Glucocorticoid enhancement of memory requires arousal-induced noradrenergic activation in the basolateral amygdala

LTP after stress: up or down?

When an organism is exposed to a stressful situation, corticosteroid levels in the brain rise. This rise has consequences for behavioral performance, including memory formation. Over the past decades, it has become clear that a rise in corticosteroid level is also accompanied by a reduction in hippocampal long-term potentiation (LTP). Recent studies, however, indicate that stress does not lead to a universal suppression of LTP. Many factors, including the type of stress, the phase of the stress response, the area of investigation, type of LTP, and the life history of the organism determine in which direction LTP will be changed.

Angst and the amygdala

Fear is an adaptation to danger, but excessive fear underlies diverse forms of mental anguish and pathology. One neural site linked to a sense of adversity is the amygdala, and one neuropeptide, corticotropin-releasing hormone (CRH), is localized within the central nucleus of the amygdala. Glucocorticoids enhance the production of CRH in this region of the brain, resulting in increased attention to external events and, when sustained for longer periods of times, perhaps contributing to anxious depression.

Involvement of noradrenergic and corticoid receptors in the consolidation of the lasting anxiogenic effects of predator stress

The roles of beta-NER (beta-noradrenergic receptor), GR (glucocorticoid) and mineral corticoid receptors (MR) in the consolidation of anxiogenic effects of predator stress were studied. One minute after predator stress, different groups of rats were injected (ip) with vehicle, propranolol (beta-NER blocker, 5 and 10 mg/kg), mifepristone (RU486, GR blocker, 20 mg/kg), spironolactone (MR blocker, 50 mg/kg), propranolol (5 mg/kg) plus RU486 (20 mg/kg) or the anxiolytic, chloradiazepoxide (CPZ, 10 mg/kg). One week later, rodent anxiety was assessed in elevated plus maze, hole board, light/dark box, social interaction and acoustic startle. Considering all tests except startle, propranolol dose dependently blocked consolidation of lasting anxiogenic effects of predator stress in all tests. GR receptor block alone was ineffective. However, GR block in combination with an ineffective dose of propranolol did blocked consolidation of predator stress effects in all tests, suggesting a synergism between beta-NER and GR. Surprisingly, MR block prevented consolidation of anxiogenic effects in all tests except the light/dark box. CPZ post stress was ineffective against the anxiogenic impact of predator stress. Study of startle was complicated by the fact that anxiogenic effects of stress on startle amplitude manifested as both an increase and a decrease in startle amplitude. Suppression of startle occurred in stressed plus vehicle injected groups handled three times prior to predator stress. In contrast, stressed plus vehicle rats handled five times prior to predator stress showed increases in startle, as did all predator stressed only groups. Mechanisms of consolidation of the different startle responses appear to differ. CPZ post stress blocked startle suppression but not enhancement of startle. Propranolol post stress had no effect on either suppression or enhancement of startle. GR block alone post stress prevented suppression of startle, but not enhancement. In contrast blocking GR and beta-NER together prevented startle enhancement. MR block also prevented startle enhancement. Effects of MR block on startle suppression were not tested. Delay of habituation to startle was found in all stressed rats. Consolidation of delay of habituation was blocked or attenuated by post stress MR block, GR plus beta-NER block and CPZ but not by post stress GR or beta-NER block alone. Taken together, present findings suggest consolidation of lasting anxiogenic effects of predator stress may share some of the same neurochemical mechanisms implicated in some forms of fear memory consolidation. Implications of these findings for the study of stress-induced changes in affect including posttraumatic stress disorder (PTSD) are discussed.

