14, however. Expression of nNOS was observed in axons extending from the stump. The number of nNOS-positive axons increased on day 21. Inducible NOS was expressed in inflammatory cells at the stump on day 1. This positive reaction subsequently weakened by day 7, however. Endothelial NOS was expressed in blood vessels at the stump on day 7, but decreased thereafter. The results of the present study suggest that NO is involved in the proliferation and migration of Schwann cells, as well as in axon regeneration at an early stage following nerve transection.

Activation of neurotrophins in lumbar dorsal root probably contributes to neuropathic pain after spinal nerve ligation

OBJECTIVES

Neurotrophins (NTs) exert various effects on neuronal system. Growing evidence indicates that NTs are involved in the pathophysiology of neuropathic pain. However, the exact role of these proteins in modulating nociceptive signaling requires being defined. Thus, the aim of this study was to evaluate the effects of spinal nerve ligation (SNL) on NTs activation in the lumbar dorsal root.

MATERIALS AND METHODS

Ten male Wistar rats were randomly assigned to two groups: tight ligation of the L5 spinal nerve (SNL: n=5) and Sham (n=5). In order to produce neuropathic pain, the L5 spinal nerve was tightly ligated (SNL). Then, allodynia and hyperalgesia tests were conducted weekly. After 4 weeks, tissue samples were taken from the two groups for laboratory evaluations. Here, Real-Time PCR quantity method was used for measuring NTs gene expression levels.

RESULTS

SNL resulted in a significant weight loss in the soleus muscle (P<0.05), mechanical allodynia and thermal hyperalgesia thresholds (respectively, P<0.05; P<0.05). Also, NGF, NT-4, NT-3, TrkA, TrkB and TrkC expression were up-regulated following spinal nerve ligation group (respectively, P=0.025, P=0.013, P=0.001, P=0.002, P<0.001, P=001) (respectively, 4.7, 5.2, 7.5, 5.1, 7.2, 6.2 folds).

CONCLUSION

The present study provides new evidence that neuropathic pain induced by spinal nerve ligation probably activates NTs and Trk receptors expression in DRG. However, further studies are needed to better elucidate the role of NTs in a neuropathic pain.

Orofacial Pain in Cancer: Part I—Mechanisms

The mechanisms involved, and possible treatment targets, in orofacial pain due to cancer are poorly understood. The aim of the first of this two-part series is to review the involved pathophysiological mechanisms and explore their possible roles in the orofacial region. However, there is a lack of relevant research in the trigeminal region, and we have therefore applied data accumulated from experiments on cancer pain mechanisms in rodent spinal models. In the second part, we review the clinical presentation of cancer-associated orofacial pain at various stages: initial diagnosis, during therapy (chemo-, radiotherapy, surgery), and in the post-therapy period. In the present article, we provide a brief outline of trigeminal functional neuro-anatomy and pain-modulatory pathways. Tissue destruction by invasive tumors (or metastases) induces inflammation and nerve damage, with attendant acute pain. In some cases, chronic pain, involving inflammatory and neuropathic mechanisms, may ensue. Distant, painful effects of tumors include paraneoplastic neuropathic syndromes and effects secondary to the release of factors by the tumor (growth factors, cytokines, and enzymes). Additionally, pain is frequent in cancer management protocols (surgery, chemotherapy, and radiotherapy). Understanding the mechanisms involved in cancer-related orofacial pain will enhance patient management.

Mechanisms of diabetic neuron damage: Molecular pathways

Diabetic polyneuropathy (DPN) is a common but intractable degenerative disorder of peripheral neurons. DPN first results in retraction and loss of sensory terminals in target organs such as the skin, whereas the perikarya (cell bodies) of neurons are relatively preserved. This is important because it implies that regrowth of distal terminals, rather than neuron replacement or rescue, may be useful clinically. Although a number of neuronal molecular abnormalities have been examined in experimental DPN, several are prominent: loss of structural proteins, neuropeptides, and neurotrophic receptors; upregulation of "stress" and "repair" proteins; elevated nitric oxide synthesis; increased AGE-RAGE signaling, NF-κB and PKC; altered neuron survival pathways; changes of pain-related ion channel investment. There is also a role for abnormalities of direct signaling of neurons by insulin, an important trophic factor for neurons that express its receptors. While evidence implicating each of these pathways has emerged, how they link together and result in neuronal degeneration remains unclear. However, several offer interesting new avenues for more definitive therapy of this condition.

Immunohistochemical Detection of Islet-1 and Neuronal Nitric Oxide Synthase in the Dorsal Root Ganglia (DRG) of Sheep Fetuses During Gestation

This study first investigated the ontogeny of Islet-1 and neuronal nitric oxide synthase (nNOS) expression and their co-localization in the DRG of sheep fetuses during gestation by immunohistochemistry (IHC). The results showed that Islet-1 and nNOS were located in the nuclei and cytoplasm of DRG neurons, respectively. The relative percentages of Islet-1-immunopositive (Islet-1+) neurons accounting for the total DRG neurons were 90%, 79%, 66%, and 53% at days 60, 90, and 120 of gestation and postnatally, respectively. The percentage of nNOS-immunopositive (nNOS+) neurons was 94% at day 60 and declined to ∼30% at day 90, with no obvious further change until the postnatal period. Dual IHC showed that ∼69% Islet-1+ neurons express nNOS at day 60 of gestation. This proportion declined to ∼24% at day 90, after which there was no significant change until birth. We also observed that most Islet-1+ and nNOS+ neurons belonged to small and medium-sized DRG neurons from day 90 of gestation to the postnatal period. These results suggest that both Islet-1 and nNOS are important for the differentiation and maintenance of some specific phenotypes of DRG neurons during late gestation of sheep fetuses, although the related mechanisms need to be further elucidated. (J Histochem Cytochem 52:797–803, 2004)

Development of nNOS-positive neurons in the rat sensory ganglia after capsaicin treatment

To gain a better understanding of the neuroplasticity of afferent neurons during postnatal ontogenesis, the distribution of neuronal nitric oxide synthase (nNOS) immunoreactivity was studied in the nodose ganglion (NG) and Th2 and L4 dorsal root ganglia (DRG) from vehicle-treated and capsaicin-treated female Wistar rats at different ages (10-day-old, 20-day-old, 30-day-old, and two-month-old). The percentage of nNOS-immunoreactive (IR) neurons decreased after capsaicin treatment in all studied ganglia in first 20 days of life, from 55.4% to 36.9% in the Th2 DRG, from 54.6% to 26.1% in the L4 DRG and from 37.1% to 15.0% in the NG. However, in the NG, the proportion of nNOS-IR neurons increased after day 20, from 11.8% to 23.9%. In the sensory ganglia of all studied rats, a high proportion of nNOS-IR neurons bound isolectin B4. Approximately 90% of the sensory nNOS-IR neurons bound to IB4 in the DRG and approximately 80% in the NG in capsaicin-treated and vehicle-treated rats. In 10-day-old rats, a large number of nNOS-IR neurons also expressed TrkA, and the proportion of nNOS(+)/TrkA(+) neurons was larger in the capsaicin-treated rats compared with the vehicle-treated animals. During development, the percentage of nNOS(+)/TrkA(+) cells decreased in the first month of life in both groups. The information provided here will also serve as a basis for future studies investigating mechanisms of sensory neuron development.

Redox and nitric oxide-mediated regulation of sensory neuron ion channel function

SIGNIFICANCE

Reactive oxygen and nitrogen species (ROS and RNS, respectively) can intimately control neuronal excitability and synaptic strength by regulating the function of many ion channels. In peripheral sensory neurons, such regulation contributes towards the control of somatosensory processing; therefore, understanding the mechanisms of such regulation is necessary for the development of new therapeutic strategies and for the treatment of sensory dysfunctions, such as chronic pain.

