Dissociating learning and performance: drug and hormone enhancement of memory storage

Using Postmeal Measures and Manipulations to Investigate Hippocampal Mnemonic Control of Eating Behavior

Episodic meal-related memories provide the brain with a powerful mechanism for tracking and controlling eating behavior because they contain a detailed record of recent energy intake that likely outlasts the physiological signals generated by feeding bouts. This review briefly summarizes evidence from human participants showing that episodic meal-related memory limits later eating behavior and then describes our research aimed at investigating whether hippocampal neurons mediate the inhibitory effects of meal-related memory on subsequent feeding. Our approach has been inspired by pioneering work conducted by Ivan Izquierdo and others who used posttraining manipulations to investigate memory consolidation. This review describes the rationale and value of posttraining manipulations, how Izquierdo used them to demonstrate that dorsal hippocampal (dHC) neurons are critical for memory consolidation, and how we have adapted this strategy to investigate whether dHC neurons are necessary for mnemonic control of energy intake. I describe our evidence showing that ingestion activates the molecular processes necessary for synaptic plasticity and memory during the early postprandial period, when the memory of the meal would be undergoing consolidation, and then summarize our findings showing that neural activity in dHC neurons is critical during the early postprandial period for limiting future intake. Collectively, our evidence supports the hypothesis that dHC neurons mediate the inhibitory effects of ingestion-related memory on future intake and demonstrates that post-experience memory modulation is not confined to artificial laboratory memory tasks.

Postmeal optogenetic inhibition of dorsal hippocampal principal neurons increases future intake in a time-dependent manner

Research involving human participants indicates that memories of recently eaten meals limit how much is eaten during subsequent eating episodes; yet, the brain regions that mediate the inhibitory effects of ingestion-related memory on future intake are largely unknown. We hypothesize that dorsal hippocampal (dHC) neurons, which are critical for episodic memories of personal experiences, mediate the inhibitory effects of ingestion-related memory on future intake. Our research program aimed at testing this hypothesis has been influenced in large part by our mentor James McGaugh and his research on posttraining manipulations. In the present study, we used an activity-guided optogenetic approach to test the prediction that if dHC glutamatergic neurons limit future intake through a process that requires memory consolidation, then inhibition should increase subsequent intake when given soon after the end of a meal but delayed inhibition should have no effect. Viral vectors containing CaMKIIα-eArchT3.0-eYFP and fiber optic probes were placed in the dHC of male Sprague-Dawley rats. Compared to intake on a day when no inhibition was given, postmeal inhibition of dHC glutamatergic neurons given for 10 min after the end of a saccharin meal increased the likelihood that rats would consume a second meal 90 min later and significantly increased the amount of saccharin solution consumed during that next meal when the neurons were no longer inhibited. Importantly, delayed inhibition given 80 min after the end of the saccharin meal did not affect subsequent intake of saccharin. Given that saccharin has minimal postingestive gastric consequences, these effects are not likely due to the timing of interoceptive visceral cues generated by the meal. These data show that dHC glutamatergic neural activity is necessary during the early postprandial period for limiting future intake and suggest that these neurons inhibit future intake by consolidating the memory of the preceding meal.

Chemogenetic Suppression of Medial Prefrontal-Dorsal Hippocampal Interactions Prevents Estrogenic Enhancement of Memory Consolidation in Female Mice

The importance of the dorsal hippocampus (DH) in mediating the memory-enhancing effects of the sex-steroid hormone 17β-estradiol (E₂) is well established. However, estrogen receptors (ERs) are highly expressed in other brain regions that support memory formation, including the medial prefrontal cortex (mPFC). The mPFC and DH interact to mediate the formation of several types of memory, and behavioral tasks that recruit the mPFC are enhanced by systemic E₂ administration, making this region a prime candidate for investigating circuit-level questions regarding the estrogenic regulation of memory. Further, infusion of E₂ directly into the DH increases dendritic spine density in both the DH and mPFC, and this effect depends upon rapid activation of cell-signaling pathways in the DH, demonstrating a previously unexplored interaction between the DH and mPFC that led us to question the role of the mPFC in object memory consolidation and the necessity of DH-mPFC interactions in the memory-enhancing effects of E₂. Here, we found that infusion of E₂ directly into the mPFC of ovariectomized mice increased mPFC apical spine density and facilitated object recognition and spatial memory consolidation, demonstrating that E₂ in the mPFC increases spinogenesis and enhances on memory consolidation. Next, chemogenetic suppression of the mPFC blocked the beneficial effects of DH-infused E₂ on memory consolidation, indicating that systems-level DH-mPFC interactions are necessary for the memory-enhancing effects of E₂. Together, these studies provide evidence that E₂ in the mPFC mediates memory formation, and reveal that the DH and mPFC act in concert to support the memory-enhancing effects of E₂ in female mice.

Chemogenetic inactivation of the dorsal hippocampus and medial prefrontal cortex, individually and concurrently, impairs object recognition and spatial memory consolidation in female mice

The dorsal hippocampus (DH) and medial prefrontal cortex (mPFC) are brain regions essential for processing and storing episodic memory. In rodents, the DH has a well-established role in supporting the consolidation of episodic-like memory in tasks such as object recognition and object placement. However, the role of the mPFC in the consolidation of episodic-like memory tasks remains controversial. Therefore, the present study examined involvement of the DH and mPFC, alone and in combination, in object and spatial recognition memory consolidation in ovariectomized female mice. To this end, we utilized two types of inhibitory Designer Receptors Exclusively Activated by Designer Drugs (DREADDs) to inactivate the DH alone, the mPFC alone, or both brain regions concurrently immediately after object training to assess the role of each region in the consolidation of object recognition and spatial memories. Our results using single and multiplexed DREADDS suggest that excitatory activity in the DH and mPFC, alone or in combination, is required for the successful consolidation of object recognition and spatial memories. Together, these studies provide critical insight into how the DH and mPFC work in concert to facilitate memory consolidation in female mice.

