Molecular mechanism of long-term depression and its role in experience-dependent ocular dominance plasticity of primary visual cortex

Primary visual cortex is a classic model to study experience-dependent brain plasticity. In early life, if one eye is deprived of normal vision, there can be a dramatic change in the ocular dominance of the striate cortex such that the large majority of neurons lose responsiveness to the deprived eye and, consequently, the ocular dominance distribution shifts in favor of the open eye. Interestingly, the visual experience dependent plasticity following monocular deprivation (MD) occurs during a transient developmental period, which is called the critical period. MD hardly induces ocular dominance plasticity beyond critical period. The mechanisms underlying ocular dominance plasticity during the critical period are not fully understood. It has been proposed that long-term depression (LTD) may underlie the loss of cortical neuronal responsiveness to the deprived eye. However, discordant results have been reported in terms of the role of LTD and LTP in visual plasticity due to the lack of specific blockers. Here we report the prevention of the normally-occurring ocular dominance (OD) shift to the open eye following MD by using a specific long-term depression (LTD) blocking peptide derived from the GluR2 subunit of the a-amino-3-hydroxy-5-methyl-isoxazole-4-propionic acid receptor (AMPAR). We were able to prevent the shift of OD to the open eye with systemic or local administration of the GluR2 peptide. Both electrophysiological and anatomical approaches were taken to demonstrate the peptide effect. Moreover, enhancing LTD with D-serine, a NMDA receptor co-agonist, brought back the ocular dominance plasticity in adult mice subject to four-day MD and, therefore, reopened the critical period. Our data indicate that LTD plays an essential role in visual plasticity during the critical period and the developmental regulation of LTD may account for the closure of critical period in adult. In an additional study, we have found anisomycin, a protein synthesis inhibitor, produces a time-dependent decline in the magnitude of the field EPSP (fEPSP) in mouse primary visual cortex and that this anisomycin-mediated fEPSP depression occludes NMDA receptor dependent LTD. In contrast, another two protein synthesis inhibitors, emetine and cycloheximide, have no effect either on baseline synaptic transmission and or on LTD. We propose that anisomycin-LTD might be mediated by p38 MAP kinase since anisomycin is also a potent activator of the P38/JNK MAPK pathway. In agreement with notion, the decline of the fEPSP caused by anisomycin can be rescued by the application of the P38 inhibitor SB203580, but not by the JNK inhibitor SP600125. The occlusion of LFS-LTD by anisomycin-induced fEPSP decline suggests that common mechanisms may be shared between the two forms of synaptic depression. Consistent with this view, bath application of the membrane permeant peptide discussed above, which specifically blocks regulated AMPA receptor endocytosis, thereby preventing the expression of LFS-LTD, prior to anisomycin treatment significantly reduced the anisomycin-induced decline of the fEPSP. In conclusion, this study indicates that anisomycin produces long-lasting depression of AMPA receptor-mediated synaptic transmission by activating P38 MAPK-mediated endocytosis of AMPA receptors in neonatal mouse visual cortex.Medicine, Faculty ofGraduat