Cellular and molecular mechanisms involved in the analgesic effects of botulinum neurotoxin: A literature review

Botulinum neurotoxins (BoNTs), derived from Clostridium botulinum, have been employed to treat a range of central and peripheral neurological disease. Some studies indicate that BoNT may be beneficial for pain conditions as well. It has been hypothesized that BoNTs may exert their analgesic effects by preventing the release of pain-related neurotransmitters and neuroinflammatory agents from sensory nerve endings, suppressing glial activation, and inhibiting the transmission of pain-related receptors to the neuronal cell membrane. In addition, there is evidence to suggest that the central analgesic effects of BoNTs are mediated through their retrograde axonal transport. The purpose of this review is to summarize the experimental evidence of the analgesic functions of BoNTs and discuss the cellular and molecular mechanisms by which they can act on pain conditions. Most of the studies reviewed in this article were conducted using BoNT/A. The PubMed database was searched from 1995 to December 2022 to identify relevant literature.

Role of the nucleoside-metabolizing enzymes on pain responses in zebrafish larvae

Purinergic signaling is a pathway related to pain underlying mechanisms. Adenosine is a neuromodulator responsible for the regulation of multiple physiological and pathological conditions. Extensive advances have been made to understand the role of adenosine in pain regulation. Here we investigated the effects of purinergic compounds able to modulate adenosine production or catabolism on pain responses induced by Acetic Acid (AA) in zebrafish larvae. We investigated the preventive role of the ecto-5'-nucleotidase inhibitor adenosine 5'-(α,β-methylene)diphosphate (AMPCP) and adenosine deaminase inhibitor erythro-9-(2-Hydroxy-3-nonyl)-adenine (EHNA) on the AA-pain induced model. The pain responses were evaluated through exploratory and aversive behaviors in zebrafish larvae. The exploratory behavior showed a reduction in the distance covered by animals exposed to 0.0025% and 0.050% AA. The movement and acceleration were reduced when compared to control. The treatment with AMPCP or EHNA followed by AA exposure did not prevent behavioral changes induced by AA for any parameter tested. There were no changes in aversive behavior after the AA-induced pain model. After AA-induced pain, the AMP hydrolysis increased on zebrafish larvae. However, the AMPCP or EHNA exposure did not prevent changes in AMP hydrolysis induced by the AA-induced pain model in zebrafish larvae. Although AMPCP or EHNA did not show differences in the AA-induced pain model, our results revealed changes in AMP hydrolysis, suggesting the involvement of the purinergic system in zebrafish larvae pain responses.

Neonatal Seizures and Purinergic Signalling

Neonatal seizures are one of the most common comorbidities of neonatal encephalopathy, with seizures aggravating acute injury and clinical outcomes. Current treatment can control early life seizures; however, a high level of pharmacoresistance remains among infants, with increasing evidence suggesting current anti-seizure medication potentiating brain damage. This emphasises the need to develop safer therapeutic strategies with a different mechanism of action. The purinergic system, characterised by the use of adenosine triphosphate and its metabolites as signalling molecules, consists of the membrane-bound P1 and P2 purinoreceptors and proteins to modulate extracellular purine nucleotides and nucleoside levels. Targeting this system is proving successful at treating many disorders and diseases of the central nervous system, including epilepsy. Mounting evidence demonstrates that drugs targeting the purinergic system provide both convulsive and anticonvulsive effects. With components of the purinergic signalling system being widely expressed during brain development, emerging evidence suggests that purinergic signalling contributes to neonatal seizures. In this review, we first provide an overview on neonatal seizure pathology and purinergic signalling during brain development. We then describe in detail recent evidence demonstrating a role for purinergic signalling during neonatal seizures and discuss possible purine-based avenues for seizure suppression in neonates.

