Mechanisms of signal transduction at the dopamine D2 receptor

Three paradoxes related to the mode of action of pramipexole: The path from D2/D3 dopamine receptor stimulation to modification of dopamine-modulated functions

Pramipexole, a D2/D3 dopamine receptor agonist, is used to treat the motor symptoms of Parkinson's disease, caused by degeneration of the dopaminergic nigrostriatal pathway. There are three paradoxes associated with its mode of action. Firstly, stimulation of D2/D3 receptors leads to neuronal inhibition, although pramipexole does not inhibit but promotes some dopamine-modulated functions, such as locomotion and reinforcement. Secondly, another dopamine-modulated function, arousal, is not promoted but inhibited by pramipexole, leading to sedation. Thirdly, pramipexole-evoked sedation is associated with an increase in pupil diameter, although sedation is expected to cause pupil constriction. To resolve these paradoxes, the path from stimulation of D2/D3 receptors to the modification of dopamine-modulated functions has been tracked. The functions considered are modulated by midbrain dopaminergic nuclei: locomotion - substantia nigra pars compacta (SNc), reinforcement/motivation - ventral tegmental area (VTA), sympathetic activity (as reflected in pupil function) - VTA; arousal - ventral periaqueductal grey (vPAG), with contributions from VTA and SNc. The application of genetics-based molecular techniques (optogenetics and chemogenetics) has enabled tracing the chains of neurones from the dopaminergic nuclei to their final targets executing the functions. The functional neuronal circuits linked to the D2/D3 receptors in the dorsal and ventral striata, stimulated by inputs from SNc and VTA, respectively, may explain how neuronal inhibition induced by pramipexole is translated into the promotion of locomotion, reinforcement/motivation and sympathetic activity. As the vPAG may increase arousal mainly by stimulating cortical D1 dopamine receptors, pramipexole would stimulate only presynaptic D2/D3 receptors on vPAG neurones, curtailing their activity and leading to sedation.

Receptor-Mediated AKT/PI3K Signalling and Behavioural Alterations in Zebrafish Larvae Reveal Association between Schizophrenia and Opioid Use Disorder

The link between substance abuse and the development of schizophrenia remains elusive. In this study, we assessed the molecular and behavioural alterations associated with schizophrenia, opioid addiction, and opioid withdrawal using zebrafish as a biological model. Larvae of 2 days post fertilization (dpf) were exposed to domperidone (DMP), a dopamine-D2 dopamine D2 receptor antagonist, and morphine for 3 days and 10 days, respectively. MK801, an N-methyl-D-aspartate (NMDA) receptor antagonist, served as a positive control to mimic schizophrenia-like behaviour. The withdrawal syndrome was assessed 5 days after the termination of morphine treatment. The expressions of schizophrenia susceptibility genes, i.e., pi3k, akt1, slc6a4, creb1 and adamts2, in brains were quantified, and the levels of whole-body cyclic adenosine monophosphate (cAMP), serotonin and cortisol were measured. The aggressiveness of larvae was observed using the mirror biting test. After the short-term treatment with DMP and morphine, all studied genes were not differentially expressed. As for the long-term exposure, akt1 was downregulated by DMP and morphine. Downregulation of pi3k and slc6a4 was observed in the morphine-treated larvae, whereas creb1 and adamts2 were upregulated by DMP. The levels of cAMP and cortisol were elevated after 3 days, whereas significant increases were observed in all of the biochemical tests after 10 days. Compared to controls, increased aggression was observed in the DMP-, but not morphine-, treated group. These two groups showed reduction in aggressiveness when drug exposure was prolonged. Both the short- and long-term morphine withdrawal groups showed downregulation in all genes examined except creb1, suggesting dysregulated reward circuitry function. These results suggest that biochemical and behavioural alterations in schizophrenia-like symptoms and opioid dependence could be controlled by common mechanisms.

Synthesis and Evaluation of Novel Tetrahydronaphthyridine CXCR4 Antagonists with Improved Drug-like Profiles

Our first-generation CXCR4 antagonist TIQ15 was rationally modified to improve drug-like properties. Introducing a nitrogen atom into the aromatic portion of the tetrahydroisoquinoline ring led to several heterocyclic variants including the 5,6,7,8-tetrahydro-1,6-naphthyridine series, greatly reducing the inhibition of the CYP 2D6 enzyme. Compound 12a demonstrated the best overall properties after profiling a series of isomeric tetrahydronaphthyridine analogues in a battery of biochemical assays including CXCR4 antagonism, CYP 2D6 inhibition, metabolic stability, and permeability. The butyl amine side chain of 12a was substituted with various lipophilic groups to improve the permeability. These efforts culminated in the discovery of compound 30 as a potent CXCR4 antagonist (IC50 = 24 nM) with diminished CYP 2D6 activity, improved PAMPA permeability (309 nm/s), potent inhibition of human immunodeficiency virus entry (IC50 = 7 nM), a cleaner off-target in vitro safety profile, lower human ether a-go-go-related gene channel activity, and higher oral bioavailability in mice (% FPO = 27) compared to AMD11070 and TIQ15.

α-Synuclein-induced dysregulation of neuronal activity contributes to murine dopamine neuron vulnerability

Pathophysiological damages and loss of function of dopamine neurons precede their demise and contribute to the early phases of Parkinson's disease. The presence of aberrant intracellular pathological inclusions of the protein α-synuclein within ventral midbrain dopaminergic neurons is one of the cardinal features of Parkinson's disease. We employed molecular biology, electrophysiology, and live-cell imaging to investigate how excessive α-synuclein expression alters multiple characteristics of dopaminergic neuronal dynamics and dopamine transmission in cultured dopamine neurons conditionally expressing GCaMP6f. We found that overexpression of α-synuclein in mouse (male and female) dopaminergic neurons altered neuronal firing properties, calcium dynamics, dopamine release, protein expression, and morphology. Moreover, prolonged exposure to the D2 receptor agonist, quinpirole, rescues many of the alterations induced by α-synuclein overexpression. These studies demonstrate that α-synuclein dysregulation of neuronal activity contributes to the vulnerability of dopaminergic neurons and that modulation of D2 receptor activity can ameliorate the pathophysiology. These findings provide mechanistic insights into the insidious changes in dopaminergic neuronal activity and neuronal loss that characterize Parkinson's disease progression with significant therapeutic implications.

Vascular tone and angiogenesis modulation by catecholamine coordinated to ruthenium

Catecholamines participate in angiogenesis, an important tumor development process. However, the way catecholamines interact with their receptors has not been completely elucidated, and doubts still remain as to whether these interactions occur between catechol and/or amine sites and particular amino acid residues on the catecholamine receptors. To evaluate how catechol and amine groups contribute to angiogenesis, we immobilized the catechol site through ruthenium ion (Ru) coordination, to obtain species with the general formula [Ru(NH₃)₄(catecholamine-R)]Cl. We then assessed the angiogenic activity of the complexes in a chorioallantoic membrane model (CAM) and examined vascular reactivity and calcium mobilization in rat aortas and vascular cells. [Ru(NH₃)₄(catecholamine-R)]Cl acted as partial agonists and/or antagonists of their respective receptors and induced calcium mobilization. [Ru(NH₃)₄(isoproterenol)]⁺ [Ru(NH₃)₄(noradrenaline)]⁺, and [Ru(NH₃)₄(adrenaline)]⁺ behaved as antiangiogenic complexes, whereas [Ru(NH₃)₄(dopamine)]⁺ proved to be a proangiogenic complex. In conclusion, catecholamines and [Ru(NH₃)₄(catecholamine-R)]Cl can modulate angiogenesis, and catechol group availability can modify the way these complexes impact the vascular tone, suggesting that catecholamines and their receptors interact differently after catecholamine coordination to ruthenium.

Methamphetamine regulation of activity and topology of ventral midbrain networks

The ventral midbrain supports a variety of functions through the heterogeneity of neurons. Dopaminergic and GABA neurons within this region are particularly susceptible targets of amphetamine-class psychostimulants such as methamphetamine. While this has been evidenced through single-neuron methods, it remains unclear whether and to what extent the local neuronal network is affected and if so, by which mechanisms. Both GABAergic and dopaminergic neurons were heavily featured within the primary ventral midbrain network model system. Using spontaneous calcium activity, our data suggest methamphetamine decreased total network output via a D2 receptor-dependent manner. Over culture duration, functional connectivity between neurons decreased significantly but was unaffected by methamphetamine. However, across culture duration, exposure to methamphetamine significantly altered changes in network assortativity. Here we have established primary ventral midbrain networks culture as a viable model system that reveals specific changes in network activity, connectivity, and topology modulation by methamphetamine. This network culture system enables control over the type and number of neurons that comprise a network and facilitates detection of emergent properties that arise from the specific organization. Thus, the multidimensional properties of methamphetamine can be unraveled, leading to a better understanding of its impact on the local network structure and function.

