A comparison of the potencies of various dopamine receptor agonists in models for pre- and postsynaptic receptor activity

Sonic hedgehog pathway as a new target of atypical antipsychotics: Revisiting of amisulpride and aripiprazole effects in a rat model of schizophrenia

OBJECTIVES

Schizophrenia is a chronic mental illness presented by cognitive deficits that precede its positive and negative symptoms. Sonic hedgehog (Shh)-pathway contributes to its pathophysiology. Shh has a role in neurogenesis as it regulates proliferation and survival of neural cells. In this study, effects of the anti-psychotics Amisulpride and/or Aripiprazole on the Shh-pathway and its relation to cognitive functions and neurogenesis in a rat model of schizophrenia were tested.

METHODS

60 male Wistar rats were allocated into the following groups: control, socially isolated, amisulpride and/or aripiprazole-treated groups. Rats were then subjected to behavioral, biochemical, and histopathological tests to assess the impact of these drugs on Shh-pathway.

KEY FINDINGS

Cognitive-dysfunction was evidenced in socially isolated group in novel object, three-chamber, and Morris water maze tests, associated by disorganised Shh-pathway proteins levels concentrations, increased glial fibrillary acidic protein (GFAP)-stained astrocytes. Treated groups favorably reversed these changes evidenced by increased Shh, transmembrane patched-1 and smoothened, glioma-associated-oncogene (GLI)-1 levels, dopamine-1 receptors and brain derived neurotrophic factor, and decreased GLI-3 protein, GFAP immune reaction in astrocytes and inflammatory markers compared to socially isolated group.

CONCLUSION

Amisulpride and/or aripiprazole have a favorable role in turning on Shh-pathway with subsequent beneficial cognitive and neurogenesis effects.

Aripiprazole, A Drug that Displays Partial Agonism and Functional Selectivity

BACKGROUND

The treatment of schizophrenia is challenging due to the wide range of symptoms (positive, negative, cognitive) associated with the disease. Typical antipsychotics that antagonize D2 receptors are effective in treating positive symptoms, but extrapyramidal side-effects (EPS) are a common occurrence. Atypical antipsychotics targeting 5-HT2A and D2 receptors are more effective at treating cognitive and negative symptoms compared to typical antipsychotics, but these drugs also result in side-effects such as metabolic syndromes.

OBJECTIVE

To identify evidence in the literature that elucidates the pharmacological profile of aripiprazole.s.

METHODS

We searched PubMed for peer reviewed articles on aripiprazole and its clinical efficacy, side-effects, pharmacology, and effects in animal models of schizophrenia symptoms.

RESULTS

Aripiprazole is a newer atypical antipsychotic that displays a unique pharmacological profile, including partial D2 agonism and functionally selective properties. Aripiprazole is effective at treating the positive symptoms of schizophrenia and has the potential to treat negative and cognitive symptoms at least as well as other atypical antipsychotics. The drug has a favorable side-effect profile and has a low propensity to result in EPS or metabolic syndromes. Animal models of schizophrenia have been used to determine the efficacy of aripiprazole in symptom management. In these instances, aripiprazole resulted in the reversal of deficits in extinction, pre-pulse inhibition, and social withdrawal. Because aripiprazole requires a greater than 90% occupancy rate at D2 receptors to be clinically active and does not produce EPS, this suggests a functionally selective effect on intracellular signaling pathways.

CONCLUSION

A combination of factors such as dopamine system stabilization via partial agonism, functional selectivity at D2 receptors, and serotonin-dopamine system interaction may contribute to the ability of aripiprazole to successfully manage schizophrenia symptoms. This review examines these mechanisms of action to further clarify the pharmacological actions of aripiprazole.

Third generation antipsychotic drugs: partial agonism or receptor functional selectivity?

Functional selectivity is the term that describes drugs that cause markedly different signaling through a single receptor (e.g., full agonist at one pathway and antagonist at a second). It has been widely recognized recently that this phenomenon impacts the understanding of mechanism of action of some drugs, and has relevance to drug discovery. One of the clinical areas where this mechanism has particular importance is in the treatment of schizophrenia. Antipsychotic drugs have been grouped according to both pattern of clinical action and mechanism of action. The original antipsychotic drugs such as chlorpromazine and haloperidol have been called typical or first generation. They cause both antipsychotic actions and many side effects (extrapyramidal and endocrine) that are ascribed to their high affinity dopamine D(2) receptor antagonism. Drugs such as clozapine, olanzapine, risperidone and others were then developed that avoided the neurological side effects (atypical or second generation antipsychotics). These compounds are divided mechanistically into those that are high affinity D(2) and 5-HT(2A) antagonists, and those that also bind with modest affinity to D(2), 5-HT(2A), and many other neuroreceptors. There is one approved third generation drug, aripiprazole, whose actions have been ascribed alternately to either D(2) partial agonism or D(2) functional selectivity. Although partial agonism has been the more widely accepted mechanism, the available data are inconsistent with this mechanism. Conversely, the D(2) functional selectivity hypothesis can accommodate all current data for aripiprazole, and also impacts on discovery compounds that are not pure D(2) antagonists.

Modification of inherent and drug-induced dopaminergic activity after exposure to benzo(alpha)pyrene

The aim of this study was to investigate the effect of benzo(alpha)pyrene (B(alpha)P), a representative polycyclic aromatic hydrocarbon (PAH), on dopaminergic activity in brain. (B(alpha)P) altered dopaminergic activity in discrete regions of the rat brain, including the hippocampus, hypothalamus, caudate putamen and nucleus accumbens. Specifically, B(alpha)P increased DA levels in the hippocampus and DA turnover in the caudate putamen. In addition, B(alpha)P suppressed DA levels in the caudate putamen and DA turnover in the nucleus accumbens. B(alpha)P also altered the effect of several dopaminergic agents, L-DOPA, sulpiride and bromocriptine, on DA activity. In particular, B(alpha)P enhanced the L-DOPA-induced increase in the DA turnover ratio in the caudate putamen and increased DA levels in the nucleus accumbens. B(alpha)P also reversed the sulpiride-induced increase of DA turnover in the nucleus accumbens and the bromocriptine-induced increase of DA turnover in the hippocampus. In addition, DA turnover was increased by B(alpha)P in the nucleus accumbens and caudate putamen and DA levels were suppressed in the nucleus accumbens of bromocriptine treated rats, though the drug alone had no effect. These changes indicate that exposure to B(alpha)P and related compounds may affect dopaminergic function in discrete brain regions that are implicated in cognitive functions, psychosis, depression and Parkinson's disease, and may possibly interfere with their pharmacological intervention.

Aminotetralin drugs and D3 receptor functions. What may partially selective D3 receptor ligands tell us about dopamine D3 receptor functions?

