Acute systemic administration of morphine selectively increases mu opioid receptor binding in the rat brain

LncRNA MRAK159688 facilitates morphine tolerance by promoting REST-mediated inhibition of mu opioid receptor in rats

Morphine tolerance (MT) caused by the long-term use of morphine is a major medical problem. The molecular mechanism of morphine tolerance remains elusive. Here, we established a morphine tolerance model in rats and verified whether the long noncoding RNA (lncRNA) MRAK159688 is involved in morphine tolerance and its specific molecular mechanism. We show the significant upregulation of MRAK159688 expression in the spinal cord of morphine-tolerant rats. Overexpression of MRAK159688 by a lentivirus reduces the analgesic efficacy of morphine and induces pain behavior. Downregulation of MRAK159688 using a small interfering RNA (siRNA) attenuates the formation of morphine tolerance, partially reverses the development of morphine tolerance and alleviates morphine-induced hyperalgesia. MRAK159688 is located in the nucleus and cytoplasm of neurons, and it colocalizes with repressor element-1 silencing transcription factor (REST) in the nucleus. MRAK159688 potentiates the expression and function of REST, thereby inhibiting the expression of mu opioid receptor (MOR) and subsequently inducing morphine tolerance. Moreover, REST overexpression blocks the effects of MRAK159688 siRNA on relieving morphine tolerance. In general, chronic morphine administration-mediated upregulation of MRAK159688 in the spinal cord contributes to morphine tolerance and hyperalgesia by promoting REST-mediated inhibition of MOR. MRAK159688 downregulation may represent a novel RNA-based therapy for morphine tolerance.

Novel Roles of Non-Coding RNAs in Opioid Signaling and Cardioprotection

Cardiovascular disease (CVD) is a significant cause of morbidity and mortality across the world. A large proportion of CVD deaths are secondary to coronary artery disease (CAD) and myocardial infarction (MI). Even though prevention is the best strategy to reduce risk factors associated with MI, the use of cardioprotective interventions aimed at improving patient outcomes is of great interest. Opioid conditioning has been shown to be effective in reducing myocardial ischemia-reperfusion injury (IRI) and cardiomyocyte death. However, the molecular mechanisms behind these effects are under investigation and could provide the basis for the development of novel therapeutic approaches in the treatment of CVD. Non-coding RNAs (ncRNAs), which are functional RNA molecules that do not translate into proteins, are critical modulators of cardiac gene expression during heart development and disease. Moreover, ncRNAs such as microRNAs (miRNAs) and long non-coding RNAs (lncRNAs) are known to be induced by opioid receptor activation and regulate opioid signaling pathways. Recent advances in experimental and computational tools have accelerated the discovery and functional characterization of ncRNAs. In this study, we review the current understanding of the role of ncRNAs in opioid signaling and opioid-induced cardioprotection.

Let-7 microRNAs and Opioid Tolerance

This chapter will focus on the role of microRNAs (miRs) in regulating the actions of opioid drugs through the opioid receptors. Opioids, such as morphine, are analgesics that are used for treating many forms of acute and chronic pain. However, their chronic use is limited by undesirable effects such as opioid tolerance. The μ opioid receptor (MOR) is the primary receptor responsible for opioids' analgesia and antinociceptive tolerance. The long 3'-untranslated region (3'-UTR) of MOR mRNA is of great interest since this region may contain elements for the post-transcriptional regulation of receptor expression, such as altering the stability of mRNA, influencing translational efficiency, and controlling mRNA transport. MicroRNAs are small non-coding RNA molecules that exert their functions through base-pairing with partially complementary sequences in the 3'-UTR of target mRNAs, resulting in decreased polypeptide formation from those mRNAs. Since the discovery of the first miR, lin-4 in Caenorhabditis elegans, hundreds of miRs have been identified from humans to viruses, which have provided a crucial and pervasive layer of post-transcriptional gene regulation. The nervous system is a rich source of miR expression, with a diversity of miR functions in fundamental neurobiological processes including neuronal development, plasticity, metabolism, and apoptosis. Recently, the let-7 family of miRs is found to be a critical regulator of MOR function in opioid tolerance. Let-7 is the first identified human miR. Its family members are highly conserved across species in sequence and function. In the review, we will present a brief review of the opioid receptors, their regulation, and opioid tolerance as well as an overview of miRs and a perspective how miRs may interact with MOR and serve as a regulator of opioid tolerance.

Regulation of opioid tolerance by let-7 family microRNA targeting the mu opioid receptor

MicroRNA has emerged as a critical regulator of neuronal functions. This study aimed to test whether let-7 microRNAs can regulate the mu opioid receptor (MOR) and opioid tolerance. Employing bioinformatics, we identified a let-7 binding site in the 3'-untranslated region (UTR) of MOR mRNA, which was experimentally confirmed as a direct target of let-7. The repressive regulation of MOR by let-7 was revealed using a LNA-let-7 inhibitor to knockdown let-7 in SH-SY5Y cells. Conversely, morphine significantly upregulated let-7 expression in SH-SY5Y cells and in a mouse model of opioid tolerance. The LNA-let-7 inhibitor decreased brain let-7 levels and partially attenuated opioid antinociceptive tolerance in mice. Although chronic morphine treatment did not change overall MOR transcript, polysome-associated mRNA declined in a let-7-dependent manner. let-7 was identified as a mediator translocating and sequestering MOR mRNA to P-bodies, leading to translation repression. These results suggest that let-7 plays an integral role in opioid tolerance.

