Effects of peripheral axotomy on neuropeptides and nitric oxide synthase in dorsal root ganglia and spinal cord of the guinea pig: an immunohistochemical study

A New Gal in Town: A Systematic Review of the Role of Galanin and Its Receptors in Experimental Pain

Galanin is a neuropeptide expressed in a small percentage of sensory neurons of the dorsal root ganglia and the superficial lamina of the dorsal horn of the spinal cord. In this work, we systematically reviewed the literature regarding the role of galanin and its receptors in nociception at the spinal and supraspinal levels, as well as in chronic pain conditions. The literature search was performed in PubMed, Web of Science, Scopus, ScienceDirect, OVID, TRIP, and EMBASE using "Galanin" AND "pain" as keywords. Of the 1379 papers that were retrieved in the initial search, we included a total of 141 papers in this review. Using the ARRIVE guidelines, we verified that 89.1% of the works were of good or moderate quality. Galanin shows a differential role in pain, depending on the pain state, site of action, and concentration. Under normal settings, galanin can modulate nociceptive processing through both a pro- and anti-nociceptive action, in a dose-dependent manner. This peptide also plays a key role in chronic pain conditions and its antinociceptive action at both a spinal and supraspinal level is enhanced, reducing animals' hypersensitivity to both mechanical and thermal stimulation. Our results highlight galanin and its receptors as potential therapeutic targets in pain conditions.

Molecular Changes in the Dorsal Root Ganglion during the Late Phase of Peripheral Nerve Injury-induced Pain in Rodents: A Systematic Review

The dorsal root ganglion is widely recognized as a potential target to treat chronic pain. A fundamental understanding of quantitative molecular and genomic changes during the late phase of pain is therefore indispensable. The authors performed a systematic literature review on injury-induced pain in rodent dorsal root ganglions at minimally 3 weeks after injury. So far, slightly more than 300 molecules were quantified on the protein or messenger RNA level, of which about 60 were in more than one study. Only nine individual sequencing studies were performed in which the most up- or downregulated genes varied due to heterogeneity in study design. Neuropeptide Y and galanin were found to be consistently upregulated on both the gene and protein levels. The current knowledge regarding molecular changes in the dorsal root ganglion during the late phase of pain is limited. General conclusions are difficult to draw, making it hard to select specific molecules as a focus for treatment.

Long-term actions of interleukin-1β on K+, Na+ and Ca2+ channel currents in small, IB4-positive dorsal root ganglion neurons; possible relevance to the etiology of neuropathic pain

Excitation of dorsal root ganglion (DRG) neurons by interleukin 1β (IL-1β) is implicated in the onset of neuropathic pain. To understand its mechanism of action, isolectin B4 positive (IB₄⁺) DRG neurons were exposed to 100pM IL-1β for 5-6d. A reversible increase in action potential (AP) amplitude reflected increased TTX-sensitive sodium current (TTX-S I_Na). An irreversible increase in AP duration reflected decreased Ca²⁺- sensitive K⁺ conductance (BK(Ca) channels). Different processes thus underlie regulation of the two channel types. Since changes in AP shape facilitated Ca²⁺ influx, this explains how IL-1β facilitates synaptic transmission in the dorsal horn; thereby provoking pain.

Injury-specific promoters enhance herpes simplex virus-mediated gene therapy for treating neuropathic pain in rodents

Chronic neuropathic pain is often difficult to treat with current pain medications. Gene therapy is presently being explored as a therapeutic approach for the treatment of neuropathic and cancer pain. In this study, we sought to use an injury-specific promoter to deliver the mu-opioid receptor (MOR) transgene such that expression would occur during the injured state only in response to release of injury-specific galanin. To determine whether an injury-specific promoter can produce neuron-specific MOR expression and enhanced antinociception, we compared animals infected with a galanin promoter virus (galMOR) or a human cytomegalovirus promoter virus (cmvMOR). In behavioral assays, we found an earlier onset and a larger magnitude of antinociception in animals infected with galMOR compared with cmvMOR. Immunohistochemical analysis of dorsal root ganglion neurons revealed a significant increase in MOR-positive staining in cmvMOR- and galMOR-treated mice. Spinal cord sections from galMOR-treated mice showed a greater increase in density but not area of MOR-positive staining. These results suggest that using injury-specific promoters to drive gene expression in primary afferent neurons can influence the onset and magnitude of antinociception in a rodent model of neuropathic pain and can be used to upregulate MOR expression in populations of neurons that are potentially injury specific.

PERSPECTIVE

An injury-specific promoter (galMOR) was used to drive MOR expression in a population- and injury-specific manner. GalMOR increased antinociception and density of MOR staining in the spinal cord. This article presents evidence that promoter selection is an important component in successful gene expression in an injury- and population-specific manner.