A Differential Hypofunctionality of Gαi Proteins Occurs in Adolescent Idiopathic Scoliosis and Correlates with the Risk of Disease Progression

Adolescent idiopathic scoliosis is the most prevalent spine deformity and the molecular mechanisms underlying its pathophysiology remain poorly understood. We have previously found a differential impairment of melatonin receptor signaling in AIS osteoblasts allowing the classification of patients into three biological endophenotypes or functional groups (FG1, FG2 and FG3). Here, we provide evidence that the defect characterizing each endophenotype lies at the level of Gαi proteins leading to a systemic and generalized differential impairment of Gi-coupled receptor signaling. The three Gαi isoforms exhibited a selective serine phosphorylation patterns for each AIS endophenotype resulting in a differential reduction in Gαi protein activity as determined by cellular dielectric spectroscopy and small interfering RNA methods. We found that one endophenotype (FG2) with phosphorylated Gαi₁ and Gαi₂ was consistently associated with a significantly high risk of spinal deformity progression when compared to the other two endophenotypes (FG1 and FG3). We further demonstrated that each endophenotype is conserved among affected family members. This study expands our understanding of the mechanism underlying the Gi-coupled receptor signaling dysfunction occurring in AIS and provides the first evidence for its hereditary nature. Collectively, our findings offers a new perspective on Gαi hypofunctionality in a human disease by revealing specific serine phosphorylation signatures of Gαi isoforms that may facilitate the identification of AIS patients at risk of spinal deformity progression.

Eckol as a Potential Therapeutic against Neurodegenerative Diseases Targeting Dopamine D₃/D₄ Receptors

The G protein-coupled receptor (GPCR) family of proteins comprises signaling proteins that mediate cellular responses to various hormones and neurotransmitters, and serves as a prime target for drug discovery. Towards our goal of discovering secondary metabolites from natural sources that can function as neuronal drugs, we evaluated the modulatory effect of eckol on various GPCRs via cell-based functional assays. In addition, we conducted in silico predictions to obtain molecular insights into the functional effects of eckol. Functional assays revealed that eckol had a concentration-dependent agonist effect on dopamine D₃ and D₄ receptors. The half maximal effective concentration (EC₅₀) of eckol for the dopamine D₃ and D₄ receptors was 48.62 ± 3.21 and 42.55 ± 2.54 µM, respectively, while the EC₅₀ values of dopamine as a reference agonist for these two receptors were 2.9 and 3.3 nM, respectively. In silico studies revealed that a low binding energy in addition to hydrophilic, hydrophobic, π⁻alkyl, and π⁻π T-shaped interactions are potential mechanisms by which eckol binds to the dopamine receptors to exert its agonist effects. Molecular dynamics (MD) simulation revealed that Phe346 of the dopamine receptors is important for binding of eckol, similar to eticlopride and dopamine. Our results collectively suggest that eckol is a potential D₃/D₄ agonist for the management of neurodegenerative diseases, such as Parkinson's disease.

Neuronal Surface Autoantibodies in Neuropsychiatric Disorders: Are There Implications for Depression?

Autoimmune diseases are affecting around 7.6-9.4% of the general population. A number of central nervous system disorders, including encephalitis and severe psychiatric disorders, have been demonstrated to associate with specific neuronal surface autoantibodies (NSAbs). It has become clear that specific autoantibodies targeting neuronal surface antigens and ion channels could cause severe mental disturbances. A number of studies have focused or are currently investigating the presence of autoantibodies in specific mental conditions such as schizophrenia and bipolar disorders. However, less is known about other conditions such as depression. Depression is a psychiatric disorder with complex etiology and pathogenesis. The diagnosis criteria of depression are largely based on symptoms but not on the origin of the disease. The question which arises is whether in a subgroup of patients with depression, the symptoms might be caused by autoantibodies targeting membrane-associated antigens. Here, we describe how autoantibodies targeting membrane proteins and ion channels cause pathological effects. We discuss the physiology of these antigens and their role in relation to depression. Finally, we summarize a number of studies detecting NSAbs with a special focus on cohorts that include depression diagnosis and/or show depressive symptoms.

New Concepts in Dopamine D2 Receptor Biased Signaling and Implications for Schizophrenia Therapy

The dopamine D₂ receptor (D₂R) is a G protein-coupled receptor that is a common target for antipsychotic drugs. Antagonism of D₂R signaling in the striatum is thought to be the primary mode of action of antipsychotic drugs in alleviating psychotic symptoms. However, antipsychotic drugs are not clinically effective at reversing cortical-related symptoms, such as cognitive deficits in schizophrenia. While the exact mechanistic underpinnings of these cognitive deficits are largely unknown, deficits in cortical dopamine function likely play a contributing role. It is now recognized that similar to most G protein-coupled receptors, D₂Rs signal not only through canonical G protein pathways but also through noncanonical beta-arrestin2-dependent pathways. We review the current mechanistic bases for this dual signaling mode of D₂Rs and how these new concepts might be leveraged for therapeutic gain to target both cortical and striatal dysfunction in dopamine neurotransmission and hence have the potential to correct both positive and cognitive symptoms of schizophrenia.

Dopamine or biopterin deficiency potentiates phosphorylation at (40)Ser and ubiquitination of tyrosine hydroxylase to be degraded by the ubiquitin proteasome system

The protein amount of tyrosine hydroxylase (TH), that is the rate-limiting enzyme for the biosynthesis of dopamine (DA), should be tightly regulated, whereas its degradation pathway is largely unknown. In this study, we analyzed how the TH protein is chemically modified and subsequently degraded under deficiencies of DA and tetrahydrobiopterin (BH4), a cofactor for TH, by using pharmacological agents in PC12D cells and cultured mesencephalic neurons. When inhibition of DA- or BH4-synthesizing enzymes greatly reduced the DA contents in PC12D cells, a marked and persistent increase in phosphorylated TH at (40)Ser (p40-TH) was concomitantly observed. This phosphorylation was mediated by D2 dopamine auto-receptor and cAMP-dependent protein kinase (PKA). Our immunoprecipitation experiments showed that the increase in the p40-TH level was accompanied with its poly-ubiquitination. Treatment of PC12D cells with cycloheximide showed that total-TH protein level was reduced by the DA- or BH4-depletion. Notably, this reduction in the total-TH protein level was sensitive not only to a 26S proteasomal inhibitor, MG-132, but also to a PKA inhibitor, H-89. These data demonstrated that DA deficiency should induce compensatory activation of TH via phosphorylation at (40)Ser through D2-autoreceptor and PKA-mediated pathways, which in turn give a rise to its degradation through an ubiquitin-proteasome pathway, resulting in a negative spiral of DA production when DA deficiency persists.

Serotonin 1A receptor agonist increases species- and region-selective adult CNS proliferation, but not through CNTF

Endogenous ciliary neurotrophic factor (CNTF)(1) regulates neurogenesis of the adult brain in the hippocampal subgranular zone (SGZ)(2) and the subventricular zone (SVZ)(3). We have previously shown that the cAMP-inhibiting D2 dopamine receptor increases neurogenesis by inducing astroglial CNTF expression. Here, we investigated the potential role of CNTF in the proliferative response to pharmacological stimulation of the serotonin 1A (5-HT1A)(4) receptor, which also inhibits cAMP, in adult mice and rats. Like others, we show that systemic treatment with the active R-enantiomer of the 5-HT1A agonist 8-Hydroxy-2-(di-n-propylamino)tetralin (8-OH-DPAT)(5) induces proliferation in the SGZ in rats using unbiased stereology of 5-Bromo-2'-deoxyuridine (BrdU)(6) positive nuclei. However, despite the bioactivity of R-8-OH-DPAT, as also shown by a decrease in hippocampal nNOS(7) mRNA levels, it did not increase CNTF mRNA as shown by highly specific quantitative RT-PCR (qPCR)(8). Surprisingly, R-8-OH-DPAT did not cause an increase in SVZ proliferation in rats or in either the SVZ or SGZ of two different strains of mice, C57BL/6J, and 129SvEv, using acute or chronic treatments. There also were no changes in CNTF mRNA, and also not in mice treated with a widely used racemic mixture of 8-OH-DPAT, higher doses or after intracerebral injection, which reduced nNOS. In contrast to the others, we propose that the 5-HT1A receptor might be non-functional in mice with regards to regulating normal neurogenesis and has region-selective activities in rats. These species- and region-specific actions raise important questions about the role of the 5-HT1A receptor in human neurogenesis and its implications for the field of depression.

