Role of dopamine D2 receptors in plasticity of stress-induced addictive behaviours

Involvement of D2-like dopamine receptors within the ventral tegmental area in the cannabidiol's inhibitory effects on the methamphetamine-seeking behavior

Cannabidiol (CBD) is a non-psychoactive substance derived from marijuana. Although a comprehensive understanding of CBD's mechanism of action is still lacking, it is well-established that CBD can effectively mitigate the addictive properties associated with drugs. This study examined how CBD inhibits the acquisition and expression of methamphetamine-induced conditioned place preference (CPP) through D2-like dopamine receptors (D2R) in the ventral tegmental area (VTA). After recovery from surgery, animals were subjected to bilateral intra-VTA administration of different dosages (0.25, 1, and 4 μg/0.3 μl DMSO per side) of a selective D2R antagonist, sulpiride before intracerebroventricular (ICV) injection of CBD, during the conditioning phase (10 μg/5 μl DMSO) or once in the post-conditioning phase (50 μg/5 μl DMSO) of methamphetamine-induced CPP (daily subcutaneous injections of methamphetamine at 1 mg/kg over 5-day conditioning period). Findings revealed that CBD inhibits the acquisition and expression of methamphetamine reward memory. At the same time, microinjection of D2R antagonists into the VTA significantly reduced CBD's suppressive effect on the acquisition (0.25 μg; P<0.05, 1 and 4 μg; P<0.001) and expression (1 and 4 μg; P<0.01) of methamphetamine place preference. Moreover, D2R blockage alone in the VTA did not affect the formation and expression of methamphetamine-induced CPP. In addition, the present study showed that administration of intra-VTA sulpiride and ICV injection of CBD together does not cause place preference in the CPP paradigm. In conclusion, pharmacological manipulation of D2Rs in the VTA may alter CBD's suppressive effects on the methamphetamine-context associations.

Cranial radiation disrupts dopaminergic signaling and connectivity in the mammalian brain

Cognitive impairment is a common and challenging side effect of cranial radiation therapy for brain tumors, though its precise mechanisms remain unclear. The mesocortical dopaminergic pathway, known to play a key role in cognitive function, is implicated in several neuropsychiatric disorders, yet its involvement in radiation-induced cognitive dysfunction is unexplored. Here, with using in vivo multi-electrode array recordings of both anesthetized and free-moving rats to monitor the firing activities of dopamine neurons in the ventral tegmental area (VTA) and local field potentials in both the prefrontal cortex (PFC) and VTA, as well as the immunofluorescence assays and western blotting, we report that cranial irradiation transiently altered VTA dopamine neuron firing patterns without affecting overall firing rates and led to sustained reductions in both "awake" and total dopamine neuron density. Additionally, radiation exposure impaired D2 receptor function and disrupted connectivity between the PFC and VTA. These multifaceted disruptions in the mesocortical dopamine signaling may underlie the development of radiation-induced cognitive dysfunction. These findings pave the way for novel research to prevent or reverse radiation-induced injury, ultimately improving the quality of life for brain tumor survivors.

Chemistry to cognition: Therapeutic potential of (m-CF3-PhSe)2 targeting rats' striatum dopamine proteins in amphetamine dependence

Amphetamine (AMPH) abuse represents a major global public health issue, highlighting the urgent need for effective therapeutic interventions to manage addiction caused by this psychostimulant. This study aimed to assess the potential of m-trifluoromethyl-diphenyldiselenide [(m-CF3-PhSe)2] in preventing the addictive effects induced by AMPH through targeting dopamine metabolism proteins. (m-CF3-PhSe)2 is of interest due to its demonstrated efficacy in mitigating opioid abuse, establishing it as a promising candidate for addiction treatment research. Initially, in silico studies examined the affinity of AMPH and (m-CF3-PhSe)2 for dopamine 1, 2, and 3 receptors (D1R, D2R, D3R), and dopamine transporter (DAT). In our experimental design, male Wistar rats were divided into four groups: I) Control; II) (m-CF3-PhSe)2; III) AMPH; IV) (m-CF3-PhSe)2 + AMPH. Animals were administered (m-CF3-PhSe)2 (0.1 mg/kg, by gavage) or canola oil (vehicle) 30 min before AMPH (4.0 mg/kg, i.p.) administration. Drug administration occurred for 8 days in the conditioned place preference (CPP) paradigm. Twenty-four hours after the last CPP conditioning section, preference for the drug-compartment was assessed, with anxiety-related effects and working memory were evaluated using the Y-maze test. Finally, animals were euthanized for striatal dissection to quantify D1R, D2R, D3R, and DAT levels in western blot. In silico findings suggest that (m-CF3-PhSe)2 may prevent AMPH activation in DAT, interacting with Asp46 and Phe319, preventing possible addictive effects of AMPH in DAT. In vivo results showed that (m-CF3-PhSe)2 attenuated AMPH effects, reducing preference for the drug-compartment in CPP test. Furthermore, (m-CF3-PhSe)2 prevented AMPH-induced anxiogenic effects in the elevated plus maze (EPM) test, similarly to light/dark test. No differences in locomotion or working memory were observed among the experimental groups in the Y-maze test. Ex vivo western blot analyses of the entire striatum indicates that (m-CF3-PhSe)2 prevented the AMPH-induced increase in D1R levels and decrease in D2R and DAT levels, with no changes in D3R levels. Overall, our study suggests that (m-CF3-PhSe)2 may interact with DAT sites similarly to AMPH, reducing drug-compartment preference and anxiogenic behaviors while maintaining dopaminergic metabolism proteins in the striatum, a key region involved in the onset and perpetuation of addiction.

Repurposing Ketamine in the Therapy of Depression and Depression-Related Disorders: Recent Advances and Future Potential

Depression represents a prevalent and enduring mental disorder of significant concern within the clinical domain. Extensive research indicates that depression is very complex, with many interconnected pathways involved. Most research related to depression focuses on monoamines, neurotrophic factors, the hypothalamic-pituitary-adrenal axis, tryptophan metabolism, energy metabolism, mitochondrial function, the gut-brain axis, glial cell-mediated inflammation, myelination, homeostasis, and brain neural networks. However, recently, Ketamine, an ionotropic N-methyl-D-aspartate (NMDA) receptor antagonist, has been discovered to have rapid antidepressant effects in patients, leading to novel and successful treatment approaches for mood disorders. This review aims to summarize the latest findings and insights into various signaling pathways and systems observed in depression patients and animal models, providing a more comprehensive view of the neurobiology of anxious-depressive-like behavior. Specifically, it highlights the key mechanisms of ketamine as a rapid-acting antidepressant, aiming to enhance the treatment of neuropsychiatric disorders. Moreover, we discuss the potential of ketamine as a prophylactic or therapeutic intervention for stress-related psychiatric disorders.

CGRP causes anxiety via HP1γ-KLF11-MAOB pathway and dopamine in the dorsal hippocampus

Calcitonin gene-related peptide (CGRP) is a neuropeptide that causes anxiety behavior; however, the underlying mechanisms remain unclear. We found that CGRP modulates anxiety behavior by epigenetically regulating the HP1γ-KLF-11-MAOB pathway and depleting dopamine in the dorsal hippocampus. Intracerebroventricular administration of CGRP (0.5 nmol) elicited anxiety-like behaviors in open field, hole-board, and plus-maze tests. Additionally, we observed an increase in monoamine oxidase B (MAOB) levels and a concurrent decrease in dopamine levels in the dorsal hippocampus of mice following CGRP administration. Moreover, CGRP increased abundance the transcriptional regulator of MAOB, Krüppel-like factor 11 (KLF11), and increased levels of phosphorylated heterochromatin protein (p-HP1γ), which is involved in gene silencing, by methylating histone H3 in the dorsal hippocampus. Chromatin immunoprecipitation assay showed that HP1γ was recruited to the Klf11 enhancer by CGRP. Furthermore, infusion of CGRP (1 nmol) into the dorsal hippocampus significantly increased MAOB expression as well as anxiety-like behaviors, which were suppressed by the pharmacological inhibition or knockdown of MAOB. Together, these findings suggest that CGRP reduces dopamine levels and induces anxiety-like behavior through epigenetic regulation in the dorsal hippocampus.

