Molecular neurobiology of dopamine receptor subtypes

Dopamine receptor 1 expressing B cells exert a proinflammatory role in female patients with rheumatoid arthritis

Rheumatoid arthritis (RA) is a chronic rheumatic disease with a clear sex-bias. Recent data indicated a role for dopamine in RA pathogenesis, while dopaminergic pathways can be modulated by estrogens. As defined mechanism of action of dopamine on B cell function in RA are unclear, we aimed to elucidate this, with special focus on sex-differences. Healthy controls (HC, n = 64) and RA patients (n = 61) were recruited. Expression of D₁-D₅ dopamine receptors (DRs) was investigated by flow cytometry on peripheral blood mononuclear cells (PBMCs). D₁-like DRs were stimulated in vitro to assess effects on B cell activation and proliferation. Secretion of cytokines and dopamine content were measured by ELISA. All DRs were expressed on PBMCs of HC and RA patients. Dopamine content in PBMCs, and frequency of D₁DR expressing B cells were significantly higher in RA females (p < 0.001). Expression of D₁DR on RA B cells correlated positively with disease duration and severity only in women. Combined B cell and D₁-like DR stimulation induced higher IL-8 and CCL-3 secretion from PBMCs of female RA patients compared to HC. These results indicate sex-specific differences in dopaminergic pathway in RA, with a proinflammatory feature of the D₁DR pathway in women.

Alteration of the Dopamine Receptors' Expression in the Cerebellum of the Lysosomal Acid Phosphatase 2 Mutant (Naked-Ataxia (NAX)) Mouse

A spontaneous mutation in the lysosomal acid phosphatase (Acp2) enzyme (nax: naked-ataxia) in experimental mice results in delayed hair appearance and severe cytoarchitectural impairments of the cerebellum, such as a Purkinje cell (PC) migration defect. In our previous investigation, our team showed that Acp2 expression plans a significant role in cerebellar development. On the other hand, the dopaminergic system is also a player in central nervous system (CNS) development, including cerebellar structure and function. In the current investigation, we have explored how Acp2 can be involved in the regulation of the dopaminergic pathway in the cerebellum via the regulation of dopamine receptor expression and patterning. We provided evidence about the distribution of different dopamine receptors in the developing cerebellum by comparing the expression of dopamine receptors on postnatal days (P) 5 and 17 between nax mice and wild-type (wt) littermates. To this aim, immunohistochemistry and Western blot analysis were conducted using five antibodies against dopamine receptors (DRD1, -2, -3, -4, and -5) accompanied by RNAseq data. Our results revealed that DRD1, -3, and -4 gene expressions significantly increased in nax cerebella but not in wt, while gene expressions of all 5 receptors were evident in PCs of both wt and nax cerebella. DRD3 was strongly expressed in the PCs' somata and cerebellar nuclei neurons at P17 in nax mice, which was comparable to the expression levels in the cerebella of wt littermates. In addition, DRD3 was expressed in scattered cells in a granular layer reminiscent of Golgi cells and was observed in the wt cerebella but not in nax mice. DRD4 was expressed in a subset of PCs and appeared to align with the unique parasagittal stripes pattern. This study contributes to our understanding of alterations in the expression pattern of DRDs in the cerebellum of nax mice in comparison to their wt littermates, and it highlights the role of Acp2 in regulating the dopaminergic system.

Advances in Dopamine D1 Receptor Ligands for Neurotherapeutics

The dopamine D1 receptor (D1R) is essential for neurotransmission in various brain pathways where it modulates key functions including voluntary movement, memory, attention and reward. Not surprisingly, the D1R has been validated as a promising drug target for over 40 years and selective activation of this receptor may provide novel neurotherapeutics for neurodegenerative and neuropsychiatric disorders. Several pharmacokinetic challenges with previously identified small molecule D1R agonists have been recently overcome with the discovery and advancement of new ligands, including drug-like non-catechol D1R agonists and positive allosteric modulators. From this, several novel molecules and mechanisms have recently entered clinical studies. Here we review the major classes of D1R selective ligands including antagonists, orthosteric agonists, non-catechol biased agonists and positive allosteric modulators, highlighting their structure-activity relationships and medicinal chemistry. Recent chemistry breakthroughs and innovative approaches to selectively target and activate the D1R also hold promise for creating pharmacotherapy for several neurological diseases.

