Prolonged dopamine D3 receptor stimulation promotes dopamine transporter ubiquitination and degradation through a PKC-dependent mechanism

Safety and tolerance of combination of monoamine oxidase inhibitors and direct dopamine agonists in adults and older adults with highly resistant depression

INTRODUCTION

Dopamine (DA) is likely to be involved in some depressive dimensions, such as anhedonia and amotivation, which account for a part of treatment-resistant forms. Monoamine oxidase inhibitors (MAOI) and direct D2 and D3 receptors agonists (D2/3r-dAG) are known to help, but we lack safety data about their combined usage. We report on safety and tolerance of the MAOI+D2r-dAG combination in a clinical series.

METHOD

All patients referred to our recourse center for depression between 2013 and 2021 were screened to select those who did receive the combo. Data were extracted from clinical files.

RESULTS

Sixteen patients of 60±17 years of age (8 women, 7 with age>65years, all suffered from treatment resistant depression, 7 with bipolar disorder) received the combo. There were no life-threatening adverse effects (AE). However, AE were reported by 14 patients (88%) most of which were mild and consisted of insomnia, nausea, nervousness, confusion, impulse control disorder and/or "sleep attacks". One patient presented a serious AE requiring a short hospitalization for confusion. Intolerance led to failure to introduce treatment in two patients (13%). The retrospective non-interventional design, the variety of molecules, and the modest sample size limited the scope of these results.

CONCLUSION

There was no life-threatening safety issue in combining MAOI and D2/3r-dAG, especially regarding cardiovascular side effects. The systematic screening of AE might account for their frequency, but these precluded the treatment in only two patients. Comparative studies are needed to assess the efficacy of this new combination.

Risperidone Decreases Expression of Serotonin Receptor-2A (5-HT2A) and Serotonin Transporter (SERT) but Not Dopamine Receptors and Dopamine Transporter (DAT) in PBMCs from Patients with Schizophrenia

Dopamine and serotonin receptors and transporters play an essential role in the pathophysiology of schizophrenia; changes in their expression have been reported in neurons and leukocytes. Each antipsychotic induces a unique pattern in leukocyte function and phenotype. However, the use of polytherapy to treat schizophrenia makes it challenging to determine the specific effects of risperidone on peripheral blood mononuclear cells (PBMCs). The aim of this study was to evaluate the changes in the expression of D3, D5, DAT, 5-HT2A, and SERT in PBMCs from healthy volunteers (HV), drug-naive patients with schizophrenia (PWS), drug-free PWS, and PWS treated with risperidone for up to 40 weeks using quantitative PCR. Our study revealed elevated mRNA levels of D3, DAT, 5-HT2A, and SERT in unmedicated PWS. Treatment with risperidone led to a reduction only in the expression of 5-HT2A and SERT. Furthermore, we observed a moderate correlation between 5-HT2A expression and the positive and negative syndrome scale (PANSS), as well as SERT expression and PANSS scale. We also found a moderate correlation between 5-HT2A and SERT expression and the positive subscale. The duration of risperidone consumption had a significant negative correlation with the expression of 5-HT2A and SERT. Our study introduces the measurement of 5-HT2A and SERT expression in PBMCs as a useful parameter for assessing the response to risperidone in PWS.

The influence of dopamine autoreceptors on temperament and addiction risk

As a major regulator of dopamine (DA), DA autoreceptors (DAARs) exert substantial influence over DA-mediated behaviors. This paper reviews the physiological and behavioral impact of DAARs. Individual differences in DAAR functioning influences temperamental traits such as novelty responsivity and impulsivity, both of which are associated with vulnerability to addictive behavior in animal models and a broad array of externalizing behaviors in humans. DAARs additionally impact the response to psychostimulants and other drugs of abuse. Human PET studies of D2-like receptors in the midbrain provide evidence for parallels to the animal literature. These data lead to the proposal that weak DAAR regulation is a risk factor for addiction and externalizing problems. The review highlights the potential to build translational models of the functional role of DAARs in behavior. It also draws attention to key limitations in the current literature that would need to be addressed to further advance a weak DAAR regulation model of addiction and externalizing risk.

