A nonextinction procedure for long-term studies of classically conditioned enhancement of acoustic startle in the rat

Abstract

In the Fear-Potentiated Startle (FPS) paradigm, the reflexive response to a noise burst is enhanced when it is presented with a stimulus (typically a light) that previously had been paired with the presentation of an aversive stimulus, usually an electric shock. In the FPS paradigm, the conditioned effect is demonstrated under conditions of extinction, i.e., the light is not paired with the shock during FPS testing. Because of this, the FPS paradigm is of somewhat limited value as a longitudinal measure for studying classically conditioned enhancement of acoustic startle. The present studies report a simple and reliable nonshock procedure for studying classically conditioned potentiation of acoustic startle in the rat that does not utilize testing under conditions of extinction. Naive rats were exposed to 5-or 3-days/week startle test sessions for up to 20 weeks. Twenty (20) startle stimuli (115 dB noise bursts; 40 ms in duration) were presented during each session. Half of these startle stimuli were presented in darkness and half were immediately preceded by a 3500-ms presentation of a 15-W light. With this paradigm, the effects of pairing the light with the startle noise burst could be studied across many test sessions in the absence of extinction training. The light did not increase startle amplitude on the first few startle trials of the first test session. By the end of the first session, however, and continuing for many weeks of testing, startle responses in the presence of the light were significantly greater (by 30–40%) than in the absence of the light. The finding that the startle stimulus itself can serve as an unconditioned stimulus (UCS) to enhance subsequent startle responses replicates an earlier finding. The persistence of this Startle-Potentiated Startle (SPS) effect across multiple weeks of testing is in contrast to that earlier report.