Search

Existing microarray gene expression profiling studies of tonic/chronic pain were subjected to meta-analysis to identify genes found to be regulated by these pain states in multiple, independent experiments. Twenty studies published from 2002 to 2008 were identified, describing the statistically significant regulation of 2254 genes. Of those, a total of 79 genes were found to be statistically significant "hits" in 4 or more independent microarray experiments, corresponding to a conservative P<0.01 overall. Gene ontology-based functional annotation clustering analyses revealed strong evidence for regulation of immune-related genes in pain states. A multi-gene quantitative real-time polymerase chain reaction experiment was run on dorsal root ganglion (DRG) and spinal cord tissue from rats and mice given nerve (sciatic chronic constriction; CCI) or inflammatory (complete Freund's adjuvant) injury. We independently confirmed the regulation of 43 of these genes in the rat-CCI-DRG condition; the genetic correlates in all other conditions were largely and, in some cases, strikingly, independent. However, a handful of genes were identified whose regulation bridged etiology, anatomical locus, and/or species. Most notable among these were Reg3b (regenerating islet-derived 3 beta; pancreatitis-associated protein) and Ccl2 (chemokine [C-C motif] ligand 2), which were significantly upregulated in every condition in the rat. [ABSTRACT FROM AUTHOR]

It is widely appreciated that there is significant inter-individual variability in pain sensitivity, yet only a handful of contributing genetic variants have been identified. Computational genetic mapping and quantitative trait locus analysis suggested that variation within the gene coding for the beta3 subunit of the Na+,K+-ATPase pump (Atp1b3) contributes to inter-strain differences in the early phase formalin pain behavior. Significant strain differences in Atp1b3 gene expression, beta3 protein expression, and biophysical properties of the Na+,K+ pump in dorsal root ganglia neurons from resistant (A/J) and sensitive (C57BL/6J) mouse strains supported the genetic prediction. Furthermore, in vivo siRNA knockdown of the beta3 subunit produced strain-specific changes in the early phase pain response, completely rescuing the strain difference. These findings indicate that the beta3 subunit of the Na+,K+-ATPase is a novel determinant of nociceptive sensitivity and further supports the notion that pain variability genes can have very selective effects on individual pain modalities. [ABSTRACT FROM AUTHOR]

Developing valid tools that assess key determinants of canine healthspan such as frailty and health-related quality of life (HRQL) is essential to characterizing and understanding aging in dogs. Additionally, because the companion dog is an excellent translational model for humans, such tools can be applied to evaluate gerotherapeutics and investigate mechanisms underlying longevity in both dogs and humans. In this multi-center, cross-sectional study, we investigated the use of a clinical questionnaire (Canine Frailty Index; CFI; Banzato et al., 2019) to assess frailty and an owner assessment tool (VetMetrica HRQL) to evaluate HRQL in 451 adult companion dogs. Results demonstrated validity of the tools by confirming expectations that frailty score increases and HRQL scores deteriorate with age. CFI scores were significantly higher (higher frailty) and HRQL scores significantly lower (worse HRQL) in old dogs (≥ 7 years of age) compared to young dogs (≥ 2 and < 6 years of age). Body size (small < 11.3 kg (25 lbs) or large > 22.7 kg (50 lbs)) was not associated with CFI or total HRQL score. However, older, larger dogs showed faster age-related decline in HRQL scores specific to owner-reported activity and comfort. Findings suggest that the clinician-assessed CFI and owner-reported VetMetrica HRQL are useful tools to evaluate two determinants of healthspan in dogs: the accumulation of frailty and the progressive decline in quality of life. Establishing tools that operationalize the assessment of canine healthspan is critical for the advancement of geroscience and the development of gerotherapeutics that benefit both human and veterinary medicine. Graphical summary of the design, results, and conclusions of the study., (© 2023. The Author(s).)

