Inflammation-induced shift in spinal GABA(A) signaling is associated with a tyrosine kinase-dependent increase in GABA(A) current density in nociceptive afferents

Peripheral GABAA receptor signaling contributes to visceral hypersensitivity in a mouse model of colitis

ABSTRACT

Pain is a common and debilitating symptom of inflammatory bowel disease (IBD). Based on evidence that peripheral GABAA receptor (GAR) inhibition plays an important role in establishing colonic afferent excitability and nociceptive threshold, we hypothesized that the increase in pain associated with IBD is due to, at least in part, a decrease in peripheral GAR-mediated inhibition. Acute colitis was induced with 5 days of dextran sodium sulfate (DSS, 3%) in the drinking water. Visceral sensitivity was assessed with the visceromotor response (VMR) evoked with balloon distention of the colon in control and DSS-treated mice before and after intracolonic administration of GAR agonist muscimol, the high-affinity GAR preferring agonist 4,5,6,7-tetrahydroisoxazolo[5,4-c]pyridine-3-ol (THIP), the GAR positive allosteric modulator diazepam, or the GAR antagonists gabazine and bicuculline. Low concentrations of muscimol or THIP increased the VMR in DSS-treated mice but not in control mice. However, high concentrations of muscimol decreased the VMR in both control and DSS-treated mice. Diazepam decreased the VMR in both DSS-treated and control mice. By contrast, at a concentration of gabazine that blocks only low-affinity GAR, there was no effect on the VMR in either DSS-treated or control mice, but at concentrations of the antagonist that block low-affinity and high-affinity GAR, the VMR was increased in control mice and decreased in DSS-treated mice. Furthermore, bicuculline increased the VMR in control mice but decreased it in DSS-treated mice. These data suggest that activating of low-affinity GAR or blocking high-affinity GAR may be effective therapeutic strategies for the management of pain in IBD.

Central sensitization of dorsal root potentials and dorsal root reflexes: An in vitro study in the mouse spinal cord

BACKGROUND

Axo-axonic contacts onto central terminals of primary afferents modulate sensory inputs to the spinal cord. These contacts produce primary afferent depolarization (PAD), which serves as a mechanism for presynaptic inhibition, and also produce dorsal root reflexes (DRRs), which may regulate the excitability of peripheral terminals and second order neurons. We aimed to identify changes in these responses as a consequence of peripheral inflammation.

METHODS

In vitro spinal cord recordings of spontaneous activities in dorsal and ventral roots were performed in control mice and following paw inflammation. We also used pharmacological assays to define the neurotransmitter systems implicated in such responses.

RESULTS

Paw inflammation increased the frequency and amplitude of spontaneous dorsal root depolarizations, the occurrence of DRRs and the amplitude of ventral roots depolarizations. PAD was classified in two different patterns based on their relation to ventral activity: time-locked and independent events. Both patterns increased in amplitude after paw inflammation, and independent events also increased in frequency. The circuits that were responsible for this activity implicated both glutamatergic and GABAergic transmission. Adrenergic modulation differentially affected both types of PAD, and this modulation changed after paw inflammation.

CONCLUSIONS

Our findings suggest the existence of independent spinal circuits at the origin of PAD and DRRs. Inflammation modulates these circuits differentially, unveiling varied mechanisms of spinal sensitization. This in vitro approach provides an isolated model for the study of the mechanisms of central sensitization and for the performance of pharmacological assays with the purpose of identifying and testing novel antinociceptive targets.

SIGNIFICANCE

Spinal circuits modulate activity of primary afferents acting on central terminals. Under in vitro conditions, dorsal roots show spontaneous activity in the form of depolarizations and action potentials. Our findings are consistent with the existence of several independent generator circuits. Experimental paw inflammation reduced mechanical withdrawal threshold and significantly increased the spontaneous activity of dorsal roots, which may be secondary to an enhanced output of spinal generators. This can be considered as a novel sign of central sensitization.

