Effects of B-HT 920 and B-HT 933 on dopamine and noradrenaline autoreceptors in the rat brain

Phenol (bio)isosteres in drug design and development

Due to their versatile properties, phenolic compounds are integral to various biologically active molecules, including many pharmaceuticals. However, their application in drug design is often hindered by issues such as poor oral bioavailability, rapid metabolism, and potential toxicity. This review explores the use of phenol bioisosteres-structurally similar compounds that can mimic the biological activity of phenols while potentially offering improved drug-like properties. We provide an extensive analysis of various phenol bioisosteres, including benzimidazolones, benzoxazolones, indoles, quinolinones, and pyridones, highlighting their impact on the pharmacokinetic and pharmacodynamic profiles of drugs. Case studies illustrate the successful application of these bioisosteres in enhancing metabolic stability, receptor selectivity, and overall therapeutic efficacy. Additionally, the review addresses the challenges associated with phenol bioisosterism, such as maintaining potency and avoiding undesirable side effects. By offering a detailed examination of current strategies and potential future directions, this review serves as a valuable resource for medicinal chemists seeking to optimize phenolic scaffolds in drug development. The insights provided herein aim to facilitate the design of more effective and safer therapeutic agents through strategic bioisosteric modifications.

Highly efficient antitumor agents of heterocycles containing sulfur atom: Linear and angular thiazonaphthalimides against human lung cancer cell in vitro

A novel series of aminothiazonaphthalimides, A(1-2) and B(1-2), has been regioselectively synthesized. The linear compounds B(1-2) were evaluated to be far more active than their angular isomers A(1-2) in antitumor evaluation. The linear compounds C-F, derived from compound B(1), all showed highly efficient antitumor activities against A549 and P388 cell lines. Also, cytotoxicities of these four analogues against two tumor cells were highly dependent on the length of the side chains. The compound A(1) or B(1), with two methylene units in the side chain, was more cytotoxic than its corresponding homologue A(2) or B(2), with one more methylene unit.

2-aminothiazole-derived opioids. Bioisosteric replacement of phenols

A series of aminothiazole-derived morphinans, benzomorphans, and morphine were synthesized. Although their affinities were somewhat lower than their phenol prototypes, one compound (9a, ATPM) has been identified possessing high affinity and selectivity at the kappa receptor. Functional assays showed that 9a was a full kappa but partial mu agonist; the efficacy at kappa was significantly greater than at mu receptors. This novel compound may be valuable for the development of long-acting analgesics and drug abuse medication.

Thiophene analogs of naphthoxazines and 2-aminotetralins: bioisosteres with improved relative oral bioavailability, as compared to 5-OH-DPAT

In the present study, a series of thiophene analogs of 2-aminotetralins and hexahydronaphthoxazines were studied in vivo for their ability to decrease striatal dopamine release, their effects on locomotor activity, and their behavioral characteristics in reserpinized rats, in order to investigate whether a thiophene moiety can act as a bioisostere for the phenol moiety. In general, the new compounds showed lower in vivo activities than 5-hydroxy-2-(N,N,-di-n-propylamino)tetralin (5-OH-DPAT). However, the introduction of the thiophene moiety gave a significant improvement of the relative oral bioavailability, compared to 5-OH-DPAT. Our results suggest that the thiophene moiety can act as a bioisostere for a phenol group in hydroxylated 2-aminotetralins. For the thianaphthoxazines it was not possible to discriminate between bioisosterism for a phenyl or a phenol moiety. The tetrahydrobenzo[b]thiophenes could be used as lead compounds for the development of novel dopamine receptor ligands with improved relative oral bioavailability.

Effects of talipexole on contraversive rotation and functional impairment in MPTP-induced chronic hemiparkinsonian monkeys

The effects of talipexole on functional motor improvement in comparison with contraversive circling were studied in five chronic (5-7 years post MPTP-lesioned) hemiparkinsonian Macaca nemestrina monkeys. Talipexole induced contraversive rotations in a dose of 32 microg/kg for about 1 hr after i.m. injection. Larger doses (56 and 100 microg/kg, i.m.) produced less effect due to sedation. Three different rating scales were used to assess functional improvement, including a clinical parkinsonism rating scale, volitional responses to fruit presentations, and number of significant hand movements. The optimal dose of talipexole was 32 microg/kg, i.m. Functional improvement by talipexole, including clinical parkinsonian rating scales and significant hand movements, as well as contraversive circling in hemiparkinsonian monkeys, confirm that this chronic animal model is useful in preclinical testing of drugs for the treatment of human parkinsonism.

Talipexole or pramipexole combinations with chloro-APB (SKF 82958) in MPTP-induced hemiparkinsonian monkeys

The effects of two predominant dopamine D2-like receptor agonists, talipexole (6-allyl-2-amino-5,6,7,8-tetrahydro-4H-thiazolo [4,5-d]-azepine dihydrochloride, B-HT 920 CL2) and pramipexole (S(-)2-amino-4,5,6,7-tetrahydro-6-propyl-aminobenzothiazole dihydrochloride, SND 919 CL2Y), were studied alone and in combination with the selective dopamine D1-like receptor agonist chloro-APB ((+/-)6-chloro-7-8-dihydroxy-3-allyl-1-phenyl-2,3,4,5-tetrahydro-1H-benz azepine hydrobromide, SKF 82958) in five chronic 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP) lesioned hemiparkinsonian Macaca nemestrina monkeys. Talipexole induced contraversive rotation in a dose-dependent manner up to 32 microg/kg, i.m. Talipexole was more potent than pramipexole (10 vs. 32 microg/kg, i.m.), but pramipexole was more efficacious in producing contraversive rotational behavior and significant hand movements in the afflicted limb. Larger doses of chloro-APB also produced contraversive rotation. Combinations of each dopamine D2-like receptor agonist in a median effective dose with chloro-APB (23.4 and 74.8 microg/kg, i.m.) had synergistic effects, producing either addition or potentiation, depending upon the dose used. The effects noted with these combinations were less than the effect of a large dose (100 microg/kg) of pramipexole. Talipexole, in the largest dose studied (100 microg/kg, i.m.), produced sedation which was not seen with the same dose of pramipexole. No significant extrapyramidal side effects were noted with either agent.