Aging is the single most important cause of disease, disability, and death in adult dogs. Contrary to the common view of aging as a mysterious and inevitable natural event, it is more usefully understood as a set of complex but comprehensible biological processes that are highly conserved across species. Although the phenotypic expression of these processes is variable, there are consistent patterns both within and between species. The purpose of this feature is to describe the patterns currently recognized in the physical and behavioral manifestations of aging in the dog and how these impact the health and welfare of companion dogs and their human caregivers. Important gaps in our knowledge of the canine aging phenotype will be identified, and current research efforts to better characterize aging in the dog will be discussed. This will help set the context for future efforts to develop clinical assessments and treatments to mitigate the negative impact of aging on dogs and humans.

We recently observed a reliable phenotypic difference in the inflammatory pain sensitivity of a congenic mouse strain compared to its background strain. By constructing and testing subcongenic strains combined with gene-expression assays, we provide evidence for the candidacy of the Yy1 gene - encoding the ubiquitously expressed and multifunctional Yin Yang 1 transcription factor - as responsible. To confirm this hypothesis, we used a Cre/lox strategy to produce mutant mice in which Yy1 expression was ablated in Nav 1.8-positive neurons of the dorsal root ganglion. These mutants also displayed reduced inflammatory pain sensitivity on the formalin test. Further testing of pain-related phenotypes in these mutants revealed robustly increased sensitivity to systemic and spinal (but not supraspinal) morphine analgesia, and greatly increased endogenous (swim stress-induced) opioid analgesia. None of the known biological roles of Yin Yang 1 were suggestive of such a phenotype, and thus a novel player in pain modulatory systems has been identified., (© 2013 John Wiley & Sons Ltd and International Behavioural and Neural Genetics Society.)

The innate immune system is increasingly appreciated to play an important role in the mediation of chronic pain, and one molecule implicated in this process is the Toll-like receptor 4 (TLR4). Here, using pharmacological and genetic manipulations, we found that activating TLR4 in the spinal cord, with the agonist lipopolysaccharide (LPS), causes robust mechanical allodynia but only in male mice. Spinal LPS had no pain-producing effect in female mice. TLR4 also has a sex-specific role in inflammatory (complete Freund's adjuvant) and neuropathic (spared nerve injury) pain: pain behaviors were TLR4 dependent in males but TLR4 independent in females. The sex differences appear to be specific to the spinal cord, as LPS administered to the brain or the hindpaw produces equivalent allodynia in both sexes, and specific to pain, as intrathecal LPS produces equivalent hypothermia in both sexes. The involvement of TLR4 in pain behaviors in male mice is dependent on testosterone, as shown by gonadectomy and hormone replacement. We found no sex differences in spinal Tlr4 gene expression at baseline or after LPS, suggesting the existence of parallel spinal pain-processing circuitry in female mice not involving TLR4.

Quantitative trait locus mapping of chemical/inflammatory pain in the mouse identified the Avpr1a gene, which encodes the vasopressin-1A receptor (V1AR), as being responsible for strain-dependent pain sensitivity to formalin and capsaicin. A genetic association study in humans revealed the influence of a single nucleotide polymorphism (rs10877969) in AVPR1A on capsaicin pain levels, but only in male subjects reporting stress at the time of testing. The analgesic efficacy of the vasopressin analog desmopressin revealed a similar interaction between the drug and acute stress, as desmopressin inhibition of capsaicin pain was only observed in nonstressed subjects. Additional experiments in mice confirmed the male-specific interaction of V1AR and stress, leading to the conclusion that vasopressin activates endogenous analgesia mechanisms unless they have already been activated by stress. These findings represent, to the best of our knowledge, the first explicit demonstration of analgesic efficacy depending on the emotional state of the recipient, and illustrate the heuristic power of a bench-to-bedside-to-bench translational strategy.

Facial expression is widely used as a measure of pain in infants; whether nonhuman animals display such pain expressions has never been systematically assessed. We developed the mouse grimace scale (MGS), a standardized behavioral coding system with high accuracy and reliability; assays involving noxious stimuli of moderate duration are accompanied by facial expressions of pain. This measure of spontaneously emitted pain may provide insight into the subjective pain experience of mice.

Interindividual variability in pain sensitivity and the response to analgesic manipulations remains a considerable clinical challenge as well as an area of intense scientific investigation. Techniques in this field have matured rapidly so that much relevant data have emerged only in the past few years. Our increasing understanding of the genetic mediation of these biological phenomena have nonetheless revealed their surprising complexity. This review provides a comprehensive picture and critical analysis of the field and its prospects.

