SKF 83959 is an antagonist of dopamine D1-like receptors in the prefrontal cortex and nucleus accumbens: a key to its antiparkinsonian effect in animals?

Task-Dependent Effects of SKF83959 on Operant Behaviors Associated With Distinct Changes of CaMKII Signaling in Striatal Subareas

BACKGROUND

SKF83959, an atypical dopamine (DA) D1 receptor agonist, has been used to test the functions of DA-related receptor complexes in vitro, but little is known about its impact on conditioned behavior. The present study examined the effects of SKF83959 on operant behaviors and assayed the neurochemical mechanisms involved.

METHODS

Male rats were trained and maintained on either a fixed-interval 30-second (FI30) schedule or a differential reinforcement of low-rate response 10-second (DRL10) schedule of reinforcement. After drug treatment tests, western blotting assayed the protein expressions of the calcium-/calmodulin-dependent protein kinase II (CaMKII) and the transcription factor cyclic AMP response element binding protein (CREB) in tissues collected from 4 selected DA-related areas.

RESULTS

SKF83959 disrupted the performance of FI30 and DRL10 behaviors in a dose-dependent manner by reducing the total number of responses in varying magnitudes. Moreover, the distinct profiles of the behavior altered by the drug were manifested by analyzing qualitative and quantitative measures on both tasks. Western-blot results showed that phospho-CaMKII levels decreased in the nucleus accumbens and the dorsal striatum of the drug-treated FI30 and DRL10 subjects, respectively, compared with their vehicle controls. The phospho-CREB levels decreased in the nucleus accumbens and the hippocampus of drug-treated FI30 subjects but increased in the nucleus accumbens of drug-treated DRL10 subjects.

CONCLUSIONS

Our results provide important insight into the neuropsychopharmacology of SKF83959, indicating that the drug-altered operant behavior is task dependent and related to regional-dependent changes of CaMKII-CREB signaling in the mesocorticolimbic DA systems.

Lack of effect of dopamine receptor blockade on SKF83959-altered operant behavior in male rats

Dopamine (DA) is important for the performance of operant behavior as revealed by psychopharmacological studies that manipulate the activity of DA subtype receptors. However, the effects of SKF83959, an atypical DA D1 receptor agonist, on operant behavior and the underlying pharmacological mechanisms remain unclear. The present study sought to determine whether blockade of DA D1- and D2-subtyped receptors would reverse the operant behavior altered by SKF83959. Male rats were trained to respond on either a fixed-interval 30 s (FI30) schedule or a differential reinforcement of low-rate response 10 s (DRL10) schedule, two timing-relevant tasks but with distinct reinforcement contingencies. Pharmacological evaluation was conducted with injection of a selective D1 (or D2) receptor antagonist alone or in combined with SKF83959 (1.0 mg/kg) following a stable baseline of behavioral performance. The results showed that SKF83959 treatment alone significantly disrupted the performance of FI30 and DRL10 behaviors mainly by showing the decreases of the response-related measures, and the distinct profiles of the behavior altered by the drug were manifested by the qualitative analysis of inter-response time data on both tasks. The effects of SKF83959 were not significantly affected/reversed by the pretreatment of either SCH23390 or eticlopride injected at the doses of 0.02 and 0.06 mg/kg; however, a subtle reversal effect was observed in the treatment of low-dose eticlopride. Despite that these results confirm the FI30 and DRL10 behaviors changed by SKF83959, the absence of pharmacological reversal effect by DA receptor antagonist suggests that either D1- or D2-subtyped receptors may not play a critical role in the alteration of timing-relevant operant behavior produced by SKF83959.

Polypharmacology of dopamine receptor ligands

Most neurological diseases have a multifactorial nature and the number of molecular mechanisms discovered as underpinning these diseases is continuously evolving. The old concept of developing selective agents for a single target does not fit with the medical need of most neurological diseases. The development of designed multiple ligands holds great promises and appears as the next step in drug development for the treatment of these multifactorial diseases. Dopamine and its five receptor subtypes are intimately involved in numerous neurological disorders. Dopamine receptor ligands display a high degree of cross interactions with many other targets including G-protein coupled receptors, transporters, enzymes and ion channels. For brain disorders like Parkinsońs disease, schizophrenia and depression the dopaminergic system, being intertwined with many other signaling systems, plays a key role in pathogenesis and therapy. The concept of designed multiple ligands and polypharmacology, which perfectly meets the therapeutic needs for these brain disorders, is herein discussed as a general ligand-based concept while focusing on dopaminergic agents and receptor subtypes in particular.

Dopamine D1 receptor stimulation modulates the formation and retrieval of novel object recognition memory: Role of the prelimbic cortex

Previous studies have shown that dopamine D1 receptor antagonists impair novel object recognition memory but the effects of dopamine D1 receptor stimulation remain to be determined. This study investigated the effects of the selective dopamine D1 receptor agonist SKF81297 on acquisition and retrieval in the novel object recognition task in male Wistar rats. SKF81297 (0.4 and 0.8 mg/kg s.c.) given 15 min before the sampling phase impaired novel object recognition evaluated 10 min or 24 h later. The same treatments also reduced novel object recognition memory tested 24 h after the sampling phase and when given 15 min before the choice session. These data indicate that D1 receptor stimulation modulates both the encoding and retrieval of object recognition memory. Microinfusion of SKF81297 (0.025 or 0.05 μg/side) into the prelimbic sub-region of the medial prefrontal cortex (mPFC) in this case 10 min before the sampling phase also impaired novel object recognition memory, suggesting that the mPFC is one important site mediating the effects of D1 receptor stimulation on visual recognition memory.