The dopamine D3 receptor gene was identified by Sokoloff and colleagues in 1990. This finding rapidly gained the interest of the scientific community because this unexpected dopamine receptor subtype may play an important role in the antipsychotic activity of neuroleptic drugs. It recognizes most neuroleptics with a high affinity, and its brain distribution is restricted mainly to the ventral part of the striatal complex. However, the characterization and the subsequent identification of functions of the D3 receptor were hampered initially by at least four important factors that are still partially unresolved: (1) the absence of selective drugs that can discriminate between the D2 and D3 receptor subtype functions in vivo, (2) the lack of apparent coupling with GTP-dependent proteins, (3) the absence of effects on second messenger systems, and (4) the low level of expression of this receptor in brain tissue; these factors have contributed to tempering the interest of scientists. However, this situation has begun to change with the identification of [3H]7-hydroxy-N,N-(di-n-propyl)-2-aminotetralin ([3H]7-OH-DPAT), the first selective ligand for the dopamine D3 receptor. Although its binding selectivity for the D3 versus the D2 receptor is somewhat artificial, the potentially important impact of identification of a function for the D3 receptor encouraged scientists to use this aminotetralin compound for in vivo studies with, however, limited success. This commentary is focused on the impact and controversies generated by the use of 7-OH-DPAT and its congeners, on new conceptual views that may arise from this research, and on what partially selective D3 receptor ligands may tell us about dopamine D3 receptor functions.

In vivo evidence for preferential role of dopamine D3 receptor in the presynaptic regulation of dopamine release but not synthesis

Brain microdialysis was used to investigate the effects of the putative dopamine D3 receptor agonist (+/-)-7-hydroxy-N,N-di-n-propyl-2-aminotetralin (7-OH-DPAT) on dopamine release, metabolism and synthesis in the dorsal striatum and nucleus accumbens of awake rats. The drug administered i.p. dose dependently decreased the release, metabolism and synthesis of dopamine in both brain areas. The potency of 7-OH-DPAT to decrease dopamine release was found to be higher in the nucleus accumbens than in the dorsal striatum (ED50 for nucleus accumbens 0.0096 mg/kg, i.p.; for dorsal striatum 0.068 mg/kg, i.p.). Dopamine metabolism, assessed by measuring 3,4-dihydroxyphenylacetic acid extracellular levels, and dopamine synthesis, determined as 3,4-dihydroxyphenylalanine output following perfusion with the L-aromatic acid decarboxylase inhibitor 3-hydroxybenzylhydrazine (10(-5) M), were decreased at higher dose ranges of 7-OH-DPAT (ED50 for decrease of 3,4-dihydroxyphenylalanine output in nucleus accumbens 0.124 mg/kg, i.p.; in dorsal striatum 0.101 mg/kg, i.p.). The hypomotility of rats induced by 7-OH-DPAT in doses of 0.002-0.25 mg/kg, i.p., was shown to correlate with the decreased dopamine release in the nucleus accumbens. Pretreatment of animals with 7-OH-DPAT at the putative dopamine D3 receptor 'selective' dose of 0.05 mg/kg, i.p., was found to prevent the increase of dopamine release but not the increase in metabolism in the dorsal striatum of freely moving rats induced by (+)-AJ76, cis (+)-(1S,2R)-5-methoxy-1-methyl-1-2-(n-propylamino)tetralin HCI (7 mg/kg, i.p.) and haloperidol (0.1 mg/kg, i.p.). Local application of 7-OH-DPAT by addition into the perfusing medium also resulted in a preferential decrease of dopamine release in the nucleus accumbens as compared with the dorsal striatum (EC50 for nucleus accumbens 1.9 nM; for dorsal striatum 11.3 nM). The present results give further support to the hypothesis that the dopamine D3 autoreceptor is preferentially involved in the presynaptic regulation of dopamine release, while the D2 autoreceptor controls dopamine synthesis.

Differential serotoninergic and dopaminergic activities of the (R)- and the (S)-enantiomers of 2-(di-n-propylamino)tetralin

Racemic 2-(di-n-propylamino)tetralin ((R,S)-DPAT), which lacks phenolic or other aromatic substituents, induces both dopaminergic (sniffing, licking and gnawing) and serotoninergic (forepaw treading and flat body posture) behavioural responses. The present study shows that s.c. administration of (R)-DPAT induces typical 5-HT1A receptor agonist behaviours. These effects are blocked by the 5-HT1A receptor antagonist (S)-5-fluoro-8-hydroxy-2-(di-n-propylamino)tetralin ((S)-UH-301). Administration of (S)-DPAT induces dopaminergic behaviours, which are fully antagonised by raclopride, a dopamine D2 receptor antagonist. Both enantiomers induce hypothermia, (R)-DPAT being antagonised by (S)-UH-301, whereas (S)-DPAT is antagonised by raclopride. The accumulation of 5-hydroxytryptophan and DOPA (3,4-dihydroxyphenylalanine) after decarboxylase inhibition that reflects presynaptic actions on 5-HT (5-hydroxytryptamine, serotonin) and dopamine neurons, respectively, are inhibited by both enantiomers of DPAT. (R)-DPAT is more potent than (S)-DPAT as an inhibitor of 5-hydroxytryptophan accumulation whereas (S)-DPAT is more potent than (R)-DPAT as an inhibitor of DOPA accumulation. Thus, in functional tests of postsynaptic actions (R)-DPAT behaves as a 5-HT1A receptor agonist and (S)-DPAT as a dopamine D2 receptor agonist. Presynaptically, (R)-DPAT shows selectivity for 5-HT1A receptors and (S)-DPAT for dopamine D2 receptors. Receptor binding studies, utilizing [3H]8-hydroxy-2-(di-n-propylamino)tetralin and [3H]quinpirole as radioligands for 5-HT1A and dopamine D2 receptors, respectively, showed (R)-DPAT to have a 3-fold higher affinity than (S)-DPAT for 5-HT1A receptors, whereas (S)-DPAT had a 6-fold higher affinity than (R)-DPAT for dopamine D2 receptors. Thus, the results from receptor binding studies support the conclusion that (R)- and (S)-DPAT are agonists showing selectivity for 5-HT1A and dopamine D2 receptors, respectively. Taken together, these findings may explain previous controversies with regard to the pharmacology of racemic DPAT and re-emphasise the necessity to study pure enantiomers of chiral compounds.

The locomotor effects of a putative dopamine D3 receptor agonist in developing rats

Dopamine receptors have been categorized into subfamilies D1 and D2, each with separate roles in dopamine-mediated behaviors. Of the D2 subfamily, the dopamine D3 receptor has been cloned, but the behavioral effects of selectively stimulating the D3 receptor are largely unknown. The purpose of this study was to quantify the locomotor responses of developing rats to the putative dopamine D3 receptor agonist, 7-hydroxy-N,N-di-n-propyl-2-aminotetralin (7-OH-DPAT). One of three doses of 7-OH-DPAT (0.01, 0.10, 1.00 mg/kg) or saline was injected subcutaneously into rats at the age of 10,20,30, or 60 days. Five minutes after the injection, rats were placed in automated activity monitors which recorded locomotor behavior at 5 min intervals for 2 h. The high dose of 7-OH-DPAT increased locomotor activity in rats of all ages. The medium and low doses increased activity in 10- and 20-day-old rats but not in 30- or 60-day-old rats. The level of drug-induced activation peaked at 20 days of age. In 30- and 60-day-old rats, but not 10- and 20-day-old rats, a period of locomotor suppression preceded the activation in response to the high dose of 7-OH-DPAT. In rats aged 20 days and older, the middle and low doses decreased locomotion early in the test session, but activation did not ensue. This dose-response pattern across ontogeny closely resembles that induced by quinpirole, an agonist at the dopamine D2 receptor subfamily.

