Involvement of GABAA receptor-associated chloride channels in the peripheral antinociceptive effect induced by GABAA receptor agonist muscimol

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.

Mechanisms underlying midazolam-induced peripheral nerve block and neurotoxicity

BACKGROUND AND OBJECTIVES

The benzodiazepine midazolam has been reported to facilitate the actions of spinally administrated local anesthetics. Interestingly, despite the lack of convincing evidence for the presence of γ-aminobutyric acid type A (GABAA) receptors along peripheral nerve axons, midazolam also has been shown to have analgesic efficacy when applied alone to peripheral nerves.These observations suggest midazolam-induced nerve block is due to another site of action. Furthermore, because of evidence indicating that midazolam has equal potency at the benzodiazepine site on the GABAA receptor and the 18-kd translocator protein (TSPO), it is possible that at least the nerve-blocking actions of midazolam are mediated by this alternative site of action.

METHODS

We used the benzodiazepine receptor antagonist flumazenil, and the TSPO antagonist PK11195, with midazolam on rat sciatic nerves and isolated sensory neurons to determine if either receptor mediates midazolam-induced nerve block and/or neurotoxicity.

RESULTS

Midazolam (300 μM)-induced block of nerve conduction was reversed by PK11195 (3 μM), but not flumazenil (30 μM). Midazolam-induced neurotoxicity was blocked by neither PK11195 nor flumazenil. Midazolam also causes the release of Ca from internal stores in sensory neurons, and there was a small but significant attenuation of midazolam-induced neurotoxicity by the Ca chelator, BAPTA. BAPTA (30 μM) significantly attenuated midazolam-induced nerve block.

CONCLUSIONS

Our results indicate that processes underlying midazolam-induced nerve block and neurotoxicity are separable, and suggest that selective activation of TSPO may facilitate modality-selective nerve block while minimizing the potential for neurotoxicity.

Peripheral neurosteroids enhance P2X receptor-induced mechanical allodynia via a sigma-1 receptor-mediated mechanism

The role of peripheral neurosteroids and their related mechanisms on nociception have not been thoroughly investigated. Based on emerging evidence in the literature indicating that neurosteroids and their main target receptors, i.e., sigma-1, GABAA and NMDA, affect P2X-induced changes in neuronal activity, this study was designed to investigate the effect of peripherally injected dehydroepiandrosterone sulphate (DHEAS) and pregnenolone sulfate (PREGS) on P2X receptor-mediated mechanical allodynia in rats. Intraplantar injection of either neurosteroids alone did not produced any detectable changes in paw withdrawal frequency to the innocuous mechanical stimulation in naïve rats. However, When DHEAS or PREGS were co-injected with a sub-effective dose of αβmeATP, mechanical allodynia was developed and this was dose dependently blocked by pre-injection of the P2X antagonist, TNP-ATP. These results demonstrates that DHEAS and PREGS potentiate the activity of P2X receptors which results in the enhancement of αβmeATP-induced mechanical allodynia. In order to investigate the potential role of peripheral sigma-1, GABAA and NMDA receptors in this facilitatory action, we pretreated animals with BD-1047 (a sigma-1 antagonist), muscimol (a GABAA agonist) or MK-801 (a NMDA antagonist) prior to DHEAS or PREGS+αβmeATP injection. Only BD-1047 effectively prevented the facilitatory effects induced by neurosteroids on αβmeATP-induced mechanical allodynia. Collectively, we have shown that peripheral neurosteroids potentiate P2X-induced mechanical allodynia and that this action is mediated by sigma-1, but not by GABAA nor NMDA, receptors.

HSV vector-mediated GAD67 suppresses neuropathic pain induced by perineural HIV gp120 in rats through inhibition of ROS and Wnt5a