An increasing amount of evidence indicates that there are significant sex differences in clinical and experimental pain sensitivity in men and women. While it is now clear that the endogenous sex steroids are involved in mediating these sex differences, the cellular and molecular mechanisms that underlie their effects on nociceptive sensitivity remain elusive. Recent studies have shown that sex steroids are potent regulators of gene expression in glial cells, particularly astrocytes. This review specifically highlights some of the evidence of sex steroid regulation of growth factor expression. Growth factors have been shown to be potent pro-nociceptive mediators in rodents. Thus, regulation of their expression by sex steroids may be a general mechanism by which sex steroids exert their effect on pain sensitivity. One such mechanism, the progesterone specific regulation of the growth factor, neuregulin-1, following nerve root injury in the rat, is described in detail. Neuregulin-1 expression is increased in spinal cord astrocytes only in female rats with circulating progesterone. Neuregulin-1 has also been shown to produce transient tactile allodynia when delivered intrathecally in rats. Our understanding of growth factor regulation by sex steroids promises to open up new avenues of investigation into the mechanisms that drive sex differences in pain sensitivity.

We have previously shown that the atypical methylxanthine, propentofylline, reduces mechanical allodynia after peripheral nerve transection in a rodent model of neuropathy. In the present study, we sought to determine whether propentofylline-induced glial modulation alters spinal glutamate transporters, glutamate transporter-1 (GLT-1) and glutamate-aspartate transporter (GLAST) in vivo, which may contribute to reduced behavioral hypersensitivity after nerve injury. In order to specifically examine the expression of the spinal glutamate transporters, a novel line of double transgenic GLT-1-enhanced green fluorescent protein (eGFP)/GLAST-Discosoma Red (DsRed) promoter mice was used. Adult mice received propentofylline (10 mg/kg) or saline via i.p. injection starting 1 h prior to L5-spinal nerve transection and then daily for 12 days. Mice receiving saline exhibited punctate expression of both eGFP (GLT-1 promoter activation) and DsRed (GLAST promoter activation) in the dorsal horn of the spinal cord, which was decreased ipsilateral to nerve injury on day 12. Propentofylline administration reinstated promoter activation on the injured side as evidenced by an equal number of eGFP (GLT-1) and DsRed (GLAST) puncta in both dorsal horns. As demonstrated in previous studies, propentofylline induced a concomitant reversal of L5 spinal nerve transection-induced expression of glial fibrillary acidic protein (GFAP). The ability of propentofylline to alter glial glutamate transporters highlights the importance of controlling aberrant glial activation in neuropathic pain and suggests one possible mechanism for the anti-allodynic action of this drug.

Sex differences in the magnitude of response to thermal and tactile stimuli have been demonstrated in both clinical and animal studies. Females typically display lower threshold responses to painful stimuli as compared to males. We have previously observed sexually dimorphic expression of the growth factor, neuregulin 1 (NRG1) following L5 nerve root ligation (LR) in male and female rats. In the present study, we sought to determine which gonadal hormones were involved in regulating NRG1 expression following L5 nerve root ligation. We observed that expression of NRG1 mRNA and the neuregulin receptors, ErbB2 and ErbB4 in the lumbar spinal cord was facilitated by the presence of progesterone in female rats following L5 nerve root ligation. An increase in NRG1 protein and NRG1 immunoreactivity was also observed in the ipsilateral spinal cord of progesterone treated female rats as compared to ovariectomized female rats and male rats at day 14 following LR. NRG1 immunoreactivity was equally colocalized with either the astrocytic marker, GFAP, and with NeuN labeled neurons 14days following L5 nerve root ligation. Intrathecal administration of recombinant NRG1-beta1 protein significantly decreased the hindpaw tactile withdrawal threshold in male rats, ovariectomized female rats, and progesterone treated female rats. These results demonstrate a role for progesterone-dependent regulation of glial and/or neuronal neuregulin 1 in female rats in mediating sex differences in nociception. Furthermore, our results suggest that NRG1 may be involved in central sensitization during the maintenance phase, but not in the initiation of persistent pain in female rats.

