The startled seahorse: is the hippocampus necessary for contextual fear conditioning?

MDMA and memory, addiction, and depression: dose-effect analysis

RATIONALE

±3,4-Methylenedioxymethamphetamine (MDMA) is a recreational drug that shows substantial promise as a psychotherapeutic agent. Still, there is some concern regarding its behavioral toxicity, and its dose-effect relationship is poorly understood. We previously explored the role of dose in the cognitive effects of MDMA in a systematic review of existing literature and found no evidence in animals that MDMA impairs memory at low doses (< 3 mg/kg) but mixed results at high doses (≥ 3 mg/kg). Since this review comprised mostly of single-dose studies and an assortment of methodologies, an empirical dose-ranging study on this topic is warranted.

OBJECTIVES

The current study aims to evaluate the conclusion from our systematic review that 3 mg/kg may be the threshold for MDMA-induced amnesia, and to further understand the dose-effect relationship of MDMA on behavioral assays of memory, addiction, and depression.

METHODS

We systematically examined the effects of 0.01 to 10 mg/kg MDMA on Pavlovian fear conditioning; behavioral sensitization, conditioned place preference, and conditioned responding; and the Porsolt forced swim test in mice.

RESULTS

High doses of MDMA (≥ 3 mg/kg) produced amnesia of fear conditioning memory, some evidence of an addictive potential, and antidepressant effects, while low doses of MDMA (≤ 1 mg/kg) had no effect on these behaviors.

CONCLUSIONS

The present dose-ranging study provides further evidence that 3 mg/kg is the threshold for MDMA-induced amnesia. These findings, in addition to our systematic review, demonstrate that careful selection of MDMA dose is critical. High doses (≥ 3 mg/kg) should likely be avoided due to evidence that they can produce amnesia and addiction. Conversely, there is little evidence to suggest that low doses, which are usually administered in clinical studies (approximately 1-2 mg/kg), will lead to these same adverse effects. Ultra-low doses (< 1 mg/kg) are likely even safer and should be investigated for therapeutic effects in future studies.

Revisiting systems consolidation and the concept of consolidation

For more than 50 years, knowledge of memory processes has been based on the consolidation hypothesis, which postulates that new memories require time to become stabilized. Two forms of the consolidation model exist. The Cellular Consolidation concept is based upon retrograde amnesia induced by amnesic treatments, the severity of which decreases as the learning to treatment increases over minutes or hours. In contrast, The Systems Consolidation model is based on post-training hippocampal lesions, which produce more severe retrograde amnesia when induced after days than after weeks. Except for the temporal parameters, Cellular and Systems Consolidation show many similarities. Here we propose that Systems consolidation, much as Cellular Consolidation (see Gisquet- Verrier and Riccio, 2018), can be explained in terms of a form of state-dependency. Accordingly, lesions of the hippocampus induce a change in the internal state of the animal, which disrupts retrieval processes. But the effect of contextual change is known to decrease with the length of the retention intervals, consistent with time-dependent retrograde amnesia. We provide evidence supporting this new view.

Pavlovian Olfactory Fear Conditioning: Its Neural Circuity and Importance for Understanding Clinical Fear-Based Disorders

Odors have proven to be the most resilient trigger for memories of high emotional saliency. Fear associated olfactory memories pose a detrimental threat of potentially transforming into severe mental illness such as fear and anxiety-related disorders. Many studies have deliberated on auditory, visual and general contextual fear memory (CFC) processes; however, fewer studies have investigated mechanisms of olfactory fear memory. Evidence strongly suggests that the neuroanatomical representation of olfactory fear memory differs from that of auditory and visual fear memory. The aim of this review article is to revisit the literature regarding the understanding of the neurobiological process of fear conditioning and to illustrate the circuitry of olfactory fear memory.

Dopamine and norepinephrine transporter inhibition for long-term fear memory enhancement

Psychostimulants are highly effective cognitive-enhancing therapeutics yet have a significant potential for abuse and addiction. While psychostimulants likely exert their rewarding and addictive properties through dopamine transporter (DAT) inhibition, the mechanisms of their procognitive effects are less certain. By one prevalent view, psychostimulants exert their procognitive effects exclusively through norepinephrine transporter (NET) inhibition, however increasing evidence suggests that DAT also plays a critical role in their cognitive-enhancing properties, including long-term memory enhancement. The present experiments test the hypothesis that combined strong NET and weak DAT inhibition will mimic the fear memory-enhancing but not the addiction-related effects of psychostimulants in mice. We examined the effects of the high affinity NET inhibitors atomoxetine or nisoxetine and the low affinity DAT inhibitor bupropion, either alone or in combination, on short- and long-term memory of Pavlovian fear conditioning. We also examined the addiction-related effects of combined strong NET and weak DAT inhibition using conditioned place preference and a locomotor activity test. While atomoxetine or nisoxetine alone enhanced short-term fear memory, the addition of bupropion was required to significantly enhance long-term fear memory. Additionally, combined atomoxetine and bupropion did not produce substantial motor stimulation or place preference. These findings suggest that combining strong NET and weak DAT inhibition could lead to the development of a highly effective cognitive enhancer that lacks the potential for addiction.

The Role of Hippocampal Replay in Memory and Planning

The mammalian hippocampus is important for normal memory function, particularly memory for places and events. Place cells, neurons within the hippocampus that have spatial receptive fields, represent information about an animal’s position. During periods of rest, but also during active task engagement, place cells spontaneously recapitulate past trajectories. Such ‘replay’ has been proposed as a mechanism necessary for a range of neurobiological functions, including systems memory consolidation, recall and spatial working memory, navigational planning, and reinforcement learning. Focusing mainly, but not exclusively, on work conducted in rodents, we describe the methodologies used to analyse replay and review evidence for its putative roles. We identify outstanding questions as well as apparent inconsistencies in existing data, making suggestions as to how these might be resolved. In particular, we find support for the involvement of replay in disparate processes, including the maintenance of hippocampal memories and decision making. We propose that the function of replay changes dynamically according to task demands placed on an organism and its current level of arousal.

