Dopamine receptor D1 but not D3 essential for morphine-induced conditioned responses

Interactions between opioids and stimulants: Behavioral pharmacology of abuse-related effects

Opioid abuse continues to be a significant public health challenge, with rates of opioid-related overdose deaths increasing continuously over the last two decades. There also has been a sharp increase in overdose deaths involving stimulant drugs, primarily cocaine and methamphetamine. Recent estimates indicate a high prevalence of co-use of opioids and stimulants, which is a particularly complex problem. Behavioral pharmacology research over the last few decades has characterized interactions between opioids and stimulants as well as evaluated potential treatments. This chapter describes interactions between opioids and stimulants, with a focus on pre-clinical studies of abuse-related behavioral effects using self-administration, reinstatement, drug discrimination, place conditioning, and intracranial self-stimulation paradigms in laboratory animals. In general, the literature provides substantial evidence of mutual enhancement between opioids and stimulants for abuse-related effects, although such results are not ubiquitous. Enhanced abuse-related effects could manifest in many ways including engaging in drug seeking and taking behaviors with greater persistence, effort, and motivation and/or increased likelihood of relapse. Moreover, studies on opioid/stimulant combinations set the stage for evaluating potential treatments for polysubstance use. Behavioral pharmacology research has proven invaluable for elucidating these relationships using rigorous experimental designs and quantitative analyses of pharmacological and behavioral data.

Inhibition of Geranylgeranylacetone on cholecystokinin-B receptor, BDNF and dopamine D1 receptor induced by morphine

Morphine is the pain releasing and abusing drug. Morphine leads to addiction by activating dopaminergic rewarding system consisted of the ventral tegmental area (VTA) and nucleus accumbens (NAc). Cholecystokinin (CCK) is a gut-brain neuropeptide and involved in morphine dependence. Brain-derived neurotrophic factor (BDNF) is a neurotrophin and plays roles in regulating addiction. Geranylgeranylacetone (GGA) is a medicine of protecting gastric mucosal injury and protecting neurons. Our previous study showed that GGA blocked morphine-induced withdrawal and relapse through inducing thioredoxin 1(Trx1). In this study, we investigated that whether cholecystokinin-B receptor (CCKB receptor) and BDNF were related to GGA inhibition on morphine addiction. At first, we made conditioned place preference (CPP) model and confirmed again that GGA blocked the expression of morphine-CPP in present study. Then, our results showed that morphine increased the expressions of dopamine D1 receptor, tyrosine hydroxylase (TH), CCKB receptor and BDNF in the VTA and NAc in mice, which was inhibited by GGA. These results suggest that CCK and BDNF in dopaminergic systems are associated with the role of GGA blocking morphine-CPP.

Identification of the Risk Genes Associated With Vulnerability to Addiction: Major Findings From Transgenic Animals

Understanding risk factors for substance use disorders (SUD) can facilitate medication development for SUD treatment. While a rich literature exists discussing environmental factors that influence SUD, fewer articles have focused on genetic factors that convey vulnerability to drug use. Methods to identify SUD risk genes include Genome-Wide Association Studies (GWAS) and transgenic approaches. GWAS have identified hundreds of gene variants or single nucleotide polymorphisms (SNPs). However, few genes identified by GWAS have been verified by clinical or preclinical studies. In contrast, significant progress has been made in transgenic approaches to identify risk genes for SUD. In this article, we review recent progress in identifying candidate genes contributing to drug use and addiction using transgenic approaches. A central hypothesis is if a particular gene variant (e.g., resulting in reduction or deletion of a protein) is associated with increases in drug self-administration or relapse to drug seeking, this gene variant may be considered a risk factor for drug use and addiction. Accordingly, we identified several candidate genes such as those that encode dopamine D₂ and D₃ receptors, mGluR₂, M₄ muscarinic acetylcholine receptors, and α₅ nicotinic acetylcholine receptors, which appear to meet the risk-gene criteria when their expression is decreased. Here, we describe the role of these receptors in drug reward and addiction, and then summarize major findings from the gene-knockout mice or rats in animal models of addiction. Lastly, we briefly discuss future research directions in identifying addiction-related risk genes and in risk gene-based medication development for the treatment of addiction.

