Effects of the substituted (S)-3-phenylpiperidine (-)-OSU6162 on PET measurements in subhuman primates: evidence for tone-dependent normalization of striatal dopaminergic activity

(-)-OSU6162 is a substituted (S)-3-phenylpiperidine derivative which exhibits some affinity to the dopamine D2 receptor family. In vivo, the compound displays a unique normalizing profile on psychomotor activity by an intriguing mixture of stimulatory and inhibitory properties. In the present investigation, some of the effects of (-)-OSU6162 on central dopaminergic function were studied by positron emission tomography (PET) and L-[11C]DOPA in anaesthetized female rhesus monkeys. (-)-OSU6162 displayed a dopaminergic tone-dependent effect with a reduction in the striatal L-[11C]DOPA influx rate in monkeys with high baseline values and an increased striatal L-[11C]DOPA influx rate in animals with low baseline values. Infusion of (-)-OSU6162 for a whole day resulted in a stable effect with no evidence of tolerance. (-)-OSU6162 also stabilized dopaminergic function by attenuating the upregulation of the striatal L-[11C]DOPA influx rate which has previously been shown to occur following 6R-BH4 or 6R-BH4 + L-tyrosine infusions. This "Protean" effect of (-)-OSU6162 on the striatal dopaminergic function corresponds to previous behavioral observations in intact animals and demonstrates a true functional correlation to the measures obtained with L-[11C]DOPA and PET. The normalizing and stabilizing profile of (-)-OSU6162 should be of value in treating a variety of disorders where an underlying dysregulation or disruption of dopaminergic function can be assumed.

MK-801-induced hyperlocomotion: differential effects of M100907, SDZ PSD 958 and raclopride

The influence of three selective monoamine receptor antagonists on spontaneous locomotion and on the hyperlocomotion induced by the un-competitive N-methyl-D-aspartate (NMDA) receptor antagonist [+]-5-methyl-10,11-dihydro-5H-dibenzo-[a,d]-cyclohepten-5,10-imine hydrogen maleate (MK-801; dizocilpine) was investigated. The selective and potent 5-hydroxytryptamine (5-HT)2A receptor antagonist R(+)-alpha(2,3-dimethoxyphenyl)-1-[2(4-fluorophenyl)ethyl)]-4-piperidine -methanol (MDL100,907; M100907) displayed a clear-cut selectivity for reduction of MK-801-induced as compared to spontaneous locomotion, in that the former was dose-dependently (0.001, 0.01, 0.1 mg/kg i.p.) blocked and even totally abolished by the highest dose, while the latter was only modestly affected. Even at high doses of M100907 (up to 9 mg/kg i.p.), spontaneous locomotion was not reduced below 40% of control. The selective dopamine D1 receptor antagonist (-)-[4aR, 10 aR]-1,2,3,4,4a,5,10,10a-octahydro-4-(4-chloro-2-methyl-phenyl)-1-methyl- benzo[g]quinoxaline-6-ol (SDZ PSD 958; 0.017, 0.15, 1.35 mg/kg i.p.) decreased both spontaneous and MK-801-induced locomotion with a slight preference for the latter; spontaneous locomotion was dose-dependently diminished to approx. 10% of controls (at 8 mg/kg i.p.). The dopamine D2 receptor antagonist raclopride ([(-)-(S)-3,5-dichloro-N-((1-ethyl-2-pyrrolidinyl) methyl)-6-methoxy-salicylamide tartrate]; 0.11, 0.33, 1.0 mg/kg i.p.) reduced both MK-801-induced and spontaneous locomotion to a similar extent. An orthogonal matrix experimental design, and multiple regression, were used to evaluate the effects of several combinations of different doses of the 5-HT2A receptor antagonist and the dopamine D1 receptor antagonist. No synergistic actions on reduction of spontaneous or MK-801-induced locomotion were detected between M100907 and SDZ PSD 958. If the hyperlocomotion elicited by acutely administered MK-801 is a valid model of at least some aspects of schizophrenia, these results indicate that the 5-HT2A receptor antagonist M100907 will have efficacy in treating this condition. The lack of effect on spontaneous locomotion, suggests that M100907, compared to dopamine receptor antagonists, will be less prone to induce psychomotor side-effects. Ongoing clinical studies will hopefully give the answers in the near future.

