Abdelkrim Alloui

The effects of total sleep deprivation, selective sleep interruption and sleep recovery on pain tolerance thresholds in healthy subjects.

The aim of this study was to compare the effects of total sleep deprivation (TSD), rapid eye movement (REM) sleep and slow wave sleep (SWS) interruption and sleep recovery on mechanical and thermal pain sensitivity in healthy adults. Nine healthy male volunteers (age 26–43 years) were randomly assigned in this double blind and crossover study to undergo either REM sleep or SWS interruption. Periods of 6 consecutive laboratory nights separated by at least 2 weeks were designed as follows: N1 Adaptation night; N2 Baseline night; N3 Total sleep deprivation (40 h); N4 and N5 SWS or REM sleep interruption; N6 Recovery. Sleep was recorded and scored using standard methods. Tolerance thresholds to mechanical and thermal pain were assessed using an electronic pressure dolorimeter and a thermode operating on a Peltier principle. Relative to baseline levels, TSD decreased significantly mechanical pain thresholds (−8%). Both REM sleep and SWS interruption tended to decrease mechanical pain thresholds. Recovery sleep, after SWS interruption produced a significant increase in mechanical pain thresholds (+ 15%). Recovery sleep after REM sleep interruption did not significantly increase mechanical pain thresholds. No significant differences in thermal pain thresholds were detected between and within periods. In conclusion this experimental study in healthy adult volunteers has demonstrated an hyperalgesic effect related to 40 h TSD and an analgesic effect related to SWS recovery. The analgesic effect of SWS recovery is apparently greater than the analgesia induced by level I (World Health Organization) analgesic compounds in mechanical pain experiments in healthy volunteers.

Silencing of the Cav3.2 T-type calcium channel gene in sensory neurons demonstrates its major role in nociception

Analgesic therapies are still limited and sometimes poorly effective, therefore finding new targets for the development of innovative drugs is urgently needed. In order to validate the potential utility of blocking T‐type calcium channels to reduce nociception, we explored the effects of intrathecally administered oligodeoxynucleotide antisenses, specific to the recently identified T‐type calcium channel family (CaV3.1, CaV3.2, and CaV3.3), on reactions to noxious stimuli in healthy and mononeuropathic rats. Our results demonstrate that the antisense targeting CaV3.2 induced a knockdown of the CaV3.2 mRNA and protein expression as well as a large reduction of ‘CaV3.2‐like’ T‐type currents in nociceptive dorsal root ganglion neurons. Concomitantly, the antisense treatment resulted in major antinociceptive, anti‐hyperalgesic, and anti‐allodynic effects, suggesting that CaV3.2 plays a major pronociceptive role in acute and chronic pain states. Taken together, the results provide direct evidence linking CaV3.2 T‐type channels to pain perception and suggest that CaV3.2 may offer a specific molecular target for the treatment of pain.

TREK‐1, a K+ channel involved in polymodal pain perception

The TREK‐1 channel is a temperature‐sensitive, osmosensitive and mechano‐gated K+ channel with a regulation by Gs and Gq coupled receptors. This paper demonstrates that TREK‐1 qualifies as one of the molecular sensors involved in pain perception. TREK‐1 is highly expressed in small sensory neurons, is present in both peptidergic and nonpeptidergic neurons and is extensively colocalized with TRPV1, the capsaicin‐activated nonselective ion channel. Mice with a disrupted TREK‐1 gene are more sensitive to painful heat sensations near the threshold between anoxious warmth and painful heat. This phenotype is associated with the primary sensory neuron, as polymodal C‐fibers were found to be more sensitive to heat in single fiber experiments. Knockout animals are more sensitive to low threshold mechanical stimuli and display an increased thermal and mechanical hyperalgesia in conditions of inflammation. They display a largely decreased pain response induced by osmotic changes particularly in prostaglandin E2‐sensitized animals. TREK‐1 appears as an important ion channel for polymodal pain perception and as an attractive target for the development of new analgesics.

