Dopamine, activation of ingestion and evaluation of response efficacy: a focus on the within-session time-course of licking burst number.

RATIONALE: Evidence on the effect of dopamine D1-like and D2-like receptor antagonists on licking microstructure and the forced swimming response led us to suggest that (i) dopamine on D1-like receptors plays a role in activating reward-directed responses and (ii) the level of response activation is reboosted based on a process of evaluation of response efficacy requiring dopamine on D2-like receptors. A main piece of evidence in support of this hypothesis is the observation that the dopamine D2-like receptor antagonist raclopride induces a within-session decrement of burst number occurring after the contact with the reward. The few published studies with a detailed analysis of the time-course of this measure were conducted in our laboratory. OBJECTIVES: The aim of this review is to recapitulate and discuss the evidence in support of the analysis of the within-session burst number as a behavioural substrate for the study of the mechanisms governing ingestion, behavioural activation and the related evaluation processes, and its relevance in the analysis of drug effects on ingestion. CONCLUSIONS: The evidence gathered so far suggests that the analysis of the within-session time-course of burst number provides an important behavioural substrate for the study of the mechanisms governing ingestion, behavioural activation and the related evaluation processes, and might provide decisive evidence in the analysis of the effects of drugs on ingestion. However, further evidence from independent sources is necessary to validate the use and the proposed interpretation of this measure.

The trace amine-associated receptor 1 agonists - non-dopaminergic antipsychotics or covert modulators of D2 receptors?

A major effort of the pharmaceutical industry has been to identify and market drug treatments that are effective in ameliorating the symptoms of psychotic illness but without the limitations of the current treatments acting at dopamine D2 receptors. These limitations include the induction of a range of adverse effects, the inadequate treatment response of a substantial proportion of people with schizophrenia, and the generally poor response to negative and cognitive features of the disease. Recently introduced drug treatments have gone some way to avoiding the first of these, with a reduced propensity for weight gain, cardiovascular risk and extrapyramidal motor effects. Despite claims of some small improvements in negative symptoms, these drugs have not demonstrated substantial increases in efficacy. Of the drugs currently in development as antipsychotic agents, several are misleadingly described as having novel 'non-dopaminergic' mechanisms that may offer improvements in addressing the limitations of adverse effects and efficacy. It will be argued, using the trace amine-associated receptor 1 agonist as an example, that several of these new drugs still act primarily through modulation of dopaminergic neurotransmission and, in not addressing the primary pathology of schizophrenia, are therefore unlikely to have the much-needed improvements in efficacy required to address the unmet need associated with resistance to current treatments.

Brain antioxidant status and gene expressions of nicotinic and dopamine receptors are improved by black seed oil administration in cigarette smoke or nicotine vapour-exposed rats.

BACKGROUND: Smoking is associated with dysregulation of the antioxidant system and addiction. AIM: This study sought to ascertain the effect of Nigella Sativa (NS) oil on the antioxidant system, nicotine/tobacco addiction as well as the expressions of α4ß2 nicotinic (nAChR) and dopamine type-2 (DRD2) receptors in selected brain regions of the rat. METHODS: Thirty male Sprague-Dawley rats were divided into 6 groups comprising of vehicle-treated control, NS oil only, Smoke only, Smoke + NS oil, Nicotine only and Nicotine + NS oil. Animals were passively exposed to cigarette smoke or nicotine vapour for 12 weeks, however, NS oil treatment commenced from 9th-12th week of the experimental duration. RESULTS: Nicotine vapour and cigarette smoke-induced increase in cotinine level were significantly ameliorated by NS treatment. Cigarette smoke or nicotine vapour exposure significantly (p<0.05) decreased the level of antioxidant enzymes while increasing malondialdehyde level in the brain homogenates of the rats.  Administration of NS oil significantly (p<0.05) reversed the reduced antioxidant level. Cigarette-smoke also significantly increased α4-nAChR expression in the frontal cortex and olfactory bulb compared to control. Nicotine vapour significantly increased DRD2 expression only in the olfactory cortex. NS oil administration reduced both the cigarette-smoke-induced increase in α4-nAChR and nicotine vapour-induced increase in DRD2 gene expression only in the olfactory cortex. CONCLUSION: Findings from this study suggest that NS oil improves brain antioxidant status while ameliorating nicotine vapour and cigarette smoke addiction through down-regulation of α4-nAChR and DRD2 gene expressions in discrete brain regions in Sprague-Dawley rats.

