Computational study on new natural compound agonists of dopamine receptor.

Dopamine receptor, a polypeptide chain composed of 7 hydrophobic transmembrane regions, is a new and vital drug target, especially Dopamine receptor 2(D2). Targeting dopamine receptors, Dopamine receptor agonists are a class of drugs similar in function and structure to dopamine and can directly act on dopamine receptors and activate it. Clinically, Dopamine receptor agonist drugs have achieved significant therapeutic effects on prolactinoma and Parkinson's Disease. In the study, we virtually screened a series of potential effective agonists of Dopamine receptor by computer techniques. Firstly, we used the Molecular Docking (LibDock) step to screen out some molecules that can dock well with the protein. Then, analysis of toxicity prediction and ADME (adsorption, distribution, metabolism and excretion) were carried out. More precise molecular docking (CDOCKER) and 3-Dimensional Quantitative Structure-Activity Relationship Modeling Study(3D-QSAR) pharmacophore generation were implemented to research and explore these compounds' binding mechanism with Dopamine receptor. Last but not least, to assess compound's binding stabilities, we carried out a molecular dynamic analysis. As the results show, two compounds (ZINC000008860530 and ZINC000004096987) from the small molecule database (ZINC database) were potential effective agonists of Dopamine receptor. These two compounds can combine with Dopamine receptor with higher affinity and proved to be no toxic. The cell experiment showed that two compounds could inhibit the proliferation and PRL secretion of MMQ cells (pituitary tumor cells). Thus, this study provided valuable information about Dopamine receptor agonist-based drug discovery. So, this study will benefit patients with prolactinoma and Parkinson's disease a lot.

Uterine adenocarcinoma in the rat induced by afidopyropen. An analysis of the lesion's induction, progression and its relevance to humans.

Afidopyropen is a novel insecticide that acts as a TRPV channel modulator in chordotonal organs of target insects. In two carcinogenicity studies with Fischer rats, an increased incidence of uterine adenocarcinomas was observed at 1000 and 3000 ppm. This finding prompted an investigation of the mechanism of the tumor formation as well as the relevance of this mechanism to humans. The mechanistic work took parallel paths: one path investigated the pharmacokinetic properties of the test substance at the doses where the tumors were found; while the second path examined the key mechanistic events that culminated in uterine adenocarcinomas. The results of the investigation indicated that the tumors only occurred at doses where excretion of test substance was saturated - indicating that homeostatic biological and/or physiological processes were overwhelmed. At the doses where these processes were overwhelmed, the test substance acted through a mechanism of dopamine agonism, triggering a cascade key events that resulted in uterine adenocarcinomas. An analysis of these mechanisms observed in rat showed that they are both quantitatively (pharmacokinetic mechanism) and qualitatively (dopamine agonism mechanism) not relevant to humans. Therefore the uterine adenocarcinomas observed in the rat associated with high doses of Afidopyropen are not expected to pose a carcinogenic risk to humans.

Effects of Environmental Enrichment on Nicotine Sensitization in Rats Neonatally Treated with Quinpirole: Analyses of Glial Cell Line-Derived Neurotrophic Factor and Implications towards Schizophrenia.

The current study analyzed the effects of environmental enrichment versus isolation housing on the behavioral sensitization to nicotine in the neonatal quinpirole (NQ; dopamine D2-like agonist) model of dopamine D2 receptor supersensitivity, a rodent model of schizophrenia. NQ treatment in rats increases dopamine D2 receptor sensitivity throughout the animal's lifetime, consistent with schizophrenia. Animals were administered NQ (1 mg/kg) or saline (NS) from postnatal day (P)1 to P21, weaned, and immediately placed into enriched housing or isolated in wire cages throughout the experiment. Rats were behaviorally sensitized to nicotine (0.5 mg/kg base) or saline every consecutive day from P38 to P45, and brain tissue was harvested at P46. Results revealed that neither housing condition reduced nicotine sensitization in NQ rats, whereas enrichment reduced sensitization to nicotine in NS-treated animals. The nucleus accumbens (NAcc) was analyzed for glial cell line-derived neurotrophic factor (GDNF), a neurotrophin important in dopamine plasticity. Results were complex, and revealed that NAcc GDNF was increased in animals given nicotine, regardless of housing condition. Further, enrichment increased GDNF in NQ rats regardless of adolescent drug treatment and in NS-treated rats given nicotine, but did not increase GDNF in NS-treated controls compared to the isolated housing condition. This study demonstrates that environmental experience has a prominent impact on the behavioral and the neural plasticity NAcc response to nicotine in adolescence.

