Potent lipophilic substituted benzamide drugs are not selective D-1 dopamine receptor antagonists in the rat

The substituted benzamide drugs YM 09151-2 and clebopride potently inhibited apomorphine-induced stereotyped behaviour in the rat and caused displacement of the specific binding of [3H]spiperone to D-2 binding sites on striatal membranes in low nanomolar concentrations. Other substituted benzamide drugs including metoclopramide, sultopride and flubepride also inhibited stereotyped behaviour to a greater or lesser degree, but were less potent in displacing [3H]spiperone from D-2 sites. YM 09151-2 and clebopride only weakly displaced specific binding of [3H]piflutixol to D-1 sites on rat striatal membranes, and only weakly inhibited striatal dopamine stimulated adenylate cyclase activity, when compared with cis-flupenthixol. The other substituted benzamide drugs did not displace [3H]piflutixol or inhibit dopamine stimulation of adenylate cyclase activity in the concentration range used. Clebopride and YM 09151-2 were highly lipophilic with apparent partition coefficient (log P') values equivalent to those of classical neuroleptic compounds, such as cis-flupenthixol. In contrast, the other substituted benzamide drugs were markedly less lipophilic. A log P' value of approximately 2 was required before inhibition of adenylate cyclase activity or displacement of [3H]piflutixol binding occurred. However, in excess of this value there was no correlation between either inhibition of adenylate cyclase activity or displacement of [3H]piflutixol binding and the lipophilicity of the various compounds. We conclude that potent lipophilic substituted benzamide drugs, like other members of the substituted benzamide series, are selective D-2 receptor antagonists. Inherent steric factors within the drug series would appear to dictate activity at D-1 and D-2 sites, although lipophilicity may contribute to actions in these environments.     

Comparison of the in-vitro receptor selectivity of substituted benzamide drugs for brain neurotransmitter receptors

The in-vitro selectivity of a group of substituted benzamide drugs for brain neurotransmitter receptors was determined to assess the most appropriate drugs for use in human PET studies. All substituted benzamide drugs studied inhibited [3H]haloperidol and [3H]spiperone binding to rat striatal membranes. The most potent compounds were YM 09151-2, clebopride and raclopride. However, these substances also interacted in differing degrees with alpha-1, alpha-2, beta-adrenergic, 5-HT-1, 5-HT-2, and opiate sites. Sulpiride, alizapride, SL 74205, TER 1546 and tiapride were specific for D-2 receptors, but these drugs were active only in the 10(-7)-10(-6) M range. Raclopride, amisulpiride and sultopride showed a 100-1000 differentiation between action on dopamine sites compared with other neurotransmitter receptors. No such selectivity was observed for clebopride or YM 09151-2. Specific substituted benzamides such as alizapride, may be appropriate in high concentrations for defining the interaction of PET ligands with brain dopamine receptors. More potent, but selective, drugs such as raclopride and amisulpiride, may be effective in low concentrations as ligands for labelling dopamine receptor sites. However, the ability of these various substituted benzamide drugs to penetrate into brain and in-vivo to identify dopamine receptors in all brain areas must be assessed.     

Definition of the in-vivo binding of [3H]spiperone in rat brain using substituted benzamide drugs

The ability of some substituted benzamide drugs to define in-vivo the binding of [3H]spiperone to brain dopamine receptors in rats was assessed using behaviourally effective doses in comparison with haloperidol. As judged using haloperidol, [3H]spiperone identified dopamine receptors in the substantia nigra, striatum, tuberculum olfactorium and hypothalamus, but not in frontal cortex or nucleus accumbens. The substituted benzamide compounds alizapride, metoclopramide, clebopride and YM 09151-2 prevented the accumulation of [3H]spiperone in the substantia nigra, striatum, tuberculum olfactorium and hypothalamus. However, YM 09151-2 also caused displacement of [3H]spiperone accumulation in the nucleus accumbens and frontal cortex. (+/- )-Sulpiride, (+/- )-sultopride, amisulpiride and prosulpride all prevented the accumulation of [3H]spiperone in the hypothalamus but were ineffective in one or more of the other regions containing dopamine receptors defined by [3H]spiperone. The isomers of sulpiride and sultopride stereoselectively defined the accumulation of [3H]spiperone in dopamine containing brain regions. The (-)-isomers of both drugs prevented the accumulation of [3H]spiperone in the substantia nigra, striatum, tuberculum olfactorium and hypothalamus. In contrast, (+)-sulpiride and (+)-sultopride were ineffective. Selected substituted benzamide drugs can be used to define the interaction of ligands with dopamine receptors in-vivo. These substances may be useful in PET studies in man. The isomers of some substituted benzamine drugs may be used to define dopamine receptors in-vivo by enantiomeric selectivity.     

