Growth-associated protein 43 immunoreactivity in the superficial dorsal horn of the rat spinal cord is localized in atrophic C-fiber, and not in sprouted A-fiber, central terminals after peripheral nerve injury

(-)-Epigallocatechin Gallate Attenuates Spinal Motoneuron Death Induced by Brachial Plexus Root Avulsion in Rats

Background:

Recent studies have indicated that epigallocatechin gallate (EGCG) benefits a variety of neurological insults. This study was performed to investigate the neuroprotective effect of EGCG after brachial plexus root avulsion in SD rats.

Methods:

One hundred twenty SD rats were randomized into the following three groups: an EGCG group, an Avulsion group, and a Sham group. There were 40 rats in each group. EGCG (100 mg/kg, i.p.) or normal saline was administered to rats immediately following the injuries. The treatment was continued from day 1 to day 7, and the animals were sacrificed on days 3, 7, 14 and 28 post-surgery for the harvesting of spinal cord samples for Nissl staining, immunohistochemistry (caspase-3, p-JNK, p-c-Jun) and western blot analysis (p-JNK, JNK, p-c-Jun, c-Jun).

Results:

EGCG treatment caused significant increases in the percentage of surviving motoneurons at days 14 and 28 (P<0.05) compared to the control animals. At days 3 and 7 after avulsion, the numbers of caspase-3-positive motoneurons in the EGCG-treated animals were significantly fewer than in the control animals (P<0.05). The numbers of p-JNK-positive motoneurons and the ratio of p-JNK/JNK were no significant differences between the Avulsion group and the EGCG-treated group after injury at any time point. The numbers of p-c-Jun-positive motoneurons and the ratio of p-c-Jun/c-Jun were significantly lower in EGCG-treated group compared with the Avulsion group at 3d and 7d after injury (p<0.05).

Conclusions:

Our results indicated that motoneurons were protected by EGCG against the cell death induced by brachial plexus root avulsion, and this effect was correlated with inhibiting c-Jun phosphorylation.

Cellular and molecular insights into neuropathy-induced pain hypersensitivity for mechanism-based treatment approaches

Neuropathic pain is currently being treated by a range of therapeutic interventions that above all act to lower neuronal activity in the somatosensory system (e.g. using local anesthetics, calcium channel blockers, and opioids). The present review highlights novel and often still largely experimental treatment approaches based on insights into pathological mechanisms, which impact on the spinal nociceptive network, thereby opening the 'gate' to higher brain centers involved in the perception of pain. Cellular and molecular mechanisms such as ectopia, sensitization of nociceptors, phenotypic switching, structural plasticity, disinhibition, and neuroinflammation are discussed in relation to their involvement in pain hypersensitivity following either peripheral neuropathies or spinal cord injury. A mechanism-based treatment approach may prove to be successful in effective treatment of neuropathic pain, but requires more detailed insights into the persistence of cellular and molecular pain mechanisms which renders neuropathic pain unremitting. Subsequently, identification of the therapeutic window-of-opportunities for each specific intervention in the particular peripheral and/or central neuropathy is essential for successful clinical trials. Most of the cellular and molecular pain mechanisms described in the present review suggest pharmacological interference for neuropathic pain management. However, also more invasive treatment approaches belong to current and/or future options such as neuromodulatory interventions (including spinal cord stimulation) and cell or gene therapies, respectively.

Brief, low frequency stimulation of rat peripheral C-fibres evokes prolonged microglial-induced central sensitization in adults but not in neonates

The sensitization of spinal dorsal horn neurones leads to prolonged enhancement of pain behaviour and can be evoked by intense C-fibre stimulation, tissue inflammation and peripheral nerve injury. Activation of central immune cells plays a key role in establishing pain hypersensitivity but the exact nature of the afferent input that triggers the activation of microglia and other glial cells within the CNS, remains unclear. Here intense but non-damaging, electrical stimulation of intact adult rat C-fibres for 5 min at 10 Hz induced central sensitization characterized by significant decreases in mechanical withdrawal thresholds 3, 24 and 48 h later. This maintained (>3 h) hypersensitivity was not observed following topical skin application of capsaicin. C-fibre evoked sensitization was accompanied by significant microglial activation, shown by increased Iba-1 immunoreactivity throughout the dorsal horn at 24 and 48 h and significant upregulation of markers of microglial activation: IL-6 and Mcp-1 at 3 h and Mmp3, CSF-1 and CD163 at 24 and 48 h. C-fibre stimulation caused no nerve damage at ultrastructural and molecular levels. Lower intensity stimulation that did not activate C-fibres or sham stimulation did not increase Iba-1 immunoreactivity or induce behavioural sensitivity. Pre-treatment with minocycline (40 mg/kg, i.p.) prevented the C-fibre evoked sensitization and microglial activation. Identical C-fibre stimulation in 10-day old rat pups failed to activate microglia or change behaviour. These results demonstrate that a brief period of low frequency C-fibre stimulation, in the absence of nerve damage, is sufficient to activate microglia resulting in behavioural hyperalgesia.

