Functional interaction between dopamine receptor subtypes for the regulation of c-fos expression

Unveiling the Differences in Signaling and Regulatory Mechanisms between Dopamine D2 and D3 Receptors and Their Impact on Behavioral Sensitization

Dopamine receptors are classified into five subtypes, with D2R and D3R playing a crucial role in regulating mood, motivation, reward, and movement. Whereas D2R are distributed widely across the brain, including regions responsible for motor functions, D3R are primarily found in specific areas related to cognitive and emotional functions, such as the nucleus accumbens, limbic system, and prefrontal cortex. Despite their high sequence homology and similar signaling pathways, D2R and D3R have distinct regulatory properties involving desensitization, endocytosis, posttranslational modification, and interactions with other cellular components. In vivo, D3R is closely associated with behavioral sensitization, which leads to increased dopaminergic responses. Behavioral sensitization is believed to result from D3R desensitization, which removes the inhibitory effect of D3R on related behaviors. Whereas D2R maintains continuous signal transduction through agonist-induced receptor phosphorylation, arrestin recruitment, and endocytosis, which recycle and resensitize desensitized receptors, D3R rarely undergoes agonist-induced endocytosis and instead is desensitized after repeated agonist exposure. In addition, D3R undergoes more extensive posttranslational modifications, such as glycosylation and palmitoylation, which are needed for its desensitization. Overall, a series of biochemical settings more closely related to D3R could be linked to D3R-mediated behavioral sensitization.

Enkephalin as a Pivotal Player in Neuroadaptations Related to Psychostimulant Addiction

Enkephalin expression is high in mesocorticolimbic areas associated with psychostimulant-induced behavioral and neurobiological effects, and may also modulate local neurotransmission in this circuit network. Psychostimulant drugs, like amphetamine and cocaine, significantly increase the content of enkephalin in these brain structures, but we do not yet understand the specific significance of this drug-induced adaptation. In this review, we summarize the neurochemical and molecular mechanism of psychostimulant-induced enkephalin activation in mesocorticolimbic brain areas, and the contribution of this opioid peptide in the pivotal neuroadaptations and long-term behavioral changes underlying psychostimulant addiction. There is evidence suggesting that adaptive changes in enkephalin content in the mesocorticolimbic circuit, induced by acute and chronic psychostimulant administration, may represent a key initial step in the long-term behavioral and neuronal plasticity induced by these drugs.

Dopamine D2-like receptor signaling suppresses human osteoclastogenesis

Dopamine, a major neurotransmitter, transmits signals via five different seven-transmembrane G protein-coupled receptors termed D1 to D5. Although the relevance of neuroendocrine system to bone metabolism has been emerging, the precise effects of dopaminergic signaling upon osteoclastogenesis remain unknown. Here, we demonstrate that human monocyte-derived osteoclast precursor cells express all dopamine-receptor subtypes. Dopamine and dopamine D2-like receptor agonists such as pramipexole and quinpirole reduced the formation of TRAP-positive multi-nucleated cells, cathepsin K mRNA expression, and pit formation area in vitro. These inhibitory effects were reversed by pre-treatment with a D2-like receptor antagonist haloperidol or a Gαi inhibitor pertussis toxin, but not with the D1-like receptor antagonist SCH-23390. Dopamine and dopamine D2-like receptor agonists, but not a D1-like receptor agonist, suppressed intracellular cAMP concentration as well as RANKL-meditated induction of c-Fos and NFATc1 mRNA expression in human osteoclast precursor cells. Finally, the dopamine D2-like receptor agonist suppressed LPS-induced osteoclast formation in murine bone marrow culture ex vivo. These findings indicate that dopaminergic signaling plays an important role in bone homeostasis via direct effects upon osteoclast differentiation and further suggest that the clinical use of neuroleptics is likely to affect bone mass.

Schizophrenia as a disorder of too little dopamine: implications for symptoms and treatment

Antipsychotics represent the first effective therapy for schizophrenia, with their benefits linked to dopamine D2 blockade. Schizophrenia was soon identified as a hyperdopaminergic disorder, and antipsychotics proved to be reasonably effective in controlling positive symptoms. However, over the years, schizophrenia has been reconceptualized more broadly, now defined as a heterogeneous disorder with multiple symptom domains. Negative and cognitive features, not particularly responsive to antipsychotic therapy, have taken on increased importance--current thinking suggests that these domains predate the onset of positive symptoms and are more closely tied to functional outcome. That they are better understood in the context of decreased dopamine activity suggests that schizophrenia may fundamentally represent a hypodopaminergic disorder. This shift in thinking has important theoretical implications from the standpoint of etiology and pathophysiology, but also clinically in terms of treatment and drug development.

