Long-term changes in the distribution of galanin in dorsal root ganglia after sciatic or spinal nerve transection in rats

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.

Local and remote immune-mediated inflammation after mild peripheral nerve compression in rats

After experimental nerve injuries that extensively disrupt axons, such as chronic constriction injury, immune cells invade the nerve, related dorsal root ganglia (DRGs), and spinal cord, leading to hyperexcitability, raised sensitivity, and pain. Entrapment neuropathies, such as carpal tunnel syndrome, involve minimal axon damage, but patients often report widespread symptoms. To understand the underlying pathology, a tube was placed around the sciatic nerve in 8-week-old rats, leading to progressive mild compression as the animals grew. Immunofluorescence was used to examine myelin and axonal integrity, glia, macrophages, and T lymphocytes in the nerve, L5 DRGs, and spinal cord after 12 weeks. Tubes that did not constrict the nerve when applied caused extensive and ongoing loss of myelin, together with compromise of small-, but not large-, diameter axons. Macrophages and T lymphocytes infiltrated the nerve and DRGs. Activated glia proliferated in DRGs but not in spinal cord. Histologic findings were supported by clinical hyperalgesia to blunt pressure and cold allodynia. Tubes that did not compress the nerve induced only minor local inflammation. Thus, progressive mild nerve compression resulted in chronic local and remote immune-mediated inflammation depending on the degree of compression. Such neuroinflammation may explain the widespread symptoms in patients with entrapment neuropathies.

Development of galanin-containing nerve fibres in rat tibia

Galanin exerts tonic inhibition of nociceptive input to the central nervous system. Recently, this peptide was demonstrated in several neuronal and non-neuronal structures in bones and joints. In this study, the time of appearance and topographic localization of galanin-containing nerve fibres in bone were studied in rats from gestational day 16 (GD16) to postnatal day 21 (PD21). The tibia was chosen as a model of developing long bone and indirect immunofluorescence combined with confocal laser scanning microscopy was used to identify galanin-immunoreactive (GAL-IR) nerve fibres. The earliest, sparse GAL-IR fibres were observed on GD21 in the perichondrium of both epiphyses and in the periosteum of the diaphysis. From PD1 onwards, GAL-IR fibres were also seen in the bone marrow cavity and in the region of the inter-condylar eminence of the knee joint. Intramedullary GAL-IR fibres in proximal and distal metaphyses appeared around PD1. Some of them accompanied blood vessels, although free fibres were also seen. GAL-IR fibres located in the cartilage canals of both epiphyses were observed from PD7, in the secondary ossification centres from PD10 and in the bone marrow of both epiphyses from PD14. The time course and localization of galanin-containing nerve fibres resemble the development of substance P- and CGRP-expressing nerve fibres, thus suggesting their sensory origin.

Immune cell involvement in dorsal root ganglia and spinal cord after chronic constriction or transection of the rat sciatic nerve

Chronic constriction injury (CCI) of the sciatic nerve in rodents produces mechanical and thermal hyperalgesia and is a common model of neuropathic pain. Here we compare the inflammatory responses in L4/5 dorsal root ganglia (DRGs) and spinal segments after CCI with those after transection and ligation at the same site. Expression of ATF3 after one week implied that 75% of sensory and 100% of motor neurones had been axotomized after CCI. Macrophage invasion of DRGs and microglial and astrocytic activation in the spinal cord were qualitatively similar but quantitatively distinct between the lesions. The macrophage and glial reactions around neurone somata in DRGs and ventral horn were slightly greater after transection than CCI while, in the dorsal horn, microglial activation (using markers OX-42(for CD11b) and ED1(for CD68)) was greater after CCI. In DRGs, macrophages positive for OX-42(CD11b), CD4, MHC II and ED1(CD68) more frequently formed perineuronal rings beneath the glial sheath of ATF3+ medium to large neurone somata after CCI. There were more invading MHC II+ macrophages lacking OX-42(CD11b)/CD4/ED1(CD68) after transection. MHC I was expressed in DRGs and in spinal sciatic territories to a similar extent after both lesions. CD8+ T-lymphocytes aggregated to a greater extent both in DRGs and the dorsal horn after CCI, but in the ventral horn after transection. This occurred mainly by migration, additional T-cells being recruited only after CCI. Some of these were probably CD4+. It appears that inflammation of the peripheral nerve trunk after CCI triggers an adaptive immune response not seen after axotomy.