Comparing the dopaminergic neurotoxic effects of benzylpiperazine and benzoylpiperazine.

Benzylpiperazine has been designated as Schedule I substance under the Controlled Substances Act by Drug Enforcement Administration. Benzylpiperazine is a piperazine derivative, elevates both dopamine and serotonin extracellular levels producing stimulatory and hallucinogenic effects, respectively, similar to methylenedioxymethamphetamine (MDMA). However, the comparative neurotoxic effects of Piperazine derivatives (benzylpiperazine and benzoylpiperazine) have not been elucidated. Here, piperazine derivatives (benzylpiperazine and benzoylpiperazine) were synthesized in our lab and the mechanisms of cellular-based neurotoxicity were elucidated in a dopaminergic human neuroblastoma cell line (SH-SY5Y). We evaluated the in vitro effects of benzylpiperazine and benzoylpiperazine on the generation of reactive oxygen species, lipid peroxidation, mitochondrial complex-I activity, catalase activity, superoxide dismutase activity, glutathione content, Bax, caspase-3, Bcl-2 and tyrosine hydroxylase expression. Benzylpiperazine and benzoylpiperazine induced oxidative stress, inhibited mitochondrial functions and stimulated apoptosis. This study provides a germinal assessment of the neurotoxic mechanisms induced by piperazine derivatives that lead to neuronal cell death.

Acute pergolide exposure stiffens engineered valve interstitial cell tissues and reduces contractility in vitro.

Medications based on ergoline-derived dopamine and serotonin agonists are associated with off-target toxicities that include valvular heart disease (VHD). Reports of drug-induced VHD resulted in the withdrawal of appetite suppressants containing fenfluramine and phentermine from the US market in 1997 and pergolide, a Parkinson's disease medication, in 2007. Recent evidence suggests that serotonin receptor activity affected by these medications modulates cardiac valve interstitial cell activation and subsequent valvular remodeling, which can lead to cardiac valve fibrosis and dysfunction similar to that seen in carcinoid heart disease. Failure to identify these risks prior to market and continued use of similar drugs reaffirm the need to improve preclinical evaluation of drug-induced VHD. Here, we present two complimentary assays to measure stiffness and contractile stresses generated by engineered valvular tissues in vitro. As a case study, we measured the effects of acute (24 h) pergolide exposure to engineered porcine aortic valve interstitial cell (AVIC) tissues. Pergolide exposure led to increased tissue stiffness, but it decreased both basal and active contractile tone stresses generated by AVIC tissues. Pergolide exposure also disrupted AVIC tissue organization (i.e., tissue anisotropy), suggesting that the mechanical properties and contractile functionality of these tissues are governed by their ability to maintain their structure. We expect further use of these assays to identify off-target drug effects that alter the phenotypic balance of AVICs, disrupt their ability to maintain mechanical homeostasis, and lead to VHD.

The glucagon-like peptide 1 (GLP-1) receptor agonist exendin-4 reduces cocaine self-administration in mice.

Glucagon-like peptide 1 (GLP-1) analogues are used for the treatment of type 2 diabetes. The ability of the GLP-1 system to decrease food intake in rodents has been well described and parallels results from clinical trials. GLP-1 receptors are expressed in the brain, including within the ventral tegmental area (VTA) and the nucleus accumbens (NAc). Dopaminergic neurons in the VTA project to the NAc, and these neurons play a pivotal role in the rewarding effects of drugs of abuse. Based on the anatomical distribution of GLP-1 receptors in the brain and the well-established effects of GLP-1 on food reward, we decided to investigate the effect of the GLP-1 analogue exendin-4 on cocaine- and dopamine D1-receptor agonist-induced hyperlocomotion, on acute and chronic cocaine self-administration, on cocaine-induced striatal dopamine release in mice and on cocaine-induced c-fos activation. Here, we report that GLP-1 receptor stimulation reduces acute and chronic cocaine self-administration and attenuates cocaine-induced hyperlocomotion. In addition, we show that peripheral administration of exendin-4 reduces cocaine-induced elevation of striatal dopamine levels and striatal c-fos expression implicating central GLP-1 receptors in these responses. The present results demonstrate that the GLP-1 system modulates cocaine's effects on behavior and dopamine homeostasis, indicating that the GLP-1 receptor may be a novel target for the pharmacological treatment of drug addiction.

