Experimental colitis-induced visceral hypersensitivity is attenuated by GABA treatment in mice

Ulcerative colitis (UC) is linked with inflammation of the large intestine due to an overactive response of the colon-immune system. UC is associated with weight loss, rectal bleeding, diarrhea, and abdominal pain. Given that γ-amino butyric acid (GABA) suppresses immune cell activity and the excitability of colonic afferents, and that there is a decrease in colonic GABA during UC, we hypothesized that UC pain is due to a decrease in the inhibition of colonic afferents. Thus, restoring GABA in the colon will attenuate inflammatory hypersensitivity. We tested this hypothesis in a mouse model of colitis. Colon inflammation was induced with seven days of dextran sodium sulfate (DSS, 3%) in the drinking water. GABA (40 mg/kg) was administered orally for the same period as DSS, and body weight, colon length, colon permeability, clinical progression of colitis (disease activity index or DAI), and colon histological score (HS) were assessed to determine the effects of GABA on colitis. A day after the end of GABA treatment, visceral sensitivity was assessed with balloon distention (of the colon)-evoked visceromotor response and colon samples were collected for the measurement of GABA and cytokines. Treatment with GABA reduced the DSS-induced increase in the colon permeability, DAI, HS, and decrease in body weight and colon length. Furthermore, GABA inhibited the DSS-induced increase in the proinflammatory cytokines tumor necrosis factor-α (TNF-α), interferon-γ (IFN-γ), interleukin-12 (IL-12), and increased the expression of the anti-inflammatory cytokine IL-10 in the colon tissue. Importantly, GABA reduced DSS-induced visceral hypersensitivity. These data suggest that increasing gastrointestinal levels of GABA may be useful for the treatment of colitis.NEW & NOTEWORTHY GABA treatment reduces the severity of colitis and inflammation and produces inhibition of visceral hypersensitivity in colon-inflamed mice. These results raise the promising possibility that GABA treatment may be an effective therapeutic strategy for the management of symptoms associated with colitis. However, clinical studies are required to corroborate whether this mouse-model data translates to human colon.

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.

Voltage-gated calcium currents in human dorsal root ganglion neurons

ABSTRACT

Voltage-gated calcium channels in sensory neurons underlie processes ranging from neurotransmitter release to gene expression and remain a therapeutic target for the treatment of pain. Yet virtually all we know about voltage-gated calcium channels has been obtained through the study of rodent sensory neurons and heterologously expressed channels. To address this, high voltage-activated (HVA) Ca2+ currents in dissociated human and rat dorsal root ganglion neurons were characterized with whole-cell patch clamp techniques. The HVA currents from both species shared basic biophysical and pharmacological properties. However, HVA currents in human neurons differed from those in the rat in at least 3 potentially important ways: (1) Ca2+ current density was significantly smaller, (2) the proportion of nifedipine-sensitive currents was far greater, and (3) a subpopulation of human neurons displayed relatively large constitutive current inhibition. These results highlight the need to for the study of native proteins in their native environment before initiating costly clinical trials.

Optogenetic inhibition of the colon epithelium reduces hypersensitivity in a mouse model of inflammatory bowel disease

ABSTRACT

Visceral pain is a prevalent symptom of inflammatory bowel disease that can be difficult to treat. Pain and hypersensitivity are mediated by extrinsic primary afferent neurons (ExPANs) that innervate the colon. Recent studies indicate that the colon epithelium contributes to initiating ExPAN firing and nociceptive responses. Based on these findings, we hypothesized that the epithelium contributes to inflammation-induced hypersensitivity. A key prediction of this hypothesis is that inhibition of the epithelium would attenuate nociceptive signaling and inflammatory hypersensitivity. To test this hypothesis, the inhibitory yellow light-activated protein archaerhodopsin was targeted to the intestinal epithelium (villin-Arch) or the ExPANs (TRPV1-Arch) that innervate the colon. Visceral sensitivity was assessed by measuring the visceromotor response (VMR) to colorectal distension (CRD), with and without yellow light illumination of the colon lumen. Inhibition of the colon epithelium in healthy villin-Arch mice significantly diminished the CRD-induced VMR. Direct inhibition of ExPANs during CRD using TRPV1-Arch mice showed that ExPAN and epithelial inhibition were similarly effective in reducing the VMR to CRD. We then investigated the effect of epithelial and ExPAN inhibition in the dextran sulfate sodium model of inflammatory bowel disease. Inhibition of the colon epithelium significantly decreased dextran sulfate sodium-induced hypersensitivity and was comparable with the inhibition of ExPANs. Together, these results reveal the potential of targeting the colon epithelium for the treatment of pain.