Dopamine D2L receptor density influences the recruitment of ß-arrestin2 and Gi1 induced by antiparkinsonian drugs.

INTRODUCTION: Brain imaging studies have highlighted that the density of dopamine D2 receptors markedly fluctuates across the stages of Parkinson's disease and in response to pharmacological treatment. Moreover, receptor density constitutes a molecular determinant for the signaling profile of D2 receptor ligands. We therefore hypothesized that variations in receptor expression could influence D2 receptor response to antiparkinsonian drugs, most notably with respect to the recruitment bias between Gi1 and ß-arrestin2. METHODS: The recruitment bias of dopamine, pramipexole, ropinirole, and rotigotine was examined using a nanoluciferase-based biosensor for probing the interactions of the D2L receptor with either Gi1 or ß-arrestin2. The characterization of the functional selectivity of these D2 receptor agonists was performed at two distinct D2L receptor densities by taking advantage of a cell model carrying an inducible system that enables the overexpression of the D2L receptor when exposed to doxycycline. RESULTS: A high receptor density oriented the balanced signaling profile of dopamine towards a preferential recruitment of Gi1. It also moderated the marked Gi1 and ß-arrestin2 biases of pramipexole and rotigotine, respectively. At variance, the Gi1 bias of ropinirole appeared as not being influenced by D2L receptor density. CONCLUSIONS: Taken together, these observations highlight receptor density as a key driver of the signaling transducer recruitment triggered by antiparkinsonian agents. Moreover, given the putative beneficial properties of ß-arrestin2 in promoting locomotion, this study provides molecular insights that position the arrestin-biased ligand rotigotine as a putatively more beneficial D2 receptor agonist for the treatment of early and late Parkinson's disease.

Activation of dopamine D2 receptors in the shell of nucleus accumbens triggers conditioned avoidance responses in rats.

Conditioned avoidance responses (CAR) behavior is a classical instrumental response paradigm, which is widely used to study aversive conditioning and defensive motivation behavior. Previous studies have shown that dopamine D1 and D2 receptors are involved in CAR behavior; however, it is unclear in which brain regions that dopamine evokes CAR behavior. The aim of the study is to investigate whether dopamine triggers CAR behavior via activating dopamine D1 or D2 receptors in the shell of nucleus accumbens or dorsolateral striatum. The present study found that infusion of the dopamine D2 receptor agonist quinpirole, but not D1 receptor agonist SKF38393, into the shell of nucleus accumbens evoked CAR behavior in reserpine-treated rats. Whereas, infusion of neither SKF38393 nor quinpirole into the dorsolateral striatum evoked CAR behavior. In addition, infusion of quinpirole into the shell of nucleus accumbens enhanced CAR behavior in the unsuccessful trained rats without affecting the motor function in the balance beam and locomotor tests. In conclusion, activation of dopamine D2, but not D1 receptors in the shell of nucleus accumbens evokes CAR behavior. However, activation of dopamine D1 and D2 receptors in the dorsolateral striatum does not evoke CAR behavior. It is suggested that the shell of nucleus accumbens is the critical brain region for dopamine to invoke CAR behavior, and activation of dopamine D2 receptors in the shell of nucleus accumbens is sufficient and necessary to evoke CAR behavior.

Heterobivalent Ligand for the Adenosine A2A-Dopamine D2 Receptor Heteromer.

