Presynaptic P2X receptors facilitate inhibitory GABAergic transmission between cultured rat spinal cord dorsal horn neurons

The Purinergic P2X7 Receptor as a Target for Adjunctive Treatment for Drug-Refractory Epilepsy

Epilepsy is one of the most common neurological diseases worldwide. Anti-seizure medications (ASMs) with anticonvulsants remain the mainstay of epilepsy treatment. Currently used ASMs are, however, ineffective to suppress seizures in about one third of all patients. Moreover, ASMs show no significant impact on the pathogenic mechanisms involved in epilepsy development or disease progression and may cause serious side-effects, highlighting the need for the identification of new drug targets for a more causal therapy. Compelling evidence has demonstrated a role for purinergic signalling, including the nucleotide adenosine 5'-triphosphate (ATP) during the generation of seizures and epilepsy. Consequently, drugs targeting specific ATP-gated purinergic receptors have been suggested as promising treatment options for epilepsy including the cationic P2X7 receptor (P27XR). P2X7R protein levels have been shown to be increased in the brain of experimental models of epilepsy and in the resected brain tissue of patients with epilepsy. Animal studies have provided evidence that P2X7R blocking can reduce the severity of acute seizures and the epileptic phenotype. The current review will provide a brief summary of recent key findings on P2X7R signalling during seizures and epilepsy focusing on the potential clinical use of treatments based on the P2X7R as an adjunctive therapeutic strategy for drug-refractory seizures and epilepsy.

Extracellular ATP inhibits excitatory synaptic input on parvalbumin positive interneurons and attenuates gamma oscillations via P2X4 receptors

BACKGROUND AND PURPOSE

P2X4 receptors (P2X4R) are ligand gated cation channels that are activated by extracellular ATP released by neurons and glia. The receptors are widely expressed in the brain and have fractional calcium currents comparable with NMDA receptors. Although P2X4Rs have been reported to modulate synaptic transmission and plasticity, their involvement in shaping neuronal network activity remains to be elucidated.

EXPERIMENTAL APPROACH

We investigated the effects of P2X receptors at network and synaptic level using local field potential electrophysiology, whole cell patch clamp recordings and calcium imaging in fast spiking parvalbumin positive interneurons (PVINs) in rat and mouse hippocampal slices. The stable ATP analogue ATPγS, selective antagonists and P2X4R knockout mice were used.

KEY RESULTS

The P2XR agonist ATPγS reversibly decreased the power of gamma oscillations. This inhibition could be antagonized by the selective P2X4R antagonist PSB-12062 and was not observed in P2X4-/- mice. The phasic excitatory inputs of CA3 PVINs were one of the main regulators of the gamma power. Associational fibre compound excitatory postsynaptic currents (cEPSCs) in CA3 PVINs were inhibited by P2X4R activation. This effect was reversible, dependent on intracellular calcium and dynamin-dependent internalization of AMPA receptors.

CONCLUSIONS AND IMPLICATIONS

The results indicate that P2X4Rs are an important source of dendritic calcium in CA3 PVINs, thereby regulating excitatory synaptic inputs onto the cells and presumably the state of gamma oscillations in the hippocampus. P2X4Rs represent an effective target to modulate hippocampal network activity in pathophysiological conditions such as Alzheimer's disease and schizophrenia.

miR-153 promotes neural differentiation by activating the cell adhesion/Ca2+ signaling pathway and targeting ion channel activity in HT-22 cells by bioinformatic analysis

MicroRNAs have been studied extensively in neurodegenerative diseases. In a previous study, miR-153 promoted neural differentiation and projection formation in mouse hippocampal HT-22 cells. However, the pathways and molecular mechanism underlying miR-153-induced neural differentiation remain unclear. To explore the molecular mechanism of miR-153 on neural differentiation, we performed RNA sequencing on miR-153-overexpressed HT-22 cells. Based on RNA sequencing, differentially expressed genes (DEGs) and pathways in miR-153-overexpressed cells were identified. The Database for Annotation, Visualization and Integrated Discovery and Gene Set Enrichment Analysis were used to perform functional annotation and enrichment analysis of DEGs. Targetscan predicted the targets of miR-153. The Search Tool for the Retrieval of Interacting Genes and Cytoscape, were used to construct protein-protein interaction networks and identify hub genes. Q-PCR was used to detect mRNA expression of the identified genes. The expression profiles of the identified genes were compared between embryonic days 9.5 (E9.5) and E11.5 in the embryotic mouse brain of the GDS3442 dataset. Cell Counting Kit-8 assay was used to determine cell proliferation and cellular susceptibility to amyloid β-protein (Aβ) toxicity in miR-153-overexpressed cells. The results indicated that miR-153 increased cell adhesion/Ca2+ (Cdh5, Nrcam, and P2rx4) and Bdnf/Ntrk2 neurotrophic signaling pathway, and decreased ion channel activity (Kcnc3, Kcna4, Clcn5, and Scn5a). The changes in the expression of the identified genes in miR-153-overexpressed cells were consistent with the expression profile of GDS3442 during neural differentiation. In addition, miR-153 overexpression decreased cellular susceptibility to Aβ toxicity in HT-22 cells. In conclusion, miR-153 overexpression may promote neural differentiation by inducing cell adhesion and the Bdnf/Ntrk2 pathway, and regulating electrophysiological maturity by targeting ion channels. MiR-153 may play an important role in neural differentiation; the findings provide a useful therapeutic direction for neurodegenerative diseases.

P2X receptor- and postsynaptic NMDA receptor-mediated long-lasting facilitation of inhibitory synapses in the rat insular cortex

Adenosine triphosphate (ATP) changes the efficacy of synaptic transmission. Despite recent progress in terms of the roles of purinergic receptors in cerebrocortical excitatory synaptic transmission, their contribution to inhibitory synaptic transmission is unknown. To elucidate the effects of α,β-methylene ATP (αβ-mATP), a selective agonist of P2X receptors (P2XRs), on inhibitory synaptic transmission in the insular cortex (IC), we performed whole-cell patch-clamp recording from IC pyramidal neurons (PNs) and fast-spiking neurons (FSNs) in either sex of VGAT-Venus transgenic rats. αβ-mATP increased the amplitude of miniature IPSCs (mIPSCs) under conditions in which NMDA receptors (NMDARs) are recruitable. αβ-mATP-induced facilitation of mIPSCs was sustained even after the washout of αβ-mATP, which was blocked by preincubation with fluorocitrate. The preapplication of NF023 (a P2X1 receptor antagonist) or AF-353 (a P2X3 receptor antagonist) blocked αβ-mATP-induced mIPSC facilitation. Intracellular application of the NMDAR antagonist MK801 blocked the facilitation. d-serine, which is an intrinsic agonist of NMDARs, mimicked αβ-mATP-induced mIPSC facilitation. The intracellular application of BAPTA a Ca2+ chelator, or the bath application of KN-62, a CaMKII inhibitor, blocked αβ-mATP-induced mIPSC facilitation, thus indicating that mIPSC facilitation by αβ-mATP required postsynaptic [Ca2+]i elevation through NMDAR activation. Paired whole-cell patch-clamp recordings from FSNs and PNs demonstrated that αβ-mATP increased the amplitude of unitary IPSCs without changing the paired-pulse ratio. These results suggest that αβ-mATP-induced IPSC facilitation is mediated by postsynaptic NMDAR activations through d-serine released from astrocytes. Subsequent [Ca2+]i increase and postsynaptic CaMKII activation may release retrograde messengers that upregulate GABA release from presynaptic inhibitory neurons, including FSNs. (250/250 words).

DHX9/DNA-tandem repeat-dependent downregulation of ciRNA-Fmn1 in the dorsal horn is required for neuropathic pain

Circular RNAs (ciRNAs) are emerging as new players in the regulation of gene expression. However, how ciRNAs are involved in neuropathic pain is poorly understood. Here, we identify the nervous-tissue-specific ciRNA-Fmn1 and report that changes in ciRNA-Fmn1 expression in spinal cord dorsal horn neurons play a key role in neuropathic pain after nerve injury. ciRNA-Fmn1 was significantly downregulated in ipsilateral dorsal horn neurons after peripheral nerve injury, at least in part because of a decrease in DNA helicase 9 (DHX9), which regulates production of ciRNA-Fmn1 by binding to DNA-tandem repeats. Blocking ciRNA-Fmn1 downregulation reversed nerve-injury-induced reductions in both the binding of ciRNA-Fmn1 to the ubiquitin ligase UBR5 and the level of ubiquitination of albumin (ALB), thereby abrogating the nerve-injury-induced increase of ALB expression in the dorsal horn and attenuating the associated pain hypersensitivities. Conversely, mimicking downregulation of ciRNA-Fmn1 in naïve mice reduced the UBR5-controlled ubiquitination of ALB, leading to increased expression of ALB in the dorsal horn and induction of neuropathic-pain-like behaviors in naïve mice. Thus, ciRNA-Fmn1 downregulation caused by changes in binding of DHX9 to DNA-tandem repeats contributes to the genesis of neuropathic pain by negatively modulating UBR5-controlled ALB expression in the dorsal horn.