The transgenic knockout mouse is one of the most important tools of modern biology, and commonly employed by pain researchers to examine the function of genes of interest. Over 400 papers, at a current rate of >60 papers per year, have been published to date describing a statistically significant behavioral pain "phenotype" resulting from the null mutation of a single gene. The standard literature review format is incapable of providing a sufficiently broad and up-to-date overview of the field. We have therefore constructed the Pain Genes Database, an interactive, web-based data browser designed to allow easy access to and analysis of the published pain-related phenotypes of mutant mice (over 200 different mutants at the date of submission). Manuscripts describing results of pain-relevant knockout studies were identified via Medline search. Manuscripts were included in the database if they described the testing of a spontaneous or genetically engineered mutant mouse with null expression of a single gene on a behavioral assay of acute or tonic nociception, injury- or stimulus-induced hypersensitivity (i.e., allodynia or hyperalgesia), or drug- or stress-induced inhibition of nociception (i.e., analgesia), and reported at least one statistically significant difference between the mutant mice and their simultaneously tested wildtype controls. The database features two levels of exploration, one allowing the identification of genes by name, acronym, genomic position or "summary" phenotype, and the other allowing in-depth browsing, paper-by-paper, of specific phenotypes and test parameters. Links to genetic databases and Medline abstracts are provided for each gene and paper. It is our intention to update the database continually based on weekly Medline searches. This database should provide pain researchers with a useful and easy-to-use tool for the generation of novel hypotheses regarding the roles of genes and their protein products in pain processing and modulation. It can be accessed at http://paingeneticslab.ca/4105/06&#95;02&#95;pain&#95;genetics&#95;database.asp (or by visiting paingeneticslab.ca and clicking on the "Pain Genes Db" link under "Resources").

Increasing evidence points to a role for spinal neuroimmune dysregulation (glial cell activation and cytokine expression) in the pathogenesis of chronic pain. Suppression of astrocytic and microglial activation with the methylxanthine derivative, propentofylline, pre-emptively attenuates the development of nerve injury-induced allodynia. Currently, we investigated the ability of systemic propentofylline to reverse existing, long-term allodynia after nerve injury-a clinically relevant paradigm. Rats received L5 spinal nerve transection or sham surgery and the development of mechanical allodynia was assessed daily for 2 weeks, at which time injured rats exhibited robust responses to non-noxious von Frey filaments. On days 14-27, rats received either saline or 101 mg/kg propentofylline by intraperitoneal (i.p.) injection. On day 28 or 42 (after a 14-day drug washout period), lumbar spinal cord sections were processed for assessment of astrocytic glial fibrillary acidic protein (GFAP) and microglial OX-42 (antibody against CR3/CD11b). Propentofylline treatment to nerve injured rats resulted in significant reversal of allodynia that lasted throughout the 14-day washout period. Spinal microglial activation was observed at days 28 and 42 post-injury at the protein level, in the absence of mRNA level changes. Less robust increases in GFAP immunoreactivity were observed at days 28 and 42 post-transection. Interestingly, propentofylline treatment suppressed microglial activation at both time points in this paradigm. Taken together, our results highlight the clinical potential of the glial modulating agent, propentofylline, for the treatment of neuropathic pain as well as a role for microglia in the long-term maintenance of allodynia.

