Chronic peripheral nerve section results in a rearrangement of the central axonal arborizations of axotomized A beta primary afferent neurons in the rat spinal cord

Functional and structural synaptic remodeling mechanisms underlying somatotopic organization and reorganization in the thalamus

The somatosensory system organizes the topographic representation of body maps, termed somatotopy, at all levels of an ascending hierarchy. Postnatal maturation of somatotopy establishes optimal somatosensation, whereas deafferentation in adults reorganizes somatotopy, which underlies pathological somatosensation, such as phantom pain and complex regional pain syndrome. Here, we focus on the mouse whisker somatosensory thalamus to study how sensory experience shapes the fine topography of afferent connectivity during the critical period and what mechanisms remodel it and drive a large-scale somatotopic reorganization after peripheral nerve injury. We will review our findings that, following peripheral nerve injury in adults, lemniscal afferent synapses onto thalamic neurons are remodeled back to immature configuration, as if the critical period reopens. The remodeling process is initiated with local activation of microglia in the brainstem somatosensory nucleus downstream to injured nerves and heterosynaptically controlled by input from GABAergic and cortical neurons to thalamic neurons. These fruits of thalamic studies complement well-studied cortical mechanisms of somatotopic organization and reorganization and unveil potential intervention points in treating pathological somatosensation.

Proposed mechanisms of low-level light therapy in the treatment of androgenetic alopecia

Androgenetic alopecia (AGA) is a global challenge, affecting a large number of people worldwide. Efficacy of the existed treatments can barely meet the demands of patients. Patients who are poorly responding to those treatments are seeking for a more effective and suitable technique to treat their disease. Low-level light therapy (LLLT) is a newly developed technique, which has been proved to stimulate hair growth. Based on the function principle of LLLT in other domains and refer to the published literatures, we write this review to neaten and elucidate the possible mechanism of LLLT in the treatment of AGA. A review of published literature which is associated with keywords LLLT, photobiomodulation, AGA, treatment, hair growth, and mechanism was performed to elucidate the proposed mechanism of LLLT in the treatment of AGA. The present study shows that LLLT can accelerate hair growth in AGA patients. The proposed mechanism of LLLT in treating AGA may vary among different specialists. But we can summarize the consensual mechanisms as follows; low-level light absorbed by chromophores can lead to the production of nitric oxide (NO) and the modulation of reactive oxygen species (ROS). These mobilized molecules subsequently activate redox-related signaling pathways in hair follicle cells and perifollicular cells. Finally, these activated cells participate in the regrowth of hair follicle. Even though the efficacy of LLLT in the treatment of AGA in both men and women has already been confirmed, the present studies focusing on discovering LLLT are still inadequate and unsystematic. More studies are needed to standardize the optimum treatment parameters applied in promoting hair growth and determine the long-term safety and efficacy of LLLT. Current recognitions about the mechanisms of LLLT, mainly focused on the molecules that may take effect, neglected different cellular components that are functional in the hair follicle macro-environment.

Role of spinal glial cells in excitability of wide dynamic range neurons and the development of neuropathic pain with the L5 spinal nerve transection in the rats: Behavioral and electrophysiological study

The activation of glial cells affects the neuronal excitability in the spinal cord. Therefore, in this study, we tried to find out the modulatory role of spinal glial cells in the excitability of wide dynamic range (WDR) neurons, induction of the long-term potentiation (LTP) and development of neuropathic pain by L5 spinal nerve transection model in the rats. Forty-eight adult male Wistar rats were used to measure the paw withdrawal threshold to mechanical stimuli and also, to carry out the spinal extracellular single unit recording experiments. In these experiments, spinal nerve ligation (SNL) and a daily injection of propentofylline (1 mg/kg, ip) as a glial cell inhibitor agent, 1 h following nerve ligation during 7-day post-SNL period, were performed. Our findings showed that the mechanical allodynia, and synaptically-evoked firing were caused LTP in the Aδ-fiber, C-fiber and lesser in the Aβ-fiber after high frequency stimulation. Daily injection of propentofylline considerably decreased LTP induction in the Aδ- and C-fibers (P < .001). It was concluded that glial cell activation mediates LTP induction in the spinal cord following peripheral nerve injury. It seems that pain modulatory role of glial cells is partly parallel to changes in neural excitability of the WDR neurons in the dorsal horn of spinal cord.

Delayed Exercise Is Ineffective at Reversing Aberrant Nociceptive Afferent Plasticity or Neuropathic Pain After Spinal Cord Injury in Rats

Neuropathic pain is a debilitating consequence of spinal cord injury (SCI) that correlates with sensory fiber sprouting. Recent data indicate that exercise initiated early after SCI prevents the development of allodynia and modulated nociceptive afferent plasticity. This study determined if delaying exercise intervention until pain is detected would similarly ameliorate established SCI-induced pain. Adult, female Sprague-Dawley rats with a C5 unilateral contusion were separated into SCI allodynic and SCI non-allodynic cohorts at 14 or 28 days postinjury when half of each group began exercising on automated running wheels. Allodynia, assessed by von Frey testing, was not ameliorated by exercise. Furthermore, rats that began exercise with no allodynia developed paw hypersensitivity within 2 weeks. At the initiation of exercise, the SCI Allodynia group displayed marked overlap of peptidergic and non-peptidergic nociceptive afferents in the C7 and L5 dorsal horn, while the SCI No Allodynia group had scant overlap. At the end of 5 weeks of exercise both the SCI Allodynia and SCI No Allodynia groups had extensive overlap of the 2 c-fiber types. Our findings show that exercise therapy initiated at early stages of allodynia is ineffective at attenuating neuropathic pain, but rather that it induces allodynia-aberrant afferent plasticity in previously pain-free rats. These data, combined with our previous results, suggest that there is a critical therapeutic window when exercise therapy may be effective at treating SCI-induced allodynia and that there are postinjury periods when exercise can be deleterious.

Lack of evidence for ectopic sprouting of genetically labeled Aβ touch afferents in inflammatory and neuropathic trigeminal pain

Background

Mechanical and in particular tactile allodynia is a hallmark of chronic pain in which innocuous touch becomes painful. Previous cholera toxin B (CTB)-based neural tracing experiments and electrophysiology studies had suggested that aberrant axon sprouting from touch sensory afferents into pain-processing laminae after injury is a possible anatomical substrate underlying mechanical allodynia. This hypothesis was later challenged by experiments using intra-axonal labeling of A-fiber neurons, as well as single-neuron labeling of electrophysiologically identified sensory neurons. However, no studies have used genetically labeled neurons to examine this issue, and most studies were performed on spinal but not trigeminal sensory neurons which are the relevant neurons for orofacial pain, where allodynia oftentimes plays a dominant clinical role.

Findings

We recently discovered that parvalbumin::Cre (Pv::Cre) labels two types of Aβ touch neurons in trigeminal ganglion. Using a Pv::CreER driver and a Cre-dependent reporter mouse, we specifically labeled these Aβ trigeminal touch afferents by timed taxomifen injection prior to inflammation or infraorbital nerve injury (ION transection). We then examined the peripheral and central projections of labeled axons into the brainstem caudalis nucleus after injuries vs controls. We found no evidence for ectopic sprouting of Pv::CreER labeled trigeminal Aβ axons into the superficial trigeminal noci-receptive laminae. Furthermore, there was also no evidence for peripheral sprouting.

