Soma-Targeted Imaging of Neural Circuits by Ribosome TetheringHighlights d Ribosome tethering restricts fluorescent proteins to the cell soma d A ribo-nanobody mouse boosts the fluorescence of existing GFP reporters d Ribo-GCaMP reduces fluorescence cross-contamination during 2P calcium imaging d Ribo-GCaMP enables whole-brain imaging of somatic calcium dynamics in the worm Authors

Descending projections from the thalamus and related structures to the midbrain are evolutionarily highly conserved. However, the basic organization of this auditory thalamotectal pathway has not yet been characterized. The purpose of this study is to obtain a better understanding of the anatomical and neurochemical features of this pathway. Analysis of the distributions of retrogradely labeled cells after focal injections of retrograde tracer into the inferior colliculus of the mouse revealed that most of the subcortical descending projections originated in the brachium of the inferior colliculus and the paralaminar portions of the auditory thalamus. In addition, the vast majority of thalamotectal cells were found to be negative for the calcium-binding proteins calbindin, parvalbumin or calretinin. Using two different strains of GAD-GFP mice, as well as immunostaining for GABA, we found that a subset of neurons in the brachium of the inferior colliculus is GABAergic, suggesting that part of this descending pathway is inhibitory. Finally, dual retrograde injections into the inferior colliculus and amygdala plus corpus striatum as well into the inferior colliculus and auditory cortex did not reveal any double labeling. These data suggest that the thalamocollicular pathway comprises a unique population of thalamic neurons that do not contain typical calcium binding proteins and do not project to other paralaminar thalamic forebrain targets, and that a previously undescribed descending GABAergic pathway emanates from the brachium of the inferior colliculus.

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