Localization and function of ATP and GABAA receptors expressed by nociceptors and other postnatal sensory neurons in rat

Presynaptic gating of monkey proprioceptive signals for proper motor action

Our rich behavioural repertoire is supported by complicated synaptic connectivity in the central nervous system, which must be modulated to prevent behavioural control from being overwhelmed. For this modulation, presynaptic inhibition is an efficient mechanism because it can gate specific synaptic input without interfering with main circuit operations. Previously, we reported the task-dependent presynaptic inhibition of the cutaneous afferent input to the spinal cord in behaving monkeys. Here, we report presynaptic inhibition of the proprioceptive afferent input. We found that the input from shortened muscles is transiently facilitated, whereas that from lengthened muscles is persistently reduced. This presynaptic inhibition could be generated by cortical signals because it started before movement onset, and its size was correlated with the performance of stable motor output. Our findings demonstrate that presynaptic inhibition acts as a dynamic filter of proprioceptive signals, enabling the integration of task-relevant signals into spinal circuits.

Gate control of sensory neurotransmission in peripheral ganglia by proprioceptive sensory neurons

Melzak and Wall's gate control theory proposed that innocuous input into the dorsal horn of the spinal cord represses pain-inducing nociceptive input. Here we show that input from proprioceptive parvalbumin-expressing sensory neurons tonically represses nociceptor activation within dorsal root ganglia. Deletion of parvalbumin-positive sensory neurons leads to enhanced nociceptor activity measured with GCaMP3, increased input into wide dynamic range neurons of the spinal cord and increased acute and spontaneous pain behaviour, as well as potentiated innocuous sensation. Parvalbumin-positive sensory neurons express the enzymes and transporters necessary to produce vesicular GABA that is known to be released from depolarized somata. These observations support the view that gate control mechanisms occur peripherally within dorsal root ganglia.

Disrupted Association of Sensory Neurons With Enveloping Satellite Glial Cells in Fragile X Mouse Model

Among most prevalent deficits in individuals with Fragile X syndrome (FXS) is hypersensitivity to sensory stimuli and somatosensory alterations. Whether dysfunction in peripheral sensory system contributes to these deficits remains poorly understood. Satellite glial cells (SGCs), which envelop sensory neuron soma, play critical roles in regulating neuronal function and excitability. The potential contributions of SGCs to sensory deficits in FXS remain unexplored. Here we found major structural defects in sensory neuron-SGC association in the dorsal root ganglia (DRG), manifested by aberrant covering of the neuron and gaps between SGCs and the neuron along their contact surface. Single-cell RNAseq analyses demonstrated transcriptional changes in both neurons and SGCs, indicative of defects in neuronal maturation and altered SGC vesicular secretion. We validated these changes using fluorescence microscopy, qPCR, and high-resolution transmission electron microscopy (TEM) in combination with computational analyses using deep learning networks. These results revealed a disrupted neuron-glia association at the structural and functional levels. Given the well-established role for SGCs in regulating sensory neuron function, altered neuron-glia association may contribute to sensory deficits in FXS.

Nicotinic receptor modulation of primary afferent excitability with selective regulation of Aδ-mediated spinal actions

Somatosensory input strength can be modulated by primary afferent depolarization (PAD) generated predominantly via presynaptic GABA_A receptors on afferent terminals. We investigated whether ionotropic nicotinic acetylcholine receptors (nAChRs) also provide modulatory actions, focusing on myelinated afferent excitability in in vitro murine spinal cord nerve-attached models. Primary afferent stimulation-evoked synaptic transmission was recorded in the deep dorsal horn as extracellular field potentials (EFPs), whereas concurrently recorded dorsal root potentials (DRPs) were used as an indirect measure of PAD. Changes in afferent membrane excitability were simultaneously measured as direct current (DC)-shifts in membrane polarization recorded in dorsal roots or peripheral nerves. The broad nAChR antagonist d-tubocurarine (d-TC) selectively and strongly depressed Aδ-evoked synaptic EFPs (36% of control) coincident with similarly depressed A-fiber DRP (43% of control), whereas afferent electrical excitability remained unchanged. In comparison, acetylcholine (ACh) and the nAChR agonists, epibatidine and nicotine, reduced afferent excitability by generating coincident depolarizing DC-shifts in peripheral axons and intraspinally. Progressive depolarization corresponded temporally with the emergence of spontaneous axonal spiking and reductions in the DRP and all afferent-evoked synaptic actions (31%-37% of control). Loss of evoked response was long-lasting, independent of DC repolarization, and likely due to mechanisms initiated by spontaneous C-fiber activity. DC-shifts were blocked with d-TC but not GABA_A receptor blockers and retained after tetrodotoxin block of voltage-gated Na⁺ channels. Notably, actions tested were comparable between three mouse strains, in rat, and when performed in different labs. Thus, nAChRs can regulate afferent excitability via two distinct mechanisms: by central Aδ-afferent actions, and by transient extrasynaptic axonal activation of high-threshold primary afferents.NEW & NOTEWORTHY Primary afferents express many nicotinic ACh receptor (nAChR) subtypes but whether activation is linked to presynaptic inhibition, facilitation, or more complex and selective activity modulation is unknown. Recordings of afferent-evoked responses in the lumbar spinal cord identified two nAChR-mediated modulatory actions: 1) selective control of Aδ afferent transmission and 2) robust changes in axonal excitability initiated via extrasynaptic shifts in DC polarization. This work broadens the diversity of presynaptic modulation of primary afferents by nAChRs.

Selected Ionotropic Receptors and Voltage-Gated Ion Channels: More Functional Competence for Human Induced Pluripotent Stem Cell (iPSC)-Derived Nociceptors

Preclinical research using different rodent model systems has largely contributed to the scientific progress in the pain field, however, it suffers from interspecies differences, limited access to human models, and ethical concerns. Human induced pluripotent stem cells (iPSCs) offer major advantages over animal models, i.e., they retain the genome of the donor (patient), and thus allow donor-specific and cell-type specific research. Consequently, human iPSC-derived nociceptors (iDNs) offer intriguingly new possibilities for patient-specific, animal-free research. In the present study, we characterized iDNs based on the expression of well described nociceptive markers and ion channels, and we conducted a side-by-side comparison of iDNs with mouse sensory neurons. Specifically, immunofluorescence (IF) analyses with selected markers including early somatosensory transcription factors (BRN3A/ISL1/RUNX1), the low-affinity nerve growth factor receptor (p75), hyperpolarization-activated cyclic nucleotide-gated channels (HCN), as well as high voltage-gated calcium channels (VGCC) of the Ca_V2 type, calcium permeable TRPV1 channels, and ionotropic GABA_A receptors, were used to address the characteristics of the iDN phenotype. We further combined IF analyses with microfluorimetric Ca²⁺ measurements to address the functionality of these ion channels in iDNs. Thus, we provide a detailed morphological and functional characterization of iDNs, thereby, underpinning their enormous potential as an animal-free alternative for human specific research in the pain field for unveiling pathophysiological mechanisms and for unbiased, disease-specific personalized drug development.

Chloride - The Underrated Ion in Nociceptors

In contrast to pain processing neurons in the spinal cord, where the importance of chloride conductances is already well established, chloride homeostasis in primary afferent neurons has received less attention. Sensory neurons maintain high intracellular chloride concentrations through balanced activity of Na⁺-K⁺-2Cl^– cotransporter 1 (NKCC1) and K⁺-Cl^– cotransporter 2 (KCC2). Whereas in other cell types activation of chloride conductances causes hyperpolarization, activation of the same conductances in primary afferent neurons may lead to inhibitory or excitatory depolarization depending on the actual chloride reversal potential and the total amount of chloride efflux during channel or transporter activation. Dorsal root ganglion (DRG) neurons express a multitude of chloride channel types belonging to different channel families, such as ligand-gated, ionotropic γ-aminobutyric acid (GABA) or glycine receptors, Ca²⁺-activated chloride channels of the anoctamin/TMEM16, bestrophin or tweety-homolog family, CLC chloride channels and transporters, cystic fibrosis transmembrane conductance regulator (CFTR) as well as volume-regulated anion channels (VRACs). Specific chloride conductances are involved in signal transduction and amplification at the peripheral nerve terminal, contribute to excitability and action potential generation of sensory neurons, or crucially shape synaptic transmission in the spinal dorsal horn. In addition, chloride channels can be modified by a plethora of inflammatory mediators affecting them directly, via protein-protein interaction, or through signaling cascades. Since chloride channels as well as mediators that modulate chloride fluxes are regulated in pain disorders and contribute to nociceptor excitation and sensitization it is timely and important to emphasize their critical role in nociceptive primary afferents in this review.