Three-month subchronic intramuscular toxicity study of rotigotine-loaded microspheres in Cynomolgus monkeys.

Continuous dopaminergic stimulation (CDS) is an important drug development strategy in the treatment of Parkinson's disease (PD). Rotigotine is a non-ergoline D(3)/D(2)/D(1) dopamine receptor agonist for treating PD. As a new treatment option for CDS, rotigotine-loaded microspheres (RoMS), long-acting sustained-release microspheres with poly(lactide-co-glycolide) as drug carrier, are now being evaluated in clinical trial. In the present study, the subchronic toxicity in Cynomolgus monkeys has been characterized via intramuscular administration with RoMS at 0, 10, 40 and 160 mg/kg, weekly for 3 months with a 1-month recovery period. The NOAEL was 10 mg/kg/week. One male at 160 mg/kg died from an extensive pulmonary embolism. The major toxicological effects were associated with dopamine agonist-related pharmacodynamic properties of rotigotine (e.g., hyperactivity and stereotype, decreased serum prolactin level) and foreign body removal reaction induced by poly(lactide-co-glycolide) and carboxymethycellulose sodium (e.g., increased mononuclear cells and neutrophils, thymus atrophy and vacuolar degeneration of adrenal cortex, foreign body granuloma with foam cells accumulation at injection sites and foam cells accumulation in spleen and multiple lymph sinuses). At the end of recovery period, above findings recovered to a normal level or to a certain degree except vacuolar degeneration of adrenal gland. RoMS has exhibited high safety on monkeys.

Estrogen treatment blocks 8-hydroxy-2-dipropylaminotetralin- and apomorphine-induced disruptions of prepulse inhibition: involvement of dopamine D1 or D2 or serotonin 5-HT1A, 5-HT2A, or 5-HT7 receptors.

Prepulse inhibition (PPI) is a measure of sensorimotor gating and an endophenotype of schizophrenia. We have shown previously in rats that estrogen treatment prevents disruption of PPI by the 5-HT(1A)/5-HT(7) receptor agonist 8-hydroxy-2-dipropylaminotetralin (8-OH-DPAT). The aim of the present study was to examine the role of dopamine D(1) and D(2) and serotonin 5-HT(1A), 5-HT(2A), and 5-HT(7) receptors in these effects. Part 1 of this study investigated the ability of estrogen treatment to reverse PPI disruption induced by 8-OH-DPAT or the dopamine D(1)/D(2) receptor agonist apomorphine. Part 2 of this study compared these effects to the ability of various antagonists in reversing the action of 8-OH-DPAT and apomorphine on PPI. Female Sprague-Dawley rats were ovariectomized (OVX), and, where appropriate, they received silastic implants containing either a low (E20) or high dose (E100) of estrogen. Two weeks later, PPI was assessed using automated startle boxes. The disruption of PPI by either treatment with 8-OH-DPAT (0.5 mg/kg) or apomorphine (0.3 mg/kg) was similarly prevented by E100 treatment. 8-OH-DPAT-induced PPI disruption was reversed by pretreatment with the 5-HT(1A) receptor antagonist N-[2-[4-(2-methoxyphenyl)-1-piperazinyl]ethyl]-N-2-pyridinylcyclohexanecarboxamide maleate salt (WAY 100,635; 1 mg/kg) and the typical antipsychotic and dopamine D(2) receptor antagonist haloperidol (0.25 mg/kg), but it was not reversed by pretreatment with the dopamine D(1) receptor antagonist R-(+)-7-chloro-8-hydroxy-3-methyl-1-phenyl-2,3,4,5-tetrahydro-1H-3-benzazepine hydrochloride (SCH 23390; 0.1 mg/kg), the 5-HT(2A/2C) receptor antagonist ketanserin (2 mg/kg), or the 5-HT(7) receptor antagonist SB-269970 (10 mg/kg). Apomorphine-induced disruptions of PPI were reversed by haloperidol and SCH 23390 only. Estrogen may prevent disruptions of PPI induced by both 8-OH-DPAT and apomorphine by an action on dopamine D(2) receptors downstream of 5-HT(1A) receptors.

In vivo model of drug-induced valvular heart disease in rats: pergolide-induced valvular heart disease demonstrated with echocardiography and correlation with pathology.

