Pharmacology and neurochemistry of apomorphine

An experimental study of consecutive administration of ropinirole and apomorphine for emesis induction in dogs

OBJECTIVE

To study the safety and effectiveness of consecutively administered ropinirole and apomorphine (both dopamine 2-like receptor agonists) for emesis induction in dogs.

DESIGN

Prospective, crossover study design.

SETTING

Institutional animal research facility.

ANIMALS

Six healthy male purpose-bred Beagle dogs.

INTERVENTIONS

Each dog received 4 treatments: (1) apomorphine infusion (21 μg/kg) over 30 minutes + ropinirole eye drops (3.75 mg/m2 ); (2) ropinirole infusion (108 μg/m2 ) over 30 minutes + apomorphine SC (100 μg/kg); (3) apomorphine SC (100 μg/kg) + ropinirole eye drops (7.5 mg/m2 ) after 30 minutes; and (4) ropinirole eye drops (7.5 mg/m2 ) + apomorphine SC (100 μg/kg) after 30 minutes. Infusions were administered via a catheter instrumented in the cephalic vein. Eye drops and SC injections were administered as described in the product inserts. Blood samples were taken for ropinirole and apomorphine concentration analysis before dosing and periodically following administrations. The washout period between the treatments was 5-7 days.

MEASUREMENTS AND MAIN RESULTS

Number of vomits and clinical signs were recorded. Alertness and heart rate were monitored in conjunction with blood sampling. The average number of vomits varied between 4.3 and 8.8 (range 1-16) following treatments. Signs of nausea, vomiting, and lethargy were seen in all individuals without significant differences between treatments. Moderate to marked, transient increase in heart rates was detected in all treatments. Infrequent noted side effects included ocular hyperemia, blepharospasms, and muscle tremors. Prior treatment with apomorphine significantly decreased the absorption of ropinirole eye drops.

CONCLUSIONS

The safety and efficacy profiles of this experimental study support that ropinirole and apomorphine could be administered consecutively in cases where the treatment using 1 substance has resulted in an incomplete evacuation of the stomach contents, and the attending veterinarian considers the use of a different agent to have benefits that outweigh the risks.

Aporphine and isoquinoline derivatives block glioblastoma cell stemness and enhance temozolomide cytotoxicity

Glioblastoma (GBM) is the most aggressive and common primary malignant brain tumor with limited available therapeutic approaches. Despite improvements in therapeutic options for GBM patients, efforts to develop new successful strategies remain as major unmet medical needs. Based on the cytotoxic properties of aporphine compounds, we evaluated the biological effect of 12 compounds obtained through total synthesis of ( ±)-apomorphine hydrochloride (APO) against GBM cells. The compounds 2,2,2-trifluoro-1-(1-methylene-3,4-dihydroisoquinolin-2(1H)-yl)ethenone (A5) and ( ±)-1-(10,11-dimethoxy-6a,7-dihydro-4H-dibenzo[de,g]quinolin-6(5H)-yl)ethenone (C1) reduced the viability of GBM cells, with 50% inhibitory concentration ranging from 18 to 48 μM in patient-derived GBM cultures. Our data show that APO, A5 or C1 modulate the expression of DNA damage and apoptotic markers, impair 3D-gliomasphere growth and reduce the expression of stemness markers. Potential activity and protein targets of A5, C1 or APO were predicted in silico based on PASS and SEA software. Dopamine receptors (DRD1 and 5), CYP2B6, CYP2C9 and ABCB1, whose transcripts were differentially expressed in the GBM cells, were among the potential A5 or C1 target proteins. Docking analyses (HQSAR and 3D-QSAR) were performed to characterize possible interactions of ABCB1 and CYP2C9 with the compounds. Notably, A5 or C1 treatment, but not temozolomide (TMZ), reduced significantly the levels of extracellular ATP, suggesting ABCB1 negative regulation, which was correlated with stronger cytotoxicity induced by the combination of TMZ with A5 or C1 on GBM cells. Hence, our data reveal a potential therapeutic application of A5 and C1 as cytotoxic agents against GBM cells and predicted molecular networks that can be further exploited to characterize the pharmacological effects of these isoquinoline-containing substances.

Structural insights into the human D1 and D2 dopamine receptor signaling complexes

The D1- and D2-dopamine receptors (D1R and D2R), which signal through Gs and Gi, respectively, represent the principal stimulatory and inhibitory dopamine receptors in the central nervous system. D1R and D2R also represent the main therapeutic targets for Parkinson's disease, schizophrenia, and many other neuropsychiatric disorders, and insight into their signaling is essential for understanding both therapeutic and side effects of dopaminergic drugs. Here, we report four cryoelectron microscopy (cryo-EM) structures of D1R-Gs and D2R-Gi signaling complexes with selective and non-selective dopamine agonists, including two currently used anti-Parkinson's disease drugs, apomorphine and bromocriptine. These structures, together with mutagenesis studies, reveal the conserved binding mode of dopamine agonists, the unique pocket topology underlying ligand selectivity, the conformational changes in receptor activation, and potential structural determinants for G protein-coupling selectivity. These results provide both a molecular understanding of dopamine signaling and multiple structural templates for drug design targeting the dopaminergic system.

Banisteriopsis caapi, a Forgotten Potential Therapy for Parkinson's Disease?

Banisteriopsis caapi, a liana indigenous to the Amazon basin with metagnomigenic properties and possible anti-depressant effects is one of the natural sources of harmala alkaloids. A summary of early trials with extracts of Banisteriopsis caapi and Peganum harmala (from which harmine was first isolated) in the 1920s and 1930s on various forms of parkinsonism is given as well as a brief overview of the known pharmacological properties of harmine. Despite its earlier abandonment because of perceived weaker efficacy than solanaceous alkaloids like scopolamine and hyoscine we propose that harmine should be reconsidered as a potential rapidly acting anti-Parkinsonian agent.

Intravenous Administration of Apomorphine Does NOT Induce Long QT Syndrome: Experimental Evidence from In Vivo Canine Models

Apomorphine is a non-selective dopamine D1/D2 receptor agonist, which has been used for patients with Parkinson's disease and reported to induce QT interval prolongation and cardiac arrest. To clarify their causal link, we assessed the cardiovascular and pharmacokinetic profile of apomorphine with the halothane-anaesthetized canine model (n = 4), whereas pro-arrhythmic potential of apomorphine was analysed with the chronic atrioventricular block canine model (n = 4). In the halothane-anaesthetized model, 0.01 mg/kg, i.v. of apomorphine hydrochloride over 10 min., providing about 10 times of its therapeutic concentration, increased the heart rate and ventricular contraction; 0.1 mg/kg over 10 min., providing about 100 times of the therapeutic, prolonged the ventricular effective refractory period; and 1 mg/kg over 10 min., providing about 1000 times of the therapeutic, decreased the ventricular contraction, mean blood pressure and cardiac output together with the intraventricular conduction delay and prolongation of the effective refractory period, whereas the left ventricular end-diastolic pressure, atrioventricular nodal conduction or ventricular repolarization were hardly affected. Meanwhile, in the atrioventricular block model, 1 mg/kg, i.v. of apomorphine hydrochloride over 10 min. neither prolonged the QT interval nor induced torsade de pointes. These results suggest that apomorphine may possess a wide margin of cardiovascular safety contrary to our expectations.