Estrogen inhibits D2S receptor-regulated Gi3 and Gs protein interactions to stimulate prolactin production and cell proliferation in lactotropic cells

The neurotransmitter dopamine (DA) is known to inhibit prolactin (PRL) secretion and the proliferation of lactotropes in the pituitary gland. Dopamine-2 (D2) receptor short (D2S) isoform is expressed in a reduced level while the D2 receptor long (D2L) isoform is expressed in an elevated level during estradiol (E(2))-induced PRL production and cell proliferation in lactotropes. To evaluate the role of these D2 receptor isoforms in E(2)-regulated lactotropic cell function, we compared E(2) effects on the level of PRL, cell proliferation, and G proteins in enriched lactotropes and lactotrope-derived PR1 cells containing only D2S isoform (D2S cells), D2L isoform (D2L cells), or no D2 receptor (V cells). Additionally, we determined the effects of G protein blockade on the E(2)-induced PRL production and cell proliferation in these cells. We here show that E(2) actions on G proteins, PRL production, and cell proliferation were maximally achieved in D2S cells, oppositely or marginally achieved in D2L cells, and absent in V cells. We also show that the DA and pertussis toxin modulations of E(2) actions on PRL, G proteins, and cell proliferation were maximally achieved in D2S cells compared with in D2L or V cells. Furthermore, we provide evidence for the existence of an inhibitory action of Gi3 on Gs that is under the control of the D2S receptor and is inhibited by E(2). These results suggest that the suppression of D2S-regulated Gi3 inhibition of Gs protein may be one of the mechanisms controlling E(2)-activated PRL synthesis and cell proliferation in lactotropes.

Maladaptive striatal plasticity in L-DOPA-induced dyskinesia

Dopamine (DA) replacement therapy with l-DOPA remains the most effective treatment for Parkinson's disease, but causes dyskinesia (abnormal involuntary movements) in the vast majority of the patients. The basic mechanisms of l-DOPA-induced dyskinesia (LID) have become the object of intense research focusing on neurochemical and molecular adaptations in the striatum. Here we review this vast literature and highlight trends that converge into a unifying pathophysiological interpretation. We propose that the core molecular alteration of striatal neurons in LID consists in an inability to turn down supersensitive signaling responses downstream of DA D1 receptors (where supersensitivity is primarily caused by DA denervation). The sustained activation of intracellular signaling pathways induced by each dose of l-DOPA leads to abnormal cellular plasticity and high bioenergetic expenditure. The over-exploitation of signaling pathways and energy reserves during treatment impairs the ability of striatal neurons to dynamically gate cortically driven motor commands. LID thus exemplifies a disorder where 'too much' molecular plasticity leads to plasticity failure in the striatum.

Lead and cognitive function in ALAD genotypes in the third National Health and Nutrition Examination Survey

The relationship between the blood lead concentration and cognitive function in children and adults with different ALAD genotypes who participated in the third National Health and Nutrition Examination Survey was investigated. The relationship between blood lead and serum homocysteine concentrations was also investigated. In children 12 to 16 years old, no difference in the relationship between cognitive function and blood lead concentration between genotypes was found. In adults 20 to 59 years old, mean reaction time decreased as the blood lead concentration increased in the ALAD rs1800435 CC/CG group. This represents an improvement in performance. In adults 60 years and older, no difference in the relationship between cognitive function and blood lead concentration between genotypes was found. The serum homocysteine concentration increased as the blood lead concentration increased in adults 20 to 59 years old and 60 years and older, but there were no differences between genotypes. The mean blood lead concentration of children with the ALAD rs1800435 CC/CG genotype was less than that of children with the GG genotype.

Persistent cognitive dysfunction after traumatic brain injury: A dopamine hypothesis

Traumatic brain injury (TBI) represents a significant cause of death and disability in industrialized countries. Of particular importance to patients the chronic effect that TBI has on cognitive function. Therapeutic strategies have been difficult to evaluate because of the complexity of injuries and variety of patient presentations within a TBI population. However, pharmacotherapies targeting dopamine (DA) have consistently shown benefits in attention, behavioral outcome, executive function, and memory. Still it remains unclear what aspect of TBI pathology is targeted by DA therapies and what time-course of treatment is most beneficial for patient outcomes. Fortunately, ongoing research in animal models has begun to elucidate the pathophysiology of DA alterations after TBI. The purpose of this review is to discuss clinical and experimental research examining DAergic therapies after TBI, which will in turn elucidate the importance of DA for cognitive function/dysfunction after TBI as well as highlight the areas that require further study.

Alteration in G proteins and prolactin levels in pituitary after ethanol and estrogen treatment

BACKGROUND

Chronic administration of ethanol increases plasma prolactin levels and enhances estradiol's mitogenic action on the lactotropes of the pituitary gland. The present study was conducted to determine the changes in the pituitary levels of G proteins during the tumor development following alcohol and ethanol treatments.

METHODS

Using ovariectomized Fischer-344 female rats, we have determined ethanol and estradiol actions at 2 and 4 weeks on pituitary weight and pituitary cell contents of prolactin, Gs. Gq11, Gi1, Gi2, and Gi3 proteins. Western blots were employed to measure protein contents.

RESULTS

Ethanol increased basal and estradiol-enhanced wet weight and the prolactin content in the pituitary in a time-dependent manner. Chronic exposure of estradiol increased the levels of Gs protein in the pituitary. Unlike estradiol, ethanol exposure did not show significant effect on the basal level of Gs protein, but moderately increased the estradiol-induced levels of this protein. Estradiol exposure enhanced Gq11 protein levels in the pituitary after 2 and 4 weeks, while ethanol treatment failed to alter these protein levels in the pituitary in control-treated or estradiol-treated ovariectomized rats. In the case of Gi1, estradiol but not ethanol increased the level of this protein at 4 weeks of treatment. However, estradiol and ethanol alone reduced the levels of both Gi2 and Gi3 proteins at 2 and 4 weeks of treatment. Ethanol also significantly reduced the estradiol-induced Gi2 levels at 4 weeks and Gi3 level at 2 and 4 weeks.

CONCLUSIONS

These results confirm ethanol's and estradiol's growth-promoting and prolactin stimulating actions on lactotropes of the pituitary and further provide evidence that ethanol and estradiol may control lactotropic cell functions by altering expression of specific group of G proteins in the pituitary.

Ciliary neurotrophic factor mediates dopamine D2 receptor-induced CNS neurogenesis in adult mice

Neurogenesis continues in the adult forebrain subventricular zone (SVZ) and the dentate gyrus of the hippocampal formation. Degeneration of dopaminergic projections in Parkinson's disease and animals reduces, whereas ciliary neurotrophic factor (CNTF) promotes, neurogenesis. We tested whether the dopaminergic system promotes neurogenesis through CNTF. Astrocytes of the SVZ and dentate gyrus expressed CNTF and were close to dopaminergic terminals. Dopaminergic denervation in adult mice reduced CNTF mRNA by approximately 60%, whereas systemic treatment with the D2 agonist quinpirole increased CNTF mRNA in the SVZ and hippocampal formation, and in cultured astrocytes by 1.5-5 fold. The effect of quinpirole in vitro was blocked by the D2 antagonist eticlopride and did not cause astroglial proliferation or hypertrophy. Systemic quinpirole injections increased proliferation in wild-type mice by approximately 25-75% but not in CNTF-/- littermates or in the SVZ of mice infused with CNTF antibodies. Quinpirole increased the number of neuroblasts in wild-type but not in CNTF-/- littermates. Neurogenesis was reduced by approximately 20% in CNTF-/- mice, confirming the endogenous role of CNTF. Nigrostriatal denervation did not affect SVZ proliferation in CNTF-/- mice, suggesting that the dopaminergic innervation normally regulates neurogenesis through CNTF. Quinpirole acted on postsynaptic receptors as it reversed the reduced proliferation seen after dopaminergic denervation in wild-type mice. Thus, CNTF mediates dopaminergic innervation- and D2 receptor-induced neurogenesis in the adult forebrain. Because CNTF is predominantly expressed in the nervous system, this mechanism and the ability to pharmacologically modulate it have implications for Parkinson's disease and cell-replacement therapies for other disorders.