A new approach to developing diagnostics and therapeutics: Aggregation-induced emission-based fluorescence turn-on

Fluorescence imaging is a promising visualization tool and possesses the advantages of in situ response and facile operation; thus, it is widely exploited for bioassays. However, traditional fluorophores suffer from concentration limits because they are always quenched when they aggregate, which impedes applications, especially for trace analysis and real-time monitoring. Recently, novel molecules with aggregation-induced emission (AIE) characteristics were developed to solve the problems encountered when using traditional organic dyes, because these new molecules exhibit weak or even no fluorescence when they are in free movement states but emit intensely upon the restriction of intramolecular motions. Inspired by the excellent performances of AIE molecules, a substantial number of AIE-based probes have been designed, synthesized, and applied to various fields to fulfill diverse detection tasks. According to numerous experiments, AIE probes are more practical than traditional fluorescent probes, especially when used in bioassays. To bridge bioimaging and materials engineering, this review provides a comprehensive understanding of the development of AIE bioprobes. It begins with a summary of mechanisms of the AIE phenomenon. Then, the strategies to realize accurate detection using AIE probes are discussed. In addition, typical examples of AIE-active materials applied in diagnosis, treatment, and nanocarrier tracking are presented. In addition, some challenges are put forward to inspire more ideas in the promising field of AIE-active materials.

Contribution of GABAergic modulation in DRGs to electroacupuncture analgesia in incisional neck pain rats

Purpose

Acupuncture therapy is effective for relieving postoperative pain. Our previous study showed that electroacupuncture (EA) at Futu (LI18) and Hegu (LI4)–Neiguan (PC6) could alleviate incisional neck pain, which was related with its effect in upregulating γ-aminobutyric acid (GABA) expression in cervical (C3–6) dorsal root ganglions (DRGs); but whether its receptor subsets GABA_Aα2R and GABA_BR1 in C3–6 DRGs are involved in EA analgesia or not, it remains unknown.

Materials and methods

Seventy-five male Sprague Dawley rats were randomized to normal control, model, LI18, LI4–PC6, and Zusanli (ST36)–Yanglingquan (GB34) groups. The incisional neck pain model was established by making a longitudinal incision along the midline of the rats’ neck, followed by repeated mechanical stimulation. EA was applied to bilateral LI18, LI4–PC6, or ST36–GB34 for 30 minutes at 4, 24, and 48 hours after operation. The thermal pain threshold of the neck was detected by a tail-flick unit, and the C3–6 DRGs were removed for assaying the immunoactivity of substance P (SP), GABA_Aα2R, glial fibrillary acidic protein (GFAP; a marker of satellite glial cells [SGCs]), and GABA_BR1 and the expression of GABA_Aα2R and GABA_BR1 mRNA and proteins using immunofluorescence, real-time PCR, and Western blotting, respectively.

Results

The cervical thermal pain threshold was significantly lower in the model group than the normal group (P<0.001), indicating hyperalgesia after neck incision, and was considerably increased in both EA-LI18 and LI4–PC6 groups (P<0.001), but not in ST36–GB34 group compared with model group (P>0.05). Immunofluorescence staining showed that GABA_Aα2 R expressed on SP⁺ neurons, and GABA_BR1 on SGCs. EA of LI18 and LI4–PC6 markedly suppressed the modeling-induced upregulation of the immunoactivity of SP (P<0.001 and P<0.01, respectively) and GFAP (P<0.01 and P<0.001, respectively) and significantly reversed neck incision–induced downregulation of the expression of GABA_Aα2R and GABA_BR1 mRNAs and proteins (P<0.05).

Conclusion

EA of LI18 and LI4–PC6 has an analgesic effect in incisional neck pain rats, which is related to its effects in upregulating GABAergic inhibitory modulation on nociceptive peptidergic neurons and SGCs in cervical DRGs.

Depolarizing Effectors of Bradykinin Signaling in Nociceptor Excitation in Pain Perception

Inflammation is one of the main causes of pathologic pain. Knowledge of the molecular links between inflammatory signals and pain-mediating neuronal signals is essential for understanding the mechanisms behind pain exacerbation. Some inflammatory mediators directly modulate the excitability of pain-mediating neurons by contacting the receptor molecules expressed in those neurons. For decades, many discoveries have accumulated regarding intraneuronal signals from receptor activation through electrical depolarization for bradykinin, a major inflammatory mediator that is able to both excite and sensitize pain-mediating nociceptor neurons. Here, we focus on the final effectors of depolarization, the neuronal ion channels, whose functionalities are specifically affected by bradykinin stimulation. Particular G-protein coupled signaling cascades specialized for each specific depolarizer ion channels are summarized. Some of these ion channels not only serve as downstream effectors but also play critical roles in relaying specific pain modalities such as thermal or mechanical pain. Accordingly, specific pain phenotypes altered by bradykinin stimulation are also discussed. Some members of the effector ion channels are both activated and sensitized by bradykinin-induced neuronal signaling, while others only sensitized or inhibited, which are also introduced. The present overview of the effect of bradykinin on nociceptor neuronal excitability at the molecular level may contribute to better understanding of an important aspect of inflammatory pain and help future design of further research on the components involved and pain modulating strategies.

Acute inflammation reveals GABAA receptor-mediated nociception in mouse dorsal root ganglion neurons via PGE2 receptor 4 signaling

Gamma‐aminobutyric acid (GABA) depolarizes dorsal root ganglia (DRG) primary afferent neurons through activation of Cl⁻ permeable GABA_A receptors but the physiologic role of GABA_A receptors in the peripheral terminals of DRG neurons remains unclear. In this study, we investigated the role of peripheral GABA_A receptors in nociception using a mouse model of acute inflammation. In vivo, peripheral administration of the selective GABA_A receptor agonist muscimol evoked spontaneous licking behavior, as well as spinal wide dynamic range (WDR) neuron firing, after pre‐conditioning with formalin but had no effect in saline‐treated mice. GABA_A receptor‐mediated pain behavior after acute formalin treatment was abolished by the GABA_A receptor blocker picrotoxin and cyclooxygenase inhibitor indomethacin. In addition, treatment with prostaglandin E2 (PGE₂) was sufficient to reveal muscimol‐induced licking behavior. In vitro, GABA induced sub‐threshold depolarization in DRG neurons through GABA_A receptor activation. Both formalin and PGE₂ potentiated GABA‐induced Ca²⁺ transients and membrane depolarization in capsaicin‐sensitive nociceptive DRG neurons; these effects were blocked by the prostaglandin E2 receptor 4 (EP4) antagonist AH23848 (10 μmol/L). Furthermore, potentiation of GABA responses by PGE₂ was prevented by the selective Na_v1.8 antagonist A887826 (100 nmol/L). Although the function of the Na⁺‐K⁺‐2Cl^‐ co‐transporter NKCC1 was required to maintain the Cl^‐ ion gradient in isolated DRG neurons, NKCC1 was not required for GABA_A receptor‐mediated nociceptive behavior after acute inflammation. Taken together, these results demonstrate that GABA_A receptors may contribute to the excitation of peripheral sensory neurons in inflammation through a combined effect involving PGE₂‐EP4 signaling and Na⁺ channel sensitization.

Basic/Translational Development of Forthcoming Opioid- and Nonopioid-Targeted Pain Therapeutics

Opioids represent an efficacious therapeutic modality for some, but not all pain states. Singular reliance on opioid therapy for pain management has limitations, and abuse potential has deleterious consequences for patient and society. Our understanding of pain biology has yielded insights and opportunities for alternatives to conventional opioid agonists. The aim is to have efficacious therapies, with acceptable side effect profiles and minimal abuse potential, which is to say an absence of reinforcing activity in the absence of a pain state. The present work provides a nonexclusive overview of current drug targets and potential future directions of research and development. We discuss channel activators and blockers, including sodium channel blockers, potassium channel activators, and calcium channel blockers; glutamate receptor-targeted agents, including N-methyl-D-aspartate, α-amino-3-hydroxy-5-methyl-4-isoxazole propionic acid, and metabotropic receptors. Furthermore, we discuss therapeutics targeted at γ-aminobutyric acid, α2-adrenergic, and opioid receptors. We also considered antagonists of angiotensin 2 and Toll receptors and agonists/antagonists of adenosine, purine receptors, and cannabinoids. Novel targets considered are those focusing on lipid mediators and anti-inflammatory cytokines. Of interest is development of novel targeting strategies, which produce long-term alterations in pain signaling, including viral transfection and toxins. We consider issues in the development of druggable molecules, including preclinical screening. While there are examples of successful translation, mechanistically promising preclinical candidates may unexpectedly fail during clinical trials because the preclinical models may not recapitulate the particular human pain condition being addressed. Molecular target characterization can diminish the disconnect between preclinical and humans' targets, which should assist in developing nonaddictive analgesics.