Early life stress causes refractoriness to haloperidol-induced catalepsy

The use of classic antipsychotic drugs is limited by the occurrence of extrapyramidal motor symptoms, which are caused by dopamine (DA) receptor blockade in the neostriatum. We examined the impact of early-life stress on haloperidol-induced catalepsy using the rat model of prenatal restraint stress (PRS). Adult "PRS rats," i.e., the offspring of mothers exposed to restraint stress during pregnancy, were resistant to catalepsy induced by haloperidol (0.5-5 mg/kg i.p.) or raclopride (2 mg/kg s.c.). Resistance to catalepsy in PRS rats did not depend on reductions in blood or striatal levels, as compared with unstressed control rats. PRS rats also showed a greater behavioral response to the DA receptor agonist, apomorphine, suggesting that PRS causes enduring neuroplastic changes in the basal ganglia motor circuit. To examine the activity of this circuit, we performed a stereological counting of c-Fos(+) neurons in the external and internal globus pallidus, subthalamic nucleus, and ventral motor thalamic nuclei. Remarkably, the number of c-Fos(+) neurons in ventral motor thalamic nuclei was higher in PRS rats than in unstressed controls, both under basal conditions and in response to single or repeated injections with haloperidol. Ventral motor thalamic nuclei contain exclusively excitatory projection neurons that convey the basal ganglia motor programming to the cerebral cortex. Hence, an increased activity of ventral motor thalamic nuclei nicely explains the refractoriness of PRS rats to haloperidol-induced catalepsy. Our data raise the interesting possibility that early-life stress is protective against extrapyramidal motor effects of antipsychotic drugs in the adult life.

Serotonin transporter inhibition attenuates l-DOPA-induced dyskinesia without compromising l-DOPA efficacy in hemi-parkinsonian rats

Long-term dopamine replacement therapy with l-DOPA in Parkinson's disease often leads to the development of abnormal involuntary movements known as l-DOPA-induced dyskinesia. Growing evidence suggests that, following dopamine cell loss, serotonin neurons acting as surrogates for dopaminergic processes take up l-DOPA, convert it to dopamine and release it in an unregulated fashion that precipitates dyskinesia. Although most studies have focused on serotonin 5-HT(1) receptor stimulation as an antidyskinetic strategy, targeting the serotonin transporter modulation of dopamine activity has been overlooked. Therefore, in the current study, selective serotonin reuptake inhibitors were tested for their ability to reduce l-DOPA- and apomorphine-induced dyskinesia. In Experiments 1 and 2, hemi-parkinsonian rats were primed with l-DOPA until stable dyskinesia developed. Rats in Experiment 1 were administered the selective serotonin reuptake inhibitors paroxetine, citalopram or fluoxetine, followed by l-DOPA. Abnormal involuntary movements and forepaw adjusting steps were recorded to determine the effects of these compounds on dyskinesia and motor performance, respectively. Brains were collected on the final test day, after which striatal and raphe monoamines were examined via high-performance liquid chromatography. In Experiment 2, dyskinesias were measured after selective serotonin reuptake inhibitors and apomorphine. Serotonin reuptake inhibitors dose-dependently attenuated l-DOPA- but not apomorphine-induced dyskinesia, and preserved l-DOPA efficacy. Neurochemically, serotonin transporter inhibition enhanced striatal and raphe serotonin levels and reduced its turnover, indicating a potential mechanism of action. The present results support targeting serotonin transporters to improve Parkinson's disease treatment and provide further evidence for the role of the serotonin system in l-DOPA's effects.

Pharmacological MRI in awake rats reveals neural activity in area postrema and nucleus tractus solitarius: relevance as a potential biomarker for detecting drug-induced emesis

Drug-induced vomiting (emesis) is a major concern in patient care and a significant hurdle in the development of novel therapeutics. With respect to the latter, rodents, such as the rat and mouse, are typically used in efficacy and safety studies; however, drug-induced emesis cannot be readily observed in these species due to the lack of an emetic reflex. It is known that emesis can be triggered by neural activity in brain regions including area postrema (AP) and nucleus tractus solitarius (NTS). In this study, using pharmacological magnetic resonance imaging (phMRI) and a blood-pool contrast agent, we imaged the hemodynamic consequences of brain activity in awake rats initiated by the administration of compounds (apomorphine 0.1, 0.3 micromol/kg i.v. and ABT-594 0.03, 0.1, 0.3 micromol/kg i.v.) that elicit emesis in other species. Regional drug-induced relative cerebral blood volume (rCBV) changes and percent activated area within the AP and NTS were calculated, in which a dose-dependent relationship was evident for both apomorphine and ABT-594. Additionally, to correlate with behavioral readouts, it was found that the activation of AP and NTS was observed at plasma concentrations consistent with those that induced emesis in ferrets for both drugs. Our data thus suggest that phMRI in awake rats may be a useful tool for predicting emetic liability of CNS-acting drugs and may provide insights into depicting the underlying emetic neural pathways in vivo.

Cardiovascular and electrocardiographic effects of the dopamine receptor agonists ropinirole, apomorphine, and PNU-142774E in conscious beagle dogs

To confirm recent in vitro findings, we examined the cardiovascular and electrocardiographic (ECG) effects of the dopamine receptor agonists ropinirole, apomorphine, and PNU-142774E in conscious dogs. Intravenous (i.v.) infusions of ropinirole totaling 20 microg/kg maximally reduced mean arterial pressure (MAP; -16 mm Hg) and the ECG PR interval (-13 milliseconds) and increased heart rate (HR; +29 b/min) and QTc length (+33 ms) at a peak plasma drug concentration (p[drug]) of 3.5 ng/ml. I.V. PNU-142774E was better tolerated through 66 microg/kg and a maximal p[drug] of 5.9 ng/ml with negligible cardiovascular changes and mild QTc reduction (13 ms). Apomorphine (25 microg/kg i.v.) was intermediate to ropinirole and PNU-142774E for emesis and peak changes in MAP (-6 mm Hg), HR (+24 b/min), and QTc (+15 milliseconds) at a mean p[drug] of 3.4 ng/ml. By comparison, the class III antiarrhythmic trecetilide (2.0 mg/kg bolus) increased QTc (+58 ms) without affecting mean arterial pressure or heart rate. This study establishes that in conscious dogs, the selective dopamine receptor agonist PNU-142774E has fewer cardiovascular and emetic effects than ropinirole and apomorphine and supports prior in vitro findings that ropinirole and apomorphine but not the PNU-142774E imidazoquinolin analog sumanirole reduces the delayed rectifier current in HERG transfected cells.

Apomorphine-induced myocardial protection is due to antioxidant and not adrenergic/dopaminergic effects

Apomorphine (Apo), a dopaminergic agonist used for treatment of Parkinson disease, is a potent antioxidant. In addition to its antioxidative effects, the dopaminergic and adrenergic effects of Apo were studied. Isolated perfused rat hearts were exposed to 25 min of no-flow global ischemia (37 degrees C) and 60 min of reperfusion (I/R, control). Drugs were introduced for the first 20 min of reperfusion. The LVDP of the control group recovered to 54.6 +/- 3.3%. Apo-treated hearts had significantly improved recovery (61.6 +/- 5%, p < 0.05). The recovery of the work index LVDP x HR was even bigger: 67.8 +/- 3.7% (Apo treatment) vs 41.7 +/- 4.6% (control, p < 0.001). Haloperidol, a dopaminergic antagonist, did not affect the recovery with Apo. Propranolol, a beta-adrenergic blocker, initially inhibited the effect of Apo. However, the recovery of the combined group (Apo + propranolol) increased and reached significance (LVDP, p < 0.05 vs control group) after cessation of propranolol perfusion. At 60 min of reperfusion this group was superior to Apo-treated hearts (LVDP, p < 0.05). Propranolol (without Apo) did not improve the hemodynamic recovery. The same pattern of recovery applies also to the recovery of the +dP/dt during the reperfusion. L-DOPA was less effective than Apo. I/R caused significant increase in carbonylation of proteins. Apomorphine inhibited the increase in carbonylation. Haloperidol did not affect this beneficial effect of Apo. L-DOPA significantly decreased the carbonylation of proteins. We conclude that the antioxidative effect of Apo is its main mechanism of cardioprotection.

