Piribedil: behavioural, neurochemical and clinical profile of a dopamine agonist

A novel arylpiperazine derivative (LQFM181) protects against neurotoxicity induced by 3- nitropropionic acid in in vitro and in vivo models

In the pursuit of novel antioxidant therapies for the prevention and treatment of neurodegenerative diseases, three new arylpiperazine derivatives (LQFM181, LQFM276, and LQFM277) were synthesized through a molecular hybridization approach involving piribedil and butylated hydroxytoluene lead compounds. To evaluate the antioxidant and neuroprotective activities of the arylpiperazine derivatives, we employed an integrated approach using both in vitro (SH-SY5Y cells) and in vivo (neurotoxicity induced by 3-nitropropionic acid in Swiss mice) models. In the in vitro tests, LQFM181 showed the most promising antioxidant activity at the neuronal membrane and cytoplasmic levels, and significant neuroprotective activity against the neurotoxicity induced by 3-nitropropionic acid. Hence, this compound was further subjected to in vivo evaluation, which demonstrated remarkable antioxidant capacity such as reduction of MDA and carbonyl protein levels, increased activities of succinate dehydrogenase, catalase, and superoxide dismutase. Interestingly, using the same in vivo model, LQFM181 also reduced locomotor behavior and memory dysfunction through its ability to decrease cholinesterase activity. Consequently, LQFM181 emerges as a promising candidate for further investigation into its neuroprotective potential, positioning it as a new therapeutic agent for neuroprotection.

Safety considerations when using non-ergot dopamine agonists to treat Parkinson's disease

INTRODUCTION

Nonergot dopamine agonists (NEDA) represent an excellent treatment option for Parkinson's disease (PD) patients, in both early and advanced stages of the disease. The post-marketing phase of NEDA has highlighted, though, the occurrence of important long-term adverse events.

AREAS COVERED

This review reports recent updates on NEDA adverse events, analyzing neurobiological bases and risk factors of these complications. A literature search has been performed using Medline and reviewing the bibliographies of selected articles.

EXPERT OPINION

NEDA represents a very important option in the treatment of PD. Criticisms on their use can be overcome through a better knowledge of these molecules and of the risk factors for adverse events which allow specialists to prevent the occurrence of undesired complications and consent a tailor-based approach. Abbreviations: PD: Parkinson's disease, DA: dopamine agonists, NEDA: non-ergot dopamine agonists, ICD: impulse control disorders, DAWS: dopamine agonist withdrawal syndrome, CYP: Cytochrome P, PK: pharmacokinetic, AUC: area under the curve, HRT: hormone replacement therapy, AV: atrioventricular, HF: heart failure, OH: orthostatic hypotension, RBD: REM behavior disorders, PDP: Parkinson's disease psychosis, DRT: dopamine replacement therapy, DDS: dopamine dysregulation syndrome, MMSE: Mini-Mental state examination, EDS: excessive daytime somnolence.

Piribedil for the Treatment of Motor and Non-motor Symptoms of Parkinson Disease

Dopamine agonists are well-established symptomatic medications for treating early and advanced Parkinson disease (PD). Piribedil was one of the first agonists to be marketed (1969) and is widely used as an extended-release oral formulation in European, Latin-American, and Asian countries. Piribedil acts as a non-ergot partial dopamine D2/D3-selective agonist, blocks alpha2-adrenoreceptors and has minimal effects on serotoninergic, cholinergic, and histaminergic receptors. Animal models support the efficacy of piribedil to improve parkinsonian motor symptoms with a lower propensity than levodopa to induce dyskinesia. In PD patients, randomized double-blind studies show that piribedil (150-300 mg/day, three times daily) is superior to placebo in improving motor disability in early PD patients. Based on such evidence, piribedil was considered in the last Movement Disorder Society Evidence-Based Medicine review as "efficacious" and "clinically useful" for the symptomatic treatment of PD, either as monotherapy or in conjunction with levodopa, in non-fluctuating early PD patients. This effect appears comparable to what is known from other D2 agonists. However, randomized controlled trials are not available to assess the effect of piribedil in managing levodopa-induced motor complications. Pilot clinical studies suggest that piribedil may improve non-motor symptoms, such as apathy, but confirmatory trials are needed. The tolerability and safety profile of piribedil fits with that of the class of dopaminergic agonists. As for other non-ergot agonists, pneumo-pulmonary, retroperitoneal, and valvular fibrotic side effects are not a concern with piribedil. The original combination of piribedil D2 dopaminergic and alpha-2 adrenergic properties deserve further investigations to better understand its antiparkinsonian profile.

