Positive allosteric modulation of GABA-A receptors reduces capsaicin-induced primary and secondary hypersensitivity in rats

Sex-dependent Cav2.3 channel contribution to the secondary hyperalgesia in a mice model of central sensitization

Central sensitization (CS) is characteristic of difficult to treat painful conditions, such as fibromyalgia and neuropathies and have sexual dimorphism involved. The calcium influx in nociceptive neurons is a key trigger for CS and the role of Cav2.1 and Cav2.2 voltage gated calcium channels (VGCC) in this role were evidenced with the use of ω-agatoxin IVA and ω-agatoxin MVIIA blockers, respectively. However, the participation of the α1 subunit of the voltage-gated channel Cav2.3, which conducts R-type currents, in CS is unknown. Furthermore, the role of sexual differences in painful conditions is still poorly understood. Thus, we investigated the role of Cav2.3 in capsaicin-induced secondary hyperalgesia in mice, which serve as a CS model predictive of the efficacy of novel analgesic drugs. Capsaicin injection in C57BL/6 mice caused secondary hyperalgesia from one to five hours after injection, and the effects were similar in male and female mice. In female but not male mice, intrathecal treatment with the Cav2.3 inhibitor SNX-482 partially and briefly reversed secondary hyperalgesia at a dose (300 pmol/site) that did not cause adverse effects. Moreover, Cav2.3 expression in the dorsal root ganglia (DRG) and spinal cord was reduced by intrathecal treatment with an antisense oligonucleotide (ASO) targeting Cav2.3 in female and male mice. However, ASO treatment was able to provide a robust and durable prevention of secondary hyperalgesia caused by capsaicin in female mice, but not in male mice. Thus, our results demonstrate that Cav2.3 inhibition, especially in female mice, has a relevant impact on a model of CS. Our results provide a proof of concept for Cav2.3 as a molecular target. In addition, the result associated to the role of differences in painful conditions linked to sex opens a range of possibilities to be explored and needs more attention. Thus, the relevance of testing Cav2.3 inhibition or knockdown in clinically relevant pain models is needed.

Thiopental Does Not Produce Hyperalgesia: A Laboratory Study Using Two Human Experimental Pain Models

Objective

Past investigations assessing the effects of thiopental on pain are conflicting. Although several studies demonstrate hyperalgesia as a result of barbiturate administration, others show analgesia. Our objective was to assess the effects of an infusion of the GABAA agonist thiopental, compared with placebo, in healthy participants on two subjective experimental pain paradigms: noxious electrical stimulation and intradermal capsaicin.

Methods

For electrical stimulation, the milliamps required to achieve pain threshold and tolerance were recorded, and the percent change from baseline was determined for each infusion condition. In the intradermal capsaicin condition, the area of hyperalgesia was determined by von Frey technique pre- and postinfusion, and the percent change in the area of hyperalgesia was calculated.

Results

Though thiopental infusion resulted in an increase in the electrical stimulation current required to elicit pain threshold or reach pain tolerance when compared with baseline, this finding was not statistically significant. In the intradermal capsaicin condition, there was a statistically significant difference in overall pre- and postinfusion pain interpretation, as measured by the McGill Pain Questionnaire (P < 0.05), but there was no significant difference in area of hyperalgesia.

Conclusions

In this human study of thiopental’s effects on two experimental pain models, our results show that thiopental does not induce hyperalgesia.

An updated review on pathophysiology and management of burning mouth syndrome with endocrinological, psychological and neuropathic perspectives

