Expression of P2X7 receptor immunoreactivity in distinct subsets of synaptic terminals in the ventral horn of rat lumbar spinal cord

Peptidergic modulation of motor neuron output via CART signaling at C bouton synapses

The intensity of muscle contraction, and therefore movement vigor, needs to be adaptable to enable complex motor behaviors. This can be achieved by adjusting the properties of motor neurons, which form the final common pathway for all motor output from the central nervous system. Here, we identify roles for a neuropeptide, cocaine- and amphetamine-regulated transcript (CART), in the control of movement vigor. We reveal distinct but parallel mechanisms by which CART and acetylcholine, both released at C bouton synapses on motor neurons, selectively amplify the output of subtypes of motor neurons that are recruited during intense movement. We find that mice with broad genetic deletion of CART or selective elimination of acetylcholine from C boutons exhibit deficits in behavioral tasks that require higher levels of motor output. Overall, these data uncover spinal modulatory mechanisms that control movement vigor to support movements that require a high degree of muscle force.

Acetylcholine and spinal locomotor networks: The insider

This article aims to review studies that have investigated the role of neurons that use the transmitter acetylcholine (ACh) in controlling the operation of locomotor neural networks within the spinal cord. This cholinergic system has the particularity of being completely intraspinal. We describe the different effects exerted by spinal cholinergic neurons on locomotor circuitry by the pharmacological activation or blockade of this propriospinal system, as well as describing its different cellular and subcellular targets. Through the activation of one ionotropic receptor, the nicotinic receptor, and five metabotropic receptors, the M1 to M5 muscarinic receptors, the cholinergic system exerts a powerful control both on synaptic transmission and locomotor network neuron excitability. Although tremendous advances have been made in our understanding of the spinal cholinergic system's involvement in the physiology and pathophysiology of locomotor networks, gaps still remain, including the precise role of the different subtypes of cholinergic neurons as well as their pre- and postsynaptic partners. Improving our knowledge of the propriospinal cholinergic system is of major relevance to finding new cellular targets and therapeutics in countering the debilitating effects of neurodegenerative diseases and restoring motor functions after spinal cord injury.

Salient brain entities labelled in P2rx7-EGFP reporter mouse embryos include the septum, roof plate glial specializations and circumventricular ependymal organs

The purinergic system is one of the oldest cell-to-cell communication mechanisms and exhibits relevant functions in the regulation of the central nervous system (CNS) development. Amongst the components of the purinergic system, the ionotropic P2X7 receptor (P2X7R) stands out as a potential regulator of brain pathology and physiology. Thus, P2X7R is known to regulate crucial aspects of neuronal cell biology, including axonal elongation, path-finding, synapse formation and neuroprotection. Moreover, P2X7R modulates neuroinflammation and is posed as a therapeutic target in inflammatory, oncogenic and degenerative disorders. However, the lack of reliable technical and pharmacological approaches to detect this receptor represents a major hurdle in its study. Here, we took advantage of the P2rx7-EGFP reporter mouse, which expresses enhanced green fluorescence protein (EGFP) immediately downstream of the P2rx7 proximal promoter, to conduct a detailed study of its distribution. We performed a comprehensive analysis of the pattern of P2X7R expression in the brain of E18.5 mouse embryos revealing interesting areas within the CNS. Particularly, strong labelling was found in the septum, as well as along the entire neural roof plate zone of the brain, except chorioidal roof areas, but including specialized circumventricular roof formations, such as the subfornical and subcommissural organs (SFO; SCO). Moreover, our results reveal what seems a novel circumventricular organ, named by us postarcuate organ (PArcO). Furthermore, this study sheds light on the ongoing debate regarding the specific presence of P2X7R in neurons and may be of interest for the elucidation of additional roles of P2X7R in the idiosyncratic histologic development of the CNS and related systemic functions.