From the cell to the clinic: a comparative review of the partial D₂/D₃receptor agonist and α2-adrenoceptor antagonist, piribedil, in the treatment of Parkinson's disease

Though L-3,4-dihydroxyphenylalanine (L-DOPA) is universally employed for alleviation of motor dysfunction in Parkinson's disease (PD), it is poorly-effective against co-morbid symptoms like cognitive impairment and depression. Further, it elicits dyskinesia, its pharmacokinetics are highly variable, and efficacy wanes upon long-term administration. Accordingly, "dopaminergic agonists" are increasingly employed both as adjuncts to L-DOPA and as monotherapy. While all recognize dopamine D(2) receptors, they display contrasting patterns of interaction with other classes of monoaminergic receptor. For example, pramipexole and ropinirole are high efficacy agonists at D(2) and D(3) receptors, while pergolide recognizes D(1), D(2) and D(3) receptors and a broad suite of serotonergic receptors. Interestingly, several antiparkinson drugs display modest efficacy at D(2) receptors. Of these, piribedil displays the unique cellular signature of: 1), signal-specific partial agonist actions at dopamine D(2)and D(3) receptors; 2), antagonist properties at α(2)-adrenoceptors and 3), minimal interaction with serotonergic receptors. Dopamine-deprived striatal D(2) receptors are supersensitive in PD, so partial agonism is sufficient for relief of motor dysfunction while limiting undesirable effects due to "over-dosage" of "normosensitive" D(2) receptors elsewhere. Further, α(2)-adrenoceptor antagonism reinforces adrenergic, dopaminergic and cholinergic transmission to favourably influence motor function, cognition, mood and the integrity of dopaminergic neurones. In reviewing the above issues, the present paper focuses on the distinctive cellular, preclinical and therapeutic profile of piribedil, comparisons to pramipexole, ropinirole and pergolide, and the core triad of symptoms that characterises PD-motor dysfunction, depressed mood and cognitive impairment. The article concludes by highlighting perspectives for clarifying the mechanisms of action of piribedil and other antiparkinson agents, and for optimizing their clinical exploitation.

The dopamine agonist piribedil with L-DOPA improves attentional dysfunction: relevance for Parkinson's disease

Cognitive deficits are often associated with motor symptoms in Parkinson's disease. This study investigates the ability of piribedil ([(methylenedioxy-3,4 benzyl)-4 pyperazinyl-1]-2 pyrimidine), a D(2)/D(3) dopamine (DA) receptor agonist with antagonist activity at alpha(2A)-adrenoceptors, to restore motor and attentional deficits in nigrostriatal 6-hydroxydopamine-lesioned rats. Subjects were trained to depress a lever, detect a stimulus occurring after variable foreperiods, and release the lever quickly afterward. Striatal DA depletions produce deficits in the timing of foreperiods and prolong reaction times. Although a subchronic treatment with piribedil (0.1-2 mg/kg) is not effective, a dose of 0.3 mg/kg administered for 3 weeks significantly reverses the akinetic deficits produced by the striatal dopamine depletion and progressively improves attentional deficits. When coadministered with the dopamine prodrug l-3,4-dihydroxyphenylalanine (l-DOPA) (3 mg/kg), piribedil (0.3 mg/kg) promotes a rapid and full recovery of preoperative performance. These results suggest that administration of l-DOPA in combination with piribedil in a chronic treatment as either initial or supplemental therapy for Parkinson's disease might improve cognitive functions while reducing the risk for motor complications.