Expression of the regeneration-associated protein SPRR1A in primary sensory neurons and spinal cord of the adult mouse following peripheral and central injury

Small proline-rich repeat protein 1A (SPRR1A) is expressed in dorsal root ganglion (DRG) neurons following peripheral nerve injury but it is not known whether SPRR1A is differentially expressed following injury to peripheral versus central DRG projections and a detailed characterization of expression in sensory neuron subpopulations and spinal cord has not been performed. Here we use immunocytochemical techniques to characterize SPRR1A expression following sciatic nerve, dorsal root, and dorsal column injury in adult mice. SPRR1A was not detected in naïve spinal cord, DRG, or peripheral nerves and there was minimal expression following injury to the centrally projecting branches of DRG neurons. However, following peripheral (sciatic) nerve injury, intense SPRR1A immunoreactivity was observed in the dorsal horn and motoneurons of the spinal cord, in L4/5 DRG neurons, and in the injured nerve. A time-course study comparing expression following sciatic nerve crush and transection revealed maximum SPRR1A levels at day 7 in both models. However, while SPRR1A was downregulated to baseline by 30 days postlesion following crush injury, it remained elevated 30 days after transection. Cell-size and double-labeling studies revealed that SPRR1A was expressed by DRG cells of all sizes and colocalized with classical markers of DRG subpopulations and their primary afferent terminals. High coexpression of SPRR1A with activating transcription factor-3 and growth-associated protein-43 was observed, indicating that it is expressed by injured and regenerating neurons. This study supports the hypothesis that SPRR1A is a regeneration-associated gene and that SPRR1A provides a valuable marker to assess the regenerative potential of injured neurons.

Reorganization of dorsal root ganglion neurons following chronic sciatic nerve constriction injury: Correlation with morphine and lidocaine analgesia

Chronic constriction injury of the sciatic nerve is an animal model for neuropathic pain. In this model, the analgesic potency of systemic morphine was significantly diminished in nerve-injured mice (ED(50) 19.4 mg/kg) compared with sham-operated mice (ED(50) 3.3 mg/kg) using a unilateral hot plate withdrawal test, with a similar reduction in sensitivity of intrathecal morphine. The sciatic nerve injury resulted in a reorganization of the dorsal root ganglion (DRG) neurons. Immunohistochemically, the chronic constriction injury triggered a withdrawal of C-fibers from the ipsilateral dorsal horn of the spinal cord. Although A-beta terminals centrally sprouted into Lamina II of the dorsal horn of the spinal cord, the peripheral A-beta fibers in the skin retracted from the epidermis to deeper layers of the dermis. To explore the functional significance of these dermal changes, we examined the topical morphine and lidocaine analgesia following chronic sciatic nerve constriction. Both morphine and lidocaine retained topical activity following chronic sciatic nerve injury, but their analgesic dose-response curves were shifted to the right when compared to sham-operated mice. Thus, the chronic nerve constriction injury model is associated with pathological changes in distribution of the central and peripheral axons of the dorsal root ganglion neurons that correspond to a decreased pharmacological sensitivity to topical analgesic agents.

Polysialic acid-induced plasticity reduces neuropathic insult to the central nervous system

Under chronic conditions of neuropathic pain, nociceptive C terminals are lost from their target region in spinal lamina II, leading to reduced thermal hyperalgesia. This region of the spinal cord expresses high levels of polysialic acid (PSA), a cell surface carbohydrate known to weaken cell-cell interactions and promote plasticity. Experimental removal of PSA from the spinal cord exacerbates hyperalgesia and results in retention of C terminals, whereas it has no effect on plasticity of touch Abeta fibers and allodynia. We propose that expression of PSA at this stress pathway relay point could serve to protect central circuitry from chronic sensory overload.

