Differential signaling of dopamine-D2S and -D2L receptors to inhibit ERK1/2 phosphorylation

Developmental prefrontal mRNA expression of D2 dopamine receptor splice variants and working memory impairments in rats after early life Interleukin-1β elevation

Long (D2L) and Short (D2S) isoforms of D2 dopamine receptor differ in their biochemical and physiological properties, which could affect functioning of prefrontal cortex. Contribution of distinct D2 dopamine receptor isoforms to cognitive dysfunctions and its developmental regulation are currently not fully elucidated. In the present study, we evaluated developmental mRNA expression of D2S/D2L dopamine receptor isoforms within the rat medial prefrontal cortex (mPFC) in the model of neurodevelopmental cognitive dysfunction. Working memory performance (Y-maze spontaneous alternations) and D2S/D2L mRNA expression in the mPFC (by qRT-PCR) were evaluated in juvenile (P27), adolescent (P42-47) and adult (P75-90) rats after chronic early life treatment with proinflammatory cytokine interleukin (IL)-1β (1 µg/kg i.p. daily P15-21). It was shown that IL-1β elevation during the 3rd week of life leads to working memory deficit originating in juvenile animals and persisting into adulthood. D2S mRNA expression was strongly downregulated during adolescence, and such downregulation was exaggerated in animals injected with IL-1β during P15-21. Early life IL-1β administrations influenced developmental changes in the D2S/D2L mRNA ratio. This measure was found to be decreased in adolescent and adult control (intact and vehicle-treated) rats compared to juvenile control, while in the case of IL-1β-treated animals, the decrease in D2S/D2L ratio was observed only in adulthood but not in adolescence compared to juvenile rats. During the adolescence, D2S mRNA expression was downregulated and D2S/D2L ratio was upregulated in the mPFC of rats treated with IL-1β during the 3rd week of life compared to controls. Based on these data we conclude that changes in the developmental expression of D2 dopamine receptor splice variants within mPFC may underlie long-lasting cognitive deficit associated with neonatal pathology.

Prefrontal mRNA expression of long and short isoforms of D2 dopamine receptor: Possible role in delayed learning deficit caused by early life interleukin-1β treatment

Long (D2L) and short (D2S) isoform of the D2 dopamine receptor are believed to play different roles in behavioral regulation. However, little is known about differential regulation of these isoforms mRNA expression during the process of learning in physiological and pathological states. In this study, we have investigated the combined effect of training in active avoidance (AA) paradigm and chronic early life treatment with pro-inflammatory cytokine interleukin (IL)-1β (1μg/kg i.p., P15-21) on D2S and D2L dopamine receptor mRNA expression in the medial prefrontal cortex (mPFC) of adult rats. We have shown differential regulation of D2 short and long mRNA isoform expression in the mPFC. There was no effect of AA-training on D2S mRNA expression, while D2L mRNA was downregulated in AA-trained control (intact and saline-treated) animals, and this effect was not observed in rats treated with IL-1β. D2S mRNA expression level negatively correlated with learning ability within control (saline-treated and intact) groups but not in IL-1β-treated animals. Thus, prefrontal expression of distinct D2 dopamine receptor splice variants is supposed to be implicated in cognitive decline caused by early life immune challenge.

Structural plasticity in mesencephalic dopaminergic neurons produced by drugs of abuse: critical role of BDNF and dopamine

Mesencephalic dopaminergic neurons were suggested to be a critical physiopathology substrate for addiction disorders. Among neuroadaptive processes to addictive drugs, structural plasticity has attracted attention. While structural plasticity occurs at both pre- and post-synaptic levels in the mesolimbic dopaminergic system, the present review focuses only on dopaminergic neurons. Exposures to addictive drugs determine two opposite structural responses, hypothrophic plasticity produced by opioids and cannabinoids (in particular during the early withdrawal phase) and hypertrophic plasticity, mostly driven by psychostimulants and nicotine. In vitro and in vivo studies identified BDNF and extracellular dopamine as two critical factors in determining structural plasticity, the two molecules sharing similar intracellular pathways involved in cell soma and dendrite growth, the MEK-ERK1/2 and the PI3K-Akt-mTOR, via preferential activation of TrkB and dopamine D3 receptors, respectively. At present information regarding specific structural changes associated to the various stages of the addiction cycle is incomplete. Encouraging neuroimaging data in humans indirectly support the preclinical evidence of hypotrophic and hypertrophic effects, suggesting a possible differential engagement of dopamine neurons in parallel and partially converging circuits controlling motivation, stress, and emotions.

