Rotation in response to selective dopamine receptor agonists in rats with electrolytic substantia nigra lesions

Rats with unilateral, electrolytic substantia nigra (ESN) lesions were tested for rotation following systemic injections of the D1 dopamine receptor agonist SKF38393 or the D2 agonist quinpirole. Only quinpirole produced significant levels of rotation, which was ipsilateral in direction. This rotation was potentiated by coadministration of the D1 agonist, and was significantly reduced by injections of either a D1 or D2 receptor antagonist. Other groups of lesioned animals were treated with reserpine for 5 days and were then tested for rotation in response to the agonists. In this case, SKF38393 produced significant levels of contralateral rotation, while quinpirole-induced rotation remained ipsilateral; coadministration of the D1 and D2 agonists resulted in pronounced ipsilateral rotation. These results stress a role for D1 receptor mechanisms in producing rotation, and suggest that different striatal efferent pathways mediate rotation in response to selective agonists following ESN lesions.     

Dopamine D1 Agonist SKF 38393 Increases the State of Phosphorylation of ARPP-21 in Substantia Nigra

Abstract: ARPP-21 is a cyclic AMP-regulated phosphoprotein (M_r= 21,000) that has a distribution in brain similar to that of DARPP-32 (dopamine- and cyclic AMP-regulated phosphoprotein, M_r= 32,000). It is enriched in the medium-sized spiny neurons in the striatum and in the striatonigral nerve terminals in the pars reticulata of the substantia nigra. The present study shows that dopamine D₁ agonist SKF 38393 increases the state of phosphorylation of ARPP-21 by 26% in nigral slices and that pretreatment of the slices with D₁ antagonist SCH 23390 blocks this effect. These results demonstrate that ARPP-21 is a dopamine-regulated phosphoprotein. Because D₁ receptors are localized on nerve terminals of striatonigral pathway, the phosphorylation of ARPP-21 is likely to mediate some of the intracellular effects of dopamine on these terminals.     

Inhibition of Dopamine Agonist-Induced Phosphoinositide Hydrolysis by Concomitant Stimulation of Cyclic AMP Formation in Brain Slices

Abstract: We examined the effects of cyclic AMP on dopamine receptor-coupled activation of phosphoinositide hydrolysis in rat striatal slices. Forskolin, dibutyryl cyclic AMP, and the protein kinase A activator Sp -cyclic adenosine monophosphothioate ( Sp -cAMPS) significantly inhibited inositol phosphate formation stimulated by the dopamine D₁ receptor agonist SKF 38393. Conversely, the protein kinase A antagonist Rp -cyclic adenosine monophosphothioate ( Rp -cAMPS) dose-dependently potentiated the SKF 38393 effect. In the presence of 200 µ M Rp -cAMPS, the dose-response curves of the dopamine D₁ receptor agonists SKF 38393 and fenoldopam were shifted to the left and maximal agonist responses were markedly increased. The agonist EC₅₀ values, however, were not significantly altered by protein kinase A inhibition. Neither Sp -cAMPS nor Rp -cAMPS significantly affected basal inositol phosphate accumulation. These findings demonstrate that dopaminergic stimulation of phosphoinositide hydrolysis is inhibited by elevations in intracellular cyclic AMP. Dopamine receptor agonists that stimulate adenylyl cyclase could suppress their activation of phosphoinositide hydrolysis by concomitantly stimulating the formation of cyclic AMP in striatal tissue. The interaction between dopamine D₁ receptor-stimulated elevations in cyclic AMP and dopaminergic stimulation of inositol phosphate formation suggests a cellular colocalization of these dopamine-coupled transduction pathways in at least some cells of the rat striatum.     