Fifth lumbar spinal nerve injury causes neurochemical changes in corresponding as well as adjacent spinal segments: a possible mechanism underlying neuropathic pain

Previous investigations of the anatomical basis of the neuropathic-like manifestations in the spinal nerve ligation animal model have shown that the central terminations of the unmyelinated primary afferents of L5 spinal nerve are not restricted to the corresponding L5 spinal segment, and rather extend to two spinal segments rostrally and one segment caudally where they intermingle with primary afferents of the adjacent L4 spinal nerve. The aim of the present study was to investigate the neurochemical changes in the dorsal horn of the spinal cord and DRGs after L5 nerve injury in rats. In the first experiment, the right L5 nerve was ligated and sectioned for 14 days, and isolectin B4 (IB4, a tracer for unmyelinated primary afferents) was injected into the left L5 nerve. The results showed that the vasoactive intestinal peptide (VIP) was up-regulated in laminae I-II of L3-L6 spinal segments on the right side in exactly the same areas where IB4 labelled terminals were revealed on the left side. In the second experiment, L5 was ligated and sectioned and the spinal cord and DRGs were stained immunocytochemically with antibodies raised against various peptides known to be involved in pain transmission and hyperalgesia. The results showed that L5 nerve lesion caused down-regulation of substance P, calcitonin-gene related peptide and IB4 binding and up-regulation of neuropeptide Y and neurokinin-1 receptor in the dorsal horn of L4 and L5 spinal segments. Similar neurochemical changes were observed only in the corresponding L5 DRG with minimal effects observed in L3, L4 and L6 DRGs. Although, L5 nerve injury caused an up-regulation in NPY, no change in SP and CGRP immunoreactivity was observed in ipsilateral garcile nucleus. These neuroplastic changes in the dorsal horn of the spinal cord, in the adjacent uninjured territories of the central terminations of the adjacent uninjured nerves, might explain the mechanism of hyperalgesia after peripheral nerve injury.

Role of neuropeptides, neurotrophins, and neurohormones in skin wound healing

Due to the close interactions between the skin and peripheral nervous system, there is increasing evidence that the cutaneous innervation is an important modulator of the normal wound healing process. The communication between sensory neurons and skin cells involves a variety of molecules (neuropeptides, neurohormones, and neurotrophins) and their specific receptors expressed by both neuronal and nonneuronal skin cells. It is well established that neurotransmitters and nerve growth factors released in skin have immunoregulatory roles and can exert mitogenic actions; they could also influence the functions of the different skin cell types during the wound healing process.

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.

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

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

Neuropathic pain modifies antioxidant activity in rat spinal cord

Oxidative stress is an important pathophysiological mechanism of many neurological diseases. Reactive oxygen and nitrogen species have been cited as molecules involved in the nociceptive process. In this study, rats were submitted to sciatic nerve transection (SNT) for induction of neuropathic pain, and enzyme activities of SOD and catalase as well as lipid peroxidation (LPO) were measured in the lumbosacral spinal cord. The results show that LPO was not changed after SNT. SOD activity was reduced 7 days after SNT, while the change in catalase activity occurred on the third and seventh days in both sham and SNT animals. Hyperalgesia in SNT group was detected at the same points in time. These results suggest that SNT was not a strong enough stimulus to deplete all antioxidant content in the spinal cord, since increase in LPO was not detected. However, the role of oxidative stress in nociception can not be excluded.

Galaninergic mechanisms at the spinal level: focus on histochemical phenotyping

The 29/30 amino acid neuropeptide galanin is present in a small population of DRG neurons under normal condition but is strongly upregulated after nerve injury. There is evidence that this upregulated galanin has trophic actions, for example promoting neurite outgrowth as well as influencing pain processing. In fact, both pro- and antinociceptive effects have been reported, probably relating to activation of different receptors. It has been proposed that presynaptic GalR2 receptors are pro-nociceptive by enhancing release of excitatory transmitters in the dorsal horn, and anti-nociceptive via an action on GalR1-positive interneurons. These neurons have recently been shown to be glutamatergic. Several other peptides and molecules are also regulated by nerve injury. Here we focus on neuropeptide tyrosine (NPY), which is upregulated in parallel with galanin. We review data reporting on coexistence between galanin and NPY and between these two peptides and the two NPY receptors Y1 and Y2. The data show considerable overlap, and it will be an important task to analyse how cross-talk between these neuropeptides can influence pain processing. It is proposed that such cross-talk can occur by release of peptides from DRGs neuron somata within dorsal root ganglia. To what extent these mechanisms shown to exist in rodents also occur in human is important, if one wants to discuss novel strategies for pain treatment on the basis of these findings. So far information is limited, but it has been demonstrated that galanin is expressed in DRGs and possibly also regulated.