Allosteric modulation of sigma-1 receptors by SKF83959 inhibits microglia-mediated inflammation

Recent studies have shown that sigma-1 receptor orthodox agonists can inhibit neuroinflammation. SKF83959 (3-methyl-6-chloro-7,8-hydroxy-1-[3-methylphenyl]-2,3,4,5-tetrahydro-1H-3-benzazepine), an atypical dopamine receptor-1 agonist, has been recently identified as a potent allosteric modulator of sigma-1 receptor. Here, we investigated the anti-inflammatory effects of SKF83959 in lipopolysaccharide (LPS)-stimulated BV2 microglia. Our results indicated that SKF83959 significantly suppressed the expression/release of the pro-inflammatory mediators, such as tumor necrosis factor-α (TNF-α), interleukin-1β (IL-1β), inducible nitric oxide synthase (iNOS), and inhibited the generation of reactive oxygen species. All of these responses were blocked by selective sigma-1 receptor antagonists (BD1047 or BD1063) and by ketoconazole (an inhibitor of enzyme cytochrome c17 to inhibit the synthesis of endogenous dehydroepiandrosterone, DHEA). Additionally, we found that SKF83959 promoted the binding activity of DHEA with sigma-1 receptors, and enhanced the inhibitory effects of DHEA on LPS-induced microglia activation in a synergic manner. Furthermore, in a microglia-conditioned media system, SKF83959 inhibited the cytotoxicity of conditioned medium generated by LPS-activated microglia toward HT-22 neuroblastoma cells. Taken together, our study provides the first evidence that allosteric modulation of sigma-1 receptors by SKF83959 inhibits microglia-mediated inflammation. SKF83959 is a potent allosteric modulator of sigma-1 receptor. Our results indicated that SKF83959 enhanced the activity of endogenous dehydroepiandrosterone (DHEA) in a synergic manner, and inhibited the activation of BV2 microglia and the expression/release of the pro-inflammatory mediators, such as tumor necrosis factor-α (TNF-α), interleukin-1β (IL-1β), inducible nitric oxide synthase (iNOS).

The dopamine D1-D2 receptor heteromer exerts a tonic inhibitory effect on the expression of amphetamine-induced locomotor sensitization

A role for the dopamine D1-D2 receptor heteromer in the regulation of reward and addiction-related processes has been previously implicated. In the present study, we examined the effects of D1-D2 heteromer stimulation by the agonist SKF 83959 and its disruption by a selective TAT-D1 peptide on amphetamine-induced locomotor sensitization, a behavioral model widely used to study the neuroadaptations associated with psychostimulant addiction. D1-D2 heteromer activation by SKF 83959 did not alter the acute locomotor effects of amphetamine but significantly inhibited amphetamine-induced locomotor responding across the 5day treatment regimen. In addition, a single injection of SKF 83959 was sufficient to abolish the expression of locomotor sensitization induced by a priming injection of amphetamine after a 72-hour withdrawal. Conversely, inhibition of D1-D2 heteromer activity by the TAT-D1 peptide enhanced subchronic amphetamine-induced locomotion and the expression of amphetamine locomotor sensitization. Treatment solely with the TAT-D1 disrupting peptide during the initial 5day treatment phase was sufficient to induce a sensitized locomotor phenotype in response to the priming injection of amphetamine. Together these findings demonstrate that the dopamine D1-D2 receptor heteromer exerts a tonic inhibitory control on neurobiological processes involved in sensitization to amphetamine, indicating that the dopamine D1-D2 receptor heteromer may be a novel molecular substrate in addiction processes involving psychostimulants.

Regulation of c-fos expression by the dopamine D1-D2 receptor heteromer

The dopamine D1 and D2 receptors form the D1-D2 receptor heteromer in a subset of neurons and couple to the Gq protein to regulate intracellular calcium signaling. In the present study the effect of D1-D2 heteromer activation and disruption on neuronal activation in the rat brain was mapped. This was accomplished using the dopamine agonist SKF 83959 to activate the D1-D2 heteromer in combination with a TAT-D1 disrupting peptide we developed, and which has been shown to disrupt the D1/D2 receptor interaction and antagonize D1-D2 heteromer-induced cell signaling and behavior. Acute SKF 83959 administration to rats induced significant c-fos expression in the nucleus accumbens that was significantly inhibited by TAT-D1 pretreatment. No effects of SKF 83959 were seen in caudate putamen. D1-D2 heteromer disruption by TAT-D1 did not have any effects in any striatal subregions, but induced significant c-fos immunoreactivity in a number of cortical regions including the orbitofrontal cortex, prelimbic and infralimbic cortices and piriform cortex. The induction of c-fos by TAT-D1 was also evident in the anterior olfactory nucleus, as well as the lateral habenula and thalamic nuclei. These findings show for the first time that the D1-D2 heteromer can differentially regulate c-fos expression in a region-dependent manner either through its activation or through tonic inhibition of neuronal activity.

SKF83959 is a novel triple reuptake inhibitor that elicits anti-depressant activity

AIM

SKF83959 (3-methyl-6-chloro-7,8-hydroxy-1-(3-methylphenyl)-2,3,4,5-tetrahydro-1H-3-benzazepine) is an atypical dopamine receptor-1 (D1 receptor) agonist, which exhibits many D1 receptor-independent effects. In the present work, we examined the effects of SKF83959 on monoaminergic transporters in vitro and its anti-depressant activity in vivo.

METHODS

Human serotonin transporter (SERT), norepinephrine transporters (NET) or dopamine transporters (DAT) were stably expressed in CHO cells. The uptake kinetics of SERT, NET, and DAT were examined using [(3)H]-serotonin, [(3)H]-norepinephrine or [(3)H]-dopamine, respectively. A triple reuptake inhibitor DOV21947 was used as the positive control. Tail suspension test and forced swimming test were conducted in mice. SKF83959 or DOV21947 (2-8 mg/kg) were intraperitoneally injected 30 min before the tests.

RESULTS

SKF83959 was a competitive inhibitor of SERT (K(i)=1.43±0.45 μmol/L), but a noncompetitive inhibitor of NET (K(i)=0.60±0.07 μmol/L) and DAT (K(i)=9.01±0.80 μmol/L). In contrast, DOV21947 was a competitive inhibitor of SERT (K(i)=0.89±0.24 μmol/L) and DAT (K(i)=1.47±0.31 μmol/L) and a noncompetitive inhibitor of NET (K(i)=0.18±0.04 μmol/L). In mice, both SKF83959 and DOV21947 elicited anti-depressant activity in a dose-dependent manner.

CONCLUSION

SKF83959 functions as a novel triple reuptake inhibitor in vitro and exerts anti-depressant effects in vivo.