Quantitative determination of some pharmaceutical piperazine derivatives through complexation with iron(III) chloride

A simple, accurate and sensitive spectrophotometric method has been developed for the determination of three pharmaceutical piperazine derivatives, namely ketoconazole (KC), trimetazidine hydrochloride (TMH) and piribedil (PD). This method is based on the formation of yellow orange complexes between iron(III) chloride and the investigated drugs. The optimum reaction conditions, spectral characteristics, conditional stability constants and composition of the water soluble complexes have been established. The method permits the determination of KC, TMH and PD over a concentration range 1-15, 1-12 and 1-12 microg ml(-1), respectively. Sandell sensitivity is found to be 0.016, 0.013 and 0.013 microg cm(-2) for KC, TMH and PD, respectively. The method was sensitive, simple, reproducible and accurate within +/-1.5%. The method is applicable to the assay of the three drugs under investigation in different dosage forms and the results are in good agreement with those obtained by the official methods (USP and JP).

Electroanalytical characteristics of piribedil and its differential pulse and square wave voltammetric determination in pharmaceuticals and human serum

The electrochemical oxidative behavior of piribedil (PR) was described. It was investigated by cyclic, linear sweep, differential pulse (DPV) and square wave (SWV) voltammetric techniques. The redox behavior of PR was found irreversible. Different parameters were tested to optimize the conditions for the determination of PR. The dependence of intensities of currents and potential on pH, concentration, scan rate, nature of the buffer was investigated. Two sensitive methods for the measurement of PR were described. For analytical purposes, a very well resolved diffusion controlled voltammetric peak was obtained in 0.1 M H(2)SO(4) and pH 5.7 acetate buffer. The determination peaks are obtained at 1.27 and 0.95 V for differential pulse and 1.29 and 0.97 V for SWV in 0.1 M H(2)SO(4) and pH 5.7 acetate buffer, respectively. The linear response was obtained in the ranges of 2 x 10(-6)-1 x 10(-3) M in 0.1 M H(2)SO(4) and 2 x 10(-6)-8 x 10(-4) M in pH 5.7 acetate buffer for both techniques. The proposed techniques were successfully applied to the determination of PR in tablet dosage forms and human serum. Excipients did not interfere in the determination. The necessary statistical validation reveals that the proposed methods are free from significant systematic errors.

Poly(vinyl chloride) ion-selective electrodes for piribedil determination

Piribedil (PD) ion-selective electrodes have been constructed from poly(vinyl chloride) matrix membrane containing piribedil-tetraphenylborate (PD-TPB) as the electroactive component with dibutylphthalate or dioctylphthalate as the plasticizing solvent mediator. The electrodes displayed a linear response over the concentration range 2.0 x 10(-5) to 10(-2) M PD. The working pH ranges of the electrodes were 3.5-6.4 and 3.0-6.0, and the isothermal coefficients of the cells were 0.00129 and 0.00096 V/degrees C respectively. The electrodes were used for the determination of the diprotonated PD species, the most successful being that based on dioctylphthalate solvent mediator. The electrodes show a linear response over the concentration range of 8.0 x 10(-6) to 10(-2) M PD, with Nernstain slope 30 mV/PD concentration decade when preconditioned by soaking in distilled water for 30 min. The electrodes exhibit good selectivity for the PD with respect to a large number of inorganic cations and organic substances of biological fluids. Piribedil is determined successfully in pure solutions and in tablets or in biological fluids using the standard additions and potentiometric titration methods. The membrane withstood soaking in distilled water for more than 5 months.

A randomized, double-blind study of a skin patch of a dopaminergic agonist, piribedil, in Parkinson's disease

This randomized, double-blind trial was designed to evaluate the efficacy of a transdermal system of piribedil on the motor symptoms of Parkinson's disease during 3 weeks of treatment administered to three different groups: placebo, one piribedil patch (1 PP), and two (2 PP) piribedil patches. Twenty-seven patients with idiopathic Parkinson's disease, treated with L-dopa but not sufficiently controlled, were included in this trial. The test treatment did not demonstrate any clinical efficacy on either the main end point (Unified Parkinson's Disease Rating Scale motor score) or the secondary end points (rigidity, bradykinesia, postural, and resting tremor scores). The main adverse events were nausea (11%), vomiting (7.4%), and malaise (7.4%) mainly observed in the placebo group (four of seven patients). The local acceptability of the transdermal system was good. Plasma piribedil concentrations at the end of treatment were 6.74+/-1.10 and 9.31+/-3.33 ng/mL in the 1 PP and 2 PP groups, respectively. These plasma levels could account for the lack of clinical efficacy, because a previous pharmacokinetics-PD study conducted in parkinsonian patients and treated with the intravenous route demonstrated that the critical limits of activity on tremor were between 10 and 30 ng/mL.

