Convergent control of synaptic GABA release from rat dorsal horn neurones by adenosine and GABA autoreceptors

Short-term plasticity in the spinal nociceptive system

ABSTRACT

Somatosensory information is delivered to neuronal networks of the dorsal horn (DH) of the spinal cord by the axons of primary afferent neurons that encode the intensity of peripheral sensory stimuli under the form of a code based on the frequency of action potential firing. The efficient processing of these messages within the DH involves frequency-tuned synapses, a phenomenon linked to their ability to display activity-dependent forms of short-term plasticity (STP). By affecting differently excitatory and inhibitory synaptic transmissions, these STP properties allow a powerful gain control in DH neuronal networks that may be critical for the integration of nociceptive messages before they are forwarded to the brain, where they may be ultimately interpreted as pain. Moreover, these STPs can be finely modulated by endogenous signaling molecules, such as neurosteroids, adenosine, or GABA. The STP properties of DH inhibitory synapses might also, at least in part, participate in the pain-relieving effect of nonpharmacological analgesic procedures, such as transcutaneous electrical nerve stimulation, electroacupuncture, or spinal cord stimulation. The properties of target-specific STP at inhibitory DH synapses and their possible contribution to electrical stimulation-induced reduction of hyperalgesic and allodynic states in chronic pain will be reviewed and discussed.

Adenosine Receptors as Potential Therapeutic Analgesic Targets

Pain represents an international burden and a major socio-economic public health problem. New findings, detailed in this review, suggest that adenosine plays a significant role in neuropathic and inflammatory pain, by acting on its metabotropic adenosine receptors (A1AR, A2AAR, A2BAR, A3AR). Adenosine receptor ligands have a practical translational potential based on the favorable efficacy and safety profiles that emerged from clinical research on various agonists and antagonists for different pathologies. The present review collects the latest studies on selected adenosine receptor ligands in different pain models. Here, we also covered the many hypothesized pathways and the role of newly synthesized allosteric adenosine receptor modulators. This review aims to present a summary of recent research on adenosine receptors as prospective therapeutic targets for a range of pain-related disorders.

Adjusting the brakes to adjust neuronal activity: Adenosinergic modulation of GABAergic transmission

About 50 years elapsed from the publication of the first full paper on the neuromodulatory action of adenosine at a 'simple' synapse model, the neuromuscular junction (Ginsborg and Hirst, 1972). In that study adenosine was used as a tool to increase cyclic AMP and for the great surprise, it decreased rather than increased neurotransmitter release, and for a further surprise, its action was prevented by theophylline, at the time only known as inhibitor of phosphodiesterases. These intriguing observations opened the curiosity for immediate studies relating the action of adenine nucleotides, known to be released together with neurotransmitters, to that of adenosine (Ribeiro and Walker, 1973, 1975). Our understanding on the ways adenosine uses to modulate synapses, circuits, and brain activity, vastly expanded since then. However, except for A_2A receptors, whose actions upon GABAergic neurons of the striatum are well known, most of the attention given to the neuromodulatory action of adenosine has been focusing upon excitatory synapses. Evidence is growing that GABAergic transmission is also a target for adenosinergic neuromodulation through A₁ and A_2A receptors. Some o these actions have specific time windows during brain development, and others are selective for specific GABAergic neurons. Both tonic and phasic GABAergic transmission can be affected, and either neurons or astrocytes can be targeted. In some cases, those effects result from a concerted action with other neuromodulators. Implications of these actions in the control of neuronal function/dysfunction will be the focus of this review.

