Involvement of neurotrophic factors in aging of noradrenergic innervations in hippocampus and frontal cortex.

In the present study, we investigated the age-dependent changes in the axon terminals of the locus coeruleus (LC) neurons in the frontal cortex and hippocampus, in which a high degree of axonal branching in the middle-aged brain was suggested to occur in our previous electrophysiological study. We used 6-, 13- and 25-month-old male F344/N rats, and performed Western blot analysis of the norepinephrine transporter (NET), brain-derived neurotrophic factor (BDNF) and glial cell line-derived neurotrophic factor (GDNF). NET expression level increased in the 13-month-old hippocampus, but was not altered by aging in the frontal cortex. BDNF expression level increased in the hippocampus, but did not change with age in the frontal cortex. On the other hand, GDNF expression level was increased with age in the frontal cortex, but was not in the hippocampus. These results suggest that the LC noradrenergic innervations may be locally regulated by different neurotrophic factors that exert their trophic actions at different target sites.