Further evidence that 3H-cis(Z)flupenthixol binds to the adenylate cyclase-associated dopamine receptor (D-1) in rat corpus striatum

The proposal that ³H-cis(Z)flupenthixol (³H-FPT) preferentially binds to striatal dopamine receptors associated with adenylate cyclase activity (D-1), distinct from dopamine receptors to which ³H-haloperidol (³H-hal) binds (D-2), has been investigated further. The dopamine agonists bromocriptine and ergotamine were 60-times more potent as displacers of ³H-hal than ³H-FPT binding. Other dopamine agonists (ergometrine, dopamine, apomorphine, 2-amino-6,7-dihydroxy-tetralin (ADTN), and ergocornine) also shared this profile, although a smaller ratio was found for these compounds. Substituted benzamides (clebopride > sultopride > sulpiride > metoclorpramide > tiapride) displace ³H-hal but have only a very slight displacing effect towards ³H-FPT, and did not inhibit dopamine-stimulated adenylate cyclase activity. The same pattern is shared by oxiperomide and molindone. Together, these results support the idea that ³H-FPT labels another class of dopamine receptors than does ³H-hal, and that the former class most likely is associated with adenylate cyclase.     

Characterization of dopaminergic compounds at hD2short, hD4.2 and hD4.7 receptors in agonist-stimulated [35S]GTPγS binding assays

Dopamine receptor agonists and antagonists have been extensively characterized in radioligand binding assays; only a limited number of laboratories have characterized them using a functional assay at multiple receptor subtypes. Experiments were designed to assess four agonists and seven antagonists at three cloned human dopamine receptors using agonist-stimulated [35S]GTPgammaS binding assays in membranes to quantify the initial cellular event following ligand/receptor interaction. In this model there is constitutive G protein activity (agonist-independent [35S]GTPgammaS binding) and potentially constitutive dopamine receptor activity. Thus, discrimination between silent antagonists, partial agonists and inverse agonists is theoretically possible. It was anticipated that distinctions could be made regarding efficacy of the seven receptor antagonists to provide insight regarding the therapeutic use of antipsychotic drugs. In membranes prepared from CHO cells transfected to express high densities of human D2short, D4.2 or D4.7 receptors, the dopamine receptor agonists apomorphine, pergolide, quinelorane and quinpirole produced concentration-dependent increases in agonist-stimulated [35S]GTPgammaS binding. At the hD2short receptor, pergolide and apomorphine were essentially equipotent and more potent than quinelorane and quinpirole; all four agonists displayed similar efficacy at this receptor. At the hD4.2 and the hD4.7 receptors apomorphine was the most potent and pergolide the least efficacious of the four drugs. The ability (both potency and efficacy) of clozapine, haloperidol, olanzapine, quetiapine, risperidone, spiperone and ziprasidone to block apomorphine-stimulated [35S]GTPgammaS binding and alter basal [35S]GTPgammaS binding was also assessed. All of the antagonists inhibited apomorphine-stimulated [35S]GTPgammaS binding with potencies (Kb values) similar to and in rank order consistent with their affinities reported in the literature using radioligand binding assays. Additionally, none of the antagonists altered basal, agonist-independent [35S]GTPgammaS binding, thus they behaved as pure, silent antagonists at D2short, D4.2 and D4.7 receptors under our conditions. In summary, the data suggest that therapeutic distinctions between typical and atypical antipsychotic drugs cannot be made based on their function at D2short, D4.2 and D4.7 subtypes of dopamine receptors.     