Dorsal BNST DRD2+ neurons mediate sex-specific anxiety-like behavior induced by chronic social isolation

The dorsal bed nucleus of stria terminalis (dBNST) is a pivotal hub for stress response modulation. Dysfunction of dopamine (DA) network is associated with chronic stress, but the roles of DA network of dBNST in chronic stress-induced emotional disorders remain unclear. We examine the role of dBNST Drd1⁺ and Drd2⁺ neurons in post-weaning social isolation (PWSI)-induced behavior deficits. We find that male, but not female, PWSI rats exhibit negative emotional phenotypes and the increase of excitability and E-I balance of dBNST Drd2⁺ neurons. More importantly, hypofunction of dBNST Drd2 receptor underlies PWSI-stress-induced male-specific neuronal plasticity change of dBNST Drd2⁺ neurons. Furthermore, chemogenetic activation of dBNST Drd2⁺ neurons is sufficient to induce anxiogenic effects, while Kir4.1-mediated chronic inhibition of dBNST Drd2⁺ neurons ameliorate PWSI-induced anxiety-like behaviors. Our findings reveal an important neural mechanism underlying PWSI-induced sex-specific behavioral abnormalities and potentially provide a target for the treatment of social stress-related emotional disorder.

Behavioral profile of adult zebrafish acutely exposed to a selective dopamine uptake inhibitor, GBR 12909

BACKGROUND

The dopamine transporter (DAT) is the main regulator of dopamine concentration in the extrasynaptic space. The pharmacological inhibition of the DAT results in a wide spectrum of behavioral manifestations, which have been identified so far in a limited number of species, mostly in rodents.

AIM

Here, we used another well-recognized model organism, the zebrafish (Danio rerio), to explore the behavioral effects of GBR 12909, a highly-affine selective DAT blocker.

METHODS

We evaluated zebrafish locomotion, novelty-related exploration, spatial cognition, and social phenotypes in the novel tank, habituation and shoaling tests, following acute 20-min water immersion in GBR 12909.

RESULTS

Our findings show hypolocomotion, anxiety-like state, and impaired spatial cognition in fish acutely treated with GBR 12909. This behavioral profile generally parallels that of the DAT knockout rodents and zebrafish, and it overlaps with behavioral effects of other DAT-inhibiting drugs of abuse, such as cocaine and D-amphetamine.

CONCLUSION

Collectively, our data support the utility of zebrafish in translational studies on DAT targeting neuropharmacology and strongly implicate DAT aberration as an important mechanisms involved in neurological and psychiatric diseases.

Current State of Modeling Human Psychiatric Disorders Using Zebrafish

Psychiatric disorders are highly prevalent brain pathologies that represent an urgent, unmet biomedical problem. Since reliable clinical diagnoses are essential for the treatment of psychiatric disorders, their animal models with robust, relevant behavioral and physiological endpoints become necessary. Zebrafish (Danio rerio) display well-defined, complex behaviors in major neurobehavioral domains which are evolutionarily conserved and strikingly parallel to those seen in rodents and humans. Although zebrafish are increasingly often used to model psychiatric disorders, there are also multiple challenges with such models as well. The field may therefore benefit from a balanced, disease-oriented discussion that considers the clinical prevalence, the pathological complexity, and societal importance of the disorders in question, and the extent of its detalization in zebrafish central nervous system (CNS) studies. Here, we critically discuss the use of zebrafish for modeling human psychiatric disorders in general, and highlight the topics for further in-depth consideration, in order to foster and (re)focus translational biological neuroscience research utilizing zebrafish. Recent developments in molecular biology research utilizing this model species have also been summarized here, collectively calling for a wider use of zebrafish in translational CNS disease modeling.

The Dopamine System in Mediating Alcohol Effects in Humans

Brain-imaging studies show that the development and maintenance of alcohol use disorder (AUD) is determined by a complex interaction of different neurotransmitter systems and multiple psychological factors. In this context, the dopaminergic reinforcement system appears to be of fundamental importance. We focus on the excitatory and depressant effects of acute versus chronic alcohol intake and its impact on dopaminergic neurotransmission. Furthermore, we describe alterations in dopaminergic neurotransmission as associated with symptoms of alcohol dependence. We specifically focus on neuroadaptations to chronic alcohol consumption and their effect on central processing of alcohol-associated and reward-related stimuli. Altered reward processing, complex conditioning processes, impaired reinforcement learning, and increased salience attribution to alcohol-associated stimuli enable alcohol cues to drive alcohol seeking and consumption. Finally, we will discuss how the neurobiological and neurochemical mechanisms of alcohol-associated alterations in reward processing and learning can interact with stress, cognition, and emotion processing.

Developmental effects of early-life stress on dopamine D2 receptor and proteins involved in noncanonical D2 dopamine receptor signaling pathway in the prefrontal cortex of male rats

OBJECTIVES

Dopamine neurotransmission is implicated in multiple neuropsychiatric disorders, most strikingly in Parkinson's disease, bipolar disorder, attention-deficit hyperactivity disorder and schizophrenia. In addition to canonical pathway, D2-receptor (D2R) exerts some of its biological actions through regulating the activity of Akt and GSK3, which in turn were found to be altered in several psychiatric illnesses. The present study examined the impacts of maternal separation, an early-life stress model which has been associated with disturbed neurodevelopment and appearance of many psychiatric disorders, on developmental changes in dopamine concentration and the expression of D2Rs, Akt and GSK-3β in the medial prefrontal cortex (PFC; a key target of stress) in adolescent and young adult male rats.

METHODS

Maternal separation was performed 3 h per day from postnatal days 2 to 11. The PFC protein and dopamine contents were determined using western blotting analysis and Eliza, respectively.

RESULTS

Results indicated long-term increases in the prefrontal dopamine levels in stressed adolescent and young adult male rats, accompanied by significant downregulation of D2R as well as upregulation of p-Akt and GSK-3β contents in stressed adolescence compared to controls, with all protein levels that returned to control values in stressed adult rats.

CONCLUSIONS

Our findings suggest that early-life stress differentially modulates prefrontal D2R/Akt/GSK-3β levels during development. Since adolescence period is susceptible to the onset of specific mental illnesses, disruption of noncanonical components of D2R signaling during this critical period may have an important role in programming neurobehavioral phenotypes in adulthood and manipulations influencing Akt/GSK-3β pathway may improve the expression of specific dopamine-related behaviors and the effects of dopaminergic drugs.

Probiotics (Bacillus clausii and Lactobacillus fermentum NMCC-14) Ameliorate Stress Behavior in Mice by Increasing Monoamine Levels and mRNA Expression of Dopamine Receptors (D1 and D2) and Synaptophysin

Stress is a physiological consequence of the body to adversity. The gut-brain axis and probiotics are gaining interest to provide better treatment for stress and other neurological disorders. Probiotic (Lactobacillus fermentum NMCC-14 and Bacillus clausii, 1010 colony-forming unit/day/animal, per oral) effects were investigated in acute (up to day 7) and subacute (days 8-14) restraint-stressed and normal mice through behavioral paradigms (elevated plus maze: EPM, light dark box/dark light box: LDB, and open field test: OFT). Time spent in the open arms of the EPM, time spent in the light compartment of the LDB, and movable time and time spent in the center of the OFT were significantly (p ≤ 0.05, n = 5) increased in probiotic-treated restraint-stressed mice. Enzyme-linked immunoassay determined blood cortisol and adrenocorticotropic hormone (ACTH) levels, which were reduced significantly (p < 0.05, n = 5) in probiotic-treated restraint-stressed mice. Hematoxylin and eosin-stained hippocampal slides also showed less or no neurodegeneration in the probiotic-treated animals. High-performance liquid chromatography and quantitative polymerase chain reaction were performed to determine the monoamine levels and mRNA expression of dopamine receptor subtypes (D1 and D2) and synaptophysin in the mice hippocampus (HC) and prefrontal cortex (PFC). The dopamine, serotonin, and norepinephrine levels were also significantly (p < 0.05, n = 5) increased in the HC and PFC of probiotic-treated animal brains. Fold expression of mRNA of D1 and D2 (except HC, LF-S, day 14) receptors and synaptophysin was also significantly (p < 0.05, n = 5) increased in the same brain parts of probiotic-treated restraint-stressed mice. Comparing mice in the Lactobacillus fermentum NMCC-14 and Bacillus clausii groups to mice in the normal group, only a significant (p < 0.05, n = 5) decrease was observed in the serum ACTH and cortisol levels on day 14 in Bacillus clausii-treated mice, where all other parameters also showed improvement. In comparison, Bacillus clausii showed greater stress suppressant activity than Lactobacillus fermentum NMCC-14. However, both probiotic bacteria can be a better and safer therapeutic alternative for ailments than currently available drugs.

