Two directions of dopamine D1/D2 receptor interaction in studies of behavioural regulation: a finding generic to four new, selective dopamine D1 receptor antagonists

Behavioural and pharmacological examinations in a transgenic mouse model of early-onset torsion dystonia

Early-onset torsion dystonia is an autosomal dominant movement disorder associated with the DYT1 gene (TOR1A) defect which results in a deletion of a glutamic acid residue in the protein torsinA. The pathophysiology of dystonia is poorly understood. Well characterized animal models can help to give insights into the underlying mechanisms and thereby to develop new therapeutics. In the present study, we further characterized transgenic DYT1 mice, which were initially described to exhibit "dystonia-like" postures. In the present study, several behavioural tests in untreated animals did not show strong differences between transgenic and control mice, but nearly all transgenic mice showed "dystonia-like" postures. However, these movements, also observed in control mice, have to be regarded as a clasping reflex. Since dystonia is thought to be related to dopaminergic dysfunctions, pharmacological investigations have been performed to clarify if dopaminergic substances alter motor behaviour in transgenic mice. Chronic treatment with L-DOPA (combined with carbidopa) enhanced the hindlimb claspings only in transgenic mice, while acute applications of drugs, which exert more selective effects on the dopaminergic system, caused similar reactions in transgenic mice and control mice. Therefore, these data do not provide clear evidence for dysfunctions of the dopaminergic system in this mouse model.

Habituation and sensitization of acoustic startle: opposite influences of dopamine D1 and D2-family receptors

The startle response evoked by repeated presentation of a loud acoustic stimulus is regulated by the independent processes of sensitization and habituation. While schizophrenia is associated with information processing impairments, there is conflicting evidence regarding the existence of habituation deficits in schizophrenic patients. Recent clinical evidence, however, indicates that patients with schizophrenia display exaggerated startle sensitization and diminished habituation. Given the linkage between dopaminergic abnormalities and schizophrenia, the goal of the present investigation was to examine the effect of deleting D1 and D2-like dopamine receptors on sensitization and habituation of the acoustic startle reflex in mice. For these experiments, the acoustic startle reflex was assessed in dopamine D1, D2, and D3 receptor wild-type (WT) and knockout (KO) mice on a C57BL/6J background, using a methodology that can measure both sensitization and habituation. Mice lacking the D1 receptor gene displayed enhanced sensitization, along with a decrease in the amount of habituation that occurs in response to repetitive presentations of a startling stimulus. Conversely, the loss of the dopamine D2 or D3 receptor gene produced a sensitization deficit and a significant increase in habituation. The behavioral phenotype exhibited by D1 receptor KO mice is clearly distinct from that of the D2 and D3 receptor KO mice. The findings in D1 receptor KO mice are reminiscent of the abnormalities observed in schizophrenic patients tested in comparable startle paradigms, and indicate that D1 agonists may possess therapeutic efficacy against the information processing deficits associated with schizophrenia.

Contributions of dopamine D1, D2, and D3 receptor subtypes to the disruptive effects of cocaine on prepulse inhibition in mice

Deficits in prepulse inhibition (PPI) of startle, an operational measure of sensorimotor gating, are characteristics of schizophrenia and related neuropsychiatric disorders. Previous studies in mice demonstrate a contribution of dopamine (DA) D(1)-family receptors in modulating PPI and DA D(2) receptors (D2R) in mediating the PPI-disruptive effects of amphetamine. To examine further the contributions of DA receptor subtypes in PPI, we used a combined pharmacological and genetic approach. In congenic C57BL/6 J wild-type mice, we tested whether the D1R antagonist SCH23390 or the D2/3R antagonist raclopride would attenuate the effects of the indirect DA agonist cocaine (40 mg/kg). Both the D1R and D2/3R antagonists attenuated the cocaine-induced PPI deficit. We also tested the effect of cocaine on PPI in wild-type and DA D1R, D2R, or D3R knockout mice. The cocaine-induced PPI deficit was influenced differently by the three DA receptor subtypes, being absent in D1R knockout mice, partially attenuated in D2R knockout mice, and exaggerated in D3R knockout mice. Thus, the D1R is necessary for the PPI-disruptive effects of cocaine, while the D2R partially contributes to these effects. Conversely, the D3R appears to inhibit the PPI-disruptive effects of cocaine. Uncovering neural mechanisms involved in PPI will further our understanding of substrates of sensorimotor gating and could lead to better therapeutics to treat complex cognitive disorders such as schizophrenia.