Rapid effects on memory consolidation and spine morphology by estradiol in female and male rodents

Contribution to Special Issue on Fast effects of steroids. Rapid, neurosteroid-like effects of estrogens on memory consolidation during recognition memory tasks in both male and female rodents are described. We discuss how these mnemonic changes are related to rapid estrogenic effects on dendritic spine density, the distribution of spine types and the expression of PSD95 and GluA2 within spines in the hippocampus and medial prefrontal cortex, two areas critical for learning and memory. Overall, these data lead to the conclusion that estrogens are capable of exerting rapid and potent influences on memory and spine morphology in both sexes. The demonstration of estrogenic effects in males, which are used in the majority of memory studies, may provide a model for better understanding how hormone dependent changes in signaling pathways mediating memory and spinogenesis are coordinated to promote memory consolidation.

Inhibition of histone deacetylase 3 via RGFP966 facilitates cortical plasticity underlying unusually accurate auditory associative cue memory for excitatory and inhibitory cue-reward associations

Epigenetic mechanisms are key for regulating long-term memory (LTM) and are known to exert control on memory formation in multiple systems of the adult brain, including the sensory cortex. One epigenetic mechanism is chromatin modification by histone acetylation. Blocking the action of histone de-acetylases (HDACs) that normally negatively regulate LTM by repressing transcription has been shown to enable memory formation. Indeed, HDAC inhibition appears to facilitate memory by altering the dynamics of gene expression events important for memory consolidation. However, less understood are the ways in which molecular-level consolidation processes alter subsequent memory to enhance storage or facilitate retrieval. Here we used a sensory perspective to investigate whether the characteristics of memory formed with HDAC inhibitors are different from naturally-formed memory. One possibility is that HDAC inhibition enables memory to form with greater sensory detail than normal. Because the auditory system undergoes learning-induced remodeling that provides substrates for sound-specific LTM, we aimed to identify behavioral effects of HDAC inhibition on memory for specific sound features using a standard model of auditory associative cue-reward learning, memory, and cortical plasticity. We found that three systemic post-training treatments of an HDAC3-inhibitor (RGPF966, Abcam Inc.) in rats in the early phase of training facilitated auditory discriminative learning, changed auditory cortical tuning, and increased the specificity for acoustic frequency formed in memory of both excitatory (S+) and inhibitory (S-) associations for at least 2 weeks. The findings support that epigenetic mechanisms act on neural and behavioral sensory acuity to increase the precision of associative cue memory, which can be revealed by studying the sensory characteristics of long-term associative memory formation with HDAC inhibitors.

Rapid actions of oestrogens and their receptors on memory acquisition and consolidation in females

Increased attention has been paid in recent years to the ways in which oestrogens and oestrogen receptors rapidly affect learning and memory. These rapid effects occur within a timeframe that is too narrow for the classical genomic mode of action of oestrogen, thus suggesting nonclassical effects as underlying mechanisms. The present review examines recent developments in the study of the rapid effects of 17β-oestradiol and oestrogen receptor (ER) agonists on learning and memory tasks in female rodents, including social recognition, object recognition, object placement (spatial memory) and social learning. By comparing studies utilising systemic or intracranial treatments, as well as pre- and post-acquisition administration of oestradiol or ER agonists, the respective contributions of individual ERs within specific brain regions to various forms of learning and memory can be determined. The first part of this review explores the effects of systemic administration of 17β-oestradiol and ER agonists on memory when administered either pre- or post-acquisition. The second part not only focuses on the effects of pre- and post-acquisition infusions of 17β-oestradiol or ER agonists into the dorsal hippocampus on memory, but also discusses the contributions of other brain regions, including the medial amygdala, medial prefrontal cortex and paraventricular nucleus of the hypothalamus. The cellular mechanisms mediating the rapid effects of 17β-oestradiol on memory, including activation of intracellular signalling cascades and epigenetic processes, are discussed. Finally, the review concludes by comparing pre- and post-acquisition findings and effects of 17β-oestradiol and ER agonists in different brain regions.

Frameworking memory and serotonergic markers

The evidence for neural markers and memory is continuously being revised, and as evidence continues to accumulate, herein, we frame earlier and new evidence. Hence, in this work, the aim is to provide an appropriate conceptual framework of serotonergic markers associated with neural activity and memory. Serotonin (5-hydroxytryptamine [5-HT]) has multiple pharmacological tools, well-characterized downstream signaling in mammals’ species, and established 5-HT neural markers showing new insights about memory functions and dysfunctions, including receptors (5-HT1A/1B/1D, 5-HT2A/2B/2C, and 5-HT3-7), transporter (serotonin transporter [SERT]) and volume transmission present in brain areas involved in memory. Bidirectional influence occurs between 5-HT markers and memory/amnesia. A growing number of researchers report that memory, amnesia, or forgetting modifies neural markers. Diverse approaches support the translatability of using neural markers and cerebral functions/dysfunctions, including memory formation and amnesia. At least, 5-HT1A, 5-HT4, 5-HT6, and 5-HT7receptors and SERT seem to be useful neural markers and therapeutic targets. Hence, several mechanisms cooperate to achieve synaptic plasticity or memory, including changes in the expression of neurotransmitter receptors and transporters.

Interaction between harmane, a class of β-carboline alkaloids, and the CA1 serotonergic system in modulation of memory acquisition

This study set to assess the involvement of dorsal hippocampus (CA1) serotonergic system on harmane induced memory acquisition deficit. We used one trial step-down inhibitory avoidancetask to evaluate memory retention and then, open field test to evaluate locomotor activity in adult male NMRI mice. The results showed that pre-training intra-peritoneal (i.p.) administration of harmane (12mg/kg) induced impairment of memory acquisition. Pre-training intra-CA1 administration of 5-HT1B/1D receptor agonist (CP94253; 0.5 and 5ng/mouse) and 5-HT2A/2B/2C receptor agonist (α-methyl 5-HT; 50ng/mouse) impaired memory acquisition. Furthermore, intra-CA1 administration of 5-HT1B/1D receptor antagonist (GR127935; 0.5ng/mouse) and 5-HT2 receptor antagonist (cinancerine; 5ng/mouse) improved memory acquisition. In addition, pre-training intra-CA1 injection of sub-threshold dose of CP94253 (0.05ng/mouse) and α-methyl 5-HT (5ng/mouse) potentiated impairment of memory acquisition induced by harmane (12mg/kg, i.p.). On the other hand, pre-training intra-CA1 infusion of sub-threshold dose of GR127935 (0.05ng/mouse) and cinancerine (0.5ng/mouse) with the administration of harmane (12mg/kg, i.p.) weakened impairment of memory acquisition. Moreover, all above doses of drugs did not change locomotor activity. The present findings suggest that there is an interaction between harmane and the CA1 serotonergic system in modulation of memory acquisition.