Differential impact of Met receptor gene interaction with early-life stress on neuronal morphology and behavior in mice

Early adversity in childhood increases the risk of anxiety, mood, and post-traumatic stress disorders in adulthood, and specific gene-by-environment interactions may increase risk further. A common functional variant in the promoter region of the gene encoding the human MET receptor tyrosine kinase (rs1858830 ‘C’ allele) reduces expression of MET and is associated with altered cortical circuit function and structural connectivity. Mice with reduced Met expression exhibit changes in anxiety-like and conditioned fear behavior, precocious synaptic maturation in the hippocampus, and reduced neuronal arbor complexity and synaptogenesis. These phenotypes also can be produced independently by early adversity in wild-type mice. The present study addresses the outcome of combining early-life stress and genetic influences that alter timing of maturation on enduring functional and structural phenotypes. Using a model of reduced Met expression (Met^+/−) and early-life stress from postnatal day 2–9, social, anxiety-like, and contextual fear behaviors in later life were measured. Mice that experienced early-life stress exhibited impairments in social interaction, whereas alterations in anxiety-like behavior and fear learning were driven by Met haploinsufficiency, independent of rearing condition. Early-life stress or reduced Met expression decreased arbor complexity of ventral hippocampal CA1 pyramidal neurons projecting to basolateral amygdala. Paradoxically, arbor complexity in Met^+/− mice was increased following early-life stress, and thus not different from arbors in wild-type mice raised in control conditions. The changes in dendritic morphology are consistent with the hypothesis that the physiological state of maturation of CA1 neurons in Met^+/− mice influences their responsiveness to early-life stress. The dissociation of behavioral and structural changes suggests that there may be phenotype-specific sensitivities to early-life stress.

Role of the bed nucleus of the stria terminalis in aversive learning and memory

Surviving threats in the environment requires brain circuits for detecting (or anticipating) danger and for coordinating appropriate defensive responses (e.g., increased cardiac output, stress hormone release, and freezing behavior). The bed nucleus of the stria terminalis (BNST) is a critical interface between the "affective forebrain"-including the amygdala, ventral hippocampus, and medial prefrontal cortex-and the hypothalamic and brainstem areas that have been implicated in neuroendocrine, autonomic, and behavioral responses to actual or anticipated threats. However, the precise contribution of the BNST to defensive behavior is unclear, both in terms of the antecedent stimuli that mobilize BNST activity and the consequent defensive reactions. For example, it is well known that the BNST is essential for contextual fear conditioning, but dispensable for fear conditioning to discrete conditioned stimuli (CSs), at least as indexed by freezing behavior. However, recent evidence suggests that there are circumstances in which contextual freezing may persist independent of the BNST. Furthermore, the BNST is involved in the reinstatement (or relapse) of conditioned freezing to extinguished discrete CSs. As such, there are critical gaps in understanding how the BNST contributes to fundamental processes involved in Pavlovian fear conditioning. Here, we attempt to provide an integrative account of BNST function in fear conditioning. We discuss distinctions between unconditioned stress and conditioned fear and the role of BNST circuits in organizing behaviors associated with these states. We propose that the BNST mediates conditioned defensive responses-not based on the modality or duration of the antecedent threat or the duration of the behavioral response to the threat-but rather as consequence the ability of an antecedent stimulus to predict when an aversive outcome will occur (i.e., its temporal predictability). We argue that the BNST is not uniquely mobilized by sustained threats or uniquely involved in organizing sustained fear responses. In contrast, we argue that the BNST is involved in organizing fear responses to stimuli that poorly predict when danger will occur, no matter the duration, modality, or complexity of those stimuli. The concepts discussed in this review are critical to understanding the contribution of the human BNST to fear and anxiety disorders.

Bridging the Gap: Towards a cell-type specific understanding of neural circuits underlying fear behaviors

Fear and anxiety-related disorders are remarkably common and debilitating, and are often characterized by dysregulated fear responses. Rodent models of fear learning and memory have taken great strides towards elucidating the specific neuronal circuitries underlying the learning of fear responses. The present review addresses recent research utilizing optogenetic approaches to parse circuitries underlying fear behaviors. It also highlights the powerful advances made when optogenetic techniques are utilized in a genetically defined, cell-type specific, manner. The application of next-generation genetic and sequencing approaches in a cell-type specific context will be essential for a mechanistic understanding of the neural circuitry underlying fear behavior and for the rational design of targeted, circuit specific, pharmacologic interventions for the treatment and prevention of fear-related disorders.

Overexpression of Protein Kinase Mζ in the Hippocampus Enhances Long-Term Potentiation and Long-Term Contextual But Not Cued Fear Memory in Rats

The persistently active protein kinase Mζ (PKMζ) has been found to be involved in the formation and maintenance of long-term memory. Most of the studies investigating PKMζ, however, have used either putatively unselective inhibitors or conventional knock-out animal models in which compensatory mechanisms may occur. Here, we overexpressed an active form of PKMζ in rat hippocampus, a structure highly involved in memory formation, and embedded in several neural networks. We investigated PKMζ's influence on synaptic plasticity using electrophysiological recordings of basal transmission, paired pulse facilitation, and LTP and combined this with behavioral cognitive experiments addressing formation and retention of both contextual memory during aversive conditioning and spatial memory during spontaneous exploration. We demonstrate that hippocampal slices overexpressing PKMζ show enhanced basal transmission, suggesting a potential role of PKMζ in postsynaptic AMPAR trafficking. Moreover, the PKMζ-overexpressing slices augmented LTP and this effect was not abolished by protein-synthesis blockers, indicating that PKMζ induces enhanced LTP formation in a protein-synthesis-independent manner. In addition, we found selectively enhanced long-term memory for contextual but not cued fear memory, underlining the theory of the hippocampus' involvement in the contextual aspect of aversive reinforced tasks. Memory for spatial orientation during spontaneous exploration remained unaltered, suggesting that PKMζ may not affect the neural circuits underlying spontaneous tasks that are different from aversive tasks. In this study, using an overexpression strategy as opposed to an inhibitor-based approach, we demonstrate an important modulatory role of PKMζ in synaptic plasticity and selective memory processing.

SIGNIFICANCE STATEMENT

Most of the literature investigating protein kinase Mζ (PKMζ) used inhibitors with selectivity that has been called into question or conventional knock-out animal models in which compensatory mechanisms may occur. To avoid these issues, some studies have been done using viral overexpression of PKMζ in different brain structures to show cognitive enhancement. However, electrophysiological experiments were exclusively done in knock-out models or inhibitory studies to show depletion of LTP. There was no study showing the effect of PKMζ overexpression in the hippocampus on behavior and LTP experiments. To our knowledge, this is the first study to combine these aspects with the result of enhanced memory for contextual fear memory and to show enhanced LTP in hippocampal slices overexpressing PKMζ.

