Effects of stress and hippocampal NMDA receptor antagonism on recognition memory in rats

17β Estradiol increases resilience and improves hippocampal synaptic function in helpless ovariectomized rats

Memory impairment is the most commonly reported cognitive symptom associated with major depressive disorder. Decreased hippocampal volume and neurogenesis in depression link hippocampal dysfunction with deficits in memory. Stress decreases hippocampal dendritic spine density and long-term potentiation (LTP) at glutamate synapses, a cellular correlate of learning and memory. However, elevated plasma levels of 17β estradiol (E2) during proestrus increase hippocampal structure and function, directly opposing the negative consequences of stress. In women, significant fluctuations in ovarian hormones likely increase vulnerability of hippocampal circuits to stress, potentially contributing to the greater incidence of depression compared to men. Using the learned helplessness model of depression and ovariectomized female rats, we investigated whether acquisition of helplessness and hippocampal synaptic dysfunction is differentially impacted by the presence or absence of plasma E2. We find that inescapable shock induces a greater incidence of helplessness in vehicle- versus E2-treated OVX rats. In the vehicle-treated group, LTP was absent at CA3-CA1 synapses in slices only from helpless rats, and CA1 spine density was decreased compared to resilient rats. In contrast, significant LTP was observed in slices from E2-treated helpless rats; importantly, spine density was not different between E2-treated helpless and resilient rats, dissociating spine density from the LTP magnitude. We also find that E2 replacement can reverse previously established helpless behavior. Thus, our results show that E2 replacement in OVX rats increases resilience and improves hippocampal plasticity, suggesting that E2 therapy may increase resilience to stress and preserve hippocampal function in women experiencing large fluctuations in plasma estrogen levels.

Effects of chronic fluoride exposure on object recognition memory and mRNA expression of SNARE complex in hippocampus of male mice

This study aimed to investigate the effects of long-term fluoride exposure on object recognition memory and mRNA expression of soluble N-ethylmaleimidesensitive fusion protein attachment protein receptors (SNARE) complex (synaptosome-associated protein of 25 kDa (SNAP-25), vesicle-associated membrane protein 2 (VAMP-2), and syntaxin 1A) in the hippocampus of male mice. Sixty sexually matured male Kunming mice were randomly divided into four groups: control group (given distilled water), low F group (25 mg/L NaF, corresponding to 11 mg/L F(-)), medium F group (50 mg/L NaF, corresponding to 22 mg/L F(-)), and high F group (100 mg/L NaF, corresponding to 45 mg/L F(-)). After 180 days, the spontaneous locomotor behavior and object recognition memory were detected by open field test and novel object recognition (NOR) test. Results showed that compared with the control group, frequency in each zone, total distance, and line crosses were significantly increased in low F and medium F groups, suggesting fluoride enhanced excitement of mice, while there were no marked changes in high F group. Twenty-four hours after training, a deficit of long-term memory (LTM) occurred only in high F group (P < 0.05), but there was no significant change of short-term memory (STM) 1.5 h later. The mRNA expression levels of SNAP-25, VAMP-2, and syntaxin 1A were detected by real-time quantitative RT-PCR, which revealed that the mRNA expression of VAMP-2 was significantly increased in medium F and high F groups (P < 0.01). Taken together, these results indicated that long-term fluoride administration can enhance the excitement of male mice, impair recognition memory, and upregulate VAMP-2 mRNA expression, which are involved in the adverse effects of fluoride on the object recognition memory of nervous system.

Glucocorticoid actions on synapses, circuits, and behavior: implications for the energetics of stress

Environmental stimuli that signal real or potential threats to homeostasis lead to glucocorticoid secretion by the hypothalamic-pituitary-adrenocortical (HPA) axis. Glucocorticoids promote energy redistribution and are critical for survival and adaptation. This adaptation requires the integration of multiple systems and engages key limbic-neuroendocrine circuits. Consequently, glucocorticoids have profound effects on synaptic physiology, circuit regulation of stress responsiveness, and, ultimately, behavior. While glucocorticoids initiate adaptive processes that generate energy for coping, prolonged or inappropriate glucocorticoid secretion becomes deleterious. Inappropriate processing of stressful information may lead to energetic drive that does not match environmental demand, resulting in risk factors for pathology. Thus, dysregulation of the HPA axis may promote stress-related illnesses (e.g. depression, PTSD). This review summarizes the latest developments in central glucocorticoid actions on synaptic, neuroendocrine, and behavioral regulation. Additionally, these findings will be discussed in terms of the energetic integration of stress and the importance of context-specific regulation of glucocorticoids.

Acute tryptophan depletion reduces nitric oxide synthase in the rat hippocampus

Acute tryptophan depletion (ATD) is extensively used to investigate the role of central serotonin (5-HT). However, several studies reported that ATD had no significant effect on central 5-HT concentration and some ATD-induced changes was independent of 5-HT in the rodent brain. Therefore, the potential mechanism of ATD might not be ascribed solely to changes in the central 5-HT system. In recent studies, evidence suggests that nitric oxide synthase (NOS) is closely associated with ATD-induced changes in modulation of cerebral blood flow and metabolism, cognitive, and locomotor activity. Thus, NOS is implicated to be an underlying factor contributing to ATD-induced changes. In the present study, the effect of ATD upon central NOS levels in the rat was evaluated. Male Sprague-Dawley (SD) rats were orally administered a tryptophan-free protein-carbohydrate mixture. Then, ATD effects upon affective behavior and spatial memory were assessed by the forced swimming test (FST) and Morris water maze test, respectively. Further, NOS activity and neuronal NOS (nNOS) protein levels in the hippocampus were measured after ATD. Our experimental results showed that ATD had no influence on affective behavior in the FST or spatial memory in SD rats. Interestingly, a significant reduction of both constitutive NOS activity and nNOS protein levels after ATD was found in the hippocampus. These findings demonstrate ATD does not influence affective behavior and spatial memory despite a direct effect on hippocampal NOS. Our study might provide a valuable clue for exploring earlier reported ATD-induced behavioral and neurochemical changes in rodents.

