Jørn Arnt

Pharmacological effects of a specific dopamine D-1 antagonist SCH 23390 in comparison with neuroleptics

Neuroleptics such as thioxanthenes (cis(Z)-flupentixol and cis(Z)-clopenthixol) and phenothiazines (fluphenazine and perphenazine), which block both dopamine (DA) D-1 and D-2 receptors and the butyrophenones (haloperidol and spiroperidol), which block D-2 receptors only, are equipotent both behaviorally and clinically. A new compound SCH 23390 which selectively blocks DA D-1 receptors, resembles many neuroleptics in its pharmacological profile: antistereotypic effects in mice, rats and dogs, cataleptogenic effect and inhibitory effect on amphetamine circling. In contrast SCH 23390 has no effect on apomorphine-induced vomiting in dogs and little effects on 6-OHDA-denervated supersensitive DA receptors, stimulated by the DA agonist 3-PPP. In a series of experiments where methylphenidate-induced stereotyped gnawing in mice was inhibited by neuroleptics, it was shown that concomitant treatment with scopolamine or diazepam attenuated the effect of butyrophenones (D-2 antagonists). The same treatment attenuated the effect of phenothiazines, to a lesser extent, and hardly attenuated the effect of thioxanthenes and SCH 23390 at all. It is concluded that DA D-1 receptors are as important as D-2 receptors for the expression of neuroleptic activity in most animal models believed to be predictive of antipsychotic and extrapyramidal side-effect potential. However, the D-1 antagonist is less sensitive than D-2 antagonists to antimuscarinic compounds and benzodiazepines.

Behavioural stimulation induced by muscimol and other GABA agonists injected into the substantia nigra

The unilateral injection of putative GABA agonists, muscimol, baclofen, imidazole acetic acid and GABA into the caudal area of substantia nigra (SN, zona reticulata) induced immediately a contralateral turning, whereas the antogonists picrotoxin and bicuculline methiodide induced ipsilateral turning. In the rostral area picrotoxin induced contralateral turning. Muscimol (10 ng bilaterally injected into SN) induced behavioural stimulation and antagonism of catelepsy in rats pretreated with high subcutaneous doses of perphenazine, haloperidol or reserpine and alpha-methyltyrosine. The muscimol behavioural stimulation seems dependent on a non-catecholaminergic neuronal pathway, probably present in the SN with GABA as the mediating neurotransmitter.

Dopamine D-1 receptor agonists combined with the selective D-2 agonist quinpirole facilitate the expression of oral stereotyped behaviour in rats

The behaviour of rats was studied after combined treatment with the selective DA D-2 agonist quinpirole and three selective D-1 agonists (SK & F 38393, SK & F 75670 and Lu 24-040). The effects on behaviour were compared with those on receptor binding and adenylate cyclase (AC). While the D-1 agonists alone did not induce stereotyped behaviour, quinpirole induced dose-dependent hyperactivity (locomotion, sniffing, head movements and rearing), whereas licking/biting was absent or seen only occasionally. Combined treatment with quinpirole and a D-1 agonist was followed by dose-dependent licking and occasional biting behaviour. The D-1 agonists had similar efficacies, but SK & F 75670 and Lu 24-040 were more potent than SK & F 38393. The maximal effects of SK & F 38393 plus quinpirole were effectively blocked by either a D-1 antagonist (SCH 23390) or a D-2 antagonist (YM 09151-2) confirming the close relation between D-1 and D-2 receptor sites in the brain. Good correspondence was found between affinities to D-1 receptors [( 3H]SCH 23390 binding) in vitro and the EC50 values for stimulation of AC activity. However, the maximal effects on DA-sensitive AC activity were less for SK & F 75670 and Lu 24-040 than for SK & F 38393. Thus, the results indicate that efficacies in the adenylate cyclase assay are dissociated from those on behaviour. Furthermore, the data indicate that in normal rats D-1 receptors are functionally relevant since D-1 agonists facilitate the expression of oral stereotyped behaviour after combination with a D-2 agonist.

