Interaction of the mu-opioid receptor with synaptophysin influences receptor trafficking and signaling

Effect of tramadol on apoptosis and synaptogenesis in hippocampal neurons: The possible role of µ-opioid receptor

Tramadol is a synthetic opioid with centrally acting analgesic activity that alleviates moderate to severe pain and treats withdrawal symptoms of the other opioids. Like other opioid drugs, tramadol abuse has adverse effects on central nervous system components. Chronic administration of tramadol induces maladaptive plasticity in brain structures responsible for cognitive function, such as the hippocampus. However, the mechanisms by which tramadol induces these alternations are not entirely understood. Here, we examine the effect of tramadol on apoptosis and synaptogenesis of hippocampal neuronal in vitro. First, the primary culture of hippocampal neurons from neonatal rats was established, and the purity of the neuronal cells was verified by immunofluorescent staining. To evaluate the effect of tramadol on neuronal cell viability MTT assay was carried out. The western blot analysis technique was performed for the assessment of apoptosis and synaptogenesis markers. Results show that chronic exposure to tramadol reduces cell viability of neuronal cells and naloxone reverses this effect. Also, the level of caspase-3 significantly increased in tramadol-exposed hippocampal neurons. Moreover, tramadol downregulates protein levels of synaptophysin and stathmin as synaptogenesis markers. Interestingly, the effects of tramadol were abrogated by naloxone treatment. These findings suggest that tramadol can induce neurotoxicity in hippocampal neuronal cells, and this effect was partly mediated through opioid receptors.

From Pleasure to Pain, and Back Again: The Intricate Relationship Between Alcohol and Nociception

AIMS

A close and bidirectional relationship between alcohol consumption and pain has been previously reported and discussed in influential reviews. The goal of the present narrative review is to provide an update on the developments in this field in order to guide future research objectives.

METHODS

We evaluated both epidemiological and neurobiological literature interrogating the relationship between alcohol use and pain for the presence of significant effects. We outlined studies on interactions between alcohol use and pain using both self-reports and objective experimental measures and discussed potential underlying mechanisms of these interactions.

RESULTS

Epidemiological, preclinical and clinical literature point to three major interactions between alcohol use and pain: (a) alcohol use leading to hyperalgesia, (b) alcohol use moderating pain and hyperalgesia and (c) chronic pain as a risk factor predisposing to alcohol relapse. Neurobiological studies using animal models to assess these interactions have transitioned from mostly involuntary modes of experimenter-controlled alcohol administration to self-administration procedures, and increasingly indicate that neuronal circuits implicated in both withdrawal and anticipation stages of alcohol use disorder also have a role in chronic pain. Mechanistically, alterations in GABA, glutamate, the corticotropin-releasing factor system, endogenous opioids and protein kinase C appear to play crucial roles in this maladaptive overlap.

CONCLUSIONS

Many of the principles explaining the interactions between alcohol and pain remain on a strong foundation, but continuing progress in modeling these interactions and underlying systems will provide a clearer basis for understanding, and ultimately treating, the damaging aspects of this interaction.

STAT3 signal that mediates the neural plasticity is involved in willed-movement training in focal ischemic rats

Willed-movement training has been demonstrated to be a promising approach to increase motor performance and neural plasticity in ischemic rats. However, little is known regarding the molecular signals that are involved in neural plasticity following willed-movement training. To investigate the potential signals related to neural plasticity following willed-movement training, littermate rats were randomly assigned into three groups: middle cerebral artery occlusion, environmental modification, and willed-movement training. The infarct volume was measured 18 d after occlusion of the right middle cerebral artery. Reverse transcription-polymerase chain reaction (PCR) and immunofluorescence staining were used to detect the changes in the signal transducer and activator of transcription 3 (STAT3) mRNA and protein, respectively. A chromatin immunoprecipitation was used to investigate whether STAT3 bound to plasticity-related genes, such as brain-derived neurotrophic factor (BDNF), synaptophysin, and protein interacting with C kinase 1 (PICK1). In this study, we demonstrated that STAT3 mRNA and protein were markedly increased following 15-d willed-movement training in the ischemic hemispheres of the treated rats. STAT3 bound to BDNF, PICK1, and synaptophysin promoters in the neocortical cells of rats. These data suggest that the increased STAT3 levels after willed-movement training might play critical roles in the neural plasticity by directly regulating plasticity-related genes.