Illuminating somatostatin analog action at neuroendocrine tumor receptors

Somatostatin analogs for the diagnosis and therapy of neuroendocrine tumors (NETs) have been used in clinical applications for more than two decades. Five somatostatin receptor subtypes have been identified and molecular mechanisms of somatostatin receptor signaling and regulation have been elucidated. These advances increased understanding of the biological role of each somatostatin receptor subtype, their distribution in NETs, as well as agonist-specific regulation of receptor signaling, internalization, and phosphorylation, particularly for the sst2 receptor subtype, which is the primary target of current somatostatin analog therapy for NETs. Various hypotheses exist to explain differences in patient responsiveness to somatostatin analog inhibition of tumor secretion and growth as well as differences in the development of tumor resistance to therapy. In addition, we now have a better understanding of the action of both first generation (octreotide, lanreotide, Octreoscan) and second generation (pasireotide) FDA-approved somatostatin analogs, including the biased agonistic character of some agonists. The increased understanding of somatostatin receptor pharmacology provides new opportunities to design more sophisticated assays to aid the future development of somatostatin analogs with increased efficacy.

The histamine autoreceptor is a short isoform of the H₃ receptor

BACKGROUND AND PURPOSE The histamine H(3) receptor was identified as the autoreceptor of brain histaminergic neurons. After its cloning, functional H(3) receptor isoforms generated by a deletion in the third intracellular loop were found in the brain. Here, we determined if this autoreceptor was the long or the short isoform. EXPERIMENTAL APPROACH We hypothesized that the deletion would affect H(3) receptor stereoselectivity. The effects of the enantiomers of two chiral ligands, N(α)-methyl-α-chloromethylhistamine (N(α) Me-αClMeHA) and sopromidine, were investigated on cAMP formation at the H(3(445)) and H(3(413)) receptor isoforms, common to all species. They were further compared with their effects at autoreceptors. They were also compared on [(35)S]GTPγ[S] binding to membranes of rat cerebral cortex, striatum and hypothalamus, the richest area in autoreceptors. KEY RESULTS The stereoselectivity of N(α) Me-αClMeHA enantiomers as agonists was similar at the H(3(413)) receptor isoform and autoreceptors, but lower at the long isoform. While (S) sopromidine did not discriminate between the isoforms, (R) sopromidine was an antagonist at the H(3(413)) receptor isoform and autoreceptors, but a full agonist at the long isoform. In rat brain, stereoselectivity of N(α) Me-αClMeHA was higher in the hypothalamus than in cerebral cortex or striatum, whereas the opposite pattern was found for sopromidine. CONCLUSIONS AND IMPLICATIONS The pharmacological profiles of H(3) receptor isoforms differed markedly, showing that the function of autoreceptors was fulfilled by a short isoform, such as the H(3(413)) receptor. Development of drugs selectively targeting autoreceptors might enhance their therapeutic efficacy and/or decrease incidence of side effects.

Antisense-mediated RNA targeting: versatile and expedient genetic manipulation in the brain

A limiting factor in brain research still is the difficulty to evaluate in vivo the role of the increasing number of proteins implicated in neuronal processes. We discuss here the potential of antisense-mediated RNA targeting approaches. We mainly focus on those that manipulate splicing (exon skipping and exon inclusion), but will also briefly discuss mRNA targeting. Classic knockdown of expression by mRNA targeting is only one possible application of antisense oligonucleotides (AON) in the control of gene function. Exon skipping and inclusion are based on the interference of AONs with splicing of pre-mRNAs. These are powerful, specific and particularly versatile techniques, which can be used to circumvent pathogenic mutations, shift splice variant expression, knock down proteins, or to create molecular models using in-frame deletions. Pre-mRNA targeting is currently used both as a research tool, e.g., in models for motor neuron disease, and in clinical trials for Duchenne muscular dystrophy and amyotrophic lateral sclerosis. AONs are particularly promising in relation to brain research, as the modified AONs are taken up extremely fast in neurons and glial cells with a long residence, and without the need for viral vectors or other delivery tools, once inside the blood brain barrier. In this review we cover (1). The principles of antisense-mediated techniques, chemistry, and efficacy. (2) The pros and cons of AON approaches in the brain compared to other techniques of interfering with gene function, such as transgenesis and short hairpin RNAs, in terms of specificity of the manipulation, spatial, and temporal control over gene expression, toxicity, and delivery issues. (3) The potential applications for Neuroscience. We conclude that there is good evidence from animal studies that the central nervous system can be successfully targeted, but the potential of the diverse AON-based approaches appears to be under-recognized.

