D1 dopamine receptor mediates dopamine-induced cytotoxicity via the ERK signal cascade

Postsynaptic striatal neurodegeneration occurs through unknown mechanisms, but it is linked to high extracellular levels of synaptic dopamine. Dopamine-mediated cytotoxicity of striatal neurons occurs through two distinct pathways: autoxidation and the D1 dopamine receptor-linked signaling pathway. Here we investigated the mitogen-activated protein kinase (MAPK) signaling pathways activated upon the acute stimulation of D1 dopamine receptors. In SK-N-MC neuroblastoma cells, endogenously expressing D1 dopamine receptors, dopamine caused activation of phosphorylated (p-)ERK1/2 and of the stress-signaling kinases, p-JNK and p-p38 MAPK, in a time- and dose-dependent manner. Selective stimulation of D1 receptors with the agonist SKF R-38393 caused p-ERK1/2, but not p-JNK or p-p38 MAPK activation, in a manner sensitive to the receptor-selective antagonist SCH 23390, protein kinase A inhibition (KT5720), and MEK1/2 inhibition (U0126 or PD98059). Activation of ERK by D1 dopamine receptors resulted in oxidative stress and cytotoxicity. In cells transfected with a catalytically defective mutant of MEK1, the upstream ERK-specific kinase, both dopamine- and SKF R-38393-mediated cytotoxicity was markedly attenuated, confirming the participation of the ERK signaling pathway. Cell fractionation studies showed that only a small amount of p-ERK1/2 was translocated to the nucleus, with the majority retained in the cytoplasm. From coimmunoprecipitation studies, p-ERK was found to form stable heterotrimeric complexes with the D1 dopamine receptor and beta-arrestin2. In cells transfected with the dominant negative mutant of beta-arrestin2, the formation of such complexes was substantially inhibited. These data provide novel mechanistic insights into the role of ERK in the cytotoxicity mediated upon activation of the D1 dopamine receptor.     

The tyrosine phosphatase Shp-2 interacts with the dopamine D(1) receptor and triggers D(1) -mediated Erk signaling in striatal neurons

We report a novel mechanism for dopamine D(1) receptor (D(1) R)-mediated extracellular signal-regulated kinases (Erk) activation in rat striatum. Erk signaling depends on phosphorylation and dephosphorylation events mediated by specific kinases and phosphatases. The tyrosine phosphatase Shp-2, that is required for Erk activation by tyrosine kinase receptors, has been recently shown to regulate signaling downstream of few G protein-coupled receptors. We show that the D(1) R interacts with Shp-2, that D(1) R stimulation results in Shp-2 tyrosine phosphorylation and activation in primary striatal neuronal cultures and that D(1) R/Shp-2 interaction is required for transmitting D(1) R-dependent signaling to Erk1/2 activation. D(1) R-mediated Erk1/2 phosphorylation in cultured striatal neurons is in fact abolished by over-expression of the inactive Shp-2(C/S) mutant and by small interfering RNA-induced Shp-2 silencing. Moreover, by using selective inhibitors we show that both D(1) R-induced Shp-2 activation and Erk1/2 phosphorylation are dependent on the cyclic AMP/protein kinase A pathway and require Src. These results, which were substantiated also in transfected human embryonic kidney 293 cells, provide a novel mechanism by which to converge D(1) R signaling to the Erk pathway and suggest that Shp-2 or the D(1) R/Shp-2 interface could represent a potential drug target for disorders of dopamine transmission involving malfunctioning of D(1) R signaling.     

Reciprocal modulation of function between the D1 and D2 dopamine receptors and the Na+,K+-ATPase