Complete sciatic nerve transection induces increase of neuropeptide Y-like immunoreactivity in primary sensory neurons and spinal cord of frogs

Neuropeptide Y (NPY) was immunohistochemically investigated in the frog spinal cord and dorsal root ganglia after axotomy. In normal ganglia, moderate NPY-like immunoreactivity (NPY-IR) prevailed in large and medium cells. In the spinal cord, the NPY-IR was densest in the dorsal part of the lateral funiculus. Other fibers and neurons NPY-IR were observed in the dorsal and ventral terminal fields and mediolateral band. NPY-IR fibers were also found in the ventral horn and in the ventral and lateral funiculi. The sciatic nerve transection increased the NPY-IR in large and medium neurons of the ipsilateral and contralateral dorsal root ganglia at 3 and 7 days, but no clear change was found at 15 days. In the spinal cord, there was a bilateral increase in the NPY-IR of the dorsal part of the lateral funiculus. In the ipsilateral side, the NPY-IR was increased at 3 and 7 days but was decreased at 15 days. In the contralateral side, a significant reduction at 15 days occurred. These findings seem to favor the role of NPY in the modulation of pain-related information in frogs, suggesting that this role of NPY may have appeared early in vertebrate evolution.

Nociceptive response to innocuous mechanical stimulation is mediated via myelinated afferents and NK-1 receptor activation in a rat model of neuropathic pain

Peripheral nerve injury in humans can produce a persistent pain state characterized by spontaneous pain and painful responses to normally innocuous stimuli (allodynia). Here we attempt to identify some of the neurophysiological and neurochemical mechanisms underlying neuropathic pain using an animal model of peripheral neuropathy induced in male Sprague-Dawley rats by placing a 2-mm polyethylene cuff around the left sciatic nerve according to the method of Mosconi and Kruger. von Frey hair testing confirmed tactile allodynia in all cuff-implanted rats before electrophysiological testing. Rats were anesthetized and spinalized for extracellular recording from single spinal wide dynamic range neurons (L(3-4)). In neuropathic rats (days 11-14 and 42-52 after cuff implantation), ongoing discharge was greater and hind paw receptive field size was expanded compared to control rats. Activation of low-threshold sensory afferents by innocuous mechanical stimulation (0.2 N for 3 s) in the hind paw receptive field evoked the typical brief excitation in control rats. However, in neuropathic rats, innocuous stimulation also induced a nociceptive-like afterdischarge that persisted 2-3 min. This afterdischarge was never observed in control rats, and, in this model, is the distinguishing feature of the spinal neural correlate of tactile allodynia. Electrical stimulation of the sciatic nerve at 4 and at 20 Hz each produced an initial discharge that was identical in control and in neuropathic rats. This stimulation also produced an afterdischarge that was similar at the two frequencies in control rats. However, in neuropathic rats, the afterdischarge produced by 20-Hz stimulation was greater than that produced by 4-Hz stimulation. Given that acutely spinalized rats were studied, only peripheral and/or spinal mechanisms can account for the data obtained; as synaptic responses from C fibers begin to fail above approximately 5-Hz stimulation [Pain 46 (1991) 327], the afterdischarge in response to 20-Hz stimulation suggests a change mainly in myelinated afferents and a predominant role of these fibers in eliciting this afterdischarge. These data are consistent with the suggestion that peripheral neuropathy induces phenotypic changes predominantly in myelinated afferents, the sensory neurons that normally respond to mechanical stimulation. The NK-1 receptor antagonist, CP-99,994 (0.5 mg/kg, i.v.), depressed the innocuous pressure-evoked afterdischarge but not the brief initial discharge of wide dynamic range neurons, and decreased the elevated ongoing rate of discharge in neuropathic rats. These results support the concept that following peripheral neuropathy, myelinated afferents may now synthesize and release substance P. A result of this is that tonic release of substance P from the central terminals of these phenotypically altered neurons would lead to ongoing excitation of NK-1-expressing nociceptive spinal neurons. In addition, these spinal neurons would also exhibit exaggerated responses to innocuous pressure stimulation. The data in this study put forth a possible neurophysiological and neurochemical basis of neuropathic pain and identify substance P and the NK-1 receptor as potential neurochemical targets for its management.

Galanin expression in adult human dorsal root ganglion neurons: initial observations

Human dorsal root ganglia (DRGs) were obtained during various procedures and processed for single and double in situ hybridisation using oligonucleotide probes complementary to three peptide mRNAs. Some postmortem ganglia were also analysed. In donor (unlesioned) DRGs 12.5% of the neuron profiles (NPs) were galanin mRNA-positive (mRNA(+)), 47.5% calcitonin gene-related peptide (CGRP) mRNA(+) and 32.7% substance P mRNA(+). The corresponding percentages for cervical/thoracic DRGs from patients suffering from severe brachial plexus injury were 32.8%, 57.4% and 34.5%, respectively. In these DRGs a high proportion of the galanin mRNA(+) NPs contained CGRP mRNA and substance P mRNA. In DRGs from a patient with migraine-like pain a comparatively small proportion expressed galanin, whereas in DRGs from a herpes zoster patient galanin mRNA(+) NPs were comparatively more frequent. The results from human postmortem DRGs revealed only weak peptide mRNA signals. The present results demonstrate that galanin is expressed in DRGs not only in a number of animal species including monkey as previously shown, but also in a considerable proportion of human DRG neurons, often together with CGRP and substance P, and mostly in small neurons. Thus, galanin may play a role in processing of sensory information, especially pain, in human DRGs and dorsal horn. However, to what extent a similarly dramatic upregulation of galanin expression can be seen after peripheral nerve lesion in man, as has been reported for rat, mouse and monkey, remains to be analysed.