Opioid-induced structural and functional plasticity of medium-spiny neurons in the nucleus accumbens

Opioid Use Disorder (OUD) is a chronic relapsing clinical condition with tremendous morbidity and mortality that frequently persists, despite treatment, due to an individual's underlying psychological, neurobiological, and genetic vulnerabilities. Evidence suggests that these vulnerabilities may have neurochemical, cellular, and molecular bases. Key neuroplastic events within the mesocorticolimbic system that emerge through chronic exposure to opioids may have a determinative influence on behavioral symptoms associated with OUD. In particular, structural and functional alterations in the dendritic spines of medium spiny neurons (MSNs) within the nucleus accumbens (NAc) and its dopaminergic projections from the ventral tegmental area (VTA) are believed to facilitate these behavioral sequelae. Additionally, glutamatergic neurons from the prefrontal cortex, the basolateral amygdala, the hippocampus, and the thalamus project to these same MSNs, providing an enriched target for synaptic plasticity. Here, we review literature related to neuroadaptations in NAc MSNs from dopaminergic and glutamatergic pathways in OUD. We also describe new findings related to transcriptional, epigenetic, and molecular mechanisms in MSN plasticity in the different stages of OUD.

Pre-exposure of adolescent mice to morphine results in stronger sensitization and reinstatement of conditioned place preference than pre-exposure to hydrocodone

BACKGROUND

Opioids are commonly prescribed to treat moderate-to-severe pain. However, their use can trigger the development of opioid use disorder. A major problem in treating opioid use disorder remains the high rate of relapse.

AIM

The purpose of this study was to determine whether there are differences among opioids in their ability to trigger relapse after pre-exposure during adolescence.

METHODS

On postnatal day 33, mice were examined for the acute locomotor response to saline, morphine, or hydrocodone (5 mg/kg). They were administered with the corresponding opioid or saline during postnatal days 34-38 (20 mg/kg) and 40-44 (40 mg/kg). On postnatal day 45, they were recorded for the development of locomotor sensitization (5 mg/kg). Starting on postnatal day 55, mice were examined for the acquisition (1, 5, 10, 20, and 40 mg/kg), extinction, and drug-induced reinstatement (1, 2.5, and 5 mg/kg) of conditioned place preference.

RESULTS

There were no significant differences in the acute locomotor response to morphine and hydrocodone. Morphine induced significantly stronger locomotor sensitization as compared to hydrocodone. Pre-exposure to morphine, but not hydrocodone, sensitized the acquisition of conditioned place preference. There were no significant differences in extinction rates. Mice pre-exposed to morphine reinstate conditioned place preference after priming with a 1 mg/kg dose. In contrast, higher priming doses were required for reinstatement in all other experimental groups.

CONCLUSIONS

Adolescent mice administered with morphine develop greater sensitization to its effects and subsequently reinstate conditioned place preference more readily than mice administered with hydrocodone. This suggests higher risk for relapse after pre-exposure to morphine during adolescence as compared to hydrocodone.

Physical exercise modifies behavioral and molecular parameters related to opioid addiction regardless of training time

Addiction is a devastating worldwide disorder that requires effective and innovative therapies. Physical exercise could be useful in addiction treatment because it shares a common neural circuit with addictive drugs. Based on this, molecular adaptations consequent to time of exercise in opioid exposed animals were evaluated. Rats were designed as sedentary (SED) or exercised (EXE). This last group was separated to perform three different periods of swimming: short-term (S-EXE), medium-term (M-EXE) and long-term (L-EXE) for 14, 28 and 42 days, respectively. On the last exercising week, one-half of the animals from SED and all animals from S-, M- and l-EXE were concomitantly exposed to morphine-conditioned place preference (CPP) paradigm and y-maze task for behavioral assessments followed by molecular assays in both Nucleus accumbens (NAc) and hippocampus. Between SED groups, morphine conditioning showed drug-CPP and increased dopamine transporter (DAT), dopamine receptor type-1 (D1R), type-2 (D2R) and glucocorticoid receptor (GR) in both brain areas in relation to saline group. Besides the small morphine-CPP in relation to SED group, all periods decreased DAT, D1R, and GR immunoreactivity in NAc, DAT and D1R in hippocampus, while D2R in both brain areas and GR in hippocampus were primarily decreased by L-EXE. Our findings show that even a short-term exercise modifies behaviors related to drug withdrawal, changing DA targets and GR, which are closely linked to addiction. Therefore, our outcomes involving physical exercise are interesting to perform a possible clinical trial, thus expanding the knowledge about drug addiction.