It is well documented that dopamine can increase or decrease the activity of the Na+,K+-ATPase (NKA, sodium pump) in an organ-specific fashion. This regulation can occur, at least partially, via receptor-mediated second messenger activation and can promote NKA insertion or removal from the plasma membrane. Using co-immunoprecipitation and mass spectrometry, we now show that, in both brain and HEK293T cells, D1 and D2 dopamine receptors (DARs) can exist in a complex with the sodium pump. To determine the impact of NKA on DAR function, biological assays were conducted with NKA and DARs co-expressed in HEK293T cells. In this system, expression of NKA dramatically decreased D1 and D2 DAR densities with a concomitant functional decrease in DAR-mediated regulation of cAMP levels. Interestingly, pharmacological inhibition of endogenous or overexpressed NKA enhanced DAR function without altering receptor number or localization. Similarly, DAR function was also augmented by small interfering RNA reduction of the endogenous NKA. These data suggest that, under basal conditions, NKA negatively regulates DAR function via protein-protein interactions. In reciprocal fashion, expression of DARs decreases endogenous NKA function in the absence of dopamine, implicating DAR proteins as regulators of NKA activity. Notably, dopamine stimulation or pertussis toxin inhibition of D2 receptor signaling did not alter NKA activity, indicating that the D2-mediated decrease in NKA function is dependent upon protein-protein interactions rather than signaling molecules. This evidence for reciprocal regulation between DARs and NKA provides a novel control mechanism for both DAR signaling and cellular ion balance.     

Dopamine D1 receptor-induced signaling through TrkB receptors in striatal neurons

In addition to its role as a neurotransmitter, dopamine can stimulate neurite outgrowth and morphological effects upon primary neurons. To investigate the signal transduction mechanisms used by dopamine in developing striatal neurons, we focused upon the effects of activating the dopamine D1 receptor. Using the D1 receptor agonist SKF38393, we found that Trk neurotrophin receptors were activated in embryonic day 18 striatal neurons. K-252a, a Trk tyrosine kinase inhibitor, and a dopamine D1 receptor antagonist could block the effects of SKF38393. The increase in TrkB phosphorylation was not the result of increased neurotrophin production. Induction of TrkB activity by SKF38393 was accompanied by the phosphorylation of several Trk signaling proteins, including phospholipase Cgamma, Akt, and MAPK. Biotinylation experiments followed by immunostaining by phospho-TrkB-specific antibodies indicated that the mechanism involved increased TrkB surface expression by dopamine D1 receptor activation. This increase in cell surface TrkB expression was dependent upon an increase in intracellular Ca(2+). These results indicate that stimulation of dopamine D1 receptors can be coupled to the neurotrophin receptor signaling to mediate the effects of dopamine upon striatal neurons.     

Differential regulation of dopamine D1 and D2 signaling by nicotine in neostriatal neurons

Nicotine, acting on nicotinic acetylcholine receptors (nAChRs) expressed at pre-synaptic dopaminergic terminals, has been shown to stimulate the release of dopamine in the neostriatum. However, the molecular consequences of pre-synaptic nAChR activation in post-synaptic neostriatal neurons are not clearly understood. Here, we investigated the effect of nAChR activation on dopaminergic signaling in medium spiny neurons by measuring phosphorylated DARPP-32 (dopamine- and cAMP-regulated phosphoprotein of Mr 32 kDa) at Thr34 (the PKA-site) in mouse neostriatal slices. Nicotine produced dose-dependent responses, with a low concentration (1 microm) causing a sustained decrease in DARPP-32 Thr34 phosphorylation and a high concentration (100 microm) causing a transient increase in DARPP-32 Thr34 phosphorylation. Depending on the concentration of nicotine, either dopamine D2 or D1 receptor signaling was predominantly activated. Nicotine at a low concentration (1 microm) activated dopamine D2 receptor signaling in striatopallidal/indirect pathway neurons, likely by activating alpha4beta2* nAChRs at dopaminergic terminals. Nicotine at a high concentration (100 microm) activated dopamine D1 receptor signaling in striatonigral/direct pathway neurons, likely by activating (i) alpha4beta2* nAChRs at dopaminergic terminals and (ii) alpha7 nAChRs at glutamatergic terminals, which, by stimulating the release of glutamate, activated NMDA/AMPA receptors at dopaminergic terminals. The differential effects of low and high nicotine concentrations on D2- and D1-dependent signaling pathways in striatal neurons may contribute to dose-dependent actions of this drug of abuse.     