Dopamine D2 receptor partial agonists display differential or contrasting characteristics in membrane and cell-based assays of dopamine D2 receptor signaling

Clinical evidence suggests that dopamine D(2) receptor partial agonists must have a sufficiently low intrinsic activity to be effective as antipsychotics. Here, we used dopamine D(2) receptor signaling assays to compare the in vitro functional characteristics of the antipsychotic aripiprazole with other dopamine D(2) receptor partial agonists (7-{3-[4-(2,3-dimethylphenyl)-piperazinyl]propoxy}-2(1H)-quinolinone [OPC-4392], (-)-3-(3-hydroxy-phenyl)-N-n-propylpiperidine [(-)3-PPP] and (+)terguride) and dopamine D(2) receptor antagonists. Aripiprazole and OPC-4392 were inactive in a guanosine-5'-O-(3-[(35)S]thio)-triphosphate ([(35)S]GTPgammaS) binding assay using Chinese Hamster Ovary (CHO) cell membranes expressing cloned human dopamine D(2Long) (hD(2L)) receptors, whereas (-)3-PPP and (+)terguride displayed low intrinsic activity. Aripiprazole also had no effect on [(35)S]GTPgammaS binding to CHO-hD(2L) cells, while OPC-4392, (-)3-PPP and (+)terguride were partial agonists. In contrast, aripiprazole, OPC-4392, (-)3-PPP, and (+)terguride were inactive in a [(35)S]GTPgammaS binding assay using rat striatal membranes. However, at a more downstream level of CHO-hD(2L) cell signalling, these drugs all behaved as dopamine hD(2L) receptor partial agonists, with aripiprazole displaying an intrinsic activity 2 to 3-fold lower (inhibition of forskolin-induced adenosine 3',5'-cyclic monophosphate accumulation) and almost half as high (enhancement of adenosine triphosphate-stimulated [(3)H]arachidonic acid release) as OPC-4392, (-)3-PPP and (+)terguride. Dopamine activity was blocked in each case by (-)raclopride, which was inactive on its own in every assay, as were the antipsychotics haloperidol, olanzapine, ziprasidone and clozapine. Together, these data, whilst preclinical in nature, are consistent with clinical evidence suggesting the favorable antipsychotic profile of aripiprazole, compared with the other clinically ineffective partial agonists, is dependent on its low intrinsic activity at dopamine D(2) receptors. This study also highlights the limitations of using [(35)S]GTPgammaS binding assays to identify dopamine D(2) receptor partial agonists.

Advances in the treatment of prolactinomas

Prolactinomas account for approximately 40% of all pituitary adenomas and are an important cause of hypogonadism and infertility. The ultimate goal of therapy for prolactinomas is restoration or achievement of eugonadism through the normalization of hyperprolactinemia and control of tumor mass. Medical therapy with dopamine agonists is highly effective in the majority of cases and represents the mainstay of therapy. Recent data indicating successful withdrawal of these agents in a subset of patients challenge the previously held concept that medical therapy is a lifelong requirement. Complicated situations, such as those encountered in resistance to dopamine agonists, pregnancy, and giant or malignant prolactinomas, may require multimodal therapy involving surgery, radiotherapy, or both. Progress in elucidating the mechanisms underlying the pathogenesis of prolactinomas may enable future development of novel molecular therapies for treatment-resistant cases. This review provides a critical analysis of the efficacy and safety of the various modes of therapy available for the treatment of patients with prolactinomas with an emphasis on challenging situations, a discussion of the data regarding withdrawal of medical therapy, and a foreshadowing of novel approaches to therapy that may become available in the future.

The role of hormones, growth factors and their receptors in pituitary tumorigenesis

Numerous factors have been shown to govern adenohypophysial cell proliferation. Human and animal models have documented that the hypothalamic trophic hormone growth hormone-releasing hormone stimulates cell proliferation, and prolonged stimulation leads to tumor formation. Similarly, lack of dopaminergic inhibition of lactotrophs and lack of feedback suppression by adrenal, gonadal or thyroid hormones are implicated, perhaps through hypothalamic stimulatory mechanisms, in pituitary adenoma formation superimposed on hyperplasia. However, most pituitary tumors are not associated with underlying hyperplasia. Overexpression of growth factors and their receptors, such as EGF, TGFalpha, EGF-R and VEGF has been identified in pituitary adenomas, and reduction of follistatin expression has been implicated in gonadotroph adenomas. Aberrant expression of members of the FGF family, an FGF antisense gene and FGF receptors have all been described in pituitary adenomas. The clonal composition of pituitary adenomas attests to the molecular basis of pituitary tumorigenesis, however, the evidence suggests that these various hypophysiotropic hormones and growth factors likely play a role as promoters of tumor cell growth in genetically transformed cells.

Mechanism of D(2) agonist-induced inhibition of GH secretion from human GH-secreting adenoma cells

The mechanism of dopamine D(2) agonist-induced inhibition of GH secretion from GH-secreting adenoma cells was investigated by measurement of intracellular calcium concentration ([Ca(2+)] (i)) and static incubation experiment. Bromocriptine decreased [Ca(2+)](i) in a concentration-dependent manner through D(2) receptor. The inhibition was abolished by pertussis toxin pretreatment. Bromocriptine did not decrease [Ca (2+)](i) after nitrendipine had decreased it. 8Br-cAMP increased [Ca(2+)](i) but application of bromocriptine decreased it, suggesting that bromocriptine-induced inhibition of [Ca(2+)](i) is not dependent on bromocriptine-induced inhibition of adenylyl cyclase. Static incubation experiment revealed that bromocriptine inhibited GH secretion in a concentration-dependent manner. The inhibition was through D(2) receptor and was abolished by pertussis toxin pretreatment. 8Br-cAMP increased GH secretion. Bromocriptine decreased GH secretion even after 8Br-cAMP pretreatment. However, the GH release from cells incubated with bromocriptine alone was significantly less than that from cells incubated with bromocriptine after 8Br-cAMP pretreatment, suggesting a modulatory action of cAMP system in bromocriptine response.

Valproate attenuates dextroamphetamine-induced subjective changes more than lithium

Dextroamphetamine administration in healthy controls produces a range of subjective and physiological effects, which have been likened to those occurring during mania. However, it is uncertain if these can be attenuated by lithium since conflicting results have been reported. To date there have been no previous studies examining the effects of valproate on dextroamphetamine-induced mood and physiological changes. The current study was a double-blind, placebo-controlled, study in which volunteers received either 1000 mg sodium valproate (n=12), 900 mg lithium (n=9), or placebo (n=12) pre-treatment for 14 days. Subjective and physiological measures were then obtained prior to administration of a 25 mg dose of dextroamphetamine, and at two time points after administration. Differences in the response to dextroamphetamine were assessed between the three treatment groups. The results of this study show that pre-treatment with lithium only significantly attenuated dextroamphetamine-induced change in happiness, while valproate pre-treatment significantly attenuated the effects of dextroamphetamine on happiness, energy, alertness and on the diastolic blood pressure. These results suggest that lithium and valproate do not have the same mechanism of action on dextroamphetamine-induced changes, and this finding may relate to differences in their mechanism of action in mood disorders.

Structural Mimicry of Two Cytochrome b562 Interhelical Loops Using Macrocycles Constrained by Oxazoles and Thiazoles

A major chemical challenge is the structural mimicry of discontinuous protein surfaces brought into close proximity through polypeptide folding. We report the design, synthesis, and solution structure of a highly functionalized saddle-shaped macrocyclic scaffold, constrained by oxazoles and thiazoles, supporting two short peptide loops projecting orthogonally from the same face of the scaffold. This structural mimetic of two interhelical loops of cytochrome b(562) illustrates a promising approach to structurally mimicking discontinuous loops of proteins.

Decreased levels of dopamine D3 receptor mRNA in schizophrenic and bipolar patients

Previous studies found an elevation of the dopamine D3 receptor (DRD3) mRNA as determined in peripheral lymphocytes in schizophrenic patients. The aim of this study was to test the hypothesis of elevated DRD3 mRNA in schizophrenia compared to bipolar disorder. Twenty-four patients, 13 schizophrenic and 11 bipolar, were included according to DSM-IV criteria. Psychometric measures were conducted using the Scale for the Assessment of Positive Symptoms, Scale for the Assessment of Negative Symptoms, Brief Psychiatric Rating Scale, Montgomery-Asberg Depression Rating Scale and Young Mania Rating Scale. mRNA was isolated from lymphocytes of venous blood samples and DRD3 mRNA was quantified using real-time reverse transcription PCR. We found a decrease in DRD3 mRNA in 13 schizophrenic (p = 0.009) and 11 bipolar (p = 0.023) patients as compared to controls. Medication history and severity of positive symptoms did not significantly influence DRD3 expression. Higher levels of DRD3 mRNA were correlated with negative schizophrenic symptoms. Interestingly, after treatment of patients with antipsychotics, DRD3 mRNA levels increased to similar levels as those of healthy controls. Bipolar patients, however, showed a slower increase in DRD3 mRNA levels after 3 weeks of therapy. Our findings suggest that the expression of DRD3 mRNA is reduced in schizophrenia and bipolar disorder, supporting the hypothesis of distorted homeostasis of dopamine receptor subtypes in psychotic disorder. The observed diminution was not specific for schizophrenia but also for bipolar disorder requiring further analysis of the regulatory factors involved in dopamine receptor subtype expression.