Distribution and injury-induced plasticity of cadherins in relationship to identified synaptic circuitry in adult rat spinal cord

Cadherins are synaptically enriched cell adhesion and signaling molecules. In brain, they function in axon targeting and synaptic plasticity. In adult spinal cord, their localization, synaptic affiliation, and role in injury-related plasticity are mostly unexplored. Here, we demonstrate in adult rat dorsal horn that E- and N-cadherin display unique patterns of localization to functionally distinct types of synapses of intrinsic and primary afferent origin. Within the nociceptive afferent pathway to lamina II, nonpeptidergic C-fiber synapses in the deeper half of lamina II (IIi) contain E-cadherin but mostly lack N-cadherin, whereas the majority of the peptidergic C-fiber synapses in the outer half of lamina II (IIo) contain N-cadherin but lack E-cadherin. Approximately one-half of the Abeta-fiber terminations in lamina III contain N-cadherin; none contain E-cadherin. Strikingly, the distribution and levels of these cadherins are differentially affected by sciatic nerve axotomy, a model of neuropathic pain in which degenerative and regenerative structural plasticity has been implicated. Within the first 7 d after axotomy, E-cadherin is rapidly and completely lost from the dorsal horn synapses with which it is affiliated, whereas N-cadherin localization and levels are unchanged; such patterns persist through 28 d postlesion. The loss of E-cadherin thus occurs before the onset of mechanical hyperalgesia (approximately 10-21 d postlesion), as reported previously. Together, the synaptic specificity displayed by these cadherins, coupled with their differential response to injury, suggests that they may proactively contribute to the maintenance of some, and incipient dismantling of other, synaptic circuits in response to nerve injury. Speculatively, such changes may ultimately contribute to subsequently emerging abnormalities in pain perception.

Skin denervation, neuropathology, and neuropathic pain in a laser-induced focal neuropathy

Small-diameter sensory nerves innervating the skin are responsive to noxious stimuli, and an injury to these nerves is presumably related to neuropathic pain. Injury-induced neuropathic pain in animals can be produced by laser irradiation, which usually requires concomitant use of photosensitive dyes, known as the photochemical approach. It is not clear whether laser irradiation alone can induce neuropathic pain. In addition, two issues are important to apply these approaches: the relationship between the extent of laser irradiation and the occurrence of neuropathic pain, and the susceptibility of small-diameter sensory nerves in the skin to laser-induced neuropathic pain. To address these issues, we designed a new model of focal neuropathy by applying a diode laser of 532 nm (100 mW) to the sciatic nerve and evaluated small-diameter nerves by quantifying skin innervation and large-diameter nerves by measuring amplitudes of the compound muscle action potential (CMAP). Immediately after laser irradiation, epineurial vessels were occluded due to the formation of thrombi, and the blood flow through these vessels was markedly reduced. On postoperative day (POD) 2, animals developed characteristic manifestations of neuropathic pain, including spontaneous pain behaviors, thermal hyperalgesia, and mechanical allodynia. These phenomena peaked during PODs 7-21, and lasted for 3-6 weeks. The neuropathology at the irradiated site of the sciatic nerve included a focal area of axonal degeneration surrounded by demyelination and endoneurial edema. The extent of damage to large-diameter motor and sensory nerves after laser irradiation was evaluated by nerve conduction studies. On the irradiated sides, amplitudes of the compound muscle action potentials and sensory nerve action potentials (SNAPs) were reduced to 65.0% (P < 0.0001) and 42.5% (P < 0.01) of those on the control sides, respectively. Motor innervation of the neuromuscular junctions (NMJs) on plantar muscles was examined by combined cholinesterase histochemistry and immunohistochemistry. The ratio of innervated NMJs on the operated sides decreased to 76.3% of that on the control side. Skin innervation in the territory of the irradiated sciatic nerves was evaluated by immunohistochemistry with neuronal markers. Among these markers, epidermal nerve densities for protein gene product (PGP) 9.5, calcitonin gene-related peptide (CGRP), and substance P (SP) were significantly lower on the irradiated sides than the control sides with a different degree of loss for each marker (42.1-53.1%, P < 0.05). Results suggest that laser-induced focal neuropathy provides a new system for studying neuropathic pain. With this approach, the extent of nerve injury can be quantified. Both small-diameter epidermal nerves and large-diameter sensory and motor nerves are susceptible to laser-induced injury of different degrees.

