Lighting conditions during suckling modify rat pupillary light responses but not entrainment to 24-hour sawtooth dim light-dark cycles

Early-life experiences have lasting consequences on future health. This study examined the impact of bright light exposure during suckling on non-image-forming system responses, specifically the pupillary light reflex (PLR) and circadian entrainment. Wistar rats were exposed to either constant bright light (SLL-rats) or 24-hour light-dark cycles (SLD-rats) during suckling. After weaning, all rats were maintained under identical conditions, experiencing different 24-hour sawtooth light-dark cycles (light: dim light, 1 µW/cm2; dark: 0.03 µW/cm2 red light) for three months. PLR was tested using 30-second blue light pulses at intensities of 3, 10, 30, 100, and 300 µW/cm2. Results showed that PLR was influenced by early-life lighting conditions. SLL-rats exhibited a linear response across all light intensities, with significantly reduced pupillary contraction compared to SLD-rats at 3, 10, 30, and 100 µW/cm2, while no differences were observed at 300 µW/cm2. Additionally, exposure to different 24-hour sawtooth cycles led to minor differences in circadian rhythm shape and entrainment, which did not correspond to the PLR differences or the manifestation of rhythms under LL in adulthood. These findings suggest that early-life lighting conditions may induce functional alterations in specific aspects of the non-image-forming system, potentially affecting physiology and behavior in adulthood.