RECENT ADVANCES

Tremendous progress in deciphering nitric oxide (NO) and ROS signaling in the nervous system has been made in recent decades. This includes the recognition of these molecules as important second messengers and the elucidation of their metabolic pathways and cellular targets. Mounting evidence suggests that these targets include many ion channels which can be directly or indirectly modulated by ROS and NO. However, the mechanisms specific to sensory neurons are still poorly understood. This review will therefore summarize recent findings that highlight the complex nature of the signaling pathways involved in redox/NO regulation of sensory neuron ion channels and excitability; references to redox mechanisms described in other neuron types will be made where necessary.

CRITICAL ISSUES

The complexity and interplay within the redox, NO, and other gasotransmitter modulation of protein function are still largely unresolved. Issues of specificity and intracellular localization of these signaling cascades will also be addressed.

FUTURE DIRECTIONS

Since our understanding of ROS and RNS signaling in sensory neurons is limited, there is a multitude of future directions; one of the most important issues for further study is the establishment of the exact roles that these signaling pathways play in pain processing and the translation of this understanding into new therapeutics.

Flexibilide obtained from cultured soft coral has anti-neuroinflammatory and analgesic effects through the upregulation of spinal transforming growth factor-β1 in neuropathic rats

Chronic neuroinflammation plays an important role in the development and maintenance of neuropathic pain. The compound flexibilide, which can be obtained from cultured soft coral, possesses anti-inflammatory and analgesic effects in the rat carrageenan peripheral inflammation model. In the present study, we investigated the antinociceptive properties of flexibilide in the rat chronic constriction injury (CCI) model of neuropathic pain. First, we found that a single intrathecal (i.t.) administration of flexibilide significantly attenuated CCI-induced thermal hyperalgesia at 14 days after surgery. Second, i.t. administration of 10-μg flexibilide twice daily was able to prevent the development of thermal hyperalgesia and weight-bearing deficits in CCI rats. Third, i.t. flexibilide significantly inhibited CCI-induced activation of microglia and astrocytes, as well as the upregulated proinflammatory enzyme, inducible nitric oxide synthase, in the ipsilateral spinal dorsal horn. Furthermore, flexibilide attenuated the CCI-induced downregulation of spinal transforming growth factor-β1 (TGF-β1) at 14 days after surgery. Finally, i.t. SB431542, a selective inhibitor of TGF-β type I receptor, blocked the analgesic effects of flexibilide in CCI rats. Our results suggest that flexibilide may serve as a therapeutic agent for neuropathic pain. In addition, spinal TGF-β1 may be involved in the anti-neuroinflammatory and analgesic effects of flexibilide.

Neuromuscular complications of critical illness

Patients admitted to intensive care units (ICUs) suffer from a wide range of neurological disorders. Some develop within the ICU rendering weakness and difficulty in weaning patients from ventilator support. ICUAW, or ICU acquired weakness, is a broad term that includes several more specific neuromuscular problems. After exclusion of other causes of weakness, ICUAW includes critical illness polyneuropathy (CIP), first described by Charles Bolton, critical illness myopathy (CIM), and disorders of neuromuscular junction transmission. This chapter reviews the clinical, electrophysiological, and pathological features of these conditions and provides clinicians with approaches toward diagnosing and investigating ICUAW.

A molecular mechanism of optic nerve regeneration in fish: the retinoid signaling pathway

The fish optic nerve regeneration process takes more than 100 days after axotomy and comprises four stages: neurite sprouting (1-4 days), axonal elongation (5-30 days), synaptic refinement (35-80 days) and functional recovery (100-120 days). We screened genes specifically upregulated in each stage from axotomized fish retina. The mRNAs for heat shock protein 70 and insulin-like growth factor-1 rapidly increased in the retinal ganglion cells soon after axotomy and function as cell-survival factors. Purpurin mRNA rapidly and transiently increased in the photoreceptors and purpurin protein diffusely increased in all nuclear layers at 1-4 days after injury. The purpurin gene has an active retinol-binding site and a signal peptide. Purpurin with retinol functions as a sprouting factor for thin neurites. This neurite-sprouting effect was closely mimicked by retinoic acid and blocked by its inhibitor. We propose that purpurin works as a retinol transporter to supply retinoic acid to damaged RGCs which in turn activates target genes. We also searched for genes involved in the second stage of regeneration. The mRNA of retinoid-signaling molecules increased in retinal ganglion cells at 7-14 days after injury and tissue transglutaminase and neuronal nitric oxide synthase mRNAs, RA-target genes, increased in retinal ganglion cells at 10-30 days after injury. They function as factors for the outgrowth of thick, long neurites. Here we present a retinoid-signaling hypothesis to explain molecular events during the early stages of optic nerve regeneration in fish.

Sensory and signaling mechanisms of bradykinin, eicosanoids, platelet-activating factor, and nitric oxide in peripheral nociceptors

Peripheral mediators can contribute to the development and maintenance of inflammatory and neuropathic pain and its concomitants (hyperalgesia and allodynia) via two mechanisms. Activation or excitation by these substances of nociceptive nerve endings or fibers implicates generation of action potentials which then travel to the central nervous system and may induce pain sensation. Sensitization of nociceptors refers to their increased responsiveness to either thermal, mechanical, or chemical stimuli that may be translated to corresponding hyperalgesias. This review aims to give an account of the excitatory and sensitizing actions of inflammatory mediators including bradykinin, prostaglandins, thromboxanes, leukotrienes, platelet-activating factor, and nitric oxide on nociceptive primary afferent neurons. Manifestations, receptor molecules, and intracellular signaling mechanisms of the effects of these mediators are discussed in detail. With regard to signaling, most data reported have been obtained from transfected nonneuronal cells and somata of cultured sensory neurons as these structures are more accessible to direct study of sensory and signal transduction. The peripheral processes of sensory neurons, where painful stimuli actually affect the nociceptors in vivo, show marked differences with respect to biophysics, ultrastructure, and equipment with receptors and ion channels compared with cellular models. Therefore, an effort was made to highlight signaling mechanisms for which supporting data from molecular, cellular, and behavioral models are consistent with findings that reflect properties of peripheral nociceptive nerve endings. Identified molecular elements of these signaling pathways may serve as validated targets for development of novel types of analgesic drugs.

Dose-dependent and combined effects of N-methyl-D-aspartate receptor antagonist MK-801 and nitric oxide synthase inhibitor nitro-L-arginine on the survival of retinal ganglion cells in adult hamsters

This study investigated the effects of daily intraperitoneal injections of N-methyl-D-aspartate receptor antagonist MK-801 and nitric oxide synthase inhibitor nitro-L-arginine (L-NA) on the survival of retinal ganglion cells (RGCs) at 1 and 2 weeks after unilateral optic nerve transection in adult hamsters. The left optic nerves of all animals were transected intraorbitally 1 mm from the optic disc and RGCs were retrogradely labeled with Fluorogold before they received different daily dosages of single MK-801 or L-NA as well as daily combinational treatments of these two chemicals. All experimental and control animals survived for 1 or 2 weeks after optic nerve transection. Our results revealed that the mean numbers of surviving RGCs increased and then decreased when the dosage of MK-801 (1.0, 3.0 and 4.5 mg/kg) and L-NA (1.5, 3.0, 4.5 and 6.0 mg/kg) increased at both 1 and 2 weeks survival time points. Daily combinational use of 1.0 mg/kg MK-801 and 1.5 mg/kg L-NA lead to a highest RGC number that was even higher than the sum of the RGC numbers in 1.0 mg/kg MK-801 and 1.5 mg/kg L-NA subgroups at 2 weeks. These findings indicated that both MK-801 and L-NA can protect axotomized RGCs in a dose-dependent manner and combinational treatment of these chemicals possesses a potentiative and protective effect.

