Role of endogenous ATP at the incision area in a rat model of postoperative pain

CD304 + adipose tissue-derived mesenchymal stem cell abundance in autologous fat grafts highly correlates with improvement of localized pain syndromes

ABSTRACT

Surgery, burns or surgery-free accident are leading causes of scars with altered tissue consistency, a reduced degree of motion and pain. Autologous fat grafting can dramatically improve tissue consistency and elasticity but less frequently results in the reduction of pain. Therefore, we analyzed different cell populations present within the adipose tissue to be engrafted and correlated them with the reduction of pain after surgery. Here, we identify a population of CD3 - CD4 - CD304 + cells present in grafted adipose tissue, whose abundance highly correlates with pain improvement shortly after surgery ( r2 = 0.7243****) as well as persistently over time (3 months later: r2 = 0.6277****, 1 year later: r2 = 0.5346***, and 4 years later: r2 = 0.5223***). These cells are characterized by the absence of the hematopoietic marker CD45, whereas they express CD90 and CD34, which characterize mesenchymal stem cells (MSCs); the concomitant presence of CD10 and CD73 in the plasma membrane supports a function of these cells in pain reduction. We deduce that the enrichment of this adipose tissue-derived MSC subset could enhance the therapeutic properties of adipose grafts and ameliorate localized pain syndromes.

Soft tissue injury in the limbs increased regional bone turnover

BACKGROUND

Pathological conditions after skeletal tissue injury such as trauma and surgical intervention are often accompanied with regional osteoporotic changes, which are recognized to be mainly caused by limb immobility after injury. However, the mechanisms for the progression of regional osteoporotic changes related to the injury remains unknown. Previous studies reported that the pathophysiological conditions related to tissue injury include the acidic micro-environment formation and increased ATP levels. In addition, we previously demonstrated that those changes in the micro-environment induced a high bone turnover state through the activation of TRPV1, ASICs and P2X expressed in bone cells. We, therefore, hypothesized that tissue injury could enhance a high bone turnover state due to those pathophysiological changes in soft tissue in the injured limb. The aim of this study was to examine whether soft tissue injury associated with cutaneous incisions in a limb affects regional bone turnover.

METHODS

Eight-week-old male C57BL/6 J mice underwent soft tissue injury associated with cutaneous incisions in the right femoral skin. During the 14 days after the incision, changes in the expression of osteoblast and osteoclast differentiation regulators and ATP were evaluated in comparison with those in uninjured mice. The pain-like behaviors and the expression of those differentiation regulators with and without treatment with bisphosphonate and Cox2 inhibitor were assessed in the injured limb.

RESULTS

Consistent with the wound healing process, the expression levels of Osterix, osteocalcin and RANKL in the femur of the incised limb were significantly increased up to 7 days, and then decreased to the same level as those in the control limbs by 14 days after the incisions. The levels of TRAP 5b and ATP were initially significantly increased, and then decreased to the same level as before injury by day 14. Bisphosphonate significantly improved the pain-like behaviors in the injured limb associated with the inhibition of osteoblast and osteoclast differentiation regulators.

CONCLUSION

We believe that the pathophysiological changes in soft tissue resulting from cutaneous incisions could be related to the induction of osteoblast and osteoclast differentiation regulators.

Nociceptive signaling mediated by P2X3, P2X4 and P2X7 receptors

Chronic pain is a debilitating condition that often occurs following peripheral tissue inflammation and nerve injury. This pain, especially neuropathic pain, is a significant clinical problem because of the ineffectiveness of clinically available drugs. Since Burnstock proposed new roles of nucleotides as neurotransmitters, the roles of extracellular ATP and P2 receptors (P2Rs) in pain signaling have been extensively studied, and ATP-P2R signaling has subsequently received much attention as it can provide clues toward elucidating the mechanisms underlying chronic pain and serve as a potential therapeutic target. This review summarizes the literature regarding the role of ATP signaling via P2X3Rs (as well as P2X2/3Rs) in primary afferent neurons and via P2X4Rs and P2X7Rs in spinal cord microglia in chronic pain, and discusses their respective therapeutic potentials.

Optimal Methods of Documenting Analgesic Efficacy in Neonatal Piglets Undergoing Castration

Analgesic products for piglet castration are critically needed. This requires extensive animal experimentation such as to meet regulatory-required proof of efficacy. At present, there are no validated methods of assessing pain in neonatal piglets. This poses challenges for investigators to optimize trial design and to meet ethical obligations to minimize the number of animals needed. Pain in neonatal piglets may be subtle, transient, and/or variably expressed and, in the absence of validated methods, investigators must rely on using a range of biochemical, physiological and behavioural variables, many of which appear to have very low (or unknown) sensitivity or specificity for documenting pain, or pain-relieving effects. A previous systematic review of this subject was hampered by the high degree of variability in the literature base both in terms of methods used to assess pain and pain mitigation, as well as in outcomes reported. In this setting we provide a narrative review to assist in determining the optimal methods currently available to detect piglet pain during castration and methods to mitigate castration-induced pain. In overview, the optimal outcome variables identified are nociceptive motor and vocal response scores during castration and quantitative sensory-threshold response testing and pain-associated behaviour scores following castration.

