To explore the effect of enriched environment on the level of NR2A and NR2B subunits of N-mehyl-D-aspartate (NMDA) receptors which belong to glutamate receptors with excitability at the 17th area of the visual cortex in amblyopia rats after the critical period, and to understand the possible mechanism of synaptic plasticity of the visual cortex in adult amblyopia rats.

Eighty Wistar rats were divided into normal group and experimental group by random number table.Right eyelids of all rats were sutured through the whole critical period in order to establish monocular deprivation (MD) amblyopia model.The rats in experimental group were divided into the amblyopia group, standard environment (SE) group and environmental enrichment (EE) group on P45 in random.The sutured right eyelids were opened on P46 in the SE group and EE group.All rats were sacrificed to get the 17th area of the left visual cortex on P60, P75 and P105.Three rats were used at different time points from each group.The Ⅰ-Ⅵ layers of the visual cortex area 17 were observed by using hematoxylin-eosin staining.The expression of NMDA-NR2A and NMDA-NR2B was detected by immunohistochemistry.Integrated optical density of NMDA-NR2A and NMDA-NR2B was detected by using special image analysis software (Image-Pro Plus 6.0). The use of animals complied with Regulation on the Managenment Experimental Ainimals from Shandong Eye Institute and Association for Research in Vision and Ophthalmology (ARVO).

The positive expression of NMDA-NR2A and NMDA-NR2B were observed in the visual cortex.The positive cells were mostly round or elliptical and mainly expressed in cell membrane.The expression of NMDA-NR2A in P60, P75 and P105 from four groups had statistical differences (all at P<0.05). There were less positive cells in amblyopia group and EE group than normal control group on P60, P75 and P105, while there were more positive cells in EE group than amblyopia group.Amblyopia can lead to reduced NMDA-NR2A expression in the visual cortex.The expression of NMDA-NR2A was stronger than that in the amblyopia group by intervention 15 days, 30 days, and 60 days with the rich environments, but did not reach the normal level (all at P<0.05). The expression of NMDA-NR2B in P60, P75 and P105 from four groups also had statistical difference (all at P<0.05). There were more positive cells in amblyopia group than those in normal control group on P60, P75 and P105.There were more positive cells in EE group than normal control group on P60, while there were equal positive cells in EE group and normal control group on P75 and P105.Amblyopia can lead to increase NMDA-NR2B expression in the visual cortex.The expression of NMDA-NR2B was weaker than that in the amblyopia group by intervention 15 days with the rich environments, but did not reach the normal level (all at P<0.05). The expression of NMDA-NR2B after intervention 30 days and 60 days reached the normal levels (all at P>0.05).

The plasticity of visual cortex exists not only in the critical period but also after the critical period of visual development.EE, as a non-invasion method, can improve and recover the synaptic plasticity in visual cortex of adult rats by the expression of NMDA-NR2A and NMDA-NR2B.

Amblyopia; Visual cortex; Synapse; Enriched environment; N-mehyl-D-aspartate receptors