Discriminative stimulus effects of a low dose of apomorphine in the rat

The discriminative stimulus (DS) effect of apomorphine was investigated in rats trained in a two-lever, food-reinforcement procedure. Rats were given subcutaneous injections of saline or 0.1 mg/kg apomorphine HCl, 15 min before training sessions. The training dose of apomorphine was chosen to activate dopamine autoreceptors selectively. Stimulus generalization studies demonstrated that the DS effects generalized completely to other directacting dopaminergic agonists such as N-n-propylnorapomorphine (NPNA), pergolide, lergotrile, and bromocriptine. The indirect-acting dopamine agonists, (+)amphetamine, cocaine, and methylphenidate produced predominantly saline-appropriate lever responses. The DS effect of apomorphine at the training dose was incompletely antagonized by haloperidol or metoclopramide. The dopaminergic antagonists tested, however, also partially generalized to apomorphine. Both enantiomers of 3-(3-hydroxyphenyl)-N-n-propylpiperidine (3-PPP) produced apomorphine-appropriate lever choice with the (-) enantiomer being slightly more potent. The discriminative property of this (0.1 mg/kg) dose of apomorphine has characteristics consistent with selective dopamine autoreceptor activation.     

Increased grooming behaviour is induced by apomorphine in mice treated with discriminant benzamide derivatives

Grooming behaviour in mice was dose dependently increased by SK&F 38393 (1.87-30 mg/kg), whereas it was dose dependently decreased by RU 24926 (2.5-10 mg/kg) or LY 171555 (0.4-1.6 mg/kg) alone or combined with SK&F 38393 or apomorphine (0.39-6 mg/kg). The inhibitory effect of 0.75 mg/kg apomorphine on grooming behaviour was not modified by SCH 23390, chlorpromazine, clozapine and thioridazine. In contrast, it was antagonized by eight other dopamine antagonists: a partial restoration to grooming scores lower or similar to those determined in control mice was obtained with flupentixol, haloperidol, metoclopramide, thioproperazine and tiapride, whereas a reversal to grooming scores higher than those determined in control mice was obtained with mice treated with (+/-)-sulpiride, amisulpride or RIV 2093. Furthermore, only SCH 23390, chlorpromazine and clozapine antagonized SK&F 38393 (1.87 mg/kg)-induced grooming behaviour, whereas the effects of flupentixol, thioridazine, metoclopramide, haloperidol and amisulpride in SK&F 38393-treated mice were parallel to those of control mice. Finally, SCH 23390 (20 micrograms/kg) antagonized the apomorphine-induced grooming in mice treated with amisulpride, (+/-)-sulpiride or tiapride. These data confirm the potential role of D-1 dopamine receptors in the expression of grooming behaviour and indicate that the dopamine receptors involved in the inhibition of grooming could be of the D-4 subtype. Our results also reveal that chlorpromazine and clozapine have D-1 antagonist properties and suggest that the modulation of apomorphine-induced grooming behaviour by dopamine antagonists in mice could be used as a test for their classification according to their activity at the different dopamine receptor subtypes.     

Assessment of the neuroleptic potential of some novel benzamide,butyrophenone, phenothiazine and indole derivatives

A series of experimental models were used to determine the activity spectra of potential neuroleptic agents on extrapyramidal and/or mesolimbic dopamine systems: catalepsy induction by the drug alone or in the presence of the acetylcholine-like agent, RS86 [spiro-(N'-methylpiperidyl-4′)-N-ethyl succinamide hydrogen fumarate], or α-methyl-p-tyrosine (α-MT), stereotypy antagonism (amphetamine) and hyperactivity antagonism (intra-accumbens and intrastriatal dopamine). Antiemetic potential was also considered. Whilst classical neuroleptics were active in all test situations, other neuroleptics including benzamide, phenothiazine, butyrophenone and indole derivatives exhibited novel activity spectra: clozapine failed to interact with α-MT, metoclopramide failed to antagonize a mesolimbic hyperactivity or interact with the acetylcholine-like agent. Other benzamide derivatives, clebopride, AHR6092, sultopride and tiapride also failed to interact with RS86, but whilst clebopride was active in all other test situations, AHR6092 was inactive in all but the antiemetic test, sultopride was only weakly cataleptogenic and tiapride only antagonized stereotypy and hyperactivity from the striatum. AHR6839, AHR6134, AHR6645, AHR6505B and AHR1709 were generally inactive in the catalepsy tests but exerted dopamine antagonistic potential in other test procedures. The antagonistic ability of AHR1709 was only revealed in the tests where dopamine function was specifically raised in the nucleus accumbens and caudate-putamen by intracerebral dopamine injections. Data are discussed in terms of a dissociation of drug effects on different cerebral dopamine systems.     

