Microinjection of a Dopamine-D1 Receptor Agonist into the Ventral Tegmental Area Reverses the Blocked Expression of Morphine Conditioned Place Preference by N-Methyl-D-Aspartate Receptor Antagonist

Relationship between the role of muscarinic M3 receptors in morphine-induced conditioned place preference and the mesolimbic dopamine system

Opioid use disorder mainly results from functional defects in the brain reward loop, which includs the ventral tegmental area (VTA) and nucleus accumbens (NAc; consisting of shell and core, NAcS and NAcC). Reward effects contribute to opioid use disorder. RMTg M₃ receptors play a role in opioid reward by regulating the γ-aminobutyric acid (GABA) neuron activity. Dopamine D₁ receptors expressed on GABA neurons regulate opioid reward by mediating the dopamine neuron activity in the VTA. Therefore, we investigated the effect of activating M₃ receptors by microinjecting pilocarpine into the RMTg along with activating D₁ receptors by microinjecting SKF38393 into the VTA on morphine-induced reward effect, using the conditioned place preference (CPP) paradigm (locomotion was also recorded). We also investigated whether the activation of M₃ receptors in the RMTg influenced dopamine release in the NAcS. The results showed that the inhibitory role of RMTg pilocarpine (60 μg/rat) infusions in morphine-induced CPP was reversed by VTA SKF38393 (4 μg/rat) infusions. Moreover, morphine (5 mg/kg, i.p.) increased dopamine release in the NAcS, which was blunted by microinjecting pilocarpine (60 μg/rat) into the RMTg. These results indicate that RMTg M₃ receptors mediate morphine-induced reward effect, which is probably related to the dopamine activity within the VTA and NAcS. The relationship between RMTg M₃ receptors and the mesolimbic dopamine system could be a potential direction for the treatment of opioid use disorder, but further verification through more comprehensive techniques is needed.

Endogenous opiates and behavior: 2020

This paper is the forty-third consecutive installment of the annual anthological review of research concerning the endogenous opioid system, summarizing articles published during 2020 that studied the behavioral effects of molecular, pharmacological and genetic manipulation of opioid peptides and receptors as well as effects of opioid/opiate agonists 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).