Descending facilitation from the rostral ventromedial medulla maintains nerve injury-induced central sensitization

Nerve injury can produce hypersensitivity to noxious and normally innocuous stimulation. Injury-induced central (i.e. spinal) sensitization is thought to arise from enhanced afferent input to the spinal cord and to be critical for expression of behavioral hypersensitivity. Descending facilitatory influences from the rostral ventromedial medulla have been suggested to also be critical for the maintenance, though not the initiation, of experimental neuropathic pain. The possibility that descending facilitation from the rostral ventromedial medulla is required for the maintenance of central sensitization was examined by determining whether ablation of mu-opioid receptor-expressing cells within the rostral ventromedial medulla prevented the enhanced expression of repetitive touch-evoked FOS within the spinal cord of animals with spinal nerve ligation injury as well as nerve injury-induced behavioral hypersensitivity. Rats received a single microinjection of vehicle, saporin, dermorphin or dermorphin-saporin into the rostral ventromedial medulla and 28 days later, underwent either sham or spinal nerve ligation procedures. Animals receiving rostral ventromedial medulla pretreatment with vehicle, dermorphin or saporin that were subjected to spinal nerve ligation demonstrated both thermal and tactile hypersensitivity, and showed significantly increased expression of touch-evoked FOS in the dorsal horn ipsilateral to nerve injury compared with sham-operated controls at days 3, 5 or 10 post-spinal nerve ligation. In contrast, nerve-injured animals pretreated with dermorphin-saporin showed enhanced behaviors and touch-evoked FOS expression in the spinal dorsal horn at day 3, but not days 5 and 10, post-spinal nerve ligation when compared with sham-operated controls. These results indicate the presence of nerve injury-induced behavioral hypersensitivity associated with nerve injury-induced central sensitization. Further, the results demonstrate the novel concept that once initiated, maintenance of nerve injury-induced central sensitization in the spinal dorsal horn requires descending pain facilitation mechanisms arising from the rostral ventromedial medulla.

Best practices in EMR implementation: a systematic review

As experience with Electronic Medical Record (EMR) implementations increases, new knowledge is gained on how to make these implementations more successful. Recently, several new conceptual frameworks described in the literature provide a richer understanding of what makes an EMR implementation successful. Using the systematic review process, we attempt to integrate the various frameworks into an over-arching framework that is comprehensive, yet pragmatic.

Quantitative measurement of neurodegeneration in an ALS–PDC model using MR microscopy

Exposure to cycad (Cycas micronesica K.D. Hill) toxins via diet has been shown to induce neurodegeneration in vivo that mimics the progressive neurological disease, amyotrophic lateral sclerosis--parkinsonism dementia complex (ALS--PDC). In previous studies, specific cortical and subcortical cell loss was measured with conventional stained sections. In the present study, magnetic resonance (MR) microscopy was used to examine neurodegeneration in three dimensions (3D) in isolated intact brains and spinal cords. Mice were fed washed cycad for 2 months and showed progressive motor deficits resembling human ALS--PDC. CNS tissue was imaged at 17.6 T. T2* scans were acquired on both spinal cord and brain samples with an isotropic resolution of 41 microm. Through MR volumetrics, cycad-fed mice showed significantly decreased volumes in lumbar spinal cord gray matter, substantia nigra, striatum, basal nucleus/internal capsule, and olfactory bulb. Cortical measurements of conventionally stained sections revealed that cycad-fed mice also showed decreased cortical thickness. These results show that MR microscopy (MRM) is sensitive enough to measure degeneration in this early stage model of a progressive neurological disease with strong correlations to behavioral deficits and histological results and may be applicable in vivo to the same model. Similar analysis may be used in the future as a diagnostic aid in tracking the early progression of neurological disorders in preclinical human subjects.

Mouse strains that lack spinal dynorphin upregulation after peripheral nerve injury do not develop neuropathic pain