ASIC3, a sensor of acidic and primary inflammatory pain

Acid‐sensing ion channels (ASICs) are cationic channels activated by extracellular acidosis that are expressed in both central and peripheral nervous systems. Although peripheral ASICs seem to be natural sensors of acidic pain (e.g., in inflammation, ischaemia, lesions or tumours), a direct demonstration is still lacking. We show that ∼60% of rat cutaneous sensory neurons express ASIC3‐like currents. Native as well as recombinant ASIC3 respond synergistically to three different inflammatory signals that are slight acidifications (∼pH 7.0), hypertonicity and arachidonic acid (AA). Moderate pH, alone or in combination with hypertonicity and AA, increases nociceptors excitability and produces pain suppressed by the toxin APETx2, a specific blocker of ASIC3. Both APETx2 and the in vivo knockdown of ASIC3 with a specific siRNA also have potent analgesic effects against primary inflammation‐induced hyperalgesia in rat. Peripheral ASIC3 channels are thus essential sensors of acidic pain and integrators of molecular signals produced during inflammation where they contribute to primary hyperalgesia.

The mechano-activated K+ channels TRAAK and TREK-1 control both warm and cold perception

The sensation of cold or heat depends on the activation of specific nerve endings in the skin. This involves heat‐ and cold‐sensitive excitatory transient receptor potential (TRP) channels. However, we show here that the mechano‐gated and highly temperature‐sensitive potassium channels of the TREK/TRAAK family, which normally work as silencers of the excitatory channels, are also implicated. They are important for the definition of temperature thresholds and temperature ranges in which excitation of nociceptor takes place and for the intensity of excitation when it occurs. They are expressed with thermo‐TRP channels in sensory neurons. TRAAK and TREK‐1 channels control pain produced by mechanical stimulation and both heat and cold pain perception in mice. Expression of TRAAK alone or in association with TREK‐1 controls heat responses of both capsaicin‐sensitive and capsaicin‐insensitive sensory neurons. Together TREK‐1 and TRAAK channels are important regulators of nociceptor activation by cold, particularly in the nociceptor population that is not activated by menthol.

A tarantula peptide against pain via ASIC1a channels and opioid mechanisms

Psalmotoxin 1, a peptide extracted from the South American tarantula Psalmopoeus cambridgei, has very potent analgesic properties against thermal, mechanical, chemical, inflammatory and neuropathic pain in rodents. It exerts its action by blocking acid-sensing ion channel 1a, and this blockade results in an activation of the endogenous enkephalin pathway. The analgesic properties of the peptide are suppressed by antagonists of the μ and δ-opioid receptors and are lost in Penk1−/− mice.

Acid-sensing ion channels in postoperative pain.

Iatrogenic pain consecutive to a large number of surgical procedures has become a growing health concern. The etiology and pathophysiology of postoperative pain are still poorly understood, but hydrogen ions appear to be important in this process. We have investigated the role of peripheral acid-sensing ion channels (ASICs), which form depolarizing channels activated by extracellular protons, in a rat model of postoperative pain (i.e., hindpaw skin/muscle incision). We report high levels of ASIC-type currents (∼77%) in sensory neurons innervating the hindpaw muscles, with a prevalence of ASIC3-like currents. The ASIC3 protein is largely expressed in lumbar DRG neurons innervating the plantar muscle, and its mRNA and protein levels are increased by plantar incision 24 h after surgery. Pharmacological inhibition of ASIC3 channels with the specific toxin APETx2 or in vivo knockdown of ASIC3 subunit by small interfering RNA led to a significant reduction of postoperative spontaneous, thermal, and postural pain behaviors (spontaneous flinching, heat hyperalgesia, and weight bearing). ASIC3 appears to have an important role in deep tissue but also affects prolonged pain evoked by skin incision alone. The specific homomeric ASIC1a blocker PcTx1 has no effect on spontaneous flinching, when applied peripherally. Together, these data demonstrate a significant role for peripheral ASIC3-containing channels in postoperative pain.

T-type calcium channels contribute to colonic hypersensitivity in a rat model of irritable bowel syndrome.