A role for the endocannabinoid system in exercise-induced spatial memory enhancement in mice

It is well known that physical exercise has positive effects on cognitive functions and hippocampal plasticity. However, the underlying mechanisms have remained to be further investigated. Here we investigated the hypothesis that the memory-enhancement promoted by physical exercise relies on facilitation of the endocannabinoid system. We observed that the spatial memory tested in the object location paradigm did not persist in sedentary mice, but could be improved by 1 week of treadmill running. In addition, exercise up-regulated CB1 receptor and BDNF expression in the hippocampus. To verify if these changes required CB1 activation, we treated the mice with the selective antagonist, AM251, before each period of physical activity. In line with our hypothesis, this drug prevented the exercise-induced memory enhancement and BDNF expression. Furthermore, AM251 reduced CB1 expression. To test if facilitating the endocannabinoid system signaling would mimic the alterations observed after exercise, we treated sedentary animals during 1 week with the anandamide-hydrolysis inhibitor, URB597. Mice treated with this drug recognized the object in a new location and have increased levels of CB1 and BDNF expression in the hippocampus, showing that potentiating the endocanabinoid system equally benefits memory. In conclusion, the favorable effects of exercise upon spatial memory and BDNF expression depend on facilitation of CB1 receptor signaling, which can be mimic by inhibition of anandamide hydrolysis in sedentary animals. Our results suggest that, at least in part, the promnesic effect of the exercise is dependent of CB1 receptor activation and is mediated by BDNF.

The memory-enhancing effects of hippocampal estrogen receptor activation involve metabotropic glutamate receptor signaling

Our laboratory has demonstrated that 17β-estradiol (E2) enhances hippocampal memory consolidation via rapid activation of multiple intracellular signaling cascades, including the ERK/MAPK cascade (Fernandez et al., 2008; Fan et al., 2010). However, the receptor mechanisms responsible for these effects of E2 remain unclear. In vitro, estrogen receptor α (ERα) signaling through metabotropic glutamate receptor 1a (mGluR1a) leads to ERK-dependent CREB phosphorylation (Boulware et al., 2005), suggesting that interactions between ERs and mGluR1a may be vital to the memory-enhancing effects of E2. As such, the present study tested the roles of classical estrogen receptors (ERα and ERβ) and mGluR1a in mediating the effects of E2 on hippocampal memory consolidation. Dorsal hippocampal (DH) infusion of ERα (PPT) or ERβ (DPN) agonists enhanced novel object recognition and object placement memory in ovariectomized female mice in an ERK-dependent manner, suggesting that these receptors influence memory by rapidly activating hippocampal cell signaling. Next, DH infusion of the mGluR1a antagonist LY367385 blocked the object and spatial memory facilitation induced by E2, PPT, and DPN, demonstrating that ER/mGluR1a signaling is critical for the memory-enhancing effects of E2. Finally, we show that ERα, ERβ, mGluR1, and ERK all reside within specialized membrane microdomains of the DH, and that ERα and ERβ physically interact with mGluR1, providing a means through which ERs may activate mGluRs and downstream signaling. Together, these findings provide the first in vivo evidence demonstrating that ER/mGluR signaling can mediate the beneficial effects of E2 on hippocampal memory consolidation.

Object-in-place associative recognition memory depends on glutamate receptor neurotransmission within two defined hippocampal-cortical circuits: a critical role for AMPA and NMDA receptors in the hippocampus, perirhinal, and prefrontal cortices

Object-in-place associative recognition memory depends on an interaction between the hippocampus (HPC), perirhinal (PRH), and medial prefrontal (mPFC) cortices, yet the contribution of glutamate receptor neurotransmission to these interactions is unknown. NMDA receptors (NMDAR) in the HPC were critical for encoding of object-in-place memory but not for single-item object recognition. Next, a disconnection procedure was used to examine the importance of “concurrent” glutamate neurotransmission in the HPC-mPFC and HPC-PRH. Contralateral unilateral infusions of NBQX (AMPAR antagonist), into the HPC-mPFC, or HPC-PRH, either before acquisition or test, impaired object-in-place performance. Thus, both circuits are necessary for encoding and retrieval. Crossed unilateral AP5 (NMDAR antagonist) infusions into the HPC-mPFC or HPC-PRH impaired encoding, but not retrieval. Specifically crossed HPC-mPFC infusions impaired both short-term (5 min) and longer term (1 h) memory while HPC-PRH infusions impaired longer term memory only. This delay-dependent effect of AP5 in the HPC-PRH on object-in-place memory, accords with its effects in the PRH, on single item object recognition memory, thereby suggesting that a single PRH synaptic plasticity mechanism underpins different recognition memory processes. Further, blocking excitatory neurotransmission in any pair of structures within the networks impaired “both” encoding and retrieval, thus object-in-place memory clearly requires network interdependency across multiple structures.

Determinants of novel object and location recognition during development

In the novel object recognition (OR) paradigm, rats are placed in an arena where they encounter two sample objects during a familiarization phase. A few minutes later, they are returned to the same arena and are presented with a familiar object and a novel object. The object location recognition (OL) variant involves the same familiarization procedure but during testing one of the familiar objects is placed in a novel location. Normal adult rats are able to perform both the OR and OL tasks, as indicated by enhanced exploration of the novel vs. the familiar test item. Rats with hippocampal lesions perform the OR but not OL task indicating a role of spatial memory in OL. Recently, these tasks have been used to study the ontogeny of spatial memory but the literature has yielded conflicting results. The current experiments add to this literature by: (1) behaviorally characterizing these paradigms in postnatal day (PD) 21, 26 and 31-day-old rats; (2) examining the role of NMDA systems in OR vs. OL; and (3) investigating the effects of neonatal alcohol exposure on both tasks. Results indicate that normal-developing rats are able to perform OR and OL by PD21, with greater novelty exploration in the OR task at each age. Second, memory acquisition in the OL but not OR task requires NMDA receptor function in juvenile rats [corrected]. Lastly, neonatal alcohol exposure does not disrupt performance in either task. Implications for the ontogeny of incidental spatial learning and its disruption by developmental alcohol exposure are discussed.

