Facilitation of conditioned motor suppression by microinjections of dopamine in the caudate nucleus of cats.

The effects of Caudate Nucleus (CN) injections of catecholamines on the suppression of a motor conditioned response (MCR), lever pressing, was investigated. Cats were trained to press a lever to obtain 0.5 ml of milk when a conditioned discriminative stimulus was on (CS-on-reward on, MCR) and to suppress the response when the light was off (CS-off-reward off, suppression of motor conditioned response, SMCR). The bilateral application of 5 or 10 micrograms of Dopamine (DA) through chronically implanted cannulae in the CN significantly decreased the lever pressing in the non rewarding situation without changing the MCR. Injection of 5 micrograms of L-DOPA caused very small effects assessed during the following 10 min. However, 10 micrograms of L-DOPA produced a significant decrement of lever pressing (CS-off) in two out of four injections. These findings further support the postulation that catecholamines in the CN have a behavioral inhibitory action upon a motor conditioned response.