Apomorphin bei der Behandlung des Morbus Parkinson

Apomorphine has long been used in many medical specialties. It is a highly potent D2-, D3- and D4-dopamine receptor agonist with a particularly high D1-dopamine receptor affinity. Due to its almost complete inactivation during liver passage it is usually applied subcutaneously. After rapid onset its effect is waning after about one hour. Apomorphine's action is not dependent on enteral dopamine resorption and on praesynaptic dopamine storage and dopamine secretion mechanisms. When applied subcutaneously its most common side effect is cutaneous nodules at the injection sites. Peripheral dopaminergic side effects, such as nausea and orthostatic hypotension, usually occur only during therapy initiation and respond well to domperidone. Hallucinations occur less frequently than with other dopamine receptor agonists. The apomorphine test determines the levodopasensitivity of a parkinsonian syndrome by application of an apomorphine bolus to distinguish between idiopathic Parkinson's disease and atypical parkinsonian syndromes. It indicates a levodopasensitivity in about 90% of the patients tested. False-negative results can occur in mild parkinsonian syndromes. A meta analysis indicates that intermittent subutaneous apomorphine applications (intermittent apomorphine therapy) with a self application device can reduce the daily duration of unpredictable off-phases by 48.8+/-8.7%. For this an average of 3.7+/-1.1 x 2.9+/-0.8 mg apomorphine has to be applied per day. Continuous subcutaneous apomorphine application with an extracorporal pump (continuous apomorhine therapy) during the wake phase reduces the duration of daily off-phases by 64.8+/-13.5% and the additional levodopa dose by 30.3+/-31.1%. 24-hour application reduces the daily duration of the off-phases by 63.5+/-19.1% and the additional levodopa dose by 65.5+/-21.9%. Levodopa-induced dyskinesias are also substantially reduced. A gradual conversion to an apomorphine monotherapy is possible. Even after prolonged treatment times no significant loss of efficacy occurs. With this profile apomorphine is a highly successful treatment option for off-phases and levodopa-induced dyskinesias in advanced cases of idiopathic Parkinson's disease which should be used before more invasive approaches are considered.

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.

A three-axes approach of subtyping the alcohol dependence syndrome

Subtyping of alcoholics according to specific characteristics has a long tradition in alcoholism research with a number of different typologies that emerged in the literature. The goal of the present study was to test a multidimensional approach of subtyping with characteristics from different axes. Therefore, male inpatients meeting ICD-10 criteria for alcohol dependence were rated on three axes by assessing their degree of sensation seeking (personality axis), age of alcoholism onset (clinical axis) and level of dopamine activity (neurobiological axis). By using a configuration frequency analysis, we identified a subtype that was characterized by high sensation seeking early age of alcoholism onset and high dopamine activity. This subtype, which is in accordance with clinical experience and cannot be explained by antisocial personality disorder, embodied a significantly greater proportion of alcoholics than expected. The result emphasizes the usefulness of multidimensional approaches integrating personality, clinical and neurobiological characteristics.

Safety of entacapone and apomorphine coadministration in levodopa-treated Parkinson's disease patients: pharmacokinetic and pharmacodynamic results of a multicenter, double-blind, placebo-controlled, cross-over study

We investigated whether administration of the catechol-O-methyl transferase (COMT) inhibitor entacapone, at doses of 200 mg and 400 mg, alters the pharmacokinetics of apomorphine in Parkinson's disease patients experiencing severe motor fluctuations. In addition, the pharmacodynamics and safety of entacapone and apomorphine coadministration in these patients were examined. The study followed a three-sequence, three-period, crossover design. Patients were randomly assigned to one of three sequences that included single oral doses of entacapone 200 mg, entacapone 400 mg, and placebo in a predefined order. On 3 separate test days, study treatment was administered before apomorphine. The study evaluations (pharmacokinetics, tapping test, and dyskinesia evaluation [Abnormal Involuntary Movements Scale - AIMS]) were performed on these days. Furthermore, Unified Parkinson Disease Rating Scale (UPDRS) scores were evaluated at baseline and study end. Pharmacokinetic parameters for apomorphine (C(max), AUC, t(max), t(1/2)) were unchanged by the administration of entacapone, and changes in both the tapping test and AIMS score were similar with all treatments (entacapone 200 mg, entacapone 400 mg, and placebo). There was no significant difference in mean total UPDRS scores between baseline and study end. The administration of entacapone did not change the pharmacokinetic or pharmacodynamic effects of apomorphine in these patients or prolong the clinical effect of apomorphine. Thus, apomorphine may be safely administered to patients receiving therapy with levodopa and entacapone, providing a useful addition to treatment for patients with advanced Parkinson's disease.

Autoimmune-induced damage of the midbrain dopaminergic system in lupus-prone mice

Spontaneous development of lupus-like disease is accompanied by impaired dopamine catabolism and degenerating axon terminals in the mesencephalon of MRL-lpr mice. We presently examine the hypothesis that systemic autoimmunity affects the central dopaminergic system in behaviorally impaired animals. The functional damage of the nigrostriatal pathway was assessed from rotational behavior after a single injection of the D1/D2-receptor agonist apomorphine. Neurodegeneration in the midbrain was estimated by Fluoro Jade B (FJB) staining. The causal role of autoimmunity was tested by comparing asymptomatic and diseased MRL-lpr mice, and by employing the immunosuppressive drug cyclophosphamide. Damage of dopaminergic neurons was assessed by tyrosine-hydroxylase (TH) staining of the midbrain. Apomorphine induced significant asymmetry in limb use, which lead to increased circling in the diseased MRL-lpr group. While FJB-positive somas were not seen in the striatum, increased staining in the substantia nigra (SN) and ventral tegmental area (VTA) were detected in behaviorally impaired MRL-lpr mice, but not in age-matched controls. Reduced brain mass and increased levels of TNF-alpha in their cerebrospinal fluid (CSF) suggested cerebral atrophy and inflammation. In addition, CSF was neurotoxic to a dopaminergic progenitor cell line. Immunosuppression attenuated CSF cytotoxicity, TNF-alpha levels, and midbrain neurodegeneration. Supportive of the notion that dying neurons were dopaminergic, the SN of autoimmune mice showed approximately a 35% reduction in the number of TH-positive cells. A three-fold increase in serum brain-reactive antibodies accompanied this loss. Although the source of toxic mediator(s) remains unknown, present results are consistent with the hypothesis that autoimmunity-induced destruction of mesonigral and mesolimbic dopaminergic pathways contributes to the etiology of aberrant behavior in an animal model of neuropsychiatric lupus.

Cardioprotective and antioxidant effects of apomorphine

Apomorphine is a potent antioxidant that infiltrates through biological membranes. We studied the effect of apomorphine (2 microM) on myocardial ischemic-reperfusion injury in the isolated rat heart. Since iron and copper ions (mediators in formation of oxygen-derived free radicals) are released during myocardial reperfusion, apomorphine interaction with iron and copper and its ability to prevent copper-induced ascorbate oxidation were studied. Apomorphine perfused before ischemia or at the commencement of reperfusion demonstrated enhanced restoration of hemodynamic function (i.e. recovery of the work index (LVDP x HR) was 69.2 +/- 4.0% with apomorphine pre-ischemic regimen vs. 43.4 +/- 9.01% in control hearts, p < 0.01, and 76.3 +/- 8.0% with apomorphine reperfusion regimen vs. 30.4 +/- 11.1% in controls, p < 0.001). This was accompanied by decreased release of proteins in the effluent and improved coronary flow recovery in hearts treated with apomorphine after the ischemia. Apomorphine forms stable complexes with copper and with iron, and inhibits the copper-induced ascorbate oxidation. It is suggested that these iron and copper chelating properties and the redox-inactive chelates formed by transition metals and apomorphine play an essential role in post-ischemic cardioprotection.

