Compounded Pain Formulations: What is the Evidence?

Pain syndromes are among the most widespread, costly, and debilitating of all neurological disorders. The number of patients living with chronic pain is expected to increase with the aging population and with the rise in obesity and diabetes across the nation. This type of pain is often insensitive to the traditional pain pharmacopeia or surgical intervention. Over the last 10 years the number of prescriptions that have been compounded by pharmacists has increased dramatically. There are a number of drugs in the area of pain management that have been formulated and compounded by pharmacists to treat conditions such as diabetic neuropathy, fibromyalgia, postherpetic neuralgia, joint pain, arthritis, and a variety of other conditions. A significant portion of these compounded analgesic preparations is made up of topical/transdermal dosage forms such as gels and creams. While the efficacy and doses of these drugs in systemic dosage forms have been widely established, little is known about the permeation and efficacy of these compounds from topical/transdermal gels. This review will provide an overview of chronic pain as a disease, the mechanisms of chronic pain, current treatment approaches to chronic pain, and a discussion of the drugs that are typically compounded into these topical formulations and studied in clinical trials.

Overexpression of µ-Opioid Receptors in Peripheral Afferents, but Not in Combination with Enkephalin, Decreases Neuropathic Pain Behavior and Enhances Opioid Analgesia in Mouse.

BACKGROUND: The current study used recombinant herpes simplex virus type I to increase expression of µ-opiate receptors and the opioid ligand preproenkephalin in peripheral nerve fibers in a mouse model of neuropathic pain. It was predicted that viral vector delivery of a combination of genes encoding the µ-opioid receptor and preproenkephalin would attenuate neuropathic pain and enhance opioid analgesia. The behavioral effects would be paralleled by changes in response properties of primary afferent neurons. METHODS: Recombinant herpes simplex virus type 1 containing cDNA sequences of the µ-opioid receptor, human preproenkephalin, a combination, or Escherichia coli lacZ gene marker (as a control) was used to investigate the role of peripheral opioids in neuropathic pain behaviors. RESULTS: Inoculation with the µ-opioid receptor viral vector (n = 13) reversed mechanical allodynia and thermal hyperalgesia and produced leftward shifts in loperamide (ED50 = 0.6 ± 0.2 mg/kg vs. ED50 = 0.9 ± 0.2 mg/kg for control group, n = 8, means ± SD) and morphine dose-response curves (ED50 = 0.3 ± 0.5 mg/kg vs. ED50 = 1.1 ± 0.1 mg/kg for control group). In µ-opioid receptor viral vector inoculated C-fibers, heat-evoked responses (n = 12) and ongoing spontaneous activity (n = 18) were decreased after morphine application. Inoculation with both µ-opioid receptor and preproenkephalin viral vectors did not alter mechanical and thermal responses. CONCLUSIONS: Increasing primary afferent expression of opioid receptors can decrease neuropathic pain-associated behaviors and increase systemic opioid analgesia through inhibition of peripheral afferent fiber activity.

Skin Permeation and Antinociception of Compounded Topical Cyclobenzaprine Hydrochloride Formulations.

Cyclobenzaprine has been commonly compounded by pharmacists into topically applied dosage forms for the treatment of pain disorders. However, the efficacy and transdermal penetration of topically applied compounded cyclobenzaprine is currently unknown. In this study, the transdermal penetration of cyclobenzaprine was studied in Franz diffusion cells using porcine skin. Cyclobenzaprine was compounded in two commercially available bases; Lipobase, Lipoderm, and a standard poloxamer lecithin organogel. In addition, cyclobenzaprine was tested in an in vivo formalin pain model. Cyclobenzaprine 5% compounded into all three bases yielded significant transdermal penetration and results in modest levels of cyclobenzaprine being retained in the skin tissue. Systemically administered cyclobenzaprine (10 mcg/kg), but not topically administered cyclobenzaprine (1% and 5%), attenuated nociception in a rodent formalin pain model.

Analgesic Efficacy and Transdermal Penetration of Topical Gabapentin Creams: Finding an Optimal Dose and Pre-treatment Time.