Inhibition of local estrogen synthesis in the hippocampus impairs hippocampal memory consolidation in ovariectomized female mice

The potent estrogen 17β-Estradiol (E2) plays a critical role in mediating hippocampal function, yet the precise mechanisms through which E2 enhances hippocampal memory remain unclear. In young adult female rodents, the beneficial effects of E2 on memory are generally attributed to ovarian-synthesized E2. However, E2 is also synthesized in the adult brain in numerous species, where it regulates synaptic plasticity and is synthesized in response to experiences such as exposure to females or conspecific song. Although de novo E2 synthesis has been demonstrated in rodent hippocampal cultures, little is known about the functional role of local E2 synthesis in mediating hippocampal memory function. Therefore, the present study examined the role of hippocampal E2 synthesis in hippocampal memory consolidation. Using bilateral dorsal hippocampal infusions of the aromatase inhibitor letrozole, we first found that blockade of dorsal hippocampal E2 synthesis impaired hippocampal memory consolidation. We next found that elevated levels of E2 in the dorsal hippocampus observed 30min after object training were blocked by dorsal hippocampal infusion of letrozole, suggesting that behavioral experience increases acute and local E2 synthesis. Finally, aromatase inhibition did not prevent exogenous E2 from enhancing hippocampal memory consolidation, indicating that hippocampal E2 synthesis is not necessary for exogenous E2 to enhance hippocampal memory. Combined, these data are consistent with the hypothesis that hippocampally-synthesized E2 is necessary for hippocampus-dependent memory consolidation in rodents.

Estradiol: Mediator of memories, spine density and cognitive resilience to stress in female rodents

Estradiol rapidly activates, within minutes, various physiological functions and behaviors including cognition in rodents. This review describes rapid effects of estradiol on hippocampal dependent learning and memory tasks in rodents. Mechanisms underlying the memory enhancements including the activation of signaling molecules and the enhancement of dendritic spinogenesis are briefly reviewed. In addition, the role of estradiol in the cognitive resilience to chronic stress exhibited only in females is discussed including contributions of ovarian as well as intra-hippocampally derived estrogens to this sex difference. Finally, speculations on possible physiologic functions for rapid mnemonic changes mediated by estrogens are made. Overall, the emergence of a novel and powerful mechanism for regulation of cognition by estradiol is described.

Effect of conditioned stimulus exposure during slow wave sleep on fear memory extinction in humans

STUDY OBJECTIVES

Repeated exposure to a neutral conditioned stimulus (CS) in the absence of a noxious unconditioned stimulus (US) elicits fear memory extinction. The aim of the current study was to investigate the effects of mild tone exposure (CS) during slow wave sleep (SWS) on fear memory extinction in humans.

DESIGN

The healthy volunteers underwent an auditory fear conditioning paradigm on the experimental night, during which tones served as the CS, and a mild shock served as the US. They were then randomly assigned to four groups. Three groups were exposed to the CS for 3 or 10 min or an irrelevant tone (control stimulus, CtrS) for 10 min during SWS. The fourth group served as controls and was not subjected to any interventions. All of the subjects completed a memory test 4 h after SWS-rich stage to evaluate the effect on fear extinction. Moreover, we conducted similar experiments using an independent group of subjects during the daytime to test whether the memory extinction effect was specific to the sleep condition.

PARTICIPANTS

Ninety-six healthy volunteers (44 males) aged 18-28 y.

MEASUREMENTS AND RESULTS

Participants exhibited undisturbed sleep during 2 consecutive nights, as assessed by sleep variables (all P > 0.05) from polysomnographic recordings and power spectral analysis. Participants who were re-exposed to the 10 min CS either during SWS and wakefulness exhibited attenuated fear responses (wake-10 min CS, P < 0.05; SWS-10 min CS, P < 0.01).

CONCLUSIONS

Conditioned stimulus re-exposure during SWS promoted fear memory extinction without altering sleep profiles.

PDE and cognitive processing: beyond the memory domain

Phosphodiesterase inhibitors (PDE-Is) enhance cAMP and/or cGMP signaling via reducing the degradation of these cyclic nucleotides. Both cAMP and cGMP signaling are essential for a variety of cellular functions and exert their effects both pre- and post-synaptically. Either of these second messengers relays and amplifies incoming signals at receptors on the cell surface making them important elements in signal transduction cascades and essential in cellular signaling in a variety of cell functions including neurotransmitter release and neuroprotection. Consequently, these processes can be influenced by PDE-Is as they increase cAMP and/or cGMP concentrations. PDE-Is have been considered as possible therapeutic agents to treat impaired memory function linked to several brain disorders, including depression, schizophrenia and Alzheimer's disease (AD). This review will, however, focus on the possible role of phosphodiesterases (PDEs) in cognitive decline beyond the memory domain. Here we will discuss the involvement of PDEs on three related domains: attention, information filtering (sensory- and sensorimotor gating) and response inhibition (drug-induced hyperlocomotion). Currently, these are emerging cognitive domains in the field of PDE research. Here we discuss experimental studies and the potential beneficial effects of PDE-I drugs on these cognitive domains, as effects of PDE-Is on these domains could potentially influence effects on memory performance. Overall, PDE4 seems to be the most promising target for all domains discussed in this review.

Estradiol and cognitive function: past, present and future

A historical perspective on estradiol's enhancement of cognitive function is presented, and research, primarily in animals, but also in humans, is reviewed. Data regarding the mechanisms underlying the enhancements are discussed. Newer studies showing rapid effects of estradiol on consolidation of memory through membrane interactions and activation of inter-cellular signaling pathways are reviewed as well as studies focused on traditional genomic mechanisms. Recent demonstrations of intra-neuronal estradiol synthesis and possible actions as a neurosteroid to promote memory are discussed. This information is applied to the critical issue of the current lack of effective hormonal (or other) treatments for cognitive decline associated with menopause and aging. Finally, the critical period hypothesis for estradiol effects is discussed along with novel strategies for hormone/drug development. Overall, the historical record documents that estradiol positively impacts some aspects of cognitive function, but effective therapeutic interventions using this hormone have yet to be realized.