Differential neurobehavioral deficits induced by apomorphine and its oxidation product, 8-oxo-apomorphine-semiquinone, in rats

Apomorphine is a potent dopamine receptor agonist, which has been used in the therapy of Parkinson's disease. It has been proposed that apomorphine and other dopamine receptor agonists might induce neurotoxicity mediated by their quinone and semiquinone oxidation derivatives. The aim of the present study was to evaluate the possible neurobehavioral effects of apomorphine and its oxidation derivative, 8-oxo-apomorphine-semiquinone (8-OASQ). Adult female Wistar rats were treated with a systemic injection of apomorphine (0.05 or 0.5 mg/kg) or 8-OASQ (0.05 or 0.5 mg/kg) 20 min before behavioral testing. Apomorphine and 8-OASQ induced differential impairing effects on short- and long-term retention of an inhibitory avoidance task. Apomorphine, but not 8-OASQ, dose-dependently impaired habituation to a novel environment. The memory-impairing effects could not be attributed to reduced nociception or other nonspecific behavioral alterations, since neither apomorphine nor 8-OASQ affected footshock reactivity or behavior during exploration of an open field. The results suggest that oxidation products of dopamine or dopamine receptor agonists might induce cognitive deficits.

Differing effects of the cannabinoid agonist, CP 55,940, in an alcohol or Tween 80 solvent, on prepulse inhibition of the acoustic startle reflex in the rat

It has been suggested that cannabinoid agonists increase dopamine (DA) transmission in the mesolimbic dopamine system. However, evidence for such an effect is inconsistent. Prepulse inhibition (PPI) of the acoustic startle reflex is a behavioural paradigm that is modulated by an increase of mesolimbic dopamine. This study sought to ascertain whether or not a cannabinoid agonist, CP 55,940, mimicked the effects of amphetamine (a drug which increases dopamine release) on PPI. The first experiment measured the PPI of 16 male Wistar rats injected (i.p.) with different doses of CP 55,940 in a Latin-square design. A second experiment replicated the effects of the first experiment in a between-subjects design, and also examined the effects of using a 5% alcohol solution as a solvent for cannabinoid agonists, in comparison to the more inert detergent, Tween 80. In both experiments, CP 55,940 in Tween 80 significantly reduced basal activity, increased startle onset latencies and increased PPI, effects opposite to those of amphetamine. These results suggest that the net behavioural effects of cannabinoids are opposite to those of amphetamine. In addition, it was found that 1 ml/kg of a 5% alcohol solution has significant behavioural effects on its own, and reverses the effects of CP 55,940 on PPI.

Serotonergic and dopaminergic activities of rigidified (R)-aporphine derivatives

Novel rigidified (R)-aporphine derivatives were synthesized from (R)-1,11-carbonylaporphine by ring expansion reactions. The structures of the novel analogues were assigned by NMR spectroscopy and X-ray crystallography. The compounds showed moderate affinities and selectivities at serotonin S-HT1A and 5-HT7 and dopamine D2A receptors.

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.

Effect of apomorphine infusion on dopamine synthesis rate relates to dopaminergic tone

The effects of apomorphine on the striatal L-[11C]DOPA influx rate was examined in anaesthetized Rhesus monkeys using positron emission tomography (PET). In comparison with baseline conditions, the addition of a continuous infusion of apomorphine produced decreases in the striatal L-[11C]DOPA influx rate in all the monkeys examined. The effect of apomorphine infusion also showed a dose-dependent trend. In individual monkeys, the magnitude of the effect showed a baseline dopaminergic tone-dependency; that is, the effect of apomorphine was most pronounced in monkeys with high baseline influx rates, and in monkeys with lower baseline values apomorphine induced a weaker effect. Studies of radiolabeled tracer and radiolabeled metabolites formed in plasma confirmed that apomorphine infusion did not induce any change in the peripheral elimination or metabolite formation of L-[11C]DOPA. The decreased striatal L-[11C]DOPA influx rate induced by apomorphine was interpreted as an agonist effect on dopamine autoreceptors regulating the dopamine synthesis rate. The observation of a baseline dopaminergic tone-dependent effect is in agreement with earlier results showing this influence on the striatal influx rate as measured with the tracer L-[11C]DOPA. A priori, it can be established that L-[11C]DOPA and PET provide a method not only to study the structural integrity of the presynaptic dopaminergic system but also to study the homeostasis-regulating mechanisms of this neurotransmitter system in vivo. The ability to measure condition-dependent effects in individuals should be of great importance in determining specific pathophysiological mechanisms underlying degenerative and functional disorders affecting the dopaminergic system.

Prenatal corticosterone treatment induces long-term changes in spontaneous and apomorphine-mediated motor activity in male and female rats

The potential influence of glucocorticoids on fetal brain development was investigated after corticosterone administration via pellets to pregnant rats during the last trimester of gestation. We examined both spontaneous motor activity and dopamine-mediated motor responses to apomorphine, a D1, D2 and D3 receptor agonist, given at a postsynaptic dose (1 mg/kg, s.c.) to both prepubertal and adult male and female offspring. Prenatal corticosterone was found to produce the following alterations in the offspring. (1) Prepubertal stage: Male offspring: a statistically significant (P < 0.05) increase was observed in spontaneous rearing, motility and locomotion (activity measured during the first 30 min) without changes in apomorphine-induced motor responses. Female offspring: a reduction (P < 0.05) only in spontaneous rearing activity was observed during the exploratory phase (activity measured during the first 10 min) without significant changes in apomorphine-induced motor responses. (2) Adult stage: Male offspring: the exploratory activity to the novel environment was increased (P < 0.05) without significant changes in apomorphine-induced motor activity. Female offspring: an increase (P < 0.05) in spontaneous locomotion was observed during the first 30 min of testing without significant changes in exploratory activity to the novel environment. However, the apomorphine-induced motility and locomotion were reduced (P < 0.05) during the first 30 min. These observations indicate that prenatal corticosterone induces both short-term and long-term changes in spontaneous motor activity as well as long-lasting alterations in dopamine receptor response in the motor network mechanisms controlled by DA receptors. These changes are in part age and sex-dependent. The possible relationship between prenatal programming of the mesolimbic and nigrostriatal dopaminergic pathways by corticosterone and the observed changes in motor function is discussed.

Effect of tolcapone on plasma and striatal apomorphine disposition in rats

The influence of tolcapone, an inhibitor of catechol-O-methyl transferase, was evaluated on the disposition of apomorphine, a dopamine agonist used to treat Parkinson's disease, to explain a previously observed increase of duration of the effect of apomorphine associated with tolcapone. Sampling was performed in rats before and at different times after administration of apomorphine and following that of tolcapone or saline. Both in plasma and striatum, times to reach maximal-concentration and maximal concentrations did not significantly differ between the two groups but the elimination half-life times and areas under the curve were significantly greater following tolcapone treatment than in the saline group. These results show that tolcapone can increase plasma apomorphine bioavailability by modifying its liver catabolism.

Alcohol withdrawal and dopamine receptor sensitivity after prolonged abstinence

1. Forty-four male inpatients suffering from moderate to severe alcohol dependence (DSM-III-R and ICD-10) as well as 14 healthy controls entered this study. Individuals were classified according to the severity of their withdrawal symptoms during detoxification i.e. group 1) no withdrawal, group 2) autonomic hyperactivity, group 3) withdrawal delirium and group 4) controls. 2. During the 6th week of treatment, that is, when all patients were recovered, controlled abstinent, and several weeks away from the end of their withdrawal syndrome, dopamine receptor sensitivity was neuroendocrinologically assessed by stimulating human growth hormone (HGH) with apomorphine (APO). 3. In a repeated measures model ANOVA, the four groups differed significantly in their HGH release. However, when excluding the controls from the analysis and focusing on alcoholics only (group 1 - 3), the significant difference disappeared. Covariates such as age, weight, quantity of drinking and duration of dependence were not related to the dependent variable. 4. In conclusion, the first significant result (with controls) reflects a blunted HGH response in alcoholics. It confirms earlier reports. The second, non significant result with the alcohol dependents only, suggests that the severity of withdrawal is not reflected by the amount of HGH released. Therefore, in alcoholics, a reduced dopamine receptor function after six weeks of abstinence, as neuro-endocrinologically assessed with apomorphine, seems to be related to alcohol dependence rather than to the severity of alcohol withdrawal.

