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Updated: Aug 29, 2026
Habituation and Prepulse Inhibition of Acoustic Startle in Rodents
Published on: September 1, 2011
High illumination levels potentiate the acoustic startle response in preweanling rats
Marianne Weber1, Nicola Watts, Rick Richardson
1School of Psychology, University of New South Wales, Sydney, New South Wales, Australia.
Insights
Young rats show fear potentiation of the acoustic startle response (FPS) to unconditioned stimuli before they do to conditioned stimuli. This suggests learned and unlearned fear develop differently in the developing rat brain.
Area of Science:
- Neuroscience
- Developmental Psychology
- Behavioral Neuroscience
Background:
- Fear potentiation of the acoustic startle response (FPS) to conditioned stimuli emerges in rats around Postnatal Day 23.
- The ontogeny of FPS to unconditioned stimuli is less understood.
Purpose of the Study:
- To investigate the developmental trajectory of FPS to an unconditioned fear stimulus in young rats.
- To determine if noradrenergic pathways are involved in early-onset FPS.
Main Methods:
- Rats aged 14, 18, and 25 days were exposed to high illumination (unconditioned stimulus) and their acoustic startle response was measured.
- Propranolol, a noradrenergic beta-receptor antagonist, was administered to 18-day-old rats before light exposure.
- Startle amplitudes were compared across age groups and treatment conditions.
Main Results:
- High illumination enhanced startle amplitudes in 18- and 25-day-old rats, but not 14-day-old rats.
- The light-induced enhancement of startle in 18-day-old rats was blocked by propranolol.
- FPS to unconditioned stimuli appears earlier than FPS to conditioned stimuli.
Conclusions:
- Conditioned and unconditioned FPS have distinct developmental timelines in rats.
- Noradrenergic systems play a role in the early emergence of unconditioned FPS.
- These findings support the hypothesis that learned and unlearned fear are mediated by separate neural systems.
Abstract:
Fear potentiation of the acoustic startle response (FPS) by aversive conditioned stimuli does not emerge in rats until Postnatal Day (P)23 (see P. S. Hunt & B. A. Campbell, 1997). However, the present study found that when presented with an unconditioned fear-eliciting stimulus, rats younger than P23 display FPS. Specifically, high illumination levels were found to enhance startle amplitudes in rats aged 18 and 25 days, but not 14 days. Furthermore, the light-enhanced startle observed in P18 rats was prevented by a systemic injection of the noradrenergic beta-receptor antagonist propranolol. These data suggest that conditioned and unconditioned FPS have different ontogenetic trajectories, and thereby provide support for the idea that learned and unlearned fear are subserved by dissociable neural systems.
