Parkinson’s disease is an age-associated progressive neurodegenerative disorder that has gained crescent social and economic impact due to the aging of the western society. All current therapies are symptomatic and fail to reverse or halt the progression of dopaminergic neurons loss. The discovery of the capability of neurotrophic factors to protect these neurons lead numerous research groups to focus their efforts in developing therapies aiming at promoting the control of Parkinson´s disease through the delivery of neurotrophic factors to the brain or by boosting their endogenous levels. Both strategies were successful in inducing protection of dopaminergic neurons and motor recovery in preclinical models of the disease. Contrariwise, very limited success was obtained in clinical studies, where glial cell line-derived neurotrophic factor and neurturin were the neurotrophic factors of choice for Parkinson’s disease therapy. These drawbacks motivate the development of novel forms of delivery or the modification of the injected molecules aiming at providing a more stable and effective administration with improved diffusion in the target tissue, and without the immune responses observed in the earliest clinical studies. Although promising results were obtained with some of these new approaches performed in experimental models of the disease, they were not yet tested in human studies. In this review, we present the current knowledge on neurotrophic factors and their role in Parkinson’s disease, focusing on the strategies that have been developed to increase their levels in target areas of the brain to achieve protection of dopaminergic neurons and motor behaviour recovery.

In this work, we studied the effect of intracellular 3',5'-cyclic adenosine monophosphate (cAMP) on Li+ transport in SH-SY5Y cells. The cells were stimulated with forskolin, an adenylate cyclase activator, or with the cAMP analogue, dibutyryl-cAMP. It was observed that under forskolin stimulation both the Li+ influx rate constant and the Li+ accumulation in these cells were increased. Dibutyryl-cAMP also increased Li+ uptake and identical results were obtained with cortical and hippocampal neurons. The inhibitor of the Na+/Ca2+ exchanger, KB-R7943, reduced the influx of Li+ under resting conditions, and completely inhibited the effect of forskolin on the accumulation of the cation. Intracellular Ca2+ chelation, or inhibition of N-type voltage-sensitive Ca2+ channels, or inhibition of cAMP-dependent protein kinase (PKA) also abolished the effect of forskolin on Li+ uptake. The involvement of Ca2+ on forskolin-induced Li+ uptake was confirmed by intracellular free Ca2+ measurements using fluorescence spectroscopy. Exposure of SH-SY5Y cells to 1 mm Li+ for 24 h increased basal cAMP levels, but preincubation with Li+, at the same concentration, decreased cAMP production in response to forskolin. To summarize, these results demonstrate that intracellular cAMP levels regulate the uptake of Li+ in a Ca2+-dependent manner, and indicate that Li+ plays an important role in the homeostasis of this second messenger in neuronal cells.http://dx.doi.org/10.1111/j.1471-4159.2004.02551.