The effect of ABT-702, a novel adenosine kinase inhibitor, on the responses of spinal neurones following carrageenan inflammation and peripheral nerve injury

Purinergic Signaling in Endometriosis-Associated Pain

Endometriosis is an estrogen-dependent gynecological disease, with an associated chronic inflammatory component, characterized by the presence of endometrial tissue outside the uterine cavity. Its predominant symptom is pain, a condition notably altering the quality of life of women with the disease. This review is intended to exhaustively gather current knowledge on purinergic signaling in endometriosis-associated pain. Altered extracellular ATP hydrolysis, due to changes in ectonucleotidase activity, has been reported in endometriosis; the resulting accumulation of ATP in the endometriotic microenvironment points to sustained activation of nucleotide receptors (P2 receptors) capable of generating a persistent pain message. P2X3 receptor, expressed in sensory neurons, mediates nociceptive, neuropathic, and inflammatory pain, and is enrolled in endometriosis-related pain. Pharmacological inhibition of P2X3 receptor is under evaluation as a pain relief treatment for women with endometriosis. The role of other ATP receptors is also discussed here, e.g., P2X4 and P2X7 receptors, which are involved in inflammatory cell–nerve and microglia–nerve crosstalk, and therefore in inflammatory and neuropathic pain. Adenosine receptors (P1 receptors), by contrast, mainly play antinociceptive and anti-inflammatory roles. Purinome-targeted drugs, including nucleotide receptors and metabolizing enzymes, are potential non-hormonal therapeutic tools for the pharmacological management of endometriosis-related pain.

Approaches for designing and discovering purinergic drugs for gastrointestinal diseases

INTRODUCTION

Purines finely modulate physiological motor, secretory, and sensory functions in the gastrointestinal tract. Their activity is mediated by the purinergic signaling machinery, including receptors and enzymes regulating their synthesis, release, and degradation. Several gastrointestinal dysfunctions are characterized by alterations affecting the purinergic system.

AREAS COVERED

The authors provide an overview on the purinergic receptor signaling machinery, the molecules and proteins involved, and a summary of medicinal chemistry efforts aimed at developing novel compounds able to modulate the activity of each player involved in this machinery. The involvement of purinergic signaling in gastrointestinal motor, secretory, and sensory functions and dysfunctions, and the potential therapeutic applications of purinergic signaling modulators, are then described.

EXPERT OPINION

A number of preclinical and clinical studies demonstrate that the pharmacological manipulation of purinergic signaling represents a viable way to counteract several gastrointestinal diseases. At present, the paucity of purinergic therapies is related to the lack of receptor-subtype-specific agonists and antagonists that are effective in vivo. In this regard, the development of novel therapeutic strategies should be focused to include tools able to control the P1 and P2 receptor expression as well as modulators of the breakdown or transport of purines.

Interleukin-1 receptor type 1 is overexpressed in neurons but not in glial cells within the rat superficial spinal dorsal horn in complete Freund adjuvant-induced inflammatory pain

Background

All known biological functions of the pro-inflammatory cytokine interleukin-1β (IL-1β) are mediated by type 1 interleukin receptor (IL-1R1). IL-1β–IL-1R1 signaling modulates various neuronal functions including spinal pain processing. Although the role of IL-1β in pain processing is generally accepted, there is a discussion in the literature whether IL-1β exerts its effect on spinal pain processing by activating neuronal or glial IL-1R1. To contribute to this debate, here we investigated the expression and cellular distribution of IL-1R1 in the superficial spinal dorsal horn in control animals and also in inflammatory pain.

Methods

Experiments were performed on rats and wild type as well as IL-1R1-deficient mice. Inflammatory pain was evoked by unilateral intraplantar injection of complete Freund adjuvant (CFA). The nociceptive responsiveness of control and CFA-treated animals were tested daily for withdrawal responses to mechanical and thermal stimuli before and after CFA injection. Changes in the expression of 48 selected genes/mRNAs and in the quantity of IL-1R1 protein during the first 3 days after CFA injection were measured with the TaqMan low-density array method and Western blot analysis, respectively. The cellular localization of IL-1R1 protein was investigated with single and double staining immunocytochemical methods.

Results

We found a six times and two times increase in IL-1R1 mRNA and protein levels, respectively, in the dorsal horn of CFA-injected animals 3 days after CFA injection, at the time of the summit of mechanical and thermal allodynia. Studying the cellular distribution of IL-1R1, we found an abundant expression of IL-1R1 on the somatodendritic compartment of neurons and an enrichment of the receptor in the postsynaptic membranes of some excitatory synapses. In contrast to the robust neuronal localization, we observed only a moderate expression of IL-1R1 on astrocytes and a negligible one on microglial cells. CFA injection into the hind paw caused a remarkable increase in the expression of IL-1R1 in neurons, but did not alter the glial expression of the receptor.

