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.
Young
B J
BJ
Department of Psychology, Dartmouth College, Hanover, NH 03755.
Helmstetter
F J
FJ
Rabchenuk
S A
SA
Leaton
R N
RN
D000161:Acoustic Stimulation; D000679:Amygdala / Q000033:anatomy & histology; D000818:Animals; D003975:Diazepam / Q000008:administration & dosage / Q000494:pharmacology*; D004305:Dose-Response Relationship, Drug; D005239:Fear / Q000187:drug effects; D006185:Habituation, Psychophysiologic / Q000187:drug effects*; D007267:Injections; D007274:Injections, Intraperitoneal; D008297:Male; D051381:Rats; D013216:Reflex, Startle / Q000187:drug effects*
D003975:Diazepam