Diminished dorsal root GABA sensitivity following chronic peripheral nerve injury

Nicotinic receptor modulation of primary afferent excitability with selective regulation of Aδ-mediated spinal actions

Somatosensory input strength can be modulated by primary afferent depolarization (PAD) generated predominantly via presynaptic GABA_A receptors on afferent terminals. We investigated whether ionotropic nicotinic acetylcholine receptors (nAChRs) also provide modulatory actions, focusing on myelinated afferent excitability in in vitro murine spinal cord nerve-attached models. Primary afferent stimulation-evoked synaptic transmission was recorded in the deep dorsal horn as extracellular field potentials (EFPs), whereas concurrently recorded dorsal root potentials (DRPs) were used as an indirect measure of PAD. Changes in afferent membrane excitability were simultaneously measured as direct current (DC)-shifts in membrane polarization recorded in dorsal roots or peripheral nerves. The broad nAChR antagonist d-tubocurarine (d-TC) selectively and strongly depressed Aδ-evoked synaptic EFPs (36% of control) coincident with similarly depressed A-fiber DRP (43% of control), whereas afferent electrical excitability remained unchanged. In comparison, acetylcholine (ACh) and the nAChR agonists, epibatidine and nicotine, reduced afferent excitability by generating coincident depolarizing DC-shifts in peripheral axons and intraspinally. Progressive depolarization corresponded temporally with the emergence of spontaneous axonal spiking and reductions in the DRP and all afferent-evoked synaptic actions (31%-37% of control). Loss of evoked response was long-lasting, independent of DC repolarization, and likely due to mechanisms initiated by spontaneous C-fiber activity. DC-shifts were blocked with d-TC but not GABA_A receptor blockers and retained after tetrodotoxin block of voltage-gated Na⁺ channels. Notably, actions tested were comparable between three mouse strains, in rat, and when performed in different labs. Thus, nAChRs can regulate afferent excitability via two distinct mechanisms: by central Aδ-afferent actions, and by transient extrasynaptic axonal activation of high-threshold primary afferents.NEW & NOTEWORTHY Primary afferents express many nicotinic ACh receptor (nAChR) subtypes but whether activation is linked to presynaptic inhibition, facilitation, or more complex and selective activity modulation is unknown. Recordings of afferent-evoked responses in the lumbar spinal cord identified two nAChR-mediated modulatory actions: 1) selective control of Aδ afferent transmission and 2) robust changes in axonal excitability initiated via extrasynaptic shifts in DC polarization. This work broadens the diversity of presynaptic modulation of primary afferents by nAChRs.

Reviewing the case for compromised spinal inhibition in neuropathic pain

A striking and debilitating property of the nervous system is that damage to this tissue can cause chronic intractable pain, which persists long after resolution of the initial insult. This neuropathic form of pain can arise from trauma to peripheral nerves, the spinal cord, or brain. It can also result from neuropathies associated with disease states such as diabetes, human immunodeficiency virus/AIDS, herpes, multiple sclerosis, cancer, and chemotherapy. Regardless of the origin, treatments for neuropathic pain remain inadequate. This continues to drive research into the underlying mechanisms. While the literature shows that dysfunction in numerous loci throughout the CNS can contribute to chronic pain, the spinal cord and in particular inhibitory signalling in this region have remained major research areas. This review focuses on local spinal inhibition provided by dorsal horn interneurons, and how such inhibition is disrupted during the development and maintenance of neuropathic pain.

Coping with Phantom Limb Pain

Phantom limb pain is a chronic neuropathic pain that develops in 45-85% of patients who undergo major amputations of the upper and lower extremities and appears predominantly during two time frames following an amputation: the first month and later about 1 year. Although in most patients the frequency and intensity of pain diminish over time, severe pain persists in about 5-10%. It has been proposed that factors in both the peripheral and central nervous systems play major roles in triggering the development and maintenance of pain associated with extremity amputations. Chronic pain is physically and mentally debilitating, affecting an individual's capacity for self-care, but also diminishing an individual's daily capacity for personal and economic independence. In addition, the pain may lead to depression and feelings of hopelessness. A National Center for Biotechnology Information study found that in the USA alone, the annual cost of dealing with neuropathic pain is more than $600 billion, with an estimated 20 million people in the USA suffering from this condition. Although the pain can be reduced by antiepileptic drugs and analgesics, they are frequently ineffective or their side effects preclude their use. The optimal approach for eliminating neuropathic pain and improving individuals' quality of life is the development of novel techniques that permanently prevent the development and maintenance of neuropathic pain, or that eliminate the pain once it has developed. What is still required is understanding when and where an effective novel technique must be applied, such as onto the nerve stump of the transected peripheral axons, dorsal root ganglion neurons, spinal cord, or cortex to induce the desired influences. This review, the second of two in this journal volume, examines the techniques that may be capable of reducing or eliminating chronic neuropathic pain once it has developed. Such an understanding will improve amputees' quality of life by blocking the mechanisms that trigger and/or maintain PLP and chronic neuropathic pain.

MHC-I promotes apoptosis of GABAergic interneurons in the spinal dorsal horn and contributes to cancer induced bone pain

Cancer induced bone pain (CIBP) remains one of the most intractable clinical problems due to poor understanding of its underlying mechanisms. Recent studies demonstrate the decline of inhibitory interneurons, especially GABAergic interneurons in the spinal cord, can evoke generation of chronic pain. It has also been reported that neuronal MHC-I expression renders neurons vulnerable to cytotoxic CD8⁺ T cells and finally lead to neurons apoptosis in a variety neurological disorders. However, whether MHC-I could induce the apoptosis of GABAergic interneurons in spinal cord and contribute to the development of CIBP remains unknown. In this study, we investigated roles of MHC-I and underlying mechanisms in CIBP on a rat model. Our results showed that increased MHC-I expression on GABAergic interneurons could deplete GABAergic interneurons by inducing their apoptosis in the spinal dorsal horn of tumor-bearing rats. Pretreatment of MHC-I RNAi-lentivirus could prevent the apoptosis of GABAergic interneurons and therefore alleviated mechanical allodynia induced by tumor cells intratibial injection. Additionally, we also found that CD8⁺ T cells were colocalized with MHC-I and GABAergic neurons and presented a significant and persistent increase in the spinal cord of tumor-bearing rats. Taken together, these findings indicated that MHC-I could evoke CIBP by promoting apoptosis of GABAergic interneurons in the dorsal horn, and this apoptosis was closely related to local CD8⁺ T cells.

