Age-related changes of auditory sensitivity across the life span of CBA/CaJ mice.

The inbred mouse strain CBA/CaJ is a frequently used animal model of age-related hearing loss in humans. These mice display significant hearing loss at a relatively advanced age, similar to most humans, with progressive loss of hearing as the mouse continues to age. While important descriptions of hearing loss in this mouse strain at multiple ages have previously been published, shortcomings persist in the data for hearing over the lifespan of the mouse. Therefore, we analyzed auditory brainstem response threshold data from records maintained by our research group to yield an extensive database of thresholds over nearly the entire life span of the CBA/CaJ mouse (from 79 to 1085 days). Data was collected from in-house bred mice of CBA/CaJ stock, initially from The Jackson Laboratory. Data was collected using BiosigRZ software and TDT System III hardware. Thresholds were routinely measured in conjunction with behavioral and electrophysiological experiments; only responses from baseline or experimentally naïve animals were analyzed. The resulting data set comprised 376 female mice and 441 males. At the lowest and highest frequencies (8 & 32 kHz), initial thresholds were just under 30 dB SPL and increased slowly until they were significantly different at 16-18 months compared to 1-3 months age, with the difference increasing over subsequent ages. At the middle frequencies (12 & 16 kHz), initial thresholds were just under 20 dB SPL and increased until they became different from initial at 16-18 months. At 24 kHz, initial thresholds were just above 20 dB and became different from initial at 13-16 months of age. The rate of change of thresholds with age were similar for all frequencies until about 30 months of age, when 32 kHz threshold changes lagged behind other frequencies. Generally, CBA/CaJ mice in our colony display relatively low thresholds until approximately 16 months of age, depending on frequency. After 16-18 months, thresholds become significantly worse. After approximately 20-22 months thresholds increase linearly with age.

Differential expression of molecular markers of synaptic plasticity in the hippocampus, prefrontal cortex, and amygdala in response to spatial learning, predator exposure, and stress-induced amnesia.

We have studied the effects of spatial learning and predator stress-induced amnesia on the expression of calcium/calmodulin-dependent protein kinase II (CaMKII), brain-derived neurotrophic factor (BDNF) and calcineurin in the hippocampus, basolateral amygdala (BLA), and medial prefrontal cortex (mPFC). Adult male rats were given a single training session in the radial-arm water maze (RAWM) composed of 12 trials followed by a 30-min delay period, during which rats were either returned to their home cages or given inescapable exposure to a cat. Immediately following the 30-min delay period, the rats were given a single test trial in the RAWM to assess their memory for the hidden platform location. Under control (no stress) conditions, rats exhibited intact spatial memory and an increase in phosphorylated CaMKII (p-CaMKII), total CaMKII, and BDNF in dorsal CA1. Under stress conditions, rats exhibited impaired spatial memory and a suppression of all measured markers of molecular plasticity in dorsal CA1. The molecular profiles observed in the BLA, mPFC, and ventral CA1 were markedly different from those observed in dorsal CA1. Stress exposure increased p-CaMKII in the BLA, decreased p-CaMKII in the mPFC, and had no effect on any of the markers of molecular plasticity in ventral CA1. These findings provide novel observations regarding rapidly induced changes in the expression of molecular plasticity in response to spatial learning, predator exposure, and stress-induced amnesia in brainregions involved in different aspects of memory processing.

The antidepressant agomelatine blocks the adverse effects of stress on memory and enables spatial learning to rapidly increase neural cell adhesion molecule (NCAM) expression in the hippocampus of rats.