NOD-like receptor protein 3 inflammasome drives postoperative mechanical pain in a sex-dependent manner

Postoperative pain management continues to be suboptimal because of the lack of effective nonopioid therapies and absence of understanding of sex-driven differences. Here, we asked how the NLRP3 inflammasome contributes to postoperative pain. Inflammasomes are mediators of the innate immune system that are responsible for activation and secretion of IL-1β upon stimulation by specific molecular signals. Peripheral IL-1β is known to contribute to the mechanical sensitization induced by surgical incision. However, it is not known which inflammasome mediates the IL-1β release after surgical incision. Among the 9 known inflammasomes, the NLRP3 inflammasome is ideally positioned to drive postoperative pain through IL-1β production because NLRP3 can be activated by factors that are released by incision. Here, we show that male mice that lack NLRP3 (NLRP3) recover from surgery-induced behavioral and neuronal mechanical sensitization faster and display less surgical site inflammation than mice expressing NLRP3 (wild-type). By contrast, female NLRP3 mice exhibit minimal attenuation of the postoperative mechanical hypersensitivity and no change in postoperative inflammation compared with wild-type controls. Sensory neuron-specific deletion of NLRP3 revealed that in males, NLRP3 expressed in non-neuronal cells and potentially sensory neurons drives postoperative pain. However, in females, only the NLRP3 that may be expressed in sensory neurons contributes to postoperative pain where the non-neuronal cell contribution is NLRP3 independent. This is the first evidence of a key role for NLRP3 in postoperative pain and reveals immune-mediated sex differences in postoperative pain.

A Novel Sex-Dependent Target for the Treatment of Postoperative Pain: The NLRP3 Inflammasome

In recent years the innate immune system has been shown to be crucial for the pathogenesis of postoperative pain. The mediators released by innate immune cells drive the sensitization of sensory neurons following injury by directly acting on peripheral nerve terminals at the injury site. The predominate sensitization signaling pathway involves the proinflammatory cytokine interleukin-1β (IL-1β). IL-1β is known to cause pain by directly acting on sensory neurons. Evidence demonstrates that blockade of IL-1β signaling decreases postoperative pain, however complete blockade of IL-1β signaling increases the risk of infection and decreases effective wound healing. IL-1β requires activation by an inflammasome; inflammasomes are cytosolic receptors of the innate immune system. NOD-like receptor protein 3 (NLRP3) is the predominant inflammasome activated by endogenous molecules that are released by tissue injury such as that which occurs during neuropathic and inflammatory pain disorders. Given that selective inhibition of NLRP3 alleviates postoperative mechanical pain, its selective targeting may be a novel and effective strategy for the treatment of pain that would avoid complications of global IL-1β inhibition. Moreover, NLRP3 is activated in pain in a sex-dependent and cell type-dependent manner. Sex differences in the innate immune system have been shown to drive pain and sensitization through different mechanisms in inflammatory and neuropathic pain disorders, indicating that it is imperative that both sexes are studied when researchers investigate and identify new targets for pain therapeutics. This review will highlight the roles of the innate immune response, the NLRP3 inflammasome, and sex differences in neuropathic and inflammatory pain.

Role of peripheral purinoceptors in the development of bee venom-induced nociception: a behavioural and electrophysiological study in rats

Colocalization of purinergic P2X and P2Y receptors in dorsal root ganglion sensory neurons implies that these receptors play an integrative role in the nociceptive transmission process under inflammatory conditions. In the present study, behavioural and in vivo electrophysiological methods were used to examine the peripheral role of P2 receptors in the persistent nociceptive responses induced by subcutaneous bee venom injection (2 mg/mL) in. Sprague-Dawley rats Local pretreatment with the wide-spectrum P2 receptor antagonist pyridoxalphosphate-6-azophenyl-2',4'-disulphonic acid (PPADS; 1 mmol/L, 50 μL) 10 min prior to s.c. bee venom injection significantly suppressed the duration of spontaneous nociceptive lifting/licking behaviour, inhibited mechanical hyperalgesia and decreased the firing of spinal dorsal horn wide dynamic range neurons in response to bee venom, without affecting primary thermal and mirror-image hyperalgesia. The localized antinociceptive action of PPADS was not due to a systemic effect, because application of the same dose of PPADS to the contralateral side was not effective. The results suggest that activation of peripheral P2 receptors is involved in the induction of nociceptive responses, mechanical hyperalgesia and the excitation of sensory spinal neurons.