Absence of mechanical allodynia and Abeta-fiber sprouting after sciatic nerve injury in mice lacking membrane-type 5 matrix metalloproteinase

Matrix metalloproteinases (MMPs) are a family of endopeptidases that degrade extracellular matrix components. Membrane-type 5 MMP (MT5-MMP/MMP-24) was identified as neuron-specific, and is believed to contribute to neuronal circuit formation and plasticity. To elucidate its function in vivo, we have generated mice lacking MT5-MMP by gene targeting. MT5-MMP-deficient mice were born without obvious morphological abnormalities. No apparent histological defects were observed in the nervous system either. However, MT5-MMP-deficient mice did not develop neuropathic pain with mechanical allodynia after sciatic nerve injury, though responses to acute noxious stimuli were normal. Neuropathic pain induced by peripheral nerve lesions is known to accompany structural reorganization of the nervous system. Intraneural injection of cholera toxin B subunit, a transganglionic tracer, into the injured sciatic nerve of wild-type mice revealed that the myelinated Abeta-fiber primary afferents sprouted from laminae III-VI of the dorsal horn of the spinal cord and invaded lamina II. However, no such sprouting and invasion of Abeta-fibers were observed in MT5-MMP-deficient mice. These findings suggest that MT5-MMP is essential for the development of mechanical allodynia and plays an important role in neuronal plasticity in this mouse model.

Phosphorylated MAP1B is induced in central sprouting of primary afferents in response to peripheral injury but not in response to rhizotomy

A peripheral nerve lesion induces sprouting of primary afferents from dorsal root ganglion (DRG) neurons into lamina II of the dorsal horn. Modifications of the environment in consequence to the axotomy provide an extrinsic stimulus. A potential neuron-intrinsic factor that may permit axonal sprouting is microtubule-associated protein 1B (MAP1B) in a specific phosphorylated form (MAP1B-P), restricted to growing or regenerating axons. We show here that both in rat and mouse, a sciatic nerve cut is rapidly followed by the appearance of MAP1B-P expression in lamina II, increasing to a maximum between 8 and 15 days, and diminishing after three months. Evidence is provided that sprouting and induction of MAP1B-P expression after peripheral injury are phenomena concerning essentially myelinated axons. This is in accordance with in situ hybridization data showing especially high MAP1B-mRNA levels in large size DRG neurons that give rise to myelinated fibers. We then employed a second lesion model, multiple rhizotomy with one spared root. In this case, unmyelinated CGRP expressing fibers do indeed sprout, but coexpression of MAP1B-P and CGRP is never observed in lamina II. Finally, because a characteristic of myelinated fibers is their high content in neurofilament protein heavy subunit (NF-H), we used NF-H-LacZ transgenic mice to verify that MAP1B-P induction and central sprouting were not affected by perturbing the axonal organization of neurofilaments. We conclude that MAP1B-P is well suited as a rapidly expressed, axon-intrinsic marker associated with plasticity of myelinated fibers.

Response of GAP-43 and p75 in human neuromas over time after traumatic injury

OBJECTIVE

GAP-43 and p75 are proteins that promote growth cone and neurite formation, elongation, and arborization in regenerating nerve axons. The objectives of this study were to determine whether GAP-43 and the low-affinity nerve growth factor receptor p75 are elevated in traumatic neuromas and whether there is a correlation between the relative amount of GAP-43 or p75 and demographic characteristics such as time elapsed between injury and repair.

METHODS

Traumatic neuromas from 21 randomly selected patients were studied, and the charts were reviewed. Specimens were collected at the time of nerve resection and grafting. Immunohistochemical analysis was performed on each sample and normal human nerve with antibodies to GAP-43 and p75. Western blot and computerized gel analyses were performed.

RESULTS

All neuroma specimens harvested within 13 months of injury exhibited markedly elevated GAP-43 levels compared with normal nerve. Specimens harvested at 14 months or more after injury showed precipitously lower GAP-43 levels, similar to or less than those of normal nerve. The correlation between the amount of intra-axonal GAP-43 and postinjury time interval was statistically significant, P = 0.0038. High GAP-43 levels were also correlated with transection injury, high postoperative sensory grade, and pain. p75 levels were elevated, without consistent variation in our population.