Agomelatine, a novel antidepressant with established clinical efficacy, acts as a melatonin receptor agonist and 5-HT(2C) receptor antagonist. As stress is a significant risk factor in the development of depression, we sought to determine if chronic agomelatine treatment would block the stress-induced impairment of memory in rats trained in the radial-arm water maze (RAWM), a hippocampus-dependent spatial memory task. Moreover, since neural cell adhesion molecule (NCAM) is known to be critically involved in memory consolidation and synaptic plasticity, we evaluated the effects of agomelatine on NCAM, and polysialylated NCAM (PSA-NCAM) expression in rats given spatial memory training with or without predator stress. Adult male rats were pre-treated with agomelatine (10 mg/kg i.p., daily for 22 d), followed by a single day of RAWM training and memory testing. Rats were given 12 training trials and then they were placed either in their home cages (no stress) or near a cat (predator stress). Thirty minutes later the rats were given a memory test trial followed immediately by brain extraction. We found that: (1) agomelatine blocked the predator stress-induced impairment of spatial memory; (2) agomelatine-treated stressed, as well as non-stressed, rats exhibited a rapid training-induced increase in the expression of synaptic NCAM in the ventral hippocampus; and (3) agomelatine treatment blocked the water-maze training-induced decrease in PSA-NCAM levels in both stressed and non-stressed animals. This work provides novel observations which indicate that agomelatine blocks the adverse effects of stress on hippocampus-dependent memory and activates molecular mechanisms of memory storage in response to a learning experience.

Acute predator stress impairs the consolidation and retrieval of hippocampus-dependent memory in male and female rats.

We have studied the effects of an acute predator stress experience on spatial learning and memory in adult male and female Sprague-Dawley rats. All rats were trained to learn the location of a hidden escape platform in the radial-arm water maze (RAWM), a hippocampus-dependent spatial memory task. In the control (non-stress) condition, female rats were superior to the males in the accuracy and consistency of their spatial memory performance tested over multiple days of training. In the stress condition, rats were exposed to the cat for 30 min immediately before or after learning, or before the 24-h memory test. Predator stress dramatically increased corticosterone levels in males and females, with females exhibiting greater baseline and stress-evoked responses than males. Despite these sex differences in the overall magnitudes of corticosterone levels, there were significant sex-independent correlations involving basal and stress-evoked corticosterone levels, and memory performance. Most importantly, predator stress impaired short-term memory, as well as processes involved in memory consolidation and retrieval, in male and female rats. Overall, we have found that an intense, ethologically relevant stressor produced a largely equivalent impairment of memory in male and female rats, and sex-independent corticosterone-memory correlations. These findings may provide insight into commonalities in how traumatic stress affects the brain and memory in men and women.

Pre-training administration of tianeptine, but not propranolol, protects hippocampus-dependent memory from being impaired by predator stress.

Extensive research has shown that the antidepressant tianeptine blocks the adverse effects of chronic stress on hippocampal functioning. The current series of experiments extended this area of investigation by examining the influence of tianeptine on acute stress-induced impairments of spatial (hippocampus-dependent) memory. Tianeptine (10 mg/kg, ip) administered to adult male rats before, but not after, water maze training blocked the amnestic effects of predator stress (occurring between training and retrieval) on memory. The protective effects of tianeptine on memory occurred in rats which had extensive pre-stress training, as well as in rats which had only a single day of training. Tianeptine blocked stress effects on memory without altering the stress-induced increase in corticosterone levels. Propranolol, a beta-adrenergic receptor antagonist (5 and 10 mg/kg, ip), in contrast, did not block stress-induced amnesia. These findings indicate that treatment with tianeptine, unlike propanolol, provides an effective means with which to block the adverse effects of stress on cognitive functions of the hippocampus.

Hyperglycemia not hypoglycemia alters neuronal dendrites and impairs spatial memory.