Activation of spinal phosphatidylinositol 3-kinase/protein kinase B mediates pain behavior induced by plantar incision in mice

The etiology of postoperative pain may be different from antigen-induced inflammatory pain and neuropathic pain. However, central neural plasticity plays a key role in incision pain. It is also known that phosphatidylinositol 3-kinase (PI3K) and protein kinase B/Akt (PKB/Akt) are widely expressed in laminae I-IV of the spinal horn and play a critical role in spinal central sensitization. In the present study, we explored the role of PI3K and Akt in incision pain behaviors. Plantar incision induced a time-dependent activation of spinal PI3K-p110γ and Akt, while activated Akt and PI3K-p110γ were localized in spinal neurons or microglias, but not in astrocytes. Pre-treatment with PI3K inhibitors, wortmannin or LY294002 prevented the activation of Akt brought on by plantar incision in a dose-dependent manner. In addition, inhibition of spinal PI3K signaling pathway prevented pain behaviors (dose-dependent) and spinal Fos protein expression caused by plantar incision. These data demonstrated that PI3K signaling mediated pain behaviors caused by plantar incision in mice.

Tissue injury and related mediators of pain exacerbation

Tissue injury and inflammation result in release of various mediators that promote ongoing pain or pain hypersensitivity against mechanical, thermal and chemical stimuli. Pro-nociceptive mediators activate primary afferent neurons directly or indirectly to enhance nociceptive signal transmission to the central nervous system. Excitation of primary afferents by peripherally originating mediators, so-called “peripheral sensitization”, is a hallmark of tissue injury-related pain. Many kinds of pro-nociceptive mediators, including ATP, glutamate, kinins, cytokines and tropic factors, synthesized at the damaged tissue, contribute to the development of peripheral sensitization. In the present review we will discuss the molecular mechanisms of peripheral sensitization following tissue injury.

Pathophysiology of astroglial purinergic signalling

Astrocytes are fundamental for central nervous system (CNS) physiology and are the fulcrum of neurological diseases. Astroglial cells control development of the nervous system, regulate synaptogenesis, maturation, maintenance and plasticity of synapses and are central for nervous system homeostasis. Astroglial reactions determine progression and outcome of many neuropathologies and are critical for regeneration and remodelling of neural circuits following trauma, stroke, ischaemia or neurodegenerative disorders. They secrete multiple neurotransmitters and neurohormones to communicate with neurones, microglia and the vascular walls of capillaries. Signalling through release of ATP is the most widespread mean of communication between astrocytes and other types of neural cells. ATP serves as a fast excitatory neurotransmitter and has pronounced long-term (trophic) roles in cell proliferation, growth, and development. During pathology, ATP is released from damaged cells and acts both as a cytotoxic factor and a proinflammatory mediator, being a universal "danger" signal. In this review, we summarise contemporary knowledge on the role of purinergic receptors (P2Rs) in a variety of diseases in relation to changes of astrocytic functions and nucleotide signalling. We have focussed on the role of the ionotropic P2X and metabotropic P2YRs working alone or in concert to modify the release of neurotransmitters, to activate signalling cascades and to change the expression levels of ion channels and protein kinases. All these effects are of great importance for the initiation, progression and maintenance of astrogliosis-the conserved and ubiquitous glial defensive reaction to CNS pathologies. We highlighted specific aspects of reactive astrogliosis, especially with respect to the involvement of the P2X(7) and P2Y(1)R subtypes. Reactive astrogliosis exerts both beneficial and detrimental effects in a context-specific manner determined by distinct molecular signalling cascades. Understanding the role of purinergic signalling in astrocytes is critical to identifying new therapeutic principles to treat acute and chronic neurological diseases.

Vatalanib decrease the positive interaction of VEGF receptor-2 and P2X2/3 receptor in chronic constriction injury rats