Effects of neuroleptic drugs on the inhibition of exploratory behaviour induced by a low dose of apomorphine: implications for the identity of dopamine receptors

Apomorphine in low doses inhibits spontaneous exploratory behaviour in rats. This effect is commonly referred to as an expression of selective stimulation of dopaminergic autoreceptors. The aim of the present study was to investigate the influence of neuroleptic drugs with different pharmacological profiles on this apomorphine induced inhibition of exploration using techniques for detailed recording of behaviour and multivariate statistical analysis of the results. By comparison with dose response analyses of apomorphine it was possible to determine whether a neuroleptic specifically antagonised the apomorphine effect or if the pattern of behaviour was qualitatively changed in some way. Apomorphine (0.05 mg/kg) was tested against cis-flupenthixol (0.01-0.5 mg/kg), haloperidol (0.01-0.1 mg/kg), metoclopramide (0.2-5 mg-kg), sulpiride (0.5-50 mg/kg) and SCH 23390 (0.005-0.05 mg/kg). Metoclopramide and haloperidol had weak antagonising effects against apomorphine while cis-flupenthixol and SCH 23390 was completely inefficient in this respect. The multivariate analysis indicated that the effects of haloperidol was restricted to only some aspects of the behavioural effects of apomorphine. Only sulpiride did selectively and dose-dependently antagonise the apomorphine induced behavioural suppression. The data provide evidence for a functional subdivision of dopamine receptors at the behavioural level.     

Neurophysiological evidence that D-1 dopamine receptor blockade attenuates postsynaptic but not autoreceptor-mediated effects of dopamine agonists

The putatively selective D-1 dopamine receptor antagonist SCH 23390 was used to study the role of the D-1 dopamine receptor in mediating the pre- and postsynaptic effects of dopamine agonists in the basal ganglia. SCH 23390 (1 mg/kg) had no significant effect on the tonic activity of substantia nigra dopamine neurons in 47% of the 19 cells studied, while the firing rates of 53% of the cells were increased. SCH 23390 did not shift the dose response of these cells to apomorphine, whereas the selective D-2 antagonist, YM-09151-2 completely blocked apomorphine's inhibitory effects on nigral dopamine cell activity. These results suggest that SCH 23390 does not interact with the D-2 dopamine autoreceptors, but does excite a subpopulation of dopamine neurons presumably through postsynaptic actions. In contrast to its inability to modify the effects of apomorphine on dopamine autoreceptors, SCH 23390 partially to fully reversed the effects of apomorphine on globus pallidus and substantia nigra pars reticulata cell activity and significantly attenuated the effects of apomorphine, pergolide, quinpirole (LY 171555) and d-amphetamine on firing rates of globus pallidus neurons. The D-1 antagonist alone had no significant effect on tonic globus pallidus neuronal activity. SCH 23390 was more potent than haloperidol in its ability to attenuate the effects of apomorphine on pallidal activity, but unlike haloperidol, was unable to totally inhibit these effects, suggesting that the two antagonists block the excitatory effects of apomorphine on pallidal cell firing rates by different mechanisms. The serotonin2 receptor antagonist, ketanserin, had no effect on pallidal or dopamine cell activity, indicating that the effects of SCH 23390 were not mediated through interactions with serotonin2 receptors. These results suggest that D-1 receptor blockade attenuates the postsynaptic, but not autoreceptor-mediated effects of dopamine agonists.     

