Synaptic modulation in pain pathways

Ketamine, a Clinically Used Anesthetic, Inhibits Vascular Smooth Muscle Cell Proliferation via PP2A-Activated PI3K/Akt/ERK Inhibition

Abnormal proliferation of vascular smooth muscle cells (VSMCs) gives rise to major pathological processes involved in the development of cardiovascular diseases. The use of anti-proliferative agents for VSMCs offers potential for the treatment of vascular disorders. Intravenous anesthetics are firmly established to have direct effects on VSMCs, resulting in modulation of blood pressure. Ketamine has been used for many years in the intensive care unit (ICU) for sedation, and has recently been considered for adjunctive therapy. In the present study, we investigated the effects of ketamine on platelet-derived growth factor BB (PDGF-BB)-induced VSMC proliferation and the associated mechanism. Ketamine concentration-dependently inhibited PDGF-BB-induced VSMC proliferation without cytotoxicity, and phosphatidylinositol 3-kinase (PI3K) and extracellular signal-regulated protein kinase (ERK) inhibitors, LY294002 and PD98059, respectively, have similar inhibitory effects. Ketamine was shown to attenuate PI3K, Akt, and ERK1/2 phosphorylation induced by PDGF-BB. Okadaic acid, a selective protein phosphatase 2A (PP2A) inhibitor, significantly reversed ketamine-mediated PDGF-BB-induced PI3K, Akt, and ERK1/2 phosphorylation; a transfected protein phosphatse 2a (pp2a) siRNA reversed Akt and ERK1/2 phosphorylation; and 3-O-Methyl-sphingomyeline (3-OME), an inhibitor of sphingomyelinase, also significantly reversed ERK1/2 phosphorylation. Moreover, ketamine alone significantly inhibited tyrosine phosphorylation and demethylation of PP2A in a concentration-dependent manner. In addition, the pp2a siRNA potently reversed the ketamine-activated catalytic subunit (PP2A-C) of PP2A. These results provide evidence of an anti-proliferating effect of ketamine in VSMCs, showing activation of PP2A blocks PI3K, Akt, and ERK phosphorylation that subsequently inhibits the proliferation of VSMCs. Thus, ketamine may be considered a potential effective therapeutic agent for reducing atherosclerotic process by blocking the proliferation of VSMCs.

Noxious mechanical heterotopic stimulation induces inhibition of the spinal dorsal horn neuronal network: analysis of spinal somatosensory-evoked potentials

Most of the endogenous pain modulation (EPM) involves the spinal dorsal horn (SDH). EPM including diffuse noxious inhibitory controls have been extensively described in oligoneuronal electrophysiological recordings but less attention had been paid to responses of the SDH neuronal population to heterotopic noxious stimulation (HNS). Spinal somatosensory-evoked potentials (SEP) offer the possibility to evaluate the neuronal network behavior, reflecting the incoming afferent volleys along the entry root, SDH interneuron activities and the primary afferent depolarization. SEP from de lumbar cord dorsum were evaluated during mechanical heterotopic noxious stimuli. Sprague-Dawley rats (n = 12) were Laminectomized (T10-L3). The sural nerve of the left hind paw was electrically stimulated (5 mA, 0.5 ms, 0.05 Hz) to induce lumbar SEP. The HNS (mechanic clamp) was applied sequentially to the tail, right hind paw, right forepaw, muzzle and left forepaw during sural stimulation. N wave amplitude decreases (-16.6 %) compared to control conditions when HNS was applied to all areas of stimulation. This effect was more intense for muzzle stimulation (-23.5 %). N wave duration also decreased by -23.6 %. HNS did not change neither the amplitude nor the duration of the P wave but dramatically increases the dispersion of these two parameters. The results of the present study strongly suggest that a HNS applied to different parts of the body is able to reduce the integrated electrical response of the SDH, suggesting that not only wide dynamic range neurons but many others in the SDH are modulated by the EPM.

Better Antiretroviral Central Nervous System Penetration is Not Associated with Reduced Chronic Pain in People Living with Human Immunodeficiency Virus

OBJECTIVE

To determine if better antiretroviral (ARV) central nervous system (CNS) penetration is associated with reduced rates of chronic pain in people living with HIV (PLWH).

