Memory modulation

Memory reconsolidation and its maintenance depend on L-voltage-dependent calcium channels and CaMKII functions regulating protein turnover in the hippocampus

Immediate postretrieval bilateral blockade of long-acting voltage-dependent calcium channels (L-VDCCs), but not of glutamatergic NMDA receptors, in the dorsal CA1 region of the hippocampus hinders retention of long-term spatial memory in the Morris water maze. Immediate postretrieval bilateral inhibition of calcium/calmodulin-dependent protein kinase (CaMK) II in dorsal CA1 does not affect retention of this task 24 h later but does hinder it 5 d later. These two distinct amnesic effects are abolished if protein degradation by proteasomes is inhibited concomitantly. These results indicate that spatial memory reconsolidation depends on the functionality of L-VDCC in dorsal CA1, that maintenance of subsequent reconsolidated memory trace depends on CaMKII, and these results also suggest that the role played by both L-VDCC and CaMKII is to promote the retrieval-dependent, synaptically localized enhancement of protein synthesis necessary to counteract a retrieval-dependent, synaptic-localized enhancement of protein degradation, which has been described as underlying the characteristic labilization of the memory trace triggered by retrieval. Thus, conceivably, L-VDCC and CaMKII would enhance activity-dependent localized protein renewal, which may account for the improvement of the long-term efficiency of the synapses responsible for the maintenance of reactivated long-term spatial memory.

About sleep's role in memory

Over more than a century of research has established the fact that sleep benefits the retention of memory. In this review we aim to comprehensively cover the field of "sleep and memory" research by providing a historical perspective on concepts and a discussion of more recent key findings. Whereas initial theories posed a passive role for sleep enhancing memories by protecting them from interfering stimuli, current theories highlight an active role for sleep in which memories undergo a process of system consolidation during sleep. Whereas older research concentrated on the role of rapid-eye-movement (REM) sleep, recent work has revealed the importance of slow-wave sleep (SWS) for memory consolidation and also enlightened some of the underlying electrophysiological, neurochemical, and genetic mechanisms, as well as developmental aspects in these processes. Specifically, newer findings characterize sleep as a brain state optimizing memory consolidation, in opposition to the waking brain being optimized for encoding of memories. Consolidation originates from reactivation of recently encoded neuronal memory representations, which occur during SWS and transform respective representations for integration into long-term memory. Ensuing REM sleep may stabilize transformed memories. While elaborated with respect to hippocampus-dependent memories, the concept of an active redistribution of memory representations from networks serving as temporary store into long-term stores might hold also for non-hippocampus-dependent memory, and even for nonneuronal, i.e., immunological memories, giving rise to the idea that the offline consolidation of memory during sleep represents a principle of long-term memory formation established in quite different physiological systems.

Time-dependent effects of corticosterone on reward-based decision-making in a rodent model of the Iowa Gambling Task

Corticosteroid hormones, released after stress, are known to change neuronal activity in two time-domains: within minutes via non-genomic pathways and with a delay of >1 h through pathways involving transcriptional regulation. Recent evidence in rodents and humans indicates that these two modes of corticosteroid action differently affect cognitive tasks. Here, we investigated whether reward-based decision-making, in a rat model of the Iowa Gambling Task (rIGT), is also differently altered by rapid versus delayed actions of corticosterone. We targeted the rapid and delayed time domain by injecting corticosterone (CORT, 1 mg/kg, s.c.) at 30 min (rapid) or 180 min (delayed) respectively prior to behavioural testing, during the final 3 days of the behavioural paradigm. In saline treated rats, the number of visits to the disadvantageous arm decreased over trial blocks, whilst this was attenuated when CORT was administered 30 min before testing. This attenuation was associated with a significantly increased c-Fos expression in the lateral orbitofrontal cortex and insular cortex, and a trend for an increase in the infralimbic cortex. The rapid corticosteroid effect contrasted with treatment 180 min before testing, where the number of visits to the disadvantageous arm as well as c-Fos labelling was not affected. These findings indicate that rapid corticosteroid actions impair reward-based decision-making.

