Ultrastructural localization of DREADD s in monkeys

Designer receptors exclusively activated by designer drugs ( DREADD s) are extensively used to modulate neuronal activity in rodents, but their use in primates remains limited. An essential need that remains is the demonstration that DREADD s are efficiently expressed on the plasma membrane of primate neurons. To address this issue, electron microscopy immunogold was used to determine the subcellular localization of the AAV vector‐induced DREADD s hM 4Di and hM 3Dq fused to different tags in various brain areas of rhesus monkeys and mice. When hM 4Di was fused to mC herry, the immunogold labelling was mostly confined to the intracellular space, and poorly expressed at the plasma membrane in monkey dendrites. In contrast, the hM 4Di‐ mC herry labelling was mostly localized to the dendritic plasma membrane in mouse neurons, suggesting species differences in the plasma membrane expression of these exogenous proteins. The lack of hM 4Di plasma membrane expression may limit the functional effects of systemic administration of DREADD ‐actuators in monkey neurons. Removing the mC herry and fusing of hM 4Di with the haemagglutinin ( HA ) tag resulted in strong neuronal plasma membrane immunogold labelling in both monkeys and mice neurons. Finally, hM 3Dq‐ mC herry was expressed mostly at the plasma membrane in monkey neurons, indicating that the fusion of mC herry with hM 3Dq does not hamper membrane incorporation of this specific DREADD . Our results suggest that the pattern of ultrastructural expression of DREADD s in monkey neurons depends on the DREADD /tag combination. Therefore, a preliminary characterization of plasma membrane expression of specific DREADD /tag combinations is recommended when using chemogenetic approaches in primates.