Several experimental models of peripheral neuropathy show that a significant upregulation of spinal dynorphin A and its precursor peptide, prodynorphin, is a common consequence of nerve injury. A genetically modified mouse strain lacking prodynorphin does not exhibit sustained neuropathic pain after nerve injury, supporting a pronociceptive role of elevated levels of spinal dynorphin. A null mutation of the gamma isoform of protein kinase C (PKCgamma KO [knockout]), as well as an inbred mouse strain, 129S6, also does not manifest behavioral signs of neuropathic pain following peripheral nerve injury. The objective of this study was to extend our observations to these genetic models to test the hypothesis that elevated levels of spinal dynorphin are essential for the maintenance of abnormal pain. In PKCgamma wild-type mice and the outbred mouse strain ICR, ligation of the L5 and L6 spinal nerves (SNL) elicited both tactile hypersensitivity and thermal hyperalgesia. Both strains showed a significant elevation in dynorphin in the lumbar spinal dorsal horn following SNL. Spinal administration of an anti-dynorphin A antiserum blocked the thermal and tactile hypersensitivity in both strains of mice. However, the PKCgamma KO mice and the 129S6 mice (which express PKCgamma) did not show abnormal pain after SNL; neither strain showed elevated levels of spinal dynorphin. The multiple phenotypic deficits in PKCgamma KO mice confound the interpretation of the proposed role of PKCgamma-expressing spinal neurons in neuropathic pain states. Additionally, the data show that the regulation of spinal dynorphin expression is a common critical feature of expression of neuropathic pain.

Allodynia and hyperalgesia produced by specific inhibition of spinal c-fos expression: lack of correlation with dynorphin content

Inhibition of spinal Fos expression increases formalin-induced nociception and decreases spinal prodynorphin messenger ribonucleic acid (mRNA), suggesting that Fos modulates nociception by inducing dynorphin synthesis. This study tests the hypothesis that Fos modulates sensitivity to other somatic stimuli, such that inhibition of Fos expression will result in tactile allodynia and thermal hyperalgesia. In addition, it correlates the somatosensory effects of inhibition of Fos expression with spinal dynorphin content. Antisense oligodeoxynucleotide (ODN) to c-fos mRNA was administered by intrathecal infusion. Tactile sensitivity was tested by probing the hindpaw with von Frey filaments. Thermal sensitivity was quantitated by using withdrawal latency to radiant heat. Two percent formalin was injected into the dorsal hindpaw, and flinches were quantitated. Fos was quantitated by counting immunoreactive cells. Dynorphin was measured by immunoassay. Intrathecal antisense, but not mismatch, ODN resulted in tactile allodynia, thermal hyperalgesia, and hyperalgesia to formalin-induced nociception. Antisense ODN decreased Fos-like immunoreactivity after formalin injection but did not alter Jun-like immunoreactivity. Antisense ODN had differing effects on spinal dynorphin content, depending on the method of administration. These experiments show a role of Fos in modulating somatosensory sensitivity and suggest that induction of dynorphin synthesis is not the sole mechanism by which Fos does so.

Glial cell line-derived neurotrophic factor normalizes neurochemical changes in injured dorsal root ganglion neurons and prevents the expression of experimental neuropathic pain

Glial cell line-derived neurotrophic factor (GDNF) is necessary for the development of sensory neurons, and appears to be critical for the survival of dorsal root ganglion (DRG) cells that bind the lectin IB4. Intrathecal infusion of GDNF has been shown to prevent and reverse the behavioral expression of experimental neuropathic pain arising from injury to spinal nerves. This effect of GDNF has been attributed to a blockade of the expression of the voltage gated, tetrodotoxin-sensitive sodium channel subtype, Na(V)1.3, in the injured DRG. Here we report that GDNF given intrathecally via osmotic-pump to nerve-injured rats (L5/L6 spinal nerve ligation) prevented the changes in a variety of neurochemical markers in the DRG upon injury. They include a loss of binding of IB4, downregulation of the purinergic receptor P2X(3), upregulation of galanin and neuropeptide Y immunoreactivity in large diameter DRG cells, and expression of the transcription factor ATF3. GDNF infusion concomitantly prevented the development of spinal nerve ligation-induced tactile hypersensitivity and thermal hyperalgesia. These observations suggest that high dose, exogenous GDNF has a broad neuroprotective role in injured primary afferent. The receptor(s) that mediates these effects of GDNF is not known. GDNF's ability to block neuropathic pain states is not likely to be specific to Na(V)1.3 expression.