Effects of repeated tiapride administration on anterior pituitary dopamine receptors and prolactin release in the rat

The substituted benzamides tiapride and sulpiride, and the classic neuroleptic haloperidol, were studied in the rat to assess their interaction with the anterior pituitary (AP) dopamine (DA) receptors both in vitro ([3H]spiperone binding) and in vivo prolactin-PRL-release). Tiapride weakly inhibited [3H]spiperone binding in both pituitary and striatal membranes with affinity 5-7 times lower than sulpiride and 400-300 times lower than haloperidol. All three drugs were more potent in displacing [3H]spiperone from striatum than from AP. In vivo, tiapride produced weak and transient stimulation of PRL release reaching a full effect at 2 mg/kg i.p. Similar doses of sulpiride produced longer-lasting effects. Haloperidol was more potent than both benzamides. In prolonged treatments (15 or 60 days), tiapride, given twice daily at 0.5 mg/kg i.p., did not modify [3H]spiperone binding in either AP or striatum, nor did it induce significant changes of basal PRL levels. The challenge with a low threshold dose of TIA (0.2 mg/kg ip) produced similar increases of PRL release in the group either treated with TIA or saline. The data indicate that the benzamides examined have low potency for interaction with DA receptors in pituitary and striatum. In particular, tiapride displayed weaker affinity for AP-DA receptors than the other drugs and induced only slight stimulation of PRL levels. Results from repeated tiapride administration indicate that the drug, at a clinically relevant dose, is unable to modify either kinetic characteristics of DA receptors in the pituitary or plasma PRL levels.     

Climbing behaviour induced by apomorphine in mice: a potential model for the detection of neuroleptic activity.

Apomorphine and the putative dopamine agonist, 2-(N, N-dipropyl)-amino-5, 6-dihydroxytetralin induced dose-dependent climbing behaviour in the mouse which was measured in wire mesh lined cages as the percentage of time spent climbing in the 30 min period following the first climb and as the maximum time spent in a single climb throughout the drug effect. These These two measures were generally found to parallel excepting when the interacting agent caused muscular hypotonia. All potential interacting agents were given as pretreatments to determine changes in motor function which may interfere with the climbing induced by 1.0 mg/kg s.c. apomorphine. The possibility of a change in the apomorphine response to a sterotyped biting, which would also interfere with climbing, was also considered. Excluding these non-specific changes, climbing behaviour was shown to be antagonised, dose-dependently, by low doses of typical and atypical neuroleptic agents (haloperidol, fluphenazine, loxapine, pimozide, oxiperomide, clozapin, thioridazine, sulpiride, tiapride and metoclopramide) but not specifically by other psychoactive agents. Climbing behaviour was modified by serotonergic agents; the agonist quipazine reduced or abolished, whilst the antagonists, methysergide and cyproheptadine, enhanced the response. Picrotoxin specifically reduced climbing behaviour but sodium valproate exerted non-specific effects, precluding conclusions as to a GABA involvement. Cholinergic and noradrenergic involvements with climbing were also apparently eliminated by the ineffectiveness of atropine, aceperone, piperoxan and propranolol. The involvement of serotonin with climbing was extended to the actions of the neuroleptics: the antagonistic effects of typical neuroleptics (haloperidol, fluphenazine, loxapine) were markedly enhanced by combination with methysergide or cyproheptadine whilst the effects of clozapine, sulpiride and thioridazine were significantly reduced. The actions of metoclopramide, oxiperomide, pimozide and tiapride were not generally modified by such combinations. These differences are discussed in terms of differential abilities to induce extrapyramidal disturbances and the mouse climbing model is forwarded as a test with potential to detect antipsychotic agents of different activity spectra.     

