Are the dorsal noradrenergic bundle projections from the locus coeruleus important for neocortical or hippocampal activation?

Three different methods were used to examine the importance of the dorsal noradrenergic bundle projections from the locus coeruleus (LC) in activation of the neocortex and hippocampus in freely moving rats. (1) After cerebral norepinephrine (NE) was depleted by systemic neonatal injections of 6-hydroxydopamine (6-OHDA) both atropine-resistant and atropine-sensitive forms of hippocampal rhythmical slow activity (RSA; theta) and neocortical low voltage fast activity (LVFA) remained intact. Compared to controls the adult rats treated with 6-OHDA in infancy reared less in a 24 h time sample of behavior and ran less in running wheels. (2) Brain dopamine and NE were also depleted by systemic injections of alpha-methyl-p-tyrosine. Following this treatment rats were very inactive behaviorally. However, normal activation of the hippocampus and neocortex was still present. (3) In normal rats, electrical stimulation of the LC was relatively ineffective, compared to stimulation of nucleus reticularis pontis caudalis, in producing behavioral changes (especially locomotion) or either atropine-resistant or atropine-sensitive hippocampal RSA or neocortical LVFA. It is concluded that the locus coeruleus is not important for cerebral activation, and that mechanisms for cerebral activation are probably diffusely represented in the reticular core. The data also show that when attempting to assess the effect of experimental manipulations on brain activity it is essential to control for the possible effects of changes in behavior.