Acute cross-tolerance to opioids in heroin delta-opioid-responding Swiss Webster mice

It is generally thought that the mu receptor actions of metabolites, 6-monoacetylmorphine (6MAM) and morphine, account for the pharmacological actions of heroin. However, upon intracerebroventricular (i.c.v.) administration in Swiss Webster mice, heroin and 6MAM act on delta receptors while morphine acts on mu receptors. Swiss Webster mice made tolerant to subcutaneous (s.c.) morphine by morphine pellet were not cross-tolerant to s.c. heroin (at 20 min in the tail flick test). Now, opioids were given in combination, s.c. (6.5 h) and i.c.v. (3 h) preceding testing the challenging agonist i.c.v. (at 10 min in the tail flick test). The combination (s.c. + i.c.v.) morphine pretreatment induced tolerance to the mu action of morphine but no cross-tolerance to the delta action of heroin, 6MAM and DPDPE and explained why morphine pelleting did not produce cross-tolerance to s.c. heroin above. Heroin plus heroin produced tolerance to delta agonists but not to mu agonists. Surprisingly, all combinations of morphine with the delta agonists produced tolerance to morphine which now acted through delta receptors (inhibited by i.c.v. naltrindole), an unusual change in receptor selectivity for morphine.     

The protective effect of zinc on morphine-induced testicular toxicity via p53 and Akt pathways: An in vitro and in vivo approach

BackgroundChronic use of morphine is associated with reproductive complications, such as hypogonadism and infertility. While the side effects of morphine have been extensively studied in the testis, much less is known regarding the effects of morphine on Sertoli cells and the effects of zinc on morphine-induced testicular injury as well as their underlying mechanisms. Therefore, the purpose of this study was to investigate the effect of morphine (alone and co-administered with zinc) on cell viability and apoptosis of the testicular (Sertoli) cells as well as the tumor suppressor p53 and phosphorylated-protein kinase B (p-Akt) protein levels in both in vitro and in vivo models.MethodsCultured Sertoli cells were exposed to morphine (23 μM), zinc (8 μM), and zinc prior to morphine and their effects on Sertoli cell viability and apoptosis were investigated. Morphine (3 mg/kg) and zinc (5 mg/kg, 1 h before morphine) were also injected intraperitoneally to rats and then the apoptotic changes in the testis were evaluated.ResultsCell viability and p-Akt protein levels decreased in morphine-treated cells, while apoptosis and p53 protein expression increased in these cells. Pretreatment with zinc recovered morphine-induced apoptotic effects, as well as over-expression of p53 and down-regulation of p-Akt. These findings were supported by a subsequent animal study.ConclusionThe present data indicated the protective effect of zinc against morphine-induced testicular (Sertoli) cell toxicity via p53/Akt pathways in both in vivo and in vitro models and suggested the clinical importance of zinc on infertility among chronic opioid users and addicted men.     

下行疼痛易化系统在吗啡耐受机制中的作用

吗啡是临床上常用的一种阿片类镇痛药物,广泛用于治疗各种类型疼痛,但是长期应用吗啡会造成吗啡耐受,从而限制了吗啡的临床应用。吗啡耐受的机制十分复杂,近年来的研究表明下行疼痛易化系统参与了吗啡耐受,本文拟对近年来此方面的研究进行综述。     

Stabilization of the μ-Opioid Receptor by Truncated Single Transmembrane Splice Variants through a Chaperone-like Action

The μ-opioid receptor gene, OPRM1, undergoes extensive alternative pre-mRNA splicing, as illustrated by the identification of an array of splice variants generated by both 5′ and 3′ alternative splicing. The current study reports the identification of another set of splice variants conserved across species that are generated through exon skipping or insertion that encodes proteins containing only a single transmembrane (TM) domain. Using a Tet-Off system, we demonstrated that the truncated single TM variants can dimerize with the full-length 7-TM μ-opioid receptor (MOR-1) in the endoplasmic reticulum, leading to increased expression of MOR-1 at the protein level by a chaperone-like function that minimizes endoplasmic reticulum-associated degradation. In vivo antisense studies suggested that the single TM variants play an important role in morphine analgesia, presumably through modulation of receptor expression levels. Our studies suggest the functional roles of truncated receptors in other G protein-coupled receptor families.     

The involvement of midbrain astrocyte in the development of morphine tolerance

Aims

Systemic administration of opiate analgesics such as morphine remains the most effective treatment for alleviating severe pain across a range of conditions including acute pain. However, chronic or repeated administration of opiate analgesics results in the development of analgesic tolerance. Glial cells such as microglia and astrocytes are known to release various inflammatory cytokines and neurotrophic factors leading to regulation of neuronal function. Recently, glial cells were reported to play important roles in the development of analgesic tolerance to morphine. Here, we focused on the involvement of midbrain glial cells, particularly astrocytes, in the development of analgesic tolerance to morphine.