Neurosteroids as positive and negative allosteric modulators of ligand-gated ion channels: P2X receptor perspective

Neurosteroids are steroids synthesized de novo in the brain from cholesterol in an independent manner from peripheral steroid sources. The term "neuroactive steroid" includes all steroids independent of their origin, and newly synthesized analogs of neurosteroids that modify neuronal activities. In vivo application of neuroactive steroids induces potent anxiolytic, antidepressant, anticonvulsant, sedative, analgesic and amnesic effects, mainly through interaction with the γ-aminobutyric acid type-A receptor (GABA_AR). However, neuroactive steroids also act as positive or negative allosteric regulators on several ligand-gated channels including N-methyl-d-aspartate receptors (NMDARs), nicotinic acetylcholine receptors (nAChRs) and ATP-gated purinergic P2X receptors. Seven different P2X subunits (P2X1-7) can assemble to form homotrimeric or heterotrimeric ion channels permeable for monovalent cations and calcium. Among them, P2X2, P2X4, and P2X7 are the most abundant within the brain and can be regulated by neurosteroids. Transmembrane domains are necessary for neurosteroid binding, however, no generic motif of amino acids can accurately predict the neurosteroid binding site for any of the ligand-gated ion channels including P2X. Here, we will review what is currently known about the modulation of rat and human P2X by neuroactive steroids and the possible structural determinants underlying neurosteroid-induced potentiation and inhibition of the P2X2 and P2X4 receptors.

Label-Free Long-Term Methods for Live Cell Imaging of Neurons: New Opportunities

Time-lapse light microscopy combined with in vitro neuronal cultures has provided a significant contribution to the field of Developmental Neuroscience. The establishment of the neuronal polarity, i.e., formation of axons and dendrites, key structures responsible for inter-neuronal signaling, was described in 1988 by Dotti, Sullivan and Banker in a milestone paper that continues to be cited 30 years later. In the following decades, numerous fluorescently labeled tags and dyes were developed for live cell imaging, providing tremendous advancements in terms of resolution, acquisition speed and the ability to track specific cell structures. However, long-term recordings with fluorescence-based approaches remain challenging because of light-induced phototoxicity and/or interference of tags with cell physiology (e.g., perturbed cytoskeletal dynamics) resulting in compromised cell viability leading to cell death. Therefore, a label-free approach remains the most desirable method in long-term imaging of living neurons. In this paper we will focus on label-free high-resolution methods that can be successfully used over a prolonged period. We propose novel tools such as scanning ion conductance microscopy (SICM) or digital holography microscopy (DHM) that could provide new insights into live cell dynamics during neuronal development and regeneration after injury.

Neuronal P2X4 receptor may contribute to peripheral inflammatory pain in rat spinal dorsal horn

Objective

Intense inflammation may result in pain, which manifests as spinal central sensitization. There is growing evidence that purinergic signaling plays a pivotal role in the orchestration of pain processing. Over the last decade the ionotropic P2X purino receptor 4 (P2X4) got into spotlight in neuropathic disorders, however its precise spinal expression was scantily characterized during inflammatory pain. Thus, we intended to analyze the receptor distribution within spinal dorsal horn and lumbar dorsal root ganglia (DRG) of rats suffering in inflammatory pain induced by complete Freund adjuvant (CFA).

Methods

CFA-induced peripheral inflammation was validated by mechanical and thermal behavioral tests. In order to ensure about the putative alteration of spinal P2X4 receptor gene expression qPCR reactions were designed, followed by immunoperoxidase and Western blot experiments to assess changes at a protein level. Colocalization of P2X4 with neuronal and glial markers was investigated by double immunofluorescent labelings, which were subsequently analyzed with IMARIS software. Transmission electronmicroscopy was applied to study the ultrastructural localization of the receptor. Concurrently, in lumbar DRG cells similar methodology has been carried out to complete our observations.

Results

The figures of mechanical and thermal behavioral tests proved the establishment of CFA-induced inflammatory pain. We observed significant enhancement of P2X4 transcript level within the spinal dorsal horn 3 days upon CFA administration. Elevation of P2X4 immunoreactivity within Rexed lamina I-II of the spinal gray matter was synchronous with mRNA expression, and confirmed by protein blotting. According to IMARIS analysis the robust protein increase was mainly detected on primary afferent axonterminals and GFAP-labelled astrocyte membrane compartments, but not on postsynaptic dendrites was also validated ultrastructurally within the spinal dorsal horn. Furthermore, lumbar DRG analysis demonstrated that peptidergic and non-peptidergic nociceptive subsets of ganglia cells were also abundantly positive for P2X4 receptor in CFA model.

Conclusion

Here we provide novel evidence about involvement of neuronal and glial P2X4 receptor in the establishment of inflammatory pain.

miRNA-22 Upregulates Mtf1 in Dorsal Horn Neurons and Is Essential for Inflammatory Pain

Chronic inflammatory pain seriously affects patients' quality of life because of a paucity of effective clinical treatments caused, at least in part, by lack of full understanding of the underlying mechanisms. miRNAs are known to be involved in inflammatory pain via silencing or degrading of target mRNA in the cytoplasm. The present study provides a novel mechanism by which miRNA-22 positively regulates metal-regulatory transcription factor 1 (Mtf1) in the nuclei of neurons in the dorsal horn of the spinal cord. We found that miRNA-22 was significantly increased in the dorsal horn of mice with either inflammatory pain induced by plantar injection of complete Freund's adjuvant (CFA) or neuropathic pain induced by unilateral sciatic nerve chronic constrictive injury (CCI). Knocking down or blocking miRNA-22 alleviated CFA-induced mechanical allodynia and heat hyperalgesia, whereas overexpressing miRNA-22 produced pain-like behaviors. Mechanistically, the increased miRNA-22 binds directly to the Mtf1 promoter to recruit RNA polymerase II and elevate Mtf1 expression. The increased Mtf1 subsequently enhances spinal central sensitization, as evidenced by increased expression of p-ERK1/2, GFAP, and c-Fos in the dorsal horn. Our findings suggest that the miRNA-22-Mtf1 signaling axis in the dorsal horn plays a critical role in the induction and maintenance of inflammatory pain. This signaling pathway may be a promising therapeutic target in inflammatory pain.

Methyltransferase-like 3 contributes to inflammatory pain by targeting TET1 in YTHDF2-dependent manner

ABSTRACT

The methyltransferase-like 3 (Mettl3) is a key component of the large N6-adenosine-methyltransferase complex in mammalian responsible for RNA N6-methyladenosine (m6A) modification, which plays an important role in gene post-transcription modulation. Although RNA m6A is enriched in mammalian neurons, its regulatory function in nociceptive information processing remains elusive. Here, we reported that Complete Freund's Adjuvant (CFA)-induced inflammatory pain significantly decreased global m6A level and m6A writer Mettl3 in the spinal cord. Mimicking this decease by knocking down or conditionally deleting spinal Mettl3 elevated the levels of m6A in ten-eleven translocation methylcytosine dioxygenases 1 (Tet1) mRNA and TET1 protein in the spinal cord, leading to production of pain hypersensitivity. By contrast, overexpressing Mettl3 reversed a loss of m6A in Tet1 mRNA and blocked the CFA-induced increase of TET1 in the spinal cord, resulting in the attenuation of pain behavior. Furthermore, the decreased level of spinal YT521-B homology domain family protein 2 (YTHDF2), an RNA m6A reader, stabilized upregulation of spinal TET1 because of the reduction of Tet1 mRNA decay by the binding to m6A in Tet1 mRNA in the spinal cord after CFA. This study reveals a novel mechanism for downregulated spinal cord METTL3 coordinating with YTHDF2 contributes to the modulation of inflammatory pain through stabilizing upregulation of TET1 in spinal neurons.

Contribution of P2X4 Receptors to CNS Function and Pathophysiology

The release and extracellular action of ATP are a widespread mechanism for cell-to-cell communication in living organisms through activation of P2X and P2Y receptors expressed at the cell surface of most tissues, including the nervous system. Among ionototropic receptors, P2X4 receptors have emerged in the last decade as a potential target for CNS disorders such as epilepsy, ischemia, chronic pain, anxiety, multiple sclerosis and neurodegenerative diseases. However, the role of P2X4 receptor in each pathology ranges from beneficial to detrimental, although the mechanisms are still mostly unknown. P2X4 is expressed at low levels in CNS cells including neurons and glial cells. In normal conditions, P2X4 activation contributes to synaptic transmission and synaptic plasticity. Importantly, one of the genes present in the transcriptional program of myeloid cell activation is P2X4. Microglial P2X4 upregulation, the P2X4+ state of microglia, seems to be common in most acute and chronic neurodegenerative diseases associated with inflammation. In this review, we summarize knowledge about the role of P2X4 receptors in the CNS physiology and discuss potential pitfalls and open questions about the therapeutic potential of blocking or potentiation of P2X4 for different pathologies.