Thyroxine administration prevents matrilineal intergenerational consequences of in utero ethanol exposure in rats

The neurodevelopmental fetal alcohol spectrum disorder (FASD) is characterized by cognitive and behavioral deficits in the offspring. Conferring the deficits to the next generation would increase overall FASD disease burden and prevention of this transmission could be highly significant. Prior studies showed the reversal of these behavioral deficits by low dose thyroxine (T4) supplementation to the ethanol-consuming mothers. Here we aim to identify whether prenatal ethanol (PE) exposure impairs hippocampus-dependent learning and memory in the second-generation (F2) progeny, and whether T4 administration to the ethanol-consuming dam can prevent it. Sprague-Dawley (S) dams received control diets (ad libitum and nutritional control) or ethanol containing liquid diet with and without simultaneous T4 (0.3mg/L diet) administration. Their offspring (SS F1) were mated with naive Brown Norway (B) males and females generating the SB F2 and BS F2 progeny. Hippocampus-dependent contextual fear memory and hippocampal expression of the thyroid hormone-regulated type 3 deiodinase, (Dio3) and neurogranin (Nrgn) were assessed. SS F1 PE-exposed females and their SB F2 progeny exhibited fear memory deficits. T4 administration to the mothers of F1 females reversed these deficits. Although SS F1 PE-exposed males also experienced fear memory deficit, this was neither transmitted to their BS F2 offspring nor reversed by prenatal T4 treatment. Hippocampal Dio3 and Nrgn expression showed similar pattern of changes. Grandmaternal ethanol consumption during pregnancy affects fear memory of the matrilineal second-generation progeny. Low dose T4 supplementation prevents this process likely via altering allele-specific and total expression of Dio3 in the hippocampus.

Contextual fear conditioning modulates hippocampal AMPA-, GluN1- and serotonin receptor 5-HT1A-containing receptor complexes

Although the roles of AMPAR (α-amino-3-hydroxyl-5-methyl-4-isoxazole propionate receptor), NMDAR (N-methyl-D-aspartate receptor) and 5HT1AR (5-hydroxytryptamine sub type 1A) in contextual fear conditioning (cFC) have been studied, information about receptor-containing complexes (RC) is not available. Moreover, there are no data on membrane or endosomal NMDA-, 5HT1A- or AMPA-RC levels, which would likely be indicative of the trafficking of these receptors. cFC was carried out in C57BL/6j mice and animals were sacrificed in the individual phases and hippocampi were taken for the determination of receptor complex and subunit levels using BN- and SDS-PAGE with subsequent Western blotting. GluA1-4, GluN1 (NMDAR subunit NR1)- and 5HT1A-RC were differentially regulated during the individual phases and differentially regulated in the membrane and endosomal fractions. GluA1-RC levels in the membrane were increased in acquisition, consolidation and retrieval phases; GluA2-RC and GluA3-RC membrane levels were reduced and modulated in early endosomes during these phases. GluA4-RC and GluN1-RC levels as well as their subunits showed the same pattern in the membrane during consolidation while 5HT1A-RC membrane and endosome levels were mainly increased during consolidation and retrieval. Taken together, the results suggest that levels of 5-HT1A-RC, NMDA-RC and AMPA-RC and subunits in membrane and endosomal preparations are paralleling individual phases of cFC. The findings from the current study suggest phase-specific receptor complex and subunit formation and propose that receptor complexes should be examined in parallel with receptor subunits to aid the interpretation of previous work and to design future work on neurotransmitter receptors in memory paradigms.

Cortical abnormalities and non-spatial learning deficits in a mouse model of CranioFrontoNasal syndrome

Eph receptors and their ephrin ligands play critical roles in the development of the nervous system, however, less is known about their functions in the adult brain. Here, we investigated the function of ephrinB1, an ephrinB family member that is mutated in CranioFrontoNasal Syndrome. We show that ephrinB1 deficient mice (EfnB1^Y/−) demonstrate spared spatial learning and memory but exhibit exclusive impairment in non-spatial learning and memory tasks. We established that ephrinB1 does not control learning and memory through direct modulation of synaptic plasticity in adults, since it is not expressed in the adult brain. Rather we show that the cortex of EfnB1^Y/− mice displayed supernumerary neurons, with a particular increase in calretinin-positive interneurons. Further, the increased neuron number in EfnB1^Y/− mutants correlated with shorter dendritic arborization and decreased spine densities of cortical pyramidal neurons. Our findings indicate that ephrinB1 plays an important role in cortical maturation and that its loss has deleterious consequences on selective cognitive functions in the adult.

Indistinguishable pattern of amygdala and hippocampus rewiring following tone or contextual fear conditioning in C57BL/6 mice

Changes in neuronal connectivity occurring upon the formation of aversive memory were examined in C57BL/6 (C57) mice 24 h after they were trained for tone fear conditioning (TFC) and contextual fear conditioning (CFC). Although TFC and CFC are amenable to distinct learning systems each involving a specific neural substrate, we found that mice trained in the two protocols showed the same increase in spine density and spine size in class I basolateral amygdala (BLA) and in dorsal hippocampus CA1 pyramidal neurons. Our findings suggest that, because of their remarkably functional hippocampus, C57 mice might engage this region in any fear situation they face. These observations raise a point relevant to aversive memory studies, i.e., how the peculiarity of memory in certain individuals impacts on the components of the fear circuitry. It is suggested that enhanced connectivity in brain regions dispensable for specific forms of learning could considerably increase the resistance of aversive memory traces to treatments aimed at disrupting them.