Effects of acute restraint stress on set-shifting and reversal learning in male rats

Exposure to acute stress alters cognition; however, few studies have examined the effects of acute stress on executive functions such as behavioral flexibility. The goal of the present experiments was to determine the effects of acute periods of stress on two distinct forms of behavioral flexibility: set-shifting and reversal learning. Male Sprague-Dawley rats were trained and tested in an operant-chamber-based task. Some of the rats were exposed to acute restraint stress (30 min) immediately before either the set-shifting test day or the reversal learning test day. Acute stress had no effect on set-shifting, but it significantly facilitated reversal learning, as assessed by both trials to criterion and total errors. In a second experiment, the roles of glucocorticoid (GR) and mineralocorticoid receptors (MR) in the acute-stress-induced facilitation of reversal learning were examined. Systemic administration of the GR-selective antagonist RU38486 (10 mg/kg) or the MR-selective antagonist spironolactone (50 mg/kg) 30 min prior to acute stress failed to block the facilitation on reversal learning. The present results demonstrate a dissociable effect of acute stress on set-shifting and reversal learning and suggest that the facilitation of reversal learning by acute stress may be mediated by factors other than corticosterone.

Prenatal loud music and noise: differential impact on physiological arousal, hippocampal synaptogenesis and spatial behavior in one day-old chicks

Prenatal auditory stimulation in chicks with species-specific sound and music at 65 dB facilitates spatial orientation and learning and is associated with significant morphological and biochemical changes in the hippocampus and brainstem auditory nuclei. Increased noradrenaline level due to physiological arousal is suggested as a possible mediator for the observed beneficial effects following patterned and rhythmic sound exposure. However, studies regarding the effects of prenatal high decibel sound (110 dB; music and noise) exposure on the plasma noradrenaline level, synaptic protein expression in the hippocampus and spatial behavior of neonatal chicks remained unexplored. Here, we report that high decibel music stimulation moderately increases plasma noradrenaline level and positively modulates spatial orientation, learning and memory of one day-old chicks. In contrast, noise at the same sound pressure level results in excessive increase of plasma noradrenaline level and impairs the spatial behavior. Further, to assess the changes at the molecular level, we have quantified the expression of functional synapse markers: synaptophysin and PSD-95 in the hippocampus. Compared to the controls, both proteins show significantly increased expressions in the music stimulated group but decrease in expressions in the noise group. We propose that the differential increase of plasma noradrenaline level and altered expression of synaptic proteins in the hippocampus are responsible for the observed behavioral consequences following prenatal 110 dB music and noise stimulation.

Differential effectiveness of tianeptine, clonidine and amitriptyline in blocking traumatic memory expression, anxiety and hypertension in an animal model of PTSD

Individuals exposed to life-threatening trauma are at risk for developing post-traumatic stress disorder (PTSD), a debilitating condition that involves persistent anxiety, intrusive memories and several physiological disturbances. Current pharmacotherapies for PTSD manage only a subset of these symptoms and typically have adverse side effects which limit their overall effectiveness. We evaluated the effectiveness of three different pharmacological agents to ameliorate a broad range of PTSD-like symptoms in our established predator-based animal model of PTSD. Adult male Sprague-Dawley rats were given 1-h cat exposures on two occasions that were separated by 10 days, in conjunction with chronic social instability. Beginning 24 h after the first cat exposure, rats received daily injections of amitriptyline, clonidine, tianeptine or vehicle. Three weeks after the second cat exposure, all rats underwent a battery of behavioral and physiological tests. The vehicle-treated, psychosocially stressed rats demonstrated a robust fear memory for the two cat exposures, as well as increased anxiety expressed on the elevated plus maze, an exaggerated startle response, elevated heart rate and blood pressure, reduced growth rate and increased adrenal gland weight, relative to the vehicle-treated, non-stressed (control) rats. Neither amitriptyline nor clonidine was effective at blocking the entire cluster of stress-induced sequelae, and each agent produced adverse side effects in control subjects. Only the antidepressant tianeptine completely blocked the effects of psychosocial stress on all of the physiological and behavioral measures that were examined. These findings illustrate the differential effectiveness of these three treatments to block components of PTSD-like symptoms in rats, and in particular, reveal the profile of tianeptine as the most effective of all three agents.

Integrin α3 is required for late postnatal stability of dendrite arbors, dendritic spines and synapses, and mouse behavior

Most dendrite branches and a large fraction of dendritic spines in the adult rodent forebrain are stable for extended periods of time. Destabilization of these structures compromises brain function and is a major contributing factor to psychiatric and neurodegenerative diseases. Integrins are a class of transmembrane extracellular matrix receptors that function as αβ heterodimers and activate signaling cascades regulating the actin cytoskeleton. Here we identify integrin α3 as a key mediator of neuronal stability. Dendrites, dendritic spines, and synapses develop normally in mice with selective loss of integrin α3 in excitatory forebrain neurons, reaching their mature sizes and densities through postnatal day 21 (P21). However, by P42, integrin α3 mutant mice exhibit significant reductions in hippocampal dendrite arbor size and complexity, loss of dendritic spine and synapse densities, and impairments in hippocampal-dependent behavior. Furthermore, gene-dosage experiments demonstrate that integrin α3 interacts functionally with the Arg nonreceptor tyrosine kinase to activate p190RhoGAP, which inhibits RhoA GTPase and regulates hippocampal dendrite and synapse stability and mouse behavior. Together, our data support a fundamental role for integrin α3 in regulating dendrite arbor stability, synapse maintenance, and proper hippocampal function. In addition, these results provide a biochemical and structural explanation for the defects in long-term potentiation, learning, and memory reported previously in mice lacking integrin α3.

Investigations into the involvement of NMDA mechanisms in recognition memory

This review will focus on evidence showing that NMDA receptor neurotransmission is critical for synaptic plasticity processes within brain regions known to be necessary for the formation of object recognition memories. The aim will be to provide evidence concerning NMDA mechanisms related to recognition memory processes and show that recognition memory for objects, places or associations between objects and places depends on NMDA neurotransmission within the perirhinal cortex, temporal association cortex medial prefrontal cortex and hippocampus. Administration of the NMDA antagonist AP5, selectively into each of these brain regions has revealed that the extent of the involvement NMDA receptors appears dependent on the type of information required to solve the recognition memory task; thus NMDA receptors in the perirhinal cortex are crucial for the encoding of long-term recognition memory for objects, and object-in-place associations, but not for short-term recognition memory or for retrieval. In contrast the hippocampus and medial prefrontal cortex are required for both long-term and short-term recognition memory for places or associations between objects and places, or for recognition memory tasks that have a temporal component. Such studies have therefore confirmed that the multiple brain regions make distinct contributions to recognition memory but in addition that more than one synaptic plasticity process must be involved.

