Role of nucleus of the solitary tract noradrenergic neurons in post-stress cardiovascular and hormonal control in male rats

<div><p></p><p>Chronic stress causes hypothalamo–pituitary–adrenal (HPA) axis hyperactivity and cardiovascular dyshomeostasis. Noradrenergic (NA) neurons in the nucleus of the solitary tract (NTS) are considered to play a role in these changes. In this study, we tested the hypothesis that NTS NA A2 neurons are required for cardiovascular and HPA axis responses to both acute and chronic stress. Adult male rats received bilateral microinjection into the NTS of 6-hydroxydopamine (6-OHDA) to lesion A2 neurons [cardiovascular study, <i>n</i> = 5; HPA study, <i>n</i> = 5] or vehicle [cardiovascular study, <i>n</i> = 6; HPA study, <i>n</i> = 4]. Rats were exposed to acute restraint stress followed by 14 d of chronic variable stress (CVS). On the last day of testing, rats were placed in a novel elevated plus maze (EPM) to test post-CVS stress responses. Lesions of NTS A2 neurons reduced the tachycardic response to acute restraint, confirming that A2 neurons promote sympathetic activation following acute stress. In addition, CVS increased the ratio of low-frequency to high-frequency power for heart rate variability, indicative of sympathovagal imbalance, and this effect was significantly attenuated by 6-OHDA lesion. Lesions of NTS A2 neurons reduced acute restraint-induced corticosterone secretion, but did not affect the corticosterone response to the EPM, indicating that A2 neurons promote acute HPA axis responses, but are not involved in CVS-mediated HPA axis sensitization. Collectively, these data indicate that A2 neurons promote both cardiovascular and HPA axis responses to acute stress. Moreover, A2 catecholaminergic neurons may contribute to the potentially deleterious enhancement of sympathetic drive following chronic stress.</p></div

Epileptic mice show anhedonia and hyperactive behaviors.

<p>A) Sucrose preference was decreased in epileptic mice relative to controls. B) Epileptic mice showed increased active behaviors in the forced swim test. (*p < 0.05). Data presented as mean ± SEM, n = 9 mice per group.</p

Epileptic mice show anxiety-like and hyperactive behaviors.

<p>Epileptic mice show reduced percent time spent in the center of the A) open field and D) open arms of the elevated plus maze. The total distance travelled by epileptic mice in the B) open field and E) elevated plus maze is decreased relative to controls. Epileptic mice spent more time freezing (immobility >2 seconds) in the C) open field and in the F) elevated plus maze compared to their control counterparts. G) 24h activity monitoring demonstrates overall hyperactivity of epileptic mice relative to control mice. Shaded region indicates the time when all behavioral tests were performed. (*p < 0.05, **p < 0.01, ***p < 0.001). Data presented as mean ± SEM, n = 9 mice per group.</p

Reduced expression of Fos in stress regulatory regions in epileptic mice.

<p>Representative micrographs showing Fos (red) and NeuN (green) immunoreactivity in A) the prelimbic region of the medial prefrontal cortex (PFC), C) the basal amygdala and E) ventral subiculum. Top panels in each set show control mice while bottom panels are from epileptic mice. Epileptic mice show decreased Fos expression in the B) prelimbic and infralimbic region of the PFC, D) the basal subdivision of the amygdaloid nucleus and F) the ventral subiculum relative to control mice. *p < 0.05, **p<0.01. Data presented as mean ± SEM, n = 6–8 mice per group.</p

Experimental design.

<p>A) Experimental time line for cohorts 1 (top) and 2 (bottom). Baseline morning corticosterone (CORT) samples were taken at 1 and 7 weeks post-SE for Cohort 1 and at week 10 for Cohort 2. At week 8, mice from Cohort 1 were exposed to restraint and blood samples were taken at 0, 30, 60, and 120 minutes from the onset of the stressor. Cohort 2 mice began behavioral testing at week 6 (SPT, sucrose preference test; EPM, elevated plus maze; FST, forced-swim test). B) Brain regions analyzed for Fos expression are based on the Paxinos and Watson brain atlas. Abbreviations. IL, infralimbic prefrontal cortex; PL, prelimbic prefrontal cortex; hippocampal subdivisions include: CA1, CA3, DG, dentate gyrus; VS, ventral subiculum; amygdaloid complex: CeA, central amygdala; LA, lateral amygdala; BA, basal amygdala and PVN, paraventricular nucleus of the hypothalamus. Figure is modified from [<a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0197955#pone.0197955.ref026" target="_blank">26</a>].</p

Corticosterone hyper-secretion is present at baseline and stress-induced states in epileptic mice.

<p>A) Epileptic mice show increase levels of corticosterone (CORT) secretion following 30 minutes of restraint stress (gray region shows time in restrainer). B) Epileptic mice in cohort 1 show increased corticosterone baseline secretion at 1 and 7 weeks compared to control mice. C) Epileptic mice in cohort 2 show increased corticosterone baseline secretion at week 10 relative to controls. D) Adrenal weight of epileptic mice in cohort 2 is greater than control mice. (*p < 0.05, epileptic vs. control; ***p < 0.001). Data presented as mean ± SEM, Cohort 1 n = 6–8 mice per group, Cohort 2 n = 7–9 mice per group).</p