D1-D2 dopamine receptor synergy promotes calcium signaling via multiple mechanisms

The D(1) dopamine receptor (D(1)R) has been proposed to form a hetero-oligomer with the D(2) dopamine receptor (D(2)R), which in turn results in a complex that couples to phospholipase C-mediated intracellular calcium release. We have sought to elucidate the pharmacology and mechanism of action of this putative signaling pathway. Dopamine dose-response curves assaying intracellular calcium mobilization in cells heterologously expressing the D(1) and D(2) subtypes, either alone or in combination, and using subtype selective ligands revealed that concurrent stimulation is required for coupling. Surprisingly, characterization of a putative D(1)-D(2) heteromer-selective ligand, 6-chloro-2,3,4,5-tetrahydro-3-methyl-1-(3-methylphenyl)-1H-3-benzazepine-7,8-diol (SKF83959), found no stimulation of calcium release, but it did find a broad range of cross-reactivity with other G protein-coupled receptors. In contrast, SKF83959 appeared to be an antagonist of calcium mobilization. Overexpression of G(qα) with the D(1) and D(2) dopamine receptors enhanced the dopamine-stimulated calcium response. However, this was also observed in cells expressing G(qα) with only the D1R. Inactivation of Gi or Gs with pertussis or cholera toxin, respectively, largely, but not entirely, reduced the calcium response in D(1)R and D(2)R cotransfected cells. Moreover, sequestration of G(βγ) subunits through overexpression of G protein receptor kinase 2 mutants either completely or largely eliminated dopamine-stimulated calcium mobilization. Our data suggest that the mechanism of D(1)R/D(2)R-mediated calcium signaling involves more than receptor-mediated G(q) protein activation, may largely involve downstream signaling pathways, and may not be completely heteromer-specific. In addition, SKF83959 may not exhibit selective activation of D(1)-D(2) heteromers, and its significant cross-reactivity to other receptors warrants careful interpretation of its use in vivo.

SKF83959 is a potent allosteric modulator of sigma-1 receptor

SKF83959 (3-methyl-6-chloro-7,8-hydroxy-1-[3-methylphenyl]-2,3,4,5-tetrahydro-1H-3-benzazepine), an atypical dopamine receptor-1 (D(1) receptor) agonist, has shown many D(1) receptor-independent effects, such as neuroprotection, blockade of Na(+) channel, and promotion of spontaneous glutamate release, which resemble the effects of the sigma-1 receptor activation. In the present work, we explored the potential modulation of SKF83959 on the sigma-1 receptor. The results indicated that SKF83959 dramatically promoted the binding of (3)H(+)-pentazocine (a selective sigma-1 receptor agonist) to the sigma-1 receptor in brain and liver tissues but produced no effect on (3)H-progesterone binding (a sigma-1 receptor antagonist). The saturation assay and the dissociation kinetics assay confirmed the allosteric effect. We further demonstrated that the SKF83959 analogs, such as SCH22390 [(R)-(1)-7-chloro-8- hydroxy-3-methyl-1-phenyl-2,3,4,5-tetrahydro-1H-3-benzazepine hydrochloride] and SKF38393 [(+/-)-1-phenyl-2,3,4,5-tetrahydro-(1H)-3-benzazepine-7,8-diol hydrobromide], also showed the similar allosteric effect on the sigma-1 receptor in the liver tissue but not in the brain tissue. Moreover, all three tested chemicals elicited no significant effect on (3)H-1,3-di(2-tolyl)-guanidine ((3)H-DTG) binding to the sigma-2 receptor. The present data uncovered a new role of SKF83959 and its analogs on the sigma-1 receptor, which, in turn, may reveal the underlying mechanism for the D(1) receptor-independent effect of the drug.

Neurobehavioral phenotyping of G(αq) knockout mice reveals impairments in motor functions and spatial working memory without changes in anxiety or behavioral despair

Many neurotransmitters, hormones, and sensory stimuli elicit their cellular responses through the targeted activation of receptors coupled to the G_αq family of heterotrimeric G proteins. Nevertheless, we still understand little about the consequences of loss of this signaling activity on brain function. We therefore examined the effects of genetic inactivation of Gnaq, the gene that encode for G_αq, on responsiveness in a battery of behavioral tests in order to assess the contribution of G_αq signaling capacity in the brain circuits mediating expression of affective behaviors (anxiety and behavioral despair), spatial working memory, and locomotor output (coordination, strength, spontaneous activity, and drug-induced responses). First, we replicated and extended findings showing clear motor deficits in G_αq knockout mice as assessed on an accelerating rotarod and the inverted screen test. We then assessed the contribution of the basal ganglia motor loops to these impairments, using open field testing and analysis of drug-induced locomotor responses to the psychostimulant cocaine, the benzazepine D₁ receptor agonists SKF83822 and SKF83959, and the NMDA receptor antagonist MK-801. We observed significant increases in drug-induced locomotor activity in G_αq knockout mice from the dopaminergic agonists but not MK-801, indicating that basal ganglia locomotor circuitry is largely intact in the absence of G_αq. Additionally, we observed normal phenotypes in both the elevated zero maze and the forced swim test indicating that anxiety and depression-related circuitry appears to be largely intact after loss of Gnaq expression. Lastly, use of the Y-maze revealed spatial memory deficits in G_αq knockout mice, indicating that receptors signaling through G_αq are necessary in these circuits for proficiency in this task.