Chronic stimulation of D1 dopamine receptors in human SK-N-MC neuroblastoma cells induces nitric-oxide synthase activation and cytotoxicity

Elevated synaptic levels of dopamine may induce striatal neurodegeneration in l-DOPA-unresponsive parkinsonism subtype of multiple system atrophy (MSA-P subtype), multiple system atrophy, and methamphetamine addiction. We examined the participation of dopamine and D1 dopamine receptors in the genesis of postsynaptic neurodegeneration. Chronic treatment of human SK-N-MC neuroblastoma cells with dopamine or H2O2 increased NO production and accelerated cytotoxicity, as indexed by enhanced nitrite levels and cell death. The antioxidant sodium metabisulfite or SCH 23390, a D1 dopamine receptor-selective antagonist, partially blocked dopamine effects but together ablated dopamine-mediated cytotoxicity, indicating the participation of both autoxidation and D1 receptor stimulation. Direct activation of D1 dopamine receptors with SKF R-38393 caused cytotoxicity, which was refractory to sodium metabisulfite. Dopamine and SKF R-38393 induced overexpression of the nitric-oxide synthase (NOS) isoforms neuronal NOS, inducible NOS (iNOS), and endothelial NOS in a protein kinase A-dependent manner. Functional studies showed that approximately 60% of total NOS activity was due to activation of iNOS. The NOS inhibitor N(G)-nitro-l-arginine methyl ester and genistein, wortmannin, or NF-kappaB SN50, inhibitors of protein tyrosine kinases phosphatidylinositol 3-kinase and NF-kappaB, respectively, reduced nitrite production by dopamine and SKF R-38393 but were less effective in attenuating H2O2-mediated effects. In rat striatal neurons, dopamine and SKF R-38393, but not H2O2, accelerated cell death through increased expression of neuronal NOS and iNOS but not endothelial NOS. These data demonstrate a novel pathway of dopamine-mediated postsynaptic oxidative stress and cell death through direct activation of NOS enzymes by D1 dopamine receptors and its associated signaling pathways.     

Association of the D2 dopamine receptor third cytoplasmic loop with spinophilin, a protein phosphatase-1-interacting protein.

Signaling through D2 class dopamine receptors is crucial to correct brain development and function, and dysfunction of this system is implicated in major neurological disorders such as Parkinson's disease and schizophrenia. To investigate potential novel mechanisms of D2 receptor regulation, the third cytoplasmic loop of the D2 dopamine receptor was used to screen a rat hippocampal yeast two-hybrid library. Spinophilin, a recently characterized F-actin and protein phosphatase-1-binding protein with a single PDZ domain was identified as a protein that specifically associates with this region of D2 receptors. A direct interaction between spinophilin and the D2 receptor was confirmed in vitro using recombinant fusion proteins. The portion of spinophilin responsible for interacting with the D2 third cytoplasmic loop was narrowed to a region that does not include the actin-binding domain, the PDZ domain, or the coiled-coil. This region is distinct from the site of interaction with protein phosphatase-1, and both D2 receptors and protein phosphatase-1 may bind spinophilin at the same time. The interaction is not mediated via the unique 29-amino acid insert in D2long; both D2long and D2short third cytoplasmic loops interact with spinophilin in vitro and in yeast two-hybrid assays. Expression of D2 receptors containing an extracellular hemagglutinin epitope in Madin-Darby canine kidney cells results in co-localization of receptor and endogenous spinophilin as determined by immunocytochemistry using antibodies directed against spinophilin and the HA tag. We hypothesize that spinophilin is important for establishing a signaling complex for dopaminergic neurotransmission through D2 receptors by linking receptors to downstream signaling molecules and the actin cytoskeleton.     

Interactions of GIPC with dopamine D2, D3 but not D4 receptors define a novel mode of regulation of G protein-coupled receptors

The C-terminus domain of G protein-coupled receptors confers a functional cytoplasmic interface involved in protein association. By screening a rat brain cDNA library using the yeast two-hybrid system with the C-terminus domain of the dopamine D(3) receptor (D(3)R) as bait, we characterized a new interaction with the PDZ domain-containing protein, GIPC (GAIP interacting protein, C terminus). This interaction was specific for the dopamine D(2) receptor (D(2)R) and D(3)R, but not for the dopamine D(4) receptor (D(4)R) subtype. Pull-down and affinity chromatography assays confirmed this interaction with recombinant and endogenous proteins. Both GIPC mRNA and protein are widely expressed in rat brain and together with the D(3)R in neurons of the islands of Calleja at plasma membranes and in vesicles. GIPC reduced D(3)R signaling, cointernalized with D(2)R and D(3)R, and sequestered receptors in sorting vesicles to prevent their lysosomal degradation. Through its dimerization, GIPC acts as a selective scaffold protein to assist receptor functions. Our results suggest a novel function for GIPC in the maintenance, trafficking, and signaling of GPCRs.     