Production of the human D2S receptor in the methylotrophic yeast P. pastoris

In order to evaluate the methylotrophic yeast Pichia pastoris as means for high-yield production of homogenous D(2S) receptor protein, we have expressed the unmodified D(2S) receptor and various D(2S) receptor fusion constructs under the transcriptional control of the highly inducible promotor of the P. pastoris alcoholoxidase 1 gene in strain SMD1163. Fusion of the D(2S) receptor gene to the alpha-factor preprosequence proved to be essential for receptor production. For the receptor fusion constructs a gene dosage of more than two copies per cell increased production levels three- to sixfold. Adding various dopaminergic ligands to the induction medium increased yields up to tenfold, reaching 51,500 +/- 5700 receptors/cell. Immunoblot analysis of the effect of tunicamycin on D(2S) receptor fusion proteins and immunoprecipitation of metabolically labeled wild-type and glycosylation-deficient D(2S) receptor fusion proteins revealed that the high-mannose-type glycosylation of the D(2S) receptor prevents cleavage of the alpha-factor prosequence by the Kex2 endopeptidase. Abolishing glycosylation restored correct processing. Immunogold electron microscopy showed that recombinant yeast cells overproducing the D(2S) receptor developed membrane stacks harboring the receptor protein. The pharmacological profile of the recombinant D(2S) receptor was similar to that reported for neuronal D(2) receptors independent of glycosylation and processing. In conclusion, the D(2S) receptor can readily be produced in P. pastoris with high yield suitable for receptor purification and future structural studies.

Endogenous dopamine maintains synchronous oscillation of intracellular calcium in primary cultured-mouse midbrain neurons

We demonstrated synchronous oscillation of intracellular Ca2+ in cultured-mouse mid-brain neurons. This synchronous oscillation was thought to result from spontaneous and synchronous neural bursts in a synaptic neural network. We also examined the role of endogenous dopamine in neural networks showing synchronous oscillation. Immunocytochemical study revealed a few tyrosine hydroxylase (TH)-positive dopaminergic neurons, and that cultured neurons expressed synaptophysin and synapsin I. Western blot analyses comfirmed synaptophysin, TH, and 2 types of dopamine receptor (DR), D1R and D2R expression. The synchronous oscillation in midbrain neurons was abolished by the application of R(-)-2-amino-5-phosphonopentanoic acid (AP-5) as an N-methyl-D-aspartate receptor (NMDAR) antagonist. This result suggests that the synchronous oscillation in midbrain neurons requires glutamatergic transmissions, as was the case in previously reported cortical neurons. SCH-12679, a D1R antagonist, inhibited synchronous oscillation in midbrain neurons, while raclopride, a D2R antagonist, induced a transient increase of intracellular Ca2+ and inhibited synchronous oscillation. We consider that endogenous dopamine maintains synchronous oscillation of intracellular Ca2+ through D1R and D2R, and that these DRs regulate intracellular Ca2+in distinctly different ways. Synchronous oscillation of midbrain neurons would be a useful tool for in vitro researches into various neural disorders directly or indirectly caused by dopaminergic neurons.

7-OH-DPAT-induced inhibition of norepinephrine release in PC12 cells

The purpose of this study was to investigate mechanisms of suppression of norepinephrine release by 7-OH-DPAT, a dopamine D(2)/D(3) receptor agonist, in PC12 cells pretreated with nerve growth factor (NGF). 7-OH-DPAT caused inhibition of basal and K(+)-evoked norepinephrine release, which could be blocked by pretreatment with raclopride, a D(2)/D(3) receptor antagonist. Moreover, dopamine D(2) and D(3 )receptors were identified by immunocytochemistry. Expression of D(2), D(3), and D(4) mRNAs and their proteins were detected using RT-PCR and immunoblotting. Furthermore, 7-OH-DPAT produced no change in cGMP levels; however, 7-OH-DPAT inhibited forskolin-stimulated cAMP accumulation that was antagonized by pretreatment with raclopride. In addition, 7-OH-DPAT inhibited carbachol-induced Ca(2+) transient, conversely, 7-OH-DPAT had no effect on 4-aminopyridine-induced Ca(2+) transient. Taken together, suppression of cAMP accumulation and calcium mobilization by 7-OH-DPAT is involved in the inhibition of norepinephrine release through activation of dopamine D(2)/D(3) receptors.

Central monoamines and their role in major depression

The role of the monoamines serotonin and noradrenaline in mental illnesses including depression is well recognized. All antidepressant drugs in clinical use increase acutely the availability of these monoamines at the synapse either by inhibiting their neuronal reuptake, inhibiting their intraneuronal metabolism, or increasing their release by blocking the alpha(2) auto- and heteroreceptors on the monoaminergic neuron. This acute increase in the amount of the monoamines at the synapse has been found to induce long-term adaptive changes in the monoamine systems that end up in the desensitization of the inhibitory auto- and heteroreceptors including the presynaptic alpha(2) and 5-HT(1B) receptors and the somatodendritic 5-HT(1A) receptors located in certain brain regions. The desensitization of these inhibitory receptors would result in higher central monoaminergic activity that coincides with the appearance of the therapeutic response. These adaptive changes responsible for the therapeutic effect depend on the availability of the specific monoamine at the synapse, as depletion of the monoamines will either reverse the antidepressant effect or causes a relapse in the state of drug-free depressed patient previously treated with antidepressant drugs. Furthermore, blocking the somatodendritic 5-HT(1A) or nerve terminal alpha(2) receptors proved to increase the response rate in the treatment of major and treatment-resistant depression, providing further support to the assumption that the antidepressant effect results from the long-term adaptive changes in the monoamine auto- and heteroregulatory receptors. On the other hand, the chronic treatment with antidepressants resulted in D(2) receptors supersensitivity in the nucleus accumbens. This supersensitivity might play a role in the mechanisms underlying antidepressant induced mood switch and rapid cycling.

Alterations in vesicular dopamine uptake contribute to tolerance to the neurotoxic effects of methamphetamine

Previous studies demonstrated that tolerance to the long-term neurotoxic effects of methamphetamine on dopamine neurons could be induced by pretreating with multiple injections of escalating doses of methamphetamine. The mechanism(s) underlying this tolerance phenomenon is unknown. Some recent studies suggested that aberrant vesicular monoamine transporter-2 (VMAT-2) and dopamine transporter function contribute to neurotoxic effects of methamphetamine. Hence, the purpose of this study was to explore the role of the VMAT-2 and dopamine transporter in the induction of tolerance to the longterm persistent dopaminergic deficits caused by methamphetamine. A second purpose was to investigate the potential role of hyperthermia and alterations in brain methamphetamine distribution in this tolerance. Results revealed that the methamphetamine pretreatment regimen attenuated both the acute methamphetamine-induced decrease in VMAT-2 function 2 h after the methamphetamine challenge administration and its resulting persistent dopamine deficits without attenuating the acute methamphetamine-induced decreases in dopamine transporter uptake. Furthermore, pretreatment with methamphetamine prior to a high-dose methamphetamine challenge administration also attenuated the acute methamphetamine-induced redistribution of VMAT-2 immunoreactivity within the nerve terminal. This protection was not due to alterations in concentration of methamphetamine in the brain because both the methamphetamine- and saline-pretreated rats had similar amounts of methamphetamine and amphetamine at 30 min to 2 h after the last methamphetamine challenge injection. In summary, these data are the first to demonstrate an association between the prevention of acute alterations in vesicular dopamine uptake and the development of tolerance to the neurotoxic effects of methamphetamine.

