D2-dopamine receptor and alpha2-adrenoreceptor-mediated analgesic response of quercetin.

Quercetin, a bioflavonoid (100-300 mg/kg) produced dose dependent increase in tail-flick latency, the analgesic effect being sensitive to reversal by naloxone (1 mg/kg). Prior treatment with haloperidol (1 mg/kg), D1/D2 receptor antagonist haloperidol, sulpiride (50 mg/kg), a selective D2 receptor antagonist, yohimbine (5 mg/kg), a alpha2-adrenoreceptor antagonist but not by SCH 23390 a, selective D1 receptor antagonist blocked this response. Apomorphine (1 mg/kg) a mixed D1/D2 dopamine receptor agonist, and quinpirole (0.5 mg/kg), a selective D2 receptor agonist also produced antinociception, that was reversed by haloperidol (1 mg/kg), sulpiride (50 mg/kg), but not by yohimbine (5 mg/kg). The antinociceptive action of quercetin (200 mg/kg) was potentiated by D2 agonist quinpirole (0.2 mg/kg). Dopamine D1 receptor agonist SKF38393 (10 and 15 mg/kg) failed to alter the antinociceptive effect of quercetin (200 mg/kg). Quercetin (200 mg/kg) reversed reserpine (2 mg/kg-4 hr) induced hyperalgesia, which was reversed by sulpiride but not by yohimbine. Thus, a role of dopamine D2 and alpha2-adrenoreceptors is postulated in the antinociceptive action of quercetin.

Dose-response functions of apomorphine, SKF 38393, LY 171555, haloperidol and clonidine on the self-stimulation evoked from lateral hypothalamus and ventral tegmentum.

The experimental animals were implanted with two bipolar electodes, one in the lateral hypothalamus including medial forebrain bundle (LH-MFB) and other in ipsilateral ventral tegmental area-substantia nigra (VTA-SN) and were trained to press a pedal for self-stimulation. This provided the scope to compare directly the effect of a given dose of a drug on the two reward regions in the same animal in the same testing situation. The current intensity was set to produce intracranial self-stimulation (ICSS) response rates of 50% less than the maximal shaping response rates for the respective animals (M60). Following systemic (intraperitoneal) administration of apomorphine (a dopamine receptor D1/D2 mixed agonist), SKF 38393 (D1 > D3 > D2 agonist), LY 17155 or quinpirole (D3 > D2 and D1) agonist), haloperidol (a DA-D2 antagonist), and clonidine (noradrenaline receptor alpha 2 agonist), the ICSS response rates evoked from LH-MFB and VTA-SN were compared with vehicle or saline-treated animals on the basis of dose-response functions. A dose-dependent inhibitory effect at M50 was observed with apomorphine (0.01-1.00 mg/kg) and haloperidol (0.05-0.30 mg/kg) for both the sites of stimulation. These doses of haloperidol did not produce any motor deficits like catalepsy and muscular rigidity. The dose-response and time-effect functions of SKF 38393 and LY 171555 at M50 showed the facilitation and suppression of ICSS of VTA-SN and LH-MFB respectively. Clonidine (0.05-0.25 mg/kg) also produced inhibitory effect on ICSS rates, but this suppression was of different magnitude with respect to the site of stimulation. These doses of clonidine were in the range that did not prevent active pedal pressing responses. ED50 (the dose required to reduce the ICSS response rate 50% of the rate after administration of vehicle) for LY 171555 was 0.8 and 4.4 mg/kg for the ICSS of VTA-SN and LH-MFB respectively and thus statistically different ED50 for apomorphine was 0.27 and 0.36 mg/kg; and for haloperidol was 0.75 and 0.90 mg/kg for LH-MFB and VTA-SN respectively and thus not different significantly. ED50 for clonidine was 0.25 and 0.08 mg/kg for VTA-SN and LH-MFB respectively and thus statistically different. The two-way analysis of variance (ANOVAR) of interaction of dose-response function of alpha 2 agonist with respect to LH-MFB and VTA-SN showed significant independence in their suppressive effects.

Dopamine receptor mediated antidepressant action of B-HT 920 in mice.