The apparent antipsychotic action of the 5-HT2a receptor antagonist M100907 in a mouse model of schizophrenia is counteracted by ritanserin. (Rapid communication)

The apparent antipsychotic action of the selective 5-HT2a receptor antagonist M100907 in MK-801-treated NMRI mice was shown to be markedly counteracted by the 5-HT2a/2c receptor antagonist ritanserin. The mechanism of action and potential implications are discussed.

Synthesis of [11C-methyl]-(-)-OSU6162, its regional brain distribution and some pharmacological effects of (-)-OSU6162 on the dopaminergic system studied in the rhesus monkey by positron emission tomography

The labelling of the presynaptic dopamine receptor antagonist (-)-OSU6162, ((S)-(-)-3-(3-(methylsulfonyl)phenyl)-1-propylpiperidine) was performed by an alkylation with [11C]methyl iodide of the thio anion (-)-OSU1281, followed by a selective oxidation to the corresponding methyl sulfone, [11C-methyl]-(-)-OSU6162. The total radiochemical yield calculated from the produced [11C]carbon dioxide to final product was about 25% and the time of synthesis was in the range of 40 min from end of bombardment. The synthesis of the precursor, (-)-OSU1281, was performed from (-)-3PPP in a three-step synthesis. The regional brain distribution of (-)-OSU6162 radiolabelled with 11C was studied in rhesus monkeys by means of positron emission tomography, PET. [11C-Methyl]-(-)-OSU6162 was rapidly and uniformly distributed to gray matters of the brain, and no decrease of radioactivity uptake in the brain was seen after pretreatment with 1 to 3 mg/kg/h of (-)-OSU6162. The effect of doses of 1 to 3 mg/kg/h of (-)-OSU6162 on the dopamine binding was studied by PET using [11C-methyl]raclopride. Radioactivity in the striatum was significantly and dose-dependently decreased by (-)-OSU6162 (r = 0.88), supporting competition with dopamine for selective binding to dopamine receptors.

Neurotransmitter aberrations in schizophrenia: new perspectives and therapeutic implications

The dopamine hypothesis has dominated schizophrenia research for decades but is now yielding to a more diversified view, where the interaction of several neurotransmitters in complex circuitries is under scrutiny. Especially, glutamatergic and serotonergic mechanisms are attracting attention. However, the role of dopamine also needs further exploration and may still turn out to have novel therapeutic applications. In the present minireview an attempt is made to integrate preclinical and clinical data on neurotransmitter aberrations in schizophrenia and to discuss their therapeutic implications.

Effects on locomotor activity after local application of (+)-UH232 in discrete areas of the rat brain

The preferential dopamine autoreceptor, and slightly D3 preferring, antagonist (+)-UH232 (cis-(+)-(1S,2R)-5-methoxy-1-methyl-2-(n-dipropylamino) tetralin) increases locomotor activity and synaptic dopamine release in the nucleus accumbens and striatum after systemic administration to the rat. As shown in this study, (+)-UH232, was unable to produce an increase in locomotor activity measured for 60 minutes after local administration into the terminal or somato-dendritic regions of the mesolimbic dopamine pathways or into the lateral ventricle. Instead, a dose dependent decrease of spontaneous locomotor activity after local application (0.05-50.0 nmol/side) of (+)-UH232 into the nucleus accumbens, was seen. A similar reduction in locomotor activity was produced by the classical dopamine antagonist raclopride. Analysis of the dose*time interactions on locomotor activity did, however, indicate that there is a significant dose*time interaction after local application of (+)-UH232 into the lateral ventricle and VTA. Raclopride, on the other hand, produced only a weak time dependent effect in the VTA. The potential problem of Leao's spreading depression in micro-injection experiments were considered, however, spreading depression does not seem to influence the effects of (+)-UH232 locally applied into the nucleus accumbens. In conclusion, both (+)-UH232 and raclopride produced a dose dependent decrease in spontaneous locomotor activity when examined as the total activity count over 60 minutes after local application into the N Acc.

