Dopamine receptor occupancy in vivo: behavioral correlates using NNC-112, NNC-687 and NNC-756, new selective dopamine D1 receptor antagonists

Motor synchronization and impulsivity in pediatric borderline personality disorder with and without attention-deficit hyperactivity disorder: an eye-tracking study of saccade, blink and pupil behavior

Shifting motor actions from reflexively reacting to an environmental stimulus to predicting it allows for smooth synchronization of behavior with the outside world. This shift relies on the identification of patterns within the stimulus - knowing when a stimulus is predictable and when it is not - and launching motor actions accordingly. Failure to identify predictable stimuli results in movement delays whereas failure to recognize unpredictable stimuli results in early movements with incomplete information that can result in errors. Here we used a metronome task, combined with video-based eye-tracking, to quantify temporal predictive learning and performance to regularly paced visual targets at 5 different interstimulus intervals (ISIs). We compared these results to the random task where the timing of the target was randomized at each target step. We completed these tasks in female pediatric psychiatry patients (age range: 11-18 years) with borderline personality disorder (BPD) symptoms, with (n = 22) and without (n = 23) a comorbid attention-deficit hyperactivity disorder (ADHD) diagnosis, against controls (n = 35). Compared to controls, BPD and ADHD/BPD cohorts showed no differences in their predictive saccade performance to metronome targets, however, when targets were random ADHD/BPD participants made significantly more anticipatory saccades (i.e., guesses of target arrival). The ADHD/BPD group also significantly increased their blink rate and pupil size when initiating movements to predictable versus unpredictable targets, likely a reflection of increased neural effort for motor synchronization. BPD and ADHD/BPD groups showed increased sympathetic tone evidenced by larger pupil sizes than controls. Together, these results support normal temporal motor prediction in BPD with and without ADHD, reduced response inhibition in BPD with comorbid ADHD, and increased pupil sizes in BPD patients. Further these results emphasize the importance of controlling for comorbid ADHD when querying BPD pathology.

Striatal D1 and D2 receptor availability are selectively associated with eye-blink rates after methylphenidate treatment

Eye-blink rate has been proposed as a biomarker of the brain dopamine system, however, findings have not been consistent. This study assessed the relationship between blink rates, measured after oral placebo) (PL) and after a challenge with oral methylphenidate (MP; 60 mg) and striatal D1 receptor (D1R) (measured at baseline) and D2 receptor (D2R) availability (measured after PL and after MP) in healthy participants. PET measures of baseline D1R ([¹¹C]NNC112) (BL-D1R) and D2R availability ([¹¹C]raclopride) after PL (PL-D2R) and after MP (MP-D2R) were quantified in the striatum as non-displaceable binding potential. MP reduced the number of blinks and increased the time participants kept their eyes open. Correlations with dopamine receptors were only significant for the eye blink measures obtained after MP; being positive for BL-D1R in putamen and MP-D2R in caudate (PL-D2R were not significant). MP-induced changes in blink rates (PL minus MP) were negatively correlated with BL-D1R in caudate and putamen. Our findings suggest that eye blink measures obtained while stressing the dopamine system might provide a more sensitive behavioral biomarker of striatal D1R or D2R in healthy volunteers than that obtained at baseline or after placebo.

PET imaging in human participants revealed that D1 and D2 dopamine receptor availability was associated with eye-blink rates following treatment with oral methylphenidate, but not a placebo.

Acute oral methylmercury exposure perturbs the gut microbiome and alters gut-brain axis related metabolites in rats

