Differential attenuation of water intake and water-rewarded operant responding by repeated administration of haloperidol and SCH 23390 in the rat.

It has previously been described that water intake in thirsty rats require higher doses of dopamine (DA) D-1 and D-2 antagonists to be attenuated than operant lever-pressing with water as reward. In the present study, effects of repeated administration of the DA D-1 antagonist SCH 23390 and the DA D-2 antagonist haloperidol were investigated in the same experimental paradigm. In agreement with previous reports, attenuation of operant responding increased progressively by haloperidol (0.05 mg/kg) given for four consecutive days. However, this attenuation was not accompanied by decreased water intake, tested for in parallel experiments. After haloperidol (0.2 mg/kg), in contrast, a progressively decreasing attenuation of water intake was found. After SCH 23390, both the initial attenuation of lever-pressing (0.02 mg/kg) and consummatory water intake (0.1 mg/kg) became less pronounced over time. The results thus show that: 1) the previously reported progressively increasing attenuation of operant responding caused by repeated administration of D-2 antagonists is not mimicked by the D-1 antagonist SCH 23390, and 2) attenuation of water intake caused by higher doses of neuroleptics is, in direct opposition, less pronounced after repeated administrations. The results also show that attenuation of operant responding by neuroleptics cannot solely be dependent upon a blunting of the impact of the reward.