Endogenous cortisol level interacts with noradrenergic activation in the human amygdala

Animal studies show that high cortisol levels exert their effect on stressful task performance via modulation of the amygdala. Availability of noradrenaline in this brain region appears to be a critical prerequisite for this effect. This relationship between noradrenaline and cortisol is explained by an animal model where the amygdala constitutes a crucial region for this interaction. In humans this model has not been extensively tested so far. In a previously reported study human subjects (aged 20.93+/-2.38) were scanned using fMRI when watching sets of emotional and neutral pictures after taking the beta-adrenergic antagonist propranolol or placebo. Stimulus sets consisted of 92 pictures, divided in four emotional categories that ranged from neutral scenes of domestic objects (CAT1) to extremely negative scenes of mutilation or accidents (CAT4). Confrontation with arousing emotional pictures, accompanied by increased noradrenaline levels, evoked increased amygdala activation under placebo but not under betablocker condition. This new and additional analysis of this data set was carried out to determine the effect of differential endogenous cortisol levels on amygdala activation. Cortisol levels during scanning were determined using salivary samples and subjects were post hoc divided in a High (n=14) and Low cortisol group (n=14). When subjects were watching emotional stimuli, presumably associated with enhanced noradrenaline (NA) levels, amygdala activation was contrasted between the two cortisol groups. We hypothesized that emotional stimuli would elicit more amygdala activation in the High than in the Low cortisol group. Here we demonstrate indeed a significant interaction effect of the endogenous cortisol level with increasing activation in the amygdala under placebo but not under betablocker condition, thereby extending the rodent based model of a synergistic effect of the two stress hormones to the human.

Adrenal stress hormones, amygdala activation, and memory for emotionally arousing experiences

Extensive evidence indicates that stress hormones released from the adrenal glands are critically involved in memory consolidation of emotionally arousing experiences. Epinephrine or glucocorticoids administered after exposure to emotionally arousing experiences enhance the consolidation of long-term memories of these experiences. Our findings indicate that adrenal stress hormones influence memory consolidation via interactions with arousal-induced activation of noradrenergic mechanisms within the amygdala. In turn, the amygdala regulates memory consolidation via its efferent projections to many other brain regions. In contrast to the enhancing effects on consolidation, high circulating levels of stress hormones impair memory retrieval and working memory. Such effects also require noradrenergic activation of the amygdala and interactions with other brain regions.

The temporal dynamics model of emotional memory processing: a synthesis on the neurobiological basis of stress-induced amnesia, flashbulb and traumatic memories, and the Yerkes-Dodson law

We have reviewed research on the effects of stress on LTP in the hippocampus, amygdala and prefrontal cortex (PFC) and present new findings which provide insight into how the attention and memory-related functions of these structures are influenced by strong emotionality. We have incorporated the stress-LTP findings into our "temporal dynamics" model, which provides a framework for understanding the neurobiological basis of flashbulb and traumatic memories, as well as stress-induced amnesia. An important feature of the model is the idea that endogenous mechanisms of plasticity in the hippocampus and amygdala are rapidly activated for a relatively short period of time by a strong emotional learning experience. Following this activational period, both structures undergo a state in which the induction of new plasticity is suppressed, which facilitates the memory consolidation process. We further propose that with the onset of strong emotionality, the hippocampus rapidly shifts from a "configural/cognitive map" mode to a "flashbulb memory" mode, which underlies the long-lasting, but fragmented, nature of traumatic memories. Finally, we have speculated on the significance of stress-LTP interactions in the context of the Yerkes-Dodson Law, a well-cited, but misunderstood, century-old principle which states that the relationship between arousal and behavioral performance can be linear or curvilinear, depending on the difficulty of the task.

Orphanin FQ/nociceptin interacts with the basolateral amygdala noradrenergic system in memory consolidation

Extensive evidence indicates that the basolateral complex of the amygdala (BLA) mediates hormonal and neurotransmitter effects on the consolidation of emotionally influenced memory and that such modulatory influences involve noradrenergic activation of the BLA. As the BLA also expresses a high density of receptors for orphanin FQ/nociceptin (OFQ/N), an opioid-like peptide with anxiolytic and amnestic properties, the present experiments investigated whether the BLA is involved in mediating OFQ/N effects on memory consolidation and whether such effects require noradrenergic activity. OFQ/N (0.01-100 pmol in 0.2 microL) administered bilaterally into the BLA of male Sprague-Dawley rats immediately after aversively motivated inhibitory avoidance training induced dose-dependent impairment on a 48-h retention trial. The beta(1)-adrenoceptor antagonist atenolol (2.0 nmol) administered concurrently into the BLA potentiated the dose-response effects of OFQ/N. In contrast, immediate post-training infusions of the peptidergic OFQ/N receptor antagonist [Nphe(1)]nociceptin(1-13)NH(2) (1-100 pmol in 0.2 microL) into the BLA enhanced 48-h retention of inhibitory avoidance training, an effect that was blocked by coadministration of atenolol. Delayed infusions of OFQ/N or [Nphe(1)]nociceptin(1-13)NH(2) into the BLA administered either 6 or 3 h after training, respectively, or immediate post-training infusions of OFQ/N into the adjacent central amygdala did not significantly alter retention performance. These findings indicate that endogenously released OFQ/N interacts with noradrenergic activity within the BLA in modulating memory consolidation.