A G protein-coupled receptor heteromer that fulfills the established criteria for its existence in vivo is the complex between adenosine A2A (A2AR) and dopamine D2 (D2R) receptors. Here, we have designed and synthesized heterobivalent ligands for the A2AR-D2R heteromer with various spacer lengths. The indispensable simultaneous binding of these ligands to the two different orthosteric sites of the heteromer has been evaluated by radioligand competition-binding assays in the absence and presence of specific peptides that disrupt the formation of the heteromer, label-free dynamic mass redistribution assays in living cells, and molecular dynamic simulations. This combination of techniques has permitted us to identify compound 26 [KDB1 (A2AR) = 2.1 nM, KDB1 (D2R) = 0.13 nM], with a spacer length of 43-atoms, as a true bivalent ligand that simultaneously binds to the two different orthosteric sites. Moreover, bioluminescence resonance energy transfer experiments indicate that 26 favors the stabilization of the A2AR-D2R heteromer.

Beta band modulation by dopamine D2 receptors in the primary motor cortex and pedunculopontine nucleus in a rat model of Parkinson's disease.

Beta band (12-30 Hz) hypersynchrony within the basal ganglia-thalamocortical network has been suggested as a hallmark of Parkinson's disease (PD) pathophysiology. Abnormal beta band oscillations are found in the pedunculopontine nucleus (PPN) and primary motor cortex (M1) and are correlated with dopamine depletion. Dopamine acts locomotion and motor performance mainly through dopamine receptors (D1 and D2). However, the precise mechanism by which dopamine receptors regulate beta band electrophysiological activities between the PPN and M1 is still unknown. Here, we recorded the neuronal activity of the PPN and M1 simultaneously by the administration of the drug (SCH23390 and raclopride), selectively blocking the dopamine D1 receptor and D2 receptor. We discovered that the increased coherent activity of the beta band (12-30 Hz) between M1 and PPN in the lesioned group could be reduced and restored by injecting raclopride in the resting and wheel running states. Our studies revealed the unique role of D2 dopamine receptor signaling in regulating ß band oscillatory activity in M1 and PPN and their relationship after the loss of dopamine, which contributes to elucidating the underlying mechanism of the pathophysiology of PD.

Cannabidiol impairs the rewarding effects of methamphetamine: Involvement of dopaminergic receptors in the nucleus accumbens.

Cannabidiol, as component of cannabis, can potentially hinder the rewarding impact of drug abuse; however, its mechanism is ambiguous. Moreover, the nucleus accumbens (NAc), as a key area in the reward circuit, extensively receives dopaminergic projections from the ventral tegmentum area. To elucidate the role of accumbal D1 and D2 dopamine receptor families in Cannabidiol's inhibitory impact on the acquisition and expression phases of methamphetamine (MET), the conditioned place preference (CPP) procedure as a common method to assay reward characteristics of drugs was carried out. Six groups of rats were treated by various doses of SCH23390 or Sulpiride (0.25, 1, and 4 µg/0.5 µL) in the NAc as D1 or D2 dopamine receptor family antagonists, respectively, prior to infusion of Cannabidiol (10 µg/5 µL) in the lateral ventricle (LV) over conditioning phase in the acquisition experiments. In the second step of the study, animals received SCH23390 or Sulpiride in the NAc before Cannabidiol (50 µg/5 µL) infusion into the LV in the expression phase of MET to illuminate the influence of SCH23390 or Sulpiride on the inhibitory impact of Cannabidiol on the expression of MET-induced CPP. Intra-NAc administration of either SCH23390 or Sulpiride impaired Cannabidiol's suppressive impact on the expression phase, while just Sulpiride could suppress the Cannabidiol's impact on the acquisition phase of the MET-induced CPP. Also, the inhibitory impact of Sulpiride was stranger in both phases of MET reward. It seems that Cannabidiol prevents the expression and acquisition phases of MET-induced CPP partly through the dopaminergic system in the NAc.

Pharmacological perturbation reveals deficits in D2 receptor responses in Thap1 null mice.