High-resolution detection of ATP release from single cultured mouse dorsal horn spinal cord glial cells and its modulation by noradrenaline

Human embryonic kidney 293 (HEK293) cells stably transfected with the rat P2X2 receptor subunit were preincubated with 200 nM progesterone (HEK293-P2X2-PROG), a potent positive allosteric modulator of homomeric P2X2 receptors, and used to detect low nanomolar concentrations of extracellular ATP. Fura-2-loaded HEK293-P2X2-PROG cells were acutely plated on top of cultured DH glial cells to quantify ATP release from single DH glial cells. Application of the α1 adrenoceptor agonist phenylephrine (PHE, 20 μM) or of a low K⁺ (0.2 mM) solution evoked reversible increases in the intracellular calcium concentration ([Ca²⁺]_i) in the biosensor cells. A reversible increase in [Ca²⁺]_i was also detected in half of the biosensor cells following the interruption of general extracellular perfusion. All increases in [Ca²⁺]_i were blocked in the presence of the P2X2 antagonist PPADS or after preloading the glial cells with the calcium chelator BAPTA, indicating that they were due to calcium-dependent ATP release from the glial cells. ATP release induced by PHE was blocked by -L-phenylalanine 2-naphtylamide (GPN) that permeabilizes secretory lysosomes and bafilomycin A1 (Baf A1), an inhibitor of the H⁺-pump of acidic secretory vesicles. By contrast, ATP release induced by application of a low-K⁺ solution was abolished by Baf A1 but not by GPN. Finally, spontaneous ATP release observed after interrupting general perfusion was insensitive to both GPN and Baf A1 pretreatment. Our results indicate that ATP is released in a calcium-dependent manner from two distinct vesicular pools and one non-vesicular pool coexisting in DH glial cells and that noradrenaline and PHE selectively target the secretory lysosome pool.

P2X3 receptors are transducers of sensory signals

Peripheral stimuli are transduced by specific receptors expressed by sensory neurons and are further processed in the dorsal horn of spinal cord before to be transmitted to the brain. While relative few receptor subtypes mediate the initial depolarisation of sensory neurons, an impressive number of molecules and ion channels integrate these inputs into coded signals. Soluble mediators and ambient conditions further shape these processes, potentially triggering peripheral and central sensitisation, or sensory downregulation. Extracellular ATP is a major signaling molecule that acts via purinergic receptors and is a powerful modulator of cell communication as well as a neurotransmitter at peripheral/central synapses. In particular, ATP-mediated signals are transduced by P2X3 receptors expressed mainly by peripheral sensory neurons. Recent evidence suggests that P2X3 receptor function not only induces neuron depolarisation and firing with consequent neurotransmitter release, but it also triggers intracellular molecular changes that amplify purinergic signaling with important consequences.

Purinergic modulation of neuronal activity in the rat prepositus hypoglossi nucleus

In the nervous system, adenosine 5'-trisphosphate (ATP) functions as a neurotransmitter and binds to ionotropic P2X receptors and metabotropic P2Y receptors. Although ATP receptors are expressed in the prepositus hypoglossi nucleus (PHN), which is a brainstem structure involved in controlling horizontal gaze, it is unclear whether ATP indeed affects the activity of PHN neurons. In this study, we investigated the effects of ATP on spontaneous firing of PHN neurons using whole-cell recordings in rat brainstem slices. Bath application of ATP increased or decreased the spontaneous firing rate of the neurons in a dose-dependent manner, indicating that ATP indeed affects PHN neuronal activity. To clarify the mechanisms of the ATP effects, we investigated the current responses of PHN neurons to a local application of ATP. The ATP application induced a fast inward (FI) current, a slow inward (SI) current, and/or a slow outward (SO) current in the neurons. The agonists of P2X and P2Y receptors induced FI and SI currents, respectively. The SO currents were not induced by the ATP agonists but were induced by adenosine, which may be extracellularly converted from ATP by ectonucleotidases. An antagonist of adenosine P1 (A₁ ) receptors abolished the adenosine-induced SO currents and bath application of adenosine decreased the spontaneous firing rate of all PHN neurons tested. These results indicate that PHN neurons express functional purinoceptors and show that the FI, SI, and SO currents were mediated via P2X, P2Y, and A₁ receptors, respectively.

Reduced expression of purinergic P2X4 receptors increases voluntary ethanol intake in C57BL/6J mice

Purinergic P2X4 receptors (P2X4Rs) belong to the P2X superfamily of ionotropic receptors that are gated by adenosine 5'-triphosphate (ATP). Accumulating evidence indicates that P2X4Rs play an important role in regulation of ethanol intake. At the molecular level, ethanol's inhibitory effects on P2X4Rs are antagonized by ivermectin (IVM), in part, via action on P2X4Rs. Behaviorally, male mice deficient in the p2rx4 gene (P2X4R knockout [KO]) have been shown to exhibit a transient increase in ethanol intake over a period of 4 days, as demonstrated by social and binge drinking paradigms. Furthermore, IVM reduced ethanol consumption in male and female rodents, whereas male P2X4R KO mice were less sensitive to the anti-alcohol effects of IVM, compared to wildtype (WT) mice, further supporting a role for P2X4Rs as targets of IVM's action. The current investigation extends testing the hypothesis that P2X4Rs play a role in regulation of ethanol intake. First, we tested the response of P2X4R KO mice to ethanol for a period of 5 weeks. Second, to gain insights into the changes in ethanol intake, we employed a lentivirus-shRNA (LV-shRNA) methodology to selectively knockdown P2X4R expression in the nucleus accumbens (NAc) core in male C57BL/6J mice. In agreement with our previous study, male P2X4R KO mice exhibited higher ethanol intake than WT mice. Additionally, reduced expression of P2X4Rs in the NAc core significantly increased ethanol intake and preference. Collectively, the findings support the hypothesis that P2X4Rs play a role in regulation of ethanol intake and that P2X4Rs represent a novel drug target for treatment of alcohol use disorder.

Extracellular adenosine 5'-triphosphate concentrations changes in rat spinal cord associated with the activation of urinary bladder afferents. A microdialysis study

Objective

To determine adenosine 5’-triphosphate levels in the interstice of spinal cord L₆-S₁ segment, under basal conditions or during mechanical and chemical activation of urinary bladder afferents.

Methods

A microdialysis probe was transversally implanted in the dorsal half of spinal cord L₆-S₁ segment in female rats. Microdialysate was collected at 15 minutes intervals during 135 minutes, in anesthetized animals. Adenosine 5’-triphosphate concentrations were determined with a bioluminescent assay. In one group of animals (n=7) microdialysate samples were obtained with an empty bladder during a 10-minutes bladder distension to 20 or 40cmH₂O with either saline, saline with acetic acid or saline with capsaicin. In another group of animals (n=6) bladder distention was performed and the microdialysis solution contained the ectonucleotidase inhibitor ARL 67156.

Results

Basal extracellular adenosine triphosphate levels were 110.9±35.34fmol/15 minutes, (mean±SEM, n=13), and bladder distention was associated with a significant increase in adenosine 5’-triphosphate levels which was not observed after bladder distention with saline solution containing capsaicin (10µM). Microdialysis with solution containing ARL 67156 (1mM) was associated with significantly higher extracellular adenosine 5’-triphosphate levels and no further increase in adenosine 5’-triphosphate was observed during bladder distension.

Conclusion

Adenosine 5’-triphosphate was present in the interstice of L₆-S₁ spinal cord segments, was degraded by ectonucleotidase, and its concentration increased following the activation of bladder mechanosensitive but not of the chemosensitive afferents fibers. Adenosine 5’-triphosphate may originate either from the central endings of bladder mechanosensitive primary afferent neurons, or most likely from intrinsic spinal neurons, or glial cells and its release appears to be modulated by capsaicin activated bladder primary afferent or by adenosine 5’-triphosphate itself.

[Effect of baicalin on ATPase and LDH and its regulatory effect on the AC/cAMP/PKA signaling pathway in rats with attention deficit hyperactivity disorder]

OBJECTIVE

To study the effect of baicalin on synaptosomal adenosine triphosphatase (ATPase) and lactate dehydrogenase (LDH) and its regulatory effect on the adenylate cyclase (AC)/cyclic adenosine monophosphate (cAMP)/protein kinase A (PKA) signaling pathway in rats with attention deficit hyperactivity disorder (ADHD).

METHODS

A total of 40 SHR rats were randomly divided into five groups: ADHD model, methylphenidate hydrochloride treatment (0.07 mg/mL), and low-dose (3.33 mg/mL), medium-dose (6.67 mg/mL), and high-dose (10 mg/mL) baicalin treatment (n=8 each). Eight WKY rats were selected as normal control group. Percoll density gradient centrifugation was used to prepare brain synaptosomes and an electron microscope was used to observe their structure. Colorimetry was used to measure the activities of ATPase and LDH in synaptosomes. ELISA was used to measure the content of AC, cAMP, and PKA.