Nitric oxide releasing nanoparticle synthesis and characterization

While the potential applications of nitric oxide for both understanding human physiology and treating disease are far reaching, the development of a reliable, cost-effective, and practical sustained delivery system for nitric oxide has yet to emerge. Using a sol-gel/glass hybrid system, we have demonstrated controlled, sustained release of nitric oxide from a stable, dry powder. Upon exposure to an aqueous environment, the material begins releasing therapeutic levels of nitric oxide over several hours to days, making it an ideal material for evaluation of nitric oxide efficacy for both clinical and research applications.

Combination of olfactory ensheathing cells with local versus systemic cAMP treatment after a cervical rubrospinal tract injury

The failure of CNS axons to regenerate following traumatic injury is due in part to a growth-inhibitory environment in CNS as well as a weak intrinsic neuronal growth response. Olfactory ensheathing cell (OECs) transplants have been reported to create a favorable environment promoting axonal regeneration, remyelination, and functional recovery after spinal cord injury. However, in our previous experiments, OEC transplants failed to promote regeneration of rubrospinal axons through and beyond the site of a dorsolateral funiculus crush in rats. Rubrospinal neurons undergo massive cell atrophy and limited expression of regeneration-associated genes after axotomy. Using the same injury model, we tested the hypothesis that treatment of the red nucleus with cAMP, known to stimulate the intrinsic growth response in other neurons, will promote rubrospinal regeneration in combination with OEC transplants. In addition, we assessed a systemic increase of cAMP using the phosphodiesterase inhibitor rolipram. OECs prevented cavity formation, attenuated astrocytic hypertrophy and the retraction of the axotomized rubrospinal axons, and tended to reduce the overall lesion size. OEC transplantation lowered the thresholds for thermal sensitivity of both forepaws. None of our treatments, alone or in combination, promoted rubrospinal regeneration through the lesion site. However, the systemic elevation of cAMP with rolipram resulted in greater numbers of OECs and axonal density within the graft and improved motor performance in a cylinder test in conjunction with enhanced rubrospinal branching and attenuated astrocytic hypertrophy.

The effect of botulinum neurotoxin A on sciatic nerve injury-induced neuroimmunological changes in rat dorsal root ganglia and spinal cord

Botulinum neurotoxin serotype A (BoNT/A) acts by cleaving synaptosome-associated-protein-25 (SNAP-25) in nerve terminals to inhibit neuronal release and shows long-lasting antinociceptive action in neuropathic pain. However, its precise mechanism of action remains unclear. Our study aimed to characterize BoNT/A-induced neuroimmunological changes after chronic constriction injury (CCI) of the sciatic nerve. In the ipsilateral lumbar spinal cords of CCI-exposed rats, the mRNA of microglial marker (complement component 1q, C1q), astroglial marker (glial fibrillary acidic protein, GFAP), and prodynorphin were upregulated, as measured by reverse transcription-polymerase chain reaction (RT-PCR). No changes appeared in mRNA for proenkephalin, pronociceptin, or neuronal and inducible nitric oxide synthase (NOS1 and NOS2, respectively). In the dorsal root ganglia (DRG), an ipsilateral upregulation of prodynorphin, pronociceptin, C1q, GFAP, NOS1 and NOS2 mRNA and a downregulation of proenkephalin mRNA were observed. A single intraplantar BoNT/A (75 pg/paw) injection induced long-lasting antinociception in this model. BoNT/A diminished the injury-induced ipsilateral spinal upregulation of C1q mRNA. In the ipsilateral DRG a significant decrease of C1q-positive cell activation and of the upregulation of prodynorphin, pronociceptin and NOS1 mRNA was also observed following BoNT/A admistration. BoNT/A also diminished the injury-induced upregulation of SNAP-25 expression in both structures. We provide evidence that BoNT/A impedes injury-activated neuronal function in structures distant from the injection site, which is demonstrated by its influence on NOS1, prodynorphin and pronociceptin mRNA levels in the DRG. Moreover, the silence of microglia/macrophages after BoNT/A administration could be secondary to the inhibition of neuronal activity, but this decrease in neuroimmune interactions could be the key to the long-lasting BoNT/A effect on neuropathic pain.

Nitroxidergic system in human trigeminal ganglia neurons: a quantitative evaluation

The trigeminal ganglia are involved in transmission of orofacial sensitivity. The free radical gas nitric oxide (NO) has recently been found to function as a messenger molecule in both central and peripheral trigeminal primary afferent neurons. NO is produced within neurons mainly by two enzymes: a constitutive (neuronal) form of NO synthase (nNOS) or an inducible form of NOS (iNOS). The aim of the study was to evaluate the distribution of trigeminal neurons according to size (small, medium and large neurons) and to correlate the percentage of NOS-immunopositive neurons with regard to neuronal size. The results showed a significant relationship between the percentage of nNOS-immunopositive neurons and the size of neurons. Evaluation of the percentage of nNOS-immunopositive neurons showed that they constitute about 50% of the total number of neurons and that they are represented mainly as large-sized neurons. The iNOS immunolabelling was very faint in all neuronal types. Since the nitroxidergic system is well represented in human trigeminal ganglia, this study indicates that it could play a relevant role in trigeminal neurotransmission.

Co-expression of GAP-43 and nNOS in avulsed motoneurons and their potential role for motoneuron regeneration

Neuronal nitric oxide synthase (nNOS) is induced after axonal injury. The role of induced nNOS in injured neurons is not well established. In the present study, we investigated the co-expression of nNOS with GAP-43 in spinal motoneurons following axonal injury. The role of induced nNOS was discussed and evaluated. In normal rats, spinal motoneurons do not express nNOS or GAP-43. Following spinal root avulsion, expression of nNOS and GAP-43 were induced and colocalized in avulsed motoneurons. Reimplantation of avulsed roots resulted in a remarkable decrease of GAP-43- and nNOS-IR in the soma of the injured motoneurons. A number of GAP-43-IR regenerating motor axons were found in the reimplanted nerve. In contrast, the nNOS-IR was absent in reimplanted nerve. These results suggest that expression of GAP-43 in avulsed motoneurons is related to axonal regeneration whereas nNOS is not.

Spinal motoneuron synaptic plasticity after axotomy in the absence of inducible nitric oxide synthase

Background

Astrocytes play a major role in preserving and restoring structural and physiological integrity following injury to the nervous system. After peripheral axotomy, reactive gliosis propagates within adjacent spinal segments, influenced by the local synthesis of nitric oxide (NO). The present work investigated the importance of inducible nitric oxide synthase (iNOS) activity in acute and late glial responses after injury and in major histocompatibility complex class I (MHC I) expression and synaptic plasticity of inputs to lesioned alpha motoneurons.

Methods

In vivo analyses were carried out using C57BL/6J-iNOS knockout (iNOS^-/-) and C57BL/6J mice. Glial response after axotomy, glial MHC I expression, and the effects of axotomy on synaptic contacts were measured using immunohistochemistry and transmission electron microscopy. For this purpose, 2-month-old animals were sacrificed and fixed one or two weeks after unilateral sciatic nerve transection, and spinal cord sections were incubated with antibodies against classical MHC I, GFAP (glial fibrillary acidic protein - an astroglial marker), Iba-1 (an ionized calcium binding adaptor protein and a microglial marker) or synaptophysin (a presynaptic terminal marker). Western blotting analysis of MHC I and nNOS expression one week after lesion were also performed. The data were analyzed using a two-tailed Student's t test for parametric data or a two-tailed Mann-Whitney U test for nonparametric data.

