Circuit dissection for causal neuroscience studies

Tadashi Isa
National Institute for Physiological Sciences, Okazaki, Japan

For causal demonstration of the structure-function relationship on the complex neural circuits in the brain, selective manipulation of individual component is necessary. Currently, such technique is possible in model animals like mouse, drosophila, zebrafish or C-elegance, in which transgenic lines can be generated using cell-specific promotors. However, it was difficult to conduct such experiments in primates, and even in rats. Recently, we succeeded in developing the techniques to enable pathway-selective and reversible blockade of synaptic transmission by combining two viral vectors in the non-human primate. In this technique, first, we inject the highly efficient retrograde gene transfer vector (HiRet) carrying tetracycline responsive element (TRE) and enhanced green fluorescent protein (eGFP) tagged with enhanced tetanus neurotoxin (eTeNT) into the projection area of targeted neurons. Then, we inject another viral vector, adeno-associated viral vector (AAV) carrying cytomegalovirus promotor and rtTAV16, a newly developed highly sensitive Tet-on transactivator into the area where the somas of the targeted neurons are located. Then, a few months later, by oral administration of doxycycline, eTeNT is expressed in the targeted neurons, cleaves VAMP-2 and synaptic transmission is halted in these neurons. By applying this technique to a group of spinal cord interneurons, namely propriospinal neurons (PNs) which are located in the mid-cervical segments (C3-C4) and project to the hand and arm motor nuclei in macaque monkeys, we could observe deficit in dexterous hand movements including precision grip and reaching. Thus, we demonstrated that these PNs are involved in the control of dexterous hand movements in normal animals. In addition, we could show that blocking the PNs resulted in impaired recovery of hand dexterity after partial spinal cord injury, suggesting that PNs play a causal role in the recovery process. Now as a new generation technology of pathway-selective manipulation, we are developing the technique for pathway selective expression of channelrhodopsin for pathway-selective optognetic control of activation with the double-vector system. I will also talk on the progress of such techniques.