Many patients with chronic neuropathic pain continue to suffer despite traditional pharmacotherapy. As a result, pharmacists commonly compound gabapentin into creams, gels, and ointments as an alternative treatment option. In this study, various concentrations (1%, 5%, and 10%) of topical gabapentin compounded in Lipoderm were applied at various pre-treatment times (30 minutes, 1 hour, and 4 hours) to investigate what gabapentin concentration and pre-treatment time best attenuates formalin-induced nociceptive behaviors in a rodent model. A 30-minute pre-treatment with 5% gabapentin demonstrated maximum attenuation of nociceptive behaviors in this in vivo preclinical pain model. Nociceptive behaviors unexpectedly increased when gabapentin concentration or pre-treatment time was increased, suggesting both antinociceptive and pronociceptive effects of transdermal gabapentin administration. Gabapentin permeation into the skin and deeper tissues of the hindpaw was measured following the in vivo study. Skin and deep tissue permeation of gabapentin was both dose and time-dependent. Maximum deep-tissue permeation occurred within 30 minutes of topical application. Skin concentrations increased with a longer 1-hour pre-treatment. Minimal skin and deeper tissue levels were found following a 4-hour pre-treatment. These results suggest that topical gabapentin may be antinociceptive in a rodent formalin model at specific doses and pre-treatment intervals.

Nonneuronal central mechanisms of pain: glia and immune response.

The role of central glial cells in the mechanisms underlying pain has been intensively studied in the last two decades. Most studies on glia and pain focused on the potential detrimental role of glial cells following noxious stimulus/insults manifested as an "activation" or a "reactive" state (increase in glial marker expression and production of proinflammatory/nociceptive molecules). Therefore, "activated" or "reactive" glial cells became a target for the future generation of drugs to treat chronic pain. Several glial modulators that reduce the activation of glial cells have shown great efficacy in multiple animal (rodents mostly) models of pain (acute, subacute, chronic, inflammatory, neuropathic, surgical, etc.). These encouraging findings inspired clinical trials that have been completed in the last 5 years. Unfortunately, all clinical trials with these glial modulators have failed to demonstrate efficacy for the treatment of pain. New lines of investigation and elegant experimental designs are shedding light on alternative glial functions, which demonstrate that "glial reactivity" is not necessarily deleterious in some pathological conditions. New strategies to validate findings through our current animal models are necessary to enhance the translational value of our preclinical studies. Also, more studies using human subjects would enhance our understanding of glial cells in the context of pain. This chapter explores the available literature to objectively ponder the potential role of glial cells in human pain conditions.

Injury-specific promoters enhance herpes simplex virus-mediated gene therapy for treating neuropathic pain in rodents.

UNLABELLED: Chronic neuropathic pain is often difficult to treat with current pain medications. Gene therapy is presently being explored as a therapeutic approach for the treatment of neuropathic and cancer pain. In this study, we sought to use an injury-specific promoter to deliver the mu-opioid receptor (MOR) transgene such that expression would occur during the injured state only in response to release of injury-specific galanin. To determine whether an injury-specific promoter can produce neuron-specific MOR expression and enhanced antinociception, we compared animals infected with a galanin promoter virus (galMOR) or a human cytomegalovirus promoter virus (cmvMOR). In behavioral assays, we found an earlier onset and a larger magnitude of antinociception in animals infected with galMOR compared with cmvMOR. Immunohistochemical analysis of dorsal root ganglion neurons revealed a significant increase in MOR-positive staining in cmvMOR- and galMOR-treated mice. Spinal cord sections from galMOR-treated mice showed a greater increase in density but not area of MOR-positive staining. These results suggest that using injury-specific promoters to drive gene expression in primary afferent neurons can influence the onset and magnitude of antinociception in a rodent model of neuropathic pain and can be used to upregulate MOR expression in populations of neurons that are potentially injury specific. PERSPECTIVE: An injury-specific promoter (galMOR) was used to drive MOR expression in a population- and injury-specific manner. GalMOR increased antinociception and density of MOR staining in the spinal cord. This article presents evidence that promoter selection is an important component in successful gene expression in an injury- and population-specific manner.

Modulation of pain in pediatric sickle cell disease: understanding the balance between endothelin mediated vasoconstriction and apelin mediated vasodilation.