Evidence that apomorphine and pergolide induce rotation in rats by different actions on D1 and D2 receptor sites

Apomorphine and the ergot derivative pergolide induced dose-dependent contralateral rotation in rats with unilateral 6-hydroxydopamine denervation of the ascending dopamine pathways. This was interpreted as an action on supersensitive receptors. However, large differences were found when comparing apomorphine and pergolide dose-response curves as well as the patterns of rotational behaviour the compounds elicited. Pergolide had a steep dose-response curve, while apomorphine had a flatter curve reaching a plateau at the dose of 1 mg/kg s.c. In doses higher than 1 mg/kg, apomorphine induced self-mutilation, while this was infrequent after pergolide. Apomorphine induced a two-peak pattern of rotation that never occurred when the same rats were tested with the ergot derivative. Both drugs induced dose-dependent ipsilateral rotation in animals with unilateral striatal kainic acid lesions but at doses 100 times higher. This effect was interpreted as an action on normosensitive receptors situated on the intact side. The differences between apomorphine and pergolide may be explained in terms of actions on different dopamine receptors, since the agonists were differently inhibited by neuroleptics acting on D1- or D2-type receptors. The D1/D2 antagonist cis-flupenthixol blocked both apomorphine and pergolide with similar potency, while sulpiride, a substituted benzamide devoid of any effect on D1 receptors, was a poor inhibitor of the apomorphine response. In contrast, sulpiride blocked pergolide rotation at doses 1000 times lower than those needed to block apomorphine rotation. Our results suggest the existence of functionally distinct sites related to the D1/D2 receptor classification.     

Alterations in different populations of striatal dopamine receptors produced by 18 months continuous administration of cis- or trans-flupenthixol to rats

Administration of cis-flupenthixol (0.8-1.2 mg/kg per day) for 18 months enhanced stereotyped behaviour induced by apomorphine, bromocriptine and lergotrile, but not that induced by amphetamine or lisuride. Catalepsy induced by acute administration of haloperidol, trifluoperazine or cis-flupenthixol was reduced by continuous chronic intake of cis-flupenthixol. The number (Bmax) and dissociation constant (KD) of specific [3H]spiperone binding sites on striatal membranes was increased by chronic administration of cis-flupenthixol, but not trans-flupenthixol. In contrast, the Bmax and KD for specific binding of [3H]N,n-propylnorapomorphine were decreased by administration of cis-flupenthixol compared to the effect of the trans-isomer. Specific binding of [3H]piflutixol was unaffected by chronic administration of cis- or trans-flupenthixol, but chronic administration of cis-flupenthixol enhanced stimulation by dopamine of the activity of striatal adenylate cyclase. As a result of chronic continuous administration of cis-flupenthixol dopamine receptors in the striatum appeared to be supersensitive to most dopamine agonists but sub-sensitive to dopamine antagonists. This was reflected by increased numbers of D-2 antagonist receptor sites of decreased affinity, but by a decreased number of agonist sites of higher affinity. The D-1 recognition sites appeared to be unaltered, but activity of adenylate cyclase stimulated by dopamine was enhanced, suggesting post-junctional changes. The D-2 receptors appear to be primarily concerned with altered function of dopamine receptors.     

Behavioural correlates of modified dopaminergic/anticholinergic responses following chronic treatment with neuroleptic agents of differing activity spectra.

An attempt was made to differentiate the mechanisms of action of these “neuroleptic” agents having antidyskinetic properties from the classical neuroleptics by studying the effects of chronic treatment on the responses to a dopamine agonist and cholinergic antagonist. Rats were chronically treated with haloperidol (2 mg/kg i.p.), oxiperomide (8 mg/kg i.p.), pimozide (8 mg/kg p.o.), tiapride (100 mg/kg i.p.), sultopride (100 mg/kg i.p.), sulpiride (100 mg/kg p.o.) and metoclopramide (100 mg/kg i.p.) for 7 consecutive days. On days 1, 3 and 7 after neuroleptic withdrawal rats were assessed for their sensitivity to the stereotypic effects of apomorphine (0.25–1.0 mg/kg s.c.) and the locomotor stimulant effects of dexetimide (0.32–1.25 mg/kg s.c.). A supersensitivity phase to apomorphine, persisting for 1–3 days, was noted in all drug-treated groups and, during this time, the responses to dexetimide were decreased (haloperidol, oxiperomide and pimozide groups), increased (metoclopramide group) or remained similar to control values (sultopride-, sulpiride- and tiapride-treated animals). The responses to dexetimide returned to control values in all treatment groups as the apomorphine sensitivity phase declined. It is suggested that chronic treatment with neuroleptic agents may enhance the sensitivity of those cerebral receptors responsible for mediating the stereotypic effects of apomorphine, and that the relationship between such dopamine function and interacting cholinergic mechanisms may differ for the different groups of neuroleptic agent. However, these differentiations did not directly correlate with the known ability of the neuroleptics to induce or antagonise dyskinetic phenomena.     