Conclusion

The results suggest that IL-1β exerts its effect on spinal pain processing primarily through neuronal IL-1R1, but it can also interact in some extent with IL-1R1 expressed by astrocytes.

ABT-702, an adenosine kinase inhibitor, attenuates inflammation in diabetic retinopathy

AIMS

This study was undertaken to determine the effect of an adenosine kinase inhibitor (AKI) in diabetic retinopathy (DR). We have shown previously that adenosine signaling via A2A receptors (A2AAR) is involved in retinal protection from diabetes-induced inflammation. Here we demonstrate that AKI-enhanced adenosine signaling provides protection from DR in mice.

MAIN METHODS

We targeted AK, the key enzyme in adenosine metabolism, using a treatment regime with the selective AKI, ABT-702 (1.5mg/kg intraperitoneally twice a week) commencing at the beginning of streptozotocin-induced diabetes at the age of eight weeks. This treatment, previously demonstrated to increase free adenosine levels in vivo, was maintained until the age of 16 weeks. Retinal inflammation was evaluated using Western blot, Real-Time PCR and immuno-staining analyses. Role of A2AAR signaling in the anti-inflammation effect of ABT-702 was analyzed in Amadori-glycated-albumin (AGA)-treated microglial cells.

KEY FINDINGS

At 16 weeks, when diabetic mice exhibit significant signs of retinal inflammation including up-regulation of oxidative/nitrosative stress, A2AAR, ENT1, Iba1, TNF-α, ICAM1, retinal cell death, and down-regulation of AK, the ABT-702 treated group showed lower signs of inflammation compared to control animals receiving the vehicle. The involvement of adenosine signaling in the anti-inflammation effect of ABT-702 was supported by the TNF-α release blocking effect of A2AAR antagonist in AGA-treated microglial cells.

SIGNIFICANCE

These results suggest a role for AK in regulating adenosine receptor signaling in the retina. Inhibition of AK potentially amplifies the therapeutic effects of site- and event-specific accumulation of extracellular adenosine, which is of highly translational impact.

Adenosine kinase: exploitation for therapeutic gain

Adenosine kinase (ADK; EC 2.7.1.20) is an evolutionarily conserved phosphotransferase that converts the purine ribonucleoside adenosine into 5'-adenosine-monophosphate. This enzymatic reaction plays a fundamental role in determining the tone of adenosine, which fulfills essential functions as a homeostatic and metabolic regulator in all living systems. Adenosine not only activates specific signaling pathways by activation of four types of adenosine receptors but it is also a primordial metabolite and regulator of biochemical enzyme reactions that couple to bioenergetic and epigenetic functions. By regulating adenosine, ADK can thus be identified as an upstream regulator of complex homeostatic and metabolic networks. Not surprisingly, ADK dysfunction is involved in several pathologies, including diabetes, epilepsy, and cancer. Consequently, ADK emerges as a rational therapeutic target, and adenosine-regulating drugs have been tested extensively. In recent attempts to improve specificity of treatment, localized therapies have been developed to augment adenosine signaling at sites of injury or pathology; those approaches include transplantation of stem cells with deletions of ADK or the use of gene therapy vectors to downregulate ADK expression. More recently, the first human mutations in ADK have been described, and novel findings suggest an unexpected role of ADK in a wider range of pathologies. ADK-regulating strategies thus represent innovative therapeutic opportunities to reconstruct network homeostasis in a multitude of conditions. This review will provide a comprehensive overview of the genetics, biochemistry, and pharmacology of ADK and will then focus on pathologies and therapeutic interventions. Challenges to translate ADK-based therapies into clinical use will be discussed critically.

