Strain differences in fear between spontaneously hypertensive and normotensive rats

J. E. Ledoux, A. Sakaguchi, D. J. Reis

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The purpose of this study was to determine whether the reduced conditioned fear response in spontaneously hypertensive rats (SHRs) relative to Wistar-Kyoto controls (WKYs): (a) reflects a decrease in fear in SHRs or an increase in fear in WKYs, relative to other strains; (b) is secondary to strain differences in cardiovascular regulation; (c) represents a weaker conditioned response or a weaker memory trace; and (d) generalizes beyond tests of conditioned fear. SHRs, WKYs and Sprague-Dawley (SD) rats were subjected to classical fear conditioning (30 trials) involving the pairing of a tone conditioned emotional stimulus (CES: 800 Hz, 82 dB, 10 s) with an electric footshock unconditioned stimulus (US: 1.5 mA, 0.5 s). Cardiovascular responses elicited by the CES (without the US) were assessed through computer assisted techniques in chronically instrumented animals. Conditioned fear behavior was assessed as the proportion of time accounted for by crouching or freezing during a 300s presentation of the CES without the US. Unconditioned fear reactivity was examined using the open field test. Conditioned fear reactivity (% freezing during 300 s CES) was reduced (P < 0.01) in SHRs relative to both WKYs and SDs, which did not differ (SHR: 57% ± 5; WKY 85% ± 3; SD: 93% ± 4). Young (7 week) and mature (14 week) animals within each strain differed in resting mean arterial pressure (MAP, in mm Hg) (WKY: young, 102 ± 1; mature, 133 ± 2; P < 0.01; SHR: young, 124 ± 3 mature, 153 ± 5; P < 0.01) but did not differ in conditioned fear reactivity (WKY: young, 91% ± 3; mature, 93% ± 3; SHR: young, 47% ± 3; mature, 62% ± 8). The conditioned pressor response (peak value in mm Hg) was greater (P < 0.05) in mature (25 ±2) than in young (16 ± 2) SHRs, although conditioned freezing did not differ (see above). In mature SHRs, conditioned freezing did not differ 15 min (49% ± 6) and 24 h (55% ± 6) after conditioning. In the open field, SHRs spent less time (in s) in the center square initially (WKY 18 ± 4; SHR: 3 ± 2, P < 0.01), crossed more squares in 3 min (WKY: 12 ± 3; SHR: 37 ± 4, P < 0.01) and defecated less (WKY 2 ± 1; SHR 0; P < 0.05) than WKYs. These findings demonstrate that the strain difference in conditioned fear represents a reduction in fear reactivity in SHRs relative to normotensive rats, is independent of strain differences in cardiovascular regulation, involves the establishment of a weaker conditioned response rather than a memory deficit, and generalizes beyond tests of conditioned fear.

Original languageEnglish (US)
Pages (from-to)137-143
Number of pages7
JournalBrain Research
Issue number1
StatePublished - Oct 24 1983


  • classical conditioning
  • emotion
  • fear
  • spontaneously hypertensive rats
  • strain differences

ASJC Scopus subject areas

  • General Neuroscience
  • Molecular Biology
  • Clinical Neurology
  • Developmental Biology


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