The Neuroendocrine Impact of Acute Stress on Synaptic Plasticity.

Stress induces changes in nervous system function on different signaling levels, from molecular signaling to synaptic transmission to neural circuits to behavior-and on different time scales, from rapid onset and transient to delayed and long-lasting. The principal effectors of stress plasticity are glucocorticoids, steroid hormones that act with a broad range of signaling competency due to the expression of multiple nuclear and membrane receptor subtypes in virtually every tissue of the organism. Glucocorticoid and mineralocorticoid receptors are localized to each of the cellular compartments of the receptor-expressing cells-the membrane, cytosol, and nucleus. In this review, we cover the neuroendocrine effects of stress, focusing mainly on the rapid actions of acute stress-induced glucocorticoids that effect changes in synaptic transmission and neuronal excitability by modulating synaptic and intrinsic neuronal properties via activation of presumed membrane glucocorticoid and mineralocorticoid receptors. We describe the synaptic plasticity that occurs in 4 stress-associated brain structures, the hypothalamus, hippocampus, amygdala, and prefrontal cortex, in response to single or short-term stress exposure. The rapid transformative impact of glucocorticoids makes this stress signal a particularly potent effector of acute neuronal plasticity.

Stress-induced glucocorticoid desensitizes adrenoreceptors to gate the neuroendocrine response to somatic stress in male mice.

Noradrenergic afferents to hypothalamic corticotropin releasing hormone (CRH) neurons provide a major excitatory drive to the hypothalamic-pituitary-adrenal (HPA) axis via α1 adrenoreceptor activation. Noradrenergic afferents are recruited preferentially by somatic, rather than psychological, stress stimuli. Stress-induced glucocorticoids feed back onto the hypothalamus to negatively regulate the HPA axis, providing a critical autoregulatory constraint that prevents glucocorticoid overexposure and neuropathology. Whether negative feedback mechanisms target stress modality-specific HPA activation is not known. Here, we describe a desensitization of the α1 adrenoreceptor activation of the HPA axis following acute stress in male mice that is mediated by rapid glucocorticoid regulation of adrenoreceptor trafficking in CRH neurons. Glucocorticoid-induced α1 receptor trafficking desensitizes the HPA axis to a somatic but not a psychological stressor. Our findings demonstrate a rapid glucocorticoid suppression of adrenergic signaling in CRH neurons that is specific to somatic stress activation, and they reveal a rapid, stress modality-selective glucocorticoid negative feedback mechanism.

Basolateral Amygdala Regulates EEG Theta-activity During Rapid Eye Movement Sleep.

Pharmacological and optogenetic studies have demonstrated that the basolateral amygdala (BLA) plays a pivotal role in regulating fear-conditioned changes in sleep, in particular, rapid eye movement sleep (REM). However, the linkage between BLA and REM regulation has been minimally examined. In this study, we optogenetically activated or inhibited BLA selectively during spontaneous REM, and determined the effects on REM amounts and on hippocampus regulated EEG-theta (θ) activity. Excitatory (CaMKIIα-hChR2 (E123A)-eYFP-WPRE) or inhibitory (CaMKIIα-eNpHR3.0-eYFP-WPRE) optogenetic constructs were stereotaxically delivered targeting glutamatergic cells in BLA (BLAGlu) of mice. Viral constructs without opsin (CaMKIIα-eYFP-WPRE) were used as controls. All mice were implanted with telemetry transmitters for monitoring electroencephalography (EEG), activity, and body temperature, and with optic cannulas for light delivery to the BLA. BLAGlu were optogenetically activated by blue light (473 nm), or inhibited by green light (532 nm), in 10 s epochs during REM, or non-REM (NREM), in undisturbed mice. Sleep amounts and EEG activity were analyzed. Projections from BLAGlu to neurons in hippocampus were immunohistochemically (IHC) examined. Brief optogenetic activation of BLAGlu during REM immediately reduced EEG theta activity (5-8 Hz, REM-θ), without affecting overall amount and propensity of sleep, while optogenetic inhibition increased REM-θ. Stimulation during NREM had no effect on EEG spectra or sleep. IHC results showed that glutamatergic and GABAergic cells in CA3 of the hippocampus received inputs from BLAGlu projection neurons. Activation of BLAGlu reduced, and inhibition increased, REM-θ without otherwise altering sleep. Optogenetic stimulation of BLAGlu may be useful for systematically manipulating sleep-related amygdalo-hippocampal interactions.

