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(+)-4-Propyl-9-hydroxynaphthoxazine ((+)PHNO) is a high affinity, preferential dopamine D 3 versus D 2 agonist employed in view of its high specificity and excellent signal-to-noise ratio as a radiotracer for positron emission tomography (PET) imaging. Surprisingly, its profile at other classes of monoamine receptor remains undocumented. In addition to hD 3 and hD 2L receptors, (+)PHNO revealed high affinity at hD 4.4 but not hD 1 or hD 5 receptors. It also revealed significant affinity for several other G protein-coupled monoaminergic receptors, in particular h5-HT 1A and h5-HT 7 . (+)PHNO behaved as a full agonist at hD 4.4 and h5-HT 1A receptors with potencies comparable to its actions at hD 3 and hD 2L receptors, and with less potency at 5-HT 7 receptors. In binding assays with membranes derived from cells co-expressing hD 3 and hD 2L receptors and labeled with [ 3 H]Nemonapride or [ 3 H]Spiperone, the proportion of high affinity binding sites recognized by (+)PHNO was higher than an equivalent mixture of membranes from cells expressing hD 3 or hD 2L receptors, suggesting that (+)PHNO promotes formation of hD 3 -hD 2L heterodimers. Further, in cells co-expressing hD 3 and hD 2L receptors, (+)PHNO showed higher efficacy for inhibiting forskolin stimulated adenylyl cyclase and inducing adenylyl cyclase super-sensitization than in cells transfected with only hD 2L receptors. In conclusion, (+)PHNO is a potent agonist at hD 4 . 4 , h5-HT 1A and h5-HT 7 as well as hD 3 and hD 2L receptors, and it potently activates dopamine hD 3 -hD 2L heterodimers. These interactions should be considered when interpreting PET studies with [ 11 C](+)PHNO and may be relevant to its functional and potential clinical properties in Parkinson's disease and other disorders., (Copyright © 2022 Elsevier B.V. All rights reserved.)

Dystrobrevin binding protein-1 (dysbindin-1), a candidate gene for schizophrenia, modulates cognition, synaptic plasticity and frontocortical circuitry and interacts with glutamatergic and dopaminergic transmission. Loss of dysbindin-1 modifies cellular trafficking of dopamine ( DA) D2 receptors to increase cell surface expression, but its influence upon signaling has never been characterized. Further, the effects of dysbindin-1 upon closely related D3 receptors remain unexplored. Hence, we examined the impact of dysbindin-1 (isoform A) co-expression on the localization and coupling of human D2L and D3 receptors stably expressed in Chinese hamster ovary or SH- SY5Y cells lacking endogenous dysbindin-1. Dysbindin-1 co-transfection decreased cell surface expression of both D3 and D2L receptors. Further, while their affinity for DA was unchanged, dysbindin-1 reduced the magnitude and potency of DA-induced adenylate cylase recruitment/ cAMP production. Dysbindin-1 also blunted the amplitude of DA-induced phosphorylation of ERK1/2 and Akt at both D2L and D3 receptors without, in contrast to cAMP, affecting the potency of DA. Interference with calveolin/clathrin-mediated processes of internalization prevented the modification by dysbindin-1 of ERK1/2 and adenylyl cyclase stimulation at D2L and D3 receptors. Finally, underpinning the specificity of the influence of dysbindin-1 on D2L and D3 receptors, dysbindin-1 did not modify recruitment of adenylyl cyclase by D1 receptors. These observations demonstrate that dysbindin-1 influences cell surface expression of D3 in addition to D2L receptors, and that it modulates activation of their signaling pathways. Accordingly, both a deficiency and an excess of dysbindin-1 may be disruptive for dopaminergic transmission, supporting its link to schizophrenia and other CNS disorders. [ABSTRACT FROM AUTHOR]