This article is part of the Special Issue entitled ‘Glutamate Receptor-Dependent Synaptic Plasticity’.

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NMDAR blockade in PRH, mPFC and HPC produces different patterns of memory deficits.

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NMDARs within these brain regions make distinct contributions to recognition memory.

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NMDARs are also critical for synaptic plasticity in the same brain regions.

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More than one synaptic plasticity process must be involved in recognition memory.

Current status on behavioral and biological markers of PTSD: a search for clarity in a conflicting literature

Extensive research has identified stereotypic behavioral and biological abnormalities in post-traumatic stress disorder (PTSD), such as heightened autonomic activity, an exaggerated startle response, reduced basal cortisol levels and cognitive impairments. We have reviewed primary research in this area, noting that factors involved in the susceptibility and expression of PTSD symptoms are more complex and heterogeneous than is commonly stated, with extensive findings which are inconsistent with the stereotypic behavioral and biological profile of the PTSD patient. A thorough assessment of the literature indicates that interactions among myriad susceptibility factors, including social support, early life stress, sex, age, peri- and post-traumatic dissociation, cognitive appraisal of trauma, neuroendocrine abnormalities and gene polymorphisms, in conjunction with the inconsistent expression of the disorder across studies, confounds attempts to characterize PTSD as a monolithic disorder. Overall, our assessment of the literature addresses the great challenge in developing a behavioral and biomarker-based diagnosis of PTSD.

Alcohol administration blocks stress-induced impairments in memory and anxiety, and alters hippocampal neurotransmitter receptor expression in male rats

Chronic exposure to stress has many deleterious effects on behavior, which can often lead to self-medication with anxiolytics, antidepressants, or alcohol. We determined the effects of alcohol administration following a stressor on established behavioral, physiological, and neural responses to stress. Male Sprague-Dawley rats received: No alcohol/No stress (CON), Alcohol alone (ALC), Stress alone (STR), or Stress plus Alcohol (STR+ALC). For seven consecutive days, two cohorts received an oral dose of 2.0 g/kg of either 20% ethanol or saline. In Cohort 1, behavioral testing began after the final treatment (day-8). Memory was tested using the object recognition (OR) and Y-maze, anxiety on the plus maze, and depression on the forced swim task. Memory on OR and Y-maze tasks was impaired in the ALC and STR groups. This deficit was reversed in the STR+ALC group, which performed not differently from the CON group. Stress alone was associated with increased anxiety, which was alleviated with alcohol treatment. No treatment effects were found in the forced swim task. In Cohort 2, hippocampal GABAα4 was upregulated in the STR+ALC group and GluN2B was upregulated in the ALC and STR+ALC groups. The STR+ALC group in Cohort 1 showed enhanced corticosterone levels after forced swim. The STR+ALC group in Cohort 2 showed increased corticosterone levels on day-1 of treatment and a habituation by day-7. In conclusion, this study found a reversal of stress-induced deficits in cognition and anxiety when alcohol was given post-stress, and changes in neurotransmitter receptor expression may contribute to these behavioral effects.

Estradiol-induced object recognition memory consolidation is dependent on activation of mTOR signaling in the dorsal hippocampus

The mammalian target of rapamycin (mTOR) signaling pathway is an important regulator of protein synthesis and is essential for various forms of hippocampal memory. Here, we asked whether the enhancement of object recognition memory consolidation produced by dorsal hippocampal infusion of 17β-estradiol (E(2)) is dependent on mTOR signaling in the dorsal hippocampus, and whether E(2)-induced mTOR signaling is dependent on dorsal hippocampal phosphatidylinositol 3-kinase (PI3K) and extracellular signal-regulated kinase (ERK) activation. We first demonstrated that the enhancement of object recognition induced by E(2) was blocked by dorsal hippocampal inhibition of ERK, PI3K, or mTOR activation. We then showed that an increase in dorsal hippocampal ERK phosphorylation 5 min after intracerebroventricular (ICV) E(2) infusion was also blocked by dorsal hippocampal infusion of the three cell signaling inhibitors. Next, we found that ICV infusion of E(2) increased phosphorylation of the downstream mTOR targets S6K (Thr-421) and 4E-BP1 in the dorsal hippocampus 5 min after infusion, and that this phosphorylation was blocked by dorsal hippocampal infusion of inhibitors of ERK, PI3K, and mTOR. Collectively, these data demonstrate for the first time that activation of the dorsal hippocampal mTOR signaling pathway is necessary for E(2) to enhance object recognition memory consolidation and that E(2)-induced mTOR activation is dependent on upstream activation of ERK and PI3K signaling.

Neurobiological effects of neonatal maternal separation and post-weaning environmental enrichment

Throughout the lifespan, the brain has a considerable degree of plasticity and can be strongly influenced by sensory input from the outside environment. Given the importance of the environment in the regulation of the brain structure, behavior and physiology, the aim of the present work was to analyze the effects of different environmental qualities during two critical ontogenic periods (early life and peripuberty) on behavior and hippocampal physiology. Male Wistar rats were separated from their mothers for 4.5h daily during the first 3 weeks of life. They were weaned on day 21 and housed under either standard or enriched conditions. At 60 d of age, all animals were then housed in same-treatment groups, two per cage, until testing began on day 74. Emotional and cognitive responses were tested using the open field, novel object recognition test and step-down inhibitory avoidance learning. In the dorsal hippocampus, glucocorticoid receptor expression and neuronal activity were examined by immunoreactivity. Grooming behavior in the open field was found to be significantly lower in maternally separated animals, but post-weaning environmental enrichment completely reversed this tendency. Inhibitory avoidance but not object recognition memory was impaired in maternally separated animals, suggesting that early maternal separation alters learning and memory in a task-specific manner. Again, environmental enrichment reversed the effects of maternal separation on the inhibitory avoidance task. Even though maternal separation did not significantly affect Fos and glucocorticoid receptor (GR) expression, environmental enrichment increased both Fos expression in the total hippocampal area and also the overall number of GR positive cells per hippocampal area, mainly due to the changes in CA1. These findings suggest that differential rearing is a useful procedure to study behavioral and physiological plasticity in response to early experience and that, although the effects of adverse experience early in life such as maternal separation can persist until adulthood, some of them can be compensated by early favorable environments, possibly through nervous system plasticity.