Involvement of dopaminergic neurotransmission in the control of goal-directed movements

Changes produced in dopamine (DA) activity, by administration of the DA-antagonists metoclopramide (10 mg/kg IM) and tiapride (16 mg/kg IM) and of the DA agonists apomorphine (0.5 and 1 mg/kg IM) and bromocriptine (8 mg/kg orally), specifically modified predatory behavior in the ferret. Sulpiride (40 mg/kg IP and 90 mg/kg IM) did not change the behavior. The number of bites necessary to kill the prey was reduced by metoclopramide and tiapride. The number of bites after the death of the prey was not changed. The latency from the first bite to the death of the prey was shortened. Apomorphine and bromocriptine increased the number of bites.The DA receptor blockers haloperidol, chlorpromazine, and clozapine had similar effects to metoclopramide and tiapride, and the DA agonist l-dopa had similar effects to apomorphine and bromocriptine. The pattern of results indicated that, considering the two major DA receptor types, D-2 receptors or D-2 in combination with D-1 but not D-1 receptors alone were involved in the control of goal-directed movements. The results also provided some evidence that blockade of these DA receptors caused a narrowing of the range of exhibited behavioral responses. Stimulation of these DA receptors had opposite effects.     

Species variation in dopamine receptor binding

Binding of 3H-spiroperidol, 3H-apomorphine and 3H-ADTN (2-amino-6,7-dihydroxytetrahydronaphthalene) associated with dopamine receptors has been evaluated in corpus striatal membranes of calf, rat and human brains. Substantial species differences are apparent for numberous agonists and antagonists in competing for receptor binding. In general, dopamine receptor antagonists are more potent in rat and agonists more potent in calf. In competing for 3H-spiroperidol binding sulpiride, molindone and metaclopramide show the most pronounced species differences, being 3--10 times more potent in rat and human than in calf. In all three species agonists compete for 3H-spiroperidol binding with Hill coefficients less than one while antagonists inhibit 3H-spiroperidol binding with Hill coefficients of about 1.0. Conversely, 3H-apomorphine and 3H-ADTN binding in all three species is inhibited by antagonists with Hill coefficients less than 1.0 while agonists display Hill coefficients of about 1.0. In general agonists are more potent in competing for binding of 3H-apomorphine and 3H-ADTN than 3H-spiroperidol. However, a small component of dopamine, apomorphine and ADTN inhibition of 3H-spiroperidol binding displays very high affinity (IC50 about 1 nM). In human amygdala 3H-spiroperidol appears to label serotonin receptors predominantly.     

A central site of action for benzamide facilitation of gastric emptying

Gastric emptying of the fed guinea-pig was measured using a non-invasive X-ray fluoroscopic technique to determine passage from the stomach of polystyrene-coated barium sulphate spheroids. Peripherally administered metoclopramide (0.1-10 mg/kg i.p.), clebopride (1-10 mg/kg i.p.), (-)-sulpiride (40 mg/kg i.p.), haloperidol (1 mg/kg i.p.) and domperidone (1-10 mg/kg i.p.) failed to modify gastric emptying. Stress inhibited emptying, and this was considered to explain the effects of eserine and high dose metoclopramide. Gastric emptying was decreased by peripherally administered atropine (0.5 mg/kg i.p.) and apomorphine (0.1-0.5 mg/kg s.c.); the apomorphine response was antagonised by pretreatment with haloperidol, domperidone, (-)-sulpiride, metoclopramide and clebopride but not by prazosin + propranolol. Gastric emptying was facilitated by intracerebroventricular (i.c.v.) administrations of metoclopramide and clebopride (40, 100 and 200 micrograms) but not by i.c.v. domperidone, haloperidol, fluphenazine or (-)-sulpiride (100, 200 micrograms) and was inhibited by i.c.v. apomorphine (100, 200 micrograms); the response to i.c.v. apomorphine was antagonised by i.c.v. pretreatments with haloperidol, domperidone, (-)-sulpiride, metoclopramide and clebopride (40-50 micrograms). Facilitation of emptying by i.c.v. metoclopramide was prevented by peripheral pretreatment with atropine (0.5 mg/kg i.p.). It is concluded that the actions of apomorphine and metoclopramide/clebopride to respectively inhibit and facilitate gastric emptying may be mediated, at least in part, via central mechanisms. Whilst apomorphine's action may be mediated via dopamine receptor mechanisms, metoclopramide and clebopride act at additional unspecified sites, metoclopramide's action being expressed via cholinergic mechanisms.     