Results

A statistical difference was shown with respect to astrogliosis between strains at the different time points studied. Also, MHC I expression by iNOS^-/- microglial cells did not increase at one or two weeks after unilateral axotomy. There was a difference in synaptophysin expression reflecting synaptic elimination, in which iNOS^-/- mice displayed a decreased number of the inputs to alpha motoneurons, in comparison to that of C57BL/6J.

Conclusion

The findings herein indicate that iNOS isoform activity influences MHC I expression by microglial cells one and two weeks after axotomy. This finding was associated with differences in astrogliosis, number of presynaptic terminals and synaptic covering of alpha motoneurons after lesioning in the mutant mice.

Immunohistochemical characterization of TH13-L2 spinal ganglia neurons in sheep (Ovis aries)

Spinal ganglia (SG) neurons are commonly classified according to various specific features. The most widespread classification based on morphological and ultrastructural features subdivides SG neurons into light and small dark neurons. Using immunohistochemical, histochemical and lectin methods, it is possible to further subdivide the small dark neurons into two subpopulations: peptidergic and nonpeptidergic neurons. The majority of studies on SG neurons were carried out on mice and rats; there are few or no studies on large mammals. In this study, some of the widely used neuronal markers, neurofilament 200 kDa (NF200), substance P (SP), calcitonin gene-related peptide (CGRP) and isolectin B4 (IB4), were employed to characterize neuronal nitric oxide synthase (nNOS)-immunoreactivity (-IR) in sheep (Ovis aries) SG (Th13-L2) neurons. The majority of the SG neurons were IB4-labeled (79 +/- 10%), followed by NF200- (45 +/- 4%), NOS- (44 +/- 10%), SP- (42 +/- 5%) and CGRP-IR (35 +/- 7%) neurons. The triple staining experiments showed that a higher percentage (75 +/- 16%) of NOS-IR neurons bound both IB4 and CGRP, or both IB4 and SP (49 +/- 6%). The IB4 marker showed an unexpected staining pattern; in fact, IB4-labeled neurons largely colocalized with NF200, usually considered a marker of light SG neurons, and with CGRP and SP. For this reason, IB4 cannot be employed in sheep to differentiate between light and dark neurons, or between peptidergic and nonpeptidergic neurons. These results suggest the importance of being cautious when comparing data among different species.

Plasticity of nodose ganglion neurons after capsaicin- and vagotomy-induced nerve damage in adult rats

Previous reports show that vagal afferent innervation of the stomach eventually regenerates from surviving nodose ganglion (NG) neurons after subdiaphragmatic vagotomy. Systemic capsaicin treatment destroys gastric vagal afferent neurons expressing vanilloid receptor 1 (VR1). However, it is not known whether gastric innervation lost after neuronal destruction can be restored. Here, we report that capsaicin-induced damage of NG neurons innervating the stomach in adult rats is followed by restoration of vagal afferent projections. Specifically, we compared measures of neuronal plasticity in NG and vagi after subdiaphragmatic vagotomy or capsaicin treatment. The numbers of VR1-immunoreactive neurons projecting to the stomach were significantly reduced 10 days after either capsaicin treatment or vagotomy. However, the VR1-immunoreactive afferent innervation of the stomach was restored to levels exceeding those of vagotomized rats by 37 days after capsaicin, whereas neither total afferent innervation nor VR1-immunoreactive innervation reached control levels, even by 67 days after vagotomy. Capsaicin treatment significantly increased NG neuronal nitric oxide synthase (nNOS) immunoreactivity at 10 days after capsaicin, and this increase was sustained for the duration of the study, indicating higher nNOS demand in restoration of vagal projections. Vagotomy was associated with a much smaller increase in the number of nNOS-immunoreactive NG neurons, detectable only at 10 days after surgery. The number of nNOS-immunopositive gastric-projecting neurons was dramatically reduced 10 days after either capsaicin treatment or vagotomy but returned to the control level in both groups at 67 days. We found a significantly higher number of growth cones in capsaicin-treated animals compared with controls. Capsaicin significantly increased the number of nNOS-immunopositive and nNOS-immunonegative growth cones in NG at all time points. Vagotomy did not increase the number of nNOS(-) growth cones in NG. We conclude that capsaicin treatment may result in more significant restorative capacities than vagotomy, mainly because of sprouting of capsaicin-insensitive nerve fibers.

How do the satellite glia cells of the dorsal root ganglia respond to stressed neurons? – nitric oxide saga from embryonic development to axonal injury in adulthood

Dorsal root ganglia (DRG) respond to peripheral nerve injury by up-regulating nitric oxide (NO) production by neurons and glia in addition to local fibroblasts, endothelium and macrophages. We hypothesise that NO produced from these cells has specific roles. We have shown that when neuronal NO synthase (nNOS) is blocked in axotomised DRG, neurons undergo degenerative changes (Thippeswamy et al., 2001, 2007a). Further, we demonstrated that increased neuronal NO production, in response to axotomy/growth factor-deprivation in vitro, signals glial cells to produce trophic factors to support neuronal survival (Thippeswamy et al., 2005a). Recently, we found that treating satellite glia–neuron co-cultures with nNOS inhibitor, 7-nitroindazole (7NI), decreases the number of nestin+ cells that show neuron-like morphology. Cultured/axotomised DRG also upregulate inducible NOS (iNOS) in non-neuronal cells. Therefore, it is plausible that degenerative changes following nNOS inhibition are also due to iNOS-mediated excessive NO production by non-neuronal cells, which indeed is cytotoxic. NG-nitro-l-arginine methylester (L-NAME), the pan NOS inhibitor did not significantly change nNOS+ neuron number in axotomised DRG compared to 7NI suggesting that iNOS-mediated NO contributes to the degenerative process. In this paper, these findings from our and others' past work on NO-mediated neuron–glia signalling in axotomised DRG are discussed.

Nitric oxide-cGMP signaling regulates axonal elongation during optic nerve regeneration in the goldfish in vitro and in vivo

Nitric oxide (NO) signaling results in both neurotoxic and neuroprotective effects in CNS and PNS neurons, respectively, after nerve lesioning. We investigated the role of NO signaling on optic nerve regeneration in the goldfish (Carassius auratus). NADPH diaphorase staining revealed that nitric oxide synthase (NOS) activity was up-regulated primarily in the retinal ganglion cells (RGCs) 5-40 days after axotomy. Levels of neuronal NOS (nNOS) mRNA and protein also increased in the RGCs alone during this period. This period (5-40 days) overlapped with the process of axonal elongation during regeneration of the goldfish optic nerve. Therefore, we evaluated the effect of NO signaling molecules upon neurite outgrowth from adult goldfish axotomized RGCs in culture. NO donors and dibutyryl cGMP increased neurite outgrowth dose-dependently. In contrast, a nNOS inhibitor and small interfering RNA, specific for the nNOS gene, suppressed neurite outgrowth from the injured RGCs. Intra-ocular dibutyryl cGMP promoted the axonal regeneration from injured RGCs in vivo. None of these molecules had an effect on cell death/survival in this culture system. This is the first report showing that NO-cGMP signaling pathway through nNOS activation is involved in neuroregeneration in fish CNS neurons after nerve lesioning.

