Miguel Á. Tejada

Tetrodotoxin (TTX) as a Therapeutic Agent for Pain

Tetrodotoxin (TTX) is a potent neurotoxin that blocks voltage-gated sodium channels (VGSCs). VGSCs play a critical role in neuronal function under both physiological and pathological conditions. TTX has been extensively used to functionally characterize VGSCs, which can be classified as TTX-sensitive or TTX-resistant channels according to their sensitivity to this toxin. Alterations in the expression and/or function of some specific TTX-sensitive VGSCs have been implicated in a number of chronic pain conditions. The administration of TTX at doses below those that interfere with the generation and conduction of action potentials in normal (non-injured) nerves has been used in humans and experimental animals under different pain conditions. These data indicate a role for TTX as a potential therapeutic agent for pain. This review focuses on the preclinical and clinical evidence supporting a potential analgesic role for TTX. In addition, the contribution of specific TTX-sensitive VGSCs to pain is reviewed.

Role of Sigma-1 Receptors in Paclitaxel-Induced Neuropathic Pain in Mice

UNLABELLED Sigma-1 (σ(1)) receptors play a role in different types of pain and in central sensitization mechanisms; however, it is unknown whether they are involved in chemotherapy-induced neuropathic pain. We compared the ability of paclitaxel to induce cold (acetone test) and mechanical (electronic Von Frey test) allodynia in wild-type (WT) and σ(1) receptor knockout (σ(1)-KO) mice. We also tested the effect on paclitaxel-induced painful neuropathy of BD-1063 (16-64 mg/kg, subcutaneously) and S1RA (32-128 mg/kg, subcutaneously), 2 selective σ(1) receptor antagonists that bind to the σ(1) receptor with high affinity and competitively. The responses to cold and mechanical stimuli were similar in WT and σ(1)-KO mice not treated with paclitaxel; however, treatment with paclitaxel (2 mg/kg, intraperitoneally, once per day during 5 consecutive days) produced cold and mechanical allodynia and an increase in spinal cord diphosphorylated extracellular signal-regulated kinase (pERK) in WT but not in σ(1)-KO mice. The administration of BD-1063 or S1RA 30 minutes before each paclitaxel dose prevented the development of cold and mechanical allodynia in WT mice. Moreover, the acute administration of both σ(1) receptor antagonists dose dependently reversed both types of paclitaxel-induced allodynia after they had fully developed. These results suggest that σ(1) receptors play a key role in paclitaxel-induced painful neuropathy. PERSPECTIVE Antagonists of the σ(1) receptor may have therapeutic value for the treatment and/or prevention of paclitaxel-induced neuropathic pain. This possibility is especially interesting in the context of chemotherapy-induced neuropathy, where the onset of nerve damage is predictable and preventive treatment could be administered.

The clinical heterogeneity of coenzyme Q10 deficiency results from genotypic differences in the Coq9 gene

Primary coenzyme Q10 (CoQ10) deficiency is due to mutations in genes involved in CoQ biosynthesis. The disease has been associated with five major phenotypes, but a genotype–phenotype correlation is unclear. Here, we compare two mouse models with a genetic modification in Coq9 gene (Coq9Q95X and Coq9R239X), and their responses to 2,4‐dihydroxybenzoic acid (2,4‐diHB). Coq9R239X mice manifest severe widespread CoQ deficiency associated with fatal encephalomyopathy and respond to 2,4‐diHB increasing CoQ levels. In contrast, Coq9Q95X mice exhibit mild CoQ deficiency manifesting with reduction in CI+III activity and mitochondrial respiration in skeletal muscle, and late‐onset mild mitochondrial myopathy, which does not respond to 2,4‐diHB. We show that these differences are due to the levels of COQ biosynthetic proteins, suggesting that the presence of a truncated version of COQ9 protein in Coq9R239X mice destabilizes the CoQ multiprotein complex. Our study points out the importance of the multiprotein complex for CoQ biosynthesis in mammals, which may provide new insights to understand the genotype–phenotype heterogeneity associated with human CoQ deficiency and may have a potential impact on the treatment of this mitochondrial disorder.

Sigma-1 receptor inhibition reverses acute inflammatory hyperalgesia in mice: role of peripheral sigma-1 receptors