Current and Future Issues in the Development of Spinal Agents for the Management of Pain

Targeting analgesic drugs for spinal delivery reflects the fact that while the conscious experience of pain is mediated supraspinally, input initiated by high intensity stimuli, tissue injury and/or nerve injury is encoded at the level of the spinal dorsal horn and this output informs the brain as to the peripheral environment. This encoding process is subject to strong upregulation resulting in hyperesthetic states and downregulation reducing the ongoing processing of nociceptive stimuli reversing the hyperesthesia and pain processing. The present review addresses the biology of spinal nociceptive processing as relevant to the effects of intrathecally-delivered drugs in altering pain processing following acute stimulation, tissue inflammation/injury and nerve injury. The review covers i) the major classes of spinal agents currently employed as intrathecal analgesics (opioid agonists, alpha 2 agonists; sodium channel blockers; calcium channel blockers; NMDA blockers; GABA A/B agonists; COX inhibitors; ii) ongoing developments in the pharmacology of spinal therapeutics focusing on less studied agents/targets (cholinesterase inhibition; Adenosine agonists; iii) novel intrathecal targeting methodologies including gene-based approaches (viral vectors, plasmids, interfering RNAs); antisense, and toxins (botulinum toxins; resniferatoxin, substance P Saporin); and iv) issues relevant to intrathecal drug delivery (neuraxial drug distribution), infusate delivery profile, drug dosing, formulation and principals involved in the preclinical evaluation of intrathecal drug safety.

Expression of glycine receptor alpha 3 in the rat trigeminal neurons and central boutons in the brainstem

Increasing evidence shows that the homomeric glycine receptor is expressed in axon terminals and is involved in the presynaptic modulation of transmitter release. However, little is known about the expression of the glycine receptor, implicated in the presynaptic modulation of sensory transmission in the primary somatosensory neurons and their central boutons. To address this, we investigated the expression of glycine receptor subunit alpha 3 (GlyRα3) in the neurons in the trigeminal ganglion and axon terminals in the 1st relay nucleus of the brainstem by light- and electron-microscopic immunohistochemistry. Trigeminal primary sensory neurons were GlyRα3-immunopositive/gephyrin-immunonegative (indicating homomeric GlyR), whereas GlyRα3/gephyrin immunoreactivity (indicating heteromeric GlyR) was observed in dendrites. GlyRα3 immunoreactivity was also found in the central boutons of primary afferents but far from the presynaptic site and in dendrites at subsynaptic sites. Boutons expressing GlyRα3 contained small round vesicles, formed asymmetric synapses with dendrites and were immunoreactive for glutamate. These findings suggest that trigeminal primary afferent boutons receive presynaptic modulation via homomeric, extrasynaptic GlyRα3, and that different subtypes of GlyR may be involved in pre- and postsynaptic inhibition.

Transient, activity dependent inhibition of transmitter release from low threshold afferents mediated by GABAA receptors in spinal cord lamina III/IV

Background

Presynaptic GABA_A receptors (GABA_ARs) located on central terminals of low threshold afferent fibers are thought to be involved in the processing of touch and possibly in the generation of tactile allodynia in chronic pain. These GABA_ARs mediate primary afferent depolarization (PAD) and modulate transmitter release. The objective of this study was to expand our understanding of the presynaptic inhibitory action of GABA released onto primary afferent central terminals following afferent stimulation.

Results

We recorded evoked postsynaptic excitatory responses (eEPSCs and eEPSPs) from lamina III/IV neurons in spinal cord slices from juvenile rats (P17–P23, either sex), while stimulating dorsal roots. We investigated time and activity dependent changes in glutamate release from low threshold A fibers and the impact of these changes on excitatory drive. Blockade of GABA_ARs by gabazine potentiated the second eEPSC during a train of four afferent stimuli in a large subset of synapses. This resulted in a corresponding increase of action potential firing after the second stimulus. The potentiating effect of gabazine was due to inhibition of endogenously activated presynaptic GABA_ARs, because it was not prevented by the blockade of postsynaptic GABA_ARs through intracellular perfusion of CsF. Exogenous activation of presynaptic GABA_ARs by muscimol depressed evoked glutamate release at all synapses and increased paired pulse ratio (PPR).

Conclusions

These observations suggest that afferent driven release of GABA onto low threshold afferent terminals is most effective following the first action potential in a train and serves to suppress the initial strong excitatory drive onto dorsal horn circuitry.

Silencing the α2 subunit of γ-aminobutyric acid type A receptors in rat dorsal root ganglia reveals its major role in antinociception posttraumatic nerve injury

BACKGROUND

Neuropathic pain (NPP) is likely the result of repetitive high-frequency bursts of peripheral afferent activity leading to long-lasting changes in synaptic plasticity in the spinal dorsal horn. Drugs that promote γ-aminobutyric acid (GABA) activity in the dorsal horn provide partial relief of neuropathic symptoms. The authors examined how in vivo silencing of the GABA receptor type A (GABAA) α2 gene in dorsal root ganglia (DRG) controls NPP.

METHODS

After crush injury to the right sciatic nerve of female rats, the α2 GABAA antisense and mismatch oligodeoxynucleotides or NO-711 (a GABA uptake inhibitor) were applied to the L5 DRG. In vivo behavioral assessment of nociception was conducted before the injury and ensuing 10 days (n = 4 to 10). In vitro quantification of α2 GABAA protein and electrophysiological studies of GABAA currents were performed on acutely dissociated L5 DRG neurons at relevant time points (n = 6 to 14).

RESULTS

NPP postcrush injury of a sciatic nerve in adult female rats coincides with significant down-regulation of the α2 subunit expression in the ipsilateral DRG (approximately 30%). Selective down-regulation of α2 expression in DRGs significantly worsens mechanical (2.55 ± 0.75 to 5.16 ± 1.16) and thermal (7.97 ± 0.96 to 5.51 ± 0.75) hypersensitivity in crush-injured animals and causes development of significant mechanical (2.33 ± 0.40 to 5.00 ± 0.33) and thermal (10.80 ± 0.29 to 7.34 ± 0.81) hypersensitivity in sham animals (data shown as mean ± SD). Conversely, up-regulation of endogenous GABA via blockade of its uptake in DRG alleviates NPP.

CONCLUSION

The GABAA receptor in the DRG plays an important role in pathophysiology of NPP caused by sciatic nerve injury and represents promising target for novel pain therapies.

Expression of gamma-aminobutyric acid type A receptor α2 subunit in the dorsal root ganglion of rats with sciatic nerve injury

The γ-aminobutyric acid neurotransmitter in the spinal cord dorsal horn plays an important role in pain modulation through primary afferent-mediated presynaptic inhibition. The weakening of γ-aminobutyric acid-mediated presynaptic inhibition may be an important cause of neuropathic pain. γ-aminobutyric acid-mediated presynaptic inhibition is related to the current strength of γ-aminobutyric acid A receptor activation. In view of this, the whole-cell patch-clamp technique was used here to record the change in muscimol activated current of dorsal root ganglion neurons in a chronic constriction injury model. Results found that damage in rat dorsal root ganglion neurons following application of muscimol caused concentration-dependent activation of current, and compared with the sham group, its current strength and γ-aminobutyric acid A receptor protein expression decreased. Immunofluorescence revealed that γ-aminobutyric acid type A receptor α2 subunit protein expression decreased and was most obvious at 12 and 15 days after modeling. Our experimental findings confirmed that the γ-aminobutyric acid type A receptor α2 subunit in the chronic constriction injury model rat dorsal root ganglion was downregulated, which may be one of the reasons for the reduction of injury in dorsal root ganglion neurons following muscimol-activated currents.

Electroacupuncture and A-317491 depress the transmission of pain on primary afferent mediated by the P2X3 receptor in rats with chronic neuropathic pain states

P2X is a family of ligand-gated ion channels that act through adenosine ATP. The P2X3 receptor plays a key role in the transmission of neuropathic pain at peripheral and spinal sites. Electroacupuncture (EA) has been used to treat neuropathic pain effectively. To determine the role of EA in neuropathic pain mediated through the P2X3 receptor in dorsal root ganglion neurons and the spinal cord, a chronic constriction injury (CCI) model was used. Sprague-Dawley rats were divided into four groups: sham CCI, CCI, CCI plus contralateral EA, and CCI plus ipsilateral EA. The mechanical withdrawal threshold (MWT) and thermal withdrawal latency (TWL) were recorded. Furthermore, the expression of the P2X3 receptor was evaluated through Western blotting and immunofluorescence. The effects of EA and A-317491 were investigated through the whole-cell patch-clamp method and intrathecal administration. Our results show that the MWT and TWL of EA groups were higher than those in the CCI group, whereas the expression of the P2X3 receptor was lower than that in the CCI group. However, no significant difference was detected between the two EA groups. EA depressed the currents created by ATP and the upregulation of the P2X3 receptor in CCI rats. Additionally, EA was more potent in reducing mechanical allodynia and thermal hyperalgesia when combined with A-317491 through intrathecal administration. These results show that both contralateral and ipsilateral EA might inhibit the primary afferent transmission of neuropathic pain induced through the P2X3 receptor. In addition, EA and A-317491 might have an additive effect in inhibiting the transmission of pain mediated by the P2X3 receptor.