Dopamine-induced apoptosis of lactotropes is mediated by the short isoform of D2 receptor

Dopamine, through D2 receptor (D2R), is the major regulator of lactotrope function in the anterior pituitary gland. Both D2R isoforms, long (D2L) and short (D2S), are expressed in lactotropes. Although both isoforms can transduce dopamine signal, they differ in the mechanism that leads to cell response. The administration of D2R agonists, such as cabergoline, is the main pharmacological treatment for prolactinomas, but resistance to these drugs exists, which has been associated with alterations in D2R expression. We previously reported that dopamine and cabergoline induce apoptosis of lactotropes in primary culture in an estrogen-dependent manner. In this study we used an in vivo model to confirm the permissive action of estradiol in the apoptosis of anterior pituitary cells induced by D2R agonists. Administration of cabergoline to female rats induced apoptosis, measured by Annexin-V staining, in anterior pituitary gland from estradiol-treated rats but not from ovariectomized rats. To evaluate the participation of D2R isoforms in the apoptosis induced by dopamine we used lactotrope-derived PR1 cells stably transfected with expression vectors encoding D2L or D2S receptors. In the presence of estradiol, dopamine induced apoptosis, determined by ELISA and TUNEL assay, only in PR1-D2S cells. To study the role of p38 MAPK in apoptosis induced by D2R activation, anterior pituitary cells from primary culture or PR1-D2S were incubated with an inhibitor of the p38 MAPK pathway (SB203850). SB203580 blocked the apoptotic effect of D2R activation in lactotropes from primary cultures and PR1-D2S cells. Dopamine also induced p38 MAPK phosphorylation, determined by western blot, in PR1-D2S cells and estradiol enhanced this effect. These data suggest that, in the presence of estradiol, D2R agonists induce apoptosis of lactotropes by their interaction with D2S receptors and that p38 MAPK is involved in this process.

From the cell to the clinic: a comparative review of the partial D₂/D₃receptor agonist and α2-adrenoceptor antagonist, piribedil, in the treatment of Parkinson's disease

Though L-3,4-dihydroxyphenylalanine (L-DOPA) is universally employed for alleviation of motor dysfunction in Parkinson's disease (PD), it is poorly-effective against co-morbid symptoms like cognitive impairment and depression. Further, it elicits dyskinesia, its pharmacokinetics are highly variable, and efficacy wanes upon long-term administration. Accordingly, "dopaminergic agonists" are increasingly employed both as adjuncts to L-DOPA and as monotherapy. While all recognize dopamine D(2) receptors, they display contrasting patterns of interaction with other classes of monoaminergic receptor. For example, pramipexole and ropinirole are high efficacy agonists at D(2) and D(3) receptors, while pergolide recognizes D(1), D(2) and D(3) receptors and a broad suite of serotonergic receptors. Interestingly, several antiparkinson drugs display modest efficacy at D(2) receptors. Of these, piribedil displays the unique cellular signature of: 1), signal-specific partial agonist actions at dopamine D(2)and D(3) receptors; 2), antagonist properties at α(2)-adrenoceptors and 3), minimal interaction with serotonergic receptors. Dopamine-deprived striatal D(2) receptors are supersensitive in PD, so partial agonism is sufficient for relief of motor dysfunction while limiting undesirable effects due to "over-dosage" of "normosensitive" D(2) receptors elsewhere. Further, α(2)-adrenoceptor antagonism reinforces adrenergic, dopaminergic and cholinergic transmission to favourably influence motor function, cognition, mood and the integrity of dopaminergic neurones. In reviewing the above issues, the present paper focuses on the distinctive cellular, preclinical and therapeutic profile of piribedil, comparisons to pramipexole, ropinirole and pergolide, and the core triad of symptoms that characterises PD-motor dysfunction, depressed mood and cognitive impairment. The article concludes by highlighting perspectives for clarifying the mechanisms of action of piribedil and other antiparkinson agents, and for optimizing their clinical exploitation.

