Early-life Stress Alters Sleep Structure and the Excitatory-Inhibitory Balance in the Nucleus Accumbens in Aged Mice
Exposure to adverse experiences during early life has profound implications for neurodevelopment, often leading to long-term behavioral and physiological alterations. Among these consequences, sleep disturbances in later life have garnered increasing attention due to their association with psychiatric disorders and cognitive decline. While clinical studies suggest a link between childhood adversity and poor sleep quality in adults, the mechanisms underlying this relationship—particularly in aging populations—remain poorly understood. This study investigates how early-life stress disrupts sleep architecture and alters neural circuitry in aged mice, focusing on the nucleus accumbens (NAc), a brain region implicated in sleep-wake regulation.
Experimental Design and Early-life Stress Model
The research utilized a cohort of twenty aged male C57BL/6 mice (16–17 months old), divided into control and early-life stress groups. The stress paradigm involved limiting nesting and bedding materials for dams from postnatal days 2 to 9 (P2–P9), a period critical for maternal care. This model mimics fragmented maternal care observed in humans under conditions of neglect or caregiver stress. Control dams received sufficient nesting material, while stressed dams were provided minimal resources, creating an unpredictable and impoverished environment for pups. Male offspring were group-housed after weaning at P28 and maintained under standard conditions until sleep and neurochemical analyses at 16–17 months of age.
Sleep-wake behaviors were recorded over 24 hours using electroencephalogram (EEG) and electromyogram (EMG) telemetry. Signals were sampled at 500 Hz, filtered (EEG: 0.5–30 Hz; EMG: 20–200 Hz), and analyzed with SleepSign software. Vigilance states—wakefulness, non-rapid eye movement (NREM) sleep, and rapid eye movement (REM) sleep—were classified in 4-second epochs based on spectral characteristics and muscle activity. Post-recording, immunohistochemistry was performed on NAc tissue to quantify presynaptic markers of excitatory (vesicular glutamate transporter-1, VGluT1) and inhibitory (vesicular GABA transporter, VGAT) neurotransmission.
Sleep Disruptions in Stressed Aged Mice
Early-life stress significantly altered sleep architecture in aged mice. Over 24 hours, stressed mice exhibited reduced wakefulness (631.33 ± 34.73 minutes vs. 697.97 ± 77.43 minutes in controls; t₁₇ = 2.376, P = 0.030) and increased NREM sleep (723.54 ± 39.21 minutes vs. 667.37 ± 62.07 minutes; t₁₇ = 2.326, P = 0.033). REM sleep duration per episode was prolonged in stressed mice (89.39 ± 12.69 minutes vs. 73.00 ± 8.98 minutes; t₁₇ = 3.277, P = 0.004), though total REM time remained unchanged.
Phase-specific analysis revealed that reduced wakefulness and increased NREM sleep occurred predominantly during the dark (active) phase. Stressed mice showed significant decreases in wakefulness during the first 3 hours of the dark phase (Figure 2C), accompanied by a corresponding rise in NREM sleep (Figure 2F). These findings suggest that early-life stress exacerbates age-related declines in arousal, particularly during periods of expected activity.
Notably, the number of sleep-wake transitions and episodes remained unaffected, indicating that stress altered sleep maintenance rather than fragmentation. The prolonged REM sleep episodes in stressed mice highlight a specific vulnerability in REM regulation, potentially linked to emotional or memory-processing circuits.
Neurochemical Imbalance in the Nucleus Accumbens
Immunohistochemical analyses revealed a disrupted excitatory-inhibitory (E/I) balance in the NAc of stressed aged mice. VGluT1 expression—a marker of glutamatergic presynaptic terminals—was significantly reduced in both the NAc core and shell (Figure 5D). Conversely, VGAT expression, reflecting GABAergic neurotransmission, was elevated across NAc subregions (Figure 5E). The resultant VGluT1/VGAT ratio, a proxy for E/I balance, showed a pronounced decrease in stressed mice (F₁,₁₆ = 81.04, P < 0.001), indicating a shift toward inhibitory dominance (Figure 5F).
The NAc integrates cortical and limbic inputs to regulate motivation and arousal. Reduced glutamatergic signaling may reflect diminished excitatory drive from prefrontal and hippocampal projections, regions known to undergo stress-induced dendritic atrophy. Increased GABAergic tone could further suppress NAc output, potentially contributing to hypoarousal and excessive sleep. These neurochemical changes align with the observed behavioral deficits, suggesting that E/I imbalance in the NAc mediates stress-induced sleep alterations.
Mechanistic Insights and Implications
The study bridges gaps between early-life stress, aging, and sleep dysregulation. Previous work in younger animals linked maternal separation to REM sleep reductions, contrasting with the prolonged REM episodes observed here. This divergence may reflect age-dependent interactions between stress and sleep circuitry. Aged mice naturally exhibit reduced wakefulness and fragmented sleep; early-life stress appears to amplify these trends, particularly during active phases.
The NAc’s role in sleep-wake regulation is increasingly recognized. Optogenetic studies demonstrate that NAc dopamine D1 receptor neurons promote wakefulness, while adenosine A2A receptor neurons induce NREM sleep. The current findings suggest that early-life stress may impair D1 receptor-mediated arousal pathways, compounded by heightened inhibition. The persistent E/I imbalance could destabilize NAc output, disrupting circadian entrainment and behavioral state transitions.
Clinically, these results underscore the lifelong consequences of early adversity. Elderly individuals with histories of childhood trauma often report sleep complaints, which may precede or exacerbate neurodegenerative and psychiatric conditions. The identification of NAc dysfunction as a potential mediator offers a target for therapeutic interventions. Pharmacological or neuromodulatory strategies to normalize E/I balance could mitigate sleep disturbances in high-risk populations.
Limitations and Future Directions
While the study establishes correlations between early-life stress, sleep alterations, and NAc neurochemistry, causal relationships remain to be tested. Chemogenetic or optogenetic manipulation of NAc subregions in stressed aged mice could clarify their role in sleep pathology. Electrophysiological recordings would provide direct evidence of E/I imbalance, complementing immunohistochemical data.
Additionally, the focus on male mice limits generalizability to females, whose stress responses and sleep patterns often differ. Future studies should explore sex-specific effects and interactions with hormonal changes during aging. Longitudinal designs tracking sleep and neural changes from adolescence to late adulthood could elucidate developmental trajectories of stress vulnerability.
Conclusion
This study demonstrates that early-life stress induces lasting disruptions in sleep architecture and NAc neurochemistry in aged mice. Stress-exposed animals exhibited reduced wakefulness, increased NREM sleep, and prolonged REM episodes, paralleled by a shift toward inhibitory dominance in the NAc. These findings highlight the NAc as a critical node in stress-sleep interactions and provide a mechanistic framework for understanding sleep disturbances in aging populations with early-life adversity.
doi.org/10.1097/CM9.0000000000000279
Was this helpful?
0 / 0