Kappa-opioid agonist U69593 potentiates locomotor sensitization to the D2/D3 agonist quinpirole: pre- and postsynaptic mechanisms

To assess whether the development and expression of behavioral sensitization to the dopamine D2/D3 agonist quinpirole (QNP) is influenced by coadministration of the kappa opioid receptor agonist U69593, rats received every 3-4 days for a total of 10 treatments an injection of U69593 (0.3 mg/kg) together with an injection of either a postsynaptic (0.5 mg/kg) or a presynaptic dose of QNP (0.05 mg/kg); locomotor activity was measured after each treatment. Control rats were injected as appropriate with QNP, U69593, and vehicle/saline. Following chronic treatment, dose-response profiles to QNP were obtained to assess the expression of sensitization; the effect of U69593 on locomotor activity in animals already sensitized to QNP was also assessed. Results showed that cotreatment of U69593 with a postsynaptic dose of QNP doubled the speed and magnitude of sensitization to QNP, while U69593 cotreatment with a presynaptic dose of QNP switched the effects of QNP from locomotor depression to locomotor sensitization. However, U69593 cotreatment with a presynaptic dose of QNP changed a different set of measures of sensitization than did cotreatment with a postsynaptic dose of the dopamine agonist. Together, findings suggest that sensitization to QNP is not a unitary phenomenon but has components that are relatively independent, mediated by distinct pre- and postsynaptic mechanisms and modulated by kappa receptor activity.

Locomotor behavior of dopamine D1 receptor transgenic/D2 receptor deficient hybrid mice

Mice that incorporate the dopamine D1 receptor transgene controlled by the D1 receptor promoter exhibit a marked increase of D1 binding in several extra-striatal brain regions and show a paradoxical hypokinetic response to D1 agonist [Exp. Neurol. 157 (1999) 169]. The agonist-induced locomotor behavior of D1 receptor transgenic mice is similar to baseline locomotor activity manifested by D2 receptor deficient mice [J. Neurosci. 18 (1998) 3470]. The similarity between these two behavioral phenotypes raised the possibility that stimulation of the over-expressed D1 receptors in the transgenic mice could cause a suppression of D2 receptor responses that manifest in hypokinesia. Alternatively, the similar phenotypes could result from altered D1/D2 receptor balance in both animal models. Two different approaches were undertaken to test these alternative hypotheses. (1) The effects of pharmacological blockade of D2 receptors on D1 agonist-stimulated hypokinesia of the D1 over-expressing animals were investigated. (2) The behavioral phenotype of hybrid D1 receptor over-expressing/D2 receptor deficient mice generated by crossbreeding the D2 knockout mice and the D1 transgenic animals was studied. The results of these studies suggested that the hypomotor response of the D1 transgenic mice was not a result of an interaction of the over-expressed D1 receptors with the native D2 receptors and that over-expressed D1 receptors likely mediate hypokinesia in the D1 transgenic animals. Considering the significance of the D1 dopamine receptor as a therapeutic target for Parkinson's disease, this D1 receptor over-expressing model provides an important experimental system to probe the basis for altered behavioral responses following stimulation of transgenetically up-regulated receptors.

Synthesis and theoretical study of 2-amino-2,3,3a,4,5,6-hexahydro-1H-phenalene and its biological evaluation on central dopaminergic system

Compound 2 considered as a rigid non-hydroxylated 2-amino tetralin was synthesized and biologically evaluated. Central administration of compound 2 (50 microg or 100 microg/10 microl) induced a reduction in urinary sodium and potassium excretion at 3 and 6 h of urine collection. We speculate that compound 2 may be acting as a dopamine receptor antagonist.