Environmental pollutants like methylmercury (MeHg) can bring devastating neurotoxicity to animals and human beings. Gut microbiota has been found to demethylate MeHg and promote the excretion of Hg through feces. However, the impacts of MeHg on gut microbiota and metabolites related to gut-brain interactions were less studied in mammals. The object of this study was to investigate the impacts of acute MeHg exposure on gut microbiome and metabolites together with its impact on gut integrity and related biological responses in rats. Rats were exposed to MeHg through oral administration and were sacrificed after 24 h 16 S rRNA gene sequencing was used to study the perturbance to gut microbiome and liquid chromatography mass spectrometry (LC-MS) was used for metabolomics profiling. It was found that gut was one of the target tissues of MeHg. MeHg induce the changes of intestinal microbial community structure and induce the regulating neuron activity change of intestinal neurotransmitters and metabolites on intestinal neurotransmitters and metabolites regulating the neuron activity. This was supported by the increased BDNF level. These findings may suggest a potential new mechanism regarding the neurotoxicity of MeHg. The protocols used in this study may also be applied to understand the neurotoxicity of other environmental neurotoxins like Pb, Mn, polychlorinated biphenyls, and pesticides, etc and to screen the neurotoxicity of emerging environmental contaminants.

The role of dopaminergic transmission through D1-like and D2-like receptors in amphetamine-induced rat ultrasonic vocalizations

RATIONALE

Systemic amphetamine (AMPH) administration increases the rate of 50-kHz ultrasonic vocalizations (USVs) in adult rats and preferentially enhances the 'trill' subtype; these effects of AMPH critically depend on noradrenergic transmission, but the possible contributions of dopamine are unclear.

OBJECTIVE

To assess the role of dopamine in 50-kHz USVs emitted drug-free and following systemic AMPH administration.

METHODS

Adult male Long-Evans rats pre-selected for high AMPH-induced calling rates were tested with AMPH (1 mg/kg, intraperitoneal (IP)) and saline following pretreatment with the following dopamine receptor antagonists: SCH 23390 (0.005-0.02 mg/kg, subcutaneous (SC)), SCH 39166 (0.03-0.3 mg/kg, SC), haloperidol (0.1, 0.2 mg/kg, IP), sulpiride (20-80 mg/kg, SC), raclopride (0.1-0.5 mg/kg, SC), clozapine (4 mg/kg, SC), risperidone (0.5 mg/kg, SC), and pimozide (1 mg/kg, IP). The dopamine and noradrenaline reuptake inhibitors (GBR 12909 and nisoxetine, respectively) were also tested, alone and in combination.

RESULTS

SCH 23390, SCH 39166, haloperidol, and raclopride dose-dependently inhibited vocalizations under AMPH and suppressed the proportion of trill calls. Sulpiride, however, had no discernable effect on call rate or profile, even at a high dose that reduced locomotor activity. Single doses of clozapine, risperidone, and pimozide all markedly decreased calling under saline and AMPH. Finally, GBR 12909 and nisoxetine failed to promote 50-kHz USVs detectably or alter the subtype profile, when tested alone or in combination.

CONCLUSIONS

The rate of 50-kHz USVs and the call subtype profile following systemic AMPH administration depends on dopaminergic neurotransmission through D1-like and D2-like receptors. However, inhibiting dopamine and/or noradrenaline reuptake appears insufficient to induce calling.

The role of D1-D2 receptor hetero-dimerization in the mechanism of action of clozapine

Clozapine is effective although still not perfect drug used to treat schizophrenia. The precise mechanism of its action is not known. Here we show that there are two binding sites for clozapine at the dopamine D1 and D2 receptors, and the affinity of D1R strongly depended on whether the receptor was present alone or together with D2R (or its genetic variant D2Ser311Cys) in the cell membrane, pointing to the role of receptor hetero-dimerization in the observed phenomenon. The use of fluorescence resonance energy transfer (FRET) technology, observed via fluorescence lifetime microscopy of the single cell, indicated that low concentration of clozapine (10(-9) M), sufficient to saturate the high affinity site, uncoupled the D1R-D2R hetero-dimers. Therefore it has been concluded that clozapine might antagonize the effect of concomitant stimulation of both dopamine receptors, which has been shown previously to enhance the formation of hetero-dimers and to stimulate the calcium signaling pathway.