Neuropathic pain can arise from a lesion affecting the peripheral nervous system. Selective P2X(3) and P2X(2/3) receptors' antagonists effectively reduce neuropathic pain. VEGF inhibitors are effective for pain relief. The present study investigated the effects of Vatalanib (VEGF receptor-2 (VEGFR-2) inhibitor) on the neuropathic pain to address the interaction of VEGFR-2 and P2X(2/3) receptor in dorsal root ganglia of chronic constriction injury (CCI) rats. Neuropathic pain symptoms following CCI are similar to most peripheral lesions as assessed by the Neuropathic Pain Symptom Inventory. Sprague-Dawley rats were randomly divided into sham group, CCI group and CCI rats treated with Vatalanib group. Mechanical withdrawal threshold and thermal withdrawal latency were measured. Co-expression of VEGFR-2 and P2X(2) or P2X(3) in L4-6 dorsal root ganglia (DRG) was detected by double-label immunofluorescence. The modulation effect of VEGF on P2X(2/3) receptor agonist-activated currents in freshly isolated DRG neurons of rats both of sham and CCI rats was recorded by whole-cell patch-clamp technique. The mechanical withdrawal threshold (MWT) and thermal withdrawal latency (TWL) in CCI group were lower than those in sham group (p<0.05). MWT and TWL in CCI rats treated with Vatalanib group were increased compared with those in CCI group (p<0.05). VEGFR-2 and P2X(2) or P2X(3) receptors were co-expressed in the cytoplasm and surface membranes of DRG. The co-expression of VEGFR-2 and P2X(2) or P2X(3) receptor in CCI group exhibited more intense staining than those in sham group and CCI rats treated with Vatalanib group, respectively. VEGF enhanced the amplitude of ATP and α,β-meATP -activated currents of both sham and CCI rats. Increment effects of VEGF on ATP and α,β-meATP -activated currents in CCI rats were higher than those in sham rats. Both ATP (100 μM) and α,β-meATP (10 μM)- activated currents enhanced by VEGF ( 1nM) were significantly blocked by Vatalanib (1 μM, an inhibitor of VEGF receptors). The stain values of VEGFR-2, P2X(2) and P2X(3) protein expression in L4/5 DRG of CCI treated with Vatalanib group were significantly decreased compared with those in CCI group (p<0.01). Vatalanib can alleviate chronic neuropathic pain by decreasing the activation of VEGF on VEGFR-2 and the positive interaction between the up-regulated VEGFR-2 and P2X(2/3) receptors in the neuropathic pain signaling.

Correlation of changes in pain intensity with synovial fluid adenosine triphosphate levels after treatment of patients with osteoarthritis of the knee with high-molecular-weight hyaluronic acid

We sought to determine whether a clinical association exists between osteoarthritis (OA)-associated knee pain and adenosine triphosphate (ATP) levels in synovial fluid (SF). A total of 28 patients with 28 primary OA knees were included. They routinely received intra-articular injection of high-molecular-weight hyaluronic acid (HA) once weekly for 5 weeks (treated group). Eight patients without knee pain who had undergone an operation for anterior or posterior cruciate ligament reconstruction 2 years ago were also examined (control group). SF and blood ATP concentrations, total amount of ATP, total SF volume, and Visual Analogue Scale (VAS) scores in all patients were measured and we compared pre-treatment values with those 1 week after the final treatment. We evaluated the correlation of change in total ATP (ΔATP) and change in VAS score (ΔVAS), ΔVAS and change in SF volume (ΔSF), and ATP concentration in SF and blood. In the treated group, SF ATP concentration, total amount of ATP, SF volume, and VAS score were all significantly lower post-treatment than pre-treatment (p = 0.0005, 0.0003, 0.0022, and < 0.0001, respectively). In treated group, ΔVAS was significantly associated with ΔATP (r = 0.56, p = 0.0032), ΔSF was significantly associated with ΔVAS (r = 0.78, p < 0.0001), and total amount of SF ATP and SF volume at pre-treatment were significantly higher than the control group (p < 0.0001, p < 0.0001) We demonstrated an association between SF ATP level changes and OA knee pain, which should facilitate a further understanding of OA pain mechanisms.

Serotonin receptors are involved in the spinal mediation of descending facilitation of surgical incision-induced increase of Fos-like immunoreactivity in rats

Background

Descending pronociceptive pathways may be implicated in states of persistent pain. Paw skin incision is a well-established postoperative pain model that causes behavioral nociceptive responses and enhanced excitability of spinal dorsal horn neurons. The number of spinal c-Fos positive neurons of rats treated intrathecally with serotonin, noradrenaline or acetylcholine antagonists where evaluated to study the descending pathways activated by a surgical paw incision.

Results

The number of c-Fos positive neurons in laminae I/II ipsilateral, lamina V bilateral to the incised paw, and in lamina X significantly increased after the incision. These changes: remained unchanged in phenoxybenzamine-treated rats; were increased in the contralateral lamina V of atropine-treated rats; were inhibited in the ipsilateral lamina I/II by 5-HT_1/2B/2C (methysergide), 5-HT_2A (ketanserin) or 5-HT_{1/2A/2C/5/6/7} (methiothepin) receptors antagonists, in the ipsilateral lamina V by methysergide or methiothepin, in the contralateral lamina V by all the serotonergic antagonists and in the lamina X by LY 278,584, ketanserin or methiothepin.

Conclusions

We conclude: (1) muscarinic cholinergic mechanisms reduce incision-induced response of spinal neurons inputs from the contralateral paw; (2) 5-HT_1/2A/2C/3 receptors-mediate mechanisms increase the activity of descending pathways that facilitates the response of spinal neurons to noxious inputs from the contralateral paw; (3) 5-HT_1/2A/2C and 5-HT_1/2C receptors increases the descending facilitation mechanisms induced by incision in the ipsilateral paw; (4) 5-HT_2A/3 receptors contribute to descending pronociceptive pathways conveyed by lamina X spinal neurons; (5) α-adrenergic receptors are unlikely to participate in the incision-induced facilitation of the spinal neurons.