Post-traumatic stress disorder in somatic disease: lessons from critically ill patients

Post-traumatic stress disorder (PTSD) is a well-recognized complication of severe illness. PTSD has been described in patients after multiple trauma, burns, or myocardial infarction with a particularly high incidence in survivors of acute pulmonary failure (Acute Respiratory Distress Syndrome) or septic shock. Many patients with evidence of PTSD after critical illness have been treated in intensive care units (ICUs). Studies in long-term survivors of ICU treatment demonstrated a clear and vivid recall of different categories of traumatic memory such as nightmares, anxiety, respiratory distress, or pain with little or no recall of factual events. A high number of these traumatic memories from the ICU has been shown to be a significant risk factor for the later development of PTSD in long-term survivors. In addition, patients in the ICU are often treated with stress hormones like epinephrine, norepinephrine, or cortisol. The number of the above-mentioned categories of traumatic memory increased with the totally administered dosages of catecholamines and cortisol, and the evaluation of these categories at different time points after discharge from the ICU showed better memory consolidation with higher dosages of stress hormones administered. Conversely, the prolonged administration of beta-adrenergic antagonists during the recovery phase after cardiac surgery resulted in a lower number of traumatic memories and a lower incidence of stress symptoms at 6 months after surgery. Findings with regard to the administration of the stress hormone cortisol were more complex, however. Several studies from our group have demonstrated that the administration of stress doses of cortisol to critically ill patients resulted in a significant reduction of PTSD symptoms measured after recovery without influencing the number of categories of traumatic memory. This can possibly be explained by a cortisol-induced temporary impairment in traumatic memory retrieval that has previously been demonstrated in both rats and humans. ICU therapy of critically ill patients can serve as a stress model that allows the delineation of stress hormone effects on traumatic memory and PTSD development. This could also result in new approaches for prophylaxis and treatment of stress-related disorders.

Dopaminergic modulation of the persistence of one-trial hippocampus-dependent memory

The persistence of new memory traces in the hippocampus, encoded following appropriate activation of glutamatergic receptors and the induction of synaptic plasticity, can be influenced by heterosynaptic activation of neuromodulatory brain systems. We therefore investigated the effects of a hippocampus-specific blockade of dopamine D1/D5 receptors on the persistence of spatial memory encoded in one trial using a delayed matching-to-place (DMP) task in a watermaze in which rats learn a new escape location each day. A within-subjects design was used such that both short (20 min) and long (6 h) retention intervals, and both drug (SCH23390, a D1/D5 receptor antagonist) and vehicle (aCSF) infusions were tested on different days in the same animals. Bilateral intrahippocampal infusion of SCH23390 (5 microg in 1 microL per side) prior to trial 1 (encoding) caused a differential impairment as a function of memory delay-with no effect during trial 2 (memory retrieval) after a 20-min interval, but a block of memory at 6 h. Further experiments revealed that infusion of SCH23390 immediately after trial 1 had no effect on retention 6 h later, and the poor memory seen at long retention intervals when the drug was present at encoding was not due to a state-dependent failure of retrieval. These results suggest that activation of D1/D5 receptors during memory encoding is necessary for the formation of a persistent memory trace in the hippocampus. The complementary effects of D1/D5 receptor blockade on the persistence of LTP and the duration of memory are consistent with the idea that changes in synaptic strength underlie memory.

