Light exposure disrupts sensorimotor gating and modulates acute stress effects in Wistar rats

Prepulse inhibition (PPI) of the acoustic startle response is a robust index of sensorimotor gating that is sensitive to emotional and environmental factors, yet the effects of acute stress on PPI in rats remain inconsistent. The present study reports findings from three experiments aimed at examining the effects of stress on the startle response and PPI in male Wistar rats using between- and within-subjects designs to disentangle the unique contributions of contextual conditioning and environmental illumination. In Experiment 1 (n = 32), acute stress was induced by the forced swim test (FST) and inescapable footshocks, and only the FST was found to reduce PPI during the stress period relative to a no-stress period, suggesting that stress effects may depend on contextual factors. Experiment 2 (n = 16) assessed contextual conditioning by pairing inescapable footshocks with either a dark or an illuminated testing context. Results showed that PPI changes were primarily driven by environmental illumination rather than by context-shock associations. Given the relatively low-to-moderate light intensity used in Experiment 2, Experiment 3 (n = 16) directly compared the startle response and PPI under dark and light conditions. Illumination significantly increased the acoustic startle response and reduced PPI relative to darkness. Together, these findings identify illumination as a powerful modulator of sensorimotor gating in nocturnal rodents, capable of masking stress-related effects. Our results highlight environmental lighting as a critical, yet often overlooked, methodological confound and underscore the necessity of controlling lighting conditions to accurately interpret stress-induced alterations in sensorimotor gating studies.