Some Observations on the Rod-Cone Character of the Behavioral Increment Threshold Function of the WT and Gnat1-/- Mice

Abstract

Purpose:: To determine the sensitivity and spectral identity of the mechanisms mediating the visual threshold in alert WT mice, and in Gnat1^–/– mice that lack functional rods, under dark- and light-adapted conditions.

Methods:: Wheel-running mice were trained to associate the presentation of a light flash with the availability of water at an electronically-monitored licking spout in their cage. Light from an interchangeable, intensity-controllable LED, mounted above the running wheel, passed through the appropriate narrow-band filter to stimulate the ventral retina with a 5 deg target, subtending ~8400 rods and ~200 cones. Flashes were randomly triggered as the mouse ran on the wheel, and it was deemed to detect the flash if it halted running immediately after flash occurrence. Controlled flood lamps that illuminate a light-diffusing box placed over the mouse cage provided "white" background light. Visual sensitivity was determined with the method of constant stimuli. Mice "ran" experiments throughout the night.

Results:: Threshold versus intensity (t.v.i.) curves were obtained from WT mice (n= 4) with flashes of either = 365 nm or 532 nm. (a) With the 365 nm flash, a rod-cone break occurred at I ~ 0.08 scot cd m^-2.or ~ 40 R* rod^-1 s^-1. (b) With a 532 nm flash there was no break in the t.v.i curve. Absolute threshold intensities were measured on WT and Gnat1^–/– mice (n = 3) with various narrow-band flashes. (a) In the dark-adapted WT mouse, the test spectral sensitivity, S(), was that of the rods. (b) In the dark-adapted Gnat1^–/– mouse, S() of the most sensitive mechanism resembled that of UV-cones. (c) In one Gnat1^–/– mouse (n=3 replications) the absolute threshold for a 532 nm flash was determined. (d) The sensitivity ratios for S_rod(500 nm)/ S_cone(365 nm) and S_rod(500 nm)/ S_cone(532 nm) were ~2000 and ~40000, respectively.

Conclusions:: (1) The scotopic sensitivity of the mouse measured with a local stimulus compares favorably with human psychophysical sensitivity measured with a small brief stimulus in the periphery, 20 deg from the fovea. (2) The high value of the sensitivity ratio S_cone(365 nm)/S_cone(532 nm) ~ 20 is consistent with very low expression of M-cone opsin in the mouse ventral retina.

Keywords: perception • adaptation: chromatic • detection