BACKGROUND

Chronic pain remains prevalent in PLWH despite widespread ARV use. Mechanisms underlying this prevalence remain unknown, though neuroinflammation from persistent CNS HIV infection and maladaptive plastic changes in the CNS have been implicated. Here we hypothesize that better CNS ARV penetration, measured using the CNS Penetration-Effectiveness (CPE) score, would decrease rates of chronic pain.

METHODS

We interviewed 254 consecutive adults from an HIV clinic in Chiang Mai, Thailand. We collected data on demographics, HIV history, ARV use, and pain characteristics. Patients were evaluated for depression using a Thai two question Patient Health Questionnaire (PHQ-2). Modified CPE score was calculated using established methods and grouped a priori into "low CPE" (≤7, poor penetration) and "high CPE" (≥8, good penetration). CPE score was compared with chronic pain scores in SPSS using appropriate statistical tests. A relationship between CPE score and a positive depression screen was tested further using multivariable binary logistic models.

RESULTS

245 of 254 subjects were on ARVs. Complete ARV data was available for 235 patients. 137 of these 235 patients (58.3%) had a CPE score ≤7, and 98 (41.7%) had a score ≥8. 49 patients had chronic pain, and 9 had neuropathic pain. Low CPE score was not associated with chronic pain (p=0.64), neuropathic pain (p=0.56), or frequent pain (p=0.80), nor was it associated with the severity of reported "worst pain" or "average pain" in the last 24 hours (p=0.18 and 0.48, respectively). Post-hoc analysis revealed that higher CPE score was a significant independent risk factor for depression measured by a positive PHQ-2 screen [OR (95% CI) = 1.29 (1.04-1.61), p=0.02]. This relationship was mediated primarily by exposure to zidovudine.

CONCLUSIONS

CPE score is not associated with chronic pain in PLWH. Post-hoc analysis demonstrated that CPE score, and zidovudine exposure in particular, predicts a positive depression screen. Given the substantial morbidity associated with chronic pain and mood disorders in PLWH, additional studies to determine preventable and treatable factors are imperative.

Cyane-carvone, a synthetic derivative of carvone, inhibits inflammatory response by reducing cytokine production and oxidative stress and shows antinociceptive effect in mice

Cyane-carvone (CC) was studied to elucidate its anti-inflammatory, antinociceptive, and antioxidant effects in Mus musculus. Anti-inflammatory (bradykinin, histamine, prostaglandin E2, serotonin, and carrageenan) and antinociceptive (acetic acid and formalin) models were utilized. Myeloperoxidase activity, interleukin (IL)-1β, tumor necrosis factor alpha (TNF-α), and glutathione (GSH) levels were evaluated. Analysis of variance followed by Student-Newman-Keuls' test was done. Results were compared with control groups (significantly when p < 0.05). In bradykinin, histamine, prostaglandin E2, and serotonin tests, 75 mg/kg CC decreased significantly paw edema (t = 30, 60, 90, and/or 120 min). In carrageenan test, 50 and 75 mg/kg CC (t = 3 h and t = 4 h) and 25 mg/kg CC (t = 4 h) decreased significantly paw edema. CC (75 mg/kg) inhibited significantly mieloperoxidase activity and decreased IL-1β and TNF-α, and all doses increased GSH levels. CC (75 mg/kg) decreased significantly the number of contortions of animals and time of licking (phase 2). CC showed anti-inflammatory, antinociceptive, and antioxidant effects in mice.

Extracellular high-mobility group box 1 protein (HMGB1) as a mediator of persistent pain

Although originally described as a highly conserved nuclear protein, high-mobility group box 1 protein (HMGB1) has emerged as a danger-associated molecular pattern molecule protein (DAMP) and is a mediator of innate and specific immune responses. HMGB1 is passively or actively released in response to infection, injury and cellular stress, providing chemotactic and cytokine-like functions in the extracellular environment, where it interacts with receptors such as receptor for advanced glycation end products (RAGE) and several Toll-like receptors (TLRs). Although HMGB1 was first revealed as a key mediator of sepsis, it also contributes to a number of other conditions and disease processes. Chronic pain arises as a direct consequence of injury, inflammation or diseases affecting the somatosensory system and can be devastating for the affected patients. Emerging data indicate that HMGB1 is also involved in the pathology of persistent pain. Here, we give an overview of HMGB1 as a proinflammatory mediator, focusing particularly on the role of HMGB1 in the induction and maintenance of hypersensitivity in experimental models of pain and discuss the therapeutic potential of targeting HMGB1 in conditions of chronic pain.