Purinergic Mechanisms and Pain

There is a brief introductory summary of purinergic signaling involving ATP storage, release, and ectoenzymatic breakdown, and the current classification of receptor subtypes for purines and pyrimidines. The review then describes purinergic mechanosensory transduction involved in visceral, cutaneous, and musculoskeletal nociception and on the roles played by receptor subtypes in neuropathic and inflammatory pain. Multiple purinoceptor subtypes are involved in pain pathways both as an initiator and modulator. Activation of homomeric P2X3 receptors contributes to acute nociception and activation of heteromeric P2X2/3 receptors appears to modulate longer-lasting nociceptive sensitivity associated with nerve injury or chronic inflammation. In neuropathic pain activation of P2X4, P2X7, and P2Y12 receptors on microglia may serve to maintain nociceptive sensitivity through complex neural-glial cell interactions and antagonists to these receptors reduce neuropathic pain. Potential therapeutic approaches involving purinergic mechanisms will be discussed.

Purinergic mechanisms and pain--an update

There is a brief summary of the background literature about purinergic signalling. The review then considers purinergic mechanosensory transduction involved in visceral, cutaneous and musculoskeletal nociception and on the roles played by P2X3, P2X2/3, P2X4, P2X7 and P2Y₁₂ receptors in neuropathic and inflammatory pain. Current developments of compounds for the therapeutic treatment of both visceral and neuropathic pain are discussed.

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.

Adenosine kinase as a target for therapeutic antisense strategies in epilepsy

PURPOSE

Given the high incidence of refractory epilepsy, novel therapeutic approaches and concepts are urgently needed. To date, viral-mediated delivery and endogenous expression of antisense sequences as a strategy to prevent seizures have received little attention in epilepsy therapy development efforts. Here we validate adenosine kinase (ADK), the astrocyte-based key negative regulator of the brain's endogenous anticonvulsant adenosine, as a potential therapeutic target for antisense-mediated seizure suppression.

METHODS

We developed adenoassociated virus 8 (AAV8)-based gene therapy vectors to selectively modulate ADK expression in astrocytes. Cell type selectivity was achieved by expressing an Adk-cDNA in sense or antisense orientation under the control of an astrocyte-specific gfaABC1D promoter. Viral vectors where injected into the CA3 of wild-type mice or spontaneously epileptic Adk-tg transgenic mice that overexpress ADK in brain. After virus injection, ADK expression was assessed histologically and biochemically. In addition, intracranial electroencephalography (EEG) recordings were obtained.

KEY FINDINGS

We demonstrate in wild-type mice that viral overexpression of ADK within astrocytes is sufficient to trigger spontaneous recurrent seizures in the absence of any other epileptogenic event, whereas ADK downregulation via AAV8-mediated RNA interference almost completely abolished spontaneous recurrent seizures in Adk-tg mice.

SIGNIFICANCE

Our data demonstrate that modulation of astrocytic ADK expression can trigger or prevent seizures, respectively. This is the first study to use an antisense approach to validate ADK as a rational therapeutic target for the treatment of epilepsy and suggests that gene therapies based on the knock down of ADK might be a feasible approach to control seizures in refractory epilepsy.

Does chronic pain alter the normal interaction between cardiovascular and pain regulatory systems? Pain modulation in the hypertensive-monoarthritic rat

Hypertension-associated hypoalgesia is widely recognized in acute pain conditions. In chronic pain states, however, the relationship between blood pressure and pain sensitivity is still ill-defined, with different authors reporting negative, positive, or even no relationship at all. This work addresses this issue, using complete Freund's adjuvant (CFA)-induced monoarthritis in different models of hypertension: Spontaneous (spontaneously hypertensive rats, SHR), induced by infusion of angiotensin II (ANG) or 1,3-dipropyl-8-sulfophenylxanthine (DPSPX, an adenosine receptors' antagonist), and renal artery ligation (RAL). Nociceptive responses associated with monoarthritis were evaluated by different behavioral tests (von Frey, ankle-bend and CatWalk) and by quantification of Fos expression at the dorsal horn upon noxious stimulation. In all hypertension models, higher von Frey thresholds and lower Fos expression were detected in hypertensive rats with chronic inflammatory pain, as compared to normotensive monoarthritic rats. In SHR and DPSPX, but not ANG or RAL models, hypertensive animals displayed lower inflammation than normotensives. Ankle-bend and CatWalk results indicated lower pain sensitivity in hypertensive rats only in SHR and DPSPX models. The present study shows the importance of using multiple models of hypertension, and evaluating pain responses by various methods, to better understand the complexity of the interactions between pain and cardiovascular regulatory systems.