Behavioural effects in the rat of the putative dopamine D3 receptor agonist 7-OH-DPAT: comparison with quinpirole and apomorphine

This study assessed the effects of IP injections of (+/-) 7-hydroxy-2(N,N-di-n-propylamino)tetralin (7-OH-DPAT), a dopamine agonist that has been reported to have preferential affinity for the dopamine D3 sub-type of receptor, on four behavioural procedures in the rat: 1) spontaneous locomotion, 2) electrical self-stimulation of the ventral tegmental area (VTA), using the curve-shift procedure 3) operant responding for food under a progressive-ratio (PR) schedule and 4) induction of stereotypies. The effects of (+/-) 7-OH-DPAT were compared to the effects of apomorphine, a non-specific DA agonist, and quinpirole, a selective D2/D3 agonist. All three dopamine agonists decreased locomotor activity at low doses (0.01-0.3 mg/kg), and only apomorphine had clear locomotor stimulant effects at the highest dose tested (3 mg/kg). The three drugs dose-dependently depressed VTA self-stimulation in a similar way, with low doses inducing a fairly parallel rightward shift of the frequency/rate curves and higher doses flattening the curves. In contrast, responding for food under the PR schedule appeared to be differentially affected by the three agonists: 7-OH-DPAT induced a biphasic effect, with a maximal decrease in lever-pressing at 0.1 mg/kg, followed by a return to baseline levels with increasing doses (0.3-3 mg/kg); quinpirole showed a tendency to decrease responding over the whole dose-range tested with a maximal effect of about 50% of baseline between 0.25 and 1 mg/kg, and apomorphine dose-dependently decreased responding, with rats ceasing to respond at 0.3 mg/kg. All three DA agonists induced stereotypies, but there was a difference in the maximal stereotypy score induced by each of the ligands: 7-OH-DPAT produced a lower maximal effect than quinpirole or apomorphine. This indicates that each of the three dopamine agonists preferentially induced different types of stereotypies. Together, these data suggest that the putative dopamine D3 agonist 7-OH-DPAT, at low doses, has depressant effects similar to those induced by low doses of the other two DA agonists. Differences in the behavioural effects of higher doses were, however, mostly observed in two procedures, PR responding and induction of stereotypies.

Effects on locomotor activity after local application of D3 preferring compounds in discrete areas of the rat brain

Compounds showing an in vitro binding preference for the dopamine D3 vs. D2 receptors were tested for effects on locomotor activity after local application in the nucleus accumbens (N Acc) and the ventral tegmental area (VTA) of the rat brain. R-(+)-7-OH-DPAT, a dopamine D3 preferring agonist, inhibited spontaneous locomotor activity over a wide dose range after injection into the N Acc. A decrease in activity over a wide dose range was also seen after local application into the VTA of both R-(+)-7-OH-DPAT and the dopamine D2 preferring agonist (+)-3-PPP. Furthermore, (+)-3-PPP produced a dose dependent increase in activity after local application into the N Acc. The putative D3 antagonist, U99194A, with a 30 fold preference for the dopamine D3 vs. D2 receptor, produced an increase in activity when injected into the N Acc. A similar pattern were seen after infusion into the lateral ventricle. Local application into the VTA did, however, not produce any significant effects. The present results support the hypothesis that dopamine D3 receptors (in contrast to the D2 receptors) are mainly postsynaptically located where they display an inhibitory action on locomotor activity.

Estimation of the interstitial free concentration of the putative dopamine D3 receptor selective agonist 7-OH-DPAT in the dorsal striatum of freely moving rats

Using the quantitative microdialysis 'point of no net flux' method, we estimated the interstitial free concentration (IFC) of (+/-)-7-hydroxy-2-(N,N-di-n-propylamino)tetralin (7-OH-DPAT) in the dorsal striatum of freely moving rats after i.p. administration of the drug at the dose of 18.3 mumol/kg. The maximal IFC of 7-OH-DPAT was found to be 1.61 microM 20 min after the injection. Due to the approximately linear relationship between dose and dialysate concentration observed, it may be inferred that the behaviourally active 7-OH-DPAT dose of 0.12 mumol/kg should give an IFC which does not exceed 10 nM. It is concluded that in vivo effects observed following 7-OH-DPAT i.p. administration at doses lower than 0.12 mumol/kg might be considered as mediated by the dopamine D3 receptor.

Motor actions of 7-OH-DPAT in normal and reserpine-treated mice suggest involvement of both dopamine D2 and D3 receptors

In non-habituated mice, 7-hydroxy-N,N-di-n-propylaminotetralin (7-OH-DPAT, 0.04-10 mg/kg s.c.) potently and rapidly suppressed species-typical behaviours and induced frozen postures, with only occasional evidence of weak behavioural stimulation occurring at 5-10 mg/kg. This inhibitory effect was reversed by the dopamine D1 receptor agonist 2,3,4,5-tetrahydro-7,8-di-hydroxy-1-phenyl-1H-3-benzazepine hydrochloride (SKF 38393, 10 mg/kg i.p.). 7-OH-DPAT (3-10 mg/kg) did not reinstate locomotion in 4 h habituated mice, either when administered alone or in conjunction with a threshold dose of SKF 38393 (3 mg/kg). By contrast, 7-OH-DPAT (0.2-10 mg/kg) dose-dependently reversed the akinesia of 24 h reserpine-treated mice. This response was blocked by the dopamine D2 receptor antagonist raclopride (10 mg/kg i.p.), but not by the dopamine D1 receptor antagonist (R)-(+)-8-chloro-2,3,4,5-tetrahydro-3-methyl-5-phenyl-1H-3- benzazepine-7-ol hemimaleate (SCH 23390, 0.05 mg/kg i.p.), and was potentiated synergistically by coinjection of SKF 38393 (3 mg/kg). These and earlier data suggest the motor inhibitory effects of 7-OH-DPAT (low doses) in normal animals are mediated by dopamine autoreceptors (D2 and/or D3), whilst its motor stimulant actions in normal (high doses) and in dopamine-depleted, supersensitive animals, are mediated by dopamine D2 receptors.