RESULTS

Compared with the normal control group, the ADHD model group had a significant reduction in the ATPase activity, a significant increase in the LDH activity, and significant reductions in the content of AC, cAMP, and PKA (P<0.05). Compared with the ADHD model group, the methylphenidate hydrochloride group and the medium- and high-dose baicalin groups had a significant increase in the ATPase activity (P<0.05), a significant reduction in the LDH activity (P<0.05), and significant increases in the content of AC, cAMP, and PKA (P<0.05). Compared with the methylphenidate hydrochloride group, the high-dose baicalin group had significantly greater changes in these indices (P<0.05). Compared with the low-dose baicalin group, the high-dose baicalin group had a significant increase in the ATPase activity (P<0.05); the medium- and high-dose baicalin groups had a significant reduction in the LDH activity (P<0.05) and significant increases in the content of AC, cAMP, and PKA (P<0.05). Compared with the medium-dose baicalin group, the high-dose baicalin group had a significant increase in the ATPase activity (P<0.05).

CONCLUSIONS

Both methylphenidate hydrochloride and baicalin can improve synaptosomal ATPase and LDH activities in rats with ADHD. The effect of baicalin is dose-dependent, and high-dose baicalin has a significantly greater effect than methylphenidate hydrochloride. Baicalin exerts its therapeutic effect possibly by upregulating the AC/cAMP/PKA signaling pathway.

Role of purinergic P2X4 receptors in regulating striatal dopamine homeostasis and dependent behaviors

Purinergic P2X4 receptors (P2X4Rs) belong to the P2X superfamily of ion channels regulated by ATP. We recently demonstrated that P2X4R knockout (KO) mice exhibited deficits in sensorimotor gating, social interaction, and ethanol drinking behavior. Dopamine (DA) dysfunction may underlie these behavioral changes, but there is no direct evidence for P2X4Rs' role in DA neurotransmission. To test this hypothesis, we measured markers of DA function and dependent behaviors in P2X4R KO mice. P2X4R KO mice exhibited altered density of pre-synaptic markers including tyrosine hydroxylase, dopamine transporter; post-synaptic markers including dopamine receptors and phosphorylation of downstream targets including dopamine and cyclic-AMP regulated phosphoprotein of 32 kDa and cyclic-AMP-response element binding protein in different parts of the striatum. Ivermectin, an allosteric modulator of P2X4Rs, significantly affected dopamine and cyclic AMP regulated phosphoprotein of 32 kDa and extracellular regulated kinase1/2 phosphorylation in the striatum. Sensorimotor gating deficits in P2X4R KO mice were rescued by DA antagonists. Using the 6-hydroxydopamine model of DA depletion, P2X4R KO mice exhibited an attenuated levodopa (L-DOPA)-induced motor behavior, whereas ivermectin enhanced this behavior. Collectively, these findings identified an important role for P2X4Rs in maintaining DA homeostasis and illustrate how this association is important for CNS functions including motor control and sensorimotor gating. We propose that P2X4 receptors (P2X4Rs) regulate dopamine (DA) homeostasis and associated behaviors. Pre-synaptic and post-synaptic DA markers were significantly altered in the dorsal and ventral striatum of P2X4R KO mice, implicating altered DA neurotransmission. Sensorimotor gating deficits in P2X4R KO mice were rescued by DA antagonists. Ivermectin (IVM), a positive modulator of P2X4Rs, enhanced levodopa (L-DOPA)-induced motor behavior. These studies highlight potential interactions between P2X4Rs and DA system.

P2X Receptor-Mediated Modulation of Sensory Transmission to the Spinal Cord Dorsal Horn

In the somatosensory system, P2X receptors are expressed on both peripheral and central terminals of primary afferent neurons. Those expressed on peripheral terminals are activated in response to both nociceptive and innocuous stimuli, whereas those at central terminals (“central terminal P2X receptors”) play an important role in modulating sensory transmission to the spinal cord dorsal horn. The author reviews recent studies on the central terminal P2X receptors. It is proposed that central terminal P2X receptors, once activated, may be involved in both central sensitization and initiation of pain. Thus, these receptors may repesent a promising target for therapeutic management of pathological pain.

Extracellular ATP and other nucleotides-ubiquitous triggers of intercellular messenger release

Extracellular nucleotides, and ATP in particular, are cellular signal substances involved in the control of numerous (patho)physiological mechanisms. They provoke nucleotide receptor-mediated mechanisms in select target cells. But nucleotides can considerably expand their range of action. They function as primary messengers in intercellular communication by stimulating the release of other extracellular messenger substances. These in turn activate additional cellular mechanisms through their own receptors. While this applies also to other extracellular messengers, its omnipresence in the vertebrate organism is an outstanding feature of nucleotide signaling. Intercellular messenger substances released by nucleotides include neurotransmitters, hormones, growth factors, a considerable variety of other proteins including enzymes, numerous cytokines, lipid mediators, nitric oxide, and reactive oxygen species. Moreover, nucleotides activate or co-activate growth factor receptors. In the case of hormone release, the initially paracrine or autocrine nucleotide-mediated signal spreads through to the entire organism. The examples highlighted in this commentary suggest that acting as ubiquitous triggers of intercellular messenger release is one of the major functional roles of extracellular nucleotides. While initiation of messenger release by nucleotides has been unraveled in many contexts, it may have been overlooked in others. It can be anticipated that additional nucleotide-driven messenger functions will be uncovered with relevance for both understanding physiology and development of therapy.

Activation of transient receptor potential vanilloid 2-expressing primary afferents stimulates synaptic transmission in the deep dorsal horn of the rat spinal cord and elicits mechanical hyperalgesia

Probenecid, an agonist of transient receptor vanilloid (TRPV) type 2, was used to evaluate the effects of TRPV2 activation on excitatory and inhibitory synaptic transmission in the dorsal horn (DH) of the rat spinal cord and on nociceptive reflexes induced by thermal heat and mechanical stimuli. The effects of probenecid were compared with those of capsaicin, a TRPV1 agonist. Calcium imaging experiments on rat dorsal root ganglion (DRG) and DH cultures indicated that functional TRPV2 and TRPV1 were expressed by essentially non-overlapping subpopulations of DRG neurons, but were absent from DH neurons and DH and DRG glial cells. Pretreatment of DRG cultures with small interfering RNAs against TRPV2 suppressed the responses to probenecid. Patch-clamp recordings from spinal cord slices showed that probenecid and capsaicin increased the frequencies of spontaneous excitatory postsynaptic currents (sEPSCs) and spontaneous inhibitory postsynaptic currents in a subset of laminae III-V neurons. In contrast to capsaicin, probenecid failed to stimulate synaptic transmission in lamina II. Intrathecal or intraplantar injections of probenecid induced mechanical hyperalgesia/allodynia without affecting nociceptive heat responses. Capsaicin induced both mechanical hyperalgesia/allodynia and heat hyperalgesia. Activation of TRPV1 or TRPV2 in distinct sets of primary afferents increased the sEPSC frequencies in a largely common population of DH neurons in laminae III-V, and might underlie the development of mechanical hypersensitivity following probenecid or capsaicin treatment. However, only TRPV1-expressing afferents facilitated excitatory and/or inhibitory transmission in a subpopulation of lamina II neurons, and this phenomenon might be correlated with the induction of thermal heat hyperalgesia.

Inherent dynamics of head domain correlates with ATP-recognition of P2X4 receptors: insights gained from molecular simulations

P2X receptors are ATP-gated ion channels involved in many physiological functions, and determination of ATP-recognition (AR) of P2X receptors will promote the development of new therapeutic agents for pain, inflammation, bladder dysfunction and osteoporosis. Recent crystal structures of the zebrafish P2X4 (zfP2X4) receptor reveal a large ATP-binding pocket (ABP) located at the subunit interface of zfP2X4 receptors, which is occupied by a conspicuous cluster of basic residues to recognize triphosphate moiety of ATP. Using the engineered affinity labeling and molecular modeling, at least three sites (S1, S2 and S3) within ABP have been identified that are able to recognize the adenine ring of ATP, implying the existence of at least three distinct AR modes in ABP. The open crystal structure of zfP2X4 confirms one of three AR modes (named AR1), in which the adenine ring of ATP is buried into site S1 while the triphosphate moiety interacts with clustered basic residues. Why architecture of ABP favors AR1 not the other two AR modes still remains unexplored. Here, we examine the potential role of inherent dynamics of head domain, a domain involved in ABP formation, in AR determinant of P2X4 receptors. In silico docking and binding free energy calculation revealed comparable characters of three distinct AR modes. Inherent dynamics of head domain, especially the downward motion favors the preference of ABP for AR1 rather than AR2 and AR3. Along with the downward motion of head domain, the closing movement of loop139-146 and loop169-183, and structural rearrangements of K70, K72, R298 and R143 enabled ABP to discriminate AR1 from other AR modes. Our observations suggest the essential role of head domain dynamics in determining AR of P2X4 receptors, allowing evaluation of new strategies aimed at developing specific blockers/allosteric modulators by preventing the dynamics of head domain associated with both AR and channel activation of P2X4 receptors.

