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D. Ladwig, H. Lu, K.A. Vessey, P.J. DeMarco, M.A. McCall; Absence of Retinal GABAC Receptor Expression Alters the Process of Dark Adaptation . Invest. Ophthalmol. Vis. Sci. 2006;47(13):395.
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© ARVO (1962-2015); The Authors (2016-present)
GABAC receptor (GABACR)–mediated inhibition plays an integral role in rod pathway function in the retina. Because the GABACR provides tonic inhibition to the rod pathway, we examined if it contributes to the process of dark–adaptation in a knockout mouse model, which lacks expression of the GABACR.
The full–field electroretinogram (ERG) was recorded from anesthetized GABACR null (Null) and wildtype C57B6/J (WT). Two age groups (young: 50–60 d, and old: 100–120 d) were used. Mice were prepared for ERG recording under room lighting, and dark–adaptation was measured using one of two protocols immediately after extinguishing room lights: 1) Constant Intensity Protocol: 10ms flashes (0.38 cd s m–2) were presented at 3 minute intervals for up to 3 hrs, or 2) Constant Response Protocol: A computerized tracking program adjusted the flash intensity of light emitting diodes to maintain a 30 µV ERG b–wave over a 50 min period.
All mice dark–adapted more quickly under the Constant Response Protocol due to the dimmer tracking flash required to elicit a 30 µV response. As has been noted previously, the amplitude of the b–wave reduces as a function of age; old Null and WT mice had the smallest b–wave amplitudes. The Constant Intensity Protocol revealed an additional age effect in which old Null mice appeared to remain in a more light–adapted state than age–similar controls. However, prolonged dark–adaptation without a tracking flash confirmed that old Null mice are able to achieve the same dark adapted plateau as WT controls. Under both protocols, young Null mice tended to reach the dark adapted plateau most quickly. Oscillatory potentials (OPs) 4–8, measureable in the Constant Intensity Protocol, were greater in young Null mice than in controls.
These data suggest that GABACR mediated feedback onto rod bipolar cells is important for the process of dark–adaptation, and that lack of this feedback increases the amplitude of certain OPs in young Null mice. These findings also suggest that older GABACR Null mice have an exaggerated response to an adapting flash.
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