Local administration of dopaminergic drugs into the ventral tegmental area modulates cataplexy in the narcoleptic canine

Cataplexy in the narcoleptic canine may be modulated by systemic administration of monoaminergic compounds. In the present study, we have investigated the effects of monoaminergic drugs on cataplexy in narcoleptic canines when perfused locally via microdialysis probes in the amygdala, globus pallidus/putamen, basal forebrain, pontine reticular formation and ventral tegmental area of narcoleptic and control Doberman pinchers. Cataplexy was quantified using the Food-Elicited Cataplexy Test and analyzed by electroencephalogram, electroculogram and electromyogram. Local perfusion with the monoaminergic agonist quinpirole, 7-OH-DPAT and BHT-920, into the ventral tegmental area produced a dose-dependent increase in cataplexy without significantly reducing basal muscle tone. Perfusion with the antagonist raclopride in the same structure produced a moderate reduction in cataplexy. Local perfusion with quinpirole, 7-OH-DPAT and BHT-920 into the globus pallidus/putamen also produced an increase, while raclopride produced a decrease, in cataplexy in narcoleptic canines. In control animals, none of the above drugs produced cataplexy or muscle atonia when perfused into either the ventral tegmental area or the globus pallidus/putamen. Other monoaminergic drugs tested in these two brain areas; prazosin, yohimbine, amphetamine, SKF 38393 and SCH 23390 had no effects on cataplexy. Local perfusion with each of the above listed drugs had no effect on cataplexy in any of the other brain regions examined. These findings show that cataplexy may be regulated by D2/D3 dopaminergic receptors in the ventral tegmental area and perhaps the globus pallidus/ putamen. It is suggested that neurons in the mesolimbic dopamine system of narcoleptics are hypersensitive to dopaminergic autoreceptor agonists.

The 5-HT1A receptor antagonist (S)-UH-301 decreases dopamine release in the rat nucleus accumbens and striatum

In this study we employed in vivo microdialysis to examine the effects of the selective 5-HT1A receptor antagonist (S)-5-fluoro-8-hydroxy-2-(dipropylamino)tetralin [(S)-UH-301] on extracellular concentrations of dopamine (DA) and its metabolites dihydroxyphenylacetic acid (DOPAC) and homovanillic acid (HVA) in the nucleus accumbens (NAC) and dorsal striatum of awake freely moving rats. Systemic administration of (S)-UH-301 (1.25, 2.5, 5.0 mg/kg s.c.) dose-dependently decreased extracellular concentrations of DA, DOPAC and HVA in the NAC. (S)-UH-301 (2.5 mg/kg s.c.) also decreased DA, but not DOPAC and HVA, concentrations in the striatum. Infusion of low concentrations (1, 10 microM) of (S)-UH-301 into either the NAC or the striatum did not affect DA levels, while only the highest concentration (1,000 microM) significantly decreased DA levels in both areas. Similarly, infusion of the selective 5-HT1A receptor agonist (R)-8-hydroxy-2-(di-n-propylamino)tetralin [(R)-8-OH-DPAT] only in high concentrations (100, 1,000 microM) decreased DA levels in both regions. These data suggest that (S)-UH-301 decreases DA release both in the NAC and the striatum probably indirectly via its purported DA-D2/D3 receptor agonistic properties. However, the observed inhibitory effect of (S)-UH-301 on DA release in the studied brain regions may also be explained, at least partly, by a serotonergic influence on the DA systems, acting at 5-HT1A receptor sites located elsewhere in the brain.

The effect of B-HT 920, a dopamine D2 agonist, on bar-press feeding in the monkey

Although the dopamine (DA) system has been shown to regulate food intake, the function of the DA receptor subtypes on behavior still remains to be elucidated. In the present study, we examined the effect of B-HT 920, a selective agonist of DA D2 receptors that preferentially affect presynaptic autoreceptors, on both food consumption and execution of a high fixed-ratio bar-press task for food reward in monkeys. Two kinds of bar-press task were used: 1) a cue-triggered bar-press task during the first 40 trials, and 2) a self-paced bar-press task in which the monkeys freely performed bar-press trials until they were satiated. A SC injection of B-HT 920 (25 micrograms/kg) increased food consumption in the home cage. The same facilitatory effect on food consumption was also observed in the operant task condition. During the cue-triggered bar-press task, however, both the latency of the bar-press responses to a cue light and the time required to complete the bar-press trials were prolonged after the injection of B-HT 920. The results suggest that the activation of D2 autoreceptors suppresses the operant food acquisition behavior and increases food consumption through an inhibition of the satiety mechanism rather than an activation of any hunger-related drive.

Talipexole and adult Gilles de la Tourette's syndrome: double-blind, placebo-controlled clinical trial

Talipexole is a new dopamine autoagonist with putative preferential activity on presynaptic dopamine receptors. In a double-blind, placebo-controlled study, we tested the drug's safety and efficacy in 13 adult men with Gilles de la Tourette's syndrome. The drug was poorly tolerated because of clinically significant sedation and dizziness. Tics did not improve at tolerable doses. These findings suggest that talipexole has no role in the regular management of tic disorders.

Hypotensive and bradycardic effects of talipexole (B-HT 920) in anaesthetized rabbits are antagonized by metoclopramide but not by yohimbine

The interactions of talipexole (B-HT 920) and clonidine with selective alpha-adrenoceptor antagonists, yohimbine (alpha 2) and prazosin (alpha 1), as well as with dopamine receptor antagonists, metoclopramide (D2), domperidone (D2) and SCH23,390 (D1) were investigated in anaesthetized rabbits after i.v. administration. Both talipexole (0.03-0.1 mg/kg) and clonidine (0.01-0.03 mg/kg) dose-dependently induced hypotension and bradycardia. Talipexole had a shorter duration of action. The hypotensive effect of the alpha 2-adrenoceptor and D2 agonist talipexole (0.03 mg/kg) was antagonized by pretreatment with metoclopramide (3 mg/kg) or domperidone (0.3-3 mg/kg), but not with yohimbine (3 mg/kg), prazosin (0.1 mg/kg) or SCH23,390 (1 mg/kg). Its bradycardic effect was antagonized only by metoclopramide (3 mg/kg). The hypotensive and bradycardic effects of clonidine (0.03 mg/kg) were most effectively antagonized by yohimbine (0.3-3 mg/kg). These findings indicate that in anaesthetized rabbits after i.v. administration, talipexole may lower blood pressure by peripheral, and heart rate by central, dopamine D2 agonism.