Binding of [3H]SKF 38393 to Dopamine D-l Receptors in Rat Striatum In Vitro; Estimation of Receptor Molecular Size by Radiation Inactivation

[3H]SKF 38393 (2,3,4,5-tetrahydro-7,8-dihydroxy-1-phenyl-1H-3-benzazepine) binds with high affinity to 3,4-dihydroxyphenylethylamine (dopamine) D-1 receptors in rat striatum in vitro (KD = 7 and 14 nM in nonfrozen and frozen striatum, respectively). The number of binding sites (Bmax) was approximately 80.0 pmol/g of original tissue, a value similar to the Bmax for the dopamine D-1 antagonist SCH 23390. Nondisplaceable [3H]SKF 38393 binding was approximately 45% of total binding. Irradiation (0-4 Mrad) of frozen whole striata decreased the number of [3H]SKF 38393 binding sites monoexponentially without changing the binding affinity. The functional molecular mass for the agonist dopamine D-1 binding site was 132,800 daltons, which is higher than the functional molecular mass of the antagonist dopamine D-1 binding site (approximately 80,000 daltons).     

Identification and characterization of functional D1 dopamine receptors in a human neuroblastoma cell line

Dopamine stimulated human neuroblastoma SK-N-MC cells to accumulated cyclic AMP. The D1 agonist SKF (R)-38393 also stimulated cyclic AMP production whereas the response to dopamine was inhibited by the D1 antagonist SCH (R)-23390. Membranes from SK-N-MC cells bound the D1 ligand [125I]SCH 23982 with a Kd of 2.1 nM and a Bmax of 102 fmol/mg protein. Binding was displaced by dopamine, SKF 38393, and SCH 23390. Up to 40% of the receptors were in an agonist high affinity, guanine nucleotide-sensitive state, compared to only 6% in rat striatum. A D1 photoaffinity probe labeled a 72 kDa protein in both SK-N-MC and rat striatal membranes. Thus, SK-N-MC human neuroblastoma cells contain D1 dopamine receptors which are similar to those found in mammalian striatum, but which are more tightly coupled to adenylate cyclase. SK-N-MC cells may be a useful model to investigate the properties and regulation of D1 dopamine receptors.     

SKF38393抑制大鼠DRG分离神经元GABA-激活电流

在大鼠新鲜分离ＤＲＧ神经元标本上应用全细胞膜片箝记录,观察了多巴胺Ｄ１受体的选择性激动剂ＳＫＦ３８３９３ＨＣＩ对ＧＡＢＡ－激活电流的作用。大部分受检细胞对ＧＡＢＡ敏感,１０＾－６－１０＾－３－ｍｏｌ／Ｌ ＧＡＢＡ可于引起呈剂量依赖性的明显去敏感作用的内向电流。     

Opposing Roles of Dopamine D1 and D2 Receptors in Nigral γ-[3H]Aminobutyric Acid Release?

This study examined the effects of dopamine D1 and D2 receptor agonists and antagonists on the spontaneous and calcium-dependent, K+-induced release of gamma-[3H]aminobutyric acid [( 3H]GABA) accumulated by slices of rat substantia nigra. SKF 38393 (D1 agonist) and dopamine (dual D1/D2 agonist) were without effect on [3H]GABA efflux by themselves (1-40 microM), or in the presence of the phosphodiesterase inhibitor isobutylmethylxanthine (IBMX) (0.5 mM), but potentiated evoked release in the presence of forskolin (0.5 microM), an adenylate cyclase activator. These increases in release were prevented by the D1 antagonist SCH 23390 (0.5 microM), but not by the D2 antagonist metoclopramide (0.5 microM). Higher concentrations of forskolin (10-40 microM) augmented stimulus-evoked [3H]GABA release directly, whereas dibutyryl cyclic AMP (100-200 microM) depressed it. Apomorphine, noradrenaline, and 5-hydroxytryptamine (1-40 microM) had no effect. The D2 stimulants lisuride, RU 24213, LY 171555, and bromocriptine dose-dependently inhibited depolarisation-induced but not basal [3H]GABA outflow. These inhibitory responses were not modified by the additional presence of SKF 38393 (10 microM) or SCH 23390 (1 microM), or by injection of 6-hydroxydopamine into the medial forebrain bundle 42 days earlier, but were attenuated by metoclopramide (0.5 microM). Higher amounts (10 microM) of SCH 23390, metoclopramide, or other D2 antagonists (loxapine, haloperidol) reduced evoked GABA release by themselves, probably by nonspecific mechanisms. These results suggest D1 and D2 receptors may have opposing effects on nigral GABA output and could explain the variable effects of mixed D1/D2 dopaminomimetics in earlier release and electrophysiological experiments.     