Relationship between neuronal nitric oxide synthase and NADPH-diaphorase in the dorsal root ganglia during neuropathic pain

BACKGROUND

Changes in nitric oxide (NO) production in the dorsal root ganglia (DRG) may contribute to allodynia after nerve injury. It is known that the histochemistry of NADPH-diaphorase (NADPH-d) is known to be not always coincident with NOS. This study was conducted to investigate the relationship between nNOS and NADPH-d expression in the DRG in a spinal nerve injury model of neuropathic pain, and to elucidate role that NO plays in neuropathic pain.

METHODS

nNOS immunohistochemistry and/or NADHP-d histochemistry were conducted in the DRG of a spinal nerve transection model of neuropathic pain, and the pain behavior was then measured by a von Frey filament test of the hindpaws of wild type and nNOS knock-out mice.

RESULTS

nNOS immunoreactive neurons and NADPH-d stained neurons were not always identical. Additionally NADPH-d increased, but nNOS did not increase significantly in the DRG after spinal nerve transection. Neuropathic pain behavior increased in the hindpaw of nNOS(-/-) mice after spinal nerve transection, but was lower than that of wild type mice after spinal nerve transection.

CONCLUSIONS

nNOS immunoreactive neurons and NADPH-d stained neurons were not always identical in the DRG, and a novel NADPH-d positive source may be involved in neuropathic pain after spinal nerve transection. Changes in nNOS expression in the DRG were not the primary cause of neuropathic pain behavior in a spinal nerve transection model of neuropathic pain.

Neurotrophin-3 significantly reduces sodium channel expression linked to neuropathic pain states

Neuropathic pain resulting from chronic constriction injury (CCI) is critically linked to sensitization of peripheral nociceptors. Voltage gated sodium channels are major contributors to this state and their expression can be upregulated by nerve growth factor (NGF). We have previously demonstrated that neurotrophin-3 (NT-3) acts antagonistically to NGF in modulation of aspects of CCI-induced changes in trkA-associated nociceptor phenotype and thermal hyperalgesia. Thus, we hypothesized that exposure of neurons to increased levels of NT-3 would reduce expression of Na(v)1.8 and Na(v)1.9 in DRG neurons subject to CCI. In adult male rats, Na(v)1.8 and Na(v)1.9 mRNAs are expressed at high levels in predominantly small to medium size neurons. One week following CCI, there is reduced incidence of neurons expressing detectable Na(v)1.8 and Na(v)1.9 mRNA, but without a significant decline in mean level of neuronal expression, and similar findings observed immunohistochemically. There is also increased accumulation/redistribution of channel protein in the nerve most apparent proximal to the first constriction site. Intrathecal infusion of NT-3 significantly attenuates neuronal expression of Na(v)1.8 and Na(v)1.9 mRNA contralateral and most notably, ipsilateral to CCI, with a similar impact on relative protein expression at the level of the neuron and constricted nerve. We also observe reduced expression of the common neurotrophin receptor p75 in response to CCI that is not reversed by NT-3 in small to medium sized neurons and may confer an enhanced ability of NT-3 to signal via trkA, as has been previously shown in other cell types. These findings are consistent with an analgesic role for NT-3.

Cellular and biochemical actions of melatonin which protect against free radicals: role in neurodegenerative disorders

Molecular oxygen is toxic for anaerobic organisms but it is also obvious that oxygen is poisonous to aerobic organisms as well, since oxygen plays an essential role for inducing molecular damage. Molecular oxygen is a triplet radical in its ground-stage (.O-O.) and has two unpaired electrons that can undergoes consecutive reductions of one electron and generates other more reactive forms of oxygen known as free radicals and reactive oxygen species. These reactants (including superoxide radicals, hydroxyl radicals) possess variable degrees of toxicity. Nitric oxide (NO*) contains one unpaired electron and is, therefore, a radical. NO* is generated in biological tissues by specific nitric oxide synthases and acts as an important biological signal. Excessive nitric oxide production, under pathological conditions, leads to detrimental effects of this molecule on tissues, which can be attributed to its diffusion-limited reaction with superoxide to form the powerful and toxic oxidant, peroxynitrite.Reactive oxygen and nitrogen species are molecular "renegades"; these highly unstable products tend to react rapidly with adjacent molecules, donating, abstracting, or even sharing their outer orbital electron(s). This reaction not only changes the target molecule, but often passes the unpaired electron along to the target, generating a second free radical, which can then go on to react with a new target amplifying their effects.This review describes the mechanisms of oxidative damage and its relationship with the most highly studied neurodegenerative diseases and the roles of melatonin as free radical scavenger and neurocytoskeletal protector.

Effect of peripheral axotomy on gene expression of NIDD in rat neural tissues

Peripheral nerve lesion-induced production of neuronal nitric oxide synthase (nNOS) was implicated to influence a range of postaxotomy processes necessary for neuronal survival and nerve regeneration (Zochodne et al., Neuroscience, 91:1515-1527, 1999; Keilhoff et al., Journal of Chemical Neuroanatomy, 24:181-187, 2002, Nitric Oxide, 10:101-111, 2004). Protein-protein interactions represent an important mechanism in the control of NOS spatial distribution or activity (Alderton et al., Biochemical Journal, 357:593-615, 2001; Dedio et al., FASEB Journal, 15:79-89, 2001; Zimmermann et al., Proceedings of the National Academy of Sciences, 99:17167-17172, 2002). As one of the nNOS-binding proteins, nNOS-interacting DHHC domain-containing protein with dendritic mRNA (NIDD) has recently been identified to increase nNOS enzyme activity by targeting nNOS to the synaptic plasma membrane in a postsynaptic density protein 95/discs-large/zona occlusens-1 domain dependent manner (Saitoh et al., Journal of Biological Chemistry, 279:29461-29468, 2004). In this paper, we established a rat model with peripheral axotomy to investigate the gene expression patterns of NIDD in neural tissues using TaqMan quantitative real-time polymerase chain reaction and in situ hybridization combined with immunofluorescence. It revealed that NIDD mRNA was upregulated after sciatic nerve transection with the similar expressing styles as that of the nNOS in the injured nerves, corresponding dorsal root ganglia, and lumbar spinal cord. These findings imply that NIDD may be involved in the different pathological conditions including nerve regeneration, neuron loss or survival, and even pain process, possibly via regulating the enzyme nNOS activity.

NO pain: potential roles of nitric oxide in neuropathic pain

The disabling human syndrome of "neuropathic pain" is an intractable complication of peripheral nerve injury or degeneration. A complex interaction between injured peripheral axons, sensory neurons and central nervous system signaling is thought to account for it. In this brief review, we present evidence that the free radical signaling molecule, nitric oxide (NO) may act at several levels of the nervous system during the development of experimental neuropathic pain. For example, NO may directly influence injured axons in the periphery, may indirectly influence pain by its role in the process of Wallerian degeneration, and may signal in the dorsal horn of the spinal cord. While it is premature to argue for therapeutic approaches that alter NO actions, it may be an important player in the cascade of events that generate neuropathic pain.