Experimental Design and Data Analysis Issues Contribute to Inconsistent Results of C-Bouton Changes in Amyotrophic Lateral Sclerosis

The possible presence of pathological changes in cholinergic synaptic inputs [cholinergic boutons (C-boutons)] is a contentious topic within the ALS field. Conflicting data reported on this issue makes it difficult to assess the roles of these synaptic inputs in ALS. Our objective was to determine whether the reported changes are truly statistically and biologically significant and why replication is problematic. This is an urgent question, as C-boutons are an important regulator of spinal motoneuron excitability, and pathological changes in motoneuron excitability are present throughout disease progression. Using male mice of the SOD1-G93A high-expresser transgenic (G93A) mouse model of ALS, we examined C-boutons on spinal motoneurons. We performed histological analysis at high statistical power, which showed no difference in C-bouton size in G93A versus wild-type motoneurons throughout disease progression. In an attempt to examine the underlying reasons for our failure to replicate reported changes, we performed further histological analyses using several variations on experimental design and data analysis that were reported in the ALS literature. This analysis showed that factors related to experimental design, such as grouping unit, sampling strategy, and blinding status, potentially contribute to the discrepancy in published data on C-bouton size changes. Next, we systematically analyzed the impact of study design variability and potential bias on reported results from experimental and preclinical studies of ALS. Strikingly, we found that practices such as blinding and power analysis are not systematically reported in the ALS field. Protocols to standardize experimental design and minimize bias are thus critical to advancing the ALS field.

Activity-dependent redistribution of Kv2.1 ion channels on rat spinal motoneurons

Homeostatic plasticity occurs through diverse cellular and synaptic mechanisms, and extensive investigations over the preceding decade have established Kv2.1 ion channels as key homeostatic regulatory elements in several central neuronal systems. As in these cellular systems, Kv2.1 channels in spinal motoneurons (MNs) localize within large somatic membrane clusters. However, their role in regulating motoneuron activity is not fully established in vivo. We have previously demonstrated marked Kv2.1 channel redistribution in MNs following in vitro glutamate application and in vivo peripheral nerve injury (Romer et al., 2014, Brain Research, 1547:1-15). Here, we extend these findings through the novel use of a fully intact, in vivo rat preparation to show that Kv2.1 ion channels in lumbar MNs rapidly and reversibly redistribute throughout the somatic membrane following 10 min of electrophysiological sensory and/or motor nerve stimulation. These data establish that Kv2.1 channels are remarkably responsive in vivo to electrically evoked and synaptically driven action potentials in MNs, and strongly implicate motoneuron Kv2.1 channels in the rapid homeostatic response to altered neuronal activity.

P2X7 Receptor Mediates Spinal Microglia Activation of Visceral Hyperalgesia in a Rat Model of Chronic Pancreatitis

BACKGROUND & AIMS

Molecular mechanisms underlying the activated spinal microglia in association with the pain in chronic pancreatitis (CP) remain unknown. We tested whether P2X7R on spinal microglia mediates the pathogenesis of visceral pain using a CP rat model.

METHODS

The CP model was induced via intraductal injection of 2% trinitrobenzene sulfonic acid into male Sprague-Dawley rats. Hyperalgesia was assessed based on the mechanical sensitivity to Von-Frey filaments (VFFs), and nocifensive behaviors were measured in response to electrical stimulation of the pancreas. Three weeks after CP induction, spinal cord samples were harvested for immunostaining, immunoblot, and real-time polymerase chain reaction analyses of the P2X7R. Changes in nocifensive behaviors and associated molecular effectors were assessed by blocking spinal cord P2X7R pharmacologically using the selective P2X7R antagonist brilliant blue G (BBG) or genetically using short interfering RNA (siRNA).

RESULTS

CP induced a significant up-regulation of spinal P2X7R expression, which colocalized with a microglial marker (OX-42). Intrathecal administration of BBG significantly attenuated CP-related visceral hyperalgesia in response to VFF-mediated or electrical stimulation of the pancreas, which was associated with suppressed spinal expression of P2X7R and inhibited activation of spinal microglia. Intrathecal injection of siRNA to knock down P2X7R expression in the spinal cord would suppress the nociceptive behaviors in CP rats.

