Local Perfusion Of Chondroitinase ABC Influences The Developmental Changes Of NR2A And NR2B In Rat Visual Cortex Around The End Of Critical Period Of Visual Plasticity

During a critical period of early postnatal development, visual system changes in shape and function adapting to the regulations of external envionment and internal gene. Visual experience refines synaptic connections such that a stable and functional network of neurons is established. Visual cortex has been proved to be the crucial impaired target of amblyopia. Due to strabismus, ptosis, and congenital cataract and so on, lacking of abundant visual experience to the normal retina resulted in a lower corrected visual acuity named amblyopia, which is often accompanied by abnormal stereo vision, movement, perception and other visual damages. Within the critical period of visual plasticity, amblyopia can be cured by removing the cause of abnormal visual experience. However, amblyopia is hard to be cured beyond the critical period of visual plasticity because of the inhibited visual plasticity. To reactivate this suppressed plasticity is the key to solving the problem.As we know, many factors such as synapic platicity in visual cortex, maturation of excitatory circuit and inhibitory circuit, extracellular matrix, neurotrophic factors and so on, have effects on the end of critical period of visual plasticity. Chondroitin solfate proteoglycans (CSPGs) are components of the extracellular matrix that inhibit axonal sprouting and growth. Their adult pattern of expression is reached during late development, when CSPGs condense around the soma and dendrites of a subset of neurons in the form of perineuronal nets (PNNs). After CSPGs degradation with chondroitinase-ABC (chABC) in adult rats, monocular deprivation caused an ocular dominance shift toward the nondeprived eye. The mature extracellular matrix is thus inhibitory to experience-dependent plasticity, and degradation of CSPGs reactivates the inhibited cortical plasticity.In rat visual cortex, glutamatergic excitatory transmission is mediated by NMDA receptor and AMPA receptor. After the peak of critical period of rat visual plasticity, the decay time of NMDA receptor mediated excitatory postsynaptic current (NMDA-EPSC) become shorter with age. NMDA receptor subunit 2A (NR2A) and NR2B are the master subunits of NMDA receptor, and their ratio increase gradually during development. Dark rearing can delay the developmental changes of NMDA-EPSC properties and NR2A/2B ratio. The developmental changes in both CSPGs and NR2A/2B ratio are associated with visual platicity, however the mechanism of reactivating the supressed visual cortical plasticity by CSPGs degradation with chABC is still unknown.We hypothesized that NR2A/2B ratio invoved in the developmental plasticity of visual cortex around the end of critical period of visual plasticity, and this ratio can be altered by CSPGs degradation with chABC. The NR2A/2B ratio is one of the important mechanisms leading to the reactivation of visual cortical plasticity after local perfusion of chABC.To identify this hypothesis, the present study was performed in the following two parts:Part 1. Developmental changes in expression of NR2A and NR2B in normal rat visual cortex around the end of critical period.Using immunofluorescence and western blot, we investigated developmental changes in NR2A and NR2B immunoreactive neurons in the visual cortex of normal rat. Results in this study revealed that: 1. The number of NR2A immunoreactive neurons in every layers of normal rat visual cortex increased gradually from postnatal week 3 (P3W) to P5W and the changes from P5W to P7W were not significant. Western blots results showd that the level of NR2A in normal rat visual cortex increased gradually from P3W to P5W and there were no significant changes from P5W to P7W. 2. The number of NR2B immunoreactive neurons in every layers of normal rat visual cortex decresed gradually from P3W to P6W and there were no significant changes from P6W to P7W. Western blots results showd that the level of NR2B in normal rat visual cortex decresed gradually from P3W to P6W and no significant changes were found from P6W to P7W. 3. Due to the developmental changes of NR2A and NR2B, the ratio of NR2A/2B in normal rat visual cortex increased gradually from P3W to P6W and there were no significant changes from P6W to P7W.Part 2. Local perfusion of chondroitinase ABC influences the developmental changes of NR2A,2B in rat visual cortex around the end of critical period of visual plasticity ChABC treated rats were made for immunofluorescence and western blots. Compared with the normal rats, the results in this part of study revealed that: 1. In the layers 2-3 and 5-6 of visual cortex of chABC treated rats, the NR2A immunoreactive neurons became less from P4W to P7W, but in the layer 4 of visual cortex of chABC treated rats, this kind of change was found from P5W to P7W. Western blots results showd that NR2A level in the visual cortex of chABC treated rats became lower from P4W to P7W. 2. Both NR2B immunoreactive neurons and NR2B level in the visual cortex of chABC treated rats had no sifnificant changes from P3W to P7W compared with normal rats at same age. 3. With the changes of NR2A, NR2A/2B ratio in the visual cortex of chABC treated rats became lower than that in normal rat visual cortex from P4W to P7W.We concluded that: 1. Around the end of critical period of visual plasticity, NR2A and NR2B involved in the changes of visual plasticity. From P3W to P7W, the NR2A/2B ratio in the visual cortex of normal rats increased gradually, and NR2A become predominant instead of NR2B. 2. ChABC treatment influenced the NR2A expression and NR2A/2B ratio in rat visual cortex from P4W to P7W. The changes of NR2A/2B ratio in rat visual cortex might be involed in the mechanism of reactivating the inhibited cortical plasticity after CSPGs degradation with the use of chABC.