AIMS: Valvular heart disease (VHD), inducing valvular regurgitation, has been described in carcinoid heart disease and recently in Parkinson's patients treated with pergolide. The aim of this study was to develop an in vivo model of drug-induced valvulopathy with pergolide in rats. METHODS AND RESULTS: Thirty male Wistar rats were given daily injections of either pergolide (0.5 mg/kg intraperitoneally) (n = 8), serotonin (20 mg/kg subcutaneously) (n = 8), or the vehicle only (n = 14) for 5 months. At 20 weeks, echocardiography demonstrated the presence of aortic regurgitation (AR) and/or mitral regurgitation (MR) in serotonin (86% AR, P = 0.0001; 57% MR, P = 0.006) and in pergolide animals (67% AR, P = 0.003; 67% MR, P = 0.003) compared with none in placebo. Pulmonary regurgitation (PR) and tricuspid regurgitation (TR) were found in the serotonin (71% PR, P = 0.19; 100% TR, P = 0.06 vs. placebo), pergolide (100% PR, P = 0.014; 83% TR, P = 0.35 vs. placebo), and placebo groups (36% PR; 57% TR). Tricuspid regurgitant area ratio (jet/atrium), however, was more severe in the serotonin [median 26.5 (range 17-42)%; P = 0.02] and pergolide animals [32 (17-39) %; P = 0.03] compared with placebo [12.5 (5-33)%]. We found a good correlation between valvular regurgitation and histologically assessed valvular thickness. Histological examination revealed the presence of diffusely thickened and myxoid aortic, mitral, and tricuspid valves in serotonin and pergolide animals as seen in VHD. CONCLUSION: We demonstrated, for the first time, that long-term pergolide administration led to VHD in rats. This small animal model will permit further in vivo investigation of drug-induced valvulopathies.

Genotoxic, neurotoxic and neuroprotective activities of apomorphine and its oxidized derivative 8-oxo-apomorphine

Apomorphine is a dopamine receptor agonist proposed to be a neuroprotective agent in the treatment of patients with Parkinson's disease. Both in vivo and in vitro studies have shown that apomorphine displays both antioxidant and pro-oxidant actions, and might have either neuroprotective or neurotoxic effects on the central nervous system. Some of the neurotoxic effects of apomorphine are mediated by its oxidation derivatives. In the present review, we discuss recent studies from our laboratory in which the molecular, cellular and neurobehavioral effects of apomorphine and its oxidized derivative, 8-oxo-apomorphine-semiquinone (8-OASQ), were evaluated in different experimental models, i.e., in vitro genotoxicity in Salmonella/microsome assay and WP2 Mutoxitest, sensitivity assay in Saccharomyces cerevisiae, neurobehavioral procedures (inhibition avoidance task, open field behavior, and habituation) in rats, stereotyped behavior in mice, and Comet assay and oxidative stress analyses in mouse brain. Our results show that apomorphine and 8-OASQ induce differential mutagenic, neurochemical and neurobehavioral effects. 8-OASQ displays cytotoxic effects and oxidative and frameshift mutagenic activities, while apomorphine shows antimutagenic and antioxidant effects in vitro. 8-OASQ induces a significant increase of DNA damage in mouse brain tissue. Both apomorphine and 8-OASQ impair memory for aversive training in rats, although the two drugs showed a different dose-response pattern. 8-OASQ fails to induce stereotyped behaviors in mice. The implications of these findings are discussed in the light of evidence from studies by other groups. We propose that the neuroprotective and neurotoxic effects of dopamine agonists might be mediated, in part, by their oxidized metabolites.

Pituitary adenylate cyclase activating polypeptide protects dopaminergic neurons and improves behavioral deficits in a rat model of Parkinson's disease.

Pituitary adenylate cyclase activating polypeptide (PACAP) is a pleiotropic neuropeptide, exerting different actions in the central and peripheral nervous systems. Among others, it has neurotrophic and neuroprotective effects. In the present study, we investigated the effects of PACAP in a rat model of Parkinson's disease. Rats were given unilateral injections of 6-hydroxydopamine (6-OHDA) into the substantia nigra. PACAP-treated animals received 0.1 microg PACAP as a pretreatment. Control animals without PACAP treatment displayed severe hypokinesia at 1 and 10 days postlesion when compared to animals receiving saline only. In only 1 day postlesion, by contrast, PACAP-treated rats showed no hypokinesia. Asymmetrical signs, such as turning, rearing and biased thigmotaxic scanning were observed in all lesioned animals 1 day postlesion. PACAP-treated animals, however, showed better recovery as they ceased to display asymmetrical signs 10 days later and showed markedly less apomorphine-induced rotations. Tyrosine-hydroxylase immunohistochemistry revealed that control animals had more than 95% loss of the dopaminergic cells in the ipsilateral substantia nigra, while PACAP-treated animals had only approximately 50% loss of dopaminergic cells. In summary, the present results show the neuroprotective effect of PACAP in 6-OHDA-induced lesion of substantia nigra, with less severe acute neurological symptoms and a more rapid amelioration of behavioral deficits.