miR-218 Promotes Dopaminergic Differentiation and Controls Neuron Excitability and Neurotransmitter Release through the Regulation of a Synaptic-Related Genes Network

In the brain, microRNAs (miRNAs) are believed to play a role in orchestrating synaptic plasticity at a higher level by acting as an additional mechanism of translational regulation, alongside the mRNA/polysome system. Despite extensive research, our understanding of the specific contribution of individual miRNA to the function of dopaminergic neurons (DAn) remains limited. By performing a dopaminergic-specific miRNA screening, we have identified miR-218 as a critical regulator of DAn activity in male and female mice. We have found that miR-218 is specifically expressed in mesencephalic DAn and is able to promote dopaminergic differentiation of embryonic stem cells and functional maturation of transdifferentiated induced DA neurons. Midbrain-specific deletion of both genes encoding for miR-218 (referred to as miR-218-1 and mir218-2) affects the expression of a cluster of synaptic-related mRNAs and alters the intrinsic excitability of DAn, as it increases instantaneous frequencies of evoked action potentials, reduces rheobase current, affects the ionic current underlying the action potential after hyperpolarization phase, and reduces dopamine efflux in response to a single electrical stimulus. Our findings provide a comprehensive understanding of the involvement of miR-218 in the dopaminergic system and highlight its role as a modulator of dopaminergic transmission.SIGNIFICANCE STATEMENT In the past decade, several miRNAs have emerged as potential regulators of synapse activity through the modulation of specific gene expression. Among these, we have identified a dopaminergic-specific miRNA, miR-218, which is able to promote dopaminergic differentiation and regulates the translation of an entire cluster of synapse related mRNAs. Deletion of miR-218 has notable effects on dopamine release and alters the intrinsic excitability of dopaminergic neurons, indicating a direct control of dopaminergic activity by miR-218.

Exploring the causes of augmentation in restless legs syndrome

Long-term drug treatment for Restless Legs Syndrome (RLS) patients can frequently result in augmentation, which is the deterioration of symptoms with an increased drug dose. The cause of augmentation, especially derived from dopamine therapy, remains elusive. Here, we review recent research and clinical progress on the possible mechanism underlying RLS augmentation. Dysfunction of the dopamine system highly possibly plays a role in the development of RLS augmentation, as dopamine agonists improve desensitization of dopamine receptors, disturb receptor interactions within or outside the dopamine receptor family, and interfere with the natural regulation of dopamine synthesis and release in the neural system. Iron deficiency is also indicated to contribute to RLS augmentation, as low iron levels can affect the function of the dopamine system. Furthermore, genetic risk factors, such as variations in the BTBD9 and MEIS1 genes, have been linked to an increased risk of RLS initiation and augmentation. Additionally, circadian rhythm, which controls the sleep-wake cycle, may also contribute to the worsening of RLS symptoms and the development of augmentation. Recently, Vitamin D deficiency has been suggested to be involved in RLS augmentation. Based on these findings, we propose that the progressive reduction of selective receptors, influenced by various pathological factors, reverses the overcompensation of the dopamine intensity promoted by short-term, low-dose dopaminergic therapy in the development of augmentation. More research is needed to uncover a deeper understanding of the mechanisms underlying the RLS symptom and to develop effective RLS augmentation treatments.

Effects of different doses of dopamine receptor agonist pramipexole on neurobehaviors and changes of mitochondrial membrane potentials in rats with global cerebral ischemia-reperfusion injury

OBJECTIVE

To explore the effects of different doses of dopamine receptor agonist pramipexole on neurobehaviors and changes of mitochondrial membrane potential in rats with global cerebral ischemia-reperfusion injury.