When pain after surgery doesn't go away…

Chronic post-surgical pain is a common, under-recognized and important clinical problem which affects millions of patients worldwide. It results from a series of neuroplastic changes associated most commonly with peripheral nerve injury at the time of surgery. Predisposing factors include the type of surgery, pre-operative and acute post-operative pain intensity, and probably psychological (e.g. pain-catastrophizing) and genetic factors [e.g. GCH1 (GTP cyclohydrolase 1) haplotype]. Preventive measures which are currently available include selection of a minimally invasive surgical technique and an aggressive multimodal perioperative analgesic regimen. Very promising therapeutic agents which target the sensitization process are currently in development.

Purinergic P2 receptors as targets for novel analgesics

Following hints in the early literature about adenosine 5'-triphosphate (ATP) injections producing pain, an ion-channel nucleotide receptor was cloned in 1995, P2X3 subtype, which was shown to be localized predominantly on small nociceptive sensory nerves. Since then, there has been an increasing number of papers exploring the role of P2X3 homomultimer and P2X2/3 heteromultimer receptors on sensory nerves in a wide range of organs, including skin, tongue, tooth pulp, intestine, bladder, and ureter that mediate the initiation of pain. Purinergic mechanosensory transduction has been proposed for visceral pain, where ATP released from epithelial cells lining the bladder, ureter, and intestine during distension acts on P2X3 and P2X2/3, and possibly P2Y, receptors on subepithelial sensory nerve fibers to send messages to the pain centers in the brain as well as initiating local reflexes. P1, P2X, and P2Y receptors also appear to be involved in nociceptive neural pathways in the spinal cord. P2X4 receptors on spinal microglia have been implicated in allodynia. The involvement of purinergic signaling in long-term neuropathic pain and inflammation as well as acute pain is discussed as well as the development of P2 receptor antagonists as novel analgesics.

Pathophysiology and therapeutic potential of purinergic signaling

The concept of a purinergic signaling system, using purine nucleotides and nucleosides as extracellular messengers, was first proposed over 30 years ago. After a brief introduction and update of purinoceptor subtypes, this article focuses on the diverse pathophysiological roles of purines and pyrimidines as signaling molecules. These molecules mediate short-term (acute) signaling functions in neurotransmission, mechanosensory transduction, secretion and vasodilatation, and long-term (chronic) signaling functions in cell proliferation, differentiation, and death involved in development and regeneration. Plasticity of purinoceptor expression in pathological conditions is frequently observed, including an increase in the purinergic component of autonomic cotransmission. Recent advances in therapies using purinergic-related drugs in a wide range of pathological conditions will be addressed with speculation on future developments in the field.

Dissociation between post-surgical pain behaviors and spinal Fos-like immunoreactivity in the rat

Previous studies have demonstrated that Fos-like immunoreactivity is increased in spinal dorsal horn neurons in several pain models, and have suggested that Fos-like immunoreactivity could be used as a marker of neurons activated by painful stimulation. In the present study, we evaluated nociceptive behaviors and spinal Fos-like immunoreactivity in a rat skin incision model of post-operative pain. In this model, evoked and non-evoked pain behaviors were observed at least for 2 days after paw surgery, an increased number of Fos-like immunoreactive neurons was observed in the spinal dorsal horn at lumbar levels 4-5 two-hour post-surgery. The number of Fos-like immunoreactive neurons was significantly greater in animals with skin-muscle incision compared to animals with skin-alone incision. Interestingly, spinal Fos-like immunoreactivity was quickly normalized in rats with paw surgery at later time points (8 and 24 h post-surgery), whereas nociceptive behaviors were still observed. Furthermore, at 24 h post-surgery, spinal Fos-like immunoreactivity induced by thermal stimulation (42, 44, 46, 48, 52 degrees C for 15 s) was not significantly different between sham animals and animals with surgery. In both groups, an increase in spinal Fos-like immunoreactive neurons was observed with increasing temperatures, with similar laminar distribution. Finally, systemic morphine reduced post-operative pain and Fos-like immunoreactivity in a naloxone reversible manner, with greater potency and efficacy on behavioral endpoints than on Fos-like immunoreactivity. These results demonstrate a different profile of nociceptive behaviors and spinal Fos-like immunoreactivity in the rat skin incision model, suggesting a limited potential of spinal Fos-like immunoreactivity to study post-surgical pain and its pharmacology.

