Optogenetic techniques are a major recent advance in tools used to understand the brain. Optogenetics combines fluorescent microscopy with genetic manipulation. The paper “Pathway-Specific Feedforward Circuits between Thalamus and Neocortex Revealed by Selective Optical Stimulation of Axons” published January 28, 2010 in Neuron is a pioneering study of thalamocortical and corticothalamic circuitry using optogenetic techniques. Projection neurons in the thalamus and in layer 6 cerebral cortex were labeled with Channelrhodopsin-2, which is a light gated ion channel, by infecting the cells with a lentivirus manipulated to express the Channelrhodopsin-2.

This study was focused on assessing the usefulness of optogenetic techniques for exploring brain circuitry and, in particular, the thalamocortical and corticothalamic circuits including the thalamic reticular nucleus. In that regard the study found the techniques to be very useful and, in particular, they showed that shining laser light onto the axon terminals of neurons expressing Channelrhodopsin-2 could be excited and release neurotransmitter.

For the most part, the research team confirmed earlier observations about circuit connectivity and dynamics from experiments that used other techniques. Nevertheless, there were some novel observations. Most surprising was that they never saw inhibitory synaptic activity between neurons in the thalamic reticular nucleus.