Children with sickle cell disease (SCD) have painful vaso-occlusive episodes (VOEs), which often reoccur across the individual's lifespan. Vaso-constrictive and vaso-dilatory molecules have been hypothesized to play a role in VOEs. Endothelin-1 (ET-1) is a potent vasoconstrictor that is released during VOEs and is correlated with pain history. Apelin is a vaso-dilatory peptide that also has a modulatory role in pain processing. We hypothesize that the ratio between vaso-dilatory and vaso-constrictive tone in children with SCD may be a marker of pain sensitization and vaso-occlusion. Plasma endothelin and apelin levels were measured in 47 children with SCD. Procedural pain and baseline pain were assessed via child- and caregiver-reports and observational distress. Pain history was assessed using retrospective chart review. Plasma apelin was related to age, with decreased levels in older children. The ratio between apelin and ET-1 was negatively correlated to observational baseline pain. The ratio between apelin and Big ET was negatively correlated to caregiver ratings of baseline pain and positively correlated to history of VOEs, which is possibly due to hydroxyurea treatment. These results suggest that an imbalance in the apelin and endothelin systems may contribute to an increasing number of VOEs and baseline pain in children with SCD.

Compounded pain formulations: what is the evidence?

Pain syndromes are among the most widespread, costly, and debilitating of all neurological disorders. The number of patients living with chronic pain is expected to increase with the aging population and with the rise in obesity and diabetes across the nation. This type of pain is often insensitive to the traditional pain pharmacopeia or surgical intervention. Over the last 10 years the number of prescriptions that have been compounded by pharmacists has increased dramatically. There are a number of drugs in the area of pain management that have been formulated and compounded by pharmacists to treat conditions such as diabetic neuropathy, fibromyalgia, postherpetic neuralgia, joint pain, arthritis, and a variety of other conditions. A significant portion of these compounded analgesic preparations is made up of topical/transdermal dosage forms such as gels and creams. While the efficacy and doses of these drugs in systemic dosage forms have been widely established, little is known about the permeation and efficacy of these compounds from topical/transdermal gels. This review will provide an overview of chronic pain as a disease, the mechanisms of chronic pain, current treatment approaches to chronic pain, and a discussion of the drugs that are typically compounded into these topical formulations and studied in clinical trials.

Construction of an affordable and easy-to-build zebrafish facility.

In vivo biomedical research is pivotal to translate in vitro findings into clinical advances. Small academic institutions with limited resources find it virtually impossible to build and maintain typical rodent facilities for research. Zebrafish research has been demonstrated to be a valuable alternative for in vivo research in pharmacology, physiology, development and genetic studies. This article demonstrates that a functional zebrafish facility can be built in an easy and affordable manner. We demonstrate that such a facility could be built in about one working day with minimal tools and expertise. The cost of the 27 1.8 L fish tank zebrafish facility constructed in this study was approximately $1,500. We estimate that the maintenance of an initial working 150 fish colony for 3 months is $1,000. This project involved students, who were introduced to aquaculturing of zebrafish for research proposes.

Endothelin-1 induced desensitization in primary afferent neurons.

Endothelin-1 (ET-1) is a known algogen that causes acute pain and sensitization in humans and spontaneous nociceptive behaviors when injected into the periphery in rats, and is elevated during vaso-occlusive episodes (VOEs) in sickle cell disease (SCD) patients. Previously, our lab has shown that a priming dose of ET-1 produces sensitization to capsaicin-induce secondary hyperalgesia. The goal of this study was to determine if the sensitization induced by ET-1 priming is occurring at the level of the primary afferent neuron. Calcium imaging in cultured dorsal root ganglion (DRG) neurons was utilized to examine the effects of ET-1 on primary afferent neurons. ET-1 induces [Ca(2+)]i transients in unprimed cells. ET-1 induced [Ca(2+)]i transients are attenuated by priming with ET-1. This priming effect occurs whether the priming dose is given 0-4 days prior to the challenge dose. Similarly, ET-1 priming decreases capsaicin-induced [Ca(2+)]i transients. At the level of the primary afferent neuron, ET-1 priming has a desensitizing effect on challenge exposures to ET-1 and capsaicin.

Evidence for the endothelin system as an emerging therapeutic target for the treatment of chronic pain.

Many people worldwide suffer from pain and a portion of these sufferers are diagnosed with a chronic pain condition. The management of chronic pain continues to be a challenge, and despite taking prescribed medication for pain, patients continue to have pain of moderate severity. Current pain therapies are often inadequate, with side effects that limit medication adherence. There is a need to identify novel therapeutic targets for the management of chronic pain. One potential candidate for the treatment of chronic pain is therapies aimed at modulating the vasoactive peptide endothelin-1. In addition to vasoactive properties, endothelin-1 has been implicated in pain transmission in both humans and animal models of nociception. Endothelin-1 directly activates nociceptors and potentiates the effect of other algogens, including capsaicin, formalin, and arachidonic acid. In addition, endothelin-1 has been shown to be involved in inflammatory pain, cancer pain, neuropathic pain, diabetic neuropathy, and pain associated with sickle cell disease. Therefore, endothelin-1 may prove a novel therapeutic target for the relief of many types of chronic pain.