Acute pain service: past, present and future

SUMMARY 

Post surgical pain is a continuing major medical challenge despite all the advances in understanding acute pain and the development of new modalities of pain management. Since 1985 when the first acute pain service (APS) was started, many organizations initiated APS protocols and guidelines and it became mandatory to have an APS in all major acute care centers. This review focuses on the stages of development of APS and the challenges of postoperative pain management to provide optimal analgesia. Further clinical studies are required to determine best medication regimen for post-operative pain management.

Long-term potentiation of glycinergic synapses triggered by interleukin 1β

Long-term potentiation (LTP) is a persistent increase in synaptic strength required for many behavioral adaptations, including learning and memory, visual and somatosensory system functional development, and drug addiction. Recent work has suggested a role for LTP-like phenomena in the processing of nociceptive information in the dorsal horn and in the generation of central sensitization during chronic pain states. Whereas LTP of glutamatergic and GABAergic synapses has been characterized throughout the central nervous system, to our knowledge there have been no reports of LTP at mammalian glycinergic synapses. Glycine receptors (GlyRs) are structurally related to GABAA receptors and have a similar inhibitory role. Here we report that in the superficial dorsal horn of the spinal cord, glycinergic synapses on inhibitory GABAergic neurons exhibit LTP, occurring rapidly after exposure to the inflammatory cytokine interleukin-1 beta. This form of LTP (GlyR LTP) results from an increase in the number and/or change in biophysical properties of postsynaptic glycine receptors. Notably, formalin-induced peripheral inflammation in vivo potentiates glycinergic synapses on dorsal horn neurons, suggesting that GlyR LTP is triggered during inflammatory peripheral injury. Our results define a previously unidentified mechanism that could disinhibit neurons transmitting nociceptive information and may represent a useful therapeutic target for the treatment of pain.

Thrombospondin-4 contributes to spinal sensitization and neuropathic pain states

Neuropathic pain is a common cause of pain after nerve injury, but its molecular basis is poorly understood. In a post-gene chip microarray effort to identify new target genes contributing to neuropathic pain development, we report here the characterization of a novel neuropathic pain contributor, thrombospondin-4 (TSP4), using a neuropathic pain model of spinal nerve ligation injury. TSP4 is mainly expressed in astrocytes and significantly upregulated in the injury side of dorsal spinal cord that correlates with the development of neuropathic pain states. TSP4 blockade by intrathecal antibodies, antisense oligodeoxynucleotides, or inactivation of the TSP4 gene reverses or prevents behavioral hypersensitivities. Intrathecal injection of TSP4 protein into naive rats is sufficient to enhance the frequency of EPSCs in spinal dorsal horn neurons, suggesting an increased excitatory presynaptic input, and to cause similar behavioral hypersensitivities. Together, these findings support that injury-induced spinal TSP4 may contribute to spinal presynaptic hypersensitivity and neuropathic pain states. Development of TSP4 antagonists has the therapeutic potential for target-specific neuropathic pain management.

Tumor suppressor menin mediates peripheral nerve injury-induced neuropathic pain through potentiating synaptic plasticity

Synaptic plasticity is a crucial step in the development of central sensitization in the pathogenesis of neuropathic hyperalgesia. Menin, the product of the multiple endocrine neoplasia type 1 (MEN1) gene, possesses the property of synaptogenesis which plays an essential role in neuronal activity. We tested the contributing role of spinal menin in peripheral nerve injury-induced neuropathic hypersensitivity through modulating neuronal synaptic plasticity. After approval by the Institutional Animal Care and Use Committee, nociceptive responses were detected with von Frey filaments and thermal plate after spared nerve injury in C57BL/6 mice who were treated with either intrathecal antisense oligonucleotide of MEN1 (ASO) or vehicle. Extracellular spontaneous discharge frequency, field excitatory postsynaptic potential (fEPSP), and monosynaptic excitatory postsynaptic currents (EPSCs) were measured electrophysiologically. Intrathecal ASO alleviated nerve injury-induced mechanical and thermal hypersensitivity. Upregulated spinal menin after nerve injury colocalized with NeuN in the superficial laminae; genetic knockdown of spinal menin reduced nerve injury induced in vivo spontaneous activity and instantaneous frequency and in vitro field potentials; ASO decreased the frequency and amplitude of monosynaptic EPSCs, and reduced synaptic strength and total charge. Collectively, these findings highlight the role of upregulated neuronal menin in the spinal cord in potentiating spinal synaptic plasticity in peripheral nerve injury-induced neuropathic hypersensitivity.