A test of context and sex-dependent dopaminergic effects on the behavior of a gregarious bird, the common waxbill Estrilda astrild

The Dopaminergic (DAergic) system has well known influences on behavioral and cognitive functions. Previous work with common waxbills (Estrilda astrild) reported context-specific DAergic effects that could have been due to social environment. Manipulating the dopamine D2-like receptor family (D2R) pathways had opposed effects on behavior depending on whether waxbills were tested alone or in a small cage with a mirror as social stimulus. Since waxbills are highly gregarious, it was hypothesized that being alone or perceiving to have a companion might explain this context-dependence. To test context-dependent DAergic effects, we compared behavioral effects of D2R manipulation in waxbills in the same familiar environment, but either alone or with a familiar, same-sex companion. We found that D2R agonism decreased movement and feeding, similarly to previous results when testing waxbills alone. However, contrary to the hypothesis of dependence on social context, we found that the behavioral effects of the D2R agonist were unchanged when waxbills were tested with a companion. The context-dependence reported earlier might thus be due to other factors, such as the stress of being in a novel environment (small cage) or with an unfamiliar social stimulus (mirror image). In tests with a companion, we also found a sex-specific social effect of D2R manipulation: D2R blocking tended to decrease aggression in males but to increase in females. Together with past work, our results suggest that DAergic effects on behavior involve different types of context- or sex-dependence.

Maternal Deprivation Increased Vulnerability to Depression in Adult Rats Through DRD2 Promoter Methylation in the Ventral Tegmental Area

OBJECTIVE

Early life adversity is a risk factor for depression in adulthood; however, the underlying mechanisms are not well understood. This study aims to investigate the effect of DNA methylation of DRD2 gene on early life stress-induced depression in adult rats.

METHODS

Newborn Sprague-Dawley rats were randomly assigned to four groups: maternal deprivation group (MD), chronic unpredictable stress (CUS) group, maternal deprivation plus chronic unpredictable stress (MD/CUS) group, and normal control group (NOR). Behaviors were measured by open field test (OFT), sucrose preference test (SPT), and Original Research Article forced swimming test (FST). Fecal CORT level was detected by ELISA. Bisulfite amplicon sequencing PCR was used to assess methylation levels of DRD2 promoter.

RESULTS

CUS and MD/CUS rats had a significantly shorter total distance, longer immobility time, and higher CORT level, while MD and MD/CUS rats had a significantly lower percentage of central distance, more feces, lower rate of sucrose preference, and lower levels of DRD2 protein and mRNA in the VTA than NOR rats. CUS rats showed a significantly higher DRD2 mRNA and protein levels in the VTA than NOR rats. CUS, MD, and MD/CUS rats showed a significantly higher level of DRD2 promoter methylation than NOR rats. CORT level was significantly correlated with the sucrose preference rate in SPT, the immobility time in FST, the total distance, and the number of fecal pellets in OFT. DRD2 protein level was significantly correlated with the sucrose preference rate and the number of fecal pellets. DRD2 mRNA level was significantly correlated with the percentage of central distance and the number of fecal pellets in OFT. The level of DRD2 promoter methylation was significantly correlated with the sucrose preference rate, immobility time, total distance, the percentage of central distance, and the number of fecal pellets.

CONCLUSIONS

Early life MD increased vulnerability to stress-induced depressive-like behavior in adult rats. Enhanced DRD2 promoter methylation in the VTA may increase the susceptibility to depression.

Chronic stress in adolescence differentially affects cocaine vulnerability in adulthood in a selectively bred rat model of individual differences: role of accumbal dopamine signaling

Stress during adolescence has profound effects on the onset and severity of substance use later in life. However, not everyone with adverse experiences during this period will go on to develop a substance use disorder in adulthood, and the factors that alter susceptibility to substance use remain unknown. Here, we investigated individual differences in response to stress and drugs of abuse using our selectively bred high-responder (bHR) and low-responder (bLR) rats. These animals model extremes of temperamental tendencies and differ dramatically in both stress responsiveness and addiction-related traits. The present study investigated how environmental interventions in the form of a chronic variable stress (CVS) regimen in early adolescence interact with the bHR/bLR phenotype to alter behavioral sensitization to cocaine in adulthood. We also determined whether accumbal dopamine signaling is involved in the interaction of stress history and cocaine by assessing the mRNA levels of dopamine D1 (D1R) and D2 (D2R) receptors. Our results showed that CVS history alone had enduring and phenotype-specific effects on accumbal dopamine signaling. Importantly, adolescent stress had opposing effects in the two lines- decreasing the locomotor response to cocaine challenge in bHRs but increasing this measure in bLRs. Moreover, these opposing effects on cocaine sensitivity following adolescent CVS were accompanied by parallel effects in the accumbal dopamine system, with prior stress and cocaine exposure interacting to decrease D2R mRNA in bHRs but increase it in bLRs. Overall, these findings indicate that environmental challenges encountered in adolescence interact with genetic background to alter vulnerability to cocaine later in life.Lay SummaryStress experienced during adolescence affects the onset and severity of drug dependence later in life. However, not everyone with adverse experiences during this period will go on to develop SUD in adulthood. Using a rat model of innate differences in emotional reactivity, this study shows that the interplay between individual temperament and previous experience of adolescent stress/trauma determines whether an individual will be vulnerable or resilient to develop SUDs later in life. In addition, the present study shows that the dopamine D2 receptor in the brain's reward center, nucleus accumbens, may be implicated in this interplay.

[Anhedonia in depression: neurobiological and genetic aspects]

Anhedonia is indeed a pathogenetically important clinical phenotype and a promising endophenotype for depressive symptoms with a very high contribution of biological and genetic factors. Neurobiological mechanisms of anhedonia are impaired functioning of the reward system of the brain, which is confirmed by many neuroimaging, genetic and experimental studies. Anhedonia has a trans-diagnoctic character and should be understood as a complex phenomenon, and it is important to correctly evaluate it within the framework of a particular research paradigm. It seems optimal to form several complementary research strategies that evaluate the most important «facets» of anhedonia, regardless of the nosological form of the disease, within the framework of one study using various methods to search for adequate biomarkers of anhedonia severity (genetic, neuroimaging, biochemical). Given the high-quality organization of such comprehensive studies based on the correct methodology of evidence-based medicine, it is likely that significant biomarker systems will be available in the near future, which, if replicated in independent samples, can be used to personalize the diagnosis and treatment of depression.