Electrophysiological effects of SKF83959 on hippocampal CA1 pyramidal neurons: potential mechanisms for the drug's neuroprotective effects

Although the potent anti-parkinsonian action of the atypical D₁-like receptor agonist SKF83959 has been attributed to the selective activation of phosphoinositol(PI)-linked D₁ receptor, whereas the mechanism underlying its potent neuroprotective effect is not fully understood. In the present study, the actions of SKF83959 on neuronal membrane potential and neuronal excitability were investigated in CA1 pyramidal neurons of rat hippocampal slices. SKF83959 (10–100 µM) caused a concentration-dependent depolarization, associated with a reduction of input resistance in CA1 pyramidal neurons. The depolarization was blocked neither by antagonists for D₁, D₂, 5-HT_2A/2C receptors and α₁-adrenoceptor, nor by intracellular dialysis of GDP-β-S. However, the specific HCN channel blocker ZD7288 (10 µM) antagonized both the depolarization and reduction of input resistance caused by SKF83959. In voltage-clamp experiments, SKF83959 (10–100 µM) caused a concentration-dependent increase of Ih current in CA1 pyramidal neurons, which was independent of D₁ receptor activation. Moreover, SKF83959 (50 µM) caused a 6 mV positive shift in the activation curve of Ih and significantly accelerated the activation of Ih current. In addition, SKF83959 also reduced the neuronal excitability of CA1 pyramidal neurons, which was manifested by the decrease in the number and amplitude of action potentials evoked by depolarizing currents, and by the increase of firing threshold and rhoebase current. The above results suggest that SKF83959 increased Ih current through a D₁ receptor-independent mechanism, which led to the depolarization of hippocampal CA1 pyramidal neurons. These findings provide a novel mechanism for the drug's neuroprotective effects, which may contributes to its therapeutic benefits in Parkinson's disease.

Dopamine D(1) receptor activation of adenylyl cyclase, not phospholipase C, in the nucleus accumbens promotes maternal behavior onset in rats

A body of evidence supports the idea that the mesolimbic dopamine (DA) system modulates the natural increase in responsiveness female rats show toward offspring (biological or foster) at birth. In the absence of the full hormonal changes associated with pregnancy and birth, female rats do not show immediate responsiveness toward foster offspring. Activation of the mesolimbic DA system can produce an immediate onset of maternal behavior in these females. For example, female rats that are hysterectomized and ovariectomized on day 15 of pregnancy (15HO) and presented with pups 48 hours later normally show maternal behavior after 2-3 days of pup exposure, but will show maternal behavior on day 0 of testing after microinjection of the DA D(1) receptor agonist, SKF 38393, into the nucleus accumbens (NA) at the time of pup presentation. DA D(1) receptor stimulation is known to activate cAMP intracellular signaling cascades via its stimulation of adenylyl cyclase (AC). However, some DA D(1) receptors are also linked to phospholipase C (PLC) and are capable of activating phosphatidylinositol signaling cascades. SKF 38393 stimulates both types of D(1) receptors. Here we provide evidence that the facilitatory effects of DA D(1) receptor stimulation in the NA on maternal behavior are mediated by AC-linked DA D(1) receptors. By examining the effects of intra-NA application of SKF 83822, a drug which selectively binds DA D(1)-AC receptors, or SKF 83959, a drug which selectively activates D(1)-PLC-linked receptors, we find that only SKF 83822 facilitates maternal behavior onset.

The limbic circuitry underlying cocaine seeking encompasses the PPTg/LDT

The direct glutamatergic projection from the medial prefrontal cortex (mPFC) to the nucleus accumbens plays a critical role in mediating the reinstatement of cocaine seeking behavior. The mPFC also sends glutamatergic projections to the pedunculopontine tegmental nucleus (PPTg) and laterodorsal tegmental nucleus (LDT), which in turn send glutamatergic and cholinergic efferents to the ventral tegmental area (VTA) where they synapse on dopaminergic cells that innervate limbic structures including the nucleus accumbens. The goal of these experiments was to examine a potential role for the PPTg/LDT in the reinstatement of cocaine seeking. All rats were trained to self-administer cocaine (0.25 mg, i.v.) on a fixed-ratio 5 schedule of reinforcement. Cocaine self-administration behavior was extinguished and a series of subsequent pharmacological experiments were performed to assess the potential role of the mPFC, PPTg/LDT and VTA in the reinstatement of cocaine seeking. Administration of the D1-like dopamine receptor agonist SKF-81297 (1.0 microg) directly into the mPFC produced a small, but statistically significant, increase in cocaine seeking behavior. Furthermore, microinjection of the ionotropic glutamate receptor antagonist CNQX (0.3 microg) into the PPTg/LDT attenuated the reinstatement of drug seeking induced by a priming injection of cocaine (10 mg/kg, i.p.). Intra-VTA administration of CNQX, the nicotinic receptor antagonist mecamylamine (10.0 microg) or the muscarinic receptor antagonist scopolamine (24.0 microg) also blocked cocaine seeking. Taken together, these results suggest that cocaine priming-induced reinstatement of drug seeking is mediated in part by a serial polysynaptic limbic subcircuit encompassing the mPFC, PPTg/LDT and VTA.

Dopamine promotes striatal neuronal apoptotic death via ERK signaling cascades

Although the mechanisms underlying striatal neurodegeneration are poorly understood, we have shown that striatal pathogenesis may be initiated by high synaptic levels of extracellular dopamine (DA). Here we investigated in rat striatal primary neurons the mobilization of the mitogen-activated protein kinase (MAPK) signaling pathways after treatment with DA. Instead of observing an elevation of the archetypical pro-cytotoxic MAPKs, p-JNK and p-p38 MAPK, we found that DA, acting through D1 DA receptors, induced a sustained stimulation of the phosphorylated form of extracellular signal-regulated kinase (p-ERK) via a cAMP/protein kinase A (PKA)/Rap1/B-Raf / MAPK/ERK kinase (MEK) pathway. Blockade of D2 DA receptors, beta-adrenergic receptors or N-methyl-D-aspartate receptors with receptor-specific antagonists had no significant effect on this process. Activation of D1 DA receptors and PKA by DA caused phosphorylation and inactivation of the striatal-enriched tyrosine phosphatase, an important phosphatase for the dephosphorylation and subsequent inactivation of p-ERK in the striatum. Interestingly, p-ERK was primarily retained in the cytoplasm, with only low amounts translocated to the nucleus. The scaffold protein beta-arrestin2 interacted with both p-ERK and D1 DA receptor, triggering the cytosolic retention of p-ERK and inducing striatal neuronal apoptotic death. These data provide unique insight into a novel role of p-ERK in striatal neurodegeneration.