Peripheral cardiac sympathetic hyperactivity in cardiovascular disease: role of neuropeptides

High levels of sympathetic drive in several cardiovascular diseases including postmyocardial infarction, chronic congestive heart failure and hypertension are reinforced through dysregulation of afferent input and central integration of autonomic balance. However, recent evidence suggests that a significant component of sympathetic hyperactivity may also reside peripherally at the level of the postganglionic neuron. This has been studied in depth using the spontaneously hypertensive rat, an animal model of genetic essential hypertension, where larger neuronal calcium transients, increased release and impaired reuptake of norepinephrine in neurons of the stellate ganglia lead to a significant tachycardia even before hypertension has developed. The release of additional sympathetic cotransmitters during high levels of sympathetic drive can also have deleterious consequences for peripheral cardiac parasympathetic neurotransmission even in the presence of β-adrenergic blockade. Stimulation of the cardiac vagus reduces heart rate, lowers myocardial oxygen demand, improves coronary blood flow, and independently raises ventricular fibrillation threshold. Recent data demonstrates a direct action of the sympathetic cotransmitters neuropeptide Y (NPY) and galanin on the ability of the vagus to release acetylcholine and control heart rate. Moreover, there is as a strong correlation between plasma NPY levels and coronary microvascular function in patients with ST-elevation myocardial infarction being treated with primary percutaneous coronary intervention. Antagonists of the NPY receptors Y1 and Y2 may be therapeutically beneficial both acutely during myocardial infarction and also during chronic heart failure and hypertension. Such medications would be expected to act synergistically with β-blockers and implantable vagus nerve stimulators to improve patient outcome.

Molecular and functional expression of cation-chloride cotransporters in dorsal root ganglion neurons during postnatal maturation

GABA depolarizes and excites central neurons during early development, becoming inhibitory and hyperpolarizing with maturation. This "developmental shift" occurs abruptly, reflecting a decrease in intracellular Cl(-) concentration ([Cl(-)](i)) and a hyperpolarizing shift in Cl(-) equilibrium potential due to upregulation of the K(+)-Cl(-) cotransporter KCC2b, a neuron-specific Cl(-) extruder. In contrast, primary afferent neurons (PANs) are depolarized by GABA throughout adulthood because of expression of NKCC1, a Na(+)-K(+)-2Cl(-) cotransporter that accumulates Cl(-) above equilibrium. The GABA(A)-mediated depolarization of PANs determines presynaptic inhibition in the spinal cord, a key mechanism gating somatosensory information. Little is known about developmental changes in Cl(-) transporter expression and Cl(-) homeostasis in PANs. Whether NKCC1 is expressed in PANs of all phenotypes or is restricted to subpopulations (e.g., nociceptors) is debatable. Likewise, whether PANs express KCC2s is controversial. We investigated NKCC1 and K(+)-Cl(-) cotransporter expression in rat and mouse dorsal root ganglion (DRG) neurons with molecular methods. Using fluorescence imaging microscopy, we measured [Cl(-)](i) in acutely dissociated rat DRG neurons (P0-P21) loaded with N-(ethoxycarbonylmethyl)-6-methoxyquinolinium bromide and classified with phenotypic markers. DRG neurons of all sizes express two NKCC1 mRNAs, one full-length and a shorter splice variant lacking exon 21. Immunolabeling with validated antibodies revealed ubiquitous expression of NKCC1 in DRG neurons irrespective of postnatal age and phenotype. As maturation progresses [Cl(-)](i) decreases gradually, persisting above equilibrium in >95% mature neurons. DRG neurons express mRNAs for KCC1, KCC3s, and KCC4, but not for KCC2s. Mechanisms underlying PANs' developmental changes in Cl(-) homeostasis are discussed and compared with those of central neurons.

The calcium-activated chloride channel anoctamin 1 acts as a heat sensor in nociceptive neurons

Nociceptors are a subset of small primary afferent neurons that respond to noxious chemical, thermal and mechanical stimuli. Ion channels in nociceptors respond differently to noxious stimuli and generate electrical signals in different ways. Anoctamin 1 (ANO1 also known as TMEM16A) is a Ca(2+)-activated chloride channel that is essential for numerous physiological functions. We found that ANO1 was activated by temperatures over 44 °C with steep heat sensitivity. ANO1 was expressed in small sensory neurons and was highly colocalized with nociceptor markers, which suggests that it may be involved in nociception. Application of heat ramps to dorsal root ganglion (DRG) neurons elicited robust ANO1-dependent depolarization. Furthermore, knockdown or deletion of ANO1 in DRG neurons substantially reduced nociceptive behavior in thermal pain models. These results indicate that ANO1 is a heat sensor that detects nociceptive thermal stimuli in sensory neurons and possibly mediates nociception.

Dorsal root ganglion application of muscimol prevents hyperalgesia and stimulates myelin protein expression after sciatic nerve injury in rats

BACKGROUND

Peripheral nerve injuries may result in debilitating pain that is poorly responsive to conventional treatment. Neuropathic pain induced by peripheral nerve injury is caused, in part, by ectopic discharges from the injury site or the dorsal root ganglia (DRG) resulting in enhanced central input and central hyperexcitability. A heterogeneous family of γ-aminobutyric acid (GABA)(A) channels is important in quieting neuronal excitability. We have recently reported that in vivo modulation of GABAergic neurons in DRG can alter the course of neuropathic pain development after peripheral nerve injury. It seems that direct application of a potent GABA(A) agonist, muscimol, to the ipsilateral DRG prevents the development of hyperalgesia in rats subjected to a sciatic nerve crush injury. In addition to potentially curtailing hyperexcitability, GABAergic stimulation upregulated expression of peripheral myelin protein 22 (PMP22), a key component of the basal lamina. PMP22 expression correlates with peripheral myelin formation and nerve regeneration.

METHODS

Because of the importance of PMP22 for the formation and stability of myelin, and the fact that PMP22 expression could be GABAergically modulated, we examined whether direct DRG application of muscimol can restore PMP22 protein expression and the integrity of nerve fibers after crush injury of a sciatic nerve.

RESULTS

Using adult female rats and a crush injury model, we found that GABAergic modulation in the ipsilateral DRG restores PMP22 protein expression in the distal segment of the sciatic nerve and improves myelin stability in the basal membrane of nerve fibers, thus giving the morphological appearance of lessened nerve injury or faster nerve fiber regeneration. Both the enhanced PMP22 protein expression and morphological improvements coincide with the abolishment of thermal and mechanical hypersensitivity.

CONCLUSIONS

The DRG could be a promising therapeutic target in nerve regeneration and pain alleviation after crush injury of a myelinated peripheral nerve.

Presynaptic alpha2-GABAA receptors in primary afferent depolarization and spinal pain control

Spinal dorsal horn GABA(A) receptors are found both postsynaptically on central neurons and presynaptically on axons and/or terminals of primary sensory neurons, where they mediate primary afferent depolarization (PAD) and presynaptic inhibition. Both phenomena have been studied extensively on a cellular level, but their role in sensory processing in vivo has remained elusive, due to inherent difficulties to selectively interfere with presynaptic receptors. Here, we address the contribution of a major subpopulation of GABA(A) receptors (those containing the α2 subunit) to spinal pain control in mice lacking α2-GABA(A) receptors specifically in primary nociceptors (sns-α2(-/-) mice). sns-α2(-/-) mice exhibited GABA(A) receptor currents and dorsal root potentials of normal amplitude in vitro, and normal response thresholds to thermal and mechanical stimulation in vivo, and developed normal inflammatory and neuropathic pain sensitization. However, the positive allosteric GABA(A) receptor modulator diazepam (DZP) had almost completely lost its potentiating effect on PAD and presynaptic inhibition in vitro and a major part of its spinal antihyperalgesic action against inflammatory hyperalgesia in vivo. Our results thus show that part of the antihyperalgesic action of spinally applied DZP occurs through facilitated activation of GABA(A) receptors residing on primary nociceptors.