Proteomic and metabolomic profiling reveals time-dependent changes in hippocampal metabolism upon paroxetine treatment and biomarker candidates

Most of the commonly used antidepressants block monoamine reuptake transporters to enhance serotonergic or noradrenergic neurotransmission. Effects besides or downstream of monoamine reuptake inhibition are poorly understood and yet presumably important for the drugs' mode of action. In the present study we aimed at identifying hippocampal cellular pathway alterations in DBA/2 mice using paroxetine as a representative Selective Serotonin Reuptake Inhibitor (SSRI). Furthermore we identified biomarker candidates for the assessment of antidepressant treatment effects in plasma. Hippocampal protein levels were compared between chronic paroxetine- and vehicle-treated animals using in vivo(15)N metabolic labeling combined with mass spectrometry. We also studied the time course of metabolite level changes in hippocampus and plasma using a targeted polar metabolomics profiling platform. In silico pathway analyses revealed profound alterations related to hippocampal energy metabolism. Glycolytic metabolite levels acutely increased while Krebs cycle metabolite levels decreased upon chronic treatment. Changes in energy metabolism were influenced by altered glycogen metabolism rather than by altered glycolytic or Krebs cycle enzyme levels. Increased energy levels were reflected by an increased ATP/ADP ratio and by increased ratios of high-to-low energy purines and pyrimidines. In the course of our analyses we also identified myo-inositol as a biomarker candidate for the assessment of antidepressant treatment effects in the periphery. This study defines the cellular response to paroxetine treatment at the proteome and metabolome levels in the hippocampus of DBA/2 mice and suggests novel SSRI modes of action that warrant consideration in antidepressant development efforts.

pH-dependent inhibition of native GABA(A) receptors by HEPES

BACKGROUND AND PURPOSE

Artificial buffers such as HEPES are extensively used to control extracellular pH (pH(e) ) to investigate the effect of H(+) ions on GABA(A) receptor function.

EXPERIMENTAL APPROACH

In neurones cultured from spinal cord dorsal horn (DH), dorsal root ganglia (DRG) and cerebellar granule cells (GC) of neonatal rats, we studied the effect of pH(e) on currents induced by GABA(A) receptor agonists, controlling pH(e) with HCO(3) (-) or different concentrations of HEPES.

KEY RESULTS

Changing HEPES concentration from 1 to 20 mM at constant pH(e) strongly inhibited the currents induced by submaximal GABA applications, but not those induced by glycine or glutamate, on DH, DRG or GC neurones, increasing twofold the EC(50) for GABA in DH neurones and GC. Submaximal GABA(A) receptor-mediated currents were also inhibited by piperazine-N,N'-bis(2-ethanesulfonic acid) (PIPES), 3-(N-morpholino)propanesulfonic acid, tris(hydroxymethyl)aminomethane or imidazole. PIPES and HEPES, both piperazine derivatives, similarly inhibited GABA(A) receptors, whereas the other buffers had weaker effects and 2-(N-morpholino)ethanesulfonic acid had no effect. HEPES-induced inhibition of submaximal GABA(A) receptor-mediated currents was unaffected by diethylpyrocarbonate, a histidine-modifying reagent. HEPES-induced inhibition of GABA(A) receptors was independent of membrane potential, HCO(3) (-) and intracellular Cl(-) concentration and was not modified by flumazenil, which blocks the benzodiazepine binding site. However, it strongly depended on pH(e) .

CONCLUSIONS AND IMPLICATIONS

Inhibition of GABA(A) receptors by HEPES depended on pH(e) , leading to an apparent H(+) -induced inhibition of DH GABA(A) receptors, unrelated to the pH sensitivity of these receptors in both low and physiological buffering conditions, suggesting that protonated HEPES caused this inhibition.

Purinergic modulation of hypoxic regulation via the rostral ventral lateral medulla in rats

Anatomical studies have demonstrated the existence of purinergic receptors in the rostral ventral lateral medulla (RVLM), a site containing some respiratory-related neurons. However, little is known about the functional role of these receptors in acute hypoxia. In the present study, we found that both the amplitude and frequency of phrenic nerve discharges were increased during hypoxia. Microinjection of adenosine 5'-triphosphate (ATP) (0.2M, 10-70nl) into the RVLM increased the hypoxic respiratory response and showed significant dose-dependency. An identical microinjection protocol of pyridoxalphosphate-6-azophenyl-2',4'-disulfonate (PPADS), a broad-spectrum P2 receptor antagonist, into the RVLM markedly attenuated the respiratory effects evoked by hypoxic ventilation. Immunohistochemical analysis showed that the P2X(2) receptor was present in the postsynaptic membrane of the RVLM neuronal cell bodies and levels of this receptor were significantly increased after acute hypoxic challenge. These results suggest that RVLM purinergic P2 receptors may contribute to respiratory control by regulating the acute hypoxic ventilatory response.

Interplay between ionotropic receptors modulates inhibitory synaptic strength

The essence of neuronal function is to generate outputs in response to synaptic potentials. Synaptic integration at a synapse determines neuronal outputs in the CNS. In a recent study, we describe that excitatory and inhibitory transmitter-gated channels physically crosstalk each other at the cellular and molecular level. Increased membrane expression of ATP P2X4 receptors by using an interference peptide competing with the intracellular endocytosis motif enhances neuronal excitability, which is further enhanced by reciprocal interaction between post-synaptic ATP- and GABA-gated channels. Molecular interaction is supported by experiments of co-immunoprecipitation and mutagenesis of P2X4 subunit.  Two amino acids in the intracellular carboxyl tail of P2X4 subunit appears to be responsible for this crosstalk. Our recent study provides molecular and electrophysiological evidence for physical interaction between excitatory and inhibitory receptors that appears to be crucial in determining synaptic strength at central synapses.

Fast synaptic inhibition in spinal sensory processing and pain control

The two amino acids GABA and glycine mediate fast inhibitory neurotransmission in different CNS areas and serve pivotal roles in the spinal sensory processing. Under healthy conditions, they limit the excitability of spinal terminals of primary sensory nerve fibers and of intrinsic dorsal horn neurons through pre- and postsynaptic mechanisms, and thereby facilitate the spatial and temporal discrimination of sensory stimuli. Removal of fast inhibition not only reduces the fidelity of normal sensory processing but also provokes symptoms very much reminiscent of pathological and chronic pain syndromes. This review summarizes our knowledge of the molecular bases of spinal inhibitory neurotransmission and its organization in dorsal horn sensory circuits. Particular emphasis is placed on the role and mechanisms of spinal inhibitory malfunction in inflammatory and neuropathic chronic pain syndromes.

Glial glutamate transporter and glutamine synthetase regulate GABAergic synaptic strength in the spinal dorsal horn

Decreased GABAergic synaptic strength ('disinhibition') in the spinal dorsal horn is a crucial mechanism contributing to the development and maintenance of pathological pain. However, mechanisms leading to disinhibition in the spinal dorsal horn remain elusive. We investigated the role of glial glutamate transporters (GLT-1 and GLAST) and glutamine synthetase in maintaining GABAergic synaptic activity in the spinal dorsal horn. Electrically evoked GABAergic inhibitory post-synaptic currents (eIPSCs), spontaneous IPSCs (sIPSCs) and miniature IPSCs were recorded in superficial spinal dorsal horn neurons of spinal slices from young adult rats. We used (2S,3S)-3-[3-[4-(trifluoromethyl)benzoylamino]benzyloxy]aspartate (TFB-TBOA), to block both GLT-1 and GLAST and dihydrokainic acid to block only GLT-1. We found that blockade of both GLAST and GLT-1 and blockade of only GLT-1 in the spinal dorsal horn decreased the amplitude of GABAergic eIPSCs, as well as both the amplitude and frequency of GABAergic sIPSCs or miniature IPSCs. Pharmacological inhibition of glial glutamine synthetase had similar effects on both GABAergic eIPSCs and sIPSCs. We provided evidence demonstrating that the reduction in GABAergic strength induced by the inhibition of glial glutamate transporters is due to insufficient GABA synthesis through the glutamate-glutamine cycle between astrocytes and neurons. Thus, our results indicate that deficient glial glutamate transporters and glutamine synthetase significantly attenuate GABAergic synaptic strength in the spinal dorsal horn, which may be a crucial synaptic mechanism underlying glial-neuronal interactions caused by dysfunctional astrocytes in pathological pain conditions.

Neurotensin inhibits background K+ channels and facilitates glutamatergic transmission in rat spinal cord dorsal horn

Neurotensin (NT) is a neuropeptide involved in the modulation of nociception. We have investigated the actions of NT on cultured postnatal rat spinal cord dorsal horn (DH) neurons. NT induced an inward current associated with a decrease in membrane conductance in 46% of the neurons and increased the frequency of glutamatergic miniature excitatory synaptic currents in 37% of the neurons. Similar effects were observed in acute slices. Both effects of NT were reproduced by the selective NTS1 agonist JMV449 and blocked by the NTS1 antagonist SR48692 and the NTS1/NTS2 antagonist SR142948A. The NTS2 agonist levocabastine had no effect. The actions of NT persisted after inactivation of G(i/o) proteins by pertussis toxin but were absent after inactivation of protein kinase C (PKC) by chelerythrine or inhibition of the MAPK (ERK1/2) pathway by PD98059. Pre- and postsynaptic effects of NT were insensitive to classical voltage- and Ca(2+) -dependent K(+) channel blockers. The K(+) conductance inhibited by NT was blocked by Ba(2+) and displayed no or little inward rectification, despite the presence of strongly rectifying Ba(2+) -sensitive K(+) conductance in these neurons. This suggested that NT blocked two-pore domain (K2P) background K(+) -channels rather than inwardly rectifying K(+) channels. Zn(2+) ions, which inhibit TRESK and TASK-3 K2P channels, decreased NT-induced current. Our results indicate that in DH neurons NT activates NTS1 receptors which, via the PKC-dependent activation of the MAPK (ERK1/2) pathway, depolarize the postsynaptic neuron and increase the synaptic release of glutamate. These actions of NT might modulate the transfer and the integration of somatosensory information in the DH.

