State of the science review: Advances in pain management in wounded service members over a decade at war.

The pain conditions and comorbidities experienced by injured service members and the challenge of pain management by the military medical system offer a unique opportunity to inform pain management and medical research. In this article, acute and chronic pain issues, current treatment options and limitations, as well as novel approaches to pain management are discussed within the context of combat casualty care, from the battlefield to hospitalization and rehabilitation. This review will also highlight the current pain management limitations that need to be addressed in future clinical and basic science research to improve care for our nation's injured service members.

The neuroanatomy of sexual dimorphism in opioid analgesia.

The influence of sex has been neglected in clinical studies on pain and analgesia, with the vast majority of research conducted exclusively in males. However, both preclinical and clinical studies indicate that males and females differ in both the anatomical and physiological composition of central nervous system circuits that are involved in pain processing and analgesia. These differences influence not only the response to noxious stimuli, but also the ability of pharmacological agents to modify this response. Morphine is the most widely prescribed opiate for the alleviation of persistent pain in the clinic; however, it is becoming increasingly clear that morphine is less potent in women compared to men. This review highlights recent research identifying neuroanatomical and physiological dimorphisms underlying sex differences in pain and opioid analgesia, focusing on the endogenous descending pain modulatory circuit.

A rat model of full thickness thermal injury characterized by thermal hyperalgesia, mechanical allodynia, pronociceptive peptide release and tramadol analgesia.

Opioid-related side effects are problematic for burn patients. Dual mechanism therapeutics targeting opioid and non-opioid mechanisms may have reduced side effects with similar analgesic efficacy. Tramadol combines mu opioid receptor agonism with norepinephrine reuptake inhibition and has been effective in treating some types of pain. The effectiveness of tramadol in treating pain associated with burns is unclear. We hypothesized that tramadol is effective in reducing thermal injury-evoked pain behaviors in a rat model. Rats were anesthetized and a 100°C metal probe was placed on the hindpaw for 30 s to induce a full thickness thermal injury. A subset of rats was perfusion fixed and hindpaw tissue and spinal cord collected for anatomical analysis. Rats received morphine (5 mg/kg; i.p.), tramadol (10-30 mg/kg; i.p.) or vehicle and latency to paw withdrawal from a noxious thermal or non-noxious mechanical stimulus was recorded every 10 min over 70 min and again at 2 h. We report that pain behaviors developed within 48 h and peaked at 1 week; paralleled by enhanced expression of pronociceptive neuropeptides in the spinal cord. Morphine and tramadol significantly attenuated hyperalgesia and allodynia, while not significantly altering motor coordination/sedation. These data indicate dual mechanism therapeutics may be effective for treating pain associated with burns.

Curcumin: a novel therapeutic for burn pain and wound healing.

INTRODUCTION: Managing burn injury-associated pain and wounds is a major unresolved clinical problem. Opioids, nonsteroidal antiinflammatory drugs (NSAIDs), antidepressants and anticonvulsants remain the most common forms of analgesic therapy to treat burn patients. However, prolonged treatment with these drugs leads to dose escalation and serious side effects. Additionally, severe burn wounds cause scarring and are susceptible to infection. Recent encouraging findings demonstrate that curcumin, a major bioactive component found in turmeric, is a natural pharmacotherapeutic for controlling both severe burn pain and for improved wound healing. AREAS COVERED: This article covers current pr-clinical and clinical studies on the analgesic and wound healing effects. Particular emphasis has been placed on studies aimed at developing improved curcumin delivery vehicles that increase its bioavailability. Based on the available evidence, a hypothesis is proposed that the dual beneficial effects of curcumin, analgesia and enhanced wound healing are mediated through common anti-inflammatory mechanisms. EXPERT OPINION: Emerging studies have demonstrated that curcumin is a promising investigational drug to treat both pain and wounds. The adequate control of severe burn pain, particularly over the long courses required for healing, as well improvements in burn wound healing are unmet clinical needs.

Serotonergic neuromodulation of peripheral nociceptors.