Apomorphine in patients with Parkinson's disease

We present a review of the recent literature and personal experience with apomorphine in patients with Parkinson's disease. Apomorphine is a potent D1 and D2 dopaminergic agonist. It has a rapid and short duration effect after subcutaneous administration at doses ranging from 15 to 180 micrograms/kg. Plasma maximal concentration is reached in 8-16 minutes, with a plasma half life of 34-70 minutes. Bioavailability is close to 100%. Repeated injections in patients show post-stimulative hyposensitivity. Apomorphine test appears very useful for the differential diagnosis between idiopathic Parkinson's disease and other Parkinson plus syndromes, and as a predictive test for dopaminergic responsiveness. Appropriate doses are able to alleviate akinesia, rigidity and tremor. Recent therapeutic trials have demonstrated the high interest of intermittent multiple subcutaneous apomorphine injections to cut the "off" motor phases in fluctuating parkinsonian patients under chronic levodopa treatment. In some cases, continuous apomorphine subcutaneous infusion with a portable pump may be required, particularly when levodopa treatment is temporarily interrupted, as after abdominal surgery. During long-term treatment, the apomorphine dose able to relieve akinesia remains stable. Peripheral side effects such as nausea and hypotension may be prevented by the co-administration of domperidone, a peripheral dopaminergic antagonist. Cutaneous fibrous nodules and psychiatric symptoms may occur, but usually at high dosages with continuous infusion. Local allergic effects have limited the use of other routes of administration, such as intranasal, sublingual, and rectal routes. Apomorphine is also used as a pharmacological tool for clinical research with the aim of a better understanding of the pathophysiology of Parkinson's disease.

Wearing-off fluctuations in Parkinson's disease: contribution of postsynaptic mechanisms

Wearing-off phenomenon that complicates levodopa therapy of Parkinson's disease has been attributed to a reduction in striatal dopamine storage due to the progressive degeneration of presynaptic dopaminergic terminals. To determine whether postsynaptic mechanisms also contribute to these response fluctuations, the duration of the antiparkinsonian response in parkinsonian patients grouped by disease severity was compared following discontinuation of a steady-state optimal-dose infusion of apomorphine. Although the plasma half-life of this dopamine receptor agonist remained constant, its mean efficacy half-time declined from 66 minutes in early, levodopa-naive patients to 33 minutes in advanced, complicated parkinsonians (p < 0.005). Since the motor effects of apomorphine do not depend on the presence of dopaminergic terminals, changes at the postsynaptic level undoubtedly contribute to the diminished response duration. The only slightly greater attenuation of levodopa's motor effects observed previously under similar conditions suggests these postjunctional alterations, possibly involving relatively plastic striatal dopaminoceptive systems, account for most of the shortening in the duration of levodopa action that underlie wearing-off fluctuations.

Stimulation of both dopamine D1 and D2 receptors facilitates in vivo acetylcholine release in the hippocampus

The effect of selective D1 and D2 dopamine (DA) receptor agonists and of a mixed D1/D2 agonist on hippocampal acetylcholine (ACh) release was investigated. LY 171555 (0.5 and 1 mg/kg, i.p.), SKF 38393 (1 to 10 mg/kg, i.p.), CY 208-243 (0.2 mg/kg, i.p.) and apomorphine (0.5 to 2 mg/kg, i.p.), at doses stimulating rat behavior, were found to increase the output of ACh in the hippocampus. Maximal increase was observed after LY 171555 1 mg/kg, SKF 38393 10 mg/kg, CY 208-243 2 mg/kg and apomorphine 2 mg/kg (85, 90, 87 and 210%, respectively). The enhancement of ACh release induced by either SKF 38393 (10 mg/kg) or LY 171555 (1 mg/kg) was prevented by the blockade of D1 receptors with SCH 23390 (0.1 mg/kg, s.c.). Co-administration of maximally active doses of LY 171555 (1 mg/kg) and SKF 38393 (10 mg/kg) produced an additive effect (about 200%). In contrast to the findings with high doses, low, presynaptic doses of LY 171555 and apomorphine reduced ACh output. Maximal reduction was observed after 0.05 mg/kg for both drugs, (43 and 52%, respectively). These results show that activation of dopaminergic transmission either at D1 and/or at D2 receptors enhances ACh output in the hippocampus.

Dose-related antagonism of the emetic effect of morphine by methylnaltrexone in dogs

Opioids administered to produce analgesia cause unwanted emesis in patients (incidence 20%-30%, depending on situation). Tests in animals show that quaternary narcotic antagonists like methylnaltrexone (MNTX) do not affect the analgesic potency of morphine, but such compounds have not been examined for their potential to antagonize morphine-induced emesis. To determine the effects of MNTX on emetic response, we assigned 85 dogs to one of 11 groups challenged with morphine alone or morphine and various doses of MNTX IM or i.v. Antagonism of the emetic response was dose related: MNTX, 0.25 mg/kg IM or 0.2 mg/kg i.v., completely blocked the emetic effect of morphine in dogs for approximately 60 minutes. If morphine-induced emesis is mediated by receptors available to a quaternary antagonist (perhaps on the peripheral side of the blood-brain barrier), MNTX may prevent opioid-induced emesis. These data indicate that opioid-induced emesis might be prevented without affecting analgesia.

Effect of chronic antidepressant treatment on responses to apomorphine in selectively bred rat strains

The purpose of this study was to verify the dopamine-sensitizing behavioral effect of chronic antidepressant treatment in two selectively bred rat strains: the hypercholinergic Flinders Sensitive Line (FSL) and control Flinders Resistant Line (FRL). Two antidepressants, desipramine HCl (DMI) and sertraline HCl, were injected IP in separate groups of FSL and FRL rats in a dose of 16.5 mumol/kg twice daily for 16 days. Twenty-four hours after withdrawal, locomotor and hypothermic responses to 0.2 mg/kg of apomorphine, SC, were examined. Attenuation of the effect of apomorphine was observed in the open field: FRLs withdrawn from sertraline were significantly less mobile than control FRLs, and the same trend was found in FSL rats. Chronic DMI resulted in similar changes in the locomotor activity. Sertraline treatment decreased apomorphine-induced hypothermia by almost half in FSLs, whereas slight hyperthermia was induced in FRL rats instead. The present results suggest that in these selectively bred strains, a serotonergic antidepressant such as sertraline may have sensitized dopaminergic autoreceptors and/or desensitized postsynaptic receptors. Apomorphine-induced hypothermia could be mediated by serotonergic neuron function that may have been altered by chronic sertraline but not DMI treatment.

