Potential Involvement of a Substantia Nigra Circuit in Opioid Reinforcement

Attenuation of morphine conditioned place preference and reinstatement by vitamin D

Opioid action in the brain involves the dopamine-reward system as well as non-dopamine pathways. Since vitamin D also modulates the brain's dopamine system, the question of this study was how vitamin D might affect the opioid influences on the reward system. Therefore, the objective of this study was to investigate the possible effect of vitamin D on the conditioned place preference (CPP) induced by morphine, as a valuable model of assessment of the reinforcing properties of opioids by associating the context to the rewarding properties of the addictive drugs. Male Wistar rats were randomly divided into two main groups that either received saline (morphine vehicle) or morphine (5 mg/kg, intraperitoneally) for CPP. Each of the main groups was divided into three vitamin D treatment subgroups: vitamin D vehicle and vitamin D (5 and 10 μg/kg, intraperitoneally). Vitamin D injections were started 1 week ahead of the experiment (two injections) or immediately after post-conditioning and in both cases, it was continued twice weekly throughout the CPP. Administration of vitamin D (10 μg/kg) before conditioning in CPP markedly attenuated morphine expression in the post-conditioning test. Receiving vitamin D (5 or 10 μg/kg) before or after conditioning significantly attenuated morphine reinstatement. Administration of vitamin D after opioid conditioning facilitated morphine memory extinction and attenuated morphine reinstatement. Vitamin D is probably a valuable addition to be considered as a part of the treatment for prevention or minimizing the dependency or relapse to opioids.

Endogenous opiates and behavior: 2021

This paper is the forty-fourth consecutive installment of the annual anthological review of research concerning the endogenous opioid system, summarizing articles published during 2021 that studied the behavioral effects of molecular, pharmacological and genetic manipulation of opioid peptides and receptors as well as effects of opioid/opiate agonizts and antagonists. The review is subdivided into the following specific topics: molecular-biochemical effects and neurochemical localization studies of endogenous opioids and their receptors (1), the roles of these opioid peptides and receptors in pain and analgesia in animals (2) and humans (3), opioid-sensitive and opioid-insensitive effects of nonopioid analgesics (4), opioid peptide and receptor involvement in tolerance and dependence (5), stress and social status (6), learning and memory (7), eating and drinking (8), drug abuse and alcohol (9), sexual activity and hormones, pregnancy, development and endocrinology (10), mental illness and mood (11), seizures and neurologic disorders (12), electrical-related activity and neurophysiology (13), general activity and locomotion (14), gastrointestinal, renal and hepatic functions (15), cardiovascular responses (16), respiration and thermoregulation (17), and immunological responses (18).

The role of enkephalinergic systems in substance use disorders

Enkephalin, an endogenous opioid peptide, is highly expressed in the reward pathway and may modulate neurotransmission to regulate reward-related behaviors, such as drug-taking and drug-seeking behaviors. Drugs of abuse also directly increase enkephalin in this pathway, yet it is unknown whether or not changes in the enkephalinergic system after drug administration mediate any specific behaviors. The use of animal models of substance use disorders (SUDs) concurrently with pharmacological, genetic, and molecular tools has allowed researchers to directly investigate the role of enkephalin in promoting these behaviors. In this review, we explore neurochemical mechanisms by which enkephalin levels and enkephalin-mediated signaling are altered by drug administration and interrogate the contribution of enkephalin systems to SUDs. Studies manipulating the receptors that enkephalin targets (e.g., mu and delta opioid receptors mainly) implicate the endogenous opioid peptide in drug-induced neuroadaptations and reward-related behaviors; however, further studies will need to confirm the role of enkephalin directly. Overall, these findings suggest that the enkephalinergic system is involved in multiple aspects of SUDs, such as the primary reinforcing properties of drugs, conditioned reinforcing effects, and sensitization. The idea of dopaminergic-opioidergic interactions in these behaviors remains relatively novel and warrants further research. Continuing work to elucidate the role of enkephalin in mediating neurotransmission in reward circuitry driving behaviors related to SUDs remains crucial.