Actions of (+/-)-7-hydroxy-N,N-dipropylaminotetralin (7-OH-DPAT) on dopamine synthesis in limbic and extrapyramidal regions of rat brain

The proposed D3-selective ligand (+/-)-7-hydroxy-N,N-dipropylaminotetralin (7-OH-DPAT) inhibited tyrosine hydroxylase in vitro (IC50 = 0.6-0.7 microM) and dihydroxyphenylalanine (DOPA) accumulation in vivo (ID50 = 4.8-6.4 mg/kg) in two autoreceptor models in extrapyramidal and limbic tissue in rat forebrain, without consistent regional selectivity. Some limbic selectivity (ID50 = 10 vs. 29 mg/kg) was found in an in vivo model permitting expression of postsynaptic D3 and D2 receptor activity. The effects were partially blocked by S(-)-eticlopride alone, and fully after reserpine pretreatment. The results suggest that 7-OH-DPAT activates D3 or D2 autoreceptors, alters dopamine storage or release, and may interact with some limbic selectivity at postsynaptic D3 and D2 receptors as a partial agonist.

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.

Subchronic treatment with the neuroleptic-like peptide desenkephalin-gamma-endorphin may decrease dopaminergic neurotransmission in the nucleus accumbens of rats

In rats, subchronic administration of desenkephalin-gamma-endorphin (DE gamma E) into the nucleus accumbens or subcutaneously for 10 days resulted in hypoactivity. Intra-accumbens administration caused a significant reduction in the nucleus accumbens tissue levels of the dopamine (DA) metabolites 3,4-dihydroxyphenylacetic acid (DOPAC) and homovanillic acid (HVA). Systemic administration of DE gamma E decreased DOPAC and 5-hydroxyindoleacetic acid (5-HIAA) levels in nucleus accumbens tissue. Subchronic subcutaneous DE gamma E treatment reduced the basal release of [3H]DA from rat nucleus accumbens slices in vitro and the basal release of endogenous DA and DOPAC in vivo as assessed with on-line dialysis in the nucleus accumbens of freely moving rats. The DA agonist N,N-dipropyl-7-hydroxy-2-aminotetralin (DP-7-ATN) was equally effective in inhibiting [3H]DA release elicited by electrical stimulation from slices of subchronically DE gamma E and placebo treated rats. Administration of a small dose of apomorphine caused similar reductions of the in vivo release of DA and DOPAC in both placebo and DE gamma E treated rats. These results indicate that subchronic DE gamma E treatment may decrease dopaminergic neurotransmission in the nucleus accumbens. This effect is probably not due to alterations in the sensitivity of presynaptically located DA autoreceptors mediating DA release in vitro and in vivo.

Modulation of dopamine D3 receptor binding by N-ethylmaleimide and neurotensin

GTP or G protein inactivation by N-ethylmaleimide reduced the Bmax value but not the KD value of 7-[3H]hydroxy-N,N-di-n-propyl-2-aminotetralin ([3H]7-OH-DPAT) binding in the rat subcortical limbic area. Neurotensin (10 nM) increased the KD and the Bmax values of [3H]7-OH-DPAT binding, and these effects persisted also following N-ethylmaleimide pretreatment. N-Propylnorapomorphine, quinpirole, raclopride, and remoxipride inhibited [3H]7-OH-DPAT binding with Ki values of 0.093, 1.97, 10.6, and 710 nM, respectively. These findings indicate that the D3 receptor is coupled to G proteins in the brain, and that neurotensin can modulate D3 agonist binding by a G protein-independent mechanism.

Effects of 7-hydroxy-N,N-di-n-propylaminotetralin on behaviour and blood pressure of spontaneously hypertensive rats

The in vivo effects of administration of the putative dopamine D3 receptor agonist 7-hydroxy-N,N-di-n-propylaminotetralin (7-OH-DPAT) were investigated in spontaneously hypertensive rats (SHR) and normotensive wistar-Kyoto controls (WKY). The i.p. injection of 7-OH-DPAT induced hyperactivity in WKY at 10 mg/kg, but only an inhibition of exploratory locomotor activity was observed in SHR at 1 mg/kg. In WKY and SHR with unilateral lesions of the nigrostriatal system, s.c. injection of 0.01-1 mg/kg of 7-OH-DPAT induced dose-dependent contralateral turning behaviour. This response was more pronounced in SHR than in WKY. The s.c. injection of 0.03, but not of 0.01 or 0.1 mg/kg, of 7-OH-DPAT induced yawning in WKY and SHR. The i.v. injection of 0.1 or 1 mg/kg of 7-OH-DPAT induced an immediate rise in blood pressure in both WKY and SHR. Pretreatment with the dopamine receptor antagonist haloperidol partially prevented this pressor response and, in addition, unmasked a late fall in blood pressure in SHR. The s.c. injection of 1 mg/kg of 7-OH-DPAT induced a decrease in body temperature, which was more pronounced in SHR than in WKY. This effect could be inhibited by pretreatment with haloperidol, but a residual hypothermia remained in SHR. These results suggest that 7-OH-DPAT induces a variety of effects in vivo, many of which may be mediated by dopamine D2 receptors or non-dopaminergic receptors. Thus, more selective dopamine D3 receptor agonists or -antagonists are needed to further explore the role of dopamine D3 receptors in vivo.(ABSTRACT TRUNCATED AT 250 WORDS)

Identification, characterization, and localization of the dopamine D3 receptor in rat brain using 7-[3H]hydroxy-N,N-di-n-propyl-2-aminotetralin

We have identified 7-[3H]hydroxy-N,N-di-n-propyl-2-aminotetralin ([3H]7-OH-DPAT) as a selective probe for the recently cloned dopamine D3 receptor and used it to assess the presence of this receptor and establish its distribution and properties in brain. In transfected Chinese hamster ovary (CHO) cells, it binds to D3 receptors with subnanomolar affinity, whereas its affinity is approximately 100-, 1000-, and 10,000-fold lower at D2, D4, and D1 receptors, respectively. Specific [3H]7-OH-DPAT binding sites, with a Kd of 0.8 nM and a pharmacology similar to those at reference D3 receptors of CHO cells, were identified in rat brain. D3 receptors differ from D2 receptors in brain by their lower abundance (2 orders of magnitude) and distribution, restricted to a few mainly phylogenetically ancient areas--e.g., paleostriatum and archicerebellum--as evidenced by membrane binding are autoradiography studies. Native D3 receptors in brain are characterized by an unusually high nanomolar affinity for dopamine and a low modulatory influence of guanyl nucleotides on agonist binding. These various features suggest that D3 receptors are involved in a peculiar mode of neurotransmission in a restricted subpopulation of dopamine neurons.

Dissociation of autoreceptor activation and behavioral consequences of low-dose apomorphine treatment

1. Low dose dopaminergic agonist effects have been used as a behavioral screen for identifying compounds with selective autoreceptor activity. 2. However, results from several recent investigations suggest that these behaviors may not be generated from an autoreceptor substrate but rather from a subpopulation of postsynaptic dopamine receptors with a high affinity for the agonist. 3. In support of this hypothesis, the present investigation reports that both hypomotility and yawning, induced in the rat with 0.07 mg/kg apomorphine, were not paralleled by autoreceptor-induced reductions in transmitter metabolism from either mesolimbic or neostriatal dopamine regions.

