Prior studies mainly focused on the effect of T on SCS plasticity. However, it has been shown that steroid-independent photostimulation can also induce SCS plasticity, but its mechanism remains unclear [9-11]. One of the proposed alternatives is the mediating effect of THs, as THs play an important role in the regulation of seasonal reproduction and are associated with neurogenesis. Surprisingly, the effect of THs on SCS plasticity has only been studied partially . In addition, it is unknown whether THs mediate SCS plasticity in a direct or an indirect manner. To this end we designed a series of experiments divided into 3 work packages (WP).