BACKGROUND/OBJECTIVE: We previously reported that chronic hyperglycemia, but not hypoglycemia, was associated with the reduction of neuronal size in the rat brain. We hypothesized that hyperglycemia-induced changes in neuronal structure would have negative consequences, such as impaired learning and memory. We therefore assessed the effects of hyperglycemia and hypoglycemia on neuronal dendritic structure and cognitive functioning in young rats. DESIGN/METHODS: Experimental manipulations were conducted on male Wistar rats for 8 wk, beginning at 4 wk of age. At the completion of the treatments, all rats were trained in the radial-arm water maze, a spatial (hippocampus-dependent) learning and memory task. Three groups of rats were tested: an untreated control group, a streptozotocin-induced diabetic (STZ-D) group, and an intermittent hypoglycemic group. Following behavioral training, the brains of all animals were examined with histologic and biochemical measurements. RESULTS: Peripheral hyperglycemia was associated with significant increases in brain sorbitol (7.5 +/- 1.6 vs. 5.84 +/- 1.0 microM/mg) and inositol (9.6 +/- 1.4 vs. 7.1 +/- 1.1 microM/mg) and reduced taurine (0.65 +/- 0.1 vs. 1.3 +/- 0.1 mg/mg). Histologic evaluation revealed neurons with reduced dendritic branching and spine density in STZ-D rats but not in control or hypoglycemic animals. In addition, the STZ-D group exhibited impaired performance on the water maze memory test. CONCLUSIONS: Hyperglycemia, but not hypoglycemia, was associated with adverse effects on the brain polyol pathway activity, neuronal structural changes, and impaired long-term spatial memory. This finding suggests that the hyperglycemic component of diabetes mellitus has a greater adverse effect on brain functioning than does intermittent hypoglycemia.

Tianeptine: an antidepressant with memory-protective properties.

The development of effective pharmacotherapy for major depression is important because it is such a widespread and debilitating mental disorder. Here, we have reviewed preclinical and clinical studies on tianeptine, an atypical antidepressant which ameliorates the adverse effects of stress on brain and memory. In animal studies, tianeptine has been shown to prevent stress-induced morphological sequelae in the hippocampus and amygdala, as well as to prevent stress from impairing synaptic plasticity in the prefrontal cortex and hippocampus. Tianeptine also has memory-protective characteristics, as it blocks the adverse effects of stress on hippocampus-dependent learning and memory. We have further extended the findings on stress, memory and tianeptine here with two novel observations: 1) stress impairs spatial memory in adrenalectomized (ADX), thereby corticosterone-depleted, rats; and 2) the stress-induced impairment of memory in ADX rats is blocked by tianeptine. These findings are consistent with previous research which indicates that tianeptine produces anti-stress and memory-protective properties without altering the response of the hypothalamic-pituitary-adrenal axis to stress. We conclude with a discussion of findings which indicate that tianeptine accomplishes its anti-stress effects by normalizing stress-induced increases in glutamate in the hippocampus and amygdala. This finding is potentially relevant to recent research which indicates that abnormalities in glutamatergic neurotransmission are involved in the pathogenesis of depression. Ultimately, tianeptine's prevention of depression-induced sequelae in the brain is likely to be a primary factor in its effectiveness as a pharmacological treatment for depression.

Atypical PKC, PKCλ/ι, activates ß-secretase and increases Aß1-40/42 and phospho-tau in mouse brain and isolated neuronal cells, and may link hyperinsulinemia and other aPKC activators to development of pathological and memory abnormalities in Alzheimer's disease.

Hyperinsulinemia activates brain Akt and PKC-λ/ι and increases Aß1-40/42 and phospho-tau in insulin-resistant animals. Here, we examined underlying mechanisms in mice, neuronal cells, and mouse hippocampal slices. Like Aß1-40/42, ß-secretase activity was increased in insulin-resistant mice and monkeys. In insulin-resistant mice, inhibition of hepatic PKC-λ/ι sufficient to correct hepatic abnormalities and hyperinsulinemia simultaneously reversed increases in Akt, atypical protein kinase C (aPKC), ß-secretase, and Aß1-40/42, and restored acute Akt activation. However, 2 aPKC inhibitors additionally blocked insulin's ability to activate brain PKC-λ/ι and thereby increase ß-secretase and Aß1-40/42. Furthermore, direct blockade of brain aPKC simultaneously corrected an impairment in novel object recognition in high-fat-fed insulin-resistant mice. In neuronal cells and/or mouse hippocampal slices, PKC-ι/λ activation by insulin, metformin, or expression of constitutive PKC-ι provoked increases in ß-secretase, Aß1-40/42, and phospho-thr-231-tau that were blocked by various PKC-λ/ι inhibitors, but not by an Akt inhibitor. PKC-λ/ι provokes increases in brain ß-secretase, Aß1-40/42, and phospho-thr-231-tau. Excessive signaling via PKC-λ/ι may link hyperinsulinemia and other PKC-λ/ι activators to pathological and functional abnormalities in Alzheimer's disease.