The role of purinergic pathways in the pathophysiology of gut diseases: pharmacological modulation and potential therapeutic applications

Gut homeostasis results from complex neuro-immune interactions aimed at triggering stereotypical and specific programs of coordinated mucosal secretion and powerful motor propulsion. A prominent role in the regulation of this highly integrated network, comprising a variety of immune/inflammatory cells and the enteric nervous system, is played by purinergic mediators. The cells of the digestive tract are literally plunged into a "biological sea" of functionally active nucleotides and nucleosides, which carry out the critical task of driving regulatory interventions on cellular functions through the activation of P1 and P2 receptors. Intensive research efforts are being made to achieve an integrated view of the purinergic system, since it is emerging that the various components of purinergic pathways (i.e., enzymes, transporters, mediators and receptors) are mutually linked entities, deputed to finely modulating the magnitude and the duration of purinergic signaling, and that alterations occurring in this balanced network could be intimately involved in the pathophysiology of several gut disorders. This review article intends to provide a critical appraisal of current knowledge on the purinergic system role in the regulation of gastrointestinal functions, considering these pathways as a whole integrated network, which is capable of finely controlling the levels of bioactive nucleotides and nucleosides in the biophase of their respective receptors. Special attention is paid to the mechanisms through which alterations in the various compartments of the purinergic system could contribute to the pathophysiology of gut disorders, and to the possibility of counteracting such dysfunctions by means of pharmacological interventions on purinergic molecular targets.

Adenosine kinase inhibition in the cochlea delays the onset of age-related hearing loss

This study was undertaken to determine the role of adenosine signalling in the development of age-related hearing loss (ARHL). We and others have shown previously that adenosine signalling via A(1) receptors is involved in cochlear protection from noise-induced cochlear injury. Here we demonstrate that enhanced adenosine signalling in the cochlea provides partial protection from ARHL in C57BL/6J mice. We targeted adenosine kinase (ADK), the key enzyme in adenosine metabolism, using a treatment regime with the selective ADK inhibitor ABT-702 (1.5mg/kg intraperitoneally twice a week) commencing at the age of three months or six months. This treatment, intended to increase free adenosine levels in the cochlea, was maintained until the age of nine months and hearing thresholds were evaluated monthly using auditory brainstem responses (ABR). At nine months, when C57BL/6J mice normally exhibit significant ARHL, both groups treated with ABT-702 showed lower ABR threshold shifts at 10 and 16kHz compared to control animals receiving the vehicle solution. The better thresholds of the ABT-702-treated mice at these frequencies were supported by increased survival of hair cells in the apical region of the cochlea. This study provides the first evidence that ARHL can be mitigated by enhancing adenosine signalling in the cochlea.

Role of adenosine kinase in cochlear development and response to noise

Adenosine signalling has an important role in cochlear protection from oxidative stress. In most tissues, intracellular adenosine kinase (ADK) is the primary route of adenosine metabolism and the key regulator of intracellular and extracellular adenosine levels. The present study provides the first evidence for ADK distribution in the adult and developing rat cochlea. In the adult cochlea, ADK was localized to the nuclear or perinuclear region of spiral ganglion neurons, lateral wall tissues, and epithelial cells lining scala media. In the developing cochlea, ADK was strongly expressed in multiple cell types at birth and reached its peak level of expression at postnatal day 21 (P21). Ontogenetic changes in ADK expression were evident in the spiral ganglion, organ of Corti, and stria vascularis. In the spiral ganglion, ADK showed a shift from predominantly satellite cell immunolabelling at P1 to neuronal expression from P14 onward. In contrast to the role of ADK in various aspects of cochlear development, the ADK contribution to the cochlear response to noise stress was less obvious. Transcript and protein levels of ADK were unaltered in the cochlea exposed to broadband noise (90-110 dBSPL, 24 hr), and the selective inhibition of ADK in the cochlea with ABT-702 failed to restore hearing thresholds after exposure to traumatic noise. This study indicates that ADK is involved in purine salvage pathways for nucleotide synthesis in the adult cochlea, but its role in the regulation of adenosine signalling under physiological and pathological conditions has yet to be established.

Mammalian target of rapamycin signaling in the spinal cord is required for neuronal plasticity and behavioral hypersensitivity associated with neuropathy in the rat

The protein kinase mammalian target of rapamycin (mTOR) regulates mRNA translation and is inhibited by rapamycin. Signaling pathways involving mTOR are implicated in physiological and pathophysiological processes. We determined the spinal effects of the rapamycin analogue cell cycle inhibitor (CCI)-779 on neuronal responses and behavioral hypersensitivity in a model of persistent neuropathic pain. We also assessed the anatomical distribution of spinal mTOR signaling pathways. Specifically, we ligated rat spinal nerves L5 and L6 to produce a model of neuropathic pain. After confirming neuropathy with behavioral testing, we obtained in vivo single-unit extracellular stimulus-evoked recordings from deep dorsal horn spinal neurons. We applied CCI-779 spinally in electrophysiological and behavioral studies and assessed its effects accordingly. We also used immunohistochemistry to probe for mTOR signaling pathways in dorsal root ganglia (DRG) and the spinal cord. We found that spinally administered CCI-779 rapidly attenuated calibrated mechanically but not thermally evoked neuronal responses and mechanically evoked behavioral responses. Immunohistochemistry showed presence of mTOR signaling pathways in nociceptive-specific C-fiber DRG and in neurons of inner lamina II of the spinal cord. We conclude that alterations in the activity of spinal mTOR signaling pathways are crucial to the full establishment of spinal neuronal plasticity and behavioral hypersensitivity associated with nerve injury.

