The Basolateral Amygdala Mediates the Role of Rapid Eye Movement Sleep in Integrating Fear Memory Responses

Abstract

The basolateral amygdala (BLA) mediates the effects of stress and fear on rapid eye movement sleep (REM) and on REM-related theta (θ) oscillatory activity in the electroencephalograph (EEG), which is implicated in fear memory consolidation. We used optogenetics to assess the potential role of BLA glutamate neurons (BLAGlu) in regulating behavioral, stress and sleep indices of fear memory, and their relationship to altered θ. An excitatory optogenetic construct targeting glutamatergic cells (AAV-CaMKIIα-hChR2-eYFP) was injected into the BLA of mice. Telemetry was used for real-time monitoring of EEG, activity, and body temperature to determine sleep states and stress-induced hyperthermia (SIH). For 3 h following shock training (ST: 20 footshocks, 0.5 mA, 0.5 s, 1 min interval), BLA was optogenetically stimulated only during REM (REM + L) or NREM (NREM + L). Mice were then re-exposed to the fear context at 24 h, 48 h, and 1 week after ST and assessed for behavior, SIH, sleep and θ activity. Control mice were infected with a construct without ChR2 (eYFP) and studied under the same conditions. REM + L significantly reduced freezing and facilitated immediate recovery of REM tested at 24 h and 48 h post-ST during contextual re-exposures, whereas NREM + L had no significant effect. REM + L significantly reduced post-ST REM-θ, but attenuated REM-θ reductions at 24 h compared to those found in NREM + L and control mice. Fear-conditioned SIH persisted regardless of treatment. The results demonstrate that BLAGlu activity during post-ST REM mediates the integration of behavioral and sleep indices of fear memory by processes that are associated with θ oscillations within the amygdalo-hippocampal pathway. They also demonstrate that fear memories can remain stressful (as indicated by SIH) even when fear conditioned behavior (freezing) and changes in sleep are attenuated.

Modeling integrated stress, sleep, fear and neuroimmune responses: Relevance for understanding trauma and stress-related disorders

“…Of these regions, the amygdala becomes activated and shows local synthesis of pro-inflammatory cytokines shortly after systemic immune challenge with administration of bacterial lipopolysaccharide ( Engler et al, 2011 ; Prager et al, 2013 ). Furthermore, our work has amply demonstrated that it regulates the effects of footshock stress and fear memories on sleep ( Liu et al, 2009 , 2011 ; Wellman et al, 2013 , 2014 , 2016 ), and, more recently, that it mediates the effects of controllable and uncontrollable stress on the neuroimmune system [Unpublished Results] and that its regulation of fear memory may be dependent on REM during the consolidation period ( Machida et al, 2021a , Machida et al, 2021b ). In summary, available evidence indicates that the complex interactions of the immune system, sleep and stress can be explored at functional, mechanistic and neurocircuit levels in ways that can lend new insight into adaptive and maladaptive stress responses.…”

The Amygdala as a Mediator of Sleep and Emotion in Normal and Disordered States

“…Brief optic activation of the basolateral amygdala during REM sleep immediately reduced REM-θ without affecting overall amount of, and propensity for, sleep, whereas optic inhibition increased REM-θ [43,45]. The reduction in REM-θ amplitude was associated with subsequent attenuated freezing and altered fear-conditioned REM-sleep responses [43]. Stimulation during NREM sleep did not affect any output measures, suggesting that the effects were REM-sleep specific.…”

“…Recent work has shown that the amygdala also regulates REM sleep-specific acitivity that appears important for emotional learning. For example, REM-dependent physiological events, including θ coherence [42], REM-θ amplitude [21,43], and phasic pontinewaves (P-waves, pontine component of PGO waves) [44] may be more accurate predictors of successful consolidation of fear memory than is the amount of REM sleep. Both REM-θ [43,45] and P-waves [37,46,47] are regulated by the amygdala, which, along with the medial prefrontal cortex and hippocampus, exhibits coordinated θ activity associated with contextual fear conditioning [48].…”

“…For example, REM-dependent physiological events, including θ coherence [42], REM-θ amplitude [21,43], and phasic pontinewaves (P-waves, pontine component of PGO waves) [44] may be more accurate predictors of successful consolidation of fear memory than is the amount of REM sleep. Both REM-θ [43,45] and P-waves [37,46,47] are regulated by the amygdala, which, along with the medial prefrontal cortex and hippocampus, exhibits coordinated θ activity associated with contextual fear conditioning [48]. Brief optic activation of the basolateral amygdala during REM sleep immediately reduced REM-θ without affecting overall amount of, and propensity for, sleep, whereas optic inhibition increased REM-θ [43,45].…”

“…Both REM-θ [43,45] and P-waves [37,46,47] are regulated by the amygdala, which, along with the medial prefrontal cortex and hippocampus, exhibits coordinated θ activity associated with contextual fear conditioning [48]. Brief optic activation of the basolateral amygdala during REM sleep immediately reduced REM-θ without affecting overall amount of, and propensity for, sleep, whereas optic inhibition increased REM-θ [43,45]. The reduction in REM-θ amplitude was associated with subsequent attenuated freezing and altered fear-conditioned REM-sleep responses [43].…”