Conclusions

CreER-based labeling prior to injury precluded the issue of phenotypic changes of neurons after injury. Our results suggest that touch allodynia in chronic orofacial pain is unlikely caused by ectopic sprouting of Aβ trigeminal afferents.

A Review of Signal Transduction of Endothelin-1 and Mitogen-activated Protein Kinase-related Pain for Nanophysiotherapy

[Purpose] An understanding of pain is very important in the study of nanophysiotherapy. In this review, we summarize the mechanisms of endothelin-1 (ET-1)- and mitogen-activated protein kinase (MAPK)-related pain, and suggest their applications in pain physiotherapy. [Method] This review focuses on the signal transduction of pain and its mechanisms. [Results] Our reviews show that mechanisms of ET-1- and MAPK-related pain exist. [Conclusions] In this review article, we carefully discuss the signal transduction in ET-1- and MAPK-related pain with reference to pain nanophysiotherapy from the perspective of nanoparticle-associated signal transduction.

A feed-forward spinal cord glycinergic neural circuit gates mechanical allodynia

Neuropathic pain is characterized by mechanical allodynia induced by low-threshold myelinated Aβ-fiber activation. The original gate theory of pain proposes that inhibitory interneurons in the lamina II of the spinal dorsal horn (DH) act as "gate control" units for preventing the interaction between innocuous and nociceptive signals. However, our understanding of the neuronal circuits underlying pain signaling and modulation in the spinal DH is incomplete. Using a rat model, we have shown that the convergence of glycinergic inhibitory and excitatory Aβ-fiber inputs onto PKCγ+ neurons in the superficial DH forms a feed-forward inhibitory circuit that prevents Aβ input from activating the nociceptive pathway. This feed-forward inhibition was suppressed following peripheral nerve injury or glycine blockage, leading to inappropriate induction of action potential outputs in the nociceptive pathway by Aβ-fiber stimulation. Furthermore, spinal blockage of glycinergic synaptic transmission in vivo induced marked mechanical allodynia. Our findings identify a glycinergic feed-forward inhibitory circuit that functions as a gate control to separate the innocuous mechanoreceptive pathway and the nociceptive pathway in the spinal DH. Disruption of this glycinergic inhibitory circuit after peripheral nerve injury has the potential to elicit mechanical allodynia, a cardinal symptom of neuropathic pain.

Targeting the affective and cognitive aspects of chronic neuropathic pain using basal forebrain neuromodulation: rationale, review and proposal

Chronic pain is a major health problem in developed countries where it may affect as much as 20% of the adult population. There have been no significant clinical breakthroughs in therapeutic options for persons with chronic neuropathic pain. These limitations underscore the importance of developing new therapies for this disabling pain syndrome. We have reviewed the limitations of the present treatment strategies for chronic pain, neurophysiology of somatosensory transmission and nociception, mechanisms of neuropathic pain, the concept of a "pain matrix" and the "top-down" modulation of pain, and the cognitive affective role in processing of the pain experience. We found that affective and cognitive aspects of pain constitute important considerations in achieving improvements in the outcomes of pain neuromodulation in patients with chronic neuropathic pain. Based on our review, we propose that future novel neuromodulatory therapeutic strategies should be directed at areas in the brain that are involved in the neural mechanisms of reward valuation and appetitive motivation such as nucleus accumbens, ventral tegmental area, and prefrontal cortex.

Sprouting of A-fibre primary afferents into lamina II in two rat models of neuropathic pain

Following peripheral nerve section, injured sensory A-fibres into lamina II of the dorsal horn and form aberrant functional synapses. Such structural changes may underlie some of the sensory abnormalities observed in nerve-injured patients, including neuropathic pain. This study compared the ability of intact and injured A-fibres to sprout in two experimental models of neuropathic pain, where the onset and presence of abnormal behaviours indicative of neuropathic pain have been well described. Rats received either a unilateral chronic constriction injury of the sciatic nerve (CCI) or lesion of the L5 spinal nerve (SNL). The central distribution of the injured and uninjured afferents labelled with choleragenoid conjugated to horseradish peroxidase (B-HRP) was examined at different postoperative survival times. In both models, the contralateral uninjured side, used for control nerve or ganglion injections, showed labelling of the L3-6 spinal segments in laminae I, III-V, leaving lamina II unlabelled. In CCI rats, injured sciatic afferents sprouted in lamina II of the L4-5 dorsal horn by 10 days postinjury. In SNL rats, injured L5 afferents sprouted into lamina II of the L4-5 dorsal horn by 24 h postinjury and were robust from 3 to 10 days. In both models, the labelling in lamina II was absent by 4 months. Labelling of the adjacent uninjured saphenous or intact L4 spinal nerve afferents did not reveal A-fibre sprouting. As the time-course of sprouting of injured A-fibres parallels the previously described behaviour interpreted as neuropathic pain in these models, this may be a phenomenon that contributes to sensory abnormalities such as ongoing pain and mechanical hypersensitivity.

Clinical prospects for transplantation of OECs in the repair of brachial and lumbosacral plexus injuries: opening a door

The reparative effects of olfactory ensheathing cells have largely been examined in lesions entirely within the CNS. There is, however, evidence that they can induce the ingrowth of severed dorsal root axons and increase the outgrowth of severed ventral root axons. The ingrowth of dorsal root axons results in reinnervation of appropriate regions in the spinal cord and dorsal column nuclei with restoration of electrical transmission and muscular control. This article discusses the further possibilities of these observations in rat studies and their potential translation to clinical injuries. This article is part of a Special Issue entitled: Understanding olfactory ensheathing glia and their prospect for nervous system repair.

Mechanical stress-induced reactive gliosis in the auditory nerve and cochlear nucleus

OBJECT

Hearing levels following microsurgical treatment gradually deteriorate in a number of patients treated for vestibular schwannoma (VS), especially in the subacute postoperative stage. The cause of this late-onset deterioration of hearing is not completely understood. The aim of this study was to investigate the possibility that reactive gliosis is a contributory factor.

METHODS

Mechanical damage to nerve tissue is a feature of complex surgical procedures. To explore this aspect of VS treatment, the authors compressed rat auditory nerves with 2 different degrees of injury while monitoring the compound action potentials of the auditory nerve and the auditory brainstem responses. In this experimental model, the axons of the auditory nerve were quantitatively and highly selectively damaged in the cerebellopontine angle without permanent compromise of the blood supply to the cochlea. The temporal bones were processed for immunohistochemical analysis at 1 week and at 8 weeks after compression.

RESULTS

Reactive gliosis was induced not only in the auditory nerve but also in the cochlear nucleus following mechanical trauma in which the general shape of the auditory brainstem response was maintained. There was a substantial outgrowth of astrocytic processes from the transitional zone into the peripheral portion of the auditory nerve, leading to an invasion of dense gliotic tissue in the auditory nerve. The elongated astrocytic processes ran in parallel with the residual auditory neurons and entered much further into the cochlea. Confocal images disclosed fragments of neurons scattered in the gliotic tissue. In the cochlear nucleus, hypertrophic astrocytic processes were abundant around the soma of the neurons. The transverse diameter of the auditory nerve at and proximal to the compression site was considerably reduced, indicating atrophy, especially in rats in which the auditory nerve was profoundly compressed.

CONCLUSIONS

The authors found for the first time that mechanical stress to the auditory nerve causes substantial reactive gliosis in both the peripheral and central auditory pathways within 1-8 weeks. Progressive reactive gliosis following surgical stress may cause dysfunction in the auditory pathways and may be a primary cause of progressive hearing loss following microsurgical treatment for VS.