Perspective

This study is consistent with growing evidence implicating mTOR signaling pathways as important modulators of persistent pain, providing novel insights into the molecular mechanisms of pain maintenance and potential for novel approaches into treating chronic pain.

Therapeutic epilepsy research: from pharmacological rationale to focal adenosine augmentation

Epilepsy is a common seizure disorder affecting approximately 70 million people worldwide. Current pharmacotherapy is neuron-centered, frequently accompanied by intolerable side effects, and fails to be effective in about one third of patients. Therefore, new therapeutic concepts are needed. Recent research suggests an astrocytic basis of epilepsy, presenting the possibility of novel therapeutic targets. In particular, dysfunction of the astrocyte-controlled, endogenous, adenosine-based seizure control system of the brain is implicated in seizure generation. Thus, astrogliosis - a pathological hallmark of the epileptic brain - is associated with upregulation of the adenosine-removing enzyme adenosine kinase (ADK), resulting in focal adenosine deficiency. Both astrogliotic upregulation of ADK in epilepsy and transgenic overexpression of ADK are associated with seizures, and inhibition of ADK prevents seizures in a mouse model of pharmacoresistant epilepsy. These findings link adenosine deficiency with seizures and predict that adenosine augmentation therapies (AATs) will likely be effective in preventing seizures. Given the wide-spread systemic and central side effects of systemically administered AATs, focal AATs (i.e., limited to the astrogliotic lesion) are a necessity. This Commentary will discuss the pharmacological rationale for the development of focal AATs. Additionally, several AAT strategies will be discussed: (1) adenosine released from silk-based brain implants; (2) adenosine released from locally implanted encapsulated cells; (3) adenosine released from stem cell-derived brain implants; and (4) adenosine augmenting gene therapies. Finally, new developments and therapeutic challenges in using focal AATs for epilepsy therapy will critically be evaluated.

Activation of ERK1/2 in the primary injury site is required to maintain melittin-enhanced wind-up of rat spinal wide-dynamic-range neurons

Peripheral modulation of wind-up enhancement induced by peripheral tissue injury is investigated in rat spinal wide-dynamic-range (WDR) neurons. After subcutaneous (s.c.) injection of melittin, a pain-related peptidergic component separated from bee venom, the responsiveness of spinal cord WDR neuron to repeated suprathreshold (1.5T, the intensity threshold) electrical stimuli is enhanced. Comparing with the less effects on early response (0-100 ms), melittin significantly increases late response (100 ms to the next stimulus artifact) and after-discharge (starting from 2s after the last stimulus artifact) with 189% and 546%, respectively. Peripheral administration of a specific MEK inhibitor, 1,4-diamino-2,3-dicyano-1,4-bis-[o-aminophenylmercapto] butadiene (U0126, 1 microg) gradually suppresses, but not completely blocks melittin-enhanced wind-up to the similar level of baseline. The inhibitions of U0126 are mainly on late response and after-discharge with 49% and 65%, respectively. Peripheral administration of three doses of U0126 (0.1, 1, 10 microg) has no effects on melittin-induced local paw edema regardless of either pre- or post-treatment of the drug. We conclude that peripheral ERKs pathway in the primary injury site is required to maintain melittin-enhanced wind-up of rat spinal cord wide-dynamic-range neurons.

4-amino-5-aryl-6-arylethynylpyrimidines: structure-activity relationships of non-nucleoside adenosine kinase inhibitors

A series of non-nucleoside adenosine kinase (AK) inhibitors is reported. These inhibitors originated from the modification of 5-(3-bromophenyl)-7-(6-morpholin-4-ylpyridin-3-yl)pyrido[2,3-d]pyrimidin-4-ylamine (ABT-702). The identification of a linker that would approximate the spatial arrangement found between the pyrimidine ring and the aryl group at C(7) in ABT-702 was a key element in this modification. A search of potential linkers led to the discovery of an acetylene moiety as a suitable scaffold. It was hypothesized that the aryl acetylenes, ABT-702, and adenosine bound to the active site of AK (closed form) in a similar manner with respect to the orientation of the heterocyclic base. Although potent acetylene analogs were discovered based on this assumption, an X-ray crystal structure of 5-(4-dimethylaminophenyl)-6-(6-morpholin-4-ylpyridin-3-ylethynyl)pyrimidin-4-ylamine (16a) revealed a binding orientation contrary to adenosine. In addition, this compound bound tightly to a unique open conformation of AK. The structure-activity relationships and unique ligand orientation and protein conformation are discussed.