Retinal dopamine D1 and D2 receptors: characterization by binding or pharmacological studies and physiological functions

1. In the retinal inner nuclear layer of the majority of species, a dopaminergic neuronal network has been visualized in either amacrine cells or the so-called interplexiform cells. 2. Binding studies of retinal dopamine receptors have revealed the existence of both D1- as well D2-subtypes. The D1-subtype was characterized by labeled SCH 23390 (Kd ranging from 0.175 to 1.6 nM and Bmax from 16 to 482 fmol/mg protein) and the D2-subtype by labelled spiroperidol (Kd ranging from 0.087 to 1.35 nM and Bmax from 12 to 1500 fmol/mg protein) and more selectively by iodosulpiride (Kd 0.6 nM and Bmax 82 fmol/mg protein) or methylspiperone (Kd 0.14 nM and Bmax 223 fmol/mg protein). 3. Retinal dopamine receptors have been also shown to be positively coupled with adenylate cyclase activity in most species, arguing for the existence of D1-subtype, whereas in some others (lower vertebrates and rats), a negative coupling (D2-subtype) has been also detected in peculiar pharmacological conditions implying various combinations of dopamine or a D2-agonist with a D1-antagonist or a D2-antagonist in the absence or presence of forskolin. 4. A subpopulation of autoreceptors of D2-subtype (probably not coupled to adenylate cyclase) also seems to be involved in the modulation of retinal dopamine synthesis and/or release. 5. Light/darkness conditions can affect the sensitivity of retinal dopamine D1 and/or D2-receptors, as studied in binding or pharmacological experiments (cAMP levels, dopamine synthesis, metabolism and release). 6. Visual function(s) of retinal dopamine receptors were connected with the regulation of electrical activity and communication (through gap junctions) between horizontal cells mediated by D1 and D2 receptor stimulation. Movements of photoreceptor cells and migration of melanin granules in retinal pigment epithelial cells as well as synthesis of melatonin in photoreceptors were on the other hand mediated by the stimulation of D2-receptors. 7. Other physiological functions of dopamine D1-receptors respectively in rabbit and in embryonic avian retina would imply the modulation of acetylcholine release and the inhibition of neuronal growth cones.

Dopamine autoreceptor agonists attenuate spontaneous motor activity but not spontaneous fighting in individually-housed mice

The present study was conducted to determine whether or not two behavioral characteristics of individually-housed mice, hyperactivity in a novel environment and intermale fighting, are attenuated by the dopamine (DA) agonists, apomorphine, (+)- and (-)-3-(3-hydroxyphenyl)-N-n-propylpiperidine (3-PPP). Autoreceptor-activating doses of these drugs which reduced spontaneous activity in a novel environment did not inhibit spontaneous fighting with conspecific olfactory bulbectomized males. Individually-housed mice were more active in a novel environment and showed a significant reduction of activity at lower doses of apomorphine, (+)- and (-)-3-PPP than group-housed mice. However, the ED50's for the inhibition of spontaneous activity in a novel environment in group- and individually-housed mice were similar: apomorphine, 0.02 vs. 0.012 mg/kg, SC; (+)-3-PPP, 0.50 vs. 0.51 mg/kg, SC; and (-)-3-PPP, 1.0 vs. 0.56 mg/kg, SC, for group- and individually-housed mice respectively. A significant proportion of individually-housed mice, but not group-housed mice, displayed catalepsy in response to high doses of (-)-3-PPP. These data suggest that DA autoreceptor agonists can modulate the hyperactivity syndrome but not spontaneous fighting behavior in individually-housed mice.

Pharmacological profile of a chromanamine analogue (DP-6OH-3CA) of the selective presynaptic dopamine agonist N,N-dipropyl-7-hydroxy-2-aminotetralin

The pharmacological profile of an oxygen isostere of the selective presynaptic dopamine agonist DP-7OH-AT, i.e. dipropyl-6-hydroxy-3-chromanamine (DP-6OH-3CA) has been evaluated in various receptor binding, neurobiochemical and behavioural experiments. The chromanamine displaced the 3H-labelled dopamine ligands, 5,6-DPAT and N-0437, with Ki values of 106 and 143 nM, respectively. In in-vivo biochemical models for presynaptic activity the chromanamine induced a half-maximal effect in the gamma-butyrolactone reversal test at 6.8 mumol kg-1 and had an ED70 value of 40 mumol kg-1 for HVA decrease in the striatum. In behavioural models for postsynaptic dopaminergic activity a half-maximal effect for the induction of stereotypy was reached at 100 mumol kg-1 and reversal of the effects of reserpine to a level of 200 counts was induced at 11 mumol kg-1. On comparison of these results with the results obtained with the carbon analogue of DP-6OH-3CA, i.e. DP-7OH-AT, it is apparent that the chromanamine has a reduced potency for dopamine D2 receptors in in-vitro and in-vivo models. The selectivity for presynaptic dopamine receptors was lower than with DP-7OH-AT and the isomeric chromanamine, DP-8OH-3CA, indicating that the optimal position of the hydroxyl group for presynaptic selectivity is in the 8 and not in the 6 position for the chromanamines.

Desenkephalin-gamma-endorphin and neuroleptics counteract the DP-7-ATN-induced hypomotility after intra-accumbens treatment

Injection of the selective presynaptic dopamine agonist N,N-dipropyl-7-hydroxy-2-aminotetralin (DP-7-ATN, 1 fg to 1 microgram) into the nucleus accumbens decreased motor activity. The reduction of motor activity was reversed by pretreatment with haloperidol (10 pg), (-)-sulpiride (10 pg) or desenkephalin-gamma-endorphin (DE gamma E) (100 pg). These results support the hypothesis that DE gamma E may interfere with presynaptically located D-2 dopamine receptor systems in the nucleus accumbens. Furthermore, the use of DP-7-ATN in the described test procedure might be a useful model for testing novel neuroleptic compounds in vivo.

The neuroleptic-like peptide desenkephalin-gamma-endorphin does not antagonize the dopamine receptor agonist-induced inhibition of the release of [3H]dopamine from rat nucleus accumbens slices in vitro

In rats, the non-opioid beta-endorphin (beta E) fragment desenkephalin-gamma-endorphin (DE gamma E, beta E6-17) antagonizes the hypomotility induced by a small dose of dopamine (DA) receptor agonists. It has been suggested that DE gamma E might act in this respect by a direct or indirect blockade of presynaptically located DA receptors in the nucleus accumbens, thereby causing an increase of DA release. Therefore in the present study the effect of DE gamma E was examined on DA receptor agonist-induced inhibition of the electrically evoked release of previously accumulated [3H]DA from rat nucleus accumbens slices in vitro. The DA receptor agonists apomorphine, LY 171555 and n,n-di-n-propyl-7-hydroxy-2-aminotetralin (DP-7-AT) inhibited in a concentration-dependent manner the electrically evoked release of [3H]DA. The selective D2 receptor antagonist (-)-sulpiride blocked the effects of apomorphine, corroborating that the DA receptor involved is of a D2 type. DE gamma E was tested at several concentrations (10(-9)-10(-6) M) and under various experimental conditions. DE gamma E, by itself, did not affect either the electrically stimulated or the basal release of [3H]DA. The inhibiting effect of DA receptor agonists was slightly reduced by DE gamma E, but this effect was present in some experiments only. It is concluded that DE gamma E does not function as an antagonist for the DA receptor mediating DA release and that the interaction observed in behavioural experiments between DA agonists and DE gamma E does not occur at the level of this receptor.

