Acquisition and performance in a rat sequential reaction time task is not affected by subtotal ventral striatal 6-OHDA lesions

Based on findings of experiments with humans, non-human primates and rodents, it is commonly accepted that dopaminergic basal ganglia processes play a crucial role in procedural and sequential learning. Primal evidence for this hypothesis came from serial reaction time tasks (SRTT) studies, demonstrating that healthy controls show increased reaction times when visual stimulus presentation switches from a previously learned sequence to random stimulus presentation. This so-called interference effect was reduced in patients with Parkinson's disease. Since ethical and methodical aspects limit neurobiological research in human subjects, we developed a rat version of the human SRTT, which can be used to study experimentally induced brain damage. In the present experiment we investigated the effects of bilateral 6-OHDA lesions of the ventral striatum on sequential learning. The lesions led to subtotal dopaminergic depletions in the ventral striatum (58–60%) and also minor depletions in the medial neostriatum (32–46%). These lesions impaired task acquisition only moderately and did not worsen sequential performance since lesion and control animals showed a comparable interference effect when the trained sequence was tested against random stimulus presentation or violated sequences. In contrast, in an earlier SRTT experiment with medial neostriatal dopaminergic lesions (58–66%), the lesion animals were clearly impaired in their sequential learning as compared to controls. Therefore, we assume that subtotal dopamine loss in the medial neostriatum, rather than the ventral striatum, has a substantial effect on sequential learning.