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.
Keywords: amygdala; fear memory; optogenetics; sleep; stress.