CONCLUSIONS

Spinal microglia P2X7R mediates central sensitization of chronic visceral pain in CP. BBG may represent an effective drug for the treatment of chronic pain in CP patients.

The purinergic neurotransmitter revisited: a single substance or multiple players?

The past half century has witnessed tremendous advances in our understanding of extracellular purinergic signaling pathways. Purinergic neurotransmission, in particular, has emerged as a key contributor in the efficient control mechanisms in the nervous system. The identity of the purine neurotransmitter, however, remains controversial. Identifying it is difficult because purines are present in all cell types, have a large variety of cell sources, and are released via numerous pathways. Moreover, studies on purinergic neurotransmission have relied heavily on indirect measurements of integrated postjunctional responses that do not provide direct information for neurotransmitter identity. This paper discusses experimental support for adenosine 5'-triphosphate (ATP) as a neurotransmitter and recent evidence for possible contribution of other purines, in addition to or instead of ATP, in chemical neurotransmission in the peripheral, enteric and central nervous systems. Sites of release and action of purines in model systems such as vas deferens, blood vessels, urinary bladder and chromaffin cells are discussed. This is preceded by a brief discussion of studies demonstrating storage of purines in synaptic vesicles. We examine recent evidence for cell type targets (e.g., smooth muscle cells, interstitial cells, neurons and glia) for purine neurotransmitters in different systems. This is followed by brief discussion of mechanisms of terminating the action of purine neurotransmitters, including extracellular nucleotide hydrolysis and possible salvage and reuptake in the cell. The significance of direct neurotransmitter release measurements is highlighted. Possibilities for involvement of multiple purines (e.g., ATP, ADP, NAD(+), ADP-ribose, adenosine, and diadenosine polyphosphates) in neurotransmission are considered throughout.

Swimming against the tide: investigations of the C-bouton synapse

C-boutons are important cholinergic modulatory loci for state-dependent alterations in motoneuron firing rate. m2 receptors are concentrated postsynaptic to C-boutons, and m2 receptor activation increases motoneuron excitability by reducing the action potential afterhyperpolarization. Here, using an intensive review of the current literature as well as data from our laboratory, we illustrate that C-bouton postsynaptic sites comprise a unique structural/functional domain containing appropriate cellular machinery (a “signaling ensemble”) for cholinergic regulation of outward K⁺ currents. Moreover, synaptic reorganization at these critical sites has been observed in a variety of pathologic states. Yet despite recent advances, there are still great challenges for understanding the role of C-bouton regulation and dysregulation in human health and disease. The development of new therapeutic interventions for devastating neurological conditions will rely on a complete understanding of the molecular mechanisms that underlie these complex synapses. Therefore, to close this review, we propose a comprehensive hypothetical mechanism for the cholinergic modification of α-MN excitability at C-bouton synapses, based on findings in several well-characterized neuronal systems.

Redistribution of Kv2.1 ion channels on spinal motoneurons following peripheral nerve injury

Pathophysiological responses to peripheral nerve injury include alterations in the activity, intrinsic membrane properties and excitability of spinal neurons. The intrinsic excitability of α-motoneurons is controlled in part by the expression, regulation, and distribution of membrane-bound ion channels. Ion channels, such as Kv2.1 and SK, which underlie delayed rectifier potassium currents and afterhyperpolarization respectively, are localized in high-density clusters at specific postsynaptic sites (Deardorff et al., 2013; Muennich and Fyffe, 2004). Previous work has indicated that Kv2.1 channel clustering and kinetics are regulated by a variety of stimuli including ischemia, hypoxia, neuromodulator action and increased activity. Regulation occurs via channel dephosphorylation leading to both declustering and alterations in channel kinetics, thus normalizing activity (Misonou et al., 2004; Misonou et al., 2005; Misonou et al., 2008; Mohapatra et al., 2009; Park et al., 2006). Here we demonstrate using immunohistochemistry that peripheral nerve injury is also sufficient to alter the surface distribution of Kv2.1 channels on motoneurons. The dynamic changes in channel localization include a rapid progressive decline in cluster size, beginning immediately after axotomy, and reaching maximum within one week. With reinnervation, the organization and size of Kv2.1 clusters do not fully recover. However, in the absence of reinnervation Kv2.1 cluster sizes fully recover. Moreover, unilateral peripheral nerve injury evokes parallel, but smaller effects bilaterally. These results suggest that homeostatic regulation of motoneuron Kv2.1 membrane distribution after axon injury is largely independent of axon reinnervation.

