In the central nervous system, basal ganglia structures are highly susceptible to infection with the human immunodeficiency virus-1 (HIV). The HIV-1 protein, Tat, which is involved in viral replication, also plays an important role in the neuronal injury and recapitulates much the pathology seen in HIV-1 infection of the brain. Patients with HIV-1 infection often abuse drugs such as methamphetamine, a drug that is well known to also cause long-term structural and functional changes of the basal ganglia. Previous studies from our laboratory have demonstrated that Tat protein and methamphetamine interacted synergistically to cause profound damage to the basal ganglia dopaminergic neurotransmitter system. Although our earlier studies implicated reactive oxygen species and the cytokine TNF-1, our preliminary data strongly suggest that ceramide-induced alterations of the vesicular monoamine transporter may play a prominent role in the enhanced toxicity of MA seen after exposure to Tat. In this proposal, we will in Specific Aim 1, determine whether HIV-1 Tat alters the compartmentalization of DA in the synaptic terminal. In Specific Aim 2, we will investigate whether the production of the second messenger ceramide is involved in Tat-induced changes in dopamine compartmentalization. Lastly, in Specific Aim 3, we will determine whether ceramide mediates the synergism between Tat and methamphetamine. Both pharmacological and genetic inhibition of ceramide synthesis will be pursued. In studying these interactions, we will examine 1) vesicular mononamine transporter function, 2) dopamine release (vesicular and tissue) 3) dopamine terminal integrity and 4) behavioral measures. The results of these experiments will elucidate a novel role for ceramide signaling in dopaminergic dysfunction, not only in HIV-1, but also in the neuronal injury associated with Parkinson's disease.