Effect of piribedil and its metabolite, S584, on brain lipid peroxidation in vitro and in vivo

We studied the effect of piribedil (1-3,4-methylendioxybenzyl-4-(2-pyrimidyl) piperazine) and its catechol metabolite, S584 (1-(3,4-dihydroxybenzyl-4-(2-pyrimidinyl)-piperazine), on rat brain lipid peroxidation (a) in vitro in rat synaptosomes and cortical slices after induction of an oxidative stress and (b) in vivo in mouse brain after short-term exposure (two and three 4-h cycles) to O2/CO2 (95%:5%). The metabolite (10[-4]-10[-5] M), but not piribedil, prevented Fe3+-stimulated lipid peroxidation in rat synaptosomes and in rat cortical slices incubated with high oxygen concentrations. Piribedil (7.5 and 30 mg/kg, orally), counteracted the increase in thiobarbituric reactive substances in the brain of mice only when these were exposed to two or three cycles of a high oxygen concentration. S584 (30 mg/kg, orally) reduced thiobarbituric acid reactive substances in brain in mice exposed either to air (control) or to three cycles of a high oxygen concentration. These results suggest that piribedil has an antiperoxidative effect in brain, which may be partly related to the in vivo formation of the catechol metabolite, S584.

In vitro affinity of piribedil for dopamine D3 receptor subtypes, an autoradiographic study

Receptor binding autoradiography, using the selective ligand [3H]7-OH-(R)DPAT (R(+)-2-dipropylamino-7-hydroxy 1,2,3,4-tetrahydronaphthalene), showed that piribedil is a potent inhibitor at dopamine D3 receptors in limbic regions (island of Calleja), with affinity (IC50) between 30 and 60 nM. The in vitro IC50 of piribedil for inhibition of [3H]spiperone binding to receptors of the dopamine D2-like family (D2, D3 and D4), ranged between 10(-7) and 10(-6) M in different brain regions (medial and lateral caudate putamen, olfactory tubercles, and nucleus accumbens). At the highest concentration tested (10(-5 M) piribedil inhibited dopamine D1 receptor binding by < 50%. It is concluded that piribedil has 20 times higher affinity for dopamine D3 than for dopamine D2-like receptors, and very low affinity for the dopamine D1 receptor subtype in rat brain. How this pattern of receptor affinity is related to the pharmacological profile of piribedil deserves further investigation.

Restoration of brain protein synthesis in mature and aged rats by a DA agonist, piribedil

Brain ageing affects numerous cerebral metabolic pathways such as cerebral glucose consumption or protein synthesis rate. The pharmacological effect of a mixed D1-D2 dopaminergic agonist, piribedil, on this last metabolism is reported. Cerebral Protein Synthesis Rate (CPSR) was measured by the [35S]L-methionine autoradiographic procedure in 38 main brain regions of 11 and 26-month-old Wistar rats after a 2-month treatment per os at 9 and 30 mg/kg/day with piribedil. Mean decrease of CPSR was -21% during the 15-month ageing we followed, with important local variations. Mean CPSR increased with the two treatments, +25% in mature and +35% in aged rats. Treatments restored CPSR of aged rats to the exact mature subjects levels in quite all the brain regions. No dose-effect or asymetrical modification was statistically revealed for the two treatments. Metabolic increases involved particularly central brain gray structures, especially some DA-targeted brain nuclei concerned with behaviour and learning. This effect argued for a general metabotrophic effect of D1-D2 dopamine stimulation of the brain. The original pattern of local ageing of brain protein synthesis in rat was also incidentally reported. This was the first direct report of a wide and effective metabolic activation of CPSR in the brain during ageing by a curative dopaminergic agonist treatment.