Blockade of dopamine D1 receptors in male rats disrupts morphine reward in pain naïve but not in chronic pain states

The rewarding effect of opiates is mediated through dissociable neural systems in drug naïve and drug-dependent states. Neuroadaptations associated with chronic drug use are similar to those produced by chronic pain, suggesting that opiate reward could also involve distinct mechanisms in chronic pain and pain-naïve states. We tested this hypothesis by examining the effect of dopamine (DA) antagonism on morphine reward in a rat model of neuropathic pain.Neuropathic pain was induced in male Sprague-Dawley rats through chronic constriction (CCI) of the sciatic nerve; reward was assessed in the conditioned place preference (CPP) paradigm in separate groups at early (4-8 days post-surgery) and late (11-15 days post-surgery) phases of neuropathic pain. Minimal effective doses of morphine that produced a CPP in early and late phases of neuropathic pain were 6 mg/kg and 2 mg/kg respectively. The DA D1 receptor antagonist, SCH23390, blocked a morphine CPP in sham, but not CCI, rats at a higher dose (0.5 mg/kg), but had no effect at a lower dose (0.1 mg/kg). The DA D2 receptor antagonist, eticlopride (0.1 and 0.5 mg/kg), had no effect on a morphine CPP in sham or CCI rats, either in early or late phases of neuropathic pain. In the CPP paradigm, morphine reward involves DA D1 mechanisms in pain-naïve but not chronic pain states. This could reflect increased sensitivity to drug effects in pain versus no pain conditions and/or differential mediation of opiate reward in these two states.

Genetic deletion of the dopamine D3 receptor increases vulnerability to heroin in mice

Despite extensive research, the neurobiological risk factors that convey vulnerability to opioid abuse are still unknown. Recent studies suggest that the dopamine D3 receptor (D3R) is involved in opioid self-administration, but it remains unclear whether altered D3R availability is a risk factor for the development of opioid abuse and addiction. Here we used dopamine D3 receptor-knockout (D3-KO) mice to investigate the role of this receptor in the different phases of opioid addiction. D3-KO mice learned to self-administer heroin faster and took more heroin than wild-type mice during acquisition and maintenance of self-administration. D3R-KO mice also displayed higher motivation to work to obtain heroin reward during self-administration under progressive-ratio reinforcement, as well as elevated heroin-seeking during extinction and reinstatement testing. In addition, deletion of the D3R induced higher baseline levels of extracellular dopamine (DA) in the nucleus accumbens (NAc), higher basal levels of locomotion, and reduced NAc DA and locomotor responses to lower doses of heroin. These findings suggest that the D3R is critically involved in regulatory processes that normally limit opioid intake via DA-related mechanisms. Deletion of D3R augments opioid-taking and opioid-seeking behaviors. Therefore, low D3R availability in the brain may represent a risk factor for the development of opioid abuse and addiction.

The role of the vasopressin system and dopamine D1 receptors in the effects of social housing condition on morphine reward

BACKGROUND

The association with opioid-abusing individuals or even the perception of opioid abuse by peers are risk factors for the initiation and escalation of abuse. Similarly, we demonstrated that morphine-treated animals housed with only morphine-treated animals (referred to as morphine only) acquire morphine conditioned place-preference (CPP) more readily than morphine-treated animals housed with drug-naïve animals (referred to as morphine cage-mates). However, the molecular mechanisms underlying these effects are still elusive.

METHODS

Mice received repeated morphine or saline while housed as saline only, morphine only, or cage-mates. Then, they were examined for the expression levels of D1 dopamine receptor (D1DR), D2 dopamine receptor (D2DR), dopamine transporter (DAT), oxytocin, and Arginine-vasopressin (AVP) in the striatum using qPCR. Additionally, we examined the effects of the AVP-V1b receptor antagonist, SSR149415, on the acquisition of morphine conditioned place-preference (CPP).

