The dopamine D1 receptor: biochemical and behavioral aspects

Effects of SCH-23390 on dopamine D1 receptor occupancy and locomotion produced by intraaccumbens cocaine infusion

This study examined the effects of both systemic and intraaccumbens administration of SCH-23390 in rats on dopamine D1 receptor occupancy and on locomotor activity produced by intraaccumbens infusion of cocaine. In experiment 1, rats received SCH-23390 (0-1 mg/kg, i.p.) 15 minutes prior to intraaccumbens infusion of cocaine (0 or 100 microg/side). In experiment 2, rats received coinfusion of SCH-23390 (0-1 microg/side) and cocaine (0 or 100 microg/side) into the nucleus accumbens (NAc). After behavioral testing, receptors occupied by SCH-23390 were quantified by injecting animals with their respective dose of SCH-23390, followed by a systemic injection of the irreversible antagonist N-ethoxycarbonyl-2-ethoxy-1,2-dihydroquinoline (EEDQ). Receptors occupied by SCH-23390, and therefore protected from EEDQ-induced inactivation, were quantified from autoradiograms of sections labeled with 3H-SCH-23390. Systemic administration of SCH-23390 dose-dependently (0.1-1.0 mg/kg) reversed cocaine-induced locomotion and occupied 72-100% of D1-like receptors in the anterior NAc. D1 receptor occupancy following systemic administration of SCH-23390 was evident as an inverted U-shaped, dose-dependent change, with the greatest occupancy observed at the intermediate dose of 0.3 mg/kg. Intraaccumbens infusion of SCH-23390 did not alter cocaine-induced locomotor activity despite occupying 40-60% of D1-like receptors in the anterior NAc core and shell. The findings that systemic, but not intraaccumbens, administration of SCH-23390 potently reversed locomotion produced by intraaccumbens cocaine infusion suggest that stimulation of D1 receptors in regions other than the NAc is involved in locomotion produced by intraaccumbens infusion of cocaine, and that stimulation of D1 receptors in the NAc is not necessary for this behavior.

Lack of effect of the dopamine D1 antagonist, NNC 01-0687, on unstimulated and stimulated release of anterior pituitary hormones in males

Dopamine in humans inhibits the secretion of luteinizing hormone (LH), follicular stimulating hormone (FSH), thyroid stimulating hormone (TSH) and prolactin (PRL), and is a stimulator of growth hormone (GH) secretion. Dopamine-D1 receptor stimulation with fenoldopam increases basal PRL levels, suppresses TSH, and increases gonadotropin releasing hormone (LHRH) induced LH release. We have investigated the effect of a dopamine D1-receptor antagonist, NNC 01-0687, on the secretion of anterior pituitary hormones. In 8 healthy males NNC 01-0687 and placebo were administered orally in a double-blind placebo controlled cross-over study for three days with a wash-out period of 14 days. Hormonal responses (PRL, LH, FSH, GH, TSH, thyroid hormones and testosterone), unstimulated and LHRH/TRH stimulated, were studied on days 1 and 3. No significant difference (p > 0.05) between placebo and active periods was found neither in unstimulated nor in stimulated hormone concentrations expressed in absolute values, percent change of before, incremental values and area under the curve. These results suggest that the neuronal DA-D1 activity is not activated during basal conditions in healthy male subjects.

Tremulous jaw movements induced by the acetylcholinesterase inhibitor tacrine: effects of antiparkinsonian drugs