REVIEW ■ : Pain Mechanisms

Over the last few years, a new synthesis has emerged concerning the neural mechanisms of acute and chronic pain. This new model deals far more successfully than do classical models with the peculiarities of chronic pain syndromes seen in the clinic. As in earlier models, Aδ- and C-nociceptive afferents detect the initial noxious event. In addition, however, this input is now known to rapidly trigger a central hyperexcitability state ("central sensitization") that amplifies sensory signals subsequently entering the CNS along other afferent fiber types. As a result, in the presence of central sensitization, pain sensation is evoked by Aβ touch input, as well as by Aδ- and C-nociceptor input. Tenderness in subacute (e.g., inflammatory) pain, for example, is due to both peripherally sensitized nociceptors and centrally amplified, low-threshold input. The new synthesis also stresses the common ground between the subacute pain of injured tissue and the chronic pain that sometimes develops after nerve injury (i.e., neuropathic pain). In the event of neuropathic pain, the affected afferent axons and their sensory cell somata in the associated dorsal root ganglia (DRGs) become hyperexcitable to applied stimuli. Some even fire spontaneously. Hyperexcitability of the afferent neuron apparently results from specific changes in the regulation of membrane channel and receptor proteins. The resulting ectopic discharge (ectopia) contributes a direct neuropathic pain signal. In addition, neuropathic ectopia sets up and maintains a central sensitization state that amplifies ongoing pain and is responsible for pain on weak stimulation of adjacent areas of skin and deep tissues with residual innervation. The discovery that normal and ectopic Aβ touch input, as well as Aδ- and C-nociceptor input, contributes to subacute and chronic pathophysiological pain states opens previously unanticipated avenues for clinical pain control. NEUROSCIENTIST 2:233-244, 1996

Peripheral nerve injury induces loss of nociceptive neuron-specific Gαi-interacting protein in neuropathic pain rat

BACKGROUND

Gαi-interacting protein (GINIP) is expressed specifically in dorsal root ganglion (DRG) neurons and functions in modulation of peripheral gamma-aminobutyric acid B receptor (GBR). Genetic deletion of GINIP leads to impaired responsiveness to GBR agonist-mediated analgesia in rodent. It is, however, not defined whether nerve injury changes GINIP expression.

RESULTS

Immunolabeling with validated antibody revealed GINIP expression in ~40% of total lumbar DRG neurons in normal adult rats. GINIP immunoreactivity was detected in ~80% of IB4-positive (nonpeptidergic) and ~30% of CGRP-positive (peptidergic) neurons. GINIP immunoreactivity in the spinal cord dorsal horn was colabeled with IB4 and partially with CGRP. In addition, GINIP was expressed in DRG neurons immunopositive for GBR1, GBR2, Gαi(s), and Gαo and was also extensively colabeled with multiple nociceptive neuronal markers, including Trpv1, NaV1.7, CaV2.2α1b, CaV3.2α1b, TrkA, and Trek2. Peripheral nerve injury by L5 spinal nerve ligation significantly decreased the proportion of GINIP immunoreactivity-positive neurons from 40 ± 8.4% to 0.8 ± 0.1% (p < 0.01, mean ± SD, four weeks after spinal nerve ligation) and the total GINIP protein to 1.3% ± 0.04% of its basal level (p < 0.01, n = 6 animals in each group, two weeks after spinal nerve ligation) in the ipsilateral L5 DRGs.

CONCLUSION

Our results show that GINIP is predominantly expressed by small nonpeptidergic nociceptive neurons and that nerve injury triggers loss of GINIP expression. Signal transduction roles of GINIP may be diverse as it colabeled with various subgroups of nociceptive neurons. Future studies may investigate details of the signaling mechanism engaged by GINIP, as well as the pathophysiological significance of lost expression of GINIP in neuropathic pain.

Cell transplants to treat the "disease" of neuropathic pain and itch

Among many mechanisms implicated in the development of neuropathic pain after nerve damage is a profound dysfunction of GABAergic inhibitory controls, manifested by ongoing pain, mechanical hypersensitivity, and thermal hyperalgesia. In some respects, neuropathic pain can be considered a "disease" of the nervous system, with features in common with trauma-induced seizures. Indeed, first-line management involves anticonvulsant therapy. An alternative to pharmacotherapy for neuropathic pain is an approach that reestablishes the inhibitory tone that is lost after nerve damage. To this end, we have transplanted embryonic cortical GABAergic precursor neurons into the spinal cord of nerve-injured mice. Using a combination of light and electron microscopic analyses, and also in vitro electrophysiological recordings from spinal cord slice preparations, we demonstrated remarkable integration of the transplants into the host, adult spinal cord. Most importantly, transplants produced a complete reversal of the hypersensitivity in a sciatic nerve injury model and in a paclitaxel-generated chemotherapy model of neuropathic pain. In related studies, we demonstrated that medial ganglionic eminence cell transplants are also effective in a chronic neuropathic itch model in which there is a significant loss of dorsal horn inhibitory interneurons. Most importantly, in contrast to systemic or intrathecal pharmacological therapies, adverse side effects are minimized when the inhibitory control, namely, γ-aminobutyric acid release, occurs in a spinal cord circuit. These studies suggest that therapy targeted at repairing the GABAergic dysfunction is a viable and novel alternative to the management of neuropathic pain and itch, particularly those that are or become refractory to traditional pharmacotherapy.

Cation-chloride cotransporters in neuronal development, plasticity and disease

Electrical activity in neurons requires a seamless functional coupling between plasmalemmal ion channels and ion transporters. Although ion channels have been studied intensively for several decades, research on ion transporters is in its infancy. In recent years, it has become evident that one family of ion transporters, cation-chloride cotransporters (CCCs), and in particular K(+)-Cl(-) cotransporter 2 (KCC2), have seminal roles in shaping GABAergic signalling and neuronal connectivity. Studying the functions of these transporters may lead to major paradigm shifts in our understanding of the mechanisms underlying brain development and plasticity in health and disease.

Potential for Cell-Transplant Therapy with Human Neuronal Precursors to Treat Neuropathic Pain in Models of PNS and CNS Injury: Comparison of hNT2.17 and hNT2.19 Cell Lines

Effective treatment of sensory neuropathies in peripheral neuropathies and spinal cord injury (SCI) is one of the most difficult problems in modern clinical practice. Cell therapy to release antinociceptive agents near the injured spinal cord is a logical next step in the development of treatment modalities. But few clinical trials, especially for chronic pain, have tested the potential of transplant of cells to treat chronic pain. Cell lines derived from the human neuronal NT2 cell line parentage, the hNT2.17 and hNT2.19 lines, which synthesize and release the neurotransmitters gamma-aminobutyric acid (GABA) and serotonin (5HT), respectively, have been used to evaluate the potential of cell-based release of antinociceptive agents near the lumbar dorsal (horn) spinal sensory cell centers to relieve neuropathic pain after PNS (partial nerve and diabetes-related injury) and CNS (spinal cord injury) damage in rat models. Both cell lines transplants potently and permanently reverse behavioral hypersensitivity without inducing tumors or other complications after grafting. Functioning as cellular minipumps for antinociception, human neuronal precursors, like these NT2-derived cell lines, would likely provide a useful adjuvant or replacement for current pharmacological treatments for neuropathic pain.