Increasing evidence points to a role for spinal neuroimmune dysregulation (glial cell activation and cytokine expression) in the pathogenesis of chronic pain. Suppression of astrocytic and microglial activation with the methylxanthine derivative, propentofylline, pre-emptively attenuates the development of nerve injury-induced allodynia. Currently, we investigated the ability of systemic propentofylline to reverse existing, long-term allodynia after nerve injury--a clinically relevant paradigm. Rats received L5 spinal nerve transection or sham surgery and the development of mechanical allodynia was assessed daily for 2 weeks, at which time injured rats exhibited robust responses to non-noxious von Frey filaments. On days 14-27, rats received either saline or 101 mg/kg propentofylline by intraperitoneal (i.p.) injection. On day 28 or 42 (after a 14-day drug washout period), lumbar spinal cord sections were processed for assessment of astrocytic glial fibrillary acidic protein (GFAP) and microglial OX-42 (antibody against CR3/CD11b). Propentofylline treatment to nerve injured rats resulted in significant reversal of allodynia that lasted throughout the 14-day washout period. Spinal microglial activation was observed at days 28 and 42 post-injury at the protein level, in the absence of mRNA level changes. Less robust increases in GFAP immunoreactivity were observed at days 28 and 42 post-transection. Interestingly, propentofylline treatment suppressed microglial activation at both time points in this paradigm. Taken together, our results highlight the clinical potential of the glial modulating agent, propentofylline, for the treatment of neuropathic pain as well as a role for microglia in the long-term maintenance of allodynia.

Glial-neuronal interactions are crucial processes in neuromodulation and synaptic plasticity. The neuregulin 1 family of growth and differentiation factors have been implicated as bidirectional signaling molecules that are involved in mediating some of these interactions. We have shown previously that neuregulin 1 expression is regulated by the gonadal hormones progesterone and 17beta-estradiol in the CNS, which might represent a novel, indirect mechanism of the neuromodulatory actions of these gonadal hormones. In the present study, we sought to determine the effects of progesterone and 17beta-estradiol on neuregulin 1 expression in rat cortical astrocytes and neurons in vitro. We observed that progesterone increased the expression of neuregulin 1 mRNA and protein in a dose-dependent manner in cultured astrocytes, which was blocked by the progesterone receptor antagonist RU-486. In contrast, 17beta-estradiol did not increase either neuregulin 1 mRNA or protein in astrocytes. We observed no effect of either progesterone or 17beta-estradiol on neuregulin 1 mRNA and protein in rat cortical neurons in vitro. Finally, we observed that treatment of cortical neurons with recombinant NRG1-beta1 caused PSD-95 to localize in puncta similar to that observed following treatment with astrocyte-conditioned medium. These results demonstrate that progesterone regulates neuregulin 1 expression, principally in astrocytes. This might represent a novel mechanism of progesterone-mediated modulation of neurotransmission through the regulation of astrocyte-derived neuregulin 1.

Reactive astrocytes display decreased glutamate transporters, such as GLT-1, and as a result synaptic glutamate clearance is impaired. In addition, these activated astrocytes are immunocompetent and release algesic mediators that can sensitize neurons in the spinal cord. Currently, we evaluated the effect of propentofylline (PPF), an experimental antiallodynic agent, on the phenotype and glutamate transporter expression of astrocytes. Primary astrocyte cultures, which represent an activated phenotype with a polygonal morphology and low GLT-1 expression, were treated for 3 or 7 days with 10, 100, or 1,000 microM PPF or dibutyryl-cAMP (db-cAMP), a known inducer of GLT-1 expression. PPF dose-dependently induced astrocytes to display a mature phenotype, with elongated processes and a stellate shape, as well as increased GLT-1 and GLAST immunoreactivity, similar to that seen with db-cAMP. Real time RT-PCR and Western blot analysis clearly demonstrated that PPF caused a potent dose-dependent induction of GLT-1 and GLAST mRNA and protein in these astrocytes. Importantly, the observed increase in glutamate transporters was found to have a functional effect, with significantly enhanced glutamate uptake in astrocytes treated with 100 or 1,000 microM PPF that was sensitive to dihydrokainate inhibition, suggesting it is GLT-1 mediated. Finally, the effect of PPF on lipopolysaccharide-induced chemokine release was investigated. Interestingly, PPF was able to dampen both MCP-1 (CCL2) and MIP-2 (CXCL2) release from astrocytes while db-cAMP significantly enhanced this chemokine expression. These findings suggest that PPF is capable of differentiating astrocytes to a homeostatic, mature phenotype, competent for glutamate clearance and distinct from that induced by db-cAMP.