Ablation of P2X7 receptor exacerbates gliosis and motoneuron death in the SOD1-G93A mouse model of amyotrophic lateral sclerosis

Amyotrophic lateral sclerosis (ALS) is a devastating neurological disorder characterized by selective degeneration of upper and lower motoneurons. The primary triggers for motoneuron degeneration are still unknown, but inflammation is considered an important contributing factor. P2X7 receptor is a key player in microglia response to toxic insults and was previously shown to increase pro-inflammatory actions of SOD1-G93A ALS microglia. We therefore hypothesized that lack of P2X7 receptor could modify disease features in the SOD1-G93A mice. Hetero- and homozygous P2X7 receptor knock-out SOD1-G93A mice were thus generated and analysed for body weight, disease onset and progression (by behavioural scores, grip and rotarod tests) and survival. Although the lifespan of P2X7(+/-) and P2X7(-/-)/SOD1-G93A female mice was extended by 6-7% with respect to SOD1-G93A mice, to our surprise the clinical onset was significantly anticipated and the disease progression worsened in both male and female P2X7(-/-)/SOD1-G93A mice. Consistently, we found increased astrogliosis, microgliosis, motoneuron loss, induction of the pro-inflammatory markers NOX2 and iNOS and activation of the MAPKs pathway in the lumbar spinal cord of end-stage P2X7(-/-)/SOD1-G93A mice. These results show that the constitutive deletion of P2X7 receptor aggravates the ALS pathogenesis, suggesting that the receptor might have beneficial effects in at least definite stages of the disease. This study unravels a complex dual role of P2X7 receptor in ALS and strengthens the importance of a successful time window of therapeutic intervention in contrasting the pathology.

Anatomy and function of cholinergic C bouton inputs to motor neurons

Motor control circuitry of the central nervous system must be flexible so that motor behaviours can be adapted to suit the varying demands of different states, developmental stages, and environments. Flexibility in motor control is largely provided by neuromodulatory systems which can adjust the output of motor circuits by modulating the properties and connectivity of neurons within them. The spinal circuitry which controls locomotion is subject to a range of neuromodulatory influences, including some which are intrinsic to the spinal cord. One such intrinsic neuromodulatory system, for which a wealth of anatomical information has recently been combined with new physiological data, is the C bouton system. C boutons are large, cholinergic inputs to motor neurons which were first described over 40 years ago but whose source and function have until recently remained a mystery. In this review we discuss how the convergence of anatomical, molecular genetic and physiological data has recently led to significant advances in our understanding of this unique neuromodulatory system. We also highlight evidence that C boutons are involved in spinal cord injury and disease, revealing their potential as targets for novel therapeutic strategies.

Expression of postsynaptic Ca2+-activated K+ (SK) channels at C-bouton synapses in mammalian lumbar -motoneurons