Dopamine D(1) and D(3) receptors oppositely regulate NMDA- and cocaine-induced MAPK signaling via NMDA receptor phosphorylation

Development of drug addiction involves complex molecular changes in the CNS. The mitogen-activated protein kinase (MAPK) signaling pathway plays a key role in mediating neuronal activation induced by dopamine, glutamate, and drugs of abuse. We previously showed that dopamine D(1) and D(3) receptors play different roles in regulating cocaine-induced MAPK activation. Although there are functional and physical interactions between dopamine and glutamate receptors, little is known regarding the involvement of D(1) and D(3) receptors in modulating glutamate-induced MAPK activation and underlying mechanisms. In this study, we show that D(1) and D(3) receptors play opposite roles in regulating N-methyl-d-aspartate (NMDA) -induced activation of extracellular signal-regulated kinase (ERK) in the caudate putamen (CPu). D(3) receptors also inhibit NMDA-induced activation of the c-Jun N-terminal kinase and p38 kinase in the CPu. NMDA-induced activation of the NMDA-receptor R1 subunit (NR1), Ca(2+)/calmodulin-dependent protein kinase II and the cAMP-response element binding protein (CREB), and cocaine-induced CREB activation in the CPu are also oppositely regulated by dopamine D(1) and D(3) receptors. Finally, the blockade of NMDA-receptor reduces cocaine-induced ERK activation, and inhibits phosphorylation of NR1, Ca(2+)/calmodulin-dependent protein kinase II, and CREB, while inhibiting ERK activation attenuates cocaine-induced CREB phosphorylation in the CPu. These results suggest that dopamine D(1) and D(3) receptors oppositely regulate NMDA- and cocaine-induced MAPK signaling via phosphorylation of NR1.     

Opposing effects of phorbol-12-myristate-13-acetate, an activator of protein kinase C, on the signaling of structurally related human dopamine D1 and D5 receptors

The ‘cross‐talk’ between different types of neurotransmitters through second messenger pathways represents a major regulatory mechanism in neuronal function. We investigated the effects of activation of protein kinase C (PKC) on cAMP‐dependent signaling by structurally related human D1‐like dopaminergic receptors. Human embryonic kidney 293 (HEK293) cells expressing D1 or D5 receptors were pretreated with phorbol‐12‐myristate‐13‐acetate (PMA), a potent activator of PKC, followed by analysis of dopamine‐mediated receptor activation using whole cell cAMP assays. Unpredictably, PKC activation had completely opposite effects on D1 and D5 receptor signaling. PMA dramatically augmented agonist‐evoked D1 receptor signaling, whereas constitutive and dopamine‐mediated D5 receptor activation were rapidly blunted. RT–PCR and immunoblotting analyses showed that phorbol ester‐regulated PKC isozymes (conventional: α, βI, βII, γ; novel: δ, ?, η, θ) and protein kinase D (PKCµ) are expressed in HEK293 cells. PMA appears to mediate these contrasting effects through the activation of Ca²⁺‐independent novel PKC isoforms as revealed by specific inhibitors, bisindolylmaleimide I, Gö6976, and Gö6983. The finding that cross‐talk between PKC and cAMP pathways can produce such opposite outcomes following the activation of structurally similar D1‐like receptor subtypes is novel and further strengthens the view that D1 and D5 receptors serve distinct functions in the mammalian nervous and endocrine systems.     

