Changes in cortical noradrenergic axon terminals of locus coeruleus neurons in aged F344 rats.

The noradrenergic innervations and noradrenaline contents of the frontal cortex in two age groups (9 and 25 months) of male F344 rats have been quantified by electrophysiological and biochemical methods. In the electrophysiological study, the percentage of locus coeruleus (LC) neurons activated antidromically from the frontal cortex decreased with age. In contrast, the percentage of LC neurons showing multiple antidromic latencies, which suggests axonal branching of individual LC neurons, increased markedly between 9 and 25 months in the frontal cortex. In the biochemical study, we found no significant difference in noradrenaline levels in the cortical terminal fields of LC neurons during aging. These results suggest that LC neurons give rise to axonal branches to retain noradrenaline levels in their target fields in the aged brain. Our findings show that LC neurons preserve a strong capability for remodeling their axon terminals even in the aged brain.