Further in vitro and in vivo studies with the putative presynaptic dopamine agonist N,N-dipropyl-7-hydroxy-2-aminotetralin

The in vitro binding of the putative dopamine autoreceptor agonist [3H]DP-7-ATN to rat striatal membrane homogenates was investigated. The maximum number of binding sites Bmax was 497.5 +/- 50.2 fmol/mg protein and the affinity constant KD was 8.3 +/- 1.5 nM using 10 microM (+) butaclamol to define non-specific binding. Lesion of the left medium forebrain bundle by 6-hydroxydopamine resulted in an almost complete loss of dopamine in the striatum but did not affect the binding of [3H]DP-7-ATN. The binding of [3H]DP-7-ATN to the homogenates of the dopaminergic cell bodies in the substantia nigra revealed a Bmax of 542.4 +/- 40.1 fmol/mg protein and a KD of 11.1 +/- 1.3 nM. The pharmacological profile of the binding was characterized as being to D-2 receptors. No direct in vitro evidence could be found for a selective binding to DA autoreceptors. The dopamine uptake inhibitor GBR 12909 interacted in a noncompetitive manner with the in vitro binding of [3H]DP-7-ATN and the latter compounds uptake into isolated synaptosomes was not through the specific dopamine uptake system but rather through diffusion. GBR 12909 failed to reveal any agonistic or antagonistic activity in the GBL model but was able to antagonize the hypomotility in rats induced by 0.25 mg/kg DP-7-ATN. The inhibitory effect of DP-7-ATN on DA release was also demonstrated using in vivo brain dialysis in conscious rats. Based on the above results, the possibility is discussed that the release regulating DA autoreceptors, which might be coupled to the reuptake complex, and the DA biosynthesis regulating autoreceptors, are different entities.

In vitro binding of the very potent and selective D-2 dopamine agonist, [3H]N-0437 to calf caudate membranes

N-0437, a non-catecholic aminotetralin has recently been described as a very potent and selective dopamine D-2 agonist. In this study the in vitro binding of [3H]N-0437 (specific activity 80.6 Ci/mmol) to calf caudate membranes is described. It was found that [3H]N-0437 binds with a high affinity (KD = 0.17 nM) and a low proportion of non-specific binding. Moreover the binding was saturable with a high number of binding sites (Bmax = 703 +/- 28 fmol/mg protein) and reversible (dissociation half-time = 68 min). Pharmacological analysis of [3H]N-0437 binding showed that it was selective for dopamine receptors and that it was also stereoselective for D-2 receptors. Non-dopaminergic drugs were without exception very poor displacers. Taken together the results suggest that [3H]N-0437 labels dopamine D-2 receptors with a high selectivity in the calf brain, and thus, that it should be a useful tool in studies of central dopamine receptors.

Release of endogenous amino acids, dopamine, and cyclic AMP from the rat brain: methodological aspects and mutual interferences

The release from the rat brain of endogenous dopamine, its metabolites, cyclic AMP, transmitter, and other amino acids was studied with push-pull cannula perfusion or dialysis. The amino acid output from the striatum with either perfusion technique was virtually identical. Relative to the tissue content, the egress of the dopamine metabolites and of the non-transmitter (metabolic) amino acids was higher than that of the transmitter substances. The intracellular second messenger cyclic AMP was also released into the perfusate, and its content was enhanced by local application of various biogenic amines. During and after electroconvulsive shocks the release (and presumably the formation) of alanine was enhanced, which may reflect increased glycolysis and transamination in the brain. Following local application of tricyclic antidepressants, the content of transmitter amino acids in push-pull perfusates of the rat thalamus and striatum was enhanced several fold. Such an increase was not found following systemic injection of high doses of mianserin and desmethylimipramine. In contrast, the effect of amphetamine on dopamine release was seen after both local and systemic applications. Locally applied dopamine produced a specific decrease in the release of gamma-aminobutyric acid in both the substantia nigra and in the striatum. Muscimol decreased, whereas picrotoxine enhanced the release of GABA from the rat striatum. Our observations emphasize that transmitter release and brain metabolism can be monitored by perfusion techniques and that the effects of drugs may depend on the route of administration. As has frequently been shown with in vitro techniques, our data indicate the importance of local control of transmitter release in vivo by both autoreceptors and alloreceptors in addition to firing activity.

Investigation of dopamine content, synthesis, and release in the rabbit retina in vitro: II. Effects of high potassium, adenylate cyclase activators, and N-n-propyl-3-(3-hydroxyphenyl) piperidine

The modulation of 3,4-dihydroxyphenylethylamine (dopamine, DA) synthesis and release in rabbit retina in vitro by high K+; adenylate cyclase activators such as forskolin, 2-chloroadenosine, vasoactive intestinal polypeptide (VIP); and the putative DA autoreceptor agonist N-n-propyl-3-(3-hydroxyphenyl) piperidine (3-PPP) has been investigated. Incubation of retinas in 50 mM K+ resulted in the activation of tyrosine hydroxylase (TH). Activation did not require the presence of extracellular Ca2+. K+ 50 mM also induced a Ca2+-dependent release of DA. Forskolin 50 microM stimulated TH but 100 microM 2-chloroadenosine and 650 nM VIP did not. Individually, (+)-3-PPP, (-)-3-PPP, and (+/-)-3-PPP reduced DA synthesis and increased its release. The effects of (+/-)-3-PPP were dose-dependent and did not require the presence of extracellular Ca2+. The activation of TH induced by 50 mM K+, but not that induced by 50 microM forskolin, was abolished by 100 microM (+/-)-3-PPP.

Pharmacological profiles of three new, potent and selective dopamine receptor agonists: N-0434, N-0437 and N-0734

A series of new dopamine (DA) receptor agonists, of the 2-aminotetralin group, i.e. N-0434, N-0437 and N-0734 were investigated in both in vivo and in vitro pharmacological test systems. In vivo, the reversal of the gamma-butyrolactone-induced increase in rat central DOPA biosynthesis rate was taken as a measure of presynaptic activity. In addition, the homovanillic acid (HVA) decrease, after intraperitoneal and after oral administration of the drugs was also taken as a measure of presynaptic activity. Postsynaptic activity was measured in two behavioural models, i.e. reserpine reversal and stereotypy induction. The effects of these drugs on noradrenaline and dopamine turnover (alpha-MpT method) were studied in addition. The displacement of [3H]N,N-dipropyl-5,6-dihydroxy-2-aminotetralin [( 3H]DP-5,6-ADTN) binding to rat striatal homogenates was studied in vitro. The results indicate that all three compounds are potent and selective DA agonists that lack significant alpha 2 activity. Because of its long duration of action and high oral activity, N-0437 seems to be a most promising candidate for further evaluation for possible therapeutic use.