P2X receptors in the rat uterine cervix, lumbosacral dorsal root ganglia, and spinal cord during pregnancy

ATP, an intracellular energy source, is released from cells during tissue stress, damage, or inflammation. The P2X subtype of the ATP receptor is expressed in rat dorsal root ganglion (DRG) cells, spinal cord dorsal horn, and axons in peripheral tissues. ATP binding to P2X receptors on nociceptors generates signals that can be interpreted as pain from damaged tissue. We have hypothesized that tissue stress or damage in the uterine cervix during late pregnancy and parturition can lead to ATP release and sensory signaling via P2X receptors. Consequently, we have examined sensory pathways from the cervix in nonpregnant and pregnant rats for the presence of purinoceptors. Antiserum against the P2X3-receptor subtype showed P2X3- receptor immunoreactivity in axon-like structures of the cervix, in small and medium-sized neurons in the L6/S1 DRG, and in lamina II of the L6/S1 spinal cord segments. Retrograde tracing confirmed the projections of axons of P2X3-receptor-immunoreactive DRG neurons to the cervix. Some P2X3-receptor-positive DRG neurons also expressed estrogen receptor-alpha immunoreactivity and expressed the phosphorylated form of cyclic AMP response-element-binding protein at parturition. Western blots showed a trend toward increases of P2X3-receptor protein between pregnancy (day 10) and parturition (day 22-23) in the cervix, but no significant changes in the DRG or spinal cord. Since serum estrogen rises over pregnancy, estrogen may influence purinoceptors in these DRG neurons. We suggest that receptors responsive to ATP are expressed in uterine cervical afferent nerves that transmit sensory information to the spinal cord at parturition.

ATP receptors in pain sensation: Involvement of spinal microglia and P2X(4) receptors

There is abundant evidence that extracellular ATP and other nucleotides have an important role in pain signaling at both the periphery and in the CNS. At first, it was thought that ATP was simply involved in acute pain, since ATP is released from damaged cells and excites directly primary sensory neurons by activating their receptors. However, neither blocking P2X/Y receptors pharmacologically nor suppressing the expression of P2X/Y receptors molecularly in sensory neurons or in the spinal cord had an effect on acute physiological pain. The focus of attention now is on the possibility that endogenous ATP and its receptor system might be activated in pathological pain states, particularly in neuropathic pain. Neuropathic pain is often a consequence of nerve injury through surgery, bone compression, diabetes or infection. This type of pain can be so severe that even light touching can be intensely painful; unfortunately, this state is generally resistant to currently available treatments. An important advance in our understanding of the mechanisms involved in neuropathic pain has been made by a recent work demonstrating the crucial role of ATP receptors (i.e., P2X₃ and P2X₄ receptors). In this review, we summarize the role of ATP receptors, particularly the P2X₄ receptor, in neuropathic pain. The expression of P2X₄ receptors in the spinal cord is enhanced in spinal microglia after peripheral nerve injury, and blocking pharmacologically and suppressing molecularly P2X₄ receptors produce a reduction of the neuropathic pain behaviour. Understanding the key roles of ATP receptors including P2X₄ receptors may lead to new strategies for the management of neuropathic pain.

A P2X receptor-mediated nociceptive afferent pathway to lamina I of the spinal cord

Of the six lamina regions in the dorsal horn of the spinal cord, lamina I is a major sensory region involved in nociceptive transmission under both physiological and pathological conditions. While P2X receptors have been shown to be involved in nociception, it remains unknown if P2X receptors are involved in nociceptive transmission to lamina I neurons. Using rat spinal cord slice preparations and patch-clamp recordings, we have demonstrated that the excitatory synaptic transmission between primary afferent fibers and lamina I neurons is significantly affected by ATP and alpha,beta-methylene-ATP. The synaptic effects of them include the increases of the frequency of both miniature excitatory postsynaptic currents (mEPSCs) and spontaneous EPSCs (sEPSCs), and decreases of evoked EPSCs (eEPSCs). These effects were blocked by pyridoxalphosphate-6-azophenyl-2', 4'-disulfonic acid (PPADS, 10 microM) and suramin (30 microM). In the neurons for which ATP and alpha,beta-methylene-ATP had effects on mEPSCs, sEPSCs and eEPSCs, capsaicin produced similar synaptic effects. Our results indicate that P2X receptors are expressed on many afferent fibers that directly synapse to lamina I neurons. Furthermore, these P2X receptor-expressing afferent fibers are capsaicin-sensitive nociceptive afferents. Thus, this study reveals a P2X receptor-mediated nociceptive afferent pathway to lamina I of the spinal cord and provides a new insight into the nociceptive functions of P2X receptors.