Inhibitory interneurons with differential plasticities at their connections tune excitatory/inhibitory balance in the spinal nociceptive system

ABSTRACT

Networks of the dorsal-horn of the spinal-cord process nociceptive information from the periphery. In these networks, the excitation/inhibition balance is critical to shape this nociceptive information and to gate it to the brain where it is interpreted as pain. Our aim was to define whether short-term plasticity of inhibitory connections could tune this inhibition/excitation balance by differentially controlling excitatory and inhibitory microcircuits. To this end, we used spinal-cord slices from adult mice expressing enhanced green fluorescent protein (eGFP) under the GAD65 promoter and recorded from both eGFP+ (putative inhibitory) and eGFP- (putative excitatory) neurons of lamina II while stimulating single presynaptic GABAergic interneurons at various frequencies. Our results indicate that GABAergic neurons of lamina II simultaneously contact eGFP- and eGFP+ neurons, but these connections display very different frequency-dependent short-term plasticities. Connections onto eGFP- interneurons displayed limited frequency-dependent changes, and strong time-dependent summation of inhibitory synaptic currents that was however subjected to a tonic activity-dependent inhibition involving A1 adenosine receptors. In contrast, GABAergic connections onto eGFP+ interneurons expressed pronounced frequency-dependent depression, thus favoring disinhibition at these synapses by a mechanism involving the activation of GABAB autoreceptors at low frequency. Interestingly, the balance favors inhibition at frequencies associated with intense pain whether it favors excitation at frequencies associated with low pain. Therefore, these target- and frequency-specific plasticities allow to tune the balance between inhibition and disinhibition while processing frequency-coded information from primary afferents. These short-term plasticities and their modulation by A1 and GABAB receptors might represent an interesting target in pain-alleviating strategies.

Adenosine Receptors in Modulation of Central Nervous System Disorders

The ubiquitous signaling nucleoside molecule, adenosine is found in different cells of the human body to provide its numerous pharmacological role. The associated actions of endogenous adenosine are largely dependent on conformational change of the widely expressed heterodimeric G-protein-coupled A1, A2A, A2B, and A3 adenosine receptors (ARs). These receptors are well conserved on the surface of specific cells, where potent neuromodulatory properties of this bioactive molecule reflected by its easy passage through the rigid blood-brainbarrier, to simultaneously act on the central nervous system (CNS). The minimal concentration of adenosine in body fluids (30-300 nM) is adequate to exert its neuromodulatory action in the CNS, whereas the modulatory effect of adenosine on ARs is the consequence of several neurodegenerative diseases. Modulatory action concerning the activation of such receptors in the CNS could be facilitated towards neuroprotective action against such CNS disorders. Our aim herein is to discuss briefly pathophysiological roles of adenosine on ARs in the modulation of different CNS disorders, which could be focused towards the identification of potential drug targets in recovering accompanying CNS disorders. Researches with active components with AR modulatory action have been extended and already reached to the bedside of the patients through clinical research in the improvement of CNS disorders. Therefore, this review consist of recent findings in literatures concerning the impact of ARs on diverse CNS disease pathways with the possible relevance to neurodegeneration.

GABAB receptors-mediated tonic inhibition of glutamate release from Aβ fibers in rat laminae III/IV of the spinal cord dorsal horn

Presynaptic GABA_B receptors (GABA_BRs) are highly expressed in dorsal root ganglion neurons and spinal cord dorsal horn. GABA_BRs located in superficial dorsal horn play an important antinociceptive role, by acting at both pre- and postsynaptic sites. GABA_BRs expressed in deep dorsal horn could be involved in the processing of touch sensation and possibly in the generation of tactile allodynia in chronic pain. The objective of this study was to characterize the morphological and functional properties of GABA_BRs expressed on Aβ fibers projecting to lamina III/IV and to understand their role in modulating excitatory synaptic transmission. We performed high-resolution electron microscopic analysis, showing that GABA_B2 subunit is expressed on 71.9% of terminals in rat lamina III-IV. These terminals were engaged in axodendritic synapses and, for the 46%, also expressed glutamate immunoreactivity. Monosynaptic excitatory postsynaptic currents, evoked by Aβ fiber stimulation and recorded from lamina III/IV neurons in spinal cord slices, were strongly depressed by application of baclofen (0.1-2.5 µM), acting as a presynaptic modulator. Application of the GABA_BR antagonist CGP 55845 caused, in a subpopulation of neurons, the potentiation of the first of two excitatory postsynaptic currents recorded with the paired-pulse protocol, showing that GABA_BRs are endogenously activated. A decrease in the paired-pulse ratio accompanied the effect of CGP 55845, implying the involvement of presynaptic GABA_BRs. CGP 55845 facilitated only the first excitatory postsynaptic current also during a train of four consecutive stimuli applied to Aβ fibers. These results suggest that GABA_BRs tonically inhibit glutamate release from Aβ fibers at a subset of synapses in deep dorsal horn. This modulation specifically affects only the early phase of synaptic excitation in lamina III-IV neurons.