Regulation of object recognition and object placement by ovarian sex steroid hormones

The ovarian hormones 17β-estradiol (E2) and progesterone (P4) are potent modulators of hippocampal memory formation. Both hormones have been demonstrated to enhance hippocampal memory by regulating the cellular and molecular mechanisms thought to underlie memory formation. Behavioral neuroendocrinologists have increasingly used the object recognition and object placement (object location) tasks to investigate the role of E2 and P4 in regulating hippocampal memory formation in rodents. These one-trial learning tasks are ideal for studying acute effects of hormone treatments on different phases of memory because they can be administered during acquisition (pre-training), consolidation (post-training), or retrieval (pre-testing). This review synthesizes the rodent literature testing the effects of E2 and P4 on object recognition (OR) and object placement (OP), and the molecular mechanisms in the hippocampus supporting memory formation in these tasks. Some general trends emerge from the data. Among gonadally intact females, object memory tends to be best when E2 and P4 levels are elevated during the estrous cycle, pregnancy, and in middle age. In ovariectomized females, E2 given before or immediately after testing generally enhances OR and OP in young and middle-aged rats and mice, although effects are mixed in aged rodents. Effects of E2 treatment on OR and OP memory consolidation can be mediated by both classical estrogen receptors (ERα and ERβ), and depend on glutamate receptors (NMDA, mGluR1) and activation of numerous cell signaling cascades (e.g., ERK, PI3K/Akt, mTOR) and epigenetic processes (e.g., histone acetylation, DNA methylation). Acute P4 treatment given immediately after training also enhances OR and OP in young and middle-aged ovariectomized females by activating similar cell signaling pathways as E2 (e.g., ERK, mTOR). The few studies that have administered both hormones in combination suggest that treatment can enhance OR and OP, but that effects are highly dependent on factors such as dose and timing of administration. In addition to providing more detail on these general conclusions, this review will discuss directions for future avenues of research into the hormonal regulation of object memory.

Recognition memory tasks in neuroendocrine research

The recognition memory tasks, novel object and novel object location, have been beneficial to neuroendocrine research concerning the effects of gonadal and adrenal hormones on cognitive function. This review discusses the advantages of these tasks in comparison with other learning and memory tasks. Experiments conducted across a number of laboratories show that gonadal hormones, both estradiol and testosterone, promote memory while the adrenal hormone, corticosterone, impairs memory. The effects of these steroid hormones on spine density in the prefrontal cortex and hippocampus are also briefly presented. Overall, results show that these steroid hormones are potent modulators of memory consolidation in rodent models.

Either the dorsal hippocampus or the dorsolateral striatum is selectively involved in consolidation of forced swim-induced immobility depending on genetic background

Healthy subjects differ in the memory system they engage to learn dual-solution tasks. Both genotype and stress experience could contribute to this phenotypic variability. The present experiments tested whether the hippocampus and the dorsal striatum, the core nodes of two different memory systems, are differently involved in 24 h retention of a stress-associated memory in two genetically unrelated inbred strains of mice. Mice from both the C57BL/6J and the DBA/2J inbred strains showed progressive increase of immobility during 10 min exposure to forced swim (FS) and retrieved the acquired levels of immobility when tested 24h later. The pattern of c-fos immunostaining promoted by FS revealed activation of a large number of brain areas in both strains, including CA1 and CA3 fields of the hippocampus. However, only DBA/2J mice showed activation of the dorsolateral striatum (DLS). In addition, FS induced a positive correlation between c-fos expression in the amygdala and CA1 and CA3 in C57BL/6J mice whereas it induced a positive correlation between c-fos expression in the amygdala and DLS in DBA/2J mice. Finally, temporary post-training inactivation of the dorsal hippocampus, by local infusion of lidocaine, prevented 24h retention of immobility in C57BL/6J mice only, whereas inactivation of the DLS prevented retention in DBA/2J mice only. These findings support the view that genetic factors can determine whether the dorsal hippocampus or the DLS are selectively engaged to consolidate stress-related memory.

The role of guanfacine as a therapeutic agent to address stress-related pathophysiology in cocaine-dependent individuals

The pathophysiology of cocaine addiction is linked to changes within neural systems and brain regions that are critical mediators of stress system sensitivity and behavioral processes associated with the regulation of adaptive goal-directed behavior. This is characterized by the upregulation of core adrenergic and corticotropin-releasing factor mechanisms that subserve negative affect and anxiety and impinge upon intracellular pathways in the prefrontal cortex underlying cognitive regulation of stress and negative emotional state. Not only are these mechanisms essential to the severity of cocaine withdrawal symptoms, and hence the trajectory of clinical outcome, but also they may be particularly pertinent to the demography of cocaine dependence. The ability of guanfacine to target overlapping stress, reward, and anxiety pathophysiology suggests that it may be a useful agent for attenuating the stress- and cue-induced craving state not only in women but also in men. This is supported by recent research findings from our own laboratory. Additionally, the ability of guanfacine to improve regulatory mechanisms that are key to exerting cognitive and emotional control over drug-seeking behavior also suggests that guanfacine may be an effective medication for reducing craving and relapse vulnerability in many drugs of abuse. As cocaine-dependent individuals are typically polydrug abusers and women may be at a greater disadvantage for compulsive drug use than men, it is plausible that medications that target catecholaminergic frontostriatal inhibitory circuits and simultaneously reduce stress system arousal may provide added benefits for attenuating cocaine dependence.

Effect of post-training administration of cocaine, diazepam and their combination on a win-stay task

According to the memory-enhancing hypothesis of addictive drugs, post-training administration of cocaine should enhance consolidation and thus facilitate learning. This hypothesis has not been tested in appetitive tasks reinforced by sucrose. The current study assessed the effect of post-training cocaine administration on the acquisition of a win-stay task, and modulation of this effect by co-administration of diazepam. Male Sprague-Dawley rats (n=63) were trained for 5 days on a win-stay task performed on an 8-arm radial maze, and were administered cocaine (0, 2.5, 7.5 or 20mg/kg), diazepam (1mg/kg), or cocaine (7.5mg/kg)+diazepam (1mg/kg) immediately following each training session. Post-training cocaine caused dose-dependent impairments that appeared linked to the development of cocaine-induced sucrose taste avoidance and/or cocaine-induced anxiety. When it was attempted to modify these learned side effects of cocaine by co-administration of diazepam, it was observed that the drug combination slowed task completion and reduced overall number of nose pokes. These findings suggest that post-training cocaine can alter behavior on appetitive tasks through learned motivational deficits rather than through a selective action on memory consolidation. The implications for the memory-enhancing hypothesis of addictive drugs are discussed.