What pharmacological interventions indicate concerning the role of the perirhinal cortex in recognition memory

Findings of pharmacological studies that have investigated the involvement of specific regions of the brain in recognition memory are reviewed. The particular emphasis of the review concerns what such studies indicate concerning the role of the perirhinal cortex in recognition memory. Most of the studies involve rats and most have investigated recognition memory for objects. Pharmacological studies provide a large body of evidence supporting the essential role of the perirhinal cortex in the acquisition, consolidation and retrieval of object recognition memory. Such studies provide increasingly detailed evidence concerning both the neurotransmitter systems and the underlying intracellular mechanisms involved in recognition memory processes. They have provided evidence in support of synaptic weakening as a major synaptic plastic process within perirhinal cortex underlying object recognition memory. They have also supplied confirmatory evidence that that there is more than one synaptic plastic process involved. The demonstrated necessity to long-term recognition memory of intracellular signalling mechanisms related to synaptic modification within perirhinal cortex establishes a central role for the region in the information storage underlying such memory. Perirhinal cortex is thereby established as an information storage site rather than solely a processing station. Pharmacological studies have also supplied new evidence concerning the detailed roles of other regions, including the hippocampus and the medial prefrontal cortex in different types of recognition memory tasks that include a spatial or temporal component. In so doing, they have also further defined the contribution of perirhinal cortex to such tasks. To date it appears that the contribution of perirhinal cortex to associative and temporal order memory reflects that in simple object recognition memory, namely that perirhinal cortex provides information concerning objects and their prior occurrence (novelty/familiarity).

Early life manipulations alter learning and memory in rats

Much research shows that early life manipulations have enduring behavioral, neural, and hormonal effects. However, findings of learning and memory performance vary widely across studies. We reviewed studies in which pre-weaning rat pups were exposed to stressors and tested on learning and memory tasks in adulthood. Tasks were classified as aversive conditioning, inhibitory learning, or spatial/relational memory. Variables of duration, type, and timing of neonatal manipulation and sex and strain of animals were examined to determine if any predict enhanced or impaired performance. Brief separations enhanced and prolonged separations impaired performance on spatial/relational tasks. Performance was impaired in aversive conditioning and enhanced in inhibitory learning tasks regardless of manipulation duration. Opposing effects on performance for spatial/relational memory also depended upon timing of manipulation. Enhanced performance was likely if the manipulation occurred during postnatal week 3 but performance was impaired if it was confined to the first two postnatal weeks. Thus, the relationship between early life experiences and adulthood learning and memory performance is multifaceted and decidedly task-dependent.

Building a better hormone therapy? How understanding the rapid effects of sex steroid hormones could lead to new therapeutics for age-related memory decline

A wealth of data collected in recent decades has demonstrated that ovarian sex-steroid hormones, particularly 17β-estradiol (E2), are important trophic factors that regulate the function of cognitive regions of the brain such as the hippocampus. The loss of hormone cycling at menopause is associated with cognitive decline and dementia in women, and the onset of memory decline in animal models. However, hormone therapy is not currently recommended to prevent or treat cognitive decline, in part because of its detrimental side effects. In this article, it is proposed that investigations of the rapid effects of E2 on hippocampal function be used to further the design of new drugs that mimic the beneficial effects of E2 on memory without the side effects of current therapies. A conceptual model is presented for elucidating the molecular and biochemical mechanisms through which sex-steroid hormones modulate memory, and a specific hypothesis is proposed to account for the rapid memory-enhancing effects of E2. Empirical support for this hypothesis is discussed as a means of stimulating the consideration of new directions for the development of hormone-based therapies to preserve memory function in menopausal women.

Persistent region-dependent neuroinflammation, NMDA receptor loss and atrophy in an animal model of penetrating brain injury

Dynamic changes in neuroinflammation and glutamate NMDA receptors (NMDAR) have been noted in traumatic and ischemic brain injury.

AIM

Here we investigate the time course and regional distribution of these changes and their relationship with atrophy in a rat model of penetrating brain injury.

MATERIALS METHODS

Quantitative autoradiography, with the neuroinflammation marker [³H]PK11195 and the NMDAR antagonist [¹²⁵I]iodoMK801, was performed on brains of animals subjected to a unilateral wireknife injury at the level of striatum and killed 3 - 60 days later. Regional atrophy was measured by morphometry.

RESULTS

The injury produced large increases in [³H]PK11195 binding density in cortical and septal regions adjacent to the knife track by day 7, with modest increases in the striatum. [¹²⁵I]iodoMK801 binding was reduced in cor tical and hippocampal regions showing marked neuroinflammation, which showed marked atrophy at subsequent time points.

CONCLUSION

These results indicate that neuroinflammaton and loss of NMDAR precede and predict tissue atrophy in cortical and hippocampal regions.

Hippocampal histone acetylation regulates object recognition and the estradiol-induced enhancement of object recognition

Histone acetylation has recently been implicated in learning and memory processes, yet necessity of histone acetylation for such processes has not been demonstrated using pharmacological inhibitors of histone acetyltransferases (HATs). As such, the present study tested whether garcinol, a potent HAT inhibitor in vitro, could impair hippocampal memory consolidation and block the memory-enhancing effects of the modulatory hormone 17β-estradiol E2. We first showed that bilateral infusion of garcinol (0.1, 1, or 10 μg/side) into the dorsal hippocampus (DH) immediately after training impaired object recognition memory consolidation in ovariectomized female mice. A behaviorally effective dose of garcinol (10 μg/side) also significantly decreased DH HAT activity. We next examined whether DH infusion of a behaviorally subeffective dose of garcinol (1 ng/side) could block the effects of DH E2 infusion on object recognition and epigenetic processes. Immediately after training, ovariectomized female mice received bilateral DH infusions of vehicle, E2 (5 μg/side), garcinol (1 ng/side), or E2 plus garcinol. Forty-eight hours later, garcinol blocked the memory-enhancing effects of E2. Garcinol also reversed the E2-induced increase in DH histone H3 acetylation, HAT activity, and levels of the de novo methyltransferase DNMT3B, as well as the E2-induced decrease in levels of the memory repressor protein histone deacetylase 2. Collectively, these findings suggest that histone acetylation is critical for object recognition memory consolidation and the beneficial effects of E2 on object recognition. Importantly, this work demonstrates that the role of histone acetylation in memory processes can be studied using a HAT inhibitor.

