IκB kinase regulates social defeat stress-induced synaptic and behavioral plasticity

Abstract

The neurobiological underpinnings of mood and anxiety disorders have been linked to the nucleus accumbens (NAc), a region important in processing the rewarding and emotional salience of stimuli. Using chronic social defeat stress, an animal model of mood and anxiety disorders, we investigated whether alterations in synaptic plasticity are responsible for the long-lasting behavioral symptoms induced by this form of stress. We hypothesized that chronic social defeat stress alters synaptic strength or connectivity of medium spiny neurons (MSNs) in the NAc to induce social avoidance. To test this, we analyzed the synaptic profile of MSNs via confocal imaging of Lucifer-yellow-filled cells, ultrastructural analysis of the postsynaptic density, and electrophysiological recordings of miniature EPSCs (mEPSCs) in mice after social defeat. We found that NAc MSNs have more stubby spine structures with smaller postsynaptic densities and an increase in the frequency of mEPSCs after social defeat. In parallel to these structural changes, we observed significant increases in IκB kinase (IKK) in the NAc after social defeat, a molecular pathway that has been shown to regulate neuronal morphology. Indeed, we find using viral-mediated gene transfer of dominant-negative and constitutively active IKK mutants that activation of IKK signaling pathways during social defeat is both necessary and sufficient to induce synaptic alterations and behavioral effects of the stress. These studies establish a causal role for IKK in regulating stress-induced adaptive plasticity and may present a novel target for drug development in the treatment of mood and anxiety disorders in humans.

Figure 1.

Chronic social defeat stress regulates spine morphology on NAc MSNs in susceptible mice. A, Representative confocal z-stack images of a Lucifer-yellow-filled NAc MSN and selected dendritic segments. B, C, Stubby spines are circled in red. Chronic social defeat stress results in an increase total spine density (B) and stubby spine density (C). D, Stubby spine density correlates with social interaction ratio. E, F, No changes were observed in mushroom (E) or thin (F) spines. G, A trend toward a decrease in spine volume in susceptible mice was found. Data are represented as group means. Error bars represent SEM. *p < 0.05, ^t p = 0.07, one-way ANOVA, 56 neurons, n = 4–5 mice per group.

Figure 2.

Chronic social defeat stress regulates the size of the PSD in susceptible mice. A, Representative photomicrographs of excitatory synapses. B, Susceptible animals have synapses with significantly smaller average PSD length. C, As well, mean PSD length correlates with social interaction ratio. Data are represented as group means. Error bars represent SEM. *p < 0.05, one-way ANOVA, n = 4–5 mice per group.

Figure 3.

Chronic social defeat stress regulates synaptic transmission in susceptible mice. A, Representative traces of mEPSCs. B, Chronic social defeat stress increases mEPSC frequency only in susceptible animals. C, No changes in amplitude were observed after chronic social defeat. Calibration: 15 pA, 100 ms. D, mEPSC frequency also correlates with social interaction ratio. Data are represented as group means. Error bars represent SEM. *p < 0.05, one-way ANOVA, 39 neurons, n = 4–6 mice per group.

Figure 4.

Chronic social defeat stress regulates levels of IKK. A, Social defeat increases total levels of IKK protein. B, Levels of IKK correlate with SI ratio. C, D, Levels of IκB (C) and phosphorylated IκB (p-IκB; D), downstream targets of IKK, are significantly increased after chronic social defeat stress. All data are expressed as mean ± SEM optical density percentage control. *p < 0.05, ^t p < 0.07, one-way ANOVA, n = 8–10 mice per group.

Figure 5.

A, Representative confocal z-stack images of viral-infected dendritic segments from MSNs. Stubby spines are circled in red. B–E, Expression of IKKdn in MSNs is sufficient to reverse the induction of stubby spines after social defeat (B) but does not affect total (C), thin (D), or mushroom (E) density. F, Expression of IKKdn causes an increase spine head volume regardless of group. Data are represented as group means. Error bars represent SEM. *p < 0.05, two-way ANOVA, 72 neurons, n = 4–6 mice per group.

Figure 6.

A, Representative expression of HSV in NAc. GFP-labeled neurons in green show the area of infection, whereas representative slices from the mouse brain atlas of Paxinos and Franklin (1997) indicate the relative spread of HSV throughout the NAc. B, Expression of IKKdn in NAc after 10 d social defeat reveres social avoidance (n = 12 mice per group. C, Expression of IKKca before a 1 d subthreshold microdefeat induces a social avoidance phenotype (n = 13 mice per group). White and black bars represent interaction time when the target aggressor mouse is absent or present, respectively. Data are represented as group means. Error bars represent SEM. *p < 0.05, two-way ANOVA.

Figure 7.

A, Representative confocal image of an MSN. Average total dendritic length (n = 5 neurons) is 948.36 ± 115.19 μm. B, Schematic representation of the effect of chronic social defeat stress on MSN spines. Small black lines in spine head represent the PSD. Social defeat results in approximately double the total number of stubby spines.