Endothelin-1-induced priming to capsaicin in young animals.

Endothelin-1 (ET-1) is a known algogen that causes acute pain and sensitization in humans and spontaneous nociceptive behaviors when injected into the periphery in rats. This study sought to examine the effect of ET-1 exposure in the neonatal period on subsequent contralateral capsaicin-induced secondary mechanical hyperalgesia. ET-1 or saline was injected into the left plantar hindpaw on postnatal day 7 (P7). On postnatal day 11 (P11), capsaicin cream or control lotion was applied to the right dorsum hind paw and mechanical paw withdrawal thresholds were measured in the plantar hind paw. In saline control males, P11 administration of capsaicin produced a secondary mechanical hyperalgesia that was still present at 2h. Neonatal priming with ET-1 did not alter the magnitude or the duration of secondary mechanical hyperalgesia in males. In contrast, in control females, P11 administration of capsaicin produced less than 40 min of mechanical hyperalgesia. Neonatal priming with ET-1 prolonged the duration of secondary mechanical hyperalgesia in females. Priming with ET-1 on P7 led to a significant increase in capsaicin-induced Fos expression in the dorsal horn of the spinal cord in both males and females compared to controls (p<0.001). These findings further suggest that pain in early life may alter future responses to painful stimuli at both the behavioral and neuronal level.

Skin permeation and antinociception of topical gabapentin formulations.

Chronic pain affects greater than 116 million Americans each year. Even with the best pain management approaches, many chronic pain patients still suffer from moderate to severe pain. An alternative therapy to treat chronic pain includes compounded topical formulations of common analgesics. Compounded dosage forms of gabapentin are commonly used for pain management, however, the penetration and efficacy of gabapentin in these compounded topical formulations has not been fully studied. In this study, the transdermal penetration of gabapentin was studied in Franz diffusion cells using porcine skin. Gabapentin was compounded in two commercially available bases; Lipobase, Lipoderm, and a standard poloxamer lecithin organogel. The penetration and retention of gabapentin in the skin was dependent upon the base. The most rapid and greatest penetration and retention of gabapentin in the skin occurred with a poloxamer lecithin organogel base. Lipobase and Lipoderm bases produced slow and smaller penetration and retention of gabapentin as compared to poloxamer lecithin organogel base. Gabapentin 1% and 5% compounded in Lipoderm were tested in the in vivo preclinical formalin pain model. Topical 5% gabapentin produced a similar reduction in nociception in both Phase 1 and Phase 2 of the formalin response as systemic subcutaneous gabapentin (100 mg/kg). Topical 1% gabapentin reduced Phase 2, but not Phase 1 formalin-induced nociceptive behaviors. These findings suggest that topical administration of gabapentin may produce local antinociception.

Information-seeking coping behaviors during painful procedures in African-American children with sickle cell disease.

This study examined the frequency of information-seeking coping behaviors in 37 African-American children (ages 5-17 years) with sickle cell disease during venipuncture. The relationships between coping behaviors and child- and parent-reported pain and observational distress were also assessed. The majority of children attended to the procedure, but did not seek information via questions. This pattern of coping was only partially effective at reducing distress and had no relation to pain. This pattern of coping is discussed within the context of cultural factors that may be important in understanding responses to procedural pain in pediatric sickle cell disease.

Tolerance develops to the antiallodynic effects of the peripherally acting opioid loperamide hydrochloride in nerve-injured rats.