Dopaminergic agonists have both presynaptic and postsynaptic effects on the guinea-pig's medial vestibular nucleus neurons

A number of studies have indicated a possible interaction between dopamine and the vestibular system. Using intracellular recordings in brainstem slices, we have tested the effects of dopamine and other dopaminergic compounds on guinea-pig medial vestibular nucleus (MVN) neurons. In normal medium, MVN neurons were depolarized by dopamine as well as by (-)quinpirole and piribedil, which are selective D2 dopaminergic agonists. The dependence of this effect on the presence of D2 receptors was confirmed by using (-)sulpiride, a D2 antagonist which blocked the depolarizing effect of dopamine. Dopaminergic D1 receptors were apparently not involved in this effect since a selective D1 agonist, SKF-38393, had no effect on MVN neurons and the D1 antagonist (+)SCH-23390 could not block the effect of dopamine. These depolarizing responses to dopamine must be due to a presynaptic action on terminals that normally release GABA spontaneously on MVN neurons, and tonically maintain them in a state of hyperpolarization. Indeed, such a spontaneous release was demonstrated to occur in the slice since application of bicuculline, a GABAA antagonist, depolarized MVN neurons in normal saline, but not in a high Mg2+/low Ca2+ solution known to block synaptic transmission. When dopamine was applied in conditions in which no GABAA-dependent transmission could occur (either in the presence of bicuculline or in a high Mg2+/low Ca2+ solution) only a hyperpolarizing, most probably postsynaptic, effect occurred. These results indicate that dopamine might exert in vivo a significant modulatory action on the vestibular system, either by a direct action on the vestibular neurons or by modulation of GABAergic transmission.

Differential regional and kinetics effects of piribedil and bromocriptine on dopamine metabolites: a brain microdialysis study in freely moving rats

Brain microdialysis coupled to HPLC was applied to freely moving rats to investigate the regional kinetics of piribedil and bromocriptine on the extracellular levels of 3,4-dihydroxyphenylacetic acid (DOPAC) and homovanillic acid (HVA) in striatum, nucleus accumbens, and frontal cortex. Both D2 agonists (20 mg/kg i.p.) decreased DOPAC and HVA in the three brain regions. The responsiveness of frontal cortex to both compounds was greater than those previously reported with other dopaminergic drugs. Regional and temporal differences were observed under piribedil: DOPAC and HVA levels decreased more in the nucleus accumbens than in striatum or frontal cortex but increased over basal values from the 5th hour in the frontal cortex suggesting a late stimulatory effect of piribedil on dopamine synthesis in this area. Such regional effects differentiate piribedil from most other D2 agonists and could explain some behavioural and therapeutic actions possibly related to involvement of nucleus accumbens or/and frontal cortex.

Are retinal or mesencephalic dopaminergic systems involved in monocular optokinetic nystagmus asymmetry in frog?

In monocular vision, frogs display a unidirectional optokinetic horizontal nystagmus (H-OKN) reacting only to temporal-nasal (T-N) stimulation. The N-T component is almost absent. The analysis of search coil recordings after administration of dopamine into the viewing eye, the occluded eye or directly into the pretectum, hardly modifies the H-OKN triggered by the viewing eye irrespective of the concentration used. Conversely, administration of Piribedil, a strong D2 dopamine agonist, provokes the appearance of a N-T component, suppressing the monocular H-OKN asymmetry, whether the drug is injected by intravitreal or intrapretectal route. It is suggested that Piribedil could also bind with receptors other than dopamine's.

Solid-phase extraction of piribedil and its metabolites from plasma and urine without and after deconjugation, by high performance liquid chromatography

A new high performance liquid chromatographic procedure has been developed for the simultaneous quantification of piribedil (PD) and its three main basic metabolites in rat plasma and urine, without and after hydrolysis. The procedure relies on isolation of the compounds from plasma and urine constituents using the Sep-Pak C18 cartridge, with satisfactory recovery and specificity, and resolution by acetonitrile gradient elution on a C18 reversed phase column coupled to a UV detector monitored at 240 nm. The assay was linear over a wide range of concentrations for all compounds in both body fluids with mean within-day and day-to-day coefficient of variation (CV) and relative error (RE) generally below 10%. Plasma concentrations of PD and its metabolites at selected intervals and urinary recoveries of all compounds before and after enzymatic hydrolysis are presented.

