Social Stress Engages Neurochemically-Distinct Afferents to the Rat Locus Coeruleus Depending on Coping Strategy

Abstract

Stress increases vulnerability to psychiatric disorders, partly by affecting brain monoamine systems, such as the locus coeruleus (LC)-norepinephrine system. During stress, LC activity is coregulated by corticotropin-releasing factor (CRF) and endogenous opioids. This study identified neural circuitry that regulates LC activity of intruder rats during the resident-intruder model of social stress. LC afferents were retrogradely labeled with Fluorogold (FG) and rats were subjected to one or five daily exposures to an aggressive resident. Sections through the nucleus paragigantocellularis (PGi) and central amygdalar nucleus (CNA), major sources of enkephalin (ENK) and CRF LC afferents, respectively, were immunocytochemically processed to detect c-fos, FG, and CRF or ENK. In response to a single exposure, intruder rats assumed defeat with a relatively short latency (SL). LC neurons, PGI-ENK LC afferents, and CNA-CRF LC afferents were activated in these rats as indicated by increased c-fos expression. With repeated stress, rats exhibited either a SL or long latency (LL) to defeat and these strategies were associated with distinct patterns of neuronal activation. In SL rats, LC neurons were activated, as were CNA-CRF LC afferents but not PGI-ENK LC afferents. LL rats had an opposite pattern, maintaining activation of PGi-ENK LC afferents but not CNA-CRF LC afferents or LC neurons. Together, these results indicate that the establishment of different coping strategies to social stress is associated with changes in the circuitry that regulates activity of the brain norepinephrine system. This may underlie differential vulnerability to the consequences of social stress that characterize these different coping strategies.

Keywords: Fluorogold; c-fos; corticotropin-releasing factor; enkephalin; locus coeruleus; resident–intruder.

Graphical abstract

Figure 1.

FG injection and retrograde labeling. Ai–Ci, Schematic diagrams adapted from the Rat Brain Atlas (Paxinos and Watson, 1998) showing the anteroposterior levels of the representative injection site (A) and retrograde labeling (B,C). Aii, Bright-field photomicrograph showing a representative FG injection within the rat LC. Bii–Cii, Bright-field photomicrographs of representative retrograde labeling in the nucleus PGi (Bii at ∼Plate 67; Paxinos and Watson, 1998) and CNA (Cii at ∼Plate 26; Paxinos and Watson, 1998) following FG injection into the LC. The arrows indicate immunoperoxidase labeled cells. Scale bars: A, 50 μm; B, C, 25 μm.

Figure 2.

Activation of locus coeruleus-projecting enkephalin neurons in the nucleus PGi. A, Scatterplot showing the percentage of FG-c-fos-labeled neurons that were also immunolabeled for ENK for individual control, SL, and LL rats. Lines through the points indicate the group mean. B, Representative immunofluorescence photomicrograph from a long latency rat showing c-fos profiles (blue), FG labeling (green), ENK-immunoreactivity (red), and triple-labeled neurons (yellow). Arrows point to single-labeled neurons and arrowheads point to triple-labeled neurons. Scale bar, 10 μm. C, High-magnification photomicrographs illustrating activated LC-projecting ENK neurons in the PGi of a LL rat. FG, c-fos, and ENK panels show single labeling for the retrograde tracer, FG (green), c-fos immunoreactivity (blue), and ENK immunoreactivity (red), respectively. The merged image shows all labels. Thin arrows point to the same triple-labeled neuron in all images. Scale bar, 10 μm.

Figure 3.

Activation of LC-projecting corticotropin-releasing factor neurons in the CNA. A, Scatterplot showing the percentage of FG-c-fos-labeled neurons that were also immunolabeled for CRF for individual control, SL, and LL rats. Lines through the points indicate the group mean. B, Representative immunofluorescence photomicrograph from a SL rat showing c-fos profiles (blue), FG labeling (green), CRF-immunoreactivity (red), and triple-labeled neurons (yellow). Arrows point to single-labeled neurons and arrowheads point to triple-labeled neurons. Scale bar, 10 μm. C, High-magnification photomicrographs illustrating activated LC-projecting CRF neurons in the CNA of a SL rat. FG, c-fos, and CRF panels show single labeling for the retrograde tracer, FG (green), c-fos immunoreactivity (blue), and CRF immunoreactivity (red), respectively. The merged image shows all labels. Thin arrows point to the same triple-labeled neuron in all images. Scale bar, 10 μm.

Figure 4:

Activation of LC neurons by a single or repeated exposure to resident–intruder stress. Ai, Aii, Representative sections from rats exposed to a single control manipulation (Ai) and a single resident–intruder stress (Aii). Aiii, Scatterplot showing the number of c-fos profiles in the LC after a single stress or control manipulation for individual control and stressed rats. Lines through the points indicate the group mean. Bi, Bii, Representative sections from a control rat exposed to five repeated manipulations (Bi) and an SL rat exposed to five repeated resident–intruder exposures (Bii). Biii, Scatterplot showing the number of c-fos profiles in the LC after the fifth stress or control manipulation for individual control, SL, and LL rats. Lines through the points indicate the group mean.

Figure 5.

Schematic depicting distinct engagement of CRF and ENK afferents to the LC and adaptations in receptors in LC neurons depending on social stress history and coping strategy. Acute social stress engages CRF inputs to the LC from the CNA and ENK inputs from the PGi to the LC. The emergence of different coping strategies with repeated social stress is associated with distinct biases toward regulation by one afferent. In rats that resist defeat (LL rats) PGi-ENK afferents to the LC remain engaged resulting in MOR internalization and upregulation in this phenotype. The CRF influence is decreased in LL rats. In contrast, in rats exhibiting a subordinate coping style (SL), amygdalar-CRF afferents are engaged and CRF1 becomes internalized in LC dendrites. The PGi-ENK influence is diminished in these rats. The selective engagement of afferents with opposing effects that are related to coping styles may be a basis for individual differences in the pathological consequences of social stress.