Serotonin in Trigeminal Ganglia of Female Rodents: Relevance to Menstrual Migraine

OBJECTIVES

We examined changes in the serotonin system across the estrous cycle in trigeminal ganglia of female rodents to determine which components are present and which are regulated by the variations in levels of ovarian steroids that occur during the estrous cycle.

BACKGROUND

Migraine is 2-3 times more prevalent in women than in men and attacks are often timed with the menstrual cycle, suggesting a mechanistic link with ovarian steroids. Serotonin has been implicated in the pathogenesis of migraine, and the effectiveness of triptans, selective 5HT-1B/D/F agonists, has provided further support for this concept. It is not known whether serotonin, its rate-limiting enzyme tryptophan hydroxylase (TPH), or its receptors are regulated by ovarian steroids in trigeminal ganglia.

METHODS

We used reverse transcription-polymerase chain reaction to examine gene expression in cycling mice, Western blots to examine protein expression, double-labeling immunohistochemistry using markers of nociceptors and nonnociceptors and confocal microscopy to identify specific types of neurons, and primary tissue culture to examine effects of estrogen on trigeminal neurons in vitro.

RESULTS

In C57/BL6 mice mRNA levels of TPH-1, the rate-limiting enzyme in serotonin synthesis, were over 2-fold higher and protein levels were 1.4-fold higher at proestrus, the high estrogen stage of the cycle than at diestrus, the low estrogen stage. TPH protein also was present in primary trigeminal cultures obtained from female Sprague-Dawley rats, but levels were not affected by 24-hour treatment with physiological levels (10(-9) M) of 17beta-estradiol. Gene expression of 5HT-1B and 5HT-1D receptors in trigeminal ganglia was not regulated by the estrous cycle. Serotonin was present in trigeminal neurons containing CGRP, a potent vasoactive neuropeptide, peripherin, an intermediate filament present in neurons with unmyelinated axons, neurofilament H, which is present in neurons with myelinated axons, and in neurons binding IB4, a marker of nonpeptidergic nociceptors. Serotonin was also present in neurons containing 5HT-1B. The serotonin-positive population was significantly larger in diameter than the serotonin-negative population. Conclusions.-Expression of the rate-limiting enzyme required for serotonin synthesis is regulated during the natural estrous cycle, and serotonin is present in larger trigeminal neurons of all the major subtypes. Colocalization of serotonin with 5HT-1B suggests that this receptor functions as an autoreceptor to regulate serotonin release. Cyclical changes in serotonin levels in trigeminal ganglia could contribute to the pathogenesis of menstrual migraine.

Specific expression of galanin in the peri-infarct zone after permanent focal cerebral ischemia in the rat

Galanin (Gal) is a neuropeptide with supposed neurotrophic-like action. In the present study, expression of Gal has been investigated in the core and peri-infarct zone at 1, 4, 24 and 72 h after middle cerebral artery occlusion (MCAo) in the rat. Three days after MCAo a small but consistent number of morphological intact Gal-positive neuronal cells were observed in the peri-infarct zone. Gal-positive cells were barely detectable in the infarct and peri-infarct zone at 24 h. No Gal immunopositive cells were detected in brain subjected to 1 and 4 h of ischemia. Gal immunoreactivity was also detected in myelinated fibers 4 and 24 h after focal ischemia. Gal may be a peptide with neurotrophic and plasticity functions under stress conditions.

Changes in galanin immunoreactivity in rat micturition reflex pathways after cyclophosphamide-induced cystitis

