Amphetamine reinstates polydipsia induced by chronic exposure to quinpirole, a dopaminergic D2 agonist, in rats

The hypothesis that the combined activation of D-1 and D-2 dopaminergic receptors is instrumental in inducing amphetamine (AMPH)-mediated hyperdipsia was tested in rats. The D-1 agonist SKF-38393 (SKF) and the D-2 agonist quinpirole (QNP) were ip injected, alone or in combination. to male rats for 10 days. After 2 days of wash-out, a single dose of AMPH (3 mg/kg) was administered. Intake of water and food and diuresis were daily measured at 2, 5 and 24 h. In two further experiments the higher dose of QNP (0.56 mg/kg) was given with two different doses of the D-1 antagonist SCH-23390 (SCH), or, respectively, of the peripheral D-2 antagonist domperidone (DMP). In a fourth experiment, the possibility that QNP, given alone or in combination with SKF, produces an AMPH-like internal state was evaluated by using a drug-discrimination paradigm. Results show that chronic administration of QNP produced a significant increase of 24 h water intake that was reinstated by AMPH. This QNP effect was only partially prevented by DMP, suggesting a main central mechanism of action. By itself D-1 receptor manipulation did not affect water intake, but influenced QNP polydipsia that, accordingly, was enhanced by the lower dose of SKF (0.3 mg/kg) and inhibited by the lower dose of SCH (0.01 mg/kg). In rats trained to discriminate AMPH from solvent, QNP partially generalized for the AMPH stimulus, an effect that was potentiated by SKF. In conclusion, a D-1-modulated sensitization of D-2 dopaminergic mechanisms is probably involved in AMPH-induced hyperdipsia