Aripiprazole and the candidate antipsychotics, S33592, bifeprunox, N-desmethylclozapine (NDMC) and preclamol, are partial agonists at D2 receptors. Herein, we examined their actions at D2L and D3 receptors expressed separately or together in COS-7 cells. In D2L receptor-expressing cells co-transfected with (D3 receptor- insensitive) chimeric adenylate cyclase-V/VI, drugs reduced forskolin-stimulated cAMP production by ∼20% versus quinpirole (48%). Further, quinpirole-induced inhibition was blunted by aripiprazole and S33592, confirming partial agonist properties. In cells co-transfected with equal amounts of D2L and D3 receptors (1 : 1), efficacies of aripiprazole and S33592 were attenuated. Further, in cells co-transfected with D2L and an excess of D3 receptors (1 : 3), aripiprazole and S33592 were completely inactive, and they abolished the actions of quinpirole. Likewise, bifeprunox, NDMC and preclamol lost agonist properties in cells co-transfected with D2L and D3 receptors. Accordingly, at split D2trunk/D3tail and D3trunk/D2tail chimeras, agonist actions of quinpirole were blocked by aripiprazole and S33592 that, like bifeprunox, NDMC and preclamol, were inactive alone. Conversely, when a 12 amino acid sequence in the third intracellular loop of D3 receptors was replaced by the homologous sequence of D2L receptors, aripiprazole, S33592, bifeprunox, NDMC and preclamol inhibited cAMP formation by ∼20% versus quinpirole (42%). Moreover, at D2L receptor-expressing cells co-transfected with modified D3i3(D2) receptors, drugs behaved as partial agonists. To summarize, low efficacy agonist actions of aripiprazole, S33592, bifeprunox, NDMC and preclamol at D2L receptors are abrogated upon co-expression of D3 receptors, probably due to physical association and weakened coupling efficacy. These findings have implications for the functional profiles of antipsychotics. [ABSTRACT FROM AUTHOR]

• LED light at 610 nm negatively impacts the survival of dopamine neurons in culture. • 610 nm LED light damages mouse mesencephalic and hiPSC-derived dopamine neurons. • Elevated dopamine in differentiated MN9D cells increases vulnerability to light. • 610 nm LED light increases ROS production in differentiated MN9D cells. • MN9D co-treatment with H2O2 boosts light induced ROS production and cytotoxicity. Our recent study revealed that fluorescent lamp light can penetrate deep into the brain of mice and rats leading to the development of typical histological characteristics associated with Parkinson's disease such as the loss of dopamine neurons in the substantia nigra. Monochromatic LED lights were thus used in this work to deepen our knowledge on the effects of the major wavelength peaks of fluorescent light on mouse and human dopaminergic cells. In particular, we exposed immortalized dopaminergic MN9D neuronal cells, primary cultures of mouse mesencephalic dopaminergic cells and human dopaminergic neurons differentiated from induced pluripotent stem cells (hiPSC) to different LED light wavelengths. We found that chronic exposure to LED light reduced overall undifferentiated MN9D cell number, with the most significant effects observed at wavelengths of 485 nm and 610 nm. Moreover, LED light especially at 610 nm was able to negatively impact on the survival of mouse mesencephalic dopaminergic cells and of human dopaminergic neurons derived from hiPSC. Notably, differentiated MN9D dopaminergic cells, which closely resemble mature dopamine neuronal phenotype, acutely exposed for 3 h at 610 nm, showed a clear increase in ROS production and cytotoxicity compared to controls undifferentiated MN9D cells. These increases were even more pronounced by the co-treatment with the oxidative agent H 2 O 2. Collectively, these findings suggest that specific wavelengths, particularly those capable of penetrating deep into the brain, could potentially pose an environmental hazard in relation to Parkinson's disease. [ABSTRACT FROM AUTHOR]