Gene and protein expressions of nitric oxide synthases in ischemia-reperfused peripheral nerve of the rat

This study examined mRNA and protein expressions of neuronal (nNOS), inducible (iNOS), and endothelial nitric oxide synthases (eNOS) in peripheral nerve after ischemia-reperfusion (I/R). Sixty-six rats were divided into the ischemia only and I/R groups. One sciatic nerve of each animal was used as the experimental side and the opposite untreated nerve as the control. mRNA levels in the nerve were quantitatively measured by competitive PCR, and protein was determined by Western blotting and immunohistochemical staining. The results showed that, after ischemia (2 h), both nNOS and eNOS protein expressions decreased. After I/R (2 h of ischemia followed by 3 h of reperfusion), expression of both nNOS and eNOS mRNA and protein decreased further. In contrast, iNOS mRNA significantly increased after ischemia and was further upregulated (14-fold) after I/R, while iNOS protein was not detected. The results reveal the dynamic expression of individual NOS isoforms during the course of I/R injury. An understanding of this modulation on a cellular and molecular level may lead to understanding the mechanisms of I/R injury and to methods of ameliorating peripheral nerve injury.

Neuropeptide Y immunoreactivity in cutaneous sympathetic and sensory neurons during development of the guinea pig

Different levels of the cutaneous vasculature are innervated selectively by subpopulations of sympathetic neurons distinguished by the presence or absence of immunoreactivity (-IR) for neuropeptide Y (NPY). This study used multiple-labelling immunohistochemistry to examine the appearance of NPY-IR in neurons innervating cutaneous vessels in the ear pinna of embryonic, fetal, and neonatal guinea pigs. NPY-immunoreactive axons were detected in the ear bud at embryonic day 25. However, these axons lacked IR for tyrosine hydroxylase (TH) and often ran in bundles with substance P (SP)-immunoreactive axons close to the epidermis. Many neuronal somata in the cervical dorsal root ganglia (DRG) at late embryonic stages contained NPY-IR with or without SP-IR, but no NPY-IR was detected in DRG or subepidermal axons by late fetal stages. IR for calcitonin gene-related peptide increased in DRG neurons from midfetal to late fetal stages, after the decrease in NPY-IR. Populations of TH-IR neurons with or without NPY-IR were present in the superior cervical ganglion (SCG) from midembryonic stages. TH-immunoreactive axons were not detected in the ear pinna until midfetal stages, when axons with TH-IR and NPY-IR innervated proximal arteries and TH-immunoreactive axons without NPY-IR innervated distal vessels. Vasoactive intestinal peptide-IR was detected transiently in most fetal SCG neurons with TH-IR and NPY-IR but was not detected in cutaneous axons. These results demonstrate that selective expression of NPY by subpopulations of sympathetic neurons occurs prior to innervation of their targets. This suggests that target contact is not required to establish appropriate patterns of expression of peptide neurotransmitters by cutaneous sympathetic neurons.

Neuronal nitric oxide synthase (nNOS) mRNA is down-regulated, and constitutive NOS enzymatic activity decreased, in thoracic dorsal root ganglia and spinal cord of the rat by a substance P N-terminal metabolite

Nitric oxide (NO) in the spinal cord plays a role in sensory and autonomic activity. Pain induced by acetic acid in the abdominal stretch (writhing) assay and hyperalgesia associated with chronic pain are highly sensitive to NO synthase (NOS) inhibitors. Because substance P (SP) is released and up-regulated in some models of chronic pain, we hypothesized that an accumulation of SP metabolites may influence NOS expression and activity. To test this hypothesis, we examined the effect of intrathecally (i.t.) injected substance P (1-7) [SP(1-7)], the major metabolite of SP in the rat, on neuronal NOS (nNOS) mRNA in the thoracic and lumbar spinal cord, dorsal root ganglia (DRG) and on the corresponding constitutive NOS (cNOS) enzyme activity. Detected using quantitative RT-PCR, nNOS mRNA content in the thoracic spinal cord was decreased 6 h after injection of 5 micromol of SP(1-7) and returned to control 2 days later. In thoracic DRG, nNOS mRNA was reduced 48 h after SP(1-7). The cNOS enzymatic activity in thoracic spinal tissue was gradually decreased to a minimum at 72 h. Down-regulation of NOS by SP(1-7) in the thoracic area appears to be highly associated with capsaicin-sensitive primary afferent neurons. No similar changes in either parameter were measured in the lumbar area after SP(1-7). These data suggest that N-terminal SP fragments, which are known to cause long-term antinociception in the writhing assay, may do so by their ability to down-regulate NO synthesis along nociceptive pathways.

