Role of the Na+-K+-2Cl- cotransporter in the development of capsaicin-induced neurogenic inflammation

Depolarization of mouse DRG neurons by GABA does not translate into acute pain or hyperalgesia in healthy human volunteers

The majority of somatosensory DRG neurons express GABAA receptors (GABAAR) and depolarise in response to its activation based on the high intracellular chloride concentration maintained by the Na-K-Cl cotransporter type 1 (NKCC1). The translation of this response to peripheral nerve terminals in people is so far unclear. We show here that GABA (EC50 = 16.67μM) acting via GABAAR produces an influx of extracellular calcium in approximately 20% (336/1720) of isolated mouse DRG neurons. In contrast, upon injection into forearm skin of healthy volunteers GABA (1mM, 100μl) did not induce any overt sensations nor a specific flare response and did not sensitize C-nociceptors to slow depolarizing electrical sinusoidal stimuli. Block of the inward chloride transporter NKCC1 by furosemide (1mg/100μl) did not reduce electrically evoked pain ratings nor did repetitive GABA stimulation in combination with an inhibited NKCC1 driven chloride replenishment by furosemide. Finally, we generated a sustained period of C-fiber firing by iontophoretically delivering codeine or histamine to induce tonic itch. Neither the intensity nor the duration of histamine or codeine itch was affected by prior injection of furosemide. We conclude that although GABA can evoke calcium transients in a proportion of isolated mouse DRG neurons, it does not induce or modify pain or itch ratings in healthy human skin even when chloride gradients are altered by inhibition of the sodium coupled NKCC1 transporter.

Bumetanide increases postsynaptic inhibition after chronic SCI and decreases presynaptic inhibition with step-training

Current anti-spastic medication significantly compromises motor recovery after spinal cord injury (SCI), indicating a critical need for alternative interventions. Because a shift in chloride homeostasis decreases spinal inhibition and contributes to hyperreflexia after SCI, we investigated the effect of bumetanide, an FDA-approved sodium-potassium-chloride intruder (NKCC1) antagonist, on presynaptic and postsynaptic inhibition. We compared its effect with step-training as it is known to improve spinal inhibition by restoring chloride homeostasis. In SCI rats, a prolonged bumetanide treatment increased postynaptic inhibition but not presynaptic inhibition of the plantar H-reflex evoked by posterior biceps and semitendinosus (PBSt) group I afferents. By using in vivo intracellular recordings of motoneurons, we further show that a prolonged bumetanide increased postsynaptic inhibition by hyperpolarizing the reversal potential for inhibitory postsynaptic potentials (IPSPs) after SCI. However, in step-trained SCI rats an acute delivery of bumetanide decreased presynaptic inhibition of the H-reflex, but not postsynaptic inhibition. These results suggest that bumetanide might be a viable option to improve postsynaptic inhibition after SCI, but it also decreases the recovery of presynaptic inhibition with step-training. We discuss whether the effects of bumetanide are mediated by NKCC1 or by off-target effects. KEY POINTS: After spinal cord injury (SCI), chloride homeostasis is dysregulated over time in parallel with the decrease in presynaptic inhibition of Ia afferents and postsynaptic inhibition of motoneurons, and the development of spasticity. While step-training counteracts these effects, it cannot always be implemented in the clinic because of comorbidities. An alternative intervention is to use pharmacological strategies to decrease spasticity without hindering the recovery of motor function with step-training. Here we found that, after SCI, a prolonged bumetanide (an FDA-approved antagonist of the sodium-potassium-chloride intruder, NKCC1) treatment increases postsynaptic inhibition of the H-reflex, and it hyperpolarizes the reversal potential for inhibitory postsynaptic potentials in motoneurons. However, in step-trained SCI, an acute delivery of bumetanide decreases presynaptic inhibition of the H-reflex, but not postsynaptic inhibition. Our results suggest that bumetanide has the potential to decrease spastic symptoms related to a decrease in postsynaptic but not presynaptic inhibition after SCI.

Inhibition of NKCC1 in spinal dorsal horn and dorsal root ganglion results in alleviation of neuropathic pain in rats with spinal cord contusion

Previous studies have confirmed the relationship between chloride homeostasis and pain. However, the role of sodium potassium chloride co-transporter isoform 1 (NKCC1) in dorsal horn and dorsal root ganglion neurons (DRGs) in spinal cord injury (SCI)-induced neuropathic pain (NP) remains inconclusive. Therefore, we aimed to explore whether suppression of NKCC1 in the spinal cord and DRGs alleviate the NP of adult rats with thoracic spinal cord contusion. Thirty adult female Sprague-Dawley rats (8 week-old, weighing 250-280 g) were randomly divided into three groups with ten animals in each group (sham, SCI, and bumetanide groups). The paw withdrawal mechanical threshold and paw withdrawal thermal latency were recorded before injury (baseline) and on post-injury days 14, 21, 28, and 35. At the end of experiment, western blotting (WB) analysis, quantitative real-time Polymerase Chain Reaction (PCR) and immunofluorescence were performed to quantify NKCC1 expression. Our results revealed that NKCC1 protein expression in the spinal cord and DRGs was significantly up-regulated in rats with SCI. Intraperitoneal treatment of bumetanide (an NKCC1 inhibitor) reversed the expression of NKCC1 in the dorsal horn and DRGs and ameliorated mechanical ectopic pain and thermal hypersensitivities in the SCI rats. Our study demonstrated the occurrence of NKCC1 overexpression in the spinal cord and DRGs in a rodent model of NP and indicated that changes in the peripheral nervous system also play a major role in promoting pain sensitization after SCI.

Improving NKCC1 Function Increases the Excitability of DRG Neurons Exacerbating Pain Induced After TRPV1 Activation of Primary Sensory Neurons

Background Our aim was to investigate the effects of the protein expression and the function of sodium, potassium, and chloride co-transporter (NKCC1) in the dorsal root ganglion (DRG) after activation of transient receptor potential vanilloid 1 receptor (TRPV1) in capsaicin-induced acute inflammatory pain and the possible mechanism of action.

Methods Male Sprague–Dawley rats were randomly divided into control, capsaicin, and inhibitor groups. The expression and distribution of TRPV1 and NKCC1 in rat DRG were observed by immunofluorescence. Thermal radiation and acetone test were used to detect the pain threshold of heat and cold noxious stimulation in each group. The expressions of NKCC1 mRNA, NKCC1 protein, and p-NKCC1 in the DRG were detected by PCR and western blotting (WB). Patch clamp and chloride fluorescent probe were used to observe the changes of GABA activation current and intracellular chloride concentration. After intrathecal injection of protein kinase C (PKC) inhibitor (GF109203X) or MEK/extracellular signal-regulated kinase (ERK) inhibitor (U0126), the behavioral changes and the expression of NKCC1 and p-ERK protein in L_4–6 DRG were observed.

Result: TRPV1 and NKCC1 were co-expressed in the DRG. Compared with the control group, the immunofluorescence intensity of NKCC1 and p-NKCC1 in the capsaicin group was significantly higher, and the expression of NKCC1 in the nuclear membrane was significantly higher than that in the control group. The expression of NKCC1 mRNA and protein of NKCC1 and p-NKCC1 in the capsaicin group were higher than those in the control group. After capsaicin injection, GF109203X inhibited the protein expression of NKCC1 and p-ERK, while U0126 inhibited the protein expression of NKCC1. In the capsaicin group, paw withdrawal thermal latency (WTL) was decreased, while cold withdrawal latency (CWL) was prolonged. Bumetanide, GF109203X, or U0126 could reverse the effect. GABA activation current significantly increased in the DRG cells of the capsaicin group, which could be reversed by bumetanide. The concentration of chloride in the DRG cells of the capsaicin group increased, but decreased after bumetanide, GF109203X, and U0126 were administered.

Conclusion Activation of TRPV1 by exogenous agonists can increase the expression and function of NKCC1 protein in DRG, which is mediated by activation of PKC/p-ERK signaling pathway. These results suggest that DRG NKCC1 may participate in the inflammatory pain induced by TRPV1.

Overexpression of chloride importer NKCC1 contributes to the sensory-affective and sociability phenotype of rats following neonatal maternal separation

BACKGROUND

Early life stress is known to affect the development of the nervous system and its function at a later age. It increases the risk to develop psychiatric disorders as well as chronic pain and its associated affective comorbidities across the lifespan. GABAergic inhibition is important for the regulation of central function and related behaviors, including nociception, anxiety or social interactions, and requires low intracellular chloride levels. Of particular interest, the oxytocinergic (OTergic) system exerts potent anxiolytic, analgesic and pro-social properties and is known to be involved in the regulation of chloride homeostasis and to be impaired following early life stress.