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

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

Enhancement of long-term spatial memory in adult rats by the noncompetitive NMDA receptor antagonists, memantine and neramexane.

Memantine and neramexane are noncompetitive NMDA receptor antagonists which have been investigated for their promising effects in aiding memory in people with dementia. Memantine is approved for the treatment of Alzheimer's disease, and neramexane is currently under development for this indication. Therefore, the present study provided a comparative assessment of the effects of equimolar doses of memantine and neramexane on spatial (hippocampus-dependent) memory. Adult male rats were given only 3 training trials to learn the location of a hidden platform in a water maze. In control (vehicle-injected) rats, this minimal amount of training produced intact short-term (15 min), but poor long-term (24 h), memory. Pre-training administration of memantine or neramexane produced a dose-dependent enhancement of long-term memory. Pharmacokinetic experiments with equimolar doses of both agents indicated that lower plasma levels of neramexane were more effective than memantine at enhancing memory. The effective doses of both agents in the current study produced plasma levels (and extrapolated brain CSF levels) within a range of activity at NMDA receptors and plasma levels seen in patients with Alzheimer's disease. These findings provide support for the use of neramexane as a pharmacological intervention in the treatment of dementia.

Influence of daily social stimulation on behavioral and physiological outcomes in an animal model of PTSD.

INTRODUCTION: We have shown in previous work that acute episodes of predator exposure occurring in the context of chronic social instability produced PTSD-like sequelae in rats. Our animal model of PTSD contained two components: (1) acute trauma, immobilization of rats in close proximity to a cat twice in 10 days, and (2) chronic social instability, 31 days of randomized housing of cage cohorts. Here we tested the hypothesis that daily social stimulation would block the development of the PTSD-like sequelae. METHODS: Beginning 24 h after the first cat exposure, adult male rats were given our established PTSD model, alone or in conjunction with daily social stimulation, in which all rats within a group interacted in a large apparatus for 2 h each day for the final 30 days of the PTSD regimen. All behavioral, for example, anxiety, memory, startle testing, and physiological assessments, for example, body growth, organ weights, and corticosterone levels, took place following completion of the psychosocial stress period. RESULTS: Daily social stimulation blocked the expression of a subset of PTSD-like effects, including predator-based cued fear conditioning, enhanced startle response, heightened anxiety on the elevated plus maze and the stress-induced suppression of growth rate. We also found that social stimulation and psychosocial stress produced equivalent outcomes in some measures, including adrenal and heart hypertrophy, thymus atrophy, and a reduction in poststress corticosterone levels. CONCLUSIONS: Daily exposure of rats to a highly social environment blocked the development of a subset of trauma-induced sequelae, particularly fear-related outcomes. It is notable that daily social stimulation normalized a subset, but not all, of the PTSD-like effects. We discuss our findings in the context of the literature demonstrating that social stimulation can counteract the adverse effects of traumatic stress on behavioral and physiological measures, as well as to produce its own stress-like outcomes.

Acute stress-induced impairment of spatial memory is associated with decreased expression of neural cell adhesion molecule in the hippocampus and prefrontal cortex.

BACKGROUND: There is an extensive literature describing how stress disturbs cognitive processing and can exacerbate psychiatric disorders. There is, however, an insufficient understanding of the molecular mechanisms involved in stress effects on brain and behavior. METHODS: Rats were given spatial memory training in a hippocampus-dependent water maze task. We investigated how a fear-provoking experience (predator exposure) would affect their spatial memory and neural cell adhesion molecule (NCAM) levels in the hippocampus, prefrontal cortex (PFC), amygdala, and cerebellum. RESULTS: Whereas the control (nonstress) group exhibited excellent memory for the hidden platform location in the water maze, the cat-exposed (stress) group exhibited a profound impairment of memory and a marked suppression of levels of the NCAM-180 isoform in the hippocampus. Predator stress produced a more global reduction of NCAM levels in the PFC but had no effect on NCAM levels in the amygdala and cerebellum. CONCLUSIONS: This work provides a novel perspective into dynamic and structure-specific changes in the molecular events involved in learning, memory, and stress. The selective suppression of NCAM-180 in the hippocampus and the more general suppression of NCAM in the PFC provide insight into the mechanisms underlying the great sensitivity of these two structures to be disturbed by stress.