Main methods

Mice were treated with morphine (10 mg/kg, s.c.) or vehicle once a day for 5 days. Pentoxifylline (an inhibitor of glial activation; 20 mg/kg, i.p. or 50 and 100 μg/mouse, i.c.v.) was administered 30 min before morphine treatment. Flavopiridol (a cyclin-dependent kinase inhibitor; 5 nmol/mouse, i.c.v.) was administered 10 min before and 10 h after morphine treatment. The analgesic effect of morphine was measured using the tail flick method.

Key findings

The development of analgesic tolerance to morphine was gradually observed during daily treatment of morphine for 5 days in mice. On days 1 and 3 after repeated morphine treatment, astrocyte marker glial fibrillary acidic protein expression levels were significantly increased, as determined by western blot analyses. These phenomena were significantly inhibited following pre-treatment with pentoxifylline or flavopiridol.

Significance

We demonstrated that midbrain astrocytes play an important role in the development of analgesic tolerance to morphine.     

Attenuation of systemic morphine-induced analgesia by central administration of ghrelin and related peptides in mice

Ghrelin, an acylated 28-amino peptide secreted in the gastric endocrine cells, has been demonstrated to stimulate the release of growth hormone, increase food intake, and inhibit pro-inflammatory cascade, etc. Ghrelin mainly combines with its receptor (GHS-R1α) to play the role in physiological and pathological functions. It has been reported that ghrelin plays important roles in the control of pain through interaction with the opioid system in inflammatory pain and acute pain. However, very few studies show the effect of supraspinal ghrelin system on antinociception induced by intraperitoneal (i.p.) administration of morphine. In the present study, intracerebroventricular (i.c.v.) injection of ghrelin (0.1, 1, 10 and 100 nmol/L) produced inhibition of systemic morphine (6 mg/kg, i.p.) analgesia in the tail withdrawal test. Similarly, i.c.v. injection GHRP-6 and GHRP-2 which are the agonists of GHS-R1α, also decreased analgesia effect induced by morphine injected intraperitoneally in mice. Furthermore, these anti-opioid activities of ghrelin and related peptides were not blocked by pretreatment with the GHS-R1α selective antagonist [d-Lys³]-GHRP-6 (100 nmol/L, i.c.v.). These results demonstrated that central ghrelin and related peptides could inhibit the analgesia effect induced by intraperitoneal (i.p.) administration of morphine. The anti-opioid effects of ghrelin and related peptides do not interact with GHS-R1a. These findings may pave the way for a new strategy on investigating the interaction between ghrelin system and opioids on pain modulation.     

青春期小鼠与成年小鼠在吗啡和食物诱导条件化位置偏爱建立上的异同

该实验通过采用吗啡诱导的条件位置偏爱(conditioned place preference,CPP)与食物诱导CPP相结合的方法来研究青春期小鼠和成年小鼠的普通学习记忆和成瘾学习记忆之间是否存在差异。结果发现:1)成年小鼠能够建立吗啡诱导CPP,而青春期小鼠不能建立;2)青春期小鼠和成年小鼠都能够建立食物诱导CPP。吗啡诱导CPP的结果提示,青春期小鼠和成年小鼠在成瘾学习记忆上有差异,青春期小鼠的成瘾记忆能力较弱。食物诱导CPP的结果提示,青春期小鼠和成年小鼠在普通学习记忆上没有差异。吗啡诱导CPP和食物诱导CPP的结果比较提示,小鼠的普通学习记忆系统和成瘾学习记忆系统发育进程是不平行的。     

Effect of Bacopasides on acquisition and expression of morphine tolerance

Opioids are extensively used for the management of both chronic malignant and non malignant pains. One major serious limitation associated with chronic use of opioids is the development of tolerance to its analgesic effect. The effect of Bacopa monnieri, a renowned ayurvedic medicine for acquisition and expression of morphine tolerance in mice, was investigated. Bacopa monnieri, n-Butanol fraction was analyzed on High performance liquid chromatography (HPLC), for Bacopaside A major components i.e. Bacoside A₃, Bacopaside ll and Bacosaponin C. Antinociceptive effect of n-Butanol extract of Bacopa monnieri (n Bt-ext BM) (5, 10 and 15 mg/kg) was assessed on hot plate. Effect of different doses of n Bt-ext BM on morphine antinociception was also assessed. n Bt-ext BM was also screened for development of tolerance to antinociceptive effect of Bacopa monnieri by administering 15 mg/kg n Bt-ext BM for seven days. Tolerance to morphine analgesia was induced in mice by administering intraperitoneally (I.P.) 20 mg/kg morphine twice daily for five days. Acute and Chronic administration of 5, 10 and 15 mg/kg n Bt-ext BM significantly reduced both expression and development of tolerance to morphine analgesia in mice. Additionally, Bacopa monnieri was found to enhance antinociceptive effect of morphine in intolerant animals. However, no tolerance to Bacopa monnieri antinociceptive effect was observed in seven days treatment schedule. These findings indicate effectiveness of Bacopa monnieri for management of morphine tolerance.