Effects of noradrenergic stimulation on memory in patients with major depressive disorder

Major depressive disorder (MDD) has been associated with alterations in the noradrenergic system and impaired memory function. In turn, enhanced memory function has been associated with noradrenergic stimulation. In this study, we examined whether noradrenergic stimulation would differentially improve memory function in patients with MDD and healthy controls. In a placebo-controlled crossover study, 20 patients with MDD and 18 age- and sex-matched healthy controls received either placebo or 5 mg of yohimbine, an alpha-2-adrenoceptor antagonist that causes increased noradrenergic activity, orally before memory testing. A word list paradigm was used to test memory consolidation. Furthermore, the autobiographical memory test assessing memory retrieval and a working memory test were administered. Salivary alpha-amylase and blood pressure were measured. Yohimbine improved memory consolidation (word list learning) across groups (main effect of yohimbine: p = 0.05). This effect was more prominent in depressed patients compared with controls (post hoc t-test: MDD p = 0.01, controls p = 0.77). Memory retrieval (autobiographical memory specificity) and working memory were not affected by yohimbine. Across groups, yohimbine administration resulted in an increase in blood pressure and alpha-amylase. In sum, these results further support the hypothesis that noradrenergic stimulation enhances memory consolidation. The mechanism by which yohimbine leads to stronger memory consolidation in depressed patients compared with healthy controls remains to be elucidated.

Physiological basis for emotional modulation of memory circuits by the amygdala

Classical experiments have demonstrated that the amygdala facilitates synaptic plasticity in other brain structures (e.g. hippocampus, basal ganglia) believed to constitute the storage sites for various types of memory. Here, we summarize new developments in our understanding of how the amygdala facilitates the formation of emotional memories. Recent insights into this question have come from studies relying on simultaneous recording of neurons in multiple brain regions during learning. This approach has revealed that in emotionally arousing conditions, whether positively or negatively valenced, the amygdala allows incoming information to be processed more efficiently in distributed cerebral networks. This review also highlights the need to understand how different brain regions act in parallel to efficiently achieve one goal.

Systemic or intra-prelimbic cortex infusion of prazosin impairs fear memory reconsolidation

The alpha-1 adrenergic antagonist prazosin has been used to alleviate the symptoms of PTSD, but the mechanism remains unclear. One possibility is that prazosin may disrupt fear memory reconsolidation, leading to attenuation of fear responses. To test this hypothesis, we administered a single systemic injection of prazosin during the reconsolidation of olfactory fear conditioning in rats. We found that a post-retrieval injection of prazosin disrupted subsequent retrieval of fear. Similarly, intra-prelimbic cortex infusion of prazosin during the reconsolidation period also disrupted subsequent retrieval of fear. These findings suggest that fear memory undergoes reconsolidation through activation of alpha-1 adrenergic receptors in the prelimbic cortex.

Memory enhancement produced by post-training exposure to sucrose-conditioned cues

A number of aversive and appetitive unconditioned stimuli (such as shock and food) are known to produce memory enhancement when they occur during the post-training period. Post-training exposure to conditioned aversive stimuli has also been shown to enhance memory consolidation processes. The present study shows for the first time that post-training exposure to conditioned stimuli previously paired with consumption of a sucrose solution also enhances memory consolidation. Male Long Evans rats were trained on a one-session conditioned cue preference (CCP) task on a radial arm maze. Immediately or 2 hours after training, rats consumed a sucrose solution or were exposed to cues previously paired with consumption of sucrose or cues previously paired with water. Twenty-four hours later, the rats were tested for a CCP. Immediate, but not delayed, post-training consumption of sucrose enhanced memory for the CCP. Immediate, but not delayed, post-training exposure to cues previously paired with sucrose, but not with water, also enhanced CCP memory. The possibility that rewarding and aversive conditioned stimuli affect memory by a common physiological process is discussed.