RationaleSigma-1 (σ1) receptor inhibition ameliorates neuropathic pain by inhibiting central sensitization. However, it is unknown whether σ1 receptor inhibition also decreases inflammatory hyperalgesia, or whether peripheral σ1 receptors are involved in this process.ObjectiveThe purpose of this study was to determine the role of σ1 receptors in carrageenan-induced inflammatory hyperalgesia, particularly at the inflammation site.ResultsThe subcutaneous (s.c.) administration of the selective σ1 antagonists BD-1063 and S1RA to wild-type mice dose-dependently and fully reversed inflammatory mechanical (paw pressure) and thermal (radiant heat) hyperalgesia. These antihyperalgesic effects were abolished by the s.c. administration of the σ1 agonist PRE-084 and also by the intraplantar (i.pl.) administration of this compound in the inflamed paw, suggesting that blockade of peripheral σ1 receptors in the inflamed site is involved in the antihyperalgesic effects induced by σ1 antagonists. In fact, the i.pl. administration of σ1 antagonists in the inflamed paw (but not in the contralateral paw) was sufficient to completely reverse inflammatory hyperalgesia. σ1 knockout (σ1-KO) mice did not develop mechanical hyperalgesia but developed thermal hypersensitivity; however, the s.c. administration of BD-1063 or S1RA had no effect on thermal hyperalgesia in σ1-KO mice, supporting on-target mechanisms for the effects of both drugs. The antiedematous effects of σ1 inhibition do not account for the decreased hyperalgesia, since carrageenan-induced edema was unaffected by σ1 knockout or systemic σ1 pharmacological antagonism.Conclusionsσ1 receptors play a major role in inflammatory hyperalgesia. Targeting σ1 receptors in the inflamed tissue may be useful for the treatment of inflammatory pain.

Melatonin enhances neural stem cell differentiation and engraftment by increasing mitochondrial function

Neural stem cells (NSCs) are regarded as a promising therapeutic approach to protecting and restoring damaged neurons in neurodegenerative diseases (NDs) such as Parkinsons disease and Alzheimers disease (PD and AD, respectively). However, new research suggests that NSC differentiation is required to make this strategy effective. Several studies have demonstrated that melatonin increases mature neuronal markers, which reflects NSC differentiation into neurons. Nevertheless, the possible involvement of mitochondria in the effects of melatonin during NSC differentiation has not yet been fully established. We therefore tested the impact of melatonin on NSC proliferation and differentiation in an attempt to determine whether these actions depend on modulating mitochondrial activity. We measured proliferation and differentiation markers, mitochondrial structural and functional parameters as well as oxidative stress indicators and also evaluated cell transplant engraftment. This enabled us to show that melatonin (25 μM) induces NSC differentiation into oligodendrocytes and neurons. These effects depend on increased mitochondrial mass/DNA/complexes, mitochondrial respiration, and membrane potential as well as ATP synthesis in NSCs. It is also interesting to note that melatonin prevented oxidative stress caused by high levels of mitochondrial activity. Finally, we found that melatonin enriches NSC engraftment in the ND mouse model following transplantation. We concluded that a combined therapy involving transplantation of NSCs pretreated with pharmacological doses of melatonin could efficiently restore neuronal cell populations in PD and AD mouse models depending on mitochondrial activity promotion.

Effects of Tetrodotoxin in Mouse Models of Visceral Pain

Visceral pain is very common and represents a major unmet clinical need for which current pharmacological treatments are often insufficient. Tetrodotoxin (TTX) is a potent neurotoxin that exerts analgesic actions in both humans and rodents under different somatic pain conditions, but its effect has been unexplored in visceral pain. Therefore, we tested the effects of systemic TTX in viscero-specific mouse models of chemical stimulation of the colon (intracolonic instillation of capsaicin and mustard oil) and intraperitoneal cyclophosphamide-induced cystitis. The subcutaneous administration of TTX dose-dependently inhibited the number of pain-related behaviors in all evaluated pain models and reversed the referred mechanical hyperalgesia (examined by stimulation of the abdomen with von Frey filaments) induced by capsaicin and cyclophosphamide, but not that induced by mustard oil. Morphine inhibited both pain responses and the referred mechanical hyperalgesia in all tests. Conditional nociceptor‑specific Nav1.7 knockout mice treated with TTX showed the same responses as littermate controls after the administration of the algogens. No motor incoordination after the administration of TTX was observed. These results suggest that blockade of TTX-sensitive sodium channels, but not Nav1.7 subtype alone, by systemic administration of TTX might be a potential therapeutic strategy for the treatment of visceral pain.

Sigma-1 receptors control immune-driven peripheral opioid analgesia during inflammation in mice

