Studies On Changes Of Primary Visual Cortex In Experimental Optic Nerve Injury Models

Whether transneuronal degeneration or neuronal death in the primary visual cortex happens after optic nerve injury has become a hot spot in the research field of neuroprotection and vision restoration. The present study was designed to investigate the characteristic changes and their potential mechanisms in primary visual cortex after acute or chronic optic nerve injuries.Part I: Early changes in the primary visual cortex of rats after acute optic nerve injuryIn the first part of the thesis, our research work was planed to investigate the differential expression of immediate-early genes c-jun and c-fos in primary visual cortex of rat early after acute optic nerve injury and the relationship between immediate-early gene expression and the injury degree of optic nerve. Furthermore, we wanted to find out if the total neuronal amount of primary visual cortex and neuronal amount of lamina IV changed early after acute optic nerve injury.Unilateral optic nerve crush or optic nerve transection was performed in Sprague-Dawley rats to obtain acute partial or complete monocular optic nerve injury models. Frozen sections through visual cortex were cut in normal rats and model rats respectively at 2h, 1d, 3d, 1w and 1m after operation. The differential expression of c-Jun and c-Fos was detected in the primary visual cortex of rats by means of immunohistochemistry. The number of NeuN labeled positive neurons in the arrowy sections of primary visual cortex and layer IV were calculated. Statistical comparisons were made using one-way ANOVA.The increased expression of c-Jun in the primary visual cortex could be observed at 2h postoperation, and reached peak value at 1d postoperation. Then it dropped and reached another peak value until lm postoperation. The extent of increase of c-Jun expression was much higher and the maintenance of fastigium was much longer in optic nerve transection models than in optic nerve crush models. The decreasedexpression of c-Fos in the primary visual cortex could be observed at 2h postoperation in optic nerve crush models, and reached valley value at 3d postoperation followed by increased expression. The extent of decrease of c-Fos expression was lower in optic nerve transection models than in optic nerve crush injury models and transient increased expression could be observed at 2h postoperation. Statistic analyses showed there were no significant changes exist between normal and model rats for total neuronal density in primary visual cortex or neuronal density in lamina IV.The results of part I indicated that the expression of immediate-early gene c-jun and c-fos changed shortly after optic nerve injury at 2h postoperation. The opposite expression of c-Jun and c-Fos in the primary visual cortex in early stage, especially 2h~3d, postoperation of optic nerve injury pointed out for us that these two groups of transcription factors may act in opposite direction in the primary visual cortex. Besides the direct influences of different injury degrees, some kind of equilibrium mechanisms may participate in accommodating the expression of these two transcription factors.Part II: Neuronal changes of primary visual cortex in chronic ocular hypertension models of monkeyPart II of this thesis was designed to investigate the metabolic and neurochemical changes in the primary visual cortex of different stages of monkey chronic ocular hypertension models along with their connection with RGC loss. Another objective of part II was to make it clear whether the amount and ultrastructure of neurons changed in the primary visual cortex of monkey glaucomatous models.The objects of our research work were 4 normal monkeys and 11 monkeys with unilateral or bilateral chronic ocular hypertension induced by laser scarification of the trabecular meshwork. Coronal and tractional sections of visual cortex were cut on a cryomicrotome. The cytochrome oxidase reaction, calculation of NeuN labeled neurons and distribution of three neurochemicals (GFAP, BDNF, TrkB) revealed by immunohistochemistry were studied. The results of cytochrome oxidase reaction were transformed into average greyness values. The spacial distribution of NeuN labeled neurons and positive area of GFAP or BDNF expression in lamina 4C of primary visual cortex were counted and the relationships between the results and RGC loss were analysed. Furthermore, the ultrastructural changes of neurons in lamina 4 of primary visual cortex were observed by means of transmission electron microscopy.Statistical comparisons were made using one-way ANOVA.Dense and continuous bands of high oxidative enzymatic activity were seen within lamina 4C of primary visual cortex in normal monkeys. A pattern of light bands alternating with dark bands in lamina 4C standing the structure of ocular dominance columns emerged in glaucomatous monkeys according to different stages of both eyes. While pale stained and continuous bands of low oxidative enzymatic activity could be seen within lamina 4C in binocular terminal-stage glaucomatous monkeys. Statistical analyses revealed there were no significant differences between the oxidative enzymatic activities in ocular dominance columns representing normal eyes and model eyes with RGC loss less than 50%. While in the ocular dominance columns representing model eyes with RGC loss more than 50%, the oxidative enzymatic activities were obviously under normal levels. There were no statistical differences of the neuronal amounts in lamina 4C between normal and glaucomatous monkeys. Similar to the results of cytochrome oxidase reaction, there were no significant differences of GFAP expression between ocular dominance columns representing normal eyes and model eyes with RGC loss less than 50%. While in the ocular dominance columns representing model eyes with RGC loss more than 50%, the GFAP expression were obviously above normal levels. The BDNF and TrkB expression was incoordinately decreased in glaucomatous monkeys, and independent of RGC loss. The ultrastructural changes of neurons in lamina 4 of glaucomatous monkeys included reduced quantity of ribosomes, swelled mitochondrias, dilated endoplasmic reticulums and increased amount of lipofuscin bodies. But there were no evidences indicated irreversible neuronal degeneration or apoptosis.These results related the neurochemical changes and glial activation in primary visual cortex to RGC loss in monkey ocular hypertension models. RGC loss of 50% might be a key-point for changes of metabolic activity at cortical level in visual centrum. With a view to this point, neuroprotection might be more effective while practiced in early to middle stage in the course of glaucoma with RGC loss less than 50%. The decreased expression of BDNF and TrkB indicated restrained neuronal activity in primary visual cortex, which might participate in the vicious circle in glaucomatous pathogenesis via lessened retrograde nutritional sustainments.Considering neither reduction of neuronal quantity in primary visual cortex nor evidences of irreversible degeneration of neuronal ultrastructure having been observed, neuronal changes of primary visual cortex in acute and chronic optic nerveinjury models were more likely representations of cortical plasticity.