D. Zamanillo

Sigma-1 receptors regulate activity-induced spinal sensitization and neuropathic pain after peripheral nerve injury

ABSTRACT Sigma‐1 receptor (σ1R) is expressed in key CNS areas involved in nociceptive processing but only limited information is available about its functional role. In the present study we investigated the relevance of σ1R in modulating nerve injury‐evoked pain. For this purpose, wild‐type mice and mice lacking the σ1R gene were exposed to partial sciatic nerve ligation and neuropathic pain‐related behaviors were investigated. To explore underlying mechanisms, spinal processing of repetitive nociceptive stimulation and expression of extracellular signal‐regulated kinase (ERK) were also investigated. Sensitivity to noxious heat of homozygous σ1R knockout mice did not differ from wild‐type mice. Baseline values obtained in σ1R knockout mice before nerve injury in the plantar, cold‐plate and von Frey tests were also indistinguishable from those obtained in wild‐type mice. However, cold and mechanical allodynia did not develop in σ1R null mice exposed to partial sciatic nerve injury. Using isolated spinal cords we found that mice lacking σ1R showed reduced wind‐up responses respect to wild‐type mice, as evidenced by a reduced number of action potentials induced by trains of C‐fiber intensity stimuli. In addition, in contrast to wild‐type mice, σ1R knockout mice did not show increased phosphorylation of ERK in the spinal cord after sciatic nerve injury. Both wind‐up and ERK activation have been related to mechanisms of spinal cord sensitization. Our findings identify σ1R as a constituent of the mechanisms modulating activity‐induced sensitization in pain pathways and point to σ1R as a new potential target for drugs designed to alleviate neuropathic pain.

Pharmacological properties of S1RA, a new sigma-1 receptor antagonist that inhibits neuropathic pain and activity-induced spinal sensitization

BACKGROUND AND PURPOSE The sigma‐1 (σ1) receptor is a ligand‐regulated molecular chaperone that has been involved in pain, but there is limited understanding of the actions associated with its pharmacological modulation. Indeed, the selectivity and pharmacological properties of σ1 receptor ligands used as pharmacological tools are unclear and the demonstration that σ1 receptor antagonists have efficacy in reversing central sensitization‐related pain sensitivity is still missing.

Molecular Cloning of α1d-Adrenergic Receptor and Tissue Distribution of Three α1-Adrenergic Receptor Subtypes in Mouse

Abstract: A partial cDNA encoding most of the third intracellular loop of the mouse α1d‐adrenergic receptor subtype was amplified from hippocampus by reverse transcription‐polymerase chain reaction (RT‐PCR) using degenerate oligodeoxynucleotide primers. This DNA fragment was used as a probe to isolate an α1d‐adrenergic receptor cDNA from a mouse brain cDNA library. The deduced amino acid sequence encodes a potential protein of 562 amino acids, and northern hybridization of poly(A)+ RNA isolated from mouse brain detected a single 3.0‐kb transcript. Partial cDNA fragments of the α1b‐ and α1a‐adrenergic receptor subtypes were also amplified from mouse brain and sequenced. Analysis of the mRNA expression by RT‐PCR indicated that the α1‐adrenergic receptors are widely distributed in mouse tissues. The α1d subtype is expressed in brain areas such as hippocampus, striatum, and brainstem and also in many extracerebral tissues, such as lung, liver, heart, kidney, and spleen. The α1a subtype is also expressed in many tissues, whereas the α1b subtype has a more restricted expression, with high levels in striatum, brainstem, and diencephalus.

Up-regulation of sigma1 receptor mRNA in rat brain by a putative atypical antipsychotic and sigma receptor ligand

Sigma(1) (sigma(1)) receptor mRNA expression was studied in the prefrontal cortex, striatum, hippocampus and cerebellum of rat brain by northern blot and in situ hybridization. The effects of a chronic treatment with antipsychotic drugs (haloperidol and clozapine), and with E-5842, a sigma(1) receptor ligand and putative atypical antipsychotic on sigma(1) receptor expression were examined. A significant increase in the levels of sigma(1) receptor mRNA in the prefrontal cortex and striatum after E-5842 administration was observed, while no apparent changes were seen with either haloperidol or clozapine. Our results suggest a long-term adaptation of the sigma(1) receptor at the level of mRNA expression in specific areas of the brain as a response to a sustained treatment with E-5842.

Role of peripheral versus spinal 5-HT7 receptors in the modulation of pain undersensitizing conditions