CONCLUSION

These preliminary data suggest that the expression of intra-axonal GAP-43 may vary over time after injury, remaining elevated for approximately the first year, then decreasing abruptly to normal or subnormal levels. These results correlate with clinical experience, indicating that peripheral nerves should be repaired relatively early if repair is indicated.

Peripheral axotomy induces only very limited sprouting of coarse myelinated afferents into inner lamina II of rat spinal cord

Peripheral axotomy-induced sprouting of thick myelinated afferents (A-fibers) from laminae III-IV into laminae I-II of the spinal cord is a well-established hypothesis for the structural basis of neuropathic pain. However, we show here that the cholera toxin B subunit (CTB), a neuronal tracer used to demonstrate the sprouting of A-fibers in several earlier studies, also labels unmyelinated afferents (C-fibers) in lamina II and thin myelinated afferents in lamina I, when applied after peripheral nerve transection. The lamina II afferents also contained vasoactive intestinal polypeptide and galanin, two neuropeptides mainly expressed in small dorsal root ganglion (DRG) neurons and C-fibers. In an attempt to label large DRG neurons and A-fibers selectively, CTB was applied four days before axotomy (pre-injury-labelling), and sprouting was monitored after axotomy. We found that only a small number of A-fibers sprouted into inner lamina II, a region normally innervated by C-fibers, but not into outer lamina II or lamina I. Such sprouts made synaptic contact with dendrites in inner lamina II. Neuropeptide Y (NPY) was found in these sprouts in inner lamina II, an area very rich in Y1 receptor-positive processes. These results suggest that axotomy-induced sprouting from deeper to superficial layers is much less pronounced than previously assumed, in fact it is only marginal. This limited reorganization involves large NPY immunoreactive DRG neurons sprouting into the Y1 receptor-rich inner lamina II. Even if quantitatively small, it cannot be excluded that this represents a functional circuitry involved in neuropathic pain.

Neurotrophin-3 antisense oligonucleotide attenuates nerve injury-induced Abeta-fibre sprouting

It is proposed that following peripheral nerve injury abnormal sprouting of Abeta-fibre primary afferent neurons in the spinal cord contributes to the allodynia that often occurs with such injury. Allodynia is characterized as pain due to a stimulus which is normally non-noxious. Our recent in vivo experiments show that intrathecal administration of neurotrophin-3 (NT-3), in normal animals, induces allodynia and sprouting of Abeta-fibres. In this study, we examine whether intrathecal administration of NT-3 antisense oligonucleotides (50 microM), via an osmotic pump for 14 days, attenuates nerve injury-induced sprouting and allodynia. The oligonucleotides used in this study were phosphorothioate modified and control experiments, using an ELISA, confirm that intrathecal administration of the antisense induces a significant decrease in NT-3 levels in the spinal cord. All surgery was conducted on anaesthetized Wistar rats (sodium pentobarbitone, i.p. 50 mg/kg). Consistent with previous studies, transganglionic labelling of Abeta-fibres with choleragenoid-horseradish peroxidase (C-HRP) shows that complete transection of the sciatic nerve induces an expansion of C-HRP label into lamina II of the spinal dorsal horn. Using image analysis, we find that intrathecal administration of NT-3 antisense attenuates the density of C-HRP labelling in lamina II in nerve injured animals. A NT-3 sense oligonucleotide (50 microM) has no effect. To test the effect of NT-3 antisense on allodynia, the nociceptive flexion reflex is examined, using an Ugo Basile Analgesymeter, in animals with partial sciatic nerve ligation. Intrathecal administration of 50 microM NT-3 antisense significantly attenuates nerve injury-induced allodynia, whereas the sense oligonucleotide has no effect. These results provide further evidence that endogenous NT-3 contributes to both nerve injury-induced Abeta-fibre sprouting and allodynia and demonstrates the potential of neurotrophin-3 antisense oligonucleotides as therapeutic agents for neuropathic pain.