Kappa Opioid Receptor Distribution and Function in Primary Afferents

Primary afferents are known to be inhibited by kappa opioid receptor (KOR) signaling. However, the specific types of somatosensory neurons that express KOR remain unclear. Here, using a newly developed KOR-cre knockin allele, viral tracing, single-cell RT-PCR, and ex vivo recordings, we show that KOR is expressed in several populations of primary afferents: a subset of peptidergic sensory neurons, as well as low-threshold mechanoreceptors that form lanceolate or circumferential endings around hair follicles. We find that KOR acts centrally to inhibit excitatory neurotransmission from KOR-cre afferents in laminae I and III, and this effect is likely due to KOR-mediated inhibition of Ca²⁺ influx, which we observed in sensory neurons from both mouse and human. In the periphery, KOR signaling inhibits neurogenic inflammation and nociceptor sensitization by inflammatory mediators. Finally, peripherally restricted KOR agonists selectively reduce pain and itch behaviors, as well as mechanical hypersensitivity associated with a surgical incision. These experiments provide a rationale for the use of peripherally restricted KOR agonists for therapeutic treatment.

Peripheral GABA receptors regulate colonic afferent excitability and visceral nociception

KEY POINTS

While the presence of GABA receptors on primary afferents has been well described, most functional analyses have focused on the regulation of transmitter release from central terminals and/or signalling in the sensory neuron cell body. Evidence that GABA receptors are transported to peripheral terminals and that there are several sources of GABA in the colon raise the possibility that GABA signalling in the periphery may influence colonic afferent excitability. GABA_A and GABA_B are present and functional in the colon, where exogenous agonists decrease the excitability of colonic afferents and suppress visceral nociception. Endogenous GABA release within the colon is sufficient to establish the resting excitability of colonic afferents as well as the behavioural response to noxious stimulation of the colon, primarily via GABA_A receptors. Peripheral GABA receptors may serve as a viable target for the treatment of visceral pain.

ABSTRACT

It is well established that GABA receptors at the central terminals of primary afferent fibres regulate afferent input to the superficial dorsal horn. However, the extent to which peripheral GABA signalling may also regulate afferent input remains to be determined. The colon was used to explore this issue because of the numerous endogenous sources of GABA that have been described in this tissue. The influence of GABA signalling on colonic afferent excitability was assessed in an ex vivo mouse colorectum pelvic nerve preparation where test compounds were applied to the receptive field. The visceromotor response (VMR) evoked by noxious colorectal distension was used to assess the impact of GABA signalling on visceral nociception, where test compounds were applied directly to the colon. Application of either GABA_A or GABA_B receptor agonists attenuated the colonic afferent response to colon stretch. Conversely, GABA_A and GABA_B receptor antagonists increased the stretch response. However, while the noxious distension-induced VMR was attenuated in the presence of GABA_A and GABA_B receptor agonists, the VMR was only consistently increased by GABA_A receptor antagonists. These results suggest that GABA receptors are present and functional in the peripheral terminals of colonic afferents and activation of these receptors via endogenous GABA release contributes to the establishment of colonic afferent excitability and visceral nociception. These results suggest that increasing peripheral GABA receptor signalling could be used to treat visceral pain.

Tonically Active α5GABAA Receptors Reduce Motoneuron Excitability and Decrease the Monosynaptic Reflex

Motoneurons, the final common path of the Central Nervous System (CNS), are under a complex control of its excitability in order to precisely translate the interneuronal pattern of activity into skeletal muscle contraction and relaxation. To fulfill this relevant function, motoneurons are provided with a vast repertoire of receptors and channels, including the extrasynaptic GABA_A receptors which have been poorly investigated. Here, we confirmed that extrasynaptic α5 subunit-containing GABA_A receptors localize with choline acetyltransferase (ChAT) positive cells, suggesting that these receptors are expressed in turtle motoneurons as previously reported in rodents. In these cells, α₅GABA_A receptors are activated by ambient GABA, producing a tonic shunt that reduces motoneurons’ membrane resistance and affects their action potential firing properties. In addition, α₅GABA_A receptors shunted the synaptic excitatory inputs depressing the monosynaptic reflex (MSR) induced by activation of primary afferents. Therefore, our results suggest that α₅GABA_A receptors may play a relevant physiological role in motor control.

