Learning in the absence of forebrain noradrenaline

Abstract

CONSIDERABLE attention has been given to suggestions that forebrain noradrenaline (NA) plays a central role in the associative learning process^1,2. For example, Kety² has proposed a general noradrenergic theory of learning in which he hypothesises that NA release associated with reinforcement facilitates the formation of learned associations. Crow³ subsequently suggested that the dorsal NA bundle which extensively innervates the cortex could be the crucial pathway involved. In support of this hypothesis are findings that learning on shock avoidance⁴ and appetitive^5–7 tasks is impaired after widespread loss of forebrain NA and dopamine. More specifically, Anlezark et al.⁸ found that bilateral electrolytic lesions to the origins of the dorsal NA bundle in the locus coeruleus impaired the acquisition of runway behaviour. Cortical forebrain NA loss was reduced to 69% of control levels in their study. We have used 6-hydroxydopamine (6-OHDA) treatments to achieve virtually total forebrain NA loss and have found no impairment on the acquisition of runway behaviour. The NA-depleted animals, were however, slow to inhibit running during extinction, suggesting that although the dorsal NA system is not involved in the acquisition of reinforced responses, it is important for the response to non-reward.