METHODS

We used behavioral measures to evaluate anxiety, social interactions and pain responses in a rat model of neonatal maternal separation (NMS). Using quantitative PCR, we investigated whether NMS was associated with alterations in the expression of chloride transporters in the cerebrum and spinal cord. Finally, we evaluated the contribution of OTergic signaling and neuro-inflammatory processes in the observed phenotype.

RESULTS

NMS animals displayed a long-lasting upregulation of chloride importer Na-K-Cl cotransporter type 1 (NKCC1) expression in the cerebrum and spinal cord. Neonatal administration of the NKCC1 inhibitor bumetanide or oxytocin successfully normalized the anxiety-like symptoms and the lack of social preference observed in NMS animals. Phenotypic alterations were associated with a pro-inflammatory state which could contribute to NKCC1 upregulation.

CONCLUSIONS

This work suggests that an impaired chloride homeostasis, linked to oxytocin signaling dysfunction and to neuro-inflammatory processes, could contribute to the sensori-affective phenotype following NMS.

Effects of α7 Nicotinic Acetylcholine Receptor Positive Allosteric Modulator on BDNF, NKCC1 and KCC2 Expression in the Hippocampus following Lipopolysaccharide-Induced Allodynia and Hyperalgesia in a Mouse Model of Inflammatory Pain

BACKGROUND & OBJECTIVES

Hyperalgesia and allodynia are frequent symptoms of inflammatory pain. Neuronal excitability induced by the Brain-Derived Neurotrophic Factor (BDNF)-tyrosine receptor kinase B (TrkB) cascade has a role in the modulation of inflammatory pain. The effects of 3a,4,5,9b-tetrahydro-4-(1-naphthalenyl)-3H-cyclopentan[c]quinoline-8-sulfonamide (TQS), an α7 nicotinic Acetylcholine Receptor Positive Allosteric Modulator (nAChR PAM), on hippocampal BDNF, cation-chloride cotransporters, NKCC1 and KCC2, expression in inflammatory pain are not known. The objective of the study was to determine the effects of TQS on BDNF, NKCC1, and KCC2 expression in the hippocampus following lipopolysaccharide (LPS)-induced allodynia and hyperalgesia in a mouse model of inflammatory pain.

METHODS

Mice were treated with TQS followed by LPS (1 mg/kg, ip) administration. The effects of TQS on mRNA and BDNF in the hippocampus were examined using qRT-PCR and Western blot, respectively. Immunoreactivity of BDNF, NKCC1, and KCC2 in the hippocampus was measured after LPS administration using immunofluorescence assay. Allodynia and hyperalgesia were determined using von Frey filaments and hot plate, respectively.

RESULTS

The LPS (1 mg/kg) upregulates mRNA of BDNF and downregulates mRNA of KCC2 in the hippocampus and pretreatment of TQS (4 mg/kg) reversed the effects induced by LPS. In addition, the TQS decreased LPS-induced upregulation of BDNF and p-NKCC1 immunoreactivity in the dentate gyrus and CA1 region of the hippocampus. BDNF receptor (TrkB) antagonist, ANA12 (0.50 mg/kg), and NKCC1 inhibitor bumetanide (30 mg/kg) reduced LPS-induced allodynia and hyperalgesia. Blockade of TrkB with ANA12 (0.25 mg/kg) enhanced the effects of TQS (1 mg/kg) against LPS-induced allodynia and hyperalgesia. Similarly, bumetanide (10 mg/kg) enhanced the effects of TQS (1 mg/kg) against allodynia and hyperalgesia.

CONCLUSION

These results suggest that antinociceptive effects of α7 nAChR PAM are associated with downregulation of hippocampal BDNF and p-NKCC1 and upregulation of KCC2 in a mouse model of inflammatory pain.

Involvement of the tuberomammillary nucleus of the hypothalamus in the modulation of nociception and joint edema in a model of monoarthritis

AIMS

Investigate the involvement of the histaminergic projections from tuberomammillary nucleus (TMN) to the spinal cord in the modulation of nociception and peripheral edema in a model of monoarthritis.

MAIN METHODS

Subacute monoarthritis was induced by an intraarticular injection of carrageenan followed by LPS 72 h later. Disability and joint edema were assessed at the 3rd hour after LPS and at every hour up to 6 h.

KEY FINDINGS

Intrathecal administration of histamine potentiated joint incapacitation and edema, while the H1R antagonist cetirizine decreased both. The H3R agonist immepip decreased both incapacitation and edema, while the H3R antagonist thioperamide had the opposite effect. The microinjection of glutamate into the ventral TMN (vTMN) caused an increase of incapacitation and articular edema, whereas the blockade of this nucleus by cobalt chloride inhibited both parameters. Intrathecal administration of cetirizine prevented the increase of incapacitation and joint edema caused by glutamate microinjection into the vTMN. Similarly, an intrathecal injection of the NKCC1 cotransporter inhibitor bumetanide prevented the effects of glutamate microinjection into the vTMN, whereas coadministration of histamine with bumetanide only inhibited the potentiation of joint edema. A microinjection of orexin B into the vTMN potentiated incapacitation and joint edema, while coadministration of the OX1/2 receptor antagonist almorexant with orexin B did not.

SIGNIFICANCE

These data support the notion that TMN participates in the modulation of a peripheral inflammatory process by means of histaminergic projections to the spinal cord, and the hypothalamus may trigger TMN activation by means of glutamate and orexin.

Differential expression of Na+/K+/Cl- cotransporter 1 in neurons and glial cells within the superficial spinal dorsal horn of rodents

Although convincing experimental evidence indicates that Na⁺/K⁺/Cl^- cotransporter 1 (NKCC1) is involved in spinal nociceptive information processing and in the generation of hyperalgesia and allodynia in chronic pain states, the cellular distribution of NKCC1 in the superficial spinal dorsal horn is still poorly understood. Because this important piece of knowledge is missing, the effect of NKCC1 on pain processing is still open to conflicting interpretations. In this study, to provide the missing experimental data, we investigated the cellular distribution of NKCC1 in the superficial spinal dorsal horn by immunohistochemical methods. We demonstrated for the first time that almost all spinal axon terminals of peptidergic nociceptive primary afferents express NKCC1. In contrast, virtually all spinal axon terminals of nonpeptidergic nociceptive primary afferents were negative for NKCC1. Data on the colocalization of NKCC1 with axonal and glial markers indicated that it is almost exclusively expressed by axon terminals and glial cells in laminae I-IIo. In lamina IIi, however, we observed a strong immunostaining for NKCC1 also in the dendrites and cell bodies of PV-containing inhibitory neurons and a weak staining in PKCγ-containing excitatory neurons. Our results facilitate further thinking about the role of NKCC1 in spinal pain processing.

Analgesic and anti-edemic properties of etifoxine in models of inflammatory sensitization

Inflammatory processes are critical promoting factors of chronic pain states, mostly by inducing peripheral and central sensitization of the nociceptive system. These processes are associated with a massive increase in glutamatergic transmission, sometimes facilitated by spinal disinhibition. In this study, we used etifoxine, a non-benzodiazepine anxiolytic known to amplify inhibition mediated by gamma-aminobutyric acid type A (GABA_A) receptors in pain processing regions, either directly (through allosteric modulation) or indirectly (through the synthesis of endogenous neurosteroids). We used different models of local inflammation to evaluate the possible direct action of etifoxine on analgesia and edema. Pain symptom and edema measurements were performed after intraplantar carrageenan injection or after topical ear inflammation. We found that etifoxine treatment was associated with reduced plantar surface temperature 24 h after intraplantar carrageenan injection. In this model, etifoxine also alleviated thermal hot and mechanical hyperalgesia. A similar finding was observed while analyzing pain symptoms in the late phase of the formalin test. In a model of ear inflammation, etifoxine appeared to have a moderate anti-edemic effect after topical application. This slight action of etifoxine on the limitation of inflammatory processes could be mediated in part by cyclo-oxygenase 1 activity inhibition. Etifoxine appears as a promising therapeutic tool contributing to the limitation of inflammatory pain symptoms. Since etifoxine is already prescribed as an anxiolytic in several countries, it could be a good candidate for the prevention of inflammatory-driven edema and hyperalgesia, although the precise mechanism of action relative to its anti-inflammatory potential remains to be elucidated.