Late direct and transneuronal effects in mice with targeted expression of a toxin gene to D1 dopamine receptor neurons

Detailed analysis of a novel transgenic model of basal ganglia disease has been undertaken. In this model the expression of an attenuated form of the diphtheria toxin gene was tightly controlled by D1 dopamine receptor regulatory domains. The behavioral and both direct toxin-mediated and transneuronal effects observed in pups in the first postnatal week have been described. Although younger pups are bradykinetic, older pups have a hyperkinetic syndrome with gait abnormality, postural instability and myoclonic jerks typical of human basal ganglia diseases such as Huntington's disease. As expected, striatal D1 dopamine receptor, dynorphin and substance P transcripts were not detected by in situ hybridization but there was a 27% increase in striatal D2 dopamine receptor messenger RNA and a 65% increase in enkephalin messenger RNA expression. Receptor autoradiographic studies confirmed the lack of D1-class binding in the mutant striatum and in contrast to young pups, a substantial increase in striatal D2-class binding. Autoradiographic quantitation also showed a 30% increase in striatal dopamine transporter binding. In addition to the changes described in the striatopallidal and nigrostriatal pathways, up-regulated dynorphin and substance P messenger RNA expression was also seen in the cortex. The capacity of the developing brain for neurochemical adaptation following injury is dramatic. The results show that primary loss of D1 dopamine receptor-positive striatonigral pathway neurons is sufficient to generate a hyperkinetic phenotype.

Behavioral effects of clozapine and dopamine receptor subtypes

The atypical neuroleptic clozapine (CLZ) is an extremely effective antipsychotic that produces relatively few motoric side effects. However, CLZ displays limited antagonism at the dopamine (DA) D2 receptor, the receptor commonly thought to mediate the antipsychotic activity of neuroleptics. The mechanism of action behind the efficacy of CLZ remains to be determined. Miller, Wickens and Beninger [Progr. Neurobiol., 34, 143-184 (1990)] propose a "D1 hypothesis of antipsychotic action" that may explain the antipsychotic effects of CLZ. This hypothesis is built on the interactions between D2, cholinergic and D1 mechanisms in the striatum. These authors assert that although typical neuroleptics block D2 receptors, it is through an indirect action on D1 receptors that their antipsychotic action is manifest. The extra-pyramidal side effects produced by typical neuroleptics are hypothesized to be due to an indirect action on cholinergic receptors. It is argued that the anticholinergic properties of CLZ negate the D2 (motor side effects) action of CLZ, allowing CLZ to diminish psychotic symptoms through a direct action on D1 receptors. Thus, CLZ may function as a D1 receptor antagonist in behavioral paradigms. The current paper reviews and compares the behavioral profile of CLZ to those produced by D2- and D1-selective antagonists with specific reference to unconditioned and conditioned behaviors in order to more fully evaluate the "D1 hypothesis of CLZ action". Although the actions of CLZ remain unique, they do share some striking similarities with D1 receptor antagonists especially in tests of unconditioned behavior, possibly implicating the D1 receptor in the action of this antipsychotic drug.

Effects of dopamine D-1 and D-2 receptors on intraocular pressure in conscious rabbits

This investigation was designed as a randomized, placebo-controlled, double-masked, crossover study in NZW rabbits with normal intraocular pressure (IOP) to investigate dopaminergic effects on IOP. SKF 38393, a selective D1-receptor agonist, increased, and SDZ PSD-958, a selective D1-receptor antagonist, decreased IOP, respectively. The selective D2-receptor agonist quinpirole decreased IOP, whereas the selective D2 receptor antagonist metoclopramide had no significant effect. Combinations of quinpirole with SDZ PSD-958 decreased IOP in an additive manner. SDZ GLC-756, a mixed D1-receptor antagonist/D2-receptor agonist, decreased IOP in a dose-dependent manner with a maximum effect greater than the maximum effects produced either by the D1-receptor antagonist SDZ PSD-958 and the D2-receptor agonist quinpirole. The effect of SDZ GLC-756 could only be partially blocked by the selective D2-receptor antagonist metoclopramide suggesting that both D1-receptor blockade and D2-receptor stimulation participate in its IOP-lowering effect. Tonography suggests that SDZ GLC-756 has no significant effect on outflow facility. Furthermore, the results suggest that both D1 and D2 receptors each play an independent role in the regulation of IOP in rabbits. Thus, simultaneous blockade of D1 receptors and stimulation of D2 receptors may provide a new pharmacological approach for the treatment of ocular hypertension frequently associated with glaucoma.