In the ventral tegmental area, progestins have actions at D1 receptors for lordosis of hamsters and rats that involve GABA A receptors

In the ventral tegmental area (VTA), progestins facilitate lordosis via actions at gamma-aminobutyric acid (GABA)(A)/benzodiazepine receptor complexes (GBRs) and dopamine type 1 receptors (D1). The relationship between progestins' actions at GBRs and D1 in the VTA for facilitating sexual behavior of hamsters and rats was examined. Ovariectomized (ovx), estradiol (E(2); 10 microg)+progesterone (P; 250 microg; SC)-primed hamsters, with bilateral guide cannulae to the VTA, were pre-tested for sexual and motor behavior and infused with the GBR antagonist bicuculline (100 ng/side) or vehicle. Thirty minutes later, hamsters were re-tested and then infused with the D1 agonist SKF38393 (100 ng/side) or vehicle. Hamsters were post-tested 30 min later. Ovx, E(2) (10 microg)-primed rats were pre-tested, infused first with bicuculline or vehicle, second with SKF38393 or vehicle, third with 5alpha-pregnan-3alpha-ol-20-one (3alpha,5alpha-THP; 0, 100, or 200 ng) and were post-tested 10 and 60 min after 3alpha,5alpha-THP infusions. VTA infusions of SKF38393 increased lordosis of hamsters or rats. Bicuculline pretreatment reduced SKF38393- and/or progesterone-mediated increases in lordosis of E2-primed hamsters. In E2-primed rats, bicuculline blocked SKF38393- and/or 3alpha,5alpha-THP-mediated increases in lordosis. There were no effects on motor behavior. Thus, in the VTA, GBR activity modulates D1-mediated actions for lordosis of hamsters and rats.     

Dopamine receptor-mediated regulation of striatal cholinergic activity: positron emission tomography studies with norchloro[18F]fluoroepibatidine

Large numbers of in vitro studies and microdialysis studies suggest that dopaminergic regulation of striatal acetylcholine (ACh) output is via inhibitory dopamine D2 receptors and stimulatory dopamine D1 receptors. Questions remain as to the relative predominance of dopamine D2 versus D1 receptor modulation of striatal ACh output under physiological conditions. Using positron emission tomography, we first demonstrate that norchloro[18F]fluoroepibatidine ([18F]NFEP), a selective nicotinic ACh receptor (nAChR) ligand, was sensitive to changes of striatal ACh concentration. We then examined the effect of quinpirole (D2 agonist), raclopride (D2 antagonist), SKF38393 (D1 agonist), and SCH23390 (D1 antagonist) on striatal binding of [18F]NFEP in the baboon. Pretreatment with quinpirole increased the striatum (ST) to cerebellum (CB) ratio by 26+/-6%, whereas pretreatment with raclopride decreased the ST/CB ratio by 22+/-2%. The ratio of the distribution volume of [18F]NFEP in striatum to that in cerebellum, which corresponds to (Bmax/K(D)) + 1 (index for nAChR availability), also showed a significant increase (29 and 20%; n = 2) and decrease (20+/-3%; n = 3) after pretreatment with quinpirole and raclopride, respectively. However, both the D1 agonist and antagonist had no significant effect. This suggests that under physiological conditions the predominant influence of endogenous dopamine on striatal ACh output is dopamine D2, not D1, receptor-mediated.     

