Age-dependent changes in axonal branching of single locus coeruleus neurons projecting to two different terminal fields.

Age-dependent changes in the axonal branching patterns of single locus coeruleus neurons, which innervate both the frontal cortex and hippocampus dentate gyrus, have been studied in male F344 rats. We used an electrophysiological approach involving antidromic activation to differentiate single from multi-threshold locus coeruleus neurons in each terminal field with age (7-27 mo of age). Most of these neurons have a single threshold in the young rats, whereas in the older brains, the neurons have multi-threshold responses. This implies an increased amount of axonal branching in the older brains. The time course of the increase differs in the two terminal fields, suggesting that the degree of plasticity or age-dependent increase in branching can differ across terminal fields.