Activity and acceptability of piribedil in Parkinson's disease: a multicentre study

Several controlled trials have shown that the dopamine agonist, Trivastal (piribedil), is active in the treatment of Parkinson's disease, particularly with regard to tremor. To determine its efficacy as monotherapy in patients previously untreated with levodopa, a 3-month multicentre study was conducted with Trivastal 50 mg LP in 113 patients with idiopathic Parkinson's disease. The study population consisted of 66 men and 47 women, aged 63.1, SD 0.6 (43-79) years with a 2.1, SD 0.2 (1-15) year history of Parkinson's disease. Mean disease stage was 1.82 (1-4) by the Hoehn and Yahr classification. Tremor was the predominant clinical feature in 42 patients; the remaining 71 patients displayed the full parkinsonian syndrom. Trivastal 50 mg LP was prescribed stepwise up to doses of 150-250 (207, SD 6.4) mg/day at the end of 3 months. No concomitant anti-parkinsonian medication was given. Patients were clinically assessed at 1, 2 and 3 months on the Webster scale, a specific tremor scale and the HARD depression scale. Mean results were as follows in the 90 patients completing the study. On the Webster scale, tremor fell from 1.7 to 1 (-41%, P less than 0.001), bradykinesia from 1.5 to 0.8 (-47%, P less than 0.001) and rigidity from 1.3 to 0.9 (-31%, P less than 0.001); on the specific scale, rest tremor decreased in daily duration and amplitude from 3.9 to 2.4 (-39%, P less than 0.001) and from 2.9 to 2.1 (-35%, P less than 0.001), respectively.(ABSTRACT TRUNCATED AT 250 WORDS)

Retinal dopaminergic receptor affinity and ocular pharmacokinetic profile of piribedil

Binding studies on retinal dopamine receptors have revealed the existence of both D1 and D2 receptors. Human retina micro-autoradiographs confirm the distribution of dopaminergic receptors in the plexiform layers. Piribedil, a dopaminergic agonist, is able to bind to D2 receptors, while its metabolite (S584) preferentially displaces D1-specific radioligands. These results demonstrate that piribedil has a dopamine-like pharmacological profile including direct interaction with receptors. When instilled into the rabbit eye, piribedil penetrates rapidly and accumulates in the pigmented epithelia--the iris ciliary body and chorioretina--before being rapidly cleared. Macro-autoradiographs confirm this distribution and show the levels to be compatible with the affinity of piribedil for retinal dopaminergic receptors.

Determination of piribedil and its basic metabolites in plasma by high-performance liquid chromatography

A high-performance liquid chromatographic method for the determination of piribedil and its p-hydroxylated, catechol and N-oxide metabolites in plasma is described. After addition of an internal standard (buspirone), the plasma samples were subjected to a three-step extraction procedure. The final extracts were evaporated to dryness under nitrogen, and the residues were reconstituted in 100 microliters of mobile phase (0.01 M phosphate buffer-acetonitrile, 50:50, v/v) and chromatographed by acetonitrile gradient elution on a C18 reversed-phase column coupled to an ultraviolet detector set at 240 nm. The method was selective for piribedil and its metabolites, and sufficiently sensitive and precise for studies aimed at elucidating the role of the metabolites in the parent drug's pharmacological effects.

Kinetics of piribedil and effects on dopamine metabolism: hepatic biotransformation is not a determinant of its dopaminergic action in rats

The importance of hepatic metabolism in relation to the central (dopaminergic) effects of piribedil (PD) is still not really known. Plasma and brain kinetics and the effects on striatal dopamine (DA) metabolism of the parent drug and its known metabolites were therefore evaluated in rats, a species widely used in neurochemical studies of this drug. PD injected intraperitoneally (IP, 15-60 mg/kg) and centrally (ICV, 100-200 micrograms/rat) lowered striatal 3,4-dihydroxyphenylacetic acid (DOPAC) and 3-methoxy-4-hydroxyphenylacetic acid (HVA) content and the intensity and time-course of the neurochemical effects were route- and dose-relatedly dependent on brain PD kinetics. The catechol (M1), p-hydroxylated (M2) and N-oxide (M3) metabolites of the drug were detectable only in trace amounts in rat brain and only at the highest IP dose tested; when administered ICV at doses equimolar to PD they caused no decrease in striatal DA metabolites, although striatal concentrations were higher than after IP PD, being comparable to or higher than those of the ICV parent drug. These data suggest that metabolites do not contribute to the dopaminergic effects of PD in rats.