Time-dependent changes in paw carrageenan-induced inflammation above and below the level of low thoracic spinal cord injury in rats

STUDY DESIGN

This was an animal study.

OBJECTIVES

Local inflammation is attenuated below high thoracic SCI, where innervation of major lymphoid organs is involved. However, whether inflammatory responses are affected after low thoracic SCI, remains undetermined. The aim of this study was to characterize the influence of low thoracic SCI on carrageenan-induced paw swelling in intact and paralyzed limbs, at acute and subacute stages.

SETTING

University and hospital-based research center, Mexico City, Mexico.

METHODS

Rats received a severe contusive SCI at T9 spinal level or sham injury. Then, 1 and 15 days after lesion, carrageenan or vehicle was subcutaneously injected in forelimb and hindlimb paws. Paw swelling was measured over a 6-h period using a plethysmometer.

RESULTS

Swelling increased progressively reaching the maximum 6 h post-carrageenan injection. Swelling increase in sham-injured rats was approximately 130% and 70% compared with baseline values of forelimbs and hindlimbs, respectively. Paws injected with saline exhibited no measurable swelling. Carrageenan-induced paw swelling 1-day post-SCI was suppressed in both intact and paralyzed limbs. Fifteen days post-injury, the swelling response to carrageenan was completely reestablished in forelimbs, whereas in hindlimbs it remained significantly attenuated compared with sham-injured rats.

CONCLUSIONS

SCI at low spinal level affects the induced swelling response in a different way depending on both, the neurological status of challenged regions and the stage of injury. These findings suggest that neurological compromise of the main immunological organs is not a prerequisite for the local swelling response to be affected after injury.

When Pain Hurts: Nociceptive Stimulation Induces a State of Maladaptive Plasticity and Impairs Recovery after Spinal Cord Injury

Spinal cord injury (SCI) is often accompanied by other tissue damage (polytrauma) that provides a source of pain (nociceptive) input. Recent findings are reviewed that show SCI places the caudal tissue in a vulnerable state that exaggerates the effects nociceptive stimuli and promotes the development of nociceptive sensitization. Stimulation that is both unpredictable and uncontrollable induces a form of maladaptive plasticity that enhances nociceptive sensitization and impairs spinally mediated learning. In contrast, relational learning induces a form of adaptive plasticity that counters these adverse effects. SCI sets the stage for nociceptive sensitization by disrupting serotonergic (5HT) fibers that quell overexcitation. The loss of 5HT can enhance neural excitability by reducing membrane-bound K+-Cl- cotransporter 2, a cotransporter that regulates the outward flow of Cl-. This increases the intracellular concentration of Cl-, which reduces the hyperpolarizing (inhibitory) effect of gamma-aminobutyric acid. Uncontrollable noxious stimulation also undermines the recovery of locomotor function, and increases behavioral signs of chronic pain, after a contusion injury. Nociceptive stimulation has a greater effect if experienced soon after SCI. This adverse effect has been linked to a downregulation in brain-derived neurotrophic factor and an upregulation in the cytokine, tumor necrosis factor. Noxious input enhances tissue loss at the site of injury by increasing the extent of hemorrhage and apoptotic/pyroptotic cell death. Intrathecal lidocaine blocks nociception-induced hemorrhage, cellular indices of cell death, and its adverse effect on behavioral recovery. Clinical implications are discussed.

The role of calcitonin gene-related peptide in peripheral and central pain mechanisms including migraine

Calcitonin gene-related peptide (CGRP) is a 37-amino acid peptide found primarily in the C and Aδ sensory fibers arising from the dorsal root and trigeminal ganglia, as well as the central nervous system. Calcitonin gene-related peptide was found to play important roles in cardiovascular, digestive, and sensory functions. Although the vasodilatory properties of CGRP are well documented, its somatosensory function regarding modulation of neuronal sensitization and of enhanced pain has received considerable attention recently. Growing evidence indicates that CGRP plays a key role in the development of peripheral sensitization and the associated enhanced pain. Calcitonin gene-related peptide is implicated in the development of neurogenic inflammation and it is upregulated in conditions of inflammatory and neuropathic pain. It is most likely that CGRP facilitates nociceptive transmission and contributes to the development and maintenance of a sensitized, hyperresponsive state not only of the primary afferent sensory neurons but also of the second-order pain transmission neurons within the central nervous system, thus contributing to central sensitization as well. The maintenance of a sensitized neuronal condition is believed to be an important factor underlying migraine. Recent successful clinical studies have shown that blocking the function of CGRP can alleviate migraine. However, the mechanisms through which CGRP may contribute to migraine are still not fully understood. We reviewed the role of CGRP in primary afferents, the dorsal root ganglion, and in the trigeminal system as well as its role in peripheral and central sensitization and its potential contribution to pain processing and to migraine.

The chloride co-transporters, NKCC1 and KCC2, in experimental autoimmune encephalomyelitis (EAE)

Patients with multiple sclerosis (MS) often complain of neuropathic pain. According to the Gate Control Theory of Pain, spinal networks of GABAergic inhibitory interneurons are important in modulating nociceptive inputs from the periphery. Na+-K+-2Cl- co-transporter 1 (NKCC1) and K+-Cl- co-transporter 2 (KCC2) generally dictate the tone of GABA/glycine inhibition by regulating intracellular chloride concentrations. In this study, we investigated the role of NKCC1 and KCC2 in neuropathic pain observed in the animal model, experimental autoimmune encephalomyelitis (EAE), a commonly used model to study the pathophysiology of MS. Quantitative real-time polymerase chain reactions (qRT-PCR) analysis revealed no change in NKCC1 mRNA transcripts in dorsal root ganglia throughout EAE disease course. However, NKCC1 and KCC2 mRNA levels in the dorsal spinal cord were significantly reduced at disease onset and peak only to recover by the chronic time point. Similarly, Western blot data revealed a significant downregulation of NKCC1 and KCC2 in the dorsal spinal cord at disease onset but an upregulation of NKCC1 protein in the dorsal root ganglia at this time point. Treatment with bumetanide, an NKCC inhibitor, had no effect on mechanical hypersensitivity seen in mice with EAE even though it reversed the changes in the levels of NKCC1 and KCC2. We noted that bumetanide treatment, while effective at reversing the changes in monomeric KCC2 levels was ineffective at reversing the changes in oligomeric KCC2 which remained repressed. These results indicate that mechanical hypersensitivity in EAE is not mediated by altered levels of NKCC1.

Combined Changes in Chloride Regulation and Neuronal Excitability Enable Primary Afferent Depolarization to Elicit Spiking without Compromising its Inhibitory Effects

The central terminals of primary afferent fibers experience depolarization upon activation of GABAA receptors (GABAAR) because their intracellular chloride concentration is maintained above electrochemical equilibrium. Primary afferent depolarization (PAD) normally mediates inhibition via sodium channel inactivation and shunting but can evoke spikes under certain conditions. Antidromic (centrifugal) conduction of these spikes may contribute to neurogenic inflammation while orthodromic (centripetal) conduction could contribute to pain in the case of nociceptive fibers. PAD-induced spiking is assumed to override presynaptic inhibition. Using computer simulations and dynamic clamp experiments, we sought to identify which biophysical changes are required to enable PAD-induced spiking and whether those changes necessarily compromise PAD-mediated inhibition. According to computational modeling, a depolarizing shift in GABA reversal potential (EGABA) and increased intrinsic excitability (manifest as altered spike initiation properties) were necessary for PAD-induced spiking, whereas increased GABAAR conductance density (ḡGABA) had mixed effects. We tested our predictions experimentally by using dynamic clamp to insert virtual GABAAR conductances with different EGABA and kinetics into acutely dissociated dorsal root ganglion (DRG) neuron somata. Comparable experiments in central axon terminals are prohibitively difficult but the biophysical requirements for PAD-induced spiking are arguably similar in soma and axon. Neurons from naïve (i.e. uninjured) rats were compared before and after pharmacological manipulation of intrinsic excitability, and against neurons from nerve-injured rats. Experimental data confirmed that, in most neurons, both predicted changes were necessary to yield PAD-induced spiking. Importantly, such changes did not prevent PAD from inhibiting other spiking or from blocking spike propagation. In fact, since the high value of ḡGABA required for PAD-induced spiking still mediates strong inhibition, we conclude that PAD-induced spiking does not represent failure of presynaptic inhibition. Instead, diminished PAD caused by reduction of ḡGABA poses a greater risk to presynaptic inhibition and the sensory processing that relies upon it.