Several studies have suggested that 5‐HT7 receptors are involved in nociceptive processing but the exact contribution of peripheral versus central 5‐HT7 receptors still needs to be elucidated. In the present study, the respective roles of peripheral and spinal 5‐HT7 receptors in the modulation of mechanical hypersensitivity were investigated under two different experimental pain conditions. In a first set of experiments, the selective 5‐HT7 receptor agonist, E‐57431, was systemically, intrathecally or peripherally (intraplantarly) administered to rats sensitized by intraplantar injection of capsaicin. Oral administration of E‐57431 (1.25–10u2009mg/kg) was found to exert a clear‐cut dose‐dependent reduction of capsaicin‐induced mechanical hypersensitivity. Interestingly, intrathecal administration of E‐57431 (100u2009μg) also inhibited mechanical hypersensitivity secondary to capsaicin injection. In contrast, a dose‐dependent enhancement of capsaicin‐induced mechanical hypersensitivity was observed after local intraplantar injection of E‐57431 (0.01–1u2009μg). In a second set of experiments, E‐57431 was systemically or intrathecally administered to rats submitted to neuropathic pain (spared nerve injury model). Significant inhibition of nerve injury‐induced mechanical hypersensitivity was found after intraperitoneal (10u2009mg/kg) as well as intrathecal (100u2009μg) administration of E‐57431 in this chronic pain model. These studies provide evidence that, under sensitizing neurogenic/neuropathic conditions, activation of 5‐HT7 receptors exerts antinociceptive effects at the level of the spinal cord and pronociceptive effects at the periphery. The antinociceptive effect mediated by central 5‐HT7 receptors seems to predominate over the pronociceptive effect at the periphery when a selective 5‐HT7 receptor agonist is systemically administered.

Operant self-administration of a sigma ligand improves nociceptive and emotional manifestations of neuropathic pain.

The treatment of neuropathic pain is unsatisfactory at the present moment and the sigma 1 receptor has been identified as a new potential target for neuropathic pain. The aim of this study was to use an operant self‐administration model to reveal the potential interest of a new sigma 1 receptor antagonist, S1RA, in chronic pain that was developed in mice by a partial ligation of the sciatic nerve.

Identification of a cyclic adenosine 3′,5′-monophosphate-dependent protein kinase phosphorylation site in the carboxy terminal tail of human D1 dopamine receptor

Each of the dopamine receptor subtypes contains several consensus sites for phosphorylation in their intracellular domains. We have used fusion proteins of the carboxy terminal tail of D1 and D5 dopamine receptors to study the phosphorylation of these proteins by cyclic adenosine 3,5 monophosphate (cAMP)-dependent protein kinase (PKA) and protein kinase C (PKC). The fusion protein of D1 dopamine receptor was efficiently phosphorylated by PKA, but not by PKC. Site-directed mutagenesis of serine 380 to an alanine residue precluded the phosphorylation by the kinase. No phosphorylation of the D5 dopamine receptor fusion protein was observed with either PKA or PKC, which indicates that these receptor subtypes might differ in their mechanisms of regulation.

Fibroblast growth factor-2 is selectively modulated in the rat brain by E-5842, a preferential sigma-1 receptor ligand and putative atypical antipsychotic.

Fibroblast growth factor‐2 (FGF‐2) is a member of a large family of trophic factors whose expression is regulated under several conditions in different areas of the brain. The goal of our experiments was to determine whether the administration of 4‐(4‐fluorophenyl)‐1,2,3,6‐tetrahydro‐1‐[4‐(1,2,4‐triazol‐1‐il)butyl] pyridine citrate (E‐5842), a sigma‐1 receptor ligand and putative atypical antipsychotic, could regulate the expression of FGF‐2. After chronic treatment with E‐5842 (21u2003days, and the animals killed 24u2003h after the last administration), an up‐regulation was observed of the expression of FGF‐2 mRNA in the prefrontal cortex and the striatum, and a down‐regulation of the expression of FGF‐2 mRNA in the hypothalamus of the rat brain. Acute treatment with E‐5842 (one single administration and animals killed 6u2003h later) up‐regulated FGF‐2 expression in the prefrontal cortex, the striatum, the hypothalamus and the hippocampus in a dose‐dependent manner. The acute up‐regulation was transient and disappeared 24u2003h after E‐5842 administration. The induction of FGF‐2 in the striatum after repeated administration has been described for clozapine, but our data concerning regulation in the prefrontal cortex suggest that this effect is unique to E‐5852 among other antipsychotics. Given the neuroprotective activity of FGF‐2, the data presented here might be relevant to the deficit in cognition and other symptoms that appear in schizophrenia.

Regulation of Ionotropic Glutamate Receptor Subunits in Different Rat Brain Areas by a Preferential Sigma1 Receptor Ligand and Potential Atypical Antipsychotic

The effect of chronic administration of the putative atypical antipsychotic E-5842, a preferential sigma1 receptor ligand, on ionotropic glutamate receptor subunit levels of mRNA and protein, was studied. The repeated administration of E-5842 differentially regulated levels of the NMDA-2A and of GluR2 subunits in a regionally specific way. Levels of immunoreactivity for the NMDA-2A subunit were up-regulated in the medial prefrontal cortex, the frontoparietal cortex, the cingulate cortex, and in the dorsal striatum, while they were down-regulated in the nucleus accumbens. Levels of the GluR2 subunit of the AMPA receptor were up-regulated in the medial prefrontal cortex and the nucleus accumbens and down-regulation was observed in the dorso-lateral striatum. Regulation of the levels of mRNA for the different subunits was also observed in some cases. The results show that E-5842, through a mechanism still unknown, is able to modify levels of several glutamate receptor subunits and these changes could be related to its antipsychotic activity in pre-clinical tests.

Effects of centrally acting analgesics on spinal segmental reflexes and wind‐up

The spinal cord is a prime site of action for analgesia. Here we characterize the effects of established analgesics on segmental spinal reflexes. The aim of the study was to look for the pattern of action or signature of analgesic effects on these reflexes.