Coexpression of delta- and mu-opioid receptors in nociceptive sensory neurons.
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ABSTRACT:
SUBMITTER: Wang HB
PROVIDER: S-EPMC2919913 | biostudies-literature |
REPOSITORIES: biostudies-literature
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Project description:μ-opioid receptor (MOR) agonists such as morphine are powerful analgesics used for pain therapy. However, the use of these drugs is limited by their side-effects, which include antinociceptive tolerance and dependence. Earlier studies reported that MOR analgesic tolerance is reduced by blockade of δ-opioid receptors (DORs) that interact with MORs. Recent studies show that the MOR/DOR interaction in nociceptive afferent neurons in the dorsal root ganglion may contribute to morphine analgesic tolerance. Further analysis of the mechanisms for regulating the trafficking of receptors, ion channels and signaling molecules in nociceptive afferent neurons would help to understand the nociceptive mechanisms and improve pain therapy.
| S-EPMC5560394 | biostudies-other
Project description:Background and objectiveThe role of peripheral mu-opioid receptors (MOPs) in chronic pain conditions is not well understood. Here, we used a combination of mouse genetics, behavioral assays, and pharmacologic interventions to investigate the contribution of primary afferent MOPs to nociceptive, inflammatory, and neuropathic pain, as well as to opioid analgesia.MethodsWe generated conditional knockout mice in which MOPs were selectively deleted in primary sensory neurons. Inflammatory and neuropathic pain states were induced in mutant and control wild-type mice and their behavioral responses to noxious stimuli were compared. Gross motor function was also evaluated. Immunohistochemistry was used to assess MOP expression in the dorsal root ganglia, periaqueductal gray, and small intestine. The effects of MOP agonists DALDA (dermorphin [D-Arg2, Lys4] (1-4) amide) and morphine were evaluated in pain behavior assays, and their effects on neuronal physiology in the dorsal root ganglia were evaluated in whole-cell patch-clamp recordings.ResultsConditional MOP knockouts and control mice exhibited similar behavioral responses to acute nociceptive stimuli and developed similar inflammation-induced hypersensitivity. Unilateral nerve injury in animals lacking peripheral MOPs induced enhanced, bilateral mechanical allodynia. Subcutaneously administered DALDA was unable to decrease the hypersensitivity induced by inflammation and nerve injury in MOP knockout animals, and morphine's antinociceptive effects were significantly attenuated in the absence of peripheral MOPs.ConclusionMOPs in primary sensory neurons contribute to the modulation of neuropathic pain behavior and opioid analgesia. Our observations highlight the clinical potential of peripherally acting opioid agonists in the management of inflammatory and neuropathic pain.
| S-EPMC8692009 | biostudies-literature
Project description:Background and purposeMu and delta opioid receptors(MOP, DOP) contribution to the manifestations of pathological pain is not understood. We used genetic approaches to investigate the opioid mechanisms modulating neuropathic pain and its comorbid manifestations.Experimental approachWe generated conditional knockout mice with MOP or DOP deletion in sensoryNav1.8-positive neurons (Nav1.8), in GABAergic forebrain neurons (DLX5/6) orconstitutively (CMV). Mutant mice and wild-type littermates were subjected topartial sciatic nerve ligation (PSNL) or sham surgery and their nociception wascompared. Anxiety-, depressivelike behaviour and cognitive performance were also measured. Opioid receptor mRNA expression, microgliosis and astrocytosis were assessed in the dorsalroot ganglia (DRG) and/or the spinal cord (SC).Key resultsConstitutive CMV-MOP knockouts after PSNL displayed reduced mechanical allodynia and enhanced heat hyperalgesia. This phenotype was accompanied by increased DOP expression in DRG and SC, and reduced microgliosis and astrocytosis in deep dorsal horn laminae. Conditional MOP knockouts and control mice developed similar hypersensitivity after PSNL, except for anenhanced heat hyperalgesia by DLX5/6-MOP male mice. Neuropathic pain-induced anxiety was aggravated in CMV-MOP and DLX5/6-MOP knockouts. Nerve-injured CMV-DOP mice showed increased mechanical allodynia, whereas Nav1.8-DOP and DLX5/8-DOP mice had partial nociceptive enhancement. CMV-DOP and DLX5/6-DOP mutants showed increased depressive-like behaviour after PSNL.Conclusions and implicationsMOP activity after nerve injury increased anxiety-like responses involving forebrain GABAergic neurons and enhanced mechanical pain sensitivity along with repression of DOP expression and spinal cord gliosis. In contrast, DOP shows a protective function limiting nociceptive and affective manifestations of neuropathic pain.