Are all spinal segments equal: intrinsic membrane properties of superficial dorsal horn neurons in the developing and mature mouse spinal cord

Neurons in the superficial dorsal horn (SDH; laminae I-II) of the spinal cord process nociceptive information from skin, muscle, joints and viscera. Most of what we know about the intrinsic properties of SDH neurons comes from studies in lumbar segments of the cord even though clinical evidence suggests nociceptive signals from viscera and head and neck tissues are processed differently. This ‘lumbar-centric' view of spinal pain processing mechanisms also applies to developing SDH neurons. Here we ask whether the intrinsic membrane properties of SDH neurons differ across spinal cord segments in both the developing and mature spinal cord. Whole cell recordings were made from SDH neurons in slices of upper cervical (C2-4), thoracic (T8-10) and lumbar (L3-5) segments in neonatal (P0-5) and adult (P24-45) mice. Neuronal input resistance (R(IN)), resting membrane potential, AP amplitude, half-width and AHP amplitude were similar across spinal cord regions in both neonates and adults (∼100 neurons for each region and age). In contrast, these intrinsic membrane properties differed dramatically between neonates and adults. Five types of AP discharge were observed during depolarizing current injection. In neonates, single spiking dominated (∼40%) and the proportions of each discharge category did not differ across spinal regions. In adults, initial bursting dominated in each spinal region, but was significantly more prevalent in rostral segments (49% of neurons in C2-4 vs. 29% in L3-5). During development the dominant AP discharge pattern changed from single spiking to initial bursting. The rapid A-type potassium current (I(Ar)) dominated in neonates and adults, but its prevalence decreased (∼80% vs. ∼50% of neurons) in all regions during development. I(Ar) steady state inactivation and activation also changed in upper cervical and lumbar regions during development. Together, our data show the intrinsic properties of SDH neurons are generally conserved in the three spinal cord regions examined in both neonate and adult mice. We propose the conserved intrinsic membrane properties of SDH neurons along the length of the spinal cord cannot explain the marked differences in pain experienced in the limbs, viscera, and head and neck.

Targeting TRPV1 as an alternative approach to narcotic analgesics to treat chronic pain conditions

In spite of intense research efforts and after the dedicated Decade of Pain Control and Research, there are not many alternatives to opioid-based narcotic analgesics in the therapeutic armamentarium to treat chronic pain conditions. Chronic opioid treatment is associated with sedation, tolerance, dependence, hyperalgesia, respiratory depression, and constipation. Since the affective component is an integral part of pain perception, perhaps it is inevitable that potent analgesics possess the property of impacting pain pathways in the supraspinal structures. The question still remains to be answered is that whether a powerful analgesic can be devoid of narcotic effect and addictive potentials. Local anesthetics are powerful analgesics for acute pain by blocking voltage-gated sodium channels that are involved in generation and propagation of action potentials. Antidepressants and anticonvulsants have proven to be useful in the treatment of certain modalities of pain. In neuropathic pain conditions, the complexity arises because of the notion that neuronal circuitry is altered, as occurs in phantom pain, in that pain is perceived even in the absence of peripheral nociceptive inputs. If the locus of these changes is in the central nervous system, commonly used analgesics may not be very useful. This review focuses on the recent advances in nociceptive transmission and nociceptive transient receptor potential vanilloid 1 channel as a target for treating chronic pain conditions with its agonists/antagonists.

Skin incision induces expression of axonal regeneration-related genes in adult rat spinal sensory neurons

Skin incision and nerve injury both induce painful conditions. Incisional and postsurgical pain is believed to arise primarily from inflammation of tissue and the subsequent sensitization of peripheral and central neurons. The role of axonal regeneration-related processes in development of pain has only been considered when there has been injury to the peripheral nerve itself, even though tissue damage likely induces injury of resident axons. We sought to determine if skin incision would affect expression of regeneration-related genes such as activating transcription factor 3 (ATF3) in dorsal root ganglion (DRG) neurons. ATF3 is absent from DRG neurons of the normal adult rodent, but is induced by injury of peripheral nerves and modulates the regenerative capacity of axons. Image analysis of immunolabeled DRG sections revealed that skin incision led to an increase in the number of DRG neurons expressing ATF3. RT-PCR indicated that other regeneration-associated genes (galanin, GAP-43, Gadd45a) were also increased, further suggesting an injury-like response in DRG neurons. Our finding that injury of skin can induce expression of neuronal injury/regeneration-associated genes may impact how clinical postsurgical pain is investigated and treated.