Psychosocial predator-based animal model of PTSD produces physiological and behavioral sequelae and a traumatic memory four months following stress onset.

We have a well-established animal model of PTSD composed of predator exposure administered in conjunction with social instability that produces PTSD-like behavioral and physiological abnormalities one month after stress initiation. Here, we assessed whether the PTSD-like effects would persist for at least 4months after the initiation of the psychosocial stress regimen. Adult male Sprague-Dawley rats were exposed to either 2 or 3 predator-based fear conditioning sessions. During each session, rats were placed in a chamber for a 3-min period that terminated with a 30-s tone, followed by 1h of immobilization of the rats during cat exposure (Day 1). All rats in the stress groups received a second fear conditioning session 10days later (Day 11). Half of the stress rats received a third fear conditioning session 3weeks later (Day 32). The two cat-exposed groups were also exposed to daily unstable housing conditions for the entire duration of the psychosocial stress regimen. The control group received stable (conventional) housing conditions and an equivalent amount of chamber exposure on Days 1, 11 and 32, without cat exposure. Behavioral testing commenced for all groups on Day 116. The stress groups demonstrated increased anxiety on the elevated plus maze, impaired object recognition memory and robust contextual and cued fear conditioned memory 3months after the last conditioning session. Combined data from the two stress groups revealed lower post-stress corticosterone levels and greater diastolic blood pressure relative to the control group. These findings indicate that predator-based psychosocial stress produces persistent PTSD-like physiological and behavioral abnormalities that may provide insight into the neurobiological and endocrine sequelae in traumatized people with PTSD.

Adverse effect of the combination of hypothyroidism and chronic psychosocial stress on hippocampus-dependent memory in rats.

Both hypothyroidism and stress interfere with cognitive function in patients. This study examined the effect of hypothyroidism and stress on hippocampus-dependent learning and memory in rats using the novel radial arm water maze (RAWM), which measures spatial working memory. Hypothyroidism was accomplished by thyroidectomy and 2 weeks later a form of intruder stress was used as the chronic psychosocial stressor. After 4-6 weeks of stress, rats were trained to learn (during the acquisition phase; four trials) and then remember (during two memory test trials occurring 15 and 120 min after the acquisition phase) the within-day location of a hidden escape platform, which was in different arm every day. The number of errors (entry into arms other than the platform arm) was noted. Within-day learning of the platform location was largely unaffected by the experimental manipulations, indicating that rats in all groups were equally capable of finding the platform to escape from the water with similar numbers of errors (P > 0.005). The number of days a rat took to reach a criterion (DTC; a maximum of one error in three consecutive days) indicated that chronic stress or hypothyroidism, alone, resulted in a mild impairment of spatial memory, and the combination of chronic stress and hypothyroidism resulted in a more severe and long-lasting memory impairment. The data indicated that the combination of stress and hypothyroidism produced more deleterious effects on hippocampal function than either chronic stress or hypothyroidism alone.

Preclinical research on stress, memory, and the brain in the development of pharmacotherapy for depression.

We have reviewed two areas of research on stress, memory, and synaptic plasticity which may be relevant toward understanding the neurobiology of major depressive disorder (MDD). First, we have presented the view that the hippocampus (HC) and prefrontal cortex (PFC) function jointly as a memory system which enables multitask processing (working memory) and consolidation of contextual information. The amygdala, by contrast, is necessary for the consolidation of emotional memories. Cognitive and neurophysiological studies have shown that HC-PFC processing is impaired, and amygdaloid processing is enhanced, by stress and in anxiety and mood disorders, including MDD. Second, we have reviewed research on the behavioral and neurophysiological actions of tianeptine, an antidepressant that is known to block the adverse effects of chronic stress on hippocampal morphology. Recent work has shown that acute tianeptine enhances cognitive and electrophysiological measures of HC-PFC functioning without interfering with the emotion-induced enhancement of amygdaloid functioning in rodents. We conclude with a synthesis of the preclinical and clinical literature on stress, memory, and tianeptine with the proposal that tianeptine should enhance HC-PFC memory-related processing in people under stress.