Evidence for dopamine 'D1-like' receptor subtypes in the behavioural effects of two new selective antagonists, LY 270411 and BW 737C

A new, chemically distinct antagonist at dopamine 'D1-like' receptors, the thienoazepine LY 270411, ([+]-2(3-chloro-6-methyl-8-phenyl-5,6,7,8 -tetrahydro-4H-thieno[2,3d]azepin-2-yl)propan-2-ol) was compared with the isoquinoline BW 737C ([S]-6-chloro-1-[2,5-dimethoxy-4- propylbenzyl]-7-hydroxy-2-methyl-1,2,3,4-tetrahydroisoquinoline) and the benzazepine SCH 23390 ([R]-7-chloro-8-hydroxy-2,3,4,5-tetrahydro-3-methyl-1-phenyl-1 H-3-benzazepine) for effects on behavioural responses to the isochroman full efficacy dopamine 'D1-like' receptor agonist A 68930 ([1R,3S]-1-aminomethyl-5, 6-dihydroxy-3-phenylisochroman) vs. the dopamine 'D2-like' receptor agonist RU 24213 (N-n-propyl-N-phenylethyl-p-3-hydroxyphenylethylamine). Grooming responses to A 68930 were readily blocked by each of LY 270411, BW 737C and SCH 23390; however, the vacuous chewing response was blocked only by BW 737C. Sniffing and locomotor responses to RU 24213 were attenuated by BW 737C and SCH 23390 but not by LY 270411; furthermore, myoclonic jerking to RU 24213 was released by BW 737C and SCH 23390 but not by LY 270411. These findings indicate that grooming induced by dopamine 'D1-like' receptor agonism is blocked by all chemical classes of dopamine 'D1-like' receptor antagonist while vacuous chewing is blocked only by isoquinoline dopamine 'D1-like' receptor antagonism; this suggests that these behaviours may be mediated via functionally and pharmacologically distinct subtypes of dopamine 'D1-like' receptor. Furthermore, LY 270411 appears unique in its activity to readily block 'D1-like' receptor agonist-induced grooming without influencing behavioural responses to dopamine 'D2-like' receptor agonism; thus, the site mediating prototypical dopamine 'D1-like' receptor agonist-induced behaviours may be dissociable pharmacologically from dopamine 'D1-like' site(s) participating in functional interactions with dopamine 'D2-like' receptors.

Fluorinated benzazepines: 1. Synthesis, radiosynthesis and biological evaluation of a series of substituted benzazepines as potential radiotracers for positron emission tomographic studies of dopamine D-1 receptors