PERSPECTIVE

Tissue injury, even without direct nerve injury, may induce a state of enhanced growth capacity in sensory neurons. Axonal regeneration-associated processes should be considered alongside nerve signal conduction and inflammatory/sensitization processes as possible mechanisms contributing to pain, particularly the transition from acute to chronic pain.

Ircinialactams: subunit-selective glycine receptor modulators from Australian sponges of the family Irciniidae

Screening an extract library of >2500 southern Australian and Antarctic marine invertebrates and algae for modulators of glycine receptor (GlyR) chloride channels identified three Irciniidae sponges that yielded new examples of a rare class of glycinyl lactam sesterterpene, ircinialactam A, 8-hydroxyircinialactam A, 8-hydroxyircinialactam B, ircinialactam C, ent-ircinialactam C and ircinialactam D. Structure-activity relationship (SAR) investigations revealed a new pharmacophore with potent and subunit selective modulatory properties against alpha1 and alpha3 GlyR isoforms. Such GlyR modulators have potential application as pharmacological tools, and as leads for the development of GlyR targeting therapeutics to treat chronic inflammatory pain, epilepsy, spasticity and hyperekplexia.

Models and mechanisms of hyperalgesia and allodynia

Hyperalgesia and allodynia are frequent symptoms of disease and may be useful adaptations to protect vulnerable tissues. Both may, however, also emerge as diseases in their own right. Considerable progress has been made in developing clinically relevant animal models for identifying the most significant underlying mechanisms. This review deals with experimental models that are currently used to measure (sect. II) or to induce (sect. III) hyperalgesia and allodynia in animals. Induction and expression of hyperalgesia and allodynia are context sensitive. This is discussed in section IV. Neuronal and nonneuronal cell populations have been identified that are indispensable for the induction and/or the expression of hyperalgesia and allodynia as summarized in section V. This review focuses on highly topical spinal mechanisms of hyperalgesia and allodynia including intrinsic and synaptic plasticity, the modulation of inhibitory control (sect. VI), and neuroimmune interactions (sect. VII). The scientific use of language improves also in the field of pain research. Refined definitions of some technical terms including the new definitions of hyperalgesia and allodynia by the International Association for the Study of Pain are illustrated and annotated in section I.

Loss of Hoxb8 alters spinal dorsal laminae and sensory responses in mice

Although Hox gene expression has been linked to motoneuron identity, a role of these genes in development of the spinal sensory system remained undocumented. Hoxb genes are expressed at high levels in the dorsal horn of the spinal cord. Hoxb8 null mutants manifest a striking phenotype of excessive grooming and hairless lesions on the lower back. Applying local anesthesia underneath the hairless skin suppressed excessive grooming, indicating that this behavior depends on peripheral nerve activity. Functional ablation of mouse Hoxb8 also leads to attenuated response to nociceptive and thermal stimuli. Although spinal ganglia were normal, a lower postmitotic neural count was found in the dorsalmost laminae at lumbar levels around birth, leading to a smaller dorsal horn and a correspondingly narrowed projection field of nociceptive and thermoceptive afferents. The distribution of the dorsal neuronal cell types that we assayed, including neurons expressing the itch-specific gastrin-releasing peptide receptor, was disorganized in the lumbar region of the mutant. BrdU labeling experiments and gene-expression studies at stages around the birth of these neurons suggest that loss of Hoxb8 starts impairing development of the upper laminae of the lumbar spinal cord at approximately embryonic day (E)15.5. Because none of the neuronal markers used was unexpressed in the adult dorsal horn, absence of Hoxb8 does not impair neuronal differentiation. The data therefore suggest that a lower number of neurons in the upper spinal laminae and neuronal disorganization in the dorsal horn underlie the sensory defects including the excessive grooming of the Hoxb8 mutant.