Influence of Pre-Training Predator Stress on the Expression of c-fos mRNA in the Hippocampus, Amygdala, and Striatum Following Long-Term Spatial Memory Retrieval.

We have studied the influence of pre-training psychological stress on the expression of c-fos mRNA following long-term spatial memory retrieval. Rats were trained to learn the location of a hidden escape platform in the radial-arm water maze, and then their memory for the platform location was assessed 24 h later. Rat brains were extracted 30 min after the 24-h memory test trial for analysis of c-fos mRNA. Four groups were tested: (1) Rats given standard training (Standard); (2) Rats given cat exposure (Predator Stress) 30 min prior to training (Pre-Training Stress); (3) Rats given water exposure only (Water Yoked); and (4) Rats given no water exposure (Home Cage). The Standard trained group exhibited excellent 24 h memory which was accompanied by increased c-fos mRNA in the dorsal hippocampus and basolateral amygdala (BLA). The Water Yoked group exhibited no increase in c-fos mRNA in any brain region. Rats in the Pre-Training Stress group were classified into two subgroups: good and bad memory performers. Neither of the two Pre-Training Stress subgroups exhibited a significant change in c-fos mRNA expression in the dorsal hippocampus or BLA. Instead, stressed rats with good memory exhibited significantly greater c-fos mRNA expression in the dorsolateral striatum (DLS) compared to stressed rats with bad memory. This finding suggests that stressed rats with good memory used their DLS to generate a non-spatial (cue-based) strategy to learn and subsequently retrieve the memory of the platform location. Collectively, these findings provide evidence at a molecular level for the involvement of the hippocampus and BLA in the retrieval of spatial memory and contribute novel observations on the influence of pre-training stress in activating the DLS in response to long-term memory retrieval.

Permissive influence of stress in the expression of a U-shaped relationship between serum corticosterone levels and spatial memory errors in rats.

The relationship between glucocorticoids (GCs) and memory is complex, in that memory impairments can occur in response to manipulations that either increase or decrease GC levels. We investigated this issue by assessing the relationship between serum corticosterone (the primary rodent GC) and memory in rats trained in the radial arm water maze, a hippocampus-dependent spatial memory task. Each day, rats learned a new location of the hidden escape platform and then 30 min later their memory of the location of the platform was tested. Under control conditions, well-trained rats had excellent spatial memory and moderately elevated corticosterone levels (approximately 26 microg/dl versus a baseline of approximately 2 microg/dl). Their memory was impaired when corticosterone levels were either reduced by metyrapone (a corticosterone synthesis inhibitor) or increased by acute stress (predator exposure), forming an overall U-shaped relationship between corticosterone levels and memory. We then addressed whether there was a causal relationship between elevated corticosterone levels and impaired memory. If elevated corticosterone levels were a sufficient condition to impair memory, then exogenously administered corticosterone, alone, should have impaired performance. However, we found that spatial memory was not impaired in corticosterone-injected rats that were not exposed to the cat. This work demonstrates that an intermediate level of corticosterone correlated with optimal memory, and either a decrease or an increase in corticosterone levels, in conjunction with strong emotionality, impaired spatial memory. These findings indicate that fear-provoking conditions, which are known to engage the amygdala, interact with stress levels of corticosterone to influence hippocampal functioning.

Influence of predator stress on the consolidation versus retrieval of long-term spatial memory and hippocampal spinogenesis.