Neuronal networks and nociceptive processing in the dorsal horn of the spinal cord

The dorsal horn (DH) of the spinal cord receives a variety of sensory information arising from the inner and outer environment, as well as modulatory inputs from supraspinal centers. This information is integrated by the DH before being forwarded to brain areas where it may lead to pain perception. Spinal integration of this information relies on the interplay between different DH neurons forming complex and plastic neuronal networks. Elements of these networks are therefore potential targets for new analgesics and pain-relieving strategies. The present review aims at providing an overview of the current knowledge on these networks, with a special emphasis on those involving interlaminar communication in both physiological and pathological conditions.

Blocking the GABA transporter GAT-1 ameliorates spinal GABAergic disinhibition and neuropathic pain induced by paclitaxel

Paclitaxel is a chemotherapeutic agent widely used for treating carcinomas. Patients receiving paclitaxel often develop neuropathic pain and have a reduced quality of life which hinders the use of this life-saving drug. In this study, we determined the role of GABA transporters in the genesis of paclitaxel-induced neuropathic pain using behavioral tests, electrophysiology, and biochemical techniques. We found that tonic GABA receptor activities in the spinal dorsal horn were reduced in rats with neuropathic pain induced by paclitaxel. In normal controls, tonic GABA receptor activities were mainly controlled by the GABA transporter GAT-1 but not GAT-3. In the spinal dorsal horn, GAT-1 was expressed at presynaptic terminals and astrocytes while GAT-3 was only expressed in astrocytes. In rats with paclitaxel-induced neuropathic pain, the protein expression of GAT-1 was increased while GAT-3 was decreased. This was concurrently associated with an increase in global GABA uptake. The paclitaxel-induced attenuation of GABAergic tonic inhibition was ameliorated by blocking GAT-1 but not GAT-3 transporters. Paclitaxel-induced neuropathic pain was significantly attenuated by the intrathecal injection of a GAT-1 inhibitor. These findings suggest that targeting GAT-1 transporters for reversing disinhibition in the spinal dorsal horn may be a useful approach for treating paclitaxel-induced neuropathic pain. Patients receiving paclitaxel for cancer therapy often develop neuropathic pain and have a reduced quality of life. In this study, we demonstrated that animals treated with paclitaxel develop neuropathic pain, have enhancements of GABA transporter-1 protein expression and global GABA uptake, as well as suppression of GABAergic tonic inhibition in the spinal dorsal horn. Pharmacological inhibition of GABA transporter-1 ameliorates the paclitaxel-induced suppression of GABAergic tonic inhibition and neuropathic pain. Thus, targeting GAT-1 transporters for reversing GABAergic disinhibition in the spinal dorsal horn could be a useful approach for treating paclitaxel-induced neuropathic pain.

Activation of toll like receptor 4 attenuates GABA synthesis and postsynaptic GABA receptor activities in the spinal dorsal horn via releasing interleukin-1 beta

