Exposure to morphine-associated cues increases mu opioid receptor mRNA expression in the nucleus accumbens of Wistar Kyoto rats

Western diet consumption does not impact the rewarding and aversive effects of morphine in male Sprague-Dawley rats

The impacts of high-fat and/or high-sugar diets on opioid-induced effects are well documented; however, little is known about the effect of such diet on the affective responses to opiates. To address this issue, in the present experiment male Sprague-Dawley rats were given ad libitum access to a western-style diet (high in saturated fat and sugar) or a standard laboratory chow diet beginning in adolescence and continuing into adulthood at which point they were trained in a combined conditioned taste avoidance (CTA)/conditioned place preference (CPP) procedure to assess the aversive and rewarding effects of morphine, respectively. On four conditioning cycles, animals were given access to a novel saccharin solution, injected with morphine (1 mg/kg or 5 mg/kg), and then placed on one side of a place preference chamber. Animals were then tested for place preference and saccharin preference. All subjects injected with morphine displayed significant avoidance of the morphine-associated solution (CTA) and preferred the side associated with the drug (CPP). Furthermore, there were no differences between the two diet groups, indicating that chronic exposure to the western diet had no impact on the affective properties of morphine (despite increasing caloric intake, body weight, body fat and lean body mass). Given previously reported increases in drug self-administration in animals with a history of western-diet consumption, this study suggests that western-diet exposure may increase drug intake via mechanisms other than changes in the rewarding or aversive effects of the drug.

Altered Intracellular Signaling Associated with Dopamine D2 Receptor in the Prefrontal Cortex in Wistar Kyoto Rats

Wistar-Kyoto rats (WKY), compared to Wistar rats, are a well-validated animal model for drug-resistant depression. Thanks to this, they can provide information on the potential mechanisms of treatment-resistant depression. Since deep brain stimulation in the prefrontal cortex has been shown to produce rapid antidepressant effects in WKY rats, we focused our study on the prefrontal cortex. Using quantitative autoradiography, we observed a decrease in the binding of [³H] methylspiperone to the dopamine D2 receptor, specifically in that brain region-but not in the striatum, nor the nucleus accumbens-in WKY rats. Further, we focused our studies on the expression level of several components associated with canonical (G proteins), as well as non-canonical, D2-receptor-associated intracellular pathways (e.g., βarrestin2, glycogen synthase kinase 3 beta-Gsk-3β, and β-catenin). As a result, we observed an increase in the expression of mRNA encoding the regulator of G protein signaling 2-RGS2 protein, which is responsible, among other things, for internalizing the D2 dopamine receptor. The increase in RGS2 expression may therefore account for the decreased binding of the radioligand to the D2 receptor. In addition, WKY rats are characterized by the altered signaling of genes associated with the dopamine D2 receptor and the βarrestin2/AKT/Gsk-3β/β-catenin pathway, which may account for certain behavioral traits of this strain and for the treatment-resistant phenotype.

Opioids in cancer: The κ‑opioid receptor (Review)

The κ‑opioid receptor (KOR) is one of the primary receptors of opioids and serves a vital role in the regulation of pain, anesthesia, addiction and other pathological and physiological processes. KOR is associated with several types of cancer and may influence cancer progression. It has been proposed that KOR may represent a new tumor molecular marker and provide a novel basis for molecular targeted therapies for cancer. However, the association between KOR and cancer remains to be explored comprehensively. The present review introduces KOR and its association with different types of cancer. Improved understanding of KOR may facilitate development of novel antitumor therapies.

Anxiety enhances pain in a model of osteoarthritis and is associated with altered endogenous opioid function and reduced opioid analgesia

Introduction

Negative affect, including anxiety and depression, is prevalent in chronic pain states such as osteoarthritis (OA) and associated with greater use of opioid analgesics, potentially contributing to present and future opioid crises.

Objectives

We tested the hypothesis that the interaction between anxiety, chronic pain, and opioid use results from altered endogenous opioid function.

Methods

A genetic model of negative affect, the Wistar-Kyoto (WKY) rat, was combined with intra-articular injection of monosodium iodoacetate (MIA; 1 mg) to mimic clinical presentation. Effects of systemic morphine (0.5-3.5 mg·kg^-1) on pain behaviour and spinal nociceptive neuronal activity were compared in WKY and normo-anxiety Wistar rats 3 weeks after MIA injection. Endogenous opioid function was probed by the blockade of opioid receptors (0.1-1 mg·kg^-1 systemic naloxone), quantification of plasma β-endorphin, and expression and phosphorylation of spinal mu-opioid receptor (MOR).