Concurrent locomotor stimulation and decrease in dopamine release in rats and mice after treatment with the competitive NMDA receptor antagonists D-CPPene and CGS 19755

The present study was aimed at investigating the effects of the competitive N-methyl-D-aspartate (NMDA) receptor antagonists D-CPPene (3-(2-carboxypiperazine-4-yl)-propenyl-1-phosphonic acid) and CGS 19755 (cis-4-(phosphonomethyl)piperidine-2-carboxylic acid) on dopamine (DA) transmission and motor activity in mice and rats. As measures of DA release we used mouse brain 3-methoxytyramine (3-MT) levels, and indirect estimate of DA release, and striatal dialysate measures of DA in conscious and freely moving rats by means of microdialysis. To obtain additional information about monoaminergic neurotransmission, brain tissue levels of DA, DOPAC, HVA, 5-HT and 5-HIAA were measured in both mice and rats. The animals were sacrificed at the time when NMDA antagonist-induced locomotor stimulation was maximal. In mice, D-CPPene and CGS 19755 decreased striatal 3-MT levels, whereas, in general, 3-MT levels in the limbic forebrain were not significantly altered. Treatment with CGS 19755 decreased rat striatal dialysate levels of DA but increased 5-HIAA at time points when locomotor activity was increased. D-CPPene and CGS 19755 have been observed to produce psychotic symptoms in man. The present study suggests that these symptoms are not a result of an increase in central dopamine release.

On the quantitative structure-activity relationships of meta-substituted (S)-phenylpiperidines, a class of preferential dopamine D2 autoreceptor ligands: modeling of dopamine synthesis and release in vivo by means of partial least squares regression

The quantitative structure-activity relationship between physicochemical properties and effects on dopamine (DA) synthesis and release in the rat brain, in a series of meta-substituted (S)-phenylpiperidines, has been investigated by means of partial least squares regression (PLS). The effect on DA synthesis caused by the drugs, in both non-pretreated and reserpine-pretreated rats, was assessed by measurements of tissue levels of L-DOPA accumulated in the striatum following treatment with a decarboxylase inhibitor. Assessment of effects on DA release was performed by analysis of perfusates collected from implanted microdialysis probes. The numerical characterization of the variation in physicochemical features of the phenylpiperidines used in the regression modeling was accomplished by using common tabulated aromatic and aliphatic substituent constants in combination with a set of property descriptors derived from molecular mechanics and semiempirical calculations. It was found that the biochemical responses could be accurately predicted by the regression models based on these molecular feature measures. The molecular features exerting influence on DA synthesis were found to be markedly different from those influencing DA release. This finding is discussed in terms of the possible existence of a dopamine receptor-mediated DA release-controlling mechanism, which may not involve the synthesis regulating DA D2 autoreceptor. Some findings regarding the impact of the piperidine N substituent on agonist properties of the drugs are reported. The regression models were also used for guidance in the search for a phenylpiperidine with a lower intrinsic activity, at the DA D2 type autoreceptor, than the partial DA agonist preclamol (3).

Neil F. Curtis

No Abstract Available.

Behavioral and neurochemical data suggest functional differences between dopamine D2 and D3 receptors

In an in vitro model for mitogenic activity in cloned Chinese hamster ovary (CHO) cells expressing rat dopamine D2 or D3 receptors, the EC50D2/EC50D3 ratios for the agonists, apomorphine, (+)-3-hydroxy-N-n-propyl-phenylpiperidine ((+)-3-PPP), quinpirole, R-(+)-7-hydroxy-2-(di-n-propylamino)tetralin (R-(+)-7-OH-DPAT) and pramipexole (SND919) were found to be 0.36, 0.41, 1.3, 3.7 and 7.0, respectively. In locomotor activity experiments with actively exploring rats, the more dopamine D3 preferring agonists, R-(+)-7-OH-DPAT and pramipexole, were most efficacious to reduce locomotion. The hypoactivity was also observed at doses that did not affect brain dopamine synthesis rate (DOPA accumulation) or release (measured in in vivo dialysis experiments). In contrast, for apomorphine, (+)-3-PPP and quinpirole there was a closer correlation between doses that reduced exploratory activity and doses that reduced brain dopamine release and synthesis. The present data support the hypothesis that the functional dopamine D3 receptor is a postsynaptic receptor inhibitory on rat locomotion.