GABA(A) Receptors: Post-Synaptic Co-Localization and Cross-Talk with Other Receptors

γ-Aminobutyric acid type A receptors (GABA_ARs) are the major inhibitory neurotransmitter receptors in the central nervous system, and importantly contribute to the functional regulation of the nervous system. Several studies in the last few decades have convincingly shown that GABA can be co-localized with other neurotransmitters in the same synapse, and can be co-released with these neurotransmitters either from the same vesicles or from different vesicle pools. The co-released transmitters may act on post-synaptically co-localized receptors resulting in a simultaneous activation of both receptors. Most of the studies investigating such co-activation observed a reduced efficacy of GABA for activating GABA_ARs and thus, a reduced inhibition of the post-synaptic neuron. Similarly, in several cases activation of GABA_ARs has been reported to suppress the response of the associated receptors. Such a receptor cross-talk is either mediated via a direct coupling between the two receptors or via the activation of intracellular signaling pathways and is used for fine tuning of inhibition in the nervous system. Recently, it was demonstrated that a direct interaction of different receptors might already occur in intracellular compartments and might also be used to specifically target the receptors to the cell membrane. In this article, we provide an overview on such cross-talks between GABA_ARs and several other neurotransmitter receptors and briefly discuss their possible physiological and clinical importance.

Inflammatory pain unmasks heterosynaptic facilitation in lamina I neurokinin 1 receptor-expressing neurons in rat spinal cord

Central sensitization in inflammatory pain conditions results in behavioral mechanical hypersensitivity. Specifically, C-fiber-driven spinal hyperexcitability enables A fibers to gain access to specific spinal circuitry, via heterosynaptic facilitatory mechanisms, to mediate mechanical hypersensitivity. However, the precise circuitry engaged is not known. Lamina I neurokinin 1 (NK1) receptor expressing (NK1R(+)) dorsal horn neurons, many of which are projection neurons, are required for the development of this hypersensitivity and are therefore likely to be a component of this circuitry. To investigate, whole-cell patch-clamp recordings were made from lamina I NK1R(+) neurons in the spinal cord slice preparation with attached dorsal root, obtained from rats with or without complete Freund's adjuvant (CFA) hindpaw inflammation. EPSCs were recorded in response to electrical stimulation of the dorsal root. Control neurons predominantly received monosynaptic C-fiber input (69%) with a smaller proportion receiving monosynaptic Aδ-fiber input (28%). In contrast, CFA inflammation significantly increased the incidence (by twofold) and magnitude (by 75% in a subset) of monosynaptic Aδ-fiber but not monosynaptic C-fiber-evoked responses. Aβ-fiber input to lamina I NK1R(+) neurons was minimal, polysynaptic in nature, and unaltered by CFA inflammation. Additional examination of control neurons revealed that a proportion received silent monosynaptic Aδ-fiber input, suggesting that these may provide the substrate for the novel Aδ inputs observed in CFA inflammation. This inflammation induced unmasking and strengthening of monosynaptic Aδ drive to lamina I NK1R(+) neurons may contribute to the heterosynaptic facilitatory mechanisms underlying mechanical hyperalgesia in inflammatory pain.

Regulation of GABA(A) receptor dynamics by interaction with purinergic P2X(2) receptors

γ-Aminobutyric acid type A receptors (GABA(A)Rs) in the spinal cord are evolving as an important target for drug development against pain. Purinergic P2X(2) receptors (P2X(2)Rs) are also expressed in spinal cord neurons and are known to cross-talk with GABA(A)Rs. Here, we investigated a possible "dynamic" interaction between GABA(A)Rs and P2X(2)Rs using co-immunoprecipitation and fluorescence resonance energy transfer (FRET) studies in human embryonic kidney (HEK) 293 cells along with co-localization and single particle tracking studies in spinal cord neurons. Our results suggest that a significant proportion of P2X(2)Rs forms a transient complex with GABA(A)Rs inside the cell, thus stabilizing these receptors and using them for co-trafficking to the cell surface, where P2X(2)Rs and GABA(A)Rs are primarily located extra-synaptically. Furthermore, agonist-induced activation of P2X(2)Rs results in a Ca(2+)-dependent as well as an apparently Ca(2+)-independent increase in the mobility and an enhanced degradation of GABA(A)Rs, whereas P2X(2)Rs are stabilized and form larger clusters. Antagonist-induced blocking of P2XRs results in co-stabilization of this receptor complex at the cell surface. These results suggest a novel mechanism where association of P2X(2)Rs and GABA(A)Rs could be used for specific targeting to neuronal membranes, thus providing an extrasynaptic receptor reserve that could regulate the excitability of neurons. We further conclude that blocking the excitatory activity of excessively released ATP under diseased state by P2XR antagonists could simultaneously enhance synaptic inhibition mediated by GABA(A)Rs.

A rapamycin-sensitive signaling pathway is essential for the full expression of persistent pain states

Translational control through the mammalian target of rapamycin (mTOR) is critical for synaptic plasticity, cell growth, and axon guidance. Recently, it was also shown that mTOR signaling was essential for the maintenance of the sensitivity of subsets of adult sensory neurons. Here, we show that persistent pain states, but not acute pain behavior, are substantially alleviated by centrally administered rapamycin, an inhibitor of the mTOR pathway. We demonstrate that rapamycin modulates nociception by acting on subsets of primary afferents and superficial dorsal horn neurons to reduce both primary afferent sensitivity and central plasticity. We found that the active form of mTOR is present in a subpopulation of myelinated dorsal root axons, but rarely in unmyelinated C-fibers, and heavily expressed in the dorsal horn by lamina I/III projection neurons that are known to mediate the induction and maintenance of pain states. Intrathecal injections of rapamycin inhibited the activation of downstream targets of mTOR in dorsal horn and dorsal roots and reduced the thermal sensitivity of A-fibers. Moreover, in vitro studies showed that rapamycin increased the electrical activation threshold of Adelta-fibers in dorsal roots. Together, our results imply that central rapamycin reduces neuropathic pain by acting both on an mTOR-positive subset of A-nociceptors and lamina I projection neurons and suggest a new pharmacological route for therapeutic intervention in persistent pain states.

Adenovector GAD65 gene delivery into the rat trigeminal ganglion produces orofacial analgesia

Background

Our goal is to use gene therapy to alleviate pain by targeting glial cells. In an animal model of facial pain we tested the effect of transfecting the glutamic acid decarboxylase (GAD) gene into satellite glial cells (SGCs) of the trigeminal ganglion by using a serotype 5 adenovector with high tropisms for glial cells. We postulated that GABA produced from the expression of GAD would reduce pain behavior by acting on GABA receptors on neurons within the ganglion.

Results

Injection of adenoviral vectors (AdGAD65) directly into the trigeminal ganglion leads to sustained expression of the GAD65 isoform over the 4 weeks observation period. Immunohistochemical analysis showed that adenovirus-mediated GAD65 expression and GABA synthesis were mainly in SGCs. GABAA and GABAB receptors were both seen in sensory neurons, yet only GABAA receptors decorated the neuronal surface. GABA receptors were not found on SGCs. Six days after injection of AdGAD65 into the trigeminal ganglion, there was a statistically significant decrease of pain behavior in the orofacial formalin test, a model of inflammatory pain. Rats injected with control virus (AdGFP or AdLacZ) had no reduction in their pain behavior. AdGAD65-dependent analgesia was blocked by bicuculline, a selective GABAA receptor antagonist, but not by CGP46381, a selective GABAB receptor antagonist.

Conclusion

Transfection of glial cells in the trigeminal ganglion with the GAD gene blocks pain behavior by acting on GABAA receptors on neuronal perikarya.

Stimulation of alpha(1)-adrenoceptors reduces glutamatergic synaptic input from primary afferents through GABA(A) receptors and T-type Ca(2+) channels

Activation of the descending noradrenergic system inhibits nociceptive transmission in the spinal cord. Although both alpha(1)- and alpha(2)-adrenoceptors in the spinal cord are involved in the modulation of nociceptive transmission, it is not clear how alpha(1)-adrenoceptors regulate excitatory and inhibitory synaptic transmission at the spinal level. In this study, inhibitory and excitatory postsynaptic currents (IPSCs and EPSCs, respectively) were recorded from lamina II neurons in rat spinal cord slices. The specific alpha(1)-adrenoceptor agonist phenylephrine significantly increased the frequency of GABAergic spontaneous IPSCs in a concentration dependent manner, and this effect was abolished by the alpha(1)-adrenoceptor antagonist 2-(2,6-dimethoxyphenoxy)ethylaminomethyl-1,4-benzodioxane (WB4101). Phenylephrine also significantly reduced the amplitude of monosynaptic and polysynaptic EPSCs evoked from primary afferents. The inhibitory effect of phenylephrine on evoked monosynaptic glutamatergic EPSCs was largely blocked by the GABA(A) receptor antagonist picrotoxin and, to a lesser extent, by the GABA(B) receptor antagonist CGP55845. Furthermore, blocking T-type Ca(2+) channels with amiloride or mibefradil diminished the inhibitory effect produced by phenylephrine or the GABA(A) receptor agonist muscimol on monosynaptic EPSCs evoked from primary afferents. Collectively, these findings suggest that activation of alpha(1)-adrenoceptors in the spinal cord increases synaptic GABA release, which attenuates glutamatergic input from primary afferents mainly through GABA(A) receptors and T-type Ca(2+) channels. This mechanism of presynaptic inhibition in the spinal cord may be involved in the regulation of nociception by the descending noradrenergic system.