Effects of the selective dopamine D(1) antagonists NNC 01-0112 and SCH 39166 on latent inhibition in the rat

Dopamine D(1) receptor blockade does not appear to be a prerequisite for antipsychotic activity since many clinically effective antipsychotics have little or no affinity for this receptor subtype. Clozapine, however, which has minimal liability for extrapyramidal symptoms, possesses affinities of similar order for D(1) and D(2) receptors. In earlier animal models used to predict antipsychotic effect, selective D(1) antagonists have shown effects similar to standard antipsychotics with preferential D(2) or mixed D(1)/D(2) antagonism. We investigated the effects of haloperidol (0.1 mg/kg) and two selective D(1) antagonists, NNC 01-0112 (0.05, 0.1 and 0.2 mg/kg) and SCH 39166 (0.02, 0.2 and 2.0 mg/kg), on latent inhibition (LI) in rats. LI is a behavioural paradigm in which repeated nonreinforced preexposure to a stimulus retards subsequent associations to that stimulus. Disrupted LI has been suggested as a model for the attentional deficits in schizophrenia. Using preexposure to a flashing light stimulus, which subsequently served as a conditioned stimulus for suppression of water licking, we demonstrated a clear LI effect with haloperidol but with neither of the two D(1) antagonists. Since selective D(1) antagonists are not clinically effective, these results add further credibility for the relevance of LI as an animal model of psychosis.

Chronic oral haloperidol and clozapine in rats: A behavioral evaluation

The present study evaluated chronic oral treatment of rats with haloperidol or clozapine. Drugs were given in the drinking water for a 23-day period. Rat behavior was analyzed once a week in an open field. Rats ingested either 1.7 mg/kg haloperidol or 40 mg/kg clozapine daily. Blood serum analysis revealed concentrations of 6 ng/ml for haloperidol and 22 ng/ml for clozapine at the end of the treatment. Haloperidol decreased overall activity from the onset of treatment. Clozapine showed similar effects only on the last test day. Control animals showed a slight habituation in exploration-related parameters. In conclusion, these results indicate that oral drug administration through the drinking water is a suitable mode of noninvasive chronic treatment that led to sufficiently high drug levels to induce specific pharmacological effects in rats.

Chronical haloperidol and clozapine treatment in rats: differential RNA display analysis, behavioral studies and serum level determination

1. Adult, female rats were treated orally for 23 days with 1.6 mg/kg haloperidol or 36 mg/kg clozapine per day, to study chronic effects of the two neuroleptics. 2. At five time points during the neuroleptic treatment, animal behavior was recorded in an open field and locomotive activity was analysed. At the end of the experiment, rats were decapitated, blood samples were collected and serum concentrations of haloperidol and clozapine were determined by a radioreceptor or HPLC assay, respectively. RNA was isolated from each brain, without cerebellum, and subjected to differential RNA display. 3. Mean serum concentrations were 8 ng/ml for haloperidol and 21 ng/ml for clozapine. Analysis of open field behavior showed that haloperidol and clozapine decreased the total distance moved and the velocity as measures of the overall activity, whereas the number of rearings and the number of entries into the center, reflecting risk assessment behavior, were differentially affected. Three neuroleptic-regulated gene fragment bands were identified in differential RNA display experiments. Two gene fragments of 281 bp and 266 bp were sequenced. 4. We conclude that our study design that used behavioral, pharmacokinetic and molecular analysis increase the likelihood of finding relevant molecular events underlying the pharmacotherapeutic effects of neuroleptics in animal models.

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.

11C- and 76Br-labelled NNC 22-0010, selective dopamine D1 receptor radioligands for PET

NNC 22-0010 is a new dopamine antagonist with a high affinity and selectivity for D1 receptors in vitro. NNC 22-0010 has both an N-methyl group and a bromine, which allows radiolabelling with either 11C or 76Br. We labelled [11C]NNC 22-0010 by N-methylation of the free base of the secondary amine with [11C]methyl iodide in a total radiochemical yield of 40%. The total synthesis time was 30 min. The specific radioactivity at time of injection of the radioligand was 48 to 55 GBq/mumol. The [76Br]NNC 22-0010 was synthesized from the iodine precursor by an exchange reaction with 76Br using a Cu(+)-assisted nucleophilic substitution reaction. The radiochemical yield was 60% after purification. Specific radioactivity at time of injection of the radioligand was 6 to 20 GBq/mumol. In PET experiments with [11C]NNC 22-0010 and [76Br]NNC 22-0010 there was a rapid uptake of radioactivity in the monkey brain. The striatum-to-cerebellum ratio was 2-2.5 after 1 h. Binding in the striatum was displaced by SCH 23390, whereas binding in the cerebellum was not reduced. Metabolite studies showed that 1 h after injection about 20% of the radioactivity in plasma represented unchanged radioligand. This value was on the same level for at least 6 h. The results indicate that radiolabelled NNC 22-0010 has potential for imaging dopamine D1 receptors selectively in the human brain.