NO-cGMP mediated galanin expression in NGF-deprived or axotomized sensory neurons

Leukaemia inhibitory factor (LIF) and nerve growth factor (NGF) are well characterized regulators of galanin expression. However, LIF knockout mice containing the rat galanin 5' proximal promoter fragment (- 2546 to + 15 bp) driving luciferase responded to axotomy in the same way as control mice. Also, LIF had no effect on reporter gene expression in vitro, neither in the presence or absence of NGF, suggesting that other factors mediate an axotomy response from the galanin promoter. We then addressed the role of nitric oxide (NO) using NGF-deprived rat dorsal root ganglion (DRG) neuron cultures infected with viral vectors containing the above-mentioned construct, and also studied endogenous galanin expression in axotomized DRG in vivo. Blocking endogenous NO in NGF-deprived DRG cultures suppressed galanin promoter activity. Consistent with this, axotomized/NGF-deprived DRG neurons expressed high levels of neuronal NO synthase (nNOS) and galanin. Further, using pharmacological NOS blockers, or adenoviral vectors expressing dominant-negative either for nNOS or soluble guanylate cyclase in vivo and in vitro, we show that the NO-cGMP pathway induces endogenous galanin in DRG neurons. We propose that both LIF and NO, acting at different promoter regions, are important for the up-regulation of galanin, and for DRG neuron survival and regeneration after axotomy.

Effect of μ and κ opioids on injury-induced microglial accumulation in leech CNS: Involvement of the nitric oxide pathway

Damage to the leech or mammalian CNS increases nitric oxide (NO) production and causes accumulation of phagocytic microglial cells at the injury site. Opioids have been postulated to modulate various parameters of the immune response. Morphine and leech morphine-like substance are shown to release NO and suppress microglial activation. Regarding the known immuno-modulatory effects of selective mu and kappa ligands, we have assessed the effect of these agents on accumulation of microglia at the site of injury in leech CNS. Leech nerve cords were dissected, crushed with fine forceps and maintained in different concentrations of opiates in culture medium for 3 h and then fixed and double stained with Hoechst 33258 and monoclonal antibody to endothelial nitric oxide synthase (NOS). Morphine and naloxone (> or =10(-3) M) but not selective mu agonist, DAMGO [d-Ala2, N-Me-Phe-Gly5(ol)-enkephalin] and antagonist, CTAP [D-Phe-Cys-Tyr-D-Trp-Arg-Thr-Pen-Thr-NH2] inhibited the microglial accumulation. The effect of morphine was abrogated by pre-treatment with naloxone and also non-selective NOS inhibitor, l-NAME [N(omega)-nitro-l-arginine-methyl-ester; 10(-3) M] implying an NO-dependent and mu-mediated mechanism. These results are similar to properties of recently found mu-3 receptor in leech, which is sensitive to alkaloids but not peptides. Both selective kappa agonist, U50,488 [3,4-dichloro-N-methyl-N-(2-(1-pyrrolidinyl)cyclohexyl)-benzeneacetamide; > or =10(-3) M], and antagonist, nor-binaltorphimine (nor-BNI; > or =10(-3) M), inhibited the accumulation. The effect of nor-BNI was reversed by l-NAME. Immunohistochemistry showed decreased endothelial NOS expression in naloxone and U50,488-treated cords. Since, NO production at the injury site is hypothesized to act as a stop signal for microglias, opioid agents may exert their effect via changing of NO gradient along the cord resulting in disruption of accumulation. These results suggest an immuno-modulatory role for mu and kappa opioid receptors on injury-induced microglial accumulation which may be mediated via NO.

Neuronal nitric oxide synthase plays a key role in CNS demyelination

Nitric oxide (NO) is a small, short-lived molecule released from a variety of cells that is implicated in a multitude of biological processes. In pathological conditions, overproduction of NO may lead to the generation of highly reactive species, such as peroxynitrite and stable nitrosothiols, that may cause irreversible cell damage. Accordingly, several studies have suggested that NO may be involved in the pathogenesis of various neuroinflammatory/degenerative diseases. Increased concentrations of NO in the CNS in such cases are usually attributed to an increase in the inducible isoform of NO synthase (iNOS) usually produced by inflammatory cells. However, recent reports have suggested that the constitutive isoforms of NOS, neuronal (nNOS) and endothelial (eNOS), can also play a role. Here we examined the role that the constitutive isoforms of NOS might play in the cuprizone-induced model of demyelination/remyelination. Our results demonstrate that demyelination was greatly prevented in mice lacking nNOS. Protection was associated with a dramatic increase in mature oligodendrocyte survival and a decrease in apoptosis. Moreover, nNOS-/- mice did not respond to cuprizone with the extensive recruitment of microglia/macrophages and astrocytes, which is a typical feature in wild-type mice. Although demyelinating less, nNOS-/- mice exhibited a delay in remyelination. In eNOS-/- mice, demyelination progressed to the same extent as in wild type, but they showed a slight delay in spontaneous remyelination. In conclusion, this study highlights the importance of considering the source of NO when assessing its role in neuroinflammation/degeneration and emphasizes the differing pathological effects driven by the different NOS isoforms.

Neurotrophin-3 attenuates galanin expression in the chronic constriction injury model of neuropathic pain

We have recently shown that exogenous neurotrophin-3 (NT-3) acts antagonistically to nerve growth factor (NGF) in regulation of nociceptor phenotype in intact neurons and suppresses thermal hyperalgesia and expression of molecules complicit in this behavioral response induced by chronic constriction injury (CCI) of the sciatic nerve. The present study examines whether there is a global influence of NT-3 in mitigating alterations in peptide and NGF receptor expression; molecules believed to also contribute to CCI-associated pain. Thus, the influence of NT-3 on phenotypic changes in dorsal root ganglion (DRG) neurons in rats coincident with CCI was examined using in situ hybridization. Seven days following injury, the incidence of expression of the neuropeptides galanin and pituitary adenylate cyclase-activating polypeptide (PACAP) was increased in L5 sensory neurons ipsilateral to the injury from 12% to 60% and 16% to 37% respectively, in addition to an increased level of expression. In contrast, there was no consistent significant change in tropomyosin-related kinase A (trkA) expression following CCI. Intrathecal infusion of NT-3 globally mitigated both the increased incidence and elevated levels of galanin messenger RNA (mRNA) expression observed following CCI, reducing the former from 60% to 39%. NT-3 infusion resulted in a limited reduction in the incidence and level of neuronal PACAP in medium to large size, but not small size, DRG neurons. NT-3 had no significant net effect on CCI-induced alterations in trkA mRNA expression.

Nitric oxide transcriptionally down-regulates specific isoforms of divalent metal transporter (DMT1) via NF-?B

Studies were performed to examine the affect of nitric oxide (NO) on expression of the divalent metal transporter (DMT1) in undifferentiated P19 embryonic carcinoma cells. DMT1 has four known isoforms which differ in both the N- and C-terminals. Results demonstrate that exposure of P19 cells to the NO precursor, sodium nitro-prusside (SNP), resulted in a decrease in expression of both positive (+) and negative (-) IRE isoforms of DMT1 with no change in the 1A species. Regulation was not as a result of decreased stability of message but was caused by reduction in transcription of the DMT1 1B isoforms. Similar results were observed in other cell lines, including PC12 and SH-SY5Y cells and rat primary sympathetic neurons. Nuclear NF-kappaB was decreased after SNP treatment, suggesting that NF-kappaB may mediate this response. Luciferase reporter assays with normal and NF-kappaB mutated constructs of the 1B promoter confirm that the NF-kappaB site between -23 to -19 upstream from the transcription start site was responsible for regulating expression. Electrophoretic mobility shift assay (EMSA) and chromatin immunoprecipitation (ChIP) assays further demonstrate that the p65 subunit of NF-kappaB and not p50 binding is specifically decreased by NO treatment. Results of these studies provide a general mechanism responsible for regulating DMT1 expression induced by stress-related signaling processes in vivo.