Toll like receptor 4 (TLR4) is an innate immune pattern recognition receptor, expressed predominantly on microglia in the CNS. Activation of spinal TLR4 plays a critical role in the genesis of pathological pain induced by nerve injury, bone cancer, and tissue inflammation. Currently, it remains unknown how synaptic activities in the spinal dorsal horn are regulated by TLR4 receptors. Through recording GABAergic currents in neurons and glial glutamate transporter currents in astrocytes in rodent spinal slices, we determined whether and how TLR4 modulates GABAergic synaptic activities in the superficial spinal dorsal horn. We found that activation of TLR4 by lipopolysaccharide (LPS) reduces GABAergic synaptic activities through both presynaptic and postsynaptic mechanisms. Specifically, LPS causes the release of IL-1β from microglia. IL-1β in turn suppresses GABA receptor activities at the postsynaptic site through activating protein kinase C (PKC) in neurons. GABA synthesis at the presynaptic site is reduced upon activation of TLR4. Glial glutamate transporter activities are suppressed by IL-1β and PKC activation induced by LPS. The suppression of glial glutamate transporter activities leads to a deficiency of glutamine supply, which results in an attenuation of the glutamate-glutamine cycle-dependent GABA synthesis. These findings shed light on understanding synaptic plasticity induced by activation of TLR4 under neuroinflammation and identify GABA receptors, glial glutamate transporters, IL-1β and PKC as therapeutic targets to abrogate abnormal neuronal activities following activation of TLR4 in pathological pain conditions.

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.

Developmental downregulation of excitatory GABAergic transmission in neocortical layer I via presynaptic adenosine A(1) receptors

Layer I of the developing cortex contains a dense GABAergic fiber plexus. These fibers provide excitatory inputs to Cajal-Retzius (CR) cells, the early born neurons in layer I. CR cells possess an extensive axonal projection and form synaptic contacts with excitatory, presumably pyramidal, neurons before birth. Interestingly, activity of CR cells declines during the first postnatal week, but mechanism(s) underlying this phenomenon is not yet known. Here we recorded inhibitory postsynaptic currents (IPSCs) in CR cells at postnatal day (P) 1-2 and P5-7. Blockade of adenosine A(1) receptors (A(1)Rs) increased the amplitude of evoked IPSCs (eIPSCs) and decreased paired-pulse ratio at P5-7 but not at P1-2. A(1)R activation decreased the mean eIPSC amplitude at P5-7, but failed to affect eIPSCs at P1-2. Ecto-adenosine triphosphatase (ATPase) inhibition completely abolished the A(1)R-mediated effects suggesting that extracellular ATP is the main source of adenosine. Because A(1)R blockade did not affect the median miniature IPSC amplitude, our results demonstrate that adenosine reduces gamma-aminiobutyric acid (GABA) release probability via presynaptic A(1)Rs at P5-7. As neuronal activity in layer I can depolarize pyramidal neurons influencing thereby glutamatergic synaptogenesis in the lower cortical layers, postnatal weakening of GABAergic transmission by adenosinergic system might reflect a developmental downregulation of this excitatory drive when glutamatergic synapses are formed.

Chronic activation of spinal adenosine A1 receptors results in hypersensitivity

Spinally administered adenosine reduces hypersensitivity in animals and humans with nerve injury, but also causes transient pain in humans and reduces tonic inhibition in spinal neurons. Nerve injury results in increased tonic spinal cord adenosine A1 receptor activation, consistent with a role for adenosine to generate hypersensitivity. Here, we demonstrate that chronic intrathecal adenosine induces hypersensitivity in normal animals and that chronic blockade of spinal adenosine A1 receptors by the A1 antagonist 8-cyclopentyl-1,3-dipropylxanthine partially prevents nerve injury-induced hypersensitivity. In contrast, chronic blockade of spinal adenosine A1 receptors failed to reduce increased tonic G-protein signaling in the spinal cord after nerve injury. These data support a role for chronic adenosine A1 receptor stimulation after nerve injury to result in hypersensitivity.

Adenosine and ATP receptors

Adenosine and ATP, via P1 and P2 receptors respectively, can modulate pain transmission under physiological, inflammatory, and neuropathic pain conditions. Such influences reflect peripheral and central actions and effects on neurons as well as other cell types. In general, adenosine A1 receptors produce inhibitory effects on pain in a number of preclinical models and are a focus of attention. In humans, i.v. infusions of adenosine reduce some aspects of neuropathic pain and can reduce postoperative pain. For P2X receptors, there is a significant body of information indicating that inhibition of P2X3 receptors may be useful for relieving inflammatory and neuropathic pain. More recently, data have begun to emerge implicating P2X4, P2X7 and P2Y receptors in aspects of pain transmission. Both P1 and P2 receptors may represent novel targets for pain relief.