Several experiments were conducted to study the effects of established or potential antiparkinsonian drugs on the tremulous jaw movements induced by the anticholinesterase tacrine (9-amino-1,2,3,4-tetrahydroaminoacridine hydrochloride). In the first group of four experiments, separate groups of animals that received 2.5 or 5.0 mg/kg tacrine showed a dose-dependent decrease in tremulous jaw movements following co-administration of the non-selective dopamine receptor agonist apomorphine, the full dopamine D2 receptor agonist bromocriptine, and the full dopamine D1 receptor agonist APB (R(+)-6-bromo-7,8-dihydroxy-3-allyl-1-phenyl-2,3,4,5-tetrahydro-1 H-3-benzazepine). Co-administration of the partial dopamine D1 receptor agonist SKF 38393 (R(+)-2,3,4,5-tetrahydro-7,8-dihydroxy-l phenyl-1 H-benzazepine: 7.5-30.0 mg/kg) did not reduce tremulous jaw movements produced by 2.5 or 5.0 mg/kg tacrine. In animals treated with 2.5 mg/kg tacrine, co-administration of SKF 38393 resulted in a dose-related trend towards a potentiation of tremulous jaw movements. In the second group of experiments, all rats received 2.5 mg/kg tacrine. The dopamine precursor L-DOPA (L-3,4-dihydroxyphenylalanine), the dopamine and norepinephrine releasing agent amantadine, and the muscarinic receptor antagonist benztropine all reduced tremulous jaw movements induced by 2.5 mg/kg tacrine. Across all experiments, it was noted that apomorphine, bromocriptine and benztropine were more potent than amantadine and L-DOPA. These results are broadly consistent with the therapeutic doses of these agents noted in the clinical literature. The results of these experiments indicate that tremulous jaw movements in rats may be a useful model for evaluating potential antiparkinsonian agents.

Anatomical localization of SKF-38393-induced behaviors in rats using the irreversible monoamine receptor antagonist EEDQ

This study was designed to localize the population of dopamine D1-like receptors involved in grooming and oral movements elicited by systemic administration of the D1-selective agonist SKF-38393. Receptors in specific dopamine terminal regions were inactivated by intracranial injection of the nonselective irreversible antagonist N-ethoxycarbonyl-2-ethoxy-1,2-dihydroquinoline (EEDQ). The effect of these injections on behaviors induced by systemic administration of SKF-38393 (10 mg/kg) was measured 48 hours later. The specific populations of D1-like receptors inactivated by the EEDQ injections were identified as a loss of 3H-SCH-23390 binding in a given region using quantitative autoradiography. EEDQ (1.5 micrograms/microliters/side) injected into the nucleus accumbens (NAc) did not alter SKF-38393-induced behaviors. Similarly, injection of EEDQ into the medial caudate-putamen (CPu) failed to alter these behaviors. In contrast, EEDQ (0.15-1.5 micrograms/microliters/side) injected into the lateral CPu decreased both SKF-38393-induced grooming and oral movements, with complete blockade of grooming observed at the highest dose. To determine whether this effect of EEDQ was due to inactivation of D1-like receptors, separate groups of animals were pretreated with SCH-23390 (3 mg/kg, S.C.) 15 min prior to injection with EEDQ. Pretreatment with SCH-23390 prevented the disruption of SKF-38393-induced behaviors, as well as the loss of 3H-SCH-23390-labeled binding sites observed after injection of EEDQ into the lateral CPu. EEDQ injections that produced disruption of SKF-38393-induced behaviors were associated with a greater loss of binding in the lateral CPu relative to other regions examined including the NAc, medial CPu, and globus pallidus. Furthermore, EEDQ injections that produced the greatest loss of 3H-SCH-23390 binding in the latter three regions did not disrupt SKF-38393-induced behavior. These results demonstrate that stimulation of D1-like receptors in the lateral CPu is necessary for behaviors induced by systemic administration of SKF-38393. The results also demonstrate the utility of this "receptor lesion" technique to localize receptor-mediated behaviors.

Locomotor activity following intra-accumbens microinjections of dopamine D1 agonist SK&F 38393 in rats

The present study examined the effects of the dopamine D1 receptor subtypes agonist SK&F 38393 on locomotor activities after bilateral microinjection (0.00, 0.01, 0.1, 1.0, 10.0 micrograms) into the nucleus accumbens (Acb). The dose of 0.1 microgram elicited the highest response rate across measures of locomotion, rearing and stereotypy behavior. On the other hand, the largest dose of 10.0 micrograms was associated with significant increase in center time behaviors. The data were supportive of the hypothesis that dose-related locomotor activities elicited by microinjections of SK&F 38393 into the Acb are independently mediated by D1 receptors.