Role of genotype in brain dopamine metabolism and dopamine-dependent behavior of mice

In mice of eight inbred strains--BALB/c, AKR/J, DBA/2, CBA, C57B1/6, DD, CC57Br, and C3H/He--brain dopamine and its metabolites, 3,4-dihydroxyphenylacetic acid (DOPAC) and homovanillic acid (HVA), in striatum and nucleus accumbens with tuberculum olfactorium, the structures of two main dopaminergic systems--nigrostriatal and mesolimbic--were determined. In both dopaminergic regions, no strain effect on either dopamine or DOPAC levels was found, while for HVA content a highly significant hereditary determination was shown. Influences of selective D1 and D2 dopamine receptor agonists--SK&F 38393 and quinpirole, respectively--as well as that of a mixed D1/D2 agonist, apomorphine, on general locomotor activity and stereotypic climbing were studied. By that, marked genotypic differences in dopamine-dependent behavior and dopamine receptor sensitivity were observed. Although both SK&F 38393 (5 mg/kg) and apomorphine (0.25 mg/kg) decreased locomotion, the effect being genotype dependent, in all strains of mice quinpirole (2.5 mg/kg) proved more potent in locomotor inhibition. SK&F 38393 (10 mg/kg) induced climbing, but 2.5 mg/kg apomorphine in most strains was much more effective. At the same time, quinpirole (up to 8 mg/kg) failed to induce this behavior. This suggests the crucial role of D1 receptors in the generation of climbing, attracting, at the same time, attention to the importance of D1/D2 interaction. The observed drastic interstrain differences in dopamine receptor sensitivity demonstrate the essential role of genotype in the effects of dopaminergic drugs.

Conditioning of apomorphine effects: simultaneous analysis of the alterations in cortical electroencephalogram and behaviour

The possible conditioning of pharmacological effects of apomorphine on the electroencephalogram was studied using telemetric recordings in rats. Previous studies have shown that apomorphine-induced stereotyped behaviour can be conditioned: after repeated pairings of defined stimuli with the drug effect, the presentation of the external stimuli alone elicited stereotype sniffing, licking, and gnawing. Since apomorphine, an agonist at dopamine receptors, also produces a characteristic EEG pattern with an increase of power in the alpha-1 band, the possibility that this effect could also be conditioned was studied. In fact, conditioning with a dose of 0.5 mg/kg apomorphine (s.c.) led to a significant increase in the number of short-lasting episodes with enhancement of the power in the alpha-1 range in the presence of the conditioned stimuli, according to a comparison of the results obtained in the conditioned group and those of the controls ("pseudoconditioned"). Moreover, behavioural studies were performed simultaneously in order to find possible correlations between conditioned effects on EEG and conditioned alterations in behaviour. In general, a fair correlation between the increase of power in the alpha-1 band and stereotyped behaviour was found. This was also the case during extinction, when the conditioned stimuli were repeatedly uncoupled from apomorphine administration: both behavioural parameters and EEG alterations showed similar time-courses and had almost disappeared during the fourth extinction session.

Absorption of apomorphine by various routes in parkinsonism

We wanted to determine the absorption and clinical effect of sublingual (SL) and transdermal apomorphine in parkinsonism. Patients received single SL apomorphine doses (N = 7) and the absorption was compared with parenteral (N = 5) and oral (N = 4) doses. One patient received a transdermal dose of apomorphine. The relative bioavailability of SL apomorphine ranged from 10 to 22% of a parenteral apomorphine dose. Oral apomorphine was less than 4% bioavailable, and the transdermal dose did not produce detectable plasma levels. Three patients with motor fluctuations responded to SL apomorphine, with a latency to effect of 20-40 min and a duration of effect of 15-100 min. One patient used SL apomorphine as an adjunct with levodopa, and during 1 month reported a large decrease in "off" periods. We conclude that apomorphine is effectively absorbed by the sublingual route.

Asymmetries in thigmotactic scanning: evidence for a role of dopaminergic mechanisms

In two experiments, the influence of spontaneous asymmetries in thigmotactic scanning was analyzed on spontaneous and drug-induced behavior in the rat. The side of the face with which an animal performed more scanning in a baseline test was defined as the dominant vibrissae side. In experiment 1, repeated testing of either spontaneous thigmotactic scanning, or scanning after apomorphine or amphetamine, yielded no evidence that rats would preferably use one side of the face for scanning, when re-exposed to the same environment. However, an asymmetry in turning was observed both under apomorphine and amphetamine, that is, turning away from the dominant vibrissae side. In experiment 2, an influence of spontaneous asymmetries in scanning was found on behavioral asymmetries induced by unilateral vibrissae removal. Only animals, in which the vibrissae of the non-dominant side had been removed, showed more scanning with the intact vibrissae side, both undrugged and after apomorphine. Turning under apomorphine was more pronounced in animals in which the vibrissae on the dominant side had been removed. These animals showed an asymmetry in turning towards the intact vibrissae side. Furthermore, in both experiments we found evidence for left/right differences in turning or scanning. The results are discussed with respect to possible endogenous substrates of asymmetry, such as within the mesostriatal dopamine system.

Apomorphine-induced emesis in the dog--routes of administration, efficacy and synergism by naloxone

Apomorphine proved to be more effective as an emetic in dogs after s.c. administration than after i.m. injection with doses of 0.04 and 0.1 mg/kg. This effect is explained by an anti-emetic effect mediated by mu-receptors in the vomiting centre in the brain, which, in contrast to the chemoreceptor trigger zone, is within the blood-brain barrier. A certain delay between the stimulation of D2-receptors in the chemoreceptor trigger zone (causing emesis) and mu-receptors in the vomiting centre (producing anti-emesis) therefore results, leading to a self-limiting emesis. Blockade of the mu-receptors by naloxone increased and prolonged the effect of apomorphine. A relatively narrow range of apomorphine concentrations on s.c. administration is then effective to stimulate the chemoreceptor trigger zone, but can hardly inhibit the vomiting centre, and must therefore be considered the most suitable route for administration of apomorphine.

Peripheral pharmacokinetics of apomorphine in humans

Apomorphine, a potent dopamine agonist, has been used in acute and chronic studies of parkinsonism and other neurological disorders. To define its peripheral pharmacokinetics, we administered apomorphine by subcutaneous injection, by subcutaneous infusion, and by intravenous infusion to 15 patients with parkinsonism and measured plasma apomorphine levels by high-performance liquid chromatography with electrochemical detection. The peak drug levels and area under the curve were closely correlated with the dose administered; time to peak was brief and was independent of dose. The variation in absorption was high between subjects but low within individual subjects. In 11 of 15 subjects, the disappearance of drug could be described by a two-compartment model, with a distribution half-life of 5 minutes and an elimination half-life of 33 minutes. The drug absorption, volume of distribution, plasma clearance, and half-lives were similar for subcutaneous injection, subcutaneous infusion, and intravenous infusion. We conclude that apomorphine is rapidly and completely absorbed from subcutaneous tissue, correlating with the rapid onset of clinical effects, and that the brief duration of clinical action of the drug is explained by its rapid clearance.

Effect of the D1 receptor agonist SKF 38393 on some behavioural effects of apomorphine in rats

The dopamine receptor agonist apomorphine in experiments on rats in low doses (0.025-0.2 mg/kg, s.c.) induced yawning which reflected a selective activation of presynaptic dopamine receptors. In high doses (0.25-1.0 mg/kg) apomorphine induced stereotyped sniffing and yawning in consequence of postsynaptic D2 receptor activation. Dopamine D1 receptor agonist SKF 38393 inhibited yawning induced by low doses of apomorphine. The inhibitory effect of SKF 38393 on apomorphine-induced yawning was attenuated by pretreatment with specific D1 receptor antagonist SCH 23390 [2-(+)-8-chloro-2,3,4,5-tetrahydro-3-methyl-5-phenyl-1H-3-benzazepine-7-ol). On the other hand however, SKF 38393 potentiated sniffing induced by the high doses of apomorphine without affecting gnawing. These data indicate that D1 receptor activation modulates both pre- and postsynaptic effects of apomorphine in opposite directions.