Memory enhancement by post-training peripheral administration of low doses of dopamine agonists: possible autoreceptor effect

These experiments examined the effect of post-training injections of low doses of dopamine (DA) agonists on the acquisition of two 8-arm radial maze tasks. On a winstay simultaneous discrimination task, a light cue signaled the location of food in four randomly selected arms on each trial, and animals were required to visit each of the lit arms twice within a trial. Animals received one food trial per day and were injected immediately after training on Day 5. The direct DA receptor agonist, apomorphine (0.05 mg/kg), and the direct D2-DA receptor agonists, LY 177555 (quinpirole: 0.05, 0.1 mg/kg) and B-HT 920 (0.05 mg/kg), all improved acquisition of winstay radial maze behavior relative to saline-injected controls. On a win-shift task, rats were allowed to obtain food from four randomly selected maze arms, followed by a delay period in which they were removed from the maze. Animals were returned to the maze for a retention test in which only those arms that had not been visited prior to the delay contained food. After training on shorter delays, a delay of 18 h was imposed between the first four and second four choices, and DA agonists were injected immediately after the first four choices. Apomorphine, LY 171555, and B-HT 920 (all at 0.05 mg/kg), all improved win-shift retention relative to saline-injected controls. On both tasks, delaying the injections for 2 h post-training eliminated the memory-improving effects of all drugs. The results indicate that post-training administration of DA agonists at doses that may preferentially stimulate autoreceptors improves memory.

Hyperthermia induced by the dopamine D1 receptor agonist SK&F38393 in combination with the dopamine D2 receptor agonist talipexole in the rat

The present experiments were performed to investigate the effects of dopamine D1 receptor agonists given alone or in combination with dopamine D2 receptor agonists on body temperature in rats. The selective dopamine D1 receptor agonist, 1-phenyl-2,3,4,5-tetrahydro-(1H)-3-benzazepine-7,8-diol (SK&F38393), produced hyperthermia. However, the dopamine D2 receptor agonist, B-HT 920 (talipexole), and the newly synthesized dopamine D2 receptor agonist, (S)-2-amino-4,5,6,7-tetrahydro-6-propylamino-benzothiazole (SND 919), did not change the temperature. Interestingly, the SK&F38393-induced hyperthermia was enhanced by talipexole and SND 919. The drastic hyperthermia induced by combined administration of dopamine D1 and D2 receptor agonists was blocked by either the dopamine D1 receptor antagonist, SCH23390, or the dopamine D2 receptor antagonist, spiperone. On the other hand, treatment with prazosin, yohimbine, propranolol, scopolamine, or methysergide failed to affect the marked hyperthermia. The present results suggest that a functional link between dopamine D1 and D2 receptors may be synergistic in the regulation of body temperature and that concurrent stimulation of both dopamine D1 and D2 receptors thereby produces marked hyperthermia in the rat.

Modulatory role of D-1 and D-2 dopamine receptor subtypes in nociception in mice

The role of D-1 and D-2 dopamine (DA) receptors in nociception in naive as well as reserpinized mice and the modulation of the nociceptive action of morphine or naloxone by the selective D-1 and D-2 DA agonists, was investigated in mice. The D-2 DA agonists, B-HT 920 and bromocriptine produced an anti-nociceptive effect in naive mice and reversed the hyperalgesic effect of reserpine (2 mg/kg, 4 h prior) pre-treatment. The D-1 DA agonist, SKF 38393 (5 mg/kg) failed to alter the nociceptive responsiveness of naive and reserpinized mice. Apomorphine, a mixed D-1/D-2 DA agonist, produced significant analgesia in naive mice and also reversed reserpine-induced hyperalgesia. SKF 38393 (5 mg/kg) enhanced the anti-nociceptive effect of B-HT 920 (0.1 mg/kg) in naive and reserpine-pre-treated mice. The anti-nociceptive response of morphine (5 mg/kg) was enhanced by B-HT 920 while SKF 38393 reduced the same. Apomorphine (0.5 mg/kg) or the combination of B-HT 920 (0.1 mg/kg) and SKF 38393 (5 mg/kg) failed to enhance the anti-nociceptive effect of morphine. Reserpine (2 mg/kg, 4 h prior) pre-treatment significantly reduced the anti-nociceptive effect of morphine. Similarly, the hyperalgesic action of naloxone (20 mg/kg) was reversed by B-HT 920, bromocriptine and apomorphine but not by SKF 38393. The reversal of the hyperalgesic action of naloxone by B-HT 920 was blocked by pre-treatment with haloperidol (0.5 mg/kg) and sulpiride (100 mg/kg). SKF 38393 (5 mg/kg) failed to potentiate the reversal action of B-HT 920 against naloxone. These data suggest a predominant role of D-2 DA receptors in anti-nociception and the possibility of the existence of an interlink between the DAergic and opioid systems.

Effects of (R)-8-OH-DPAT and the enantiomers of UH-301 on motor activities in the rat: antagonism of (R)-8-OH-DPAT-induced effects

The effects of the enantiomers of 5-fluoro-8-hydroxy-2-(dipropylamino)tetralin, UH-301 and the potent 5-HT1A-receptor agonist (R)-8-hydroxy-2-(dipropylamino)tetralin, (R)-8-OH-DPAT, on locomotion, rearing and total activity were studied in rats. The experiments were performed as tests either of exploratory activity in non-habituated rats or of motor activity of rats habituated to the environment for 2 h before drug injection. (R)-8-OH-DPAT increased locomotion and total activity and decreased rearing in both conditions. (R)-UH-301 increased locomotion and slightly also total activity in habituated rats and decreased rearing in both conditions. (S)-UH-301 decreased locomotion and rearing in both conditions but only in doses of 10 mumol/kg and above. Lower doses of (S)-UH-301 (10 mumol/kg) antagonized (R)-8-OH-DPAT-induced increases of locomotion and total activity. As (S)-UH-301 decreased rearing, per se, it was not able to antagonize the (R)-8-OH-DPAT induced decrease. These results further support previous data that (S)-UH-301 is a 5-HT1A antagonist while (R)-UH-301 is a 5-HT1A agonist.

5 Centrally Acting Dopamine D2 Receptor Ligands: Agonists

The collected amount of research on D2 agonists is immense. The D2 research field has been, and still is, very dynamic. Despite this fact, only a few compounds have reached the clinic so far. At present there are clinical trials ongoing with partial D2 agonists possessing a range of intrinsic efficacies. Some of these agonists are tested for their potential effects in Parkinson's disease (high intrinsic efficacy), while others are tested for potential anti-psychotic effects (low intrinsic efficacy). An interesting possibility has arisen through the research on the synergism between D1 and D2 receptors. This could possibly be utilized in the alleviation of Parkinsonian symptomatology. The near future will show whether the compounds under evaluation hold promise for being new, valuable medicines for treating major diseases like Parkinson's disease and schizophrenia.