Behavioural stimulation is induced by separate dopamine D-1 and D-2 receptor sites in reserpine-pretreated but not in normal rats

The dopamine (DA) D-1 agonist SK&F 38393 as well as the D-2 agonist pergolide and the mixed D-1/D-2 agonist apomorphine induced strong hypermotility and oral stereotypy in rats pretreated with a daily dose of reserpine for 2 and in particular for 4 days (3 and 5 injections, respectively). SK&F 38393 had no behavioural stimulant effect in saline-pretreated rats, whereas pergolide and apomorphine produced stimulation, although only after higher doses. Agonists at 5-HT and muscarinic receptors and at alpha 1-adrenoceptors were ineffective in reserpine-pretreated rats whereas the alpha 2-adrenoceptor agonist, clonidine, and the muscarinic antagonist, scopolamine, produced weak locomotor stimulation. The hypermotility induced by SK&F 38393 in reserpinized rats was blocked by pretreatment with the DA D-1 antagonists, SCH 23390 and SK&F 83566c, whereas the DA D-2 antagonists, YM 09151-2, clebopride and spiroperidol were weak or ineffective. In contrast pergolide-induced hypermotility was blocked by low doses of the D-2 antagonists but was weakly or not influenced by the D-1 antagonists. Selectivity ratios between drug potencies in the two models ranged from 65 to more than 600. The mixed D-1/D-2 antagonists, cis(Z)-flupentixol and cis(Z)-clopenthixol, blocked the effect of both SK&F 38393 and pergolide. The alpha 1-adrenoceptor antagonist, prazosin, and the 5-HT2 receptor antagonist, ketanserin, did not modify the effect of SK&F 38393 or pergolide. Stereotyped behaviour induced by a high pergolide dose in normal rats was, in contrast to the effect in reserpinized rats, blocked by low doses of either SCH 23390 or spiroperidol. Finally, the hypermotility induced by apomorphine in reserpinized rats was markedly antagonized by both SCH 23390 and spiroperidol. The results suggest a close relation between D-1 and D-2 receptor sites in normal rats. After prolonged reserpine treatment, the D-1 agonist acquires full DA agonist efficacy. Furthermore, behavioural stimulation under these conditions is mediated by two separate D-1 and D-2 receptor sites which can be manipulated independently by antagonists. The mechanism by which this phenomenon occurs is unknown but the adaptational changes show close similarities to those observed after 6-hydroxyDA-induced denervation.

Biochemical effects and drug levels in rats after long-term treatment with the specific 5-HT-uptake inhibitor, citalopram

The effects in rats of long-term administration of the potent, specific 5-HT uptake inhibitor citalopram have been investigated. Citalopram hydrobromide (MW=405) was given in the diet, 99 or 25 μmol/kg daily, for 13 days or orally, 49 μmol/kg twice a day, for 14 days. High plasma and brain levels of citalopram were found during the treatment period, whereas negligible amounts were found 24 h after withdrawal. The 5-HT uptake mechanism in blood platelets was completely blocked, since levels of whole blood 5-HT during and shortly (2 days) after treatment were decreased by 75–90%. The drug load after the two highest doses in terms of plasma drug levels was the same as in depressed patients treated with citalopram. Receptor binding technique ex vivo was applied to different brain parts to measure receptor parameters for several neurotransmitters. All data were evaluated by Eadie-Hoffstee analysis. No changes were seen in Bmax and Kd for β-receptors (3H-dihydroalprenolol) in frontal cortex, occipital+temporal cortex, whole cortex and limbic structures, 5-HT2 receptors (3H-spiroperidol) in frontal and whole cortex, α1-receptors (3H-prazosin) in “rest of brain” and DA D-2 receptors (3H-spiroperidol) in corpus striatum and limbic structures. The uptake mechanism for 5-HT as well as the inhibitory effect of citalopram on this uptake remained unaffected in brain synaptosomes derived from control and from citalopram (99 μmol/kg)-treated rats. Thus long-term treatment with citalopram does not induce changes in neurotransmitter receptors as seen with most tricyclic as well as newer “atypical” antidepressants. Most striking is the lack of β- and 5-HT2 receptor down-regulation. Since citalopram clinically shows clear antidepressant activity, this down-regulation does not seem to be a prerequisite of antidepressant activity.