Gastrin-releasing peptide receptor signaling in the integration of stress and memory

Neuropeptides act as signaling molecules that regulate a range of aspects of brain function. Gastrin-releasing peptide (GRP) is a 27-amino acid mammalian neuropeptide, homolog of the amphibian peptide bombesin. GRP acts by binding to the GRP receptor (GRPR, also called BB2), a member of the G-protein coupled receptor (GPCR) superfamily. GRP produced by neurons in the central nervous system (CNS) plays a role in synaptic transmission by activating GRPRs located on postsynaptic membranes, influencing several aspects of brain function. Here we review the role of GRP/GRPR as a system mediating both stress responses and the formation and expression of memories for fearful events. GRPR signaling might integrate the processing of stress and fear with synaptic plasticity and memory, serving as an important component of the set of neurobiological systems underlying the enhancement of memory storage by aversive information.

Neurokinin3 receptor as a target to predict and improve learning and memory in the aged organism

Impaired learning and memory performance is often found in aging as an early sign of dementia. It is associated with neuronal loss and reduced functioning of cholinergic networks. Here we present evidence that the neurokinin3 receptors (NK3-R) and their influence on acetylcholine (ACh) release may represent a crucial mechanism that underlies age-related deficits in learning and memory. Repeated pharmacological stimulation of NK3-R in aged rats was found to improve learning in the water maze and in object-place recognition. This treatment also enhanced in vivo acetylcholinergic activity in the frontal cortex, hippocampus, and amygdala but reduced NK3-R mRNA expression in the hippocampus. Furthermore, NK3-R agonism incurred a significantly higher increase in ACh levels in aged animals that showed superior learning than in those that were most deficient in learning. Our findings suggest that the induced activation of ACh, rather than basal ACh activity, is associated with superior learning in the aged. To test whether natural variation in NK3-R function also determines learning and memory performance in aged humans, we investigated 209 elderly patients with cognitive impairments. We found that of the 15 analyzed single single-nucleotide ploymorphism (SNPs) of the NK3-R-coding gene, TACR3, the rs2765 SNP predicted the degree of impairment of learning and memory in these patients. This relationship could be partially explained by a reduced right hippocampus volume in a subsample of 111 tested dementia patients. These data indicate the NK3-R as an important target to predict and improve learning and memory performance in the aged organism.

Short-term social isolation induces depressive-like behaviour and reinstates the retrieval of an aversive task: mood-congruent memory in male mice?

BACKGROUND

Although mood-congruent memory (MCM), or the tendency to recall information consistent with one's mood, is a robust phenomenon in human depression, to our knowledge, it has never been demonstrated in animals.

METHODS

Mice were subjected to social isolation (SI) or crowding for 12 hours and had their depressive-like behaviour (evaluated by the forced swim, tail suspension, sucrose preference and splash tests) or their serum corticosterone concentrations evaluated. In addition, we determined the temporal forgetting curve of the plus-maze discriminative avoidance task (PM-DAT) and examined the effects of SI or crowding on memory retrieval in the PM-DAT. Finally, we verified the effects of metyrapone pretreatment on reinstatement of memory retrieval or on the increase of corticosterone levels induced by SI.

RESULTS

Twelve hours of SI produced depressive-like behaviour, enhanced corticosterone concentration and reinstated retrieval of a forgotten discriminative aversive (i.e., negatively valenced) task. Depressive-like behaviour was critical for this facilitative effect of SI because 12 hours of crowding neither induced depressive-like behaviour nor enhanced retrieval, although it increased corticosterone levels at the same magnitude as SI. However, corticosterone increase was a necessary condition for MCM in mice, in that the corticosterone synthesis inhibitor metyrapone abolished SI-induced retrieval reinstatement.

LIMITATIONS

Our study did not investigate the effects of the social manipulations proposed here in a positively valenced task.

CONCLUSION

To our knowledge, the present paper provides the first evidence of MCM in animal models.

Epinephrine and glucose modulate training-related CREB phosphorylation in old rats: relationships to age-related memory impairments

Epinephrine enhances memory in young adult rats, in part, by increasing blood glucose levels needed to modulate memory. In old rats, epinephrine is deficient at raising blood glucose levels and thus is only moderately effective at enhancing memory. In contrast, systemic glucose injections improve memory in old rats, with resulting memory performance equal to that of young rats. The diminished response of glucose to training in old rats may blunt downstream neurochemical and molecular mechanisms needed to upregulate memory processes. In the first experiment, young adult and old rats were trained on an inhibitory avoidance task with immediate post-training injections of aCSF or glucose into the dorsal hippocampus. Old rats had significant memory impairments compared to young rats 7 days after training. Intrahippocampal injections of glucose reversed age-related deficits, improving memory scores in old rats to values seen in young rats. A second experiment examined age-related changes in activation of the transcription factor CREB, which is widely implicated in memory formation and may act downstream of hormonal and metabolic signals. Activation was assessed in response to training with systemic injections of epinephrine and glucose at doses known to enhance memory. Young adult and old rats were trained on inhibitory avoidance with immediate post-training systemic injections of saline, epinephrine, or glucose. After training, old rats had significant impairments in CREB phosphorylation in area CA1 and the dentate gyrus region of the hippocampus, and in the basolateral and lateral amygdala. Epinephrine and glucose attenuated age-related deficits in CREB phosphorylation, but were more effective in the amygdala and hippocampus, respectively. Together, these results support the view that age-related changes in blood glucose responses to epinephrine contribute to memory impairments, which may be related to alterations in regional patterns of CREB phosphorylation.