Nociception, the encoding and processing of noxious environmental stimuli by sensory neurons, functions to protect an organism from bodily damage. Activation of the terminal endings of certain sensory neurons, termed nociceptors, triggers a train of impulses to neurons in the spinal cord. Signals are integrated and processed in the dorsal spinal cord and then projected to the brain where they elicit the perception of pain. A number of neuromodulators that can affect nociceptors are released in the periphery during the inflammation that follows an initial injury. Serotonin (5-HT) is a one such proinflammatory mediator. This review discusses our current understanding of the neuromodulatory role of 5-HT, and specifically how this monoamine activates and sensitizes nociceptors. Potential therapeutic targets to treat pain are described.

Sex differences in serotonin enhancement of capsaicin-evoked calcitonin gene-related peptide release from human dental pulp.

Serotonin (5HT) is a pronociceptive mediator in the periphery, and evidence implicates involvement in trigeminal pain processing. However, the mechanism(s) by which 5HT modulates trigeminal nociceptors remains unclear. Trigeminal pain can be evoked by the transient receptor potential V1 channel (TRPV1), which is expressed by nociceptive trigeminal neurons and induces release of proinflammatory calcitonin gene-related peptide (CGRP). In our preclinical models, 5HT evoked thermal hyperalgesia and enhanced calcium influx and CGRP release from the TRPV1 population of trigeminal nociceptors. Whether this occurs in humans is unknown. As dental pulp is densely innervated by trigeminal nociceptors, routine tooth extractions offer a unique opportunity to examine whether 5HT enhances CGRP release from human nociceptors. Pulpal tissue was collected from 140 extracted molar teeth from men and women, and basal release samples were collected before treatment with saline or 5HT 100µmol/L. CGRP release was then stimulated with the TRPV1 agonist capsaicin 1µmol/L and quantitated by enzyme immunoassay. Additional samples were collected for Western blots to examine 5HT receptor expression. We report that 5HT induced a significant increase in capsaicin-evoked CGRP release, and that this enhancement was observed only in female dental pulp, with no effect of 5HT on male dental pulp. The greatest amount of CGRP release occurred in dental pulp from women in the luteal phase of the menstrual cycle. These results indicate that 5HT enhances capsaicin-evoked CGRP release from human trigeminal nociceptors in a sexually dimorphic manner providing a mechanistic basis for prevalence of trigeminal pain disorders in women.

Serotonin increases the functional activity of capsaicin-sensitive rat trigeminal nociceptors via peripheral serotonin receptors.

Peripheral serotonin (5HT) has been implicated in migraine and temporomandibular pain disorders in humans and animal models and yet the mechanism(s) by which 5HT evokes pain remains unclear. Trigeminal pain can be triggered by activation of the transient receptor potential V1 channel (TRPV1), expressed by a subset of nociceptive trigeminal ganglia (TG) neurons and gated by capsaicin, noxious heat, and other noxious stimuli. As 5HT is released in the periphery during inflammation and evokes thermal hyperalgesia, and TRPV1 is essential for thermal hyperalgesia, we hypothesized that 5HT increases the activity of capsaicin-sensitive trigeminal neurons and that this increase can be attenuated by pharmacologically targeting peripheral 5HT receptors. TG cultures were pretreated with 5HT (10 nM-100 µM), sumatriptan (5HT(1B/1D) agonist), ketanserin (5HT(2A) antagonist), granisetron (5HT(3) antagonist), or vehicle prior to capsaicin (30-50 nM). Single-cell accumulation of intracellular calcium was recorded or calcitonin gene-related peptide (CGRP) release was measured following each treatment. In addition, using in situ hybridization and immunohistochemistry, we detected the colocalization of 5HT(1B), 5HT(1D), 5HT(2A), and 5HT(3A), but not 5HT(2C) mRNA with TRPV1 in TG cells. 5HT pretreatment evoked a significant increase in calcium accumulation in capsaicin-sensitive trigeminal neurons and enhanced capsaicin-evoked CGRP release, but had no significant effect when given alone. Sumatriptan, ketanserin, and granisetron treatment attenuated calcium accumulation and 5HT enhancement of capsaicin-evoked CGRP release. Together these results indicate that 5HT increases the activity of capsaicin-sensitive peripheral nociceptors, which can be attenuated by pharmacologically targeting peripheral 5HT receptors, thereby providing a mechanistic basis for peripheral craniofacial pain therapy.