Neurochemical plasticity and the role of neurotrophic factors in bladder reflex pathways after spinal cord injury

Transection of the spinal cord that interrupts the spinobulbospinal micturition reflex pathway, abolishes voluntary voiding and initially produces an areflexic bladder with complete urinary retention. However, depending upon the species, reflex bladder activity slowly recovers over the course of weeks or months. In chronic spinal animals, reflex mechanisms in the lumbosacral spinal cord are capable of duplicating many of the functions performed by reflex pathways in animals with an intact spinal cord and can induce bladder hyperreflexia. However, the bladder does not empty efficiently due to a loss of bladder-sphincter coordination (bladder-sphincter dyssynergia). In contrast to normal animals in which the sphincter relaxes during voiding, animals with a spinal cord injury exhibit sphincter contractions during voiding, an increase in urethral outlet resistance, urinary retention, bladder hyperreflexia, bladder overdistension, and an increase in bladder afferent cell size. Changes in electrophysiological or neurochemical properties of bladder afferent cells in the dorsal root ganglia and of spinal pathways could contribute to the emergence of the spinal micturition reflex, bladder hyperreflexia and changes in the pharmacologic responses of reflex pathways in the lumbosacral spinal cord after spinal cord injury. Urinary bladder hyperreflexia after spinal cord injury may reflect a change in the balance of neuroactive compounds in bladder reflex pathways. This review will detail: (1) changes in the neurochemical phenotype of bladder afferent neurons and of spinal neurons mediating micturition reflexes after spinal cord injury, with an emphasis on three neuroactive compounds, neuronal nitric oxide synthase (nNOS), galanin, and pituitary adenylate cyclase activating polypeptide (PACAP); (2) possible functional consequences on bladder reflexes of changes in spinal cord neurochemistry after spinal cord injury, and (3) the potential role of neurotrophic factors expressed in the urinary bladder or spinal cord after spinal cord injury in mediating these neurochemical changes.

Phylogenesis of constitutively formed nitric oxide in non-mammals

It is widely recognized that nitric oxide (NO) in mammalian tissues is produced from L-arginine via catalysis by NO synthase (NOS) isoforms such as neuronal NOS (nNOS) and endothelial NOS (eNOS) that are constitutively expressed mainly in the central and peripheral nervous system and vascular endothelial cells, respectively. This review concentrates only on these constitutive NOS (cNOS) isoforms while excluding information about iNOS, which is induced mainly in macrophages upon stimulation by cytokines and polysaccharides. The NO signaling pathway plays a crucial role in the functional regulation of mammalian tissues and organs. Evidence has also been accumulated for the role of NO in invertebrates and non-mammalian vertebrates. Expression of nNOS in the brain and peripheral nervous system is widely determined by staining with NADPH (reduced nicotinamide adenine dinucleotide phosphate) diaphorase or NOS immunoreactivity, and functional roles of NO formed by nNOS are evidenced in the early phylogenetic stages (invertebrates and fishes). On the other hand, the endothelium mainly produces vasodilating prostanoids rather than NO or does not liberate endothelium-derived relaxing factor (EDRF) (fishes), and the ability of endothelial cells to liberate NO is observed later in phylogenetic stages (amphibians). This review article summarizes various types of interesting information obtained from lower organisms (invertebrates, fishes, amphibians, reptiles, and birds) about the properties and distribution of nNOS and eNOS and also the roles of NO produced by the cNOS as an important intercellular signaling molecule.

Nitric oxide and its modulators in chronic constriction injury-induced neuropathic pain in rats

This study was conducted to examine the role of nitric oxide (NO) in peripheral neuropathy induced by chronic constriction injury of sciatic nerve of rats by using NO precursor, NO donors and nitric oxide synthase (NOS) inhibitors. Chronic constriction injury of sciatic nerve of rats resulted in peripheral neuropathy as confirmed by nociceptive behavioural tests using mechanical, thermal and cold allodynia. NO precursor, L-arginine and NO donors sodium nitroprusside, S-nitroso-N-acetylpenicillamine potentiated the hyperalgesia and allodynia significantly suggesting proalgesic effect in neuropathic rats. Intracerebroventricular (i.c.v.) administration of rats with NOS inhibitors such as L-N(G)-nitroarginine methyl ester, N-iminoethyl lysine and 7-nitroindazole did not show any effect but i.p. administration of NOS inhibitors aminoguanidine, L-N(G)-nitroarginine methyl ester and 7-nitroindazole caused alleviation of pain. The study confirms the involvement of endogenously synthesized and exogenously administered NO in chronic constriction injury-induced neuropathy in rats. Significant increase in the levels of nitrate and nitrite in ligated sciatic nerve suggest that local up regulation of NO in the production and maintenance of neuropathic pain. In conclusion, initial attempt to manipulate L-arginine: NO pathway is indicative of therapeutic potential of these interventions in the management of neuropathic pain.

GW274150, a novel and highly selective inhibitor of the inducible isoform of nitric oxide synthase (iNOS), shows analgesic effects in rat models of inflammatory and neuropathic pain

Nitric oxide (NO), synthesised by different isoforms of nitric oxide synthase (NOS), has been linked with the development and maintenance of nociception. We studied the role of the inducible isoform, iNOS, in two different rat pain models with an inflammatory component. iNOS was immunohistochemically detected locally in the paw 6h after Freund's Complete Adjuvant (FCA) injection, showing a plateau at 24-72 h and falling slowly in the following weeks. This correlated with the late phase of the hypersensitivity to pain revealed in the behavioural tests. A highly selective iNOS inhibitor GW274150 (1-30 mg/kg orally, 24h after FCA) suppressed the accumulation of nitrite in the inflamed paw indicating substantial iNOS inhibition. At the same time it partially reversed FCA-induced hypersensitivity to pain and edema in a dose-dependent manner. After Chronic Constriction Injury (CCI) surgery to the sciatic nerve, iNOS presence was only detected locally in the region of the nerve (inflammatory cells). GW274150 (3-30 mg/kg orally, 21 days after surgery) also reversed significantly the CCI-associated hypersensitivity to pain. No iNOS was detectable in dorsal root ganglia, spinal cord or brain in either model. This study demonstrates a role for peripherally-expressed iNOS in pain conditions with an inflammatory component and the potential value of iNOS inhibitors in such conditions.

The physiology and pathophysiology of nitric oxide in the brain

Nitric oxide (NO) is a molecule with pleiotropic effects in different tissues. NO is synthesized by NO synthases (NOS), a family with four major types: endothelial, neuronal, inducible and mitochondrial. They can be found in almost all the tissues and they can even co-exist in the same tissue. NO is a well-known vasorelaxant agent, but it works as a neurotransmitter when produced by neurons and is also involved in defense functions when it is produced by immune and glial cells. NO is thermodynamically unstable and tends to react with other molecules, resulting in the oxidation, nitrosylation or nitration of proteins, with the concomitant effects on many cellular mechanisms. NO intracellular signaling involves the activation of guanylate cyclase but it also interacts with MAPKs, apoptosis-related proteins, and mitochondrial respiratory chain or anti-proliferative molecules. It also plays a role in post-translational modification of proteins and protein degradation by the proteasome. However, under pathophysiological conditions NO has damaging effects. In disorders involving oxidative stress, such as Alzheimer's disease, stroke and Parkinson's disease, NO increases cell damage through the formation of highly reactive peroxynitrite. The paradox of beneficial and damaging effects of NO will be discussed in this review.