Antiparkinsonian dopamine agonists: a review of the pharmacokinetics and neuropharmacology in animals and humans

With the intention of compensating for the deficit of endogenous dopamine (DA) in the basal ganglia of Parkinsonian patients by substitution with agents which directly stimulate central DA receptors, synthetic DA agonists have been introduced almost 20 years ago for the symptomatic treatment of Parkinson's disease. The original expectation that DA agonists would be able to completely restore extrapyramidal motor function in Parkinsonian patients has turned out as too mechanistic and simplicative. However, undoubtedly DA agonists have improved therapeutic possibilities in Parkinson's disease. Thus, clinical evidence from controlled chronic studies in patients indicates that the therapeutic results following the early application of DA agonists in combination with L-DOPA on a long-term base are superior to the respective monotherapy. However, none of the DA agonists currently employed for antiparkinsonian treatment i.e. apomorphine and the ergoline derivatives bromocriptine, lisuride and pergolide, is optimal with respect to pharmacokinetic properties (poor oral bioavailability with considerable intra- and interindividual variation) or pharmacological profiles (low selectivity for DA receptors in case of the ergot agonists). The pathophysiology underlying Parkinson's disease which turned out more complex than initially expected might provide another explanation for the limited therapeutic potential of DA agonists. Therefore, apart from summarizing the pharmacokinetics, biotransformation, neuropharmacology and neurobiochemistry of the DA agonists employed clinically, the present article also reviews physiological aspects of (a) central dopaminergic neurotransmission including the topographical distribution of DA receptor subtypes and their functional significance, (b) the intracellular signal processing in striatal output neurons and (c) the intraneuronal mechanisms which integrate the various neurotransmitter signals converging on the striatal output neuron to a demand-adjusted effector cell response via the cross-talk between the different second messenger systems. Based on these considerations, potential pharmacological approaches for the development of improved antiparkinsonian drugs are outlined. There is a therapeutic demand for more selective and better bioavailable DA agonists. In particular, selective D-1 receptor agonists are highly desirable to provide a more specific probe than SKF 38 393 for clarifying the current controversy on the disparate findings in nonprimate species and monkeys or Parkinsonian patients, respectively, regarding the functional significance of D-1 receptors for the antiparkinsonian action of DA agonists or L-DOPA. The therapeutic importance of D-2 receptor activation is generally accepted; whether DA agonists combining a balanced affinity to both D-1 and D-2 receptors within one molecule (to some extent a property of apomorphine) might be superior to subtype-specific DA agonists remains to be tested clinically.(ABSTRACT TRUNCATED AT 400 WORDS)

Dopamine receptor mediated hypothermic action of B-HT 920 in rats

The hypothermic action of the thiazoloazepine derivative B-HT 920, an alpha 2-adrenoceptor agonist has been investigated in rats. B-HT 920 (6-allyl-2-amino-5,6,7,8-tetrahydro-4H-thiazolo-(4,5-d)-azepine dihydrochloride) (0.25-1.0 mg kg-1 i.p.) induced a dose-dependent hypothermia. This peak effect was seen within 60-90 min and lasted up to 120 min. Its action was potentiated by the selective D1-dopamine agonist SKF 38393 and inhibited by the D2-antagonists haloperidol (1 mg kg-1) and sulpiride (100 mg kg-1). The hypothermic action of B-HT 920 was centrally mediated; i.c.v. administration of 10 micrograms produced a significant fall in rectal temperature which was sensitive to blockade by haloperidol. B-HT 920 also potentiated the hypothermic action of apomorphine (0.1 and 0.5 mg kg-1) in a haloperidol sensitive manner. Reserpine (5 mg kg-1 i.p.) pretreatment reduced the hypothermic response of B-HT 920 (0.5 mg kg-1) but sensitized the response due to the combination of B-HT 920 (0.5 mg kg-1) and apomorphine (0.1 mg kg-1). Neither the selective alpha 2-adrenoceptor antagonists, yohimbine (1 mg kg-1) or idazoxan (1 mg kg-1), the histamine antagonist mepyramine (10 mg kg-1) nor the 5-HT antagonist cyproheptadine (5 mg kg-1) inhibited B-HT 920-induced hypothermia. Similarly, the selective alpha 1-antagonist prazosin (1 mg kg-1) and the beta-antagonist propranolol (10 mg kg-1) failed to modify the hypothermic action of B-HT 920. These observations demonstrated hypothermia induced by B-HT 920 is mediated by postsynaptic D2-receptors and D1- and D2-receptor interplay is essential for the full expression of hypothermia in rats.