The primary dystonia DYT6 is caused by mutations in the transcription factor Thanatos-associated protein 1 (THAP1). To understand THAP1's functions, we generated mice lacking THAP1 in the nervous system. THAP1 loss causes locomotor deficits associated with transcriptional changes. Since many of the genes misregulated involve dopaminergic signaling, we pharmacologically challenged the two striatal canonical dopamine pathways: the direct, regulated by the D1 receptor, and the indirect, regulated by the D2 receptor. We discovered that depleting THAP1 specifically interferes with the D2 receptor responses, pointing to a selective misregulation of the indirect pathway in DYT6 with implications for pathogenesis and treatment.

Chiral Cyclic Aliphatic Linkers as Building Blocks for Selective Dopamine D2 or D3 Receptor Agonists.

Linkers are emerging as a key component in regulating the pharmacology of bitopic ligands directed toward G-protein coupled receptors (GPCRs). In this study, the role of regio- and stereochemistry in cyclic aliphatic linkers tethering well-characterized primary and secondary pharmacophores targeting dopamine D2 and D3 receptor subtypes (D2R and D3R, respectively) is described. We introduce several potent and selective D2R (rel-trans-16b; D2R Ki = 4.58 nM) and D3R (rel-cis-14a; D3R Ki = 5.72 nM) agonists while modulating subtype selectivity in a stereospecific fashion, transferring D2R selectivity toward D3R via inversion of the stereochemistry around these cyclic aliphatic linkers [e.g., (-)-(1S,2R)-43 and (+)-(1R,2S)-42]. Pharmacological observations were supported with extensive molecular docking studies. Thus, not only is it an innovative approach to modulate the pharmacology of dopaminergic ligands described, but a new class of optically active cyclic linkers are also introduced, which can be used to expand the bitopic drug design approach toward other GPCRs.

Early life exposure to poly I:C impairs striatal DA-D2 receptor binding, myelination and associated behavioural abilities in rats.

Early-life viral infections critically influence the brain development and have been variously reported to cause neuropsychiatric diseases such as Schizophrenia, Parkinson's diseases, demyelinating diseases, etc. To investigate the alterations in the dopaminergic system, myelination and associated behavioral impairments following neonatal viral infection, the viral immune activation model was created by an intraperitoneal injection of Poly I:C (5 mg/kg bw/ip) to neonatal rat pups on PND-7. The DA-D2 receptor binding was assessed in corpus striatum by using 3H-Spiperone at 3, 6 and 12 weeks of age. MOG immunolabelling was performed to check myelination stature and myelin integrity, while corpus callosum calibre was assessed by Luxol fast blue staining. Relative behavioral tasks i.e., motor activity, motor coordination and neuromuscular strength were assessed by open field, rotarod and grip strength meter respectively at 3, 6 and 12 weeks of age. Following Poly I:C exposure, a significant decrease in DA-D2 receptor binding, reduction in corpus callosum calibre and MOG immunolabelling indicating demyelination and a significant decrease in locomotor activity, neuromuscular strength and motor coordination signify motor deficits and hypokinetic influence of early life viral infection. Thus, the findings suggest that early life poly I:C exposure may cause demyelination and motor deficits by decreasing DA-D2 receptor binding affinity.

Long-lasting behavioral effects of quinpirole exposure on zebrafish.