Dopamine receptors, controlling dopamine levels in rat adrenal glands-comparison with central dopaminergic autoreceptors

Previous work in this laboratory, as well as observations reported in the literature, indicate that the adrenal medulla contains dopamine (DA) receptors of the D-2 subtype, which among other things are capable of controlling the DA level in rat adrenal glands. To further characterize the DA receptors involved in the control of the adrenal DA level, the effects of 9 DA receptor agonists with various intrinsic activities were compared. After various periods of drug administration the rats were killed by decapitation and the DA content of the adrenal glands and the DOPAC content of the forebrain were measured by high-performance liquid chromatography with electrochemical detection. All the investigated DA receptors agonists caused an increase in adrenal DA level, although statistical significance was not reached in one case [(-)-HW 165]. Domperidone, a DA D-2 receptor antagonist which does not readily cross the blood brain barrier, blocked the DA-elevating effects of apomorphine, quinpirole, B-HT 920 and both enantiomers of 3-PPP. For the two ergolines terguride and SDZ 208-920 the blockade by domperidone was not complete, suggesting that their effects are mediated not only through DA, but also through other receptor systems. The dose of domperidone used (3 mg/kg) had but a marginal influence on brain DOPAC levels, supporting the almost exclusively peripheral effect of this agent. Our data indicate that the DA D-2 receptors which control the DA level in the adrenal medulla in rats, have characteristics similar to, though not identical with the autoreceptors in the forebrain.

Pramipexole, a dopamine D2 autoreceptor agonist, decreases the extracellular concentration of dopamine in vivo

Pramipexole (SND 919) is a dopamine D2 autoreceptor agonist which is structurally related to talipexole (B-HT 920), a potential antipsychotic agent. The aim of this study was to investigate the effects of pramipexole on the extracellular concentration of dopamine in vivo. Dopamine and its metabolites, 3,4-dihydrophenylacetic acid and homovanillic acid, were measured in the anterior striatum of freely moving rats by microdialysis and high-performance liquid chromatography with electrochemical detection. Pramipexole (30 and 100 micrograms/kg) caused long-lasting decreases in the extracellular concentrations of dopamine and its metabolites. Talipexole (30 micrograms/kg) produced similar effects. Sulpiride (5 mg/kg), a selective dopamine D2 antagonist, caused a transient increase in the concentration of dopamine and long-lasting increases in the concentrations of its metabolites; it also reversed the effects of pramipexole. SCH-23390 (100 micrograms/kg), a selective dopamine D1 receptor antagonist, caused a transient increase in the concentration of dopamine but did not affect the concentrations of the metabolites. SCH-23390 failed to reverse the effects of pramipexole. These results indicate that pramipexole reduces the extracellular concentrations of dopamine and its metabolites in vivo through a reversible interaction with the dopamine D2 receptor.

Dopamine receptor mediated hypothermic action of B-HT 920 in rats

The hypothermic action of the thiazoloazepine derivative B-HT 920, an alpha 2-adrenoceptor agonist has been investigated in rats. B-HT 920 (6-allyl-2-amino-5,6,7,8-tetrahydro-4H-thiazolo-(4,5-d)-azepine dihydrochloride) (0.25-1.0 mg kg-1 i.p.) induced a dose-dependent hypothermia. This peak effect was seen within 60-90 min and lasted up to 120 min. Its action was potentiated by the selective D1-dopamine agonist SKF 38393 and inhibited by the D2-antagonists haloperidol (1 mg kg-1) and sulpiride (100 mg kg-1). The hypothermic action of B-HT 920 was centrally mediated; i.c.v. administration of 10 micrograms produced a significant fall in rectal temperature which was sensitive to blockade by haloperidol. B-HT 920 also potentiated the hypothermic action of apomorphine (0.1 and 0.5 mg kg-1) in a haloperidol sensitive manner. Reserpine (5 mg kg-1 i.p.) pretreatment reduced the hypothermic response of B-HT 920 (0.5 mg kg-1) but sensitized the response due to the combination of B-HT 920 (0.5 mg kg-1) and apomorphine (0.1 mg kg-1). Neither the selective alpha 2-adrenoceptor antagonists, yohimbine (1 mg kg-1) or idazoxan (1 mg kg-1), the histamine antagonist mepyramine (10 mg kg-1) nor the 5-HT antagonist cyproheptadine (5 mg kg-1) inhibited B-HT 920-induced hypothermia. Similarly, the selective alpha 1-antagonist prazosin (1 mg kg-1) and the beta-antagonist propranolol (10 mg kg-1) failed to modify the hypothermic action of B-HT 920. These observations demonstrated hypothermia induced by B-HT 920 is mediated by postsynaptic D2-receptors and D1- and D2-receptor interplay is essential for the full expression of hypothermia in rats.

B-HT 920 is a full agonist at both pre- and postsynaptic D-2 dopamine receptors

Stimulation of presynaptic D-2 dopamine receptors by B-HT 920 or by apomorphine inhibited the synthesis of dopamine in the corpus striatum of gammabutyrolactone-treated mice to about the same extent. Stimulation of postsynaptic D-2 dopamine receptors by B-HT 920 given in combination with the D-1 receptor agonist SKF38393 enhanced the motor activity of reserpine-treated mice at least as much as observed following the combined D-1/D-2 receptor agonist apomorphine. Since B-HT 920 is as effective as apomorphine in these models, B-HT 920 appears to be a full agonist at both pre- and post-synaptic D-2 dopamine receptors.

Decrease of prolactin secretion via stimulation of pituitary dopamine D-2 receptors after application of talipexole and SND 919

The present experiments were performed to investigate the effects of talipexole (B-HT 920) and SND 919 on prolactin release from the anterior pituitary glands of rats both in vivo and in vitro. The basal serum prolactin levels were reduced dose dependently by s.c. administration of talipexole or SND 919 at doses of 5-100 micrograms/kg. Daily treatment with estradiol (35 micrograms/kg for 3 days) increased serum prolactin levels in male rats to levels 4-fold higher than those of non-primed rats. This increase was suppressed by administration of talipexole or SND 919. In vitro, the spontaneous prolactin release into perfusates from isolated anterior pituitary was inhibited by talipexole or SND 919 added at concentrations ranging from 10(-9) to 10(-6) M. This inhibitory effect of SND 919 was blocked by concurrent application of a dopamine D-2 receptor antagonist, YM-09151-2. The spontaneous prolactin release from the anterior pituitary isolated from estradiol-primed rats was 2-fold higher than that from non-primed rats. This increased release was also inhibited by application of either drug. The inhibitory effects of these drugs were greater in estradiol-primed rats than in non-primed rats when expressed as percent inhibition of control prolactin release. The results suggest that talipexole and SND 919 have a selective dopamine D-2 receptor agonistic property and are almost completely effective to counteract the enhancement of prolactin release induced by estrogens via stimulation of dopamine D-2 receptors in the anterior pituitary.