Small-conductance calcium-activated potassium (SK) channels mediate medium after-hyperpolarization (AHP) conductances in neurons throughout the central nervous system. However, the expression profile and subcellular localization of different SK channel isoforms in lumbar spinal α-motoneurons (α-MNs) is unknown. Using immunohistochemical labelling of rat, mouse and cat spinal cord, we reveal a differential and overlapping expression of SK2 and SK3 isoforms across specific types of α-MNs. In rodents, SK2 is expressed in all α-MNs, whereas SK3 is expressed preferentially in small-diameter α-MNs; in cats, SK3 is expressed in all α-MNs. Function-specific expression of SK3 was explored using post hoc immunostaining of electrophysiologically characterized rat α-MNs in vivo. These studies revealed strong relationships between SK3 expression and medium AHP properties. Motoneurons with SK3-immunoreactivity exhibit significantly longer AHP half-decay times (24.67 vs. 11.02 ms) and greater AHP amplitudes (3.27 vs. 1.56 mV) than MNs lacking SK3-immunoreactivity. We conclude that the differential expression of SK isoforms in rat and mouse spinal cord may contribute to the range of medium AHP durations across specific MN functional types and may be a molecular factor distinguishing between slow- and fast-type α-MNs in rodents. Furthermore, our results show that SK2- and SK3-immunoreactivity is enriched in distinct postsynaptic domains that contain Kv2.1 channel clusters associated with cholinergic C-boutons on the soma and proximal dendrites of α-MNs. We suggest that this remarkably specific subcellular membrane localization of SK channels is likely to represent the basis for a cholinergic mechanism for effective regulation of channel function and cell excitability.

Spinal P2X(7) receptor mediates microglia activation-induced neuropathic pain in the sciatic nerve injury rat model

P2X(7) receptor is an important member of ATP-sensitive ionotropic P2X receptors family, which includes seven receptor subtypes (P2X(1)-P2X(7)). Recent evidence indicates that P2X(7)R participates in the onset and persistence of neuropathic pain. In tetanic stimulation of the sciatic nerve model, P2X(7)R was involved in the activation of microglia, but whether this happens in other neuropathic pain models remains unclear. In this study we used immunohistochemistry and Western blot to explore the relationship of P2X(7)R expression with microglia activation, and with mechanical allodynia and thermal hypersensitivity in the chronic constriction of the sciatic nerve (CCI) rat model. The results show that following nerve ligature, mechanical allodynia and thermal hypersensitivity were developed within 3 days (d), peaked at 14d and persisted for 21d on the injured side. P2X(7)R levels in the ipsilateral L4-6 spinal cord were increased markedly after injury and the highest levels were observed on day 14, significant difference was observed at I-IV layers of the dorsal horn. The change in P2X(7)R levels in the spinal cord was consistent with the development of mechanical allodynia and thermal hypersensitivity. Intrathecal administration of the P2X(7)R antagonist Brilliant Blue G (BBG) reversed CCI-induced mechanical allodynia and thermal hypersensitivity. Double-labeled immunofluorescence showed that P2X(7)R expression were restricted to microglia, spinal microglia were activated after nerve injury, which was inhibited by BBG. These results indicated that spinal P2X(7)R mediate microglia activation, this process may play an important role in development of mechanical allodynia and thermal hypersensitivity in CCI model.

Purinergic activation of dorsal root ganglion neurones in vivo

Functional relevance of non-synaptic purinergic receptors on dorsal root ganglion cells was tested in vivo by the influence of ATP using 2P-LSM and Ca imaging. Within a few seconds after local application of ATP, neurones in dorsal root ganglion were activated indicated by an increase of their calcium signal. The signal reached its maximum within a few seconds and declined to control values after about 30 s. Purinergic action seems to include non-synaptic cell-to-cell communication within dorsal root ganglia.

Involvement of microglial P2X7 receptors and downstream signaling pathways in long-term potentiation of spinal nociceptive responses