The human brain matures into a complex structure, and to reach its complete development, connections must occur along exact paths. If at any stage, the processes are altered, interrupted, or inhibited, the consequences can be permanent. Dopaminergic signaling participates in the control of physiological functions and behavioral processes, and alterations in this signaling pathway are related to the pathogenesis of several neurological disorders. For this reason, the use of pharmacological agents able to interact with the dopaminergic signaling may elucidate the biological bases of such disorders. We investigated the long-lasting behavioral effects on adult zebrafish after quinpirole (a dopamine D2/D3 receptor agonist) exposure during early life stages of development (24 h exposure at 5 days post-fertilization, dpf) to better understand the mechanisms underlying neurological disorders related to the dopaminergic system. Quinpirole exposure at the early life stages of zebrafish led to late behavioral alterations. When evaluated at 120 dpf, zebrafish presented increased anxiety-like behaviors. At the open tank test, fish remained longer at the bottom of the tank, indicating anxiety-like behavior. Furthermore, quinpirole-treated fish exhibited increased absolute turn angle, likely an indication of elevated erratic movements and a sign of increased fear or anxiety. Quinpirole-treated fish also showed altered swimming patterns, characterized by stereotypic swimming. During the open tank test, exposed zebrafish swims from corner to corner in a repetitive manner at the bottom of the tank. Moreover, quinpirole exposure led to memory impairment compared to control fish. However, quinpirole administration had no effects on social and aggressive behavior. These findings demonstrate that dopaminergic signaling altered by quinpirole administration in the early life stages of development led to late alterations in behavioral parameters of adult zebrafish.

Dopaminergic mechanisms underlying the expression of antipsychotic-induced dopamine supersensitivity in rats.

Antipsychotic treatment can produce a dopamine-supersensitive state, potentiating the response to dopamine receptor stimulation. In both schizophrenia patients and rats, this is linked to tolerance to ongoing antipsychotic treatment. In rodents, dopamine supersensitivity is often confirmed by an exaggerated psychomotor response to d-amphetamine after discontinuation of antipsychotic exposure. Here we examined in rats the dopaminergic mechanisms mediating this enhanced behavioural response, as this could uncover pathophysiological processes underlying the expression of antipsychotic-evoked dopamine supersensitivity. Rats received 0.5 mg/kg/day haloperidol via osmotic minipump for 2 weeks, before treatment was discontinued. After cessation of antipsychotic treatment, rats showed a supersensitive psychomotor response to the D2 agonist quinpirole, but not to the D1 partial agonist SKF38393 or the dopamine reuptake blocker GBR12783. Furthermore, acute D1 receptor blockade (using SCH39166) decreased the exaggerated psychomotor response to d-amphetamine in haloperidol-pretreated rats, whereas acute D2 receptor blockade (using sulpiride) enhanced it. Thus, after discontinuation of antipsychotic treatment, D1- and D2-mediated transmission differentially modulate the expression of a supersensitive response to d-amphetamine. This supersensitive behavioural response was accompanied by enhanced GSK3ß activity and suppressed ERK1/2 activity in the nucleus accumbens (but not caudate-putamen), suggesting increased mesolimbic D2 transmission. Finally, after discontinuing haloperidol treatment, neither increasing ventral midbrain dopamine impulse flow nor infusing d-amphetamine into the cerebral ventricles triggered the expression of already established dopamine supersensitivity, suggesting that peripheral effects are required. Thus, while dopamine receptor-mediated signalling regulates the expression of antipsychotic-evoked dopamine supersensitivity, a simple increase in central dopamine neurotransmission is insufficient to trigger this supersensitivity.

Distinct dose-dependent effects of methamphetamine on real-time dopamine transmission in the rat nucleus accumbens and behaviors.

Methamphetamine (METH) is a potent psychostimulant that exerts many of its physiological and psychomotor effects by increasing extracellular dopamine (DA) concentrations in limbic brain regions. While several studies have focused on how potent, neurotoxic doses of METH augment or attenuate DA transmission, the acute effects of lower and behaviorally activating doses of METH on modulating DA regulation (release and clearance) through DA D2 autoreceptors and transporters remain to be elucidated. In this study, we investigated how systemic administration of escalating, subneurotoxic doses of METH (0.5-5 mg/kg, IP) alter extracellular DA regulation in the nucleus accumbens (NAc), in both anesthetized and awake-behaving rats through the use of in vivo fast-scan cyclic voltammetry. Pharmacological, electrochemical, and behavioral evidence show that lower doses (≤2.0 mg/kg, IP) of METH enhance extracellular phasic DA concentrations and locomotion as well as stereotypies. In contrast, higher doses (≥5.0 mg/kg) further increase both phasic and baseline DA concentrations and stereotypies but decrease horizontal locomotion. Importantly, our results suggest that acute METH-induced enhancement of extracellular DA concentrations dose dependently activates D2 autoreceptors. Therefore, these different METH dose-dependent effects on mesolimbic DA transmission may distinctly impact METH-induced behavioral changes. This study provides valuable insights regarding how low METH doses alter DA transmission and paves the way for future clinical studies on the reinforcing effects of METH.