Burning mouth syndrome (BMS) is a chronic oro-facial pain disorder of unknown cause. It is more common in peri- and post-menopausal women, and sex hormone dysregulation is believed to be an important causative factor. Psychosocial events often trigger or exacerbate symptoms, and persons with BMS appear to be predisposed towards anxiety and depression. Atrophy of small nerve fibres in the tongue epithelium has been reported, and potential neuropathic mechanisms for BMS are now widely investigated. Historically, BMS was thought to comprise endocrinological, psychosocial and neuropathic components. Neuroprotective steroids and glial cell line-derived neurotrophic factor family ligands may have pivotal roles in the peripheral mechanisms associated with atrophy of small nerve fibres. Denervation of chorda tympani nerve fibres that innervate fungiform buds leads to alternative trigeminal innervation, which results in dysgeusia and burning pain when eating hot foods. With regard to the central mechanism of BMS, depletion of neuroprotective steroids alters the brain network-related mood and pain modulation. Peripheral mechanistic studies support the use of topical clonazepam and capsaicin for the management of BMS, and some evidence supports the use of cognitive behavioural therapy. Hormone replacement therapy may address the causes of BMS, although adverse effects prevent its use as a first-line treatment. Selective serotonin reuptake inhibitors (SSRIs) and serotonin and noradrenaline reuptake inhibitors (SNRIs) may have important benefits, and well-designed controlled studies are expected. Other treatment options to be investigated include brain stimulation and TSPO (translocator protein 18 kDa) ligands.

A randomised, placebo-controlled clinical trial with the α2/3/5 subunit selective GABAA positive allosteric modulator PF-06372865 in patients with chronic low back pain

The effect of PF-06372865, a subtype-selective positive allosteric modulator of the γ-aminobutyric acid type A (GABAA) receptor, on chronic low back pain was investigated in a randomised, placebo- and active-controlled phase 2 clinical trial. The parallel treatment group trial consisted of a 1-week single-blind placebo run in the phase, followed by 4-week double-blind treatment. Patients were randomised to receive either PF-06372865, naproxen, or placebo twice a day for 4 weeks. The primary end point was the numerical rating score of low back pain intensity after 4 weeks of active treatment. Secondary end points included the Roland Morris Disability Questionnaire and the Hopkins Verbal Learning Test-Revised. The trial had predefined decision rules based on the probability that PF-06372865 was better than placebo. The study was stopped at the interim analysis for futility. At this time, a total of 222 patients were randomised and the mean PF-06372865 4-week response on the low back pain intensity was 0.16 units higher (worse) than placebo (90% confidence interval -0.28 to 0.60). There were small, statistically significant reductions in the delayed recall test score with PF-06372865, as measured by Hopkins Verbal Learning Test-Revised. The effects of naproxen were in line with expectations. PF-06372865 was well tolerated. The most common treatment-related adverse events in the PF-06372865 arm were somnolence (5 mild and 4 moderate), dizziness (2 mild and 3 moderate), and nausea (2 mild). Although the reason for the lack of analgesic effect is not completely clear, it may be a result of not achieving sufficient receptor occupancy to drive efficacy.

Valnoctamide: The effect on relieving of neuropathic pain and possible mechanisms

The purpose of this study is to assess the possible anti-allodynic and antihyperalgesic effect of valnoctamide, an amide derivative of valproic acid, at the doses of 40, 70 and 100 mg/kg (i.p.) in neuropathic pain model induced by chronic constriction injury in rats, by using dynamic plantar test and plantar test (Hargreaves method), and to evaluate that the possible role of certain serotonin, noradrenergic, opioid and GABAergic receptors by pre-treatment with 1 mg/kg (i.p.) ketanserin, yohimbine, naloxone and 0.5 mg/kg (i.p.) bicuculline, respectively. 70 and 100 mg/kg valnoctamide significantly increased the mechanical and thermal thresholds decreasing with the development of neuropathy and demonstrated anti-allodynic and antihyperalgesic activity. Limited contribution of serotonin 5-HT_2A/2C receptors and α2-adrenoceptors, and significant contribution of GABA_A and opioid receptors to the anti-allodynic activity have been identified whereas remarkable contribution of opioid receptors and significant contribution of serotonin 5-HT_2A/2C receptors, α2-adrenoceptors, GABA_A receptors to the antihyperalgesic activity have been identified. Based upon these findings and considering that valnoctamide has safer side-effect profile, it is possible to say that valnoctamide is a potential agent that might be used alone or in combination with the other effective therapies in the alleviating of neuropathic pain.