Tetanic stimulation of the sciatic nerve (TSS) produces long-term potentiation (LTP) of C-fiber-evoked field potentials in the spinal cord. This potentiation is considered to be a substrate for long-lasting sensitization in the spinal pain pathway. Because microglia have previously been shown to regulate the induction of spinal LTP, we hypothesize that P2X7 receptors (P2X7R), which are predominantly expressed in microglia and participate in the communication between microglia and neurons, may play a role in this induction. This study investigated the potential roles of P2X7Rs in spinal LTP and persistent pain induced by TSS in rats. OxATP or BBG, a P2X7R antagonist, prevented the induction of spinal LTP both in vivo and in spinal cord slices in vitro and alleviated mechanical allodynia. Down-regulation of P2X7Rs with P2X7-siRNA blocked the induction of spinal LTP and inhibited mechanical allodynia. Double immunofluorescence showed colocalization of P2X7Rs with the microglial marker OX-42, but not with the astrocytic marker GFAP or the neuronal marker NeuN. Intrathecal injection of BBG suppressed the up-regulation of microglial P2X7Rs and increased expression of Fos in the spinal superficial dorsal horn. Further, pre-administration of BBG inhibited increased expression of the microglial marker Iba-1, phosphorylated p38 (p-p38), interleukin 1β (IL-1β) and GluR1 following TSS. Pre-administration of the IL-1 receptor antagonist (IL-1ra) blocked both the induction of spinal LTP and the up-regulation of GluR1. These results suggest that microglial P2X7Rs and its downstream signaling pathways play a pivotal role in the induction of spinal LTP and persistent pain induced by TSS.

High concentration of glucose enhances the expression of P2X7 purine receptor in interstitial cells of Cajal in vitro

AIM: To investigate the effects of high concentration of glucose on the expression of P2X₇ purine receptor in the interstitial cells of Cajal (ICC) in vitro and to explore the mechanisms underlying gastrointestinal dysmotility in diabetic mellitus.

METHODS: ICC were isolated from the intestine of newborn mice by enzymatic dissociation and centrifugation and cultured in an incubator containing 50 mL/L CO₂. Cultured ICC were identified by immunofluorescence staining using antibodies directed against c-Kit receptor and P2X₇ receptor. ICC were then divided into two groups: control group and experimental group, which were treated with normal and high concentrations of glucose, respectively. After treatment, cell morphology was observed under an inverted light microscope. The expression of P2X₇ receptor and c-Kit receptor mRNAs in ICC was detected by reverse transcription-polymerase chain reaction (RT-PCR).

RESULTS: Immunofluorescence staining demonstrated that both P2X₇ receptor and c-Kit receptor were positive on ICC cells. After treatment with high concentration of glucose, ICC became bigger, and cell processes became shorter. RT-PCR analysis proved the expression of P2X₇ receptor in ICC. The expression level of c-Kit receptor mRNA was weaker and that of P2X₇ receptor mRNA was stronger in the experimental group than in the control group.

CONCLUSION: P2X₇ receptor is expressed in ICC. Hyperglycemia may alter cell morphology, decrease the expression of c-Kit receptor, enhance the expression of P2X₇ receptor in ICC, and thereby play a role in the pathogenesis of gastrointestinal dysmotility in diabetic mellitus.

Physiological and pathological functions of P2X7 receptor in the spinal cord

ATP-mediated signaling has widespread actions in the nervous system from neurotransmission to regulation of proliferation. In addition, ATP is released during injury and associated to immune and inflammatory responses. Still, the potential of therapeutic intervention of purinergic signaling during pathological states is only now beginning to be explored because of the large number of purinergic receptors subtypes involved, the complex and often overlapping pharmacology and because ATP has effects on every major cell type present in the CNS. In this review, we will focus on a subclass of purinergic-ligand-gated ion channels, the P2X7 receptor, its pattern of expression and its function in the spinal cord where it is abundantly expressed. We will discuss the mechanisms for P2X7R actions and the potential that manipulating the P2X7R signaling pathway may have for therapeutic intervention in pathological events, specifically in the spinal cord.