Enhanced evoked excitatory transmitter release in experimental neuropathy requires descending facilitation

Nerve injury-induced afferent discharge is thought to elicit spinal sensitization and consequent abnormal pain. Experimental neuropathic pain, however, also depends on central changes, including descending facilitation arising from the rostral ventromedial medulla (RVM) and upregulation of spinal dynorphin. A possible intersection of these influences at the spinal level was explored by measuring evoked, excitatory transmitter release in tissues taken from nerve-injured animals with or without previous manipulation of descending modulatory systems. Spinal nerve ligation (SNL) produced expected tactile and thermal hyperesthesias. Capsaicin-evoked calcitonin gene-related peptide (CGRP) release was markedly enhanced in lumbar spinal tissue from SNL rats when compared with sham-operated controls. Enhanced, evoked CGRP release from SNL rats was blocked by anti-dynorphin A(1-13) antiserum; this treatment did not alter evoked release in tissues from sham-operated rats. Dorsolateral funiculus lesion (DLF) or destruction of RVM neurons expressing mu-opioid receptors with dermorphin-saporin, blocked tactile and thermal hypersensitivity, as well as SNL-induced upregulation of spinal dynorphin. Spinal tissues from these DLF-lesioned or dermorphin-saporin-treated SNL rats did not exhibit enhanced capsaicin-evoked CGRP-IR release. These data demonstrate exaggerated release of excitatory transmitter from primary afferents after injury to peripheral nerves, supporting the likely importance of increased afferent input as a driving force of neuropathic pain. The data also show that modulatory influences of descending facilitation are required for enhanced evoked transmitter release after nerve injury. Thus, convergence of descending modulation, spinal plasticity, and afferent drive in the nerve-injured state reveals a mechanism by which some aspects of nerve injury-induced hyperesthesias may occur.

Design of novel peptide ligands which have opioid agonist activity and CCK antagonist activity for the treatment of pain

Disease states such as neuropathic pain offer special challenges in drug design due to the system changes which accompany these diseases. In this manuscript we provide an example of a new approach to drug design in which we have modified a potent and selective peptide ligand for the CCK-2 receptor to a peptide which has potent agonist binding affinity and bioactivity at delta and mu opioid receptors, and simultaneous antagonist activity at CCK receptors. De novo design based on the concept of overlapping pharmacophores was a central hypothesis of this design, and led to compounds such as H-Tyr-DPhe-Gly-DTrp-NMeNle-Asp-Phe-NH(2) (i.e., RSA 601) which have the designed properties.

Analysis of the candidate 8p21 tumour suppressor, BNIP3L, in breast and ovarian cancer

Loss of heterozygosity (LOH) on the short arm of chromosome 8, at 8p12-p23, is one of the most frequent genetic events in both breast and ovarian cancer, suggesting the location of a shared tumour suppressor gene. Microcell-mediated chromosome transfer of chromosome 8 suppresses tumorigenicity and growth of colorectal and prostate cancer cell lines, further supporting the presence of a tumour suppressor gene on 8p. We have taken a candidate gene approach to try to identify this tumour suppressor gene at 8p12-p23. BNIP3L, which has sequence homology to pro-apoptotic proteins and the ability to suppress colony formation in soft agar, is located at 8p21, within a region of ovarian cancer LOH, breast cancer LOH and prostate cancer metastasis suppression. BNIP3L expression was assessed by both RT-PCR and Northern blot analysis in breast and ovarian cancer cell lines and found to be expressed at similar levels relative to expression in their respective normal epithelial cell lines. Genetic analysis of BNIP3L in 40 primary ovarian and 25 primary breast tumours identified one somatic, intronic mutation in one ovarian tumour, as well as several polymorphisms, including one resulting in an amino-acid substitution. These data suggest that BNIP3L is unlikely to be the target of 8p LOH in ovarian or breast cancer.

Dynorphin-independent spinal cannabinoid antinociception

Spinal antinociception produced by delta 9-tetrahydro-cannabinol (Delta(9)-THC) and other cannabinoid agonists has been suggested to be mediated by the release of dynorphin acting at the kappa opioid receptor. Alternatively, as cannabinoid receptors are distributed appropriately in the pain transmission pathway, cannabinoid agonists might act directly at the spinal level to inhibit nociception, without requiring dynorphin release. Here, these possibilities were explored using mice with a deletion of the gene encoding prodynorphin. Antinociceptive dose-response curves were constructed for spinal Delta(9)-THC and WIN 55,212-2 in prodynorphin knock-out mice and in wild-type littermates. WIN 55,212-2 and Delta(9)-THC were equipotent in the wild-type and prodynorphin knock-out mice. Spinal pretreatment with a kappa opioid receptor antagonist, nor-binaltorphimine (nor-BNI), did not alter the dose-response curves for either WIN 55,212-2 or Delta(9)-THC in prodynorphin knock-out and wild-type mice. However, the same dose of nor-BNI used blocked U50,488H-induced antinociception in both wild-type and prodynorphin knock-out mice, confirming kappa opioid receptor activity. Pretreatment with SR141716A, a cannabinoid receptor antagonist blocked the antinociceptive actions of both WIN 55,212-2 and Delta(9)-THC. These data support the conclusion that antinociception produced by spinal cannabinoids are likely to be mediated directly through activation of cannabinoid receptors without the requirement for dynorphin release or activation of kappa opioid receptors.