G protein-mediated mitogen-activated protein kinase activation by two dopamine D2 receptors

Two isoforms of dopamine D2 receptor, D2L (long) and D2S (short), differ by the insertion of 29 amino acids specific to D2L within the putative third intracellular loop of the receptor, which appears to be important in selectivity for G-protein coupling. We have generated D2L- and D2S-expressing Chinese hamster ovary (CHO) cells, and regulation of the mitogen-activated protein kinase (MAPK) pathway was examined in these cells. Both D2L and D2S mediated a rapid and transient activation of MAPK with dominant activation of p42-kDa MAPK. Pertussis toxin treatment completely abrogated stimulation of MAPK mediated by D2L and D2S, demonstrating that both receptors couple to pertussis toxin-sensitive G proteins in this signaling. Stimulation of MAPK mediated by both D2L and D2S receptor was markedly attenuated by coexpression of the C-terminus of beta-adrenergic receptor kinase (betaARKct), which selectively inhibits Gbetagamma-mediated signal transduction. Further analysis of D2L- and D2S-mediated MAPK activation demonstrated that D2L-mediated MAPK activation was not significantly affected by PKC depletion or partially affected by genistein. In contrast, D2S-mediated MAPK activation was potentially inhibited by PKC depletion and genistein was capable of completely inhibiting D2S-mediated MAPK activation. Together, these results suggest that D2L- and D2S-mediated MAPK activation is predominantly Gbetagamma subunit-mediated signaling and that protein kinase C and tyrosine phosphorylations are involved in these signaling pathways.     

Novel Dopamine D2 Receptor Signaling through Proteins Interacting with the Third Cytoplasmic Loop

The diverse activities of dopamine D2-like receptors, including D2, D3, and D4 receptors, are mediated by proteins that interact with the third cytoplasmic loop and regulate receptor signaling, receptor trafficking, and apoptosis. Such interacting proteins include calmodulin, the N-methyl-d-aspartate receptor 2B subunit, calcium/calmodulin-dependent protein kinase II, prostate apoptosis response-4, and β-arrestins, which regulate receptor signaling and the pharmacological action through D2 receptor. The gene encoding the D2 receptor gives rise to two isoforms, termed the dopamine D2 receptor long isoform (D2L) and the dopamine D2 receptor short isoform; the latter lacks 29 amino acids of the D2L receptor within the third cytoplasmic loop. In this review, we first focus on novel functions of the hetero-oligomeric D1/D2 and D2/adenosine A_2A receptors. We next discuss novel signaling through proteins interacting with the D2 receptor third cytoplasmic loop and define the function of a novel binding protein, heart-type fatty acid binding protein, which interacts with the D2L third cytoplasmic loop.     

Signaling via dopamine D1 and D3 receptors oppositely regulates cocaine-induced structural remodeling of dendrites and spines

Repeated exposure to cocaine can induce persistent alterations in the brain. The structural remodeling of dendrites and dendritic spines is thought to play a critical role in cocaine addiction. We previously demonstrated that signaling via dopamine D1 and D3 receptors have opposite effects on cocaine-induced gene expression. Here, we show that cocaine-induced structural remodeling in the nucleus accumbens (NAc) and caudoputamen (CPu) is mediated by D1 receptors and inhibited by D3 receptors. In addition, chronic exposure to cocaine results in an altered number of asymmetric spine synapses via the actions of both D1 and D3 receptors. The contradictory effects of D1 and D3 receptor signaling on cocaine-induced structural remodeling is associated with NMDA-receptor R1 subunit (NR1) phosphorylation, and is dependent upon the activation of extracellular signal-regulated kinase (ERK). In addition, we found that D1 and D3 receptor signaling has contradictory effects upon the activation of the myocyte enhancer factor 2 (MEF2), which is involved in the dendritic remodeling after cocaine treatment. Together, these data suggest that dopamine D1 and D3 receptors differentially regulate the cocaine-induced structural remodeling of dendrites and spines via mechanisms involving the consecutive actions of NR1 phosphorylation, ERK activation, and MEF2 activity in the NAc and CPu.     