Peripherally acting opioids are potentially attractive drugs for the clinical management of certain chronic pain states due to the lack of centrally mediated adverse effects. However, it remains unclear whether tolerance develops to peripheral opioid analgesic effects under neuropathic pain conditions. We subjected rats to L5 spinal nerve ligation (SNL) and examined the analgesic effects of repetitive systemic and local administration of loperamide hydrochloride, a peripherally acting opioid agonist. We found that the inhibition of mechanical hypersensitivity, an important manifestation of neuropathic pain, by systemic loperamide (1.5mg/kg subcutaneously) decreased after repetitive drug treatment (tolerance-inducing dose: 0.75 to 6.0mg/kg subcutaneously). Similarly, repeated intraplantar injection of loperamide (150 µg/50 µL intraplantarly) and D-Ala(2)-MePhe(4)-Glyol(5) enkephalin (300 µg/50 µL), a highly selective mu-opioid receptor (MOR) agonist, also resulted in decreased inhibition of mechanical hypersensitivity. Pretreatment with naltrexone hydrochloride (5mg/kg intraperitoneally) and MK-801 (0.2mg/kg intraperitoneally) attenuated systemic loperamide tolerance. Western blot analysis showed that repetitive systemic administration of morphine (3mg/kg subcutaneously), but not loperamide (3mg/kg subcutaneously) or saline, significantly increased MOR phosphorylation in the spinal cord of SNL rats. In cultured rat dorsal root ganglion neurons, loperamide dose-dependently inhibited KCl-induced increases in [Ca(2+)]i. However, this drug effect significantly decreased in cells pretreated with loperamide (3 µM, 72 hours). Intriguingly, in loperamide-tolerant cells, the delta-opioid receptor antagonist naltrindole restored loperamide's inhibition of KCl-elicited [Ca(2+)]i increase. Our findings indicate that animals with neuropathic pain may develop acute tolerance to the antiallodynic effects of peripherally acting opioids after repetitive systemic and local drug administration.

Sensitization to acute procedural pain in pediatric sickle cell disease: modulation by painful vaso-occlusive episodes, age, and endothelin-1.

UNLABELLED: The impact of pain early in life is a salient issue for sickle cell disease (SCD), a genetic condition characterized by painful vaso-occlusive episodes (VOEs) that can begin in the first year of life and persist into adulthood. This study examined the effects of age and pain history (age of onset and frequency of recent VOEs) on acute procedural pain in children with SCD. Endothelin-1, a vaso-active peptide released during VOEs and acute tissue injury, and its precursor, Big Endothelin, were explored as markers of pain sensitization and vaso-occlusion. Sixty-one children with SCD (ages 2 to 18) underwent venipuncture at routine health visits. Procedural pain was assessed via child and caregiver reports and observational distress. Pain history was assessed using retrospective chart review. Three primary results were found: 1) younger age was associated with greater procedural pain across pain outcomes; 2) higher frequency of VOEs was associated with greater procedural pain based on observational distress (regardless of age); and 3) age was found to moderate the relationship between VOEs and procedural pain for child-reported pain and observational distress for children 5 years of age and older. Associations between the endothelin variables and pain prior to venipuncture were also observed. PERSPECTIVE: For children with SCD, the child's age and recent pain history should be considered in procedural pain management. The endothelin system may be involved in preprocedure pain, but additional research is needed to understand the role of endothelins in pain sensitization.

Dynamic temporal and spatial regulation of mu opioid receptor expression in primary afferent neurons following spinal nerve injury.

Despite using prescribed pain medications, patients with neuropathic pain continue to experience moderate to severe pain. There is a growing recognition of a potent peripheral opioid analgesia in models of inflammatory and neuropathic pain. The goal of this study was to characterize the temporal and spatial expression of mu opioid receptor (mOR) mRNA and protein in primary afferent neurons in a rat L5 spinal nerve ligation model of persistent neuropathic pain. Bilateral L4 and L5 dorsal root ganglia (DRGs), L4 and L5 spinal cord segments, and hind paw plantar skins were collected on days 0 (naïve), 3, 7, 14, and 35 post-spinal nerve ligation or post-sham surgery. We found that expression of mOR mRNA and protein in primary afferent neurons changed dynamically and site-specifically following L5 spinal nerve ligation. Real-time RT-PCR, immunohistochemistry, and Western blot analysis demonstrated a down-regulation of mOR mRNA and protein in the injured L5 DRG. In contrast, in the uninjured L4 DRG, mOR mRNA transiently decreased on day 7 and then increased significantly on day 14. Western blot analysis revealed a persistent increase in mOR protein expression, although immunohistochemistry showed no change in number of mOR-positive neurons in the uninjured L4 DRG. Interestingly, mOR protein expression was reduced in the skin on days 14 and 35 post-nerve injury and in the L4 and L5 spinal cord on day 35 post-nerve injury. These temporal and anatomically specific changes in mOR expression following nerve injury are likely to have functional consequences on pain-associated behaviors and opioid analgesia.

Propentofylline: glial modulation, neuroprotection, and alleviation of chronic pain.