Purinergic component of mechanosensory transduction is increased in a rat model of colitis

ATP contributes to mechanosensory transduction in the rat colorectum. P2X3 receptors are present on dorsal root ganglia (DRG) neurons that supply this area of the gut. Previous studies have shown an increased role for ATP in inflamed tissues. We aimed to investigate whether an increased purinergic component exists during mechanosensory transduction in a rat model of colitis. An in vitro rat colorectal preparation was used to investigate whether distension increased ATP release and to evaluate the role of purinergic antagonists in distension-evoked sensory discharges in the pelvic nerve in normal and colitis preparations. DRG neuron purinoceptors were also studied. Distension-evoked responses in the colitis model were attenuated to a significantly greater extent by 2',3'-O-trinitrophenyl-ATP and pyridoxyl 5-phosphate 6-azophenyl-2',4'-disulfonic acid. Inflammation caused augmented distension-evoked sensory nerve excitation after application of ATP and alpha,beta-methylene ATP. Single-fiber analysis confirmed that mean firing per unit was increased. Distension-evoked increases in ATP release from epithelial cells were substantially higher. The number of DRG neurons responding to ATP and the number of those staining for the P2X3 receptor, particularly those containing calcitonin gene-related peptide, were increased. Adenosine, after ectoenzymatic breakdown of ATP, is involved to a lesser degree in the longer-lasting distension-evoked sensory discharge, suggesting reduced ATPase activity. It was therefore concluded that ATP has an enhanced role in mechanosensory transduction in the inflamed rat colorectum. The underlying mechanisms appear to involve increased distension-evoked release of ATP as well as an increase in the number of DRG neurons supplying the colorectum expressing P2X3 receptors, especially those containing calcitonin gene-related peptide.

The Effects of Pretreatment with Lidocaine or Bupivacaine on the Spatial and Temporal Expression of c-Fos Protein in the Spinal Cord Caused by Plantar Incision in the Rat

We investigated the spatial and temporal patterns of c-Fos protein (Fos) expression in the dorsal horn of the spinal cord caused by plantar incision in the rat and the effects of pretreatment with local anesthetics. Bupivacaine (0.5%), lidocaine (2%), or saline for control was injected for nerve block and local infiltration before the plantar incision was made under anesthesia. Pain behavior and Fos expression in the L4-L5 segments of the spinal cord were assessed at 1, 3, 6, 24, 48, 72, and 120 h after the incision. The withdrawal threshold to mechanical stimulation decreased significantly at 1 h until 120 h (1-72 h, P < 0.01;120 h, P < 0.05), and pretreatment with local anesthetics increased the threshold significantly at 1 h (both groups: P < 0.01), 3 h (both groups: P < 0.01), and 6 h (bupivacaine, P < 0.01; lidocaine, P < 0.05) in comparison with that in the saline group. In the saline group, Fos expression was detected predominantly in laminae I-II and V-VI, and the total Fos expression was maximal at 1 h and then decreased gradually. Pretreatment with local anesthetics suppressed Fos expression significantly in all layers, and this suppression continued for several days. This study provides evidence of spatial and temporal changes in Fos expression induced by plantar incision. Our results indicate that although pretreatment with local anesthetics suppresses Fos expression for several days in the postoperative period, the analgesic effect is observed only for the expected duration of the local anesthetic used.

IMPLICATIONS

Prevention of early pain by pretreatment with local anesthetics provides little benefit for postoperative pain relief in the plantar incision model, although c-Fos expression is suppressed. The number of c-Fos-expressing neurons is not necessarily correlated with pain behavior.

Pharmacological characterisation of a rat model of incisional pain

1. Both clinical and preclinical models of postsurgical pain are being used more frequently in the early evaluation of new chemical entities. In order to assess the validity and reliability of a rat model of postincisional pain, the effects of different classes of clinically effective analgesic drugs were evaluated against multiple behavioural end points. 2. Following surgical incision, under general anaesthesia, of the plantar surface of the rat hind paw, we determined the time course of mechanical hyperalgesia, tactile allodynia and hind limb weight bearing using the Randall-Selitto (paw pressure) assay, electronic von Frey and dual channel weight averager, respectively. Behavioural evaluations began 24 h following surgery, and were continued for 9-14 days. 3. Mechanical hyperalgesia, tactile allodynia and a decrease in weight bearing were present on the affected limb within 1 day of surgery with maximum sensitivity 1-3 days postsurgery. Accordingly, we examined the effect of nonsteroidal antiinflammatory drugs (NSAIDs), morphine and gabapentin, on established hyperalgesia and allodynia, 1 day following plantar incision.4. In accordance with previous reports, both systemic morphine and gabapentin administration reversed mechanical hyperalgesia and tactile allodynia in the incised rat hind paw. Both drugs were more potent against mechanical hyperalgesia than tactile allodynia. 5. All of the NSAIDs tested, including cyclooxygenase 2 selective inhibitors, reversed mechanical hyperalgesia and tactile allodynia in the incised rat hind paw. The rank order of potency for both hyperalgesia and allodynia was indomethacin > celecoxib > etoricoxib > naproxen. 6. We have investigated the potency and efficacy of different classes of analgesic drugs in a rat model of postincisional pain. The rank order of potency for these drugs reflects their utility in treating postoperative pain in the clinic. As these compounds showed reliable efficacy across two different behavioural end points, the Randall-Selitto (paw pressure) assay and electronic von Frey, these methods may prove useful in the study of postsurgical pain and the assessment of novel treatments.