Tyrosine Kinase Inhibitors Reduce NMDA NR1 Subunit Expression, Nuclear Translocation, and Behavioral Pain Measures in Experimental Arthritis

In the lumbar spinal cord dorsal horn, release of afferent nerve glutamate activates the neurons that relay information about injury pain. Here, we examined the effects of protein tyrosine kinase (PTK) inhibition on NMDA receptor NR1 subunit protein expression and subcellular localization in an acute experimental arthritis model. PTK inhibitors genistein and lavendustin A reduced cellular histological translocation of NMDA NR1 in the spinal cord occurring after the inflammatory insult and the nociceptive behavioral responses to heat. The PTK inhibitors were administered into lumbar spinal cord by microdialysis, and secondary heat hyperalgesia was determined using the Hargreaves test. NMDA NR1 cellular protein expression and nuclear translocation were determined by immunocytochemical localization with light and electron microscopy, as well as with Western blot analysis utilizing both C- and N-terminal antibodies. Genistein and lavendustin A (but not inactive lavendustin B or diadzein) effectively reduced (i) pain related behavior, (ii) NMDA NR1 subunit expression increases in spinal cord, and (iii) the shift of NR1 from a cell membrane to a nuclear localization. Genistein pre-treatment reduced these events that occur in vivo within 4 h after inflammatory insult to the knee joint with kaolin and carrageenan (k/c). Cycloheximide reduced glutamate activated upregulation of NR1 content confirming synthesis of new protein in response to the inflammatory insult. In addition to this in vivo data, genistein or staurosporin inhibited upregulation of NMDA NR1 protein and nuclear translocation in vitro after treatment of human neuroblastoma clonal cell cultures (SH-SY5Y) with glutamate or NMDA (4 h). These studies provide evidence that inflammatory activation of peripheral nerves initiates increase in NMDA NR1 in the spinal cord coincident with development of pain related behaviors through glutamate non-receptor, PTK dependent cascades.

Peripheral GABAA receptor-mediated signaling facilitates persistent inflammatory hypersensitivity

Unlike in the central nervous system (CNS), in the adult peripheral nervous system (PNS), activation of GABA_A receptors (GABA_AR) is excitatory because of the relatively high concentration of intracellular chloride in these neurons. Indeed, exogenous GABA and muscimol, a GABA_AR agonist, exacerbate acute inflammatory hypersensitivity in rodents. However, it remains unclear whether peripheral GABA_AR and the endogenous GABA play an important role in persistent inflammatory hypersensitivity. In this study, we thus investigated how peripheral GABA_AR affects pain hypersensitivity by using the complete Freund's adjuvant (CFA)-induced persistent inflammatory pain mouse model. We found that intraplantar (i.pl.) administration of GABA_AR antagonists, picrotoxin, and 1(S),9(R)-(-)-bicuculline methiodide significantly inhibited both spontaneous nociceptive (paw licking and flinching) behavior and mechanical hypersensitivity in CFA-injected mice at day 3 (D3), but not in naïve mice. Interestingly, CFA-induced mechanical hypersensitivity was significantly reversed by anti-GABA antibody (anti-GABA, i.pl.). In addition, RT-qPCR revealed that glutamate decarboxylase Gad1 (GAD 67) and Gad2 (GAD 65) mRNA expression was also upregulated in the ipsilateral hind paw of CFA-injected mice at D3. Finally, 5α-pregnan-3α-ol-20-one (3α,5α-THP), a selective positive allosteric modulator of GABA_AR, produced mechanical hypersensitivity in naïve mice in a dose-dependent manner. Taken together, our results indicate that peripheral GABA_AR and endogenous GABA, possibly produced by the inflamed tissue, potentiate CFA-induced persistent inflammatory hypersensitivity, suggesting that they can be used as a therapeutic target for alleviating inflammatory pain.

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.

The α5 subunit containing GABAA receptors contribute to chronic pain

It has been recently proposed that α5-subunit containing GABAA receptors (α5-GABAA receptors) that mediate tonic inhibition might be involved in pain. The purpose of this study was to investigate the contribution of α5-GABAA receptors in the loss of GABAergic inhibition and in formalin-induced, complete Freund's adjuvant (CFA)-induced and L5 and L6 spinal nerve ligation-induced long-lasting hypersensitivity. Formalin or CFA injection and L5 and L6 spinal nerve ligation produced long-lasting allodynia and hyperalgesia. Moreover, formalin injection impaired the rate-dependent depression of the Hofmann reflex. Peripheral and intrathecal pretreatment or post-treatment with the α5-GABAA receptor antagonist, L-655,708 (0.15-15 nmol), prevented and reversed, respectively, these long-lasting behaviors. Formalin injection increased α5-GABAA receptor mRNA expression in the spinal cord and dorsal root ganglia (DRG) mainly at 3 days. The α5-GABAA receptors were localized in the dorsal spinal cord and DRG colabeling with NeuN, CGRP, and IB4 which suggests their presence in peptidergic and nonpeptidergic neurons. These receptors were found mainly in small and medium sized neurons. Formalin injection enhanced α5-GABAA receptor fluorescence intensity in spinal cord and DRG at 3 and 6 days. Intrathecal administration of L-655,708 (15 nmol) prevented and reversed formalin-induced impairment of rate-dependent depression. These results suggest that α5-GABAA receptors play a role in the loss of GABAergic inhibition and contribute to long-lasting secondary allodynia and hyperalgesia.