Spatiotemporal and anatomical analyses of P2X receptor-mediated neuronal and glial processing of sensory signals in the rat dorsal horn

Extracellularly released adenosine triphosphate (ATP) modulates sensory signaling in the spinal cord. We analyzed the spatiotemporal profiles of P2X receptor-mediated neuronal and glial processing of sensory signals and the distribution of P2X receptor subunits in the rat dorsal horn. Voltage imaging of spinal cord slices revealed that extracellularly applied ATP (5-500 μM), which was degraded to adenosine and acting on P1 receptors, inhibited depolarizing signals and that it also enhanced long-lasting slow depolarization, which was potentiated after ATP was washed out. This post-ATP rebound potentiation was mediated by P2X receptors and was more prominent in the deep than in the superficial layer. Patch clamp recording of neurons in the superficial layer revealed long-lasting enhancement of depolarization by ATP through P2X receptors during the slow repolarization phase at a single neuron level. This depolarization pattern was different from that in voltage imaging, which reflects both neuronal and glial activities. By immunohistochemistry, P2X(1) and P2X(3) subunits were detected in neuropils in the superficial layer. The P2X(5) subunit was found in neuronal somata. The P2X(6) subunit was widely expressed in neuropils in the whole gray matter except for the dorsal superficial layer. Astrocytes expressed the P2X(7) subunit. These findings indicate that extracellular ATP is degraded into adenosine and prevents overexcitation of the sensory system, and that ATP acts on pre- and partly on postsynaptic neuronal P2X receptors and enhances synaptic transmission, predominantly in the deep layer. Astrocytes are involved in sensitization of sensory network activity more importantly in the superficial than in the deep layer.

Facilitation of glutamate and GABA release by P2X receptor activation in supraoptic neurons from freshly isolated rat brain slices

The supraoptic nuclei (SON), the hypothalamic release site of vasopressin and oxytocin, receive a non-glutamatergic, excitatory input from the caudal medulla that uses noradrenaline and ATP as neurotransmitters. Here, we studied the actions of extracellular ATP on SON neurons in hypothalamic slices isolated from the brains of 16- to 24-day-old rats. Whole-cell current clamp recordings performed 1-6 h after isolation showed that exogenous ATP application increased the frequency of action potentials and induced the depolarization of resting membranes. Voltage clamp recordings showed that ATP increased the frequency of GABAergic or glutamatergic spontaneous synaptic currents without changing their amplitude and evoked inward current (126±13 pA) in about 80% of SON neurons. The application of ATPγS and 2MeSATP mimicked the effects of ATP, but 2MeSADP, 2MeSAMP and αβmeATP had no effect. The P2X7 receptor agonist, BzATP, did not induce an inward current, but it increased intracellular calcium concentration in non-neuronal SON cells in slices. Suramin and pyridoxalphosphate-6-azophenyl-2',4'-disulfonic acid (PPADS) inhibited ATP-induced currents, whereas pH 6.5 and ivermectin, a specific allosteric modulator of the P2X4 receptor, potentiated ATP-induced currents. The P2Y1-selective antagonist, 2'-deoxy-N⁶-methyladenosine 3',5'-bisphosphate tetrasodium salt (MRS 2179), had no effect on ATP-induced responses. Quantitative real-time PCR showed that P2X2>P2X7>P2X4 purinergic receptor mRNAs were expressed in the SON tissue, but the levels of P2X1, P2X3, P2X5, P2X6, P2Y1, P2Y2 and P2Y12 mRNA were minor. These results show that SON neurons express functional presynaptic and extrasynaptic P2X2 and P2X4 receptors that modulate glutamate and GABA release and control the electrical excitability of SON neurons.

Cross-talk between P2X4 and gamma-aminobutyric acid, type A receptors determines synaptic efficacy at a central synapse

The essence of neuronal function is to generate outputs in response to synaptic potentials. Synaptic integration at postsynaptic sites determines neuronal outputs in the CNS. Using immunohistochemical and electrophysiological approaches, we first reveal that steroidogenic factor 1 (SF-1) green fluorescent protein (GFP)-positive neurons in the ventromedial nucleus of the hypothalamus express P2X4 subunits that are activated by exogenous ATP. Increased membrane expression of P2X4 channels by using a peptide competing with P2X4 intracellular endocytosis motif enhances neuronal excitability of SF-1 GFP-positive neurons. This increased excitability is inhibited by a P2X receptor antagonist. Furthermore, increased surface P2X4 receptor expression significantly decreases the frequency and the amplitude of GABAergic postsynaptic currents of SF-1 GFP-positive neurons. Co-immunopurification and pulldown assays reveal that P2X4 receptors complex with aminobutyric acid, type A (GABA(A)) receptors and demonstrate that two amino acids in the carboxyl tail of the P2X4 subunit are crucial for its physical association with GABA(A) receptors. Mutation of these two residues prevents the physical association, thereby blocking cross-inhibition between P2X4 and GABA(A) receptors. Moreover, disruption of the physical coupling using competitive peptides containing the identified motif abolishes current inhibition between P2X4 and GABA(A) receptors in recombinant system and P2X4 receptor-mediated GABAergic depression in SF-1 GFP-positive neurons. Our present work thus provides evidence for cross-talk between excitatory and inhibitory receptors that appears to be crucial in determining GABAergic synaptic strength at a central synapse.

Axon Physiology

Axons are generally considered as reliable transmission cables in which stable propagation occurs once an action potential is generated. Axon dysfunction occupies a central position in many inherited and acquired neurological disorders that affect both peripheral and central neurons. Recent findings suggest that the functional and computational repertoire of the axon is much richer than traditionally thought. Beyond classical axonal propagation, intrinsic voltage-gated ionic currents together with the geometrical properties of the axon determine several complex operations that not only control signal processing in brain circuits but also neuronal timing and synaptic efficacy. Recent evidence for the implication of these forms of axonal computation in the short-term dynamics of neuronal communication is discussed. Finally, we review how neuronal activity regulates both axon morphology and axonal function on a long-term time scale during development and adulthood.

Pharmacologically targeting the P2rx4 gene on maintenance and reinstatement of alcohol self-administration in rats

Genetic studies indicate that alcohol consumption associates with expression of the P2rx4 gene, a gene that codes for the P2X(4) receptor. This receptor is a subtype in the purinergic system of ligand-gated ion channels that when activated exerts excitatory effects in CNS. P2X(4) function is inhibited by alcohol and P2X(4) receptors are modulated positively by the antiparasitic agent, ivermectin. Two experiments were performed to test the ability of ivermectin to alter the behavioral effects of alcohol in rats. After alcohol exposure was achieved via the "drinking in the dark" procedure, separate groups of Sprague-Dawley rats were trained to lever press for either alcohol (10% ethanol/2% sucrose) or sucrose (3%) solutions in operant chambers. Rats were tested for maintenance of operant self-administration under a progressive ratio condition (Experiment 1) and for reinstatement of extinguished responding induced by solution presentation (Experiment 2) after ivermectin (0; 1-10mg/kg; IP) administration. Ivermectin decreased the amount of work that the animal performed to obtain reinforcers in the maintenance study, particularly in the group reinforced with alcohol, and tended to decrease reinstated lever press responding. Conditioned approach behavior (head entries) was significantly reduced by ivermectin in both experiments. Reduction in motor activity was seen during the longer maintenance sessions but not in the shorter reinstatement sessions. Results suggest some support for ivermectin-like drugs as potential treatment agents for alcohol dependence. Caution is warranted due to modest specificity on behavior reinforced by alcohol, some reduction in general activity levels, and the lack of dose-response effects.

P2 receptor signaling in neurons and glial cells of the central nervous system

Purine and pyrimidine nucleotides are extracellular signaling molecules in the central nervous system (CNS) leaving the intracellular space of various CNS cell types via nonexocytotic mechanisms. In addition, ATP is a neuro-and gliotransmitter released by exocytosis from neurons and neuroglia. These nucleotides activate P2 receptors of the P2X (ligand-gated cationic channels) and P2Y (G protein-coupled receptors) types. In mammalians, seven P2X and eight P2Y receptor subunits occur; three P2X subtypes form homomeric or heteromeric P2X receptors. P2Y subtypes may also hetero-oligomerize with each other as well as with other G protein-coupled receptors. P2X receptors are able to physically associate with various types of ligand-gated ion channels and thereby to interact with them. The P2 receptor homomers or heteromers exhibit specific sensitivities against pharmacological ligands and have preferential functional roles. They may be situated at both presynaptic (nerve terminals) and postsynaptic (somatodendritic) sites of neurons, where they modulate either transmitter release or the postsynaptic sensitivity to neurotransmitters. P2 receptors exist at neuroglia (e.g., astrocytes, oligodendrocytes) and microglia in the CNS. The neuroglial P2 receptors subserve the neuron-glia cross talk especially via their end-feets projecting to neighboring synapses. In addition, glial networks are able to communicate through coordinated oscillations of their intracellular Ca(2+) over considerable distances. P2 receptors are involved in the physiological regulation of CNS functions as well as in its pathophysiological dysregulation. Normal (motivation, reward, embryonic and postnatal development, neuroregeneration) and abnormal regulatory mechanisms (pain, neuroinflammation, neurodegeneration, epilepsy) are important examples for the significance of P2 receptor-mediated/modulated processes.

