Scopolamine and KCl injections into the caudate nucleus. Overtraining-induced protection against deficits of learning

To test the hypothesis that extended training of an instrumental task prevents the performance impairments seen after cholinergic and generalized blockade of caudate-putamen complex (NC) activity in animals with a relatively low degree of training, groups of rats were trained to press a lever under a continuous reinforcement schedule for 5, 15 or 25 sessions; The effects of microinjections of scopolamine and potassium chloride into the CN were then assessed. In agreement with early studies in cats, a significant deficit in performance was produced in the animals with a low or medium degree of training, while no changes in learned behavior were seen in the overtained rats. These results show that: (a) normal neural activity of the CN is essential for performance of instrumental behavior during acquisition and early maintenance stages but not after overtraining, and (b) that after extended training the encoding necessary for performance may be transferred to another neural system outside the CN.