The neurotrophins brain-derived neurotrophic factor (BDNF) and neurotrophin-3 (NT-3) have been shown to promote survival and differentiation of midbrain dopaminergic (DAergic) neurons in vitro and in vivo. This is consistent with their expression and that of their cognate receptors, trkB and trkC, in the nigrostriatal system. Degeneration of DAergic neurons of the substantia nigra and alpha-synuclein-positive aggregates in the remaining substantia nigra (SN) neurons are hallmarks of Parkinson's disease (PD). Reduced expression of BDNF has been reported in the SN from PD patients. Moreover, mutations in the BDNF gene have been found to play a role in the development of familial PD. We show now that haploinsufficiencies of the neurotrophin receptors trkB and/or trkC cause a reduction in numbers of SN neurons in aged (21-23 month old) mice, which is accompanied by a reduced density in striatal tyrosine hydroxylase immunoreactive (TH-ir) fibers. These aged mutant mice, in contrast to wild-type littermates, display an accumulation of alpha-synuclein in the remaining TH-positive neurons of the SN. We conclude that impairment in trkB and/or trkC signaling induces a phenotype in the aged SN, which includes two hallmarks of PD, losses of TH positive neurons and axons along with massive neuronal deposits of alpha-synuclein.