Dorsal root ganglion application of muscimol prevents hyperalgesia and stimulates myelin protein expression after sciatic nerve injury in rats

BACKGROUND

Peripheral nerve injuries may result in debilitating pain that is poorly responsive to conventional treatment. Neuropathic pain induced by peripheral nerve injury is caused, in part, by ectopic discharges from the injury site or the dorsal root ganglia (DRG) resulting in enhanced central input and central hyperexcitability. A heterogeneous family of γ-aminobutyric acid (GABA)(A) channels is important in quieting neuronal excitability. We have recently reported that in vivo modulation of GABAergic neurons in DRG can alter the course of neuropathic pain development after peripheral nerve injury. It seems that direct application of a potent GABA(A) agonist, muscimol, to the ipsilateral DRG prevents the development of hyperalgesia in rats subjected to a sciatic nerve crush injury. In addition to potentially curtailing hyperexcitability, GABAergic stimulation upregulated expression of peripheral myelin protein 22 (PMP22), a key component of the basal lamina. PMP22 expression correlates with peripheral myelin formation and nerve regeneration.

METHODS

Because of the importance of PMP22 for the formation and stability of myelin, and the fact that PMP22 expression could be GABAergically modulated, we examined whether direct DRG application of muscimol can restore PMP22 protein expression and the integrity of nerve fibers after crush injury of a sciatic nerve.

RESULTS

Using adult female rats and a crush injury model, we found that GABAergic modulation in the ipsilateral DRG restores PMP22 protein expression in the distal segment of the sciatic nerve and improves myelin stability in the basal membrane of nerve fibers, thus giving the morphological appearance of lessened nerve injury or faster nerve fiber regeneration. Both the enhanced PMP22 protein expression and morphological improvements coincide with the abolishment of thermal and mechanical hypersensitivity.

CONCLUSIONS

The DRG could be a promising therapeutic target in nerve regeneration and pain alleviation after crush injury of a myelinated peripheral nerve.

Models and mechanisms of hyperalgesia and allodynia

Hyperalgesia and allodynia are frequent symptoms of disease and may be useful adaptations to protect vulnerable tissues. Both may, however, also emerge as diseases in their own right. Considerable progress has been made in developing clinically relevant animal models for identifying the most significant underlying mechanisms. This review deals with experimental models that are currently used to measure (sect. II) or to induce (sect. III) hyperalgesia and allodynia in animals. Induction and expression of hyperalgesia and allodynia are context sensitive. This is discussed in section IV. Neuronal and nonneuronal cell populations have been identified that are indispensable for the induction and/or the expression of hyperalgesia and allodynia as summarized in section V. This review focuses on highly topical spinal mechanisms of hyperalgesia and allodynia including intrinsic and synaptic plasticity, the modulation of inhibitory control (sect. VI), and neuroimmune interactions (sect. VII). The scientific use of language improves also in the field of pain research. Refined definitions of some technical terms including the new definitions of hyperalgesia and allodynia by the International Association for the Study of Pain are illustrated and annotated in section I.

Subarachnoid Transplant of a Human Neuronal Cell Line Attenuates Chronic Allodynia and Hyperalgesia After Excitotoxic Spinal Cord Injury in the Rat

The relief of neuropathic pain after spinal cord injury (SCI) remains daunting, because pharmacologic intervention works incompletely and is accompanied by multiple side effects. Transplantation of human cells that make specific biologic agents that can potentially modulate the sensory responses that are painful would be very useful to treat problems such as pain. To address this need for clinically useful human cells, the human neuronal NT2 cell line was used as a source to isolate a unique human neuronal cell line that synthesizes and secretes/releases the inhibitory neurotransmitters gamma-aminobutyric acid (GABA) and glycine. This new cell line, hNT2.17, expresses an exclusively neuronal phenotype, does not incorporate bromodeoxyuridine during differentiation, and does not express the tumor-related proteins fibroblast growth factor 4 and transforming growth factor-alpha during differentiation after 2 weeks of treatment with retinoic acid and mitotic inhibitors. The transplant of predifferentiated hNT2.17 cells was used in the excitotoxic SCI pain model, after intraspinal injection of the mixed AMPA/metabotropic receptor agonist quisqualic acid (QUIS). When hNT2.17 cells were transplanted into the lumbar subarachnoid space, tactile allodynia and thermal hyperalgesia induced by the injury were quickly and potently reversed. Control cell transplants of nonviable hNT2.17 cells had no effect on the hypersensitivity induced by QUIS. The effects of hNT2.17 cell grafts appeared 1 week after transplants and did not diminish during the 8-week course of the experiment when grafts were placed 2 weeks after SCI. Immunohistochemistry and quantification of the human grafts were used to ensure that many grafted cells were still present and synthesizing GABA at the end of the study. These data suggest that the human neuronal hNT2.17 cells can be used as a "biologic minipump" for antinociception in models of SCI and neuropathic pain.

PERSPECTIVE

This study describes the initial characterization and use of a human-derived cell line to treat neuropathic pain that would be suitable for clinical application, once further tested for safety and approved by the Food and Drug Administration. A dose of these human cells could be delivered with a spinal tap and affect the intrathecal spinal environment for sensory system modulation.

Activity-dependent phosphorylation of Akt/PKB in adult DRG neurons

The serine/threonine kinase Akt/PKB has been implicated in cell survival signalling in many cell types, including the dorsal root ganglion (DRG). However, little is known about its role in physiological and pathophysiological conditions in the adult sensory and nociceptive system. In this study, we show that in naive animals almost all cells express Akt but only a subset of small-diameter neurons expresses a high level of phospho-Akt (p-Akt Ser 473). Activation of peripheral nociceptors in vivo using intraplantar injections of capsaicin in anaesthetized rats induced a rapid onset and time-dependent increase in p-Akt Ser 473 in small- and medium-sized DRG, predominantly TRPV1-positive neurons. In addition, electrical stimulation of 'A and C' fibres in the sciatic nerve induced an increase in the cytoplasmic staining of p-Akt Ser 473 in small- and medium-size DRG neurons. Blocking neuronal activity in the sciatic nerve using tetrodotoxin reduced the basal level of p-Akt Ser 473. Cultured DRG neurons confirmed that phosphorylation of Akt in different cellular compartments is triggered by depolarization or receptor activation, and suggested that this effect is mediated in part by phosphatidylinositol 3-kinase. Our results show that p-Akt Ser 473 is a marker of nociceptor activation and suggest a novel role for Akt in the transduction of intracellular signals in adult DRG neurons.

Similar electrophysiological changes in axotomized and neighboring intact dorsal root ganglion neurons

We investigated electrophysiological changes in chronically axotomized and neighboring intact dorsal root ganglion (DRG) neurons in rats after either a peripheral axotomy consisting of an L5 spinal nerve ligation (SNL) or a central axotomy produced by an L5 partial rhizotomy (PR). SNL produced lasting hyperalgesia to punctate indentation and tactile allodynia to innocuous stroking of the foot ipsilateral to the injury. PR produced ipsilateral hyperalgesia without allodynia with recovery by day 10. Intracellular recordings were obtained in vivo from the cell bodies (somata) of axotomized and intact DRG neurons, some with functionally identified peripheral receptive fields. PR produced only minor electrophysiological changes in both axotomized and intact somata in L5 DRG. In contrast, extensive changes were observed after SNL in large- and medium-sized, but not small-sized, somata of intact (L4) as well as axotomized (L5) DRG neurons. These changes included (in relation to sham values) higher input resistance, lower current and voltage thresholds, and action potentials with longer durations and slower rising and falling rates. The incidence of spontaneous activity, recorded extracellularly from dorsal root fibers in vitro, was significantly higher (in relation to sham) after SNL but not after PR, and occurred in myelinated but not unmyelinated fibers from both L4 (9.1%) and L5 (16.7%) DRGs. We hypothesize that the changes in the electrophysiological properties of axotomized and intact DRG neurons after SNL are produced by a mechanism associated with Wallerian degeneration and that the hyperexcitability of intact neurons may contribute to SNL-induced hyperalgesia and allodynia.

