Changes in the strength of prepulse inhibition with variation in the startle baseline associated with individual differences and with old age in rats and mice

Abstract

We examined the relationship between variation in the peak level and temporal course of prepulse inhibition (PPI) of the acoustic startle reflex (ASR) and between-subject variation in ASR control baselines. Prepulses were light flashes (for young rats, N = 155), silent gaps in noise (young mice, N = 46), or noise pulses in quiet (young, middle aged, and aged mice, N = 20). Differences in the baseline ASR accounted for up to 99% of the variance in difference-score measures of PPI (ASRcontrol - ASRprepulse), but not 10% of the variance in relative measures [1 - (ASRp/ASRc)]. The temporal course of relative PPI did not differ with baseline ASR, nor did peak values of relative PPI in Experiments 2 and 3, but were greater (p < .05) in low than in high ASRc subgroups in Experiment 1. These findings (1) indicate that the neural substrates for ASR elicitation and for PPI are largely independent; (2) reveal that, unlike the ASR, PPI does not decline with age; and (3) suggest the outlines of a formal model for interpreting changes in relative versus difference-score measures of PPI when baseline ASR varies.