D2-class dopamine receptor inhibition of NMDA currents in prefrontal cortical neurons is platelet-derived growth factor receptor-dependent

NMDA receptor function is modulated by both G-protein-coupled receptors and receptor tyrosine kinases. In acutely isolated rat hippocampal neurons, direct activation of the platelet-derived growth factor (PDGF) receptor or transactivation of the PDGF receptor by D4 dopamine receptors inhibits NMDA-evoked currents in a phospholipase C (PLC)-dependent manner. We have investigated further the ability of D2-class dopamine receptors to modulate NMDA-evoked currents in isolated rat prefrontal cortex (PFC). We have demonstrated that, similar to isolated hippocampal neurons, the application of PDGF-BB or quinpirole to isolated PFC neurons induces a slow-onset and long-lasting inhibition of NMDA-evoked currents. However, in contrast to hippocampal neurons, the inhibition of NMDA-evoked currents by quinpirole in PFC neurons is dependent upon D2/3, rather than D4, dopamine receptors. In PFC slices, application of both PDGF-BB and quinpirole induced a phosphorylation of the PDGF receptor at the PLCgamma binding and activation site, Tyr1021. The PDGF receptor kinase inhibitor, tyrphostin A9, and the D2/3 dopamine receptor antagonist, raclopride, inhibited quinpirole-induced Tyr1021 phosphorylation. These finding suggest that quinpirole treatment inhibits NMDAR signaling via PDGF receptor transactivation in both the hippocampus and the PFC, and that the effects of quinpirole in these regions are mediated by D4 and D2/3 dopamine receptors, respectively.     

Reduced D2-mediated signaling activity and trans-synaptic upregulation of D1 and D2 dopamine receptors in mice overexpressing the dopamine transporter

The dopamine transporter (DAT) regulates the temporal and spatial actions of dopamine by reuptaking this neurotransmitter into the presynaptic neurons. We recently generated transgenic mice overexpressing DAT (DAT-tg) that have a 3-fold increase in DAT protein levels which results in a 40% reduction of the extracellular DA concentration in the striatum. The aim of this study was to examine the effect of this reduction in dopaminergic tone on postsynaptic responses mediated by dopamine receptors. We report here that DAT-tg mice have increased levels of striatal D1 (30%) and D2 (approximately 60%) dopamine receptors with D1 receptor signaling components not significantly altered, as evidenced by unaffected basal or stimulated levels of phospho-GluR1 (Ser845) and phospho-ERK2. However, the novel D2 mediated Akt signaling is markedly altered in DAT-tg animals. In particular, there is a 300% increase in the basal levels of phospho-Akt in the striatum of DAT-tg, reflecting the reduced extracellular dopamine tone in these animals. This increase in basal pAkt levels can be pharmacologically recapitulated by partial dopamine depletion in WT mice treated with the selective tyrosine hydroxylase inhibitor alpha-methyl-para-tyrosine (alpha-MPT). Behaviorally, DAT-tg animals demonstrate an augmented synergistic interaction between up-regulated D1 and D2 receptors, which results in increased climbing behavior in transgenic mice after stimulation with either apomorphine or a co-administration of selective D1 and D2 receptor agonists. In sum, our study reveals that hypodopaminegia caused by up-regulation of DAT results in significant alterations at postsynaptic receptor function with most notable dysregulation at the level of D2 receptor signaling.     

Distribution of D(5) dopamine receptor mRNA in rat ventromedial hypothalamic nucleus

Estrogen induces lordosis through, in part, estrogen receptor (ER)-mediated synthesis of progesterone receptors (PR) in the ventromedial nucleus (VMN). In vitro, PR is activated by the neurotransmitter dopamine through D1-like receptors (1). In vivo, lordosis is induced by dopamine, an effect mediated in part by PR and D(5) dopamine receptors. The purpose of the present study was to determine mRNA distribution of D1-like receptors in the female rat brain using RT-PCR combined with punchout microdissection techniques. Employing specific primers to D(5) and D(1) dopamine receptors, we found detectable expression levels of D(5) dopamine receptor mRNA in VMN as well as the arcuate nucleus/median eminence (ArcN/ME). In contrast, D(1) dopamine receptor mRNA was detected only in VMN. By using this highly sensitive and specific RT-PCR methodology, we have confirmed the presence of D(5) dopamine receptor mRNA in an area of the brain that regulates reproductive behavior through PR. The data support the previous observation that D(5) dopamine receptors in VMN contribute to facilitation of female reproductive behavior by D1-like agonists.     