Potentiation of the glutamatergic synaptic input to rat locus coeruleus neurons by P2X7 receptors

Locus coeruleus (LC) neurons in a rat brain slice preparation were superfused with a Mg(2+)-free and bicuculline-containing external medium. Under these conditions, glutamatergic spontaneous excitatory postsynaptic currents (sEPSCs) were recorded by means of the whole-cell patch-clamp method. ATP, as well as its structural analogue 2-methylthio ATP (2-MeSATP), both caused transient inward currents, which were outlasted by an increase in the frequency but not the amplitude of the sEPSCs. PPADS, but not suramin or reactive blue 2 counteracted both effects of 2-MeSATP. By contrast, α,β-methylene ATP (α,β-meATP), UTP and BzATP did not cause an inward current response. Of these latter agonists, only BzATP slightly facilitated the sEPSC amplitude and strongly potentiated its frequency. PPADS and Brilliant Blue G, as well as fluorocitric acid and aminoadipic acid prevented the activity of BzATP. Furthermore, BzATP caused a similar facilitation of the miniature (m)EPSC (recorded in the presence of tetrodotoxin) and sEPSC frequencies (recorded in its absence). Eventually, capsaicin augmented the frequency of the sEPSCs in a capsazepine-, but not PPADS-antagonizable, manner. In conclusion, the stimulation of astrocytic P2X7 receptors appears to lead to the outflow of a signalling molecule, which presynaptically increases the spontaneous release of glutamate onto LC neurons from their afferent fibre tracts. It is suggested, that the two algogenic compounds ATP and capsaicin utilise separate receptor systems to potentiate the release of glutamate and in consequence to increase the excitability of LC neurons.

Selective potentiation of homomeric P2X2 ionotropic ATP receptors by a fast non-genomic action of progesterone

P2X receptors are ligand-gated ion channels activated by ATP that are widely expressed in the organism and regulate many physiological functions. We have studied the effect of progesterone (PROG) on native P2X receptors present in rat dorsal root ganglion (DRG) neurons and on recombinant P2X receptors expressed in HEK293 cells or Xenopus laevis oocytes. The effects of PROG were observed and already maximal during the first coapplication with ATP and did not need any preincubation of the cells with PROG, indicating a fast mechanism of action. In DRG neurons, PROG rapidly and reversibly potentiated submaximal but not saturating plateau-like currents evoked by ATP, but had no effect on the currents activated by alpha,beta-methylene ATP, an agonist of homomeric or heteromeric receptors containing P2X1 or P2X3 subunits. In cells expressing homomeric P2X2 receptors, responses to submaximal ATP, were systematically potentiated by PROG in a dose-dependent manner with a threshold between 1 and 10 nM. PROG had no effect on ATP currents carried by homomeric P2X1, P2X3, and P2X4 receptors or by heteromeric P2X1/5 and P2X2/3 receptors. We conclude that PROG selectively potentiates homomeric P2X2 receptors and, in contrast with dehydroepiandrosterone (DHEA), discriminates between homomeric and heteromeric P2X2-containing receptors. This might have important physiological implications since the P2X2 subunit is the most widely distributed P2X subunit in the organism. Moreover, DHEA and PROG might be useful tools to clarify the distribution and the role of native homo- and heteromeric P2X2 receptors.

A point mutation in the ectodomain-transmembrane 2 interface eliminates the inhibitory effects of ethanol in P2X4 receptors

ATP-gated P2X4 receptors (P2X4R) are abundantly expressed in the CNS. However, little is known about the molecular targets for ethanol action in P2X4Rs. The current investigation tested the hypothesis that the ectodomain-transmembrane (TM) interface contains residues that are important for the action of ethanol in P2X4Rs. Wild type (WT) and mutant P2X4R were expressed in Xenopus oocytes. ATP concentration-response curves and ethanol (10-200 mM)-induced changes in ATP EC(10)-gated currents were determined using two-electrode voltage clamp (-70 mV). Alanine substitution at the ectodomain-TM1 interface (positions 50-61) resulted in minimal changes in ethanol response. On the other hand, alanine substitution at the ectodomain-TM2 interface (positions 321-337) identified two key residues (D331 and M336) that significantly reduced ethanol inhibition of ATP-gated currents without causing marked changes in ATP I(max), EC(50), or Hill's slope. Other amino acid substitutions at positions 331 and 336 significantly altered or eliminated the modulatory effects of ethanol. Linear regression analyses revealed a significant relationship between hydropathy and polarity, but not molecular volume/molecular weight of the residues at these two positions. The results support the proposed hypothesis and represent an important step toward developing ethanol-insensitive receptors for investigating the role of P2X4Rs in mediating behavioral effects of ethanol.

Genetical genomic determinants of alcohol consumption in rats and humans

BACKGROUND

We have used a genetical genomic approach, in conjunction with phenotypic analysis of alcohol consumption, to identify candidate genes that predispose to varying levels of alcohol intake by HXB/BXH recombinant inbred rat strains. In addition, in two populations of humans, we assessed genetic polymorphisms associated with alcohol consumption using a custom genotyping array for 1,350 single nucleotide polymorphisms (SNPs). Our goal was to ascertain whether our approach, which relies on statistical and informatics techniques, and non-human animal models of alcohol drinking behavior, could inform interpretation of genetic association studies with human populations.

RESULTS

In the HXB/BXH recombinant inbred (RI) rats, correlation analysis of brain gene expression levels with alcohol consumption in a two-bottle choice paradigm, and filtering based on behavioral and gene expression quantitative trait locus (QTL) analyses, generated a list of candidate genes. A literature-based, functional analysis of the interactions of the products of these candidate genes defined pathways linked to presynaptic GABA release, activation of dopamine neurons, and postsynaptic GABA receptor trafficking, in brain regions including the hypothalamus, ventral tegmentum and amygdala. The analysis also implicated energy metabolism and caloric intake control as potential influences on alcohol consumption by the recombinant inbred rats. In the human populations, polymorphisms in genes associated with GABA synthesis and GABA receptors, as well as genes related to dopaminergic transmission, were associated with alcohol consumption.

CONCLUSION

Our results emphasize the importance of the signaling pathways identified using the non-human animal models, rather than single gene products, in identifying factors responsible for complex traits such as alcohol consumption. The results suggest cross-species similarities in pathways that influence predisposition to consume alcohol by rats and humans. The importance of a well-defined phenotype is also illustrated. Our results also suggest that different genetic factors predispose alcohol dependence versus the phenotype of alcohol consumption.

Miniature EPSCs in the lateral superior olive before hearing onset: regional and cell-type-specific differences and heterogeneous neuromodulatory effects of ATP

Spontaneous activity occurs in the mammalian auditory system prior to hearing onset and is relevant for neuronal differentiation. Growing evidence indicates that miniature events, i.e., action potential-independent synaptic activity, also have some developmental relevance. An intriguing question is whether these events are purely stochastic or rather display specific characteristics. We addressed this question and studied miniature excitatory postsynaptic currents (mEPSCs) in morphologically defined neurons of the rat lateral superior olive (LSO) during early neonatal life. To do so, whole-cell recordings from neurons in acute slices were combined with Lucifer yellow fillings. mEPSCs were identified by their TTX insensitivity and their blockade by glutamate receptor antagonists. Altogether, 60% of the LSO neurons displayed mEPSCs, and their presence correlated with the cell location and morphology. Their percentage was highest in the medial limb (86%) and lowest in the lateral limb (14%). Seventy-seven percent of the neurons with mEPSCs were bipolar cells, whereas 77% of those without mEPSCs were multipolar cells. The neuromodulator ATP affected the frequency of mEPSCs in 61% of the LSO neurons in a heterogeneous manner: both frequency increases and decreases occurred. These data provide further evidence for the specificity of mEPSCs. Finally, we investigated whether missing cochlear input changes mEPSCs characteristics. Characterizing LSO neurons of Ca(V)1.3(-/-) mice, which lack cochlea-driven nerve activity, we observed higher mEPSC frequencies and peak amplitudes, indicative of a compensatory response to deprivation. Together, our results demonstrate specific, rather than stochastic, characteristics of mEPSCs in the neonatal LSO, in accordance with their potential developmental significance.