Immune and inflammatory mechanisms in neuropathic pain

Tissue damage, inflammation or injury of the nervous system may result in chronic neuropathic pain characterised by increased sensitivity to painful stimuli (hyperalgesia), the perception of innocuous stimuli as painful (allodynia) and spontaneous pain. Neuropathic pain has been described in about 1% of the US population, is often severely debilitating and largely resistant to treatment. Animal models of peripheral neuropathic pain are now available in which the mechanisms underlying hyperalgesia and allodynia due to nerve injury or nerve inflammation can be analysed. Recently, it has become clear that inflammatory and immune mechanisms both in the periphery and the central nervous system play an important role in neuropathic pain. Infiltration of inflammatory cells, as well as activation of resident immune cells in response to nervous system damage, leads to subsequent production and secretion of various inflammatory mediators. These mediators promote neuroimmune activation and can sensitise primary afferent neurones and contribute to pain hypersensitivity. Inflammatory cells such as mast cells, neutrophils, macrophages and T lymphocytes have all been implicated, as have immune-like glial cells such as microglia and astrocytes. In addition, the immune response plays an important role in demyelinating neuropathies such as multiple sclerosis (MS), in which pain is a common symptom, and an animal model of MS-related pain has recently been demonstrated. Here, we will briefly review some of the milestones in research that have led to an increased awareness of the contribution of immune and inflammatory systems to neuropathic pain and then review in more detail the role of immune cells and inflammatory mediators.

Increased nociceptive response in mice lacking the adenosine A1 receptor

The role of the adenosine A1 receptor in nociception was assessed using mice lacking the A1 receptor (A1R-/-) and in rats. Under normal conditions, the A1R-/- mice exhibited moderate heat hyperalgesia in comparison to the wild-type mice (A1R+/+). The mechanical and cold sensitivity were unchanged. The antinociceptive effect of morphine given intrathecally (i.t.), but not systemically, was reduced in A1R-/- mice and this reduction in the spinal effect of morphine was not associated with a decrease in binding of the mu-opioid ligand DAMGO in the spinal cord. A1R-/- mice also exhibited hypersensitivity to heat, but not mechanical stimuli, after localized inflammation induced by carrageenan. In mice with photochemically induced partial sciatic nerve injury, the neuropathic pain-like behavioral response to heat or cold stimulation were significantly increased in the A1R-/-mice. Peripheral nerve injury did not change the level of adenosine A1 receptor in the dorsal spinal cord in rats and i.t. administration of R-PIA effectively alleviated pain-like behaviors after partial nerve injury in rats and in C57/BL/6 mice. Taken together, these data suggest that the adenosine A1 receptor plays a physiological role in inhibiting nociceptive input at the spinal level in mice. The C-fiber input mediating noxious heat is inhibited more than other inputs. A1 receptors also contribute to the antinociceptive effect of spinal morphine. Selective A1 receptor agonists may be tested clinically as analgesics, particularly under conditions of neuropathic pain.

The adenosine kinase inhibitor ABT-702 augments EEG slow waves in rats

ABT-702 is a novel and selective non-nucleoside adenosine kinase (AK) inhibitor that produces increases in endogenous extracellular adenosine. Adenosine (ADO) is thought to be an important neuromodulator of sleep, therefore, the effects of ABT-702 and AK inhibition were examined on rat EEG and sleep, and compared to ADO receptor agonists to further evaluate the role of ADO receptor activation on sleep related EEG patterns. ABT-702 (10.0-30.0 micromol/kg, i.p.) increased the amplitude of the 1-4 Hz band (Fast Fourier Transform (FFT) analysis, p<0.05), which is indicative of augmented sleep-related slow waves. Theophylline (5.0 micromol/kg, i.p.), a centrally active, non-selective adenosine receptor antagonist, attenuated the effects of ABT-702 (20.0 micromol/kg, i.p.) on EEG, whereas 8-(p-sulfophenyl)-theophylline (8-PST, 150.0 micromol/kg, i.p.), a peripherally active antagonist, did not, indicating that the EEG effects of ABT-702 are mediated by a central ADO receptor mechanism. The selective A(1) agonist N6-cyclopentyladenosine (CPA, 30.0 micromol/kg, i.p.) also increased the amplitude of 1-4 Hz band, but was not as efficacious as ABT-702. In contrast, the A(2A) agonist CGS-21680 (1.0-10.0 micromol/kg, i.p.) and the non-selective agonist, N(6)-ethylcarboximidoadenosine (NECA, 0.03-0.1 micromol/kg, ip.), lowered 1-4 Hz amplitude for 2 h after injection. Finally, ABT-702 (10.0 micromol/kg, i.p.) was found to significantly increase slow wave sleep and decrease REM sleep in rats implanted with both EEG and EMG electrodes for evaluation of sleep. These studies demonstrate that increased extracellular adenosine through AK inhibition can elicit modulatory effects on EEG slow waves via an interaction with central ADO receptor subtypes.

