Enhancement of μ-opioid receptor desensitization by nitric oxide in rat locus coeruleus neurons: involvement of reactive oxygen species

Transactivation of the EGF receptor as a novel desensitization mechanism for G protein-coupled receptors, illustrated by dopamine D2-like and β2 adrenergic receptors

Transactivation of epidermal growth factor receptors (EGFR) provides intricate control over multiple regulatory cellular processes that merge the diversity of G protein-coupled receptors (GPCRs) with the robust signaling capacities of receptor tyrosine kinases. Contrary to the typical assertions, our findings demonstrate that EGFR transactivation contributes to the desensitization of GPCRs. Repeated agonist stimulation of certain GPCRs enhanced EGFR transactivation, triggering a series of cellular events associated with GPCR desensitization. This effect was observed in receptors undergoing desensitization (D3R, K149C-D2R, β2AR) but not in those resistant to desensitization (D2R, C147K-D3R, D4R, β2AR mutants lacking GRK2 or GRK6 phosphorylation sites). The EGFR inhibitor AG1478 prevented both desensitization and the associated cellular events. Similarly, these cellular events were also observed when cells were treated with EGF, but only in GPCRs that undergo desensitization. These findings suggest that EGFR transactivation diversifies pathways involved in ERK activation through the EGFR signaling system and also mediates GPCR desensitization. Alongside the widely accepted steric hindrance model, these findings offer new insights into understanding the mechanisms of GPCR desensitization, which occurs through complex cellular processes.

Mu opioid receptor-mediated release of endolysosome iron increases levels of mitochondrial iron, reactive oxygen species, and cell death

Objectives

Opioids including morphine and DAMGO activate mu-opioid receptors (MOR), increase intracellular reactive oxygen species (ROS) levels, and induce cell death. Ferrous iron (Fe2+) through Fenton-like chemistry increases ROS levels and endolysosomes are "master regulators of iron metabolism" and contain readily-releasable Fe2+ stores. However, mechanisms underlying opioid-induced changes in endolysosome iron homeostasis and downstream-signaling events remain unclear.

Methods

We used SH-SY5Y neuroblastoma cells, flow cytometry, and confocal microscopy to measure Fe2+ and ROS levels and cell death.

Results

Morphine and DAMGO de-acidified endolysosomes, decreased endolysosome Fe2+ levels, increased cytosol and mitochondria Fe2+ and ROS levels, depolarized mitochondrial membrane potential, and induced cell death; effects blocked by the nonselective MOR antagonist naloxone and the selective MOR antagonist β-funaltrexamine (β-FNA). Deferoxamine, an endolysosome-iron chelator, inhibited opioid agonist-induced increases in cytosolic and mitochondrial Fe2+ and ROS. Opioid-induced efflux of endolysosome Fe2+ and subsequent Fe2+ accumulation in mitochondria were blocked by the endolysosome-resident two-pore channel inhibitor NED-19 and the mitochondrial permeability transition pore inhibitor TRO.

Conclusions

Opioid agonist-induced increases in cytosolic and mitochondrial Fe2+ and ROS as well as cell death appear downstream of endolysosome de-acidification and Fe2+ efflux from the endolysosome iron pool that is sufficient to affect other organelles.

Synthesis of the Mechanisms of Opioid Tolerance: Do We Still Say NO?

The use of morphine as a first-line agent for moderate-to-severe pain is limited by the development of analgesic tolerance. Initially opioid receptor desensitization in response to repeated stimulation, thought to underpin the establishment of tolerance, was linked to a compensatory increase in adenylate cyclase responsiveness. The subsequent demonstration of cross-talk between N-methyl-D-aspartate (NMDA) glutamate receptors and opioid receptors led to the recognition of a role for nitric oxide (NO), wherein blockade of NO synthesis could prevent tolerance developing. Investigations of the link between NO levels and opioid receptor desensitization implicated a number of events including kinase recruitment and peroxynitrite-mediated protein regulation. Recent experimental advances and the identification of new cellular constituents have expanded the potential signaling candidates to include unexpected, intermediary compounds not previously linked to this process such as zinc, histidine triad nucleotide-binding protein 1 (HINT1), micro-ribonucleic acid (mi-RNA) and regulator of G protein signaling Z (RGSZ). A further complication is a lack of consistency in the protocols used to create tolerance, with some using acute methods measured in minutes to hours and others using days. There is also an emphasis on the cellular changes that are extant only after tolerance has been established. Although a review of the literature demonstrates a lack of spatio-temporal detail, there still appears to be a pivotal role for nitric oxide, as well as both intracellular and intercellular cross-talk. The use of more consistent approaches to verify these underlying mechanism(s) could provide an avenue for targeted drug development to rescue opioid efficacy.