MicroRNAs in Basolateral Amygdala Associated with Stress and Fear Memories Regulate Rapid Eye Movement Sleep in Rats.

Stress-related sleep disturbances are distressing clinical symptoms in posttraumatic stress disorder patients. Intensely stressful events and their memories change rapid eye movement (REM) sleep in animal models. REM sleep varies with individual differences of stress resilience or vulnerability. The basolateral amygdala (BLA) is a primary mediator of the effects of stress and fear memories on sleep. However, the molecular mechanisms in BLA regulating the effects of fear conditioning, shock training (ST) and context re-exposure (CTX) on REM sleep are not well known. MicroRNAs (miRNAs) are small, non-coding RNAs and posttranscriptional gene regulators of diverse biological processes. The aim of this study is to investigate ST- and CTX-altered miRNAs in the BLA of resilience and vulnerable animals and on REM sleep regulation. MiRNAs expression profiles in BLA were generated following ST and CTX using the Taqman Low Density rodent microRNA array. The altered BLA miRNAs expression and REM sleep reduction observed in ST and CTX vulnerable animals. AntagomiR-221 microinjection into BLA for one of the upregulated miRNAs, miR-221 in BLA, attenuated the REM sleep reduction. This study suggests that miRNAs in the BLA may play a significant role in mediating the effects of stress and fear memories on REM sleep.

Group II metabotropic glutamate receptor activation in the basolateral amygdala mediates individual differences in stress-induced changes in rapid eye movement sleep.

Group II metabotropic glutamate receptors (mGluR2/3s) have been implicated in stress and trauma related disorders including post-traumatic stress disorder (PTSD). PTSD is characterized by flashbacks, anxiety, and sleep disturbances. While many people are exposed to trauma in their lifetime, only a small percentage go on to develop PTSD, indicating individual differences in stress and emotional processing. Wistar strain rats display directionally different rapid-eye movement sleep (REM) responses to footshock stress, with resilient rats having no change or an increase in REM and vulnerable rats having a significant reduction in REM compared to baseline. The basolateral nucleus of the amygdala (BLA) is key in regulating individual differences in stress-induced alterations in sleep. Group II metabotropic glutamate receptors (mGluR2/3s) negatively modulate glutamate and are implicated in fear, fear memory, and sleep. The current study evaluated the effect of mGluR2/3 agonist LY379268 (LY37) in BLA on stress and fear memory induced changes in sleep, EEG spectra, behavioral fear expression and physiological stress. These data indicate that vulnerable rats treated with LY37 have an attenuation of the REM reductions generally seen in vulnerable rats. Furthermore, LY37 altered EEG spectra in the delta (0.5-4.5 Hz) and theta (5-9.5 Hz) frequency. LY37 did not impact behavioral fear expression or physiological stress. Therefore, mGluR2/3s within BLA are implicated in regulating individual differences in sleep responses to fear- and stress-related memories.

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

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.

Predicting stress resilience and vulnerability: brain-derived neurotrophic factor and rapid eye movement sleep as potential biomarkers of individual stress responses.

STUDY OBJECTIVES: To examine the rapid eye movement sleep (REM) response to mild stress as a predictor of the REM response to intense stress and brain-derived neurotrophic factor (BDNF) as a potential biomarker of stress resilience and vulnerability. METHODS: Outbred Wistar rats were surgically implanted with electrodes for recording electroencephalography (EEG) and electromyogram (EMG) and intraperitoneal Data loggers to record body temperature. Blood was also obtained to measure circulating BDNF. After recovery, rats were exposed to mild stress (novel chamber, NC) and later intense stress (shock training, ST), followed by sleep recording. Subsequently, rats were separated into resilient (Res; n=27) or vulnerable (Vul; n = 15) based on whether or not there was a 50% or greater decrease in REM after ST compared to baseline. We then compared sleep, freezing, and the stress response (stress-induced hyperthermia, SIH) across groups to determine the effects of mild and intense stress to determine if BDNF was predictive of the REM response. RESULTS: REM totals in the first 4 hours of sleep after exposure to NC predicted REM responses following ST with resilient animals having higher REM and vulnerable animals having lower REM. Resilient rats had significantly higher baseline peripheral BDNF compared to vulnerable rats. CONCLUSIONS: These results show that outbred rats display significant differences in post-stress sleep and peripheral BDNF identifying these factors as potential markers of resilience and vulnerability prior to traumatic stress.