The thalamus receives input from 3 distinct cortical layers, but input from only 2 of these has been well characterized. We therefore investigated whether the third input, derived from layer 6b, is more similar to the projections from layer 6a or layer 5. We studied the projections of a restricted population of deep layer 6 cells ("layer 6b cells") taking advantage of the transgenic mouse Tg(Drd1a-cre)FK164Gsat/Mmucd (Drd1a-Cre), that selectively expresses Cre-recombinase in a subpopulation of layer 6b neurons across the entire cortical mantle. At P8, 18% of layer 6b neurons are labeled with Drd1a-Cre::tdTomato in somatosensory cortex (SS), and some co-express known layer 6b markers. Using Cre-dependent viral tracing, we identified topographical projections to higher order thalamic nuclei. VGluT1+ synapses formed by labeled layer 6b projections were found in posterior thalamic nucleus (Po) but not in the (pre)thalamic reticular nucleus (TRN). The lack of TRN collaterals was confirmed with single-cell tracing from SS. Transmission electron microscopy comparison of terminal varicosities from layer 5 and layer 6b axons in Po showed that L6b varicosities are markedly smaller and simpler than the majority from L5. Our results suggest that L6b projections to the thalamus are distinct from both L5 and L6a projections.
Journal article
Cereb Cortex
01/05/2018
28
1882 - 1897
Adaptor Proteins, Signal Transducing, Animals, Animals, Newborn, Biotin, Brain Mapping, Cerebral Cortex, Dextrans, Embryo, Mammalian, Eye Proteins, Green Fluorescent Proteins, Mice, Mice, Inbred C57BL, Mice, Transgenic, Microscopy, Electron, Mutation, Myelin Basic Protein, Nerve Tissue Proteins, Neurons, Receptors, Dopamine D1, Synapses, Thalamic Nuclei, Transduction, Genetic, Vesicular Glutamate Transport Protein 1