The symptoms of irritable bowel syndrome (IBS) include significant abdominal pain and bloating. Current treatments are empirical and often poorly efficacious, and there is a need for the development of new and efficient analgesics aimed at IBS patients. T-type calcium channels have previously been validated as a potential target to treat certain neuropathic pain pathologies. Here we report that T-type calcium channels encoded by the CaV3.2 isoform are expressed in colonic nociceptive primary afferent neurons and that they contribute to the exaggerated pain perception in a butyrate-mediated rodent model of IBS. Both the selective genetic inhibition of CaV3.2 channels and pharmacological blockade with calcium channel antagonists attenuates IBS-like painful symptoms. Mechanistically, butyrate acts to promote the increased insertion of CaV3.2 channels into primary sensory neuron membranes, likely via a posttranslational effect. The butyrate-mediated regulation can be recapitulated with recombinant CaV3.2 channels expressed in HEK cells and may provide a convenient in vitro screening system for the identification of T-type channel blockers relevant to visceral pain. These results implicate T-type calcium channels in the pathophysiology of chronic visceral pain and suggest CaV3.2 as a promising target for the development of efficient analgesics for the visceral discomfort and pain associated with IBS.

Group III metabotropic glutamate receptors inhibit hyperalgesia in animal models of inflammation and neuropathic pain

&NA; Glutamate plays a key role in modulation of nociceptive processing. This excitatory amino acid exerts its action through two distinct types of receptors, ionotropic and metabotropic glutamate receptors (mGluRs). Eight mGluRs have been identified and divided in three groups based on their sequence similarity, pharmacology and G‐protein coupling. While the role of group I and II mGluRs is now well established, little is known about the part played by group III mGluRs in pain. In this work, we studied comparatively the involvement of spinal group III mGluR in modulation of acute, inflammatory and neuropathic pain. While intrathecal injection of ACPT‐I, a selective group III mGluR agonist, failed to induce any change in vocalization thresholds of healthy animals submitted to mechanical or thermal stimuli, it dose‐dependently inhibited the nociceptive behavior of rats submitted to the formalin test and the mechanical hyperalgesia associated with different animal models of inflammatory (carrageenan‐treated and monoarthritic rats) or neuropathic pain (mononeuropathic and vincristine‐treated rats). Similar effects were also observed following intrathecal injection of PHCCC, a positive allosteric modulator of mGlu4. Antihyperalgesia induced by ACPT‐I was blocked either by LY341495, a nonselective antagonist of mGluR, by MAP4, a selective group III antagonist. This study provide new evidences supporting the role of spinal group III mGluRs in the modulation of pain perception in different pathological pain states of various etiologies but not in normal conditions. It more particularly highlights the specific involvement of mGlu4 in this process and may be a useful therapeutic approach to chronic pain treatment.

T-type calcium channel inhibition underlies the analgesic effects of the endogenous lipoamino acids.

Lipoamino acids are anandamide-related endogenous molecules that induce analgesia via unresolved mechanisms. Here, we provide evidence that the T-type/Cav3 calcium channels are important pharmacological targets underlying their physiological effects. Various lipoamino acids, including N-arachidonoyl glycine (NAGly), reversibly inhibited Cav3.1, Cav3.2, and Cav3.3 currents, with potent effects on Cav3.2 [EC50 ∼200 nm for N-arachidonoyl 3-OH-γ-aminobutyric acid (NAGABA-OH)]. This inhibition involved a large shift in the Cav3.2 steady-state inactivation and persisted during fatty acid amide hydrolase (FAAH) inhibition as well as in cell-free outside-out patch. In contrast, lipoamino acids had weak effects on high-voltage-activated (HVA) Cav1.2 and Cav2.2 calcium currents, on Nav1.7 and Nav1.8 sodium currents, and on anandamide-sensitive TRPV1 and TASK1 currents. Accordingly, lipoamino acids strongly inhibited native Cav3.2 currents in sensory neurons with small effects on sodium and HVA calcium currents. In addition, we demonstrate here that lipoamino acids NAGly and NAGABA-OH produced a strong thermal analgesia and that these effects (but not those of morphine) were abolished in Cav3.2 knock-out mice. Collectively, our data revealed lipoamino acids as a family of endogenous T-type channel inhibitors, suggesting that these ligands can modulate multiple cell functions via this newly evidenced regulation.