Effects of thienorphine on synaptic structure and synaptophysin expression in the rat nucleus accumbens

The partial opioid agonist thienorphine is currently in Phase II clinical trials in China as a candidate drug for the treatment of opioid dependence. However, its effect on synaptic plasticity in the NAc (nucleus accumbens) remains unclear. In the present study, we measured structural parameters of the synaptic interface to investigate the effect of thienorphine, morphine or a combination of both on synaptic morphology in the NAc of rats. Expression of synaptophysin was also examined. Ultrastructural observation showed that synaptic alterations were less pronounced after chronic thienorphine administration than after chronic morphine administration. Animals that received thienorphine had thinner postsynaptic densities and shorter active zones in the NAc compared with those in the saline group, but the active zone was larger, and the cleft narrower, than those in the morphine group. Furthermore, synaptophysin expression in the NAc was significantly greater after chronic administration of thienorphine, morphine, or both, than after saline. These results identified interesting differences between thienorphine and morphine in their effects on synaptic structure and synaptophysin expression in the rat NAc. Further study is deserved to investigate thienorphine as a new treatment for opioid dependence.

Role of FK506 binding protein 12 in morphine-induced μ-opioid receptor internalization and desensitization

Agonist-activated μ-opioid receptor (OPRM1) undergoes robust receptor phosphorylation by G protein-coupled receptor kinases and subsequent β-arrestin recruitment, triggering receptor internalization and desensitization. Morphine, a widely prescribed opioid, induces receptor phosphorylation inefficiently. Previously we reported that FK506 binding protein 12 (FKBP12) specifically interacts with OPRM1 and such interaction attenuates receptor phosphorylation and facilitates morphine-induced recruitment and activation of protein kinase C. In the current study, we demonstrated that the association of FKBP12 with OPRM1 also affects morphine-induced receptor internalization and G protein-dependent adenylyl cyclase desensitization. Morphine induced faster receptor internalization and adenylyl cyclase desensitization in cells expressing OPRM1 with Pro(353) mutated to Ala (OPRM1P353A), which does not interact with FKBP12, or in the presence of FK506 which dissociates the receptor-FKBP12 interaction. Furthermore, knockdown of cellular FKBP12 level by siRNA accelerated morphine-induced receptor internalization and adenylyl cyclase desensitization. Our study further demonstrated that peptidyl prolyl cis-trans isomerase activity of FKBP12 probably plays a role in inhibition of receptor phosphorylation. In the view that internalized receptor recycles and thus counteracts the development of analgesic tolerance, receptor's association with FKBP12 could also contribute to the development of morphine tolerance through modulation of receptor trafficking.

δ-opioid receptor activation leads to neurite outgrowth and neuronal differentiation via a STAT5B-Gαi/o pathway