Significance New pain medications with novel mechanisms of action are needed. Here we show that sigma-1 antagonism decreases inflammatory pain hypersensitivity by enhancing the actions of endogenous opioid peptides produced by leukocytes in mice. Sigma-1 antagonism results in opioid analgesia only at the inflamed site, where immune cells naturally accumulate. This mechanism, which maximizes the analgesic potential of immune cells in painful inflamed sites, differs from that of conventional analgesics. Sigma-1 antagonism potentiates the antinociceptive effects of opioid drugs, so sigma-1 receptors constitute a biological brake to opioid drug-induced analgesia. The pathophysiological role of this process is unknown. We aimed to investigate whether sigma-1 antagonism reduces inflammatory pain through the disinhibition of the endogenous opioidergic system in mice. The selective sigma-1 antagonists BD-1063 and S1RA abolished mechanical and thermal hyperalgesia in mice with carrageenan-induced acute (3 h) inflammation. Sigma-1–mediated antihyperalgesia was reversed by the opioid antagonists naloxone and naloxone methiodide (a peripherally restricted naloxone analog) and by local administration at the inflamed site of monoclonal antibody 3-E7, which recognizes the pan-opioid sequence Tyr–Gly–Gly–Phe at the N terminus of most endogenous opioid peptides (EOPs). Neutrophils expressed pro-opiomelanocortin, the precursor of β-endorphin (a known EOP), and constituted the majority of the acute immune infiltrate. β-endorphin levels increased in the inflamed paw, and this increase and the antihyperalgesic effects of sigma-1 antagonism were abolished by reducing the neutrophil load with in vivo administration of an anti-Ly6G antibody. The opioid-dependent sigma-1 antihyperalgesic effects were preserved 5 d after carrageenan administration, where macrophages/monocytes were found to express pro-opiomelanocortin and to now constitute the majority of the immune infiltrate. These results suggest that immune cells harboring EOPs are needed for the antihyperalgesic effects of sigma-1 antagonism during inflammation. In conclusion, sigma-1 receptors curtail immune-driven peripheral opioid analgesia, and sigma-1 antagonism produces local opioid analgesia by enhancing the action of EOPs of immune origin, maximizing the analgesic potential of immune cells that naturally accumulate in painful inflamed areas.

Targeting immune-driven opioid analgesia by sigma-1 receptors: Opening the door to novel perspectives for the analgesic use of sigma-1 antagonists

Graphical abstract Figure. No caption available. ABSTRACT Immune cells have a known role in pronociception, since they release a myriad of inflammatory algogens which interact with neurons to facilitate pain signaling. However, these cells also produce endogenous opioid peptides with analgesic potential. The sigma‐1 receptor is a ligand‐operated chaperone that modulates neurotransmission by interacting with multiple protein partners, including the &mgr;‐opioid receptor. We recently found that sigma‐1 antagonists are able to induce opioid analgesia by enhancing the action of endogenous opioid peptides of immune origin during inflammation. This opioid analgesia is seen only at the inflamed site, where immune cells naturally accumulate. In this article we review the difficulties of targeting the opioid system for selective pain relief, and discuss the dual role of immune cells in pain and analgesia. Our discussion creates perspectives for possible novel therapeutic uses of sigma‐1 antagonists as agents able to maximize the analgesic potential of the immune system.

Modality-specific peripheral antinociceptive effects of μ-opioid agonists on heat and mechanical stimuli: Contribution of sigma-1 receptors

&NA; Morphine induces peripherally &mgr;‐opioid‐mediated antinociception to heat but not to mechanical stimulation. Peripheral sigma‐1 receptors tonically inhibit &mgr;‐opioid antinociception to mechanical stimuli, but it is unknown whether they modulate &mgr;‐opioid heat antinociception. We hypothesized that sigma‐1 receptors might play a role in the modality‐specific peripheral antinociceptive effects of morphine and other clinically relevant &mgr;‐opioid agonists. Mechanical nociception was assessed in mice with the paw pressure test (450 g), and heat nociception with the unilateral hot plate (55 °C) test. Local peripheral (intraplantar) administration of morphine, buprenorphine or oxycodone did not induce antinociception to mechanical stimulation but had dose‐dependent antinociceptive effects on heat stimuli. Local sigma‐1 antagonism unmasked peripheral antinociception by &mgr;‐opioid agonists to mechanical stimuli, but did not modify their effects on heat stimulation. TRPV1+ and IB4+ cells are segregated populations of small neurons in the dorsal root ganglia (DRG) and the density of sigma‐1 receptors was higher in IB4+ cells than in the rest of small nociceptive neurons. The in vivo ablation of TRPV1‐expressing neurons with resiniferatoxin did not alter IB4+ neurons in the DRG, mechanical nociception, or the effects of sigma‐1 antagonism on local morphine antinociception in this type of stimulus. However, it impaired the responses to heat stimuli and the effect of local morphine on heat nociception. In conclusion, peripheral opioid antinociception to mechanical stimuli is limited by sigma‐1 tonic inhibitory actions, whereas peripheral opioid antinociception to heat stimuli (produced in TRPV1‐expressing neurons) is not. Therefore, sigma‐1 receptors contribute to the modality‐specific peripheral effects of opioid analgesics. Highlights&mgr;‐opioid agonists induce peripheral antinociception to heat stimulus.&mgr;‐opioid agonists do not induce peripheral antinociception to mechanical stimulus.&sgr;1 receptors do not modulate peripheral &mgr;‐opioid antinociception to heat stimulus.&sgr;1 receptors limit peripheral &mgr;‐opioid antinociception to mechanical stimulus.&sgr;1 receptors contribute to the modality‐specific peripheral effects of opioids.