MHC class I immune proteins are critical for hippocampus-dependent memory and gate NMDAR-dependent hippocampal long-term depression

Memory impairment is a common feature of conditions that involve changes in inflammatory signaling in the brain, including traumatic brain injury, infection, neurodegenerative disorders, and normal aging. However, the causal importance of inflammatory mediators in cognitive impairments in these conditions remains unclear. Here we show that specific immune proteins, members of the major histocompatibility complex class I (MHC class I), are essential for normal hippocampus-dependent memory, and are specifically required for NMDAR-dependent forms of long-term depression (LTD) in the healthy adult hippocampus. In β2m^−/−TAP^−/−mice, which lack stable cell-surface expression of most MHC class I proteins, NMDAR-dependent LTD in area CA1 of adult hippocampus is abolished, while NMDAR-independent forms of potentiation, facilitation, and depression are unaffected. Altered NMDAR-dependent synaptic plasticity in the hippocampus of β2m^−/−TAP^−/−mice is accompanied by pervasive deficits in hippocampus-dependent memory, including contextual fear memory, object recognition memory, and social recognition memory. Thus normal MHC class I expression is essential for NMDAR-dependent hippocampal synaptic depression and hippocampus-dependent memory. These results suggest that changes in MHC class I expression could be an unexpected cause of disrupted synaptic plasticity and cognitive deficits in the aging, damaged, and diseased brain.

RO4938581, a GABAAα5 modulator, displays strong CYP1A2 autoinduction properties in rats

Autoinduction in drug metabolism is a known phenomenon observed when a drug induces the enzymes responsible for its own metabolism. The potency, rate and extent of autoinduction following a given treatment paradigm may have therapeutic implications in clinic as well as for in vivo pharmacological assessments in animals. RO4938581, an imidazo-triazolo-benzodiazepine, is a novel GABAAα5 negative modulator recently pursued for the treatment of cognitive dysfunctions. As circulating plasma levels of RO4938581 were shown to decrease rapidly after repeated dosing in rats, with CYP1A2 being involved in the metabolism of the compound, we examined the potential role of RO4938581-mediated autoinduction of CYP1A2. Incubation of rat hepatocytes with RO4938581 revealed potent CYP1A2 induction with significant increase in enzymatic activity at concentrations of 0.1nM and RO4938581 was shown to be 700-fold more potent than β-napththoflavone. Ex vivo studies revealed a 7-fold increase in metabolic CYP1A2 activity in liver microsomes prepared from rats administered with 0.1mg/kg of RO4938581 24h before. This induction profile was reflected in vivo in pharmacokinetic studies in rats where an 8-fold reduction in plasma exposure was observed after a second dose. The reduction in plasma exposures due to CYP1A2 autoinduction were confirmed functionally in contextual fear conditioning paradigm in rats, where a positive pharmacological effect observed after acute drug administration disappeared completely after sub-chronic dosing. Together, these findings suggest that RO4938581 possesses potent CYP1A2 autoinductive properties in rats and may serve as a tool for mechanistic metabolism or drug-drug interaction studies encircling this enzyme in rats.

Identification of functional circuitry between retrosplenial and postrhinal cortices during fear conditioning

The retrosplenial cortex (RSP) and postrhinal cortex (POR) are heavily interconnected with medial temporal lobe structures involved in learning and memory. Previous studies indicate that RSP and POR are necessary for contextual fear conditioning, but it remains unclear whether these regions contribute individually or instead work together as a functional circuit to modulate learning and/or memory. In Experiment 1, learning-related neuronal activity was assessed in RSP from home cage, shock-only, context-only, or fear-conditioned rats using real-time PCR and immunohistochemical methods to quantify immediate-early gene expression. A significant increase in activity-regulated cytoskeleton-associated protein (Arc) mRNA and Arc and c-Fos protein expression was detected in RSP from fear-conditioned rats compared with all other groups. In Experiment 2, retrograde tracing combined with immunohistochemistry revealed that, compared with controls, a significant proportion of cells projecting from RSP to POR were immunopositive for c-Fos in fear-conditioned rats. These results demonstrate that neurons projecting from RSP to POR are indeed active during fear conditioning. In Experiment 3, a functional disconnection paradigm was used to further examine the interaction between RSP and POR during fear conditioning. Compared with controls, rats with unilateral lesions of RSP and POR on opposite sides of the brain exhibited impaired contextual fear memory, whereas rats with unilateral lesions in the same hemisphere displayed intermediate levels of freezing compared with controls and rats with contralateral lesions. Collectively, these results are the first to show that RSP and POR function as a cortical network necessary for contextual fear learning and memory.

Exposure to kynurenic acid during adolescence produces memory deficits in adulthood

The glia-derived molecule kynurenic acid (KYNA) is an antagonist of α7 nicotinic acetylcholine receptors and the glycine(B) binding site on n-methyl-d-aspartateglutamate receptors, both of which have critical roles in neural plasticity as well as learning and memory. KYNA levels are increased in the brains and cerebral spinal fluid of persons with schizophrenia, leading to the notion that changes in KYNA concentration might contribute to cognitive dysfunction associated with this disorder. Indeed, recent studies indicate that increasing endogenous KYNA concentration by administering l-kynurenine (L-KYN, the precursor of KYNA) impairs spatial as well as contextual learning and memory in adult rats. In the present study, rats were treated with L-KYN (100 mg/kg) throughout adolescence to increase endogenous KYNA concentration during this critical time in brain development. Rats were then tested drug-free as adults to test the hypothesis that exposure to elevated levels of KYNA during development may contribute to cognitive dysfunction later in life. Consistent with prior studies in which adult rats were treated acutely with L-KYN, juvenile rats exposed to increased KYNA concentration during adolescence exhibited deficits in contextual fear memory, but cue-specific fear memory was not impaired. In addition, rats treated with L-KYN as adolescents were impaired on a novel object recognition memory task when tested as adults. The memory deficits could not be explained by drug-induced changes in locomotor activity or shock sensitivity. Together, these findings add to the growing literature supporting the notion that exposure to increased concentration of KYNA may contribute to cognitive deficits typically observed in schizophrenia.

