Human immunodeficiency virus-1 Tat protein and methamphetamine interact synergistically to impair striatal dopaminergic function William F. Maragos,* ,  Kristie L. Young,* Jadwiga T. Turchan,* Masha Guseva,§ James R. Pauly,§ Avi Nath* , à and Wayne A. Cass  Departments of *Neurology,  Anatomy and Neurobiology, àMicrobiology and Immunology, and §College of Pharmacy, University of Kentucky, Lexington, Kentucky, USA Abstract The human immunodeficiency virus (HIV)-1 transactivating protein Tat may be pathogenically relevant in HIV-1-induced neuronal injury. The abuse of methamphetamine (MA), which is associated with behaviors that may transmit HIV-1, may damage dopaminergic afferents to the striatum. Since Tat and MA share common mechanisms of injury, we examined whether co-exposure to these toxins would lead to enhanced dopaminergic toxicity. Animals were treated with either saline, a threshold dose of MA, a threshold concentration of Tat injected directly into the striatum, or striatal injections of Tat followed by exposure to MA. Threshold was defined as the highest concentration of toxin that would not result in a sig- nificant loss of striatal dopamine levels. One week later, MA-treated animals demonstrated a 7% decline in striatal dopamine levels while Tat-treated animals showed an 8% reduction. Exposure to both MA + Tat caused an almost 65% reduction in striatal dopamine. This same treatment caused a 56% reduction in the binding capacity to the dopamine transporter. Using human fetal neurons, enhanced toxicity was also observed when cells were exposed to both Tat and MA. Mitochondrial membrane potential was disrupted and could be prevented by treatment with antioxidants. This study demonstrates that the HIV-1 ÔvirotoxinÕ Tat enhances MA- induced striatal damage and suggests that HIV-1-infected individuals who abuse MA may be at increased risk of basal ganglia dysfunction. Keywords: dopamine, dopamine transporter, human immu- nodeficiency virus, methamphetamine, mitochondria, Tat. J. Neurochem. (2002) 83, 955–963. Several lines of evidence indicate that the basal ganglia is vulnerable to damage by human immunodeficiency virus (HIV)-1 infection. During active infection, these structures containamongthebrain’shighestviralburdenaswellasHIV- 1-infected macrophages and multinucleated giant cells (Kure et al. 1990), which cause loss of dopaminergic neurons in the substantia nigra (Reyes et al. 1991) and striatal atrophy (Berger and Nath 1997). Signs of parkinsonism have also been observed in the Ôacquired immune deficiency sydrome (AIDS)–dementia complexÕ (Mirsattari et al. 1998). Under- lying these abnormalities are decreased levels of dopamine in the CSF (Berger et al. 1994) and reduced levels of both dopamine and homovanillic acid in the caudate nucleus of patientswithAIDS(Sardar et al.1996).Ithasbeensuggested that products released from HIV-1-infected cells (e.g. Ôviro- toxinsÕ) are responsible for neuronal damage (Kolson and Pomerantz1996;NathandGeiger1998).Onesuchproductis the HIV-1 gene regulatory protein Tat. Tat is a trans-activating non-structural nuclear regulatory protein composed of 86–104 amino acids. When presented extracellularly, it is cytotoxic to neurons (Sabatier et al. 1991; Magnuson et al. 1995; New et al. 1997; Gavriil et al. 2000). Intraparenchymal injections of Tat damage both striatal efferent (Hayman et al. 1993; Philippon et al. 1994) Received July 11, 2002; revised manuscript received August 14, 2002; accepted August 29, 2002. Address correspondence and reprint requests to Dr William F. Maragos, University of Kentucky Medical Center, Department of Neur- ology, Kentucky Clinic, Room L-445, Lexington, KY 40536-0284, USA. E-mail: maragos@pop.uky.edu Abbreviations used: AIDS, acquired immune deficiency syndrome; DAT, dopamine transporter; DOPAC, dihydroxyphenylacetic acid; FBS, fetal bovine serum; HIV, human immunodeficiency virus; 5HIAA, 5-hydroxyindoleacetic acid; HVA, homovanillic acid; MA, metham- phetamine; PSLD, protected least-significant difference; ROS, reactive oxygen species. Journal of Neurochemistry , 2002, 83, 955–963 Ó 2002 International Society for Neurochemistry, Journal of Neurochemistry , 83, 955–963 955