Neuroprotective effects of PACAP against paraquat-induced oxidative stress in the Drosophila central nervous system.

Parkinson's disease (PD) is a progressive neurodegenerative movement disorder that can arise after long-term exposure to environmental oxidative stressors, such as the herbicide paraquat (PQ). Here we investigated the potential neuroprotective action of vertebrate pituitary adenylate cyclase-activating polypeptide (PACAP) against PQ in Drosophila. We found that pre-treatment with this neuropeptide applied to the ventral nerve cord (VNC) at low doses markedly extended the survival of wild-type decapitated flies exposed to neurotoxic levels of PQ or dopamine (DA). In contrast and interestingly, application of a PACAP receptor antagonist, PACAP-6-38, had opposite effects, significantly decreasing the resistance of flies to PQ. PACAP also reduced PQ-induced caspase activation and reactive oxygen species (ROS) accumulation in the VNC. We then searched for the endogenous neuropeptide receptor potentially involved in PACAP-mediated neuroprotection in Drosophila. Knocking down the gene encoding the receptor Han/PDFR of the neuropeptide pigment-dispersing factor (PDF) in all neurons conferred to flies higher resistance to PQ, whereas PDFR downregulation restricted to PDF or DA neurons did not increase PQ resistance, but remarkably suppressed the neuroprotective action of PACAP. Further experiments performed with Pdf and Pdfr-deficient mutant strains confirmed that PDF and its receptor are required for PACAP-mediated neuroprotection in flies. We also provide evidence using split-green fluorescent protein (split-GFP) reconstitution that PDF neurons make synaptic contacts onto DA neurons in the abdominal VNC. Our results therefore suggest that the protective action of PACAP against PQ-induced defects in the Drosophila nervous system involves the modulation of PDFR signaling in a small number of interconnected neurons.