Amelioration of chronic neuropathic pain after partial nerve injury by adeno-associated viral (AAV) vector-mediated over-expression of BDNF in the rat spinal cord

Changing the levels of neurotrophins in the spinal cord micro-environment after nervous system injury has been proposed to recover normal function, such that behavioral response to peripheral stimuli does not lead to chronic pain. We have investigated the effects of recombinant adeno-associated viral (rAAV)-mediated over-expression of brain-derived neurotrophic factor (BDNF) in the spinal cord on chronic neuropathic pain after unilateral chronic constriction injury (CCI) of the sciatic nerve. The rAAV-BDNF vector was injected into the dorsal horn at the thirteenth thoracic spinal cord vertebra (L(1) level) 1 week after CCI. Allodynia and hyperalgesia induced by CCI in the hindpaws were permanently reversed, beginning 1 week after vector injection, compared with a similar injection of a control rAAV-GFP vector (green fluorescent protein) or saline. In situ hybridization for BDNF demonstrated that both dorsal and ventral lumbar spinal neurons contained an intense signal for BDNF mRNA, at 1 to 8 weeks after vector injection. There was no similar BDNF mRNA over-expression associated with either injections of saline or rAAV-GFP. These data suggest that chronic neuropathic pain is sensitive to early spinal BDNF levels after partial nerve injury and that rAAV-mediated gene transfer could potentially be used to reverse chronic pain after nervous system injuries in humans.

Neuropathic pain: what do we do with all these theories?

Only a generation ago there were few ideas as to what might cause neuropathic pain, and even fewer relevant data. In contrast, we can currently point to hundreds of distinct cellular changes that are triggered by nerve injury and that might be relevant to the emergence of pain symptomatology. The number may soon increase to thousands. It is essential, therefore, to redirect efforts towards the development of experimental strategies for testing which of these are essential parts of the pain process and which are tangential. In this paper I point out four such strategies: timing, deletion, prevention and genetic heterogeneity, and summarize how one neuropathic pain theory, the ectopic pacemaker hypothesis, holds up to scrutiny.

Only early intervention with gamma-aminobutyric acid cell therapy is able to reverse neuropathic pain after partial nerve injury

Pharmacological treatment for neuropathic pain, although often effective for brief periods, can result in intractable persistent pain with certain patients. Cell therapy for neuropathic pain is a newly developing technology useful for an examination of enhanced normal sensory function after nerve injury with the placement of cells near the spinal cord, and grafts of immortalized cells bioengineered to chronically supply the inhibitory neurotransmitter gamma-aminobutyric acid (GABA) have been used to reverse the chronic pain behaviors. However, it is not known whether there is a therapeutic window for the use of intervention with cell therapy after partial nerve injury. To investigate whether neuropathic pain is sensitive to the timing of placement of cell grafts, neuronal cells bioengineered to synthesize GABA were transplanted in the lumbar subarachnoid space one to four weeks after unilateral chronic constriction injury (CCI) of the sciatic nerve and sensory behaviors were evaluated before and after CCI and transplants. Both thermal hyperalgesia and tactile allodynia were reversed when transplants were placed either one or two weeks after partial nerve injury, compared to maintenance of these behaviors with the injury alone. However, if GABA cells were placed any later than 2 weeks after nerve injury, such intervention was ineffective to reverse the thermal and tactile hypersensitivities induced by the injury. This suggests that altered spinal GABA levels may contribute to the early development of chronic neuropathic pain and that early intervention with cellular therapy to restore GABA may prevent the development of that pain.

Emerging cell and molecular strategies for the study and treatment of painful peripheral neuropathies

Pharmacologic treatment for the symptoms of painful neuropathy has been problematic, because there has been limited understanding of the underlying etiologies and systemic levels that an effective dose can have on multiple side effects. The use of molecular methods, such as gene deletion from knockout mice and cellular minipumps for delivery of biologic antinociceptive molecules, has led to a better understanding of the underlying mechanisms of the induction of intractable neuropathic pain. The initiation of an excitatory cascade after injury or disease leads to the induction of various second messenger systems, loss or down-regulation of the endogenous inhibitory spinal GABA system and central sensitization, causing such pain. The development and use of cellular minipumps, immortalized cell lines bioengineered to secrete various antinociceptive molecules for the reversal of neuropathic pain, makes cellular therapy a strategy for clinical use in the next few years. The development of molecular "disimmortalization" technologies will make the use of such engineered cell lines safe for human use. Direct somatic gene transfer for neuropathic pain will eventually overcome the problems associated with transplantation of non-autologous and xenogenic cells. These virus-mediated methods, although at the early stages of evolution and use, offer large-scale production of biologic agents that can be conveniently and confidently used for the long-term relief of chronic neuropathic pain in a clinical setting, without systemic effects or surgical interventions.

Lumbar transplant of neurons genetically modified to secrete brain-derived neurotrophic factor attenuates allodynia and hyperalgesia after sciatic nerve constriction

Chronic delivery of anti-nociceptive molecules by means of cell grafts near the pain processing centers of the spinal cord is a newly developing technique for the treatment of neuropathic pain. The rat neuronal cell line, RN33B, derived from E13 rat brainstem raphe and immortalized with the SV40 temperature-sensitive allele of large T antigen (tsTag), was transfected with rat brain-derived neurotrophic factor cDNA (BDNF), and the BDNF-synthesizing cell line, 33BDNF.4, was isolated. The 33BDNF.4 cells synthesized mature BDNF protein at permissive temperature (33 degrees C), when the cells were proliferating, and during differentiation at non-permissive temperature (39 degrees C) in vitro. The bio-active BDNF protein was also secreted by the cells during both growth conditions, as measured by ELISA analysis of BDNF content and secretion. The bio-activity of the BDNF in 33BDNF.4 cell conditioned media was assessed by neurite outgrowth from E15 dorsal root ganglion (DRG) cultures. A control cell line, 33V1, transfected with the vector alone, did not synthesize or secrete any significant BDNF at either growth condition. Both cell lines were used as grafts in a model of chronic neuropathic pain induced by unilateral chronic constriction injury (CCI) of the sciatic nerve. Pain-related behaviors, including cold and tactile allodynia and thermal and tactile hyperalgesia, were evaluated after CCI in the affected hindpaw. When 33BDNF.4 and 33V1 cells were transplanted in the lumbar subarachnoid space of the spinal cord 1 week after CCI, they survived greater than 7 weeks on the pia mater around the spinal cord and the 33BDNF.4 cells continued to synthesize BDNF in vivo. Furthermore, the tactile and cold allodynia and tactile and thermal hyperalgesia induced by CCI was significantly reduced during the 2-7 week period after grafts of 33BDNF.4 cells. The maximal effect on chronic pain behaviors with the BDNF grafts occurred 2-3 weeks after transplant and the anti-nociceptive effects of the BDNF cell grafts was permanent. Transplants of the control 33V1 cells had no effect on the allodynia and hyperalgesia induced by CCI and these cells did not synthesize BDNF in vivo. These data suggest that a chronically applied, low local dose of BDNF supplied by transplanted cells near the spinal dorsal horn was able to reverse the development of chronic neuropathic pain following CCI. The use of neural cell lines that are able to deliver anti-nociceptive molecules, such as BDNF, in a model of chronic pain offers a novel approach to pain management and such 'biologic minipumps' can be developed for safe use in humans.

