Force requirements in lever-pressing and responding after haloperidol.

One hypothesis regarding the actions of neuroleptic drugs on operant responding is that they interfere in some manner with the motoric capability of the animal. To further explore this possibility, we investigated the effects of haloperidol on a bar press response after animals were trained on levers with different force requirements. In the first experiment, two groups of rats were trained to press levers having either low (30 g) or high (100 g) force requirements. The effects of haloperidol on bar pressing when both groups were responding on the light lever were then examined. Under these conditions, the groups showed similar declines in response rates, indicating little transfer between previous experience on the heavy lever and responding in the presence of haloperidol. In the second experiment, the same groups of rats received further training exclusively on either the heavy or the light lever; the rate of responding was similar for the two groups. The effects of haloperidol on pressing either the heavy or the light lever were then examined. It was found that the rate-decreasing effects of haloperidol were similar in both groups. However, when the lever-pressing response was subsequently extinguished, rats working on the heavy lever were significantly more resistant to extinction than the light lever groups. The results of these experiments fail to indicate that the putative motoric effects of haloperidol interact significantly with response force demands. Furthermore, the data provide yet further evidence that blockade of dopamine receptors and removal of reinforcement are not equivalent.