Effects of pretreatment with luteinizing hormone releasing hormone (LHRH) on behaviors induced by apomorphine in rats

The influence of LHRH on the behavioral effects induced by apomorphine (APO) was studied in male rats. Several doses of apomorphine (31.25, 62.5, 125, 250 and 500 micrograms/kg) were administered subcutaneously (SC) after LHRH 100 micrograms/kg or solvent. Low doses of apomorphine induced hypomotility and impaired acquisition of a conditioned avoidance response (CAR). High doses produced hypermotility, stereotyped sniffing and a short lasting increase, followed by a decrease in the acquisition of CARs. Pretreatment with LHRH potentiated the hypomotility induced by low doses of apomorphine (62.5 and 125 micrograms/kg) and the hypermotility, stereotyped sniffing and the enhancement in acquisition of CARs produced by higher doses of apomorphine (250 and 500 micrograms/kg). These findings suggest that LHRH could indirectly regulate dopamine activity through an increase in sensitivity of dopamine receptors (pre- and postsynaptic), which mediate the behavioral effects of APO. It is postulated that this hypersensitivity of DA receptors could be the consequence of an inhibition of presynaptic dopaminergic transmission, induced by LHRH.

Behavioural responses to the selective D1-dopamine receptor agonist R-SK&F 38393 and the selective D2-agonist RU 24213 in young compared with aged rats

1. In aged male Sprague-Dawley rats (22 months) with a selective loss of D2- but not of D1-dopamine receptors, stereotyped behaviour induced by 0.5 mg kg-1 apomorphine was increased and prolonged in comparison with young (4 month) counterparts. This suggested a pharmacokinetic effect rather than a pharmacodynamic change. 2. The syndrome of non-stereotyped behavioural responses to the selective D1-agonist R-SK&F 38393, 1.25-20.0 mg kg-1, was unchanged in aged vs young animals, but the topography of individual behaviours constituting this overall syndrome was altered with aging. 3. Neither the overall syndrome of low intensity stereotyped behaviour nor the topography of individual behaviours induced by the selective D2-agonist RU 24213, 1.25-20.0 mg kg-1, were altered in aged vs young animals. 4. Loss of D2- but not D1-receptors with aging was therefore found to be associated with no change in responsivity to a D2-receptor agonist. The decreased intense grooming and increased vacuous chewing responses to the D1-agonist with aging parallel the previously demonstrated effects of selective D2-antagonists on these D1-stimulated behaviours. 5. It is suggested that age-related decline in D2-receptor activity may have greater functional consequences in relation to D1-:D2-interactions than in simply influencing responsivity to a D2-agonist. Such interactive effects should be taken into account when considering the pathophysiology and treatment of age-related extrapyramidal movement disorders.

Apomorphine and pergolide induce hypothermia by stimulation of dopamine D-2 receptors

Studies in the male rat have shown that the dopamine D-2 receptor antagonists sulpiride and eticlopride, produce a dose-dependent prevention of the hypothermia induced by the D-1/D-2 receptor agonist apomorphine and the relatively selective D-2 agonist pergolide in the rat. In contrast, the D-1 antagonist SCH 23390 (given by the s.c. and i.p. route of administration) failed to prevent the hypothermic effect induced by both DA agonists, but tended to enhance the hypothermia caused by the two DA agonists. Thus, D-2 dopamine receptors appear to play a decisive role in the mediation of the hypothermic response of apomorphine and pergolide. There may also exist an interaction between D-1 and D-2 receptors in the expression of DA-agonist-induced hypothermia.

Apomorphine in the evaluation of dopaminergic function in man

1. Apomorphine (Apo), a short acting dopamine (DA) receptor agonist, stimulates growth hormone (GH) secretion, decreases prolactin secretion, induces yawning, penile erections and other physiological effects in man. An effect on behavior, movement disorders and alcoholism has also been described. 2. Apo-mediated responses are used to evaluate DA function in psychiatric and neurological disorders. Many of the studies in schizophrenia using the GH response to Apo as an index of central DA function are difficult to interpret because of failure to control for key variables. 3. The GH response to Apo is a useful system to evaluate the effects of various drugs including peptides which may not cross the blood brain barrier on DA function in man. 4. Apo is a potent sedative. Specific antimanic, antischizophrenic, and anticraving effects in alcoholics have not been convincingly demonstrated. Side effects of Apo and failure to use active placebo make double-blind studies difficult. 5. Apo improves parkinsonian symptoms and certain forms of reflex epilepsy but beneficial effects in other involuntary movement disorders requires further documentation. 6. Apo may be a useful agent to evaluate DA function in impotent patients and predict a therapeutic response to long-acting dopaminergic agents. 7. Impairment of DA function may play a role in diabetic impotence. 8. The development of a simple polygraphic method to monitor the yawning response to Apo may facilitate clinical studies on the basic physiology of yawning in man and the use of the yawning response as a measure of central DA function in schizophrenia and other clinical disorders. 9. The use of Apo with 18F-fluorodeoxyglucose positron emission tomography to examine regional DA function in man opens up a promising area of research. 10. Though long-acting orally active aporphine DA agonists and antagonists have been developed the problem of tolerance may limit their therapeutic potential.

Disposition of apomorphine in rat brain areas: relationship to stereotypy

Apomorphine-induced stereotyped behavior and apomorphine levels in plasma, striatum and nucleus accumbens were determined in rats at various intervals after a single i.p. injection of serial doses of apomorphine hydrochloride (0.625, 1.25, 2.5 and 5 mg/kg). Apomorphine disappeared from plasma in a mono-exponential mode with a half-life of about 10 min. In striatum and nucleus accumbens apomorphine concentrations peaked 10 min after administration, declining thereafter with a half-life comparable to that in plasma. Apomorphine was concentrated and distributed similarly in the two brain regions; the brain areas/plasma ratio was approximately seven for all doses tested. The rise of apomorphine levels in brain areas slightly preceded the behavioral response, whereas after the peak effect (20-30 min) the intensity of stereotypy declined almost parallel with the log drug concentrations. Plotting apomorphine levels in the tissues assayed against the drug response at the same interval for individual rats, regardless of dose, indicated a highly significant relation between the degree of behavioral effects and brain apomorphine levels. The threshold apomorphine concentrations for inducing stereotyped behaviour were 108 and 95 ng/g respectively in striatum and nucleus accumbens. These findings show that the time course and magnitude of the behavioral effects of apomorphine corresponded with its brain levels in 'dopaminergic' areas, suggesting that apomorphine-induced stereotyped behavior in rats can be described by a direct mechanism. Reserpine (5 mg/kg s.c.) enhanced the apomorphine stereotypy but did not affect apomorphine's disposition in brain and plasma.(ABSTRACT TRUNCATED AT 250 WORDS)

The emetic effect of B-HT 920 and apomorphine in the dog: antagonism by haloperidol

Recent investigations have suggested that the alpha 2-adrenoreceptor agonist B-HT 920 is also a dopamine (DA) agonist with a selectivity for presynaptic receptors. In the present study, the emetic effect of B-HT 920 was investigated. Intravenous injections of B-HT 920 (0.32-10.0 micrograms/kg) and a DA2-agonist apomorphine (3.2-100.0 micrograms/kg) caused dose-dependent emesis. The ED50 of B-HT 920 and apomorphine were 3.2 and 12.3 micrograms/kg, respectively. When haloperidol (10.0-24.5 micrograms/kg i.v.), a DA2-antagonist, was given 5 minutes before B-HT 920 (10 micrograms/kg) or apomorphine (32 micrograms/kg), it caused a dose-dependent prevention of B-HT 920- and apomorphine-induced emesis. The ED50 of haloperidol in preventing the emetic effect of both drugs was identical (13.5 micrograms/kg). In contrast, haloperidol (32 micrograms/kg i.v.) did not prevent the emetic effect of ouabain (40 micrograms/kg i.v.). Neither did yohimbine (0.1 mg/kg i.v.), an alpha 2-adrenoreceptor antagonist, prevent the emetic effect of B-HT 920 (10 micrograms/kg). These results suggest that B-HT 920, acting like apomorphine, induces emesis by activating DA2-receptors probably in the chemoreceptor trigger zone of the area postrema.