A2A antagonist prevents dopamine agonist-induced motor complications in animal models of Parkinson's disease.

Adenosine A(2A) receptors, abundantly expressed on striatal medium spiny neurons, appear to activate signaling cascades implicated in the regulation of coexpressed ionotropic glutamatergic receptors. To evaluate the contribution of adenosinergic mechanisms to the pathogenesis of the response alterations induced by dopaminergic treatment, we studied the ability of the selective adenosine A(2A) receptor antagonist KW-6002 to prevent as well as palliate these syndromes in rodent and primate models of Parkinson's disease. In rats, KW-6002 reversed the shortened motor response produced by chronic levodopa treatment while reducing levodopa-induced hyperphosphorylation at S845 residues on AMPA receptor GluR1 subunits. In primates, KW-6002 evidenced modest antiparkinsonian activity when given alone. Once-daily coadministration of KW-6002 with apomorphine prevented the development of dyskinesias, which appeared in control animals 7-10 days after initiating apomorphine treatment. Animals initially given apomorphine plus KW-6002 for 3 weeks did not begin to manifest apomorphine-induced dyskinesias until 10-12 days after discontinuing the A(2A) antagonist. These results suggest that KW-6002 can attenuate the induction as well as the expression of motor response alterations to chronic dopaminergic stimulation in parkinsonian animals, possibly by blocking A(2A) receptor-stimulated signaling pathways. Our findings strengthen the rationale for developing A(2A) antagonists as an early treatment strategy for Parkinson's disease.

Toxic effects of apomorphine on rat cultured neurons and glial C6 cells, and protection with antioxidants.

Many catechol derivatives are currently used as drugs, even if they produce reactive oxygen species that may cause tissue damage. Among them, apomorphine, a potent dopamine agonist, displays efficient anti-parkinsonian properties, but the consequences of its oxidant and toxic properties have been poorly investigated on in vitro models. In the present work, we investigated apomorphine cytotoxicity by incubating cultures of rat glioma C6 cells and primary cultures of neurons with different concentrations of the drug. Apomorphine-promoted cell death was proportional to its concentration and was time-dependent. The ED(50) of apomorphine on C6 cell death after 48 hr was about 200 microM. The cytotoxic effects induced by apomorphine were correlated to its autoxidation, which leads to the formation of reactive oxygen species, semiquinones, quinones, and a melanin-like pigment. C6 cells that underwent treatment with 400 microM apomorphine for 6 hr displayed features of necrosis, including loss of membrane integrity, degeneration of mitochondria, and DNA fragmentation. Thiols, such as cysteine, N-acetyl-L-cysteine, and glutathione, significantly protected cultured neurons and C6 cells against apomorphine-induced cytotoxicity. Thiols also inhibited apomorphine autoxidation. These data strongly suggest that apomorphine cytotoxicity towards neurons and C6 cells results from an intracellular oxidative stress.

Nasal toxicological investigations of Carbopol 971P formulation of apomorphine: effects on ciliary beat frequency of human nasal primary cell culture and in vivo on rabbit nasal mucosa.

