Cruz Miguel Cendán

Pharmacology and Therapeutic Potential of Sigma1 Receptor Ligands

Sigma (σ) receptors, initially described as a subtype of opioid receptors, are now considered unique receptors. Pharmacological studies have distinguished two types of σ receptors, termed σ1 and σ2. Of these two subtypes, the σ1 receptor has been cloned in humans and rodents, and its amino acid sequence shows no homology with other mammalian proteins. Several psychoactive drugs show high to moderate affinity for σ1 receptors, including the antipsychotic haloperidol, the antidepressant drugs fluvoxamine and sertraline, and the psychostimulants cocaine and methamphetamine; in addition, the anticonvulsant drug phenytoin allosterically modulates σ1 receptors. Certain neurosteroids are known to interact with σ1 receptors, and have been proposed to be their endogenous ligands. These receptors are located in the plasma membrane and in subcellular membranes, particularly in the endoplasmic reticulum, where they play a modulatory role in intracellular Ca2+ signaling. Sigma1 receptors also play a modulatory role in the activity of some ion channels and in several neurotransmitter systems, mainly in glutamatergic neurotransmission. In accordance with their widespread modulatory role, σ1 receptor ligands have been proposed to be useful in several therapeutic fields such as amnesic and cognitive deficits, depression and anxiety, schizophrenia, analgesia, and against some effects of drugs of abuse (such as cocaine and methamphetamine). In this review we provide an overview of the present knowledge of σ1 receptors, focussing on σ1 ligand neuropharmacology and the role of σ1 receptors in behavioral animal studies, which have contributed greatly to the potential therapeutic applications of σ1 ligands.

Sigma-1 receptors are essential for capsaicin-induced mechanical hypersensitivity: Studies with selective sigma-1 ligands and sigma-1 knockout mice

ABSTRACT We evaluated the role of σ1 receptors on capsaicin‐induced mechanical hypersensitivity and on nociceptive pain induced by punctate mechanical stimuli, using wild‐type and σ1 receptor knockout (σ1‐KO) mice and selective σ1 receptor‐acting drugs. Mutation in σ1‐KO mice was confirmed by PCR analysis of genomic DNA and, at the protein level, by [3H](+)‐pentazocine binding assays. Both wild‐type and σ1‐KO mice not treated with capsaicin showed similar responses to different intensities of mechanical stimuli (0.05–8 g force), ranging from innocuous to noxious, applied to the hind paw. This indicates that σ1 gene inactivation does not modify the perception of punctate mechanical stimuli. The intraplantar (i.pl.) administration of capsaicin induced dose‐dependent mechanical allodynia in wild‐type mice (markedly reducing both the threshold force necessary to induce paw withdrawal and the latency to paw withdrawal induced by a given force). In contrast, capsaicin was completely unable to induce mechanical hypersensitivity in σ1‐KO mice. The high‐affinity and selective σ1 antagonists BD‐1063, BD‐1047 and NE‐100, administered subcutaneously (s.c.), dose‐dependently inhibited mechanical allodynia induced by capsaicin (1 μg,i.pl.), yielding ED50 (mg/kg) values of 15.80 ± 0.93, 29.31 ± 1.65 and 40.74 ± 7.20, respectively. The effects of the σ1 antagonists were reversed by the σ1 agonist PRE‐084 (32 mg/kg, s.c.). None of the drugs tested modified the responses induced by a painful mechanical punctate stimulus (4 g force) in nonsensitized animals. These results suggest that σ1 receptors are essential for capsaicin‐induced mechanical hypersensitivity, but are not involved in mechanical nociceptive pain.