Evidence for biochemical heterogeneity in schizophrenia: a multivariate study of monoaminergic indices in human post-mortal brain tissue

A previously performed post-mortem study comparing monoaminergic indices in the brains of 14 schizophrenic patients and 10 patients with psychosis not diagnosed as schizophrenia, with age-matched control cases without any known neuropsychiatric illness, was re-investigated, using multivariate analysis. The monoaminergic patterns showing up in this analysis suggested the existence of at least two different forms of the disease, both of which could be distinguished from the controls as well as from each other. One of the schizophrenic groups consisted of paranoid cases, and had a relatively mild family history, whereas the other group, mainly consisting of hebephrenic cases, had a severe family history. The former group showed low levels of dopamine and high levels of serotonergic precursor and metabolite, whereas the latter group in some respects tended to show the opposite aberrations. Neuroleptic treatment did not seem to account for the different biochemical profiles, unless one assumes that this treatment can cause completely different monoaminergic aberrations in different individuals. Instead, one could argue that the different biochemical profiles found are characteristic of the disease.

Substituted (S)-phenylpiperidines and rigid congeners as preferential dopamine autoreceptor antagonists: synthesis and structure-activity relationships

A series of (S)-phenylpiperidines in which the substituents on the aromatic ring and nitrogen have been varied has been prepared. They have been evaluated pharmacologically to explore the importance of these substituents for the interaction with central dopamine (DA) receptors. On the basis of biochemical and behavioral data in rats, several of these compounds are characterized as centrally acting DA autoreceptor antagonists. (S)-Phenylpiperidines having an aromatic substituent with a high group dipole moment in the 3-position, i.e., meta with respect to the piperidine ring, and being N-substituted with a propyl group were found to be highly active in vivo on the synthesis and turnover of dopamine. However, they do not induce strong hypoactivity or catalepsy. Interestingly, the most active compounds in vivo were found to display only low affinity for DA D2 and D3 receptors in vitro. In addition, 7-triflate-substituted octahydrobenzo[f]quinolines and 6-triflate-substituted hexahydro-1H-benz[e]indoles have been prepared and pharmacologically evaluated. The trans isomers of these rigid structures were found to display a pharmacological profile similar to that of the flexible phenylpiperidines. The corresponding cis isomers were found to be inactive in vivo.

Differential effects of dopamine D2 and D3 receptor antagonists in regard to dopamine release, in vivo receptor displacement and behaviour

To establish possible functional differences between the dopamine D2 and D3 receptor we investigated the relation between the ability, for a set of nine mixed dopamine D2 and D3 receptor antagonists, to displace N, N-dipropyl-2-amino-5,6-dihydroxy tetralin (DP-5,6-ADTN) from striatal binding sites and the subsequent behavioural consequences in vivo. Dopamine D2 receptor preferring antagonists are powerful displacers of DP-5,6-ADTN from the striatum. Maximal displacement is followed by strong hypomotility. Displacement of the agonist by the D3 preferring antagonist U99194A is only partial and results in synergistic increases in locomotor activity. Superimposing haloperidol upon GBR12909 leads to a synergistic increase in striatal dialysate dopamine concentrations. This effect is absent when combining GBR12909 with the putative D3 antagonist U99194A. These data give support for the hypothesis that the dopamine D3 receptor is functionally relevant at the postsynaptic level. Here, in contrast to the D2 receptor, it is proposed to exert an inhibitory influence on psychomotor functions.

Locomotor inhibition by the D3 ligand R-(+)-7-OH-DPAT is independent of changes in dopamine release

The dopamine D3 preferring ligand R-(+)-7-OH-DPAT induced strong hypolocomotion in rats. Doses producing reduction of locomotion failed to affect dopamine release or synthesis rate. These data support the hypothesis that the dopamine D3 receptor is a postsynaptic receptor with an inhibitory influence on rat locomotor activity.