Improved oral bioavailability of capsaicin via liposomal nanoformulation: preparation, in vitro drug release and pharmacokinetics in rats

This study innovatively prepared an effective capsaicin-loaded liposome, a nanoformulation with fewer irritants, for oral administration. The in vitro and in vivo properties of the liposomal encapsulation were investigated and the potential possibility of oral administration evaluated. The liposomal agent composed of phospholipid, cholesterol, sodium cholate and isopropyl myristate was prepared using film-dispersion method. A level A in vitro-in vivo correlation (IVIVC) was established for the first time, which demonstrated an excellent IVIVC of both formulated and free capsaicin in oral administration. Physicochemical characterizations including mean particle size, zeta (ζ) potential and average encapsulation efficiency of capsaicin-loaded liposome were found to be 52.2 ± 1.3 nm, -41.5 ± 2.71 mv and 81.9 ± 2.43 %, respectively. In vivo, liposomal encapsulation allowed a 3.34-fold increase in relative bioavailability compared to free capsaicin. The gastric mucosa irritation studies indicated that the liposomal system was a safe carrier for oral administration. These results support the fact that capsaicin, an effective drug for the treatment of neuropathic pain, could be encapsulated in liposome for improved oral bioavailability. The excellent IVIVC of capsaicin-loaded liposome could also be a promising tool in liposomal formulation development with an added advantage of reduced animal testing.

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.

Acetazolamide attenuates chemical-stimulated but not thermal-stimulated acute pain in mice

AIM

Acetazolamide (AZA), a carbonic anhydrase (CA) inhibitor, has been found to alleviate inflammatory and neuropathic pain in rats. In the present study, we investigated the effects of AZA on thermal- and chemical-stimulated acute pain in mice and the possible mechanisms underlying the effects.

METHODS

Five acute pain models based on thermal and chemical stimuli were established to investigate the effects of AZA on different types of nociception in mice. The antinociceptive effects of methazolamide (another CA inhibitor) and diazepam (a positive allosteric modulator of GABAA receptor) were also examined. The drugs were administered either intraperitoneally (ip) or intrathecally.

RESULTS

AZA (50-200 mg/kg, ip) did not produce analgesia in two thermal-stimulated acute pain models, ie, mouse tail-flick and hot-plate tests. In contrast, AZA (50-200 mg/kg, ip) dose-dependently reduced paw licking time in both capsaicin and formalin tests in mice. A similar result was observed in a mouse acetic acid-induced writhing test. However, AZA (10 nmol/mouse, intrathecally) did not produce significant analgesia in the 3 chemical-stimulated acute pain models. In addition, methazolamide (50-200 mg/kg, ip) and diazepam (0.25-1.0 mg/kg, ip) did not produce significant analgesia in either thermal- or chemical-stimulated acute pain.

CONCLUSION

AZA produces analgesia in chemical-stimulated, but not thermal-stimulated acute pain in mice. The attenuation of chemical-stimulated acute pain by AZA may not be due to enhancement of GABAA receptor-mediated inhibition via inhibiting CA activity but rather a peripheral ion channel-related mechanism.

GABA(A) receptor modulation: potential to deliver novel pain medicines?

GAB(A) (γ-aminobutyric acid) is abundantly expressed within the brain, and spinal cord pain circuits where it acts as the principal mediator of fast inhibitory neurotransmission. However, drugs that target GABA(A) receptor function such as the classical benzodiazepines have not been optimised to promote analgesia, are limited by side effects and are not routinely used for this purpose in humans. Compounds such as NS11394, L-838,417, HZ166 and TPA023 all bind to the same benzodiazepine site on the GABA(A) receptor to allosterically modulate receptor function and enhance the actions of GABA. By virtue of their ability to activate selected subtypes of GABA(A) receptors (principally those containing α2, α3 and α5 subunits) these compounds have been shown to possess excellent tolerability profiles in animals. Importantly, a number of these molecules also mediate profound analgesia in animal models of inflammatory and neuropathic pain. Other modulators such as neurosteroids bind to distinct sites on GABA(A) receptor α subunits, possess a unique pharmacology and are capable of targeting alternative GABA(A) receptor expressing populations. Moreover, neurosteroids also have pronounced analgesic actions in animal pain models. The continuing call for novel mechanism of action analgesics to target specific pathologies, especially in clinical neuropathic conditions, emphasizes the need to test modulators of GABA(A) receptor function in both human experimental pain models and pain patients.