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F. A. Rocha, D. F. Ventura, C. C. Pazos-Moura, T. M. Ortiga-Carvalho, L. A. Santiago, D. A. Santiago, P. F. Gardino, C. N. Pessoa; M-Cone Absence in ERG-Measured Spectral Sensitivity of Mice With a Trß2 Gene Point Mutation. Invest. Ophthalmol. Vis. Sci. 2007;48(13):1293. doi: https://doi.org/.
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Morphological data has indicated that M-opsin differentiation depends on levels of thyroid hormones and the integrity of the thyroid hormone receptor beta 2 (TRß2). This study analyzes the M-opsin retinal expression in transgenic mice with a TRß gene point mutation (Δ337T) using the electroretinogram. This mutation reduces the affinity of the receptor for thyroid hormones.
We used wild-type mice (wt), and transgenic mice with the 337T mutation homozygote (hm) TRßΔ337T/ Δ337T and heterozygote (ht) TRßΔ337T/+) from both genders. Mice were anesthetized with an intramuscular injection of a mixture of xylazine hydrochloride (21 mg/kg) and ketamine hydrochloride (108 mg/kg) and the pupil was dilated by topical application of atropine sulfate (0.04%). ERGs were recorded with DTL fiber electrodes contacting the corneal surface through a layer of 1% methylcellulose. The signal was filtered with a bandpass set at 0.3 - 1000 Hz, monitored and continuously digitized at a rate of 1 kHz by a computer equipped with a data acquisition board. Peak-to-peak amplitudes of the a- and b-waves were measured for 14 different wavelengths (340, 360, 380, 400, 420, 440, 460, 480, 500, 520, 540, 560, 580, 600 nm) with same number of quanta (4.6 x 1014 q/s/cm2).
Electroretinograms of wt mice revealed high sensitivity in the UV and green regions of the wavelength spectrum. Wt mice gave clear responses to UV light (mean amplitude = 129 µV ± 31,56 at 360 nm), while in hm mice responses were substantially reduced (61 µV ± 18,75) and in ht mice responses intermediate amplitudes were found (91 µV ± 38,44). Responses to the middle wavelength region (500 nm) recorded in wt mice had mean amplitude of 91 µV (±12,5) and were of similar amplitude for ht mice (80 µV ± 33,44). On the other hand, hm mice produced only very small or inconsistent responses at 500 nm.
ERGs recorded in mice without a functional thyroid hormone receptor ß (hm) had normal amplitude of responses to short wavelengths but lacked responses to the middle (green) wavelength region of the spectrum. This result corroborates the morphological finding of direct participation of the gene TRß in the expression of M opsin.
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