Spinal dis-inhibition in inflammatory pain

Inflammatory diseases and neuropathic insults trigger signaling cascades, which frequently lead to intense and long-lasting pain syndromes in affected patients. Such pain syndromes are characterized not only by an increased sensitivity to painful stimuli (hyperalgesia), but also by a qualitative change in the sensory perception of other, tactile stimuli (allodynia) and the occurrence of spontaneous pain in the absence of any sensory input. Long-term potentiation (LTP)-like changes in synaptic transmission between nociceptive C-fibers and spino-periaqueductal grey projection neurons as well as a loss of inhibitory control by GABAergic and glycinergic spinal dorsal horn neurons have repeatedly been proposed as underlying principles. While considerable evidence supports a significant contribution of C-fiber LTP to hyperalgesia, such monosynaptic plasticity cannot explain the occurrence of allodynia and spontaneous pain. In this review, we focus on mechanisms of synaptic dis-inhibition in inflammatory pain and propose that pathologically heightened pain sensitivity can be reversed by restoring synaptic inhibition with drugs that target specific spinal GABAA receptor subtypes.

Peripheral cannabinoids attenuate carcinoma-induced nociception in mice

We investigated the cannabinoid receptor (CBr) agonists Win55,212-2 (non-selective) and AM1241 (CBr2 selective) and the peripheral receptor (CBr1) in carcinoma-induced pain using a mouse model. Tumors were induced in the hind paw of female mice by local injection of a human oral squamous cell carcinoma (SCC). Significant pain, as indicated by reduction in withdrawal thresholds in response to mechanical stimulation, began at 4 days after SCC inoculation and lasted to 18 days. Local administration of Win55,212-2 (10 mg/kg) and AM1241 (10 mg/kg) significantly elevated withdrawal thresholds, indicating an antinociceptive effect. Ipsilateral expression of CBr1 protein in L5 DRG was significantly upregulated compared to ipsilateral L4 DRG and in normal tissue. These findings support the suggestion that cannabinoids are capable of producing antinociception in carcinoma-induced pain.

Sensitization of rat dorsal horn neurons by NGF-induced subthreshold potentials and low-frequency activation. A study employing intracellular recordings in vivo

Intramuscular injection of NGF in human subjects has been reported not to elicit pain, whereas 5% NaCl does. On the other hand, NGF injections induce a long-lasting hyperalgesia. In the present study, the possible neuronal basis of these effects was studied at the spinal level. In anesthetized rats, neurons in the segment L4 were recorded intracellularly before (n=65), during (n=15), and after injections of NGF (n=50) as well as during and after 5% NaCl (during: n=12, after: n=39) into the gastrocnemius-soleus (GS) muscle. The neuronal responses to electrical and mechanical stimuli were tested and possible changes caused by the stimulants recorded. Of those neurons that responded to the NGF injections (7 out of 15), the majority exhibited subthreshold excitatory postsynaptic potentials (EPSPs). Only 3 out of 15 neurons reacted with action potentials (APs) at a low frequency. Already 5 to 30 min after NGF injection, some of these neurons showed signs of a sensitization. In comparison to NGF, hypertonic saline i.m. elicited APs at a higher frequency in a larger number of neurons (9 out of 12). One day after NGF i.m., the proportion of dorsal horn neurons responding with APs to electrical stimulation of the GS nerves had increased significantly from 4.6% to 28.0%. Despite the stronger excitatory effect of 5% NaCl, the sensitization of the dorsal horn neurons after hypertonic saline was less than that after NGF (15.3%). Behavioral experiments showed that NGF injections induced stronger mechanical allodynia and hyperalgesia than hypertonic saline i.m. The data demonstrate that low-frequency activation or even subthreshold potentials in dorsal horn neurons evoked by unmyelinated muscle afferents are an effective means of sensitizing these neurons.

Moving from an averaged to specific view of spinal cord pain processing circuits

Neurons in the superficial dorsal horn (SDH) of the spinal cord play a critical role in processing potentially painful or noxious signals from skin, muscle, and viscera. Many acute pain therapies are based on the notion that altering the excitability of SDH neurons can block or gate these signals and reduce pain. This same notion also underlies treatments for certain chronic pain states. Basic scientists are now beginning to identify a number of potential molecular targets for spinal cord-based pain therapies with a focus on ion channels and receptors that can alter neuronal excitability. The current challenge in pain research is to identify which are the most promising targets and how their manipulation alters pain processing. In this review, we propose that our understanding of spinal pain processing mechanisms and translation of these discoveries into pain therapies could be improved by 1) better appreciating and understanding neuronal heterogeneity in the SDH; 2) establishing connectivity patterns among SDH neuron types; and 3) testing and extending findings made in vitro to intact (in vivo) animal models. As this information becomes available, it will be possible to determine the precise distribution of potential therapeutic targets on various SDH neuron types within specific circuits known to be functionally important in spinal pain processing.