The effects of dexmedetomidine and halothane on Fos expression in the spinal dorsal horn using a rat postoperative pain model

We investigated the effect of an intrathecal injection of a selective alpha2 adrenergic receptor agonist, dexmedetomidine (Dex), and halothane anesthesia on Fos expression in the lumbar spinal dorsal horn after skin incision of the plantar surface of the hind paw, a postoperative pain model using rats. Fos immunoreactivity was induced particularly in the superficial layers of the spinal cord 2 h after surgery. Halothane anesthesia (0.5-1.5%) partially reversed Fos induction, but not in a dose-dependent manner. Preoperative spinal Dex (0.1-10 microg) dose-dependently reduced Fos immunoreactivity, while a relatively high dose of Dex (10 microg) was necessary to produce a profound effect. When used with halothane anesthesia, relatively low doses of Dex (1-3 microg) completely suppressed Fos induction in the superficial spinal layers. These findings indicate that preoperative Dex treatment may provide anesthesia that does not induce stress on spinal neurons.

Analgesic effects of intrathecal administration of P2Y nucleotide receptor agonists UTP and UDP in normal and neuropathic pain model rats

Recent electrophysiological, behavioral, and biochemical studies revealed that ATP plays a role in facilitating spinal pain transmission via ionotropic P2X nucleotide receptors, although the involvement of metabotropic P2Y nucleotide receptors remains unclear. In the present study, we examined the effects of i.t. administration of P2Y receptor agonists UTP, UDP, and related compounds on nociception in normal rats and tactile allodynia in a neuropathic pain model. In the paw pressure test using normal rats, i.t. administration of UTP (30 and 100 nmol/rat) and UDP (30 and 100 nmol/rat), but not UMP (100 nmol/rat) or uridine (100 nmol/rat), significantly elevated the mechanical nociceptive thresholds, whereas ATP (30 and 100 nmol/rat) and alpha,beta-methylene-ATP (10 and 30 nmol/rat) lowered them. Similarly, in the tail-flick test, UTP (10, 30, and 100 nmol/rat) and UDP (100 nmol/rat) significantly prolonged the thermal nociceptive latency. In the von Frey filament test on normal rats, UTP (100 nmol/rat) and UDP (100 nmol/rat) produced no allodynia to the tactile stimulus, whereas ATP (100 nmol/rat) induced a significant and long-lasting tactile allodynia. In the neuropathic pain model, in which the sciatic nerves of rats were partially ligated, UTP (30 and 100 nmol/rat) and UDP (30 and 100 nmol/rat) produced significant antiallodynic effects. Furthermore, UTP (100 nmol/rat) and UDP (100 nmol/rat) caused no motor deficit in the inclined plane test. Taken together, these results suggest that the activation of UTP-sensitive P2Y(2) and/or P2Y(4) receptors and the UDP-sensitive P2Y(6) receptor, in contrast to P2X receptors, produces inhibitory effects on spinal pain transmission.

Downregulation of P2X3 receptor-dependent sensory functions in A/J inbred mouse strain

There is large variability in the various pain responses including those to tissue injury among inbred mouse strains. However, the determinant factors for the strain-specific differences remain unknown. The P2X3 sensory-specific ATP-gated channel has been implicated as a damage-sensing molecule that evokes a pain sensation by receiving endogenous ATP from injured tissue. In this study, to clarify the contribution of the sensory P2X3 signalling to strain-specific differences in tissue injury pain, we examined whether the P2X3-mediated in vivo and in vitro responses in dorsal root ganglion (DRG) neurons are changed in the A/J inbred mouse strain, which is known to be resistant to tissue injury pain caused by formalin. Here we found that A/J mice exhibited a low magnitude of nocifensive behaviour induced by the P2X agonist alpha,beta-methylene ATP (alpha beta meATP) into the hindpaw compared with C57BL/6 J mice. This behaviour was blocked by P2X3 antisense oligodeoxynucleotides. The low magnitude of the in vivo pain sensation could be observed similarly in the in vitro response; the increase in the intracellular Ca(2+) increase by alpha beta meATP in capsaicin-sensitive DRG neurons from A/J mice was significantly lower than that from C57BL/6 J mice. In A/J DRG neurons the P2X3 protein level was significantly lower compared with C57BL/6 J DRG neurons. The change in P2X3 protein was selective because P2X2 protein was expressed equally in both strains. The present study suggests that the downregulation of sensory P2X3 could be one of the molecular predispositions to low sensitivity to tissue injury pain in the A/J inbred mouse strain.