We have prepared N-alkyl, aryl, fluoroalkyl, fluoroaryl and iodoaryl derivatives of 7-chloro-8-hydroxy-3-methyl-1-(3'-aminophenyl)-2,3,4,5-tetrahydro-1 H-3-benzazepine (SCH 38548) as high-affinity ligands for the dopamine D1 receptor. Binding affinities of the compounds for dopamine D1, D2, and serotonin 5-HT2 receptor sites in rat brain homogenates were measured. The affinity of SCH 38548 for dopamine D1 receptors was found to be 0.53 +/- 0.46 nM, whereas lower affinities (in the micromolar range) for dopamine D2 and serotonin 5-HT2 receptors were found. Alkylation (ethyl, n-propyl and benzyl) and acylation (benzoyl) of the amino group of SCH 38548 did not decrease affinities for the D1 receptors significantly. The fluoroethyl, fluoropropyl, and fluorobenzyl derivatives showed approximately an 8-fold, 9-fold, and 3-fold decrease in affinity for the D1 sites compared to SCH 38548. The N-4-fluorobenzoyl derivative, however, showed a similar affinity for the D1 sites as for SCH 38548. All four fluorinated derivatives exhibited weak binding at D2 and serotonin 5-HT2 receptors. The N-(4-18F-fluorobenzoyl)SCH 38548 was prepared by reacting SCH 38548 with 4-18F-fluorobenzoyl fluoride in 2-5% radiochemical yield with a specific radioactivity of approximately 600-700 Ci/mmol. The N-(3-18F-fluoropropyl)SCH 38548 was prepared by reacting SCH 38548 with 18F-fluoropropyl iodide in 2-5% radiochemical yield with a specific radioactivity of approximately 600-700 Ci/mmol. N-(4-18F-fluorobenzoyl)SCH 38548 failed to localize in the dopaminergic sites in the rat and rhesus monkey brain. Biodistribution of N-(3-18F-fluoropropyl)SCH 38548 in rats showed specific uptake and retention (0.64% injected dose/g at 30 min) of the radiotracer in the striata, with striata-to-cerebellum ratios reaching 12 at 2 h postinjection (p.i.). Positron emission tomography scans in rheusus monkeys indicate selective uptake of the radiotracer in the striata. After IV injection of N-(3-18F-fluoropropyl)SCH 38548, a rapid brain uptake of the tracer from blood was observed. Initial uptake in the striata and cerebellum was approximately 0.02% of injected dose/cc. Nonspecific uptake from the tissue surrounding the striata cleared slowly. The striata-to-cerebellum ratio increased from 1.20 to 3.5 min postinjection to approximately 2.5 at 120 min p.i. The specific uptake of N-(3-18F-fluoropropyl)SCH 38548 in the striata was displaced by IV administration of SCH 24518 (2 mg/kg).

The temporal evolution of striatal dopamine receptor binding and mRNA expression following hypoxia-ischemia in the neonatal rat

Neonatal hypoxic-ischemic (HI) brain injury in the rat alters dopamine receptors. To determine whether such changes are permanent, dopamine receptors and corresponding mRNA were examined at various time points after neonatal HI using receptor autoradiography and in situ hybridization. Rat pups underwent ligation of the left common carotid artery followed by hypoxic exposure (8.5% O2 for 3 h). Controls underwent sham surgery alone. Animals surviving for 2-80 days following HI were studied. Striatal D1 receptors (labeled by [3H]SCH23390) were reduced as early as 2 days following HI, remained depressed for 21 days, but recovered to control levels by young adulthood (3 months of age). D2 receptors (labeled by [125I] iodosulpride) did not decline until 10 days after HI, and remained uniformly depressed throughout the caudate-putamen thereafter. Changes in D1 receptor mRNA transcripts closely paralleled alterations in receptors: early reductions in D1 mRNA signal recovered by young adulthood. D2 mRNA exhibited a unique temporal profile with an early decrease (2 days following HI), and prompt, persistent recovery. Dopamine receptors and transcripts are differentially affected by HI injury early in development. Whereas D1 receptor expression recovers from neonatal HI injury, D2 receptors remain permanently affected despite the presence of normal levels of D2 receptor transcripts. A persistent, post-transcriptional effect of HI on D2 receptor expression is suggested.

Comparison of the new atypical antipsychotics olanzapine and ICI 204,636 with clozapine on behavioural responses to the selective "D1-like" dopamine receptor agonist A 68930 and selective "D2-like" agonist RU 24213

The effects of the putative atypical antipsychotics olanzapine and ICI 204,636 on behavioural responses to the selective "D2-like" dopamine receptor agonist RU 24213 and to the selective "D1-like" agonist A 68930 were compared with those of the prototype atypical antipsychotic clozapine, the selective D1-like antagonist SCH 23390 and the selective D2-like antagonist YM 09151-2. Olanzapine (0.4-2.0 mg/kg) and ICI 204,636 (4.0-36.0 mg/kg), like clozapine (4.0-36.0 mg/kg) and SCH 23390 (0.01-1.0 mg/kg), effected at best modest reduction in typical sniffing and locomotor responses and, with the exception of ICI 204,636, released episodes of atypical myoclonic jerking to RU 24213 (12.5 mg/kg); a high dose of olanzapine (10.0 mg/kg), like YM 09151-2 (0.005-0.5 mg/kg), blocked all responsivity to RU 24213. Conversely, olanzapine (0.4-2.0 mg/kg) and ICI 204,636 (4.0-36.0 mg/kg), like clozapine (4.0-12.0 mg/kg) and SCH 23390 (0.01-0.1 mg/kg), readily blocked typical grooming responses to A 68930 (0.5 mg/kg); YM 09151-2 failed to block grooming and exerted more variable effects. Olanzapine and, to a lesser extent, ICI 204,636 share with clozapine a preferential action to attenuate D1-mediated function; given their lack of selective affinity for D1-like receptors, this common effect may be exerted at an alternative level of synaptic function. The action of olanzapine and particularly ICI 204,636 to release additional episodes of atypical vacuous chewing to A 68930 indicates some deviation from a wholly clozapine-like profile, the clinical significance of which remains to be specified.