Neuropeptide expression in the ferret trigeminal ganglion following ligation of the inferior alveolar nerve

Previous studies have found changes in neuropeptide expression in trigeminal ganglion cells after inferior alveolar nerve (IAN) section. These changes may play a part in the persistent sensory abnormalities that can be experienced after trigeminal nerve injuries. Here, neuropeptide expression after IAN ligation was studied, as this type of injury is thought to be more likely to result in sensory disturbances. The neuropeptides investigated were substance P, calcitonin gene-related peptide, enkephalin (ENK), galanin (GAL), neuropeptide Y (NPY) and vasoactive intestinal polypeptide. In anaesthetised adult female ferrets the left IAN was sectioned and the central stump tightly ligated. Recovery was allowed for 3 days, 3 or 12 weeks before perfusion-fixation. In a second procedure, 1 week before perfusion, the IAN was exposed and an injection made central to the injury site, using a mixture of 4% Fluorogold and 4% Isolectin B4 conjugated to horseradish peroxidase, to identify cell bodies with axons in the inferior alveolar nerve and cells with unmyelinated axons within this population, respectively. Control experiments involved tracer injection alone. After harvesting the tissue, sagittal sections were taken from both the right and left ganglia and immunohistochemical staining used to reveal the presence of peptides and Isolectin B4 tracer. The results showed a significant decrease in GAL expression after injury and an increase in ENK and NPY expression. No significant differences were seen in the expression of the other peptides or in the proportion of lectin-positive cells at any time after injury. When compared with previous data, significant differences were found between peptide expression following nerve ligation and nerve section. These results reveal that the changes in neuropeptide expression in the trigeminal ganglion that follow IAN injury are dependent upon the type of injury. The extent to which changes in the central neuropeptide levels contribute to the development of sensory disorders remains to be established.

[Primary sensory neurons expressing histamine H1-receptor mRNA]

Pharmacological studies have suggested that a subgroup of primary sensory neurons is responsive to histamine via the H1 receptor. However, which type of primary sensory neurons express H1 receptor is not known. We addressed this issue using in situ hybridization histochemistry with a cRNA probe for the guinea pig H1 receptor mRNA. H1 receptor mRNA was expressed in about 15-20% of the trigeminal and lumbar dorsal root ganglion (DRG) neurons, but none of the nodose ganglion neurons. The positive neurons in DRG were exclusively small in size and were labeled by isolectin B4, suggesting that these neurons have unmyelinated fibers. However, H1-receptor mRNA-expressing DRG neurons were not immunoreactive to substance P (SP) or calcitonin gene-related peptide (CGRP), which are implicated in the nociceptive transmission of the primary sensory system. Moreover, in guinea pigs neonatally treated with capsaicin (50 mg/kg), few CGRP-immunoreactive neurons were seen in DRG, but the percentage of H1-receptor mRNA-expressing neurons (15%-20%) and the intensity of the mRNA signals in these neurons were not affected by neonatal capsaicin treatment, suggesting that H1 receptor-expressing neurons are not sensitive to capsaicin. These findings suggest that H1-receptor-expressing neurons are involved in the transmission of a unique sensory modality such as itch. A marked increase in the number of mRNA-positive DRG neurons was observed 1-5 days after a crush injury of the sciatic nerve (3-4-fold of the control value). These neurons that turned mRNA-positive after the nerve crush were also mainly small-sized. The mRNA signals were detected in many peptidergic (SP/CGRP) neurons, in contrast to the normal condition. On the other hand, mRNA signals were decreased in the neurons that showed intense labeling in the normal condition. These results suggest that the gene expression of H1 receptors up-regulated in injured afferents may be involved in neuropathic pain.

Distribution of cGMP in guinea pig autonomic ganglia after stimulation with sodium nitroprusside

Nitric oxide (NO) is an intercellular messenger molecule in the nervous system and exerts its action in many regions by generating cyclic GMP (cGMP) via soluble guanylyl cyclase. In this study, on the male guinea pig, we have analyzed the localization of cGMP in some autonomic ganglia with immunohistochemistry after stimulation with sodium nitroprusside (SNP) as NO donor, and made correlations with the NO synthesizing enzyme NO synthase (NOS), tyrosine hydroxylase (TH) and some neuropeptides. The putative target neurons for NO were examined in the anterior pelvic ganglia (APGs), as well as some pre- and paravertebral sympathetic ganglia. The results show that cGMP-like immunoreactivity (LI) in the APG was in most cases observed in the TH-positive, NOS-negative neuron population after SNP stimulation, whereas the NOS-expressing cholinergic population mostly lacked detectable cGMP-LI. In the pre- and paravertebral ganglia, SNP stimulation increased cGMP levels to a much lesser extent than in the APGs. cGMP was also observed in blood vessels, in the ganglion capsule, and in some cases. possibly in satellite cells. We propose, as one alternative, that there is a functional, intraganglionic regulatory loop between the parasympathetic and sympathetic divisions of the APG, using the NO/cGMP pathway.