Intracerebral infusion of (+)-AJ76 and (+)-UH232: effects on dopamine release and metabolism in vivo

The dopamine D3- and autoreceptor preferring antagonists (+)-AJ76 and (+)-UH232 were administered locally in the striatum and the nucleus accumbens. Their effects on dialysate dopamine and 3,4-di-hydroxyphenylacetic acid (DOPAC) were measured and compared with the effects of raclopride. (+)-AJ76 and (+)-UH232 but not raclopride seem to interact primarily with dopamine receptors in the terminal regions of the A9 and A10 dopaminergic fibers to exert their maximal effect on dopamine release in vivo. Thus, (+)-AJ76 and (+)-UH232 seem to recruit different dopamine receptor populations as compared to raclopride. Though the dopamine receptor antagonist-induced effects on dopamine release seem to be mediated mainly by dopamine receptors in the terminal areas, the effects on DOPAC by the different antagonists seem to be mediated mainly via effects elsewhere, presumably at the somatodendritic autoreceptors. Thus, it is suggested that the regulation of extracellular dopamine and DOPAC after treatment with dopamine receptor antagonists are subjected to different control mechanisms.

Substituted 3-phenylpiperidines: new centrally acting dopamine autoreceptor antagonists

The (+)-and (-)-enantiomer of compounds 4 and 5 were synthesized and tested for central dopamine (DA) receptor stimulating activity, using biochemical and behavioral tests in rats. Based on the available data the (-)-enantiomers of 4 and 5 are characterized as centrally acting DA autoreceptor antagonists with oral activity. They display a similar pharmacological profile as the prototype DA autoreceptor antagonists (+)-1 and (+)-2 and show a certain preference for the D3 DA receptor antagonist binding site.

The dopamine D3 receptor and autoreceptor preferring antagonists (+)-AJ76 and (+)-UH232; a microdialysis study

The in vivo neurochemical profiles of haloperidol, raclopride and the dopamine D3 and autoreceptor preferring dopamine receptor antagonists (+)-UH232 and (+)-AJ76 on dopamine release and metabolism in the dorsal striatum and in the nucleus accumbens are described. It is shown that both (+)-UH232 and especially (+)-AJ76 have different effects on brain dialysate dopamine and 3,4-dihydroxyphenylacetic acid (DOPAC) as compared to haloperidol or raclopride. It is suggested that the relative increase in dialysate dopamine over the relative increase in DOPAC is a neurochemical fingerprint, unique for different dopamine receptor antagonists. As a consequence the increased release and metabolism of dopamine after systemic administration of dopamine receptor antagonists may be controlled by different receptors and different dopamine antagonists can partly distinguish between these receptors. This may be due to their different interactions with different dopamine D2 type receptors. It is finally concluded that (+)-UH232 and especially (+)-AJ76 seem to prefer release regulating autoreceptors at the level of the axon terminals.

The dopamine D3-receptor: a postsynaptic receptor inhibitory on rat locomotor activity

We report on the pharmacological effects of the 20 fold D3 vs. D2 dopamine receptor preferring compound U99194A. It is shown that U99194A increases rat locomotor activity at doses that do not increase release or utilisation of dopamine in the striatum or the nucleus accumbens significantly. The data do not support any direct agonist action of U99194A at dopamine receptors. It is suggested that U99194A can antagonise a population of postsynaptic dopamine receptors involved in the suppression of some aspects of psychomotor activity. These postsynaptic receptors presumably belong to the D3 receptor subtype.

The influence of serotoninergic drugs on dopaminergic neurotransmission in rat substantia nigra, striatum and limbic forebrain in vivo

The effects of serotoninergic drugs on dopaminergic neurotransmission in the substantia nigra, the striatum and the limbic forebrain of rat have been investigated. The accumulation of 3-methoxytyramine (3-MT) following inhibition of monoamine oxidase with pargyline was used as an indirect measure of dopamine (DA) activity in vivo. The effects of the following serotoninergic drugs were tested: the 5-HT1A receptor agonist 8-OH-DPAT, the 5-HT1B receptor agonist trifluoromethyl-phenylpiperazine (TFMPP), CGS 12066 B and RU 24969, the 5-HT1A/1B antagonist (+/-)pindolol, the 5-HT2/1C receptor antagonist ritanserin, the 5-HT2/1C receptor agonist DL-1-(2,5-dimethoxy-4-iodophenyl)-2-aminopropane (DOI), the 5-HT3 receptor antagonist BRL 43694, the unselective 5-HT receptor antagonist methiothepin, and carbidopa + L-5-hydroxytryptophan (L-5-HTP) to achieve a general, unselective stimulation of multiple 5-HT receptors. In the substantia nigra, carbidopa + 5-HTP treatment increased the 3-MT accumulation by 26% and decreased the DA concentration to 67% of controls, tentatively suggesting a 5-HTP-induced displacement of nigral DA. A minor, non dose-related reduction in nigral 3-MT was seen after the 5-HT1A receptor agonist 8-OH-DPAT. None of the other serotonin receptor acting drugs induced any pronounced effect on the nigral 3-MT accumulation. Taken together, the findings provide little support for the idea that one single 5-HT receptor subtype serves a modulatory function on DA activity in the substantia nigra. In the striatum and the limbic forebrain, trifluoromethyl-phenylpiperazine dose-dependently increased the 3-MT accumulation to maximally 200%-220% of controls.(ABSTRACT TRUNCATED AT 250 WORDS)