Pharmacotherapy for self-injurious behavior: preliminary tests of the D1 hypothesis

1. The D1 dopamine hypersensitivity model of self-injurious behavior leads to a testable clinical hypothesis: that the mixed D1/D2 dopamine antagonist fluphenazine may improve the symptoms of self-injurious patients. 2. The hypothesis was tested in an open pilot trial in six patients and a partially controlled trial in nine patients. 3. Some degree of clinical improvement was observed in eleven of the fifteen. 4. The trials represent a partial affirmation of the D1 hypothesis. However, it is also clear that conventional methodology for psychopharmacologic research is inappropriate for the proper clinical evaluation of self-injurious patients. The proper method should include the following elements: i) An epidemiologically representative sample ii) A naturalistic study environment iii) A longitudinal design with long-term follow-up iv) Concurrent behavioral ratings using direct observations and a reliable, treatment-sensitive rating scale. 5. Before subjects enter a clinical trial of an experimental medication, a neuropsychiatric differential diagnosis should be applied to limit the diversity of the sample.

Clozapine antagonism of D-1 and D-2 dopamine receptor-mediated behaviors

When tested in rats supersensitive to dopamine agonists, the atypical neuroleptic clozapine displayed pharmacological properties expected of both a D-1 and D-2 receptor antagonist. The locomotor response induced by the D-1 receptor agonist SKF-38393 in neonatal-6-hydroxydopamine (6-OHDA)-lesioned rats was reversed in a dose-related fashion, although a complete blockade of this behavior was not observed indicative for only a partial antagonism of D-1 receptor function. Clozapine also blocked the self mutilation resulting from L-dihydroxyphenylalanine (L-DOPA) administration to neonatal-6-OHDA-lesioned rats, an effect previously linked to D-1 receptor activation. At higher doses, clozapine blocked the locomotor activity elicited by the D-2 agonist LY-171555 in adult-6-OHDA-lesioned rats. Therefore, the action of clozapine on D-1 as well as D-2 receptor-mediated behaviors contributes to its pharmacological effects. The ability of clozapine to stop self-mutilatory behavior in neonatal-6-OHDA-lesioned rats suggests that this drug might be an effective treatment for self-injurious behavior associated with the Lesch-Nyhan syndrome and mental retardation.

Interaction between accumbens D1 and D2 receptors regulating rat locomotor activity

The effect of intra-accumbens injections of various dopaminergic agonists and antagonists on the rat locomotor activity has been evaluated in automated open fields. Locomotor stimulation has been observed after local administration of d-amphetamine (10 micrograms), apomorphine (10 micrograms), as well as of solution containing the D1 agonist SKF 38 393 and D2 receptor agonist LY 171 555 (quinpirole) in doses (10 and 4 micrograms, respectively) which were inactive when both drugs were administered separately. On the other hand separate injections of metoclopramide (0.1 microgram) and SCH 23 390 (0.5 microgram) (D2 and D1 receptor antagonists) very potently inhibited animals' locomotor activity. The data indicate that concomitant stimulation of both accumbens D1- and D2-receptor related mechanisms is a necessary condition to increase rat motility. Moreover, it seems that accumbens D1 receptors may be differently involved in the control of facilitatory versus inhibitory motor processes.

Studies on the discriminative stimulus properties of apomorphine in rhesus monkeys

Four rhesus monkeys were trained to discriminate the effect of apomorphine (0.1 mg/kg IM) from that of saline injections. The discriminative stimulus (DS) effect of apomorphine generalized to the dopamine D2 receptor agonist quinpirole. The D1 dopamine receptor agonist SKF 38393 elicited responses only on the saline-appropriate lever. Stimulus generalization of the dopamine autoreceptor agonist 3-PPP exhibited stereospecificity favoring the (+) over the (-) isomer. d-Amphetamine, phencyclidine, 8-hydroxy-2-(di-n-propylamino)tetralin (8-OH-DPAT), and clonidine did not share the DS effect of apomorphine. The D2-selective antagonists sulpiride and metoclopramide reversed both the DS effect and the response rate reduction produced by the training dose of apomorphine. Chlorpromazine and the D1 antagonist Sch 23390 also antagonized the DS effect, but the antagonism was accompanied by a further rate reduction. Haloperidol and clozapine antagonized the DS effect incompletely. The DS effect produced by apomorphine in this study appears to be mediated predominantly by post-synaptic D2 receptor activation, with contribution also from the D1 receptor.