17beta-estradiol activates estrogen receptor beta-signalling and inhibits transient receptor potential vanilloid receptor 1 activation by capsaicin in adult rat nociceptor neurons

There is mounting evidence that estrogens act directly on the nervous system to affect the severity of pain. Estrogen receptors (ERs) are expressed by sensory neurons, and in trigeminal ganglia, 17beta-estradiol can indirectly enhance nociception by stimulating expression and release of prolactin, which increases phosphorylation of the nociceptor transducer transient receptor potential vanilloid receptor 1 (TRPV1). Here, we show that 17beta-estradiol acts directly on dorsal root ganglion (DRG) sensory neurons to reduce TRPV1 activation by capsaicin. Capsaicin-induced cobalt uptake and the maximum TRPV1 current induced by capsaicin were inhibited when isolated cultured DRGs neurons from adult female rats were exposed to 17beta-estradiol (10-100 nm) overnight. There was no effect of 17beta-estradiol on capsaicin potency, TRPV1 activation by protons (pH 6-4), and P2X currents induced by alpha,beta-methylene-ATP. Diarylpropionitrile (ERbeta agonist) also inhibited capsaicin-induced TRPV1 currents, whereas propylpyrazole triol (ERalpha agonist) and 17alpha-estradiol (inactive analog) were inactive, and 17beta-estradiol conjugated to BSA (membrane-impermeable agonist) caused a small increase. TRPV1 inhibition was antagonized by tamoxifen (1 microm), but ICI182870 (10 microm) was a potent agonist and mimicked 17beta-estradiol. We conclude that TRPV1 in DRG sensory neurons can be inhibited by a nonclassical estrogen-signalling pathway that is downstream of intracellular ERbeta. This affects the vanilloid binding site targeted by capsaicin but not the TRPV1 activation site targeted by protons. These actions could curtail the nociceptive transducer functions of TRPV1 and limit chemically induced nociceptor sensitization during inflammation. They are consistent with clinical reports that female pelvic pain can increase after reductions in circulating estrogens.

Na+,K+,2Cl- cotransport and intracellular chloride regulation in rat primary sensory neurons: thermodynamic and kinetic aspects

Adult primary afferent neurons are depolarized by GABA throughout their entire surface, including their somata located in dorsal root ganglia (DRG). Primary afferent depolarization (PAD) mediated by GABA released from spinal interneurons determines presynaptic inhibition, a key mechanism in somatosensory processing. The depolarization is due to Cl(-) efflux through GABA(A) channels; the outward Cl(-) gradient is generated by a Na+,K+,2Cl(-) cotransporter (NKCC) as first established in amphibians. Using fluorescence imaging microscopy we measured [Cl(-)]i and cell water volume (CWV) in dissociated rat DRG cells (P0-P21) loaded with N-(ethoxycarbonylmethyl)-6-methoxyquinolinium bromide and calcein, respectively. Basal [Cl(-)]i was 44.2 +/- 1.2 mM (mean +/- SE), Cl(-) equilibrium potential (E Cl) was -27.0 +/- 0.7 mV (n = 75). This [Cl(-)]i is about four times higher than electrochemical equilibrium. On isosmotic removal of external Cl(-), cells lost Cl(-) and shrank. On returning to control solution, cells reaccumulated Cl(-) and recovered CWV. Cl(-) reaccumulation had Na+-dependent (SDC) and Na+-independent (SIC) components. The SIC stabilized at [Cl(-)]i = 13.2 +/- 1.2 mM, suggesting that it was passive (E(Cl) = -60.5 +/- 3 mV). Bumetanide blocked CWV recovery and most (65%) of the SDC (IC50 = 5.7 microM), indicating that both were mediated by NKCC. Active Cl(-) uptake fell with increasing [Cl(-)]i and became negligible when [Cl(-)]i reached basal levels. The kinetics of active Cl(-) uptake suggests a negative feedback system in which intracellular Cl(-)regulates its own influx thereby keeping [Cl(-)]i constant, above electrochemical equilibrium but below the value that would attain if NKCC reached thermodynamic equilibrium.

New insights into purinergic receptor signaling in neuronal differentiation, neuroprotection, and brain disorders

Ionotropic P2X and metabotropic P2Y purinergic receptors are expressed in the central nervous system and participate in the synaptic process particularly associated with acetylcholine, GABA, and glutamate neurotransmission. As a result of activation, the P2 receptors promote the elevation of free intracellular calcium concentration as the main signaling pathway. Purinergic signaling is present in early stages of embryogenesis and is involved in processes of cell proliferation, migration, and differentiation. The use of new techniques such as knockout animals, in vitro models of neuronal differentiation, antisense oligonucleotides to induce downregulation of purinergic receptor gene expression, and the development of selective inhibitors for purinergic receptor subtypes contribute to the comprehension of the role of purinergic signaling during neurogenesis. In this review, we shall discuss the participation of purinergic receptors in developmental processes and in brain physiology, including neuron-glia interactions and pathophysiology.

Physiology and pathophysiology of purinergic neurotransmission

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

An intracellular motif of P2X(3) receptors is required for functional cross-talk with GABA(A) receptors in nociceptive DRG neurons

Functional cross-talk between structurally unrelated P2X ATP receptors and members of the 'cys-loop' receptor-channel superfamily represents a recently-discovered mechanism for rapid modulation of information processing. The extent and the mechanism of the inhibitory cross-talks between these two classes of ionotropic receptors remain poorly understood, however. Both ionic and molecular coupling were proposed to explain cross-inhibition between P2X subtypes and GABA(A) receptors, suggesting a P2X subunit-dependent mechanism. We show here that cross-inhibition between neuronal P2X(3) or P2X(2+3) and GABA(A) receptors does not depend on chloride and calcium ions. We identified an intracellular QST(386-388) motif in P2X(3) subunits which is required for the functional coupling with GABA(A) receptors. Moreover the cross-inhibition between native P2X(3) and GABA receptors in cultured rat dorsal root ganglia (DRG) neurons is abolished by infusion of a peptide containing the QST motif as well as by viral expression of the main intracellular loop of GABA(A)beta3 subunits. We provide evidence that P2X(3) and GABA(A) receptors are colocalized in the soma and central processes of nociceptive DRG neurons, suggesting that specific intracellular P2X(3)-GABA(A) subunit interactions underlie a pre-synaptic cross-talk that might contribute to the regulation of sensory synaptic transmission in the spinal cord.

Kinetics of ATP release following compression injury of a peripheral nerve trunk

Compression and/or contusion of a peripheral nerve trunk can result in painful sensations. It is possible that release of ATP into the extracellular space may contribute to this symptom. In the present study, we used real-time measurements of ATP-induced bioluminescence together with electrophysiological recordings of compound action potentials to follow changes in the extracellular ATP concentration of isolated rat spinal roots exposed to mechanical stimuli. Nerve compression for about 8 s resulted in an immediate release of ATP into the extracellular space and in a decrease in the amplitude of compound action potentials. On average, a rise in ATP to 60 nM was observed when nerve compression blocked 50% of the myelinated axons. After the compression, the extracellular concentration of ATP returned to the resting level within a few minutes. The importance of ecto-nucleotidases for the recovery period was determined by exposure of isolated spinal roots to high concentrations of ATP and by use of inhibitors of ecto-nucleotidases. It was observed that spinal roots have a high capacity for ATP hydrolysis which is only partially blocked by betagamma-methylene ATP and ARL 67156. In conclusion, acute nerve compression produces an increase in the extracellular concentration of ATP and of its metabolites which may be sufficient for activation of purinergic P2 and/or P1 receptors on axons of nociceptive afferent neurons.