Acute and chronic sectioning of fifth lumbar spinal nerve has equivalent effects on the primary afferents of sciatic nerve in rat spinal cord

The mechanism of neuropathic pain may be associated with sprouting of uninjured primary afferents of peripheral nerves into regions of the spinal cord denervated through peripheral injury. However, this remains controversial. Therefore, the purpose of the present investigation was, first, to determine in detail the central distributions of the unmyelinated primary afferents of each of the L4, L5, and L6 components of sciatic nerve, then to assess the distribution of afferent sciatic terminals following acute and chronic injury to (L5) nerve. First, we injected isolectin B4 (IB4), into the sciatic nerves in three groups of rats, each of which had two of the three L4, L5, or L6 components ligated and cut, and the one remaining, uninjured. Although the terminal labelling found in the L5 segment of the spinal cord originated from the L5 component, some terminal labelling remained in cases when either the L4 or L6 component was intact. Second, tracers transported in predominantly unmyelinated (IB4 and WGA-HRP) or myelinated (cholera toxin subunit B) nerves were injected into the sciatic nerve following acute or chronic (21-day) injury restricted to the L5 component. In each case, the central distribution of nerve terminals in the spinal dorsal horn was equivalent following either acute or chronic injury to the L5 component. Consequently, these data provide no support for the suggestion that neuropathic pain in spinal ligation model results from uninjured L4 and L6 components sprouting to occupy sites vacated by the injured L5 component of the sciatic nerve.

Chapter 27: Neural plasticity after nerve injury and regeneration

Injuries to the peripheral nerves result in partial or total loss of motor, sensory, and autonomic functions in the denervated segments of the body due to the interruption of axons, degeneration of distal nerve fibers, and eventual death of axotomized neurons. Functional deficits caused by nerve injuries can be compensated by reinnervation of denervated targets by regenerating injured axons or by collateral branching of undamaged axons, and remodeling of nervous system circuitry related to the lost functions. Plasticity of central connections may compensate functionally for the lack of adequate target reinnervation; however, plasticity has limited effects on disturbed sensory localization or fine motor control after injuries, and may even result in maladaptive changes, such as neuropathic pain and hyperreflexia. After axotomy, neurons shift from a transmitter to a regenerative phenotype, activating molecular pathways that promote neuronal survival and axonal regeneration. Peripheral nerve injuries also induce a cascade of events, at the molecular, cellular, and system levels, initiated by the injury and progressing throughout plastic changes at the spinal cord, brainstem nuclei, thalamus, and brain cortex. Mechanisms involved in these changes include neurochemical changes, functional alterations of excitatory and inhibitory synaptic connections, sprouting of new connections, and reorganization of sensory and motor central maps. An important direction for research is the development of therapeutic strategies that enhance axonal regeneration, promote selective target reinnervation, and are also able to modulate central nervous system reorganization, amplifying positive adaptive changes that improve functional recovery and also reducing undesirable effects.

Identity of myelinated cutaneous sensory neurons projecting to nocireceptive laminae following nerve injury in adult mice

It is widely thought that, after peripheral injury, some low-threshold mechanoreceptive (LTMR) afferents "sprout" into pain-specific laminae (I-II) of the dorsal horn and are responsible for chronic pain states such as mechanical allodynia. Although recent studies have questioned this hypothesis, they fail to account for a series of compelling results from single-fiber analyses showing extensive projections from large-diameter myelinated afferents into nocireceptive layers after nerve injury. Here we show that, in the thoracic spinal cord of naïve adult mouse, all myelinated nociceptors gave rise to terminal projections throughout the superficial dorsal horn laminae (I-II). Most (70%) of these fibers had large-diameter axons with recurving flame-shaped central arbors that projected throughout the dorsal horn laminae I-V. This morphology was reminiscent of that attributed to sprouted LTMRs described in previous studies. After peripheral nerve axotomy, we found that LTMR afferents with narrow, uninflected somal action potentials did not sprout into superficial laminae of the dorsal horn. Only myelinated noiceptive afferents with broad, inflected somal action potentials were found to give rise to recurving collaterals and project into superficial "pain-specific" laminae after axotomy. We conclude that the previously undocumented central morphology of large, myelinated cutaneous nociceptors may very well account for the morphological findings previously thought to require sprouting of LTMRs.

Modulation of Trigeminal Spinal Subnucleus Caudalis Neuronal Activity Following Regeneration of Transected Inferior Alveolar Nerve in Rats

Modulation of trigeminal spinal subnucleus caudalis neuronal activity following regeneration of transected inferior alveolar nerve in rats. To clarify the neuronal mechanisms of abnormal pain in the face innervated by the regenerated inferior alveolar nerve (IAN), nocifensive behavior, trigeminal ganglion neuronal labeling following Fluorogold (FG) injection into the mental skin, and trigeminal spinal subnucleus caudalis (Vc) neuronal properties were examined in rats with IAN transection. The mechanical escape threshold was significantly higher at 3 days and lower at 14 days after IAN transection, whereas head withdrawal latency to heat was significantly longer at 3, 14, and 60 days after IAN transection. The number of FG-labeled ganglion neurons was significantly reduced at 3 days after IAN transection but increased at 14 and 60 days. The number of wide dynamic range (WDR) neurons with background (BG) activity was significantly higher at 14 and 60 days after IAN transection compared with naïve rats, and the number of WDR and low-threshold mechanoreceptive (LTM) neurons with irregularly bursting BG activity was increased at these two time points. Mechanically evoked responses were significantly larger in WDR and LTM neurons 14 days after IAN transection compared with naïve rats. Heat- and cold-evoked responses in WDR neurons were significantly lower at 14 days after transection compared with naïve rats. Mechanoreceptive fields were also significantly larger in WDR and LTM neurons at 14 and 60 days after IAN transection. These findings suggest that these alterations may be involved in the development of mechanical allodynia in the cutaneous region innervated by the regenerated IAN.

Immediate electrical stimulation enhances regeneration and reinnervation and modulates spinal plastic changes after sciatic nerve injury and repair

We have studied whether electrical stimulation immediately after nerve injury may enhance axonal regeneration and modulate plastic changes at the spinal cord level underlying the appearance of hyperreflexia. Two groups of adult rats were subjected to sciatic nerve section followed by suture repair. One group (ES) received electrical stimulation (3 V, 0.1 ms at 20 Hz) for 1 h after injury. A second group served as control (C). Nerve conduction, H reflex, motor evoked potentials, and algesimetry tests were performed at 1, 3, 5, 7 and 9 weeks after surgery, to assess muscle reinnervation and changes in excitability of spinal cord circuitry. The electrophysiological results showed higher levels of reinnervation, and histological results a significantly higher number of regenerated myelinated fibers in the distal tibial nerve in group ES in comparison with group C. The monosynaptic H reflex was facilitated in the injured limb, to a higher degree in group C than in group ES. The amplitudes of motor evoked potentials were similar in both groups, although the MEP/M ratio was increased in group C compared to group ES, indicating mild central motor hyperexcitability. Immunohistochemical labeling of sensory afferents in the spinal cord dorsal horn showed prevention of the reduction in expression of substance P at one month postlesion in group ES. In conclusion, brief electrical stimulation applied after sciatic nerve injury promotes axonal regeneration over a long distance and reduces facilitation of spinal motor responses.