Climbing and stereotyped behaviours in mice require the stimulation of D-1 dopamine receptors

Apomorphine-induced climbing and sniffing behaviours in mice were antagonize by low doses of SCH 23390 and metoclopramide. The selective D-2 dopamine receptor agonists, LY 171555 and RU 24926, and some other dopamine agonists (piribedil, lergotrile, bromocriptine) exerted only inhibitory effects on spontaneous behaviours. The selective D-1 dopamine receptor agonist, SK&F 38393, did not modify the climbing score but increased the sniffing and decreased the gnawing scores compared to the scores of control mice. Typical climbing and stereotyped behaviours were produced by the combination of SK&F 39383 with LY 171555, RU 24926 or with the other dopamine agonists tested. These data suggest that the stimulation of D-1 dopamine receptors is required for the induction of climbing and stereotyped behaviours in mice.

Differential effects of selected dopaminergic agents on locomotor activity in normotensive and spontaneously hypertensive rats

Spontaneously hypertensive rats (SHR) exhibit a significantly higher level of spontaneous locomotor activity than age-matched normotensive controls (WKY). The direct-acting dopamine agonists, apomorphine and pergolide, produced a biphasic effect on locomotor activity levels in normotensive controls. Low doses of these agonists decreased activity levels, while higher doses of these agonists dramatically stimulated activity. In marked contrast to these results was the effect observed in the SHR, in which these agonists at all doses tested decreased activity. Amphetamine, a dopamine releaser, stimulated activity levels in both the WKY and SHR; however, the magnitude of the increase was somewhat attenuated in the SHR.

Yawning elicited by systemic and intrastriatal injection of piribedil and apomorphine in the rat

The behavioural effects of systemic and intrastriatal injections of the dopamine agonists piribedil and apomorphine in male rats were examined. Bilateral application of piribedil (50 and 100 micrograms) or apomorphine (5, 10 and 20 micrograms) to the striatum produced yawning and chewing mouth movements accompanied by intermittent stretching and sexual arousal. Low doses of piribedil (1.25 and 2.5 mg/kg) and apomorphine (0.1 and 0.2 mg/kg) injected SC produced an identical yawning syndrome. Previous work has suggested that yawning elicited by systemic dopamine agonist treatment is a consequence of dopamine autoreceptor stimulation. Similarly, the most likely explanation of the present data is that yawning elicited by systemic and central dopamine agonist treatment was due to activation of dopamine autoreceptors. Systemic injection of haloperidol and scopolamine abolished yawning induced by intrastriatal piribedil and these data provide tentative support for the proposal that a dopamine-acetylcholine link may be involved in the expression of yawning.

Effects of piribedil and of d-amphetamine on locomotor activity in immature rats

The effects of piribedil, a direct dopamine (DA) agonist, and of d-amphetamine, an indirect DA agonist, on locomotor activity in 28-day old rats have been investigated. d-Amphetamine (1,2 and 4 mg/kg) increased activity in a dose dependent manner whereas only a 40 mg/kg dose of piribedil significantly increased activity; low doses of piribedil (1.25-10.0 mg/kg) had no effect on activity. Since hyperactivity induced by a direct DA agonist, such as piribedil, is thought to reflect post-synaptic DA receptor activation, the results appear to indicate that post-synaptic DA receptor mechanisms are fully developed in 28-day old rats. In contrast, as low doses of piribedil which cause sedation in adult animals failed to decrease locomotor activity under the present experimental conditions, it seems that DA autoreceptors may not be well developed in 28-day old rats.