RESULTS

Increased striatal expression of D1DR and AVP was observed in morphine only animals, but not morphine cage-mates. No significant effects were observed on the striatal expression of D2DR, DAT, or oxytocin. Antagonizing the AVP-V1b receptors decreased the acquisition of morphine CPP in the morphine only mice, but did not alter the acquisition of morphine CPP in the morphine cage-mate mice.

CONCLUSIONS

Housing with drug-naïve animals protects against the increase in striatal expression of D1DR and AVP elicited by morphine exposure. Moreover, our studies suggest that the protective effect of housing with drug-naïve animals on the acquisition of morphine reward might be, at least partially, mediated by AVP.

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.

The Role of Dopamine D1 and D3 Receptors in N-Methyl-D-Aspartate (NMDA)/GlycineB Site-Regulated Complex Cognitive Behaviors following Repeated Morphine Administration

BACKGROUND

Opiate addiction is associated with complex cognitive impairment, which contributes to the development of compulsive drug use and relapses. Dopamine and N-methyl-D-aspartate receptors play critical roles in opiate-induced cognitive deficits. However, the roles of D1 and D3 receptors in the N-methyl-D-aspartate/glycineB receptor-regulated cognitive behaviors induced by morphine remain unknown.

METHODS

The 5-choice serial reaction time task was used to investigate the cognitive profiles associated with repeated morphine administration in D1 (D1-/-)- and D3 (D3-/-)-receptor knockout mice. The expression of phosphorylated NR1, Ca2+/calmodulin-dependent protein kinase II (CaMKII), and cAMP response element-binding protein (CREB) in the brain was examined by western blotting. D1-/- and D3-/- mice were treated with the N-methyl-D-aspartate/glycineB site agonist l-aminocyclopropanecarboxylic acid and the antagonist L-701,324 to chronically disrupt N-methyl-D-aspartate receptor function and investigate their effects on morphine-induced cognitive changes.

RESULTS

Repeated morphine administration impaired attentional function and caused impulsive and compulsive behaviors. D1-/- mice exhibited hardly any premature nosepokes. D3-/- mice showed robustly increased morphine-induced impulsive behavior. The numbers of premature responses were decreased by L-701,324 administration and increased by ACPC administration; these effects were completely abolished in D1-/- mice due to their inability to perform reward-based tasks. In contrast, the inhibitory effects of L-701,324 on impulsive behavior were significantly augmented in D3-/- mice.

CONCLUSIONS

N-methyl-D-aspartate/glycineB site functions may contribute to morphine-induced cognitive deficits, especially those related to impulsive behavior. D1 and D3 receptors may have contrasting effects with respect to modulating impulsive behavior. D3 receptors have inhibitory effects on impulsive behaviors, and these effects are clearly mediated by N-methyl-D-aspartate/glycineB receptor and μ-opioid receptor interactions.

Dopamine D1 receptor agonist treatment attenuates extinction of morphine conditioned place preference while increasing dendritic complexity in the nucleus accumbens core

The dopamine D1 receptor (D1R) has a role in opioid reward and conditioned place preference (CPP), but its role in CPP extinction is undetermined. We examined the effect of D1R agonist SKF81297 on the extinction of opioid CPP and associated dendritic morphology in the nucleus accumbens (NAc), a region involved with reward integration and its extinction. During the acquisition of morphine CPP, mice received morphine and saline on alternate days; injections were given immediately before each of eight daily conditioning sessions. Mice subsequently underwent six days of extinction training designed to diminish the previously learned association. Mice were treated with either 0.5mg/kg SKF81297, 0.8mg/kg SKF81297, or saline immediately after each extinction session. There was a dose-dependent effect, with the highest dose of SKF81297 attenuating extinction, as mice treated with this dose had significantly higher CPP scores than controls. Analysis of medium spiny neuron morphology revealed that in the NAc core, but not in the shell, dendritic arbors were significantly more complex in the morphine conditioned, SKF81297-treated mice compared to controls. In separate experiments using mice conditioned with only saline, SKF81297 administration after extinction sessions had no effect on CPP and produced differing effects on dendritic morphology. At the doses used in our experiments, SKF81297 appears to maintain previously learned opioid conditioned behavior, even in the face of new information. The D1R agonist's differential, rather than unidirectional, effects on dendritic morphology in the NAc core suggests that it may be involved in encoding reward information depending on previously learned behavior.