Changes in the noradrenergic innervation of the area dentata after axotomy of coeruleohippocampal projections or unilateral lesion of the locus coeruleus.

Lesions of the septal region result in a significant decrease in the norepinephrine content of the area dentata. Over a period of one year, norepinephrine levels return to normal, presumably due to the proliferation of remaining locus coeruleus fibers. Unilateral lesions of the locus coeruleus produce reductions in [3H]norepinephrine uptake values of about 70% and 30% in the ipsilateral and contralateral area dentata, respectively. By 12 weeks after lesion, the noradrenergic fiber density in the contralateral area dentata is within the range of control measurements, whereas the area dentata ipsilateral to the lesion remains significantly depleted of noradrenergic fibers. At 26 weeks postlesion, no further change was observed in the noradrenergic fiber density of either area dentata. These results are interpreted in light of the hypothesis that locus coeruleus axon proliferation is induced by axotomy and represents the expansion of intact collaterals of damaged fibers.