DOUBLY DISSOCIABLE EFFECTS OF MEDIAN-RAPHE AND DORSAL-RAPHE LESIONS ON THE PERFORMANCE OF THE 5-CHOICE SERIAL REACTION-TIME TEST OF ATTENTION IN RATS

Six experiments examined the effects of selective median (MRN)- and do rsal (DRN)-raphe nucleus lesions on the performance of the live-choice serial reaction time task. In this test rats are required to localize brief visual stimuli presented randomly in one of five locations in a pproximately 30 min sessions of 100 trials. Both accuracy and latency to respond are measured, as well as the incidence of premature and per severative responding. Selective 5-HT lesions were induced by intra-ra phe infusions of 5,7-dihydroxytryptamine following pretreatment with b oth a noradrenergic and a dopaminergic re-uptake inhibitor. Analysis o f tissue monoamine content demonstrated that the MRN lesion profoundly depleted hippocampal 5-HT (by about 90%) without affecting noradrenal ine and dopamine, whereas the DRN lesion primarily depleted (by about 80%) nucleus accumbens and caudate-putamen 5-HT. Rats with 5-HT lesion s of the MRN performed the task with a similar degree of accuracy to t hat exhibited by sham-operated controls. Although the MRN lesion did n ot affect the latency to respond correctly to the visual targets the l esioned animals collected the food reward significantly faster than th e controls. A transient increase in the number of premature responses also resulted from this lesion. In contrast the DRN lesion produced a transient but significant increase in the accuracy of performance, and increased both the speed and the probability of responding. The simil arity of the effects following global forebrain 5-HT depletion and the selective DRN lesion suggests that the 5-HT projections of the DRN ra ther than the MRN may play an important role in impulsive behaviour fo llowing 5-HT depletion. (C) 1997 Elsevier Science B.V.