SDZ GLC 756, a novel octahydrobenzo[g]quinoline derivative exerts opposing effects on dopamine D1 and D2 receptors

SDZ GLC-756, a novel octahydrobenzo[g]quinoline derivative, is equipotent in displacing [3H]SCH23390 from dopamine D1 receptors and [3H]205-501 from dopamine D2 receptor binding sites. It blocks dopamine sensitive adenylate cyclase with the same potency as SCH23390, indicating antagonist properties at dopamine D1 receptors. On the other hand, SDZ GLC 756 inhibits electrically evoked acetylcholine release from rat striatal slices with the same potency as the selective dopamine D2 receptor agonist bromocriptine. This effect is blocked by spiperone suggesting that it is mediated by dopamine D2 receptor activation. The opposing action of SDZ GLC 756 on dopamine D1 and D2 receptors is also evident in vivo. SDZ GLC 756, like SCH23390, blocks apomorphine-induced rearing in mice. On the other hand, it inhibits prolactin secretion and produces circling in unilateral 6-OHDA-lesioned rats, which is compatible with stimulant properties at dopamine D2 receptors. This drug might be a new tool to study linkage between dopamine D1 and D2 receptors.

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.

Behavioural pharmacology of 'D-1-like' dopamine receptors: further subtyping, new pharmacological probes and interactions with 'D-2-like' receptors

1. D-1 receptors are now recognised to play a critical psychopharmacological role in the regulation of unconditioned motor and numerous other aspects of behaviour. 2. There appears to exist a broad family of 'D-1-like' receptors in terms both of differential coupling to distinct messenger/transduction mechanisms and of gene cloning, whose behavioural roles remain to be clarified. 3. The adenylyl cyclase-inhibiting benzazepine SK&F 83959 induces behavioural responses in rats that are similar to those induced by the full efficacy cyclase-stimulating isochroman A 68930 but not to those induced by its high efficacy partial agonist benzazepine congener R-6-Br-APB; these data indicate roles for individual 'D-1-like' receptors in mediating distinct elements of dopaminergic behaviour. 4. The putative D-1 autoreceptor agonist B-HT 920 and the putative D-3 agonist 7-OH-DPAT demonstrate different behavioural profiles when given both alone and in combination with the selective 'D-1-like' antagonist BW 737C; D-3 receptors may participate in cooperative/synergistic but not in oppositional 'D-1-like': 'D-2-like' interactions. 5. Such interactions apparent at the level of behaviour are complemented by evidence for similar interactions at numerous alternative levels of function, though these may differ between rodent and primate species. 6. A broader range of more selective agonists and antagonists, able to distinguish between individual members of the 'D-1-like' and of the 'D-2-like' receptor families are needed to clarify these issues.

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.

