Different mechanisms of homologous and heterologous μ-opioid receptor phosphorylation

The efficiency of μ-opioid receptor signalling is tightly regulated and ultimately limited by the coordinated phosphorylation of intracellular serine and threonine residues. Here, we review and discuss recent progress in the generation and application of phosphosite-specific μ-opioid receptor antibodies, which have proved to be excellent tools for monitoring the spatial and temporal dynamics of receptor phosphorylation and dephosphorylation. Agonist-induced phosphorylation of μ-opioid receptors occurs at a conserved 10 residue sequence ³⁷⁰TREHPSTANT³⁷⁹ in the receptor''s carboxyl-terminal cytoplasmic tail. Diverse opioids induce receptor phosphorylation at S375, present in the middle of this sequence, but only high-efficacy opioids have the ability to drive higher order phosphorylation on flanking residues (T370, T376 and T379). S375 is the initiating residue in a hierarchical phosphorylation cascade. In contrast, agonist-independent heterologous μ-opioid receptor phosphorylation occurs primarily at T370. The combination of phosphosite-specific antibodies and siRNA knockdown screening also facilitated the identification of relevant kinases and phosphatases. In fact, morphine induces a selective S375 phosphorylation that is predominantly catalysed by GPCR kinase 5 (GRK5), whereas multisite phosphorylation induced by high-efficacy opioids specifically requires GRK2/3. By contrast, T370 phosphorylation stimulated by phorbol esters or heterologous activation of G_q-coupled receptors is mediated by PKCα. Rapid μ-opioid receptor dephosphorylation occurs at or near the plasma membrane and is catalysed by protein phosphatase 1γ (PP1γ). These findings suggest that there are distinct phosphorylation motifs for homologous and heterologous regulation of μ-opioid receptor phosphorylation. However, it remains to be seen to what extent different μ-opioid receptor phosphorylation patterns contribute to the development of tolerance and dependence in vivo.

LINKED ARTICLES

This article is part of a themed section on Opioids: New Pathways to Functional Selectivity. To view the other articles in this section visit http://dx.doi.org/10.1111/bph.2015.172.issue-2     

Orexin/hypocretin role in reward: implications for opioid and other addictions

Addiction is a devastating disorder that affects 15.3 million people worldwide. While prevalent, few effective treatments exist. Orexin receptors have been proposed as a potential target for anti-craving medications. Orexins, also known as hypocretins, are neuropeptides produced in neurons of the lateral and dorsomedial hypothalamus and perifornical area, which project widely throughout the brain. The absence of orexins in rodents and humans leads to narcolepsy. However, orexins also have an established role in reward seeking. This review will discuss some of the original studies describing the roles of the orexins in reward seeking as well as specific works that were presented at the 2013 International Narcotics Research Conference. Orexin signalling can promote drug-induced plasticity of glutamatergic synapses onto dopamine neurons of the ventral tegmental area (VTA), a brain region implicated in motivated behaviour. Additional evidence suggests that orexin signalling can also promote drug seeking by initiating an endocannabinoid-mediated synaptic depression of GABAergic inputs to the VTA, and thereby disinhibiting dopaminergic neurons. Orexin neurons co-express the inhibitory opioid peptide dynorphin. It has been proposed that orexin in the VTA may not mediate reward per se, but rather occludes the ‘anti-reward’ effects of dynorphin. Finally, orexin signalling in the prefrontal cortex and the central amygdala is implicated in reinstatement of reward seeking. This review will highlight recent work describing the role of orexin signalling in cellular processes underlying addiction-related behaviours and propose novel hypotheses for the mechanisms by which orexin signalling may impart drug seeking.

LINKED ARTICLES

This article is part of a themed section on Opioids: New Pathways to Functional Selectivity. To view the other articles in this section visit http://dx.doi.org/10.1111/bph.2015.172.issue-2     

Opioid receptor trafficking and interaction in nociceptors

Opiate analgesics such as morphine are often used for pain therapy. However, antinociceptive tolerance and dependence may develop with long-term use of these drugs. It was found that μ-opioid receptors can interact with δ-opioid receptors, and morphine antinociceptive tolerance can be reduced by blocking δ-opioid receptors. Recent studies have shown that μ- and δ-opioid receptors are co-expressed in a considerable number of small neurons in the dorsal root ganglion. The interaction of μ-opioid receptors with δ-opioid receptors in the nociceptive afferents is facilitated by the stimulus-induced cell-surface expression of δ-opioid receptors, and contributes to morphine tolerance. Further analysis of the molecular, cellular and neural circuit mechanisms that regulate the trafficking and interaction of opioid receptors and related signalling molecules in the pain pathway would help to elucidate the mechanism of opiate analgesia and improve pain therapy.