Effects of B-HT 920 on nigrostriatal and mesolimbic dopamine systems in normosensitive and supersensitive rats

1. B-HT 920, a D2 dopamine receptor agonist, was tested for its ability to exert presynaptic actions in normosensitive rats, and for possible postsynaptic actions in rats made 'supersensitive' to apomorphine. 2. In normosensitive rats, B-HT 920 (0.01-0.3 mg kg-1, i.p.) increased dopamine concentrations and lowered metabolite levels to a similar extent in all four terminal regions examined (medial prefrontal cortex, olfactory tubercle, nucleus accumbens, caudate-putamen). Analogous effects were seen for 5-hydroxytryptamine and its metabolite 5-hydroxyindoleacetic acid. 3. Rats which received bilateral 6-hydroxydopamine (6-OHDA) infusions into the caudate-putamen showed signs of postsynaptic dopamine receptor activation (stereotyped behaviour) in response to B-HT 920 (0.1 and 1.0 mg kg-1, i.p.) and to apomorphine (0.2 mg kg-1, s.c.). Similarly, B-HT 920 (0.1 mg kg-1) induced contralateral circling in rats that had received unilateral 6-OHDA infusions into the medial forebrain bundle; the rate of circling increased gradually over several weeks. 4. In contrast, bilateral 6-OHDA infusions into the nucleus accumbens resulted in a supersensitive (locomotor stimulant) response to a low dose of apomorphine (0.1 mg kg-1, s.c.), but not to B-HT 920 (0.01 and 0.1 mg kg-1). 5. In intact rats, withdrawal of chronic haloperidol treatment induced behavioural supersensitivity to apomorphine but not to B-HT 920.

Potentiation of yawning responses to the dopamine receptor agonists B-HT 920 and SND 919 by pindolol in the rat

Subcutaneous injection of B-HT 920, a dopamine D2-receptor agonist, in doses ranging from 5 to 100 micrograms/kg, induced yawning behavior in rats. Yawning was also elicited by low doses (25-500 micrograms/kg sc) of SND 919, a newly synthesized dopamine receptor agonist. The yawning evoked by B-HT 920 or SND 919 was increased by the beta-adrenoceptor antagonist pindolol (20 mg/kg ip) which alone did not induce yawning. Stereotyped behavior did not appear after B-HT 920 or SND 919 given alone or in combination with pindolol. The results suggest that SND 919 as well as B-HT 920 has stimulatory activity at dopamine D2-receptors, and that pindolol may exert its enhancement of the yawning response to dopamine receptor agonists via blockade of beta-adrenoceptors.

Possible involvement of differing classes of dopamine D-2 receptors in yawning and stereotypy in rats

The present experiments were performed to investigate differences in the properties of the dopamine D-2 receptors related to yawning and stereotypy. Subcutaneous injection of talipexole (B-HT 920) (10-250 micrograms/kg) or SND 919 ((S)-2-amino-4,5,6,7-tetrahydro-6-propylamino-benzothiazole) (25-500 micrograms/kg) evoked yawning behavior with bell-shaped responses. However, SK&F 38393 (1-phenyl-2,3,4,5-tetrahydro-(1H)-3-benzazepine-7,8-diol) (0.5-10 mg/kg SC) did not elicit yawning and decreased yawning responses to low doses of talipexole (25 micrograms/kg SC) or SND 919 (100 micrograms/kg SC). These low but effective doses for inducing yawning of talipexole or SND 919 in combination with SK&F 38393 (0.5-10 mg/kg SC) did not elicit stereotypy. In contrast, yawning behavior was not produced after very high doses of talipexole (500 micrograms/kg SC) or SND 919 (1000 micrograms/kg SC) given alone or in combination with SK&F 38393 (0.5-10 mg/kg SC). These extremely high doses of talipexole or SND 919 evoked slight stereotypy, which was enhanced by the combined treatment with SK&F 38393. The present results suggest that the dopamine D-2 receptors related to yawning are more sensitive to dopamine receptor agonists than those related to stereotypy, and that concurrent stimulation of postsynaptic dopamine D-1 receptors with D-2 receptors reduces the incidence of yawning but enhances that of stereotypy.

The alpha 2-adrenoceptor antagonists idazoxan and yohimbine can unmask the postsynaptic dopamine agonist effects of B-HT 920

Administration of B-HT 920 alone produced weak stereotypy in a small percentage (4-8%) of rats. In contrast, after combined administration of idazoxane and B-HT 920, stereotyped behaviors (sniffing and licking) were consistently produced in all rats. The appearance of stereotypy after combined treatment with yohimbine and B-HT 920 was inversely related to the dose of yohimbine. These data suggest that the alpha 2-adrenoceptor agonist effects of B-HT 920 can mask it's postsynaptic dopamine agonist effects.

Locomotor behaviour of selective dopamine agonists in mice: is endogenous dopamine the only catecholamine involved?

The purpose of the study was to determine the effect alone and in combination of the selective dopamine (DA) agonists SKF 38393 (D1-) and B-HT 920 (D2-) on the locomotor activity of reserpine pretreated mice (5 mg kg-1 i.p.). After 4 h, reserpine-B-HT 920 (up to 20 mg kg-1 s.c.) did not induce locomotor activity whereas SKF 38393 was markedly effective at high doses (greater than or equal to 30 mg kg-1 s.c.). In contrast, at 24 h, reserpine-B-HT 920 (0.2-6 mg kg-1 s.c.) elicited considerable locomotor activity and SKF 38393 (1-100 mg kg-1 s.c.) was effective at lower doses when compared with the corresponding 4 h reserpine experiments. When, however, these animals additionally received alpha-methyl-p-tyrosine (alpha MPT; 300 mg kg-1 i.p., at 4 h before the DA agonists) neither B-HT 920 (0.2-20 mg kg-1 s.c.) nor SKF 38393 (1-100 mg kg-1 s.c.) had an effect of their own. When B-HT 920 was tested in the presence of a fixed-dose of SKF 38393 (10 or 3 mg kg-1 s.c., combination experiments) B-HT 920 (0.6-20 mg kg-1 s.c.) induced considerable locomotor activity at 4 h post reserpine. At 24 h post reserpine the dose-response curve of B-HT 920 (0.06-20 20 mg kg-1 s.c.) was shifted to the left and the maximum effect was greatly increased. When additional alpha MPT was given, the dose response curve was the same but the maximum effect was markedly reduced.(ABSTRACT TRUNCATED AT 250 WORDS)

