Chronic social stress alters levels of corticotropin-releasing factor and arginine vasopressin mRNA in rat brain

Abstract

In the visible burrow system model of chronic social stress, male rats housed in mixed-sex groups quickly form a dominance hierarchy in which the subordinates appear to be severely stressed. A subgroup of subordinates have an impaired corticosterone response after presentation of a novel restraint stressor, leading to their designation as nonresponsive subordinates. To examine the mechanism underlying the blunted corticosterone response in these animals, in situ hybridization histochemistry was used to quantify corticotropin-releasing factor (CRF) and arginine vasopressin (AVP) mRNA expression in the brain. In two separate visible burrow system experiments, the nonresponsive subordinates expressed a significantly lower average number of CRF mRNA grains per cell in the paraventricular hypothalamic nucleus compared with stress-responsive subordinates, dominants (DOM), or cage-housed control (CON) rats. The number of CRF mRNA labeled cells was also significantly lower in nonresponders than in responsive subordinates or DOM. In the central amygdala, CRF mRNA levels were increased in both groups of subordinates compared with CON rats, whereas responsive subordinates exhibited higher levels than the DOM rats as well. AVP mRNA levels did not vary with behavioral rank in any subdivision of the paraventricular hypothalamic nucleus. In the medial amygdala, the number of cells expressing AVP mRNA was significantly greater in CON rats compared with both groups of subordinates, although the average number of AVP mRNA grains per cell did not vary with rank. In addition, the number of AVP-positive cells significantly correlated with plasma testosterone level.

Fig. 1.

Plasma CORT responses to a novel restraint stressor in animals from both VBS experiments. In experiment 1, prestress CORT was measured in tail blood samples taken immediately after removal from the VBS; in experiment 2, prestress samples were taken after animals had been removed from the VBS and placed in their home cages for 1 hr. In both studies, stress CORT levels were measured after 1 hr in a plexiglass restrainer, and recovery CORT was measured 1 hr after termination of the stressor. Recovery CORT level was significantly lower for the DOM group in experiment 2 than the CON rat recovery value. CON animals were combined from both studies.DOM, Dominant rats; NRS, nonresponder subordinate rats; SRS, stress-responsive subordinate rats. Values represent the mean ± SEM; *p < 0.0001 versus NRS rats; ⁺p < 0.05 versus CON rats; ^Δp < 0.05 versus the DOM group in experiment 2.

Fig. 2.

Dark-field photographs of CRF mRNA expression in the PVN of the hypothalamus. In both experiment 1 (top row) and experiment 2 (bottom row), NRS rats exhibit significantly lower levels of CRF mRNA compared with DOM and SRS rats. In experiment 2, NRS rats also have significantly less CRF mRNA compared with CON rats. Scale bar, 100 μm.

Fig. 3.

CRF mRNA expression in the PVN of the hypothalamus of VBS animals. A, CRF mRNA in experiment 1 expressed as grains per cell. B, CRF mRNA in experiment 2 expressed as grains per cell. C, Number of CRF mRNA-labeled cells in experiment 1. Values represent the mean ± SEM; *p < 0.05 versus NRS;⁺p < 0.05 versus CON.

Fig. 4.

Dark-field photographs of CRF mRNA expression in the ACE. NRS and SRS rats exhibit significantly higher levels of CRF mRNA than do CON rats. Additionally, SRS rats show greater CRF mRNA expression than DOM rats. Scale bar, 50 μm.

Fig. 5.

Population results of the relative optical density of CRF mRNA levels in the ACE across the behavioral groups. Values represent the mean ± SEM; *p < 0.05 versus CON; ⁺p < 0.05 versus DOM.

Fig. 6.

Dark-field photographs of AVP mRNA expression in the AME. NRS and SRS rats exhibit significantly fewer AVP mRNA-labeled cells than do CON rats in the AME. Scale bar, 50 μm.

Fig. 7.

Population results showing AVP mRNA expression in the AME. A, Average number of AVP mRNA grains expressed per cell. B, Average number of AVP mRNA-labeled cells in the AME as a function of behavioral rank; *p < 0.05 versus CON.

Fig. 8.

Number of AVP mRNA-labeled cells in the AME correlates with plasma testosterone level. Data from individual rats are charted showing the positive relationship between testosterone level and number of AVP mRNA cells in the AME (Y = 0.54× + 11.05, r² = 0.153,F_(1,24) = 4.32, p < 0.05).