Effects of transcranial direct current stimulation on consolidation of fear memory

It has been shown that applying transcranial direct current stimulation (tDCS) over the dorsolateral prefrontal cortex (DLPFC) influences declarative memory processes. This study investigates the efficacy of tDCS on emotional memory consolidation, especially experimental fear conditioning. We applied an auditory fear-conditioning paradigm, in which two differently colored squares (blue and yellow) were presented as conditioned stimuli (CS) and an auditory stimulus as unconditioned stimulus (UCS). Sixty-nine participants were randomly assigned into three groups: anodal, cathodal, and sham stimulation. The participants of the two active groups (i.e., anodal and cathodal) received tDCS over the left DLPFC for 12 min after fear conditioning. The effect of fear conditioning and consolidation (24 h later) was measured by assessing the skin conductance response (SCR) to the CS. The results provide evidence that cathodal stimulation of the left DLPFC leads to an inhibitory effect on fear memory consolidation compared to anodal and sham stimulation, as indicated by decreased SCRs to CS+ presentation during extinction training at day 2. In conclusion, current work suggests that cathodal stimulation interferes with processes of fear memory consolidation.

General Stress Responses in the Honey Bee

The biological concept of stress originated in mammals, where a “General Adaptation Syndrome” describes a set of common integrated physiological responses to diverse noxious agents. Physiological mechanisms of stress in mammals have been extensively investigated through diverse behavioral and physiological studies. One of the main elements of the stress response pathway is the endocrine hypothalamo-pituitary-adrenal (HPA) axis, which underlies the “fight-or-flight” response via a hormonal cascade of catecholamines and corticoid hormones. Physiological responses to stress have been studied more recently in insects: they involve biogenic amines (octopamine, dopamine), neuropeptides (allatostatin, corazonin) and metabolic hormones (adipokinetic hormone, diuretic hormone). Here, we review elements of the physiological stress response that are or may be specific to honey bees, given the economical and ecological impact of this species. This review proposes a hypothetical integrated honey bee stress pathway somewhat analogous to the mammalian HPA, involving the brain and, particularly, the neurohemal organ corpora cardiaca and peripheral targets, including energy storage organs (fat body and crop). We discuss how this system can organize rapid coordinated changes in metabolic activity and arousal, in response to adverse environmental stimuli. We highlight physiological elements of the general stress responses that are specific to honey bees, and the areas in which we lack information to stimulate more research into how this fascinating and vital insect responds to stress.

Aversive priming: cognitive processing of threatening stimuli is facilitated by aversive primes

It would be reasonable to expect that our previous experience regarding a stimulus that predicts harm would make the subsequent identification of that stimulus easier when harm happens again. Forty-eight volunteers were submitted to both phases of this sequence of events: learning of the predictive relationship and later priming. A face with neutral expression (CS+) was paired with a moderately aversive electric shock and another (CS-) with a neutral tone. Subsequently, these two faces, as well as other known and new faces, were presented for familiarity judgments. Both the CS+ and the CS- faces were preceded by an aversive stimulus (aversive prime) in one occasion and by a neutral stimulus (neutral prime) in another. The familiarity judgment regarding the CS+ was faster after the aversive prime than after the neutral prime, but there was no difference regarding the CS-. The differential effect of the aversive prime over the CS+ and the CS- showed a significant but small correlation with the differential skin conductance response to CS+ and CS- (signal learning), and with the differential evaluation of those stimuli in terms of like-dislike (evaluative learning). The scope of these results, as well as the usefulness of this methodological model, is discussed.