Transmission pathways and mediators as the basis for clinical pharmacology of pain

INTRODUCTION

Mediators in pain transmission are the targets of a multitude of different analgesic pharmaceuticals. This review explores the most significant mediators of pain transmission as well as the pharmaceuticals that act on them. Areas covered: The review explores many of the key mediators of pain transmission. In doing so, this review uncovers important areas for further research. It also highlights agents with potential for producing novel analgesics, probes important interactions between pain transmission pathways that could contribute to synergistic analgesia, and emphasizes transmission factors that participate in transforming acute injury into chronic pain. Expert commentary: This review examines current pain research, particularly in the context of identifying novel analgesics, highlighting interactions between analgesic transmission pathways, and discussing factors that may contribute to the development of chronic pain after an acute injury.

Expanding Spectrum of Sodium Potassium Chloride Co-transporters in the Pathophysiology of Diseases

Sodium potassium chloride co-transporter (NKCC) belongs to cation-dependent chloride co-transporter family, whose activation allows the entry of Na(+), K(+) and 2Cl(-) inside the cell. It acts in concert with K(+) Cl(-) co-transporter (KCC), which extrudes K(+) and Cl(-) ions from cell. NKCC1 is widely distributed throughout the body, while NKCC2 is exclusively present in kidney. Protein kinase A, protein kinase C, Ste20-related proline-alanine-rich kinase, oxidative stress responsive kinases, With No K=lysine kinase and protein phosphatase type 1 control the phosphorylation/dephosphorylation of key threonine residues of in regulatory domain of NKCC1. The selective inhibitors of NKCC1 including bumetanide and furosemide are conventionally employed as diuretics. However, recent studies have indicated that NKCC1 may be involved in the pathophysiology of anxiety, cerebral ischemia, epilepsy, neuropathic pain, fragile X syndrome, autism and schizophrenia. The inhibitors of NKCC1 are shown to produce anxiolytic effects; attenuate cerebral ischemia-induced neuronal injury; produce antiepileptic effects and attenuate neuropathic pain. In the early developing brain, GABAA activation primarily produces excitatory actions due to high NKCC1/KCC2 ratio. However, as the development progresses, the ratio of NKCC1/KCC2 ratio reverses and there is switch in the polarity of GABAA actions and latter acquires the inhibitory actions. The recapitulation of developmental-like state during pathological state may be associated with increase in the expression and functioning of NKCC1, which decreases the strength of inhibitory GABAergic neurotransmission. The present review describes the expanding role and mechanism of NKCC1 in the pathophysiology of different diseases.

The Role of Regulatory Transporters in Neuropathic Pain

Neuropathic pain arises from an injury or disease of the somatosensory nervous system rather than stimulation of pain receptors. As a result, the fine balance between excitation and inhibition is perturbed leading to hyperalgesia and allodynia. Various neuropathic pain models provide considerable evidence that changes in the glutamatergic, GABAergic, and monoaminergic systems. Neurotransmitter reuptake transporter proteins have the potential to change the temporal and spatial profile of various neurotransmitters throughout the nervous system. This, in turn, can affect the downstream effects of these neurotransmitters and hence modulate pain. This chapter explores various reuptake transporter systems and implicates their role in pain processing. Understanding the transporter systems will enhance drug discovery targeting different facets of neuropathic pain.

Oxytocin in the paraventricular nucleus attenuates incision-induced mechanical allodynia

Oxytocin (OT) neurons localized in the paraventricular nucleus (PVN) and supraoptic nucleus (SON) send fibers to the brain and spinal cord. While most previous studies have looked at the role of OT in chronic pain, few have investigated the role of OT in acute pain, particularly postoperative pain. In the present study, the role of OT in incision-induced allodynia was explored for the first time, using a rat incisional pain model. Immunohistochemical staining showed that, compared with the baseline (prior to incision) measurements, the OT content in the PVN was significantly decreased at 0.5, 1.0 and 3.0 h post-incision and returned to the baseline level at 6.0 h post-incision. By contrast, there was no significant difference in the OT content in the SON prior to and subsequent to incision. A dose-dependent inhibition of mechanical hypersensitivity was detected 30 min after intracerebroventricular injection of OT (100, 400 or 600 ng) and lasted for 3.0 h. No significant difference was noted, however, between the intrathecal OT injection group (600 ng) and the control group. In conclusion, the present study provides the first in vivo evidence that OT in the PVN predominantly attenuates incision-induced mechanical allodynia at the supraspinal, rather than the spinal, level. This suggests that OT is involved in supraspinal analgesia for postoperative pain.

Spinal presynaptic inhibition in pain control

The gate control theory proposed that the nociceptive sensory information transmitted to the brain relies on an interplay between the inputs from nociceptive and non-nociceptive primary afferent fibers. Both inputs are normally under strong inhibitory control in the spinal cord. Under healthy conditions, presynaptic inhibition activated by non-nociceptive fibers modulates the afferent input from nociceptive fibers onto spinal cord neurons, while postsynaptic inhibition controls the excitability of dorsal horn neurons, and silences the non-nociceptive information flow to nociceptive-specific (NS) projection neurons. However, under pathological conditions, this spinal inhibition may be altered and lead to chronic pain. This review summarizes our knowledge of presynaptic inhibition in pain control, with particular focus on how its alteration after nerve or tissue injury contributes to neuropathic or inflammatory pain syndromes, respectively.

Metaplasticity and behavior: how training and inflammation affect plastic potential within the spinal cord and recovery after injury

Research has shown that spinal circuits have the capacity to adapt in response to training, nociceptive stimulation and peripheral inflammation. These changes in neural function are mediated by physiological and neurochemical systems analogous to those that support plasticity within the hippocampus (e.g., long-term potentiation and the NMDA receptor). As observed in the hippocampus, engaging spinal circuits can have a lasting impact on plastic potential, enabling or inhibiting the capacity to learn. These effects are related to the concept of metaplasticity. Behavioral paradigms are described that induce metaplastic effects within the spinal cord. Uncontrollable/unpredictable stimulation, and peripheral inflammation, induce a form of maladaptive plasticity that inhibits spinal learning. Conversely, exposure to controllable or predictable stimulation engages a form of adaptive plasticity that counters these maladaptive effects and enables learning. Adaptive plasticity is tied to an up-regulation of brain derived neurotrophic factor (BDNF). Maladaptive plasticity is linked to processes that involve kappa opioids, the metabotropic glutamate (mGlu) receptor, glia, and the cytokine tumor necrosis factor (TNF). Uncontrollable nociceptive stimulation also impairs recovery after a spinal contusion injury and fosters the development of pain (allodynia). These adverse effects are related to an up-regulation of TNF and a down-regulation of BDNF and its receptor (TrkB). In the absence of injury, brain systems quell the sensitization of spinal circuits through descending serotonergic fibers and the serotonin 1A (5HT 1A) receptor. This protective effect is blocked by surgical anesthesia. Disconnected from the brain, intracellular Cl^- concentrations increase (due to a down-regulation of the cotransporter KCC2), which causes GABA to have an excitatory effect. It is suggested that BDNF has a restorative effect because it up-regulates KCC2 and re-establishes GABA-mediated inhibition.

Role of TRPV1 and ASIC3 in formalin-induced secondary allodynia and hyperalgesia

BACKGROUND

In the present study we determined the role of transient receptor potential V1 channel (TRPV1) and acid-sensing ion channel 3 (ASIC3) in chronic nociception.

METHODS

1% formalin was used to produce long-lasting secondary allodynia and hyperalgesia in rats. Western blot was used to determine TRPV1 and ASIC3 expression in dorsal root ganglia.

RESULTS

Peripheral ipsilateral, but not contralateral, pre-treatment (-10min) with the TRPV1 receptor antagonists capsazepine (0.03-0.3μM/paw) and A-784168 (0.01-1μM/paw) prevented 1% formalin-induced secondary mechanical allodynia and hyperalgesia in the ipsilateral and contralateral paws. Likewise, peripheral ipsilateral, but not contralateral, pre-treatment with the non-selective and selective ASIC3 blocker benzamil (0.1-10μM/paw) and APETx2 (0.02-2μM/paw), respectively, prevented 1% formalin-induced secondary mechanical allodynia and hyperalgesia in both paws. Peripheral ipsilateral post-treatment (day 6 after formalin injection) with capsazepine (0.03-0.3μM/paw) and A-784168 (0.01-1μM/paw) reversed 1% formalin-induced secondary mechanical allodynia and hyperalgesia in both paws. In addition, peripheral ipsilateral post-treatment with benzamil (0.1-10μM/paw) and APETx2 (0.02-2μM/paw), respectively, reversed 1% formalin-induced secondary mechanical allodynia and hyperalgesia in both paws. TRPV1 and ASIC3 proteins were expressed in dorsal root ganglion in normal conditions, and 1% formalin injection increased expression of both proteins in this location at 1 and 6 days compared to naive rats.