Excitability changes of dorsal root axons following nerve injury: implications for injury-induced changes in axonal Na(+) channels

Electrophysiological recordings were obtained from rat dorsal roots in a sucrose gap chamber to study changes in Na(+) currents following nerve injury. Application of 4-aminopyridine unmasks a prominent and well-characterized depolarization (delayed depolarization) following the action potential. In our previous studies, this potential, which is only present in cutaneous afferent axons, has been shown to correlate with activation of a slow Na(+) current. The delayed depolarization in the dorsal root was reduced 1 week after sciatic nerve ligation, suggesting a reduction in the kinetically slow Na(+) currents on dorsal root axons [control: 44. 2+/-7.3% (n=5); injury: 7.3+/-4.7% (n=5), P<0.001]. The refractory period of the action potential was reduced following nerve injury, in agreement with biophysical studies indicating faster "repriming" of fast Na(+) currents on cutaneous afferent cell bodies. Dorsal root ligation near the spinal cord also results in a reduction in the delayed depolarization. These results indicate that changes in Na(+) channel organization occur on dorsal root axons following either central or peripheral target disconnection, suggesting trophic support can be derived from either the CNS or the PNS.

Drug sensitivities of the primary afferents and their changes after inflammation

This study explored the possibility that altered sensitivities of the dorsal root and dorsal root ganglion to neuroactive substances released in inflamed tissue may be involved in radicular pain. The chemical sensitivities of the dorsal root and ganglion of rats were examined by monitoring nerve membrane potential. Endogenous pain inducing substances such as bradykinin, serotonin, acetylcholine, and histamine caused depolarizations of the dorsal root and the ganglion. Application of bradykinin or capsaicin to the dorsal root and ganglion on the isolated spinal cord preparation evoked spinal reflex activities in the lumbar ventral root. These results suggest that, when pain inducing substances are released at the dorsal root or its ganglion, they may initiate action potentials and cause pain. As an inflammation model, chromic gut was tied loosely around the lower lumbar nerve root. The dorsal root of the surgically treated rats showed an increased sensitivity to bradykinin when compared with sham operated rats. In contrast, the sensitivity of the dorsal root to gamma-aminobutyric acid, a major inhibitory transmitter in the spinal cord, was decreased. This result suggests that these reciprocal changes in the sensitivities of the dorsal root may play an important role in the pathogenesis of chemical radiculitis.

Peripheral GABA(A) receptors: evidence for peripheral primary afferent depolarization

We propose that the primary afferent depolarization that follows GABA(A) receptor activation in the spinal cord also occurs in the periphery. As evidence, the present study localizes beta2/beta3 and alpha1 subunits of the GABA(A) receptor on 10-14% of the unmyelinated primary afferents axons in the glabrous skin of the cat paw. Behavioral studies demonstrate that local peripheral injection of the GABA(A) agonist muscimol at a low concentration (2.0 microM) attenuates, and at a high concentration (1 mM) enhances, formalin-induced nociceptive behaviors. Intraplantar injection of muscimol alone at a high dose evokes thermal hyperalgesia. Bicuculline, a GABA(A) antagonist, prevents these muscimol-induced changes in behavior. The muscimol-induced effects are due to local rather than systemic or central activation of GABA(A) receptors, as such effects are not observed in the contralateral paw. We interpret these findings to indicate that activation of GABA(A) receptors by low concentrations of muscimol depolarizes peripheral primary afferent terminals, a phenomenon we call peripheral primary afferent depolarization, in turn reducing the size of the peripheral action potentials and concomitantly reducing the amount of algogenic substances released from the peripheral terminals of these fibers. This sequence of events presumably results in a reduction in nociceptor activation. Higher concentrations of muscimol further depolarize GABA(A) receptor-containing terminals, which then initiates action potentials in nociceptors analogous to the appearance of dorsal root reflexes that arise following activation of GABA(A) receptors on central primary afferent terminals. These latter events reverse the analgesic effects of GABA(A) ligands and lead to potentiation of nociceptive input. Thus, the present study provides anatomical and behavioral evidence supporting a bimodal role for GABA(A) receptors in the modulation of peripheral nociceptive transmission.

A single intrathecal injection of GABA permanently reverses neuropathic pain after nerve injury

To investigate whether neuropathic pain is sensitive to spinal GABA levels, GABA was injected intrathecally after nerve injury and sensory behaviors were evaluated. Both thermal and tactile hypersensitivities were permanently reversed at the highest doses of GABA. However, if GABA was injected any later than 2-3 weeks after nerve injury, it was ineffective to prevent such hypersensitivity. This suggests that altered spinal GABA levels contribute to the induction phase of chronic neuropathic pain and that early intervention to restore GABA may prevent the development of that pain.

Transplants of neuronal cells bioengineered to synthesize GABA alleviate chronic neuropathic pain

The use of cell lines utilized as biologic "minipumps" to provide antinociceptive molecules, such as GABA, in animal models of pain is a newly developing area in transplantation biology. The neuronal cell line, RN33B, derived from E13 brain stem raphe and immortalized with the SV40 temperature-sensitive allele of large T antigen (tsTag), was transfected with rat GAD67 cDNA (glutamate decarboxylase, the synthetic enzyme for GABA), and the GABAergic cell line, 33G10.17, was isolated. The 33G10.17 cells transfected with the GAD67 gene expressed GAD67 protein and synthesized low levels of GABA at permissive temperature (33 degrees C), when the cells were proliferating, and increased GAD67 and GABA during differentiation at nonpermissive temperature (39 degrees C) in vitro, because GAD67 protein expression was upregulated with differentiation. A control cell line, 33V1, transfected with the vector alone, contained no GAD67 or GABA at either temperature. These cell lines were used as grafts in a model of chronic neuropathic pain induced by unilateral chronic constriction injury (CCI) of the sciatic nerve. Pain-related behaviors, including cold and tactile allodynia and thermal and tactile hyperalgesia, were evaluated after CCI in the affected hind paw. When 33G10.17 and 33V1 cells were transplanted in the lumbar subarachnoid space of the spinal cord 1 week after CCI, they survived greater than 7 weeks on the pia mater around the spinal cord. Furthermore, the tactile and cold allodynia and tactile and thermal hyperalgesia induced by CCI was significantly reduced during the 2-7-week period after grafts of 33G10.17 cells. The maximal effect on chronic pain behaviors with the GABAergic grafts occurred 2-3 weeks after transplantation. Transplants of 33V1 control cells had no effect on the allodynia and hyperalgesia induced by CCI. These data suggest that a chronically applied, low local dose of GABA presumably supplied by transplanted cells near the spinal dorsal horn was able to reverse the development of chronic neuropathic pain following CCI. The use of neural cell lines that are able to deliver inhibitory neurotransmitters, such as GABA, in a model of chronic pain offers a novel approach to pain management.

