Reduction in TrkA-immunoreactive neurons is not associated with an overexpression of galaninergic fibers within the nucleus basalis in Down's syndrome

Down's syndrome (DS) individuals develop neuropathological features similar to Alzheimer's disease (AD), including degeneration of cholinergic basal forebrain (CBF) neurons. In AD a reduction in CBF/trkA-containing neurons has been suggested to trigger a hyperexpression of galaninergic fibers within the nucleus basalis subfield of the basal forebrain. The present study examined the interrelationship between reductions in CBF/trkA-containing neurons and the overexpression of galaninergic fibers within the nucleus basalis in DS. Within the nucleus basalis stereologic evaluation revealed a 46% reduction in the number of trkA-immunopositive neurons, whereas optical density measurements displayed a nonsignificant 18% reduction in neuronal trkA immunoreactivity in DS as compared with age-matched controls. Western blot analysis also showed a significant reduction in cortical trkA protein levels in DS. A semiquantitative examination of galaninergic fibers in the nucleus basalis revealed only a modest hypertrophy of galaninergic fibers within the nucleus basalis in DS. The present findings indicate a significant reduction in trkA within the nucleus basalis and cortex with only a moderate hypertrophy of galaninergic fibers in DS. These observations suggest that DS may not be an exact genetic model for investigation of changes in the AD basal forebrain.