Changes in neuropeptide expression in the trigeminal ganglion following inferior alveolar nerve section in the ferret

Changes in neuropeptide expression in afferent nerve fibres may play a role in the persistent sensory abnormalities that can be experienced following trigeminal nerve injuries. We have therefore studied changes in the expression of the neuropeptides substance P, calcitonin gene-related peptide, enkephalin, galanin, neuropeptide Y and vasoactive intestinal polypeptide in the trigeminal ganglion following peripheral nerve injury. In anaesthetised adult female ferrets, the left inferior alveolar nerve was sectioned and recovery allowed for three days, three weeks or 12 weeks prior to perfusion-fixation. During a second procedure, one week prior to perfusion, the inferior alveolar nerve was exposed and an injection made central to the injury site using a mixture of 4 % Fluorogold and 4 % isolectin B4 conjugated to horseradish peroxidase to identify cell bodies with axons in the inferior alveolar nerve and cells with unmyelinated axons within this population, respectively. Control animals received tracer injection alone. After harvesting the tissue, sagittal sections were taken from both the right and left ganglia and immunohistochemical staining was used to reveal the presence of peptides and isolectin B4-horseradish peroxidase tracer. Within the Fluorogold-labelled population, cell counts revealed a significant reduction in the proportion of substance P-containing cells at three days (P = 0.0025), three weeks (P = 0.0094) and three months (P = 0.0149) after nerve section, and a significant reduction in the proportion of calcitonin gene-related peptide-containing cells at three days (P = 0.0003) and three weeks (P = 0.007). No significant changes were seen in the expression of the other peptides, or at other time periods. A significant reduction in the number of isolectin B4-horseradish peroxidase-positive cells (with unmyelinated axons) was seen at three days (P = 0.0025), three weeks (P = 0.0074) and three months after the injury (P = 0.0133). These results demonstrate a significant reduction in the expression of some neuropeptides in the early stages after inferior alveolar nerve section. Some of the results differ markedly from those reported previously in other systems, and may be related to the specific nerve studied, species variations or differences between spinal and trigeminal nerves.

Histamine H1 receptor mRNA is expressed in capsaicin-insensitive sensory neurons with neuropeptide Y-immunoreactivity in guinea pigs

Histamine H1 receptor mRNA-expressing sensory neurons in guinea pigs are unmyelinated and are not immunoreactive to substance P and calcitonin gene-related peptide (CGRP) [Mol. Brain Res. 66 (1999) 24], which are implicated in the nociceptive transmission of the primary sensory system. In this study, we examined whether these H1 receptor mRNA-expressing neurons are sensitive to capsaicin by using in situ hybridization histochemistry. Of lumbar dorsal root ganglion (DRG) neurons in control animals, 17% were positive for CGRP. In guinea pigs neonatally treated with capsaicin (50 mg/kg), few CGRP-immunoreactive neurons were seen in the DRGs. However, the percentages of H1 receptor mRNA-expressing neurons (15-20%) and the intensity of the mRNA signals in these neurons were not affected by neonatal capsaicin treatment. We also revealed the presence of both capsaicin-sensitive and insensitive neuropeptide Y (NPY)-immunoreactive neurons in the DRGs. These neurons were exclusively small. H1 receptor mRNA was expressed in NPY-immunoreactive neurons in naive guinea pig DRGs. These results suggest that H1 receptor mRNA is expressed in capsaicin-insensitive DRG neurons with NPY-immunoreactivity in guinea pigs.

Distinct sprouting responses of sympathetic and peptidergic sensory axons proximal to a sciatic nerve transection in guinea pigs and rats

Sprouting of sympathetic and peptidergic sensory neurones proximal to nerve lesions may reflect upregulation of growth factors around damaged dorsal root ganglion (DRG) cells. Axons containing noradrenaline or calcitonin gene-related peptide were visualized in DRGs and spinal roots of guinea pigs and rats. After sciatic transection in rats, varicose terminals of both types appeared around large DRG somata. These neurones were surrounded by proliferated satellite cells expressing glial fibrillary acidic protein (GFAP) and p75. This did not occur in guinea pigs. Instead, sympathetic axons grew through the DRG and centrally along the dorsal roots (which contained p75-positive glia), avoiding the DRG somata. Thus the glial reaction to peripheral injury differs between species such that, in guinea pigs, the environment in the spinal roots rather than in the DRGs favours sympathetic sprouting.

An interaction of opioids and galanin in dorsal horn of the spinal cord in mononeuropathic rats

The present study was performed in rats with experimentally induced mononeuropathy after common sciatic nerve ligation. The hind-paw withdrawal latencies to thermal and mechanical stimulation were increased significantly after intrathecal injection of 3 nmol of galanin. The increased hind-paw response latencies induced by galanin were attenuated by following intrathecal injection of 22 nmol, but not 11 or 2.75 nmol of the opioid receptor antagonist naloxone. Further, the increased hind-paw response latencies induced by galanin were prevented by following intrathecal injection of 10 nmol of mu-opioid receptor antagonist, beta-funaltrexamine (beta-FNA), but not by 10 nmol of delta-opioid receptor antagonist, natrindole or 10 nmol of kappa-opioid receptor antagonist, nor-binaltorphimine (nor-BNI). Intrathecal 10 nmol of beta-FNA alone had no significant effects on the hind-paw withdrawal responses. These results demonstrate the existence of a specific interaction between galanin and opioids in the transmission of presumed nociceptive information in the spinal cord of mononeuropathic rats. This interaction involves the activation of mu-opioid receptor.