PERSPECTIVE

This study used different models of hypertension to investigate whether chronic pain alters the normal integration of cardiovascular and pain regulatory systems. A complete understanding of the mechanisms underlying the complex interactions between these systems may disclose future therapeutic approaches to treat hypertension/chronic pain comorbidity states.

Amitriptyline does not block the action of ATP at human P2X4 receptor

BACKGROUND AND PURPOSE

Amitriptyline is a tricyclic antidepressant that is also widely used to treat neuropathic pain in humans, but the mechanism of this anti-hyperalgesic effect is unknown. Microglia in the mouse spinal cord become activated in neuropathic pain, and expression of P2X4 receptors by these microglia is increased. Antisense RNA targeting P2X4 receptors suppresses the development of tactile allodynia in rats. This suggests that blockade of P2X4 receptors might be the mechanism by which amitriptyline relieves neuropathic pain.

EXPERIMENTAL APPROACH

We expressed human, rat and mouse P2X receptors (P2X2, P2X4, P2X7) in human embryonic kidney cells and evoked inward currents by applying ATP. We compared the action of ATP on control cells and cells treated with amitriptyline.

KEY RESULTS

Amitriptyline (10 microM), either applied acutely or by pre-incubation for 2-6 h, had no effect on inward currents evoked by ATP (0.3-100 microM) at human P2X4 receptors. At rat and mouse receptors, amitriptyline (10 microM) caused a modest reduction in the maximum responses to ATP, without changes in EC(50) values, but it had no effect at 1 microM. Amitriptyline also had no effects on currents evoked by ATP at rat P2X2 receptors, or at rat or human P2X7 receptors.

CONCLUSION AND IMPLICATIONS

The results do not support the view that amitriptyline owes its pain-relieving actions in man to the direct blockade of P2X4 receptors.

Effects of protons on macroscopic and single-channel currents mediated by the human P2X7 receptor

Human P2X7 receptors (hP2X7Rs) belong to the P2X family, which opens an intrinsic cation channel when challenged by extracellular ATP. hP2X7Rs are expressed in cells of the inflammatory and immune system. During inflammation, ATP and protons are secreted into the interstitial fluid. Therefore, we investigated the effect of protons on the activation of hP2X7Rs. hP2X7Rs were expressed in Xenopus laevis oocytes and activated by the agonists ATP or benzoyl-benzoyl-ATP (BzATP) at different pH values. The protons reduced the hP2X7R-dependent cation current amplitude and slowed the current deactivation depending on the type and concentration of the agonist used. These effects can be explained by (i) the protonation of ATP, which reduces the effective concentration of the agonist ATP(4-) at the high- and low-affinity ATP activation site of the hP2XR, and (ii) direct allosteric inhibition of the hP2X7R channel opening that follows ATP(4-) binding to the low-affinity activation site. Due to the hampered activation via the low-affinity activation site, a low pH (as observed in inflamed tissues) leads to a relative increase in the contribution of the high-affinity activation site for hP2X7R channel opening.

The neural-glial purinergic receptor ensemble in chronic pain states

Chronic pain is characterized by enhanced sensory neurotransmission that underlies increased sensitivity to noxious stimuli and the perception of non-noxious stimuli as painful. Evidence from neurophysiological and pharmacological studies demonstrates that ATP produces pain by directly enhancing neuronal excitability via the activation of specific ligand-gated ion channels, the P2X3 and P2X2/3 receptors. In addition, ATP activates CNS glial cells (e.g. microglia) in response to persistent nociceptive stimulation. This latter effect involves several distinct receptor-mediated signaling pathways linked to the P2X4, P2X7 and P2Y(12) receptors. This review summarizes new data that places these purinergic signaling events in a mechanistic context that illustrates the ability of ATP to initiate and maintain states of heightened sensory neuron excitability associated with persistent pain.