Dopamine D-2 receptor agonist-induced behavioural depression: critical dependence upon postsynaptic dopamine D-1 function. A behavioural and biochemical study

The dopamine (DA) D-2 receptor agonists quinpirole (threshold dose, 0.01 mg/kg IP), pergolide (0.025 mg/kg), B-HT 920 (0.003 mg/kg) and (-)-3-PPP (4 mg/kg) produced dose-dependent locomotor depression (immobility) in mice as assessed by a subjective scoring system, with the immobility being characterized by a frozen posture. The animals were still but had their eyes open. The immobility was accompanied by reductions in sniffing, rearing and grooming. The depression (and the associated reduction in the various behaviours) produced by quinpirole (0.1 mg/kg), pergolide (0.1 mg/kg) and B-HT 920 (0.1 mg/kg) was substantially (but not always completely) reversed by the selective D-1 receptor agonist SKF38393 (up to 12 mg/kg) and the non-selective D-1 receptor agonist CY208243 (up to 3 mg/kg). The immobility induced by (-)-3-PPP (16 mg/kg) was also reversed by CY208243 and SKF38393, but the reversal was due to an increase in grooming behaviour in mice challenged with the D-1 receptor agonists, whether or not the animals had also received (-)-3-PPP. There was no reversal of the depression of rearing or sniffing. In contrast, CY208243 and SKF38393 also antagonized the immobility induced by B-HT 920, but the reversal was accompanied by at least partial reversals of the depression of sniffing, rearing and grooming. The reversal of quinpirole-induced immobility by SKF38393 and CY208243 was antagonized by SCH23390 (0.1 mg/kg). The selective D-2 receptor antagonist raclopride (0.025 to 0.4 mg/kg) could not reverse quinpirole-induced immobility. High doses of either raclopride (0.4 mg/kg) or SCH23390 (greater than 0.1 mg/kg) significantly increased immobility. Although raclopride itself (0.2 mg/kg) produced a substantial increase in DOPAC and homovanillic acid (HVA) levels in the striatum, it did not antagonize the autoreceptor mediated effects of quinpirole (0.1 mg/kg) in reducing the striatal dihydroxyphenylacetic acid (DOPAC) to DA ratio. However, the same dose of raclopride was partly effective in reducing the effects of lower doses of quinpirole (0.01 and 0.03 mg/kg) on the striatal DOPAC to DA ratio. Raclopride (0.2 mg/kg) also partially but significantly reduced the locomotor stimulant effects of d-amphetamine in reserpinized mice. Biochemical analyses in the striata indicated that CY208243 slightly retarded DA turnover (as assessed by the DOPAC/DA ratio). SKF38393 itself also slightly reduced DA turnover. In automated activity cages, using mice depleted of DA with reserpine and alpha-methyltyrosine, all the D-2 receptor agonists tested, in combination with SKF38393, produced an increase in activity.(ABSTRACT TRUNCATED AT 400 WORDS)

Stimulation of postsynaptic D2- dopamine receptors by B-HT 958 is revealed by co-treatment with the D1- receptor agonist SKF 38393

The motor activity of reserpine-treated mice was used to study effects of B-HT 958 (2-amino-6-(p-chlorobenzyl)-4H-5,6,7,8-tetrahydrothiazolo-[5,4-d]- azepine) on postsynaptic dopamine and noradrenaline receptors. The motor activity was only slightly stimulated by B-HT 958 or by the D1- receptor agonist SKF 38393 but it was markedly increased by the two drugs given in combination. The effect of B-HT 958 peaked earlier following low rather than high doses. The enhanced motor activity was inhibited by the D2- receptor antagonist sulpiride or the D1- receptor antagonist SCH 23390, indicating that it was caused by stimulation of both receptor types. The results suggest that B-HT 958 stimulates postsynaptic D2- receptors in addition to D2- autoreceptors and that its blockade of postsynaptic alpha 2-adrenoceptors is of no importance for the motor activity.

Central regulation of adrenal tyrosine hydroxylase: interaction between dopamine and GABA systems

It has been previously demonstrated that nigrostriatal dopaminergic fibres participate in the neural regulation of the activity of adrenal tyrosine hydroxylase, specifically in its induction. To determine whether activation or inhibition of these fibres is responsible for this induction, the role of presynaptic dopamine receptors was investigated. Apomorphine (0.2 mg/kg), (+)3-PPP (10 mg/kg) and BHT 920 (1-3 mg/kg), drugs that are reported to bind to presynaptic dopamine receptors and thereby inhibit the release of that neurotransmitter, caused significant increases in the activity of the enzyme. As a central GABA (gamma-aminobutyric acid) system is believed to exert inhibitory control over the release of dopamine, GABA agonists were also tested for their effects. Muscimol (3 mg/kg), gamma-hydroxybutyrate (500 mg/kg) and HA-966 (150 mg/kg) produced significant induction of the adrenal enzyme; this induction was not blocked by dopamine postsynaptic receptor antagonists. After intraventricular administration (5 micrograms/rat) in normal animals, HA-966 produced significant induction of tyrosine hydroxylase. Its systemic administration did not induce the enzyme in animals with the adrenal denervated. When administered together at submaximal doses, HA-966 and BHT 920 produced an additive effect in the induction of adrenal tyrosine hydroxylase.

Potentiation by serotonergic inhibition of yawning induced by dopamine receptor agonists in rats

Low doses of the dopamine D2-receptor agonist, B-HT 920 (25 micrograms/kg, SC), and the dopamine D1/D2-receptor agonists, apomorphine (50 micrograms/kg, SC) and piribedil (1 mg/kg, SC), evoked yawning. However, the dopamine D1-receptor agonist, SK&F 38393 (2 mg/kg, SC), failed to induce yawning. The yawning responses induced by B-HT 920, apomorphine or piribedil were markedly increased without eliciting stereotypy by pretreatment with reserpine (5 mg/kg, IP, 24 hr). These yawning responses were also enhanced by p-chlorophenylalanine (PCPA) (300 mg/kg, IP, 72 hr), but not by alpha-methyl-p-tyrosine (300 mg/kg, IP, 6 hr). The yawning induced by B-HT 920 given alone and in combination with reserpine or PCPA was inhibited by spiperone (0.5 mg/kg, IP) or scopolamine (0.5 mg/kg, IP), but not by SCH 23390 (0.5 mg/kg, IP). The present results suggest that yawning is evoked by stimulation of dopamine D2-receptors having a high affinity and consequent muscarinic activation, and that the yawning induced by dopamine receptor agonists is potentiated by decreases in serotonergic neuron activity.