CONCLUSIONS

Data suggest that TRPV1 and ASIC3 participate in the development and maintenance of long-lasting secondary allodynia and hyperalgesia induced by formalin in rats. The use of TRPV1 and ASIC3 antagonists by peripheral administration could prove useful to treat chronic pain.

Inhibition of peripheral anion exchanger 3 decreases formalin-induced pain

We determined the role of chloride-bicarbonate anion exchanger 3 in formalin-induced acute and chronic rat nociception. Formalin (1%) produced acute (first phase) and tonic (second phase) nociceptive behaviors (flinching and licking/lifting) followed by long-lasting evoked secondary mechanical allodynia and hyperalgesia in both paws. Local peripheral pre-treatment with the chloride-bicarbonate anion exchanger inhibitors 4,4'-diisothiocyanatostilbene-2,2'-disulfonic acid and 4-acetamido-4'-isothiocyanato-2,2'-stilbenedisulfonic acid prevented formalin-induced nociception mainly during phase 2. These drugs also prevented in a dose-dependent fashion long-lasting evoked secondary mechanical allodynia and hyperalgesia in both paws. Furthermore, post-treatment (on day 1 or 6) with 4,4'-diisothiocyanatostilbene-2,2'-disulfonic acid reversed established hypersensitivity. Anion exchanger 3 was expressed in dorsal root ganglion neurons and it co-localized with neuronal nuclei protein (NeuN), substance P and purinergic P2X3 receptors. Furthermore, Western blot analysis revealed a band of about 85 kDa indicative of anion exchanger 3 protein expression in dorsal root ganglia of naïve rats, which was enhanced at 1 and 6 days after 1% formalin injection. On the other hand, this rise failed to occur during 4,4'-diisothiocyanatostilbene-2,2'-disulfonic acid exposure. These results suggest that anion exchanger 3 is present in dorsal root ganglia and participates in the development and maintenance of short and long-lasting formalin-induced nociception.

Anoctamin 1 mediates thermal pain as a heat sensor

Vertebrates can sense and avoid noxious heat that evokes pain. Many thermoTRP channels are associated with temperature sensation. TRPV1 is a representative ion channel that is activated by noxious heat. Anoctamin 1 (ANO1) is a Cl- channel activated by calcium that is highly expressed in small sensory neurons, colocalized with markers for nociceptors, and most surprisingly, activated by noxious heat over 44oC. Although ANO1 is a Cl- channel, opening of this channel leads to depolarization of sensory neurons, suggesting a role in nociception. Indeed, the functional deletion of ANO1 in sensory neurons triggers the reduction in thermal pain sensation. Thus, it seems clear that ANO1 is a heat sensor in a nociceptive pathway. Since ANO1 modulators are developed for the purpose of treating chronic diseases such as cystic fibrosis, this finding is likely to predict unwanted effects and provide a guide for better developmental strategy

Trigeminal ganglion neurons of mice show intracellular chloride accumulation and chloride-dependent amplification of capsaicin-induced responses

Intracellular Cl⁻ concentrations ([Cl⁻]_i) of sensory neurons regulate signal transmission and signal amplification. In dorsal root ganglion (DRG) and olfactory sensory neurons (OSNs), Cl⁻ is accumulated by the Na⁺-K⁺-2Cl⁻ cotransporter 1 (NKCC1), resulting in a [Cl⁻]_i above electrochemical equilibrium and a depolarizing Cl⁻ efflux upon Cl⁻ channel opening. Here, we investigate the [Cl⁻]_i and function of Cl⁻ in primary sensory neurons of trigeminal ganglia (TG) of wild type (WT) and NKCC1^−/− mice using pharmacological and imaging approaches, patch-clamping, as well as behavioral testing. The [Cl⁻]_i of WT TG neurons indicated active NKCC1-dependent Cl⁻ accumulation. Gamma-aminobutyric acid (GABA)_A receptor activation induced a reduction of [Cl⁻]_i as well as Ca²⁺ transients in a corresponding fraction of TG neurons. Ca²⁺ transients were sensitive to inhibition of NKCC1 and voltage-gated Ca²⁺ channels (VGCCs). Ca²⁺ responses induced by capsaicin, a prototypical stimulus of transient receptor potential vanilloid subfamily member-1 (TRPV1) were diminished in NKCC1^−/− TG neurons, but elevated under conditions of a lowered [Cl⁻]_o suggesting a Cl⁻-dependent amplification of capsaicin-induced responses. Using next generation sequencing (NGS), we found expression of different Ca²⁺-activated Cl⁻ channels (CaCCs) in TGs of mice. Pharmacological inhibition of CaCCs reduced the amplitude of capsaicin-induced responses of TG neurons in Ca²⁺ imaging and electrophysiological recordings. In a behavioral paradigm, NKCC1^−/− mice showed less avoidance of the aversive stimulus capsaicin. In summary, our results strongly argue for a Ca²⁺-activated Cl⁻-dependent signal amplification mechanism in TG neurons that requires intracellular Cl⁻ accumulation by NKCC1 and the activation of CaCCs.

Inflammatory mediators potentiate high affinity GABA(A) currents in rat dorsal root ganglion neurons

Following acute tissue injury action potentials may be initiated in afferent processes terminating in the dorsal horn of the spinal cord that are propagated back out to the periphery, a process referred to as a dorsal root reflex (DRR). The DRR is dependent on the activation of GABA(A) receptors. The prevailing hypothesis is that DRR is due to a depolarizing shift in the chloride equilibrium potential (E(Cl)) following an injury-induced activation of the Na(+)-K(+)-Cl(-)-cotransporter. Because inflammatory mediators (IM), such as prostaglandin E(2) are also released in the spinal cord following tissue injury, as well as evidence that E(Cl) is already depolarized in primary afferents, an alternative hypothesis is that an IM-induced increase in GABA(A) receptor mediated current (I(GABA)) could underlie the injury-induced increase in DRR. To test this hypothesis, we explored the impact of IM (prostaglandin E(2) (1 μM), bradykinin (10 μM), and histamine (1 μM)) on I(GABA) in dissociated rat dorsal root ganglion (DRG) neurons with standard whole cell patch clamp techniques. IM potentiated I(GABA) in a subpopulation of medium to large diameter capsaicin insensitive DRG neurons. This effect was dependent on the concentration of GABA, manifest only at low concentrations (<10 μM). THIP evoked current were also potentiated by IM and GABA (1 μM) induced tonic currents enhanced by IM were resistant to gabazine (20 μM). The present data are consistent with the hypothesis that an acute increase in I(GABA) contributes to the emergence of injury-induced DRR.

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.

Inhibition of KCC2 in mouse spinal cord neurons leads to hypersensitivity to thermal stimulation

BACKGROUND

KCC2, a neuronal-specific K-Cl cotransporter, is involved in pain perception physiology through its effects on postsynaptic inhibition in spinal cord neurons. We injected a newly identified, highly potent and selective inhibitor of KCC2 (D4), an inactive structural variant (D4.14), and the Na-K-2Cl cotransporter (NKCC1) inhibitor, bumetanide, into the intrathecal space of mice to measure their effect on heat-evoked nociceptive responses.

METHODS

Commercially available intrathecal catheters were modified and surgically placed into 2 cohorts of 10 mice. After recovery from the procedure, the mice were injected with D4, D4.14, and bumetanide through this catheter. Nociceptive measurements (hotplate assay, tail flick assay) were performed after injection of each of the test drugs and compared with vehicle controls.

RESULTS

Two mice in each cohort were omitted because of postprocedure complications. There was a statistically significant decrease (P < 0.01) in withdrawal latency after injection of the active KCC2 inhibitor but not after injection of the inactive compound (P = 0.78), as measured by hotplate assay at 55°C. Injection of bumetanide significantly increased withdrawal latency (P = 0.02) at the same temperature. These results were confirmed using tail flick assays performed at 49°C.

