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Abstract

Optogenetics is a potentially powerful tool to manipulate and map specific neural circuits. The few studies that have so far implemented this method in primates focused on the neocortex. Here, we transduced cells in multiple thalamic nuclei of one rhesus macaque with a DNA construct encoding the microbial proton-pump ArchT and the green fluorescent protein (GFP). A constitutively active promoter (CAG) was used to ensure high-level protein expression. Adeno associated virus (AAV2 or AAV5) was used to deliver the gene. Electrophysiological recordings were carried out under anesthesia six to eight weeks after the AAV injections. Continuous illumination with green light (532 nm) through an optic fiber (110 µm diameter) placed in the injected thalamic regions markedly and reliably reduced the local ongoing spiking activity (60 on average) with fast recovery to baseline firing after light offset. Post-mortem stereological microscope examination indicated that ~25 of the neurons in the thalamic injection sites were GFP-labeled and exhibited the typical large soma size and radial dendritic arborization characteristic of thalamocortical projection (TC) neurons. We also found dense GFP-labeled axon terminals in layers 3-4 in the cortical targets of the injected thalamic regions. In the TC soma, the GFP labeling was mostly localized at membrane sites with no accumulation in the cytoplasm and cell nucleus. Based on morphological analysis there was no obvious GFP labeling in local interneurons or in the glia. However, besides the apparently healthy TC neurons, we also observed a small percentage (~5) of round GFP-labeled formations that had roughly the same diameter as the TC dendrite arborization (~85 µm) but no recognizable neuropil or perikaryal morphology. These formations could be the remains of cells that degenerated after overexpressing the construct. These results show that AAV vectors can be used in the monkey thalamus for intra-neuronal delivery of opsin-encoding DNA sequences and reliable manipulation of neuronal activity. While further characterization of the extent and specificity of gene expression is necessary, intrathalamic injections of AAV vectors could provide the much-needed tool to examine separately and in great physiological and anatomical detail the intricately mingled components of the primate thalamocortical circuitry.