, 2008). These results suggest that

increased excitatory spine dynamics following sensory deprivation are not simply caused by reduced cortical activity levels, but rather depend on competition between deprived and non-deprived inputs. To distinguish between these two alternative explanations for spine changes in inhibitory neurons, we performed complete bilateral retinal lesions, removing all visually evoked input, thus preventing the functional reorganization that is observed following focal retinal lesions. As expected, these mice were unresponsive to visual stimuli and demonstrated no functional recovery over the months following the complete retinal lesion (Keck et al., 2008). The density (Figures 3C and 3D) as well as the survival fraction (Figure 3E) of spines on inhibitory neurons decreased in the 48 hr following complete retinal lesions to the same Entinostat cell line degree as we had found after focal lesions. Inhibitory neuron spine density decreased significantly 6 hr after focal lesions but only 48 hr following

complete lesions, indicating that the exact timing of structural changes depends on the nature of the deprivation MDV3100 price (see Discussion). So far, we have shown that inhibitory neurons lose a substantial fraction of their excitatory inputs, suggesting a lower level of inhibition in the visual cortex after sensory deprivation. Is this also reflected on the output side (i.e., axons and boutons) of these cells? In control animals, chronic two-photon imaging did not reveal any changes in the overall axonal architecture

over a period of 6 days, but we observed a baseline turnover of axonal boutons. Similar to boutons on excitatory axons (De Paola et al., 2006 and Stettler et al., 2006), the overall density of boutons on inhibitory axons remains constant over time (Figures 4A and 4C, red curve), but boutons are constantly added and lost over time (Figure 4D, red curve). To determine if baseline structural dynamics are altered by sensory deprivation, we measured very changes in inhibitory axons and boutons in the 72 hr before and after a focal retinal lesion (Figure 4B). Using intrinsic signal imaging, we localized the LPZ (Keck et al., 2008) and performed two-photon imaging of inhibitory neurons in layers 1 and 2/3 located in the center of the deprived cortical region. Examination of axonal branches did not reveal any change to the axonal architecture in lesioned animals. In contrast, we found clear and rapid changes of inhibitory boutons. Similar to dendritic spines of inhibitory neurons, inhibitory cell bouton density dropped massively within 24 hr of the lesion (Figures 4B and 4C; 24 hr: 0.44 ± 0.02 boutons/μm axon; corresponding to 84% ± 2% of the original value measured before lesions).