Cellular mechanisms of hyperalgesia and spontaneous pain in a spinalized rat model of peripheral neuropathy: changes in myelinated afferent inputs implicated

Various hypotheses have been proposed to account for the mechanical hyperalgesia and spontaneous pain seen in animal models of peripheral neuropathy. The purpose of the present study was to determine whether there exists a spinal neuronal correlate to these properties. An experimental neuropathy was induced in male Sprague-Dawley rats by placing a 2-mm PE-90 polyethylene cuff around the sciatic nerve. All rats were subsequently confirmed to exhibit mechanical allodynia in the von Frey test. After induction of anaesthesia with pentobarbital and acute spinalization at T9, electrophysiological experiments were performed, recording extracellular single unit activity from ipsi- and contralateral wide dynamic range dorsal horn neurons in spinal segments L1-4. On-going activity was greater in short-term (11-22 days after cuff implantation) and long-term (42-52 days) cuff-implanted rats; 38 spikes/s in short-term versus 19 spikes/s in controls; 29 spikes/s in long-term ipsi- and contralateral neurons. Receptive fields in controls were always restricted, but in almost all cuff-implanted rats extended over the whole hind paw. Responses to noxious mechanical (pinch) and noxious heat stimulation of the cutaneous receptive field in controls consisted of the typical fast initial discharge followed by an afterdischarge. In all neurons from cuff-implanted rats the initial discharge resembled that in controls. However, the afterdischarge, particularly that in response to noxious pinch, was markedly greater in both magnitude and duration. It is suggested that the greater on-going discharge is the cellular correlate of spontaneous pain, and the potentiation of the afterdischarge in response to noxious stimulation is the correlate of hyperalgesia. Given that acutely spinalized rats were tested, only peripheral and/or spinal mechanisms can be considered to explain these data. Considering all the data, it can be concluded that there is a greater change in fibres mediating noxious mechanical than noxious thermal inputs. Among different hypotheses, the one with which the present data are most compatible is that which proposes that chronic nerve injury or inflammation induces phenotypic changes predominantly in myelinated afferents. There may be a redistribution of membrane-bound ion channels, predominantly sodium channels, which leads to ectopic activity and thus spontaneous discharge of dorsal horn neurons. With regard to mechanical stimulation-evoked synaptic input, the central terminals of myelinated afferents expand into regions of the spinal cord which normally receive their predominant input from unmyelinated nociceptive afferents. This may be coupled with a change in these myelinated afferents so that they now synthesize and release peptides, primarily substance P, from their central terminals with the result that the effects of their chemical mediators of synaptic transmission add to the effects of nociceptive inputs leading to exaggerated responses to painful stimuli, thus the basis of clinical hyperalgesia.

Increased expression of growth-associated protein (GAP-43) in lower urinary tract pathways following cyclophosphamide (CYP)-induced cystitis

Alterations in the expression of growth-associated protein 43 (GAP-43) were examined in lower urinary tract micturition reflex pathways in a chronic model of cyclophosphamide (CYP)-induced cystitis. In control animals, expression of GAP-43 was present in specific regions of the gray matter in the rostral lumbar and caudal lumbosacral spinal cord, including: (1) the dorsal commissure; (2) the dorsal horn and (3) the regions of the intermediolateral cell column (L1-L2) and the sacral parasympathetic nucleus (L6-S1) and (4) in the lateral collateral pathway of Lissauer in L6-S1 spinal segments. Densitometry analysis has demonstrated significant increases (p</=0.001; 1.5-4.0-fold increase) in GAP-43-immunoreactivity (IR) in these regions of the rostral lumbar (L1-L2) and caudal lumbosacral (L6-S1) spinal cord following CYP-induced urinary bladder inflammation. Changes in GAP-43-IR were restricted to those segmental levels examined (L1-L2 and L6-S1) that are involved in lower urinary tract reflexes. Changes in GAP-43-IR were not observed at the L5 segmental level. In contrast to significant increases in GAP-43-IR in specific regions of the rostral lumbar and caudal lumbosacral spinal cord, no changes in GAP-43-IR were observed in the L1, L2 or L6, S1 dorsal root ganglia (DRG). In control animals, virtually all retrogradely labeled (Fast Blue) bladder afferent cells in the L1, L2 and L6, S1 DRG expressed GAP-43-IR. This percentage (approximately 100%) of bladder afferent cells expressing GAP-43-IR was unchanged following CYP-induced urinary bladder inflammation. Alterations in GAP-43-IR following chronic cystitis may suggest a reorganization of bladder afferent projections and spinal elements involved in bladder reflexes consistent with alterations in bladder function observed in animal models of cystitis.

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.