Decrease of lymphoproliferative response by amphetamine is mediated by dopamine from the nucleus accumbens: influence on splenic met-enkephalin levels

Despite the mesocorticolimbic dopaminergic pathway being one of the main substrates underlying stimulating and reinforcing effects induced by psychostimulant drugs, there is little information regarding its role in their effects at the immune level. We have previously demonstrated that acute exposure to amphetamine (5 mg/kg, i.p.) induced an inhibitory effect on the splenic T-cell proliferative response, along with an increase in the methionine(met)-enkephalin content at limbic and immune levels, 4 days after drug administration. In this study, we investigated if a possible dopamine mechanism underlies these amphetamine-induced effects by administering D1 and D2 dopaminergic antagonists or a dopaminergic terminal neurotoxin before the drug. Pre-treatment with either SCH-23390 (0.1 mg/kg, i.p.) or raclopride (0.1 mg/kg, i.p.), a D1 or D2 dopaminergic receptor antagonist, respectively, abrogated the effects of amphetamine on the lymphoproliferative response and on met-enkephalin levels of the spleen. The amphetamine-induced increase in limbic met-enkephalin content was suppressed by SCH-23390 but not by raclopride pre-treatment. Finally, an intra-accumbens 6-hydroxy-dopamine injection administered 2 weeks previously prevented amphetamine-induced effects on the lymphoproliferative response and on met-enkephalin levels in the prefrontal cortex and spleen. These findings strongly suggest that D1 and D2 dopaminergic receptors are involved in amphetamine-induced effects at immune level as regards the lymphoproliferative response and the changes in spleen met-enkephalin content, whereas limbic met-enkephalin levels were modulated only by the D1 dopaminergic receptors. In addition, this study showed that a mesolimbic component modulated amphetamine-induced effects on the immune response, as previously shown at a behavioral level.

PiggyBac transposon-mediated, reversible gene transfer in human embryonic stem cells

Permanent and reversible genetic modifications are important approaches to study gene function in different cell types. They are also important for stem cell researchers to explore and test the therapeutic potential of stem cells. The piggyBac transposon from insects is a rising nonviral system that efficiently mutagenizes and mediates gene transfer into the mammalian genome. It is also characterized by its precise excision, leaving no trace sequence behind so that the genomic integrity of the mutated cell can be restored. Here, we use an optimized piggyBac transposon system to mediate gene transfer and expression of a bifunctional fluorescent reporter in human embryonic stem (ES) cells. We provide molecular evidence for transposase-mediated piggyBac integration events and functional evidence for successful expression of a transferred fluorescent protein genes in human ES cells and their in vitro differentiated derivatives. We also demonstrate that the integrated piggyBac transposon can be removed and an undisrupted insertion site can be restored, which implies potential applications for its use in gene therapy and genetics studies.

Trpc2 gene impacts on maternal aggression, accessory olfactory bulb anatomy and brain activity

The Trpc2 gene codes for an ion channel found in the vomeronasal organ (VNO). Studies using the Trpc2(-/-) (KO) mouse have exploited the gene's role in signal transduction to explore the VNO's role in pheromonally mediated behaviors. To date, no study has evaluated the impact of the Trpc2 gene on activity within the brain. In this study, we examine the gene's effect on brain regions governing maternal aggression. We intruder-tested lactating dams and then quantified Fos immunoreactivity (Fos-IR) in the vomeronasal amygdala, hypothalamus, olfactory regions and accessory olfactory bulb (AOB). Our data confirm previous reports that loss of the Trpc2 gene severely diminishes maternal aggression. We also show that deletion of the gene results in differential hypotrophy of the glomerular layer (GlA) of the AOB, with the anterior portion the GlA resembling that of wild-type mice, and the posterior portion reduced or absent. This anatomy is suggestive of residual functioning in the apical VNO of these animals. Our Fos study describes an impact of the deletion on a network of 21 brain regions involved in emotion, aggression and olfaction, suggesting that signals from the VNO mediate activity throughout the brain. Home-cage observations of KO dams show specific deficits in nest-building, suggesting a role for pup pheromones in inducing and maintaining pup-directed maternal behaviors as well as maternal aggression.

Plasticity of addiction: a mesolimbic dopamine short-circuit?