Inflammatory mediator-induced modulation of GABAA currents in human sensory neurons

The purpose of the present study was to characterize the properties of A-type GABA receptor (GABAA receptor) currents in human sensory neurons. Neurons were obtained from adult organ donors. GABAA currents were recorded in isolated neurons. Both large inactivating low-affinity currents and smaller persistent high-affinity currents were present in all of the 129 neurons studied from 15 donors. The kinetics of human GABAA currents were slower than those in rat sensory neurons. GABA currents were completely blocked by bicuculline (10 μM), and persistent currents were activated by the δ-subunit-preferring agonist, 4,5,6,7-tetrahydroisoxazolo[5,4-c]pyridine-3-ol (THIP). The GABA current equilibrium potential was ∼ 20 mV more hyperpolarized than in rat neurons. Both low- and high-affinity currents were increased by inflammatory mediators but via different second messenger pathways. These results highlight potentially important species differences in the properties of ion channels present in their native environment and suggest the use of human sensory neurons may be a valuable tool to test compounds prior to use in humans.

GABA pharmacology: the search for analgesics

Decades of research have been devoted to defining the role of GABAergic transmission in nociceptive processing. Much of this work was performed using rigid, orthosteric GABA analogs created by Povl Krogsgaard-Larsen and his associates. A relationship between GABA and pain is suggested by the anatomical distribution of GABA receptors and the ability of some GABA agonists to alter nociceptive responsiveness. Outlined in this report are data supporting this proposition, with particular emphasis on the anatomical localization and function of GABA-containing neurons and the molecular and pharmacological properties of GABAA and GABAB receptor subtypes. Reference is made to changes in overall GABAergic tone, GABA receptor expression and activity as a function of the duration and intensity of a painful stimulus or exposure to GABAergic agents. Evidence is presented that the plasticity of this receptor system may be responsible for the variability in the antinociceptive effectiveness of compounds that influence GABA transmission. These findings demonstrate that at least some types of persistent pain are associated with a regionally selective decline in GABAergic tone, highlighting the need for agents that enhance GABA activity in the affected regions without compromising GABA function over the long-term. As subtype selective positive allosteric modulators may accomplish these goals, such compounds might represent a new class of analgesic drugs.

Activity-dependent hyperpolarization of EGABA is absent in cutaneous DRG neurons from inflamed rats

A shift in GABA(A) signaling from inhibition to excitation in primary afferent neurons appears to contribute to the inflammation-induced increase in afferent input to the CNS. An activity-dependent depolarization of the GABA(A) current equilibrium potential (E(GABA)) has been described in CNS neurons which drives a shift in GABA(A) signaling from inhibition to excitation. The purpose of the present study was to determine if such an activity-dependent depolarization of E(GABA) occurs in primary afferents and whether the depolarization is amplified with persistent inflammation. Acutely dissociated retrogradely labeled cutaneous dorsal root ganglion (DRG) neurons from naïve and inflamed rats were studied with gramicidin perforated patch recording. Rather than a depolarization, 200 action potentials delivered at 2 Hz resulted in a ∼10 mV hyperpolarization of E(GABA) in cutaneous neurons from naïve rats. No such hyperpolarization was observed in neurons from inflamed rats. The shift in E(GABA) was not blocked by 10 μM bumetanide. Furthermore, because activity-dependent hyperpolarization of E(GABA) was fully manifest in the absence of HCO₃⁻ in the bath solution, this shift was not dependent on a change in HCO₃⁻-Cl⁻ exchanger activity, despite evidence of HCO₃⁻-Cl⁻ exchangers in DRG neurons that may contribute to the establishment of E(GABA) in the presence of HCO₃⁻. While the mechanism underlying the activity-dependent hyperpolarization of E(GABA) has yet to be identified, because this mechanism appears to function as a form of feedback inhibition, facilitating GABA-mediated inhibition of afferent activity, it may serve as a novel target for the treatment of inflammatory pain.