The role of the periaqueductal gray in the modulation of pain in males and females: are the anatomy and physiology really that different?

Anatomical and physiological studies conducted in the 1960s identified the periaqueductal gray (PAG) and its descending projections to the rostral ventromedial medulla (RVM) and spinal cord dorsal horn, as a primary anatomical pathway mediating opioid-based analgesia. Since these initial studies, the PAG-RVM-spinal cord pathway has been characterized anatomically and physiologically in a wide range of vertebrate species. Remarkably, the majority of these studies were conducted exclusively in males with the implicit assumption that the anatomy and physiology of this circuit were the same in females; however, this is not the case. It is well established that morphine administration produces greater antinociception in males compared to females. Recent studies indicate that the PAG-RVM pathway contributes to the sexually dimorphic actions of morphine. This manuscript will review our anatomical, physiological, and behavioral data identifying sex differences in the PAG-RVM pathway, focusing on its role in pain modulation and morphine analgesia.

Sex differences in micro-opioid receptor expression in the rat midbrain periaqueductal gray are essential for eliciting sex differences in morphine analgesia.

Opioid-based narcotics are the most widely prescribed therapeutic agent for the alleviation of persistent pain; however, it is becoming increasingly clear that morphine is significantly less potent in women compared with men. Morphine primarily binds to mu-opioid receptors (MORs), and the periaqueductal gray (PAG) contains a dense population of MOR-expressing neurons. Via its descending projections to the rostral ventromedial medulla and the dorsal horn of the spinal cord, the PAG is considered an essential neural substrate for opioid-based analgesia. We hypothesized that MOR expression in the PAG was sexually dimorphic, and that these sex differences contribute to the observed sex differences in morphine potency. Using immunohistochemistry, we report that males had a significantly higher expression of MOR in the ventrolateral PAG compared with cycling females, whereas the lowest level of expression was observed in proestrus females. CFA-induced inflammatory pain produced thermal hyperalgesia in both males and females that was significantly reversed in males with a microinjection of morphine into the ventrolateral PAG; this effect was significantly greater than that observed in proestrus and estrus females. Selective lesions of MOR-expressing neurons in the ventrolateral PAG resulted in a significant reduction in the effects of systemic morphine in males only, and this reduction was positively correlated with the level of MOR expression in the ventrolateral PAG. Together, these results provide a mechanism for sex differences in morphine potency.

Androgen and estrogen (alpha) receptor localization on periaqueductal gray neurons projecting to the rostral ventromedial medulla in the male and female rat.

The periaqueductal gray (PAG) is involved in many gonadal steroid-sensitive behaviors, including responsiveness to pain. The PAG projects to the rostral ventromedial medulla (RVM), comprising the primary circuit driving pain inhibition. Morphine administered systemically or directly into the PAG produces greater analgesia in male compared to female rats, while manipulation of gonadal hormones alters morphine potency in both sexes. It is unknown if these alterations are due to steroidal actions on PAG neurons projecting to the RVM. The expression of androgen (AR) and estrogen (ERalpha) receptors in the PAG of female rats and within this descending inhibitory pathway in both sexes is unknown. The present study used immunohistochemical techniques (1) to map the distribution of AR and ERalpha across the rostrocaudal axis of the PAG; and (2) to determine whether AR and/or ERalpha were colocalized on PAG neurons projecting to the RVM in male and female rats. AR and ERalpha immunoreactive neurons (AR-IR, ERalpha-IR) were densely distributed within the caudal PAG of male rats, with the majority localized in the lateral/ventrolateral PAG. Females had significantly fewer AR-IR neurons, while the quantity of ERalpha was comparable between the sexes. In both sexes, approximately 25-50% of AR-IR neurons and 20-50% of ERalpha-IR neurons were retrogradely labeled. This study provides direct evidence of the expression of steroid receptors in the PAG and the descending pathway driving pain inhibition in both male and female rats and may provide a mechanism whereby gonadal steroids modulate pain and morphine potency.