The advantage of using positron emission tomography in drug research

A large problem in drug discovery is to find relevant in vitro or in vivo animal models and to be able to extrapolate the results obtained to humans. Drug research now benefits from the fast development of imaging technologies that trace radiolabelled molecules directly in the human brain. Positron emission tomography (PET) and allied techniques use molecules that are labelled with short-lived radioisotopes and injected intravenously. The most straightforward approach is to radiolabel a new potential drug and then to trace its anatomical distribution and binding in the brain. An indirect approach is to study how the unlabelled drug inhibits specific radioligand binding. The demonstration of quantitative relationships between drug binding in vivo and drug effects in patients is used to validate targets for drug action and to optimize clinical treatment.

Evaluation of SCH 39166 as PET ligand for central D1 dopamine receptor binding and occupancy in man

SCH 39166 is the first selective D1-dopamine receptor antagonist developed for clinical trials in schizophrenia. SCH 39166 was evaluated as a radioligand for PET, labeled with 11C, and as a D1-dopamine receptor antagonist after single oral doses in healthy men. After intravenous injection of [11C]SCH 39166 distribution of radioactivity in brain grossly reflected D1-dopamine receptor density. The putamen to cerebellum ratio at equilibrium was low (1.54 +/- 0.18 SD), which makes [11C]SCH 39166 less suitable as a radioligand for applied PET studies. Saturability of specific binding was demonstrated after IV injection of [11C]SCH 39166 with low specific radioactivity. Stereospecificity of binding was examined using the stereoisomer [11C]SCH 39165. D1-Receptor occupancy was demonstrated with [11C]SCH 39166 2 h after administration of single oral doses of unlabeled SCH 39166 to each of three healthy subjects (25, 100 and 400 mg). There was a substantial reduction of specific [11C]SCH 39166 uptake in the putamen after all doses. Single oral doses of 100 mg induced approximately 70% D1-dopamine receptor occupancy in the basal ganglia, which should be sufficient to investigate the antipsychotic potential of D1-dopamine receptor antagonism in clinical studies.

Tolerability, safety and pharmacokinetics of single dose and multiple dosing of the selective D1 antagonist NNC 01-0687 in healthy subjects

Selective dopamine D1-receptor antagonists have been shown to exhibit similar effects in animal models for antipsychotic action as the selective D2 antagonists. NNC 01-0687, a benzazepine with selective and high affinity to the D1-receptor, was well tolerated by healthy subjects allocated to double blind, placebo controlled studies. Complaints of moderate restlessness and drowsiness were reported after administration of 25 mg NNC 01-0687, indicating the dose to be the maximum tolerated single dose. The highest multiple dose level of a daily dose of 45 mg NNC 01-0687 administered t.i.d. for 14 days was assessed as safe and well-tolerated with few reports of adverse events. Some alanine aminotransferase (ALT) elevations appeared in both treatment groups (active and placebo) and no evident influence of NNC 01-0687 on the liver function could be derived. No statistically significant or clinically relevant effects were observed in haematological parameters, urinalyses, blood pressure, heart rate, ECG or plasma levels of prolactin, cortisol or growth hormone. The plasma drug concentration curves indicated a fast absorption with tmax at 0.5-1 h and an apparent elimination half-life of 3-4 h. Both AUC and Cmax appeared to be linearly correlated to the dose, indicating linear pharmacokinetics. With similar Cmax and AUC on day 1 and day 10 no accumulation was observed. When administered just after lunch, the Cmax was reduced by 50-60% and the tmax increased to 3 h, but without change of AUC.