Increased dopamine release by the autoreceptor antagonist (+)-AJ 76 is Ca2(+)-dependent

The effects of the preferential autoreceptor antagonist (+)-AJ 76 on dopamine release and metabolism were studied in the brain microdialysis model. The Ca2+ dependence of the effects of (+)-AJ 76 and d-amphetamine were compared. We found that (+)-AJ 76 increased the release and metabolism of dopamine and that the release was saturable. The release of dopamine by (+)-AJ 76 was dependent on extracellular Ca2+. However, the effects of (+)-AJ 76 on dopamine metabolism were independent of extracellular Ca2+. The effects of d-amphetamine on dopamine release and metabolism were independent of Ca2+. We conclude that the dopamine released by (+)-AJ 76 is dependent on neuronal impulse flow and that the dopamine released is of vesicular origin. The effects on dopamine metabolism and release may be exerted via different mechanisms. In contrast, the release and metabolism of dopamine by d-amphetamine were independent of impulse flow and extracellular Ca2+. We suggest that (+)-AJ 76 and d-amphetamine release dopamine from different pools.

The putatively antipsychotic agent amperozide produces behavioural stimulation in the rat. A behavioural and biochemical characterization

Amperozide (FG 5606; N-ethyl-4-[4',4'-bis(p-fluorophenyl)butyl]-1-piperazinecarboximide ) is a new putatively antipsychotic compound with a postulated 5-HT2 antagonistic profile. Somewhat surprisingly amperozide dose dependently induced a behavioural stimulation in reserpinized and in nonpretreated rats. The behaviour consisted of both forward and backward locomotion as well as forepaw circling and a grooming like behaviour. Since the behavioural pattern clearly differ from that produced by classical dopaminergic or serotonergic agonists (e.g. apomorphine or 8-hydroxy-2-(di-n-propylamino)tetralin, 8-OH-DPAT), and has not been previously reported, we decided to investigate the origin of this effect. In the behavioural paradigms it was not possible to antagonize the amperozide stimulation in reserpinized rats with the dopamine receptor blockers haloperidol, raclopride or R(+)-7-chloro-8-hydroxy-3-methyl-1-phenyl-2,3,4,5-tetrahydro-1 H-3-benzazepine, SCH 23390. Neither the 5-HT2 receptor blocking agent ritanserin nor the tryptophan and tyrosine hydroxylase inhibitor DL-3,4-dihydroxy-phenyl-alpha-propylacetamide, H22/54, could block the motoric stimulation or the forepaw circling behaviour produced by amperozide. However, the noradrenaline synthesis inhibitor bis-(4-methyl-1-homopiperazinylthiocarbonyl)-disulfide, FLA 63, as well as the alpha-adrenoceptor antagonist phenoxybenzamine, could partly inhibit the locomotor stimulation. Hence, noradrenaline seems to be, at least in part, involved in the behavioural stimulatory effect of amperozide. Biochemically amperozide had no effect on the dopamine synthesis rate (DOPA formation) in normal or reserpinized animals in the striatal or the limbic brain regions. In reserpinized animals amperozide also failed to antagonize the decrease in DOPA formation after apomorphine and 3-hydroxy-benzylhydrazine HCl, NSD 1015, in these regions.(ABSTRACT TRUNCATED AT 250 WORDS)