Arylbenzazepines are potent modulators for the delayed rectifier K+ channel: a potential mechanism for their neuroprotective effects

(+/-) SKF83959, like many other arylbenzazepines, elicits powerful neuroprotection in vitro and in vivo. The neuroprotective action of the compound was found to partially depend on its D(1)-like dopamine receptor agonistic activity. The precise mechanism for the (+/-) SKF83959-mediated neuroprotection remains elusive. We report here that (+/-) SKF83959 is a potent blocker for delayed rectifier K(+) channel. (+/-) SKF83959 inhibited the delayed rectifier K(+) current (I(K)) dose-dependently in rat hippocampal neurons. The IC(50) value for inhibition of I(K) was 41.9+/-2.3 microM (Hill coefficient = 1.81+/-0.13, n = 6), whereas that for inhibition of I(A) was 307.9+/-38.5 microM (Hill coefficient = 1.37+/-0.08, n = 6). Thus, (+/-) SKF83959 is 7.3-fold more potent in suppressing I(K) than I(A). Moreover, the inhibition of I(K) by (+/-) SKF83959 was voltage-dependent and not related to dopamine receptors. The rapidly onset of inhibition and recovery suggests that the inhibition resulted from a direct interaction of (+/-) SKF83959 with the K(+) channel. The intracellular application of (+/-) SKF83959 had no effects of on I(K), indicating that the compound most likely acts at the outer mouth of the pore of K(+) channel. We also tested the enantiomers of (+/-) SKF83959, R-(+) SKF83959 (MCL-201), and S-(-) SKF83959 (MCL-202), as well as SKF38393; all these compounds inhibited I(K). However, (+/-) SKF83959, at either 0.1 or 1 mM, exhibited the strongest inhibition on the currents among all tested drug. The present findings not only revealed a new potent blocker of I(K) , but also provided a novel mechanism for the neuroprotective action of arylbenzazepines such as (+/-) SKF83959.

Pharmacological characterization of the effects of dopamine D(1) agonists on eye blinking in rats

Dopamine D(1)-like partial agonists antagonize some abuse-related effects of cocaine and have been proposed as candidate medications for psychostimulant abuse. Earlier studies have showed that D(1)-like agonists increase eye blinking in monkeys and that the magnitude of this effect may be related to agonist efficacy. These studies characterized the effects of D(1)-like agonists on eye blinking in female Sprague-Dawley rats placed in customized restraint tubes. After vehicle injections, eye blink rates averaged 2.1+/-0.25 blinks/min, or 31+/-4 blinks/15 min. The D(1)-like agonists SKF 82958 and R(+)-6Br-APB dose-dependently increased eye blinking to 136 and 124/15 min, respectively. The selective D(1)-like antagonist SCH 23390 decreased eye blinking and the peripherally selective D(1)-like agonist fenoldopam, the D(2)-like agonist (+)PHNO, and the indirect dopamine agonist methamphetamine all failed to alter eye blink rates relative to vehicle levels. Additional studies with unique congeners and isomers of the D(1)-like partial agonist SKF 83959, MCL 202, MCL 204, MCL 206, MCL 207 and MCL 209, resulted in only moderate increases in eye blink rates (27-84 blinks/15 min). These effects were dose-related for one compound, MCL 209 (max 84+/-19 blinks/15 min) and plateaued at the highest doses, suggestive of partial agonist effects. Additionally, the agonist MCL 206, like the D(1)antagonist SCH 23390, antagonized the effects of SKF 82958 on eye blinking. The findings suggest that D(1)-like agonists increase eye blinking in rats and that these effects may provide a simple measure that can be used to distinguish partial D(1)-like ligands. Further studies with novel D(1)-like partial agonists may be useful in the development of pharmacotherapies for cocaine abuse.

Neuroprotective effects of atypical D1 receptor agonist SKF83959 are mediated via D1 receptor-dependent inhibition of glycogen synthase kinase-3 beta and a receptor-independent anti-oxidative action

3-methyl-6-chloro-7,8-hydroxy-1-(3-methylphenyl)-2,3,4,5-tetrahydro-1H-3-benzazepine (SKF83959), a selective agonist for the putative phosphatidylinositol (PI)-linked dopamine receptor (DAR), has been shown to possess potent anti-Parkinson disease effects but produces less dyskinesia and motor fluctuation that are frequently observed in Parkinson disease drug therapies. The present study was designed to detect the neuroprotection of SKF83959 and its potential mechanism for the effect in cultured rat cortical cells. The presence of SKF83959 with a dose range of 0.1-30 micromol/L improved H2O2-reduced cell viability in a dose-dependent manner. The anti-apoptotic action of SKF83959 was partially abolished by pre-application of the D1 antagonist SCH23390 (30 micromol/L) and the PI 3-kinase (PI 3-K) inhibitor LY294002 but not by the MEK1/2 inhibitor PD98059 (30 micromol/L). Moreover, SKF83959 treatment significantly inhibited H2O2-activated glycogen synthase kinase-3beta (GSK-3beta) which was associated with the drug's neuroprotective effect, but this inhibition was attenuated by SCH23390 and a selective PI 3-K inhibitor. Moreover, the application of either SKF83959 or a pharmacological inhibitor of GSK-3beta attenuated the inhibition by H2O2 on the expression of inducible NO synthase and production of NO. This indicates that D1-like receptor, presumably PI-linked D1 receptor, -mediated alteration of PI 3-K/Akt/GSK-3beta pathway is involved in the neuroprotection by SKF83959. In addition, SKF83959 also effectively decreased the level of the lipid peroxidation and increased the activity of GSH-peroxidase altered by H2O2. These results suggest that SKF83959 exerts its neuroprotective effect through both receptor-dependent and independent mechanisms: Inhibition of GSK-3beta and consequently increasing the expression of inducible NO synthase via putative PI-linked DAR; and its anti-oxidative activity which is independent of DAR.

