Possible involvement of β-endorphin in a loss of the coordinated balance of μ-opioid receptors trafficking processes by fentanyl

Effect of chronic ethanol treatment on μ-opioid receptor function, interacting proteins and morphine-induced place preference

RATIONALE

Both the acute and chronic consumption of ethanol have been reported to modify several molecular events in the central nervous system, and the endogenous μ-opioid receptor system is involved in the reinforcing/rewarding effects of ethanol.

OBJECTIVES

The present study was designed to clarify the effects of chronic ethanol treatment on cellular processes involving μ-opioid receptor and the development of morphine-induced rewarding effects.

METHODS

Male C57BL/6J mice were continuously treated with a liquid diet containing 3.0 w/v ethanol. The direct involvement of μ-opioid receptor functions in the activation of G-proteins and changes in protein levels in the lower midbrain of mice after chronic treatment with ethanol were investigated by a [(35)S] GTPγS binding assay and Western blotting, respectively. The rewarding effects of morphine (5 mg/kg) under treatment with ethanol were measured by the conditioned place preference paradigm.

RESULTS

The function of μ-opioid receptor was increased by treatment with ethanol in the lower midbrain using [(35)S] GTPγS binding assay. Furthermore, the GRK2 protein level was significantly increased by treatment with ethanol. Chronic treatment with ethanol enhanced the rewarding effects of morphine. On the other hand, this enhancement of the rewarding effects of morphine by ethanol treatment was significantly inhibited by the GRK2 inhibitor β-adrenergic receptor kinase 1 inhibitor.

CONCLUSIONS

The present study demonstrated that chronic treatment with ethanol enhanced the rewarding effects of morphine by up-regulating functional changes in μ-opioid receptor, mediated by GRK2.

Oxycodone-induced conditioned place preference and sensitization of locomotor activity in adolescent and adult mice

Nonmedical use of the prescription opioid oxycodone has become a major public health problem in the United States, with special concern for adolescents. Although adults and adolescents have different sensitivities for drugs, little is known about the rewarding effects of oxycodone in adolescents compared to adults, even in rodent models. Here, we investigate sensitivity to oxycodone by the conditioned place preference assay of conditioned reward, and effect on the locomotor activity in adolescent (4 weeks old) and adult (10 weeks old) C57BL/6J mice. Mice of both ages were trained with multiple doses of oxycodone (0, 0.3, 1, and 3 mg/kg) and showed conditioned preference in a dose-dependent manner. The adult mice developed conditioned preference to the lowest dose tested (0.3 mg/kg), but adolescent mice did not. Dose-dependent oxycodone-induced increases in locomotor activity were observed across the conditioning session. Interestingly, adolescent mice developed greater sensitization to the locomotor-activating effects of oxycodone than adult mice. Thus differences in sensitivity to oxycodone, such as the lower initial sensitivity for conditioned preference but greater locomotor sensitization in adolescent mice, may indicate contributing factors in oxycodone abuse and later addiction in human adolescents.

Endogenous opiates and behavior: 2011

This paper is the thirty-fourth consecutive installment of the annual review of research concerning the endogenous opioid system. It summarizes papers published during 2011 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 (Section 16); and immunological responses (Section 17).