Neuroleptic-induced acute dyskinesias in rhesus monkeys

Rhesus monkeys, previously subjected to twiceweekly injections of various neuroleptics, subsequently respond to acute IM injections of haloperidol with marked bucco-lingual and whole body movement disturbances consisting of mouth opening, protrusion, retraction or curling of the tongue together with writhing movements of the neck, trunk and/or limbs. These phenomena, which closely resemble the acute dyskinetic or dystonic reactions described in patients at the beginning of neuroleptic treatment, were also observed after acute IM injections of other neuroleptics such as fluphenazine, metoclopramide, oxiperomide, sulpiride, sultopride and tiapride. No dyskinesias were observed after chlorpromazine, chlordiazepoxide, clozapine, RMI 81582 or thioridazine at doses which otherwise had marked behavioural effects. The dyskinesias induced by haloperidol could be suppressed by prior treatment with the anticholinergic scopolamine.These observations, which correlate well with clinical findings, suggest that neuroleptic induced acute dyskinesias in the Rhesus monkey might be a useful model for predicting the liability of new anti-psychotics for inducing acute dyskinetic reactions in man.     

Dopamine D2 receptors selectively labeled by a benzamide neuroleptic: [3H]-YM-09151-2

Summary In order to label dopamine D₂ receptors selectively we tritiated the potent benzamide neuroleptic, YM-09151-2 (26.7 Ci/mmol). The binding of [³H]-YM-09151-2 to canine striatal membranes was saturable and specific with a K _D of 57 pmol/l and B _max of 36 pmol/g tissue as determined by Scatchard analysis. The K _D, but not the B _max, of [³H]-YM-09151-2 increased 6-fold in the absence of sodium chloride. [³H]-YM-09151-2 labeled 40% more sites than [³H]-spiperone in the same tissue homogenate. [³H]-YM-09151-2 binding was inhibited by dopaminergic drugs in a concentration and stereoselective manner with the appropriate dopamine D₂ receptor profile. Thus, dopamine agonists inhibited [³H]-YM-09151-2 binding to canine striatal membranes with the following rank order of potency: (–)-N-n-propylnorapomorphine > apomorphine > (±)-6,7-dihydroxy-2-aminotetralin > (+)-N-n-propylnorapomorphine > dopamine > (–)-noradrenaline > serotonin > (–)-isoprenaline. Dopaminergic antagonists competed for [³H]-YM-09151-2 binding with the following order of potency: spiperone > (+)-butaclamol > haloperidol > clebopride > (–)-sulpiride > SCH-23390 > (–)-butaclamol. Furthermore, dopamine agonists recognized 2 states of the receptor labeled by [³H]-YM-09151-2, D ₂ ^high and D ₂ ^low . The D ₂ ^high state of the receptor could be converted to D ₂ ^low by guanine nucleotides and sodium ions as is the case for [³H]-spiperone binding to D₂ receptors. [³H]-YM-09151-2 appears to be a more selective ligand for dopamine D₂ receptors than [³H]-spiperone, since YM-09151-2 displays approximately 9-fold lower affinity than spiperone for cortical serotonergic (S₂) receptors. [³H]-YM-09151-2 may become a useful tool for the selective characterization of dopamine D₂ receptors.Abbreviations used (±)ADTN (±)-2-amino-6,7-dihydroxy-1,2,3,4-tetrahydronaphthalene - NPA N-n-propylnorapomorphine - Gpp(NH)p 5-guanylylimidodiphosphate