2. Use of Optogenetics Techniques to Discover Pathway-specific Feedforward Circuits Between Thalamus and Neocortex
All brain functions involve coordinated neural activities in many brain regions. Therefore, it is important to understand how different brain regions interact with each other. One particularly important coupled system in the brain is the neocortex and the thalamus. The neocortex, the thalamus, and the axonal tracts that interconnect these structures comprise the vast majority of the mammalian brain and are crucial for sensation, perception and consciousness. Thalamocortical (TC) pathways provide the major extrinsic input to neocortex, and corticothalamic (CT) pathways are a principal source of synaptic input to thalamus. These pathways are entwined, making their study challenging by conventional electrical stimulation methods. We used cell- specific expression of channelrhodopsin-2 (ChR2), a light-sensitive cation channel, in either thalamocortical or corticothalamic projection cells to manipulate the activity on these tracts and study their effects on their target locations in mouse brain slices.
Viral delivery of ChR2
Lentiviruses carrying fusion genes for ChR2 and fluorescent proteins (pLenti-Synapsin-hChR2(H134R)-EYFP-WPRE) were injected into ventrobasal thalamic complex (VB) or the barrel cortex of ICR or GIN mice in vivo, between postnatal days 8 and 15. Typical viral titers were 1010 IU/ml. Injection volumes were between 0.3 and 2 µl. After allowing 1–3 weeks for ChR2 expression, acute somatosensory thalamocortical or horizontal brain slices (300 µm thick) were prepared for in vitro recording and stimulation (Figure 1).
Selective labeling of TC and CT pathways
On the other hand, injections into barrel cortex produced ChR2/EYFP expression in cortical neurons, including CT projection cells and their axons within ventrobasal thalamus and thalamic reticular nucleus (TRN) (Figure 3A and B). The optical stimulation of neurons in barrel cortex generated spike responses due to their ChR2 expression (Figure 3C and D).
Effects of TC activity on cortical cells
These excitatory thalamic synapses onto cortical neurons can drive spiking in cortical inhibitory interneurons. Since these interneurons synapse make many local synapses, TC input is able to produce powerful feedforward inhibition in surrounding cells. In accordance with this, we found that laser stimulation of ChR2-expressing TC arbors nearly always produced feedforward inhibition (inhibition was observed in 63/67 cortical cells tested; Figure 5).