Integrin β1 signals through Arg to regulate postnatal dendritic arborization, synapse density, and behavior

Integrins are heterodimeric extracellular matrix receptors that are essential for the proper development of the vertebrate nervous system. We report here that selective loss of integrin β1 in excitatory neurons leads to reductions in the size and complexity of hippocampal dendritic arbors, hippocampal synapse loss, impaired hippocampus-dependent learning, and exaggerated psychomotor sensitivity to cocaine in mice. Our biochemical and genetic experiments demonstrate that the intracellular tail of integrin β1 binds directly to Arg kinase and that this interaction stimulates activity of the Arg substrate p190RhoGAP, an inactivator of the RhoA GTPase. Moreover, genetic manipulations that reduce integrin β1 signaling through Arg recapitulate the integrin β1 knock-out phenotype in a gene dose-sensitive manner. Together, these results describe a novel integrin β1-Arg-p190RhoGAP pathway that regulates dendritic arbor size, promotes synapse maintenance, supports proper hippocampal function, and mitigates the behavioral consequences of cocaine exposure.

Spontaneous object recognition and its relevance to schizophrenia: a review of findings from pharmacological, genetic, lesion and developmental rodent models

RATIONALE

Spontaneous (novel) object recognition (SOR) is one of the most widely used rodent behavioural tests. The opportunity for rapid data collection has made SOR a popular choice in studies that explore cognitive impairment in rodent models of schizophrenia, and that test the efficacy of drugs intended to reverse these deficits.

OBJECTIVES

We provide an overview of the many recent studies that have used SOR to explore the mnemonic effects of manipulation of the key transmitter systems relevant to schizophrenia-the dopamine, glutamate, GABA, acetylcholine, serotonin and cannabinoid systems-alone or in combination. We also review the use of SOR in studying memory in genetically modified mouse models of schizophrenia, as well as in neurodevelopmental and lesion models. We end by discussing the construct and predictive validity, and translational relevance, of SOR with respect to cognitive impairment in schizophrenia.

RESULTS

Perturbation of the dopamine or glutamate systems can generate robust and reliable impairment in SOR. Impaired performance is also seen following antagonism of the muscarinic acetylcholine system, or exposure to cannabinoid agonists. Cognitive enhancement has been reported using alpha7-nicotinic acetylcholine receptor agonists and 5-HT(6) antagonists. Among non-pharmacological models, neonatal ventral hippocampal lesions and maternal immune activation can impair SOR, while mixed results have been obtained with mice carrying mutations in schizophrenia risk-associated genes, including neuregulin and COMT.

CONCLUSIONS

While SOR is not without its limitations, the task represents a useful method for studying manipulations with relevance to cognitive impairment in schizophrenia, as well as the interactions between them.

The progesterone-induced enhancement of object recognition memory consolidation involves activation of the extracellular signal-regulated kinase (ERK) and mammalian target of rapamycin (mTOR) pathways in the dorsal hippocampus

Although much recent work has elucidated the biochemical mechanisms underlying the modulation of memory by 17β-estradiol, little is known about the signaling events through which progesterone (P) regulates memory. We recently demonstrated that immediate post-training infusion of P into the dorsal hippocampus enhances object recognition memory consolidation in young ovariectomized female mice (Orr et al., 2009). The goal of the present study was to identify the biochemical alterations that might underlie this mnemonic enhancement. We hypothesized that the P-induced enhancement of object recognition would be dependent on activation of the ERK and mTOR pathways. In young ovariectomized mice, we found that bilateral dorsal hippocampal infusion of P significantly increased levels of phospho-p42 ERK and the mTOR substrate S6K in the dorsal hippocampus 5 min after infusion. Phospho-p42 ERK levels were downregulated 15 min after infusion and returned to baseline 30 min after infusion, suggesting a biphasic effect of P on ERK activation. Dorsal hippocampal ERK and mTOR activation were necessary for P to facilitate memory consolidation, as suggested by the fact that inhibitors of both pathways infused into the dorsal hippocampus immediately after training blocked the P-induced enhancement of object recognition. Collectively, these data provide the first demonstration that the ability of P to enhance memory consolidation depends on the rapid activation of cell signaling and protein synthesis pathways in the dorsal hippocampus.

Object recognition memory and BDNF expression are reduced in young TgCRND8 mice

The TgCRND8 mouse model of Alzheimer's disease exhibits progressive cortical and hippocampal β-amyloid accumulation, resulting in plaque pathology and spatial memory impairment by 3 months of age. We tested whether TgCRND8 cognitive function is disrupted prior to the appearance of macroscopic plaques in an object recognition task. We found profound deficits in 8-week-old mice. Animals this age were not impaired on the Morris water maze task. TgCRND8 and littermate controls did not differ in their duration of object exploration or optokinetic responses. Thus, visual and motor dysfunction did not confound the phenotype. Object memory deficits point to the frontal cortex and hippocampus as early targets of functional disruption. Indeed, we observed altered levels of brain-derived neurotrophic factor (BDNF) messenger ribonucleic acid (mRNA) in these brain regions of preplaque TgCRND8 mice. Our findings suggest that object recognition provides an early index of cognitive impairment associated with amyloid exposure and reduced brain-derived neurotrophic factor expression in the TgCRND8 mouse.

Effects of Peripheral and Intra-hippocampal Administration of Sodium Salicylate on Spatial Learning and Memory of Rats

OBJECTIVES

Cyclooxygenases (COXs) are known to play some roles in physiological mechanisms related to learning and memory. Since sodium salicylate is an inhibitor of COX, we have evaluated the effect of peripheral and intra-hippocampal administration of sodium salicylate on spatial learning and memory in male rats.