It remains unclear how opioid receptors (δ, μ, κ) are implicated in mechanisms controlling differentiation, cell proliferation, and survival. Opioid receptors are coupled to Gi/Go proteins and recent findings have shown that opioid receptors can form a multicomponent signaling complex, consisting of members of G protein and the signal transducer and activator of transcription (STAT)5B. We thus wondered whether activation of the opioid receptors could direct differentiation and neurite outgrowth through a molecular pathway involving STAT5B and other signaling intermediates. We demonstrate that prolonged δ-opioid receptor (δ-OR) activation with opioid agonists induces STAT5B phosphorylation in Neuro-2A cells. Moreover, [D-Ser2, Leu5, Thr6]-enkephalin-activation of δ-OR triggers neurite outgrowth and neuronal survival; these effects are blocked by the selective antagonist naltrindole, by treatment with pertussis toxin, and after expression of a dominant negative mutant of STAT5B (DN-STAT5B), suggesting that the signaling pathway participating in this mechanism involves Gi/o proteins and p-STAT5B. Additional studies have shown that while [D-Ser(2) , Leu(5) , Thr(6) ]-enkephalin exposure of neuroblastoma cells induces a marked increase in the differentiation marker proteins, βIII-tubulin (Tuj-1), synaptophysin, and neural cell adhesion molecule, over-expression of the DN-STAT5B attenuated significantly their expression levels. Taken together, our findings demonstrate that δ-OR activation leads to a number of neurotropic events via a Gαi/o-linked and STAT5B-dependent manner. We propose a novel signalling pathway for δ-opioid receptor (δ-ΟR)-mediated neurotropic events. STAT5B interacts with the δ-ΟR and upon prolonged receptor activation phosphorylates STAT5B in a Gi/Go dependent manner leading to increased neuronal survival, neurite outgrowth and differentiation. These findings contribute to a better understanding of the molecular and cellular events following δ-OR activation and suggest a possible neuroprotective role opioids could exert.

Neonatal administration of thimerosal causes persistent changes in mu opioid receptors in the rat brain

Thimerosal added to some pediatric vaccines is suspected in pathogenesis of several neurodevelopmental disorders. Our previous study showed that thimerosal administered to suckling rats causes persistent, endogenous opioid-mediated hypoalgesia. Here we examined, using immunohistochemical staining technique, the density of μ-opioid receptors (MORs) in the brains of rats, which in the second postnatal week received four i.m. injections of thimerosal at doses 12, 240, 1,440 or 3,000 μg Hg/kg. The periaqueductal gray, caudate putamen and hippocampus were examined. Thimerosal administration caused dose-dependent statistically significant increase in MOR densities in the periaqueductal gray and caudate putamen, but decrease in the dentate gyrus, where it was accompanied by the presence of degenerating neurons and loss of synaptic vesicle marker (synaptophysin). These data document that exposure to thimerosal during early postnatal life produces lasting alterations in the densities of brain opioid receptors along with other neuropathological changes, which may disturb brain development.

The association of dynamin with synaptophysin regulates quantal size and duration of exocytotic events in chromaffin cells

Although synaptophysin is one of the most abundant integral proteins of synaptic vesicle membranes, its contribution to neurotransmitter release remains unclear. One possibility is that through its association with dynamin it controls the fine tuning of transmitter release. To test this hypothesis, we took advantage of amperometric measurements of quantal catecholamine release from chromaffin cells. First, we showed that synaptophysin and dynamin interact in chromaffin granule-rich fractions and that this interaction relies on the C terminal of synaptophysin. Experimental maneuvers that are predicted to disrupt the association between these two proteins, such as injection of antibodies against dynamin or synaptophysin, or peptides homologous to the C terminal of synaptophysin, increased the quantal size and duration of amperometric spikes. In contrast, the amperometric current that precedes the spike remained unchanged, indicating that synaptophysin/dynamin association does not regulate the initial fusion pore, but it appears to target a later step of exocytosis to control the amount of catecholamines released during a single vesicle fusion event.

