Dopamine D₄ receptor counteracts morphine-induced changes in µ opioid receptor signaling in the striosomes of the rat caudate putamen

Dopamine D4 Receptor Is a Regulator of Morphine-Induced Plasticity in the Rat Dorsal Striatum

Long-term exposition to morphine elicits structural and synaptic plasticity in reward-related regions of the brain, playing a critical role in addiction. However, morphine-induced neuroadaptations in the dorsal striatum have been poorly studied despite its key function in drug-related habit learning. Here, we show that prolonged treatment with morphine triggered the retraction of the dendritic arbor and the loss of dendritic spines in the dorsal striatal projection neurons (MSNs). In an attempt to extend previous findings, we also explored whether the dopamine D4 receptor (D4R) could modulate striatal morphine-induced plasticity. The combined treatment of morphine with the D4R agonist PD168,077 produced an expansion of the MSNs dendritic arbors and restored dendritic spine density. At the electrophysiological level, PD168,077 in combination with morphine altered the electrical properties of the MSNs and decreased their excitability. Finally, results from the sustantia nigra showed that PD168,077 counteracted morphine-induced upregulation of μ opioid receptors (MOR) in striatonigral projections and downregulation of G protein-gated inward rectifier K+ channels (GIRK1 and GIRK2) in dopaminergic cells. The present results highlight the key function of D4R modulating morphine-induced plasticity in the dorsal striatum. Thus, D4R could represent a valuable pharmacological target for the safety use of morphine in pain management.

Underlying Susceptibility to Eating Disorders and Drug Abuse: Genetic and Pharmacological Aspects of Dopamine D4 Receptors

The dopamine D4 receptor (DRD4) has a predominant expression in the prefrontal cortex (PFC), brain area strictly involved in the modulation of reward processes related to both food and drug consumption. Additionally, the human DRD4 gene is characterized by a variable number of tandem repeats (VNTR) in the exon 3 and, among the polymorphic variants, the 7-repeat (7R) allele appears as a contributing factor in the neurobiological mechanisms underlying drug abuse, aberrant eating behaviors and related comorbidities. The 7R variant encodes for a receptor with a blunted intracellular response to dopamine, and carriers of this polymorphism might be more tempted to enhance dopamine levels in the brain, through the overconsumption of drugs of abuse or palatable food, considering their reinforcing properties. Moreover, the presence of this polymorphism seems to increase the susceptibility of individuals to engage maladaptive eating patterns in response to negative environmental stimuli. This review is focused on the role of DRD4 and DRD4 genetic polymorphism in these neuropsychiatric disorders in both clinical and preclinical studies. However, further research is needed to better clarify the complex DRD4 role, by using validated preclinical models and novel compounds more selective for DRD4.

On the G Protein-Coupled Receptor Neuromodulation of the Claustrum

G protein-coupled receptors modulate the synaptic glutamate and GABA transmission of the claustrum. The work focused on the transmitter–receptor relationships in the claustral catecholamine system and receptor–receptor interactions between kappa opioid receptors (KOR) and SomatostatinR2 (SSTR2) in claustrum. Methods used involved immunohistochemistry and in situ proximity ligation assay (PLA) using confocal microscopy. Double immunolabeling studies on dopamine (DA) D1 receptor (D1R) and tyrosine hydroxylase (TH) immunoreactivities (IR) demonstrated that D1R IR existed in almost all claustral and dorsal endopiriform nucleus (DEn) nerve cell bodies, known as glutamate projection neurons, and D4R IR in large numbers of nerve cell bodies of the claustrum and DEn. However, only a low to moderate density of TH IR nerve terminals was observed in the DEn versus de few scattered TH IR terminals found in the claustrum. These results indicated that DA D1R and D4R transmission in the rat operated via long distance DA volume transmission in the rat claustrum and DEn to modulate claustral-sensory cortical glutamate transmission. Large numbers of these glutamate projection neurons also expressed KOR and SSTR2 which formed KOR-SSTR2 heteroreceptor complexes using PLA. Such receptor–receptor interactions can finetune the activity of the glutamate claustral-sensory cortex projections from inhibition to enhancement of their sensory cortex signaling. This can give the sensory cortical regions significant help in deciding on the salience to be given to various incoming sensory stimuli.

Pharmacological activation of dopamine D4 receptor modulates morphine-induced changes in the expression of GAD65/67 and GABAB receptors in the basal ganglia

Dopamine D₄ receptor (D₄R) stimulation, in a putative D₄R/μ opioid heteroreceptor (MOR) complex, counteracts the molecular, cellular and behavioural actions of morphine which are associated with morphine addiction, without any effect on its analgesic properties. In the present work, we have evaluated the role of D₄R in modulating the effects of a continuous treatment with morphine on the GABAergic system in the basal ganglia. It has been demonstrated that the co-administration of a D₄R agonist together with morphine leads to a restoration of GABA signaling by preventing drug-induced changes in GAD_65/67 expression in the caudate putamen, globus palidus and substantia nigra. Results from GABA_BR1 and GABA_BR2 expression suggest a role of D₄R in modulation of the GABA_B heteroreceptor complexes along the basal ganglia, especially in the functional divisions of the caudate putamen. These results provide a new proof of the functional interaction between D₄R and MOR and we postulate this putative heteroreceptor complex as a key target for the development of a new strategy to prevent the addictive effects of morphine in the treatment of pain. This article is part of the Special Issue entitled 'Receptor heteromers and their allosteric receptor-receptor interactions'.