MATERIALS AND METHODS

Male rats were studied in two groups; the first group received different intraperitoneal (i.p.) sodium salicylate doses (0, 200, 300, and 400 mg/kg) and the second group received intra-hippocampal doses of the drug (0, 30, 50, and 100 μg/0.5 μl/side). The spatial performance of rats was tested using Morris water maze (MWM) task. The spatial learning and memory parameters were analyzed using ANOVA.

RESULTS

Peripheral and intra-hippocampal administration of sodium salicylate did not lead to a statistically significant change in the mean time (escape latency), and also the distance traveled for finding the hidden platform during the training days, compared with the control group. But at the probe trial, the percentage of time spent in the target quadrant by rats which received the highest doses of drug significantly increased.

CONCLUSION

We found that both peripheral and intra-hippocampal administration of sodium salicylate facilitates the process of spatial memory consolidation in the MWM.

Increased sensitivity to alcohol induced changes in ERK Map kinase phosphorylation and memory disruption in adolescent as compared to adult C57BL/6J mice

Adolescence is a critical period of brain development that is accompanied by increased probability of risky behavior, such as alcohol use. Emerging research indicates that adolescents are differentially sensitive to the behavioral effects of acute ethanol as compared to adults but the neurobiological mechanisms of this effect remain to be fully elucidated. This study was designed to evaluate effects of acute ethanol on extracellular signal-regulated kinase phosphorylation (p-ERK1/2) in mesocorticolimbic brain regions. We also sought to determine if age-specific effects of ethanol on p-ERK1/2 are associated with ethanol-induced behavioral deficits on acquisition of the hippocampal-dependent novel object recognition (NOR) test. Adolescent and adult C57BL/6J mice were administered acute ethanol (0 0.5, 1, or 3g/kg, i.p.). Brains were removed 30-min post injection and processed for analysis of p-ERK1/2 immunoreactivity (IR). Additional groups of mice were administered ethanol (0 or 1g/kg) prior to the NOR test. Analysis of p-ERK1/2 IR showed that untreated adolescent mice had significantly higher levels of p-ERK1/2 IR in the nucleus accumbens shell, basolateral amygdala (BLA), central amygdala (CeA), and medial prefrontal cortex (mPFC) as compared to adults. Ethanol (1g/kg) selectively reduced p-ERK1/2 IR in the dentate gyrus and increased p-ERK1/2 IR in the BLA only in adolescent mice. Ethanol (3g/kg) produced the same effects on p-ERK1/2 IR in both age groups with increases in CeA and mPFC, but a decrease in the dentate gyrus, as compared to age-matched saline controls. Pretreatment with ethanol (1g/kg) disrupted performance on the NOR test specifically in adolescents, which corresponds with the ethanol-induced inhibition of p-ERK1/2 IR in the hippocampus. These data show that adolescent mice have differential expression of basal p-ERK1/2 IR in mesocorticolimbic brain regions. Acute ethanol produces a unique set of changes in ERK1/2 phosphorylation in the adolescent brain that are associated with disruption of hippocampal-dependent memory acquisition.

Hippocampal neurotransmitter efflux during one-trial novel object recognition in rats

Several lines of evidence point to a role for the hippocampal formation and contiguous temporal lobe structures in a variety of learning and memory paradigms. Presumably, these cognitive phenomena are mediated (and accompanied) by dynamic changes in neurochemical transmission that may differ between learning and recall phases. However, the neurotransmitter correlates of most memory-related tasks have not been thoroughly investigated. Here we used a one-trial object recognition paradigm paired with in vivo microdialysis to assess hippocampal acetylcholine (ACh), glutamate and GABA efflux when rats were exposed to familiar objects, and when given the option to explore familiar and novel objects. Rats preferentially explored the novel object over the familiar one when presented with the option. Regardless of object familiarity, object exploration was accompanied by an increase in hippocampal ACh efflux, while GABA efflux was unaffected. However, glutamate efflux was not increased above baseline levels by presentation of familiar objects, but was significantly enhanced in the presence of the novel object. These data suggest that the hippocampus, and in particular, hippocampal glutamate, may be involved in memory processes during novelty recognition paradigms.

Aging impairs hippocampus-dependent long-term memory for object location in mice

The decline in cognitive function that accompanies normal aging has a negative impact on the quality of life of the elderly and their families. Studies in humans and rodents show that spatial navigation and other hippocampus-dependent functions are particularly vulnerable to the deleterious effects of aging. However, reduced motor activity and alterations in the stress response that accompany normal aging can hinder the ability to study certain cognitive behaviors in aged animals. In an attempt to circumvent these potential confounds, we used a hippocampus-dependent object-place recognition task to show that long-term spatial memory is impaired in aged mice. Aged animals performed similarly to young adult mice on an object recognition task that does not rely on hippocampal function.

Zinc signaling through glucocorticoid and glutamate signaling in stressful circumstances

Humans and animals are constantly exposed to environmental stress. The hypothalamic-pituitary-adrenal (HPA) axis responds to stress, followed by glucocorticoid secretion from the adrenal glands. This response serves to maintain homeostasis in the living body through energy mobilization or to restore it. The brain is an important target for glucocorticoids. The hippocampus participates in the regulation of the HPA axis. Stress activates glutamatergic neurons in the hippocampus, and serious stress induces dyshomeostasis of extracellular glutamate. This dyshomeostasis, which is potentiated by glucocorticoids, modifies cognitive and emotional behavior. On the other hand, zinc is necessary for glucocorticoid signaling and is released from glutamatergic (zincergic) neurons to modulate synaptic glutamate signaling. Stress also induces dyshomeostasis of extracellular zinc, which may be linked to dyshomeostasis of extracellular glutamate. Thus, glucocorticoid signaling might also contribute to dyshomeostasis of extracellular zinc. It is likely that zinc signaling participates in cognitive and emotional behavior through glucocorticoid and glutamate signaling under stressful circumstances. This Mini-Review analyzes the relationship among signals of glucocorticoid, glutamate, and zinc under stressful circumstances to elucidate the significance of the zinc signaling in response to stress.

