Stress-induced activity in the locus coeruleus is not sensitive to stressor controllability

Abstract

An important factor in determining the adverse consequences of a stress experience is the degree to which an individual can exert control over the stressor. Stressor controllability is known to influence brain norepinephrine levels, but its impact on activity in noradrenergic cell bodies is unknown. In the present study we investigated whether noradrenergic neurons within the locus coeruleus (LC), the major source of forebrain norepinephrine, are sensitive to stressor controllability. We exposed adult male Sprague-Dawley rats to escapable or yoked inescapable tailshock and assessed LC activity by measuring changes in the immediate early gene c-fos and the enzyme tyrosine hydroxylase (TH). We used in situ hybridization to measure levels of c-fos mRNA, TH mRNA, and TH primary transcript in the LC. In all three cases stress exposure increased expression relative to an unstressed homecage control group, but expression did not differ between controllable and uncontrollable stress. To further examine whether stressor controllability influences the number of stress-responsive LC neurons we performed double-label immunohistochemistry for TH and Fos. Again we detected an overall effect of stress, which did not differ between controllable and uncontrollable stress. We conclude that exposure to stress robustly increases expression of TH and c-fos in the LC, but this effect is not influenced by stressor controllability. To the extent that the expression of these genes reflects degree of neuronal activation, our results suggest that stress-induced activity of noradrenergic cell bodies in the LC is not sensitive to stressor controllability.

Figure 1

Autoradiographic images of locus coeruleus tissue. A, Location of LC tissue analyzed in this study, with enlargened detail of peri-LC region. Image represents −10.04 mm bregma from the atlas of Paxinos & Watson [47]. B–E, sample autoradiographic images of TH (B, C) and c-fos (D, E). Tissue is from unstressed rats (B, D) and inescapably shocked rats (C, E). Stress time points depicted in TH and c-fos images are 24 h and 1 h, respectively. Scale bar = 500 μm.

Figure 2

TH mRNA in the locus coeruleus 24 h after stress. Graph shows optical density of in situ hybridization autoradiograms. Data are expressed as mean + SEM of unstressed homecage control (HC), escapable shock (ES), and inescapable shock (IS). n = 5 for HC, n = 6 for both ES and IS. * p < 0.05 vs. homecage control.

Figure 3

TH primary transcript in the locus coeruleus 1 h after stress. Sample in situ hybridization autoradiograms in tissue from unstressed (A), escapably shocked (B), and inescapably shocked (C) rats. Scale bar = 500 μm. D, Optical density of autoradiograms expressed as mean + SEM of unstressed homecage control (HC), escapable shock (ES), and inescapable shock (IS). n = 7 per group. ** p < 0.001 vs. homecage control.

Figure 4

c-fos mRNA in the locus coeruleus 1 h after stress. Graph shows optical density of in situ hybridization autoradiograms. Data are expressed as mean + SEM of unstressed homecage control (HC), escapable shock (ES), and inescapable shock (IS). n = 7 per group. ** p < 0.001 vs. homecage control.

Figure 5

Fos (A, C) and TH (B, D) immunoreactivity in the locus coeruleus 2h after exposure to inescapable shock. Top row: 20X magnification, scale bar = 250μm. Bottom row: 100X magnification, scale bar = 50 μm. White arrow denotes Fos/TH double-labeled cell, black arrow denotes Fos single-labeled cell.

Figure 6

Number of Fos / TH double-labeled cells per LC region following escapable shock (ES) and inescapable shock (IS). Data are expressed as mean + SEM. n = 7 / group.