Short and long term axotomy-induced ERG changes in albino and pigmented rats

Abstract

Purpose: To investigate the different components of full-field flash electroretinogram (ERG) responses in adult albino and pigmented rats at various time intervals following optic nerve transection (ONT).

Methods: In adult Sprague-Dawley (SD, albino) and Piebald-Viral-Glaxo (PVG, pigmented) rats, the left optic nerve was transected intraorbitally to induce retinal ganglion cell (RGC) death. ERG responses were recorded simultaneously from both eyes beforehand and at 1, 2, 4, and 12 week intervals after ONT. The ERG a- and b-waves and the scotopic threshold responses (STR) were analyzed in scotopic conditions. White light stimuli of intensities ranging from 10(-6) to 10(-4) cd.s.m(-2) were used to record the positive and negative scotopic threshold responses (pSTR and nSTR), while stimulus light intensities ranging from 10(-4) to 10(2) cd.s.m(-2) were used to analyze the a- and b-wave amplitudes of standard ERG recordings.

Results: In the albino rats, one week after intraorbital ONT, the STR mean amplitude decreased significantly, to approximately 60% of the values registered for the contralateral eye (p<0.05), which had not been operated on; standard ERG a- and b-waves showed a small reduction in amplitude-to approximately 85%. By two weeks after ONT, the STR mean amplitude was approximately 40% that of the contralateral eye, while the a- and b-wave amplitudes had further decreased to approximately 75%. Four weeks after ONT, the STR had fallen to 60% of that of the contralateral eyes, whereas the a- and b-waves reached values of approximately 90%. Twelve weeks after ONT, the STR remained significantly reduced to approximately 45%, whereas the a- and b-waves reached values of approximately 90%. In the pigmented rats, one week after intraorbital ONT, the mean amplitude had decreased significantly, to approximately 60% for the pSTR and to 80% for the nSTR of the values registered for the intact contralateral eye (p<0.05); while the standard ERG a- and b-waves showed a small reduction in amplitude to approximately 90%. Two weeks after ONT, the STR mean amplitude was approximately 55%, while the a- and b-wave amplitudes had further decreased to approximately 65%. Four weeks after ONT, the STR also was significantly reduced, to only 40%, whereas the a- and b-waves reached values of approximately 60%. Twelve weeks after ONT, the pSTR and nSTR remained significantly reduced to approximately 40% and 70%, respectively; whereas the a- and b-waves reached values of approximately 80%.

Conclusions: Optic nerve injury results in transient reductions of the major ERG components, the a- and b-waves, as well as permanent reductions of the early components of the ERG, the negative and positive scotopic threshold responses. Because ONT induces massive RGC loss, it is likely that permanent reduction in the STR represents a lack of the RGC population, thus highlighting the importance of the STR recordings as an electrophysiological tool for the assessment of RGC function.

Figure 1

Scotopic electroretinographic recordings in albino and pigmented rats. Examples of the ERG traces recorded in an albino (A) and pigmented (B) rats in response to flash stimuli of increasing intensity. Thin traces correspond to recordings obtained from the right eye and bold traces correspond to recordings obtained from the left eye. The intensity of the flash stimuli is indicated in log cd·s/m² units at the left of the recording traces. The scotopic threshold responses were elicited by weak light stimuli from −4.60 to −5.40 log cd·s/m². Rod and mixed responses were elicited by light intensities from −3.96 to 2.03 log cd·s/m². No significant difference in the ERG amplitudes between left and right eyes was observed in any of the studied animals. Examples for the measurement of wave amplitudes are shown in the albino rat recordings.

Figure 2

Electroretinographic amplitude measurements in albino and pigmented rats. Averaged data (mean±SEM) of ERG amplitudes versus stimulus intensities both from control albino (A, C; n=6) and pigmented rats (B, D; n=8) is shown for the right eye (open symbols) and left eye (filled symbols). A, B: Positive scotopic threshold response (circles) and negative scotopic threshold responses (triangles). C, D: Triangles correspond to the a-wave amplitude and circles correspond to the b-wave amplitudes. When the amplitudes obtained from the albino group were compared to those of the pigmented, there were no significant differences (t-test; p>0.05) for any of the waves analyzed, except the pSTR (t-test; p<0.05).

Figure 3

Scotopic electroretinographic recordings two weeks after optic nerve transection. Examples of the ERG traces recorded in an albino (A) and pigmented (B) rat in response to flash stimuli of increasing intensity for the unoperated right eye (thin traces) and for the operated left eye (bold traces) two weeks after optic nerve transection. The intensity of the flash stimuli is indicated to the left of the recording traces. Reduction in the pSTR and nSTR responses from operated eyes versus unoperated eyes is clearly evident in both albino and pigmented rats.