Characterization of functional μ opioid receptor turnover in rat locus coeruleus: an electrophysiological and immunocytochemical study

BACKGROUND AND PURPOSE

Regulation of μ receptor dynamics such as its trafficking is a possible mechanism underlying opioid tolerance that contributes to inefficient recycling of opioid responses. We aimed to characterize the functional turnover of μ receptors in the noradrenergic nucleus locus coeruleus (LC).

EXPERIMENTAL APPROACH

We measured opioid effect by single-unit extracellular recordings of LC neurons from rat brain slices. Immunocytochemical techniques were used to evaluate μ receptor trafficking.

KEY RESULTS

After near-complete, irreversible μ receptor inactivation with β-funaltrexamine (β-FNA), opioid effect spontaneously recovered in a rapid and efficacious manner. In contrast, α₂ -adrenoceptor-mediated effect hardly recovered after receptor inactivation with the irreversible antagonist EEDQ. When the recovery of opioid effect was tested after various inactivating time schedules, we found that the longer the β-FNA pre-exposure, the less efficient and slower the functional μ receptor turnover became. Interestingly, μ receptor turnover was slower when β-FNA challenge was repeated in the same cell, indicating constitutive μ receptor recycling by trafficking from a depletable pool. Double immunocytochemistry confirmed the constitutive nature of μ receptor trafficking from a cytoplasmic compartment. The μ receptor turnover was slowed down when LC neuron calcium- or firing-dependent processes were prevented or vesicular protein trafficking was blocked by a low temperature or transport inhibitor.

CONCLUSIONS AND IMPLICATIONS

Constitutive trafficking of μ receptors from a depletable intracellular pool (endosome) may account for its rapid and efficient functional turnover in the LC. A finely-tuned regulation of μ receptor trafficking and endosomes could explain neuroadaptive plasticity to opioids in the LC.

Enhancement of μ-opioid receptor desensitization by orexin-A in rat locus coeruleus neurons

Opioids have always been used in clinical practice for pain management. However, development of tolerance to their effects following long term administration, seriously restricts further clinical use of these drugs. In this regard, μ-opioid receptor (MOR) desensitization, as an initial step in development of opioid tolerance, is of particular significance. Previous studies support the involvement of orexinergic system in development of opioid tolerance. Locus coeruleus (LC) nucleus has been shown to modulate pain and development of tolerance. Opioid receptors (particularly μ) are densely expressed within the LC. Moreover, it receives widespread orexinergic inputs and orexin type 1 receptors (OX1Rs) are also highly expressed in this brain region. In the present study, the effect of orexin-A (OXA) on met-enkephalin (ME)-induced MOR desensitization was investigated in locus coeruleus neurons of male Wistar rats (2-3weeks of age). ME (30μM), as a potent MOR agonist, was applied for 10min and the outward K⁺ current was recorded using whole cell patch clamp recording. The percentage of decrease in ME-induced K⁺ current was considered as the degree of MOR desensitization. Results indicated that OXA (100nM) enhances ME-induced MOR desensitization via affecting OX1Rs in rat locus coeruleus neurons and this effect is mediated by a protein kinase C dependent mechanism within the LC. The activity of orexinergic system might potentiate the signaling pathways underlying opioid-induced receptor desensitization.

Contribution of nitric oxide-dependent guanylate cyclase and reactive oxygen species signaling pathways to desensitization of μ-opioid receptors in the rat locus coeruleus

Nitric oxide (NO) is involved in desensitization of μ-opioid receptors (MOR). We used extracellular recordings in vitro to unmask the NO-dependent pathways involved in MOR desensitization in the rat locus coeruleus (LC). Perfusion with ME (3 and 10 μM) concentration-dependently reduced subsequent ME effect, indicative of MOR desensitization. ME (3 μM)-induced desensitization was enhanced by a NO donor (DEA/NO 100 μM), two soluble guanylate cyclase (sGC) activators (A 350619 30 μM and BAY 418543 1 μM) or a cGMP-dependent protein kinase (PKG) activator (8-pCPT-cGMP 30 μM). DEA/NO-induced enhancement was blocked by the sGC inhibitor NS 2028 (10 μM). A 350619 effect was also blocked by NS 2028, but not by the antioxidant Trolox. ME (10 μM)-induced desensitization was blocked by the neuronal NO synthase inhibitor 7-NI (100 μM) and restored by the PKG activator 8-Br-cGMP (100-300 μM). Paradoxically, ME (10 μM)-induced desensitization was not modified by sGC inhibitors (NS 2028 and ODQ), PKG inhibitors (H8 and Rp-8-Br-PET-cGMP) or antioxidant agents (Trolox, U-74389G and melatonin), but it was attenuated by a combination of NS 2028 and Trolox. In conclusion, MOR desensitization in the LC may be mediated or regulated by NO through sGC and reactive oxygen species signaling pathways.