Selective dopamine antagonist pretreatment on the antiparkinsonian effects of benzazepine D1 dopamine agonists in rodent and primate models of Parkinson's disease--the differential effects of D1 dopamine antagonists in the primate

In rats with unilateral 6-hydroxydopamine (6-OHDA) lesions of the medial forebrain bundle, pretreatment with the D1 DA antagonists, SCH 23390 (7-chloro-8-hydroxy-2,3,4,5-tetrahydro-3-methyl-1-phenyl-1H-3-benzazepin e) and A66359 (1- 2-bromo-4,5-dimethoxybenzyl]-7-hydroxy-6-methoxy-2-methyl- 1,2,3,4 tetrahydroisoquinoline), but not the D2 DA antagonist raclopride inhibited the contralateral circling induced by the benzazepine D1 DA agonists SKF 38393 (7-H, 3-H analogue of SCH 23390), SKF 80723 (7-H, 3-H, 6-Br analogue) and SKF 83959 (7-H, 6-Cl, 3'-CH3 analogue). In MPTP (1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine) treated common marmosets, administration of SKF 80723 and SKF 83959 increased locomotor activity and reversed the motor disability. Grooming and oral activities were also increased. Pretreatment with SCH 23390 and A66359 inhibited all the behavioural changes induced by both D1 DA agonists. In general, higher doses of A66359 and more especially SCH 23390 were needed to inhibit SKF 83959 and SKF 80723 induced increases in oral activity and grooming than locomotor activity. Raclopride pretreatment did not affect SKF 83959 and SKF 80723 induced oral activity and grooming, though it reduced the duration of the locomotor changes induced by the D1 DA agonists. These findings demonstrate that the behavioural effects of benzazepine D1 DA agonists in the 6-OHDA lesioned rat and MPTP-treated marmoset are mediated by D1 DA receptor sites, although in the primate, stimulation of D2 DA receptors by endogenous DA may be necessary in facilitating the antiparkinsonian effects of D1 DA agonists. The differential sensitivities of locomotor/motor disability and oral/grooming behaviours to antagonism by D1 DA antagonists may indicate the involvement of multiple D1 DA receptor subtypes in mediating benzazepine D1 DA agonist induced behaviours in the MPTP-treated marmoset.

The effects of clozapine on behavioural responses to the selective 'D1-like' dopamine receptor agonist, A 68930, and to the selective 'D2-like' agonist, RU 24213

1. The influence of the atypical antipsychotic clozapine on D1 dopamine receptor-mediated function was examined in terms of its effects on behavioural responses to the new isochroman selective D1 agonist, A 68930, and to the selective D2 agonist, RU 24213. 2. In rat striatal membrane preparations, radioligand binding studies with [3H]-SCH 23390 and [3H]-spiperone confirmed clozapine to show weak and non-selective affinity for both D1 and D2 receptors. 3. Using a rapid time-sampling behavioural check list technique, clozapine (4.0-36.0 mg kg-1) exerted only modest antagonism of RU 24213 (15.0 mg kg-1)-induced sniffing and locomotion, and weakly released some episodes of myoclonic jerking; such antagonism with release of jerking has been shown previously to occur only during concurrent stimulation of D2 receptors and attenuation of D1 function. 4. Over the same dose-range, clozapine completely blocked A 68930 (0.25 mg kg-1)-induced intense grooming but failed to influence the vacuous chewing response; this profile was similar to that demonstrated previously for selective D1 antagonists. 5. On the basis of complete blockade of typical D1 agonist-induced grooming and weak release of atypical jerking to D2 agonism in the face of modest reduction in typical D2-stimulated behaviours, clozapine appears to exert some preferential but not selective attenuation of D1 receptor-mediated function. Clozapine may attenuate activity through a classical D1 receptor at a level beyond the recognition site, for which it has little affinity, or by way of new, putative 'D1-like' site(s) that subserve distinct elements of dopaminergic behaviour.

Role of the nucleus accumbens and the striatum in the production of turning behaviour in intact rats

Recent knowledge of the mechanisms underlying turning or circling behaviour in intact rats is reviewed. Most interest has been directed towards the striatum because of the classical hypothesis that turning behaviour results from lateral differences in the activity of the bilateral nigrostriatal pathway. However, the assumption that asymmetrical activation of the striatum is a necessary condition for dopamine-dependent turning behaviour has been questioned by several studies showing that unilateral injection of amphetamine or dopamine receptor agonists into the nucleus accumbens, a target of the mesolimbic dopaminergic system, also produces reliable circling away from the side of injection. Apart from discussing differences in stepping patterns of turning and discussing the role of the dopamine D1/D2 receptor interaction, the present survey focuses attention upon the two-component hypothesis, especially in relation to our recent studies in which activities of dopamine D1 and D2 receptors in the striatum and the nucleus accumbens have been manipulated separately in intact rats. It is hypothesized that turning behaviour is produced by asymmetry within nucleus accumbens circuits which involve neuronal connections from the nucleus accumbens to the A9 cell area, which in turn projects to the ventrolateral striatum that determines the direction of turning.