METHODS

A total of 75 SPF Sprague-Dawley male rats were randomly divided into sham group (n=20), model group (n=20), pramipexole administration group (n=35). The rat model of global cerebral ischemia-reperfusion injury was prepared by the modified Pulsinelli's four-vessel occlusion method. Pramipexole administration group was administered intraperitoneally in rats with global cerebral ischemia-reperfusion injury at different doses of pramipexole 0.25 mg/kg, 0.5 mg/kg, 1 mg/kg, 2 mg/kg, once a day for 14 consecutive days. Based on the results of modified neurological severity scores, open field test and morphology by Nissl's staining to determine the optimal dose of pramipexole. Mitochondrial membrane potential in the optimal dose of pramipexole administration group were measured by the JC-1 fluorescent probe staining method.

RESULTS

1. Different doses of pramipexole 0.25 mg/kg, 0.5 mg/kg, 1 mg/kg, and 2 mg/kg, were used as drug administration in rats with global cerebral ischemia-reperfusion injury for 14 consecutive days, and we found that all four doses of pramipexole could improve the modified neurological severity scores of rats with global cerebral ischemia-reperfusion injury to varying degrees, but only 0.5 mg/kg pramipexole at 1, 3, 7 and 14 days consistently reduced modified neurological severity scores and improved neurological function in rats with global cerebral ischemia-reperfusion injury. In the open-field test, only 0.5 mg/kg pramipexole increased the number of entries into the central zone, duration spent in the central zone, total distance travelled in the open field and average velocity, which improved the spontaneous activities and reduced anxiety and depression of rats with global cerebral ischemia-reperfusion injury. 2. Different doses of pramipexole 0.25 mg/kg, 0.5 mg/kg, 1 mg/kg, and 2 mg/kg for 14 consecutive days significantly increased the number of surviving neurons in the hippocampal CA1 subfield in rats with global cerebral ischemia-reperfusion injury to varying degrees. Based on these results, we tentatively found that 0.5 mg/kg pramipexole may be the optimal dose in all of the above. 3. We found that 0.5 mg/kg pramipexole significantly increased the mitochondrial membrane potential in rats after global cerebral ischemia-reperfusion injury.

CONCLUSION

Different doses of dopamine receptor agonist pramipexole improved neurological function of rats with global cerebral ischemia-reperfusion injury to varying degrees, and 0.5 mg/kg pramipexole may be the optimal dose in all of the above. Pramipexole may produce neuroprotective effects by protecting neurons in the hippocampus and improving the mitochondrial membrane potential after global cerebral ischemia-reperfusion injury.

Electroacupuncture alleviated depression‐like behaviors in ventromedial prefrontal cortex of chronic unpredictable mild stress‐induced rats: Increasing synaptic transmission and phosphorylating dopamine transporter

AIMS

Electroacupuncture (EA) shows advantages in both clinical practice and depression animal models. Dopaminergic-related dysfunction in the prefrontal cortex (PFC) may be a hidden antidepressant mechanism of EA, where dopamine transporter (DAT) plays an essential role. This study aimed to investigate the synaptic transmission and DAT-related changes of EA in depression.

METHODS

Male Sprague-Dawley rats were subjected to 3-week chronic unpredictable mild stress (CUMS). The successfully modeled rats were then randomly and equally assigned to CUMS, selective serotonin reuptake inhibitor (SSRI), and EA or SSRI + EA groups, followed by a 2-week treatment respectively. After monitoring body weight and behavioral tests of all rats, the ventromedial PFC (vmPFC) tissue was collected for electrophysiology and the expression detection of DAT, phosphorylated DAT (p-DAT), cyclic adenosine monophosphate (cAMP), protein kinase A (PKA), and trace amine-associated receptor 1 (TAAR1).