The delayed depolarization in rat cutaneous afferent axons is reduced following nerve transection and ligation, but not crush: implications for injury-induced axonal Na+ channel reorganization

Two distinct populations of Na+ channels (kinetically fast and slow) are present on the cell bodies and axons of cutaneous afferent neurons; the fast current is increased and the slow current reduced in amplitude following nerve injury. The present study was undertaken to determine if similar changes occur on the axons of these neurons following peripheral nerve injury. The compound action potentials from rat sural nerves were recorded in a sucrose gap chamber. Following application of 4-aminopyridine, a prominent and well-characterized depolarization (the delayed depolarization) followed the action potential. This potential, only present on cutaneous afferent axons, has been correlated with activation of a slow Na+ current. The delayed depolarization was reduced after nerve transection. The refractory period of transmission of the action potential was shortened in the transected nerves, but that of the delayed depolarization was prolonged. The changes were largest when the sural nerve was cut and ligated [control: 38.1 +/- 1.7% (n = 5); injury: 24.5 +/- 2.8% (n = 5), P < 0.05], which prevented reconnection to its peripheral target. When the nerve was crushed and allowed to reestablish peripheral target connections, the delayed depolarization was minimally effected. These results indicate that the changes in Na+ channel organization following peripheral target disconnection observed on cutaneous afferent cell bodies also occur on their axons.

Somatotopic redistribution of c-fos expressing neurons in the superficial dorsal horn after peripheral nerve injury

The functional somatotopic reorganization of the lumbar spinal cord dorsal horn after nerve injury was studied in the rat by mapping the stimulus-evoked distribution of neurons expressing proto-oncogene c-fos. In three different nerve injury paradigms, the saphenous nerve was electrically stimulated at C-fibre strength at survival times ranging from 40 h to more than six months: 1) Saphenous nerve stimulation from three weeks onwards after ipsilateral sciatic nerve transection resulted in an increase in the number of Fos-immunoreactive neurons within the dorsal horn saphenous territory in laminae I-II, and an expansion of the saphenous territory into the denervated sciatic territory until 14 weeks postinjury. 2) Saphenous nerve stimulation from five days onwards after ipsilateral sciatic nerve section combined with saphenous nerve crush resulted in an increase in the number of Fos-immunoreactive neurons within the dorsal horn saphenous nerve territory, and an expansion of the saphenous nerve territory into the denervated sciatic nerve territory. 3) Stimulation of the crushed nerve (without previous adjacent nerve section) at five days, but not at eight months resulted in a temporary increase in the number of Fos-immunoreactive neurons within the territory of the injured nerve, and no change in area at either survival time. The results indicate that nerve injury results in an increased capacity of afferents in an adjacent uninjured, or regenerating nerve, to excite neurons both in its own and in the territory of the permanently injured nerve in the dorsal horn. The onset and duration of the increased postsynaptic excitability and expansion depends on the types of nerve injuries involved. These findings indicate the complexity of the central changes that follows in nerve injuries that contain a mixture of uninjured, regenerating and permanently destroyed afferents.

Evidence for physiologically active axonal adenosine receptors in the rat corpus callosum

Several neurotransmitter receptors have been identified on axons, and emerging evidence suggests that central axonal conduction may be modulated by neurotransmitters. We have recently demonstrated the presence of extra-synaptic adenosine Al receptors along rat hippocampal axons. We now present immunocytochemical evidence for Al receptors on rat corpus callosum axons and show that these receptors actively modulate axon physiology. Using rat brain coronal slices, we stimulated the corpus callosum and recorded the evoked extracellular compound action potential. The lipid-soluble, Al-specific adenosine receptor agonist cyclopentyladenosine, dose-dependently decreased the compound action potential amplitude, an effect reversed by the specific Al antagonist 8-cyclopentyl-1, 3-dipropylxanthine. These data provide the first direct evidence that axonal Al adenosine receptors modulate axon physiology in the adult mammalian brain. Influencing axonal transmission is a potentially powerful mechanism of altering information processing in the nervous system.

Differential effects of NGF and BDNF on axotomy-induced changes in GABA(A)-receptor-mediated conductance and sodium currents in cutaneous afferent neurons

The effects of nerve growth factor (NGF) and brain-derived neurotrophic factor (BDNF) on injury-induced changes in the electrophysiological properties of adult rat cutaneous afferent dorsal root ganglion (DRG) neurons were examined. Whole cell patch-clamp techniques were used to study gamma-aminobutyric acid-A (GABA(A))-receptor-mediated conductance, voltage-dependent sodium currents, and action potential waveform in cutaneous afferent neurons (35-60 microm diam) cultured from control and axotomized animals. Cutaneous afferent neurons were identified by retrograde labeling with hydroxy-stilbamidine (Fluoro-gold, a fluorescent retrograde axonal tracer); the sciatic nerve was transected 1 wk after Fluoro-gold injection and L4/L5 DRG neurons were cultured 2-3 wk after axotomy. NGF, BDNF, or Ringer (vehicle) solution was delivered in vivo directly to the transected sciatic nerve stump in axotomized rats via an osmotic pump. Recordings were obtained from neurons 5-24 h after culture. Axotomized neurons from rats treated with vehicle solution displayed a twofold increase in GABA-induced conductance and a prominent reduction in the proportion of neurons expressing action potentials that had inflections on the falling phase. The expression of kinetically slow tetrodotoxin (TTX)-resistant sodium current was markedly reduced and an increased expression of kinetically fast TTX-sensitive current was observed in neurons from vehicle-treated, axotomized rats. Treatment with NGF (0.25 microg/microl at 12 microl/day for 14 days) in axotomized animals resulted in an increase in the proportion of neurons expressing TTX-resistant, slow sodium currents and inflected action potentials, but had no effect on GABA-induced conductance. Treatment with BDNF (0.5 microg/microl at 12 microl/day for 14 days) attenuated the axotomy-induced increase in GABA(A)-receptor-mediated conductance while minimally affecting action potential waveform. The observed neurotrophin effects occurred independently of cell size changes. These findings indicate a differential regulation of GABA(A) receptor and sodium channel properties in axotomized rat cutaneous afferent neurons by specific neurotrophic factors.

GABA and its receptors in the spinal cord

The importance of the inhibitory neurotransmitter, GABA, within higher centres of the mammalian brain is unquestionable. However, its role within the spinal cord is of equal significance. There have been numerous studies over the past two decades that have established GABA as a neurotransmitter at both post- and presynaptic sites in the cord. Here, Marzia Malcangio and Norman Bowery review the current status of GABA in relation to nociception and skeletal muscle tone, and indicate that its contribution to spinal cord function should not be overlooked.

