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Phillip Thomas Yuhas, Andy Hartwick; Comparison of in vivo pupil responses and in vitro ipRGC spiking responses to repeated light stimuli in rats. Invest. Ophthalmol. Vis. Sci. 2018;59(9):5040.
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© ARVO (1962-2015); The Authors (2016-present)
Repeated exposure to red and blue lights elicits progressively greater pupil constriction and slower re-dilation in humans. The physiological basis for this phenomenon is unknown, but we hypothesize that repeated light exposure potentiates the responses of the intrinsically photosensitive retinal ganglion cells (ipRGCs) that regulate pupil size. Here, we investigate this photopotentiation effect in rats using in vivo and in vitro approaches.
In vivo pupil size recordings were obtained from adult rats (n = 4) using two bright (2.5 x 1013 and 8.8 x 1013 phot/s/cm2), blue (470 nm) stimuli that slowly flashed (0.05 Hz) for 2 min. In vitro multielectrode array ipRGC recordings were obtained from the retinas of postnatal (P) rats at age P11 (n = 151 cells) and P18 (n = 56 cells). Action potentials evoked by a bright (1015 phot/s/cm2) blue stimulus, which flashed (0.1 Hz) for 1 min, were counted in 10 s bins (5 s during light on, then 5 s post-light offset).
When evoked by the dimmer stimulus, mean normalized (100% = smallest pupil size) pupil constriction during and after the last two pulses (91.4 ± 4.9% constriction over 40 s period) was not significantly larger (p = 0.15) than that of the first two pulses (76.5 ± 4.7%). When the brighter stimulus was used, however, the last two pulses elicited significantly more (p = 0.006) pupil constriction (93.1 ± 1.4%) than the first two pulses (86.8 ± 2.0%). In contrast, a progressive reduction in ipRGC spike counts occurred in the retinal recordings obtained in response to the flashing stimulus. In the P11 cohort, the ipRGC spike count during and after the last two pulses (110.7 ± 80.1 spikes/cell over 20 s period) was significantly less (p < 0.001) than that for the first two pulses (227.9 ± 122.2). In the P18 cohort, ipRGC spiking was also significantly less (p < 0.001) during and after the last two pulses (125.3 ± 113.7) than the first two pulses (227.9 ± 150.3).
In response to a bright, flashing blue light, rat pupils exhibit increasingly greater constriction as the stimulus progresses. However, in recordings obtained from young rat retinas, ipRGC spike counts progressively decrease over time in response to a similar flashing blue stimulus. These results suggest that changes in the intrinsic melanopsin-based responses of ipRGCs to repeated light exposure are not responsible for pupil response photopotentiation.
This is an abstract that was submitted for the 2018 ARVO Annual Meeting, held in Honolulu, Hawaii, April 29 - May 3, 2018.
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