Nerve constriction in the rat: model of neuropathic, surgical and central pain

In preparation for a series of electrophysiological experiments in a model of neuropathic pain, the present spinal reflex study was done to determine the optimal time after sciatic nerve constriction in the rat for tactile allodynia and to determine also the appropriate 'control' for the nerve constriction model. Therefore, this study focused on the magnitude and time course of change in paw withdrawal threshold following unilateral sciatic nerve constriction in the rat. Male Sprague-Dawley rats (375-425g) were used. Nerve constriction was done by placing a 2 mm polyethylene cuff (PE-90) around the left sciatic nerve (n=8). A second group of rats (n=8) received unilateral sham surgery and a third group (n=8) was unoperated. The ipsi- and contralateral hind paw withdrawal thresholds in each of the 3 groups were measured using von Frey hairs. In unoperated rats, the withdrawal threshold of each of the hind paws remained unchanged at approximately 50 g throughout the entire time course of the study, which lasted 145 days. However, in cuff-implanted rats, the withdrawal threshold of the nerve-injured hind paw decreased as soon as 1 day after surgery, reached as low as 1 to 2 g by 5 days and remained low throughout the test period. Threshold in sham-operated rats showed a bilateral decrease starting on days 1-3, which stabilised at about 30 g until about day 40, after which values returned gradually toward the unoperated withdrawal thresholds. In nerve-constricted rats the withdrawal threshold of the hind paw contralateral to the cuff followed the same change seen in sham-operated rats until about day 37, after which the withdrawal threshold matched that of the cuff-implanted hind paw. The data show that the cuff-induced sciatic nerve constriction produces a sustained hypersensitivity to normally innocuous tactile sensory input and that a relatively constant ipsilateral mechanical hyperalgesia can be found from days 5-27. It is also demonstrated that the contralateral hind paw and either hind paw in sham-operated rats are inappropriate as 'controls'. The data in this study suggest that three distinct types of allodynia are expressed. Ipsilateral allodynia may be representative of a model of neuropathic pain. The contralateral allodynia may be a model of central pain, as it likely arises from changes in central sensory processing. Allodynia in sham-operated rats was also expressed bilaterally and may be a model of long-term postoperative pain.

Neuropeptide Y (NPY) expression is increased in explanted guinea pig parasympathetic cardiac ganglia neurons

While expression of neuropeptides by sympathetic neurons is altered by decentralization and axotomy, it is not known whether similar experimental paradigms also modulate the chemical phenotype of parasympathetic cardiac ganglia neurons. The present study tested whether guinea pig parasympathetic neuron neuropeptide Y (NPY) expression was altered when cardiac ganglia preparations were maintained as organ explants in the presence or absence of colchicine. Two experimental approaches were used to examine NPY expression. First, immunocytochemical techniques were used to quantitate numbers of neurons within the cardiac ganglia exhibiting NPY-immunoreactivity; second, reverse transcription PCR was used to examine proNPY mRNA expression. In control cardiac ganglia preparations, approximately 4% of ganglia neurons exhibited NPY-immunoreactivity. The percentage of NPY-immunopositive neurons in 30- and 72-h explanted cardiac ganglia preparations, maintained in the absence of colchicine, increased to 11 and 16%, respectively. Colchicine treatment of explanted preparations further increased the percentage of NPY-positive ganglia cells 24% (30 h) and 32% (72 h). All NPY-immunoreactive neurons from control ganglia and explanted ganglia were choline acetyltransferase(ChAT)-immunoreactive, indicating retention of the cholinergic phenotype. ProNPY mRNA also was increased following ganglia explantation, consistent with the increase in the numbers of NPY-immunoreactive neurons. NPY transcripts were further increased after 30 h, but not after 72 h in colchicine-treated, explanted cardiac ganglia preparations. These results demonstrate that NPY expression is altered in explanted cardiac ganglia preparations, providing evidence that the chemical phenotype of parasympathetic cardiac neurons can be modulated.

