The basolateral amygdala can mediate the effects of fear memory on sleep independently of fear behavior and the peripheral stress response

Abstract

Fear conditioning associated with inescapable shock training (ST) and fearful context re-exposure (CR) alone can produce significant behavioral fear, a stress response and alterations in subsequent REM sleep. These alterations may vary among animals and are mediated by the basolateral nucleus of the amygdala (BLA). Here, we used the GABAA agonist, muscimol (Mus), to inactivate BLA prior to CR and examined the effects on sleep, freezing and stress-induced hyperthermia (SIH). Wistar rats (n=28) were implanted with electrodes for recording sleep, data loggers for recording core body temperature, and with cannulae aimed bilaterally into BLA. After recovery, the animals were habituated to the injection procedure and baseline sleep was recorded. On experimental day 1, rats received ST (20 footshocks, 0.8mA, 0.5s duration, 60s interstimulus interval). On experimental day 7, the rats received microinjections (0.5ul) into BLA of either Mus (1.0uM; n = 13) or vehicle (Veh; n = 15) prior to CR (CR1). On experimental day 21, the animals experienced a second CR (CR2) without Mus. For analysis, the rats were separated into 4 groups: (Veh-vulnerable (Veh-Vul; n=8), Veh-resilient (Veh-Res; n=7), Mus-vulnerable (Mus-Vul; n=7), and Mus-resilient (Mus-Res; n=6)) based on whether or not REM was decreased, compared to baseline, during the first 4 h following ST. Pre-CR1 inactivation of BLA did not alter freezing or SIH, but did block the reduction in REM in the Mus-Vul group compared to the Veh-Vul group. These data indicate that BLA is an important region for mediating the effects of fearful memories on sleep.

Antagonism of corticotropin releasing factor in the basolateral amygdala of resilient and vulnerable rats: Effects on fear-conditioned sleep, temperature and freezing

“…Using a relatively intense fear conditioning paradigm, we have found that virtually identical footshock stressors and virtually identical fear-conditioned behavioral (freezing) and stress (SIH) responses can be followed by either increases or decreases in REM (e.g., (Wellman et al, 2017; Wellman et al, 2016)). Thus, while studies have reported that sleep has a role in the consolidation of contextual fear memory associated with brief or mild fearful experiences (Reviewed in (Havekes et al, 2015)), our work demonstrates that REM is not necessary for the formation of contextual fear memory associated with intensely stressful experiences, such as those modeled by our experiments.…”

“…Post-training inactivation of BLA did block the subsequent fear-conditioned reduction in REM in Vul rats whereas REM in the Res rats was not altered. Pre-context inactivation of BLA also attenuated the reduction in REM in the Vul rats, but did not significantly alter REM in the Res rats (Wellman et al, 2017). The changes in REM were independent of freezing and SIH in both Vul and Res animals regardless of whether BLA was inactivated after shock training or prior to context re-exposure.…”

“…Evoking contextual fear memories produce virtually identical directional changes in REM to those observed in response to the initial stressor. These differences in REM in putatively vulnerable (Vul, reduced REM) and resilient (Res, normal/increased REM) rats occur even though indices of fear (behavioral freezing) and stress (stress-induced hyperthermia (SIH)) are virtually identical across groups (Sanford et al, 2010; Wellman et al, 2017; Wellman et al, 2016; Yang et al, 2011). The importance of differences in REM after stress is not fully understood; however, reduced and fragmented REM have been linked to the onset of posttraumatic stress disorder (PTSD) (Mellman et al, 2002; Mellman et al, 2007a), which is viewed as arising from abnormal functioning in the brain’s fear system (Shvil et al, 2013).…”

“…For example, rapid eye movement sleep (REM) can be reduced after training with inescapable shock (IS) and increased after training with escapable shock (ES) (Sanford et al, 2010; Yang et al, 2011). Some outbred Wistar rats show reductions in REM during the first 4 h after training with IS whereas others show normal amounts or even increased REM shortly after training with IS (Wellman et al, 2017; Wellman et al, 2016). Evoking contextual fear memories produce virtually identical directional changes in REM to those observed in response to the initial stressor.…”

“…It also is not known whether CRFR1 in BLA plays a role in mediating individual differences in the effects of fear memory on REM. To assess these possibilities, we separated Wistar rats into Vul and Res groups based on REM amounts observed in the first 4 h after training with IS and examined stress-induced and fear-conditioned changes in sleep as previously described (Wellman et al, 2017; Wellman et al, 2016). In a subset of animals, we microinjected ANT into BLA immediately after ST to determine whether CRFR1 in BLA has a role in mediating individual differences in ST-induced and fear-conditioned changes in sleep.…”