Lack of apparent antipsychotic effect of the D1-dopamine receptor antagonist SCH39166 in acutely ill schizophrenic patients

SCH 39166 is the first selective D1 dopamine receptor antagonist developed for the treatment of schizophrenic patients. To examine potential antipsychotic effect, tolerability and safety, SCH 39166 was given orally to 17 acutely ill drug free schizophrenic patients (DSMIIIR) in an open 4-week study. Doses were escalated from 10 to 100 mg b.i.d. according to a fixed schedule over 17 days and remained at 100 mg b.i.d. for another 11 days. The drug was withdrawn prematurely in ten patients because of deterioration or refusal to take SCH 39166. In the nine patients participating for more than 2 weeks, none had an apparent reduction of BPRS or CGI scores. Side effects were agitation, akathisia and emesis in single patients. After withdrawal of SCH 39166 of the patients improved when treated with classical neuroleptics or clozapine. The result of the study does not support the prediction that selective D1 dopamine receptor antagonism will produce antipsychotic effects in schizophrenia.

Discriminative stimulus effects of cocaine in squirrel monkeys: lack of antagonism by the dopamine D2 partial agonists terguride, SDZ 208-911, and SDZ 208-912

The effects of cocaine alone and after pretreatment with the dopamine D2 partial agonists terguride, SDZ 208-911, and SDZ 208-912 were determined in squirrel monkeys trained to discriminate cocaine from saline using a two-lever drug discrimination procedure. When tested alone, cocaine engendered dose-related increases in the percentage of responses on the cocaine-associated lever, reaching virtually exclusive cocaine-appropriate responding after a dose of 1.0 mg/kg. Pretreatment with terguride (0.003-0.03 mg/kg), SDZ 208-911 (0.001-0.01), and SDZ 208-912 (0.003-0.018 mg/kg) did not consistently alter the discriminative stimulus effects of cocaine. Although some doses of each D2 partial agonist either increased (notably SDZ 208-911) or decreased (notably SDZ 208-912) the level of cocaine-appropriate responding engendered by low to intermediate doses of cocaine, none of the drugs reduced the percentage of cocaine-appropriate responding engendered by low to intermediate doses of cocaine, none of the drugs reduced the percentage of cocaine-appropriate responses engendered by 1.0 mg/kg cocaine. The results do not support the view that terguride, SDZ 208-911, or SDZ 208-912 would serve as functional antagonists of the subjective effects of cocaine.

Characterization of benzazepine UDP-glucuronosyl-transferases in laboratory animals and man

1. The O-glucuronidation of two dopamine D1 receptor antagonists, Odapipam and Berupipam, were studied in hepatic microsomal fractions from mouse, rat, rabbit, dog, pig, and man using 14C-UDP-glucuronic acid. 2. The influence of pH, detergent, gender, drug-metabolizing enzyme inducers, and age were examined. Detergents like the zwitterionic CHAPS and non-ionic Tween 20, Triton X-100, and Brij 35 stimulated the glucuronidation rate by up to 600% of native activity with the latter being most effective. Both apparent Km and Vmax increased following detergent treatment in rat hepatic microsomes. Less marked activation of UDP glucuronosyltransferase activity was observed with Brij 35 in mouse, rabbit, dog, and pig compared with rat. In contrast, human hepatic microsomes were not stimulated by detergent treatment. 3. Marked species-dependent UDP-glucuronosyltransferase activity were observed for the two compounds. In general, Odapipam exhibited higher Vmax and Km compared with Berupipam with the exception of rabbit where the reverse was true. Similar kinetic parameters were, however, observed in human hepatic microsomes. Highest glucuronidation rate (in general) was observed in mouse followed by dog, pig, rabbit, man, and rat. 4. UGT activity in human livers showed up to a seven-fold variation. Conjugation of each compound were highly correlated (r = 0.92; n = 20) suggesting that identical isoform(s) were involved in this reaction. A significant age-related decrease in UDP-glucuronosyltranferase activity was observed, which partly could be explained by a preponderance in elderly female donor liver samples.