Transcriptomic integration of D4R and MOR signaling in the rat caudate putamen

Morphine binding to opioid receptors, mainly to μ opioid receptor (MOR), induces alterations in intracellular pathways essential to the initial development of addiction. The activation of the dopamine D₄ receptor (D₄R), which is expressed in the caudate putamen (CPu), mainly counteracts morphine-induced alterations in several molecular networks. These involve transcription factors, adaptive changes of MOR signaling, activation of the nigrostriatal dopamine pathway and behavioural effects, underlining functional D₄R/MOR interactions. To shed light on the molecular mechanisms implicated, we evaluated the transcriptome alterations following acute administration of morphine and/or PD168,077 (D₄R agonist) using whole-genome microarrays and a linear regression-based differential expression analysis. The results highlight the development of a unique transcriptional signature following the co-administration of both drugs that reflects a countereffect of PD168,077 on morphine effects. A KEGG pathway enrichment analysis using GSEA identified 3 pathways enriched positively in morphine vs control and negatively in morphine + PD168,077 vs morphine (Ribosome, Complement and Coagulation Cascades, Systemic Lupus Erythematosus) and 3 pathways with the opposite enrichment pattern (Alzheimer’s Disease, Neuroactive Ligand Receptor Interaction, Oxidative Phosphorilation). This work supports the massive D₄R/MOR functional integration at the CPu and provides a gateway to further studies on the use of D₄R drugs to modulate morphine-induced effects.

Dopamine D4 receptor stimulation prevents nigrostriatal dopamine pathway activation by morphine: relevance for drug addiction

Morphine is one of the most effective drugs used for pain management, but it is also highly addictive. Morphine elicits acute and long-term adaptive changes at cellular and molecular level in the brain, which play a critical role in the development of tolerance, dependence and addiction. Previous studies indicated that the dopamine D₄ receptor (D₄ R) activation counteracts morphine-induced adaptive changes of the μ opioid receptor (MOR) signaling in the striosomes of the caudate putamen (CPu), as well as the induction of several Fos family transcription factors. Thus, it has been suggested that D₄ R could play an important role avoiding some of the addictive effects of morphine. Here, using different drugs administration paradigms, it is determined that the D₄ R agonist PD168,077 prevents morphine-induced activation of the nigrostriatal dopamine pathway and morphological changes of substantia nigra pars compacta (SNc) dopamine neurons, leading to a restoration of dopamine levels and metabolism in the CPu. Results from receptor autoradiography indicate that D₄ R activation modulates MOR function in the substantia nigra pars reticulata (SNr) and the striosomes of the CPu, suggesting that these regions are critically involved in the modulation of SNc dopamine neuronal function through a functional D₄ R/MOR interaction. In addition, D₄ R activation counteracts the rewarding effects of morphine, as well as the development of hyperlocomotion and physical dependence without any effect on its analgesic properties. These results provide a novel role of D₄ R agonist as a pharmacological strategy to prevent the adverse effects of morphine in the treatment of pain.

Endogenous Opiates and Behavior: 2015

This paper is the thirty-eighth consecutive installment of the annual review of research concerning the endogenous opioid system. It summarizes papers published during 2015 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.

G-protein-coupled receptor type A heteromers as an emerging therapeutic target

INTRODUCTION

The discovery of receptor-receptor interactions (RRIs) in the early 1980s provided evidence that G-protein-coupled receptors (GPCRs) operate not only as monomers but also as heteromers, in which integration of the incoming signals takes place already at the plasma membrane level through allosteric RRIs. These integrative mechanisms give sophisticated dynamics to the structure and function of these receptor assemblies in terms of modulation of recognition, G-protein signaling and selectivity and switching to β-arrestin signaling.

AREAS COVERED

The present review briefly describes the concept of direct RRI and the available data on the mechanisms of oligomer formation. Further, pharmacological data concerning the best characterized heteromers involving type A GPCRs will be analyzed to evaluate their profile as possible targets for the treatment of various diseases, in particular of impacting diseases of the CNS.

EXPERT OPINION

GPCR heteromers have the potential to open a completely new field for pharmacology with likely a major impact in molecular medicine. Novel pharmacological strategies for the treatment of several pathologies have already been proposed. However, several challenges still exist to accurately characterize the role of the identified heteroreceptor complexes in pathology and to develop heteromer-specific ligands capable of efficiently exploiting their pharmacological features.