Both Ca2+-permeable and -impermeable AMPA receptors contribute to primary synaptic drive onto rat dorsal horn neurons

Blockade of Ca2+-permeable AMPA receptors in the rat spinal cord diminishes the development of hyperalgesia and allodynia associated with peripheral injury. Cobalt uptake studies reveal that Ca2+-permeable AMPA receptors are expressed by some substance P receptor-expressing (NK1R+) neurons in lamina I, as well as other neurons throughout the superficial dorsal horn. Selective elimination of NK1R+ neurons in lamina I and lamina III/IV of the dorsal horn also suppresses development of hyperalgesia and allodynia. These observations raise the possibility that Ca2+-permeable AMPA receptors contribute to excitatory synaptic drive onto the NK1R+ neurons associated with allodynia and hyperalgesia. The first synapse in the pain pathway is the glutamatergic excitatory drive from the primary afferent fibres onto dorsal horn neurons. Therefore, we tested whether Ca2+-permeable AMPA receptors are located on lamina I and lamina III/IV NK1R+ neurons postsynaptic to primary afferent fibres, using inward rectification and polyamine toxins for receptor identification. We examined three different populations of dorsal horn neurons; lamina I NK1R+ neurons, including projection neurons, and non-NK1R+ (NK1R-) neurons including interneurons, and lamina III/IV NK1R+ neurons, believed to contribute to the low-threshold mechanosensory pathway. The majority of synapses in all three groups had rectification indices less than 1.0 and greater than 0.4, indicating that the AMPA receptors at these synapses are a mixture of Ca2+-permeable and -impermeable forms. Lamina III/IV NK1R+ neurons and lamina I NK1R- neurons have a significantly higher proportion of postsynaptic Ca2+-permeable AMPA receptors than lamina I NK1R+ neurons. Thus synaptically positioned Ca2+-permeable AMPA receptors directly contribute to low-threshold sensory afferent drive into the dorsal horn, and can mediate afferent input onto interneurons such as GABAergic neurons. These receptors also contribute to high-threshold primary afferent drive onto NK1R+ neurons in the superficial dorsal horn, but do so less consistently.

Purinergic P2 receptors as targets for novel analgesics

Following hints in the early literature about adenosine 5'-triphosphate (ATP) injections producing pain, an ion-channel nucleotide receptor was cloned in 1995, P2X3 subtype, which was shown to be localized predominantly on small nociceptive sensory nerves. Since then, there has been an increasing number of papers exploring the role of P2X3 homomultimer and P2X2/3 heteromultimer receptors on sensory nerves in a wide range of organs, including skin, tongue, tooth pulp, intestine, bladder, and ureter that mediate the initiation of pain. Purinergic mechanosensory transduction has been proposed for visceral pain, where ATP released from epithelial cells lining the bladder, ureter, and intestine during distension acts on P2X3 and P2X2/3, and possibly P2Y, receptors on subepithelial sensory nerve fibers to send messages to the pain centers in the brain as well as initiating local reflexes. P1, P2X, and P2Y receptors also appear to be involved in nociceptive neural pathways in the spinal cord. P2X4 receptors on spinal microglia have been implicated in allodynia. The involvement of purinergic signaling in long-term neuropathic pain and inflammation as well as acute pain is discussed as well as the development of P2 receptor antagonists as novel analgesics.

P2Y1and P2X7receptors induce calcium/calmodulin-dependent protein kinase II phosphorylation in cerebellar granule neurons

The activation of nucleotide receptors-- both ionotropic, P2X, and most of metabotropic, P2Y-- increases intracellular calcium concentration, resulting in calcium/calmodulin-dependent protein kinase II (CaMKII) activation. Stimulation of cerebellar granule neurons in culture-- with different P2X and P2Y agonists and their effect on CaMKII phosphorylation-- was studied using immunocytochemical and microfluorimetrical techniques. P2X agonist: 2'-3'-o-(4-benzoylbenzoyl)-adenosine 5'-triphosphate (BzATP), alpha,beta-methylene adenosine 5'-triphosphate (alpha,beta-meATP) and diadenosine pentaphosphate (Ap(5)A); and P2Y agonists: 2-(methylthyo)-adenosine diphosphate (2MeSADP) and uridine 5'-bisphosphate (UDP); tested induced a CaMKII phosphorylation but with a different immunostaining pattern in each group. Stimulation with 2MeSADP induced a Ca(2+) release from intracellular stores and a significant CaMKII phosphorylation in cell somas and neurites. This agrees with the subcellular distribution of P2Y(1). MRS 2179, a specific P2Y(1) inhibitor, antagonized the 2MeSADP effect. On the other hand, cerebellar granule neuron stimulation with BzATP, in Mg(2+)-free conditions, produced extracellular calcium entrance and, as a result, a significant increase in CaMKII phosphorylation mostly in fibres, which correspond with P2X(7) subdistribution. Immunocytochemical and microfluorimetrical experiments, using Zn(2+) and Brilliant Blue G (BBG), as a specific P2X(7) antagonist, confirmed that BzATP was acting through the P2X(7) receptor. These results indicate that P2Y(1) and P2X(7) produce a significant increase in CaMKII phosphorylation, but show important differences in subcellular distribution and in effect duration. P2X(7) activation in granule neurons is not associated with pore formation, according to the absence of YO-PRO-1 fluorescence. The abundant presence of P2X(7) at the synaptic structures suggests a relevant role played by this receptor in synaptic plasticity.

GABAA-Receptor blockade reverses the injury-induced sensitization of nociceptor-specific (NS) neurons in the spinal dorsal horn of the rat

Single-unit electrical activity was recorded from 80 nociceptor-specific (NS) neurons in the dorsal horn of the lumbar spinal cord of pentobarbital anesthetized rats. Their responses to low- and high-intensity mechanical stimulation of their receptive fields (RFs) were recorded before and after the application of irritant agents [capsaicin (CAP) or mustard oil (MO)] to the RF. Before the applications of the irritants the neurons responded only to high-intensity stimuli, but after this procedure 20 of 28 neurons tested were sensitized, i.e., gave increased responses to high-intensity stimuli and showed novel responses to low-intensity mechanical stimulation as well as an Abeta-fiber afferent drive. CAP was more likely to induce sensitization than MO and the majority of sensitized neurons were located in the superficial dorsal horn. No relationship was found between the magnitude of the response to the sensitizing agent and the presence or absence of sensitization. Cumulative doses of two gamma-aminobutyric acid type A (GABA(A))-receptor antagonists, picrotoxin and bicuculline, were administered systemically or applied directly over the spinal cord. The GABA(A) antagonists reversed the sensitization of the neurons by reducing the novel low-threshold responses. These results show that NS neurons in the spinal dorsal horn can be sensitized by a sustained afferent discharge in peripheral nociceptors and that this sensitization can be reduced or reversed by low doses of GABA(A)-receptor antagonists. This provides evidence for a mechanism in which an enhanced GABAergic transmission can lead to hyperexcitability and sensitization of NS neurons in the dorsal horn.

P2X purinoceptors and sensory transmission

The involvement of P2X purinoreceptors (P2X receptors) in somatosensory transmission is herein reviewed with a focus on those receptors that are expressed on sensory neurons to elucidate their roles in the initiation of sensory excitation from primary afferent neurons, in modulating synaptic transmission at the first sensory synapses formed between primary afferent central terminals and dorsal horn neurons, in directly mediating sensory synaptic transmission to the spinal cord dorsal horn, and in modulating synaptic transmission among spinal cord dorsal horn neurons. Research on P2X receptors has indicated that these receptors play a significant role in both physiological and pathological pain states. As a result, P2X receptors may serve as therapeutic targets for the treatment of pathological pain conditions associated with nerve injury, tissue inflammation, cancer, and other diseases.

The effect of P2X receptor activity on GABAA receptor-mediated inhibition in the gerbil hippocampus

In the present study, to elucidate the effect of altered P(2)X receptor transmission on GABA(A) receptor expression and its transmission, we studied the morphological and electrophysiological responses of GABA(A) receptor in the gerbil hippocampus following P(2)X receptor antagonist/agonist treatment. Suramin or pyridoxal phosphate-6-azophenyl-2',4'-disulfonic acid (PPADS) treatment did not affect GABA(A) receptor immunoreactivities and paired-pulse responses in the gerbil hippocampus. In addition, ATP treatment did not significantly affect population spike amplitude ratios and EPSP slope ratios in the gerbil dentate gyrus. Co-application, but not pretreatment, of PPADS or suramin enhanced the effect of muscimol on paired-pulse inhibition in the dentate gyrus. In contrast, co-application of ATP reduced the effect of muscimol in the dentate gyrus. These findings indicate that the blockade of P(2)X receptor did not affect GABA(A) receptor immunoreactivities, and P(2)X receptor may modulate GABA(A) receptor-mediated inhibition when in co-activation with GABA(A) receptor. Therefore, our findings suggest that the relationship between GABA(A) receptor and P(2)X receptor may not be reciprocal, although GABA(A) receptor activity affects P(2)X receptor functionality and its expression.