Purines and sensory nerves

P2X and P2Y nucleotide receptors are described on sensory neurons and their peripheral and central terminals in dorsal root, nodose, trigeminal, petrosal, retinal and enteric ganglia. Peripheral terminals are activated by ATP released from local cells by mechanical deformation, hypoxia or various local agents in the carotid body, lung, gut, bladder, inner ear, eye, nasal organ, taste buds, skin, muscle and joints mediating reflex responses and nociception. Purinergic receptors on fibres in the dorsal spinal cord and brain stem are involved in reflex control of visceral and cardiovascular activity, as well as relaying nociceptive impulses to pain centres. Purinergic mechanisms are enhanced in inflammatory conditions and may be involved in migraine, pain, diseases of the special senses, bladder and gut, and the possibility that they are also implicated in arthritis, respiratory disorders and some central nervous system disorders is discussed. Finally, the development and evolution of purinergic sensory mechanisms are considered.

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.

Adenosine augmentation therapies (AATs) for epilepsy: prospect of cell and gene therapies

Deficiencies in the brain's own adenosine-based seizure control system contribute to seizure generation. Consequently, reconstitution of adenosinergic neuromodulation constitutes a rational approach for seizure control. This review will critically discuss focal adenosine augmentation strategies and their potential for antiepileptic and disease modifying therapy. Due to systemic side effects of adenosine focal adenosine augmentation--ideally targeted to an epileptic focus--becomes a therapeutic necessity. This has experimentally been achieved in kindled seizure models as well as in post-status epilepticus models of spontaneous recurrent seizures using three different therapeutic strategies that will be discussed here: (i) polymer-based brain implants that were loaded with adenosine; (ii) brain implants comprised of cells engineered to release adenosine and embedded in a cell-encapsulation device; (iii) direct transplantation of stem cells engineered to release adenosine. To meet the therapeutic goal of focal adenosine augmentation, genetic disruption of the adenosine metabolizing enzyme adenosine kinase (ADK) in rodent and human cells was used as a molecular strategy to induce adenosine release from cellular brain implants, which demonstrated antiepileptic and neuroprotective properties. New developments and therapeutic challenges in using AATs for epilepsy therapy will critically be evaluated.

Recent improvements in the development of A(2B) adenosine receptor agonists

Adenosine is known to exert most of its physiological functions by acting as local modulator at four receptor subtypes named A(1), A(2A), A(2B) and A(3) (ARs). Principally as a result of the difficulty in identifying potent and selective agonists, the A(2B) AR is the least extensively characterised of the adenosine receptors family. Despite these limitations, growing understanding of the physiological meaning of this target indicates promising therapeutic perspectives for specific ligands. As A(2B) AR signalling seems to be associated with pre/postconditioning cardioprotective and anti-inflammatory mechanisms, selective agonists may represent a new therapeutic group for patients suffering from coronary artery disease. Herein we present an overview of the recent advancements in identifying potent and selective A(2B) AR agonists reported in scientific and patent literature. These compounds can be classified into adenosine-like and nonadenosine ligands. Nucleoside-based agonists are the result of modifying adenosine by substitution at the N (6)-, C(2)-positions of the purine heterocycle and/or at the 5'-position of the ribose moiety or combinations of these substitutions. Compounds 1-deoxy-1-{6-[N'-(furan-2-carbonyl)-hydrazino]-9H-purin-9-yl}-N-ethyl-beta-D-ribofuranuronamide (19, hA(1) K (i) = 1050 nM, hA(2A) K (i) = 1550 nM, hA(2B) EC(50) = 82 nM, hA(3) K (i) > 5 muM) and its 2-chloro analogue 23 (hA(1) K (i) = 3500 nM, hA(2A) K (i) = 4950 nM, hA(2B) EC(50) = 210 nM, hA(3) K (i) > 5 muM) were confirmed to be potent and selective full agonists in a cyclic adenosine monophosphate (cAMP) functional assay in Chinese hamster ovary (CHO) cells expressing hA(2B) AR. Nonribose ligands are represented by conveniently substituted dicarbonitrilepyridines, among which 2-[6-amino-3,5-dicyano-4-[4-(cyclopropylmethoxy)phenyl]pyridin-2-ylsulfanyl]acetamide (BAY-60-6583, hA(1), hA(2A), hA(3) EC(50) > 10 muM; hA(2B) EC(50) = 3 nM) is currently under preclinical-phase investigation for treating coronary artery disorders and atherosclerosis.