Estrogens facilitate memory processing through membrane mediated mechanisms and alterations in spine density

Estrogens exert sustained, genomically mediated effects on memory throughout the female life cycle, but here we review new studies documenting rapid effects of estradiol on memory, which are exerted through membrane-mediated mechanisms. Use of recognition memory tasks in rats shows that estrogens enhance memory consolidation within 1h. 17α-Estradiol is more potent than 17β-estradiol, and the dose response relationship between estrogens and memory is an inverted U shape. Use of specific estrogen receptor (ER) agonists suggests mediation by an ERβ-like membrane receptor. Enhanced memory is associated with increased spine density and altered noradrenergic activity in the medial prefrontal cortex and hippocampus within 30 min of administration. The environmental chemical, bisphenol-A, rapidly antagonizes enhancements in memory in both sexes possibly through actions on spines. Thus, estradiol and related compounds exert rapid alterations in cognition through non-genomic mechanisms, a finding which may provide a basis for better understanding and treating memory impairments.

The effects of acute administration of the hydroalcoholic extract of rosemary (Rosmarinus officinalis L.) (Lamiaceae) in animal models of memory

Rosmarinus officinalis (Rosemary) demonstrates antioxidant, antidepressant, diuretic, antinociceptive and antiulcerogenic activities. The present study was designed to examine the effects of the hydroalcoholic extract of R. officinalis on the memory of male mice. The behavioral tasks employed were social recognition (SR), the Morris water maze (MWM) and an inhibitory avoidance task (IA). The treatment with 150 and 300 mg/kg of R. officinalis improved the acquisition phase of learning of a new social memory in the SR task because a decrease was observed in the duration of social investigation. In the Morris water maze, no significant effect was observed on spatial memory when the groups were compared for the time spent in the correct quadrant. In the inhibitory avoidance task, the decrease in the step-down latencies in the test session indicate that 150 mg/kg of R. officinalis improved long-term memory when administered in the consolidation phase of learning. In conclusion, the present study showed that, the hydroalcoholic extract of R. officinalis at 150 and 300 mg/kg modulated the short- and long-term memories of mice, in a social recognition and inhibitory avoidance task, respectively. This modulator effect was shown to improve learning and memory processes.

Cholinergic influence on memory stages: A study on scopolamine amnesic mice

Objectives:

The study was planned to determine cholinergic influence on different stages of memory - acquisition, consolidation and recall in scopolamine-induced amnesia (memory impairment) in mice.

Materials and Methods:

To study acquision, consolidation and recall stages of memory, we administered scopolamine (0.75, 1.5 and 3 mg/kg ip) 30 minutes and five minutes prior to first trial acquisition and consolidation and 30 minutes prior to second trial recall of passive avoidance (PA) test, respectively, in separate groups. Tacrine (5 mg/kg po) and rivastigmine (5 mg/kg po) were administered one hour prior to first trial in separate groups which received scopolamine (3 mg/kg ip) 30 minutes and five minutes prior to first trial where as the control group received vehicle only.

Results:

In the control group, there was a significant (P < 0.01) increase in transfer latency time (TLT) in the second trial compared to first indicating successful learning. In scopolamine treated groups, administering scopolamine 30 minutes or five minutes prior to first trial did not show any significant (P > 0.05) change in TLT whereas mice treated with scopolamine 30 minutes prior to second trial showed significant (P < 0.01) increase in TLT in second trial as compared to the first. Both tacrine and rivastigmine administration in scopolamine treated mice showed significant (P < 0.05-0.01) increase in TLT in second trial as compared to first trial while the rivastigmine treated group showed greater percentage retention compared to tacrine treated group.

Conclusion:

Results show that acquisition and consolidation are more susceptible to the scopolamine effects than recall. Thus, it may be concluded that cholinergic influence is more on acquisition and consolidation as compared to recall.

A Valepotriate Fraction of Valeriana glechomifolia Shows Sedative and Anxiolytic Properties and Impairs Recognition But Not Aversive Memory in Mice

Plants of the genus Valeriana (Valerianaceae) are used in traditional medicine as a mild sedative, antispasmodic and tranquilizer in many countries. This study was undertaken to explore the neurobehavioral effects of systemic administration of a valepotriate extract fraction of known quantitative composition of Valeriana glechomifolia (endemic of southern Brazil) in mice. Adult animals were treated with a single intraperitoneal injection of valepotriate fraction (VF) in the concentrations of 1, 3 or 10 mg kg⁻¹, or with vehicle in the pre-training period before each behavioral test. During the exploration of an open field, mice treated with 10 mg kg⁻¹ of VF showed reduced locomotion and exploratory behavior. Although overall habituation sessions for locomotion and exploratory behavior among vehicle control and doses of VF were not affected, comparison between open-field and habituation sessions within each treatment showed that VF administration at 1 and 10 mg kg⁻¹ impaired habituation. In the elevated plus-maze test, mice treated with VF (10 mg kg⁻¹) showed a significant increase in the percentage of time spent in the open arms without significant effects in the number of total arm entries. VF at 3 mg kg⁻¹ produced an impairment of novel-object recognition memory. In contrast, VF did not affect fear-related memory assessed in an inhibitory avoidance task. The results indicate that VF can have sedative effects and affect behavioral parameters related to recognition memory.

Cognitive enhancers: focus on modulatory signaling influencing memory consolidation

Biological research has unraveled many of the molecular and cellular mechanisms involved in the formation of long-lasting memory, providing new opportunities for the development of cognitive-enhancing drugs. Studies of drug enhancement of cognition have benefited from the use of pharmacological treatments given after learning, allowing the investigation of mechanisms regulating the consolidation phase of memory. Modulatory systems influencing consolidation processes include stress hormones and several neurotransmitter and neuropeptide systems. Here, we review some of the findings on memory enhancement by drug administration in animal models, and discuss their implications for the development of cognitive enhancers.

Lipopolysaccharide-induced experimental immune activation does not impair memory functions in humans

Systemic immune activation occurring together with release of peripheral cytokines can affect behavior and the functioning of the central nervous system (CNS). However, it remains unknown whether and to what extent cognitive functions like memory and attention are affected during transient immune activation. We employed a human endotoxemia model and standardized neuropsychological tests to assess the cognitive effects of an experimental inflammation in two groups of 12 healthy young men before and after intravenous injection of lipopolysaccharide (LPS, Escherichia coli, 0.4 ng/kg) or physiological saline. Endotoxin administration caused a profound transient physiological response with elevations in body temperature, number of circulating neutrophils, and increases in plasma cytokine levels [interleukin (IL)-6, IL-10, tumor necrosis factor (TNF)-α], and concentrations of norepinephrine, ACTH and cortisol. However, these changes in immune and neuroendocrine parameters were not associated with alterations of memory performance, selective attention or executive functions.

