Retrograde labeling of locus coeruleus neurons after lesion-induced sprouting of the coeruleohippocampal projection.

Locus coeruleus axons have been observed to proliferate and reinnervate targets after the partial removal of normal locus coeruleus inputs. Sixteen weeks after a large electrolytic lesion of the septal region, wheat germ agglutinin-horseradish peroxidase (WGA-HRP) injections of the dorsal area dentata resulted in the labeling of neurons of the dorsal half of the locus coeruleus. Similar injections made at earlier postlesion times failed to produce visible HRP labeling in the locus coeruleus. These observations are consistent with our previous report of the time course for the noradrenergic reinnervation of the dorsal area dentata after septal lesions. Using double-labeling techniques, we have also demonstrated that collaterals of locus coeruleus axons that originally innervated the dorsal area dentata sprout to reinnervate that region 4 months after a lesion of the septal region. Thus the homotypic compensatory proliferation of locus coeruleus projections appears to be mediated, at least in part, by a pruning effect.