D1 dopamine receptor stimulation enables the postsynaptic, but not autoreceptor, effects of D2 dopamine agonists in nigrostriatal and mesoaccumbens dopamine systems

Possible functional interactions between D1 and D2 dopamine (DA) receptors were examined using extracellular single-cell recording with microiontophoretic application of selective D1 and D2 receptor agonists both postsynaptically, in the rat nucleus accumbens (NAc) and caudate-putamen (CPu), and presynaptically, at impulse-regulating somatodendritic DA autoreceptors in the ventral tegmental area (A10) and substantia nigra pars compacta (A9). In addition, synthesis-modulating nerve terminal DA autoreceptors were studied in both the CPu and NAc using the gamma-butyrolactone (GBL) neurochemical model of isolated nerve terminal autoreceptor function in vivo. In both the NAc and CPu, the inhibition of neurons produced by iontophoresis of the D2 receptor agonists quinpirole or RU-24213 was attenuated by acute DA depletion via the tyrosine hydroxylase inhibitor alpha-methyl-p-tyrosine (AMPT). However, during iontophoresis of the selective D1 DA receptor agonist SKF 38393, the inhibitory effects of the D2 agonists were again evident, suggesting that the attenuation of D2 agonist-induced inhibition was due to decreased D1 receptor activation. In contrast, the inhibitory effects produced by the non-selective D1/D2 agonist apomorphine or by SKF 38393 were unaffected by AMPT pretreatment. Thus, D1 receptor activation appears necessary for D2 receptor-mediated inhibition of NAc and CPu neurons, whereas D2 receptor activation is not required for the inhibition produced by D1 receptor stimulation. In contrast to postsynaptic D2 receptors, the ability of DA agonists to stimulate D2 DA autoreceptors was not altered by manipulations of D1 receptor occupation. Enhancing D1 receptor stimulation with SKF 38393 or reducing D1 receptor occupation with either the selective D1 receptor antagonist SCH 23390 or AMPT failed to alter the rate-inhibitory effect of i.v. quinpirole on A9 or A10 DA neurons. Similarly, iontophoresis of SKF 38393 failed to alter the inhibitory effects of iontophoretic quinpirole. SKF 38393 also failed to affect the inhibition of GBL-induced increases in DOPA accumulation (tyrosine hydroxylase activity) produced by quinpirole in either the NAc or CPu. Furthermore, reversal of GBL-induced increases in DOPA accumulation by apomorphine or quinpirole was unaffected by pretreatment with SCH 23390. Therefore, D1 receptor occupation appears to be necessary for the expression of the functional effects of postsynaptic D2 receptor stimulation but not presynaptic D2 DA autoreceptor stimulation.

Relative dopamine D1 and D2 receptor affinity and efficacy determine whether dopamine agonists induce hyperactivity or oral stereotypy in rats