Fear conditioning is disrupted by damage to the postsubiculum

The hippocampus plays a central role in spatial and contextual learning and memory, however relatively little is known about the specific contributions of parahippocampal structures that interface with the hippocampus. The postsubiculum (PoSub) is reciprocally connected with a number of hippocampal, parahippocampal and subcortical structures that are involved in spatial learning and memory. In addition, behavioral data suggest that PoSub is needed for optimal performance during tests of spatial memory. Together, these data suggest that PoSub plays a prominent role in spatial navigation. Currently it is unknown whether the PoSub is needed for other forms of learning and memory that also require the formation of associations among multiple environmental stimuli. To address this gap in the literature we investigated the role of PoSub in Pavlovian fear conditioning. In Experiment 1 male rats received either lesions of PoSub or Sham surgery prior to training in a classical fear conditioning procedure. On the training day a tone was paired with foot shock three times. Conditioned fear to the training context was evaluated 24 hr later by placing rats back into theconditioning chamber without presenting any tones or shocks. Auditory fear was assessed on the third day by presenting the auditory stimulus in a novel environment (no shock). PoSub-lesioned rats exhibited impaired acquisition of the conditioned fear response as well as impaired expression of contextual and auditory fear conditioning. In Experiment 2, PoSub lesions were made 1 day after training to specifically assess the role of PoSub in fear memory. No deficits in the expression of contextual fear were observed, but freezing to the tone was significantly reduced in PoSub-lesioned rats compared to shams. Together, these results indicate that PoSub is necessary for normal acquisition of conditioned fear, and that PoSub contributes to the expression of auditory but not contextual fear memory.

Post-training unilateral amygdala lesions selectively impair contextual fear memories

The basolateral amygdala (BLA) and the dorsal hippocampus (dHPC) are both structures with key roles in contextual fear conditioning. During fear conditioning, it is postulated that contextual representations of the environment are formed in the hippocampus, which are then associated with foot shock in the amygdala. However, it is not known to what extent a functional connection between these two structures is required. This study investigated the effect on contextual and cued fear conditioning of disconnecting the BLA and dHPC, using asymmetrically placed, excitotoxic unilateral lesions. Post-training lesions selectively impaired contextual, but not cued, fear, while pretraining lesions resulted in a similar but nonsignificant pattern of results. This effect was unexpectedly observed in both the contralateral disconnection group and the anticipated ipsilateral control, which prompted further examination of individual unilateral lesions of BLA and dHPC. Post-training unilateral dHPC lesions had no effect on contextual fear memories while bilateral dHPC lesions and unilateral BLA lesions resulted in a near total abolition of contextual fear but not cued conditioned fear. Again, pretraining unilateral BLA lesions resulted in a strong but nonsignificant trend to the impairment of contextual fear. Furthermore, an analysis of context test-induced Fos protein expression in the BLA contralateral to the lesion site revealed no differences between post-training SHAM and unilateral BLA lesioned animals. Therefore, post-training unilateral lesions of the BLA are sufficient to severely impair contextual, but not cued, fear memories.

Sustained expression of brain-derived neurotrophic factor is required for maintenance of dendritic spines and normal behavior

Brain-derived neurotrophic factor (BDNF) plays important roles in the development, maintenance, and plasticity of the mammalian forebrain. These functions include regulation of neuronal maturation and survival, axonal and dendritic arborization, synaptic efficacy, and modulation of complex behaviors including depression and spatial learning. Although analysis of mutant mice has helped establish essential developmental functions for BDNF, its requirement in the adult is less well documented. We have studied late-onset forebrain-specific BDNF knockout (CaMK-BDNF(KO)) mice, in which BDNF is lost primarily from the cortex and hippocampus in early adulthood, well after BDNF expression has begun in these structures. We found that although CaMK-BDNF(KO) mice grew at a normal rate and can survive more than a year, they had smaller brains than wild-type siblings. The CaMK-BDNF(KO) mice had generally normal behavior in tests for ataxia and anxiety, but displayed reduced spatial learning ability in the Morris water task and increased depression in the Porsolt swim test. These behavioral deficits were very similar to those we previously described in an early-onset forebrain-specific BDNF knockout. To identify an anatomical correlate of the abnormal behavior, we quantified dendritic spines in cortical neurons. The spine density of CaMK-BDNF(KO) mice was normal at P35, but by P84, there was a 30% reduction in spine density. The strong similarities we find between early- and late-onset BDNF knockouts suggest that BDNF signaling is required continuously in the CNS for the maintenance of some forebrain circuitry also affected by developmental BDNF depletion.

Enhanced intrinsic excitability and EPSP-spike coupling accompany enriched environment-induced facilitation of LTP in hippocampal CA1 pyramidal neurons

Environmental enrichment (EE) is a well-established paradigm for studying naturally occurring changes in synaptic efficacy in the hippocampus that underlie experience-induced modulation of learning and memory in rodents. Earlier research on the effects of EE on hippocampal plasticity focused on long-term potentiation (LTP). Whereas many of these studies investigated changes in synaptic weight, little is known about potential contributions of neuronal excitability to EE-induced plasticity. Here, using whole-cell recordings in hippocampal slices, we address this gap by analyzing the impact of EE on both synaptic plasticity and intrinsic excitability of hippocampal CA1 pyramidal neurons. Consistent with earlier reports, EE increased contextual fear memory and dendritic spine density on CA1 cells. Furthermore, EE facilitated LTP at Schaffer collateral inputs to CA1 pyramidal neurons. Analysis of the underlying causes for enhanced LTP shows EE to increase the frequency but not amplitude of miniature excitatory postsynaptic currents. However, presynaptic release probability, assayed using paired-pulse ratios and use-dependent block of N-methyl-d-aspartate receptor currents, was not affected. Furthermore, CA1 neurons fired more action potentials (APs) in response to somatic depolarization, as well as during the induction of LTP. EE also reduced spiking threshold and after-hyperpolarization amplitude. Strikingly, this EE-induced increase in excitability caused the same-sized excitatory postsynaptic potential to fire more APs. Together, these findings suggest that EE may enhance the capacity for plasticity in CA1 neurons, not only by strengthening synapses but also by enhancing their efficacy to fire spikes—and the two combine to act as an effective substrate for amplifying LTP.

Age sensitivity of behavioral tests and brain substrates of normal aging in mice

Knowledge of age sensitivity, the capacity of a behavioral test to reliably detect age-related changes, has utility in the design of experiments to elucidate processes of normal aging. We review the application of these tests in studies of normal aging and compare and contrast the age sensitivity of the Barnes maze, eyeblink classical conditioning, fear conditioning, Morris water maze, and rotorod. These tests have all been implemented to assess normal age-related changes in learning and memory in rodents, which generalize in many cases to age-related changes in learning and memory in all mammals, including humans. Behavioral assessments are a valuable means to measure functional outcomes of neuroscientific studies of aging. Highlighted in this review are the attributes and limitations of these measures in mice in the context of age sensitivity and processes of brain aging. Attributes of these tests include reliability and validity as assessments of learning and memory, well-defined neural substrates, and sensitivity to neural and pharmacological manipulations and disruptions. These tests engage the hippocampus and/or the cerebellum, two structures centrally involved in learning and memory that undergo functional and anatomical changes in normal aging. A test that is less well represented in studies of normal aging, the context pre-exposure facilitation effect (CPFE) in fear conditioning, is described as a method to increase sensitivity of contextual fear conditioning to changes in the hippocampus. Recommendations for increasing the age sensitivity of all measures of normal aging in mice are included, as well as a discussion of the potential of the under-studied CPFE to advance understanding of subtle hippocampus-mediated phenomena.

