Abstract
Purpose:
Rods and cones control 80-85% of the maximum constriction of the mouse pupil. We seek to disentangle their unique and overlapping contributions to this control while recognizing that pupil size is modulated by diurnal rhythms, and that optimal photoreceptor function co-depends on “time of day”. We designed a pupillometer to measure light-induced pupil changes in unrestrained mice, throughout the day.
Methods:
Mice were trained in a modified “RunningMouse” apparatus (Naarendorp et al., 2010, JN) to visit a water spout inside a sphere abutting the cage. Steady “white” or 500 nm light was applied to the sphere and video images of one eye were recorded. Pupillometric data were collected on WT (n=5), Trα-/- (no rod function; n=2) and Gnat-2cpfl3 (no cone function; n=3) mice at two “times of day”, around Zeitgeber times: ZT22 and ZT10. Mice were entrained with 470 nm light.
Results:
(1) Around ZT22, in darkness, the pupils of all genotypes reached maximum size (~ 6.5 mm2). On weakly to moderately intense fields, WT and Trα-/- pupils remained fully dilated but began to constrict, just above the threshold intensity for cone vision. On the most intense field, ~2000 scot cd m-2, their pupil sizes decreased to ~40% of the dark value. The Gnat-2cpfl3 pupil showed, at low light levels, high sensitivity to small increments. In the photopic range, its size changed from 35% to 30% of the dark value and on the most intense field to ~8%. (2) Around ZT10, in darkness, WT and Trα-/- pupils were constricted to ~98% and ~55%, resp.; the Gnat-2cpfl3 pupil was 100% dilated. At low intensities, the WT pupil showed high sensitivity, the reduced Trα-/- pupil was unresponsive. At photopic levels, pupils of both genotypes constricted to ~15% and on the most intense fields to 3-4% (of dark size). The Gnat-2cpfl3 pupil data were largely similar to those obtained around ZT22. In all three genotypes, field intensities > 200 scot cd m-2 stimulated ipRGCs initiating the final drive of the pupil size to its minimum.
Conclusions:
In daytime, WT rods are responsible for 70% of the maximum pupil constriction; cones bring the combined contribution up to 85%. Between 2 - 5 scot cd m-2 rods and cones simultaneously reduce the pupil size by 20%. At night, cones in WT mice almost exclusively control the pupil suggesting that the scotopic neural circuitry allowing rods to influence pupil size is temporarily switched off or reorganized.