I tried to visualize the dendritic spines of dopaminergic neurons in ventrolateral periaqueductal gray (vlPAG) by injected retrograde virus AAV.rTH.PI.Cre.SV40 to Rostral ventromedial medulla (RVM) and also AAV5-CAG-FLEX-EGFP-WPRE to vlPAG (dilution ratio 1:2000) in C57BL6 mouse, then sacrificed at 3 weeks post-injection. However, I only can see the soma body and cant see dendritic spines. I wonder why I cant see the dendritic spines?