Modulation of opioid-induced feeding behavior by endogenous nitric oxide in neonatal layer-type chicks

The current study was designed to evaluate the effects of central administration of L-arginine (The precursor of nitric oxide), N(G)-nitro-L-arginine methyl ester (L-NAME), a nitric oxide (NO) synthase inhibitor, selective opioid receptor agonists and involvement of central nitrergic/opioidergic systems on feeding behavior in neonatal layer-type chicks. The results of this study showed that the intracerebroventricular (ICV) injection of L-arginine (400 and 800 nmol) significantly decreased food intake (P < 0.001) but the injection of 200 nmol L-arginine had no effect on cumulative food intake in FD3 chickens (P > 0.05). The ICV injection of L-NAME (200 and 400 nmol) increased food intake (P < 0.001) but 100 nmol of L-NAME had no significant effect (P > 0.05). On the other hand, the co-injection of 100 nmol L-NAME significantly attenuated the anorexigenic effect of 800 nmol L-arginine (P < 0.001). Moreover, the food intake of chicks was significantly decreased by ICV injection of DAMGO (μ-opioid receptor agonist, 125 pmol) (P < 0.001) while both DPDPE (δ-opioid receptor agonist, 40 pmol) and U-50488H (κ-opioid receptor agonist, 30 nmol) significantly stimulated food intake (P < 0.001). In addition, the hypophagic effect of DAMGO was significantly amplified by administration of L-arginine (P < 0.001) while the administration of L-NAME attenuated the hypophagic effect of DAMGO (P < 0.001). In contrast, co-injection of L-arginine or L-NAME with DPDPE had no effect on the hyperphagia induced by DPDPE as well as the hyperphagic effect of U-50488H on food intake was not affected by concurrent injection of L-arginine or L-NAME (P > 0.05). These results suggest that nitrergic and opioidergic systems have an important role on feeding behavior in the CNS of neonatal layer-type chicks and it seems that interaction between them is mediated by μ-opioid receptor.

Involvement of neuronal nitric oxide synthase in desensitisation of µ-opioid receptors in the rat locus coeruleus

Nitric oxide (NO) has been recently shown to enhance µ-opioid receptor (MOR) desensitisation in locus coeruleus (LC) neurons. The aim of this study was to evaluate by single-unit extracellular recordings in rat brain slices whether the neuronal NO synthase is involved in MOR desensitisation in LC neurons. As expected, a high concentration of the opioid agonist Met(5)-enkephalin (ME; 10 µM, 10 min) strongly desensitised the inhibition induced by a test application of ME (0.8 µM, 1 min), whereas lower ME concentrations (1 and 3 µM) only weakly desensitised it. The neuronal NO synthase inhibitors 7-nitroindazole (10-100 µM), S-methyl-L-thiocitrulline (0.01-10 µM) and N(ω)-propyl-L-arginine (1-10 µM) attenuated ME (10 µM)-induced opioid desensitisation, although the endothelial NO synthase inhibitor N(5)-(1-iminoethyl)-L-ornithine (3-30 µM) failed to change it. The NO donor sodium nitroprusside (1 mM), but not its inactive analog potassium ferricyanide (1 mM), enhanced the ME (3 µM)-induced desensitisation and prevented the effect of S-methyl-L-thiocitrulline (10 µM). Sodium nitroprusside (1 mM) failed to change the desensitisation of α2-adrenoceptors by noradrenaline (100 µM, 10 min). These results suggest the contribution of NO and a neuronal type of NO synthase in homologous MOR desensitisation in rat LC neurons.

Endogenous opiates and behavior: 2012

This paper is the thirty-fifth consecutive installment of the annual review of research concerning the endogenous opioid system. It summarizes papers published during 2012 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 (Section 2), and the roles of these opioid peptides and receptors in pain and analgesia (Section 3); stress and social status (Section 4); tolerance and dependence (Section 5); learning and memory (Section 6); eating and drinking (Section 7); alcohol and drugs of abuse (Section 8); sexual activity and hormones, pregnancy, development and endocrinology (Section 9); mental illness and mood (Section 10); seizures and neurologic disorders (Section 11); electrical-related activity and neurophysiology (Section 12); general activity and locomotion (Section 13); gastrointestinal, renal and hepatic functions (Section 14); cardiovascular responses (Section 15); respiration and thermoregulation (Section 16); and immunological responses (Section 17).