The development of drug addiction progresses along a continuum from acute drug use to compulsive use and drug seeking behavior. Many researchers have focused on identifying the physiological mechanisms involved in drug addiction in order to develop effective pharmacotherapies. Neuroplasticity, the putative mechanism underlying learning and memory, is modified by drugs of abuse and may contribute to the development of the eventual addicted state. Innovative treatments directly targeting these drug-induced changes in brain reward components and circuits may be efficacious in reducing drug use and relapse.

Dopamine, kidney, and hypertension: studies in dopamine receptor knockout mice

Dopamine is important in the pathogenesis of hypertension because of abnormalities in receptor-mediated regulation of renal sodium transport. Dopamine receptors are classified into D(1)-like (D(1), D(5)) and D(2)-like (D(2), D(3), D(4)) subtypes, all of which are expressed in the kidney. Mice deficient in specific dopamine receptors have been generated to provide holistic assessment on the varying physiological roles of each receptor subtype. This review examines recent studies on these mutant mouse models and evaluates the impact of individual dopamine receptor subtypes on blood pressure regulation.

Roles of conserved intracellular sequences regions for the proper expression of dopamine D(2) and D(3) receptors

The dopamine D(2) receptor and D(3) receptor (D(2)R, D(3)R) have high homology in both their amino acid composition and signaling pathways. Virtually all signaling pathways reported thus far overlap between the two receptors with the exception that the D(3)R signals are 2 approximately 5 times less efficient than D(2)R. Previous studies have suggested that conformational constraints of D(3)R might be responsible for the poor coupling with the G protein. To this hypothesis, point mutations were introduced into some of the conserved regions between D(2)R and D(3)R, and their effects on receptor expression were investigated. Among the four conserved intracellular receptor regions examined (TTT motif in the 1(st) intracellular loop, SS motif in the 2(nd) intracellular loop, YxxL and TxxS/xS motifs in the 3(rd) intracellular loop), a mutation of the Thr-Thr-Thr (TTT) motif in the first intracellular loop or the LxxY motif in the 3(rd) intracellular loop markedly decreased the level of D(3)R expression compared with D(2)R. The TTT motif was further mutated individually or in combination to test which residue plays a critical role on the expression of the receptor proteins. Different amino acids between D(2)R and D(3)R in the 1(st) intracellular loop were exchanged to determine if the adjacent amino acid residues are responsible for the differences between D(2)R and D(3)R. The first two threonine residues become more important when the individual threonine residue is mutated. However, all three intact threonine residues are essential for proper expression of the receptor proteins. The neighboring sequences around the triplet threonine residues in the 1(st) loop of D(3)R are not important for proper positioning of the receptor proteins on the plasma membrane. It was concluded that D(2)R has a more flexible overall conformation that can accept mutated residues in the intracellular region than D(3)R, which might be partly responsible for the quantitative differences in the signaling efficiency between D(2)R and D(3)R.

Involvement of extracellular signal-regulated protein kinase in acute cocaine-induced c-fos in nucleus accumbens

It is well known that acute cocaine administration increases c-Fos expression that is involved in cocaine-induced persistent changes in the central nervous system. In the present study, we investigated a possible involvement of extracellular signal-regulated protein kinase (ERK) in induction of c-fos expression in response to acute cocaine treatment in nucleus accumbens (NAc). We found that inhibition of ERK activation significantly attenuated cocaine-induced c-fos expression at both protein and mRNA levels in the NAc. Furthermore, using an immunofluorescent staining approach, we found that inhibition of ERK activation completely abolished cocaine-induced increase in number of c-Fos-positive cells in the core region of NAc, whereas, in shell region of NAc, inhibition of ERK activation partially attenuated cocaine-induced c-Fos expression. Our findings suggest that ERK might participate in cocaine-induced c-fos expression in the NAc, particularly in the core region of NAc.

Dysregulation of dopamine-dependent mechanisms as a determinant of hypertension: studies in dopamine receptor knockout mice

Dopamine plays an important role in the pathogenesis of hypertension by regulating epithelial sodium transport and by interacting with vasoactive hormones/humoral factors, such as aldosterone, angiotensin, catecholamines, endothelin, oxytocin, prolactin pro-opiomelancortin, reactive oxygen species, renin, and vasopressin. Dopamine receptors are classified into D(1)-like (D(1) and D(5)) and D(2)-like (D(2), D(3), and D(4)) subtypes based on their structure and pharmacology. In recent years, mice deficient in one or more of the five dopamine receptor subtypes have been generated, leading to a better understanding of the physiological role of each of the dopamine receptor subtypes. This review summarizes the results from studies of various dopamine receptor mutant mice on the role of individual dopamine receptor subtypes and their interactions with other G protein-coupled receptors in the regulation of blood pressure.