Stimulation of D1 dopamine receptors reveals direct effects of the preferential dopamine autoreceptor agonist B-HT 920 on postsynaptic dopamine receptors

Possible postsynaptic effects of the preferential dopamine autoreceptor agonist B-HT 920 were studied by means of the mouse motor activity. In reserpine-treated mice, B-HT 920 did not cause any motor activity by itself but it markedly potentiated the slight stimulating effect of the D1 dopamine agonist SKF 38 393. The effect was blocked by either the D2-receptor antagonist sulpiride or the D1-receptor antagonist SCH 23 390, indicating that motor activity is dependent on simultaneous activation of both dopamine receptor types. The hyperactivity produced by 0.1 mg kg-1 B-HT 920 in combination with SKF 38 393 in reserpine-treated mice was at least as great as that following a maximal dose of apomorphine, indicating that B-HT 920 is a full agonist at postsynaptic D2 receptors. The effect of 0.1 mg kg-1 B-HT 920 peaked earlier than those of 1 mg kg-1 and particularly, 10 mg kg-1 suggesting additional effects of the later two doses. B-HT 920 stimulates dopamine autoreceptors almost maximally following 0.1 or 1 mg kg-1 but only the latter dose (with or without SKF 38 393) caused hyperactivity of mice not treated with reserpine. This finding indicates that the postsynaptic D2 receptors are less sensitive to B-HT 920 than the D2 dopamine autoreceptors.

The dopamine autoreceptor agonist, B-HT 920, preferentially reduces brain dopamine release in vivo: biochemical indices of brain dopamine, noradrenaline and serotonin in ventriculocisternal perfusates in the cat

B-HT 920 (6-allyl-2-amino-5,6,7,8-tetrahydro-4H-thiazolo[4,5-d]azepine), a candidate for selective dopamine (DA) autoreceptor agonist activity, was tested for its interactions with biochemical parameters of brain dopaminergic, noradrenergic and serotoninergic systems as measured in ventriculocisternal perfusates of chloralose-anaesthetized cats. DA, 3,4-dihydroxyphenylacetic acid (DOPAC), homovanillic acid (HVA), noradrenaline (NA) and 5-hydroxyindolic acid (5-HIAA) were measured in samples of 30 min collection periods by high-pressure liquid chromatography with electrochemical detection. B-HT 920, in the dose range of 0.03-1 mg/kg i.v., promptly inhibited the efflux of DA and DOPAC in a dose-dependent manner. The 1 mg/kg dose of B-HT 920 reduced the DA levels below 25% of control levels for the whole length of the experiments. The HVA levels were reduced less and in a protracted manner. Only the highest dose of B-HT 920 tested (1 mg/kg) had a significant effect on the level of NA (marked, prompt reduction) and 5-HIAA (delayed, moderate reduction), reflecting its well known alpha 2-adrenoceptor agonist property. The effects of B-HT 920 on the dopaminergic indices were DA receptor-mediated as they were reversed by a low dose (0.05 mg/kg i.v.) of haloperidol. In contrast, the alpha 2-adrenoceptor blocking drug, idazoxan, 4 mg/kg i.v., while it reversed the NA and 5-HIAA reductions did not modify the effect of B-HT 920 on DA, DOPAC and HVA. Thus B-HT 920, in the dose range between 0.03-0.1 mg/kg, selectively affected brain dopaminergic parameters. Our experiments demonstrated that B-HT 920 causes an effective, long lasting and selective suppression of extracellular brain DA levels in vivo. B-HT 920 represents a promising compound for clinical use in pathological conditions known to be ameliorated by a reduction of brain DA activity, such as Huntington's disease, mania and schizophrenia.

Dopamine-mediated behaviour following chronic treatment with B-HT 920

Following subchronic (5-day) dosing with B-HT 920 (2-amino-6-allyl-5,6,7,8-tetrahydro-4H-thiazolo(4,5-d)azepine (1 mg kg-1 day-1 i.p.) in rats there was a significant increase in both apomorphine-induced motor activity and stereotypy. On continued B-HT 920 treatment, however, the enhancement of apomorphine motor activity faded into insignificance but the increase in stereotypy persisted beyond 15 days. The results are discussed in terms of dopamine autoreceptor tolerance, postsynaptic D2 supersensitivity and possible differential effects in different brain loci on the above two receptor sub-classes.

Preferential effects of caffeine on limbic and cortical dopamine systems

In this study, we investigated the effects of acute caffeine administration on the activity of midbrain dopamine neurons. Caffeine significantly depressed the firing rates of dopamine neurons in the ventral tegmental area (A10 group), but had no significant effect on the firing rates of dopamine neurons in the substantia nigra zona compacta (A9 group). The action of caffeine in A10 was completely blocked by pretreatment with the adenosine agonist L-phenyl-isopropyl-adenosine (L-PIA), confirming numerous lines of evidence that caffeine and other xanthines act as competitive antagonists at adenosine receptors. The dopamine antagonist haloperidol also antagonized the effects of caffeine. This finding is consistent with a mechanism of caffeine-induced depression of dopamine neuron activity involving dopamine release, similar to that observed during amphetamine administration. Finally, the benzodiazepine diazepam also antagonized the dopaminergic effects of caffeine. It appears that, in the rat, caffeine administration inhibits mesolimbic and mesocortical projecting dopamine neurons, but has no effect on dopamine neurons that project to the striatum.