The induction of pain: an integrative review

The highly disagreeable sensation of pain results from an extraordinarily complex and interactive series of mechanisms integrated at all levels of the neuroaxis, from the periphery, via the dorsal horn to higher cerebral structures. Pain is usually elicited by the activation of specific nociceptors ('nociceptive pain'). However, it may also result from injury to sensory fibres, or from damage to the CNS itself ('neuropathic pain'). Although acute and subchronic, nociceptive pain fulfils a warning role, chronic and/or severe nociceptive and neuropathic pain is maladaptive. Recent years have seen a progressive unravelling of the neuroanatomical circuits and cellular mechanisms underlying the induction of pain. In addition to familiar inflammatory mediators, such as prostaglandins and bradykinin, potentially-important, pronociceptive roles have been proposed for a variety of 'exotic' species, including protons, ATP, cytokines, neurotrophins (growth factors) and nitric oxide. Further, both in the periphery and in the CNS, non-neuronal glial and immunecompetent cells have been shown to play a modulatory role in the response to inflammation and injury, and in processes modifying nociception. In the dorsal horn of the spinal cord, wherein the primary processing of nociceptive information occurs, N-methyl-D-aspartate receptors are activated by glutamate released from nocisponsive afferent fibres. Their activation plays a key role in the induction of neuronal sensitization, a process underlying prolonged painful states. In addition, upon peripheral nerve injury, a reduction of inhibitory interneurone tone in the dorsal horn exacerbates sensitized states and further enhance nociception. As concerns the transfer of nociceptive information to the brain, several pathways other than the classical spinothalamic tract are of importance: for example, the postsynaptic dorsal column pathway. In discussing the roles of supraspinal structures in pain sensation, differences between its 'discriminative-sensory' and 'affective-cognitive' dimensions should be emphasized. The purpose of the present article is to provide a global account of mechanisms involved in the induction of pain. Particular attention is focused on cellular aspects and on the consequences of peripheral nerve injury. In the first part of the review, neuronal pathways for the transmission of nociceptive information from peripheral nerve terminals to the dorsal horn, and therefrom to higher centres, are outlined. This neuronal framework is then exploited for a consideration of peripheral, spinal and supraspinal mechanisms involved in the induction of pain by stimulation of peripheral nociceptors, by peripheral nerve injury and by damage to the CNS itself. Finally, a hypothesis is forwarded that neurotrophins may play an important role in central, adaptive mechanisms modulating nociception. An improved understanding of the origins of pain should facilitate the development of novel strategies for its more effective treatment.

Activated eosinophils elicit substance P release from cultured dorsal root ganglion neurons

This study was performed to test the hypothesis that activated eosinophils or their secretory products can directly stimulate sensory neurons to release their neuropeptides. Neurons derived from neonatal rat dorsal root ganglia (DRG), which synthesize and store sensory neuropeptides, were placed in primary cell culture and were exposed to eosinophils or their bioactive mediators. The resultant release of substance P (SP) was measured by enzyme-linked immunosorbent assay and was expressed as a percent (mean +/- SE) of total neuronal SP content. Eosinophils were isolated from human volunteers with a history of allergic rhinitis and/or mild asthma and were activated by incubation with cytochalasin B (5 micrograms/ml) and N-formyl-methionyl-leucyl-phenylalanine (FMLP, 1 microM). Activated eosinophils [6 x 10(6)/ml, suspended in Hanks' buffered salt solution (HBSS)] applied to cultured DRG neurons for 30 min increased basal SP release 2.4-fold compared with HBSS-exposed neurons (activated eosinophils 11.10 +/- 2.48% vs. HBSS 4.59 +/- 0.99%; P = 0.002), whereas neither nonactivated eosinophils nor cytochalasin B and FMLP in HBSS influenced SP release. Additional cultured DRG neurons were exposed to soluble products made by eosinophils. Compared with SP release under control conditions (2.37 +/- 0.34%), major basic protein (MBP) increased release in a concentration-related fashion (e.g., 3 microM MBP: 6.23 +/- 0.67%, P = 0.006 vs. control), whereas neither eosinophil cationic protein (3 microM), eosinophil-derived neurotoxin (3 microM), leukotriene D4 (500 nM), platelet-activating factor (100 nM), nor H2O2 (100 microM) affected SP release. These studies demonstrate that activated eosinophils can stimulate cultured DRG neurons directly and suggest that MBP may be the responsible mediator.

Effects of peripheral nerve ligation on expression of mu-opioid receptor in sensory ganglion neurons: an immunohistochemical study in dorsal root and nodose ganglion neurons of the rat

The present study was attempted to examine if mu-opioid receptor (MOR) might be transported by axonal flow peripherally through peripheral axons of somatic sensory ganglion neurons. After unilateral ligation of the sciatic nerve or the vagus nerve distal to the dorsal root ganglion (DRG) or nodose ganglion (NG), MOR-like immunoreactivity (MOR-LI) of neuronal cell bodies in the DRG of the fourth and fifth lumbar nerves, NG, ambiguus nucleus (Amb) and dorsal motor nucleus of the vagus nerve (DMV) on the side of the ligation was apparently reduced within 1 week after the nerve ligation. However, within 24 h after the nerve ligation, a transient enhancement of MOR-LI was observed in cell bodies of DRG neurons, sciatic nerve stump proximal to the ligature, and cell bodies of NG neurons on the side of the ligation; such a transient enhancement of MOR-LI was not detected in the Amb and DMV. The results suggest that MOR undergoes centrifugal axonal flow in peripheral axons of somatic and visceral sensory ganglion neurons, and that MOR synthesis in sensory ganglion neurons is vulnerable to damage of the peripheral axons.