Inhibitory avoidance memory deficit induced by scopolamine: Interaction of cholinergic and glutamatergic systems in the ventral tegmental area

Interaction of cholinergic and glutamatergic inputs in the ventral tegmental area (VTA) influencing a learned behavior is a topic of great interest. In the present study the effect of intra-VTA administration of a nonselective muscarinic acetylcholine antagonist, scopolamine, and N-methyl-d-aspartate (NMDA) receptor agents by themselves as well as their interactions on consolidation and retrieval of inhibitory avoidance (IA) memory have been investigated. A step-through inhibitory avoidance task was used for memory assessment in male Wistar rats. The results showed that intra-VTA administration of scopolamine (1 and 2microg/rat) and NMDA receptor antagonist, MK-801 (0.75 and 1microg/rat) immediately after training, impaired consolidation of IA memory. Interestingly, co-administration of an ineffective dose of MK-801 (0.5microg/rat) with ineffective doses of scopolamine (0.25 and 0.5microg/rat) significantly decreased the consolidation process. Post-training intra-VTA injections of NMDA (0.001 and 0.01microg/rat) had no effects by itself, whereas its co-administration with scopolamine (2microg/rat) prevented the effect of the later drug. The results also showed that pre-test intra-VTA administration of scopolamine (3 and 4microg/rat) and MK-801 (1 and 2microg/rat) impaired retrieval of the IA memory. Moreover, co-administration of an ineffective dose of MK-801 (0.5microg/rat) with ineffective doses of scopolamine (1 and 2microg/rat) increasingly reduced the retrieval of the IA memory. On the contrary to its post-training treatment, pre-test administration of NMDA either alone or in combination with scopolamine caused no significant effect on retrieval of IA memory. It can be concluded that muscarinic acetylcholine and NMDA glutamate receptors in the VTA are involved in the mechanism(s) underlying consolidation and retrieval of the IA memory.

Autoradiographic study of serotonin transporter during memory formation

Serotonin transporter (SERT) has been associated with drugs of abuse like d-methamphetamine (METH). METH is well known to produce effects on the monoamine systems but it is unclear how METH affects SERT and memory. Here the effects of METH and the serotonin reuptake inhibitor fluoxetine (FLX) on autoshaping and novel object recognition (NOR) were investigated. Notably, both memory tasks recruit different behavioral, neural and cognitive demand. In autoshaping task a dose-response curve for METH was determined. METH (1.0mg/kg) impaired short-term memory (STM; lasting less of 90min) in NOR and impaired both STM and long-term memory (LTM; lasting 24 and 48h) in autoshaping, indicating that METH had long-lasting effects in the latter task. A comparative autoradiography study of the relationship between the binding pattern of SERT in autoshaping new untrained vs. trained treated (METH, FLX, or both) animals was made. Considering that hemispheric dominance is important for LTM, hence right vs. left hemisphere of the brain was compared. Results showed that trained animals decreased cortical SERT binding relative to untrained ones. In untrained and trained treated animals with the amnesic dose (1.0mg/kg) of METH SERT binding in several areas including hippocampus and cortex decreased, more remarkably in the trained animals. In contrast, FLX improved memory, increased SERT binding, prevented the METH amnesic effect and re-established the SERT binding. In general, memory and amnesia seemed to make SERT more vulnerable to drugs effects.

Low dose dexamethasone reverses depressive-like parameters and memory impairment in rats submitted to sepsis

Sepsis is characterized by a systemic inflammatory response of the immune system against an infection, presenting with hypothalamic-pituitary-adrenal (HPA) axis dysfunction, behavior alterations, and high mortality. In this study, we aimed to evaluate the effects of dexamethasone on mortality, anhedonia, circulating corticosterone and adrenocorticotropin hormone (ACTH) levels, body and adrenal gland weight, and aversive memory in sepsis survivor rats. Male Wistar rats underwent sham operation or cecal ligation and perforation (CLP) procedure. Rats subjected to CLP were treated with "basic support" and dexamethasone (at 0.2 and 2mg/kg daily for 7 days after CLP, intraperitonially) or saline. After 10 days of sepsis procedure, it was evaluated aversive memory, sweet food consumption, and body and adrenal gland weight. Serum and plasma were also obtained. It was observed that low dose dexamethasone reverted anhedonia, normalized adrenal gland and body weight, corticosterone and ACTH levels, and decreased mortality and avoidance memory impairment, demonstrating that low doses of dexamethasone for moderate periods may be beneficial for sepsis treatment and its sequelae-depressive-like parameters and memory impairment.

Enhancing effect of heroin on social recognition learning in male Sprague-Dawley rats: modulation by heroin pre-exposure

RATIONALE

There is evidence that pre-exposure to drugs of abuse can induce sensitization to several of their effects.

OBJECTIVE

Four experiments were conducted to investigate the effect of heroin pre-exposure on modulation of memory consolidation as indexed by heroin's action on rate of learning.

MATERIALS AND METHODS

Male Sprague-Dawley rats were tested on a social recognition learning task which assesses changes in investigation during repeated exposure to the same rat (habituation training: four sessions) and during exposure to a novel rat (dishabituation test). In the first experiment, rats received 0, 0.3, or 1 mg/kg heroin s.c. immediately following each training session, or 1 mg/kg heroin 2 h post-training. In experiments 2 and 3, rats received 1 mg/kg heroin post-training after a 7-day drug-free period from heroin pre-exposure achieved through conditioned place preference (1 mg/kg s.c., 1 injection/day x 4 days) or intravenous self-administration (0.05 mg/kg/infusion i.v., 3 h/day x 9 days) training. In experiment 4, rats received 0, 0.03, 0.3, or 3 mg/kg heroin post-training after a 7-day drug-free period from a regimen of heroin administration (i.e., 1 mg/kg heroin/day s.c. x 7 days) that induced locomotor sensitization.

RESULTS

Post-training administration of heroin enhanced social recognition learning in a dose- and time-dependent manner. Importantly, no regimen of heroin pre-exposure significantly altered this effect of heroin.

CONCLUSIONS

These results do not support the hypothesis that heroin pre-exposure leads to sensitization to its effect on memory consolidation of non-drug-related learning. However, this requires further testing using alternative heroin pre-exposure regimens, a wider range of post-training heroin doses, as well as other types of learning tasks.