RESULTS

Depressive-like behaviors induced by CUMS were alleviated by EA, SSRI, and SSRI + EA treatments through behavioral tests. Compared with CUMS group, EA improved synaptic transmission in vmPFC by upregulating spontaneous excitatory postsynaptic currents amplitude. Molecularly, EA reversed the increased total DAT and p-DAT expression as well as the decreased ratio of p-DAT/total DAT along with the activation of TAAR1, cAMP, and PKA in vmPFC.

CONCLUSION

We speculated that the antidepressant effect of EA was associated with enhanced synaptic transmission in vmPFC, and the upregulated phosphorylation of DAT relevant to TAAR1, cAMP, and PKA may be the potential mechanism.

Sexual satiety modifies methamphetamine-induced locomotor and rewarding effects and dopamine-related protein levels in the striatum of male rats

RATIONALE

Drug and natural rewarding stimuli activate the mesolimbic dopaminergic system. Both methamphetamine (Meth) and copulation to satiety importantly increase dopamine (DA) release in the nucleus accumbens (NAc), but with differences in magnitude. This paper analyzes the interaction between Meth administration and the intense sexual activity associated with sexual satiety.

OBJECTIVES

To evaluate possible changes in Meth-induced behavioral effects and striatal DA-related protein expression due to sexual satiety.

METHODS

Meth-induced locomotor activity and conditioned place preference (CPP) were tested in sexually experienced male rats that copulated to satiety (S-S) or ejaculated once (1E) the day before or displayed no sexual activity (control group; C). DA receptors and DA transporter expression were determined by western blot in the striatum of animals of all sexual conditions treated with specific Meth doses.

RESULTS

Meth's locomotor and rewarding effects were exacerbated in S-S animals, while in 1E rats, only locomotor effects were enhanced. Sexual activity, by itself, modified DA-related protein expression in the NAc core and in the caudate-putamen (CPu), while Meth treatment alone changed their expression only in the NAc shell. Meth-induced changes in the NAc shell turned in the opposite direction when animals had sexual activity, and additional changes appeared in the NAc core and CPu of S-S rats.

CONCLUSION

Sexual satiety sensitizes rats to Meth's behavioral effects and the Meth-induced striatal DA-related protein adaptations are modified by sexual activity, evidencing cross-sensitization between both stimuli.

Behaviorally penetrant, anomalous dopamine efflux exposes sex and circuit dependent regulation of dopamine transporters

Virtually all neuropsychiatric disorders display sex differences in prevalence, age of onset, and/or clinical symptomology. Although altered dopamine (DA) signaling is a feature of many of these disorders, sex-dependent mechanisms uniquely responsive to DA that drive sex-dependent behaviors remain unelucidated. Previously, we established that anomalous DA efflux (ADE) is a prominent feature of the DA transporter (DAT) variant Val559, a coding substitution identified in two male-biased disorders: attention-deficit/hyperactivity disorder and autism spectrum disorder. In vivo, Val559 ADE induces activation of nigrostriatal D2-type DA autoreceptors (D2ARs) that magnifies inappropriate, nonvesicular DA release by elevating phosphorylation and surface trafficking of ADE-prone DAT proteins. Here we demonstrate that DAT Val559 mice exhibit sex-dependent alterations in psychostimulant responses, social behavior, and cognitive performance. In a search for underlying mechanisms, we discovered that the ability of ADE to elicit D2AR regulation of DAT is both sex and circuit-dependent, with dorsal striatum D2AR/DAT coupling evident only in males, whereas D2AR/DAT coupling in the ventral striatum is exclusive to females. Moreover, systemic administration of the D2R antagonist sulpiride, which precludes ADE-driven DAT trafficking, can normalize DAT Val559 behavioral changes unique to each sex and without effects on the opposite sex or wildtype mice. Our studies support the sex- and circuit dependent capacity of D2ARs to regulate DAT as a critical determinant of the sex-biased effects of perturbed DA signaling in neurobehavioral disorders. Moreover, our work provides a cogent example of how a shared biological insult drives alternative physiological and behavioral trajectories as opposed to resilience.