Oral administration of NNC 756--a placebo controlled PET study of D1-dopamine receptor occupancy and pharmacodynamics in man

NNC 756 is a new benzazepine with high affinity and selectivity for D1-dopamine receptors. In a double-blind, placebo controlled, cross-over study, positron emission tomography and the radioligand [11C]SCH 23390 were used to determine central D1-dopamine receptor occupancy after a single oral dose of 80 mg NNC 756 in three healthy men. NNC 756 induced 75, 66 and 47% occupancy of D1-dopamine receptors in the putamen of at 1.5 h after drug administration and 46, 36 and 24% after 7.5 h. There was a hyperbolic relationship between the occupancy values and the serum concentrations. The Ki value for the hyperbola was 6.4 ng/ml (+/- SD 1.4). The occupancy at 1.5 h is on the same level as that shown to induce effects in animal models for prediction of antipsychotic effect. Restlessness (akathisia) appeared in two subjects and nausea in one subject at time of peak drug concentration in serum. The oral dose level of 80 mg should be appropriate to investigate the potential antipsychotic effect of NNC 756.

Long-term treatment with low doses of the D1 antagonist NNC 756 and the D2 antagonist raclopride in monkeys previously exposed to dopamine antagonists

Eight Cebus apella monkeys previously exposed to D1 and D2 antagonists were treated subcutaneously for 8 weeks with the D1 antagonist NNC 756 (0.01 mg/kg), followed by a wash-out period of 4 weeks and treatment with the D2 antagonist raclopride for 8 weeks (end doses 0.01 mg/kg). NNC 756 induced no dystonia, while marked dystonia was induced by raclopride. Mild tolerance to the dystonia-inducing effect of raclopride slowly developed. Both drugs induced significant sedation and mild bradykinesia. Sedation induced by NNC 756 was stronger than that of raclopride, while no differences were found regarding bradykinesia. The sedative effect of both NNC 756 and raclopride increased over time during chronic treatment. No changes in bradykinesia developed. No significant dyskinesia was induced by NNC 756, while raclopride significantly induced both acute and tardive oral dyskinesia. Furthermore, raclopride-induced acute dyskinesia worsened during chronic treatment. Concomitant treatment with NNC 756 tended to reduce the D1 agonist SKF 81297-induced dyskinesia and grooming, while concomitant treatment with raclopride increased SKF 81297-induced dyskinesia and tended to decrease SKF 81297-induced grooming. Chronic treatment with raclopride induced supersensitivity to both the D2/D3 agonist LY 171555 and SKF 81297, while chronic NNC 756 treatment only induced supersensitivity to SKF 81297. The findings indicate that D1 antagonists may induce less dystonia and oral dyskinesia as compared with D2 antagonists and support the hypothesis of both a permissive and an inhibitory interaction between D1 and D2 receptor systems.

Dopamine receptors: molecular biology, biochemistry and behavioural aspects

The description of new dopamine (DA) receptor subtypes, D1-(D1 and D5) and D2-like (D2A, D2B, D3, D4), has given an impetus to DA research. While selective agonists and antagonists are not generally available yet, the receptor distribution in the brain suggests that they could be new targets for drug development. Binding characteristics and second messenger coupling has been explored in cell lines expressing the new cloned receptors. The absence of selective ligands has meant that in vivo studies have lagged behind. However, progress has been made in understanding the function of DA-containing discrete brain nuclei and the functional consequence of the DA's interaction with other neurotransmitters. This review explores some of the latest advances in these various areas.

[11C]NNC 687 and [11C]NNC 756, dopamine D-1 receptor ligands. Preparation, autoradiography and PET investigation in monkey