Spread of excitation across modality borders in spinal dorsal horn of neuropathic rats

Under physiological conditions, nociceptive information is mainly processed in superficial laminae of the spinal dorsal horn, whereas non-nociceptive information is processed in deeper laminae. Neuropathic pain patients often suffer from touch-evoked pain (allodynia), suggesting that modality borders are disrupted in their nervous system. We studied whether excitation evoked in deep dorsal horn neurons either via stimulation of primary afferent Abeta-fibres, by direct electrical stimulation or via glutamate microinjection leads to activation of neurons in the superficial dorsal horn. We used Ca(2+)-imaging in transversal spinal cord slices of neuropathic and control animals to monitor spread of excitation from the deep to the superficial spinal dorsal horn. In neuropathic but not control animals, a spread of excitation occurred from the deep to the superficial dorsal horn. The spread of excitation was synaptically mediated as it was blocked by the AMPA receptor antagonist CNQX. In contrast, block of NMDA receptors was ineffective. In control animals, the violation of modality borders could be reproduced by bath application of GABA(A) and glycine receptor antagonists. Furthermore, we could show that neuropathic animals were more prone to synchronous network activity than control animals. Thus, following peripheral nerve injury, excitation generated in dorsal horn areas which process non-nociceptive information can invade superficial dorsal horn areas which normally receive nociceptive input. This may be a spinal mechanism of touch-evoked pain.

Neural plasticity after peripheral nerve injury and regeneration

Injuries to the peripheral nerves result in partial or total loss of motor, sensory and autonomic functions conveyed by the lesioned nerves to the denervated segments of the body, due to the interruption of axons continuity, degeneration of nerve fibers distal to the lesion and eventual death of axotomized neurons. Injuries to the peripheral nervous system may thus result in considerable disability. After axotomy, neuronal phenotype switches from a transmitter to a regenerative state, inducing the down- and up-regulation of numerous cellular components as well as the synthesis de novo of some molecules normally not expressed in adult neurons. These changes in gene expression activate and regulate the pathways responsible for neuronal survival and axonal regeneration. Functional deficits caused by nerve injuries can be compensated by three neural mechanisms: the reinnervation of denervated targets by regeneration of injured axons, the reinnervation by collateral branching of undamaged axons, and the remodeling of nervous system circuitry related to the lost functions. Plasticity of central connections may compensate functionally for the lack of specificity in target reinnervation; plasticity in human has, however, limited effects on disturbed sensory localization or fine motor control after injuries, and may even result in maladaptive changes, such as neuropathic pain, hyperreflexia and dystonia. Recent research has uncovered that peripheral nerve injuries induce a concurrent cascade of events, at the systemic, cellular and molecular levels, initiated by the nerve injury and progressing throughout plastic changes at the spinal cord, brainstem relay nuclei, thalamus and brain cortex. Mechanisms for these changes are ubiquitous in central substrates and include neurochemical changes, functional alterations of excitatory and inhibitory connections, atrophy and degeneration of normal substrates, sprouting of new connections, and reorganization of somatosensory and motor maps. An important direction for ongoing research is the development of therapeutic strategies that enhance axonal regeneration, promote selective target reinnervation, but are also able to modulate central nervous system reorganization, amplifying those positive adaptive changes that help to improve functional recovery but also diminishing undesirable consequences.

Study of Cutaneous Reflex Compensation During Locomotion After Nerve Section in the Cat

In the cat, section of all cutaneous nerves of the hindfeet except the tibial (Tib) nerve supplying the plantar surface results in a long-lasting decrease in the intensity of Tib stimulation needed for a threshold response in flexor muscles and an increase in the amplitude of the phase-dependent responses recorded in various muscles during locomotion. Stimulating through chronically implanted nerve cuffs ensured a stable stimulation over time. The increase in reflex amplitude was well above the small increase in the amplitude of the locomotor bursts themselves that results from the denervation. Short latency responses (P1) were seen in flexor muscles, especially at the knee (semitendinosus) and ankle (tibialis anterior and extensor digitorum longus), with stimuli applied in the swing phase and also to a lesser degree in the later part of the cycle. Longer latency responses (P2) were increased in hip, knee, and ankle flexors, as well as in a contralateral extensor (vastus lateralis) when applied in late stance. Responses evoked from stimulating the proximal end of sectioned nerves were not larger than before neurectomy. This suggests that the increased responsiveness to Tib stimulation is not simply caused by an increase in motoneuron excitability, because this would have resulted in a nonspecific increase of responses to stimulation of any nerve. It is concluded that the adult locomotor system is capable of central reorganization to enhance specific remaining cutaneous reflex pathways after a partial cutaneous denervation of the paw.

Synaptic plasticity in the adult spinal dorsal horn: The appearance of new functional connections following peripheral nerve regeneration

Peripherally regenerated fibers were impaled in the dorsal columns. Each impaled fiber's adequate stimulus was determined and the fiber was activated by passing brief (200 ms) current pulses through the microelectrode. Cord dorsum potentials (CDPs) elicited by fiber stimulation were recorded at 8 sites, and then the fiber was injected with Neurobiotin (NB). In the same preparations, dorsal horn cells were impaled and their receptive fields (RFs) mapped; areas of skin from which the most vigorous responses were elicited were noted. Needle electrodes inserted into these cutaneous "hot spots" were used to electrically activate minimal numbers of peripherally regenerated fibers while simultaneously recording the resulting CDPs and any intracellular EPSPs. This allowed determination of connectivity between regenerated fibers and dorsal horn cells with overlapping RFs. In agreement with findings in intact animals, NB revealed long-ranging collaterals which were not seen using intraaxonally injected horseradish peroxidase (HRP). Although there was no qualitative difference in their morphology compared to those seen in controls, the correlation between spatial distribution of boutons and amplitudes of the monosynaptic CDPs of peripherally regenerated fibers revealed significant shifts in the functional efficacy of many central connections. Transcutaneous electrical stimulation revealed a significantly higher incidence of connectivity between regenerated fibers and cells with overlapping RFs at 9-12 months (86%) than at 5-6 months (34%). Although there was no obvious anatomical reorganization of afferent projections in the dorsal horn, the observed functional changes with time following transection show the formation of new functional central connections.

Changes in magnesium concentration in the serum and cerebrospinal fluid of neuropathic rats

BACKGROUND

Central sensitization of neuropathic pain is associated with an influx of extracellular calcium via the opening of N-methyl-D-aspartate (NMDA) receptor-gated ion channels, which are usually blocked by magnesium plugs. As magnesium-deficient rats develop a mechanical hyperalgesia and intrathecal or intraperitoneal magnesium suppresses neuropathic pain, the magnesium concentrations in serum and cerebrospinal fluid may be altered in neuropathic pain. We therefore compared the magnesium concentrations in serum and cerebrospinal fluid of neuropathic rats with those in injured rats without symptoms of neuropathic pain and normal rats.

METHODS

Mechanical allodynia was induced in male Sprague-Dawley rats by tight ligature of the left lumbar fifth and sixth spinal nerves. The threshold of paw withdrawal was evaluated by the up-down method using withdrawal response to stimulus with a von Frey filament on the third, seventh and 14th days. Rats with a threshold of less than 4 g were selected as the symptomatic group and compared with an asymptomatic group, an unoperated control group and a sham-operated group. On the 16th day, the Mg2+ concentrations in serum and cerebrospinal fluid were measured.

RESULTS

The magnesium concentrations in the serum and cerebrospinal fluid of symptomatic neuropathic rats did not differ from those in the injured rats without symptoms of neuropathic pain, sham-operated rats and normal rats.