Figure 4

Electroretinographic amplitude measurements two weeks after optic nerve transection. Averaged data (mean±SEM) of ERG amplitudes versus stimulus intensity both from albino (A, C; n=7) and pigmented rats (B, D; n=5) from recordings obtained two weeks after ONT. Open symbols show data averaged from unoperated right eyes and filled symbols show data averaged from operated left eyes. A, B: Data corresponding to positive scotopic threshold responses (pSTR) are shown as circles and that corresponding to negative scotopic threshold responses (nSTR) are shown as triangles. C, D: Amplitudes corresponding to the a-wave are shown as triangles and those corresponding to b-wave are shown as circles. A significant reduction in the wave amplitudes of the pSTR, nSTR, b-wave scotopic response, and a- and b-wave was observed in the operated eyes (t-test; p<0.001) when compared to the unoperated control eyes.

Figure 5

Scotopic electroretinographic recordings four weeks after optic nerve transection. Examples of the ERG traces recorded in an albino (A) and pigmented (B) rats in response to flash stimuli of increasing intensity four weeks after optic nerve transaction. Thin traces correspond to the unoperated right eye and bold traces correspond to the operated left eye. The intensity of the flash stimuli is indicated to the left of the recording traces. Reduced pSTR and nSTR responses from operated eyes versus control eyes were clearly observed for both strains, while an apparent recovery of the ERG wave amplitudes was observed for the scotopic and mixed responses in the albino rats.

Figure 6

Electroretinographic amplitude measurements four weeks after optic nerve transection. Averaged data (mean±SEM) of ERG amplitudes versus stimulus intensity both from albino (A, C; n=9) and pigmented rats (B, D; n=6) is shown from recordings obtained four weeks after ONT. Open symbols show data averaged from unoperated right eyes and filled symbols show data averaged from operated left eyes. A, B: Data corresponding to positive scotopic threshold responses (pSTR) are shown as circles and that corresponding to negative scotopic threshold responses (nSTR) are shown as triangles. C, D: Amplitudes corresponding to the a-wave are shown as triangles and those corresponding to b-wave are shown as circles. A significant reduction in the wave amplitudes of the pSTR, nSTR, b-wave scotopic response, and a- and b-wave was observed in the operated eyes (t-test; p<0.001) when compared with the unoperated eyes. The significant reduction of ERG wave amplitudes in operated eyes versus unoperated eyes is evident only in the pSTR and nSTR responses (p<0.001) in albino rats. A significant reduction for the pSTR, nSTR, b-wave scotopic response, and a- and b-wave of the mixed response was observed in pigmented rats (p<0.001).

Figure 7

Scotopic electroretinographic recordings 12 weeks after optic nerve transection. Examples of the ERG traces recorded in an albino (A) and pigmented (B) rats in response to flash stimuli of increasing intensity for the unoperated right eye (thin traces) and for the operated left eye (bold traces) 12 weeks after optic nerve transection. The intensity of the flash stimuli is indicated to the left of the recording traces. Reduction in the pSTR and nSTR responses from operated eyes versus unoperated eyes was clearly seen 12 weeks after ONT, while no other apparent difference in ERG scotopic and mixed responses was evident between the operated and unoperated animals for both rat strains.

Figure 8

Electroretinographic amplitude measurements 12 weeks after optic nerve transection. Averaged data (mean±SEM) of ERG amplitudes versus stimulus intensity both from albino (A, C; n=9) and pigmented rats (B, D; n=5) from recordings obtained twelve weeks after ONT. Open symbols show data averaged from unoperated right eyes and filled symbols show data averaged from operated left eyes. A, B: Data corresponding to positive scotopic threshold responses (pSTR) are shown as circles and that corresponding to negative scotopic threshold responses (nSTR) are shown as triangles. C, D: Amplitudes corresponding to the a-wave are shown as triangles and those corresponding to b-wave are shown as circles. A significant reduction of ERG wave amplitudes in operated eyes versus unoperated eyes is observed for the pSTR and nSTR responses (p<0.001) in both strains of rat. There was a small reduction in the b-wave scotopic response and a- and b-wave of the mixed response in pigmented rats (p<0.001).

Figure 9

Evolution of electroretinographic wave amplitudes over a 12-week time period. Average data (mean±SEM) of the reduction in the ERG wave amplitudes is represented as the percentage between operated and unoperated eyes for 1, 2, 4, and 12 weeks after ONT, both from albino (A, C) and pigmented rats (B, D). A, B: Close triangles show averaged amplitudes for the positive scotopic threshold response (pSTR) and open triangles show averaged amplitudes for the negative scotopic threshold responses (nSTR). C, D: Open circles show averaged data for the a-wave amplitudes and closed circles show averaged data for the b-wave amplitude. A permanent reduction in the pSTR and nSTR was observed for the 12-week time period both for pigmented and albino rats. The reduction in the a-wave and b-wave amplitudes recovered earlier in albino rats than in pigmented rats.