Recent improvements in the development of A(2B) adenosine receptor agonists

Adenosine is known to exert most of its physiological functions by acting as local modulator at four receptor subtypes named A(1), A(2A), A(2B) and A(3) (ARs). Principally as a result of the difficulty in identifying potent and selective agonists, the A(2B) AR is the least extensively characterised of the adenosine receptors family. Despite these limitations, growing understanding of the physiological meaning of this target indicates promising therapeutic perspectives for specific ligands. As A(2B) AR signalling seems to be associated with pre/postconditioning cardioprotective and anti-inflammatory mechanisms, selective agonists may represent a new therapeutic group for patients suffering from coronary artery disease. Herein we present an overview of the recent advancements in identifying potent and selective A(2B) AR agonists reported in scientific and patent literature. These compounds can be classified into adenosine-like and nonadenosine ligands. Nucleoside-based agonists are the result of modifying adenosine by substitution at the N (6)-, C(2)-positions of the purine heterocycle and/or at the 5'-position of the ribose moiety or combinations of these substitutions. Compounds 1-deoxy-1-{6-[N'-(furan-2-carbonyl)-hydrazino]-9H-purin-9-yl}-N-ethyl-beta-D-ribofuranuronamide (19, hA(1) K (i) = 1050 nM, hA(2A) K (i) = 1550 nM, hA(2B) EC(50) = 82 nM, hA(3) K (i) > 5 muM) and its 2-chloro analogue 23 (hA(1) K (i) = 3500 nM, hA(2A) K (i) = 4950 nM, hA(2B) EC(50) = 210 nM, hA(3) K (i) > 5 muM) were confirmed to be potent and selective full agonists in a cyclic adenosine monophosphate (cAMP) functional assay in Chinese hamster ovary (CHO) cells expressing hA(2B) AR. Nonribose ligands are represented by conveniently substituted dicarbonitrilepyridines, among which 2-[6-amino-3,5-dicyano-4-[4-(cyclopropylmethoxy)phenyl]pyridin-2-ylsulfanyl]acetamide (BAY-60-6583, hA(1), hA(2A), hA(3) EC(50) > 10 muM; hA(2B) EC(50) = 3 nM) is currently under preclinical-phase investigation for treating coronary artery disorders and atherosclerosis.

Purinergic signalling and disorders of the central nervous system

Purines have key roles in neurotransmission and neuromodulation, with their effects being mediated by the purine and pyrimidine receptor subfamilies, P1, P2X and P2Y. Recently, purinergic mechanisms and specific receptor subtypes have been shown to be involved in various pathological conditions including brain trauma and ischaemia, neurodegenerative diseases involving neuroimmune and neuroinflammatory reactions, as well as in neuropsychiatric diseases, including depression and schizophrenia. This article reviews the role of purinergic signalling in CNS disorders, highlighting specific purinergic receptor subtypes, most notably A(2A), P2X(4) and P2X(7), that might be therapeutically targeted for the treatment of these conditions.

