Behavioral responses to visual stimuli in Brn3b knockout mice

Abstract

Purpose : Transcription factor Brn3b is essential to retinal development, with Brn3b knockout mice missing ~70% of retinal ganglion cells. The physiological effects of Brn3b loss are well documented, but individual losses of visual function in those mice are poorly quantified. In this experiment, we use visual stimuli to better describe the visual capabilities of Brn3b ^KO/KO (global knockout; KO), Rax:Cre; Brn3b^CKOAP/WT (retina-specific Brn3b heterozygote with phenotypically wild-type retina; Het), and Rax:Cre; Brn3b^CKOAP/KO (retina-specific knockout with retinas phenotypically resembling KO; CKO) mice as compared to Brn3b^WT/WT (wild-type; WT) mice.

Methods : Mice were placed in square arena (53cm sides, 32cm height, nest at perimeter, stimulus screen above) and allowed to adapt for 7 min. Mice were recorded for 20s and stimulus was triggered at 10s mark. Three trials were recorded per mouse per stimulus condition. Loom stimulus was a black disk expanding on a gray screen from 0cm (0° visual angle) to 31.5cm (49.3° visual angle) for 0.25s, remaining onscreen for 0.5s. Loom stimulus tested at 0.55, 0.38, 0.29, and 0.2 contrast levels. Sweep stimulus was a 2.6cm (4.3° visual angle) black disk moving diagonally across the gray screen (0.55 contrast) at 21.8°/s. Sweep stimulus also tested at 10.9°/s speed, 43.5°/s speed, 1.92° visual angle, and 7.67° visual angle.

Results : Loom: Loss of Brn3b ablates fleeing as a response to loom stimulus. KO mice exhibit significantly reduced fleeing behavior (p <0.001), and show a moderate but not statistically significant (p = 0.35), increase in freezing behavior. CKO mice show no fleeing during stimulus period. KO mice that flee show a delayed reaction (p < 0.001). Decreasing contrast appears to increase reaction time.
Sweep: Loss of Brn3b mutes responses to sweep stimulus. KO mice show significantly less freezing than WT mice (p < 0.05). Interestingly, CKO mice are more likely than WT mice to flee from sweep stimulus (p < 0.01). There is a significant effect of stimulus speed on percent time spent freezing for WT mice (p < 0.001).

Conclusions : Loss of Brn3b drastically reduces fleeing as a response to loom stimuli while leaving freezing intact, indicating that Brn3b is not necessary to spot a stimulus but is necessary to correctly interpret the stimulus. Since responses to sweeping stimuli are only modestly affected, Brn3b loss may differentially affect RGC types involved in the reactions to moving stimuli.

This abstract was presented at the 2019 ARVO Annual Meeting, held in Vancouver, Canada, April 28 - May 2, 2019.