l-DOPA promotes striatal dopamine release through D1 receptors and reversal of dopamine transporter.

Previous studies have pointed out that l-DOPA can interact with D1 or D2 receptors independent of its conversion to endogenous dopamine. The present study was set to investigate whether l-DOPA modulates dopamine release from striatal nerve terminals, using a preparation of synaptosomes preloaded with [3H]DA. Levodopa (1 µM) doubled the K+-induced [3H]DA release whereas the D2/D3 receptor agonist pramipexole (100 nM) inhibited it. The l-DOPA-evoked facilitation was mimicked by the D1 receptor agonist SKF38393 (30-300 nM) and prevented by the D1/D5 antagonist SCH23390 (100 nM) but not the DA transporter inhibitor GBR12783 (300 nM) or the aromatic l-amino acid decarboxylase inhibitor benserazide (1 µM). Higher l-DOPA concentrations (10 and 100 µM) elevated spontaneous [3H]DA efflux. This effect was counteracted by GBR12783 but not SCH23390. Binding of [3H]SCH23390 in synaptosomes (in test tubes) revealed a dense population of D1 receptors (2105 fmol/mg protein). Both SCH23390 and SKF38393 fully inhibited [3H]SCH23390 binding (Ki 0.42 nM and 29 nM, respectively). l-DOPA displaced [3H]SCH23390 binding maximally by 44% at 1 mM. This effect was halved by addition of GBR12935 and benserazide. We conclude that l-DOPA facilitates exocytotic [3H]DA release through SCH23390-sensitive D1 receptors, independent of its conversion to DA. It also promotes non-exocytotic [3H]DA release, possibly via conversion to DA and reversal of DA transporter. These data confirm that l-DOPA can directly interact with dopamine D1 receptors and might extend our knowledge of the neurobiological mechanisms underlying l-DOPA clinical effects.

Involvement of dopamine D2 and glutamate NMDA receptors in the antidepressant-like effect of amantadine in mice.

The present study investigated the pharmacological mechanisms of the antidepressant-like effects of amantadine in mice and their influence on hippocampal neurogenesis. To improve the translational validity of preclinical results, reproducibility across laboratories and replication in other animal models and species are crucial. Single amantadine administration at doses of 50 and 75 mg/kg resulted in antidepressant-like effects in mice in the tail suspension test (TST), reflected by an increase in immobility time. The effects of amantadine were seen at doses that did not alter locomotor activity. The tyrosine hydroxylase inhibitor α-methyl-ρ-tyrosine did not influence the anti-immobility effect of amantadine in the TST. Pretreatment with the α1 adrenergic receptor antagonist prazosin, ß adrenergic receptor antagonist propranolol, α2 adrenergic receptor antagonist yohimbine, and α2 adrenergic receptor agonist clonidine did not alter the antidepressant-like effect of amantadine. However, amantadine's effect was blocked by the dopamine D2 receptor antagonist haloperidol and glutamate receptor agonist N-methyl-D-aspartate (NMDA). Repeated amantadine administration (50 mg/kg) also exerted an antidepressant-like effect, paralleled by an increase in hippocampal neurogenesis. The present results demonstrate that the antidepressant-like effects of amantadine may be mediated by its actions on D2 and NMDA receptors and likely involve hippocampal neurogenesis.

D1- and D2-like receptors differentially mediate the effects of dopaminergic transmission on cost-benefit evaluation and motivation in monkeys.