Evidence for D1 dopamine receptor-mediated modulation of the synaptic transmission from motor axon collaterals to Renshaw cells in the rat spinal cord

The possible modulatory role of D1 dopamine receptors on the excitability of lumbar spinal Renshaw cells was studied in anesthetized rats spinalized at T4 level. Burst responses elicited by single electrical shocks to ipsilateral ventral roots L6 (frequency 0.5 Hz, stimulus width 0.1 ms) and spontaneous activity were recorded extracellularly using conventional 3 M KCl filled glass micropipettes. The specific D1 agonist SKF 38393 (0.5-1 mg/kg i.v.) enhanced Renshaw cell burst responses by 20-60% (n = 7) and increased their spontaneous discharge rate (n = 3). This effect was clearly antagonized by the specific D1 antagonist SCH 23390 (1 mg/kg i.v.) although SCH 23390 proved ineffective per se. We conclude that SKF 38393 induced facilitation was due to activation of the specific D1 receptors which could be the functional counterpart of the presynaptic D2 receptors described earlier by us in the same synapse.

Behavioural effects of the putative D-3 dopamine receptor agonist 7-OH-DPAT in relation to other "D-2-like" agonists

The putative D-3 dopamine receptor agonist 7-OH-DPAT (10 micrograms/kg, s.c.) reduced spontaneous activity in rats, without inducing yawning; higher doses (0.1-10.0 mg/kg, s.c.) stimulated non-stereotyped sniffing, locomotion and chewing, which were attenuated by the selective D-1 antagonist BW 737C (5.0 mg/kg, s.c.) without release of any atypical behaviours. Low doses of 7-OH-DPAT may act on inhibitory D-3 receptors, while higher doses may act at stimulatory D-3 or other "D-2-like" receptors that participate in cooperative but not oppositional interactions with D-1 receptors.

Grooming and vacuous chewing induced by SK&F 83959, an agonist of dopamine 'D1-like' receptors that inhibits dopamine-sensitive adenylyl cyclase

The benzazepine SK&F 83959 demonstrates high affinity for dopamine D1 receptors and potently inhibits the stimulation of adenylyl cyclase by dopamine. This compound induced the same intense grooming behaviour identified previously as a characteristic response to typical dopamine D1 receptor agonists such as SK&F 77434 that stimulate adenylyl cyclase; it also induced vacuous chewing. These findings constitute direct evidence for a behaviourally relevant subtype of 'D1-like' receptor that is not linked to adenylyl cyclase.

Behavioural evidence for "D-1-like" dopamine receptor subtypes in rat brain using the new isochroman agonist A 68930 and isoquinoline antagonist BW 737C

The full efficacy, high potency isochroman D-1 agonist A 68930 demonstrated greater than 220-fold selectivity for D-1 over D-2 receptors. A 68930 (0.06 and 0.25 mg/kg) readily induced intense grooming, together with vacuous chewing; these responses became less evident following higher doses (1.0 and 4.0 mg/kg) and sniffing became prominent. Intense grooming was blocked by three D-1 antagonists, the benzazepines SCH 23390 (0.01-1.0 mg/kg) and NNC-756 (0.01-1.0 mg/kg), and the isoquinoline BW 737C (0.2-5.0 mg/kg); however, vacuous chewing was not antagonised by SCH 23390 and NNC-756, but was blocked by BW 737C. Intense grooming was attenuated by the D-2 antagonist YM 09151 (0.005-0.5 mg/kg) while vacuous chewing was enhanced. These data suggest that intense grooming is mediated by a "D-1 like" receptor that recognises all known chemical classes of D-1-selective compounds, while vacuous chewing may be mediated by a pharmacologically distinct subtype of "D-1-like" receptor that recognises preferentially the isochromans and isoquinolines.