Behavior performed at onset of drug action and apomorphine stereotypy

This study tests the hypothesis that stereotyped behavior induced by apomorphine reflects strengthening of behavior observed at the onset of drug action. Rats (n = 26) were injected with apomorphine (1.25 mg/kg) while being engaged in copulation. Contrary to the hypothesis, apomorphine did not strengthen ongoing copulation but arrested it. Instead of mating the males became engaged in behavior typically exhibited by rats injected with the drug. It is speculated that stereotyped behavior may be an expression of a drug-induced shift of attention or responsiveness to some particular range of stimuli.

Effects of antidepressant drugs, selective noradrenaline-or 5-hydroxytryptamine uptake inhibitors, on apomorphine-induced hypothermia in mice

Antidepressant drugs which are selective noradrenaline (NA) uptake inhibitors (desipramine, maprotiline, oxaprotiline, talsupram: 0.625-10 mg/kg) antagonized dose-dependently hypothermia induced by 16 mg/kg apomorphine (APO) in mice. Of the two stereoisomers of oxaprotiline, only that inhibiting NA uptake was active. Antidepressants which are selective 5-hydroxytryptamine (5-HT) uptake inhibitors (citalopram, fluvoxamine: 2.5-40 mg/kg) did not affect APO (16 mg/kg)-induced hypothermia. Neither NA nor 5-HT uptake inhibitors counteracted hypothermia induced by 1 mg/kg APO, a dose which is easily antagonized by low doses of dopamine receptor blockers. The antagonistic action of desipramine towards APO (16 mg/kg)-induced hypothermia was prevented by phenoxybenzamine, prazosin and (-)-propranolol, while (+)-propranolol and cyproheptadine were inactive. St. 587 (an alpha 1-adrenoceptor agonist) or salbutamol (an agonist of beta-adrenoceptors) attenuated APO (16 mg/kg)-induced hypothermia: given jointly, the drugs completely reversed it. m-CPP, a 5-HT receptor agonist, did not affect APO (16 mg/kg)-induced hypothermia. In conclusion, the antagonistic action of antidepressant drugs towards APO (16 mg/kg)-induced hypothermia in mice did not reflect their "antidepressant properties", dopamine antagonism or their action on 5-HT receptors, only their effects on the NA uptake and/or NA transmission. Both alpha 1 and beta-adrenoceptors are involved in this antagonistic action.

Concentration-response relations for apomorphine effects on heart rate in conscious rats

The pharmacokinetics of apomorphine in plasma and brain tissue have been studied in relation to the time courses of effects on heart rate in conscious rats. The kinetic behaviour was investigated after 2 mg kg-1 i.v. and 5 mg kg-1 s.c., respectively. Apomorphine showed a high total plasma clearance (165-207 ml min-1 kg-1) and, despite a relatively large volume of distribution (3.4-4.1 litre kg-1), a biological half-life of about 14 min was obtained irrespective of route of administration. The kinetics in whole brain were identical with those in plasma. Apomorphine produced biphasic effects on the heart rate during the time courses of subcutaneous single doses: a low dose (50 micrograms kg-1) induced pure bradycardia while the doses of 100 micrograms kg-1 and 5 mg kg-1 produced responses oscillating between bradycardia and tachycardia. When we evaluated the relation between apomorphine concentrations and effects on the heart frequency with a composed sigmoid Emax model, apomorphine exhibited a U-shaped steady-state plasma concentration-response curve. Bradycardia appeared after low concentrations, reached a maximum and then decreased with increasing concentrations. A further augmentation of apomorphine concentration resulted in the opposite effect, i.e. tachycardia. Separate concentration-response curves for bradycardia and tachycardia were calculated. The changes in biophase concentration that occur during the absorption and disposition may thus cause the fluctuations between contrasting effects seen during the time course of a single dose.

Antagonism of apomorphine-induced yawning by SCH 23390: evidence against the autoreceptor hypothesis

The ability of apomorphine to induce yawning (YWG) in normal and reserpinized rats and its interaction with SCH 23390, a potent and specific D-1 receptor antagonist, was studied. Apomorphine was more potent in inducing YWG in reserpine-pretreated as compared to control rats. SCH 23390, in low doses (0.05 mg/kg SC), was able to significantly reduce the YWG evoked by apomorphine both in control and in reserpine-pretreated rats. The results indicate that D-1 receptors contribute to YWG elicited by apomorphine and contradict the idea that this effect is mediated by DA autoreceptors.

Differential effects of 5-hydroxytryptaminergic antagonists upon apomorphine- and lergotrile-induced hypothermia and stereotyped behaviour in rats

The dopaminergic stimulants apomorphine and lergotrile both evoked hypothermia and stereotyped behaviour in rats. These drug effects were sensitive to antagonism by haloperidol, a dopaminergic receptor blocker. In rats pretreated with 5-hydroxytryptaminergic receptor blockers, cinanserin reduced apomorphine-induced hypothermia but cyproheptadine did not. Both cinanserin and cyproheptadine significantly potentiated lergotrile-induced hypothermia. Similarly, the stereotypic effects of apomorphine were partly reduced by cyproheptadine, although higher doses of cyproheptadine did potentiate lergotrile-induced stereotyped behaviour. These findings of different influences of 5-HT antagonists upon the effects of apomorphine and lergotrile indicate that these two dopaminergic stimulants may not work in identical manner to produce outwardly similar drug effects.

Pipradrol-induced hyperthermia in the rabbit

Intravenous administration of the stimulant drug pipradrol evoked a dose-dependent hyperthermic response in rabbits. This drug effect was significantly reduced by pretreatment with chlorpromazine, phenoxybenzamine, intravenous BE-2254 or intracerebroventricular BE-2254. In animals pretreated with cyproheptadine or atropine, the hyperthermic effect was significantly potentiated. However, haloperidol, pimozide, diphenhydramine and pentobarbital failed to appreciably influence the drug response. These findings suggest that pipradrol-induced hyperthermia in the rabbit involves activation of central alpha-adrenergic receptors and is apparently enhanced by blockade of serotonergic or cholinergic receptors.

Systematic comparison of apomorphine-induced behavioral changes in two mouse strains with inherited differences in brain dopamine receptors

Dosage and time dependencies of apomorphine-induced changes in stereotyped behaviors (climbing, gnawing and sniffing), locomotor activity and rearing activity were compared in young adult male mice of two inbred strains, DBA/2 and C57BL/6. These two strains are known to differ in their genetically specified brain dopamine receptor number. Apomorphine administered intraperitoneally at dosages of 0.5-20 mg/kg failed to induced stereotyped climbing in DBA/2 at any of the doses tested but markedly increased climbing in C57BL/6 at higher dosages. Apomorphine-induced stereotyped gnawing occurred in both strains at higher dosages although the latency was shorter and maximal effect greater in C57BL/6. Stereotyped sniffing was induced in both strains to a comparable degree at doses greater than or equal to 2.0 mg/kg, and the duration of this stereotypy was indistinguishable between strains. Locomotor activity was inhibited maximally in DBA/2 at an apomorphine dosage of 2 mg/kg and was inhibited to a greater extent than was C57BL/6. Rearing was inhibited in both strains by doses of apomorphine greater than or equal to 0.5 mg/kg; however the duration of the effect was considerably greater in DBA/2 than in C57BL/6. These data suggest that genetically determined differences in central dopamine receptors may have profound and selective effects on behaviors mediated by dopamine pathways; that complex behavioral patterns, e.g., apomorphine-induced stereotypy, may be dissected in to individual components by identifying neuropharmaco genetic differences between strains; that marked strain-specific, inherited differences in dopamine agonist-induced behavioral changes do occur among inbred, non-mutant mouse strains and that their occurrence in other mammalian species including man should be considered.