OBJECTIVE: The objective of this study was to investigate the nasal toxicity of a mucoadhesive Carbopol 971P formulation of apomorphine. MATERIALS AND METHODS: The effects of different concentrations of Carbopol 971P and apomorphine on ciliary beat frequency (CBF) were studied in suspension cultures of human nasal epithelial cells. The rabbit nasal mucosal tolerance of the formulation and its components were investigated using light microscopy. Different groups of the rabbits received twice daily, air puffs, glucose, glucose/apomorphine, Carbopol 971P or Carbopol 971P/apomorphine for 1 week (glucose-treated rabbits) or 1, 2 and 4 weeks (other treatments). RESULTS: Both Carbopol 971P and apomorphine showed both concentration- and time-dependent inhibitory effects on the CBF. The effects on CBF were: apomorphine, 1.0% w/v, irreversible ciliostasis; 0.1 and 0.5% w/v, reversible cilio-inhibition; 0.01%w/v, irreversible cilio-stimulation; and Carbopol 971P, 0.1 and 0.25% w/v, partially-reversible cilio-inhibition. Glucose and glucose/apomorphine physical mixture caused mild inflammation. Carbopol 971P (both with and without apomorphine) caused severe inflammation, which increased with duration of treatment. Necrosis, squamous metaplasia or ciliary degeneration was not observed. CONCLUSIONS: Due to the severe inflammation caused by Carbopol 971P with and without apomorphine, we conclude that this polymer is not a suitable carrier for intranasal administration of apomorphine. This is in spite of the reversible effects of Carbopol 971P (0.1 and 0. 25% w/v) and apomorphine (0.1 and 0.5% w/v) on CBF.

Lactoferrin is synthesized by mouse brain tissue and its expression is enhanced after MPTP treatment.

The presence of iron in brain tissue in increased concentrations in Parkinson's disease cases, where it might be responsible for oxidative stress, and the parallel observation that the iron transporter lactoferrin (Lf) was present in increased amounts in surviving neurons, led us to study the synthesis of Lf in a mouse model of Parkinson's disease. In this context, the origin and expression of brain Lf in normal, aged and MPTP (1-methyl-4-phenyl-1, 2,3,6-tetrahydropyridine)-treated mice were investigated. Lf immunostaining was observed mainly on microvessels in the cerebral cortex of the adult mice and to a greater extent in older mice. Lf immunoreactivity was also present in the hippocampus only in the aged mouse brains, associated with structures which seemed to be pyramidal neurons and fibers. After RT-PCR (polymerase chain reaction), Lf transcripts were found in mouse brain tissue whatever the age of the animals studied but the level of their expression was very low. No up-regulation of Lf was detectable during aging. Lf distribution and expression in the MPTP-induced Parkinsonian mouse model were also investigated. A marked depletion of dopamine (DA) occurred in the high dose MPTP-treated mice. The level of Lf expression was found to be markedly increased in the same animals and this up-regulation occurred on the first day after MPTP administration. When the brain was stressed by the neurotoxin MPTP, Lf expression increased in line with antioxidant enzymes such as catalase and gamma-glutamylcysteine synthetase, which may permit the protection of brain tissue from oxidative damage induced by the drug.

Effect of a dopamine agonist on luteinizing hormone receptors, cyclic AMP production and steroidogenesis in rat Leydig cells.

Dopamine agonists are known to increase the incidence of Leydig cell hyperplasia/adenomas when administered to rats over periods of 1-2 years. We have examined the early changes in factors affecting luteinizing hormone (LH)-controlled signal transduction pathways and steroidogenesis in Leydig cells in vitro after chronic oral administration of one of these dopamine agonists, Mesulergine (CU327-085) (N-(1-6,dimethylergolin-8a-yl)-N',N'-dimethylsulphamide hydrochloride) to Sprague-Dawley (SD) rats. Eight-week-old rats were given this dopamine agonist (2 mg/kg body wt/day) in food for 1, 5, or 12 weeks. The Leydig cells from control and treated rats were purified by elutriation and density gradient centrifugation. The dopamine agonist treatment was found to decrease the specific binding of 125I-human chorionic gonadotrophin (hCG) binding to the Leydig cells: a decrease was detected as early as 1 week after treatment and was more pronounced after 5 and 12 weeks. This was found to be due to a decrease in the LH/hCG receptor numbers and not to a decrease in LH/hCG-receptor binding affinity. Both basal and LH-stimulated cAMP and testosterone production were also decreased; cAMP production was decreased by approximately 50% by all concentrations of LH added whereas testosterone production was only decreased with submaximum stimulating concentrations of LH. The formation of testosterone in response to dibutyryl cAMP was also decreased by approximately 50%, indicating additional lesions in the signal transduction pathway. The addition of the cell permeant 22R-hydroxycholesterol (22R) demonstrated that testosterone but not pregnenolone production was decreased by treatment with the dopamine agonist, thus indicating that the 17 alpha-hydroxylase/C17-20 lyase may have been inhibited. Supporting evidence for this was found because the dopamine agonist also increased aromatase activity in the Leydig cells and thus the potential to produce estrogens; previous studies have shown that estradiol is an inhibitor of the 17-20 lyase enzyme. The addition of the dopamine agonist directly to the Leydig cells did not inhibit cAMP production or testosterone production except at high concentrations. It is concluded that treatment of rats with the dopamine agonist indirectly (i.e., via the pituitary) affects Leydig cell function resulting in a rapid decrease in LH receptors and cAMP and testosterone production. Aromatase activity is increased and thus the capacity to produce estrogens. These early changes in the signal transduction pathways and steroidogenesis may be involved in the Leydig cell hyperplasia/adenoma formation that subsequently occurs.