| S-EPMC7042108 | biostudies-literature
Project description:The peripheral δ opioid receptor (DOR) is an attractive target for analgesic drug development. There is evidence that DOR can form heteromers with the κ-opioid receptor (KOR). As drug targets, heteromeric receptors offer an additional level of selectivity and, because of allosteric interactions between protomers, functionality. Here we report that selective KOR antagonists differentially altered the potency and/or efficacy of DOR agonists in primary cultures of adult rat peripheral sensory neurons and in a rat behavioral model of thermal allodynia. In vitro, the KOR antagonist nor-binaltorphimine (nor-BNI) enhanced the potency of [D-Pen(2,5)]-enkephalin (DPDPE), decreased the potency of [D-Ala(2),D-Leu(5)]-enkephalin (DADLE), and decreased the potency and efficacy of 4-[(R)-[(2S,5R)-4-allyl-2,5-dimethylpiperazin-1-yl](3-methoxyphenyl)methyl]-N,N-diethylbenzamide (SNC80) to inhibit prostaglandin E(2) (PGE(2))-stimulated adenylyl cyclase activity. In vivo, nor-BNI enhanced the effect of DPDPE and decreased the effect of SNC80 to inhibit PGE(2)-stimulated thermal allodynia. In contrast to nor-BNI, the KOR antagonist 5'-guanidinonaltrindole (5'-GNTI) reduced the response of DPDPE both in cultured neurons and in vivo. Evidence for DOR-KOR heteromers in peripheral sensory neurons included coimmunoprecipitation of DOR with KOR, a DOR-KOR heteromer selective antibody augmented the antinociceptive effect of DPDPE in vivo, and the DOR-KOR heteromer agonist 6'-GNTI inhibited adenylyl cyclase activity in vitro as well as PGE(2)-stimulated thermal allodynia in vivo. Taken together, these data suggest that DOR-KOR heteromers exist in rat primary sensory neurons and that KOR antagonists can act as modulators of DOR agonist responses most likely through allosteric interactions between the protomers of the DOR-KOR heteromer.
| S-EPMC3263945 | biostudies-literature
Project description:Opioid neuropeptides and their receptors are evolutionarily conserved neuromodulatory systems that profoundly influence behavior. In dorsal striatum, which expresses the endogenous opioid enkephalin, patches (or striosomes) are limbic-associated subcompartments enriched in mu opioid receptors. The functional implications of opioid signaling in dorsal striatum and the circuit elements in patches regulated by enkephalin are unclear. Here, we examined how patch output is modulated by enkephalin and identified the underlying circuit mechanisms. We found that patches are relatively devoid of parvalbumin-expressing interneurons and exist as self-contained inhibitory microcircuits. Enkephalin suppresses inhibition onto striatal projection neurons selectively in patches, thereby disinhibiting their firing in response to cortical input. The majority of this neuromodulation is mediated by delta, not mu-opioid, receptors, acting specifically on intra-striatal collateral axons of striatopallidal neurons. These results suggest that enkephalin gates limbic information flow in dorsal striatum, acting via a patch-specific function for delta opioid receptors.
| S-EPMC4698909 | biostudies-literature
Project description:The role of Mu opioid receptor (MOR)-mediated regulation of GABA transmission in opioid reward is well established. Much less is known about MOR-mediated regulation of glutamate transmission in the brain and how this relates to drug reward. We previously found that MORs inhibit glutamate transmission at synapses that express the Type 2 vesicular glutamate transporter (vGluT2). We created a transgenic mouse that lacks MORs in vGluT2-expressing neurons (MORflox-vGluT2cre) to demonstrate that MORs on the vGluT2 neurons themselves mediate this synaptic inhibition. We then explored the role of MORs in vGluT2-expressing neurons in opioid-related behaviors. In tests of conditioned place preference, MORflox-vGluT2cre mice did not acquire place preference for a low dose of the opioid, oxycodone, but displayed conditioned place aversion at a higher dose, whereas control mice displayed preference for both doses. In an oral consumption assessment, these mice consumed less oxycodone and had reduced preference for oxycodone compared with controls. MORflox-vGluT2cre mice also failed to show oxycodone-induced locomotor stimulation. These mice displayed baseline withdrawal-like responses following the development of oxycodone dependence that were not seen in littermate controls. In addition, withdrawal-like responses in these mice did not increase following treatment with the opioid antagonist, naloxone. However, other MOR-mediated behaviors were unaffected, including oxycodone-induced analgesia. These data reveal that MOR-mediated regulation of glutamate transmission is a critical component of opioid reward.