Tetrodotoxin inhibits the development and expression of neuropathic pain induced by paclitaxel in mice

&NA; We evaluated the effect of low doses of systemically administered tetrodotoxin (TTX) on the development and expression of neuropathic pain induced by paclitaxel in mice. Treatment with paclitaxel (2 mg/kg, i.p., once daily during 5 days) produced long‐lasting (2–4 weeks) heat hyperalgesia (plantar test), mechanical allodynia (electronic Von Frey test) and cold allodynia (acetone drop method), with maximum effects observed on days 7, 10 and 10–14, respectively. Acute subcutaneous treatment with 1 or 3 μg/kg of TTX reduced the expression of mechanical allodynia, whereas higher doses (3 or 6 μg/kg) were required to reduce the expression of cold allodynia and heat hyperalgesia. In contrast, TTX (3 or 6 μg/kg, s.c.) did not affect the response to the same thermal and mechanical stimuli in control animals, which indicates that the antihyperalgesic and antiallodynic effects of TTX were not due to unspecific inhibition of the perception of these stimuli. Administration of TTX (6 μg/kg, s.c.) 30 min before each of the 5 doses of paclitaxel did not modify the development of heat hyperalgesia produced by the antineoplastic, but abolished the development of mechanical and cold allodynia. Coadministration of a lower dose of TTX (3 μg/kg) also prevented the development of mechanical allodynia. No signs of TTX‐induced toxicity or motor incoordination were observed. These data suggest that low doses of TTX can be useful to prevent and treat paclitaxel‐induced neuropathic pain, and that TTX‐sensitive subtypes of sodium channels play a role in the pathogenesis of chemotherapy‐induced neuropathic pain.

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.

Effects of serine/threonine protein phosphatase inhibitors on morphine-induced antinociception in the tail flick test in mice

The aim of this study was to evaluate the effects of serine/threonine protein phosphatase (PP) inhibitors on morphine-induced antinociception in the tail flick test in mice, and on [3H]naloxone binding to the forebrain crude synaptosome fraction. Neither okadaic acid nor cantharidin (1-10000 nM) displaced [3H]naloxone from its specific binding sites, which indicates that they do not interact at the opioid receptor level. The i.c.v. administration of very low doses of okadaic acid (0.001-1 pg/mouse) and cantharidin (0.001-1 ng/mouse), which inhibit PP2A, produced a dose-dependent antagonism of the antinociception induced by morphine (s.c.). However, L-nor-okadaone (0.001 pg/mouse-1 ng/mouse, i.c.v.), an analogue of okadaic acid lacking activity against protein phosphatases, did not affect the antinociceptive effect of morphine. On the other hand, high doses of okadaic acid (10 ng/mouse, i.c.v.) and cantharidin (1 microg/mouse, i.c.v.), which also block PP1, and calyculin-A (0.1 fg/mouse-1 ng/mouse, i.c.v.), which inhibits equally both PP1 and PP2A, did not modify the morphine-induced antinociception. These results suggest that the activation of type 2A serine/threonine protein phosphatases may play a role in the antinociceptive effect of morphine, and that PP1 might counterbalace this activity.

Visceral and somatic pain modalities reveal NaV1.7‐independent visceral nociceptive pathways

Voltage‐gated sodium channels play a fundamental role in determining neuronal excitability. Specifically, voltage‐gated sodium channel subtype NaV1.7 is required for sensing acute and inflammatory somatic pain in mice and humans but its significance in pain originating from the viscera is unknown. Using comparative behavioural models evoking somatic and visceral pain pathways, we identify the requirement for NaV1.7 in regulating somatic (noxious heat pain threshold) but not in visceral pain signalling. These results enable us to better understand the mechanisms underlying the transduction of noxious stimuli from the viscera, suggest that the investigation of pain pathways should be undertaken in a modality‐specific manner and help to direct drug discovery efforts towards novel visceral analgesics.

Inhibitors of serine/threonine protein phosphatases antagonize the antinociception induced by agonists of α2 adrenoceptors and GABAB but not κ-opioid receptors in the tail flick test in mice