The effects of acute treatment with escitalopram on the different stages of contextual fear conditioning are reversed by atomoxetine

RATIONALE

Although the antidepressant and anxiolytic effects of selective serotonin reuptake inhibitors and serotonin-noradrenaline reuptake inhibitors are well-documented, less is known about their cognitive effects.

OBJECTIVE

Escitalopram, a selective serotonin reuptake inhibitor, and atomoxetine, a selective noradrenaline reuptake inhibitor, were used to evaluate the interaction between noradrenergic and serotonergic neurotransmission in the modulation of contextual fear conditioning in rats.

METHODS

Contextual fear-conditioning test was used to investigate the acute effects of escitalopram, alone or in combination with atomoxetine, in different stages of learning and memory in rats. Furthermore, microdialysis in freely moving animals was used to investigate the effect of escitalopram on serotonin, dopamine, and noradrenaline levels in the rat hippocampus.

RESULTS

Escitalopram significantly increased conditioned responses when applied before the acquisition, but decreased responses, when applied before the recall test. When administered during memory consolidation, escitalopram dose-dependently enhanced conditioned responding. These effects were blocked by atomoxetine. Escitalopram (at a dose that affects memory consolidation) increased hippocampal serotonin levels fourfold without changing dopamine or noradrenaline. Atomoxetine, at dose levels that blocked the effects of escitalopram on contextual fear conditioning, increased the extracellular levels of noradrenaline eightfold but did not change dopamine or serotonin. A combined treatment of escitalopram and atomoxetine caused a significant attenuation of escitalopram-induced increase in serotonin levels, while noradrenaline levels were not affected.

CONCLUSIONS

These findings indicate that escitalopram affects fear memory in rats, likely modulated by increases in serotonin levels in the brain. This effect is impaired by atomoxetine, probably due to a noradrenaline-mediated decrease in serotonin levels. Further studies are warranted to study the effects of potential differences among antidepressant therapies on long-term cognitive outcomes.

Automated assessment of pavlovian conditioned freezing and shock reactivity in mice using the video freeze system

The Pavlovian conditioned freezing paradigm has become a prominent mouse and rat model of learning and memory, as well as of pathological fear. Due to its efficiency, reproducibility and well-defined neurobiology, the paradigm has become widely adopted in large-scale genetic and pharmacological screens. However, one major shortcoming of the use of freezing behavior has been that it has required the use of tedious hand scoring, or a variety of proprietary automated methods that are often poorly validated or difficult to obtain and implement. Here we report an extensive validation of the Video Freeze system in mice, a “turn-key” all-inclusive system for fear conditioning in small animals. Using digital video and near-infrared lighting, the system achieved outstanding performance in scoring both freezing and movement. Given the large-scale adoption of the conditioned freezing paradigm, we encourage similar validation of other automated systems for scoring freezing, or other behaviors.

Time-limited involvement of dorsal hippocampus in unimodal discriminative contextual conditioning

Converging evidence examining the effects of post-training manipulations of the hippocampus suggests that the hippocampus may play a time-limited role in the maintenance of a variety of forms of memory. In particular, either lesions or inactivation of the dorsal hippocampus results in many cases in a time-limited retrograde impairment in nondiscriminative contextual conditioning paradigms. However, the extent to which hippocampal manipulations result in a time-limited retrograde amnesia for a variety of forms of learning has recently been called into question (reviewed in Sutherland, Sparks, & Lehmann (2010)). The present study examined the effect of inactivation of the dorsal hippocampus either 7, 28, or 42 days following training in an explicitly nonspatial, discriminative contextual conditioning paradigm (Otto & Poon, 2006; Parsons & Otto, 2008). Inactivation of the dorsal hippocampus resulted in a significant deficit in the expression of contextual conditioning at 7 and 28 days, but not 42 days, following training. Importantly, inactivation of the hippocampus did not affect either baseline freezing levels or conditioning to an explicit CS. Together with previous data exploring hippocampal contributions to discriminative unimodal contextual conditioning, these data suggest that the hippocampus may play a particularly prominent role in the temporary maintenance of memory in discriminative contextual paradigms.

Sleep deprivation and Pavlovian fear conditioning

Sleep has been suggested to play a role in memory consolidation. Prior rodent studies have used sleep deprivation to examine this relationship. First, we reexamined the effects of sleep deprivation on Pavlovian fear conditioning. We found that the deprivation method itself (i.e., gentle handling) induced deficits independent of sleep. Second, we examined an alternative method of sleep deprivation using amphetamine and found that this method failed to induce amnesia. These data indicate that sleep deprivation is a problematic way to examine the role of sleep in memory consolidation, and an alternative paradigm is proposed.

Contextual, but not auditory, fear conditioning is disrupted by neurotoxic selective lesion of the basal nucleus of amygdala in rats

The basolateral amygdala complex (BLA) is involved in acquisition of contextual and auditory fear conditioning. However, the BLA is not a single structure but comprises a group of nuclei, including the lateral (LA), basal (BA) and accessory basal (AB) nuclei. While it is consensual that the LA is critical for auditory fear conditioning, there is controversy on the participation of the BA in fear conditioning. Hodological and neurophysiological findings suggest that each of these nuclei processes distinct information in parallel; the BA would deal with polymodal or contextual representations, and the LA would process unimodal or elemental representations. Thus, it seems plausible to hypothesize that the BA is required for contextual, but not auditory, fear conditioning. This hypothesis was evaluated in Wistar rats submitted to multiple-site ibotenate-induced damage restricted to the BA and then exposed to a concurrent contextual and auditory fear conditioning training followed by separated contextual and auditory conditioning testing. Differing from electrolytic lesion and lidocaine inactivation, this surgical approach does not disturb fibers of passage originating in other brain areas, restricting damage to the aimed nucleus. Relative to the sham-operated controls, rats with selective damage to the BA exhibited disruption of performance in the contextual, but not the auditory, component of the task. Thus, while the BA seems required for contextual fear conditioning, it is not critical for both an auditory-US association, nor for the expression of the freezing response.