Differential involvement of dopamine D-1 and D-2 receptors in the circling behaviour induced by apomorphine, SK & F 38393, pergolide and LY 171555 in 6-hydroxydopamine-lesioned rats

The antagonistic effect of dopamine (DA) D-1 and D-2 antagonists against circling behaviour induced by various DA agonists in 6-OHDA-lesioned rats has been investigated. DA D-1/D-2 selectivity of agonists in vitro was measured by the stimulatory effect on DA-sensitive adenylate cyclase in rat striatal homogenates (D-1), the inhibitory effect on electrically-induced release of 3H-DA in rabbit striatal slices (D-2) and the affinity to 3H-piflutixol (D-1) and 3H-spiroperidol (D-2) binding sites in rat striatal membranes. The contralateral circling behaviour induced by the DA D-1 agonist SK & F 38393 was blocked by the DA D-1 antagonist, SCH 23390, and by the mixed DA D-1/D-2 antagonist cis(Z)-flupentixol, but was not influenced by the DA D-2 antagonists spiroperidol and clebopride. In contrast, circling behaviour induced by the preferential DA D-2 agonists pergolide and LY 171555 was blocked by clebopride, spiroperidol, and cis(Z)-flupentixol, but weakly or not influenced by SCH 23390. Apomorphine-induced circling behaviour was blocked by cis(Z)-flupentixol, partially antagonized by SCH 23390 and clebopride but not inhibited by spiroperidol, although the time-course of circling was changed. Combinations of SCH 23390 with spiroperidol or clebopride in low doses completely blocked the effect of apomorphine. These results indicate that DA D-1 and D-2 receptors mediate circling behaviour through separate mechanisms which can be independently manipulated with respective agonists and antagonists. Furthermore, the results indicate that both DA D-1 and D-2 receptors are involved in the effect of apomorphine, since selective antagonists induced maximally 50% inhibition. Complete blockade was only found in combination experiments and by the mixed D-1/D-2 antagonists cis(Z)-flupentixol, cis(Z)-clopenthixol, and clozapine.

Differential inhibition by dopamine D-1 and D-2 antagonists of circling behaviour induced by dopamine agonists in rats with unilateral 6-hydroxydopamine lesions

The selective dopamine (DA) D-1 antagonist SCH 23390 antagonized the contralateral circling behaviour induced by the DA D-1 agonist SK & F 38393 in rats lesioned unilaterally with 6-hydroxy-DA but had a 600 times weaker effect against the preferential DA D-2 agonist pergolide. The D-2 antagonists clebopride and spiroperidol had the reverse selectivity. The mixed D-1/D-2 antagonists cis(Z)-flupentixol and cis(Z)-clopenthixol blocked the circling induced by either agonist. It is concluded that circling behaviour is mediated by closely related but independent DA D-1 and D-2 receptor sites.

Differential effects of classical and newer antipsychotics on the hypermotility induced by two dose levels of d-amphetamine

The inhibitory effects of a variety of established and putative antipsychotic compounds on the hypermotility induced by D-amphetamine at two dose levels (0.5 and 2.0 mg/kg) have been studied. Classical antipsychotics (haloperidol, fluphenazine and cis(Z)-flupentixol) and the selective dopamine D2 receptor antagonist remoxipride inhibit hypermotility in the two conditions with similar potencies, whereas sertindole, clozapine, risperidone, ziprasidone and olanzapine preferentially inhibit the effect of the low dose of D-amphetamine (selectivity ratios between 6.5 and 18). Seroquel, amperozide and the selective 5-HT2A receptor antagonist MDL 100.151 ((+/-)-alpha-(2,3-dimethoxyphenyl)-1-[2-(4-fluorophenylethyl)]-4-p iperidine - methanol) have no effect on D-amphetamine 2.0 mg/kg, but inhibit the response to D-amphetamine 0.5 mg/kg. The alpha 1-adrenoceptor antagonist prazosin inhibits the motility response to D-amphetamine 0.5 mg/kg with slightly higher potency than that to D-amphetamine 2.0 mg/kg, whereas the 5-HT2A/2C receptor antagonist ritanserin selectively inhibits the effect of D-amphetamine 0.5 mg/kg. The histamine H1 receptor antagonist mepyramine is ineffective in both models. All compounds, except remoxipride, MDL 100.151 and ritanserin (which are ineffective) inhibit spontaneous locomotor activity at dose levels close to those inhibiting the response to D-amphetamine 2.0 mg/kg. Prazosin has partial inhibitory effect. In conclusion, dopamine antagonism has similar inhibitory effect on hyperactivity induced by low and high D-amphetamine dosages, alpha 1-adrenoceptor antagonism also contributes to both effects, whereas 5-HT2 receptor antagonism selectively interacts with the low D-amphetamine dose. This indicates that the responses to D-amphetamine 0.5 and 2.0 mg/kg are differently modulated by these neurotransmitters.(ABSTRACT TRUNCATED AT 250 WORDS)