Dopamine D2R is Required for Hippocampal-dependent Memory and Plasticity at the CA3-CA1 Synapse

Dopamine receptors play an important role in motivational, emotional, and motor responses. In addition, growing evidence suggests a key role of hippocampal dopamine receptors in learning and memory. It is well known that associative learning and synaptic plasticity of CA3-CA1 requires the dopamine D1 receptor (D1R). However, the specific role of the dopamine D2 receptor (D2R) on memory-related neuroplasticity processes is still undefined. Here, by using two models of D2R loss, D2R knockout mice (Drd2-/-) and mice with intrahippocampal injections of Drd2-small interfering RNA (Drd2-siRNA), we aimed to investigate how D2R is involved in learning and memory as well as in long-term potentiation of the hippocampus. Our studies revealed that the genetic inactivation of D2R impaired the spatial memory, associative learning, and the classical conditioning of eyelid responses. Similarly, deletion of D2R reduced the activity-dependent synaptic plasticity in the hippocampal CA1-CA3 synapse. Our results demonstrate the first direct evidence that D2R is essential in behaving mice for trace eye blink conditioning and associated changes in hippocampal synaptic strength. Taken together, these results indicate a key role of D2R in regulating hippocampal plasticity changes and, in consequence, acquisition and consolidation of spatial and associative forms of memory.

Behavioral sensitization and cellular responses to psychostimulants are reduced in D2R knockout mice

Repeated cocaine exposure causes long-lasting neuroadaptations that involve alterations in cellular signaling and gene expression mediated by dopamine in different brain regions, such as the striatum. Previous studies have pointed out to the dopamine D1 receptor as one major player in psychostimulants-induced behavioral, cellular, and molecular changes. However, the role of other dopamine receptors has not been fully characterized. Here we used dopamine D2 receptor knockout (D2^-/- ) mice to explore the role of D2 receptor (D2R) in behavioral sensitization and its associated gene expression after acute and chronic cocaine and amphetamine administration. We also studied the impact of D2R elimination in D1R-mediated responses. We found that cocaine- and amphetamine-induced behavioral sensitization is deficient in D2^-/- mice. The expression of dynorphin, primarily regulated by D1R and a marker of direct-pathway striatal neurons, is attenuated in naïve- and in cocaine- or amphetamine-treated D2^-/- mice. Moreover, c-Fos expression observed in D2^-/- mice was reduced in acutely but not in chronically treated animals. Interestingly, inactivation of D2R increased c-Fos expression in neurons of the striatopallidal pathway. Finally, elimination of D2R blunted the locomotor and striatal c-Fos response to the full D1 agonist SKF81297. In conclusion, D2R is critical for the development of behavioral sensitization and the associated gene expression, after cocaine administration, and it is required for the locomotor responses promoted by D1R activation.

A new GABAergic somatostatin projection from the BNST onto accumbal parvalbumin neurons controls anxiety

The prevailing view is that parvalbumin (PV) interneurons play modulatory roles in emotional response through local medium spiny projection neurons (MSNs). Here, we show that PV activity within the nucleus accumbens shell (sNAc) is required for producing anxiety-like avoidance when mice are under anxiogenic situations. Firing rates of sNAc^PV neurons were negatively correlated to exploration time in open arms (threatening environment). In addition, sNAc^PV neurons exhibited high excitability in a chronic stress mouse model, which generated excessive maladaptive avoidance behavior in an anxiogenic context. We also discovered a novel GABAergic pathway from the anterior dorsal bed nuclei of stria terminalis (adBNST) to sNAc^PV neurons. Optogenetic activation of these afferent terminals in sNAc produced an anxiolytic effect via GABA transmission. Next, we further demonstrated that chronic stressors attenuated the inhibitory synaptic transmission at adBNST^GABA → sNAc^PV synapses, which in turn explains the hyperexcitability of sNAc PV neurons on stressed models. Therefore, activation of these GABAergic afferents in sNAc rescued the excessive avoidance behavior related to an anxious state. Finally, we identified that the majority GABAergic input neurons, which innervate sNAc^PV cells, were expressing somatostatin (SOM), and also revealed that coordination between SOM- and PV- cells functioning in the BNST → NAc circuit has an inhibitory influence on anxiety-like responses. Our findings provide a potentially neurobiological basis for therapeutic interventions in pathological anxiety.

Dual Effect of 5-HT1B/1D Receptors on Dopamine Neurons in Ventral Tegmental Area: Implication for the Functional Switch After Chronic Cocaine Exposure

BACKGROUND

Serotonin (5-HT) 1B/1D receptor (5-HT_1B/1DR) agonists undergo an abstinence-induced switch in their effects on cocaine-related behaviors, which may involve changes in modulation of dopamine (DA) neurons in the ventral tegmental area (VTA). However, it is unclear how 5-HT_1B/1DRs affect VTA DA neuronal function and whether modulation of these neurons mediates the abstinence-induced switch after chronic cocaine exposure.

METHODS

We examined the ability of 5-HT_1B/1DRs to modulate D₂ autoreceptors (D₂ARs) and synaptic transmission in the VTA by slice recording and single unit recording in vivo in naïve mice and in mice with chronic cocaine treatment.

RESULTS

We report a bidirectional modulation of VTA DA neuronal firing through the interaction of VTA 5-HT_1B/1DRs and D₂ARs. In both VTA slices and the VTA of anesthetized mice, the 5-HT_1B/1DR agonist CP94253 decreased DA neuronal firing rate and evoked excitatory postsynaptic currents to DA neurons in slice. Paradoxically, CP94253 decreased quinpirole-induced inhibition of DA neurons by reducing D₂AR-mediated G protein-gated inwardly rectifying potassium current. This manifested decreased GABA_A (gamma-aminobutyric acid A) receptor-mediated evoked inhibitory postsynaptic currents in slice, resulting in disinhibition of DA neurons, in opposition to the 5-HT_1B/1DR-induced inhibition. This dual effect was verified in chronic cocaine-treated and mild stress-treated, male mice on days 1 and 20 posttreatment.

CONCLUSIONS

This study revealed dual effects of CP94253 on VTA DA neurons that are dependent on D₂AR sensitivity, with anti-inhibition under normal D₂AR sensitivity and inhibition under low D₂AR sensitivity. These dual effects may underlie the ability of CP94253 to both enhance and inhibit cocaine-induced behaviors.

The neurobehavioral effects of the designer drug naphyrone - an experimental investigation with pharmacokinetics and concentration/effect relationship in mice

RATIONALE

The recreational use of naphyrone, a potent synthetic cathinone with a pyrovalerone structure, has raised questions about possible deleterious neurobehavioral consequences.

OBJECTIVE

To investigate naphyrone-induced neurobehavioral effects and alterations in brain monoamines using two patterns of abuse, i.e., single and repeated (binge) use.

METHODS

We studied naphyrone dose/induced locomotor activity relationship at 3, 10, 30, and 100 mg/kg in mice. We investigated the effects of single (30 mg/kg; acute injection) versus repeated (30 mg/kg ×3/day for 3 days; binge injection) intraperitoneal naphyrone administration on locomotor activity, anxiety-like behavior, spatial recognition memory, anhedonia, behavioral despair, and social interaction. We measured post-mortem prefrontal cortex levels of monoamines and modeled naphyrone pharmacokinetics and concentration/locomotor effect relationship.

RESULTS

Both naphyrone administration patterns induced time-dependent increases in locomotor activity (p < 0.001 and p < 0.0001, respectively) and social interaction (p < 0.05 and p < 0.001, respectively) but did not alter spatial recognition memory or anhedonia. Acute naphyrone injection induced anxiety-like behavior (p < 0.01) and reduced resignation (p < 0.01) whereas binge administration induced non-anxiety-like behavior (p < 0.05) and did not alter behavioral despair. Both patterns increased the prefrontal cortex dopamine (p < 0.0001) and norepinephrine (p < 0.05 and p < 0.01, respectively) but not serotonin content. Naphyrone pharmacokinetics followed a two-compartment model with an overall elimination half-life of 0.3 h. The naphyrone concentration/locomotor effect relationship was described by an additive E_max model with an EC₅₀ of 672 μg/L.

CONCLUSIONS

Single naphyrone administration increases locomotor activity according to a direct concentration/effect relationship. The neurobehavioral effects after binge differs from those after single administration and are not explained by drug accumulation given the relatively fast elimination.