Inhibitory effects of bromocriptine on corticosterone secretion in male rats

Bromocriptine, a dopamine D2 receptor agonist, is widely used for treating prolactinoma, Parkinson's disease and galactorrhea. However, the influence of bromocriptine on the endocrine system, especially adrenal function, is not clear. The present study was aimed to investigate the effects of bromocriptine on corticosterone production in rats. Male rats were treated or not treated by bromocriptine (5 mg/kg, s.c.) twice per day for 2 days before decapitation. The adrenal zona fasciculata-reticularis cells were prepared and incubated with adrenocorticotropic hormone (ACTH), forskolin (an adenylyl cyclase activator), 8-bromo-adenosine 3':5' cyclic monophosphate (8-Br-cAMP, a membrane-permeable analogue of cAMP), and steroidogenic precursors including 25-OH-cholesterol and pregnenolone. The concentrations of prolactin, corticosterone and pregnenolone in the plasma and/or medium were measured by radioimmunoassay (RIA). The protein expression of cytochrome P450 side-chain cleavage (P450scc) enzyme and steroidogenic acute regulatory protein (StAR) was analyzed by Western blotting. Administration of bromocriptine in vivo resulted in a decrease in the levels of plasma prolactin and corticosterone. Basal--and ACTH--as well as forskolin-stimulated corticosterone secretion by zona fasciculata-reticularis cells was also lower in bromocriptine-treated rats than in control animals. The decreased production of corticosterone in zona fasciculata-reticularis cells could be reversed by administration of 8-Br-cAMP. The corticosterone and pregnenolone release induced by 25-OH-cholesterol in zona fasciculata-reticularis cells was reduced by administration of bromocriptine. The protein expression of both StAR protein and P450scc in zona fasciculata-reticularis cells was inhibited in the bromocriptine-treated group. Administration of bromocriptine in vitro reduced the release of corticosterone stimulated by ACTH and forskolin in rat zona fasciculata-reticularis cells. These results suggested that bromocriptine caused adrenal dysfunction through inhibition of ACTH action and of the activity of adenylyl cyclase, and impaired the early steps of corticosterone biosynthesis.

Dopaminergic inhibition of secretin-stimulated choleresis by increased PKC-gamma expression and decrease of PKA activity

To determine the role and mechanisms of action by which dopaminergic innervation modulates ductal secretion in bile duct-ligated rats, we determined the expression of D1, D2, and D3 dopaminergic receptors in cholangiocytes. We evaluated whether D1, D2 (quinelorane), or D3 dopaminergic receptor agonists influence basal and secretin-stimulated choleresis and lumen expansion in intrahepatic bile duct units (IBDU) and cAMP levels in cholangiocytes in the absence or presence of BAPTA-AM, chelerythrine, 1-(5-isoquinolinylsulfonyl)-2-methyl piperazine (H7), or rottlerin. We evaluated whether 1) quinelorane effects on ductal secretion were associated with increased expression of Ca(2+)-dependent PKC isoforms and 2) increased expression of PKC causes inhibition of PKA activity. Quinelorane inhibited secretin-stimulated choleresis in vivo and IBDU lumen space, cAMP levels, and PKA activity in cholangiocytes. The inhibitory effects of quinelorane on secretin-stimulated ductal secretion and PKA activity were blocked by BAPTA-AM, chelerythrine, and H7. Quinelorane effects on ductal secretion were associated with activation of the Ca(2+)-dependent PKC-gamma but not other PKC isoforms. The dopaminergic nervous system counterregulates secretin-stimulated ductal secretion in experimental cholestasis.

Dopamine D2-like receptor-mediated opening of K+ channels in opossum kidney cells

(1) This study examined the effects of dopamine D(1)- and D(2)-like receptor activation upon basolateral K(+) (I(K)) currents and changes in membrane potential in opossum kidney (OK) cells. (2) The addition of amphotericin B (3 micro g ml(-1)) to the apical side resulted in a rapid increase in I(K), this effect being markedly inhibited by the addition of the K(+) channel blockers barium chloride (1 mM) or glibenclamide (10 micro M), but not apamin (1 micro M). The K(+) channel opener pinacidil increased the amphotericin B-induced I(K). The selective D(2)-like receptor agonist quinerolane increased, in a concentration dependent manner (EC(50)=136 nM), I(K) across the basolateral membrane, this effect being abolished by pre-treatment with pertussis toxin (PTX), S-sulpiride (selective D(2)-like receptor antagonist) and glibenclamide. The selective D(1)-like receptor agonist SKF 38393 did not change I(K). Both H-89 (PKA inhibitor) and chelerythrine (PKC inhibitor) failed to prevent the stimulatory effect of quinerolane upon I(K). (3) Quinerolane did not change basal levels of cyclic AMP and also failed to affect the forskolin-induced increase in cyclic AMP levels. (4) The stimulation of D(2)-like receptor was associated with a rapid hyperpolarizing effect, whereas D(1)-like receptor activation was accompanied by increases in cell membrane potential. The hyperpolarizing effect of quinerolane (EC(50)=129 nM) was prevented by pre-treatment with PTX, S-sulpiride and glibenclamide. (5) It is concluded that stimulation of dopamine D(2)-like, but not D(1)-like, receptors coupled to PTX-sensitive G proteins of the G(i/o) class produce membrane hyperpolarization through opening of K(ATP) channels.

Methylphenidate redistributes vesicular monoamine transporter-2: role of dopamine receptors

It is well accepted that methylphenidate (MPD) inhibits dopamine (DA) transporter function. In addition to this effect, this study demonstrates that MPD increases vesicular [3H]DA uptake and binding of the vesicular monoamine transporter-2 (VMAT-2) ligand dihydrotetrabenazine (DHTBZ) in a dose- and time-dependent manner in purified striatal vesicles prepared from treated rats. This change did not result from residual MPD introduced by the original in vivo treatment, because application of MPD in vitro (< or =1 miccrom) was without effect, and higher concentrations decreased vesicular [3H]DA uptake. In addition, MPD treatment increased and decreased VMAT-2 immunoreactivity in striatal vesicle subcellular and plasmalemmal membrane fractions, respectively. The MPD-induced increase in both VMAT-2 immunoreactivity and DHTBZ binding was attenuated by pretreatment in vivo with either the DA D(1) receptor antagonist SCH23390 or the DA D2 receptor antagonist eticlopride. Coadministration of these antagonists in vivo inhibited completely the MPD-induced increase in DHTBZ binding in the purified vesicular preparation. These observations suggest a role for DA in the MPD-induced redistribution of VMAT-2. The implications of this phenomenon will be discussed.

Ciproxifan, a histamine H3-receptor antagonist/inverse agonist, potentiates neurochemical and behavioral effects of haloperidol in the rat

By using double in situ hybridization performed with proenkephalin and H3-receptor riboprobes on the same sections from rat brain, we show that histamine H3 receptors are expressed within striatopallidal neurons of the indirect movement pathway. The majority ( approximately 70%) of striatal enkephalin neurons express H3-receptor mRNAs. This important degree of coexpression of proenkephalin and H3-receptor mRNAs prompted us to explore the effect of H3-receptor ligands on the regulation of enkephalin mRNA expression in the striatum. Acute administration of ciproxifan, a H3-receptor antagonist/inverse agonist, did not modify the expression of the neuropeptide by itself but strongly increased the upregulation of its expression induced by haloperidol. This potentiation (1) was suppressed by the administration of (R)-alpha-methylhistamine, a H3-receptor agonist, (2) occurred both in the caudate-putamen and nucleus accumbens, and (3) was also observed with a similar pattern on c-fos and neurotensin mRNA expression. Similarly, whereas it was devoid of any motor effect when used alone, ciproxifan strongly potentiated haloperidol-induced locomotor hypoactivity and catalepsy, two behaviors in which striatal neurons are involved. The strong H3-receptor mRNA expression in enkephalin neurons suggests that the synergistic neurochemical and motor effects of ciproxifan and haloperidol result from direct H3/D2-receptor interactions, leading to an enhanced activation of striatopallidal neurons of the indirect movement pathway. The potentiation of the effects of haloperidol by ciproxifan strengthens the potential interest of H3-receptor antagonists/inverse agonists to improve the symptomatic treatment of schizophrenia.