Effects of the dopamine stabilizer, OSU-6162, on brain stimulation reward and on quinpirole-induced changes in reward and locomotion

Dysregulation of limbic dopamine (DA) neurotransmission results in abnormal positive or negative emotional states that characterize several mental disorders. Drugs that restore DA homeostasis are most likely to constitute effective treatments for such emotional disturbances. In this study, we investigated the effects of several doses of OSU-6162, a drug that belongs to a new class named "DA stabilizers", on brain stimulation reward. Because quinpirole produces, depending on the dose, a pre-synaptic depressant and a post-synaptic stimulatory effect on reward and locomotor activity, we also compared the ability of OSU-6162 and haloperidol to prevent these effects of the full DA agonist. Results show that OSU-6162 produced a dose-orderly reduction of reward with no change in the capacity of the animals to produce the operant response, and prevented, like haloperidol, both stimulatory and depressant effects of quinpirole on locomotor activity but only its reward stimulatory effect. The observed functional antagonism of OSU-6162 on these DA-dependent behaviors suggests that it may constitute an effective treatment for abnormal positive emotional state, and that it would be exempt of motor side-effects.

GAP1(IP4BP)/RASA3 mediates Galphai-induced inhibition of mitogen-activated protein kinase

The dopamine D2S receptor (short isoform) couples to inhibitory Galphai/o proteins to inhibit thyrotropin-releasing hormone (TRH)-stimulated p42/p44 mitogen-activated protein kinase (ERK1/2) phosphorylation in GH4ZR7 rat pituitary cells, consistent with its actions to inhibit prolactin gene transcription and cell proliferation. However, the underlying mechanism is unclear. To identify novel Galphai effectors, yeast two-hybrid screening of a GH4ZR7 cDNA library was done using constitutively active Galphai3-Q204L, and multiple clones of the RasGAP cDNA GAP1(IP4BP)/RASA3 were identified. In yeast mating assay, RASA3 preferentially interacted with activated forms of Galphai/o/z proteins, but not with Galphas. A direct interaction was indicated by in vitro pull-down assay, in which S-His-RASA3 preferentially bound guanosine 5'-O-(gamma-thio)triphosphate-activated Galphai3 and Galphai2 compared with guanosine 5'-O-(beta-thio)diphosphate-inactivated proteins. Similarly, in co-immunoprecipitation studies in HEK-293 cells, FLAG-tagged RASA3 preferentially interacted with activated mutants of Galphai3 and Galphai2 compared with wild type proteins. In GH4ZR7 cells, co-immunoprecipitation studies of endogenous proteins demonstrated a Galphai3-RASA3 complex that was induced upon TRH/D2S receptor co-activation. To address RASA3 function in dopamine D2S receptor-induced inhibition of ERK1/2 activity, endogenous RASA3 protein expression was suppressed (70% knockdown) in GH4ZR7 cells stably transfected with full-length antisense cDNA of RASA3. The selected antisense clones had similar levels of dopamine D2S receptor binding and D2S-induced inhibition of cAMP formation compared with parental GH4ZR7 cells. In these clones, D2S-mediated inhibition of TRH-induced phospho-ERK1/2 was reversed by 70-80% compared with parental GH4ZR7 cells. Our results provide a novel mechanism for dopamine D2S-induced inhibition of ERK1/2 and indicate that RASA3 links Galphai proteins to inhibit Gq-induced Ras/ERK1/2 activation.