Human immunodeficiency virus (HIV)-related neuropathic pain is a debilitating chronic condition that is severe and unrelenting. Despite the extensive research, the exact neuropathological mechanisms remain unknown, which hinders our ability to develop effective treatments. Loss of GABAergic tone may play an important role in the neuropathic pain state. Glutamic acid decarboxylase 67 (GAD67) is one of isoforms that catalyze GABA synthesis. Here, we used recombinant herpes simplex virus (HSV-1) vectors that encode gad1 gene to evaluate the therapeutic potential of GAD67 in peripheral HIV gp120-induced neuropathic pain in rats. We found that 1) subcutaneous inoculation of the HSV vectors expressing GAD67 attenuated mechanical allodynia in the model of HIV gp120-induced neuropathic pain, 2) the anti-allodynic effect of GAD67 was reduced by GABA-A and-B receptors antagonists, 3) HSV vectors expressing GAD67 reversed the lowered GABA-IR expression, and 4) the HSV vectors expressing GAD67 suppressed the upregulated mitochondrial superoxide and Wnt5a in the spinal dorsal horn. Taken together, our studies support the concept that recovering GABAergic tone by the HSV vectors may reverse HIV-associated neuropathic pain through suppressing mitochondrial superoxide and Wnt5a. Our studies provide validation of HSV-mediated GAD67 gene therapy in the treatment of HIV-related neuropathic pain.

Effect of adjuvant drugs on the action of local anesthetics in isolated rat sciatic nerves

BACKGROUND AND OBJECTIVES

There is increasing clinical use of adjuvant drugs to prolong the duration of local anesthetic-induced block of peripheral nerves. However, the mechanistic understanding regarding drug interactions between these compounds in the periphery is quite limited. Accordingly, we undertook this study to determine whether selected adjuvant drugs are efficacious in blocking action potential propagation in peripheral nerves at concentrations used clinically and whether these drugs influence peripheral nerve block produced by local anesthetics.

METHODS

Isolated rat sciatic nerves were used to assess (1) the efficacy of buprenorphine, clonidine, dexamethasone, or midazolam, alone and in combination, on action potential propagation; and (2) their influence on the blocking actions of local anesthetics ropivacaine and lidocaine. Compound action potentials (CAPs) from A- and C-fibers were studied before and after drug application.

RESULTS

At estimated clinical concentrations, neither buprenorphine nor dexamethasone affected either A- or C-waves of the CAP. Clonidine produced a small but significant attenuation of the C-wave amplitude. Midazolam attenuated both A- and C-wave amplitudes, but with greater potency on the C-wave. The combination of clonidine, buprenorphine, and dexamethasone had no influence on the potency or duration of local anesthetic- or midazolam-induced block of A- and C-waves of the CAP.

CONCLUSIONS

These results suggest that the reported clinical efficacy of clonidine, buprenorphine, and dexamethasone influences the actions of local anesthetics via indirect mechanisms. Further identification of these indirect mechanisms may enable the development of novel approaches to achieve longer-duration, modality-specific peripheral nerve block.

Activation of GABA-A receptor ameliorates homocysteine-induced MMP-9 activation by ERK pathway

Hyperhomocysteinemia (HHcy) is a risk factor for neuroinflammatory and neurodegenerative diseases. Homocysteine (Hcy) induces redox stress, in part, by activating matrix metalloproteinase-9 (MMP-9), which degrades the matrix and leads to blood-brain barrier dysfunction. Hcy competitively binds to gamma-aminbutyric acid (GABA) receptors, which are excitatory neurotransmitter receptors. However, the role of GABA-A receptor in Hcy-induced cerebrovascular remodeling is not clear. We hypothesized that Hcy causes cerebrovascular remodeling by increasing redox stress and MMP-9 activity via the extracellular signal-regulated kinase (ERK) signaling pathway and by inhibition of GABA-A receptors, thus behaving as an inhibitory neurotransmitter. Hcy-induced reactive oxygen species production was detected using the fluorescent probe, 2'-7'-dichlorodihydrofluorescein diacetate. Hcy increased nicotinamide adenine dinucleotide phosphate-oxidase-4 concomitantly suppressing thioredoxin. Hcy caused activation of MMP-9, measured by gelatin zymography. The GABA-A receptor agonist, muscimol ameliorated the Hcy-mediated MMP-9 activation. In parallel, Hcy caused phosphorylation of ERK and selectively decreased levels of tissue inhibitors of metalloproteinase-4 (TIMP-4). Treatment of the endothelial cell with muscimol restored the levels of TIMP-4 to the levels in control group. Hcy induced expression of iNOS and decreased eNOS expression, which lead to a decreased NO bioavailability. Furthermore muscimol attenuated Hcy-induced MMP-9 via ERK signaling pathway. These results suggest that Hcy competes with GABA-A receptors, inducing the oxidative stress transduction pathway and leading to ERK activation.