Activation of P2X7 receptors in glial satellite cells reduces pain through downregulation of P2X3 receptors in nociceptive neurons

Purinergic ionotropic P2X7 receptors (P2X7Rs) are closely associated with excitotoxicity and nociception. Inhibition of P2X7R activation has been considered as a potentially useful strategy to improve recovery from spinal cord injury and reduce inflammatory damage to trauma. The physiological functions of P2X7Rs, however, are poorly understood, even though such information is essential for making the P2X7R an effective therapeutic target. We show here that P2X7Rs in satellite cells of dorsal root ganglia tonically inhibit the expression of P2X3Rs in neurons. Reducing P2X7R expression using siRNA or blocking P2X7R activity by antagonists elicits P2X3R up-regulation, increases the activity of sensory neurons responding to painful stimuli, and evokes abnormal nociceptive behaviors in rats. Thus, contrary to the notion that P2X7R activation is cytotoxic, P2X7Rs in satellite cells play a crucial role in maintaining proper P2X3R expression in dorsal root ganglia. Studying the mechanism underlying the P2X7R-P2X3R control, we demonstrate that activation of P2X7Rs evokes ATP release from satellite cells. ATP in turn stimulates P2Y1 receptors in neurons. P2Y1 receptor activation appears to be necessary and sufficient for the inhibitory control of P2X3R expression. We further determine the roles of the P2X7R-P2Y1-P2X3R inhibitory control under injurious conditions. Activation of the inhibitory control effectively prevents the development of allodynia and increases the potency of systemically administered P2X7R agonists in inflamed rats. Thus, direct blocking P2X7Rs, as proposed before, may not be the best strategy for reducing pain or lessening neuronal degeneration because it also disrupts the protective function of P2X7Rs.

P2Y1 receptor-mediated glutamate release from cultured dorsal spinal cord astrocytes

P2 receptors have been implicated in the release of neurotransmitter and proinflammatory cytokines by the response to neuroexcitatory substances in astrocytes. In the present study, we examined the mechanisms of ADP and adenosine 5'-O-2-thiodiphosphate (ADPbetaS, ADP analogue) on glutamate release from cultured dorsal spinal cord astrocytes by using confocal laser scanning microscopy and HPLC. Immunofluorescence activity showed that P2Y(1) receptor protein is expressed in cultured astrocytes. ADP and ADPbetaS-induced [Ca(2+)](i) increase and glutamate release are mediated by P2Y(1) receptor. Ca(2+) release from IP(3)-sensitive calcium stores and protein kinase C (PKC) activation is important for glutamate release from astrocytes. Furthermore, P2Y(1) receptor-evoked glutamate release is regulated by volume-sensitive Cl(-) channels and anion co-transporter, which open up the possibility that P2Y(1) receptor activation causes the increase of cell volume. Release of glutamate by ADPbetaS was abolished by 5-nitro-2 (3-phenyl propy lamino)-benzoate plus furosemide but was unaffected by botulinum toxin A. These observations indicate that P2Y(1) receptor-evoked glutamate may be mediated via volume-sensitive Cl(-) channel but not via exocytosis of glutamate containing vesicles. We speculate that P2Y(1) receptors-evoked glutamate efflux, occurring under pathological condition, may modulate the activity of synapses in spinal cord.

Synaptic terminals from mice midbrain exhibit functional P2X7 receptor

P2X(7) receptor has been recently localized in mice cerebellar granule neuron fibers. Here, the expression of this subunit has been detected in wild type mice midbrain, by quantitative real time-polymerase chain reaction, immunocytochemistry and Western blot assays. The functionality of this P2X(7) subunit has been confirmed using microfluorimetric experiments in isolated synaptic terminals from mice midbrain. 2'-3'-O-(4-benzoylbenzoyl)-ATP (BzATP) was 30-fold more potent than ATP and EC(50) values were 20 microM and 630 microM respectively. Brilliant Blue G (BBG) and 1-[N,O-bis(5-isoquinolinesulfonyl)-N-methyl-L-tyrosyl]-4-phenylpiperazine (KN-62) produced an inhibition in the responses induced by BzATP, with IC(50) values of 0.027 nM and 2.23 nM, respectively. In addition, P2X(7) inhibitors as ZnSO(4), BBG and suramin abolished partially or totally the responses induced by the physiological agonist ATP. According to immunochemical and PCR assays the presence of a "P2X(7)-like" protein in synaptosomes from validated P2X(7) knockout (KO) model have been detected. In KO animals, BzATP was sixfold more potent than ATP and the EC(50) values were 87 microM and 590 microM respectively. BBG and KN-62 also produced an inhibition in the responses induced by BzATP, with IC(50) value of 0.61 nM and 118 nM respectively, both of them higher than in wild type mice. Moreover, the calcium mobilization ability of native P2X(7) receptors was higher in control compared with KO mice. These biochemical and pharmacological experiments are consistent with the presence of a functional P2X(7) receptor in wild type mice midbrain, and the existence of a less efficient "P2X(7)-like" receptor in the KO model.