Pronociceptive effects of spinal dynorphin promote cannabinoid-induced pain and antinociceptive tolerance

Recent studies indicate that sustained opioid administration produces increased expression of spinal dynorphin, which promotes enhanced sensitivity to non-noxious and noxious stimuli. Such increased "pain" may manifest behaviorally as a decrease in spinal antinociceptive potency. Here, the possibility of similar mechanisms in the antinociception of spinal cannabinoids was explored. Response thresholds to non-noxious mechanical and noxious thermal stimuli were assessed. Antinociception was determined using the 52 degrees C tail-flick test. Mice received repeated WIN 55,212-2, its inactive enantiomer, WIN 55,212-3 or vehicle (i.th., bid, 5 days). WIN 55,212-2, but not WIN 55,212-3 or vehicle, produced a time-related increased sensitivity to non-noxious and noxious stimuli. WIN 55,212-2, but not WIN 55,212-3 or vehicle, elicited a significant increase in lumbar spinal dynorphin content at treatment day 5. Increased sensitivity to mechanical and thermal stimuli produced by WIN 55,212-2 was reversed to baseline levels by i.th. MK-801 or dynorphin antiserum; control serum had no effect. WIN 55,212-2, but not WIN 55,212-3 or vehicle, produced dose-related antinociception and repeated administration resulted in antinociceptive tolerance. While MK-801 and dynorphin antiserum did not alter acute antinociception produced by WIN 55,212-2, these substances significantly blocked antinociceptive tolerance when given immediately prior to WIN 55,212-2 challenge on day 5. Daily MK-801 pretreatments, prior to WIN 55,212-2 injection, also produced a significant block of antinociceptive tolerance. These data suggest that like opioids, repeated spinal administration of a cannabinoid CB1 agonist elicits abnormal pain, which results in increased expression of spinal dynorphin. Manipulations that block cannabinoid-induced pain also block the behavioral manifestation of cannabinoid tolerance.

CYP gene polymorphisms and early menarche

Early age at menarche is a risk factor for breast cancer. A previous study reported a significant positive association between the CYP3A4*1B variant allele and early puberty. We investigated whether polymorphisms of the CYP3A4, CYP17, CYP1B1, and CYP1A2 genes predict the age at onset of menarche. Five hundred eighty-three nulliparous women between ages 17 and 35, of various ethnic backgrounds, completed a questionnaire that included information about menstrual history. Samples of DNA were provided and used to genotype these women for polymorphic variants in the four genes. There was no significant difference in mean age at menarche between women who carried two variant CYP17 A2 alleles (12.5 years) and women who carried one or no variant allele (12.5 years) (P = 0.8, adjusted for ethnic group and year of birth). Similar results were found for the CYP1B1*3 variant allele and for the CYP1A2*1F variant allele. Women who carried two variant CYP3A4*1B alleles had an earlier mean age at menarche (12.0 years) than women who carried one or no variant allele (12.6 years) (P = 0.02). However, after adjusting for ethnic group and year of birth, no significant differences in mean age at menarche were found. The polymorphic variants of the CYP3A4, CYP17, CYP1B1, and CYP1A2 genes are unlikely to influence age of menarche.

RT-PCR reveals muscarinic acetylcholine receptor mRNA in the pre-Bötzinger complex

Muscarinic receptors mediate the postsynaptic excitatory effects of acetylcholine (ACh) on inspiratory neurons in the pre-Bötzinger complex (pre-BötC), the hypothesized site for respiratory rhythm generation. Because pharmacological tools for identifying the subtypes of the muscarinic receptors that underlie these effects are limited, we probed for mRNA for these receptors in the pre-BötC. We used RT-PCR to determine the expression of muscarinic receptor subtypes in tissue punches of the pre-BötC taken from rat medullary slices. Cholinergic receptor subtype M(2) and M(3) mRNAs were observed in the first round of PCR amplification. All five subtypes, M(1)-M(5), were observed in the second round of amplification. Our results suggest that the majority of muscarinic receptor subtypes in the pre-BötC are M(2) and M(3), with minor expression of M(1), M(4), and M(5).

