Amaury François

Oxaliplatin-induced cold hypersensitivity is due to remodelling of ion channel expression in nociceptors.

Cold hypersensitivity is the hallmark of oxaliplatin‐induced neuropathy, which develops in nearly all patients under this chemotherapy. To date, pain management strategies have failed to alleviate these symptoms, hence development of adapted analgesics is needed. Here, we report that oxaliplatin exaggerates cold perception in mice as well as in patients. These symptoms are mediated by primary afferent sensory neurons expressing the thermoreceptor TRPM8. Mechanistically, oxaliplatin promotes over‐excitability by drastically lowering the expression of distinct potassium channels (TREK1, TRAAK) and by increasing the expression of pro‐excitatory channels such as the hyperpolarization‐activated channels (HCNs). These findings are corroborated by the analysis of TREK1‐TRAAK null mice and use of the specific HCN inhibitor ivabradine, which abolishes the oxaliplatin‐induced cold hypersensibility. These results suggest that oxaliplatin exacerbates cold perception by modulating the transcription of distinct ionic conductances that together shape sensory neuron responses to cold. The translational and clinical implication of these findings would be that ivabradine may represent a tailored treatment for oxaliplatin‐induced neuropathy.

State-dependent properties of a new T-type calcium channel blocker enhance CaV3.2 selectivity and support analgesic effects

Summary This study deciphers the mechanism of inhibition of T‐type calcium channels by TTA‐A2, demonstrating that TTA‐A2 affinity for CaV3.2‐inactivated state confers a preferential analgesic efficacy toward pathological pain. ABSTRACT T‐type calcium channels encoded by the CaV3.2 isoform are expressed in nociceptive primary afferent neurons where they contribute to hyperalgesia and thus are considered as a potential therapeutic target to treat pathological pain. Here we report that the small organic state‐dependent T‐type channel antagonist TTA‐A2 efficiently inhibits recombinant and native CaV3.2 currents. Although TTA‐A2 is a pan CaV3 blocker, it demonstrates a higher potency for CaV3.2 compared to CaV3.1. TTA‐A2 selectivity for T‐type currents was demonstrated in sensory neurons where it lowered cell excitability uniquely on neurons expressing T‐type channels. In vivo pharmacology in CaV3.2 knockout and wild type mice reveal that TTA‐A2‐mediated antinociception critically depends on CaV3.2 expression. The pathophysiology of irritable bowel syndrome (IBS) was recently demonstrated to involve CaV3.2 in a rat model of this disease. Oral administration of TTA‐A2 produced a dose‐dependent reduction of hypersensitivity in an IBS model, demonstrating its therapeutic potential for the treatment of pathological pain. Overall, our results suggest that the high potency of TTA‐A2 in the depolarized state strengthen its analgesic efficacy and selectivity toward pathological pain syndromes. This characteristic would be beneficial for the development of analgesics targeting T‐type channels, in particular for the treatment of pain associated with IBS.

TAFA4, a Chemokine-like Protein, Modulates Injury-Induced Mechanical and Chemical Pain Hypersensitivity in Mice

C-low-threshold mechanoreceptors (C-LTMRs) are unique among C-unmyelinated primary sensory neurons. These neurons convey two opposite aspects of touch sensation: a sensation of pleasantness, and a sensation of injury-induced mechanical pain. Here, we show that TAFA4 is a specific marker of C-LTMRs. Genetic labeling in combination with electrophysiological recordings show that TAFA4+ neurons have intrinsic properties of mechano-nociceptors. TAFA4-null mice exhibit enhanced mechanical and chemical hypersensitivity following inflammation and nerve injury as well as increased excitability of spinal cord lamina IIi neurons, which could be reversed by intrathecal or bath application of recombinant TAFA4 protein. In wild-type C57/Bl6 mice, intrathecal administration of TAFA4 strongly reversed carrageenan-induced mechanical hypersensitivity, suggesting a potent analgesic role of TAFA4 in pain relief. Our data provide insights into how C-LTMR-derived TAFA4 modulates neuronal excitability and controls the threshold of somatic sensation.