CONCLUSIONS

Inhibition of KCC2 by D4 led to decreased heat-evoked withdrawal latency in mice, as measured by hotplate and tail flick assays, whereas inhibition of NKCC1 by bumetanide resulted in increased response latencies to heat stimuli as measured by both of these nociceptive tests.

Deleterious impact of a γ-aminobutyric acid type A receptor preferring general anesthetic when used in the presence of persistent inflammation

BACKGROUND

Experimental data suggest general anesthetics preferring γ-aminobutyric acid receptor type A may increase postoperative pain in patients with persistent inflammation. The current study was designed to begin to test this hypothesis.

METHODS

Groups of rats were defined by the presence of inflammation, surgical intervention, and/or the type of general anesthetic used for a 3-h period of anesthesia. Persistent inflammation was induced with complete Freund adjuvant. The surgical intervention was a plantar incision. Three mechanistically distinct general anesthetics were used: pentobarbital, ketamine/xylazine, and isoflurane. Ongoing pain and hypersensitivity were assessed with guarding behavior analysis and the von Frey test, respectively.

RESULTS

There was no influence of general anesthetic type on the magnitude or time course of recovery from postoperative hypersensitivity in the absence of persistent inflammation. However, in the presence of persistent inflammation, recovery from hypersensitivity was significantly slower in the pentobarbital group than in the ketamine/xylazine or isoflurane groups. The pentobarbital effect was significant within 3 days of surgery and persisted through the remainder of the testing period. A comparable delay in recovery was observed in pentobarbital-anesthetized inflamed rats not subjected to hind paw incision. The time to 50% recovery in the pentobarbital-treated inflamed groups was almost double that in the other groups. No differences were observed between ketamine/xylazine and isoflurane. Pentobarbital exposure did not increase guarding scores.

CONCLUSIONS

These results suggest that general anesthetics preferring γ-aminobutyric acid receptor type A may have deleterious consequences when used in the presence of persistent inflammation.

Newborn Analgesia Mediated by Oxytocin during Delivery

The mechanisms controlling pain in newborns during delivery are poorly understood. We explored the hypothesis that oxytocin, an essential hormone for labor and a powerful neuromodulator, exerts analgesic actions on newborns during delivery. Using a thermal tail-flick assay, we report that pain sensitivity is two-fold lower in rat pups immediately after birth than 2 days later. Oxytocin receptor antagonists strongly enhanced pain sensitivity in newborn, but not in 2-day-old rats, whereas oxytocin reduced pain at both ages suggesting an endogenous analgesia by oxytocin during delivery. Similar analgesic effects of oxytocin, measured as attenuation of pain-vocalization induced by electrical whisker pad stimulation, were also observed in decerebrated newborns. Oxytocin reduced GABA-evoked calcium responses and depolarizing GABA driving force in isolated neonatal trigeminal neurons suggesting that oxytocin effects are mediated by alterations of intracellular chloride. Unlike GABA signaling, oxytocin did not affect responses mediated by P2X3 and TRPV1 receptors. In keeping with a GABAergic mechanism, reduction of intracellular chloride by the diuretic NKCC1 chloride co-transporter antagonist bumetanide mimicked the analgesic actions of oxytocin and its effects on GABA responses in nociceptive neurons. Therefore, endogenous oxytocin exerts an analgesic action in newborn pups that involves a reduction of the depolarizing action of GABA on nociceptive neurons. Therefore, the same hormone that triggers delivery also acts as a natural pain killer revealing a novel facet of the protective actions of oxytocin in the fetus at birth.

Role of peripheral 5-HT(4), 5-HT(6), and 5-HT(7) receptors in development and maintenance of secondary mechanical allodynia and hyperalgesia

The role of 5-hydroxytryptamine (5-HT)(4), 5-HT(6), and 5-HT(7) receptors in formalin-induced secondary allodynia and hyperalgesia in rats was assessed. Formalin produced acute nociceptive behaviors (flinching and licking/lifting) followed by long-term secondary mechanical allodynia and hyperalgesia. Pretreatment (-10min) with cromoglycate (195-1950nmol/paw) partially inhibited acute nociceptive behaviors and completely prevented secondary allodynia and hyperalgesia on day 6 after injection. Ipsilateral peripheral pretreatment with the selective 5-HT(4) (ML-10302, 1-100nmol/paw), 5-HT(6) (EMD-386088, 0.001-0.01nmol/paw), and 5-HT(7) (LP-12, 0.01-100nmol/paw) receptor agonists significantly increased secondary allodynia and hyperalgesia in both paws. In contrast, ipsilateral peripheral pretreatment with the selective 5-HT(4) (GR-125487, 1-100nmol/paw), 5-HT(6) (SB-258585, 0.00001-0.001nmol/paw), and 5-HT(7) (SB-269970, 0.1-10nmol/paw) receptor antagonists significantly prevented formalin-induced secondary allodynia and hyperalgesia in both paws. The pronociceptive effect of ML-10302 (100nmol/paw), EMD-386088 (0.01nmol/paw), and LP-12 (100nmol/paw) were completely prevented by GR-125487 (5-HT(4) antagonist, 1nmol/paw), SB-258585 (5-HT(6) antagonist, 0.00001nmol/paw), and SB-269970 (5-HT(7), antagonist, 0.01nmol/paw), respectively. Ipsilateral peripheral posttreatment with cromoglycate or GR-125487 (1-100nmol/paw), SB-258585 (0.001-0.1nmol/paw), and SB-269970 (0.1-10nmol/paw) reversed formalin-induced secondary allodynia and hyperalgesia in both paws. Results suggest that a barrage of afferent input induced by 5-HT at peripheral 5-HT(4), 5-HT(6), and 5-HT(7) receptors participate in the development and maintenance of formalin-induced long-term secondary allodynia and hyperalgesia in the rat. 5-hydroxytryptamine (5-HT) released in peripheral tissues after formalin injection sensitized primary afferent neurons via 5-HT(4), 5-HT(6), and 5-HT(7) receptors, leading to development and maintenance of secondary allodynia and hyperalgesia.

Role of the NKCC1 co-transporter in sensitization of spinal nociceptive neurons

The Na(+), K(+), 2Cl(-) co-transporter type 1 (NKCC1) plays a pivotal role in hyperalgesia associated with inflammatory stimuli. NKCC1 contributes to maintain high [Cl(-)](i) in dorsal root ganglia (DRG) neurons which cause primary afferent depolarization (PAD) when GABA(A) receptors are activated. Enhanced GABA-induced depolarization, through increased NKCC1 activity, has been hypothesized to produce orthodromic spike activity of sufficient intensity to account for touch-induced pain. In the present study, we investigate this hypothesis using in vivo electrophysiology on rat dorsal horn neurons; the effects of spinal blockade of NKCC1 on intraplantar capsaicin-induced sensitization of dorsal horn neurons were examined. Single wide dynamic range (WDR) and nociceptive specific (NS) neuron activity in the dorsal horn was recorded using glass microelectrodes in anesthetized rats. Dorsal horn neurons with a receptive field on the plantar surface of the hindpaw were studied. Neuronal responses to mechanical stimuli (brush, von Frey filaments) were recorded ten minutes before intraplantar injection of 0.3 ml 0.1% capsaicin (CAP), 40 min after CAP and 15 min after local application of the NKCC1 blocker bumetanide (BTD; 500 μM) on the spinal cord. After CAP, low and high threshold stimulation of the cutaneous receptive field produced a significant enhancement in spike frequency over pre-CAP values in both WDR and NS neurons. Spinal BTD application reduced the spike frequency to baseline levels as well as attenuated the CAP-induced increases in background activity. Our data support the hypothesis that NKCC1 plays an important role in the sensitization of dorsal horn neurons following a peripheral inflammatory insult.