Haloperidol regulates the phosphorylation level of the MEK-ERK-p90RSK signal pathway via protein phosphatase 2A in the rat frontal cortex

Haloperidol, a classical antipsychotic drug, affects the extracellular signal-regulated kinase (ERK) pathway in the brain. However, findings are inconsistent and the mechanism by which haloperidol regulates ERK is poorly understood. Therefore, we examined the ERK pathway and the related protein phosphatase 2A (PP2A) in detail after haloperidol administration. Haloperidol (0.5 and 1 mg/kg) induced biphasic changes in the phosphorylation level of mitogen-activated protein kinase kinase (MEK), ERK, and p90 ribosomal S6 kinase (p90RSK) without changing Raf-1 phosphorylation. Fifteen minutes after haloperidol administration, MEK-ERK-p90RSK phosphorylation increased, whilst PP2A activity decreased. At 60 min, the reverse was observed and the binding of PP2A to MEK and ERK increased. Higher dosages of haloperidol (2 and 4 mg/kg), affected neither MEK-ERK-p90RSK phosphorylation nor PP2A activity. Accordingly, PP2A regulates acute dose- and time-dependent changes in MEK-ERK-p90RSK phosphorylation after haloperidol treatment. These findings suggest the involvement of a dephosphorylating mechanism in the acute action of haloperidol.

Differential desensitization of dopamine D2 receptor isoforms by protein kinase C: The importance of receptor phosphorylation and pseudosubstrate sites

Altered regulation of dopamine D(2) receptors is implicated in addiction, schizophrenia and movement disorders, as well as lactotroph growth and regulation. Dopamine D(2S) and dopamine D(2L) receptors are alternately-spliced variants that differ by 29 amino acids in the third intracellular (i3) domain and display different sensitivity to desensitization by protein kinase C (PKC). In the present studies we determined the specific phosphorylation sites on the dopamine D(2S) receptor that confer PKC-mediated desensitization. In dopamine D(2L) receptors, we identified a PKC pseudosubstrate site responsible for the relative insensitivity of the receptor to PKC-induced uncoupling. In transiently transfected Ltk(-) fibroblast cells, 2-min preactivation of PKC with 12-O-tetradecanoyl 4beta-phorbol 13alpha-acetate (TPA) completely inhibited calcium mobilization induced by the dopamine D(2S) receptor, but not the dopamine D(2L) variant. Point mutation of i3 PKC sites Ser228/229Gly rendered the dopamine D(2S) receptor resistant to PKC action, with lesser effects of other Ser and Thr mutations. Inactivation of the PKC pseudosubstrate motif in the dopamine D(2L) receptor sensitized the receptor to PKC, and this was reversed by mutation of i3 PKC sites Ser228/229. A phospho-specific antibody generated against phospho-Ser228/229 demonstrated PKC-induced phosphorylation at these sites of dopamine D(2S), but not D(2L) receptors, in Ltk(-) cells. Conversely, the pseudosubstrate dopamine D(2L) receptor mutant displayed PKC-induced phosphorylation at Ser228/229, which was abolished when these sites were mutated. Similar phosphorylation results were observed using GH4 cells stably transfected with dopamine D(2) receptors and mutants. Thus the relative location of phosphorylation and pseudosubstrate sites provides an important determinant substrate sensitivity to PKC.

A Nurr1 point mutant, implicated in Parkinson's disease, uncouples ERK1/2-dependent regulation of tyrosine hydroxylase transcription

The orphan nuclear receptor NURR1 is critical for the development of mesencephalic dopamine neurons and directly regulates tyrosine hydroxylase (TH) via specific NGFI-B response elements (NBRE). We identified a Parkinson's disease patient with a NURR1 mutation, resulting in a p.Ser125Cys change, immediately adjacent to the putative ERK1/2 phosphorylation site. Here we show, in dopaminergic SK-N-AS human neuroblastoma cells, that this substitution markedly attenuated NURR1-induced transcriptional activation through a human TH promoter NBRE. Furthermore, in SK-N-AS cells co-transfected with the dopamine-D2S receptor and NURR1, the dopamine-D2 agonist quinpirole stimulated ERK1/2 phosphorylation and enhanced transcriptional activation by wild-type NURR1 but not the p.Ser125Cys NURR1 mutant, and these actions were blocked by the specific MEK1/2 inhibitor PD98059. These results indicate that Ser125 is critical for basal and ERK1/2-induced NURR1 activity and suggest a role for this and other NURR1 mutations in the regulation of dopamine synthesis and predisposition to Parkinson's disease.