Involvement of D2 receptor in the NAc in chronic unpredictable stress-induced depression-like behaviors

D2 receptors (D2Rs) located in both pre- and postsynaptic membranes of medium spiny neurons (MSNs) in the nucleus accumbens (NAc) are involved in the stress response and associated behaviors. The role of D2Rs in chronic unpredictable stress (CUS)-induced depression-like behaviors is not clear. Quinpirole (a D2R agonist) and eticlopride (a D2R antagonist) were stereotactically delivered into the NAc before Sprague Dawley rats underwent CUS. CUS-induced depression-like behaviors were accompanied by a significant decrease in both the dopamine (DA) level and D2R expression in the NAc. Eticlopride reversed CUS-induced depression-like behavior and rescued the DA levels in the NAc, and microinjection of DA into the NAc of CUS individuals had the same effect as eticlopride. By contrast, delivery of quinpirole into the NAc of control animals induced depression-like behaviors accompanied by a decrease in the DA level in the NAc. These results show that DA plays a key role in CUS-induced depression-like behaviors and the D2R exerts a presynaptic negative feedback on DA levels during CUS. Microinjection of quinpirole into the NAc also decreased the level of the kalirin-7 protein in the NAc of both control and stressed animals, while eticlopride increased its level in the NAc of rats. In agreement with these results, intraperitoneal injection of eticlopride in mice also caused an increase in both the kalirin-7 protein level in the NAc and spine density in MSNs, while quinpirole reduced them. These results suggest that regulation of kalirin-7 through D2R in the NAc is a general pathway in rats and mice, and is involved in CUS-induced depression-like behaviors. Kalirin-7 may be directly regulated through the D2R postsynaptic pathway or indirectly through the presynaptic pathway in the NAc. The interaction between D2R and kalirin-7 needs to be investigated further.

Genetics of resilience: Implications from genome-wide association studies and candidate genes of the stress response system in posttraumatic stress disorder and depression

Resilience is the ability to cope with critical situations through the use of personal and socially mediated resources. Since a lack of resilience increases the risk of developing stress-related psychiatric disorders such as posttraumatic stress disorder (PTSD) and major depressive disorder (MDD), a better understanding of the biological background is of great value to provide better prevention and treatment options. Resilience is undeniably influenced by genetic factors, but very little is known about the exact underlying mechanisms. A recently published genome-wide association study (GWAS) on resilience has identified three new susceptibility loci, DCLK2, KLHL36, and SLC15A5. Further interesting results can be found in association analyses of gene variants of the stress response system, which is closely related to resilience, and PTSD and MDD. Several promising genes, such as the COMT (catechol-O-methyltransferase) gene, the serotonin transporter gene (SLC6A4), and neuropeptide Y (NPY) suggest gene × environment interaction between genetic variants, childhood adversity, and the occurrence of PTSD and MDD, indicating an impact of these genes on resilience. GWAS on PTSD and MDD provide another approach to identifying new disease-associated loci and, although the functional significance for disease development for most of these risk genes is still unknown, they are potential candidates due to the overlap of stress-related psychiatric disorders and resilience. In the future, it will be important for genetic studies to focus more on resilience than on pathological phenotypes, to develop reasonable concepts for measuring resilience, and to establish international cooperations to generate sufficiently large samples.

Dopamine D1 and D2 Receptors Differentially Regulate Rac1 and Cdc42 Signaling in the Nucleus Accumbens to Modulate Behavioral and Structural Plasticity After Repeated Methamphetamine Treatment

BACKGROUND

Methamphetamine (METH) is a highly addictive psychostimulant that strongly activates dopamine receptor signaling in the nucleus accumbens (NAc). However, how dopamine D₁ and D₂ receptors (D₁Rs and D₂Rs, respectively) as well as downstream signaling pathways, such as those involving Rac1 and Cdc42, modulate METH-induced behavioral and structural plasticity is largely unknown.

METHODS

Using NAc conditional D₁R and D₂R deletion mice, Rac1 and Cdc42 mutant viruses, and a series of behavioral and morphological methods, we assessed the effects of D₁Rs and D₂Rs on Rac1 and Cdc42 in modulating METH-induced behavioral and structural plasticity in the NAc.

RESULTS

D₁Rs and D₂Rs in the NAc consistently regulated METH-induced conditioned place preference, locomotor activation, and dendritic and spine remodeling of medium spiny neurons but differentially regulated METH withdrawal-induced spatial learning and memory impairment and anxiety. Interestingly, Rac1 and Cdc42 signaling were oppositely modulated by METH, and suppression of Rac1 signaling and activation of Cdc42 signaling were crucial to METH-induced conditioned place preference and structural plasticity but not to locomotor activation. D₁Rs activated Rac1 and Cdc42 signaling, while D₂Rs inhibited Rac1 signaling but activated Cdc42 signaling to mediate METH-induced conditioned place preference and structural plasticity but not locomotor activation. In addition, NAc D₁R deletion aggravated METH withdrawal-induced spatial learning and memory impairment by suppressing Rac1 signaling but not Cdc42 signaling, while NAc D₂R deletion aggravated METH withdrawal-induced anxiety without affecting Rac1 or Cdc42 signaling.

CONCLUSIONS

D₁Rs and D₂Rs differentially regulate Rac1 and Cdc42 signaling to modulate METH-induced behavioral plasticity and the structural remodeling of medium spiny neurons in the NAc.

Dopamine D2L Receptor Deficiency Causes Stress Vulnerability through 5-HT1A Receptor Dysfunction in Serotonergic Neurons

Mental disorders are caused by genetic and environmental factors. We here show that deficiency of an isoform of dopamine D₂ receptor (D₂R), D_2LR, causes stress vulnerability in mouse. This occurs through dysfunction of serotonin [5-hydroxytryptamine (5-HT)] 1A receptor (5-HT_1AR) on serotonergic neurons in the mouse brain. Exposure to forced swim stress significantly increased anxiety- and depressive-like behaviors in D_2LR knock-out (KO) male mice compared with wild-type mice. Treatment with 8-OH-DPAT, a 5-HT_1AR agonist, failed to alleviate the stress-induced behaviors in D_2LR-KO mice. In forced swim-stressed D_2LR-KO mice, 5-HT efflux in the medial prefrontal cortex was elevated and the expression of genes related to 5-HT levels was upregulated by the transcription factor PET1 in the dorsal raphe nucleus. Notably, D_2LR formed a heteromer with 5-HT_1AR in serotonergic neurons, thereby suppressing 5-HT_1AR-activated G-protein-activated inwardly rectifying potassium conductance in D_2LR-KO serotonergic neurons. Finally, D_2LR overexpression in serotonergic neurons in the dorsal raphe nucleus alleviated stress vulnerability observed in D_2LR-KO mice. Together, we conclude that disruption of the negative feedback regulation by the D_2LR/5-HT_1A heteromer causes stress vulnerability.SIGNIFICANCE STATEMENT Etiologies of mental disorders are multifactorial, e.g., interactions between genetic and environmental factors. In this study, using a mouse model, we showed that genetic depletion of an isoform of dopamine D₂ receptor, D_2LR, causes stress vulnerability associated with dysfunction of serotonin 1A receptor, 5-HT_1AR in serotonergic neurons. The D_2LR/5-HT_1AR inhibitory G-protein-coupled heteromer may function as a negative feedback regulator to suppress psychosocial stress.

In vivo imaging of D2 receptors and corticosteroids predict behavioural responses to captivity stress in a wild bird

Individual physiological variation may underlie individual differences in behaviour in response to stressors. This study tested the hypothesis that individual variation in dopamine and corticosteroid physiology in wild house sparrows (Passer domesticus, n = 15) would significantly predict behaviour and weight loss in response to a long-term stressor, captivity. We found that individuals that coped better with captivity (fewer anxiety-related behaviours, more time spent feeding, higher body mass) had lower baseline and higher stress-induced corticosteroid titres at capture. Birds with higher striatal D₂ receptor binding (examined using positron emission tomography (PET) with ¹¹C-raclopride 24 h post-capture) spent more time feeding in captivity, but weighed less, than birds with lower D₂ receptor binding. In the subset of individuals imaged a second time, D₂ receptor binding decreased in captivity in moulting birds, and larger D₂ decreases were associated with increased anxiety behaviours 2 and 4 weeks post-capture. This suggests changes in dopaminergic systems could be one physiological mechanism underlying negative behavioural effects of chronic stress. Non-invasive technologies like PET have the potential to transform our understanding of links between individual variation in physiology and behaviour and elucidate which neuroendocrine phenotypes predict stress resilience, a question with important implications for both humans and wildlife.