T-type calcium channels in chronic pain: mouse models and specific blockers

Pain is a quite frequent complaint accompanying numerous pathologies. Among these pathological cases, neuropathies are retrieved with identified etiologies (chemotherapies, diabetes, surgeries…) and also more diffuse syndromes such as fibromyalgia. More broadly, pain is one of the first consequences of the majority of inherited diseases. Despite its importance for the quality of life, current pain management is limited to drugs that are either old or with a limited efficacy or that possess a bad benefit/risk ratio. As no new pharmacological concept has led to new analgesics in the last decades, the discovery of medications is needed, and to this aim the identification of new druggable targets in pain transmission is a first step. Therefore, studies of ion channels in pain pathways are extremely active. This is particularly true with ion channels in peripheral sensory neurons in dorsal root ganglia (DRG) known now to express unique sets of these channels. Moreover, both spinal and supraspinal levels are clearly important in pain modulation. Among these ion channels, we and others revealed the important role of low voltage-gated calcium channels in cellular excitability in different steps of the pain pathways. These channels, by being activated nearby resting membrane potential have biophysical characteristics suited to facilitate action potential generation and rhythmicity. In this review, we will review the current knowledge on the role of these channels in the perception and modulation of pain.

T-type calcium channels in neuropathic pain.

Abstract Pain is a quite frequent complaint accompanying numerous pathologies. Among these pathological cases, numerous neuropathies are retrieved with identified etiologies (chemotherapies, diabetes, surgeries…) and also more diffuse syndromes such as fibromyalgia. More broadly, pain is one of the first consequences of most inherited diseases. Despite its importance for the quality of life, current pain management is limited to drugs that are either old or with a limited efficacy or that possess a bad risk benefit ratio. As no new pharmacological concept has led to new analgesics in the last decades, the discovery of new medications is needed, and to this aim, the identification of new druggable targets in pain transmission is a first step. Therefore, studies of ion channels in pain pathways are extremely active. This is particularly true with ion channels in peripheral sensory neurons in dorsal root ganglia known how to express unique sets of these channels. Moreover, both spinal and supraspinal levels are clearly important in pain modulation. Among these ion channels, we and others revealed the important role of low voltage-gated calcium channels in cellular excitability in different steps of the pain pathways. These channels, by being activated nearby resting membrane potential, have biophysical characteristics suited to facilitate action potential generation and rhythmicity. In this review, we will present the current knowledge on the role of these channels in the perception and modulation of pain.

Cav3.2 calcium channels: The key protagonist in the supraspinal effect of paracetamol

Summary Supraspinal Cav3.2 calcium channels are involved in analgesic effect of paracetamol through their inhibition following the activation of supraspinal TRPV1 receptors by AM404. ABSTRACT To exert its analgesic action, paracetamol requires complex metabolism to produce a brain‐specific lipoamino acid compound, AM404, which targets central transient receptor potential vanilloid receptors (TRPV1). Lipoamino acids are also known to induce analgesia through T‐type calcium‐channel inhibition (Cav3.2). In this study we show that the antinociceptive effect of paracetamol in mice is lost when supraspinal Cav3.2 channels are inhibited. Therefore, we hypothesized a relationship between supraspinal Cav3.2 and TRPV1, via AM404, which mediates the analgesic effect of paracetamol. AM404 is able to activate TRPV1 and weakly inhibits Cav3.2. Interestingly, activation of TRPV1 induces a strong inhibition of Cav3.2 current. Supporting this, intracerebroventricular administration of AM404 or capsaicin produces antinociception that is lost in Cav3.2−/− mice. Our study, for the first time, 1) provides a molecular mechanism for the supraspinal antinociceptive effect of paracetamol; 2) identifies the relationship between TRPV1 and the Cav3.2 channel; and 3) suggests supraspinal Cav3.2 inhibition as a potential pharmacological strategy to alleviate pain.

Functional Divergence of Delta and Mu Opioid Receptor Organization in CNS Pain Circuits

Summary Cellular interactions between delta and mu opioid receptors (DORs and MORs), including heteromerization, are thought to regulate opioid analgesia. However, the identity of the nociceptive neurons in which such interactions could occur in vivo remains elusive. Here we show that DOR-MOR co-expression is limited to small populations of excitatory interneurons and projection neurons in the spinal cord dorsal horn and unexpectedly predominates in ventral horn motor circuits. Similarly, DOR-MOR co-expression is rare in parabrachial, amygdalar, and cortical brain regions processing nociceptive information. We further demonstrate that in the discrete DOR-MOR co-expressing nociceptive neurons, the two receptors internalize and function independently. Finally, conditional knockout experiments revealed that DORs selectively regulate mechanical pain by controlling the excitability of somatostatin-positive dorsal horn interneurons. Collectively, our results illuminate the functional organization of DORs and MORs in CNS pain circuits and reappraise the importance of DOR-MOR cellular interactions for developing novel opioid analgesics.