Painful purinergic receptors

Multiple P2 receptor-mediated mechanisms exist by which ATP can alter nociceptive sensitivity following tissue injury. Evidence from a variety of experimental strategies, including genetic disruption studies and the development of selective antagonists, has indicated that the activation of P2X receptor subtypes, including P2X(3), P2X(2/3), P2X(4) and P2X(7), and P2Y (e.g., P2Y(2)) receptors, can modulate pain. For example, administration of a selective P2X(3) antagonist, A-317491, has been shown to effectively block both hyperalgesia and allodynia in different animal models of pathological pain. Intrathecally delivered antisense oligonucleotides targeting P2X(4) receptors decrease tactile allodynia following nerve injury. Selective antagonists for the P2X(7) receptor also reduce sensitization in animal models of inflammatory and neuropathic pain, providing evidence that purinergic glial-neural interactions are important modulators of noxious sensory neurotransmission. Furthermore, activation of P2Y(2) receptors leads to sensitization of polymodal transient receptor potential-1 receptors. Thus, ATP acting at multiple purinergic receptors, either directly on neurons (e.g., P2X(3), P2X(2/3), and P2Y receptors) or indirectly through neural-glial cell interactions (P2X(4) and P2X(7) receptors), alters nociceptive sensitivity. The development of selective antagonists for some of these P2 receptors has greatly aided investigations into the nociceptive role of ATP. This perspective highlights some of the recent advances to identify selective P2 receptor ligands, which has enhanced the investigation of ATP-related modulation of pain sensitivity.

Physiology and pathophysiology of purinergic neurotransmission

This review is focused on purinergic neurotransmission, i.e., ATP released from nerves as a transmitter or cotransmitter to act as an extracellular signaling molecule on both pre- and postjunctional membranes at neuroeffector junctions and synapses, as well as acting as a trophic factor during development and regeneration. Emphasis is placed on the physiology and pathophysiology of ATP, but extracellular roles of its breakdown product, adenosine, are also considered because of their intimate interactions. The early history of the involvement of ATP in autonomic and skeletal neuromuscular transmission and in activities in the central nervous system and ganglia is reviewed. Brief background information is given about the identification of receptor subtypes for purines and pyrimidines and about ATP storage, release, and ectoenzymatic breakdown. Evidence that ATP is a cotransmitter in most, if not all, peripheral and central neurons is presented, as well as full accounts of neurotransmission and neuromodulation in autonomic and sensory ganglia and in the brain and spinal cord. There is coverage of neuron-glia interactions and of purinergic neuroeffector transmission to nonmuscular cells. To establish the primitive and widespread nature of purinergic neurotransmission, both the ontogeny and phylogeny of purinergic signaling are considered. Finally, the pathophysiology of purinergic neurotransmission in both peripheral and central nervous systems is reviewed, and speculations are made about future developments.

P2X7-related modulation of pathological nociception in rats

Growing evidence supports a role for the immune system in the induction and maintenance of chronic pain. ATP is a key neurotransmitter in this process. Recent studies demonstrate that the glial ATP receptor, P2X7, contributes to the modulation of pathological pain. To further delineate the endogenous mechanisms that are involved in P2X7-related antinociception, we utilized a selective P2X7 receptor antagonist, A-438079, in a series of in vivo and in vitro experiments. Injection of A-438079 (10-300 micromol/kg, i.p.) was anti-allodynic in three different rat models of neuropathic pain and it attenuated formalin-induced nocifensive behaviors. Using in vivo electrophysiology, A-438079 (80 micromol/kg, i.v.) reduced noxious and innocuous evoked activity of different classes of spinal neurons (low threshold, nociceptive specific, wide dynamic range) in neuropathic rats. The effects of A-438079 on evoked firing were diminished or absent in sham rats. Spontaneous activity of all classes of spinal neurons was also significantly reduced by A-438079 in neuropathic but not sham rats. In vitro, A-438079 (1 microM) blocked agonist-induced (2,3-O-(4-benzoylbenzoyl)-ATP, 30 microM) current in non-neuronal cells taken from the vicinity of the dorsal root ganglia. Furthermore, A-438079 dose-dependently (0.3-3 microM) decreased the quantity of the cytokine, interleukin-1beta, released from peripheral macrophages. Thus, ATP, acting through the P2X7 receptor, exerts a wide-ranging influence on spinal neuronal activity following a chronic injury. Antagonism of the P2X7 receptor can in turn modulate central sensitization and produce antinociception in animal models of pathological pain. These effects are likely mediated through immuno-neural interactions that affect the release of endogenous cytokines.