Stress changes amphetamine response, D2 receptor expression and epigenetic regulation in low-anxiety rats

The aim of this study was to assess the influence of chronic restraint stress on amphetamine (AMPH)-related appetitive 50-kHz ultrasonic vocalisations (USVs) in rats differing in freezing duration in a contextual fear test (CFT), i.e. HR (high-anxiety responsive) and LR (low-anxiety responsive) rats. The LR and the HR rats, previously exposed to an AMPH binge experience, differed in sensitivity to AMPH's rewarding effects, measured as appetitive vocalisations. Moreover, chronic restraint stress attenuated AMPH-related appetitive vocalisations in the LR rats but had no influence on the HR rats' behaviour. To specify, the restraint LR rats vocalised appetitively less in the AMPH-associated context and after an AMPH challenge than the control LR rats. This phenomenon was associated with a decrease in the mRNA level for D2 dopamine receptor in the amygdala and its protein expression in the basal amygdala (BA) and opposite changes in the nucleus accumbens (NAc) - an increase in the mRNA level for D2 dopamine receptor and its protein expression in the NAc shell, compared to control conditions. Moreover, we observed that chronic restraint stress influenced epigenetic regulation in the LR and the HR rats differently. The contrasting changes were observed in the dentate gyrus (DG) of the hippocampus - the LR rats presented a decrease, but the HR rats showed an increase in H3K9 trimethylation. The restraint LR rats also showed higher miR-494 and miR-34c levels in the NAc than the control LR group. Our study provides behavioural and biochemical data concerning the role of differences in fear-conditioned response in stress vulnerability and AMPH-associated appetitive behaviour. The LR rats were less sensitive to the rewarding effects of AMPH when previously exposed to chronic stress that was accompanied by changes in D2 dopamine receptor expression and epigenetic regulation in mesolimbic areas.

Early-Life Stress in D2 Heterozygous Mice Promotes Autistic-like Behaviors through the Downregulation of the BDNF-TrkB Pathway in the Dorsal Striatum

A number of specific genetic variants including gene mutations and single nucleotide variations have been identified in genomewide association studies of autism spectrum disorder (ASD). ASD phenotypes in individuals carrying specific genetic variations are manifest mostly in a heterozygous state. Furthermore, individuals with most genetic variants show incomplete penetrance and phenotypic variability, suggesting that non-genetic factors are also involved in developing ASD. However, the mechanisms of how genetic and environmental factors interactively promote ASD are not clearly understood. In the present study, we investigated whether early-life stress (ELS) in D2 dopamine receptor heterozygous knockout (D2^+/−) mice induces ASD-like symptoms. To address that, we exposed D2 heterozygous pups to maternal separation stress for 3 h daily for 13 days beginning on postnatal day 2. D2^+/− adult mice that had experienced ELS exhibited impaired sociability in the three-chamber test and home-cage social interaction test and increased grooming behavior, whereas wildtype littermates exposed to ELS did not show those phenotypes. ELS-exposed D2^+/− mice had decreased levels of BDNF, TrkB, phospho-ERK1/2 and phospho-CREB in the dorsal striatum. Administration of the TrkB agonist 7,8-dihydroxyflavone (7,8-DHF) to ELS-exposed D2^+/− mice rescued the sociability deficits and repetitive behavior. In contrast, behavioral rescue by 7,8-DHF in ELS-exposed D2^+/− mice was blocked when TrkB expression in the dorsal striatum was locally inhibited by the injection of TrkB-siRNA. Together, our results suggest that the interaction between ELS and defective D2 gene function promotes autistic-like behaviors by downregulating the BDNF-TrkB pathway in the dorsal striatum.

Acute and chronic neurobehavioral effects of the designer drug and bath salt constituent 3,4-methylenedioxypyrovalerone in the rat

BACKGROUND

The substantial increase in use of 3,4-methylenedioxypyrovalerone (MDPV), a popular recreational synthetic cathinone, has raised legitimate questions about its behavioral consequences and abuse liability.

AIMS

The aim of this study was to study MDPV-induced neurobehavioral effects in the rat, using different paradigms traditionally developed to study drug-attributed addictive properties.

METHODS

Different patterns of intraperitoneal 3 mg/kg MDPV administration were investigated. Consequences on rat horizontal locomotion and behavior of acute, intermittent (once daily dosing over 10 days), and binge (three-time daily dosing for 3 days) MDPV administration as well as challenge after 10 day MDPV withdrawal were studied. The dopamine receptor-D1 antagonist, SCH23390, was bilaterally infused in the nucleus accumbens to determine the role of D1-receptors in MDPV-related effects on the associative memory recall using the conditioned place preference paradigm. In addition, in a separate experience using western blot, we investigated the effects of chronic MDPV administration (four injections during 24 h) on ΔFosB expression in the nucleus accumbens, caudate putamen, and prefrontal cortex.

RESULTS

Acute MDPV administration increased stereotypies and open arm entries in the elevated plus maze while SCH23390 abolished MDPV-induced enhancing effects on memory consolidation. Intermittent MDPV administration resulted in sensitization of MDPV-induced locomotor effects and tolerance during the following challenge. With binge MDPV administration, locomotor activity was not altered despite tolerance onset after challenge. SCH23390 abolished MDPV-induced conditioned place preference. Chronic MDPV administration induced ΔFosB accumulation in the nucleus accumbens, caudate putamen, and prefrontal cortex.

CONCLUSIONS

Our findings clearly show that MDPV produces profound behavioral alterations mediated by the activation of the dopaminergic system similarly to other amphetamines.

Sex-Specific Gene Expression in the Mouse Nucleus Accumbens Before and After Cocaine Exposure

The nucleus accumbens plays a major role in the response of mammals to cocaine. In animal models and human studies, the addictive effects of cocaine and relapse probability have been shown to be greater in females. Sex-specific differential expression of key transcripts at baseline and after prolonged withdrawal could underlie these differences. To distinguish between these possibilities, gene expression was analyzed in four groups of mice (cycling females, ovariectomized females treated with estradiol or placebo, and males) 28 days after they had received seven daily injections of saline or cocaine. As expected, sensitization to the locomotor effects of cocaine was most pronounced in the ovariectomized mice receiving estradiol, was greater in cycling females than in males, and failed to occur in ovariectomized/placebo mice. After the 28-day withdrawal period, RNA prepared from the nucleus accumbens of the individual cocaine- or saline-injected mice was subjected to RNA sequencing analysis. Baseline expression of 3% of the nucleus accumbens transcripts differed in the cycling female mice compared with the male mice. Expression of a similar number of transcripts was altered by ovariectomy or was responsive to estradiol treatment. Nucleus accumbens transcripts differentially expressed in cycling female mice withdrawn from cocaine exhibited substantial overlap with those differentially expressed in cocaine-withdrawn male mice. A small set of transcripts were similarly affected by cocaine in the placebo- or estradiol-treated ovariectomized mice. Sex and hormonal status have profound effects on RNA expression in the nucleus accumbens of naive mice. Prolonged withdrawal from cocaine alters the expression of a much smaller number of common and sex hormone-specific transcripts.

