Gene delivery using adeno-associated virus (AAV) vectors is a widely used method to transduce neurons in the brain, especially due to its safety, efficacy, and long-lasting expression. In addition, by varying AAV serotype, promotor, and titer, it is possible to affect the cell specificity of expression or the expression levels of the protein of interest. Dopamine neurons in the substantia nigra projecting to the striatum, comprising the nigrostriatal pathway, are involved in movement control and degenerate in Parkinson's disease. AAV-based gene targeting to the projection area of these neurons in the striatum has been studied extensively to induce the production of neurotrophic factors for disease-modifying therapies for Parkinson's disease. Much less emphasis has been put on AAV-based gene therapy targeting dopamine neurons in substantia nigra. We will review the literature related to targeting striatum and/or substantia nigra dopamine neurons using AAVs in order to express neuroprotective and neurorestorative molecules, as well as produce animal disease models of Parkinson's disease. We discuss difficulties in targeting substantia nigra dopamine neurons and their vulnerability to stress in general. Therefore, choosing a proper control for experimental work is not trivial. Since the axons along the nigrostriatal tract are the first to degenerate in Parkinson's disease, the location to deliver the therapy must be carefully considered. We also review studies using AAV--synuclein (-syn) to target substantia nigra dopamine neurons to produce an -syn overexpression disease model in rats. Though these studies are able to produce mild dopamine system degeneration in the striatum and substantia nigra and some behavioural effects, there are studies pointing to the toxicity of AAV-carrying green fluorescent protein (GFP), which is often used as a control. Therefore, we discuss the potential difficulties in overexpressing proteins in general in the substantia nigra.

Parkinson's disease is associated with the loss of dopamine (DA) neurons in ventral mesencephalon. We have previously reported that no single neurotrophic factor we tested protected DA neurons from the dopaminergic toxin 1-methyl-4-phenylpyridinium (MPP+) in dissociated cultures isolated from the P0 rat substantia nigra, but that a combination of five neurotrophic factors was protective. We now report that cerebral DA neurotrophic factor (CDNF) and a variant of neurturin (NRTN), N4, were also not protective when provided alone but were protective when added together. In cultures isolated from the substantia nigra, MPP+ (10 mu M) decreased tyrosine hydroxylase-positive cells to 41.7 +/- 5.4% of vehicle control. Although treatment of cultures with 100 ng/ml of either CDNF or N4 individually before and after toxin exposure did not significantly increase survival in MPP+-treated cultures, when the two trophic factors were added together at 100 ng/ml each, survival of cells was increased 28.2 +/- 6.1% above the effect of MPP+ alone. In cultures isolated from the ventral tegmental area, another DA rich area, a higher dose of MPP+ (1 mM) was required to produce an EC50 in TH-positive cells but, as in the substantia nigra, only the combination of CDNF and N4 (100 ng/ml each) was successful at increasing the survival of these cells compared to MPP+ alone (by 22.5 +/- 3.5%). These data support previous findings that CDNF and N4 may be of therapeutic value for treatment of PD, but suggest that they may need to be administered together.