Abstract
BackgroundThe G protein-coupled receptor 40 (GPR40), broadly expressed in various tissues such as the spinal cord, exerts multiple physiological functions including pain regulation. This study aimed to elucidate the mechanisms underlying GPR40 activation-induced antinociception in neuropathic pain, particularly related to the spinal glial expression of IL-10 and subsequent β-endorphin.MethodsSpinal nerve ligation-induced neuropathic pain model was used in this study. β-Endorphin and IL-10 levels were measured in the spinal cord and cultured primary microglia, astrocytes, and neurons. Double immunofluorescence staining of β-endorphin with glial and neuronal cellular biomarkers was also detected in the spinal cord and cultured primary microglia, astrocytes, and neurons.ResultsGPR40 was expressed on microglia, astrocytes, and neurons in the spinal cords and upregulated by spinal nerve ligation. Intrathecal injection of the GPR40 agonist GW9508 dose-dependently attenuated mechanical allodynia and thermal hyperalgesia in neuropathic rats, with Emax values of 80% and 100% MPE and ED50 values of 6.7 and 5.4 μg, respectively. Its mechanical antiallodynia was blocked by the selective GPR40 antagonist GW1100 but not GPR120 antagonist AH7614. Intrathecal GW9508 significantly enhanced IL-10 and β-endorphin immunostaining in spinal microglia and astrocytes but not in neurons. GW9508 also markedly stimulated gene and protein expression of IL-10 and β-endorphin in cultured primary spinal microglia and astrocytes but not in neurons, originated from 1-day-old neonatal rats. The IL-10 antibody inhibited GW9508-stimulated gene expression of the β-endorphin precursor proopiomelanocortin (POMC) but not IL-10, whereas the β-endorphin antibody did not affect GW9508-stimulated IL-10 or POMC gene expression. GW9508 increased phosphorylation of mitogen-activated protein kinases (MAPKs) including p38, extracellular signal-regulated kinase (ERK), and c-Jun N-terminal kinase (JNK), and its stimulatory effects on IL-10 and POMC expression were blocked by each MAPK isoform inhibitor. Spinal GW9508-induced mechanical antiallodynia was completely blocked by intrathecal minocycline, IL-10 neutralizing antibody, β-endorphin antiserum, and μ-opioid receptor-preferred antagonist naloxone.ConclusionsOur results illustrate that GPR40 activation produces antinociception via the spinal glial IL-10/β-endorphin antinociceptive pathway.
Highlights
The orphan G protein-coupled receptor 40 (GPR40), known as free fatty acid (FFA) receptor 1, was first discovered in 1997 [1] and deorphanized in 2003 [2]
Intrathecal GW9508 exerted mechanical antiallodynia and thermal antihyperalgesia in neuropathic pain via GPR40 activation The antinociceptive effects of the GPR40 agonist GW9508 were assessed in neuropathic rats induced by L5/L6 spinal nerve ligation approximately 2 weeks after surgery
The notion is supported by the following findings: (1) Intrathecal injection of GW9508 stimulated the expression of IL-10 and β-endorphin in microglia and astrocytes, but not in neurons, in both contralateral and ipsilateral spinal cords of neuropathic rats, directly identified by double immunofluorescence staining of IL-10 and β-endorphin with cellular biomarkers, i.e., ionized calcium binding adaptor molecule-1 (Iba-1), glial fibrillary acid protein (GFAP), and neuronal specific nuclear protein (NeuN). (2) Treatment with GW9508 stimulated the gene and protein expression of IL-10 and β-endorphin in primary cultures of microglia and astrocytes, but not of neurons, in a GW1100-reversible manner
Summary
The orphan G protein-coupled receptor 40 (GPR40), known as free fatty acid (FFA) receptor 1, was first discovered in 1997 [1] and deorphanized in 2003 [2]. Belonging to the family A of the G protein-coupled receptor, it is activated by saturated/unsaturated medium- and long-chain FFA [2, 3]. GPR40 activation exerts multiple physiological functions in glucose metabolism, including regulation of the secretion of insulin [3], incretin [6], and glucagon [7]. GPR40 activation directly led to ERK activation, which subsequently performed profound functions in neuronal plasticity and long-term memory [15, 16]. The G protein-coupled receptor 40 (GPR40), broadly expressed in various tissues such as the spinal cord, exerts multiple physiological functions including pain regulation. This study aimed to elucidate the mechanisms underlying GPR40 activation-induced antinociception in neuropathic pain, related to the spinal glial expression of IL-10 and subsequent β-endorphin
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