Augmented fear bradycardia in rats with heart failure

Abstract

In congestive heart failure (CHF), while resting parasympathetic activity becomes reduced, parasympathetically-mediated responses to stressors have not been described. This study aimed to (1) elucidate the effect of CHF on fear bradycardia, a parasympathetically-mediated response, and (2) examine if brain oxidative stress of CHF mediates fear bradycardia. White noise sound (WNS) exposure to conscious rats induced freezing behavior and elicited bradycardia. WNS exposure-elicited bradycardia was greater in rats with CHF than in controls. Superoxide dismutase mimetics administered in the lateral/ventrolateral midbrain periaqueductal gray (l/vlPAG), a region that contributes to the generation of fear bradycardia, had no effect on the bradycardia response in control and CHF rats. Dihydroethidium staining in situ showed that superoxide generation in the l/vlPAG of CHF rats was increased as compared to controls. These results demonstrate that CHF leads to the augmentation of fear bradycardia. Moreover, oxidative stress in the l/vlPAG of CHF unlikely mediates the augmented fear bradycardia.

Keywords: Fear; Heart failure; Heart rate; Midbrain periaqueductal gray; Parasympathetic nervous system.

Fig. 1

Top. Examples of confocal images of DHE fluorescence taken in the PAG of sham-operated (Sham) and congestive heart failure (CHF) rats. Scale bars: 50 μm. Bottom. Comparisons of DHE fluorescence intensities in the dorsolateral, lateral, and ventrolateral parts of the PAG (dlPAG, lPAG, and vlPAG) between 6 Sham and 6 CHF rats. Values represent mean ± SEM. *P < 0.05 vs. Sham rats, detected by two-sample t tests

Fig. 2

a Microinjection sites mapped on standard sections using the atlas of Paxinos and Watson [16] in 12 Sham and 27 CHF rats. These sites were marked with injected Indian ink after data collection. The areas outlined with thick lines represent the lateral/ventrolateral periaqueductal gray (l/vlPAG). In the l/vlPAG at − 7.8 mm caudal to the bregma for CHF rats, successful microinjection sites of 2 CHF rats were hidden behind the sites presented. b Examples of electrocardiogram tracings obtained from one Sham rat and one CHF rat before and during white noise sound (WNS) exposure. c Thirty-second averaged time courses of mean arterial pressure (MAP) and heart rate (HR) while conscious, free-moving Sham (black circle) and CHF (white circle) rats were exposed to 5 min of WNS at 90 dB 30–40 min after microinjection of saline into the brain, irrespective of microinjection site. Values are mean ± SEM. *P < 0.05, vs. baseline, detected by the Dunnett’s post hoc test following one-way repeated-measures ANOVA. d Comparisons of 5 min averaged changes from baseline in MAP and HR during WNS exposure between Sham and CHF rats. ^†P < 0.05, Sham vs. CHF. NS no significant differences between Sham and CHF rats. The statistic differences were examined by two-sample t tests

Fig. 3

a, c Thirty-second averaged time courses of MAP and HR while conscious, free-moving Sham (a, n = 7) and CHF (c, n = 8) rats, of which microinjection sites were located within the l/vlPAG, were exposed to 5-min of WNS at 90 dB. At 30–40 min prior to the onset of WNS exposure, saline (100 nl) or Tempol (10 mM diluted in 100 nl saline) was bilaterally microinjected into the l/vlPAG. Values are mean ± SEM. *P < 0.05, vs. baseline, detected by the Dunnett’s post hoc test following one-way repeated-measures ANOVA. b, d Comparisons of 5 min averaged changes from baseline in MAP and HR between saline and Tempol microinjections into the l/vlPAG in the Sham (b) and CHF (d) rats. NS no significant differences between saline and Tempol microinjections, examined by paired t tests

Fig. 4

a Thirty-second averaged time courses of MAP and HR while conscious, free-moving CHF (n = 8) rats, of which microinjection sites were located within the l/vlPAG, were exposed to 5-min of WNS at 90 dB at 5–15 or 30–40 min after bilateral microinjection into the l/vlPAG of saline (100 nl) or Tiron (200 mM diluted in 100 nl saline). Values represent mean ± SEM *P < 0.05, vs. baseline, detected by the Dunnett’s post hoc test following one-way repeated-measures ANOVA. b Comparisons of 5 min averaged changes from baseline in MAP and HR during WNS exposure between saline and Tiron microinjections into the l/vlPAG and between 5–15 and 30–40 min after microinjection in the CHF rats. NS no significant differences between saline and Tiron microinjections and between 5–15 and 30–40 min after administration, examined by two-way repeated-measures ANOVA followed by the Tukey’s post hoc tests