Origin and functional role of the extracellular serotonin in the midbrain raphe nuclei

There is considerable interest in the regulation of the extracellular compartment of the transmitter serotonin (5-hydroxytryptamine, 5-HT) in the midbrain raphe nuclei because it can control the activity of ascending serotonergic systems and the release of 5-HT in terminal areas of the forebrain. Several intrinsic and extrinsic factors of 5-HT neurons that regulate 5-HT release in the dorsal (DR) and median (MnR) raphe nucleus are reviewed in this article. Despite its high concentration in the extracellular space of the raphe nuclei, the origin of this pool of the transmitter remains to be determined. Regardless of its origin, is has been shown that the release of 5-HT in the rostral raphe nuclei is partly dependent on impulse flow and Ca(2+) ions. The release in the DR and MnR is critically dependent on the activation of 5-HT autoreceptors in these nuclei. Yet, it appears that 5-HT autoreceptors do not tonically inhibit 5-HT release in the raphe nuclei but rather play a role as sensors that respond to an excess of the endogenous transmitter. Both DR and MnR are equally responsive to the reduction of 5-HT release elicited by the local perfusion of 5-HT(1A) receptor agonists. In contrast, the effects of selective 5-HT(1B) receptor agonists are more pronounced in the MnR than in the DR. However, the cellular localization of 5-HT(1B) receptors in the raphe nuclei remains to be established. Furthermore, endogenous noradrenaline and GABA tonically regulate the extracellular concentration of 5-HT although the degree of tonicity appears to depend upon the sleep/wake cycle and the behavioral state of the animal. Glutamate exerts a phasic facilitatory control over the release of 5-HT in the raphe nuclei through ionotropic glutamate receptors. Overall, it appears that the extracellular concentration of 5-HT in the DR and the MnR is tightly controlled by intrinsic serotonergic mechanisms as well as afferent connections.

Dopamine D3 receptor antagonist effects on the motivational effects of ethanol

Dopaminergic systems are thought to play important roles in the motivational effects of ethanol. In the present experiments, we examined the effects of U99194A, a putative dopamine D(3) receptor antagonist, on ethanol-induced conditioned place preference, locomotor stimulation, taste aversion, and self-administration. In two separate studies with the use of a place conditioning procedure, adult male Swiss-Webster mice received six pairings of a tactile stimulus with ethanol (1 or 3 g/kg, i.p.), U99194A (20 mg/kg, i.p.), or ethanol + U99194A. For determination of ethanol-stimulated activity, subjects received U99194A at a dose of 0, 10, 20, or 30 mg/kg 15 min before ethanol at 0, 1, or 2 g/kg immediately before a 30-min locomotor activity test. In a taste conditioning procedure, subjects received five 1-h access periods to 0.2 M NaCl. After the first four access periods, subjects received ethanol at 0, 2, or 4 g/kg and U99194A at 0, 10, or 20 mg/kg. In an oral self-administration procedure, male C57BL/6J mice received U99194A at 0, 10, or 20 mg/kg, followed by 30-min access to 10% (wt./vol.) sucrose or 10% (vol./vol.) ethanol in 10% sucrose. The acquisition of ethanol-induced conditioned place preference was enhanced by U99194A. However, U99194A did not produce significant preference alone. U99194A did not alter locomotor stimulation produced by an injection of ethanol at 2 g/kg. U99194A also did not alter the acquisition of ethanol-induced conditioned taste aversion and did not change oral ethanol self-administration. These results support the suggestion that dopamine D(3) receptors have specific involvement in ethanol reward, as measured by place conditioning, but are not important for ethanol-stimulated activity, ethanol taste aversion, or ethanol intake.

Inhibitory mechanism of bromocriptine on catecholamine release evoked by cholinergic stimulation and membrane depolarization from the rat adrenal medulla

The purpose of this study was to determine whether bromocriptine affects the catecholamines (CA) secretion evoked in isolated perfused rat adrenal glands, by cholinergic stimulation, membrane depolarization and calcium mobilization, and to establish the mechanism of its action. The perfusion of bromocriptine (1-10 microM) into an adrenal vein, for 60 min, produced relatively dose-dependent inhibition in the secretion of catecholamines (CA) evoked by acetylcholine (ACh, 5.32 mM), DMPP (100 microM for 2 min), McN-A-343 (100 microM for 2 min), cyclopiazonic acid (CPA, 10 microM for 4 min) and Bay-K-8644 (10 microM for 4 min). High K+ (56 mM)-evoked CA release was also inhibited, although not in a dose-dependent fashion. Also, in the presence of apomorphine (100 microM), which is also known to be a selective D2-agonist, the CA secretory responses evoked by ACh, high potassium, DMPP, McN-A-343, Bay-K-8644 and cyclopiazonic acid were also significantly depressed. However, in adrenal glands preloaded with bromocriptine (3 microM) in the presence of metoclopramide (15 microM), a selective D2-antagonist, the CA secretory responses evoked by ACh, high potassium, DMPP, McN-A-343, Bay-K-8644 and cyclopiazonic acid considerably recovered as compared to that of bromocriptine only. Taken together, these results suggest that bromocriptine can inhibit the CA secretion evoked by stimulation of cholinergic receptors, as well as by membrane depolarization, in the perfused rat adrenal medulla. It is thought this inhibitory effect of bromocriptine may be mediated by inhibiting the influx of extracellular calcium and the release from intracellular calcium stores, through the activation of dopaminergic D2-receptors located in the rat adrenomedullary chromaffin cells. Furthermore, these findings also suggest that the dopaminergic D2-receptors may play an important role in regulating adrenomedullary CA secretion.

G protein preferences for dopamine D2 inhibition of prolactin secretion and DNA synthesis in GH4 pituitary cells

Dopamine is the primary inhibitory regulator of lactotroph proliferation and prolactin (PRL) secretion in vivo, acting via dopamine D2 receptors (short D2S and long D2L forms). In GH4C1 pituitary cells transfected with D2S or D2L receptor cDNA, dopamine inhibits PRL secretion and DNA synthesis. These actions were blocked by pertussis toxin, implicating G(i)/G(o) proteins. To address roles of specific G(i)/G(o)4 proteins in these actions a series of GH4C1 cell lines specifically depleted of individual Galpha subunits was examined. D2S-mediated inhibition of BayK8644-stimulated PRL secretion was primarily dependent on G(o) over G(i), as observed for BayK8644-induced calcium influx. By contrast, inhibitory coupling of the D2S receptor to TRH-induced PRL secretion was partially impaired by depletion of any single G protein, but especially G(i)3. Inhibitory coupling of D2L receptors to PRL secretion required G(o), but not G(i)2, muscarinic receptor coupling was resistant to depletion of any G(i)/G(o) protein, whereas the 5-HT1A and somatostatin receptors required G(i)2 or G(i)3 for coupling. The various receptors also demonstrated distinct G protein requirements for inhibition of DNA synthesis: depletion of any G(i)/G(o) subunit completely uncoupled the D2S receptor, the D2L receptor was uncoupled by depletion of G(i)2, and muscarinic and somatostatin receptors were resistant to depletion of G(i)2 only. These results demonstrate distinct receptor-G protein preferences for inhibition of TRH-induced PRL secretion and DNA synthesis.

D2-like receptor-mediated inhibition of Na+-K+-ATPase activity is dependent on the opening of K+ channels

This study examined the effects of D2-like dopamine receptor activation on Na+-K+-ATPase activity while apical-to-basal, ouabain-sensitive, amphotericin B-induced increases in short-circuit current and basolateral K+ (I(K)) currents in opossum kidney cells were measured. The inhibitory effect of dopamin on Na+-K+-ATPase activity was completely abolished by either D1- or D2-like receptor antagonists and mimicked by D1- and D2-like receptor agonists SKF-38393 and quinerolane, respectively. Blockade of basolateral K+ channels with BaCl2 (1 mM) or glibenclamide (10 microM), but not apamin (1 microM), totally prevented the inhibitory effects of quinerolane. The K+ channel opener pinacidil decreased Na+-K+-ATPase activity. The inhibitory effect of quinerolane on Na+-K+- ATPase activity was abolished by pretreatment of opossum kidney cells with pertussis toxin (PTX). Quinerolane increased I(K) across the basolateral membrane in a concentration-dependent manner; this effect was abolished by pretreatment with PTX, S-sulpiride, and glibenclamide. SKF-38393 did not change I(K). Both H-89 (protein kinase A inhibitor) and chelerythrine (protein kinase C inhibitor) failed to prevent the stimulatory effect of quinerolane on I(K). The stimulation of the D2-like receptor was associated with a rapid hyperpolarizing effect, whereas D1-like receptor activation was accompanied by increases in cell membrane potential. It is concluded that stimulation of D2-like receptors leads to inhibition of Na+-K+-ATPase activity and hyperpolarization; both effects are associated with the opening of K+ channels.

Localization of peripheral dopamine D1 and D2 receptors in rat corpus cavernosum

OBJECTIVE

To detect and locate anatomically peripheral dopamine D1 and D2 receptors in rat cavernosa, as dopamine is important in sexual drive and penile erection through receptors located in the central nervous system.