Background: Neuropathic pain and radicular low back pain both have a major impact on human health worldwide. Microarray gene analysis on central nervous system tissues holds great promise for discovering novel targets for persistent pain modulation., Methods: Rat models of lumbar radiculopathy (L5 nerve root ligation) and neuropathy (L5 spinal nerve transection) were used for these studies. The authors measured mechanical allodynia followed by analysis of global gene expression in the lumbar spinal cord at two time points (7 days and 14 days) after surgery using the Affymetrix RAE230A GeneChip(R) (Santa Clara, CA). The expression patterns of several genes of interest were subsequently confirmed using real-time reverse transcriptase polymerase chain reaction., Results: The authors observed similarly robust mechanical allodynia in both models. Second, they observed significant differences in lumbar spinal cord gene expression across chronic pain models. There was little overlap between genes altered in each injury model, suggesting that the site and type of injury produce distinct spinal mechanisms mediating the observed mechanical allodynia. The authors further confirmed a subset of the genes using reverse transcriptase polymerase chain reaction and identified several genes as either neuropathy-associated genes or radiculopathy-associated genes., Conclusions: These two models of persistent pain produce similar allodynic outcomes but produce differential gene expression. These results suggest that diverging mechanisms lead to a common behavioral outcome in these pain models. Furthermore, these distinct pathophysiologic mechanisms in neuropathic versus radicular pain may implicate unique drug therapies for these types of chronic pain syndromes.

Sex differences in the magnitude of response to thermal and tactile stimuli have been demonstrated in both clinical and animal studies. Female rats typically display lower thresholds to painful stimuli and display more robust responses following nerve injury as compared with males. There is a body of evidence implicating the sex hormones in mediating this sex difference. In the present study, we sought to determine which gonadal hormones were involved in mediating the observed female hypersensitivity in female rats both prior to and following experimental nerve root injury using a chronic hormone replacement paradigm. Female rats were ovariectomized and hormone pellets containing 17beta-estradiol, progesterone (P), 17beta-estradiol+progesterone or placebo were implanted s.c. Our results demonstrate that only the group of female rats that received progesterone alone maintained the hypersensitive phenotype following ovariectomy, compared with gonadally intact male rats. This result was observed both in response to thermal stimuli in non-injured female rats and to thermal and tactile stimuli following L5 nerve root ligation, a model of low back pain associated with lumbar radiculopathy. Postmortem analysis of serum gonadal hormone concentrations demonstrates that the hormonal manipulations were successful and the exogenous hormones were similar to physiological levels observed in the sham-ovariectomized controls. Taken together, these results demonstrate the critical role for progesterone in mediating enhanced female tactile and thermal hypersensitivity following L5 nerve root ligation.

Considerable evidence indicates that there are sex-related differences in clinical and experimental pain sensitivity. In the present study, we sought to determine what genes were expressed in the spinal cord in a sexually dimorphic manner. We first analyzed global gene expression in the lumbar spinal cord of uninjured male and female rats using the Affymetrix RAE230A GeneChip platform in order to identify genes that are selectively expressed in male and female rats at a basal level. We subsequently analyzed global gene expression in the lumbar spinal cord of male and female rats at two time points (7 days and 14 d) following a rodent model of lumbar radiculopathy (L5 nerve root ligation) in order to determine what genes were regulated in a sexually dimorphic manner following nerve root injury. We utilized a linear regression analysis method to identify genes that were significantly different from the corresponding sham surgical controls. The expression patterns of several genes of interest were subsequently confirmed using RT-PCR. Our findings demonstrate significant differences in lumbar spinal cord gene expression in both uninjured and injured (L5 nerve root ligation) male and female rats. Further confirmation of a subset of the genes identified Neuregulin 1 and its high affinity receptor, ErbB4, Tachykinin 1, and Metabotropic glutamate receptor 6 as female specific genes upregulated following L5 nerve root injury. These findings provide several target genes for further study that may elucidate the neurochemical mechanisms underlying sex differences in pain sensitivity and lead to improved treatments for chronic pain syndromes.