CONCLUSION

Our results suggest that physiologic homeostasis is maintained by active transport through the blood-brain barrier despite the activation of NMDA receptor-gated ion channels. However, rats with neuropathic pain may be in a magnesium-deficient condition at the effector site, such that magnesium treatment can decrease neuropathic pain.

The Effect of Ting Point (Tendinomuscular Meridians) Electroacupuncture on Thermal Pain: A Model for Studying the Neuronal Mechanism of Acupuncture Analgesia

OBJECTIVE

The aim of this study was to characterize the role of Ting points (TP) in acute pain management and its potential use in functional imaging studies by quantitatively assessing: (1) the change in peripheral thermal thresholds before and after the electroacupuncture (EA); and (2) the corresponding behavioral feedback of thermal pain stimulation and the de qi sensation of EA.

DESIGN

The study design was prospective.

SETTINGS/LOCATION

Healthy subjects were recruited for the study at the University of California, San Diego Medical Center.

SUBJECTS/INTERVENTIONS

Thirteen (13) healthy subjects were studied. Baseline thermal thresholds (cold and warm sensations and cold and hot pain) were measured at premarked testing sites along the medial aspects of bilateral lower extremities. Five (5) seconds of hot pain (HP) was delivered to the testing sites and the corresponding pain visual analog scale (VAS) scores were recorded. Thirty (30) seconds of EA was delivered via the SP1 and LR1 on the left lower extremities at 5 Hz via a 6-V square-wave stimulator.

OUTCOME MEASURES

The VAS scores of the HP and de qi sensation (tingling) during the EA were recorded. The thermal thresholds and VAS scores for the HP and de qi were obtained immediately and both 30 and 60 minutes after the EA. Adaptation testing was also carried out to assess the change in thermal thresholds and the VAS scores of HP without EA.

RESULTS

The warm thresholds of bilateral medial calves significantly increased (p < 0.01) after 30 seconds of EA stimulation. The HP VAS score reduced significantly at the ipsilateral calf during EA in comparison to preacupuncture and postacupuncture (p < 0.01) measurements. No significant change in thermal thresholds was noted in the adaptation paradigm.

CONCLUSIONS

EA at the TP has an inhibitory effect on the C-fiber afferents. The analgesic benefit observed is most likely A-delta afferent mediated. Further correlation studies in functional imaging may provide defining data for the observed analgesic mechanism.

Treatment of postherpetic neuralgia: a review of therapeutic options

Postherpetic neuralgia (PHN) is a disabling consequence of the reactivation of the varicella zoster infection. The observation that patients with PHN experience various types of pain suggests that multiple pathophysiologic mechanisms are involved, which may include the peripheral and central nervous systems. A reasonable initial strategy would involve selecting from among multiple agents that have complementary mechanisms of action and that have been proven effective in controlled clinical trials, such as the lidocaine patch 5%, gabapentin, tricyclic antidepressants, and opioids. Based on initial assessment and ongoing reassessment, sequential trials should be undertaken until adequate pain relief is achieved. This may ultimately lead to therapy with more than one medication. Safety and tolerability are important considerations in choosing initial therapy, particularly in older patients. Physicians can either add another agent to the current regimen or switch to a new type of monotherapy if there is inadequate response to initial therapy. Alternative therapies, (i.e., ketamine, intrathecal corticosteroid injections) have not been adequately studied. Well-designed, multicenter, controlled clinical trials are needed to develop a treatment algorithm that provides an evidence-based, rational approach to treating PHN.

Enhancement of stimulation-induced ERK activation in the spinal dorsal horn and gracile nucleus neurons in rats with peripheral nerve injury

It has been suggested that low-threshold sensory pathways have an important role in the formation and maintenance of sensory abnormalities which are observed after peripheral nerve injury. In the present study, we examined the involvement of these pathways in the development of hyperexcitability after sciatic nerve injury (SNI) by detecting the intracellular signal molecule. The rats that received a transection of the sciatic nerve 7 days before were electrically stimulated at 0.1 mA and 3 mA in the proximal region of the nerve injury site. We found a small number of phosphorylated extracellular signal-regulated kinase (pERK)-labelled neurons in laminae I-II and III-IV of the spinal dorsal horn in the control rats after 0.1 mA stimulation. By contrast, there was a marked increased of pERK-labelled neurons both in the superficial laminae and laminae III-IV after the same stimulation in the SNI rats. Enhancement of ERK activation induced by 3 mA stimulation was also observed. Immunoreactivity of pERK in gracile nucleus neurons was also dramatically increased after 0.1 mA stimulation to the injured nerve. These data suggest that the rats with peripheral nerve injury had an increased responsiveness to the low- or high-threshold peripheral stimuli in I-II, III-IV and gracile nucleus neurons. Furthermore, SNI rats that received neonatal capsaicin treatment showed a decreased number of pERK neurons after 0.1 mA stimulation in the dorsal horn and gracile nucleus neurons compared to the control rats. Thus, C-fibres may contribute to the enhanced excitability of the low-threshold sensory neurons after peripheral nerve injury.

Territorial and extra-territorial distribution of Fos protein in the lumbar spinal dorsal horn neurons in rats with chronic constriction nerve injuries

This study aimed to examine the relationship between temporal and spatial expression patterns of Fos protein in the spinal dorsal horn neurons and thermal hyperalgesia behaviors in rats with chronic constriction injury (CCI) to the sciatic nerve. Our results demonstrated that Fos protein expression in the spinal dorsal horn neurons at L5 segment ipsilateral and contralateral to CCI of the sciatic nerve was significantly greater than in sham rats from days 10 to 30 postoperatively (PO 10d to 30d), and was concentrated on the injury (ipsilateral) side. Unlike the short-lived expression after tissue inflammation, laminae I to VI (especially laminae III/IV) displayed a persistent greater number of Fos-like immunoreactive (Fos-LI) neurons for at least 30 days after CCI of the sciatic nerve. After the increase in laminae III/IV, Fos-LI neurons tended to gradually increase in laminae I/II and V/VI at L5 segment from PO 2d to 30d, which were correlated with the heat hyperalgesia (48 degrees C) behaviors measured by paw withdrawal latency in CCI rats but not in sham rats. Interestingly, a persistent increase of Fos-LI neurons in laminae I to VI at L5 segment of the ipsilateral and contralateral sides and at the L1 segment that was out of the normal central terminations of the sciatic nerve suggested the probable presence of territorial and extra-territorial central sensitization or inadequate central nervous system (CNS) adaptive mechanisms. These findings may partly explain why abnormal pain sensations are sometimes distributed in a pattern that does not coincide with the territories of nerves or with the posterior roots of the peripheral nerve after injury.

Do central terminals of intact myelinated primary afferents sprout into the superficial dorsal horn of rat spinal cord after injury to a neighboring peripheral nerve?