Chronic stress increases susceptibility to food addiction by increasing the levels of DR2 and MOR in the nucleus accumbens

Background: Stress-related obesity might be related to the suppression of the hypothalamic-pituitary- adrenocortical axis and dysregulation of the metabolic system. Chronic stress also induces the dysregulation of the reward system and increases the risk of food addiction, according to recent clinical findings. However, few studies have tested the effect of chronic stress on food addiction in animal models. Purpose: The objective of this study was to identify whether chronic stress promotes food addiction or not and explore the possible mechanisms. Method: We applied adaily 2 hrsflashing LED irradiation stress to mice fed chow or palatable food to mimic the effect of chronic stress on feeding. After 1 month of chronic stress exposure, we tested their binge eating behaviors, cravings for palatable food, responses for palatable food, and compulsive eating behaviors to evaluate the effect of chronic stress on food addiction-like behaviors. We detected changes in the levels of various genes and proteins in the nucleus accumbens (NAc), ventral tegmental area (VTA) and lateral hypothalamus using qPCR and immunofluorescence staining, respectively. Results: Behaviors results indicated chronic stress obviously increased food addiction score (FAS) in the palatable food feeding mice. Moreover, the FAS had astrong relationship with the extent of the increase in body weight. Chronic stress increased the expression of corticotropin-releasing factor receptor 1(CRFR1) was increased in the NAc shell and core but decreased in the VTA of the mice fed with palatable food. Chronic stress also increased expression of both dopamine receptor 2 (DR2) and mu-opioid receptor (MOR) in the NAc. Conclusion: Chronic stress aggravates the FAS and contributed to the development of stress-related obesity. Chronic stress drives the dysregulation of the CRF signaling pathway in the reward system and increases the expression of DR2 and MOR in the nucleus accumbens.

Dopamine D1 receptor activation improves adult hippocampal neurogenesis and exerts anxiolytic and antidepressant-like effect via activation of Wnt/β-catenin pathways in rat model of Parkinson's disease

Parkinson's disease (PD) is primarily characterized by midbrain dopamine depletion. Dopamine acts through dopamine receptors (D1 to D5) to regulate locomotion, motivation, pleasure, attention, cognitive functions and formation of newborn neurons, all of which are likely to be impaired in PD. Reduced hippocampal neurogenesis associated with dopamine depletion has been demonstrated in patients with PD. However, the precise mechanism to regulate multiple steps of adult hippocampal neurogenesis by dopamine receptor(s) is still unknown. In this study, we tested whether pharmacological agonism and antagonism of dopamine D1 and D2 receptor regulate nonmotor symptoms, neural stem cell (NSC) proliferation and fate specification and explored the cellular mechanism(s) underlying dopamine receptor (D1 and D2) mediated adult hippocampal neurogenesis in rat model of PD-like phenotypes. We found that single unilateral intra-medial forebrain bundle administration of 6-hydroxydopamine (6-OHDA) reduced D1 receptor level in the hippocampus. Pharmacological agonism of D1 receptor exerts anxiolytic and antidepressant-like effects as well as enhanced NSC proliferation, long-term survival and neuronal differentiation by positively regulating Wnt/β-catenin signaling pathway in hippocampus in PD rats. shRNA lentivirus mediated knockdown of Axin-2, a negative regulator of Wnt/β-catenin signaling potentially attenuated D1 receptor antagonist induced anxiety and depression-like phenotypes and impairment in adult hippocampal neurogenesis in PD rats. Our results suggest that improved nonmotor symptoms and hippocampal neurogenesis in PD rats controlled by D1-like receptors that involve the activation of Wnt/β-catenin signaling.

Dopamine D2 receptor-mediated circuit from the central amygdala to the bed nucleus of the stria terminalis regulates impulsive behavior

Impulsivity is closely associated with addictive disorders, and changes in the brain dopamine system have been proposed to affect impulse control in reward-related behaviors. However, the central neural pathways through which the dopamine system controls impulsive behavior are still unclear. We found that the absence of the D2 dopamine receptor (D2R) increased impulsive behavior in mice, whereas restoration of D2R expression specifically in the central amygdala (CeA) of D2R knockout mice (Drd2 ^-/- ) normalized their enhanced impulsivity. Inhibitory synaptic output from D2R-expressing neurons in the CeA underlies modulation of impulsive behavior because optogenetic activation of D2R-positive inhibitory neurons that project from the CeA to the bed nucleus of the stria terminalis (BNST) attenuate such behavior. Our identification of the key contribution of D2R-expressing neurons in the CeA → BNST circuit to the control of impulsive behavior reveals a pathway that could serve as a target for approaches to the management of neuropsychiatric disorders associated with impulsivity.

A competitive model for striatal action selection

The direct and indirect pathway striatal medium spiny neurons (dMSNs and iMSNs) have long been linked to action selection, but the precise roles of these neurons in this process remain unclear. Here, we review different models of striatal pathway function, focusing on the classic "go/no-go" model which posits that dMSNs facilitate movement while iMSNs inhibit movement, and the "complementary" model, which argues that dMSNs facilitate the selection of specific actions while iMSNs inhibit potentially conflicting actions. We discuss the merits and shortcomings of these models and propose a "competitive" model to explain the contribution of these two pathways to behavior. The "competitive" model argues that rather than inhibiting conflicting actions, iMSNs are tuned to the same actions that dMSNs facilitate, and the two populations "compete" to determine the animal's behavioral response. This model provides a theoretical explanation for how these pathways work together to select actions. In addition, it provides a link between action selection and behavioral reinforcement, via modulating synaptic strength at inputs onto dMSNs and iMSNs. Finally, this model makes predictions about how imbalances in the activity of these pathways may underlie behavioral traits associated with psychiatric disorders. Understanding the roles of these striatal pathways in action selection may help to clarify the neuronal mechanisms of decision-making under normal and pathological conditions.

A review of nonhuman primate models of early life stress and adolescent drug abuse

Adolescence represents a developmental stage in which initiation of drug use typically occurs and is marked by dynamic neurobiological changes. These changes present a sensitive window during which perturbations to normative development lead to alterations in brain circuits critical for stress and emotional regulation as well as reward processing, potentially resulting in an increased susceptibility to psychopathologies. The occurrence of early life stress (ELS) is related to a greater risk for the development of substance use disorders (SUD) during adolescence. Studies using nonhuman primates (NHP) are ideally suited to examine how ELS may alter the development of neurobiological systems modulating the reinforcing effects of drugs, given their remarkable neurobiological, behavioral, and developmental homologies to humans. This review examines NHP models of ELS that have been used to characterize its effects on sensitivity to drug reinforcement, and proposes future directions using NHP models of ELS and drug abuse in an effort to develop more targeted intervention and prevention strategies for at risk clinical populations.

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ELS has long-lasting neurobiological and behavioral consequences.

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ELS is a major risk factor for the initiation of adolescent drug use.

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Sex differences are apparent in the consequences of ELS, including drug use.

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Nonhuman primate models of ELS are critical for understanding ELS effects on neurobiology and risk for adolescent drug use.

Role of Dopamine D2 Receptor in Stress-Induced Myelin Loss

Dopaminergic systems play a major role in reward-related behavior and dysregulation of dopamine (DA) systems can cause several mental disorders, including depression. We previously reported that dopamine D2 receptor knockout (D2R^-/-) mice display increased anxiety and depression-like behaviors upon chronic stress. Here, we observed that chronic stress caused myelin loss in wild-type (WT) mice, while the myelin level in D2R^-/- mice, which was already lower than that in WT mice, was not affected upon stress. Fewer mature oligodendrocytes (OLs) were observed in the corpus callosum of stressed WT mice, while in D2R^-/- mice, both the control and stressed group displayed a decrease in the number of mature OLs. We observed a decrease in the number of active β-catenin (ABC)-expressing and TCF4-expressing cells among OL lineage cells in the corpus callosum of stressed WT mice, while such regulation was not found in D2R^-/- mice. Administration of lithium normalized the behavioral impairments and myelin damage induced by chronic stress in WT mice, and restored the number of ABC-positive and TCF4-positive OLs, while such effect was not found in D2R^-/- mice. Together, our findings indicate that chronic stress induces myelin loss through the Wnt/β-catenin signaling pathway in association with DA signaling through D2R.