Negative cross-talk between presynaptic adenosine and acetylcholine receptors

Functional interactions between presynaptic adenosine and acetylcholine (ACh) autoreceptors were studied at the frog neuromuscular junction by recording miniature end-plate potentials (MEPPs) during bath or local application of agonists. The frequency of MEPPs was reduced by adenosine acting on presynaptic adenosine A1 receptors (EC(50) = 1.1 microm) or by carbachol acting on muscarinic M2 receptors (EC(50) = 1.8 microm). However, carbachol did not produce the depressant effect when it was applied after the action of adenosine had reached its maximum. This phenomenon implied that the negative cross-talk (occlusion) had occurred between A1 and M2 receptors. Moreover, the occlusion was receptor-specific as ATP applied in the presence of adenosine continued to depress MEPP frequency. Muscarinic antagonists [atropine or 1-[[2-[(diethylamino)methyl)-1-piperidinyl]acetyl]-5,11-dihydro-6H-pyrido [2,3-b][1,4]benzodiazepine-6-one) (AFDX-116)] had no effect on the inhibitory action of adenosine and adenosine antagonists [8-(p-sulfophenyl)theophylline (8-SPT) or 1,3-dipropyl-8-cyclopentylxanthine (DPCPX)] had no effect on the action of carbachol. These data suggested that membrane-delimited interactions did not occur between A1 and M2 receptors. Both carbachol and adenosine similarly inhibited quantal release triggered by high potassium, ionomycin or sucrose. These results indicated a convergence of intracellular pathways activated by M2 and A1 receptors to a common presynaptic effector located downstream of Ca(2+) influx. We propose that the negative cross-talk between two major autoreceptors could take place during intense synaptic activity and thereby attenuate the presynaptic inhibitory effects of ACh and adenosine.

Synaptic modulation in pain pathways

All higher organisms possess a sensory system that allows them to detect potentially tissue-damaging (or noxious) stimuli. The proper functioning of this system is essential to protect their bodies from tissue damage. However, under pathological conditions after severe tissue injury and in inflammatory or neuropathic diseases, this system can become sensitized, and pain can then turn into a disease. Such exaggerated pain sensation (or hyperalgesia) can arise at different levels of integration. It can originate from an increased responsiveness of primary nociceptors, specialized nerve cells, which sense noxious stimuli, or from changes in the central processing of nociceptive input. Like other sensory input, nociceptive signals are relayed in the central nervous system by neurons, which communicate with each other mainly through chemical synapses. Changes in the excitability of these neurons or in the strength of their synaptic coupling provide the cellular basis for many forms of pathological pain. This review focuses on the synaptic processing of pain-related signals in the spinal cord dorsal horn, the first site of synaptic integration in the pain pathway. Particular emphasis is paid to synaptic processes underlying the generation of pathological pain evoked by inflammation or neuropathic diseases.

Plasticity of purine release during cerebral ischemia: clinical implications?

Adenosine is a powerful modulator of neuronal function in the mammalian central nervous system. During a variety of insults to the brain, adenosine is released in large quantities and exerts a neuroprotective influence largely via the A(1) receptor, which inhibits glutamate release and neuronal activity. Using novel enzyme-based adenosine sensors, which allow high spatial and temporal resolution recordings of adenosine release in real time, we have investigated the release of adenosine during hypoxia/ischemia in the in vitro hippocampus. Our data reveal that during the early stages of hypoxia adenosine is likely released per se and not as a precursor such as cAMP or an adenine nucleotide. In addition, repeated hypoxia results in reduced production of extracellular adenosine and this may underlie the increased vulnerability of the mammalian brain to repetitive or secondary hypoxia/ischemia.