In order to investigate whether normal myelinated primary afferent axons sprout into the territories of adjacent injured peripheral nerve fibers in the superficial dorsal horn of the spinal cord, adult rats underwent either sectioning of the saphenous or femoral nerves on one side, or else unilateral denervation of the skin of the posterior thigh. Two weeks later cholera toxin B subunit (CTb), which is normally transported selectively by myelinated somatic primary afferents, was injected into the ipsilateral (intact) sciatic nerve. The relationship between CTb, vasoactive intestinal peptide (VIP), and binding of Bandeiraea simplicifolia isolectin B4 (IB4) was then examined in the ipsilateral dorsal horn of the second to fifth lumbar spinal segments (L2-L5). Sectioning of the femoral or saphenous nerves resulted in a reduction of IB4 binding in laminae I-II in the medial third of the dorsal horn of L2, L3, and the upper part of L4. VIP-immunoreactivity was upregulated in exactly the same regions in which IB4-binding was reduced. These correspond to the areas that were previously innervated by unmyelinated afferents in the sectioned nerves. CTb-labeling was detected in regions known to receive input from myelinated sciatic afferents: lamina I and a band extending from the inner part of lamina II (IIi) to lamina V in the L3-5 segments, and the deepest part of the dorsal horn in L2. Importantly, no CTb-labeling was detected in the outer part of lamina II (IIo) in the denervated areas. Sectioning of branches of the posterior cutaneous nerve of the thigh resulted in a reduction of IB4-binding and upregulation of VIP-immunoreactivity in the lateral part of the superficial dorsal horn of caudal L4 and L5. Again, CTb-immunoreactivity showed the normal sciatic pattern in L4-L5, with no labeling detected in lamina IIo in the denervated region. These results do not support the suggestion that the central terminals of intact myelinated afferents sprout into regions of lamina II occupied by adjacent nerves that have been axotomized peripherally.

Lack of evidence for sprouting of Abeta afferents into the superficial laminas of the spinal cord dorsal horn after nerve section

The central arborizations of large myelinated cutaneous afferents normally extend as far dorsally as the ventral part of lamina II in rat spinal cord. Woolf et al. (1992) reported that after nerve injury some of these afferents sprouted into lamina I and the dorsal part of lamina II, and it has been suggested that this could contribute to allodynia associated with neuropathic pain. Part of the evidence for sprouting was on the basis of the use of cholera toxin B subunit as a selective tracer for A-fibers, and the validity of this approach has recently been questioned; however, sprouting was also reported in experiments involving intra-axonal labeling of chronically axotomized afferents. We have used intra-axonal labeling in the rat to examine central terminals of 58 intact sciatic afferents of presumed cutaneous origin and 38 such afferents axotomized 7-10 weeks previously. Both normal and axotomized populations included axons with hair follicle afferent-like morphology and arbors that entered the ventral half of lamina II; however, none of these extended farther dorsally. We also performed bulk labeling of myelinated afferents by injecting biotinylated dextran into the lumbar dorsal columns bilaterally 8-11 weeks after unilateral sciatic nerve section. We observed that both ipsilateral and contralateral to the sectioned nerve, arbors of axons with hair follicle afferent-like morphology in the sciatic territory extended only as far as the ventral half of lamina II. Therefore these results do not support the hypothesis that Abeta afferents sprout into the superficial laminas after nerve section.

Phosphorylation of extracellular signal-regulated kinases 1/2 is predominantly enhanced in the microglia of the rat spinal cord following dorsal root transection

The present study was initiated to investigate the role of extracellular signal-regulated kinases (ERK) 1/2 signaling pathway in the early response of spinal cord and associated dorsal root ganglion (DRG) to rhizotomy by using Western blotting and immunohistochemical techniques in a rat model of L3 and L4 dorsal root transection. The results showed that there were a considerable amount of total and phosphorylated ERK 1/2 protein in both spinal cord and DRG in normal animals killed under pentobarbital anesthesia. The total ERK 1/2 distributed in both glia and neurons, while phosphorylated ERK 1/2 dominantly existed in the latter in the gray matter of spinal cord, as demonstrated with double immunofluorescent staining. Twenty-four and forty-eight hours after axotomy, the phosphorylation level of ERK 1/2 in the operation side of dorsal spinal cord was much higher than that in the contralateral side, while the total ERK 1/2 level seemed unchanged. The increased expression of Fos protein was also seen in the dorsal spinal cord at lesion side twelve and twenty-four hours after axotomy. Double fluorescent staining proved that the phosphorylated ERK 1/2 positive cells in the ipsilateral dorsal spinal cord after axotomy predominantly were microglia and small portion was oligodendrocytes, whereas the Fos expression was mainly in neurons. In normal DRG, most neurons, especially the medium and small-sized ones, and the satellite cells contained total ERK 1/2-like immunoreactivity, whereas only a small portion of neurons and satellite cells contained phosphorylated ERK 1/2. After unilateral dorsal rhizotomy, there were no detectable changes for the phosphorylation of ERK 1/2 in either neurons or satellite cells in DRG.Collectively, the present results suggest that both ERK and Fos signal pathways involve the cellular activation in the spinal cord following dorsal rhizotomy, with ERK mainly in microglia and Fos in neurons. The increase of phosphorylation of ERK 1/2 in microglia of spinal cord after rhizotomy implicates that ERK signaling pathway involves intracellular activity of microglia responding to the experimental injury.

The challenge of chronic pain

Chronic pain is a complex problem with staggering negative health and economic consequences. The complexity of chronic pain is presented within Cervero and Laird's model that describes three phases of pain, including pain without tissue damage, pain with tissue damage and inflammation, and neuropathic pain. The increased afferent input in phases 2 and 3 of chronic pain produces marked changes in primary afferents, dorsal root ganglia, and spinal cord dorsal horn. These changes promote the symptoms of chronic pain, including spontaneous pain, hyperalgesia, and allodynia. Increased afferent input also evokes supraspinal input to the dorsal horn, including biphasic innervation from the ventromedial medulla and A7 catecholamine cell group, that promotes hyperalgesia and allodynia. More rostral brain structures, such as the lateral hypothalamus, amygdala, and hippocampus, may also play a role in chronic pain. Although much has been discovered about the multiple pathological mechanisms involved in chronic pain, further research is needed to fully comprehend these mechanisms.

Electrophysiological changes in adult rat dorsal horn neurons after neonatal peripheral inflammation

Neonatal peripheral inflammation has been shown to produce profound anatomical changes in the dorsal horn of adult rats. In this study, we explored whether parallel physiological changes exist. Neonatal rats were injected with complete Freund's adjuvant (CFA) into the left hind paw. At 8-10 wk of age, single dorsal horn neurons were recorded in response to graded intensities of mechanical stimuli delivered to the receptive field. In addition, cord dorsum potentials, produced by electrical stimuli delivered to the left sciatic nerve at 2.5x threshold, were recorded bilaterally from L2 to S3. There were significant increases in background activity and responses to brush and pinch in neonatal rats that were treated with CFA, as compared with control rats. Further analysis showed similar significant changes when dorsal horn neurons were categorized into wide dynamic range (WDR), high-threshold (HT), and low-threshold (LT) groups. The receptive field was significantly larger in neonatally treated rats as compared with control rats. Additionally, there was a significant increase in the response to a 49 degrees C heat stimulus in neonatally treated rats as compared with control rats. There was also a trend for the amplitudes of N1, N2, and P waves of the cord dorsum potential to increase and latencies to decrease in neonatally treated rats, but no significant differences were detected between different levels of the spinal cord (L2 to S3). These data further support the notion that anatomical and physiological plasticity changes occurred in the spinal cord following early neonatal CFA treatment.

