Monoamine roles in retention and reversal of delayed response in cats

Two experiments were performed to study the effects of manipulation of central monoamines upon behavior in a delayed response (DR) situation. In the first study, serotonin (5-HT) levels were increased by administration of 5-hydroxy-L-tryptophan (5-HTP) and RO4-4602, a decarboxylase inhibitor, to cats that had overlearned the DR. This intervention had no significant effect upon performance in the 0 sec delay condition, but significantly increase error and non-response scores during delay trials; the effect is specific to an "information holding" demand upon the animal and according to the neurochemical analysis appears to be due to a central effect of 5-HT. In a second experiment, dopamine levels were raised by L-DOPA administration during a reversal of the original DR situation, and the effect of L-DOPA on the evolution of response strategies was observed. All animals developed a position habit that proved impossible to correct but L-DOPA animals developed a significant position habit more quickly than controls, thus suggesting a possible relationship between the neostriatal dopamine accumulation and behavioral plasticity.