Dopamine denervation of specific striatal subregions differentially affects preparation and execution of a delayed response task in the rat

In the present study, the effects of unilateral or bilateral dopamine dener vation of either the dorsal or ventral striatum on the preparation and exec ution of a delayed response task in the rat were investigated. Animals were instructed to hold a lever pressed down by the presentation of a visual an d/or acoustic signal, and were required to hold the lever until a trigger s timulus occurred after an unpredictable delay ranging from 2 to 4 s. The tr igger stimulus required animals to release the lever and to press a second lever for food reinforcement. The time between instruction and trigger sign al represented the preparation phase preceding movement. The motor performa nce was evaluated by using reaction and movement times in addition to corre ct responses in each session. Dopaminergic denervation of either the dorsal or ventral striatum ipsilaterally to the side in which the second lever to be pressed was located did not significantly change reaction and movement times, although it reduced the percentage of correct trials. A significant increase of both reaction and movement times was recorded only after bilate ral denervation of the ventral striatum. The analysis of incorrect response s indicated that dopaminergic innervation of the two striatal subregions ha d different functions in the correct execution of the behavioral paradigm. In the group of animals with dorsal lesions the most frequent incorrect res ponse was represented by a lack of the conditioned response to the presenta tion of the instruction stimulus starting the trial. If the animals reacted properly to this signal, the performance thereafter was correct in the maj ority of trials. Conversely, animals with ventral lesions exhibited a large repertoire of incorrect responses throughout the paradigm, including prema ture release or delayed press of levers, and omission of the second lever p ress. Histological verification of brain coronal sections by tyrosine-hydro xylase immunoreactivity showed that the lesions were confined in either the dorsal or ventral striatum, sparing the lateral region. The data support t he hypothesis that dopaminergic innervation enables the two striatal region s to differently participate in the preparation and execution of complex de layed sensorimotor tasks. Indeed, the dorsal striatum seems to be involved in the correct utilization of external sensory information for the initiati on of conditioned behavior, whereas, the ventral striatum appears to be mai nly concerned with the temporal expectation of impending stimuli that trigg er reward-reinforced movements. (C) 1999 Elsevier Science B.V. All rights r eserved.