Crystal structure of the ATP-gated P2X(4) ion channel in the closed state

P2X receptors are cation-selective ion channels gated by extracellular ATP, and are implicated in diverse physiological processes, from synaptic transmission to inflammation to the sensing of taste and pain. Because P2X receptors are not related to other ion channel proteins of known structure, there is at present no molecular foundation for mechanisms of ligand-gating, allosteric modulation and ion permeation. Here we present crystal structures of the zebrafish P2X(4) receptor in its closed, resting state. The chalice-shaped, trimeric receptor is knit together by subunit-subunit contacts implicated in ion channel gating and receptor assembly. Extracellular domains, rich in beta-strands, have large acidic patches that may attract cations, through fenestrations, to vestibules near the ion channel. In the transmembrane pore, the 'gate' is defined by an approximately 8 A slab of protein. We define the location of three non-canonical, intersubunit ATP-binding sites, and suggest that ATP binding promotes subunit rearrangement and ion channel opening.

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.

Spinal P2X receptor modulates muscle pressor reflex via glutamate

Static contraction of skeletal muscle evokes reflex increases in blood pressure and heart rate. Previous studies showed that P2X receptors located at the dorsal horn of the spinal cord play a role in modulating the muscle pressor reflex. P2X stimulation can alter release of the excitatory amino acid, glutamate (Glu). In this report, we tested the hypothesis that stimulation of P2X receptors enhances the concentrations of Glu ([Glu]) in the dorsal horn, and that blocking P2X receptors attenuates contraction-induced Glu increases and the resultant reflex pressor response. Contraction was elicited by electrical stimulation of the L(7) and S(1) ventral roots of 14 cats. Glu samples were collected from microdialysis probes inserted in the L(7) level of the dorsal horn of the spinal cord, and dialysate [Glu] was determined using the HPLC method. First, microdialyzing alpha,beta-methylene ATP (0.4 mM) into the dorsal horn significantly increased [Glu]. In addition, contraction elevated [Glu] from baseline of 536 +/- 53 to 1,179 +/- 192 nM (P < 0.05 vs. baseline), and mean arterial pressure by 39 +/- 8 mmHg in the control experiment. Microdialyzing the P2X receptor antagonist pyridoxal phosphate-6-azophenyl-2',4'-disulfonic acid (10 mM) into the dorsal horn attenuated the contraction induced-Glu increase (610 +/- 128 to 759 +/- 147 nM; P > 0.05) and pressor response (16 +/- 3 mmHg, P < 0.05 vs. control). Our findings demonstrate that P2X modulates the cardiovascular responses to static muscle contraction by affecting the release of Glu in the dorsal horn of the spinal cord.

Endogenous ATP potentiates only vasopressin secretion from neurohypophysial terminals

Exogenous ATP induces inward currents and causes the release of arginine-vasopressin (AVP) from isolated neurohypophysial terminals (NHT); both effects are inhibited by the P2X2 and P2X3 antagonists, suramin and PPADS. Here we examined the role of endogenous ATP in the neurohypophysis. Stimulation of NHT caused the release of both AVP and ATP. ATP induced a potentiation in the stimulated release of AVP, but not of oxytocin (OT), which was blocked by the presence of suramin. In loose-patch clamp recordings, from intact neurohypophyses, suramin or PPADS produces an inhibition of action potential currents in a static bath, that can be mimicked by a hyperpolarization of the resting membrane potential (RMP). Correspondingly, in a static versus perfused bath there is a depolarization of the RMP of NHT, which was reduced by either suramin or PPADS. We measured an accumulation of ATP (3.7 +/- 0.7 microM) released from NHT in a static bath. Applications of either suramin or PPADS to a static bath decreased burst-stimulated capacitance increases in NHT. Finally, only vasopressin release from electrically stimulated intact neurohypophyses was reduced in the presence of Suramin or PPADS. These data suggest that there was sufficient accumulation of ATP released from the neurohypophysis during stimulations to depolarize its nerve terminals. This would occur via the opening of P2X2 and P2X3 receptors, inducing an influx of Ca2+. The subsequent elevation in [Ca2+](i) would further increase the stimulated release of only vasopressin from NHT terminals. Such purinergic feedback mechanisms could be physiologically important at most CNS synapses.

Na+ modulates anion permeation and block of P2X7 receptors from mouse parotid glands

We previously reported that mouse parotid acinar cells display anion conductance (I(ATPCl)) when stimulated by external ATP in Na+-free extracellular solutions. It has been suggested that the P2X7 receptor channel (P2X7R) might underlie I(ATPCl). In this work we show that I (ATPCl) can be activated by ATP, ADP, AMP-PNP, ATPgammaS and CTP. This is consistent with the nucleotide sensitivity of P2X7R. Accordingly, acinar cells isolated from P2X7R( -/- ) mice lacked I(ATPCl). Experiments with P2X7R heterologously expressed resulted in ATP-activated currents (I(ATP-P2X7)) partially carried by anions. In Na(+)-free solutions, I (ATP-P2X7) had an apparent anion permeability sequence of SCN(-) > I(-) congruent with NO3(-) > Br(-) > Cl(-) > acetate, comparable to that reported for I(ATPCl) under the same conditions. However, in the presence of physiologically relevant concentrations of external Na+, the Cl(-) permeability of I(ATP-P2X7) was negligible, although permeation of Br(-) or SCN(-) was clearly resolved. Relative anion permeabilities were not modified by addition of 1 mM: carbenoxolone, a blocker of Pannexin-1. Moreover, cibacron blue 3GA, which blocks the Na(+) current activated by ATP in acinar cells but not I(ATPCl), blocked I(ATP-P2X7) in a dose-dependent manner when Na+ was present but failed to do so in tetraethylammonium containing solutions. Thus, our data indicate that P2X7R is fundamental for I(ATPCl) generation in acinar cells and that external Na+ modulates ion permeability and conductivity, as well as drug affinity, in P2X7R.

Roles of ectodomain and transmembrane regions in ethanol and agonist action in purinergic P2X2 and P2X3 receptors

The present work investigated sites of ethanol action in ATP-gated P2X receptors (P2XRs) using chimeric strategies that exploited the differences in ethanol response between P2X2R (inhibition) and P2X3R (potentiation). We tested ethanol (10-200mM) effects on ATP- and alpha,beta-methylene-ATP (alpha,beta-meATP)-induced currents in wildtype P2X2, P2X3 and chimeric P2X2/P2X3Rs expressed in Xenopus oocytes using two-electrode voltage-clamp (-70mV). Exchanging ectodomain regions of P2X2 and P2X3Rs reversed wildtype ethanol responses. Substituting back portions of the P2X2R ectodomain at TM interfaces in chimeras that contained the P2X3R ectodomain restored wildtype P2X2R-like ethanol response. Point mutations that replaced non-conserved ectodomain residues at TM interfaces of P2X3Rs with homologous P2X2R residues identified positions that reversed the direction (304) or changed the magnitude (53, 55 and 313) of ethanol response. Homologous substitutions in P2X2Rs did not significantly alter wildtype P2X2R-like ethanol responses. These findings suggest that ectodomain segments at TM interfaces play key roles in determining qualitative and quantitative responses to ethanol of P2X2 and P2X3Rs. Studies that substituted TM regions of P2X3R with respective P2X2R TMs indicate that the TM1, but not the TM2, region plays a role in determining the magnitude of ethanol response. Studies with ATP and alpha,beta-meATP support prior indications that TM regions are important in agonist desensitization and suggest that both ectodomain and TM regions play roles in determining agonist potency and selectivity. Overall, these findings are the first to identify potential targets for ethanol in P2X2 and P2X3Rs and should provide insight into the sites of ethanol action in other P2XRs.

Purinergic P2X receptors facilitate inhibitory GABAergic and glycinergic neurotransmission to cardiac vagal neurons in the nucleus ambiguus

This study examined whether adenosine 5'-triphosphate (ATP) modulated inhibitory glycinergic and GABAergic neurotransmission to cardiac vagal neurons. Inhibitory activity to cardiac vagal neurons was isolated and examined using whole-cell patch-clamp recordings in an in vitro brain slice preparation in rats. ATP (100 microM) evoked increases in the frequency of glycinergic and GABAergic miniature inhibitory postsynaptic currents (mIPSCs) in cardiac vagal neurons which were blocked by the broad P2 receptor antagonist pyridoxal-phosphate-6-azophenyl-2',4'-disulphonic acid (100 microM). Application of the P2Y agonists uridine triphosphate (15 microM) and adenosine 5'-0-(Z-thiodiphosphate) (60 microM) did not enhance inhibitory neurotransmission to cardiac vagal neurons however, application of the selective P2X; receptor agonist, alpha, beta-methylene ATP (100 microM), increased glycinergic and GABAergic mIPSC neurotransmission to cardiac vagal neurons. The increase in inhibitory neurotransmission evoked by alpha, beta-methylene ATP was abolished by the selective P2X receptor antagonist 2',3'-O-(2,4,6-Trinitrophenyl) adenosine 5'-triphosphate (100 microM) indicating P2X receptors enhance the release of inhibitory neurotransmitters to cardiac neurons. The voltage-gated calcium channel blocker cadmium chloride did not alter the evoked increase in inhibitory mIPSCs. This work demonstrates that P2X receptor activation enhances inhibitory neurotransmission to parasympathetic cardiac vagal neurons and demonstrates an important functional role for ATP mediated purinergic signaling to cardiac vagal neurons.