Cannabinoid Receptor-Regulated Cyclic AMP Accumulation in the Rat Striatum

The present study demonstrates that desacetyllevonantradol, a synthetic cannabinoid analog, reduces cyclic AMP levels in rat striatal slices stimulated with vasoactive intestinal peptide or SKF 38393, a D1-dopamine agonist. Desacetyllevonantradol and the D2 agonist LY 171555 both inhibited D1-stimulated cyclic AMP accumulation in the striatum. Spiperone, a specific D2-dopamine antagonist, fully reversed the inhibitory effect of LY 171555 but not that of desacetyllevonantradol, indicating that this cannabinoid response is not occurring through a D2-dopaminergic mechanism. Morphine also inhibited cyclic AMP accumulation in striatal slices stimulated with either SKF 38393 or vasoactive intestinal peptide. Naloxone, an opioid antagonist, fully reversed the effect of morphine but not that of desacetyllevonantradol, indicating that cannabinoid drugs are not acting via a mechanism involving opioid receptors. The response to maximally effective concentrations of desacetyllevonantradol was not additive to that of maximally effective concentrations of either morphine or LY 171555, suggesting that dopaminergic, opioid, and cannabinoid receptors may be present on the same populations of cells.     

Forskolin Potentiates the Stimulation of Rat Striatal Adenylate Cyclase Mediated by D-1 Dopamine Receptors, Guanine Nucleotides, and Sodium Fluoride

We report here that forskolin acts in a synergistic manner with dopaminergic agonists, guanine nucleotides, or sodium fluoride to potentiate the stimulation of rat striatal adenylate cyclase mediated by these reagents. In the presence of 100 microM GTP, 100 microM guanyl-5'-yl imidodiphosphate [Gpp(NH)p], or 10 mM NaF, there is a greater than additive increase in forskolin-stimulated enzyme activity as well as a concomitant decrease (two- to fourfold) in the EC50 value for forskolin stimulation of striatal enzyme activity. In the presence of various concentrations of forskolin (10 nM-100 microM), the stimulation of adenylate cyclase elicited by GTP, Gpp(NH)p, and NaF is potentiated 194-1,825%, 122-1,141%, and 208-938%, respectively, compared with the stimulation by these agents above basal activity in the absence of forskolin. With respect to 3,4-dihydroxyphenylethylamine (dopamine) receptor-mediated stimulation of striatal enzyme activity, the stimulation of enzyme activity by dopaminergic agonists, in the absence or presence of forskolin, was GTP-dependent and could be antagonized by the selective D-1 antagonist SCH23390 (100 nM), indicating that these effects are mediated by D-1 dopamine receptors. In the presence of 100 microM GTP, forskolin at various concentrations markedly potentiates the stimulation elicited by submaximal as well as a maximally effective concentrations of dopamine (100 microM) and SKF38393 (1 microM). At higher concentrations of forskolin (10-100 microM) the stimulation elicited by the partial agonist SKF38393 is comparable to that of the full agonist dopamine.(ABSTRACT TRUNCATED AT 250 WORDS)     

Parallel Decrease of Glutamic Acid Decarboxylase and Preproenkephalin mRNA in the Rat Striatum Following Chronic Treatment with a Dopaminergic D1 Antagonist and D2 Agonist

The levels of mRNA encoding glutamic acid decarboxylase (GAD) and preproenkephalin (PPE) were measured by Northern blot analysis, in the dorsal and the ventral part of the striatum, following long-term treatments with drugs acting selectively on D1 or D2 dopaminergic receptors. Chronic injection of the selective D1 antagonist SCH 23390 elicited a significant decrease in level of both GAD and PPE mRNA (-30%) in the dorsal striatum, whereas no significant change was observed in the ventral striatum. Chronic administration of both SCH 23390 and RU 24926, a D2 agonist, decreased the GAD and PPE mRNA levels in the dorsal (-38 and -57%, respectively) as well as in the ventral (-70 and -60%, respectively) striatum. In the ventral striatum the marked reduction of GAD mRNA levels was paralleled by a significant decrease of Vmax values of GAD enzymatic activity (-41%). These results suggest that the decrease in content of both GAD and PPE mRNA, promoted by the chronic blockade of D1 receptors, is mainly due to the action of dopamine acting on unaffected D2 receptors. Indeed, this decrease is further amplified when the D2 agonist and the D1 antagonist are administered together. Our results substantiate further the molecular mechanisms by which dopamine acts on different populations of GABAergic and enkephalinergic neurons in the two striatal regions examined.     