LINKED ARTICLES

This article is part of a themed section on Opioids: New Pathways to Functional Selectivity. To view the other articles in this section visit http://dx.doi.org/10.1111/bph.2015.172.issue-2     

Age‐dependent differences in morphine‐induced taste aversions

Adolescence is a developmental period of particular importance given the host of neurobiological changes that occur during this stage of development. Drug use and abuse is said to be a function of the balance of its rewarding and aversive effects, and any age‐dependent differences in morphine's aversive effects could impact drug intake. The present experiments examined the ability of morphine sulfate (0, 3.2, 10, and 18 mg/kg) to induce taste aversions in adolescent and adult rats under high (20‐min fluid access each day; Experiment 1A/B) and low (50% of ad libitum access; Experiment 2A/B) deprivation conditions. In both studies, adolescent and adult rats were given a novel saccharin solution to drink and were subsequently injected with morphine. Independent of the deprivation condition, adults acquired stronger aversions than adolescents and did so at a faster rate. On a subsequent two‐bottle aversion test, all morphine‐injected subjects drank a significantly lower percentage of saccharin than vehicle‐injected controls with adults exhibiting stronger aversions than adolescents. These age‐dependent differences in morphine‐induced CTAs extend the findings with other drugs of abuse for which adolescents exhibit weaker aversions. The possible basis for and implications of these differences were discussed. © 2012 Wiley Periodicals, Inc. Dev Psychobiol 55: 415–428, 2013     

川芎嗪通过抑制脊髓小胶质细胞活化缓解吗啡耐受

探讨川芎嗪对脊髓小胶质细胞活化和慢性吗啡耐受的影响并考察其作用机制。运用水浴甩尾法检测慢性吗啡耐受小鼠甩尾痛阈;免疫荧光法检测小鼠脊髓水平小胶质细胞标记分子(ionized calcium binding adapter molecule 1,IBA1)的变化情况;Western blot法检测p38 MAPK(mitogen-activated protein kinase,MAPK)磷酸化水平的改变,以及RT-PCR法检测川芎嗪对慢性吗啡耐受过程中白细胞介素-1β(IL-1β)和肿瘤坏死因子-α(TNF-α)mRNA表达水平的影响。实验结果发现,川芎嗪(15,30,60 mg/kg)能够剂量依赖性地抑制吗啡引起的脊髓IBA-1、p-p38 MAPK、p38、TNF-α和IL-1β水平的升高,改善慢性吗啡耐受。研究结果表明,川芎嗪能显著改善慢性吗啡耐受,其机制可能与抑制小胶质细胞p38 MAPK信号通路有关。     

Effects of morphine on associative memory and locomotor activity in the honeybee (Apis mellifera)

Morphine can modulate the processes underlying memory in vertebrates. However, studies have shown various modulations by morphine: positive, negative and even neutral. The honeybee is a potential platform for evaluating the effects of drugs, especially addictive drugs, on the nervous system. However, the involvement of morphine in learning and memory in insects or other invertebrates is poorly understood. The current work evaluated whether morphine affects memory acquisition, consolidation and retrieval in honeybees, using the proboscis extension response (PER) paradigm. We demonstrated that morphine treatment (5 μg/bee) before training decreased the percentage of correct PERs and the response latency related to aversive rather than rewarding odors when tested after 1 or 24 h. Morphine treatment after training also caused a decrease in this latency when tested after 24 h. Meanwhile, morphine treatment reduced the ambulation distance when tested after 30 min. Our findings suggest that morphine impairs the acquisition of short- and long-term associative memory and slightly disrupts the consolidation of long-term memory in honeybees. These negative effects cannot be explained by reduced locomotion but by impaired memory associated with aversion.     

A dopamine D1 receptor agonist improved learning and memory in morphine-treated rats

ABSTRACT

Objective: The objective of this article is to study the role of the dopamine (DA) D1 receptor in the midbrain periaqueductal grey (PAG) on learning and memory in morphine-addicted rats.

Methods: DA D1 receptor agonist SKF81297 and D1 receptor antagonist SCH SCH23390 were administrated into the PAG, respectively, and the learning and memory behavioral changes of morphine addicted rats were detected by water maze. Western blot and immunohistochemistry were used to detect glutamate decarboxylase 67 (GAD67) and tyrosine receptor kinase B (TrkB) in PAG.

Results: D1 receptor agonist shortened the latency to platform and increased the number of platform crossings, indicating improved learning and memory ability of morphine addict rat. D1 receptor agonist increased GAD67 expression and decreased TrkB in PAG.

Conclusion: (1) The PAG is involved in the learning and memory changes of the addicted rats; (2) the activation of DA D1 receptor will increase the GAD67, reduce the damage to peripheral neurons, and improve the learning and memory of the addicted rats; and (3) D1 receptor agonists further reduced TrkB expression in morphine-addicted rats, whereas TrkB levels deviated from changes in rat behavior.     

患者自控皮下镇痛在中重度癌性疼痛患者中应用

目的探讨患者自控皮下镇痛(PCSA)在中重度癌性疼痛患者中应用的有效性和安全性。方法回顾性分析自2010年9月至2014年8月中国医科大学附属第一医院和第四医院收治的121例中重度癌性疼痛患者的临床资料。根据镇痛方法不同,分为PCSA镇痛(PCSA组)31例,口服吗啡缓释片镇痛(吗啡组)50例,芬太尼透皮贴镇痛(芬太尼组)40例。对比分析各组患者镇痛效果和镇痛相关并发症的发生率。结果 PCSA组患者恶心呕吐、便秘、爆发性疼痛发生率明显低于吗啡组及芬太尼组,差异均有统计学意义(P<0.05)。PCSA组患者镇痛后1周VAS评分与吗啡组及芬太尼组比较,差异均有统计学意义(P<0.05)。结论 PCSA尽管便携度略逊一筹,但其可为中重度癌性疼痛患者提供恒定满意的镇痛效果,值得在临床中推广。