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.

Glutamate-stimulated ATP release from spinal cord astrocytes is potentiated by substance P

ATP has recently emerged as a key molecule mediating pathological pain. The aim of this study was to examine whether spinal cord astrocytes could be a source of ATP in response to the nociceptive neurotransmitters glutamate and substance P. Glutamate stimulated ATP release from these astrocytes and this release was greatly potentiated by substance P, even though substance P alone did not elicit ATP release. Substance P also potentiated glutamate-induced inward currents, but did not cause such currents alone. When glutamate was applied alone it acted exclusively through alpha-amino-3-hydroxy-5-methylisoxazole-4-proprionate receptors to stimulate Ca(2+) influx-dependent ATP release. However, when substance P was co-applied with glutamate, ATP release could be elicited by activation of NMDA and metabotropic glutamate receptors. Activation of neurokinin receptor subtypes, protein kinase C and phospholipases A(2), C and D were needed for substance P to bring about its effects. These results suggest that astrocytes may be a major source of ATP in the spinal cord on activation of nerve fibres that release substance P and glutamate.

P2X(7) receptors: properties and relevance to CNS function

Among seven members of P2X ionotropic receptors activated by extracellular ATP, the P2X(7) subtype is unique in that it can function as a cation channel, a nonselective pore, or even a signaling complex coupled with multiple downstream components. Several roles of P2X(7) receptors have been described in CNS cells in the past decade, including release of cytokines and transmitters, modulation of presynaptic transmitter release, and activation of multiple signaling pathways. The finding that P2X(7) pores may directly mediate efflux of cytosolic glutamate, GABA, and ATP in glial cells is particularly interesting, as it provides a novel mechanism of glial transmitter release that may play important roles not only in physiological intercellular communication but also in pathological neural injury.

Emerging challenges of assigning P2X7 receptor function and immunoreactivity in neurons

Currently available antibodies to the P2X(7) receptor are unreliable determinants of neuronal P2X(7) immunoreactivity, owing to staining of a "P2X(7)-like" protein that is not eliminated by legitimate P2X(7) gene-knockout approaches. Despite this, compelling electrophysiological and pharmacological data strongly support a role for P2X(7) receptors in neuronal function and injury. A major priority for the field now is to identify the neuronal "P2X(7)-like" protein and develop effective antibodies selective for neuronal P2X(7) immunoreactivity. Until this occurs, we are dependent on rigorous application of multiple pharmacological criteria for attribution of neuronal function to P2X(7) receptors in non-human tissues, including greater activity in response to BzATP than to ATP, sensitivity to blockade by nanomolar concentrations of Brilliant Blue-G, irreversible antagonism by periodate-oxidized ATP, and lack of inhibition by suramin.

Characterization of a functional P2X7-like receptor in cerebellar granule neurons from P2X7knockout mice

The presence of ionotropic P2X(7) receptor has been studied in mice brain from wild type and P2X(7) receptor knockout animals. Western blot and immunocytochemical assays show the presence of a protein containing the P2X(7) immunogenic epitopes in the brain of knockout model. Reverse transcriptase polymerase chain reaction experiments demonstrate the absence of the disrupted sequence, but other sequences of P2X(7) specific mRNA expression have been detected. Functional calcium imaging experiments in cultured granule neurons from P2X(7) knockout cerebella show the existence of a functional P2X(7)-like receptor that keeps some of the properties of the genuine receptor.