Comparison of the pain suppressive effects of clinical and experimental painful conditioning stimuli

Studies in healthy volunteers suggested that the classical counterirritation phenomenon (i.e. pain inhibits pain effect) might depend on diffuse noxious inhibitory controls (DNIC), which modulate the spinal transmission of nociceptive signals. In the present study, we sought to determine whether similar mechanisms were at play in patients with different subtypes of neuropathic pain. Ten patients presenting with a traumatic peripheral nerve injury associated with dynamic mechano-allodynia (i.e. pain triggered by brushing) or static mechano-allodynia (i.e. pain triggered by light pressure stimuli) were included in this study. To investigate counterirritation mechanisms in these patients, we analysed the RIII nociceptive flexion reflex and concomitant painful sensation elicited by electrical stimulation of the sural nerve. We compared the effects of heterotopic 'clinical' conditioning stimuli (i.e. pain evoked by brushing or pressure within the allodynic area located in the upper limb or chest) to those of experimental heterotopic noxious stimuli (HNCS) consisting of a cold pressor test or tourniquet test applied to the normal upper limb. Static mechano-allodynia induced inhibitions of both the RIII reflex and the concomitant painful sensation. These effects were similar to those induced by HNCS and were probably due to an increased activation of DNIC. In contrast, in patients with dynamic allodynia, brushing within the allodynic area reduced the pain sensation at the foot, but did not inhibit the electrophysiological responses, suggesting that in this case the counterirritation effect may take place at the supraspinal level. Thus, the mechanisms of counterirritation are not univocal, but depend on the pathophysiological mechanisms of clinical pain.

Diabetic neuropathy and nerve regeneration

Diabetic neuropathy is the most common peripheral neuropathy in western countries. Although every effort has been made to clarify the pathogenic mechanism of diabetic neuropathy, thereby devising its ideal therapeutic drugs, neither convinced hypotheses nor unequivocally effective drugs have been established. In view of the pathologic basis for the treatment of diabetic neuropathy, it is important to enhance nerve regeneration as well as prevent nerve degeneration. Nerve regeneration or sprouting in diabetes may occur not only in the nerve trunk but also in the dermis and around dorsal root ganglion neurons, thereby being implicated in the generation of pain sensation. Thus, inadequate nerve regeneration unequivocally contributes to the pathophysiologic mechanism of diabetic neuropathy. In this context, the research on nerve regeneration in diabetes should be more accelerated. Indeed, nerve regenerative capacity has been shown to be decreased in diabetic patients as well as in diabetic animals. Disturbed nerve regeneration in diabetes has been ascribed at least in part to all or some of decreased levels of neurotrophic factors, decreased expression of their receptors, altered cellular signal pathways and/or abnormal expression of cell adhesion molecules, although the mechanisms of their changes remain almost unclear. In addition to their steady-state changes in diabetes, nerve injury induces injury-specific changes in individual neurotrophic factors, their receptors and their intracellular signal pathways, which are closely linked with altered neuronal function, varying from neuronal survival and neurite extension/nerve regeneration to apoptosis. Although it is essential to clarify those changes for understanding the mechanism of disturbed nerve regeneration in diabetes, very few data are now available. Rationally accepted replacement therapy with neurotrophic factors has not provided any success in treating diabetic neuropathy. Aside from adverse effects of those factors, more rigorous consideration for their delivery system may be needed for any possible success. Although conventional therapeutic drugs like aldose reductase (AR) inhibitors and vasodilators have been shown to enhance nerve regeneration, their efficacy should be strictly evaluated with respect to nerve regenerative capacity. For this purpose, especially clinically, skin biopsy, by which cutaneous nerve pathology including nerve regeneration can be morphometrically evaluated, might be a safe and useful examination.

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.

Alteration of medullary dorsal horn neuronal activity following inferior alveolar nerve transection in rats

The effects of inferior alveolar nerve (IAN) transection on escape behavior and MDH neuronal activity to noxious and nonnoxious stimulation of the face were precisely analyzed. Relative thresholds for escape from mechanical stimulation applied to the whisker pad area ipsilateral to the transection were significantly lower than that for the contralateral and sham-operated whisker pad until 28 days after the transection, then returned to the preoperative level at 40 days after transection. A total of 540 neurons were recorded from the medullary dorsal horn (MDH) of the nontreated naive rats [low-threshold mechanoreceptive (LTM), 27; wide dynamic range (WDR), 31; nociceptive specific (NS), 11] and sham-operated rats with skin incision (LTM, 34; WDR, 30; NS, 23) and from the ipsilateral (LTM, 82; WDR, 82; NS, 31) and contralateral MDH relative to the IAN transection (LTM, 77; WDR, 82; NS, 33). The electrophysiological properties of these neurons were precisely analyzed. Background activity of WDR neurons on the ipsilateral side relative to the transection was significantly increased at 2-14 days after the operation as compared with that of naive rats. Innocuous and noxious mechanical-evoked responses of LTM and WDR neurons were significantly enhanced at 2-14 days after IAN transection. The mean area of the receptive fields of WDR neurons was significantly larger on the ipsilateral MDH at 2-7 days after transection than that of naive rats. We could not observe any modulation of thermal responses of WDR and NS neurons following IAN transection. Also, no MDH neurons were significantly affected in the rats with sham operations. The present findings suggest that the increment of neuronal activity of WDR neurons in the MDH following IAN transection may play an important role in the development of the mechano-allodynia induced in the area adjacent to the area innervated by the injured nerve.

Neurogenesis in postnatal mouse dorsal root ganglia

Neurogenesis continues in various regions of the central nervous system (CNS) throughout life. As the mitogen basic fibroblast growth factor (bFGF) can proliferate neuronal precursors of CNS neurons in culture, and is also upregulated within adult dorsal root ganglia following axotomy, it is possible that the postnatal dorsal root ganglia contain bFGF-responsive neuronal precursors. We undertook cell culture of postnatal mouse dorsal root ganglia to demonstrate neurogenesis. Basic FGF induced a cellular proliferative response in dorsal root ganglia cell culture. After 2 weeks in serum-free medium containing bFGF, neurons were rarely observed. However, following removal of bFGF and addition of trophic factors, many cells were observed that morphologically resembled dorsal root ganglia neurons, stained for neuronal markers, and generated action potentials. Furthermore, bromodeoxyuridine, used as a marker of cytogenesis, was detected in neurofilament-160(+) and/or microtubule-associated protein-2(+) cells that morphologically resembled neurons. In addition to bFGF, epidermal growth factor, nerve growth factor, and sonic hedgehog were also capable of generating spherical cell clusters that contained cells that stained for neuronal markers following the addition of trophic factors. These results suggest that early postnatal dorsal root ganglia contain neural precursors that appear to proliferate in response to various factors and can then be induced to differentiate into neurons. In conclusion, the existence of neural precursors and the possibility of neurogenesis in postnatal dorsal root ganglia may provide a greater range of plasticity available to somatosensory systems during growth or following injury, perhaps to replace ineffectual or dying neurons.

Pericellular Griffonia simplicifolia I isolectin B4-binding ring structures in the dorsal root ganglia following peripheral nerve injury in rats

Patients with a peripheral nerve injury often suffer from persistent chronic pain, but the underlying mechanism remains largely unknown. The persistent nature of the pain suggests injury-induced profound structural changes along the sensory pathways. In the present study, using the plant Griffonia simplicifolia I isolectin B4 (IB4) as a marker for nonpeptidergic small sensory neurons, we sought to examine whether these neurons sprout in the dorsal root ganglia (DRG) in response to peripheral nerve injury. The lumbar 5 (L5) spinal nerve was transected, and rats were allowed to survive for varying lengths of time before IB4 histology was performed. We found that a subpopulation of IB4-positive sensory neurons sprouted robustly after spinal nerve injury. Twelve weeks after spinal nerve injury, the IB4-positive ring structures became dramatic and encircled both large and small neurons in the DRG. The aberrant sprouting of small sensory neurons was also demonstrated by retrograde labeling. The processes of satellite cells surrounding large sensory neurons also became IB4 positive, and 87.8% of perineuronal IB4-positive ring structures intermingled and/or coexpressed with glial fibrillary acidic protein-positive satellite cells. Thus, the sprouting axons of IB4-positive neurons were intermingled with IB4-positive satellite cells, forming perineuronal ring structures surrounding large-diameter neurons. Ultrastructural examinations further confirmed that IB4-positive nerve terminals were entangled with satellite cells and IB4-negative unmyelinated sprouting fibers around sensory neurons. These studies have provided the first evidence that a subpopulation of IB4-binding small sensory neurons sprouts and forms perineuronal ring structures together with IB4-positive satellite cells in response to nerve injury. The significance of the sprouting of IB4-positive neurons remains to be determined.