Possible involvement of dopaminergic (DAergic) system in forced swimming-induced immobility (despair behaviour) was investigated in mice. B-HT 920 (0.05 and 0.1 mg/kg), a post-synaptic DAergic agonist, produced a dose dependent reduction in immobility period, which was sensitive to blockade by haloperidol (0.5 mg/kg) and sulpiride (100 mg/kg). This effect was also blocked by alpha 2 antagonist yohimbine (5 mg/kg). SKF 38393 (5 mg/kg), a D1-DA agonist potentiated the action of B-HT 920. Reserpinization (2 mg/kg, 24 hr prior) produced despair immobility in mice. When a low dose of B-HT 920 (0.05 mg/kg) was given to reserpinized animals, the duration of immobility period was further increased. But on the other hand, a higher dose (0.1 mg/kg) of it reduced reserpine-induced immobility. Desipramine (5 and 10 mg/kg), elicited a dose dependent reduction in the immobility period, which was sensitive to blockade by sulpiride (100 mg/kg). Desipramine (10 mg/kg) showed a diphasic response in combination with B-HT 920, i.e., a potentiation of the response due to a low dose of B-HT 920 (0.05 mg/kg) and an antagonism of the response due to a higher dose of B-HT 920 (0.1 mg/kg), respectively. SKF 38393 (5 mg/kg), potentiated the action of desipramine (5 mg/kg). SKF 38393 (5 mg/kg) further potentiated the action of desipramine (5 mg/kg) and B-HT 920 (0.05 mg/kg). These observations suggests that B-HT 920 reduces behavioural immobility by DAergic mechanism and desipramine also modulates D2-DA receptors in its anti-depressant action.

On the characterization of dopamine-induced contractile response of rat anococcygeus muscle preparation.

Presence of specific dopamine (DA) receptors and their characterization was attempted in rat anococcygeus muscle preparation. Dopamine (10(-6) M) and B-HT 920 (10(-6) M) produced concentration dependent contractions of the rat anococcygeus muscle preparation. The response of DA was shifted towards right in presence of haloperidol (10(-6) M; pA2 = 6.8) and sulpiride (10(-4) M) in a competitive manner. Alpha 2 antagonists yohimbine (10(-5) M) and idazoxan (10(-5) M) blocked the response to DA in a competitive manner, while alpha 1 antagonist prazosin (10(-5) M) completely blocked the response to DA. SCH 23390 (10(-5) M), a D1 DA antagonist potentiated the response to DA. Reserpinization (5 mg/kg, 24 hr prior) brought about a shift towards the right, and this response was similarly blocked by haloperidol, sulpiride and yohimbine without affecting the maximum response. Desipramine (10(-5) M) blocked the response of DA in a non-competitive manner. Pretreatment of animals with desipramine (10 mg/kg) followed by reserpine, brought about a reversal of action of reserpine. The response to B-HT 920 (10(-6) M), was blocked similarly by haloperidol and yohimbine. However, the effect of desipramine was more pronounced when compared to DA per se. SKF 38393, a D1 DA agonist, potentiated the response to B-HT 920. The findings suggest the presence of both D1 and D2 DA receptors in rat anococcygeus muscle and that DA also acts on adrenergic receptors to produce a contractile response of this muscle preparation.

Concomitant D1 dopamine receptor activation (SKF 38393) unmasks D2 dopaminergic actions of B-HT 920 in mice.

The present study attempts to demonstrate D1/D2 dopamine (DA) receptor interactions during stereotyped behaviour in mice. B-HT 920 [2-amino-6-allyl-5, 6, 7, 8-tetrahydro-4H-thiazolo-(4, 5-d)-azepine] (0.05-1.0 mg/kg), a selective D2-DA agonist, induced mild per se stereotypy consisting mainly of sniffing and rearing responses. Apomorphine, a mixed D1/D2 agonist, also produced typical stereotypic response in mice. The stereotypic response of B-HT 920 was blocked by D2-DA antagonist, sulpiride (50 mg/kg). The effect of apomorphine was not influenced by co-treatment with SKF 38393. Simultaneous administration of B-HT 920 (0.1-0.5 mg/kg) with SKF 38393 (5 mg/kg), a selective D1-DA agonist, elicited dramatic increase in stereotyped behaviours in naive as well as in 24 hr reserpinised (2 mg/kg) mice. Co-treatment of apomorphine (0.5 mg/kg) with B-HT 920 (0.1, 0.25 mg/kg) also resulted in a clearly synergistic effect on stereotyped behaviour. These potentiated responses were reduced or blocked by haloperidol, a D2-DA antagonist. The data suggest that in presence of concomitant stimulation of D1-DA receptors. B-HT 920 exhibits full expression of postsynaptic D2-DA receptor mediated behavioural effects.