Environmental exposure to some neurotoxicants produce damage to the basal ganglia. Dopaminergic neurons of the substantia nigra are most vulnerable to the action of specific intracellular toxins. We hypothesize that genetic determinants render dopaminergic neurons susceptible to neurotoxicant damage thus producing the phenomenon of selective vulnerability. We will model such environmental-genetic interactions to determine whether modification of a single gene within a subpopulation of substantia nigra dopaminergic neurons increases their susceptibility to neurotoxicants. To test our hypothesis we will employ an approach we recently developed, somatic mosaic analysis in the mouse, to dysregulate the expression of the dopamine transporter (DAT) in dopaminergic neurons. Somatic mosaic analysis is a binary cre/loxP molecular genetic method utilizing a germline transmitted recombinational substrate containing a dormant transcription unit and somatic gene transfer of a herpes virus vector that expresses cre recombinase and "activates" the gene of interest. In this application we will construct the DAT XAT (Cat eXcision Activation Transgene) dormant transcription unit which is designed to direct transcription only in dopaminergic neurons of the DAT gene and co-transcribed reporter gene, beta-galactosidase, following cre recombinase "activation". DAT XAT activation efficiency will be tested in vitro, mice harboring DAT XAT generated and in vivo activation studied. Vulnerability to systemically administered neurotoxicants MPTP and paraquat will be examined in mice in which a subset of nigral dopaminergic neurons have increased DAT expression. Increased vulnerability will be manifest by increased cell death in neurons carrying the "activated" DAT XAT transgene. Recombination-mediated gene activation permanently changes the genetic constitution of that subset of dopaminergic neurons thus allowing us to study neuronal vulnerability during chronic low dose exposure to paraquat as a model of environmental exposure. Taken together, these studies will provide the opportunity to model environmental neurotoxicant-genetic interactions as a cause of basal ganglia damage and provide a paradigm to explore etiologic mechanisms underlying idiopathic Parkinson's disease.