L-kynurenine treatment alters contextual fear conditioning and context discrimination but not cue-specific fear conditioning

The glia-derived molecule kynurenic acid (KYNA) is an antagonist of alpha7 nicotinic acetylcholine receptors and the glycine(B) binding site on NMDA receptors. KYNA levels are elevated in the brain and cerebrospinal fluid of persons with schizophrenia and Alzheimer's disease, both of which are characterized by deficits in contextual learning and memory. The present study tested the hypothesis that increases in KYNA concentration would impair contextual fear conditioning but spare cue-specific fear conditioning. Rats in each experiment received injections of vehicle solution or l-kynurenine (L-KYN, 100mg/kg), the precursor for KYNA. Administration of L-KYN has been shown to produce clinically relevant increases in KYNA concentration. In Experiment 1, L-KYN-treated rats exhibited impaired contextual fear memory compared to control rats, while fear conditioning to a discrete auditory cue was unaffected. In Experiment 2, rats were trained to discriminate between two different training environments, one in which foot shock was delivered and one that was not paired with foot shock. Although both groups of rats eventually learned the discrimination, learning was slower in L-KYN-treated rats. The results of Experiment 3 demonstrated that the deficits in context discrimination could not be explained by the preferential use of an elemental learning strategy by L-KYN-treated rats. Together, these findings indicate that elevated concentration of endogenous KYNA interferes with contextual learning and memory and support the notion that increased concentration of KYNA may contribute to cognitive dysfunction. In addition, these data provide new insight into how novel 'gliotransmitters' may modulate neuronal function and behavior.

Pavlovian fear conditioning as a behavioral assay for hippocampus and amygdala function: cautions and caveats

Pavlovian fear conditioning has become an important model for investigating the neural substrates of learning and memory in rats, mice and humans. The hippocampus and amygdala are widely believed to be essential for fear conditioning to contexts and discrete cues, respectively. Indeed, this parsing of function within the fear circuit has been used to leverage fear conditioning as a behavioral assay of hippocampal and amygdala function, particularly in transgenic mouse models. Recent work, however, blurs the anatomical segregation of cue and context conditioning and challenges the necessity for the hippocampus and amygdala in fear learning. Moreover, nonassociative factors may influence the performance of fear responses under a variety of conditions. Caution must therefore be exercised when using fear conditioning as a behavioral assay for hippocampal- and amygdala-dependent learning.

Memory and psychostimulants: modulation of Pavlovian fear conditioning by amphetamine in C57BL/6 mice

RATIONALE AND OBJECTIVES

With the use of prescription stimulants on the rise, it is important to examine the cognitive effects of low and moderate doses of stimulants rather than only those typical of addicts.

MATERIALS AND METHODS

The present study examined the effects a range of doses (0.005-8 mg/kg) of D: -amphetamine sulfate on cued and contextual Pavlovian fear conditioning in mice.

RESULTS

In agreement with previous research, subjects administered with a moderately high dose of amphetamine (8 mg/kg) pre-training, typical of what addicts might take, displayed impaired conditioned freezing when tested off-drug. Alternately, subjects injected with a very low dose of amphetamine (0.005, 0.025, or 0.05 mg/kg) pre-training, similar to the therapeutic doses for attention deficit hyperactivity disorder, displayed enhanced memory when tested off-drug. A control study showed that these effects were not due to state-dependent learning.

CONCLUSIONS

Thus, dose is a critical determinant of the cognitive effects of psychostimulants.

Calorie restriction alters physical performance but not cognition in two models of altered neuroendocrine signaling

A major neuroendocrinological effect of calorie restriction (CR) is induction of neuropeptide Y (NPY) in the arcuate nucleus (ARC). Aside from its appetite-stimulating effects, NPY is thought to be involved in the modulation of behavioral processes including anxiety and learning and memory. In the present study physical fitness, anxiety, and learning/memory-related tasks were assessed in mice lacking NPY or a functional ARC after dietary manipulation by CR. Physical fitness was improved by CR when measured by inclined screen and rotarod, and this diet effect was not affected by NPY or ARC status. As has been observed previously, the NPY knockout mice displayed heightened anxiety in an open field. This phenotype was not fully recapitulated in the ARC-lesioned model. CR affected neither total locomotor activity in the open field nor thigmotaxic behavior in these models. Neither NPY nor CR had a significant effect on Morris water maze performance; however, ARC-damaged mice were unable to learn the task, and this deficit was not corrected by CR. We conclude that despite established effects of CR on ARC signaling, our results suggest a mechanistic separation between the two where behavior is concerned.

Retrieval of emotional memories

Long-term memories are influenced by the emotion experienced during learning as well as by the emotion experienced during memory retrieval. The present article reviews the literature addressing the effects of emotion on retrieval, focusing on the cognitive and neurological mechanisms that have been revealed. The reviewed research suggests that the amygdala, in combination with the hippocampus and prefrontal cortex, plays an important role in the retrieval of memories for emotional events. The neural regions necessary for online emotional processing also influence emotional memory retrieval, perhaps through the reexperience of emotion during the retrieval process.

Cholinergic modulation of Pavlovian fear conditioning in rats: differential effects of intrahippocampal infusion of mecamylamine and methyllycaconitine

The cholinergic system has consistently been implicated in Pavlovian fear conditioning. Considerable work has been done to localize specific nicotinic receptor subtypes in the hippocampus and determine their functional importance; however, the specific function of many of these subtypes has yet to be determined. An alpha7 nicotinic antagonist methyllycaconitine (MLA) (35 microg), and a broad spectrum non-alpha7 nicotinic antagonist mecamylamine (35 microg) was injected directly into the dorsal hippocampus or overlying cortex either 15 min pre-, 1 min post-, or 6h post-fear conditioning. One week after conditioning, retention of contextual and cue (tone) conditioning were assessed. A significant impairment in retention of contextual fear was observed when mecamylamine was injected 15 min pre- and 1 min post-conditioning. No significant impairment was observed when mecamylamine was injected 6h post-conditioning. Likewise, a significant impairment in retention of contextual fear was observed when MLA was injected 1 min post-conditioning; however, in contrast, MLA did not show any significant impairments when injected 15 min pre-conditioning, but did show a significant impairment when injected 6h post-conditioning. There were no significant impairments observed when either drug was injected into overlying cortex. No significant impairments were observed in cue conditioning for either drug. In general, specific temporal dynamics involved in nicotinic receptor function were found relative to time of receptor dysfunction. The results indicate that the greatest deficits in long-term retention (1 week) of contextual fear are produced by central infusion of MLA minutes to hours post-conditioning or mecamylamine within minutes of conditioning.