MATERIALS AND METHODS

Corpora cavernosa were obtained from Sprague-Dawley rats; total RNA and membrane proteins were extracted and cryostat sections prepared. The rat brain hypothalamus was used as a control for dopamine D1 and D2 receptors. The presence and expression of peripheral dopamine D1 and D2 receptor mRNAs in rat corpus cavernosa was assessed using reverse transcription and polymerase chain reaction (RT-PCR), and Northern blot hybridization using (32)P-UTP-labelled RNA probes. Concurrently, corresponding proteins from D1 and D2 receptors were assayed and detected by a Western blotting technique. The anatomical location of dopamine D1 and D2 receptor mRNAs in rat penile tissues was identified by in situ hybridization using (35)S-UTP-labelled RNA probes in cryostat sections. Immunohistochemical staining was used to locate peripheral dopamine D1 and D2 receptor proteins in rat corpora cavernosa.

RESULTS

Dopamine D1 and D2 receptor gene expression was detected in rat corpora cavernosa. In situ hybridization signals for dopamine D1 and D2 receptor mRNAs were localized to corpus cavernosal tissues and dorsal vessels in the rat penis. Western blot analyses showed peripheral dopamine D1 and D2 receptor proteins in rat corpora cavernosa. Immunohistochemically, peripheral dopamine D1 and D2 receptor proteins were detected in dorsal nerves, dorsal vessels and corpus cavernosal smooth muscle of the rat penile tissues.

CONCLUSIONS

Peripheral dopamine D1 and D2 receptors are present in the corpora cavernosa of rats. The functional significance of these receptors and signal transduction pathways in modulating the vascular tone of the penis warrants further investigation.

betagamma Dimers mediate synergy of dopamine D2 and adenosine A2 receptor-stimulated PKA signaling and regulate ethanol consumption

Dopamine release is activated by ethanol and addicting drugs, but molecular mechanisms linking dopaminergic signaling to neuronal responses and drinking behavior are poorly understood. We report that dopamine-D2 receptors induce PKA Calpha translocation and increase CRE-regulated gene expression. Ethanol also activates PKA signaling. Subthreshold concentrations of the D2 agonist NPA and ethanol, without effect alone, together cause synergistic PKA translocation and CRE-mediated gene transcription. D2 or adenosine A2 receptor blockade, pertussis toxin, Rp-cAMPS, or overexpression of dominant-negative peptides that sequester betagamma dimers prevent synergy. Importantly, overexpression of a betagamma inhibitor peptide in the nucleus accumbens strikingly reduces sustained alcohol consumption. We propose that synergy of D2 and A2 confers ethanol hypersensitivity and that betagamma dimers are required for voluntary drinking.

Functional selectivity of dopamine receptor agonists. II. Actions of dihydrexidine in D2L receptor-transfected MN9D cells and pituitary lactotrophs

D(2)-like dopamine receptors mediate functional changes via activation of inhibitory G proteins, including those that affect adenylate cyclase activity, and potassium and calcium channels. Although it is assumed that the binding of a drug to a single isoform of a D(2)-like receptor will cause similar changes in all receptor-mediated functions, it has been demonstrated in brain that the dopamine agonists dihydrexidine (DHX) and N-n-propyl-DHX are "functionally selective". The current study explores the underlying mechanism using transfected MN9D cells and D(2)-producing anterior pituitary lactotrophs. Both dopamine and DHX inhibited adenylate cyclase activity in a concentration-dependent manner in both systems, effects blocked by D(2), but not D(1), antagonists. In the MN9D cells, quinpirole and R-(-)-N-propylnorapomorphine (NPA) also inhibited the K(+)-stimulated release of [(3)H]dopamine in a concentration-responsive, antagonist-reversible manner. Conversely, neither DHX, nor its analogs, inhibited K(+)-stimulated [(3)H]dopamine release, although they antagonized the effects of quinpirole. S-(+)-NPA actually had the reverse functional selectivity profile from DHX (i.e., it was a full agonist at D(2L) receptors coupled to inhibition of dopamine release, but a weak partial agonist at D(2L) receptor-mediated inhibition of adenylate cyclase). In lactotrophs, DHX had little intrinsic activity at D(2) receptors coupled to G protein-coupled inwardly rectifying potassium channels, and actually antagonized the effects of dopamine at these D(2) receptors. Together, these findings provide compelling evidence for agonist-induced functional selectivity with the D(2L) receptor. Although the underlying molecular mechanism is controversial (e.g., "conformational induction" versus "drug-active state selection"), such data are irreconcilable with the widely held view that drugs have "intrinsic efficacy".

Functional and autoradiographic characterization of dopamine D2-like receptors in the guinea pig heart

Dopamine receptors include the D1- (D1 and D5 subtypes) and D2-like (D2, D3, and D4 subtypes) families. D1-like receptors are positively and D2-like receptors negatively coupled to the adenylyl cyclase. Dopamine D2-like (D4 subtype) receptors have been identified in human and rat hearts. However the presence of D2 and D3 receptor subtypes is unclear. Furthermore, their role in cardiac functions is unknown. By autoradiographic studies of guinea pig hearts, we identified D3 and D4 receptors, using the selective radioligands [3H]-7-OH-DPAT and [3H]emonapride (YM-09151-2 plus raclopride). Western blot analysis confirmed D3 and D4 receptors in the right and left ventricle of the same species. Selective agonists of D3 and D4 receptors (+/-)-7-OH-DPAT and PD 168 077 (10(-9) to 10(-5) M, respectively) induced a significant negative chronotropic and inotropic effect in the isolated guinea pig heart preparation. Negative inotropic effect induced by PD 168 077 was associated with an inhibition in cyclase activity. No changes in cyclase activity were found with (+/-)-7-OH-DPAT. The aim of this study is to support the presence of D3 and D4 receptors in the heart. Although our results suggest that D3 and D4 receptors are functionally active in the heart, we need additional information with an antagonist and an agonist of improved potency and selectivity to understand the respective roles of D3 and D4 receptors in the cardiac functions.

Dopamine receptor agonists for treating prolactinomas

Prolactinomas are the most common hormone-secreting pituitary tumours and cause infertility and gonadal and sexual dysfunction in both sexes. The approach to prolactinomas has changed in the last 25 years thanks to the availability of dopaminergic drugs characterised by a potent prolactin-inhibitory effect, a tumour shrinking effect associated with a satisfactory tolerability. In more recent years, cabergoline 1-[(6-allelylergolin-8beta-yl)carbonyl]-1-[3-(dimethylamino) propyl]-3-ethyl-urea an ergoline derivative with potent, selective and long-lasting inhibitory activity on prolactin release, has been used to suppress prolactin secretion in women with hyperprolactinaemia. Cabergoline was shown to be significantly more effective than bromocriptine in inducing a complete biochemical response and clinical efficacy and was better tolerated than bromocriptine in the majority of patients. Notable tumour shrinkage until tumour disappearance was observed during cabergoline treatment in most patients with macroprolactinoma and it was also proven effective in patients resistant to or with a poor response to bromocriptine. In view of the limited data on cabergoline-associated pregnancies and the long half-life of the drug, it is currently recommended that women hoping to become pregnant, once ovulatory cycles have been established, should discontinue cabergoline therapy 1 month before they intend to conceive. However, no data concerning negative effects on pregnancy or offspring have been reported. The great efficacy of this compound together with its excellent tolerability makes this drug the current treatment of choice for the majority of patients with hyperprolactinaemic disorders.

Pharmacological identification of inward current evoked by dopamine in rat subthalamic neurons in vitro

Dopaminergic mechanisms in the subthalamic nucleus (STN) are implicated in the pathophysiology of Parkinson's disease. Here, electrophysiological responses of STN neurons to dopamine (DA) were investigated by using whole-cell patch-clamp recordings in the rat brain slice preparation. Under current-clamp, DA depolarized membrane potential and increased the frequency of spontaneous action potentials of STN neurons. Under voltage-clamp, DA (3-300 microM) produced a reversible concentration-dependent inward current (I(DA); 6-40 pA) with an EC(50) of 13 microM. This DA-induced current had a negative slope conductance which reversed at -102 mV. It was partially reduced by barium and by superfusion with an elevated concentration of extracellular K(+). Moreover, TTX and glutamate receptor antagonists (CNQX and AP5) did not significantly affect the DA responses, indicating that I(DA) is not dependent upon afferent synaptic activity in the STN. Quinpirole, a D(2) receptor agonist, mimicked the DA action more effectively than did the D(1) agonist SKF-38393. The D(2) antagonist sulpiride, but not the D(1) antagonist SCH-23390, blocked responses induced by DA. Intracellular application of G-protein inhibitor GDP-beta-S also suppressed I(DA). GTP-gamma-S, added to the pipette solution, evoked a sustained inward shift in the absence of DA. These results suggest that DA increases the activity of STN neurons via activation of G-protein-coupled D(2)-like receptors which reduce a K(+) conductance.