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JT Erichsen, SN Haamedi, WJ Kuenzel, W Hodos; Myopia is Induced by Lesions of the Isthmo-Optic Nucleus in Chicks . Invest. Ophthalmol. Vis. Sci. 2002;43(13):192.
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Purpose: The isthmo-optic nucleus (ION) projects to the contralateral retina in birds, and is reported to play an important role in modulating retinal activity, especially in ground foraging species. Previous studies of experimental myopia in domestic chicks indicate that eye growth is affected by retinal activity. We investigated the effect of a unilateral lesion of ION upon the subsequent growth of the contralateral eye. Methods: Electrolytic lesions (25mA for 10s) of the right ION were made in 12 chicks at age 2-3 weeks. A further 8 chicks were used as untreated, age-matched controls. After one week, both eyes of all chicks were refracted using retinoscopy. Brains were then fixed, frozen, serially sectioned, and analysed to provide information about the size and location of the lesions. Eyes from 8 of the experimental chicks were photographed whole as well as hemisected to determine their dimensions. Results: The extent of the right ION destroyed ranged from 0 to 100% with an average of 6611%. In the 12 experimental chicks, the left eye was significantly more myopic than the right by an average value of -2.480.64D (p<0.01). This relative myopia was observed in 11 of the 12 animals. In the control chicks, the refractive states of the two eyes were not significantly different (+0.430.54D) and displayed no systematic left/right effects. In the 8 pairs of photographed eyes, the vitreous chamber depth of the left eye was significantly longer (0.4mm) than the right (t=2.73, p<0.01). Conclusions: Disturbance of the ION input resulted in a relative myopia of the contralateral eye. The ION provides a largely excitatory input to the retinal amacrine and ganglion cells. Our results suggest that removal of the ION input may produce a reduction in retinal activity which in turn induces increased eye growth.
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