Aquaporins in spinal cord injury: the janus face of aquaporin 4

Although malfunction of spinal cord water channels (aquaporins, AQP) likely contributes to severe disturbances in ion/water homeostasis after spinal cord injury (SCI), their roles are still poorly understood. Here we report and discuss the potential significance of changes in the AQP4 expression in human SCI that generates glial fibrillary acidic protein (GFAP)-labeled astrocytes devoid of AQP4, and GFAP-labeled astroglia that overexpress AQP4. We used a rat model of contusion SCI to study observed changes in human SCI. AQP4-negative astrocytes are likely generated during the process of SCI-induced replacement of lost astrocytes, but their origin and role in SCI remains to be investigated. We found that AQP4-overexpression is likely triggered by hypoxia. Our transcriptional profiling of injured rat cords suggests that elevated AQP4-mediated water influx accompanies increased uptake of chloride and potassium ions which represents a protective astrocytic reaction to hypoxia. However, unbalanced water intake also results in astrocytic swelling that can contribute to motor impairment, but likely only in milder injuries. In severe rat SCI, a low abundance of AQP4-overexpressing astrocytes was found during the motor recovery phase. Our results suggest that severe rat contusion SCI is a better model to analyze AQP4 functions after SCI. We found that AQP4 increases in the chronic post-injury phase are associated with the development of pain-like behavior in SCI rats, while possible mechanisms underlying pain development may involve astrocytic swelling-induced glutamate release. In contrast, the formation and size of fluid-filled cavities occurring later after SCI does not appear to be affected by the extent of increased AQP4 levels. Therefore, the effect of therapeutic interventions targeting AQP4 will depend not only on the time interval after SCI or animal models, but also on the balance between protective role of increased AQP4 in hypoxia and deleterious effects of ongoing astrocytic swelling.

Kinase regulation of Na+-K+-2Cl- cotransport in primary afferent neurons

The Na(+)-K(+)-2Cl(-) cotransporter NKCC1 is expressed in sensory neurons where it accumulates intracellular Cl(-) and facilitates primary afferent depolarization. Depolarization of primary afferent fibre terminals interferes with the gating of incoming sensory signals to the spinal cord. The cotransporter belongs to a family of ion transporters which are sensitive to changes in cell volume. Cell shrinkage, through mechanisms that are still unknown, leads to the phosphorylation and activation of NKCC1. Similarly, axotomy results in increased NKCC1 phosphorylation in dorsal root ganglion (DRG) neurons. This review summarizes the work on the kinases that directly mediate NKCC1 activation. These are the sterile-20-like kinases SPAK and OSR1. Upon their activation through phosphorylation by upstream kinases, SPAK and OSR1 bind to specific peptides located in the cytosolic N-terminal tail of NKCC1, phosphorylate, and stimulate cotransport activity. Expression of SPAK and OSR1 varies from tissue to tissue, but in DRG neurons and in spinal cord, SPAK and OSR1 expression levels are similar. In DRG neurons, both kinases participate in the modulation of NKCC1, as the knockdown of one kinase only results in a partial decrease of NKCC1 function, while the knockdown of both kinases is additive. The identity of the kinases (e.g. WNK kinases) that possibly act upstream of SPAK and OSR1 is also discussed.

Role of opioid receptors in the reduction of formalin-induced secondary allodynia and hyperalgesia in rats

This study assesses the effects of peripheral or intrathecal pre-treatment or post-treatment with micro, delta, kappa and nociceptin/orphanin FQ (NOP) opioid receptor agonists (morphine, U-50488 [trans-(+/-)-3,4-dichloro-N-methyl-N-[2-(1-pyrrolidinyl)cyclohexyl]benzeneacetamide hydrochloride], DADLE [D-Ala2-Leu5-enkephalin] and nociceptin, respectively) on formalin-induced secondary mechanical allodynia and hyperalgesia in rats. 1% Formalin injection produced acute nociceptive behaviors (flinching and licking/lifting) followed by long-term tactile secondary allodynia and hyperalgesia. Neither peripheral (into the formalin-injected paw) nor intrathecal morphine post-treatment reversed formalin-induced secondary allodynia and hyperalgesia. In contrast, morphine pre-treatment prevented the development of these pain behaviors. Intrathecal and peripheral post- but not pre-treatment with U-50488 or DADLE significantly reduced secondary allodynia and hyperalgesia. Interestingly, nociceptin reduced both pain behaviors regardless of the administration site or treatment time. Local antinociceptive effects of morphine, DADLE, U-50488 or nociceptin were blocked by naltrexone, naltrindole, 5-guanidinonaltrindole and [Nphe(1)]nociceptin(1-13)NH(2), respectively. These results suggest that the long-term nociceptive behaviors induced by formalin are differentially modulated by selective opioid receptor agonists. In addition, data suggest that peripheral and spinal delta and kappa opioid receptors are important when nociceptive behaviors are established. In contrast, micro opioid receptors are more important at the beginning of the injury when the sensory system has not changed. NOP receptors participate diminishing both the development and maintenance of nociceptive behaviors. Results suggest that a barrage of afferent input induced by formalin injection initiates a long-term differential change in peripheral and spinal processing that affect the efficacy of opioid receptor agonists.

Increased capsaicin receptor TRPV1 in the peritoneum of women with chronic pelvic pain

OBJECTIVE

To investigate the expression of capsaicin receptor [transient receptor potential vanilloid type-1 (TRPV1)] in the peritoneum of women with chronic pelvic pain (CPP).

METHODS

A case-control study was conducted on 25 women with CPP and 10 controls. Samples of the rectouterine excavation (2 cm) were obtained by laparoscopy, fixed in 4% formaldehyde, and underwent immunohistochemistry analysis using rabbit anti-TRPV1 (1:400) polyclonal antibodies and anti-protein gene product 9.5 (PGP 9.5) (1:2000) as a neuronal marker. Ten sequential images of high magnification fields (x40) were captured from each slide and the area identified with the antibody was calculated with Kontron V2.0 software.

RESULTS

Immunoreactivity to TRPV1 was sparsely detected in the nervous tissue and epithelium of endometriotic lesions. The percent area of immunoreactivity for TRPV1 [expressed as median (range)] was greater in specimens from women with CPP, 1.02% (0.54 to 2.93), than from women without the disease, 0.14% (0.07 to 1.12) (P<0.0001). This greater expression was not secondary to an increase in neuronal fibers because there was also a significant difference in the percent area TRPV1:PGP 9.5 ratio between women with CPP, 1.18 (0.26 to 4.63), and controls, 0.15 (0.06 to 0.95) (P=0.0003).

DISCUSSION

TRPV1 may play an important role in the maintenance and perpetuation of symptoms in women with CPP. In view of the immunoreactivity detected for TRPV1, the endometriotic lesion may have the ability to interfere with nociception or with the inflammatory peritoneal environment in women with CPP. Further studies are needed to elucidate the participation of TRPV1 in CPP and its association with endometriosis.

Sex differences in inflammation evoked by noxious chemical, heat and electrical stimulation

Neurogenic inflammation (NI) is a feature of several inflammatory pain conditions in which females are overrepresented. Therefore, we asked if there are sex differences in the inflammatory response evoked by well known neurogenic stimuli. We compared the amount of plasma extravasation (PE), a measure of inflammation, in the hindpaw skin of male and female rats caused by subcutaneous injection of capsaicin, application of noxious heat (51 degrees C water bath) or electrical stimulation of the saphenous nerve. We also compared the amount of PE in males and females evoked by substance P (SP), the principal neurogenic mediator of PE. PE was quantified using a video camera and digital image analysis to measure changes in reflectance (pixel intensity, PI) of skin due to accumulation of extravasated Evans blue (EB) dye. The increase in PI induced by capsaicin was significantly greater in females compared to males (p<0.001) and in estrus, diestrus, and metestrus females compared to proestrus females. The time to reach maximal capsaicin-induced PE was two times longer in estrus, diestrus, and metestrus females compared to males (p<0.05). PE induced by heat was also significantly greater in females compared to males (p<0.001), however, there was no sex-related difference in PE induced by electrical stimulation or by injection of SP. These findings show that females have a greater inflammatory response when inflammation is induced by capsaicin and noxious heat suggesting possible sex-related changes in TRPV-1 receptor mediated mechanisms. These results add to the growing list of sex difference responses to noxious somatic stimulation.

Chloride regulation in the pain pathway

Melzack and Wall's Gate Control Theory of Pain laid the theoretical groundwork for a role of spinal inhibition in endogenous pain control. While the Gate Control Theory was based on the notion that spinal inhibition is dynamically regulated, mechanisms underlying the regulation of inhibition have turned out to be far more complex than Melzack and Wall could have ever imagined. Recent evidence indicates that an exquisitely sensitive form of regulation involves changes in anion equilibrium potential (E(anion)), which subsequently impacts fast synaptic inhibition mediated by GABA(A), and to a lesser extent, glycine receptor activation, the prototypic ligand gated anion channels. The cation-chloride co-transporters (in particular NKCC1 and KCC2) have emerged as proteins that play a critical role in the dynamic regulation of E(anion) which in turn appears to play a critical role in hyperalgesia and allodynia following peripheral inflammation or nerve injury. This review summarizes the current state of knowledge in this area with particular attention to how such findings relate to endogenous mechanisms of hyperalgesia and allodynia and potential applications for therapeutics based on modulation of intracellular Cl(-) gradients or pharmacological interventions targeting GABA(A) receptors.