A conserved cysteine residue in the third transmembrane domain is essential for homomeric 5-HT3 receptor function

The cysteine (Cys) residue at position 312 in the third transmembrane domain (M3) is conserved among 5-hydroxytryptamine type 3 (5-HT(3)) receptor subunits and many other subunits of the nicotinic acetylcholine (nACh) related Cys-loop receptor family, including most of the gamma-aminobutyric acid type A (GABA(A)) and glycine receptor subunits. To elucidate a possible role for the Cys-312 in human 5-HT(3)A receptors, we replaced it with alanine and expressed the 5-HT(3)A(C312A) mutant in HEK293 cells. The mutation resulted in an absence of 5-HT-induced whole-cell current without reducing homopentamer formation, surface expression or 5-HT binding. The 5-HT(3)A(C312A) mutant, when co-expressed with the wild-type 5-HT(3)A subunit, did not affect functional expression of receptors, suggesting that the mutant is not dominant negative. Interestingly, co-expression of 5-HT(3)A(C312A) with 5-HT(3)B led to surface expression of heteropentamers that mediated small 5-HT responses. This suggests that the Cys-312 is essential for homomeric but not heteromeric receptor gating. To further investigate the relationship between residue 312 and gating we replaced it with amino acids located at the equivalent position within other Cys-loop subunits that are either capable or incapable of forming functional homopentamers. Replacement of 5-HT(3)A Cys-312 by Gly or Leu (equivalent residues in the nACh receptor delta and gamma subunits) abolished and severely attenuated function, respectively, whereas replacement by Thr or Ser (equivalent residues in nACh receptor alpha7 and GABA(A) subunits) supported robust function. Thus, 5-HT(3)A residue 312 and equivalent polar residues in the M3 of other Cys-loop subunits are essential determinants of homopentameric gating.

Modulation of opioid receptor function by protein-protein interactions

Opioid receptors, MORP, DORP and KORP, belong to the family A of G protein coupled receptors (GPCR), and have been found to modulate a large number of physiological functions, including mood, stress, appetite, nociception and immune responses. Exogenously applied opioid alkaloids produce analgesia, hedonia and addiction. Addiction is linked to alterations in function and responsiveness of all three opioid receptors in the brain. Over the last few years, a large number of studies identified protein-protein interactions that play an essential role in opioid receptor function and responsiveness. Here, we summarize interactions shown to affect receptor biogenesis and trafficking, as well as those affecting signal transduction events following receptor activation. This article also examines protein interactions modulating the rate of receptor endocytosis and degradation, events that play a major role in opiate analgesia. Like several other GPCRs, opioid receptors may form homo or heterodimers. The last part of this review summarizes recent knowledge on proteins known to affect opioid receptor dimerization.

Endogenous opiates and behavior: 2007

This paper is the thirtieth consecutive installment of the annual review of research concerning the endogenous opioid system. It summarizes papers published during 2007 that studied the behavioral effects of molecular, pharmacological and genetic manipulation of opioid peptides, opioid receptors, opioid agonists and opioid antagonists. The particular topics that continue to be covered include the molecular-biochemical effects and neurochemical localization studies of endogenous opioids and their receptors related to behavior, and the roles of these opioid peptides and receptors in pain and analgesia; stress and social status; tolerance and dependence; learning and memory; eating and drinking; alcohol and drugs of abuse; sexual activity and hormones, pregnancy, development and endocrinology; mental illness and mood; seizures and neurologic disorders; electrical-related activity and neurophysiology; general activity and locomotion; gastrointestinal, renal and hepatic functions; cardiovascular responses; respiration and thermoregulation; and immunological responses.

Membrane glycoprotein M6A promotes mu-opioid receptor endocytosis and facilitates receptor sorting into the recycling pathway