GABAA-receptor-mediated conductance and action potential waveform in cutaneous and muscle afferent neurons of the adult rat: differential expression and response to nerve injury

1. Whole cell patch-clamp recordings were obtained from identified cutaneous and muscle afferent neurons (33-60 microns diam) in dissociated L4 and L5 dorsal root ganglia (DRGs) from normal rats and from rats 2-3 wk after sciatic nerve ligation or crush injury. gamma-Aminobutyric acid (GABA)-induced conductance was compared in normal and injured neurons from both functional classes of sensory neurons. 2. Control cutaneous afferent neurons had a peak GABA-mediated conductance of 287 +/- 27 (SE) nS compared with 457 +/- 42 nS for control muscle afferent neurons. 3. An inflection on the downslope of the action potential was observed in 47% of cutaneous afferent neurons compared with 20% of muscle afferent neurons. 4. After ligation and transection of the sciatic nerve there was no change in the GABA-mediated conductance of muscle afferent neurons or in the action potential waveform (23% inflected). However, the cutaneous afferent neurons displayed a greater than two-fold increase in their GABA-mediated conductance and displayed a prominent reduction in the number of neurons with inflected action potentials (13% inflected). Input resistance was similar in cutaneous and muscle afferent neurons and decreased after ligation in cutaneous but not muscle afferents. Resting potential averaged from -50 to -56 mV in normal and ligated groups for both cutaneous and muscle afferent neurons. 5. After crush injury in cutaneous afferent neurons where the transected axons were allowed to regenerate into the distal nerve stump, GABAA-receptor-mediated conductance was elevated compared with controls. However, action potential waveform was not altered by crush injury, suggesting a differential regulation of these two properties in cutaneous afferent neurons. 6. These data indicate that injury-induced plasticity of GABAA-receptor-mediated conductance and action potential waveform occurs in cutaneous but not muscle afferent DRG neurons. It appears that peripherally derived influences are critical in maintaining the electrophysiological phenotype of cutaneous afferent neurons but not muscle afferent neurons.

Enhancement of GABAA receptor-mediated conductances induced by nerve injury in a subclass of sensory neurons

1. The effects of axotomy on the electrophysiologic properties of adult rat dorsal root ganglion (DRG) neurons were studied to understand the changes in excitability induced by traumatic nerve injury. Nerve injury was induced in vivo by sciatic nerve ligation with distal nerve transection. Two to four weeks after nerve ligation, a time when a neuroma forms, lumbar (L4 and L5) DRG neurons were removed and placed in short-term tissue culture. Whole cell patch-clamp recordings were made 5-24 h after plating. 2. DRG neurons were grouped into large (43-65 microns)-, medium (34-42 microns)-, and small (20-32 microns)- sized classes. Large neurons had short duration action potentials with approximately 60% having inflections on the falling phase of their action potentials. In contrast, action potentials of medium and small neurons were longer in duration and approximately 68% had inflections. 3. Pressure microejection of gamma-aminobutyric acid (GABA, 100 microM) or muscimol (100 microM) onto voltage-clamped DRG neurons elicited a rapidly desensitizing inward current that was blocked by 200 microM bicuculline. To measure the peak conductance induced by GABA or muscimol, neurons were voltage-clamped at a holding potential of -60 mV, and pulses to -80 mV and -100 mV were applied at a rate of 2.5 or 5 Hz during drug application. Slope conductances were calculated from plots of whole cell current measured at each of these potentials. 4. GABA-induced currents and conductances of control DRG neurons increased progressively with cell diameter. The mean GABA conductance was 36 +/- 10 nS (mean +/- SE) in small neurons, 124 +/- 21 nS in medium neurons, and 527 +/- 65 nS in large neurons. 5. After axotomy, medium neurons had significantly larger GABA-induced conductances compared with medium control neurons (390 +/- 50 vs. 124 +/- 21; P < 0.001). The increase in GABA conductance of medium neurons was associated with a decrease in duration of action potentials. In contrast, small neurons had no change in GABA conductance or action potential duration after ligation. The GABA conductance of large control neurons was highly variable, and ligation resulted in an increase that was significant only for neurons > 50 microns. The mean action potential duration in large neurons was not significantly changed, but neurons with inflections on the falling phase of the action potential were less common after ligation. There was no difference in resting potential or input resistance between control and ligated groups, except that the resting potential was less negative in small cells after axotomy.(ABSTRACT TRUNCATED AT 400 WORDS)

The resolution of neuropathic hyperalgesia following motor and sensory functional recovery in sciatic axonotmetic mononeuropathies

Nerve lesions producing extensive axonal loss can induce painful hyperalgesic states in man. The affect of axonal regeneration and end-organ reinnervation on hyperalgesia and pain is controversial. This study used two axonotmetic models, the sciatic crush injury (CI) and the sciatic chronic constrictive injury (CCI), to investigate the affects of nerve regeneration and reinnervation on hyperalgesia and presumed painful behavior in rats. The sciatic CI resulted in a transient loss of both sciatic motor function and the withdrawal response to pinch and heat in the sciatic distribution. Extensive recovery of motor function, pinch and heat response occurred over days 23-38 post-crush injury. This temporally corresponded with a plateau in the hindpaw autotomy score and a resolution of the saphenous-mediated pressure and heat hyperalgesia (adjacent neuropathic hyperalgesia; ANH) which developed over the medial dorsum of the hindpaw following the sciatic CI. In contrast, with sciatic transection and distal stump excision, no motor recovery occurs, large areas of the hindpaw remain unresponsive to heat and pinch, and the saphenous mediated ANH fails to resolve over a period of 3 months. When sciatic CI was compared to contralateral sciatic transection within the same rat, the bilateral saphenous-mediated pressure and heat thresholds were initially identical, but by 23-27 days post-crush, the crush side thresholds became hypoalgesic relative to the section side. This demonstrates an attenuation of the crush-induced ANH which temporally corresponds to the recovery of motor and sensory function. When the sciatic nerve was proximally crushed and distally transected (3 cm below the crush site), the saphenous-mediated pressure and heat threshold changes were identical (over 6 weeks of serial testing) to those produced by a contralateral sciatic transection within the same rat. This indicates that the microenvironments surrounding the regenerating axon tips did not differentially affect the development of ANH following sciatic CI or transection. The sciatic CCI resulted in a transient loss of hindpaw motor function without the loss of pinch or heat withdrawal responses in the sciatic distribution. Motor function recovery occurred primarily over days 23-59 post-ligature. During this prolonged period of motor function recovery there was a resolution of the sciatic-mediated plantar surface heat hyperalgesia and the saphenous-mediated heat ANH. The above data support the hypothesis that the successful regeneration of distal axons after axonotmetic lesions can initiate the resolution of neuropathic hyperalgesia.

