Facilitation of the suppressing effect of dopamine upon a motor conditioned response by 6-hydroxydopamine applied into the caudate nucleus in cats.

It has been described that the application of DA in the Caudate Nucleus improves the learning of an inhibitory motor conditioned response (SMCR). However, the application of 6-hydroxydopamine (6-OHDA) at low doses of 5, 10 and 20 micrograms produced a clear increment of the inhibition, while higher doses of 80 or 160 micrograms diminished markedly the suppression of the MCR. We postulated that although low doses of 6-OHDA produce lesions in the dopaminergic structures they cause hypersensitization by denervation. To prove this hypothesis we observed the effect of DA application into the CN before and after applying one of the lower doses of 6-OHDA. Cats were trained to press a lever (MCR) to obtain 0.5 ml of milk when a conditioned discriminative stimulus (light) was on, and to suppress the response when the light was off (SMCR). The various applications of 10 micrograms of DA in both CNs, always produced a decrement in the lever pressing rate only in the SMCR situation. The effects of the DA applications after the 6-OHDA application were more significant (p < 0.01) than when applied before the 6-OHDA. These findings further support the postulation that catecholamines in the CN improve the inhibitory actions required for the suppression of motor responses.