The localization of GABA-Transaminase in the striato-nigral system

The localization of gamma-aminobutyric acid transaminase (GABA-T), the degrading enzyme for γ-aminobutyric acid, was examined in the striatum and substantia nigra using biochemical techniques. Selective destruction of the nigrostriatal dopaminergic system with 6-hydroxydopamine had no effect on the activity of GABA-T in either the striatum or the substantia nigra, although striatal tyrosine hydroxylase activity was reduced by half. Intrastriatal injection of kainic acid in adult rats resulted in a significant dose-dependent decrease in GABA-T activity in both the striatum and the substantia nigra. The decrease in both of these regions was significantly correlated with the decrease in the GABA synthetic enzyme glutamate decarboxylase (GAD). The intrastriatal injection of kainic acid in ten day old rats did not affect striatal GAD or GABA-T activities, although striatal choline acetyl-transferase activity was reduced by half.It is concluded that the GABA-T activity in the striatum is predominantly localized in neuronal elements, although not, apparently, in cholinergic neurons. Some GABA-T activity is also present in the terminals of the striatonigral neurons. However, the dopaminergic nigrostriatal neurons do not appear to contain GABA-T. It is suggested that high GABA-T activity may be characteristic of GABA neurons.     

Physicochemical modulation of agonist-induced [35s]GTPgammaS binding: implications for coexistence of multiple functional conformations of dopamine D1-like receptors

Dopamine agonist-stimulated [35S]GTPgammaS binding to membrane G proteins was studied in select brain regions under experimental conditions that permit the activation of receptor coupling to the G proteins Gi, Gs, or Gq. Agents studied were agonists known to be effective at various dopamine receptor/effector systems and included quinelorane (D2-like/Gi), SKF38393 (D1-like/Gq, D1-like/Gs), SKF85174 (D1-like/Gs), and SKF83959 (D1-like/Gq). Dopamine and SKF38393 significantly stimulated [35S]GTPgammaS binding to normal striatal membranes by 161% and 67% above controls. Deoxycholate, which enhances agonist-induced phospholipase C (PLC) stimulation, markedly enhanced the agonistic effects of dopamine and SKF38393 to 530% and 637% above controls, respectively. The enhancing effects of deoxycholate were reversed if it was washed off the membranes before agonist addition. The thiol-reducing agent, dithiothreitol, completely abolished the effects of SKF38393 and SKF83959, whereas SKF85174 effects were augmented. Agonist responses were concentration-related, and highest efficacies were obtained in the hippocampus, thus paralleling both the brain regional distribution and agonist efficacies previously observed in phosphoinositide hydrolysis assays. These findings suggest that D1-like receptor conformations that mediate agonist stimulation of Gs/adenylylcyclase may be structurally different from those that mediate Gq/PLC activation. Although the exact mechanism of deoxycholate's effect awaits elucidation, the results are consistent with the emerging concept of functional selectivity whereby deoxycholate could create a membrane environment that facilitates the transformation of the receptor from a conformation that activates Gs/adenylylcyclase to one that favors Gq/PLC signaling.     

Pertussis Toxin Attenuates D2 Inhibition and Enhances D1 Stimulation of Adenylate Cyclase by Dopamine in Rat Striatum

The response of adenylate cyclase to GTP and to dopamine (DA) was investigated in synaptic plasma membranes isolated from rat striatum injected with pertussis toxin, which inactivates the inhibitory guanine nucleotide-binding regulatory protein (Ni) of adenylate cyclase. Pertussis toxin treatment reverted the inhibitory effects on the enzyme activity elicited by micromolar concentrations of GTP and reduced by 50% the DA inhibition of cyclase activity via D2 receptors. The toxin treatment enhanced the net stimulation of enzyme activity by DA in the presence of micromolar concentrations of GTP. However, the stimulatory effect of the selective D1 receptor agonist SKF 38393 was not significantly affected. The data indicate that Ni mediates D2 inhibition of striatal adenylate cyclase and participates in the modulation of D1 stimulation of the enzyme activity by DA.     