Stimulated D(1) dopamine receptors couple to multiple Galpha proteins in different brain regions

Previous studies have revealed that activation of rat striatal D(1) dopamine receptors stimulates both adenylyl cyclase and phospholipase C via G(s) and G(q), respectively. The differential distribution of these systems in brain supports the existence of distinct receptor systems. The present communication extends the study by examining other brain regions: hippocampus, amygdala, and frontal cortex. In membrane preparations of these brain regions, selective stimulation of D(1) dopamine receptors increases the hydrolysis of phosphatidylinositol/phosphatidylinositol 4,5-biphosphate. In these brain regions, D(1) dopamine receptors couple differentially to multiple Galpha protein subunits. Antisera against Galpha(q) blocks dopamine-stimulated PIP(2) hydrolysis in hippocampal and in striatal membranes. The binding of [(35)S]GTPgammaS or [alpha-(32)P]GTP to Galpha(i) was enhanced in all brain regions. Dopamine also increased the binding of [(35)S]GTPgammaS or [alpha-(32)P]GTP to Galpha(q) in these brain regions: hippocampus = amygdala > frontal cortex. However, dopamine-stimulated binding of [(35)S]GTPgammaS to Galphas only in the frontal cortex and striatum. This differential coupling profile in the brain regions was not related to a differential regional distribution of the Galpha proteins. Dopamine induced increases in GTPgammaS binding to Galpha(s) and Galpha(q) was blocked by the D(1) antagonist SCH23390 but not by D(2) receptor antagonist l-sulpiride, suggesting that D(1) dopamine receptors couple to both Galpha(s) and Galpha(q) proteins. Co-immunoprecipitation of Galpha proteins with receptor-binding sites indicate that in the frontal cortex, D(1) dopamine-binding sites are associated with both Galpha(s) and Galpha(q) and, in hippocampus or amygdala, D(1) dopamine receptors couple solely to Galpha(q). The results indicate that in addition to the D(1)/G(s)/adenylyl cyclase system, brain D(1)-like dopamine receptor sites activate phospholipase C through Galpha(q) protein.     

D2 Dopamine Receptors Stimulate Mitogenesis Through Pertussis Toxin-Sensitive G Proteins and Ras-Involved ERK and SAP/JNK Pathways in Rat C6-D2L Glioma Cells

Abstract: Dopamine D2 receptors are members of the G protein-coupled receptor superfamily and are expressed on both neurons and astrocytes. Using rat C6 glioma cells stably expressing the rat D2L receptor, we show here that dopamine (DA) can activate both the extracellular signal-regulated kinase (ERK) and c-Jun NH₂-terminal kinase (JNK) pathways through a mechanism involving D2 receptor-G protein complexes and the Ras GTP-binding protein. Agonist binding to D2 receptors rapidly activated both kinases within 5 min, reached a maximum between 10 and 15 min, and then gradually decreased by 60 min. Maximal activation of both kinases occurred with 100 nM DA, which produced a ninefold enhancement of ERK activity and a threefold enhancement of JNK activity. DA-induced kinase activation was prevented by either (+)-butaclamol, a selective D2 receptor antagonist, or pertussis toxin, an uncoupler of G proteins from receptors, but not by (?)-butaclamol, the inactive isomer of (+)-butaclamol. Cotransfection of RasN17, a dominant negative Ras mutant, prevented DA-induced activation of both ERK and JNK. PD098059, a specific MEK1 inhibitor, also blocked ERK activation by DA. Transfection of SEK1(K → R) vector, a dominant negative SEK1 mutant, specifically prevented DA-induced JNK activation and subsequent c-Jun phosphorylation without effect on ERK activation. Furthermore, stimulation of D2 receptors promoted [³H]thymidine incorporation with a pattern similar to that for kinase activation. DA mitogenesis was tightly linked to Ras-dependent mitogen-activated protein kinase (MAPK) and JNK pathways. Transfection with RasN17 and application of PD098059 blocked DA-induced DNA synthesis. Transfection with FlagΔ169, a dominant negative c-Jun mutant, also prevented stimulation of [³H]thymidine incorporation by DA. The demonstration of D2 receptor-stimulated MAPK pathways may help to understand dopaminergic physiological functions in the CNS.