&NA; We previously reported that serine/threonine protein phosphatases (PPs) play a role in the antinociception induced by the μ‐opioid receptor agonist morphine. In this study we evaluated the possible involvement of PPs on the antinociception induced by agonists of others G protein‐coupled receptors in the tail flick test in mice. The subcutaneous administration of clonidine (0.25–4 mg/kg), baclofen (2–32 mg/kg) or U50,488H (2–16 mg/kg) (agonists of α2 adrenoceptors, GABAB and κ‐opioid receptors, respectively) produced dose‐dependent antinociception. The antinociceptive effects of clonidine and baclofen were antagonized in a dose‐dependent way by the protein phosphatase inhibitors okadaic acid (0.001–10 pg/mouse, i.c.v.) and cantharidin (0.001–10 ng/mouse, i.c.v.), and okadaic acid was 1000 times more potent than cantharidin in producing this effect. The effects of these drugs appear to be specifically due to the blockade of PPs, since L‐norokadaone (an analogue of okadaic acid that has no effect on PPs) did not modify clonidine‐ or baclofen‐induced antinociception over the wide range of doses used (0.001–1000 pg/mouse, i.c.v.). On the other hand, the antinociception induced by activation of κ‐opioid receptors with U50,488H was not modified by okadaic acid or cantharidin. In conclusion, our data support the idea that serine/threonine PPs are differentially involved in the antinociceptive effects of several agonists of G protein‐coupled receptors in mice.

Hypoalgesia Induced by Reward Devaluation in Rats

Reduced sensitivity to physical pain (hypoalgesia) has been reported after events involving reward devaluation. Reward devaluation was implemented in a consummatory successive negative contrast (cSNC) task. Food-deprived Wistar rats had access to 32% sucrose during 16 sessions followed by access to 4% sucrose during 3 additional sessions. An unshifted control group had access to 4% sucrose throughout the 19 sessions. Pain sensitivity was measured using von Frey filaments (Experiment 1) and Hargreaves thermal stimuli (Experiment 2) in pretraining baseline, 5 min, and 300 min after either the first (session 17) or second (session 18) devaluation session in the cSNC situation. Sucrose consumption was lower in downshifted groups relative to unshifted groups during postshift sessions—the cSNC effect. Hypoalgesia was observed in downshifted groups relative to unshifted controls when pain sensitivity was assessed 5 min after either the first or second devaluation session, regardless of the pain sensitivity test used. Both pain sensitivity tests yielded evidence of hypoalgesia 300 min after the second downshift session, but not 300 min after the first devaluation session. Whereas hypoalgesia was previously shown only after the second devaluation session, here we report evidence of hypoalgesia after both the first and second devaluation sessions using mechanical and thermal nociceptive stimuli. Moreover, the hypoalgesia observed 300 min after the second devaluation session in both experiments provides unique evidence of the effects of reward loss on sensitivity to physical pain 5 hours after the loss episode. The underlying neurobehavioral mechanisms remain to be identified.

245 ROLE OF SIGMA-1 RECEPTORS IN COLD ALLODYNIA INDUCED BY PACLITAXEL

This substance regulates various systems including excitatory glutamatergic nervous system activation of the N-methyl-Daspartate (NMDA) receptor related to nitric oxide (NO) production. Hyperalgesia caused by peripheral tissue damage is involved in glutamate-related neuronal plasticity of the spinal cord. However, its mechanism of action still is unknown. Therefore, we determined the analgesic effect of agmatine in relation to modification of glutamate-related neuronalplasticity using a rat inflammatory pain model. Methods: SD rats implanted with an intrathecal catheter were subjected to formalin-induced hyperalgesia. Pain behavior was assessed by counting spontaneous flinches per min after formalin injection into paws under halothane anesthesia. The rats were separated into six groups as follows, (1) Non-treated (saline, it. inj.); (2) Agmatine (30ug, it. inj.); (3) MK-801 (15ug, it. inj.); (4) AVS (OH radical scavenger, 100ug, it. inj); (5) Agmatine + MK-801; (6) Agmatine + AVS. Results: Rats showed biphasic pain behavior – the sum of the first (0–5 min) is 14/min and the sum of the second (20–60 min) is 147/min. All drugs diminished the sum of the second phase 32–68% compared with non-treated animals but did not affect the first phase. An additive effect of agmatine was obtained with MK801 but not with AVS. Conclusion: We suggest that the synergistic analgesia effect of agmatine may involve the inhibition of NMDA receptor(s) related to NO production, and that agmatine acts on multiple sites of glutamatergic neuronal activation in developing hyperalgesia.