A vehicle injection into the right core of the nucleus accumbens both reverses the region-specificity and alters the type of contralateral turning elicited by unilateral stimulation of dopamine D2/D3 and D1 receptors in the left core of the nucleus accumbens

The goal of the present study was to analyse to what extent variables such as (1) injected volume, (2) nature of the solvent of drugs (saline versus distilled water) and (3) placement of an additional cannula to inject the solvent of the drugs at the opposite side of the brain, influenced the behavioural effects of the combined administration of the dopamine D(1) receptor agonist (+/-)-1-phenyl-2,3,4,5-tetrahydro-[1H]-3-benzazepine-7,8-diol (SKF 38393, 5.0 microg) and the dopamine D(2)/D(3) receptor agonist quinpirole (10.0 microg) into the shell or core of the nucleus accumbens of freely moving rats. First, we found that increasing the injected volume from 0.2 microl to 0.5 microl significantly increased the amount of contralateral turning after injection of the drugs into the shell and, especially, the core of rats equipped with one cannula. More importantly, the type of turning behaviour changed: instead of a predominance of pivoting, both pivoting and circling appeared. Second, replacing the solvent saline by distilled water resulted in a minor, but significant, decrease of the amount of contralateral turning elicited from either the shell or the core of the nucleus accumbens of rats equipped with one cannula. The type of turning was not changed by this new solvent. Third, and most importantly, this study showed that the vehicle injection into the right core exerted a potentiating effect on the number of contralateral rotations elicited by injections of SKF 38393+quinpirole into the left core, whereas such a vehicle injection into the right shell did not affect the number of contralateral rotations elicited by injections of SKF 38393+quinpirole into the left shell. The type of turning in these rats was not changed when compared to rats equipped with one cannula. It is hypothesized that the fluid injected into the core, directly or indirectly, enhanced the dopaminergic asymmetry between the left and the right brain, implying that this manipulation anyhow reduced the dopaminergic activity in the region under discussion. In conclusion, subtle changes in the methodology used to study both the behaviour-specificity and the region-specificity of drug injections into the brain significantly directs the outcome of such studies.

Dopamine D1 receptor coupling to Gs/olf and Gq in rat striatum and cortex: a scintillation proximity assay (SPA)/antibody-capture characterization of benzazepine agonists

Cloned, human dopamine D(1) receptors recruit multiple effectors but the G-protein subtype(s) activated by cerebral populations remain poorly defined, a question addressed using a rapid immunocapture technique. In rat striatum, dopamine (DA) and four selective, benzazepine agonists at D(1) receptors concentration-dependently enhanced [(35)S]GTPgammaS binding to Galphas/olf. For all drugs, Galphaq was also recruited with similar potencies and efficacies. Comparable observations were made in the cortex wherein profiles of Galphas/olf vs Galphaq activation were also highly correlated. In contrast to Galphas/olf and Galphaq, Galphao and Galphai were activated neither in the striatum nor in the cortex, except for SKF82958. As compared to DA, both SKF81297 and SKF82958 were full agonists at Gs/olf and Gq in cortex and striatum, whereas SKF38393 behaved as a partial agonist. Likewise, the "atypical" agonist, SKF83959 only partially activated Galphaq and also Gs/olf in these two regions. In both striatum and cortex, the selective D(1) receptor antagonist, SCH23390, abolished the recruitment of Galphaq and Galphas by DA, and the action of DA was partially attenuated by SKF83959. These findings demonstrate that, in native CNS tissue, DA and other D(1) receptor agonists activate Galphas and Galphaq with similar potencies and efficacies, suggesting their recruitment via pharmacologically-indistinguishable populations of D(1) receptors, and show that SPA technology is well-adapted to study the coupling of native DA receptors.

Behavioral effects of the R-(+)- and S-(-)-enantiomers of the dopamine D(1)-like partial receptor agonist SKF 83959 in monkeys

Dopamine D(1)-like partial receptor agonists such as SKF 83959 have been proposed as potential candidates for the treatment of cocaine addiction. The present studies were conducted to further characterize SKF 83959 by pharmacologically evaluating effects of its R-(+)- and S-(-)-enantiomers, MCL 202 and MCL 201, respectively, on overt behavior (eye blinking) and schedule-controlled performance in squirrel monkeys. MCL 202, like the D(1) full receptor agonist SKF 82958, produced dose-related increases in eye blinking and decreases in rates of fixed-ratio responding. However, the magnitude of effects of MCL 202 on eye blinking was less than observed with SKF 82958. In contrast to the effects of its R-(+) enantiomer, MCL 201 was relatively devoid of behavioral activity up to doses that were approximately 10-fold greater than MCL 202. Pretreatment with the selective D(1)-like receptor antagonist SCH 39166 dose-dependently antagonized increases in eye blinking produced by MCL 202, confirming the involvement of D(1) mechanisms in its effects. A dose-ratio analysis of the antagonism of effects of MCL 202 by SCH 39166 revealed an apparent pA(2) value of 7.675 with a slope of -0.78+/-0.04. In further studies, pretreatment with MCL 202 antagonized the effects of SKF 82958 on eye blinking and, like SCH 39166, schedule-controlled behavior in a dose-related manner. A dose-ratio analysis of the antagonist effects of MCL 202 on the SKF 82958-induced increases in eye blinking revealed ratios of 2.7, 4.8 and 31.1 for 0.1, 0.3 and 1.0 mg/kg dose of the antagonist, respectively, indicative of a significant change in the potency of SKF 82958. These results suggest that MCL 202, like its parent compound SKF 83959, has both D(1) receptor-mediated agonist and antagonist properties, consistent with its characterization as a partial agonist at the D(1)-like receptor. In addition, the inactivity of MCL 201, the S-(-)-enantiomer, suggests that the behavioral effects of SKF 83959 can be attributed primarily to the activity of its R-(+)-enantiomer.

Modulation of Ca2+ signals by phosphatidylinositol-linked novel D1 dopamine receptor in hippocampal neurons

Recent evidence indicates the existence of a putative novel phosphatidylinositol-linked D1 dopamine receptor in brain that mediates phosphatidylinositol hydrolysis via activation of phospholipase Cbeta. The present work was designed to characterize the Ca(2+) signals regulated by this phosphatidylinositol-linked D(1) dopamine receptor in primary cultures of hippocampal neurons. The results indicated that stimulation of phosphatidylinositol-linked D1 dopamine receptor by its newly identified selective agonist SKF83959 induced a long-lasting increase in basal [Ca(2+)](i) in a time- and dose-dependent manner. Stimulation was observable at 0.1 microm and reached the maximal effect at 30 microm. The [Ca(2+)](i) increase induced by 1 microm SKF83959 reached a plateau in 5 +/- 2.13 min, an average 96 +/- 5.6% increase over control. The sustained elevation of [Ca(2+)](i) was due to both intracellular calcium release and calcium influx. The initial component of Ca(2+) increase through release from intracellular stores was necessary for triggering the late component of Ca(2+) rise through influx. We further demonstrated that activation of phospholipase Cbeta/inositol triphosphate was responsible for SKF83959-induced Ca(2+) release from intracellular stores. Moreover, inhibition of voltage-operated calcium channel or NMDA receptor-gated calcium channel strongly attenuated SKF83959-induced Ca(2+) influx, indicating that both voltage-operated calcium channel and NMDA receptor contribute to phosphatidylinositol-linked D(1) receptor regulation of [Ca(2+)](i).