Dendritic Spines in Depression: What We Learned from Animal Models

Depression, a severe psychiatric disorder, has been studied for decades, but the underlying mechanisms still remain largely unknown. Depression is closely associated with alterations in dendritic spine morphology and spine density. Therefore, understanding dendritic spines is vital for uncovering the mechanisms underlying depression. Several chronic stress models, including chronic restraint stress (CRS), chronic unpredictable mild stress (CUMS), and chronic social defeat stress (CSDS), have been used to recapitulate depression-like behaviors in rodents and study the underlying mechanisms. In comparison with CRS, CUMS overcomes the stress habituation and has been widely used to model depression-like behaviors. CSDS is one of the most frequently used models for depression, but it is limited to the study of male mice. Generally, chronic stress causes dendritic atrophy and spine loss in the neurons of the hippocampus and prefrontal cortex. Meanwhile, neurons of the amygdala and nucleus accumbens exhibit an increase in spine density. These alterations induced by chronic stress are often accompanied by depression-like behaviors. However, the underlying mechanisms are poorly understood. This review summarizes our current understanding of the chronic stress-induced remodeling of dendritic spines in the hippocampus, prefrontal cortex, orbitofrontal cortex, amygdala, and nucleus accumbens and also discusses the putative underlying mechanisms.

Dopamine signaling regulates the projection patterns in the mouse chiasm

Ocular albinism (OA) is characterized by inadequate L-3, 4-dihydroxyphenylalanine (L-DOPA) and dopamine (DA) in the eyes. This study investigated DA-related signaling pathways in mouse chiasm projection patterns and the potential role of ocular albinism type 1 (OA1) and dopamine 1A (D1A) receptors in the optic pathway. In embryonic day (E) E13-E15 retina, most L-DOPA and OA1-positive cells were distributed among Müller glial cells on E13 and retinal ganglion cells (RGC) on E14. In the ventral diencephalon, OA1 and L-DOPA were strongly expressed on the optic chiasm (OC) and optic tract (OT), respectively, but weak on the optic stalk (OS). At E13-E15, DA and D1A staining was predominately expressed in radially arranged cells with a neuronal expression pattern. In the ventral diencephalon, DA and D1A were strongly expressed on the OC, OT and OS. Furthermore, L-DOPA significantly inhibited retinal axon outgrowth in both the dorsal nasal (DN) and ventral temporal (VT) groups. DA inhibited retinal axon outgrowth, which was abolished by the D1A antagonist SCH23390. Brain slice cultures indicated that L-DOPA inhibited axons that crossed at the OC of E13 embryos, which was not abolished by DA. L-DOPA also inhibited axons that crossed at the OC of albino mice. Albino mice exhibited reduced ipsilateral retinal projections compared with C57 pigmented mice. No significant difference was identified in the uncrossed projections of albino mice following L-DOPA and DA expression. Furthermore, transcription factor Zic family member 2 (Zic2) upregulated OA1 mRNA expression. Our findings provide critical insights into DA-related signaling in retinal development.

Discovery and characterization of a G protein-biased agonist that inhibits β-arrestin recruitment to the D2 dopamine receptor

A high-throughput screening campaign was conducted to interrogate a 380,000+ small-molecule library for novel D2 dopamine receptor modulators using a calcium mobilization assay. Active agonist compounds from the primary screen were examined for orthogonal D2 dopamine receptor signaling activities including cAMP modulation and β-arrestin recruitment. Although the majority of the subsequently confirmed hits activated all signaling pathways tested, several compounds showed a diminished ability to stimulate β-arrestin recruitment. One such compound (MLS1547; 5-chloro-7-[(4-pyridin-2-ylpiperazin-1-yl)methyl]quinolin-8-ol) is a highly efficacious agonist at D2 receptor-mediated G protein-linked signaling, but does not recruit β-arrestin as demonstrated using two different assays. This compound does, however, antagonize dopamine-stimulated β-arrestin recruitment to the D2 receptor. In an effort to investigate the chemical scaffold of MLS1547 further, we characterized a set of 24 analogs of MLS1547 with respect to their ability to inhibit cAMP accumulation or stimulate β-arrestin recruitment. A number of the analogs were similar to MLS1547 in that they displayed agonist activity for inhibiting cAMP accumulation, but did not stimulate β-arrestin recruitment (i.e., they were highly biased). In contrast, other analogs displayed various degrees of G protein signaling bias. These results provided the basis to use pharmacophore modeling and molecular docking analyses to build a preliminary structure-activity relationship of the functionally selective properties of this series of compounds. In summary, we have identified and characterized a novel G protein-biased agonist of the D2 dopamine receptor and identified structural features that may contribute to its biased signaling properties.