Alterations in the expression of the neuropeptide, galanin, were examined in micturition reflex pathways of rat after cyclophosphamide (CYP)-induced cystitis of variable duration: acute (4 h), intermediate (48 h), or chronic (10 days). In control animals, galanin expression was present in specific regions of the gray matter in the rostral lumbar and caudal lumbosacral spinal cord, including: (1) the dorsal commissure (DCM); (2) superficial dorsal horn; (3) the regions of the intermediolateral cell column (L1-L2) and the sacral parasympathetic nucleus (SPN, L6-S1); and (4) the lateral collateral pathway (LCP) in lumbosacral spinal segments. Densitometry analysis demonstrated significant decreases (P< or =0.01) in galanin immunoreactivity (IR) in these regions of the L1-S1 spinal cord after acute or intermediate CYP-induced cystitis. In contrast, increases (P< or =0.01) in galanin-IR were observed in the DCM, SPN, or LCP regions in the L6-S1 spinal segments in rats with chronic cystitis. No changes in the number of galanin-immunoreactive cells were observed in the L1-S1 dorsal root ganglia (DRG) after CYP-induced cystitis of any duration. A small percentage of bladder afferent cells (Fast-blue-labeled) in the DRG expressed galanin-IR in control rats; this was not altered with cystitis. Galanin-IR was observed encircling DRG cells after chronic cystitis. These changes may contribute to urinary bladder dysfunction, altered sensation, and referred somatic hyperalgesia after cystitis.

Sciatic nerve regeneration in mice and rats: recovery of sensory innervation is followed by a slowly retreating neuropathic pain-like syndrome

Peripheral nerve regeneration has been studied extensively in the sciatic nerve crush model, at the level of both function and gene expression. The crush injury allows full recovery of sensory and motor function in about 3 weeks as assessed by the foot reflex withdrawal test and De Medinacelli walking patterns. We used the recently developed CatWalk paradigm to study walking patterns in more detail in mice and rats. We found that, following the recovery of sensory function, the animals developed a state of mechanical allodynia, which retreated slowly over time. The motor function, although fully recovered with the conventional methods, was revealed to be still impaired because the animals did not put weight on their previously injured paw. The development of neuropathic pain following successful sensory recovery has not been described before in crush-lesioned animals and may provide an important new parameter to assess full sensory recovery.

Selective reactions of cutaneous and muscle afferent neurons to peripheral nerve transection in rats

To determine whether peripheral nerve injury has similar effects on all functional types of afferent neuron, we retrogradely labeled populations of neurons projecting to skin and to muscle with FluoroGold and lesioned various peripheral nerves in the rat. Labeled neurons were counted after different periods and related to immunohistochemically identified ectopic terminals and satellite cells in lumbar dorsal root ganglia. After 10 weeks, 30% of cutaneous afferent somata labeled from transected sural nerves had disappeared but, if all other branches of the sciatic nerve had also been cut, 60% of cutaneous neurons were lost. Small-diameter sural neurons preferentially disappeared. In contrast, the number of muscle afferent somata was not affected by transection of various nerves. p75 was downregulated in axotomized cutaneous neurons but in not axotomized muscle neurons. Conversely, p75 was upregulated in satellite cells around cutaneous but not muscle neurons. Consistent with this, perineuronal rings containing tyrosine hydroxylase, calcitonin gene-related peptide, galanin, or synaptophysin were formed preferentially around cutaneous neurons. Selective lesions of predominantly cutaneous nerves triggered the formation of rings, but none were detected after selective lesions of muscle nerves. We conclude that cutaneous neurons are both more vulnerable and more associated with ectopic nerve terminals than muscle neurons in dorsal root ganglia after transection and ligation of peripheral nerves.

Expression of galanin and galanin receptor-1 in normal bone and during fracture repair in the rat