Role of anoctamin-1 and bestrophin-1 in spinal nerve ligation-induced neuropathic pain in rats

Background

Calcium-activated chloride channels (CaCCs) activation induces membrane depolarization by increasing chloride efflux in primary sensory neurons that can facilitate action potential generation. Previous studies suggest that CaCCs family members bestrophin-1 and anoctamin-1 are involved in inflammatory pain. However, their role in neuropathic pain is unclear. In this investigation we assessed the involvement of these CaCCs family members in rats subjected to the L5/L6 spinal nerve ligation. In addition, anoctamin-1 and bestrophin-1 mRNA and protein expression in dorsal root ganglion (DRG) and spinal cord was also determined in the presence and absence of selective inhibitors.

Results

L5/L6 spinal nerve ligation induced mechanical tactile allodynia. Intrathecal administration of non-selective CaCCs inhibitors (NPPB, 9-AC and NFA) dose-dependently reduced tactile allodynia. Intrathecal administration of selective CaCCs inhibitors (T16A_inh-A01 and CaCC_inh-A01) also dose-dependently diminished tactile allodynia and thermal hyperalgesia. Anoctamin-1 and bestrophin-1 mRNA and protein were expressed in the dorsal spinal cord and DRG of naïve, sham and neuropathic rats. L5/L6 spinal nerve ligation rose mRNA and protein expression of anoctamin-1, but not bestrophin-1, in the dorsal spinal cord and DRG from day 1 to day 14 after nerve ligation. In addition, repeated administration of CaCCs inhibitors (T16A_inh-A01, CaCC_inh-A01 or NFA) or anti-anoctamin-1 antibody prevented spinal nerve ligation-induced rises in anoctamin-1 mRNA and protein expression. Following spinal nerve ligation, the compound action potential generation of putative C fibers increased while selective CaCCs inhibitors (T16A_inh-A01 and CaCC_inh-A01) attenuated such increase.

Conclusions

There is functional anoctamin-1 and bestrophin-1 expression in rats at sites related to nociceptive processing. Blockade of these CaCCs suppresses compound action potential generation in putative C fibers and lessens established tactile allodynia. As CaCCs activity contributes to neuropathic pain maintenance, selective inhibition of their activity may function as a tool to generate analgesia in nerve injury pain states.

Electronic supplementary material

The online version of this article (doi:10.1186/s12990-015-0042-1) contains supplementary material, which is available to authorized users.

α5GABAA receptors mediate primary afferent fiber tonic excitability in the turtle spinal cord

γ-Amino butyric acid (GABA) plays a key role in the regulation of central nervous system by activating synaptic and extrasynaptic GABAA receptors. It is acknowledged that extrasynaptic GABAA receptors located in the soma, dendrites, and axons may be activated tonically by low extracellular GABA concentrations. The activation of these receptors produces a persistent conductance that can hyperpolarize or depolarize nerve cells depending on the Cl− equilibrium potential. In an in vitro preparation of the turtle spinal cord we show that extrasynaptic α5GABAA receptors mediate the tonic state of excitability of primary afferents independently of the phasic primary afferent depolarization mediated by synaptic GABAA receptors. Blockade of α5GABAA receptors with the inverse agonist L-655,708 depressed the dorsal root reflex (DRR) without affecting the phasic increase in excitability of primary afferents. Using RT-PCR and Western blotting, we corroborated the presence of the mRNA and the α5GABAA protein in the dorsal root ganglia of the turtle spinal cord. The receptors were localized in primary afferents in dorsal root, dorsal root ganglia, and peripheral nerve terminals using immunoconfocal microscopy. Considering the implications of the DRR in neurogenic inflammation, α5GABAA receptors may serve as potential pharmacological targets for the treatment of pain.