Efficient viral expression of a chemogenetic receptor in the old-world monkey amygdala

Genetically encoded synthetic receptors, such as the chemogenetic and optogenetic proteins, are powerful tools for functional brain studies in animals. In the primate brain, with its comparatively large, intricate anatomical structures, it can be challenging to express transgenes, such as the hM4Di chemogenetic receptor, in a defined anatomical structure with high penetrance. Here, we compare parameters for lentivirus vector injections in the rhesus monkey amygdala. We find that four injections of 20 μl, infused at 0.5 μl/min, can achieve neuronal hM4Di expression in 50–100% of neurons within a 60 mm³ volume, without observable damage from overexpression. Increasing the number of hM4Di_CFP lentivirus injections to up to 12 sites per hemisphere, resulted in 30%–40% neuronal coverage of the overall amygdala volume, with coverage reaching 60% in some subnuclei. Manganese Chloride was mixed with lentivirus and used as an MRI marker to verify targeting accuracy and correct unsuccessful injections in these experiments. In a separate monkey we visualized, in vivo, viral expression of the hM4Di receptor protein in the amygdala, using Positron Emission Tomography. Together, these data show efficient and verifiable expression of a chemogenetic receptor in old-world monkey amygdala.