Results

Monosodium iodoacetate-treated WKY rats had enhanced OA-like pain, blunted morphine-induced analgesia, and greater mechanical hypersensitivity following systemic naloxone, compared with Wistar rats, and elevated plasma β-endorphin levels compared with saline-treated WKY controls. Increased MOR phosphorylation at the master site (serine residue 375) in the spinal cord dorsal horn of WKY rats with OA-like pain (P = 0.0312) indicated greater MOR desensitization.

Conclusions

Reduced clinical analgesic efficacy of morphine was recapitulated in a model of high anxiety and OA-like pain, in which endogenous opioid tone was altered, and MOR function attenuated, in the absence of previous exogenous opioid ligand exposure. These findings shed new light on the mechanisms underlying the increased opioid analgesic use in high anxiety patients with chronic pain.

Validation of chronic mild stress in the Wistar-Kyoto rat as an animal model of treatment-resistant depression

A recent review proposed four criteria for an animal model of treatment-resistant depression (TRD): a phenotypic resemblance to a risk factor for depression; enhanced response to stress; nonresponse to antidepressant drugs and response to treatments effective in TRD, such as deep brain stimulation (DBS) of the prefrontal cortex or ketamine. Chronic mild stress (CMS) provides a valid model of depression; the Wistar-Kyoto (WKY) rat is considered to be nonresponsive to antidepressant drugs. Here, we applied CMS to WKY rats. WKY and Wistar rats were exposed to CMS, then treated with saline, imipramine, citalopram or venlafaxine. After 5 weeks of CMS and 3 weeks of drug treatment, all WKY groups were implanted unilaterally with DBS electrodes in the prefrontal cortex, and examined in sucrose intake, elevated plus maze (EPM; decreased entries and time in the open arms) and novel object recognition (decreased exploration) tests, following 2×2 h of DBS. CMS decreased sucrose intake, open arm entries on the EPM, and object recognition. Relative to Wistars, WKY rats showed evidence of increased emotionality in the EPM and novel object recognition tests, and a greater impact of CMS on body weight gain and open arm entries. Wistars responded to drug treatment with an increase in sucrose intake but WKY were nonresponsive to drug treatment on all three behavioural tests. With one exception, DBS reversed the anhedonic, anxiogenic and dyscognitive effects of CMS in all groups of WKY rats. In a further experiment, subacute ketamine (10 mg/kg) also normalized behaviour on all three tests. We conclude that WKY rats subjected to CMS meet all four criteria for a valid model of TRD, and provide a basis for studying the mechanism of action of DBS.

Endogenous Opiates and Behavior: 2016

This paper is the thirty-ninth consecutive installment of the annual review of research concerning the endogenous opioid system. It summarizes papers published during 2016 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, drug abuse and alcohol, 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.

Intravenous morphine self-administration alters accumbal microRNA profiles in the mouse brain

A significant amount of evidence indicates that microRNAs (miRNAs) play an important role in drug addiction. The nucleus accumbens (NAc) is a critical part of the brain's reward circuit and is involved in a variety of psychiatric disorders, including depression, anxiety, and drug addiction. However, few studies have examined the expression of miRNAs and their functional roles in the NAc under conditions of morphine addiction. In this study, mice were intravenously infused with morphine (0.01, 0.03, 0.3, 1 and 3 mg/kg/infusion) and showed inverted U-shaped response. After morphine self-administration, NAc was used to analyze the functional networks of altered miRNAs and their putative target mRNAs in the NAc following intravenous self-administration of morphine. We utilized several bioinformatics tools, including Kyoto Encyclopedia of Genes and Genomes (KEGG) pathway mapping and CyTargetLinker. We found that 62 miRNAs were altered and exhibited differential expression patterns. The putative targets were related to diverse regulatory functions, such as neurogenesis, neurodegeneration, and synaptic plasticity, as well as the pharmacological effects of morphine (receptor internalization/endocytosis). The present findings provide novel insights into the regulatory mechanisms of accumbal molecules under conditions of morphine addiction and identify several novel biomarkers associated with morphine addiction.