Neuropharmacological profile of a new series of dopamine agonists: N-n-propyl-hexahydronaphthoxazines

It has been demonstrated that within the series of hydroxylated (7-OH, 9-OH) and non-hydroxylated (N-0498) hexahydronaphthoxazines the 9-OH (N-0500) analogue is a very potent centrally acting DA receptor agonist. In in vitro [3H]DP-5,6-ADTN binding experiments, reflecting D-2 dopaminergic activity, N-0500 was equipotent with apomorphine and RU-29717, whereas both the 7-OH (N-0499) and N-0498 were much less effective. In in vivo tests related to DA receptor stimulation N-0500 was found to be the most active compound. In the gamma-butyrolactone model, a test for DA autoreceptor activation, N-0500 was 10 times as potent as apomorphine, but 3 times less active than RU-29717. The locomotor activity of mice was inhibited more strongly by N-0500 than by N-0499. Striatal concentrations of 3,4-dihydroxyphenylacetic acid and homovanillic acid were rapidly reduced by N-0500 both after intraperitoneal and oral administration, indicating that this compound is well absorbed from the gastrointestinal tract and passes the blood-brain barrier to activate DA autoreceptors. In models for postsynaptic DA receptor stimulation (induction of stereotypy in rats, reversal of reserpine-induced immobility of mice) N-0500 was found to be as effective as RU-29717 in inducing stereotyped behaviors in rats, but was much less effective than RU-29717 in restoring the mobility of reserpinized mice, suggesting a selectivity for D-2 DA receptors by N-0500 in contrast to the mixed D-1/D-2 receptor activity of RU-29717. In in vitro binding experiments for evaluating the affinity towards other receptor types, N-0500 exhibited only a weak affinity towards 5-HT1 and alpha 2 binding sites and possessed a very weak affinity for 5-HT2 and alpha 1 receptor sites. It was concluded from these in vitro binding experiments that N-0500, has not only a very high affinity for D-2 DA receptors, but is more selective than RU-29717 and much more selective than the ergot bromocriptine. On the basis of its very potent in vivo central D-2 dopamine receptor activities and its in vitro selectivity, N-0500, being the most potent compound within the series, is a much more specifically acting drug than many of the dopaminergic ergolines and might therefore be a good candidate for the treatment of Parkinson's disease.

S(-)DP-5,6-ADTN as an in vivo dopamine receptor ligand: relation between displacement by dopamine agonists and their pharmacological effects

The use of (-)DP-5,6-ADTN as a non-radioactively labeled ligand for an in vivo DA receptor assay is described and compared with racemic DP-5,6-ADTN, previously used for that purpose. The effects of four DA agonists (NPA, bromocriptine, DP-7-OH-ATN and 3-PPP) on the specific (-)DP-5,6-ADTN binding are related to their potencies to decrease striatal HVA concentrations and to induce stereotypy in rats. NPA and DP-7-OH-ATN caused a maximal decrease in HVA levels, when only a fraction of the receptors were occupied, while the occurrence of stereotypy was associated with a high receptor occupation, reflecting the higher affinity of these agonists for presynaptic than for postsynaptic receptors. Bromocriptine did not show this effect, as the dose-response relationships for HVA decrease, for induction of stereotypy and for the decrease in specific (-)DP-5,6-ADTN binding were all virtually equal to each other. While NPA and bromocriptine behaved as full postsynaptic agonists, in that maximal stereotyped behavior was observed after high doses, DP-7-OH-ATN was found to be a partial postsynaptic agonist, as it did not induce maximal stereotypy at a maximal receptor occupation. Racemic 3-PPP only caused a state of hypoactivity, but did neither affect specific (-)DP-5,6-ADTN binding nor striatal HVA levels. Our results are discussed in view of theories on the relation between receptor occupation and pharmacological effects and it is concluded that the in vivo receptor binding method using (-)DP-5,6-ADTN is a very useful tool for such investigations.

Apomorphine enantiomers' effects on dopamine metabolism: receptor and non-receptor related actions

The enantiomers of apomorphine (APO) inhibited dopamine synthesis in rat striatal synaptosomes, with R(-)-APO being about twice as potent as S(+)-APO. Sulpiride, a DA receptor antagonist, partially antagonized the inhibitory effects of only (-)-APO, suggesting that (-)-APO's, but not (+)-APO's, effects on dopamine synthesis may be at least partially receptor-mediated. The addition of 6-methyl-5,6,7,8-tetrahydrobiopterine (6-MPH4), an artificial cofactor for tyrosine hydroxylase, partially antagonized the inhibitory effects of both enantiomers, being considerably more effective against the (+)enantiomer. These data suggest that the APO enantiomers may directly inhibit enzymes within the synaptosome which regulate dopamine synthesis. Furthermore, investigations measuring DA synthesis rates in synaptosomes that had been pre-incubated with (-)-APO and then washed to remove the (-)-APO in the medium, indicate that (-)-APO may be retained by synaptosomes. Preliminary studies measuring the accumulation of [3H](-)-APO by synaptosomes also suggest that synaptosomes can accumulate APO. Although both APO enantiomers suppressed DA synthesis in vitro, only (-)-APO reduced striatal DA metabolite concentrations in vivo, and this reduction was prevented by haloperidol, a DA receptor antagonist. In addition, 6-MPH4 prevented the decrease in the DA metabolite 3,4-dihydroxyphenylacetic acid (DOPAC) produced by (-)-APO but not the decrease in the DA metabolite homovanillic acid (HVA).

Brain dopamine autoreceptors in aging rats

The function of dopamine (DA) autoreceptors is evaluated in vivo in striatum and mesolimbic regions of young adult (4 months), mature (14 months) and old (26 months) male Wistar rats. gamma-Butyrolactone (GBL)-induced dihydroxyphenylalanine (DOPA) accumulation in rats also treated with an inhibitor of aromatic L-aminoacid decarboxylase was used to determine the presence of synthesis-modulating nerve terminal autoreceptors while its reversal with apomorphine served as an index of autoreceptor stimulation. GBL-induced DOPA accumulation in striatum is very high at all three ages (130-150% increase in comparison with controls) as is its reversal by apomorphine (65-80% decrease in comparison with GBL alone). In mesolimbic regions, GBL has much less effect than in striatum (31% rise at 4 and 26 months, 12% rise at 14 months), but apomorphine's effect is of the same order of magnitude (down 60-80%). The conclusion can be drawn that aging does not significantly affect DA autoreceptor function in striatum and mesolimbic areas.

Synthesis and radioreceptor binding activity of N-0437, a new, extremely potent and selective D2 dopamine receptor agonist

The synthesis of a new, potent and selective D2 dopamine receptor agonist, N-0437, of the 2-aminotetralin group is described. The results of a radioreceptor binding assay using a homogenate of porcine anterior pituitary as a tissue source for D2 dopamine receptors and 3H-spiperone as radioligand demonstrate that this compound is one of the most potent compounds so far evaluated in this test system.