Toxins in anti-nociception and anti-inflammation

The use of toxins as novel molecular probes to study the structure-function relationship of ion-channels and receptors as well as potential therapeutics in the treatment of wide variety of diseases is well documented. The high specificity and selectivity of these toxins have attracted a great deal of interest as candidates for drug development. This review highlights the involvement of the proteins and peptide toxins as well as non-proteinaceous compounds derived from both venomous and non-venomous animals, in anti-nociception and anti-inflammation. The possible mechanisms of these potential therapeutic agents and possible clinical applications in the treatment of pain and inflammation are also summarized.

Descending serotonergic facilitation mediated through rat spinal 5HT3 receptors is unaltered following carrageenan inflammation

A descending facilitatory drive originating from superficial dorsal horn NK1-expressing neurones and relaying through parabrachial and rostroventral medial medulla to act on deep dorsal horn neurones, mediated through spinal 5HT3 receptors, was recently documented. To determine if this pathway plays a role in the pathophysiology of inflammation, we investigated the effects of spinally administered ondansetron (a selective 5HT3 receptor antagonist) on deep dorsal horn neuronal responses in carrageenan inflamed and naïve animals using in vivo electrophysiology. The mechanical and thermal evoked responses of spinal neurones were dose dependently attenuated by ondansetron to a similar degree in both groups. In contrast, the electrically evoked responses (Abeta-, Adelta-, C-fibre evoked response and post-discharge) remained unaltered in both groups. Thus 5HT3 receptor mediated descending facilitation remains unaltered at this stage after tissue injury.

Alterations in dorsal horn neurones in a rat model of cancer-induced bone pain

Cancer-induced bone pain is a major clinical problem. A rat model based on intra-tibial injection of MRMT-1 mammary tumour cells was used to mimic progressive cancer-induced bone pain. At the time of stable behavioural changes (decreased thresholds to mechanical and cold stimuli) and bone destruction, in vivo electrophysiology was used to characterize natural (mechanical, thermal, and cold) and electrical-evoked responses of superficial and deep dorsal horn neurones in halothane-anaesthetized rats. Receptive field size was significantly enlarged for superficial neurones in the MRMT-1 animals. Superficial cells were characterised as either nociceptive specific (NS) or wide dynamic range (WDR). The ratio of WDR to NS cells was substantially different between sham operated (growth media alone) (26:74%) and MRMT-1 injected rats (47:53%). NS cells showed no significant difference in their neuronal responses in MRMT-1-injected compared to sham rats. However, superficial WDR neurones in MRMT-1-injected rats had significantly increased responses to mechanical, thermal and electrical (A beta-, C fibre-, and post-discharge evoked response) stimuli. Deep WDR neurones showed less pronounced changes to the superficial dorsal horn, however, the response to thermal and electrical stimuli, but not mechanical, were significantly increased in the MRMT-1-injected rats. In conclusion, the spinal cord is significantly hyperexcitable with previously superficial NS cells becoming responsive to wide-dynamic range stimuli possibly driving this plasticity via ascending and descending facilitatory pathways. The alterations in superficial dorsal horn neurones have not been reported in neuropathy or inflammation adding to the evidence for cancer-induced bone pain reflecting a unique pain state.