Hypodynamia--hypokinesia induced variations in expression of fos protein in structures related to somatosensory system in the rat

There have been many reports describing modifications of the sensory and motor cortex following various types of disuse. Hypodynamia--hypokinesia is characterized by the absence of weight-bearing and by a decrease in motor activity. We have shown a reorganization of the cortical cartography after hypodynamia--hypokinesia. In order to give an anatomical account for this cortical plasticity, we set out to determine whether cerebral and spinal structures exhibited variations of their neuronal activation. For this purpose, immunocytochemical detection of Fos protein was performed in the rat brain and spinal cord. Following stimulation of the sciatic nerve, Fos protein was detected in the primary and secondary somatosensory cortex in control rats and in rats submitted to an episode of 14 days of hypodynamia--hypokinesia. Results showed that the stimulation of the sciatic nerve induced an increase in the number of Fos-immunoreactive neurons in all these structures. Moreover, after hypodynamia--hypokinesia, the number of Fos-immunoreactive neurons was increased in the primary and secondary somatosensory cortex and in the spinal cord. These results provide evidence for a higher activation of cortical cells after hypodynamia--hypokinesia in comparison to controls. These data support the hypothesis that hypodynamia--hypokinesia contributes to the development of functional plasticity.

Effects of red nucleus ablation and exogenous neurotrophin-3 on corticospinal axon terminal distribution in the adult rat

Collateral sprouting of undamaged descending axons is one potential mechanism for recovery of function after incomplete spinal cord injury. In this study, we have investigated whether terminals of the intact corticospinal tract in the rat would sprout following ablation of a parallel descending pathway, the rubrospinal tract. No sprouting was detected after this injury alone. However, the combination of rubrospinal tract ablation with administration of 100ng neurotrophin-3 to neurons of the corticospinal tract resulted in marked increased density of corticospinal innervation in the superficial dorsal horn. There was no effect of administration of neurotrophin-3 alone and increase in axon density was not detected in the deep dorsal horn. These results imply that spontaneous sprouting of undamaged corticospinal axons does not occur following ablation of a parallel tract system, although collateral sprouting can be induced through a combination of the lesion plus exogenous growth factor. Induced change in corticospinal terminal density is detected in the superficial dorsal horn only, supporting the hypothesis that this is an area particularly supportive of circuit reorganisation.

Activation of diffuse noxious inhibitory controls (DNIC) in rats with an experimental peripheral mononeuropathy

Diffuse noxious inhibitory controls (DNIC), which involve supraspinal structures and modulate the transmission of nociceptive signals, were investigated in rats with chronic constriction injury of the sciatic nerve. Nerve-injured rats with increased sensitivity to mechanical and thermal stimulation on the operated side were anesthetized and recordings were made from trigeminal convergent neurons. Inhibitions of C-fiber-evoked neuronal responses during and after the application of nociceptive conditioning stimuli to the hindpaw, were measured to evaluate DNIC. The conditioning stimuli consisted of graded natural (pressure and heat) and electrical stimuli and were applied alternately to non-operated and operated hindpaws. Compared with the non-operated paw, inhibitions elicited by pressure on the operated hindpaw were increased significantly at all stimulus intensities. Albeit to a lesser extent, inhibitions elicited by thermal stimulation of the operated paw were also increased in the nerve-injured animals. Such exacerbation of DNIC-induced inhibitions produced by mechanical and thermal stimulation of the operated paw can be explained by an increase in the afferent input to the spinal cord. In contrast to the results obtained with natural stimulations, inhibitions evoked from the operated and non-operated paws were similar when graded electrical stimulation was used as the conditioning stimulus. This was true regardless of the intensity and frequency of stimulation and regardless of whether the stimuli were applied transcutaneously or directly to the sciatic nerve. The clear-cut difference between the results obtained with natural and electrical conditioning stimuli suggests that the nociceptive neurons involved in the triggering of DNIC may not be sensitized at the central level. Peripheral mechanisms such as the sensitization of nerve injured fibers and/or sprouting of nerve terminals may thus be the main causes of DNIC increase in this model of neuropathic pain.

Functional reorganization of sensory pathways in the rat spinal dorsal horn following peripheral nerve injury

Functional reorganization of sensory pathways in the rat spinal dorsal horn following sciatic nerve transection was examined using spinal cord slices with an attached dorsal root. Slices were obtained from animals whose sciatic nerve had been transected 2-4 weeks previously and compared to sham-operated controls. Whole-cell recordings from substantia gelatinosa neurones in sham-operated rats, to which nociceptive information was preferentially transmitted, revealed that dorsal root stimulation sufficient to activate A afferent fibres evoked a mono- and/or polysynaptic EPSC in 111 of 131 (approximately 85%) neurones. This is in contrast to the response following A fibre stimulation, where monosynaptic EPSCs were observed in 2 of 131 (approximately 2%) neurones and polysynaptic EPSCs were observed in 18 of 131 (approximately 14%) neurones. In sciatic nerve-transected rats, however, a polysynaptic EPSC following stimulation of A afferents was elicited in 30 of 37 (81%) neurones and a monosynaptic EPSC evoked by A afferent stimulation was detected in a subset of neurones (4 of 37, approximately 11%). These observations suggest that, following sciatic nerve transection, large myelinated A afferent fibres establish synaptic contact with interneurones and transmit innocuous information to substantia gelatinosa. This functional reorganization of the sensory circuitry may constitute an underlying mechanism, at least in part, for sensory abnormalities following peripheral nerve injuries.

Changes in tactile stimuli-induced behavior and c-Fos expression in the superficial dorsal horn and in parabrachial nuclei after sciatic nerve crush

Neurons in the superficial laminae of the dorsal horn are dominated by input from peripheral nociceptors. Following peripheral nerve injury, low threshold mechanoreceptive Abeta-fibers sprout from their normal termination site in laminae III/IV into laminae I-II and this structural reorganization may contribute to neuropathic tactile pain hypersensitivity. We have now investigated whether a sciatic nerve crush injury alters the behavioral response in rats to tactile stimuli and whether this is associated with a change in the pattern of c-Fos expression in the dorsal horn and the parabrachial area of the brainstem. Sciatic nerve crush resulted in a patchy but marked tactile allodynia manifesting first at 3 weeks and persisting for up to 52 weeks. C-Fos expression in the dorsal horn and parabrachial region was never observed on brushing the skin of the sciatic nerve territory in animals with intact nerves, but was found after sciatic nerve crush with peripheral regeneration. We conclude that after nerve injury, low threshold mechanoreceptor fibers may play a major role in producing pain-related behavior by activating normally nociceptive-specific regions of the central nervous system such as the superficial laminae of the dorsal horn and the parabrachial area.

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.