NNC 687 and NNC 756 [(+)-5-(2,3-dihydrobenzofuran-7-yl)-7-hydroxy-3-methyl- 8-nitro-2,3,4,5-tetrahydro-1H-3-benzazepine and (+)-8-chloro-5-(2,3-dihydrobenzofuran-7-yl)-7-hydroxy-3-methyl-2,3,4,5- tetrahydro-1H-3-benzazepine] are two new potent dopamine D-1 receptor antagonists. [11C]NNC 687 and [11C]NNC 756 were both prepared by N-methylation of the corresponding desmethyl compounds with [11C]methyl iodide. The reactions were performed in acetone with subsequent normal-phase semi-preparative HPLC and resulting in 50-60% radiochemical yield (from EOB and decay-corrected) with a total synthesis time of 30-35 min and a radiochemical purity higher than 99%. The specific radioactivity obtained at time of injection was about 1500 Ci/mmol (55 GBq/mumol). Autoradiographic examination of [11C]NNC 687 and [11C]NNC 756 binding in post-mortem human brain sections showed specific binding in the striatum, a region with high density of dopamine D-1 receptors. PET examination of the radioligands in a Cynomolgus monkey demonstrated accumulation of radioactivity predominantly in the striatum. The ratio between radioactivities in the striatum and the cerebellum was about 2 and 8 for [11C]NNC 687 and [11C]NNC 756 after 60 min. [11C]NNC 756 should have potential as PET ligand for examination of central dopamine D-1 receptors in man.

Effects of chronic discontinuous and continuous treatment of rats with a dopamine D1 receptor antagonist (NNC-756)

Rats were treated intermittently or continuously with the dopamine D1 receptor antagonist NNC-756 for 15 weeks. Two weeks after withdrawal they were challenged with the dopamine D1 receptor agonist SK&F 38393, either alone or after pretreatment with NNC-756. Neither treatment regimen resulted in irreversible increases in oral activities when treated rats were compared with controls; however, transient elevations were observed in the beginning of treatment in the continuously treated group and in the withdrawal phase in the discontinuously treated group. Furthermore, discontinuous treatment resulted in within-group elevations in vacuous chewing movements and tongue protrusions after withdrawal. Dopamine D1 receptor supersensitivity was not observed after challenge with the dopamine D1 receptor agonist. NNC-756 efficiently blocked the behavioural response to stimulation with SK&F 38393. Both treatment regimens resulted in the development of rigidity and catalepsy. The present study suggests that treatment with selective dopamine D1 receptor antagonists is less likely to cause irreversible oral dyskinesia than is treatment with classical neuroleptic drugs.

PET examination of [11C]NNC 687 and [11C]NNC 756 as new radioligands for the D1-dopamine receptor

The benzazepines NNC 687 and NNC 756 have in animal studies been described as selective D1-dopamine receptor antagonists. Both compounds have been labeled with 11C for examination by positron emission tomography (PET). In the present study central receptor binding was studied in monkeys and healthy men. After IV injection of both radioligands in Cynomolgus monkeys radioactivity accumulated markedly in the striatum, a region with a high density of D1-dopamine receptors. This striatal uptake was displaced by high doses of the selective D1-antagonist SCH 23390 (2 mg/kg) but not by the 5HT2-antagonist ketanserin (1.5 mg/kg) or the selective D2-antagonist raclopride (3 mg/kg). The cortical uptake after injection of [11C]NNC 687 was not reduced in displacement experiments with ketanserin. The cortical uptake of [11C]NNC 756 was reduced in displacement and protection experiments with ketanserin by 24-28% (1.5 mg/kg), whereas no reduction could be demonstrated on striatal uptake. In healthy males both compounds accumulated markedly in the striatum. For [11C]NNC 687 the ratio of radioactivity in the putamen to cerebellum was about 1.5. For [11C]NNC 756 the ratio was about 5. This ratio of 5 for [11C]NNC 756 is the highest obtained so far for PET radioligands for the D1-dopamine receptor.

Chronic treatment with the D1 receptor antagonist, SCH 23390, and the D2 receptor antagonist, raclopride, in cebus monkeys withdrawn from previous haloperidol treatment. Extrapyramidal syndromes and dopaminergic supersensitivity