Local amplification of glucocorticoids in the aging brain and impaired spatial memory

The hippocampus is a prime target for glucocorticoids (GCs) and a brain structure particularly vulnerable to aging. Prolonged exposure to excess GCs compromises hippocampal electrophysiology, structure, and function. Blood GC levels tend to increase with aging and correlate with impaired spatial memory in aging rodents and humans. The magnitude of GC action within tissues depends not only on levels of steroid hormone that enter the cells from the periphery and the density of intracellular receptors but also on the local metabolism of GCs by 11β-hydroxysteroid dehydrogenases (11β-HSD). The predominant isozyme in the adult brain, 11β-HSD1, locally regenerates active GCs from inert 11-keto forms thus amplifying GC levels within specific target cells including in the hippocampus and cortex. Aging associates with elevated hippocampal and neocortical 11β-HSD1 and impaired spatial learning while deficiency of 11β-HSD1 in knockout (KO) mice prevents the emergence of cognitive decline with age. Furthermore, short-term pharmacological inhibition of 11β-HSD1 in already aged mice reverses spatial memory impairments. Here, we review research findings that support a key role for GCs with special emphasis on their intracellular regulation by 11β-HSD1 in the emergence of spatial memory deficits with aging, and discuss the use of 11β-HSD1 inhibitors as a promising novel treatment in ameliorating/improving age-related memory impairments.

The endocannabinoid transport inhibitor AM404 differentially modulates recognition memory in rats depending on environmental aversiveness

Cannabinoid compounds may influence both emotional and cognitive processes depending on the level of environmental aversiveness at the time of drug administration. However, the mechanisms responsible for these responses remain to be elucidated. The present experiments investigated the effects induced by the endocannabinoid transport inhibitor AM404 (0.5-5 mg/kg, i.p.) on both emotional and cognitive performances of rats tested in a Spatial Open Field task and subjected to different experimental settings, named High Arousal (HA) and Low Arousal (LA) conditions. The two different experimental conditions influenced emotional reactivity independently of drug administration. Indeed, vehicle-treated rats exposed to the LA condition spent more time in the center of the arena than vehicle-treated rats exposed to the HA context. Conversely, the different arousal conditions did not affect the cognitive performances of vehicle-treated animals such as the capability to discriminate a spatial displacement of the objects or an object substitution. AM404 administration did not alter locomotor activity or emotional behavior of animals exposed to both environmental conditions. Interestingly, AM404 administration influenced the cognitive parameters depending on the level of emotional arousal: it impaired the capability of rats exposed to the HA condition to recognize a novel object while it did not induce any impairing effect in rats exposed to the LA condition. These findings suggest that drugs enhancing endocannabinoid signaling induce different effects on recognition memory performance depending on the level of emotional arousal induced by the environmental conditions.

Involvement of the insular cortex in regulating glucocorticoid effects on memory consolidation of inhibitory avoidance training

Glucocorticoids are known to enhance the consolidation of memory of emotionally arousing experiences by acting upon a network of interconnected brain regions. Although animal studies typically do not consider the insular cortex (IC) to be part of this network, the present findings indicate that the IC is importantly involved in regulating glucocorticoid effects on memory consolidation of emotionally arousing inhibitory avoidance training. The specific glucocorticoid receptor (GR) agonist RU 28362 (3 or 10 ng in 0.5 μl) infused bilaterally into the IC of male Sprague-Dawley rats immediately after one-trial inhibitory avoidance training dose-dependently enhanced 48 h retention performance. Moreover, training on the inhibitory avoidance task increased neuronal activity of the IC, as assessed by an increased number of cells expressing immunoreactivity for phosphorylated extracellular signal-regulated kinase 1/2 (pERK1/2). However, systemic administration of a memory-enhancing dose of corticosterone (1 mg/kg) after inhibitory avoidance training rapidly reduced the number of pERK1/2-positive cells in the IC, suggesting that glucocorticoid administration reduces overall neuronal activity of the IC. To investigate which components of the inhibitory avoidance training experience were influenced by the intra-IC glucocorticoid administration, in the last experiment rats were trained on a modified inhibitory avoidance task in which context exposure and footshock training occur on two sequential days. RU 28362 administration into the IC enhanced later retention when infused immediately after either the context or footshock training. Thus, these findings indicate that the IC mediates glucocorticoid effects on the consolidation of memory of different components of inhibitory avoidance training and suggest that the IC might be an important element of the rodent brain network involved in emotional regulation of learning and memory.