From Malthus to motive: how the HPA axis engineers the phenotype, yoking needs to wants

The hypothalamo-pituitary-adrenal (HPA) axis is the critical mediator of the vertebrate stress response system, responding to environmental stressors by maintaining internal homeostasis and coupling the needs of the body to the wants of the mind. The HPA axis has numerous complex drivers and highly flexible operating characterisitics. Major drivers include two circadian drivers, two extra-hypothalamic networks controlling top-down (psychogenic) and bottom-up (systemic) threats, and two intra-hypothalamic networks coordinating behavioral, autonomic, and neuroendocrine outflows. These various networks jointly and flexibly control HPA axis output of periodic (oscillatory) functions and a range of adventitious systemic or psychological threats, including predictable daily cycles of energy flow, actual metabolic deficits over many time scales, predicted metabolic deficits, and the state-dependent management of post-prandial responses to feeding. Evidence is provided that reparation of metabolic derangement by either food or glucocorticoids results in a metabolic signal that inhibits HPA activity. In short, the HPA axis is intimately involved in managing and remodeling peripheral energy fluxes, which appear to provide an unidentified metabolic inhibitory feedback signal to the HPA axis via glucocorticoids. In a complementary and perhaps a less appreciated role, adrenocortical hormones also act on brain to provide not only feedback, but feedforward control over the HPA axis itself and its various drivers, as well as coordinating behavioral and autonomic outflows, and mounting central incentive and memorial networks that are adaptive in both appetitive and aversive motivational modes. By centrally remodeling the phenotype, the HPA axis provides ballistic and predictive control over motor outflows relevant to the type of stressor. Evidence is examined concerning the global hypothesis that the HPA axis comprehensively induces integrative phenotypic plasticity, thus remodeling the body and its governor, the brain, to yoke the needs of the body to the wants of the mind. Adverse side effects of this yoking under conditions of glucocorticoid excess are discussed.

Emotional enhancement of memory via amygdala-driven facilitation of rhinal interactions

Emotions generally facilitate memory, an effect mediated by the basolateral amygdala (BLA). To study the underlying mechanisms, we recorded BLA, perirhinal and entorhinal neurons during an appetitive trace-conditioning task. We focused on the rhinal cortices because they constitute the interface between the hippocampus, a mediator of memory consolidation, and the neocortex, the storage site of declarative memories. We found that, after unexpected rewards, BLA activity increased impulse transmission from perirhinal to entorhinal neurons and that this effect decayed as the association between conditioned stimuli and rewards was learned. At this late phase of learning, the BLA effect occurred when the animals were anticipating the reward. By enhancing the processing of sensory cues, the BLA-mediated facilitation of rhinal interactions may explain how the amygdala promotes memory formation in emotional conditions.

Angst and the amygdala

Fear is an adaptation to danger, but excessive fear underlies diverse forms of mental anguish and pathology. One neural site linked to a sense of adversity is the amygdala, and one neuropeptide, corticotropin-releasing hormone (CRH), is localized within the central nucleus of the amygdala. Glucocorticoids enhance the production of CRH in this region of the brain, resulting in increased attention to external events and, when sustained for longer periods of time, perhaps contributing to anxious depression.

Effects of stress and corticosterone on activity and plasticity in the amygdala

The basolateral amygdala (BLA) has been repeatedly shown to mediate the effects of stress on memory-related processes. However, the way in which stress influences BLA itself has not been fully explored. We studied the effects of stress and corticosterone (CORT) on activity and plasticity in the BLA in the rat, using the electrophysiological procedure of long-term potentiation (LTP) induction in vivo. Rats were exposed to an acute elevated-platform stress or administered vehicle or 5 mg/kg, 10 mg/kg, or 25 mg/kg of CORT systemically, after which they were anesthetized and prepared for field potential recording in the BLA, in response to stimulation of the entorhinal cortex. The elevated platform stress enhanced baseline responses in BLA and plasma CORT but inhibited amygdalar LTP. Systemic injections of CORT enhanced baseline responses in BLA in a dose-dependent manner but did not influence amygdalar LTP. Posttetanic potentiation (PTP) was similarly reduced in CORT- and vehicle-injected groups, possibly because of an additional stress from the injection, thus implying that PTP and LTP in the amygdala differentially react to stress. These results suggest that the increase in amygdalar baseline activity following the exposure to stress may be mediated by the concomitant increase in plasma CORT. However, the suppression of amygdalar LTP is not a result of elevated levels of CORT, suggesting that activity and plasticity in the amygdala might be mediated by different mechanisms.