Characterization of D3 Autoreceptor Function in Whole Zebrafish Brain with Fast-Scan Cyclic Voltammetry

Zebrafish (Danio rerio) are ideal model organisms for investigating nervous system function, both in health and disease. Nevertheless, functional characteristics of dopamine (DA) release and uptake regulation are still not well-understood in zebrafish. In this study, we assessed D3 autoreceptor function in the telencephalon of whole zebrafish brains ex vivo by measuring the electrically stimulated DA release ([DA]max) and uptake at carbon fiber microelectrodes with fast-scan cyclic voltammetry. Treatment with pramipexole and 7-OH-DPAT, selective D3 autoreceptor agonists, sharply decreased [DA]max. Conversely, SB277011A, a selective D3 antagonist, nearly doubled [DA]max and decreased k, the first-order rate constant for the DA uptake, to about 20% of its original value. Treatment with desipramine, a selective norepinephrine transporter blocker, failed to increase current, suggesting that our electrochemical signal arises solely from the release of DA. Furthermore, blockage of DA uptake with nomifensine-reversed 7-OH-DPAT induced decreases in [DA]max. Collectively, our data show that, as in mammals, D3 autoreceptors regulate DA release, likely by inhibiting uptake. The results of this study are useful in the further development of zebrafish as a model organism for DA-related neurological disorders such as Parkinson's disease, schizophrenia, and drug addiction.

Potential Mechanisms for Why Not All Antipsychotics Are Able to Occupy Dopamine D3 Receptors in the Brain in vivo

Dysfunctions of the dopaminergic system are believed to play a major role in the core symptoms of schizophrenia such as positive, negative, and cognitive symptoms. The first line of treatment of schizophrenia are antipsychotics, a class of medications that targets several neurotransmitter receptors in the brain, including dopaminergic, serotonergic, adrenergic and/or muscarinic receptors, depending on the given agent. Although the currently used antipsychotics display in vitro activity at several receptors, majority of them share the common property of having high/moderate in vitro affinity for dopamine D₂ receptors (D₂Rs) and D₃ receptors (D₃Rs). In terms of mode of action, these antipsychotics are either antagonist or partial agonist at the above-mentioned receptors. Although D₂Rs and D₃Rs possess high degree of homology in their molecular structure, have common signaling pathways and similar in vitro pharmacology, they have different in vivo pharmacology and therefore behavioral roles. The aim of this review, with summarizing preclinical and clinical evidence is to demonstrate that while currently used antipsychotics display substantial in vitro affinity for both D₃Rs and D₂Rs, only very few can significantly occupy D₃Rs in vivo. The relative importance of the level of endogenous extracellular dopamine in the brain and the degree of in vitro D₃Rs receptor affinity and selectivity as determinant factors for in vivo D₃Rs occupancy by antipsychotics, are also discussed.

Pramipexole regulates depression-like behavior via dopamine D3 receptor in a mouse model of Parkinson's disease

Depression is one of the strongest predictors of quality of life in patients with Parkinson's disease (PD). Despite the high prevalence of depression, there is no clear guidance for its treatment in PD because the evidence for the efficacy of most antidepressants remains insufficient. Pramipexole, a dopamine agonist, is one of the few drugs that has proven to be clinically useful. However, the underlying mechanisms of antidepressive effects of pramipexole are still unknown. A 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP)-induced mouse model, dopamine D2 receptor (DRD2) and D3 receptor (DRD3) knockout mice were used in our study. Compared with other dopamine D2-like receptor agonists and madopar, pramipexole improved depression-like behavior and alleviate bradykinesia in an MPTP-induced mouse model of PD. Pramipexole significantly improved depression-like behavior in DRD2^-/- mice but not in DRD3^-/- mice. These results demonstrate that the antidepressive effect of pramipexole is mediated by DRD3 but not DRD2. Our findings highlight the need to develop novel dopamine agonists specifically targeting DRD3 for the treatment of depression in PD in the future.