Second-order olfactory-mediated fear-potentiated startle

Abstract

Recently, we reported that discrete (4-sec) olfactory cues paired with footshock serve as effective conditioned stimuli (CSs) for potentiating the acoustic startle response in rats using the fear-potentiated startle paradigm. Because odors are such salient cues for the rat, and because of the robust olfactory conditioning observed previously, the current studies investigated second-order fear conditioning using olfactory and visual cues. In Experiments 1 and 2, we used a small number of first-order and second-order training trials on separate days to investigate second-order fear-potentiated startle. Significant potentiated startle was observed in animals receiving Paired/Paired training in both studies, but surprisingly, control animals in the Unpaired/Paired group (Exp. 1) also showed significant potentiated startle to a light S2 at testing. These findings are addressed in the Discussion. Overall, the results of both experiments suggest that olfactory cues serve as efficient S1 and S2 stimuli in second-order fear-potentiated startle paradigms when only a small number of first and second-order training trials are presented.

Figure 1

Mean (± SEM) percent potentiation [(Light-noise trials minus Noise-Alone trials) ÷ Noise-Alone × 100] following second-order conditioning using an odor as S1 and a light as S2. Asterisks denote significant potentiation to S2 in the Paired/Paired (PRD/PRD) and Unpaired/Paired (UNP/PRD) groups (P < .05).

Figure 2

Mean (± SEM) percent potentiation [(Odor-noise trials minus Noise-Alone trials) ÷ Noise-Alone × 100] following second-order conditioning using a light as S1 and an odor as S2. Asterisks denote significant potentiation to S2 only in the Paired/Paired (PRD/PRD) group (P < .05).