Study Design: This study examined the differences in tactile hypersensitivity across 6 different strains of male mice, and between male and female rats of 3 different strains in a rodent model of low back pain associated with lumbar radiculopathy., Objective: We investigated the possibility that differences in tactile allodynia following the same nerve root injury are affected by genotype and sex in rodents., Summary of Background Data: Low back pain associated with radiculopathy affects countless people throughout the world, encompassing a wide range of individual pain susceptibility. The roles of genetics and sex on differences in nociceptive sensitivities following lumbar nerve root injury have yet to be fully characterized., Methods: Six strains of mice (BALB/cJ, CBA/J, C57BL/6J, 129P3/J, C3H/HeJ, and C58/J; all males) and male and female Sprague Dawley, Holtzman, and Long-Evans rats underwent a lumbar nerve root injury followed by assessment of tactile allodynia., Results: The most sensitive mouse strains following nerve root injury were: 129P3/J, C58/J, and BALB/cJ; and the less sensitive strains were: C57BL/6J, C3H/HeJ, and CBA/J. Female Sprague Dawley and Long-Evans rats displayed increased hypersensitivity following nerve root injury compared to males. No sex differences were observed in Holtzman rats., Conclusions: Different mouse strains, and male and female rats that are exposed to identical nerve root injuries have diverse levels of tactile hypersensitivity, supporting the hypothesis that genetic factors and sex play a key role in radicular pain. Our results correlate with data compiled in identical mouse and rat strains after L5-L6 nerve ligation, suggesting that the precise nature of the injury is not relevant to the inheritance of neuropathic symptom sensitivity.

Glial cells function in maintenance of homeostasis as well as in pathophysiology. In this study, we determined the time course of spinal glial cell activation during the development of morphine analgesic tolerance in an L5 spinal nerve transection rodent model of neuropathic pain. We also sought to assess whether the method of morphine administration affected neuroimmune activation at the levels of transcription and translation. Rats received L5 spinal nerve transection or no surgery on day 0. On day 6 post-transection, osmotic minipumps were implanted to deliver saline or morphine s.c. (1 or 10 mg/kg) or i.t. (5 or 20 nmol/h). Mechanical allodynia developed immediately after spinal nerve transection; this hypersensitivity was reversed with both low- and high-dose morphine by either route. Tolerance to antiallodynia developed after 3 days of i.t. morphine and after 6 days of s.c. morphine, indicating hastened tolerance following i.t. delivery. Analysis of mRNA revealed that s.c. morphine treatment did not lead to increases in glial activation markers. In contrast, i.t. morphine caused a biphasic alteration in glial fibrillary acidic protein (GFAP) and integrin alpha M mRNA. Protein levels for GFAP were elevated after s.c. and i.t. administration of morphine; however, induction was further enhanced in the latter group. Here, we show for the first time that there is differential recruitment of transcriptional and translational mechanisms of glial activation by systemic and i.t. morphine. Furthermore, we suggest that enhanced neuroimmune activation after i.t. dosing contributes to the hastened development of analgesic tolerance seen in these animals.

Considerable evidence exists for sex differences in human pain sensitivity. Women typically report a higher incidence of various painful conditions and report that the conditions are more painful when compared to men. In the present study, we sought to determine whether sex differences in pain sensitivity are observed using a lumbar radiculopathy model of low back pain in the rat and whether removal or alteration of gonadal hormones at specific timepoints can modulate these sex differences. Pubertal and adult male and female Sprague-Dawley rats were castrated 2 or 6 weeks prior to L5 nerve root injury to determine the activational hormonal effects. In a separate study, neonatal male and female Sprague-Dawley rats were either castrated or injected with testosterone, respectively, on postnatal day one to determine the organizational effects of gonadal hormones on L5 nerve root injury-induced behavioral hypersensitivity. Our results demonstrate that there was a statistically significant sex difference in the magnitude of mechanical allodynia and thermal hyperalgesia following experimentally induced radiculopathy in the rat: females demonstrated decreased thresholds to tactile and thermal stimuli as compared to males. Furthermore, the enhanced female hypersensitivity was reversed in pubertal and adult animals ovariectomized 6 weeks, but not 2 weeks prior to L5 nerve root injury. Our results demonstrate that the activational effects of gonadal hormones mediate the enhanced female tactile and thermal hypersensitivity following L5 nerve root injury. These results suggest that manipulation of gonadal hormones may be a potential source for novel therapies for chronic pain in women.