In contrast, we know relatively little about the GPR35 orphan receptor and its relation to PD, and ZAP could be acting not only as a PDE inhibitor but also as a GPR35 activator in our PD model human cells Studies have suggested a potential role for GPR35 in regulating neuronal excitability and synaptic release [78, 79] and controlling inflammation and the immune system [72, 80]. This evidence concerns the gene GPR35 and Parkinson disease.