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

Postsynaptic dopamine (D(2))-mediated behavioural effects of high acute doses of artemisinin in rodents

Artemisinin or qinghaosu is the active principle of quinghao (Artemisia annua L.) developed from Chinese traditional medicine, which is now widely used around the world against falciparum malaria. Behavioural effects of high acute doses of artemisinin were studied on spontaneous motor activity (SMA), exploratory behavior, apomorphine-induced stereotype behavior and pentobarbital sleeping time in mice and rats in order to provide additional evidence on its safety profile on the central nervous system (CNS). Effects of the drug on bromocriptine-induced hyperactivity in short term reserpinised mice were also evaluated. Intraperitoneal (i.p.) injection of artemisinin at doses of 50 and 100mg/kg, significantly (P<0.05) reduced the SMA in mice, prolonged the pentobarbital sleeping time in rats, and attenuated the apomorphine-induced stereotypy in mice. Mice pretreated with reserpine, showed a significant decrease in locomotor activity compared to the saline-treated group. Bromocriptine, a D(2) receptor agonist, induced locomotor activity in mice pretreated with reserpine which was attenuated by artemisinin. The results suggest that artemisinin possesses sedative property, which may be mediated via postsynaptic dopamine (D(2)) receptor in the CNS.

5 Centrally Acting Dopamine D2 Receptor Ligands: Agonists

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

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

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

Postsynaptic dopamine/adenosine interaction: II. Postsynaptic dopamine agonism and adenosine antagonism of methylxanthines in short-term reserpinized mice

Caffeine and its first-stage metabolites (paraxanthine, theophylline and theobromine) caused a significant potentiation of the locomotor activity induced by bromocriptine, 5 mg/kg, in mice pretreated with reserpine, 5 mg/kg (4h prior to the start of motor activity recordings). None of these substances significantly enhanced locomotor activity in reserpinized mice when administered alone. The rank order of potency was caffeine greater than paraxanthine greater than theophylline greater than theobromine. A high dose of a D-2 antagonist (sulpiride 100 mg/kg) caused a marked inhibition of the locomotor activity induced by bromocriptine, 5 mg/kg, plus 25 mg/kg of caffeine, paraxanthine or theophylline. However, a high dose of a D-1 antagonist (SCH-23390 1 mg/kg) caused a significant decrease of the locomotor activity induced by bromocriptine 5 mg/kg, plus 25 mg/kg of caffeine or paraxanthine, but did not change the locomotor activity caused by bromocriptine, 5 mg/kg, plus theophylline 25 mg/kg. The inhibitory effect of 5'-(N-ethyl)carboxamido-adenosine (NECA), 0.025 mg/kg, on bromocriptine-induced locomotor activation in reserpinized mice was reversed by the simultaneous administration of 10, 25 and 50 mg/kg of caffeine, paraxanthine or theophylline. The rank order of potency for reversal was theophylline greater than paraxanthine = caffeine. We suggest that methylxanthines act postsynaptically by potentiating the effects of D-2 stimulation and that this potentiation can be produced by D-1 agonism (paraxanthine or caffeine) and by adenosine antagonism (theophylline, paraxanthine or caffeine), most probably involving A-2 receptors.

Postsynaptic dopamine/adenosine interaction: I. Adenosine analogues inhibit dopamine D2-mediated behaviour in short-term reserpinized mice

Mice pretreated with reserpine 5 mg/kg (4 h prior to the start of motor activity recording) showed locomotor activation after the administration of the D-2 agonist bromocriptine (5 mg/kg). This bromocriptine-induced locomotor activity was dose dependently inhibited by the co-administration of a D-2 antagonist (sulpiride) and dose dependently potentiated by a D-1 agonist (CY 208-243). The potentiating effect of the D-1 agonist could be inhibited by either a D-1 or a D-2 antagonist (SCH 23390 1 mg/kg or sulpiride 100 mg/kg, respectively). The bromocriptine-induced locomotor activity was not altered by either blockade of D-1 dopaminergic receptors (SCH 23390 1 mg/kg) or by co-administration of a greater dose of reserpine (10 mg/kg) plus the dopamine synthesis inhibitor, alpha-methyl-p-tyrosine (200 mg/kg). The adenosine agonists, L-PIA (a preferentially A-1 adenosine agonist) and NECA (an A-1 and A-2 adenosine agonist with above 10-fold greater affinity for A-2 than L-PIA) inhibited in a dose-dependent manner the effect of bromocriptine, NECA being above ten times more potent than L-PIA. The findings show that bromocriptine stimulates postsynaptic D-2 receptors in dopamine-depleted mice and that this effect can be inhibited by adenosine stimulation. The existence of a postsynaptic D-2/A-2 interaction is suggested, the stimulation of A-2 receptors causing an inhibition of responses elicited by postsynaptic D-2 stimulation.