The neuropeptide galanin (GAL) has recognized physiological actions in the nervous system and other tissues, but there is no documented evidence of GAL influencing normal or pathological bone metabolism. GAL expression, however, is upregulated in central and peripheral nerves following axotomy and is known to influence neural regeneration. Thus, severance of skeletal-associated nerves during fracture could similarly increase local GAL concentrations and thereby influence fracture healing. The initial aim of this study was therefore to identify the presence of GAL in normal bone and/or fracture callus by assessing the concentration and cellular localization of GAL in intact and/or fractured rat rib, using radioimmunoassay and immunohistochemistry, respectively. Groups of Sprague-Dawley rats (13 weeks old) had their left sixth ribs surgically fractured or underwent sham surgery and then calluses and nonfractured rib samples were analyzed at 1 and 2 weeks postsurgery (n = 5-6 per group). Low (basal) concentrations of GAL were detected in control ribs, whereas at 1 and 2 weeks postfracture, callus samples contained markedly increased levels of peptide ( approximately 32- and 18-fold increase, respectively, relative to controls; P < 0.01), revealing a strong upregulation during bone healing. Plasma GAL concentrations were also increased at 2 weeks postfracture (P < 0.005). In normal (nonfractured) rib, minimal levels of GAL-like immunoreactivity (LI) were present in cortical bone, periosteum, endosteum, and surrounding skeletal muscle. In costal cartilage plates, intense GAL-LI was present in all chondrocytes of the hypertrophic zone and in a population of chondrocytes in the reserve zone. GAL-LI was not present, however, in chondrocytes in the proliferative zone of costal cartilage or skeletal muscle fibers. In fracture callus, levels of GAL-LI were moderate to intense in osteoprogenitor cells and osteoblasts, in some chondrocytes, and in cartilaginous, osseous, and periosteal matrices. Subsequent studies revealed the presence of galanin receptor-1-like immunoreactivity (GALR1-LI) in most cell types shown to contain GAL-LI, although the distribution of GALR1-LI was more extensive in reserve zone chondrocytes than that of GAL-LI; and GALR1-LI also appeared in late proliferative zone chondrocytes of costal cartilage. In summary, GAL concentrations were significantly increased in fracture callus and plasma of rats that underwent rib fracture. In addition, GAL- and GALR1-LI was also detected in specific cells and structures within costal cartilage, bone, and fracture callus. These results strongly implicate GAL in aspects of cartilage growth plate physiology and fracture repair, possibly acting in an autocrine/paracrine fashion via GALR1.

Effects and consequences of nerve injury on the electrical properties of sensory neurons

Nociceptive pain alerts the body to potential or actual tissue damage. By contrast, neuropathic or "noninflammatory" pain, which results from injury to the nervous system, serves no useful purpose. It typically continues for years after the original injury has healed. Sciatic nerve lesions can invoke chronic neuropathic pain that is accompanied by persistent, spontaneous activity in primary afferent fibers. This activity, which reflects changes in the properties and functional expression of Na+, K+, and Ca2+ channels, initiates a further increase in the excitability of second-order sensory neurons in the dorsal horn. This change persists for many weeks. The source of origin of the pain thus moves from the peripheral to the central nervous system. We hypothesize that this centralization of pain involves the inappropriate release of peptidergic neuromodulators from primary afferent fibers. Peptides such as substance P, neuropeptide Y (NPY), calcitonin-gene-related peptide (CGRP), and brain-derived neurotrophic factor (BDNF) may promote enduring changes in excitability as a consequence of neurotrophic actions on ion channel expression in the dorsal horn. Findings that form the basis of this hypothesis are reviewed. Study of the neurotrophic control of ion channel expression by spinal peptides may thus provide new insights into the etiology of neuropathic pain.

Macrophage and lymphocyte invasion of dorsal root ganglia after peripheral nerve lesions in the rat

The distribution of major histocompatibility complex class II (MHC II)-positive non-neuronal cells and T-lymphocytes was examined immunohistochemically in dorsal root ganglia (DRGs) up to 12 weeks following transection of one sciatic or lumbar spinal nerve in adult rats. Unlike within the brain, MHC II immunopositive (+) and T-cells are normally present within DRGs. After nerve transection, MHC II+ cell density increased (by about four times after each lesion) in DRGs projecting into lesioned nerves. Subsequently the number declined after sciatic but not spinal nerve transection. MHC II+ cells did not contain glial markers, even when these were up-regulated after the lesions. Initially, MHC II+ cells lay outside the satellite glia but, by 11 weeks, they had moved through them to lie against the somata. T-cells invaded the lesioned DRGs earlier than the MHC II+ cells. They achieved greater numbers after spinal (30 x control) than after sciatic (12 x control) nerve transection. They also increased in undamaged ganglia adjacent to the spinal nerve injury. T-cell density progressively declined after spinal but not sciatic nerve transection. Both cell types appeared to invade the DRGs initially through blood vessels and the meninges, particularly near the subarachnoid angle. At later stages, occasional neurones had dense aggregations of T-cell receptor+ and MHC II+ cells associated with them. We conclude that the magnitude and time course of changes in MHC II expression and T-cell numbers in lesioned DRGs differ from the responses within motor nuclei after axotomy. The influx of inflammatory cells may contribute to neurone survival in the short term. Their long-term presence has implications for patients. These cells have the potential to release excitatory cytokines that may generate ectopic impulse activity in sensory neurones after nerve injury and so may play a role in the generation of chronic neuropathic pain.