BHT-920 and LY-171555 (quinpirole) have similar affinities for striatal D-2 dopamine receptors, and similar affinities for striatal D-1 dopamine receptors

Antiparkinsonian activity of talipexole in MPTP-treated monkeys: in combination with L-dopa and as chronic treatment

We examined whether or not the antiparkinsonian activity of talipexole (B-HT 920, 6-allyl-2-amino-5,6,7,8-tetrahydro-4H-thiazolo[4,5-d]-azepine) could be optimised by combination with L-3,4-dihydroxyphenylalanine (L-dopa). Additionally, the effects of chronic treatment with talipexole on motor behavior were investigated using 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP)-treated and normal common marmosets. Administration of MPTP (0.5 mg/animal i.v. once or twice) to marmosets induced persistent parkinsonian motor deficits. The antiparkinsonian activity of talipexole (40 micrograms/kg s.c.) was significantly enhanced by its combination with L-dopa (30 mg/kg i.p.). This may further support the postulated postsynaptic dopamine D2 receptor agonist properties of talipexole. Chronic treatment with talipexole (a daily dose of 40 micrograms/kg s.c. for 21 days) did not lead to tolerance to the antiparkinsonian activity in MPTP-treated animals. No obvious dyskinesia was seen throughout the chronic treatment. In contrast, in normal marmosets, talipexole at a dose of 80 micrograms/kg which is a dose sufficient to induce hyperactivity did not increase motor activity during the treatment repeated for 21 days. These results suggest that talipexole is a selective dopamine D2 receptor agonist drug of potential use in the treatment of Parkinson's disease.

Stimulation of adenosine A2 receptors induces catalepsy

The central administration of the adenosine A2 agonist CGS 21680 induced catalepsy in the rat. This effect was counteracted by the previous systemic administration of the adenosine antagonist theophylline or the D2 agonist BHT-920. These results are in agreement with the view that adenosine A2 receptors regulate central dopamine D2 transmission and underline the potential antipsychotic activity of A2 agonists.

Electroencephalographic correlates of the sedative effects of dopamine agonists presumably acting on autoreceptors

Alterations both in the activity of the cortical EEG and behaviour were studied after administration of dopamine receptor agonists. In addition to apomorphine, which provided contrasting effects, both on the EEG and behaviour, when small and large doses were compared, alterations elicited by the D2 agonist, quinpirole and another agonist, with preference for dopamine D2 autoreceptors, talipexole (B-HT 920), were evaluated by using telemetric EEG recordings in rats. Similarly to apomorphine, quinpirole produced opposite effects after small and large doses: a small dose (0.05 mg/kg) led to sedation and an increase of EEG power spectra in all of the bands, except beta 2, whereas a larger dose (0.5 mg/kg) elicited stereotypy and desynchronization of the EEG, with a characteristic increase of power in the alpha 1 band. The effects on the EEG and on behaviour, obtained with the small dose of quinpirole were very similar to those of a small dose of apomorphine (0.05 mg/kg) and a small dose of talipexole (0.02 mg/kg) but even the large dose of talipexole (0.5 mg/kg) produced similar effects: all of these treatments produced behavioural sedation and an increase of power in the EEG in all of the bands, except beta 2; such increases appeared most pronounced in the delta band. The present study provides further evidence that drugs, which are assumed to activate dopamine autoreceptors, are effective in inducing sedation. This sedation was accompanied by a characteristic pattern, observed in EEG power spectra analysis.