Reduced retrograde labelling with fluorescent tracer accompanies neuronal atrophy of basal forebrain cholinergic neurons in aged rats.

During ageing, basal forebrain cholinergic neurons are prone to degeneration for unknown reasons. In this study we morphometrically evaluated the retrograde labelling of basal forebrain neurons obtained after injection of FluoroGold into multiple sites in the cerebral neocortex in aged (24-33 months) as compared with young adult (four to six months) male Sprague-Dawley rats. In addition, we looked for differences in the distribution of degenerative changes in topographic subdivisions of the basal forebrain cholinergic complex of neurons identified by immunohistochemical detection of the cholinergic markers choline acetyltransferase or low-affinity neurotrophin receptor. After injection of FluoroGold into the cerebral neocortex, the number of retrogradely labelled neurons in the horizontal diagonal band/ substantia innominata and basal nucleus was significantly lower in aged rats, by 41% and 48%, respectively. In aged rats injected with FluoroGold as well as in non-injected aged rats, the numbers of neurons immunoreactive for choline acetyltransferase and low-affinity neurotrophin receptor were significantly lower, by 23-27% in the basal forebrain system as a whole, with no significant difference in the degree of decline amongst different subdivisions (i.e. medial septum, diagonal band, substantia innominata and basal nucleus). The ratios of the number of neurons labelled with FluoroGold as compared with the number of neurons immunoreactive for either cholinergic marker were significantly lower in aged rats, by 32-37%, indicating that the decline in the number of neurons retrogradely transporting tracer was greater than the decline in the number of immunoreactive neurons in aged animals. Immunoreactive as well as retrogradely labelled neurons showed a significant shrinkage of cell surface area of 6-13% in different subdivisions of the basal forebrain cholinergic system in aged rats. These findings confirm significant loss and atrophy of basal forebrain cholinergic neurons in aged rats, and demonstrate significantly reduced retrograde labelling of these neurons with fluorescent tracer applied to their target cortex. This reduced retrograde labelling suggests an impairment of either uptake or retrograde transport mechanisms in these neurons in aged rats. Such an impairment may contribute to the degenerative changes of basal forebrain cholinergic neurons observed in ageing and age-related degenerative conditions such as Alzheimer's disease.