The epigenetics of estrogen: epigenetic regulation of hormone-induced memory enhancement

Epigenetic processes have been implicated in everything from cell proliferation to maternal behavior. Epigenetic alterations, including histone alterations and DNA methylation, have also been shown to play critical roles in the formation of some types of memory, and in the modulatory effects that factors, such as stress, drugs of abuse and environmental stimulation, have on the brain and memory function. Recently, we demonstrated that the ability of the sex-steroid hormone 17β-estradiol (E(2)) to enhance memory formation is dependent on histone acetylation and DNA methylation, a finding that has important implications for understanding how hormones influence cognition in adulthood and aging. In this article, we provide an overview of the literature demonstrating that epigenetic processes and E(2) influence memory, describe our findings indicating that epigenetic alterations regulate E(2)-induced memory enhancement, and discuss directions for future work on the epigenetics of estrogen.

Visual perception and memory systems: from cortex to medial temporal lobe

Visual perception and memory are the most important components of vision processing in the brain. It was thought that the perceptual aspect of a visual stimulus occurs in visual cortical areas and that this serves as the substrate for the formation of visual memory in a distinct part of the brain called the medial temporal lobe. However, current evidence indicates that there is no functional separation of areas. Entire visual cortical pathways and connecting medial temporal lobe are important for both perception and visual memory. Though some aspects of this view are debated, evidence from both sides will be explored here. In this review, we will discuss the anatomical and functional architecture of the entire system and the implications of these structures in visual perception and memory.

Glucocorticoids act on glutamatergic pathways to affect memory processes

Glucocorticoids can acutely affect memory processes, with both facilitating and impairing effects having been described. Recent work has revealed that glucocorticoids may affect learning and memory processes by interacting with glutamatergic mechanisms. In this opinion article I describe different glutamatergic pathways that glucocorticoids can affect to modulate memory processes. Furthermore, glucocorticoid-glutamatergic interactions during information processing are proposed as a potential model to explain many of the diverse actions of glucocorticoids on cognition. The model suggests that direct modulation of glutamatergic pathways by glucocorticoids could serve as an important mechanism for these hormones to directly alter cognitive functions.

HDAC inhibition modulates hippocampus-dependent long-term memory for object location in a CBP-dependent manner

Transcription of genes required for long-term memory not only involves transcription factors, but also enzymatic protein complexes that modify chromatin structure. Chromatin-modifying enzymes, such as the histone acetyltransferase (HAT) CREB (cyclic-AMP response element binding) binding protein (CBP), are pivotal for the transcriptional regulation required for long-term memory. Several studies have shown that CBP and histone acetylation are necessary for hippocampus-dependent long-term memory and hippocampal long-term potentiation (LTP). Importantly, every genetically modified Cbp mutant mouse exhibits long-term memory impairments in object recognition. However, the role of the hippocampus in object recognition is controversial. To better understand how chromatin-modifying enzymes modulate long-term memory for object recognition, we first examined the role of the hippocampus in retrieval of long-term memory for object recognition or object location. Muscimol inactivation of the dorsal hippocampus prior to retrieval had no effect on long-term memory for object recognition, but completely blocked long-term memory for object location. This was consistent with experiments showing that muscimol inactivation of the hippocampus had no effect on long-term memory for the object itself, supporting the idea that the hippocampus encodes spatial information about an object (such as location or context), whereas cortical areas (such as the perirhinal or insular cortex) encode information about the object itself. Using location-dependent object recognition tasks that engage the hippocampus, we demonstrate that CBP is essential for the modulation of long-term memory via HDAC inhibition. Together, these results indicate that HDAC inhibition modulates memory in the hippocampus via CBP and that different brain regions utilize different chromatin-modifying enzymes to regulate learning and memory.

Assessment of spatial memory in mice

Improvements in health care have greatly increased life span in the United States. The focus is now shifting from physical well-being to improvement in mental well-being or maintenance of cognitive function in old age. It is known that elderly people suffer from cognitive impairment, even without neurodegeneration, as a part of 'normal aging'. This 'age-associated memory impairment' (AAMI), can have a devastating impact on the social and economic life of an individual as well as the society. Scientists have been experimenting to find methods to prevent the memory loss associated with aging. The major factor involved in these experiments is the use of animal models to assess hippocampal-based spatial memory. This review describes the different types of memory including hippocampal-based memory that is vulnerable to aging. A detailed overview of various behavioral paradigms used to assess spatial memory including the T-maze, radial maze, Morris water maze, Barnes maze and others is presented. The review also describes the molecular basis of memory in hippocampus called as 'long-term potentiation'. The advantages and limitations of the behavioral models in assessing memory and the link to the long-term potentiation are discussed. This review should assist investigators in choosing suitable methods to assess spatial memory in mice.

An animal model of recognition memory and medial temporal lobe amnesia: history and current issues

The medial temporal lobe includes a system of anatomically connected structures that are essential for declarative memory (conscious memory for facts and events). A prominent form of declarative memory is recognition memory (the ability to identify a recently encountered item as familiar). Recognition memory has been frequently assessed in humans and in the experimental animal. This article traces the successful development of an animal model of human medial temporal lobe amnesia, which eventually identified the structures in the medial temporal lobe important for memory. Attention is given to two prominent behavioral paradigms (delayed nonmatching to sample and tests of spontaneous novelty preference).

Converging effects of acute stress on spatial and recognition memory in rodents: a review of recent behavioural and pharmacological findings

The heterogeneous effects of acute stress on learning and memory depend on numerous parameters related to the stressor, the time the stressor is experienced, and the nature of the stimuli or task examined. In the present review, we systematically summarize the rodent literature examining the effects of acute extrinsic stress on spatial and recognition memory. Converging evidence from a number of behavioural tasks suggests acute stress disrupts the retrieval of spatial and recognition memory regardless of whether the stress is experienced before or after learning. Few studies have attempted to discern whether these effects are due to specific failures in consolidation or retrieval of task relevant information. Recent studies demonstrate that diverse mechanisms related to activation of the hypothalamic-pituitary-adrenal axis and alterations in glutamatergic synaptic plasticity mediate the effects of acute stress on spatial and recognition memory. Taken together, these findings have significantly advanced our understanding of the neural mechanisms mediating learning and memory and may stimulate the search for novel therapeutics to treat stress-related psychiatric disorders.