The interaction of mu-opioid receptor (MOPr) with the neuronal membrane glycoprotein M6a is known to facilitate MOPr endocytosis in human embryonic kidney 293 (HEK293) cells. To further study the role of M6a in the post-endocytotic sorting of MOPr, we investigated the agonist-induced co-internalization of MOPr and M6a and protein targeting after internalization in HEK293 cells that co-expressed HA-tagged MOPr and Myc-tagged M6a. We found that M6a, MOPr, and Rab 11, a marker for recycling endosomes, co-localized in endocytotic vesicles, indicating that MOPr and M6a are primarily targeted to recycling endosomes after endocytosis. Furthermore, co-expression of M6a augmented the post-endocytotic sorting of delta-opioid receptors into the recycling pathway, indicating that M6a might have a more general role in opioid receptor post-endocytotic sorting. The enhanced post-endocytotic sorting of MOPr into the recycling pathway was accompanied by a decrease in agonist-induced receptor down-regulation of M6a in co-expressing cells. We tested the physiological relevance of these findings in primary cultures of cortical neurons and found that co-expression of M6a markedly increased the translocation of MOPrs from the plasma membrane to intracellular vesicles at steady state and significantly enhanced both constitutive and agonist-induced receptor endocytosis. In conclusion, our results strongly indicate that M6a modulates MOPr endocytosis and post-endocytotic sorting and has an important role in receptor regulation.

Role of receptor internalization in the agonist-induced desensitization of cannabinoid type 1 receptors

Agonist-induced internalization of G protein-coupled receptors (GPCRs) is an important mechanism for regulating signaling transduction of functional receptors at the plasma membrane. We demonstrate here that both caveolae/lipid-rafts- and clathrin-coated-pits-mediated pathways were involved in agonist-induced endocytosis of the cannabinoid type 1 receptor (CB1R) in stably transfected human embryonic kidney (HEK) 293 cells and that the internalized receptors were predominantly sorted into recycling pathway for reactivation. The treatment of CB1 receptors with the low endocytotic agonist Delta9-THC induced a faster receptor desensitization and slower resensitization than the high endocytotic agonist WIN 55,212-2. In addition, the blockade of receptor endocytosis or recycling pathway markedly enhanced agonist-induced CB1 receptor desensitization. Furthermore, co-expression of phospholipase D2, an enhancer of receptor endocytosis, reduced CB1 receptor desensitization, whereas co-expression of a phospholipase D2 negative mutant significantly increased the desensitization after WIN 55,212-2 treatment. These findings provide evidences for the importance of receptor endocytosis in counteracting CB1 receptor desensitization by facilitating receptor reactivation. Moreover, in primary cultured neurons, the low endocytotic agonist Delta9-THC or anandamide exhibited a greater desensitization of endogenous CB1 receptors than the high endocytotic agonist WIN 55,212-2, CP 55940 or 2-arachidonoyl glycerol, indicating that cannabinoids with high endocytotic efficacy might cause reduced development of cannabinoid tolerance to some kind cannabinoid-mediated effects.

Membrane glycoprotein M6a interacts with the micro-opioid receptor and facilitates receptor endocytosis and recycling

Using a yeast two-hybrid screen, the neuronal membrane glycoprotein M6a, a member of the proteolipid protein family, was identified to be associated with the mu-opioid receptor (MOPr). Bioluminescence resonance energy transfer and co-immunoprecipitation experiments confirmed that M6a interacts agonist-independently with MOPr in human embryonic kidney 293 cells co-expressing MOPr and M6a. Co-expression of MOPr with M6a, but not with M6b or DM20, exists in many brain regions, further supporting a specific interaction between MOPr and M6a. After opioid treatment M6a co-internalizes and then co-recycles with MOPr to cell surface in transfected human embryonic kidney 293 cells. Moreover, the interaction of M6a and MOPr augments constitutive and agonist-dependent internalization as well as the recycling rate of mu-opioid receptors. On the other hand, overexpression of a M6a-negative mutant prevents mu-opioid receptor endocytosis, demonstrating an essential role of M6a in receptor internalization. In addition, we demonstrated the interaction of M6a with a number of other G protein-coupled receptors (GPCRs) such as the delta-opioid receptor, cannabinoid receptor CB1, and somatostatin receptor sst2A, suggesting that M6a might play a general role in the regulation of certain GPCRs. Taken together, these data provide evidence that M6a may act as a scaffolding molecule in the regulation of GPCR endocytosis and intracellular trafficking.