The effects of a range of dopamine (DA) agonists on stereotyped behaviour in rats were analysed and compared both with the affinity of the compounds for D1 and D2 receptor binding sites in vitro and their ability to stimulate the adenylate cyclase activity in rat striatal homogenates. Full and partial agonists at the D1 receptor coupled to adenylate cyclase do not induce sterotypies when given alone, whereas full D2 agonists (e.g. quinpirole) induce hyperactivity but not oral sterotypies. Partial D2 agonists (e.g. (-)-3-PPP) only induce sedation. Mixed D1/D2 agonists (e.g. apomorphine) induce both hyperactivity and oral stereotypies. Maximum stereotypies were induced by combination of SK & F 38393 and a series of D2 agonists, including full agonists and the partial D2 agonist B-HT 920, whereas partial agonists with low intrinsic activity (e.g. (-)-3-PPP, EMD 23448) did not induce stereotypies when given together with SK & F 38393. However, these partial agonists reduced the maximum effect of apomorphine, whereas the full agonists (e.g. quinpirole, (-)-NPA) and B-HT 920 had no apomorphine antagonistic activity. The mixed D1/D2 agonists apomorphine and N,N-dipropyl-5,6-ADTN were only weakly influenced by SK & F 38393, or not at all. D1 agonists with central effects, including SK & F 38393, SK & F 81297 (with relatively high efficacies), and the partial agonist SK & F 75670 with low efficacy, changed the hyperactivity induced by quinpirole into maximum oral stereotypy, whereas the peripheral D1 agonist fenoldopam had no such effect. Inhibition of DA and NA synthesis with alpha-methyl-p-tyrosine depleted striatal DA levels by 72 per cent and antagonized the hyperactivity induced by the D2 agonists quinpirole and (-)-NPA, but not that of apomorphine. Combination of SK & F 38393 and quinpirole induced maximum stereotypy in DA-depleted animals. These results suggest that D1 receptor tonus is a necessary prerequisite for the expression of a DA agonist's effect. The hyperactivity induced by full D2 agonists appears to be mediated by D1 tonus provided by endogenous DA activity, but stronger D1 stimulation is necessary to induce oral stereotypy. A high degree of D1 receptor activation increases the ability of partial D2 agonists to induce hyperactivity or oral stereotypies since treatment with both SK & F 38393 and B-HT 920 had marked effects while B-HT 920 was ineffective.(ABSTRACT TRUNCATED AT 400 WORDS)

Human dopamine receptor subtypes--in vitro binding analysis using 3H-SCH 23390 and 3H-raclopride

Affinities and regional densities of the D1- and D2-dopamine receptor subtypes were studied in the human post-mortem brain in vitro using the two selective radioligands 3H-SCH 23390 and 3H-raclopride. 3H-Raclopride binding was confined to the caudate nucleus, the putamen and the substantia nigra, while 3H-SCH 23390 bound to cortical regions as well. The binding of 3H-SCH 23390 was reduced by a low concentration of ketanserin, indicating binding to 5-HT2 receptors in addition to the D1-dopamine receptors. The endogenous neurotransmitter dopamine interacted potently both with the D1-dopamine receptor and the D2-dopamine receptor, displaying two affinity states for each subtype. The distribution of the dopamine receptor subtypes obtained in the present in vitro investigation is in agreement with data obtained with 11C-SCH 23390 and 11C-raclopride in positron emission tomographic studies in human volunteers.

Distribution of dopamine D1 receptors in rat cortical areas, neostriatum, olfactory bulb and hippocampus in relation to endogenous dopamine contents

The tritiated dopamine D1 antagonist SCH23390 was employed to determine the densities of D1 receptors in seven discrete and functionally identified cortical areas (cingulate, frontal, parietal, primary somatosensory, primary visual, retrosplenial and entorhinal-piriform) as well as in the neostriatum, hippocampus and olfactory bulbs. In addition, the tissue levels of the catecholamines NA, AD, DA, the indoleamine 5-HT and their main metabolites (MHPG, DOPAC, HVA, 3-MT, 5-HTP and 5-HIAA) were measured in the different regions by HPLC with electrochemical detection. The Scatchard analysis of saturation curves revealed the highest density of [3H]SCH23390 binding sites for the neostriatum, while the densities were 10-20 times lower for total cerebral cortex and hippocampus respectively. For the olfactory bulb and other cortical areas, D1 receptor densities were determined by equilibrium binding at a fixed radioligand concentration approaching saturation. The distribution of D1 receptors was heterogeneous with the greatest densities in entorhinal-piriform and cingulate cortices. The endogenous DA levels measured for all regions correlated with their metabolite (DOPAC, HVA and 3-MT) contents (r = 0.999; P less than 0.001). There was also a very good correlation (r = 0.981; P less than 0.001) between tissue DA and D1 receptor densities. This quantitative information reflects particular features of the organization of the DA systems and is discussed in relation to turnover and recently established aspects of the DA innervation.