C-terminal di-leucine motif of dopamine D₁ receptor plays an important role in its plasma membrane trafficking

The dopamine D₁ receptor (D₁R), a G protein-coupled receptor, plays a critical role in regulating blood pressure through its actions on renal hemodynamics and epithelial ion transport, which are highly linked to its intracellular trafficking. In this study, we generated a series of C-terminal mutants of D₁R that were tagged with or without enhanced yellow fluorescent protein, and analyzed the consequences of these mutants on the plasma membrane trafficking of D₁R and cyclic AMP response to D₁R stimulation. D₁R with mutations within the endocytic recycling signal (amino acid residues 360–382) continued to be functional, albeit decreased relative to wild-type D₁R. Mutation of the palmitoylation site (347C>S) of D₁R did not impair its trafficking to the plasma membrane, but abolished its ability to increase cyclic AMP accumulation. In contrast, replacement of di-leucines (344–345L>A) by alanines resulted in the retention of D₁R in the early endosome, decreased its glycosylation, and prevented its targeting to the plasma membrane. Our studies suggest that di-L motif at the C-terminus of D₁R is critical for the glycosylation and cell surface targeting of D₁R.

Dopamine D2 receptor stimulation inhibits cold-initiated thermogenesis in brown adipose tissue in conscious rats

Dopamine D(2)-like receptor agonists cause hypothermia. We investigated whether inhibiting heat production by interscapular brown adipose tissue (iBAT), a major thermogenic organ in rats, contributes to hypothermia caused by dopamine D(2)-like receptor agonists. Temperature of iBAT and tail artery blood flow were measured in conscious rats. Activity in postganglionic sympathetic nerves supplying iBAT was assessed in anesthetized rats. Conscious rats were housed in a warm cage maintained at 26-28 degrees C and then transferred to a cold cage at 5-10 degrees C to induce iBAT thermogenesis. Cold exposure increased iBAT temperature (+0.7+/-0.1 degrees C, 30 min after transferring to the cold cage, P<0.01, n=54). The mixed dopamine D(2)/D(3) receptor agonist, 7-hydroxy-2-(di-n-propylamino)tetralin (7-OH-DPAT, 0.5 mg/kg s.c.) reversed the cold-induced increase in iBAT temperature (-2.8+/-0.2 degrees C at 30 min after 7-OH-DPAT treatment during cold exposure vs. +0.3+/-0.1 degrees C at 30 min after vehicle treatment during cold exposure, n=8). These temperature changes were blocked by pre-treatment with the D(2) receptor antagonists spiperone (20 microg/kg i.p.) and L-741,626 (2.5 mg/kg i.p.), but not by the selective D(3) receptor antagonist SB-277011A (10 mg/kg i.p.). Another mixed dopamine D(2)/D(3) receptor agonist, quinpirole (0.5 mg/kg s.c.) also reversed cold-induced iBAT thermogenesis, and this effect was also prevented by pre-treatment with spiperone, but not with a peripherally acting dopamine receptor antagonist, domperidone (2 mg/kg s.c.). Neither 7-OH-DPAT nor quinpirole reversed cutaneous vasoconstriction elicited by cold exposure. In anesthetized rats, quinpirole (0.5 mg/kg i.v.) abolished iBAT sympathetic nerve discharge elicited by cooling the trunk, and this change was reversed by spiperone (20 microg/kg i.v.). These results demonstrate that activation of CNS dopamine D(2) receptors inhibits sympathetically-mediated iBAT thermogenesis in response to cold exposure. Furthermore, they suggest that in rats hypothermia induced by dopamine D(2) receptor agonists in cold environments is mainly due to decreased heat production rather than to increased heat loss.

Food reinforcement

The reinforcing value of food, measured by how hard someone is willing to work to obtain food, is influenced by food palatability, food deprivation and food variety, and may be a more powerful determinant of food intake than hedonics or liking. The reinforcing value of food is mediated in part by dopaminergic activity. Genotypes that influence dopamine transport and the density of dopamine D2 receptors interact with food reinforcement to influence eating behavior, and D2 receptor genotypes may influence food reinforcement and weight gain after smoking cessation. Inhibition of dopamine transport increases brain dopamine concentrations, which may influence weight gain after smoking cessation and can reduce energy intake in obese adults.