Propentofylline is a unique methylxanthine with clear cyclic AMP, phosphodiesterase, and adenosine actions, including enhanced synaptic adenosine signaling. Both in vitro and in vivo studies have demonstrated profound neuroprotective, antiproliferative, and anti-inflammatory effects of propentofylline. Propentofylline has shown efficacy in preclinical models of stroke, opioid tolerance, and acute and chronic pain. Clinically, propentofylline has shown efficacy in degenerative and vascular dementia, and as a potential adjuvant treatment for schizophrenia and multiple sclerosis. Possible mechanisms of action include a direct glial modulation to decrease a reactive phenotype, decrease glial production and release of damaging proinflammatory factors, and enhancement of astrocyte-mediated glutamate clearance. This chapter reviews the literature that supports a myriad of protective actions of this small molecule and implicates propentofylline as a potential therapeutic for the treatment of chronic pain. From these studies, we propose a CNS multipartite synaptic action of propentofylline that includes modulation of pre- and postsynaptic neurons, astrocytes, and microglia in the treatment of chronic pain syndromes, including, but not limited to, neuropathic pain.

Incorporating Scientific Publishing into an Undergraduate Neuroscience Course: A Case Study Using IMPULSE.

The journal IMPULSE offers undergraduates worldwide the opportunity to publish research and serve as peer reviewers for the submissions of others. Undergraduate faculty have recognized the journal's value in engaging students working in their labs in the publication process. However, integration of scientific publication into an undergraduate laboratory classroom setting has been lacking. We report here on a course at Ursinus College where 20 students taking Molecular Neurobiology were required to submit manuscripts to IMPULSE. The syllabus allowed for the laboratory research to coincide with the background research and writing of the manuscript. Students completed their projects on the impact of drugs on the Daphnia magna nervous system while producing manuscripts ready for submission by week 7 of the course. Findings from a survey completed by the students and perceptions of the faculty member teaching the course indicated that students spent much more time writing, were more focused on completing the assays, completed the assays with larger data sets, were more engaged in learning the scientific concepts and were more thorough with their revisions of the paper knowing that it might be published. Further, the professor found she was more thorough in critiquing students' papers knowing they would be externally reviewed. Incorporating journal submission into the course stimulated an in depth writing experience and allowed for a deeper exploration of the topic than students would have experienced otherwise. This case study provides evidence that IMPULSE can be successfully used as a means of incorporating scientific publication into an undergraduate laboratory science course. This approach to teaching undergraduate neuroscience allows for a larger number of students to have hands-on research and scientific publishing experience than would be possible with the current model of a few students in a faculty member's laboratory. This report illustrates that IMPULSE can be incorporated as an integral part of an academic curriculum with positive outcomes on student engagement and performance.

Metabotropic glutamate receptor-5 and protein kinase C-epsilon increase in dorsal root ganglion neurons and spinal glial activation in an adolescent rat model of painful neck injury.

There is growing evidence that neck pain is common in adolescence and is a risk factor for the development of chronic neck pain in adulthood. The cervical facet joint and its capsular ligament is a common source of pain in the neck in adults, but its role in adolescent pain remains unknown. The aim of this study was to define the biomechanics, behavioral sensitivity, and indicators of neuronal and glial activation in an adolescent model of mechanical facet joint injury. A bilateral C6-C7 facet joint distraction was imposed in an adolescent rat and biomechanical metrics were measured during injury. Following injury, forepaw mechanical hyperalgesia was measured, and protein kinase C-epsilon (PKCɛ) and metabotropic glutamate receptor-5 (mGluR5) expression in the dorsal root ganglion and markers of spinal glial activation were assessed. Joint distraction induced significant mechanical hyperalgesia during the 7 days post-injury (p < 0.001). Painful injury significantly increased PKCɛ expression in small- and medium-diameter neurons compared to sham (p < 0.05) and naïve tissue (p < 0.001). Similarly, mGluR5 expression was significantly elevated in small-diameter neurons after injury (p < 0.05). Spinal astrocytic activation after injury was also elevated over sham (p < 0.035) and naïve (p < 0.0001) levels; microglial activation was only greater than naïve levels (p < 0.006). Mean strains in the facet capsule during injury were 32.8 ± 12.9%, which were consistent with the strains associated with comparable degrees of hypersensitivity in the adult rat. These results suggest that adolescents may have a lower tissue tolerance to induce pain and associated nociceptive response than do adults.