Motor response to a dopamine D3 receptor preferring agonist compared to apomorphine in levodopa-primed 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine monkeys.

The profile of dopamine receptor subtype activation contributing to the therapeutic efficacy and motor response complications of levodopa (nonselective pro-agonist) in Parkinson's disease remains unclear. Potent, selective, short-acting dopamine D2 receptor subfamily agonists show good antiparkinsonian efficacy but produce dyskinesias comparable to levodopa. Nonetheless, agonists displaying higher affinity for dopamine receptors other than the D2 subtype may have a better therapeutic index. To clarify this issue, we compared the nonselective dopamine D1/D2 receptor subfamilies agonist apomorphine to the dopamine D3 receptor preferring agonist [R-(+)-trans-3,4,4a,10b-tetrahydro-4-propyl-2H,5H-[1]benzopyrano[4 , 3-b]-1,4-oxazin-9-ol] (PD 128,907) in 6 levodopa-primed , 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine-lesioned parkinsonian monkeys with reproducible dyskinesias. Single s.c. dosing with the lowest fully effective dose of apomorphine (averaging 27.9 +/- 4.5 microg/kg) and PD 128,907 (averaging 41.7 +/- 4.4 microg/kg) yielded equivalent antiparkinsonian efficacy on the behavioral scale and portable activity monitoring used. A comparable significant dose-dependent increase in the response magnitude and duration was seen with two higher doses. The severity of dyskinesia was also similar between the two drugs. When the lower dose for each drug was administered six times at a fixed 90-min interval, both drugs remained efficacious with no significant tolerance observed. The D3 receptor preferring antagonist U-99194A significantly reduced the motor effects of both apomorphine and PD 128,907. Thus, increased D3 receptor tone does not acutely ameliorate dyskinesias in levodopa-primed parkinsonian monkeys. Given the reported lack of affinity of PD 128,907 for central D1 receptors, our data support the concept that the pharmacological activation of D1 receptors is not mandatory for relief of parkinsonism and production of dyskinesia.

Effect of a dopamine agonist on the development of Leydig cell hyperplasia in Sprague-Dawley rats.

Dopamine agonists are known to increase the incidence of Leydig cell hyperplasia/adenomas when administered to rats over periods of 1-2 years. We have examined the early changes in factors affecting Leydig cell growth/hyperplasia after chronic oral administration of one of these dopamine agonists, Mesulergine (CU32-085) [N-(1-6-dimethylergolin-8 alpha-yl)-N,N-dimethylsulphamide hydrochloride), to Sprague-Dawley (SD) rats. Eight-week-old rats were given the dopamine agonist (2 mg/kg body weight/day) in food for 5 or 57 weeks. The dopamine agonist treatment had no significant effect on food intake, body weight, and testis and seminal vesicle size, but significantly decreased testicular interstitial fluid volume at 5 weeks (by 51%). Leydig cells isolated from rats treated with the dopamine agonist for 5 weeks exhibited an increase in the rate of protein synthesis compared with the controls (by 28%). This treatment, however, had no significant effect on the number of Leydig cells or macrophages as assessed by histological examination of testicular sections. Treatment with the dopamine agonist for 57 weeks caused a 36 and 28% increase in the number of Leydig cells and macrophages, respectively. Nodules of Leydig cells, indicating the first signs of tumor development, were present in testes from the 57- but not the 5-week-treated animals or the controls of both groups, although an increase in the number of Leydig cells occurred with aging. Thick-walled arterioles were found in the intertubular spaces of the testis sections from rats treated for 57 weeks. These findings suggest that chronic treatment of male SD rats with the dopamine agonist causes hypertrophy of Leydig cells within 5 weeks (as assessed by [3H]methionine incorporation), followed by hyperplasia within 2 years, prior to the development of Leydig cell adenomas, which occur within 1-2 years after the initiation of treatment.