Disruption of orofacial movement topographies in congenic mutants with dopamine D5 but not D4 receptor or DARPP-32 transduction 'knockout'

The role of D(1)-like [D(1), D(5)] and D(2)-like [D(2), D(3), D(4)] dopamine receptors and dopamine transduction via DARPP-32 in topographies of orofacial movement was assessed in restrained mice with congenic D(4) vs. D(5) receptor vs. DARPP-32 'knockout'. D(4) and DARPP-32 mutants evidenced no material phenotype; also, there were no alterations in topographical responsivity to either the selective D(2)-like agonist RU 24213 or the selective D(1)-like agonist SK and F 83959. In contrast, D(5) mutants evidenced an increase in spontaneous vertical jaw movements, which habituated more slowly than in wildtypes, and a decrease in horizontal jaw movements; topographical responsivity to SK and F 83959 and RU 24213 was unaltered. D(5) receptors regulate distinct topographies of vertical and horizontal jaw movement in an opposite manner. In assuming that the well-recognised role of the D(1)-like family in regulating orofacial movements involves primarily D(1) receptors, a role for their D(5) counterparts may have been overlooked.

Cooperative activation of D1-like and D2-like dopamine receptors in the nucleus accumbens shell is required for the reinstatement of cocaine-seeking behavior in the rat

Activation of D1-like (D1, D5) or D2-like (D1, D3, D4) dopamine receptors in the nucleus accumbens shell is sufficient to reinstate cocaine-seeking behavior in rats. The goal of these experiments was to assess whether cooperative activation of D1-like and D2-like dopamine receptors in the accumbens shell is required to promote cocaine reinstatement. Rats were initially trained to self-administer cocaine (0.25 mg, i.v.) using a fixed-ratio schedule of reinforcement for approximately 21 days. Animals subsequently underwent an extinction phase during which saline was substituted for cocaine. Once cocaine self-administration behavior was extinguished (defined as <15% of the total responses maintained during self-administration), dopamine receptor agonist-induced reinstatement of cocaine seeking was assessed. Administration of the selective D1/5 agonist R-(+)-6-chloro-7,8-dihydroxy-1-phenyl-2,3,4,5-tetrahydro-1H-3-benzazepine hydrobromide (SKF-81297) (1.0 microg) or the D2/3 receptor agonist trans-(-)-(4aR)-4,4a,5,6,7,8,8a,9-octahydro-5-propyl-1H-pyrazolo[3,4-g]quinoline hydrochloride (quinpirole) (3.0 microg) directly into the nucleus accumbens shell promoted reinstatement of cocaine seeking. In order to determine if endogenous dopamine tone in the accumbens shell is required for dopamine receptor agonist-induced reinstatement of cocaine seeking, D1/5 or D2/3 dopamine receptor antagonists were administered into the nucleus accumbens shell prior to a selective dopamine receptor agonist. Microinfusion of the D2/3 dopamine receptor antagonist sulpiride ((S)-5-aminosulfonyl-N-[(1-ethyl-2-pyrrolidinyl)methyl]-2-methoxybenzamide) (1.0 microg) into the nucleus accumbens shell 10 minutes prior to SKF-81297 (1.0 microg) blocked the ability of this D1-like dopamine receptor agonist to reinstate cocaine seeking. Similarly, administration of the selective D1/5 dopamine receptor antagonist R(+)-7-chloro-8-hydroxy-3-methyl-1-phenyl-2,3,4,5-tetrahydro-1H-3-benzazepine hydrochloride (SCH-23390) (1.0 microg) into the nucleus accumbens shell prior to quinpirole (3.0 microg) blocked reinstatement of drug-seeking behavior elicited by this D2/3 dopamine receptor agonist. Moreover, intra-accumbal shell co-administration of subthreshold doses of quinpirole (1.5 microg) and SKF-81297 (0.1 microg) promoted cocaine-seeking behavior. Collectively, these results indicate that cooperative activation of D1-like and D2-like dopamine receptors in the nucleus accumbens shell is necessary to reinstate cocaine seeking in rats.

Topographical resolution of jaw movements mediated by cyclase- vs. non-cyclase-coupled dopamine D1-like receptors: Studies with SK&F 83822

This study examined the effects on orofacial movement topography of SK&F 83822 ([R/S]-6-chloro-7,8-dihydroxy-3-allyl-1-[3-methylphenyl]-2,3,4,5-tetrahydro-1H-3-benzazepine), which stimulates dopamine D(1)-like receptors coupled to stimulation of adenylyl cyclase (AC) but not phosphoinositide (PI) hydrolysis, in comparison with SK&F 83959 ([R/S]-3-methyl-6-chloro-7,8-dihydroxy-1-[3-methyl-phenyl]-2,3,4,5-tetrahydro-1H-3-benzazepine), which stimulates PI hydrolysis but not AC. SK&F 83822 alone induced chattering, while SK&F 83959 alone exerted little effect. SK&F 83822 and SK&F 83959 each in combination with the dopamine D(2)-like agonist quinpirole resulted in synergistic induction of non-chattering movements with tongue protrusions. These effects were blocked by the dopamine D(1)-like receptor antagonist SCH 23390 ([R]-3-methyl-7-chloro-8-hydroxy-1-phenyl-2,3,4,5-tetrahydro-1H-3-benzazepine). However, the dopamine D(2)-like receptor antagonist YM 09151-2 (cis-N-[1-benzyl-2-methyl-pyrrolidin-3-yl]-5-chloro-2-methoxy-4-methylaminobenzamide) exerted a biphasic effect on synergism with SK&F 83822: chattering was initially released but antagonised thereafter. Only antagonism was seen for synergism with SK&F 83959. While both AC- and PI-coupled dopamine D(1)-like receptors participate in synergistic dopamine D(1)-like:D(2)-like receptor interactions, topographically specific synergistic and oppositional dopamine D(1)-like:D(2)-like interactions evident with SK&F 83822 reflect the involvement primarily of D(1)-like receptors coupled to AC rather than PI.