Sexually dimorphic activation of the periaqueductal gray-rostral ventromedial medullary circuit during the development of tolerance to morphine in the rat.

The midbrain periaqueductal gray (PAG) and its descending projections to the rostral ventromedial medulla (RVM) provides an essential neural circuit for the antinociceptive effects of opiates, and has been implicated in the development of tolerance to morphine. Systemic morphine activates a greater proportion of PAG-RVM neurons in male vs female rats, and induces tolerance to a greater degree in males. The present studies tested the hypothesis that if the PAG-RVM pathway is essential for the development of tolerance, then: (i) morphine activation of the PAG-RVM pathway should decline as tolerance develops; and (ii) sex differences in the development of tolerance to morphine should be reflected as a greater decline in the activation of this pathway in males. These hypotheses were tested in male and female rats using behavioral testing (hot-plate) and immunohistochemistry to map the activation of the PAG-RVM pathway following repeated morphine administration (5 mg/kg; s.c.). In males, morphine potency decreased from 3.0 to 6.3 mg/kg, indicating tolerance, and this was paralleled by a steady decline in the percentage of PAG-RVM output neurons activated by morphine. In contrast, in females the shift in morphine potency was significantly attenuated (D(50) 6-8.3 mg/kg), and no significant difference in the activity of PAG-RVM output neurons was noted. These results demonstrate that the greater development of tolerance to morphine administration in male rats corresponds with a significant reduction in the activation of the PAG-RVM circuit and suggest a central role for the PAG in the development of tolerance to morphine.

Sex differences in the anatomical and functional organization of the periaqueductal gray-rostral ventromedial medullary pathway in the rat: a potential circuit mediating the sexually dimorphic actions of morphine.

Previous studies have demonstrated that morphine, administered systemically or directly into the periaqueductal gray (PAG), produces a significantly greater degree of antinociception in males in comparison with females. Because the midbrain PAG and its descending projections to the rostral ventromedial medulla (RVM) constitute an essential neural circuit for opioid-based analgesia, the present studies were conducted to determine whether sex differences in the anatomical organization of the PAG-RVM pathway, and its activation during persistent inflammatory pain, could account for sex-based differences in opioid analgesia. In the rat, retrograde tracing was combined with Fos immunocytochemistry to investigate sexual dimorphism in the organization of the PAG-RVM circuit and its activation by persistent inflammatory pain induced by intraplantar injection of complete Freund's adjuvant (CFA). The ability of morphine to suppress the activation of the PAG-RVM circuit was also examined. Sexually dimorphic retrograde labeling was observed within the dorsomedial and lateral/ventrolateral PAG at all rostrocaudal levels, with females having significantly more PAG-RVM output neurons in comparison with males. While no sex differences were noted in the activation of the PAG by persistent inflammatory pain, significantly more PAG-RVM cells were activated in males in comparison with females. Systemic administration of morphine significantly suppressed CFA-induced Fos in the PAG in males only. The results of these studies demonstrate that both the anatomical organization and the functional activation of the PAG-RVM circuit are sexually dimorphic and may provide the anatomical substrate for sex-based differences in morphine analgesia.

Lead exposure alters the development of agonistic behavior in golden hamsters.

We tested the effects of exposure to different doses of lead acetate (either 0, 25, 100, or 400 ppm) on the development of aggressive behavior in male golden hamsters. Pups were tested for offensive responses across puberty, as they were maturing from play fighting to adult aggression. Our data show a dose-specific effect of lead exposure on the development of aggression during puberty at doses resulting in blood levels well below 20 microg/dl. Animals exposed to 25 ppm lead acetate were faster and performed more than twice as many attacks on intruders by late puberty. They were also twice as likely to initiate adult instead of play-fighting attacks around mid-puberty. These observations were independent of any effect on growth. Thus, exposure to low doses of lead enhanced aggression and accelerated its maturation. As such, our data support the association between exposure to low doses of lead and aggressive behavior in boys.