Capsaicin‐sensitive sensory neurons regulate myocardial function and gene expression pattern of rat hearts: a DNA microarray study

We have previously shown that capsaicin-sensitive sensory nerves contribute to the regulation of normal cardiac function and to the development of cardiac adaptation to ischemic stress; however, the underlying molecular mechanisms remain unknown. Therefore, here we assessed cardiac functional alterations and relative gene expression changes by DNA microarray analysis of 6400 genes in rat hearts 7 days after the end of systemic capsaicin treatment protocol leading to selective sensory chemodenervation. Capsaicin pretreatment resulted in a cardiac dysfunction characterized by elevation of left ventricular end-diastolic pressure and led to altered expression of 80 genes of known function or homology to known sequences. Forty-seven genes exhibited significant up-regulation and 33 genes were down-regulated (changes ranged from -3.9 to +4.8-fold). The expression changes of 10 selected genes were verified, and an additional 11 genes were examined by real-time quantitative PCR. This is the first demonstration that gene expression changes in the heart due to capsaicin pretreatment included vanilloid receptor-1 (capsaicin receptor), transient receptor potential protein, GABA receptor rho-3 subunit, 5-hydroxytryptamine 3 receptor B, neurokinin receptor 2, endothelial nitric oxide synthase, matrix metalloproteinase-13, cytochrome P450, farnesyl-transferase, ApoB, and leptin. None of the genes have been previously shown to be involved in the mechanism of the cardiac functional effects of sensory chemodenervation by capsaicin. We conclude that capsaicin-sensitive sensory nerves play a significant role in the regulation of a variety of neuronal and non-neuronal genes in the heart and possibly in other tissues as well.

Presence of diverse functional P2X receptors in rat cerebellar synaptic terminals

Studies in individual synaptic terminals have demonstrated the presence of diverse functional P2X receptors in rat cerebellum. No immunolabelling for P2X1, P2X4, P2X5 and P2X6, and scarce presence of P2X2 were found at the cerebellar synaptic terminals. P2X3 immunolabelling was present in 28% of isolated synaptosomes. At these synaptic terminals, nucleotides as ATP or alpha,beta-meATP induced Ca2+ transients in the presence of extracellular Ca2+, showing homologous and heterologous receptor desensitization in 60% of cases. Ip5I 10 nM did not block responses to alpha,beta-meATP, but inhibition occurred when antagonist concentrations were equal or higher than 100 nM. These data agree with the presence of abundant P2X3 homomeric receptors. P2X7 immunolabelling was present in 60% of terminals and P2X7 receptor hallmarks in Ca2+ responses have been found. BzATP was more potent than ATP and responses were potentiated when assayed in Mg2+-free medium. EC50 values were, respectively, 39.4+/-0.4 and 0.3+/-0.1 microM for ATP in the presence or absence of Mg2+. Maximal values of synaptosomal calcium transients, in the presence or absence of Mg2+, were, respectively, 91.6+/-11.9 and 132.9+/-12.9 nM for ATP; and 104.3+/-9.4 and 169.7+/-17.1 nM for BzATP. In addition, Zn2+ inhibited ATP responses in the absence of Mg2+ and the P2X7 specific antagonist Brilliant Blue G completely blocked these responses in one half of synaptosomes. This study reports the presence of functional P2X3 and P2X7 receptors at synaptic sites, which provides complexity and regulatory possibilities to the cerebellar neurotransmission.

Involvement of P2 receptors in the growth and survival of neurons in the CNS

Extracellular adenosine 5'-triphosphate (ATP) has been recognized as a ubiquitous, unstable signalling molecule, acting as a fast neurotransmitter and modulator of transmitter release and neuronal excitability. Recent findings have demonstrated that ATP is a growth factor participating in differentiation, cell proliferation, and survival, as well as a toxic agent that mediates cellular degeneration and death. Potential sources of extracellular purines in the nervous system include neurons, glia, endothelium, and blood. A complex family of ectoenzymes rapidly hydrolyzes or interconverts extracellular nucleotides, thereby either terminating their signalling action or producing an active metabolite of altered purinoceptor selectivity. Most effects are mediated through the 2 main subclasses of specific cell surface receptors, P2X and P2Y. Members of these P2X/Y receptor families are widely expressed in the central nervous system (CNS) and are involved in glia-glia and glia-neuron communications, whereby they play important physiological and pathophysiological roles in a variety of biological processes. After different kinds of "acute" CNS injury (e.g., ischemia, hypoxia, mechanical stress, axotomy), extracellular ATP can reach high concentrations, up to the millimolar range, flowing out from cells into the extracellular space, exocytotically, via transmembrane transport, or as a result of cell damage. In this review, P2 receptor activation as a cause or a consequence of neuronal cell activation or death and/or glial activation is described. The involvement of P2 receptors is also described under different "chronic" pathological conditions, such as pain, epilepsia, toxic influence of ethanol or amphetamine, retinal diseases, Alzheimer's disease (AD), and possibly, Parkinson's disease. The relationship between changes in P2 receptor expression and the specific response of different cell types to injury is extremely complex and can be related to detrimental and/or beneficial effects. The present review therefore considers ATP acting via P2 receptors as a potent regulator of normal physiological and pathological processes in the brain, with a focus on pathophysiological implications of P2 receptor functions.

Enhanced thermal avoidance in mice lacking the ATP receptor P2X3

P2X3 is an ATP-gated cation channel subtype expressed by a subpopulation of primary sensory neurons. In vivo spinal cord recordings in mice lacking P2X3 (P2X3-/-) have suggested that this protein may be important for the coding of peripheral warm stimuli. To explore this possibility more thoroughly, we examined behavioral and electrophysiological responses to thermal stimuli in P2X3-/- mice. As previously reported, recording from the spinal cord dorsal horn of anesthetized P2X3-/- mice revealed a blunted response of wide dynamic range neurons to hind paw heating. When placed in a thermal gradient, however, P2X3-/- mice exhibited an unexpectedly enhanced avoidance of both hot and cold temperatures, relative to controls. In the tail immersion test, mutant mice exhibited shorter withdrawal latencies at temperatures above and below thermoneutrality. Consistent with these changes, P2X3-/- mice exhibited enhanced induction of spinal cord c-FOS following hind paw heating to 45 degrees C. Thus, gain- and loss-of-function thermosensory phenotypes coexist in P2X3-/- mice. No changes in thermal preference were observed in wild-type mice injected subcutaneously with the P2X3 antagonist, A317491 or intrathecally with the P2X3 and P2X1 antagonist TNP-ATP. The reason for this apparent discrepancy is unclear, but we cannot exclude the possibility that compensatory events contribute, at least in part, to the P2X3-/- phenotype. Regardless, this study illustrates the utility of thermal preference assays as part of a comprehensive approach to the analysis of mouse thermosensation.

Evidence for peripherally antinociceptive action of propofol in rats: Behavioral and spinal neuronal responses to subcutaneous bee venom

In the present study, behavioral and in vivo electrophysiological methods were used to examine the peripheral effects of propofol on tonic ongoing pain-related responses produced by subcutaneous bee venom-induced inflammatory pain state. Local administration of 0.5 microg propofol produced significant suppression of the well-established ongoing pain responses in both conscious rats and dorsal horn nociceptive neurons. The locally antinociceptive action of propofol is not caused by systemic effect, because contralateral administration of the same dose of drug did not produce any effect. This result indicates that besides central actions, propofol has peripherally antinociceptive action as well.

Role of cation-chloride-cotransporters (CCC) in pain and hyperalgesia

The importance of the GABAergic system in spinal nociceptive processing has long been appreciated but we have only recently begun to understand how this system is modulated by the regulation of anion gradients. In neuronal tissues, cation-chloride cotransporters regulate Cl- homeostasis and the activity and/or expression of these transporters has important implications for the direction and magnitude of anion flow through GABA-A channels. Here we review recent evidence that two cation-chloride cotransporters, NKCC1 and KCC2 are involved in pain and enhanced nociception. On the one hand, NKCC1 activity is upregulated in primary afferents following an inflammatory insult and this produces excessive GABAergic depolarization in primary afferents leading to cross excitation between low and high threshold afferents. On the other hand, KCC2 expression is reduced in dorsal horn neurons following peripheral nerve injury resulting in a loss of GABA-/glycinergic inhibitory tone and, in some cases, inverting its action into net excitation. Pharmacological targeting of these cation chloride cotransporters to restore normal GABA-/glycinergic transmission in the spinal cord represents an entirely novel approach to the development of analgesics.