Presynaptic modulation of spinal reflexes

Recent evidence suggests that independent sets of interneurons mediate presynaptic inhibition of primary and secondary muscle spindles and of tendon organ afferents. There is also evidence that the information which flows through different intraspinal collaterals of a single muscle spindle or tendon organ afferent fiber is selectively affected by electrical stimulation of the motor cortex. These studies suggest that presynaptic inhibition plays an important role in the selection of the sensory signals required for the execution of a specific motor task.

Dorsal root potentials and afferent input to the spinal cord in rats with an experimental peripheral neuropathy

Compound action potentials (CAPs), dorsal root potentials (DRPs) and cord dorsum potentials evoked by stimulating the sciatic nerves have been measured in 4 control rats and in 19 rats with a constriction injury of one sciatic nerve produced by loose ligation of the nerve at mid-thigh level 5 days (n = 8) or 10 days (n = 11) before the acute experiments. The contralateral nerve was exposed but not ligated in a sham procedure. In all cases, the nerve was stimulated proximal to the lesion. At 5 days post-operative (PO) the maximal A-fibre CAPs on the nerve-injured side were not significantly different from those on the sham-operated side. At 10 days PO all animals showed a decrease in the CAP on the nerve-injured side. The mean CAP area on the nerve-injured side was 74.0% +/- 4.2 of the sham-operated side, which was significantly different (P less than 0.005). The sciatic nerves and L5 dorsal roots from 4 of the 10 day PO animals were examined histologically and showed no signs of demyelination or degeneration. The amplitude and area of the maximal DRPs were significantly smaller on the nerve-injured side than on the sham-operated side in all of the nerve-injured animals (P less than 0.01 at 5 days PO; P less than 0.05 at 10 days PO). The mean area of DRPs from the nerve-injured side was 61.7% +/- 10.1 and 46.8% +/- 7.5 of the DRPs from the sham-operated side in the 5 and 10 day PO animals, respectively. The DRPs evoked by sub-maximal afferent volleys were also measured. In all of the nerve-injured animals the CAP-DRP curve on the nerve-injured side was shifted to the right compared to that of the sham-operated side, such that a given size of CAP evoked a smaller DRP on the nerve-injured side than on the sham-operated side. We conclude that the constriction injury produces a decrease in the DRP generated by a volley in the injured nerve and that this change is independent of the decrease in the CAP seen in the injured nerve. We propose that the constriction injury affects the central mechanism responsible for generating primary afferent depolarization (PAD), and thus the pre-synaptic inhibitory control of the afferent input from the injured nerve is impaired.

Effects of nerve growth factor on TTX- and capsaicin-sensitivity in adult rat sensory neurons

We have investigated the effects of nerve growth factor (NGF, 2.5 ng/ml for 1-2 weeks) on enriched adult rat dorsal root ganglion (DRG) neurons maintained in cell culture in defined media. Whole-cell recordings in cells cultured in the absence and presence of NGF revealed no significant difference in resting membrane potential and input resistance. However, the threshold for spike generation was significantly lower in untreated cells than in treated cells; -25 +/- 1.1 mV vs -19 +/- 2.2 mV, respectively. The sensitivity of the Na+ spike to tetrodotoxin (TTX, 1 microM) was different in cells cultured in the absence or presence of NGF. For example, spikes were abolished by TTX in 100% of untreated cells, while in NGF-treated cells the spike was abolished in only 41% of the neurons. Chemosensitivity of DRG neurons was also different in the absence and presence of NGF. For example, the percent of neurons in which a current activated by 8-methyl-N-vanillyl-6-nonenamide (capsaicin, 500 nM) was detected, increased from 18% in untreated cells to 55% in NGF-treated cells. NGF did not influence the number of cells surviving. The results indicate that NGF can regulate TTX and capsaicin sensitivity in these adult rat sensory neurons. Our experimental protocol indicates that this effect is not mediated by a factor in the serum or released from non-neuronal cells.

Trophic influence of the distal nerve segment on GABAA receptor expression in axotomized adult sensory neurons

The depolarizing action of gamma-aminobutyric acid (GABA), or the GABAA receptor agonist muscimol, on rat dorsal root (L4 and L5) fibers is attenuated following transection, but not crush, of the sciatic nerve. Following discrete nerve crush, axons actively regenerate and contact both the distal nerve segment and the peripheral target tissues. The aim of the present study was to distinguish between these two regions as possible sources of trophic support for retrograde maintenance of dorsal root GABA receptor sensitivity. A surgical procedure was employed to permit a delimited segment of axonal regeneration while prohibiting reestablishment of end organ innervation; the sciatic nerve was crushed and a ligature was placed 3 cm distal to the crush site. Under these conditions, the injury-induced decrement in the dorsal root GABA response, observed between 12 and 21 postoperative days, was significantly attenuated relative to that of ligated nerves, in which regeneration into the distal stump does not occur. The data suggest that nerve transection by ligation restricts trophic support for maintenance of GABA receptor expression in dorsal root ganglion (DRG) neurons. Furthermore, during regeneration the denervated distal nerve segment assumes a neurotrophic role in the maintenance of dorsal root GABA sensitivity, consistent with the hypothesis that growth factors derived from reactive Schwann cells may positively regulate the expression of receptors on axotomized sensory neurons.

The development of chronic mechanical hyperalgesia, autotomy and collateral sprouting following sciatic nerve section in rat

The development of mechanical hyperalgesia over the tarsometatarsal joints subsequent to chronic sciatic section was studied in rats. Mechanical thresholds began to drop within 7 days of sciatic section and continued to decline for 2-3 weeks, resulting in significant hyperalgesia over the original saphenous and sciatic border zones lasting up to 310 days. Contralateral hyperalgesia, which is seen in tissue injury mechanical hyperalgesia, failed to develop. Collateral sprouting of high-threshold mechanoreceptors from the saphenous nerve into the denervated deep and cutaneous tissue was observed. The extent of cutaneous sprouting did not significantly correlate with the severity of hyperalgesia. The simultaneous uniform reduction of mechanical thresholds across all tarsometatarsal areas after chronic sciatic section is consistent with a central modulation of nociceptive afferent input. The severity and temporal development of the hyperalgesia strongly correlated with the extent of autotomy behavior. This animal model of chronic mechanical hyperalgesia following peripheral nerve section closely resembles a human neuropathic pain syndrome and may provide an experimental opportunity to advance our understanding of the neuropathology subserving chronic pain.

Morphological and physiological properties of neurons after long-term axonal regeneration: observations on chronic and delayed sequelae of peripheral nerve injury

Axonal regeneration has been the focus of extensive investigation of mechanisms which mediate structural and functional recovery after injury to mammalian peripheral nerves and has proven to be a valuable model for development and plasticity in the nervous system. Although details of the acute morphological and physiological responses to nerve injury are well-described, less information is available to nerve injury are well-described, less information is available about long-term alterations which persist or develop after regenerated axons have established connections with their targets. The present paper briefly discusses the mammalian neuron's initial response to peripheral nerve injury and subsequent events which occur during regeneration. Morphological and physiological alterations observed in neurons after long-term axonal regeneration are described and are considered in the context of their potential implications for clinical recovery after nerve injury, as well as their potential contribution to the appearance of delayed neurological dysfunction. Selective responses to neuronal injury during development and in different fiber populations are discussed.