Delta Opioid Receptor Expression and Function in Primary Afferent Somatosensory Neurons

The functional diversity of primary afferent neurons of the dorsal root ganglia (DRG) generates a variety of qualitatively and quantitatively distinct somatosensory experiences, from shooting pain to pleasant touch. In recent years, the identification of dozens of genetic markers specifically expressed by subpopulations of DRG neurons has dramatically improved our understanding of this diversity and provided the tools to manipulate their activity and uncover their molecular identity and function. Opioid receptors have long been known to be expressed by discrete populations of DRG neurons, in which they regulate cell excitability and neurotransmitter release. We review recent insights into the identity of the DRG neurons that express the delta opioid receptor (DOR) and the ion channel mechanisms that DOR engages in these cells to regulate sensory input. We highlight recent findings derived from DORGFP reporter mice and from in situ hybridization and RNA sequencing studies in wild-type mice that revealed DOR presence in cutaneous mechanosensory afferents eliciting touch and implicated in tactile allodynia. Mechanistically, we describe how DOR modulates opening of voltage-gated calcium channels (VGCCs) to control glutamatergic neurotransmission between somatosensory neurons and postsynaptic neurons in the spinal cord dorsal horn. We additionally discuss other potential signaling mechanisms, including those involving potassium channels, which DOR may engage to fine tune somatosensation. We conclude by discussing how this knowledge may explain the analgesic properties of DOR agonists against mechanical pain and uncovers an unanticipated specialized function for DOR in cutaneous mechanosensation.

Optical Activation of TrkA Signaling

Nerve growth factor/tropomyosin receptor kinase A (NGF/TrkA) signaling plays a key role in neuronal development, function, survival, and growth. The pathway is implicated in neurodegenerative disorders including Alzheimers disease, chronic pain, inflammation, and cancer. NGF binds the extracellular domain of TrkA, leading to the activation of the receptors intracellular kinase domain. As TrkA signaling is highly dynamic, mechanistic studies would benefit from a tool with high spatial and temporal resolution. Here we present the design and evaluation of four strategies for light-inducible activation of TrkA in the absence of NGF. Our strategies involve the light-sensitive protein Arabidopsis cryptochrome 2 and its binding partner CIB1. We demonstrate successful recapitulation of native NGF/TrkA functions by optical induction of plasma membrane recruitment and homo-interaction of the intracellular domain of TrkA. This approach activates PI3K/AKT and Raf/ERK signaling pathways, promotes neurite growth in PC12 cells, and supports survival of dorsal root ganglion neurons in the absence of NGF. This ability to activate TrkA using light bestows high spatial and temporal resolution for investigating NGF/TrkA signaling.

T-Type Calcium Channels in Pain Neuronal Circuits

Pain is a quite frequent complaint accompanying numerous pathologies. Among these pathological cases numerous neuropathies are retrieved with identified etiologies (chemotherapy induced peripheral neuropathies (CIPN), diabetes, surgeries) and also more diffuse syndromes such as fibromyalgia, migraine. More broadly, pain is one of the first and dramatic consequences of the majority of inherited diseases. Despite their importance for the quality of life, current therapies in symptomatic pain management are limited to drugs that are either old, or with a limited efficacy or that possess a bad benefit/risk ratio. Morphine and opioids for example have severe side effects. As no new pharmacological concept has led to new analgesics in the Past, the discovery of new medications is needed. It is necessary to identify new targets (such as ionic channels, the primary molecules of cellular excitability) in pain transmission before hoping to find specific molecules to treat different kinds of pain. Therefore studies of ion channels in pain pathways are extremely active. This is particularly true with ion channels in peripheral sensory neurons in dorsal root ganglia known now to express unique sets of these channels. Moreover, both spinal and supra spinal levels are clearly important in pain modulation with a key role of limbic areas such as thalamus filtering upcoming noxious information in their way to the cortex in the so called pain matrix representing a network of cerebral regions involved in the building of pain sensation, which comprises cortical somatic areas, insula, cingulate cortex, and associated with multiple other regions. Among these ion channels, we and others revealed the important role of low voltage-gated calcium channels (T-type channels) in cellular excitability in different steps of the pain pathways. These channels, by being activated nearby resting membrane potential have biophysical characteristic suited to facilitate action potential generation and rhythmicity. In this chapter we will review the current knowledge on the role of these channels in the perception and modulation of pain.