The different populations of dopaminergic neurons located in the ventral mesencephalon have long been associated with distinct functional roles. The nigrostriatal projection is considered necessary for efficient motor performance, while the mesolimbocortical projection is usually associated with reward signalling. However, a number of recent studies in our laboratory suggest that the divergence between these two functions of dopamine is not as delineated as it may once have seemed. In these experiments, we have been developing improved behavioural methods for assessing the nature of the deficit in rats with unilateral dopamine lesions, as well as the efficacy of various experimental cell and gene therapies for Parkinson's disease. The behavioural task we selected is a lateralized nose-poking task in which rats are trained to respond to stimulus lights on either side of their heads. This task not only allows us to accurately measure aspects of motor performance, but, because it requires extensive training, it also allows us to assess aspects of motor learning. The concurrence of motor performance parameters (which are considered to be dependent on striatal dopamine) and motor learning parameters (which are thought to be dependent on mesolimbocortical reward signalling) within the same task has revealed some surprising consequences of dopamine lesions and neuroprotective/neuroreparative approaches to repair in rat models of Parkinson's disease. The data generated using this task suggest that the motor deficits that occur as a consequence of dopamine lesions may be downstream of a deficit in reward signalling. If so, this could redefine our perception of the role of dopamine in controlling motor function.