Distinct potassium channels on pain-sensing neurons

Differential expression of ion channels contributes functional diversity to sensory neuron signaling. We find nerve injury induced by the Chung model of neuropathic pain leads to striking reductions in voltage-gated K(+) (Kv) channel subunit expression in dorsal root ganglia (DRG) neurons, suggesting a potential molecular mechanism for hyperexcitability of injured nerves. Moreover, specific classes of DRG neurons express distinct Kv channel subunit combinations. Importantly, Kv1.4 is the sole Kv1 alpha subunit expressed in smaller diameter neurons, suggesting that homomeric Kv1.4 channels predominate in A delta and C fibers arising from these cells. These neurons are presumably nociceptors, because they also express the VR-1 capsaicin receptor, calcitonin gene-related peptide, and/or Na(+) channel SNS/PN3/Nav1.8. In contrast, larger diameter neurons associated with mechanoreception and proprioception express high levels of Kv1.1 and Kv1.2 without Kv1.4 or other Kv1 alpha subunits, suggesting that heteromers of these subunits predominate on large, myelinated afferent axons that extend from these cells.

Effect of photodynamic therapy (PDT) on the expression of pro-apoptotic protein Bak in nasopharyngeal carcinoma (NPC)

BACKGROUND AND OBJECTIVE

To investigate the effect of photodynamic therapy (PDT) on expression of the pro-apoptotic gene Bak in nasopharyngeal carcinoma (NPC).

STUDY DESIGN/MATERIALS AND METHODS

Apoptosis and expression of the pro-apoptotic gene Bak on the tumor tissues from both pre- and post-PDT were determined using the in situ end labeling (ISEL), standard immunohistochemistry technique and western blot, respectively, in 24 patients with either persistent or recurrent NPC after radiotherapy.

RESULTS

Before PDT, apoptotic index (AI) in tumor tissue was 1.2 +/- 0.6. At 6, 12, 24 and 48 hours after PDT, AI were 6.5 +/- 3.1, 23.6 +/- 8.3, 67.2 +/- 14.2 and 89.3 +/- 8.1, respectively. PDT caused apoptosis in a time-dependent fashion. Immunohistochemical assay indicated that 75% (18/24) of the patients had an upgrade expression of Bak protein in their tumor tissues after PDT. Increases in expression of Bak from PDT were also confirmed by western blot analysis.

CONCLUSIONS

PDT probably causes NPC cell apoptosis through an upregulation of the pro-apoptotic protein Bak expression.

Muscarinic versus nicotinic modulation of a visual task. a pet study using drug probes

Little is known about acetylcholine (ACh) modulation of central visual processing in humans. Receptor densities in visual brain regions are differentially distributed suggesting that receptor subtypes have different functions. Using PET, we have previously described the brain regions activated by a simple pattern-flash stimulus in healthy elderly subjects. To evaluate muscarinic and nicotinic contributions to ACh modulation of visual processing, we scanned elderly subjects watching the pattern-flash stimulus during no drug, during physostigmine augmentation, and during scopolamine antagonism of physostigmine's action. These manipulations of ACh significantly altered regional cerebral blood flow (rCBF) in brain regions activated by the task. The pattern of rCBF values across drug conditions suggested that muscarinic and nicotinic effects were dissociated. Muscarinic action predominated in striate cortex (Brodmann Area, BA 17) and lateral visual association areas (BA 18, 19), while nicotinic action predominated in the thalamus and inferior parietal regions (BA 39/40). Both muscarinic and nicotinic actions increased rCBF in some regions while decreasing it in others. A parsimonious reconciliation of these results with functional anatomy suggests that muscarinic action modulates visual attribute processing, while nicotinic action modulates arousal and selective attention to the visual task.

Characterization and humanization of a monoclonal antibody that neutralizes human leukocyte interferon: a candidate therapeutic for IDDM and SLE

We have developed a panel of murine monoclonal antibodies that recognize human interferon alpha. One of these mononclonal antibodies binds and neutralizes, with high affinity, all of seven tested recombinant human interferon alphas. This mononclonal antibody also neutralizes the interferon activity present in two independent pools of interferon alphas prepared following stimulation of human peripheral blood leukocytes. The complementary determining regions from this murine mononclonal antibody were transferred to a human IgG2 heavy chain and to a human kappa1 light chain. In addition, six (heavy chain) and two (light chain) amino acids were transferred from the framework regions. This generated a humanized mononclonal antibody that retained the specificity of the mouse parent. The humanized anti-interferon alpha antibody is a candidate therapeutic for those diseases, such as insulin-dependent diabetes, systemic lupus erythematosis, psoriasis and Crohn's disease, which are all characterized by pathological expression of interferon alpha.