| S-EPMC7854952 | biostudies-literature
Project description:Molecular cloning has identified three opioid receptors: mu (MOR), delta (DOR) and kappa (KOR). Yet, cloning of these receptor types has offered little clarification to the diverse pharmacological profiles seen within the growing number of novel opioid ligands, which has led to the proposal of multiple subtypes. In the present study, utilizing in vitro and in vivo methods including the use of opioid receptor knockout mice, we find that certain antinociceptive effects of the KOR-1 and KOR-2 subtype-selective ligands (+)-(5?,7?,8?)-N-Methyl-N-[7-(1-pyrrolidinyl)-1-oxaspiro[4.5]dec-8-yl]-benzene-acetamide (U69, 593) and 4-[(3,4-Dichlorophenyl)acetyl]-3-(1-pyrrolidinylmethyl)-1-piperazine-carboxylic acid methyl ester fumarate (GR89, 696), respectively, are potentiated by antagonism of MOR and DOR receptors. We believe that our findings can be best explained by the existence of KOR-DOR and KOR-MOR heteromers. We only find evidence for the existence of these heteromers in neurons mediating mechanical nociception, but not thermal nociception. These findings have important clinical ramifications as they reveal new drug targets that may provide avenues for more effective pain therapies.
| S-EPMC4086711 | biostudies-literature
Project description:Heteromerization of opioid receptors has been shown to alter opioid receptor pharmacology. However, how receptor heteromerization affects the processes of endocytosis and postendocytic sorting has not been closely examined. This question is of particular relevance for heteromers of the μ-opioid receptor (MOR) and δ-opioid receptor (DOR), because the MOR is recycled primarily after endocytosis and the DOR is degraded in the lysosome. Here, we examined the endocytic and postendocytic fate of MORs, DORs, and DOR/MOR heteromers in human embryonic kidney 293 cells stably expressing each receptor alone or coexpressing both receptors. We found that the clinically relevant MOR agonist methadone promotes endocytosis of MOR but also the DOR/MOR heteromer. Furthermore, we show that DOR/MOR heteromers that are endocytosed in response to methadone are targeted for degradation, whereas MORs in the same cell are significantly more stable. It is noteworthy that we found that the DOR-selective antagonist naltriben mesylate could block both methadone- and [D-Ala2,NMe-Phe4,Gly-ol5]-enkephalin-induced endocytosis of the DOR/MOR heteromers but did not block signaling from this heteromer. Together, our results suggest that the MOR adopts novel trafficking properties in the context of the DOR/MOR heteromer. In addition, they suggest that the heteromer shows "biased antagonism," whereby DOR antagonist can inhibit trafficking but not signaling of the DOR/MOR heteromer.
| S-EPMC3097170 | biostudies-literature
Project description:Enkephalin analogues with a 4-anilidopiperidine scaffold have been designed and synthesized to achieve therapeutic benefit for the treatment of pain due to mixed mu and delta opioid agonist activities. Ligand 16, in which a Dmt-substituted enkephalin-like structure was linked to the N-phenyl-N-piperidin-4-yl propionamide moiety, showed very high binding affinities (0.4 nM) at mu and delta receptors with an increased hydrophobicity (aLogP = 2.96). This novel lead compound was found to have very potent agonist activities in MVD (1.8 nM) and GPI (8.5 nM) assays.
| S-EPMC2678914 | biostudies-literature
Project description:Although opioids still remain the most powerful pain-killers, the chronic use of opioid analgesics is largely limited by their numerous side-effects, including opioid dependence. However, the mechanism underlying this dependence is largely unknown. In this study, we used the withdrawal symptoms precipitated by naloxone to characterize opioid dependence in mice. We determined the functional role of mu-opioid receptors (MORs) expressed in different subpopulations of neurons in the development of morphine withdrawal. We found that conditional deletion of MORs from glutamatergic neurons expressing vesicular glutamate transporter 2 (Vglut2+) largely eliminated the naloxone-precipitated withdrawal symptoms. In contrast, conditional deletion of MORs expressed in GABAergic neurons had a limited effect on morphine withdrawal. Consistently, mice with MORs deleted from Vglut2+ glutamatergic neurons also showed no morphine-induced locomotor hyperactivity. Furthermore, morphine withdrawal and morphine-induced hyperactivity were not significantly affected by conditional knockout of MORs from dorsal spinal neurons. Taken together, our data indicate that the development of morphine withdrawal is largely mediated by MORs expressed in Vglut2+ glutamatergic neurons.
| S-EPMC7532259 | biostudies-literature