We have studied the influence of predator stress (30 min of cat exposure) on long-term (24 h) spatial memory and the density of spines in basilar dendrites of CA1 neurons. Predator stress occurred either immediately before water maze training (Stress Pre-Training) or before the 24 h memory test (Stress Pre-Retrieval). The Control (nonstress) group exhibited excellent long-term spatial memory and a robust increase in the density of stubby, but not mushroom, shaped spines. The Stress Pre-Training group had impaired long-term memory and did not exhibit any changes in spine density. The Stress Pre-Retrieval group was also impaired in long-term memory performance, but this group exhibited an increase in the density of stubby, but not mushroom, shaped spines, which was indistinguishable from the control group. These findings indicate that: (1) A single day of water maze training under control conditions produced intact long-term memory and an increase in the density of stubby spines in CA1; (2) Stress before training interfered with the consolidation of information into long-term memory and suppressed the training-induced increase in spine density; and (3) Stress immediately before the 24 h memory test trial impaired the retrieval of the stored memory, but did not reverse the training-induced increase in CA1 spine density. Overall, this work provides evidence of structural plasticity in dendrites of CA1 neurons which may be involved in the consolidation process, and how spinogenesis and memory are modulated by stress.

Competitive interactions between endogenous LTD and LTP in the hippocampus underlie the storage of emotional memories and stress-induced amnesia.

This speculative review serves two purposes. First, it as an extension of the ideas we developed in a previous review (Diamond et al., Hippocampus, 2004;14:281-291), and second, it is a rebuttal to Abraham's (Hippocampus, 2004;14:675-676) critique of that review. We had speculated on the functional significance of the finding that post-training LTP induction produces retrograde amnesia. We noted the similarities between the findings that strong tetanizing stimulation can produce LTP and retrograde amnesia, and that a strong emotional experience can produce a long-lasting memory and retrograde amnesia, as well. The commonalities between LTP induction and emotional learning provided the basis of our hypothesis that an emotional experience generates endogenous LTD/depotentiation, which reverses synaptic plasticity formed during previous learning experiences, and endogenous LTP, which underlies the storage of new information. Abraham raised several concerns with our review, including the criticism that our speculation "falters because there is no evidence that stress causes LTD or depotentiation," and that research on stress and hippocampus has "failed to report any LTP-like changes." Abraham's points are well-taken because stress, in isolation, does not appear to generate long-lasting changes in baseline measures of hippocampal excitability. Here, within the context of a reply to Abraham's critique, we have provided a review of the literature on the influence of stress, novelty, fear conditioning, and the retrieval of emotional memories on cognitive and physiological measures of hippocampal functioning. An emphasis of this review is our hypothesis that endogenous forms of depotentiation, LTD and LTP are generated only when arousing experiences occur in conjunction with memory-related activation of the hippocampus and amygdala. We conclude with speculation that interactions among the different forms of endogenous plasticity underlie a form of competition by synapses and memories for access to retrieval resources.

Stress generates emotional memories and retrograde amnesia by inducing an endogenous form of hippocampal LTP.

Models of the neurobiology of memory have been based on the idea that information is stored as distributed patterns of altered synaptic weights in neuronal networks. Accordingly, studies have shown that post-training treatments that alter synaptic weights, such as the induction of long-term potentiation (LTP), can interfere with retrieval. In these studies, LTP induction has been relegated to the status of a methodological procedure that serves the sole purpose of disturbing synaptic activity in order to impair memory. This perspective has been expressed, for example, by Martin and Morris (2002: Hippocampus 12:609-636), who noted that post-training LTP impairs memory by adding "behaviorally meaningless" noise to hippocampal neural networks. However, if LTP truly is a memory storage mechanism, its induction should represent more than just a means with which to disrupt memory. Since LTP induction produces retrograde amnesia, the formation of a new memory should also produce retrograde amnesia. In the present report, we suggest that one type of learning experience, the storage of fear-related (i.e., stressful) memories, is consistent with this prediction. Studies have shown that stress produces potent effects on hippocampal physiology, generates long-lasting memories, and induces retrograde amnesia, all through mechanisms in common with LTP. Based on these findings, we have developed the hypothesis that a stressful experience generates an endogenous form of hippocampal LTP that substitutes a new memory representation for preexisting representations. In summary, our hypothesis implicates the induction of endogenous synaptic plasticity by stress in the formation of emotional memories and in retrograde amnesia.