Increases in transient receptor potential vanilloid-1 mRNA and protein in primary afferent neurons stimulated by protein kinase C and their possible role in neurogenic inflammation

A recent study by our group demonstrates pharmacologically that the transient receptor potential vanilloid-1 (TRPV(1)) is activated by intradermal injection of capsaicin to initiate neurogenic inflammation by the release of neuropeptides in the periphery. In this study, expression of TRPV(1), phosphorylated protein kinase C (p-PKC), and calcitonin gene-related peptide (CGRP) in dorsal root ganglion (DRG) neurons was visualized by using immunofluorescence, real-time PCR, and Western blots to examine whether increases in TRPV(1) mRNA and protein levels evoked by capsaicin injection are subject to modulation by the activation of PKC and to analyze the role of this process in the pathogenesis of neurogenic inflammation. Capsaicin injection into the hindpaw skin of anesthetized rats evoked increases in the expression of TRPV(1), CGRP and p-PKC in mRNA and/or protein levels and in the number of single labeled TRPV(1), p-PKC, and CGRP neurons in ipsilateral L4-5 DRGs. Coexpressions of TRPV(1) with p-PKC and/or CGRP in DRG neurons were also significantly increased after CAP injection. These evoked expressions at both molecular and cellular levels were significantly inhibited after TRPV(1) receptors were blocked by 5'-iodoresiniferatoxin (5 microg) or PKC was inhibited by chelerythrine chloride (5 microg). Taken together, these results provide evidence that up-regulation of TRPV(1) mRNA and protein levels under inflammatory conditions evoked by capsaicin injection is subject to modulation by the PKC cascade in which increased CGRP level in DRG neurons may be related to the initiation of neurogenic inflammation. Thus, up-regulation of TRPV(1) receptors in DRG neurons seems critical for initiating acute neurogenic inflammation.

Differential contribution of the Na(+)-K(+)-2Cl(-) cotransporter NKCC1 to chloride handling in rat embryonic dorsal root ganglion neurons and motor neurons

Plasma membrane chloride (Cl(-)) pathways play an important role in neuronal physiology. Here, we investigated the role of NKCC1 cotransporters (a secondary active Cl(-) uptake mechanism) in Cl(-) handling in cultured rat dorsal root ganglion neurons (DRGNs) and motor neurons (MNs) derived from fetal stage embryonic day 14. Gramicidin-perforated patch-clamp recordings revealed that DRGNs accumulate intracellular Cl(-) through a bumetanide- and Na(+)-sensitive mechanism, indicative of the functional expression of NKCC1. Western blotting confirmed the expression of NKCC1 in both DRGNs and MNs, but immunocytochemistry experiments showed a restricted expression in dendrites of MNs, which contrasts with a homogeneous expression in DRGNs. Both MNs and DRGNs could be readily loaded with or depleted of Cl(-) during GABA(A) receptor activation at depolarizing or hyperpolarizing membrane potentials. After loading, the rate of recovery to the resting Cl(-) concentration (i.e., [Cl(-)](i) decrease) was similar in both cell types and was unaffected by lowering the extracellular Na(+) concentration. In contrast, the recovery on depletion (i.e., [Cl(-)](i) increase) was significantly faster in DRGNs in control conditions but not in low extracellular Na(+). The experimental observations could be reproduced by a mathematical model for intracellular Cl(-) kinetics, in which DRGNs show higher NKCC1 activity and smaller Cl(-)-handling volume than MNs. On the basis of these results, we conclude that embryonic DRGNs show a higher somatic functional expression of NKCC1 than embryonic MNs. The high NKCC1 activity in DRGNs is important for maintaining high [Cl(-)](i), whereas lower NKCC1 activity in MNs allows large [Cl(-)](i) variations during neuronal activity.

Sensitization of primary afferent nociceptors induced by intradermal capsaicin involves the peripheral release of calcitonin gene-related Peptide driven by dorsal root reflexes

Neuropeptides released from axons of primary afferent nociceptive neurons are the key elements for the incidence of neurogenic inflammation and their release is associated with dorsal root reflexes (DRRs). However, whether the release is due to the triggering of DRRs and plays a role in inflammation-induced pain still remain to be determined. The present study assessed the role of calcitonin gene-related peptide (CGRP) in sensitization of primary afferent nociceptors induced by activation of transient receptor potential vanilloid-1 (TRPV(1)) after intradermal injection of capsaicin and determined if this release is due to activation of primary afferent neurons antidromically by triggering of DRRs. Under dorsal root intact conditions, primary afferent nociceptive fibers recorded in anesthetized rats could be sensitized by capsaicin injection, as shown by an increase in afferent responses and lowering of the response threshold to mechanical stimuli. After DRRs were removed by dorsal rhizotomy, the capsaicin-evoked sensitization was significantly reduced. In dorsal root intact rats, peripheral pretreatment with a CGRP receptor antagonist could dose-dependently reduce the capsaicin-induced sensitization. Peripheral post-treatment with CGRP could dose-dependently restore the capsaicin-induced sensitization under dorsal rhizotomized conditions. Capsaicin injection evoked increases in numbers of single and double labeled TRPV(1) and CGRP neurons in ipsilateral dorsal root ganglia (DRG). After dorsal rhizotomy, these evoked expressions were significantly inhibited.

PERSPECTIVE

These data indicate that the DRR-mediated neurogenic inflammation enhances sensitization of primary afferent nociceptors induced by capsaicin injection. The underlying mechanism involves antidromic activation of DRG neurons via upregulation of TRPV(1) receptors whereby CGRP is released peripherally.

Modulation of chloride homeostasis by inflammatory mediators in dorsal root ganglion neurons

Background

Chloride currents in peripheral nociceptive neurons have been implicated in the generation of afferent nociceptive signals, as Cl^- accumulation in sensory endings establishes the driving force for depolarizing, and even excitatory, Cl^- currents. The intracellular Cl^- concentration can, however, vary considerably between individual DRG neurons. This raises the question, whether the contribution of Cl^- currents to signal generation differs between individual afferent neurons, and whether the specific Cl^- levels in these neurons are subject to modulation. Based on the hypothesis that modulation of the peripheral Cl^- homeostasis is involved in the generation of inflammatory hyperalgesia, we examined the effects of inflammatory mediators on intracellular Cl^- concentrations and on the expression levels of Cl^- transporters in rat DRG neurons.

Results

We developed an in vitro assay for testing how inflammatory mediators influence Cl^- concentration and the expression of Cl^- transporters. Intact DRGs were treated with 100 ng/ml NGF, 1.8 μM ATP, 0.9 μM bradykinin, and 1.4 μM PGE₂ for 1–3 hours. Two-photon fluorescence lifetime imaging with the Cl^--sensitive dye MQAE revealed an increase of the intracellular Cl^- concentration within 2 hours of treatment. This effect coincided with enhanced phosphorylation of the Na⁺-K⁺-2Cl^- cotransporter NKCC1, suggesting that an increased activity of that transporter caused the early rise of intracellular Cl^- levels. Immunohistochemistry of NKCC1 and KCC2, the main neuronal Cl^- importer and exporter, respectively, exposed an inverse regulation by the inflammatory mediators. While the NKCC1 immunosignal increased, that of KCC2 declined after 3 hours of treatment. In contrast, the mRNA levels of the two transporters did not change markedly during this time. These data demonstrate a fundamental transition in Cl^- homeostasis toward a state of augmented Cl^- accumulation, which is induced by a 1–3 hour treatment with inflammatory mediators.

Conclusion

Our findings indicate that inflammatory mediators impact on Cl^- homeostasis in DRG neurons. Inflammatory mediators raise intracellular Cl^- levels and, hence, the driving force for depolarizing Cl^- efflux. These findings corroborate current concepts for the role of Cl^- regulation in the generation of inflammatory hyperalgesia and allodynia. As the intracellular Cl^- concentration rises in DRG neurons, afferent signals can be boosted by excitatory Cl^- currents in the presynaptic terminals. Moreover, excitatory Cl^- currents in peripheral sensory endings may also contribute to the generation or modulation of afferent signals, especially in inflamed tissue.

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.