Adenosine suppresses the response of neurons to gaba in the superficial laminae of the rat spinal dorsal horn

With the nystatin-perforated whole-cell patch-clamp recording technique, the modulatory effects of adenosine on GABA-activated whole-cell currents were investigated in neurons acutely dissociated from the superficial laminae (laminae I and II) of the rat spinal dorsal horn. The results showed that: (1) GABA acted on GABA(A) receptor and elicited inward Cl(-) currents (I(GABA)) at a holding potential (V(H)) of -40 mV; (2) adenosine suppressed GABA-induced Cl(-) current with affecting neither the reversal potential of I(GABA) nor the apparent affinity of GABA to its receptor; (3) N6-cyclo-hexyladenosine, a selective A(1) adenosine receptor agonist, mimicked the suppressing effect of adenosine on I(GABA), whereas 8-cyclopentyl-1,3-dipropylxanthine, a selective A(1) adenosine receptor antagonist, blocked the suppressing effect of adenosine; (4) chelerythrine, an inhibitor of protein kinase C, reduced the suppressing effect of adenosine on I(GABA); (5) pretreatment with 1,2-bis-(2-aminophenoxy) ethane-N,N,N',N'-tetraacetic acid tetrakis (acetoxy-methyl) ester, a Ca(2+) chelator, did not affect adenosine-induced suppression of I(GABA). The results indicate that: (1) the suppression of adenosine on I(GABA) is mediated by adenosine A(1) receptor and through a Ca(2+)-independent protein kinase C transduction pathway; (2) the interactions between adenosine and GABA might be involved in the modulation of nociceptive information transmission at spinal cord level.

Adenosine in the spinal cord and periphery: release and regulation of pain

In the central nervous system (CNS), adenosine is an important neuromodulator and regulates neuronal and non-neuronal cellular function (e.g. microglia) by actions on extracellular adenosine A(1), A(2A), A(2B) and A(3) receptors. Extracellular levels of adenosine are regulated by synthesis, metabolism, release and uptake of adenosine. Adenosine also regulates pain transmission in the spinal cord and in the periphery, and a number of agents can alter the extracellular availability of adenosine and subsequently modulate pain transmission, particularly by activation of adenosine A(1) receptors. The use of capsaicin (which activates receptors selectively expressed on C-fibre afferent neurons and produces neurotoxic actions in certain paradigms) allows for an interpretation of C-fibre involvement in such processes. In the spinal cord, adenosine availability/release is enhanced by depolarization (K(+), capsaicin, substance P, N-methyl-D-aspartate (NMDA)), by inhibition of metabolism or uptake (inhibitors of adenosine kinase (AK), adenosine deaminase (AD), equilibrative transporters), and by receptor-operated mechanisms (opioids, 5-hydroxytryptamine (5-HT), noradrenaline (NA)). Some of these agents release adenosine via an equilibrative transporter indicating production of adenosine inside the cell (K(+), morphine), while others release nucleotide which is converted extracellularly to adenosine by ecto-5'-nucleotidase (capsaicin, 5-HT). Release can be capsaicin-sensitive, Ca(2+)-dependent and involve G-proteins, and this suggests that within C-fibres, Ca(2+)-dependent intracellular processes regulate production and release of adenosine. In the periphery, adenosine is released from both neuronal and non-neuronal sources. Neuronal release from capsaicin-sensitive afferents is induced by glutamate and by neurogenic inflammation (capsaicin, low concentration of formalin), while that from sympathetic postganglionic neurons (probably as adenosine 5'-triphosphate (ATP) with NA) occurs following more generalized inflammation. Such release is modified differentially by inhibitors of AK and AD. Following nerve injury, there is an alteration in capsaicin-sensitive adenosine release, as spinal release now is less responsive to opioids, while peripheral release is less responsive to inhibitors of metabolism. Following inflammation, adenosine is released from a variety of cell types in addition to neurons (e.g. endothelial cells, neutrophils, mast cells, fibroblasts). ATP is released both spinally and peripherally following inflammation or injury, and may be converted to adenosine by ecto-5'-nucleotidase contributing an additional source of adenosine. Release of adenosine from both spinal and peripheral compartments has inhibitory effects on pain transmission, as methylxanthine adenosine receptor antagonists reduce analgesia produced by agents which augment extracellular levels of adenosine spinally (morphine, 5-HT, substance P, AK inhibitors) and peripherally (AK inhibitors, AD inhibitors). Increases in extracellular adenosine availability also may contribute to antiinflammatory effects of certain agents (methotrexate, sulfasalazine, salicylates, AK inhibitors), and this could have secondary effects on pain signalling in chronic inflammation. The purpose of the present review is to consider: (a). the factors that regulate the extracellular availability of adenosine in the spinal cord and at peripheral sites; and (b). the extent to which this adenosine affects pain signalling in these two distinct compartments.