Restructuring of basal ganglia circuitry and associated behaviors triggered by low striatal D2 receptor expression: implications for substance use disorders

Dopamine D2 receptors (D2Rs) consistently emerge as a critical substrate for the etiology of some major psychiatric disorders. Indeed, a central theory of substance use disorders (SUDs) postulates that a reduction in D2R levels in the striatum is a determining factor that confers vulnerability to abuse substances. A large number of clinical and preclinical studies strongly support this link between SUDs and D2Rs; however, identifying the mechanism by which low D2Rs facilitate SUDs has been hindered by the complexity of circuit connectivity, the heterogeneity of D2R expression and the multifaceted constellation of phenotypes observed in SUD patient. Animal models are well‐suited for understanding the mechanisms because they allow access to the circuitry and the genetic tools that enable a dissection of the D2R heterogeneity. This review discusses recent findings on the functional role of D2Rs and highlights the distinctive contributions of D2Rs expressed on specific neuronal subpopulations to the behavioral responses to stimulant drugs. A circuit‐wide restructuring of local and long‐range inhibitory connectivity within the basal ganglia is observed in response to manipulation of striatal D2R levels and is accompanied by multiple alterations in dopamine‐dependent behaviors. Collectively, these new findings provide compelling evidence for a critical role of striatal D2Rs in shaping basal ganglia connectivity; even among neurons that do not express D2Rs. These findings from animal models have deep clinical implications for SUD patients with low levels D2R availability where a similar restructuring of basal ganglia circuitry is expected to take place.

Selective Vulnerability of Striatal D2 versus D1 Dopamine Receptor-Expressing Medium Spiny Neurons in HIV-1 Tat Transgenic Male Mice

Despite marked regional differences in HIV susceptibility within the CNS, there has been surprisingly little exploration into the differential vulnerability among neuron types and the circuits they underlie. The dorsal striatum is especially susceptible, harboring high viral loads and displaying marked neuropathology, with motor impairment a frequent manifestation of chronic infection. However, little is known about the response of individual striatal neuron types to HIV or how this disrupts function. Therefore, we investigated the morphological and electrophysiological effects of HIV-1 trans-activator of transcription (Tat) in dopamine subtype 1 (D1) and dopamine subtype 2 (D2) receptor-expressing striatal medium spiny neurons (MSNs) by breeding transgenic Tat-expressing mice to Drd1a-tdTomato- or Drd2-eGFP-reporter mice. An additional goal was to examine neuronal vulnerability early during the degenerative process to gain insight into key events underlying the neuropathogenesis. In D2 MSNs, exposure to HIV-1 Tat reduced dendritic spine density significantly, increased dendritic damage (characterized by swellings/varicosities), and dysregulated neuronal excitability (decreased firing at 200-300 pA and increased firing rates at 450 pA), whereas insignificant morphologic and electrophysiological consequences were observed in Tat-exposed D1 MSNs. These changes were concomitant with an increased anxiety-like behavioral profile (lower latencies to enter a dark chamber in a light-dark transition task, a greater frequency of light-dark transitions, and reduced rearing time in an open field), whereas locomotor behavior was unaffected by 2 weeks of Tat induction. Our findings suggest that D2 MSNs and a specific subset of neural circuits within the dorsal striatum are preferentially vulnerable to HIV-1.SIGNIFICANCE STATEMENT Despite combination antiretroviral therapy (cART), neurocognitive disorders afflict 30-50% of HIV-infected individuals and synaptodendritic injury remains evident in specific brain regions such as the dorsal striatum. A possible explanation for the sustained neuronal injury is that the neurotoxic HIV-1 regulatory protein trans-activator of transcription (Tat) continues to be expressed in virally suppressed patients on cART. Using inducible Tat-expressing transgenic mice, we found that dopamine subtype 2 (D2) receptor-expressing medium spiny neurons (MSNs) are selectively vulnerable to Tat exposure compared with D1 receptor-expressing MSNs. This includes Tat-induced reductions in D2 MSN dendritic spine density, increased dendritic damage, and disruptions in neuronal excitability, which coincide with elevated anxiety-like behavior. These data suggest that D2 MSNs and specific circuits within the basal ganglia are preferentially vulnerable to HIV-1.

The GABAergic Hypothesis for Cognitive Disabilities in Down Syndrome

Down syndrome (DS) is a genetic disorder caused by the presence of a third copy of chromosome 21. DS affects multiple organs, but it invariably results in altered brain development and diverse degrees of intellectual disability. A large body of evidence has shown that synaptic deficits and memory impairment are largely determined by altered GABAergic signaling in trisomic mouse models of DS. These alterations arise during brain development while extending into adulthood, and include genesis of GABAergic neurons, variation of the inhibitory drive and modifications in the control of neural-network excitability. Accordingly, different pharmacological interventions targeting GABAergic signaling have proven promising preclinical approaches to rescue cognitive impairment in DS mouse models. In this review, we will discuss recent data regarding the complex scenario of GABAergic dysfunctions in the trisomic brain of DS mice and patients, and we will evaluate the state of current clinical research targeting GABAergic signaling in individuals with DS.

Presynaptic G Protein-Coupled Receptors: Gatekeepers of Addiction?

Drug abuse and addiction cause widespread social and public health problems, and the neurobiology underlying drug actions and drug use and abuse is an area of intensive research. Drugs of abuse alter synaptic transmission, and these actions contribute to acute intoxication as well as the chronic effects of abused substances. Transmission at most mammalian synapses involves neurotransmitter activation of two receptor subtypes, ligand-gated ion channels that mediate fast synaptic responses and G protein-coupled receptors (GPCRs) that have slower neuromodulatory actions. The GPCRs represent a large proportion of neurotransmitter receptors involved in almost all facets of nervous system function. In addition, these receptors are targets for many pharmacotherapeutic agents. Drugs of abuse directly or indirectly affect neuromodulation mediated by GPCRs, with important consequences for intoxication, drug taking and responses to prolonged drug exposure, withdrawal and addiction. Among the GPCRs are several subtypes involved in presynaptic inhibition, most of which are coupled to the Gi/o class of G protein. There is increasing evidence that these presynaptic Gi/o-coupled GPCRs have important roles in the actions of drugs of abuse, as well as behaviors related to these drugs. This topic will be reviewed, with particular emphasis on receptors for three neurotransmitters, Dopamine (DA; D1- and D2-like receptors), Endocannabinoids (eCBs; CB1 receptors) and glutamate (group II metabotropic glutamate (mGlu) receptors). The focus is on recent evidence from laboratory animal models (and some evidence in humans) implicating these receptors in the acute and chronic effects of numerous abused drugs, as well as in the control of drug seeking and taking. The ability of drugs targeting these receptors to modify drug seeking behavior has raised the possibility of using compounds targeting these receptors for addiction pharmacotherapy. This topic is also discussed, with emphasis on development of mGlu2 positive allosteric modulators (PAMs).

Melanocortin 4 Receptor and Dopamine D2 Receptor Expression in Brain Areas Involved in Food Intake

BACKGROUND

The melanocortin 4 receptor (MC4R) is involved in the regulation of homeostatic energy balance by the hypothalamus. Recent reports showed that MC4R can also control the motivation for food in association with a brain reward system, such as dopamine. We investigated the expression levels of MC4R and the dopamine D2 receptor (D2R), which is known to be related to food rewards, in both the hypothalamus and brain regions involved in food rewards.

METHODS

We examined the expression levels of D2R and MC4R by dual immunofluorescence histochemistry in hypothalamic regions and in the bed nucleus of the stria terminalis (BNST), the central amygdala, and the ventral tegmental area of transgenic mice expressing enhanced green fluorescent protein under the control of the D2R gene.

RESULTS

In the hypothalamic area, significant coexpression of MC4R and D2R was observed in the arcuate nucleus. We observed a significant coexpression of D2R and MC4R in the BNST, which has been suggested to be an important site for food reward.

CONCLUSION

We suggest that MC4R and D2R function in the hypothalamus for control of energy homeostasis and that within the brain regions related with rewards, such as the BNST, the melanocortin system works synergistically with dopamine for the integration of food motivation in the control of feeding behaviors.