Dopaminergic Modulation of Glutamate Release in Striatum as Measured by Microdialysis

Glutamate and aspartate are the primary neurotransmitters of projections from motor and premotor cortices to the striatum. Release of glutamate may be modulated by dopamine receptors located on corticostriatal terminals. The present study used microdialysis to investigate the dopaminergic modulation of in vivo striatal glutamate and aspartate release in the striatum of awake-behaving rats. Local perfusion with a depolarizing concentration of K+ through a dialysis probe into the rat striatum produced a significant increase in the release of glutamate, aspartate, and taurine. The D2 agonist LY171555 blocked the K(+)-induced release of glutamate and aspartate, but not taurine, in a concentration-dependent manner. The D1 agonist SKF 38393 did not alter K(+)-induced release of glutamate and taurine, but did significantly decrease aspartate release. Neither agonist had any effect on basal amino acid release. The D2 antagonist (-)-sulpiride reversed the inhibitory effects of LY 171555 on K(+)-induced glutamate release. These results provide in vivo evidence for a functional interaction between dopamine, the D2 receptor, and striatal glutamate release.     

Interactions of novel dopaminergic ligands with D-1 and D-2 dopamine receptors

The interactions of three novel dopaminergic ligands, SKF38393, SKF82526 and SKF83742, with D-1 and D-2 dopamine (DA) receptors have been investigated using radioligand binding techniques and computer modeling procedures. Using the bovine anterior pituitary D-2 DA receptor system, SKF38393 and SKF82526 behave as agonists demonstrating biphasic agonist/3H-antagonist competition curves. For both drugs, the high affinity phase comprised 30% of the total displacement curve. Such findings are atypical as previously tested classical dopamine agonists demonstrated high and low affinity displacement phases in equal proportions. Such behavior exhibited by the SKF agonists may be related to their activity as partial agonists. In contrast, SKF83742 behaves as an antagonist exhibiting homogeneous monophasic competition curves. Similar results are obtained in the rat striatal membrane D-2 DA receptor system. Both SKF38393 and SKF82526 also demonstrate shallow biphasic displacement curves on rat striatal D-1 receptors labeled with 3H-flupentixol whereas SKF83742/3H-flupentixol curves are uniphasic. Of all the ligands, only SKF38393 clearly demonstrates higher affinity for 3H-flupentixol labeled D-1 receptors.     

Differential regulation of somatodendritic and nerve terminal dopamine release by serotonergic innervation of substantia nigra

Nigrostriatal dopaminergic neurons release dopamine from dendrites in substantia nigra and axon terminals in striatum. The cellular mechanisms for somatodendritic and axonal dopamine release are similar, but somatodendritic and nerve terminal dopamine release may not always occur in parallel. The current studies used in vivo microdialysis to simultaneously measure changes in dendritic and nerve terminal dopamine efflux in substantia nigra and ipsilateral striatum respectively, following intranigral application of various drugs by reverse dialysis through the nigral probe. The serotonin releasers (+/-)-fenfluramine (100 micro m) and (+)-fenfluramine (100 micro m) significantly increased dendritic dopamine efflux without affecting extracellular dopamine in striatum. The non-selective serotonin receptor agonist 1-(m-chlorophenyl)-piperazine (100 micro m) elicited a similar pattern of dopamine release in substantia nigra and striatum. NMDA (33 micro m) produced an increase in nigral dopamine of a similar magnitude to mCPP or either fenfluramine drug. However, NMDA also induced a concurrent increase in striatal dopamine. The D2 agonist quinpirole (100 micro m) had a parallel inhibitory effect on dopamine release from dendritic and terminal sites as well. Taken together, these data suggest that serotonergic afferents to substantia nigra may evoke dendritic dopamine release through a mechanism that is uncoupled from the impulse-dependent control of nerve terminal dopamine release.     

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.