On the role of the dorsal noradrenergic bundle in learning and habituation to novelty.

In Experiment 1, the performance of vehicle control rats and rats with 6-hydroxydopamine-induced lesions of the dorsal noradrenergic bundle (DB) was examined in acquisition and extinction of bar pressing and in spontaneous and food-reinforced alternation in a T-shape maze. Plasma corticosterone levels in basal conditions, after chronic food restriction, after transportation to a novel environment, and after sessions of either rewarded or nonrewarded bar pressing were assayed. DB lesions produced a significant decrease of spontaneous alternation and a significant but small resistance to extinction, without reliably altering either corticosterone responses or instrumental spatial alternation. In Experiment 2, bar-press extinction and instrumental alternation were reexamined in new groups of control rats and rats with DB lesions without any blood collection procedures. The DB lesions did not reliably alter either behaviors on any measures. Taken together, these data indicate no consistent effects of forebrain noradrenaline depletion on either extinction or spatial memory or pituitary-adrenocortical function. However, the impairment of spontaneous alternation found in a previous study was confirmed. These findings are discussed in terms of the proposed roles of the dorsal noradrenergic bundle in learning and habituation to novelty.