Scopolamine reverses haloperidol-attenuated lever-pressing for water but not haloperidol-attenuated water intake in the rat.

The operant lever-pressing response has previously (Ljungberg, Pharmacol Biochem Behav 27: 341-350, 1987) been found to be inhibited by lower doses of haloperidol than the corresponding consummatory act, i.e., water intake. In the present study it was found that the attenuation of the lever-pressing response caused by the neuroleptic, but not the attenuation of the water intake, could be counteracted by scopolamine. The results support the notion that blockade of operant responding by low doses of neuroleptics are probably related to the extra-pyramidal side-effects of neuroleptics seen in the clinic, as both phenomena can be counteracted by anticholinergics. These results therefore conflict with the anhedonia hypothesis put forward as an explanation of the attenuating effects of neuroleptics in operant settings. The findings also have a clear bearing on the role of dopamine in feeding and drinking behavior, as the results implies that different aspects of the control of water intake (i.e., the operant vs. the consummatory phase) are governed by different mechanisms in the CNS.