in the visual system, the afferent axons from the dorsal lateral geniculate nucleus (dLGN) to the primary visual cortex (V1) show significant activity-dependent plasticity in early postnatal life. To determine whether activity-de pendent plasticity operates also in feedback projections from V1 to dLGN, we inactivated cortical inputs pharmacologically and examined possible changes in the density of synaptic proteins, vesicular glutamate transporter 1 (VGluT1) and type 1 metabotropic glutamate receptor alpha (mGluR1 alpha), which locate pre- and postsynaptically at feedback projections, respectively in dLGN of rats. The intensity of the immunohistochemical signal of mGluR1 alpha in dLGN significantly decreased following the cortical inactivation for at least 2 days, and the decrease was maintained under cortical inactivation until 28 days. On the other hand, the signal intensity of VGluT1 showed a significant increase following 14 or 28 days of cortical inactivation. In adult rats, however, we found no significant change in VGluT1 signal intensity and only a small and transient downregulation of mGluR1 alpha following 7-day inactivation. Thus, the decrease in presynaptic activity induces a rapid downregulation of postsynaptic mGluR1 alpha followed by a delayed upregulation of presynaptic VGluT1 in young rats. These results suggest that feedback synapses are regulated by neural activity during development. (C) 2009 Elsevier Ireland Ltd and the Japan Neuroscience Society. All rights reserved.