Recently, employing radioligand displacement and functional coupling studies, we demonstrated that SB269,652 (N-[(1r,4r)−4-[2-(7-cyano-1,2,3,4-tetrahydroisoquinolin-2-yl)ethyl]cyclohexyl]−1H-indole-2-carboxamide) interacts in an atypical manner with dopamine D 3 receptor displaying a unique profile reminiscent of a negative allosteric ligand. Here, we characterized the binding of radiolabelled [ 3 H]SB269,652 to human dopamine D 3 receptor stably expressed in Chinese Hamster Ovary cells. Under saturating conditions, SB269,652 showed a KD value of ≈ 1 nM. Consistent with high selectivity for human dopamine D 3 receptor, [ 3 H]SB269,652 binding was undetectable in cells expressing human dopamine D 1 , D 2L or D 4 receptors and absent in synaptosomes from dopamine D 3 receptor knockout vs . wild-type mice. In contrast to saturation binding experiments, the dissociation kinetics of [ 3 H]SB269,652 from human dopamine D 3 receptors initiated with an excess of unlabelled ligand were best fitted by a bi-exponential binding model. Supporting the kinetic data, competition experiments with haloperidol, S33084 (a dopamine D 3 receptor antagonist) or dopamine, were best described by a two-site model. In co-transfection experiments binding of SB269,652 to dopamine D 3 receptor was able to influence the functional coupling of dopamine D 2 receptor, supporting the notion that SB269,652 is a negative allosteric modulator across receptor dimers. However, because SB269,652 decreases the rate of [ 3 H]nemonapride dissociation, the present data suggest that SB269,652 behaves as a bitopic antagonist at unoccupied dopamine D 3 receptor, binding simultaneously to both orthosteric and allosteric sites, and as a pure negative allosteric modulator when receptors are occupied and it can solely bind to the allosteric site. [ABSTRACT FROM AUTHOR]

We investigated the effects of continuous artificial light exposure on the mouse substantia nigra (SN). A three month exposure of C57Bl/6J mice to white fluorescent light induced a 30% reduction in dopamine (DA) neurons in SN compared to controls, accompanied by a decrease of DA and its metabolites in the striatum. After six months of exposure, neurodegeneration progressed slightly, but the level of DA returned to the basal level, while the metabolites increased with respect to the control. Three month exposure to near infrared LED light (∼710 nm) did not alter DA neurons in SN, nor did it decrease DA and its metabolites in the striatum. Furthermore mesencephalic cell viability, as tested by [ 3 H]DA uptake, did not change. Finally, we observed that 710 nm LED light, locally conveyed in the rat SN, could modulate the firing activity of extracellular-recorded DA neurons. These data suggest that light can be detrimental or beneficial to DA neurons in SN, depending on the source and wavelength. [ABSTRACT FROM AUTHOR]

We previously found that the dopamine D3 receptor can be split at the third cytoplasmic loop into two fragments (D3trunk and D3tail), and that the mixture of the two fragments retains the binding and functional activity of the wild type receptor. The dopamine D3 receptor gene gives rise to several inactive receptor splice variants, one of which is the D3nf. Since this gene variant very closely resembles our D3trunk fragment, in this study we investigated if the transfection of D3nf with D3tail could result in the rescue of a functional dopamine receptor. Our experiments showed that D3tail can indeed rescue the activity of D3nf, and that the pharmacological profile of this split D3nf/D3tail receptor is identical to that of the wild type D3 receptor. [Copyright &y& Elsevier]

In this report we show that dextromethorphan, a non-opioid cough suppressant, prevents the neurodegeneration of dopaminergic neurons in the substantia nigra of mice treated with diethyldithiocarbamate (DDC) and 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP). This effect is further substantiated by the assessment of dopamine (DA) content in the striatum of these animals. Dextromethorphan does not attenuate the striatal DA fall induced by MPTP alone but completely prevents DDC-induced enhancement after the combined treatment. Moreover, a study of DA metabolites has confirmed this neuroprotective property. The striatal levels of serotonin, which were studied as a control neuronal marker, did not change with any of the treatments administered. Furthermore, we show that dextromethorphan reduces the toxicity of glutamate against dopamine neurons in mesencephalic cell cultures. In line with previous data suggesting that dextromethorphan can prevent neuronal damage, our observations supply new evidence regarding the possibility of this compound being of therapeutic use in neurodegenerative diseases. [Copyright &y& Elsevier]