Enhanced release of adenosine in rat hind paw following spinal nerve ligation: involvement of capsaicin-sensitive sensory afferents

Modulation of endogenous adenosine levels by inhibition of adenosine metabolism produces a peripheral antinociceptive effect in a neuropathic pain model. The present study used microdialysis to investigate the neuronal mechanisms modulating extracellular adenosine levels in the rat hind paw following tight ligation of the L5 and L6 spinal nerves. Subcutaneous injection of 50 microl saline into the nerve-injured paw induced a rapid and short-lasting increase in extracellular adenosine levels in the subcutaneous tissues of the rat hind paw ipsilateral to the nerve injury. Saline injection did not increase adenosine levels in sham-operated rats or non-treated rats. The adenosine kinase inhibitor 5'-amino-5'-deoxyadenosine and the adenosine deaminase inhibitor 2'-deoxycoformycin, at doses producing a peripheral antinociceptive effect, did not further enhance subcutaneous adenosine levels in the nerve-injured paw. Systemic pretreatment with capsaicin, a neurotoxin selective for small-diameter sensory afferents, markedly reduced the saline-evoked release of adenosine in rat hind paw following spinal nerve ligation. Systemic pretreatment with 6-hydroxydopamine, a neurotoxin selective for sympathetic afferent nerves, did not affect release. These results suggest that following nerve injury, peripheral capsaicin-sensitive primary sensory afferent nerve terminals are hypersensitive, and are able to release adenosine following a stimulus that does not normally evoke release in sham-operated or intact rats. Sympathetic postganglionic afferents do not appear to be involved in such release. The lack of effect on such release by the inhibitors of adenosine metabolism suggests an altered peripheral adenosine system following spinal nerve ligation.

Neurobiology of neuropathic pain: mode of action of anticonvulsants

Anticonvulsants are widely used for the treatment of neuropathic pain. Here we review the evidence for a number of peripheral and central changes after nerve injury that may provide a basis for the mechanisms of action of anticonvulsant therapies. The roles of sodium channels, calcium channels, and central glutamate mechanisms are emphasized as the main targets for anticonvulsant drugs in neuropathic pain states. The focus of this article is on anticonvulsants; however, opioids and antidepressants can also be effective in increasing inhibitions to control of pain in a manner similar to that of the enhancement of gamma-aminobutyric acid (GABA) function by antiepileptic drugs. A brief account of these approaches to neuropathic pain is also given.

Descending control of pain

Upon receipt in the dorsal horn (DH) of the spinal cord, nociceptive (pain-signalling) information from the viscera, skin and other organs is subject to extensive processing by a diversity of mechanisms, certain of which enhance, and certain of which inhibit, its transfer to higher centres. In this regard, a network of descending pathways projecting from cerebral structures to the DH plays a complex and crucial role. Specific centrifugal pathways either suppress (descending inhibition) or potentiate (descending facilitation) passage of nociceptive messages to the brain. Engagement of descending inhibition by the opioid analgesic, morphine, fulfils an important role in its pain-relieving properties, while induction of analgesia by the adrenergic agonist, clonidine, reflects actions at alpha(2)-adrenoceptors (alpha(2)-ARs) in the DH normally recruited by descending pathways. However, opioids and adrenergic agents exploit but a tiny fraction of the vast panoply of mechanisms now known to be involved in the induction and/or expression of descending controls. For example, no drug interfering with descending facilitation is currently available for clinical use. The present review focuses on: (1) the organisation of descending pathways and their pathophysiological significance; (2) the role of individual transmitters and specific receptor types in the modulation and expression of mechanisms of descending inhibition and facilitation and (3) the advantages and limitations of established and innovative analgesic strategies which act by manipulation of descending controls. Knowledge of descending pathways has increased exponentially in recent years, so this is an opportune moment to survey their operation and therapeutic relevance to the improved management of pain.

Recent developments in the discovery of novel adenosine kinase inhibitors: mechanism of action and therapeutic potential

Adenosine (ADO) is an endogenous inhibitory neuromodulator that limits cellular excitability in response to tissue trauma and inflammation. Adenosine kinase (AK; EC 2.7.1.20) is the primary metabolic enzyme regulating intra- and extracellular concentrations of ADO. AK inhibitors have been shown to significantly increase ADO concentrations at sites of tissue injury and to provide effective antinociceptive, antiinflammatory, and anticonvulsant activity in animal models. Structurally novel nucleoside and non-nucleoside AK inhibitors that demonstrate high specificity for the AK enzyme compared with other ADO metabolic enzymes, transporters, and receptors have recently been synthesized. These compounds have also demonstrated improved cellular and tissue penetration compared with earlier tubercidin analogs. These compounds have been shown to exert beneficial effects in animal models of pain, inflammation and epilepsy with reduced cardiovascular side effects compared with direct acting ADO receptor (P1) agonists, thus supporting the hypothesis that AK inhibitors can enhance the actions of ADO in a site- and event-specific fashion.