The effects of chronic treatment with dopamine (DA) D1 and D2 receptor antagonists were evaluated in eight cebus apella monkeys with mild oral dyskinesia after previous haloperidol treatment. SCH 23390 (D1 antagonist) was given daily to investigate the direct behavioural effect during long-term treatment and the subsequent supersensitivity to DA agonists. Raclopride (D2 antagonist) was investigated for comparison. All drugs were given subcutaneously. SCH 23390 and raclopride induced dystonic syndromes, catalepsy, sedation and reduced locomotor activity. The monkeys developed marked tolerance to the dystonic effect of SCH 23390, while they showed increased sensibility to the dystonic effect of raclopride. Baseline oral dyskinesia (24 h after injection) remained unchanged during D1 antagonist treatment, while it increased during D2 antagonist treatment. SCH 23390 induced supersensitivity to the oral dyskinesia- and grooming-inducing effects of SKF 81297 (D1 agonist) after 9 weeks, while the subsequent treatment with raclopride induced supersensitivity to the reactivity- and stereotypy-inducing effects of quinpirole (D2 receptor agonist) after 3 weeks. Because of the possibility of a carry-over effect (SKF 81297-induced oral hyperkinesia and grooming), other changes in raclopride-induced behaviours cannot be ruled out. The development of tolerance to the dystonic effect of SCH 23390 and the unchanged baseline oral dyskinesia during SCH 23390 treatment indicate an advantageous profile of side effects of DA D1 receptor blockade.

Effect of chronic treatment with NNC 756, a new D-1 receptor antagonist, or raclopride, a D-2 receptor antagonist, in drug-naive Cebus monkeys: dystonia, dyskinesia and D-1/D-2 supersensitivity

When given subcutaneously in gradually increasing doses, up to 1 mg/kg, NNC 756, a dopamine (DA) D-1 antagonist, failed to produce dystonia in eight drug-naive Cebus monkeys. In contrast, raclopride, a DA D-2 antagonist, produced dystonia at low doses (0.010-0.015 mg/kg). Following pre-treatment with raclopride, NNC 756 also induced dystonia at low doses (0.015-0.025 mg/kg), but continued treatment caused tolerance, and increasing doses of NNC 756 could be administered without induction of dystonia. NNC 756 induced a dose-dependent parkinsonism (slow, stiff movements and tremor), and more sedation than raclopride. After treatment for 14 weeks, withdrawal of raclopride (0.01 mg/kg) led to mild oral dyskinesia (tardive dyskinesia), while withdrawal of NNC 756 (1.0 mg/kg) led to a special grooming syndrome, but no dyskinesia. Withdrawal of raclopride as well as NNC 756 led to behavioural D-1 and D-2 dopamine supersensitivity in the form of increased dyskinesia (including grooming after NNC 756) induced by D-1 agonist (SKF 81297) and increased arousal induced by D-2 agonist (quinpirole). These results indicate that D-1 antagonists such as NNC 756 elicit fewer extrapyramidal symptoms (both acute and tardive) than D-2 antagonists such as raclopride, although extremely high doses may cause a special grooming withdrawal syndrome.

NNC-112, NNC-687 and NNC-756, new selective and highly potent dopamine D1 receptor antagonists

The neurochemical properties of three novel benzazepine derivatives NNC-112, NNC-687 and NNC-756 were assessed. These compounds inhibited dopamine D1 receptor binding in vitro with low nanomolar to picomolar dissociation constants whereas those for the D2 receptor were in the micromolar range. Contrary to classical neuroleptics, but similar to the atypical neuroleptics, clozapine and fluperlapine, NNC-112, NNC-687 and NNC-756 were relatively more potent in inhibiting dopamine-stimulated adenylyl cyclase than [3H]SCH 23390 binding. Both NNC-112 and NNC-756 had high affinity for the 5-HT2 receptor whereas NNC-687 had low affinity for this receptor. The affinity for other receptors or neurotransmitter transporters was very low. In vivo, the dopamine D1 receptor selective profile of NNC-112, NNC-687 and NNC-756 was evident from the potent inhibition of D1 receptor binding whereas no effect on D2 receptor binding was apparent. In addition, the compounds blocked D1 receptor-mediated rotation in unilaterally 6-hydroxydopamine-lesioned rats, but had no effect on D2-induced rotation. Thus, NNC-112, NNC-687 and NNC-756 are potent and selective dopamine D1 receptor antagonists that may be useful in the treatment of schizophrenia.