Visualizing sensory transmission between dorsal root ganglion and dorsal horn neurons in co-culture with calcium imaging

Sensory information is conveyed to the central nervous system by primary afferent neurons within dorsal root ganglia (DRG), which synapse onto neurons of the dorsal horn of the spinal cord. This synaptic connection is central to the processing of both sensory and pain stimuli. Here, we describe a model system to monitor synaptic transmission between DRG neurons and dorsal horn neurons that is compatible with high-throughput screening. This co-culture preparation comprises DRG and dorsal horn neurons and utilizes Ca(2+) imaging with the indicator dye Fura-2 to visualize synaptic transmission. Addition of capsaicin to co-cultures stimulated DRG neurons and led to activation of dorsal horn neurons as well as increased intracellular Ca(2+) concentrations. This effect was dose-dependent and absent when DRG neurons were omitted from the culture. NMDA receptors are a critical component of synapses between DRG and dorsal horn neurons as MK-801, a use-dependent non-competitive antagonist, prevented activation of dorsal horn neurons following capsaicin treatment. This model system allows for rapid and efficient analysis of noxious stimulus-evoked Ca(2+) signal transmission and provides a new approach both for investigating synaptic transmission in the spinal cord and for screening potential analgesic compounds.

Pain-relieving effects of intravenous ATP in chronic intractable orofacial pain: an open-label study

PURPOSE

Chronic orofacial pain is often refractory to conventional pain therapies. We conducted an open-label study to determine whether adenosine 5'-triphosphate (ATP) could alleviate chronic intractable orofacial pain, and if so, which type of pain could respond to ATP.

METHODS

In 8 and 16 patients with non-neuropathic and neuropathic intractable orofacial pain, respectively, ATP was intravenously infused at a rate of 100 microgxkg(-1)xmin(-1) over 120 min. The magnitudes of spontaneous pain and brush-evoked allodynia were graded with a visual analog scale (VAS). When a VAS score for spontaneous pain was decreased by 50% or more by ATP, the patient was classified as a responder.

RESULTS

The patients could be clearly divided into 10 responders and 14 non-responders. Ten of the 16 patients (62.5%) with neuropathic pain, but none of the 8 patients with non-neuropathic pain, responded to ATP. In particular, all of 8 patients with neuropathic pain following pulpectomy, with or without subsequent tooth extraction, responded to ATP. In the 10 responders, VAS scores for spontaneous pain decreased slowly but progressively during the infusion period, and eventually, ATP reduced the VAS scores for spontaneous pain and allodynia by 82 +/- 15% and 74 +/- 9%, respectively. In these responders, the analgesic and anti-allodynic effects of ATP outlasted the infusion period for medians of 7 and 12 h, respectively.

CONCLUSION

Intravenous ATP did not relieve non-neuropathic orofacial pain. However, it exerted slowly expressed but long-lasting analgesic and anti-allodynic effects in patients with neuropathic orofacial pain, especially in those suffering from neuropathic pain following pulpectomy and/or tooth extraction.

Analgesic strategies beyond the inhibition of cyclooxygenases

Blocking the formation of prostaglandins with cyclooxygenase (COX) inhibitors has been the treatment of choice for inflammatory pain for more than a century. Although these agents provide profound pain relief, their long-term use is hampered by severe side-effects, mainly ulceration of the upper gastrointestinal tract. The development of COX-2-selective inhibitors ("coxibs") has significantly reduced gastrointestinal toxicity, but evidence from controlled clinical trials and experimental studies indicates that the use of coxibs has a significant cardiovascular risk. Recently, signalling elements downstream of COX-2 inhibition have been identified, which offer a great diversity of possible targets. This review focuses on prostaglandin E synthases, prostaglandin receptors and downstream effectors of prostaglandins in the PNS and CNS, including transient receptor potential channels, tetrodotoxin-resistant Na(+) channels and inhibitory glycine receptors. These novel targets should enable inflammatory pain to be treated with improved specificity and, possibly, fewer side-effects.