PDE9A and Huntington disease: Finally, the PDE5 inhibitor sildenafil has been found to reduce striatal projection neurons loss, increase CREB activity and BDNF levels, as well as improve the phenotype of the 3‐nitroproprionic neurotoxic rat model of HD,54 which further points to a significant role of cGMP signaling in HD pathogenesis, as also suggested by a recent study that used the selective PDE9A inhibitor PF‐04447943 in transgenic HD rats and demonstrated a normalization of cortically evoked firing of striatal MSNs.27