Monoamine neuron innervation of the hippocampal formation: alteration by neonatal irradiation.

Previous studies showed that postnatal irradiation of the hippocampal formation in the rat results in a marked reduction in the granule cell population of the area dentata with an attendant disruption of its normal cytoarchitecture. The present study was carried out to determine the effect of postnatal hippocampal irradiation on the pattern of hippocampal innervation by locus ceruleus norepinephrine neuron axons and midbrain raphe serotonin neuron axons. Irradiated animals exhibit a normal innervation of Ammon’s horn by both norepinephrine and serotonin neuron axons. In the area dentata, however, the normal pattern of innervation is markedly altered, with both an apparent decrease in total innervation and a redistribution of the innervation occurring. These effects are interpreted as representing a failure of development of the growth-promoting factors that determine the normal innervation pattern.