Renal dopaminergic mechanisms and hypertension: a chronology of advances

Dopamine (DA) has been shown to influence kidney function through endogenous synthesis and subsequent interaction with locally expressed dopamine receptor subtypes (D1, D5 as D1-like and D2, D3, and D4 as D2-like). DA, and DA-receptor specific agonists and antagonists can alter renal water and electrolyte excretion along with renin release when infused systemically or intrarenally. Such effects are brought about by a combination of renal hemodynamic and direct tubular effects evoked along the full length of the nephron. The cellular mechanisms that direct these dopamine-mediated renal electrolyte fluxes have recently been clarified and include alterations in adenylyl cyclase, phospholipase C, and phospholipase A1 activity. The dopaminergic system also interacts directly with the renal kallikrein-kinin, prostaglandin and other neurohumoral systems. Aberrant renal dopamine production and/or dopamine receptor function have been reported in salt-dependent and low-renin forms of human primary hypertension as well as in genetic models of animal hypertension, including the SHR and Dahl SS rat. DA D1 or D3 receptor knockout mice have been shown to develop hypertension.

Postnatal development of dopamine D1 receptor immunoreactivity in the rat retina

Dopamine, an important neuromodulator in the retina, controls the balance of rod cone photoreceptor activity and influences the activity of several interneurons. The postnatal development of dopaminergic neurons, visualized immunocytochemically, was compared to the development of dopamine D1 receptor immunoreactivity. Expression of D1 receptors was monitored throughout the postnatal development of the rat retina using a subtype-specific monoclonal antibody. D1 receptors are expressed in the inner plexiform layer beginning at birth. Labeling of the inner plexiform layer changed from a diffuse pattern, staining the entire layer, to the typical adult punctate staining, that was organized in layered bands and occurred in the second postnatal week. The staining did not co-localize with dopaminergic cells; instead, it colocalized with cells in the inner nuclear layer or the ganglion cell layer. Within these cells, D1 receptors were most heavily expressed in processes stratifying in the inner plexiform layer. Staining in the outer plexiform layer and in horizontal cells was found beginning in the second postnatal week. Clustering of the D1 receptor within plexiform layers, a process typical for the well-described function of dopamine modulation in the adult, occurred late in postnatal development. A possible function of D1 receptors in neuronal development is discussed.

Molecular components of striatal plasticity: the various routes of cyclic AMP pathways

Neuroplasticity serves an important role for normal striatal function and in disease states. One route to neuroplasticity involves activation of the transcription factor cyclic 3', 5'-adenosine monophosphate (cyclic AMP) response element binding protein (CREB) by phosphorylation of the amino acid 133Ser. Dopamine and glutamate, the two predominant neurotransmitters in the striatum, induce CREB phosphorylation in primary cultures of rat striatum through cyclic AMP and Ca2+ pathways. Here we present the role of N-methyl-D-aspartate receptors and Ca2+ in cyclic AMP-mediated CREB phosphorylation.

Site of dopamine D1 receptor binding to Gs protein mapped with synthetic peptides

We have synthesized peptides corresponding to the known sequence of the rat dopamine D1 receptor and measured the effects on binding of the Gs protein to the activated receptor. Peptides corresponding to the second and third cytoplasmic loop (amino acids 160-180 and 260-276), as well as the N-terminal part of the carboxy terminus (amino acids 375-391) were effective in preventing Gs binding to the activated receptor. Peptides representing other surface regions, e.g., the first cytoplasmic loop (87-100), the C-terminus (400-420 and 440-460) and all extracellular surface peptides (6-22, 120-137, 210-224 and 338-352) are not involved in rat dopamine D1 receptor-Gs protein interaction.

Regulation of dopamine D2 receptor mRNA expression in the olfactory tubercle by cocaine

The influence of acute and chronic cocaine treatment on the expression of dopamine D2 receptor (D2R) mRNA was examined in the rat basal ganglia by in situ hybridization histochemistry. No significant alterations in D2R mRNA levels were observed in the striatum or the nucleus accumbens following acute or chronic cocaine. In the olfactory tubercle, acute cocaine (20 and 40 mg/kg, i.p.) was found to decrease D2R mRNA expression. Chronic cocaine (20 mg/kg, i.p. x 15 days), however, produced an increase in D2R mRNA levels in this region which was detected 24 h, but not 7 days after withdrawal. Acute and chronic cocaine have different effects on the regulation of D2R gene expression in the olfactory tubercle.