Abstract
Purpose: :
All 3 photoreceptor types (rods, cones and intrinsically photosensitive retinal ganglion cells [ipRGCs]) contribute the photic information that mediates the pupillary light reflex. In previous studies, the role of ipRGCs in the pupil response was mostly inferred either through the use of transgenic animals or by using chromatic stimuli designed to target differences in photoreceptor spectral sensitivity. In this study, the aim was to investigate whether the slow temporal characteristics (sluggishness) of ipRGC responses could be utilized to distinguish the contribution of these neurons to the rat pupillary light reflex.
Methods: :
Brown Norway rats were lightly anesthetized through isoflurane inhalation and positioned with an LED light source in front of the left eye, which was dilated with tropicamide. An infrared-sensitive video camera monitored the rat’s right pupil to record the consensual pupil response. Rats were dark adapted for at least 15 min prior to light stimulations. Using a blue LED light (peak = 470 nm), rats were exposed to flickering light (at 0.05, 0.1 or 0.5 Hz) at different intensities (range: 1013 to 1015 photons/s/cm2). In additional in vitro experiments, ipRGC action potential firing in response to these flickering light stimuli was measured by obtaining multi-electrode array (MEA) recordings from isolated neonatal rat retinas.
Results: :
In response to 5 s pulses of light, the rate of pupil dilation during 10 s after light offset was slower as light intensity increased, consistent with an increased contribution by ipRGCs with brighter light stimuli. The slope of pupil dilation (%) over time (s) was 3.2, 1.5, and 0.2 for light pulses of 1013, 1014 and 1015 phots/s/cm2 (n=3 each group). For each of the 3 light flickering rates, a significant flicker in pupil size at 1013 to 1014 phots/s/cm2 was observed (n=3 each group). By 1015 phots/s/cm2, the pupil response had reached the critical flicker fusion threshold (absence of pupil flicker), as confirmed by Fourier analysis. The MEA recordings demonstrated that rat ipRGCs responses to flickering (0.05, 0.1, 0.5 Hz) bright (1015 phots/s/cm2) light were not significantly different from responses to continuous light.
Conclusions: :
The in vitro recordings confirmed that ipRGCs have poor temporal sensitivity, relative to rods and cones, in responding to closely spaced light stimuli. The in vivo experiments indicate that the critical flicker fusion threshold of the pupil response has potential in unmasking the threshold light intensity at which ipRGCs significantly contribute to the pupillary light reflex.
Keywords: pupillary reflex • ganglion cells • photoreceptors