N-methyl,N-propargyl-2-aminotetralins:novel dopamine agonists with monoamine oxidase inhibiting properties

A series of mono- (5 and 7) and dihydroxylated (5,6 and 6,7)N-methyl,N-propargyl-2-aminotetralins were studied with respect to their dopamine agonistic and monoamine oxidase inhibitory activities. MAO inhibition was found to be reduced by hydroxylation of the aromatic ring. Among the hydroxylated compounds the 7-OH analogue was the most potent inhibitor in in vitro and ex vivo experiments. Both catecholic structures were equipotent with apomorphine as displacers of the specific in vitro binding of [3H]NPA to rat striatal homogenates. Moreover, the catecholic analogues had a potency comparable to that of apomorphine in the gamma-butyrolactone model whereas the monohydroxy analogues were less active. On the basis of their effectiveness to induce stereotypy in rats and to reverse reserpine-induced hypomotility in mice (both used as indices of postsynaptic dopamine receptor stimulation) the catecholic compounds were more potent than the monohydroxy analogues but much less active than apomorphine. Dopamine agonistic activity was also reflected in decreased HVA levels in the striatum whilst effects on striatal 3-MT levels reflected the balance between dopamine agonistic (decrease in 3-MT) and MAO inhibitory (increase in 3-MT) activity of the various compounds. It was concluded that both the mono- and dihydroxylated compounds have MAO inhibiting and dopamine agonistic activities. The MAO inhibitory activity predominated within the monohydroxy structures whereas the dopamine agonistic effect was predominant for the catecholic compounds. It would thus appear that, at least for the 2-aminotetralins, it is difficult to prepare an analogue which combines a high degree of both MAO inhibitory and DA agonistic activity.

The effectiveness of yohimbine in blocking rat central dopamine autoreceptors in vivo

The influence of various alpha-adrenoceptor antagonists (10 mg/kg i.p.) upon the rate of turnover of dopamine (DA) in the rat brain was investigated. Taking the levels of the DA metabolites 3,4-dihydroxyphenylacetic acid (DOPAC) and homovanillic acid (HVA) as a measure of the rate of DA turnover, it was found that prazosin and phenoxybenzamine decreased, whereas piperoxane and yohimbine increased the turnover rate both in the corpus striatum and in the tuberculum olfactorium. Azapetine, phentolamine and tolazoline as well as the beta-adrenoceptor antagonist propranolol were without a significant effect, whereas the DA antagonist haloperidol increased DOPAC and HVA levels and decreased the levels of DA itself. The possibility that the yohimbine-induced increase in the DA turnover rate was produced by a direct blockade of DA autoreceptors, was investigated under conditions where influences other than those elicited via DA autoreceptors are thought to be eliminated, i.e. in rats treated with reserpine or gamma-butyrolactone (GBL). In rats that were pretreated with reserpine, yohimbine (10 mg/kg i.p.) was found to be ineffective in antagonizing the reduction of the accumulation of 3,4-dihydroxyphenylalanine (DOPA) following decarboxylase inhibition, that was produced by the DA agonist apomorphine (2.0 mg/kg i.p.). In rats pretreated with reserpine, yohimbine (10 mg/kg i.p.) was also ineffective in antagonizing the reduction of the DOPAC and HVA levels produced by apomorphine (2.0 mg/kg i.p.), but it was effective in antagonizing the reduction of the HVA level that was produced by the selective DA autoreceptor agonist N,N-di-n-propyl-7-hydroxy-2-aminotetralin (DP-7-AT, 1.0 mg/kg i.p.).(ABSTRACT TRUNCATED AT 250 WORDS)

Dopamine synthesis in synaptosomes: relation of autoreceptor functioning to pH, membrane depolarization, and intrasynaptosomal dopamine content

Factors affecting dopamine (DA) synthesis in rat striatal synaptosomes were examined by measuring the conversion of [3H]tyrosine (Tyr) to [3H]DA. Any [3H]DA that was synthesized was extracted into a toluene-based scintillation cocktail and quantitated by liquid scintillation spectrometry. The extraction was facilitated using di-(2-ethylhexyl) phosphoric acid (DEHP), a liquid cation exchanger. DA, apomorphine, and other DA agonists were much less potent inhibitors of DA synthesis in striatal synaptosomes at pH 6.2 than at pH 7.2. 3-(3-Hydroxyphenyl)-N-n-propylpiperidine (3-PPP), a putative DA autoreceptor agonist, was inactive at pH 6.2. However, at pH 7.2, 3-PPP did inhibit DA synthesis. This inhibition was reversed by sulpiride, a DA receptor antagonist, but not by benztropine, a DA uptake blocker, suggesting that 3-PPP inhibits DA synthesis by stimulating the DA autoreceptor. DA release from synaptosomes was much greater at pH 6.2 than at pH 7.2, most probably because the synaptosomal membrane appears to be depolarized at pH 6.2, as measured by the accumulation of [3H]tetraphenylphosphonium ions. Since tyrosine hydroxylase is inhibited by DA, this finding suggested that low assay buffer pH (i.e., pH 6.2) might interfere with the ability of 3-PPP and other DA agonists to inhibit DA synthesis, by promoting DA release. Likewise, reserpine and tetrabenazine, compounds which disrupt vesicular DA storage, were much less effective inhibitors of DA synthesis at pH 6.2 (high basal DA release). Moreover, D-amphetamine and high buffer potassium concentrations, treatments which promote DA release, also interfered with the ability of 3-PPP to inhibit DA synthesis. Thus, modulation of the release of DA in equilibrium with tyrosine hydroxylase may be a mechanism by which the DA autoreceptor regulates DA synthesis.

The hydroxy-hexanydronaphthoxazines: a new group of very potent and selective dopamine agonists

Replacement of the pyrrole ring system in the dopaminergic oxaergolines (e.g. RU 29717) by a phenolic OH group leads to the hydroxy-hexahydronaphthoxazines, a new group of potent dopamine agonists which exhibit increased selectivity due to their lower affinity for adrenergic and 5-HT receptors.

In vivo dopamine autoreceptor selectivity appears to be critically dependent upon the aromatic hydroxyl position in a series of N,N-disubstituted 2-aminotetralins

The potencies of a number of 2-aminotetralin derivatives as centrally acting dopamine (DA) receptor agonists were investigated using the reversal of the gamma-butyrolactone-induced increase in rat central DA biosynthesis rate as a measure of potency at DA autoreceptors and the reversal of the reserpine-induced immobility of mice as a measure of postsynaptic DA receptor stimulating potency. The results indicated that the compounds fell into two separate groups depending on their effectiveness in the postsynaptic test model. High postsynaptic effectiveness was achieved with compounds bearing a hydroxyl group at the 5 position of the aminotetralin structure, whereas aminotetralins lacking this substitution pattern were found to possess high DA autoreceptor selectivity. The observed dichotomy of DA agonists is discussed in relation to the possible involvement of multiple DA receptors and alpha-adrenoceptors.