A double-blind, placebo-controlled study of the use of amphetamine in the treatment of aphasia

BACKGROUND AND PURPOSE

A number of studies suggest that drugs which increase the release of norepinephrine promote recovery when administered late (days to weeks) after brain injury in animals. A small number of clinical studies have investigated the effects of the noradrenergic agonist dextroamphetamine in patients recovering from motor deficits following stroke. To determine whether these findings extend to communication deficits subsequent to stroke, we administered dextroamphetamine, paired with speech/language therapy, to patients with aphasia.

METHODS

In a prospective, double-blind study, 21 aphasic patients with an acute nonhemorrhagic infarction were randomly assigned to receive either 10 mg dextroamphetamine or a placebo. Patients were entered between days 16 and 45 after onset and were treated on a 3-day/4-day schedule for 10 sessions. Thirty minutes after drug/placebo administration, subjects received a 1-hour session of speech/language therapy. The Porch Index of Communicative Ability was used at baseline, at 1 week off the drug, and at 6 months after onset as the dependent language measure.

RESULTS

Although there were no differences between the drug and placebo groups before treatment (P=0.807), by 1 week after the 10 drug treatments ended there was a significant difference in gain scores between the groups (P=0.0153), with the greater gain in the dextroamphetamine group. The difference was still significant when corrected for initial aphasia severity and age. At the 6-month follow-up, the difference in gain scores between the groups had increased; however, the difference was not significant (P=0.0482) after correction for multiple comparisons.

CONCLUSIONS

Administration of dextroamphetamine paired with 10 1-hour sessions of speech/language therapy facilitated recovery from aphasia in a small group of patients in the subacute period after stroke. Neuromodulation with dextroamphetamine, and perhaps other drugs that increase central nervous system noradrenaline levels, may facilitate recovery when paired with focused behavioral treatment.

Neuropathic pain: the paradox of dynorphin

One of the curious but common consequences of opioid administration in the clinical setting is the induction, at sites uninvolved in the original presentation of discomfort, of pain itself. The induction of pain is also a reliable, measurable phenomenon in animals receiving continuous delivery of opioid. Such pain induction is associated with the expression of spinal dynorphin, a finding that is especially intriguing in light of dynorphin's ability to recapitulate many of the characteristics of chronic, neuropathic pain when administered intrathecally (i.e., into the spine). The effective treatment of chronic pain syndromes-and of tolerance to antinociceptive therapies-may thus rest on an understanding of the biological roles of dynorphin in neurotransmission.

Inhibition of neuropathic pain by selective ablation of brainstem medullary cells expressing the mu-opioid receptor

Neurons in the rostroventromedial medulla (RVM) project to spinal loci where the neurons inhibit or facilitate pain transmission. Abnormal activity of facilitatory processes may thus represent a mechanism of chronic pain. This possibility and the phenotype of RVM cells that might underlie experimental neuropathic pain were investigated. Cells expressing mu-opioid receptors were targeted with a single microinjection of saporin conjugated to the mu-opioid agonist dermorphin; unconjugated saporin and dermorphin were used as controls. RVM dermorphin-saporin, but not dermorphin or saporin, significantly decreased cells expressing mu-opioid receptor transcript. RVM dermorphin, saporin, or dermorphin-saporin did not change baseline hindpaw sensitivity to non-noxious or noxious stimuli. Spinal nerve ligation (SNL) injury in rats pretreated with RVM dermorphin-saporin failed to elicit the expected increase in sensitivity to non-noxious mechanical or noxious thermal stimuli applied to the paw. RVM dermorphin or saporin did not alter SNL-induced experimental pain, and no pretreatment affected the responses of sham-operated groups. This protective effect of dermorphin-saporin against SNL-induced pain was blocked by beta-funaltrexamine, a selective mu-opioid receptor antagonist, indicating specific interaction of dermorphin-saporin with the mu-opioid receptor. RVM microinjection of dermorphin-saporin, but not of dermorphin or saporin, in animals previously undergoing SNL showed a time-related reversal of the SNL-induced experimental pain to preinjury baseline levels. Thus, loss of RVM mu receptor-expressing cells both prevents and reverses experimental neuropathic pain. The data support the hypothesis that inappropriate tonic-descending facilitation may underlie some chronic pain states and offer new possibilities for the design of therapeutic strategies.