Effects of systemic and intra-amygdaloid diazepam on long-term habituation of acoustic startle in rats

Two experiments were conducted to examine the effects of the anxiolytic drug, diazepam, on long-term habituation of the acoustic startle response. The experiments were based upon the hypothesis that manipulations that reduce fear should enhance long-term response decrements by reducing a fear-like sensitization process. In Experiment 1 rats given intraperitoneal injections of 0.5, 1.2, or 2.5 mg/kg showed larger decrements of startle amplitude than vehicle-injected controls both over trials within sessions and over days. In Experiment 2 rats injected with 35 micrograms of diazepam bilaterally into the amygdala showed larger decrements of startle amplitudes over days than vehicle-injected controls. No within-session startle effects were detected in Experiment 2. Freezing behavior was measured in Experiment 2 as an index of fear, and the amygdala injections of diazepam retarded the development of fear in the startle chamber. This index of fear was not possible in Experiment 1 because of the sedating effects of systemic diazepam. We conclude that diazepam, acting at least in part through the amygdala, attenuates the fear-like sensitization process associated with the acoustic startle stimulus. By attenuating sensitization diazepam produces larger than normal reductions in startle amplitudes over trials and days without significantly affecting initial responsiveness.