It has been widely accepted that dopamine (DA) plays a major role in motivation, yet the specific contribution of DA signaling at D1-like receptor (D1R) and D2-like receptor (D2R) to cost-benefit trade-off remains unclear. Here, by combining pharmacological manipulation of DA receptors (DARs) and positron emission tomography (PET) imaging, we assessed the relationship between the degree of D1R/D2R blockade and changes in benefit- and cost-based motivation for goal-directed behavior of macaque monkeys. We found that the degree of blockade of either D1R or D2R was associated with a reduction of the positive impact of reward amount and increasing delay discounting. Workload discounting was selectively increased by D2R antagonism. In addition, blocking both D1R and D2R had a synergistic effect on delay discounting but an antagonist effect on workload discounting. These results provide fundamental insight into the distinct mechanisms of DA action in the regulation of the benefit- and cost-based motivation, which have important implications for motivational alterations in both neurological and psychiatric disorders.

Effects of intrastriatal injection of the dopamine receptor agonist SKF38393 and quinpirole on locomotor behavior in hemiparkinsonism rats.

Dopamine (DA) in the striatum is essential to influence motor behavior and may lead to movement impairment in Parkinson's disease (PD). The present study examined the different functions of the DA D1 receptor (D1R) and DA D2 receptor (D2R) by intrastriatal injection of the D1R agonist SKF38393 and the D2R agonist quinpirole in 6-hydroxydopamine (6-OHDA)-lesioned and control rats. All rats separately underwent dose-response behavior testing for SKF38393 (0, 0.5, 1.0, and 1.5 µg/site) or quinpirole (0, 1.0, 2.0, and 3.0 µg/site) to determine the effects of the optimal modulating threshold dose. Two behavior assessment indices, the time of latency to fall and the number of steps on a rotating treadmill, were used as reliable readouts of motor stimulation variables for quantifying the motor effects of the drugs. The findings indicate that at threshold doses, SKF38393 (1.0 µg/site) and quinpirole (1.0 µg/site) produce a dose-dependent increase in locomotor activity compared to vehicle injection. The ameliorated behavioral responses to either SKF38393 or quinpirole in lesioned rats were greater than those in unlesioned control rats. Moreover, the dose-dependent increase in locomotor capacity for quinpirole was greater than that for SKF38393 in lesioned rats. These results can clarify several key issues related to DA receptors directly and may provide a basis for exploring the potential of future selective dopamine therapies for PD in humans.

Dopamine receptor agonists ameliorate bleomycin-induced pulmonary fibrosis by repressing fibroblast differentiation and proliferation.

Idiopathic pulmonary fibrosis (IPF) is the most common fatal interstitial lung disease, with limited therapeutic options. The abnormal and uncontrolled differentiation and proliferation of fibroblasts have been confirmed to play a crucial role in driving the pathogenesis of IPF. Therefore, effective and well-tolerated antifibrotic agents that interfere with fibroblasts would be an ideal treatment, but no such treatments are available. Remarkably, we found that dopamine (DA) receptor D1 (D1R) and DA receptor D2 (D2R) were both upregulated in myofibroblasts in lungs of IPF patients and a bleomycin (BLM)-induced mouse model. Then, we explored the safety and efficacy of DA, fenoldopam (FNP, a selective D1R agonist) and sumanirole (SMR, a selective D2R agonist) in reversing BLM-induced pulmonary fibrosis. Further data showed that DA receptor agonists exerted potent antifibrotic effects in BLM-induced pulmonary fibrosis by attenuating the differentiation and proliferation of fibroblasts. Detailed pathway analysis revealed that DA receptor agonists decreased the phosphorylation of Smad2 induced by TGF-ß1 in primary human lung fibroblasts (PHLFs) and IMR-90 cells. Overall, DA receptor agonists protected mice from BLM-induced pulmonary fibrosis and may be therapeutically beneficial for IPF patients in a clinical setting.