The occurrence of nitric oxide synthase-containing axonal baskets surrounding large neurons in rat dorsal root ganglia after sciatic nerve ligation

To clarify the possible role of nitric oxide (NO) induced in primary sensory neurons after peripheral axotomy, NO synthase (NOS) immunohistochemistry was carried out on rat L5 dorsal root ganglia after sciatic nerve ligation. The results were compared with the expression of 27-kDa heat shock protein (HSP27), a neuroprotective molecule. In intact animals, NOS-immunoreactive neurons represented about 2% of all dorsal root ganglion (DRG) neurons, whereas HSP27-immunoreactive neurons comprised about 14%. After sciatic nerve ligation, both neurons increased, in number and immunoreactivity, reaching a maximum at 2 weeks, when NOS- and HSP27-immunoreactive neurons represented about 33 and 66%, respectively. NOS-immunoreactive neurons then remained unchanged until 7 weeks although HSP27-immunoreactive neurons showed a slight decline. The increased NOS-immunoreactive neurons were preferentially small (100-500 microm(2)) and coexpressed with HSP27 (about 87%). On the other hand, in the proximal stump of sciatic nerves, numerous NOS-immunoreactive fibers with a regenerative profile appeared transiently (2-4 weeks). At higher magnification, an axonal sprout from the NOS-immunoreactive small DRG neurons was found to form a basket-like structure (or basket) mostly around the cell body of NOS-negative large neurons. Retrograde labeling with a fluorescent tracer showed that both neurons sent peripheral axon collaterals to the sciatic nerve. The appearance of this unique structure was most prominent after depletion of the NOS-immunoreactive regenerating fibers in the sciatic nerve (at 7-9 weeks). The findings suggest that NO might be involved in not only axonal regeneration but also the rewiring of two classes of DRG neurons after peripheral nerve injury.

Reduction in Induced Pain by Ultrasound May Be Caused by Altered Expression of Spinal Neuronal Nitric Oxide Synthase-Producing Neurons

OBJECTIVE

To examine the peripheral influence of therapeutic ultrasound (US) on central spinal nociceptive modulation.

DESIGN

Controlled, experimental animal trial.

SETTING

Neuroscience laboratory of a medical university in Taiwan.

ANIMALS

Ten male Wistar rats weighing 250 to 300 g.

INTERVENTION

To induce inflammatory arthritis, the rats were injected intracapsularly with complete Freund's adjuvant (CFA) into the right tibiotarsal joint. Eighteen hours later, at the inflammatory phase of arthritis, US or sham-operated treatment was applied to the arthritic limb.

MAIN OUTCOME MEASURES

The numbers and distributional proportions of immunoreactive spinal neuronal nitric oxide synthase-like (nNOS-LI) neurons were assessed.

RESULTS

The nNOS-LI neurons were abundant bilaterally in the L1 and L2 regions of the spinal cord areas after CFA-induced arthritis with sham-operated treatment. US treatment significantly suppressed this increase in the numbers of nNOS-LI neurons bilaterally in the superficial laminae (laminae I-II, P < .001), nucleus proprius (laminae III-IV, P < .01), deep laminae (laminae V-VI, P < .001), and ventral horn (laminae VII-X, P < .001) of the spinal cord. When expressed as a percentage of the total labeled cells, the proportions of nNOS-LI neurons showed significant differences in laminae III-IV (P < .001) and laminae V-VI (P < .01) between sham-treated rats and those treated with US.

CONCLUSIONS

US treatment may modulate the CFA insult-induced increase in total and regional nNOS-LI neurons. I propose that the peripheral influences of US on central modulation of the spinal nociceptive processing system is important and may reflect the neuroplasticity of the spinal cord in response to peripheral input.

Satellite glial cells in sensory ganglia: from form to function

Current information indicates that glial cells participate in all the normal and pathological processes of the central nervous system. Although much less is known about satellite glial cells (SGCs) in sensory ganglia, it appears that these cells share many characteristics with their central counterparts. This review presents information that has been accumulated recently on the physiology and pharmacology of SGCs. It appears that SGCs carry receptors for numerous neuroactive agents (e.g., ATP, bradykinin) and can therefore receive signals from other cells and respond to changes in their environment. Activation of SGCs might in turn influence neighboring neurons. Thus SGCs are likely to participate in signal processing and transmission in sensory ganglia. Damage to the axons of sensory ganglia is known to contribute to neuropathic pain. Such damage also affects SGCs, and it can be proposed that these cells have a role in pathological changes in the ganglia.

A neuronal isoform of protein kinase G couples mitogen-activated protein kinase nuclear import to axotomy-induced long-term hyperexcitability in Aplysia sensory neurons

The induction of a long-term hyperexcitability (LTH) in vertebrate nociceptive sensory neurons (SNs) after nerve injury is an important contributor to neuropathic pain in humans, but the signaling cascades that induce this LTH have not been identified. In particular, it is not known how injuring an axon far from the cell soma elicits changes in gene expression in the nucleus that underlie LTH. The nociceptive SNs of Aplysia (ap) develop an LTH with electrophysiological properties after axotomy similar to those of mammalian neurons and are an experimentally useful model to examine these issues. We cloned an Aplysia PKG (cGMP-dependent protein kinase; protein kinase G) that is homologous to vertebrate type-I PKGs and found that apPKG is activated at the site of injury in the axon after peripheral nerve crush. The active apPKG is subsequently retrogradely transported to the somata of the SNs, but apPKG activity does not appear in other neurons whose axons are injured. In the soma, apPKG phosphorylates apMAPK (Aplysia mitogen-activated protein kinase), resulting in its entry into the nucleus. Surprisingly, studies using recombinant proteins in vivo and in vitro indicate that apPKG directly phosphorylates the threonine moiety in the T-E-Y activation site of apMAPK when the -Y- site contains a phosphate. We used inhibitors of nitric oxide synthase, soluble guanyl cyclase, or PKG after nerve injury, and found that each prevented the appearance of the LTH. Moreover, blocking apPKG activation prevented the nuclear import of apMAPK. Consequently, the nitric oxide-PKG-MAPK pathway is a potential target for treatment of neuropathic pain.

Neurotrophin-3 suppresses thermal hyperalgesia associated with neuropathic pain and attenuates transient receptor potential vanilloid receptor-1 expression in adult sensory neurons

Neurotrophin-3 (NT-3) negatively modulates nerve growth factor (NGF) receptor expression and associated nociceptive phenotype in intact neurons, suggesting a beneficial role in treating aspects of neuropathic pain mediated by NGF. We report that NT-3 is effective at suppressing thermal hyperalgesia associated with chronic constriction injury (CCI); however, NT-3 does not alter the mechanical hypersensitivity that also develops with CCI. Thermal hyperalgesia is critically linked to expression and activation of the capsaicin receptor, transient receptor potential vanilloid receptor-1 (TRPV1). Thus, its modulation by NT-3 after CCI was examined. CCI results in elevated TRPV1 expression at both the mRNA and protein levels in predominantly small-to-medium neurons, with the percentage of neurons expressing TRPV1 remaining unchanged at approximately 56%. Attenuation of thermal hyperalgesia mediated by NT-3 correlates with decreased TRPV1 expression such that only approximately 26% of neurons ipsilateral to CCI expressed detectable TRPV1 mRNA. NT-3 effected a decrease in expression of the activated component of the signaling pathway linked to regulation of TRPV1 expression, phospho-p38 MAPK (Ji et al., 2002), in neurons ipsilateral to CCI. Exogenous NT-3 could both prevent the onset of thermal hyperalgesia and reverse established thermal hyperalgesia and elevated TRPV1 expression 1 week after CCI. Continuous infusion is required for suppression of both thermal hyperalgesia and TRPV1 expression, because removal of NT-3 resulted in a prompt reestablishment of the hyperalgesic state and corresponding CCI-associated TRPV1 phenotype. In conclusion, although NGF drives inflammation-associated thermal hyperalgesia via its regulation of TRPV1 expression, NT-3 is now identified as a potent negative modulator of this state.