Partial and full dopamine D1 receptor agonists in mice and rats : relation between behavioural effects and stimulation of adenylate cyclase activity in vitro

The dopamine (DA) D1 agonists, SK&F 83959, SK&F 75670, SK&F 38993, SK&F 81297 and SK&F 80723, had variable abilities to stimulate adenylate cyclase activity in rat striatal homogenates. Their efficacies, in relation to the effect of 100 microM DA were 0, 33, 69, 68 and 81%, respectively. In rats, all compounds induced (1) contralateral circling behaviour after unilateral 6-hydroxy-DA lesions, (2) ipsilateral circling behaviour after midbrain hemitransection after cotreatment with the D2 agonist quinpirole and (3) oral stereotypies after their combination with quinpirole. Maximum effects and rank order of potencies were similar in the three test models. In mice SK&F 83959, SK&F 75670 and SK&F 38393 inhibited methylphenidate-induced gnawing behaviour and induced no or only weak hypermotility. SK&F 81297 induced marked hypermotility which was partially inhibited by SK&F 83959 and SK&F 75670 and was completely blocked by the D1 antagonist, SCH 23390. It is concluded that no relation could be demonstrated between the efficacy to stimulate adenylate cyclase and to induce circling behaviours and stereotypies in rats. In contrast, a relation between biochemical and behavioural efficacies was found in the mouse models. The results suggest that different subtypes of D1 receptors mediate the behavioural effects reported in this study.

The role of serotonergic mechanisms in inhibition of isolation-induced aggression in male mice

The role of serotonergic (5-HT) receptor subtypes in mediation of aggressive behaviour in isolated male mice has been studied. Increase of attack latency was used as a simple measure of antiaggressive behaviour. 5-HT1A agonists (BAY R 1531, 8-OHDPAT, flesinoxan, gepirone, 5MeO DMT, buspirone, ipsapirone, BMY 14802) completely inhibit the aggressive behaviour irrespective of their intrinsic activities. Also the putative antagonists spiroxatrine and NAN 190 as well as the non-selective 5-HT1 agonists RU 24969, TFMPP, mCPP and eltoprazine have an antiaggressive effect. The mixed 5-HT1A andβ-adrenoceptor antagonists (−)-alprenolol and pindolol are ineffective and do not inhibit the effect of 8-OHDPAT. Neither does the non-selective 5-HT antagonist metergoline. The antiaggressive effect correlates with 5-HT1A receptor affinity in vitro and with generalization to the 8-OHDPAT-induced discriminative stimulus. The selective 5-HT uptake inhibitor citalopram does not inhibit aggressive behaviour. The 5-HT2 agonist DOI has an antiaggressive effect only at high doses, whereas the 5-HT2 antagonist ritanserin and the 5-HT3 antagonist ondansetron are ineffective. Prazosin (α1-adrenoceptor antagonist), clonidine (α2-adrenoceptor agonist), clenbuterol (β-adrenoceptor agonist), ketanserin (5-HT2 receptor andα1-adrenoceptor antagonist), clozapine and (−)-octoclothepin (dopamine (DA), 5-HT2 receptor andα1-adrenoceptor antagonist) all show an antiaggressive effect. SCH 23390 (DA D1 receptor antagonist) and emonapride (DA D2 receptor antagonist) are ineffective. In conclusion, 5-HT1A receptors are involved in mediation of isolation-induced aggressive behaviour in mice. The involvement of other 5-HT receptor subtypes needs further clarification. The adrenergic system may also be involved. DA antagonists are ineffective.