Relative dopamine D1 and D2 receptor affinity and efficacy determine whether dopamine agonists induce hyperactivity or oral stereotypy in rats

The effects of a range of dopamine (DA) agonists on stereotyped behaviour in rats were analysed and compared both with the affinity of the compounds for D1 and D2 receptor binding sites in vitro and their ability to stimulate the adenylate cyclase activity in rat striatal homogenates. Full and partial agonists at the D1 receptor coupled to adenylate cyclase do not induce sterotypies when given alone, whereas full D2 agonists (e.g. quinpirole) induce hyperactivity but not oral sterotypies. Partial D2 agonists (e.g. (-)-3-PPP) only induce sedation. Mixed D1/D2 agonists (e.g. apomorphine) induce both hyperactivity and oral stereotypies. Maximum stereotypies were induced by combination of SK & F 38393 and a series of D2 agonists, including full agonists and the partial D2 agonist B-HT 920, whereas partial agonists with low intrinsic activity (e.g. (-)-3-PPP, EMD 23448) did not induce stereotypies when given together with SK & F 38393. However, these partial agonists reduced the maximum effect of apomorphine, whereas the full agonists (e.g. quinpirole, (-)-NPA) and B-HT 920 had no apomorphine antagonistic activity. The mixed D1/D2 agonists apomorphine and N,N-dipropyl-5,6-ADTN were only weakly influenced by SK & F 38393, or not at all. D1 agonists with central effects, including SK & F 38393, SK & F 81297 (with relatively high efficacies), and the partial agonist SK & F 75670 with low efficacy, changed the hyperactivity induced by quinpirole into maximum oral stereotypy, whereas the peripheral D1 agonist fenoldopam had no such effect. Inhibition of DA and NA synthesis with alpha-methyl-p-tyrosine depleted striatal DA levels by 72 per cent and antagonized the hyperactivity induced by the D2 agonists quinpirole and (-)-NPA, but not that of apomorphine. Combination of SK & F 38393 and quinpirole induced maximum stereotypy in DA-depleted animals. These results suggest that D1 receptor tonus is a necessary prerequisite for the expression of a DA agonist's effect. The hyperactivity induced by full D2 agonists appears to be mediated by D1 tonus provided by endogenous DA activity, but stronger D1 stimulation is necessary to induce oral stereotypy. A high degree of D1 receptor activation increases the ability of partial D2 agonists to induce hyperactivity or oral stereotypies since treatment with both SK & F 38393 and B-HT 920 had marked effects while B-HT 920 was ineffective.(ABSTRACT TRUNCATED AT 400 WORDS)

Depression of rat brain tryptophan hydroxylase activity following the acute administration of methylenedioxymethamphetamine

The psychotomimetic agent, methylenedioxymethamphetamine, produced a rapid, persistent and dose-dependent reduction in cortical tryptophan hydroxylase activity when administered acutely to rats. This effect did not occur in vitro and did not require N-demethylase activity in the whole animal. Kinetic analysis revealed the loss of enzyme activity to be due to an alteration in Vmax with no change in the affinity of the enzyme for either its cofactor or substrate. Coadministration of the serotonin (5-HT) uptake inhibitor, citalopram, only partially antagonized the loss of tryptophan hydroxylase activity 3 hr after methylenedioxymethamphetamine, but completely prevented the loss of cortical 5-HT. Recovery of enzyme activity did occur by 1 week if the neurotoxic effect of methylenedioxymethamphetamine was blocked by fluoxetine. The effect of methylenedioxymethamphetamine on 5-HT synthesis was not affected by pretreatment with alpha-methyl-p-tyrosine, reserpine or yohimbine. Ketanserine and methiothepin, 5-HT receptor antagonists, did partially block the methylenedioxymethamphetamine-induced loss of tryptophan hydroxylase activity, suggesting a possible role for neurotransmitter release in the acute effects of the drug on enzyme activity.

Synergistic interaction between dopamine D-1 and D-2 receptor agonists: circling behaviour of rats with hemitransection

Circling behaviour induced by dopamine (DA) agonists with different D-1/D-2 receptor selectivity was studied in rats with hemitransection at a level caudal to the striatum. The mixed D-1/D-2 agonist apomorphine induced ipsilateral circling behaviour after administration of doses similar to those that induced stereotyped behaviour in unlesioned rats. The effect of apomorphine was not influenced by co-treatment with SK & F 38393 or quinpirole, indicating that apomorphine induces a comparable D-1 and D-2 receptor stimulation in vivo also. Three selective D-1 agonists, SK & F 38393, SK & F 75670 and Lu 24-040 had no effects alone, while the preferential D-2 agonists quinpirole, pergolide and (-)-N-propylnorapomorphine induced ipsilateral circling of weaker intensity than did apomorphine. After co-treatment with SK & F 38393 the effects of these compounds were markedly increased. Combination of SK & F 38393, SK & F 75670 or Lu 24-040 with quinpirole induced circling with intensities similar to those seen after apomorphine. Pretreatment with the D-1 antagonist SCH 23390 or the D-2 antagonist YM 09151-2 completely antagonized the ipsilateral circling induced by either apomorphine or quinpirole + SK & F 38393. A range of partial (autoreceptor) D-2 agonists, i.e. (-)-3-PPP, (+)-3-phenethyl-PP, terguride, EMD 23448 and B-HT 920 were all ineffective as was the alpha 2-adrenoceptor agonist clonidine. However, B-HT 920 induced strong ipsilateral circling after combination with SK & F 38393, whereas (-)-3-PPP was ineffective.(ABSTRACT TRUNCATED AT 250 WORDS)

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.

Inhibitory role of D-1 dopamine receptors for the jerks induced by B-HT 920 in rats

Jerks of the head and upper trunk produced by the dopamine agonist B-HT 920 in reserpine-treated rats were abolished by the D-1 dopamine receptor agonist SKF 38393. The combined treatment with SKF 38393 and B-HT 920 instead resulted in stereotypies and locomotion. B-HT 920 also caused jerks when given after the D-1 receptor antagonist SCH 23390 to rats not pretreated with reserpine. The results indicate that B-HT 920 induces jerks by stimulation of postsynaptic D-2-like dopamine receptors provided that the D-1 receptors are not activated.

Update on dopamine agonists in Parkinson's disease: "beyond bromocriptine"

Since the initiation of bromocriptine therapy for Parkinson's disease several newer dopamine agonists have been developed. Pergolide has reached the stage of Phase 3 clinical trials and will probably be available for general use sometime in the foreseeable future. Lisuride shows most promise in its parenteral form for infusion therapy of patients with severe fluctuations. Mesulergine, another ergot-derivative and ciladopa, a new non-ergot agonist, have been withdrawn from further clinical use due to tumorogenesis in rats. It is questionable how applicable these findings are to the use of the drugs in elderly humans with parkinsonism. Recently a small number of drugs have been found to have postsynaptic dopamine agonist properties only in the setting of denervated supersensitive dopamine receptors. These agents may be particularly effective in the early treatment of patients with Parkinson's disease. This paper will review a number of the dopamine agonists which have been developed since the introduction of bromocriptine therapy. Several of these have shown beneficial effects in early clinical trials while others show promise in preclinical studies of animal models of parkinsonism.