Modulation of long-term memory for object recognition via HDAC inhibition

Histone acetylation is a chromatin modification critically involved in gene regulation during many neural processes. The enzymes that regulate levels of histone acetylation are histone acetyltransferases (HATs), which activate gene expression and histone deacetylases (HDACs), that repress gene expression. Acetylation together with other histone and DNA modifications regulate transcription profiles for specific cellular functions. Our previous research has demonstrated a pivotal role for cyclicAMP response element binding protein (CREB)-binding protein (CBP), a histone acetyltransferase, in long-term memory for novel object recognition (NOR). In fact, every genetically modifiedCbp mutant mouse characterized thus far exhibits impaired long-term memory for NOR. These results suggest that long-term memory for NOR is especially sensitive to alterations in CBP activity. Thus, in the current study, we examined the role of HDACs in memory for NOR. We found that inducing a histone hyperacetylated state via HDAC inhibition transforms a learning event that would not normally result in long-term memory into an event that is now remembered long-term. We have also found that HDAC inhibition generates a type of long-term memory that persists beyond a point at which normal memory for NOR fails. This result is particularly interesting because one alluring aspect of examining the role of chromatin modifications in modulating transcription required for long-term memory processes is that these modifications may provide potentially stable epigenetic markers in the service of activating and/or maintaining transcriptional processes.

Estrogens and age-related memory decline in rodents: what have we learned and where do we go from here?

The question of whether ovarian hormone therapy can prevent or reduce age-related memory decline in menopausal women has been the subject of much recent debate. Although numerous studies have demonstrated a beneficial effect of estrogen and/or progestin therapy for certain types of memory in menopausal women, recent clinical trials suggest that such therapy actually increases the risk of cognitive decline and dementia. Because rodent models have been frequently used to examine the effects of age and/or ovarian hormone deficiency on mnemonic function, rodent models of age-related hormone and memory decline may be useful in helping to resolve this issue. This review will focus on evidence suggesting that estradiol modulates memory, particularly hippocampal-dependent memory, in young and aging female rats and mice. Various factors affecting the mnemonic response to estradiol in aging females will be highlighted to illustrate the complications inherent to studies of estrogen therapy in aging females. Avenues for future development of estradiol-based therapies will also be discussed, and it is argued that an approach to drug development based on identifying the molecular mechanisms underlying estrogenic modulation of memory may lead to promising future treatments for reducing age-related mnemonic decline.

Drug enhancement of memory consolidation: historical perspective and neurobiological implications

INTRODUCTION

Studies of drug enhancement of cognition began with Lashley's (Psychobiology 1:141-170, 1917) report that strychnine administered before daily training trials enhanced rats' maze learning. Many subsequent studies confirmed that finding and found that stimulant drugs also enhance the learning of a wide range of tasks.

DISCUSSION

A central problem in interpreting such findings is that of distinguishing the drug effects on brain processes underlying memory formation from many other possible effects of the drugs on the behavior used to assess learning. The subsequent finding that comparable learning enhancement can be obtained by posttraining drug administration provided compelling evidence that drugs can enhance memory by acting on memory consolidation processes. Such evidence stimulated the investigation of endogenous regulation of memory consolidation by arousal-released adrenal stress hormones.

CONCLUSION

Considerable evidence now indicates that such hormones regulate memory consolidation via activation of the basolateral amygdala and subsequent influences on many efferent brain regions involved in processing recent experiences. The implications of these findings for the development of cognitive enhancing drugs are discussed.

Selective phosphodiesterase inhibitors: a promising target for cognition enhancement

RATIONALE

One of the major complaints most people face during aging is an impairment in cognitive functioning. This has a negative impact on the quality of daily life and is even more prominent in patients suffering from neurodegenerative and psychiatric disorders including Alzheimer's disease, schizophrenia, and depression. So far, the majority of cognition enhancers are generally targeting one particular neurotransmitter system. However, recently phosphodiesterases (PDEs) have gained increased attention as a potential new target for cognition enhancement. Inhibition of PDEs increases the intracellular availability of the second messengers cGMP and/or cAMP.

OBJECTIVE

The aim of this review was to provide an overview of the effects of phosphodiesterase inhibitors (PDE-Is) on cognition, the possible underlying mechanisms, and the relationship to current theories about memory formation.

MATERIALS AND METHODS

Studies of the effects of inhibitors of different PDE families (2, 4, 5, 9, and 10) on cognition were reviewed. In addition, studies related to PDE-Is and blood flow, emotional arousal, and long-term potentiation (LTP) were described.

RESULTS

PDE-Is have a positive effect on several aspects of cognition, including information processing, attention, memory, and executive functioning. At present, these data are likely to be explained in terms of an LTP-related mechanism of action.

CONCLUSION

PDE-Is are a promising target for cognition enhancement; the most suitable candidates appear to be PDE2-Is or PDE9-Is. The future for PDE-Is as cognition enhancers lies in the development of isoform-specific PDE-Is that have limited aversive side effects.

Memory-enhancing treatments reverse the impairment of inhibitory avoidance retention in sepsis-surviving rats

Introduction

Survivors from sepsis have presented with long-term cognitive impairment, including alterations in memory, attention, concentration, and global loss of cognitive function. Thus, we evaluated the effects of memory enhancers in sepsis-surviving rats.

Methods

The rats underwent cecal ligation and perforation (CLP) (sepsis group) with 'basic support' (saline at 50 mL/kg immediately and 12 hours after CLP plus ceftriaxone at 30 mg/kg and clindamycin at 25 mg/kg 6, 12, and 18 hours after CLP) or sham-operated (control group). After 10 or 30 days, rats were submitted to an inhibitory avoidance task. After task training, animals received injections of saline, epinephrine, naloxone, dexamethasone, or glucose. Twenty-four hours afterwards, animals were submitted to the inhibitory avoidance test.

Results

We demonstrated that memory enhancers reversed impairment in the sepsis group 10 and 30 days after sepsis induction. This effect was of lower magnitude when compared with sham animals 10 days, but not 30 days, after sepsis.

Conclusions

Using different pharmacologic approaches, we conclude that the adrenergic memory formation pathways are responsive in sepsis-surviving animals.