Stimulation of D1-like or D2 dopamine receptors in the shell, but not the core, of the nucleus accumbens reinstates cocaine-seeking behaviour in the rat

Although increases in dopamine transmission in the brain are clearly involved in the reinstatement of cocaine seeking, the role of nucleus accumbens dopamine in cocaine priming-induced reinstatement remains controversial. The goal of these experiments was to evaluate the relative contributions of D1-like and D2-like dopamine receptors in the nucleus accumbens core and shell in the reinstatement of cocaine-seeking behaviour. Initially, rats were trained to press a lever for cocaine (0.25 mg, i.v.) using a fixed-ratio 5 (FR5) schedule of reinforcement. Responding was then extinguished by substituting saline for cocaine. During the reinstatement phase, subtype-specific dopamine receptor agonists were microinjected into the nucleus accumbens core or medial shell in order to assess their ability to induce cocaine seeking. Administration of the D1/D5 dopamine receptor agonist SKF-81297 (1.0 microg) into the nucleus accumbens shell, but not core, reinstated drug-seeking behaviour. Similarly, microinjection of quinpirole (3.0 microg), a D2/D3 dopamine receptor agonist, into the nucleus accumbens shell and not core reinstated drug-seeking behaviour. In contrast, administration of the D3- or D4-preferring dopamine receptor agonists PD 128,907 (1.5 and 3.0 microg) and PD 168,077 (0.3 and 3.0 microg), respectively, did not promote reinstatement when administered into either the core or the shell. Taken together, these results indicate that activation of D1/D5 or D2 dopamine receptors, in the limbic shell subregion of the nucleus accumbens but not the basal ganglia-orientated accumbens core, promotes the reinstatement of cocaine-seeking behaviour.

The atypical dopamine D1 receptor agonist SKF 83959 induces striatal Fos expression in rats

The effects of dopamine D1 receptor agonists are often presumed to result from an activation of adenylyl cyclase, but dopamine D1 receptors may also be linked to other signal transduction cascades and the relative importance of these various pathways is currently unclear. SKF 83959 is an agonist at dopamine D1 receptors linked to phospholipase C, but has been reported to be an antagonist at receptors linked to adenylyl cyclase. The current report demonstrates that SKF 83959 induces pronounced, nonpatchy, expression of the immediate-early gene product Fos in the striatum of intact rats which can be converted to a patchy pattern by pretreatment with the dopamine D2-like receptor agonist quinpirole. In rats with unilateral 6-hydroxydopamine lesions SKF 83959 induces strong behavioral rotation and a greatly potentiated Fos response. All of the responses to SKF 83959, in both intact and dopamine-depleted animals, can be blocked by pretreatment with the dopamine D1 receptor antagonist SCH-23390. In intact subjects, SKF 83959 induced Fos expression less potently than the standard dopamine D1 receptor agonist SKF 82958, but the two drugs were approximately equipotent in deinnervated animals. These results demonstrate for the first time that possession of full efficacy at dopamine D1 receptors linked to adenylyl cyclase is not a necessary requirement for the induction of striatal Fos expression in intact animals and suggest that alternative signal transduction pathways may play a role in dopamine agonist induced Fos expression, especially in dopamine-depleted subjects.

The role of D1 dopamine receptors and phospho-ERK in mediating cytotoxicity. Commentary

Striatal neurodegeneration observed in several neurological diseases, occurs through unknown mechanisms. Recent evidence suggests that its pathogenesis may be linked, in part, to high synaptic levels of dopamine (DA), which can then cause neurotoxicity of striatal neurons through mitogen-activated protein kinases (MAPKs). Here we comment on the role of extracellular signal-regulated kinase (ERK) activation in the cytotoxicity mediated upon activation of the D1 DA receptor, and describe a possible mechanism for phospho-ERK (p-ERK) in inducing cytotoxicity.

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.

Prefrontal, accumbal [shell] and ventral striatal mechanisms in jaw movements and non-cyclase-coupled dopamine D1-like receptors

The effect on jaw movements of intracerebral injections of the dopamine D1-like receptor agents SK&F 83959 (3-methyl-6-chloro-7,8-dihydroxy-1-[3-methylphenyl]-2,3,4,5-tetrahydro-1H-3-benzazepine), SK&F 38393 ([R]-7,8-dihydroxy-1-phenyl-2,3,4,5-tetrahydro-1H-3-benzazepine) and SCH 23390 ([R]-3-methyl-7-chloro-8-hydroxy-1-phenyl-2,3,4,5-tetrahydro-1H-3-benzazepine) and of injections of the dopamine D2-like receptor agonist quinpirole into the ventrolateral striatum, accumbens shell or prefrontal cortex were studied. SK&F 38393 and SK&F 83959 injected into the ventrolateral striatum synergised with i.v. quinpirole; in the shell of accumbens, SK&F 38393 evidenced weaker synergism with quinpirole, while SK&F 83959 did not synergise with it; neither agent synergised with quinpirole in the prefrontal cortex. Co-injection of SCH 23390 or SK&F 83959 into the prefrontal cortex antagonised jaw movements induced by injection of SK&F 83959 into the ventrolateral striatum in combination with i.v. quinpirole. Injection of SK&F 83959 + quinpirole into the ventrolateral striatum, but not into the accumbens shell, resulted in synergism. These findings indicate a primary, but not exclusive, role for ventral striatal, non-cyclase-coupled dopamine D1-like receptors in the induction of jaw movements. These processes appear to require tonic activity of prefrontal cyclase-linked dopamine D1A [and/or D1B] receptors.