Up-regulation of mu-opioid receptors in the spinal cord of morphine-tolerant rats

Though morphine remains the most powerful drug for treating pain, its effectiveness is limited by the development of tolerance and dependence. The mechanism underlying development of tolerance to morphine is still poorly understood. One of the factors could be an alteration in the number of micro-receptors within specific parts of the nervous system. However, reports on changes in the micro-opioid receptor density in the spinal cord after chronic morphine administration are conflicting. Most of the studies have used subcutaneously implanted morphine pellets to produce tolerance. However, it does not simulate clinical conditions, where it is more common to administer morphine at intervals, either by injections or orally. In the present study, rats were made tolerant to morphine by injecting increasing doses of morphine (10-50 mg/kg, subcutaneously) for five days. In vitro tissue autoradiography for localization of micro-receptor in the spinal cord was done using [3H]-DAMGO. As compared to the spinal cord of control rats, the spinal cord of tolerant rats showed an 18.8% increase or up-regulation in the density of micro-receptors in the superficial layers of the dorsal horn. This up-regulation of micro-receptors after morphine tolerance suggests that a fraction of the receptors have been rendered desensitized, which in turn could lead to tolerance

Tissue slices in radioligand binding assays: studies in brain, pineal and muscle

The use of tissue homogenates in receptor binding assays raises serious questions as to the physiological value of a preparation which examines receptors (binding sites) in disrupted tissue. In order to usefully study the regulatory properties of neurotransmitter receptors under physiological conditions, the necessity for tissue preparations which retain some degree of cellular integrity is clear. We review here the experiments which have utilized intact tissue - largely in the form of thick slices - to perform radioligand binding assays. There are many reports which note marked differences between studies in intact versus broken cell preparations. For example, significant discrepancies in KD and Bmax values are apparent for [3H] quinuclidinyl benzilate (muscarinic) and [3H] ouabain (Na+/K+-ATP ase, sodium pump) sites in brain and muscle respectively. A further example is the well-described stimulatory effect of GABA on benzodiazepine binding sites which is not seen in tissue slices. Other examples are highlighted. For all ligands so far examined, binding to slices is reversible, stereospecific, saturable, displaceable by appropriate drugs and of high affinity (nM). The method developed in our own laboratory is inexpensive, rapid and involves a minimum of tissue preparation. The technique is so simple as to allow many workers to enter this field who would not otherwise have done so. We suggest that metabolically active tissue slices offer the simplest approach to the study of cell-surface receptor regulation in living tissue.

Morphine-induced opioid receptor down-regulation detected in intact adult rat brain cells

Intact brain cells dissociated from the brains of adult rats were used to study the regulation of opioid receptors by in vivo chronic morphine treatment. When the specific binding of [3H]naloxone in a physiological iso-osmotic buffer was compared in cells obtained from sham-operated rats and in those treated for 6 days with four (2 X 2) 75 mg morphine pellets, it was found that morphine treatment resulted in a significant reduction in the density of [3H]naloxone binding sites. This receptor down-regulation was not accompanied by a change in the receptor affinity for the ligand. This effect of morphine was reversed upon the removal of morphine pellets for 18 h after tolerance induction. When similar experiments were performed using brain homogenates prepared and assayed in the same physiological buffer, there was an increase in the number of [3H]naloxone binding sites in morphine-treated animals compared to controls. On the other hand, when the binding experiments were conducted in 50 mM Tris-HCl buffer, no difference in ligand binding was apparent between control and morphine-treated groups. The present results demonstrate opioid receptor down-regulation by chronic morphine treatment measured in intact brain cells, and suggest that different conclusions may be reached when other tissue preparations are used to assess the receptor density.

Morphine analgesia is potentiated in adult rats prenatally exposed to ethanol

Exposure to inescapable, intermittent footshock elicits an opioid-mediated stress-induced analgesia in rats. We have previously shown that this response is markedly potentiated in adult rats, prenatally exposed to ethanol. To further investigate our hypothesis that endogenous opioid pain-inhibitory systems are modified by prenatal ethanol exposure, we have measured the analgesic response to morphine, in vitro brain opiate receptor binding characteristics, and occupation of brain opiate receptors following systemic administration of morphine. Compared to controls, rats prenatally exposed to ethanol had significantly enhanced morphine analgesia. This enhancement, however, does not appear attributable to changes in number or affinity of mu or delta opiate receptors, or to altered occupation of receptors by morphine.