Cocaine and Pavlovian fear conditioning: dose-effect analysis

Emerging evidence suggests that cocaine and other drugs of abuse can interfere with many aspects of cognitive functioning. The authors examined the effects of 0.1-15mg/kg of cocaine on Pavlovian contextual and cued fear conditioning in mice. As expected, pre-training cocaine dose-dependently produced hyperactivity and disrupted freezing. Surprisingly, when the mice were tested off-drug later, the group pre-treated with a moderate dose of cocaine (15mg/kg) displayed significantly less contextual and cued memory, compared to saline control animals. Conversely, mice pre-treated with a very low dose of cocaine (0.1mg/kg) showed significantly enhanced fear memory for both context and tone, compared to controls. These results were not due to cocaine's anesthetic effects, as shock reactivity was unaffected by cocaine. The data suggest that despite cocaine's reputation as a performance-enhancing and anxiogenic drug, this effect is seen only at very low doses, whereas a moderate dose disrupts hippocampus and amygdala-dependent fear conditioning.

Impaired trace fear conditioning following neonatal ethanol: reversal by choline

Neonatal ethanol exposure in animals results in performance deficits on tests of hippocampus-dependent spatial memory, and recent studies have shown that extra dietary choline can ameliorate some of these impairments. In this experiment, rats were administered 5.25 g/kg ig ethanol per day or sham intubations on Postnatal Days (PD) 4-9 and choline (0.1 ml of an 18.8 mg/ml solution) or saline subcutaneously on PD 4-20. On PD 30, rats were given delay or trace fear conditioning trials and were tested for conditioned stimulus-elicited freezing 24 hr later. Neonatal ethanol produced a profound impairment in trace conditioning that was reversed by choline. Groups did not differ in delay conditioned responding, indicating that neonatal ethanol produces a relatively selective cognitive deficit that can be alleviated with supplemental choline.

Neonatal treatment with a competitive NMDA antagonist results in response-specific disruption of conditioned fear in preweanling rats

RATIONALE

The N-methyl-D-aspartate (NMDA) receptor has been implicated in processes of neurodevelopment, including cell proliferation, synaptogenesis, and apoptosis. Several studies have reported that administration of NMDA antagonists early in development can cause long-lasting changes in behavior. For example, Gould and Cameron [Behav Neurosci 111:49-56 (1997a)] have shown that a single injection of the competitive NMDA antagonist CGP 43487 on postnatal day (PD) 5 affected behavioral immobility in young rats exposed to the odor of a natural predator.

OBJECTIVES

This experiment was undertaken to determine whether the behavioral effects previously reported would also be seen with conditioned cues. Both stimulus-elicited behavioral immobility (freezing) and changes in heart rate were recorded to examine impairments in responding across multiple measures.

METHODS

Animals were given a single injection of 0, 2.5, or 5.0 mg/kg CGP 43487 on PD 5. On PD 20 subjects were given paired or unpaired presentations of either an olfactory or auditory conditioned stimulus (CS) with a 110-dB-white-noise unconditioned stimulus. CS-elicited freezing and changes in heart rate were measured.

RESULTS

Pups treated with CGP exhibited impairments in conditioned freezing, but were unaffected in their expression of conditioned changes in heart rate, to both olfactory and auditory stimuli.

CONCLUSIONS

These results indicate that neonatal treatment with an NMDA antagonist affects the expression of fear in a response-specific manner. The data suggest that antagonist-induced alterations in neural systems involved in the expression of freezing are affected by NMDA receptor blockade early in life.

Dorsal hippocampus NMDA receptors differentially mediate trace and contextual fear conditioning

The dorsal hippocampus (DH) is critically involved in the acquisition and expression of trace and contextual fear conditioning. NMDA/glutamate receptor-mediated transmission is thought to be one mechanism mediating the plastic changes that support long-term memories in the DH. However, their precise involvement in acquisition and expression processes has not been defined. To examine this issue, the NMDA receptor antagonist, D,L-2-amino-5-phosphonovaleric acid (APV; 10 microg/microl; 0.5 microl), was infused into the DH prior to conditioning and/or testing, using a trace fear conditioning procedure. All rats were tested for freezing to both tone and context in separate, counterbalanced sessions. The three sessions (1 training and 2 test) were separated by approximately 24 h. Using this design, it was possible to assess the role for DH NMDA receptors in the acquisition versus expression of trace and contextual fear conditioning. APV disrupted acquisition, but not expression, of contextual fear conditioning. By contrast, APV attenuated both acquisition and expression of trace fear memories. Thus, DH NMDA receptors appear to contribute to retrieval of some, but not all, fear memories.

Electrolytic lesions of the dorsal hippocampus disrupt renewal of conditional fear after extinction

There is a growing body of evidence that the hippocampus is critical for context-dependent memory retrieval. In the present study, we used Pavlovian fear conditioning in rats to examine the role of the dorsal hippocampus (DH) in the context-specific expression of fear memory after extinction (i.e., renewal). Pre-training electrolytic lesions of the DH blunted the expression of conditional freezing to an auditory conditional stimulus (CS), but did not affect the acquisition of extinction to that CS. In contrast, DH lesions impaired the context-specific expression of extinction, eliminating the renewal of fear normally observed to a CS presented outside of the extinction context. Post-extinction DH lesions also eliminated the context dependence of fear extinction. These results are consistent with those using pharmacological inactivation of the DH and suggest that the DH is required for using contextual stimuli to regulate the expression of fear to a Pavlovian CS after extinction.

Perirhinal and postrhinal contributions to remote memory for context

The perirhinal (PER) and postrhinal (POR) cortices, two components of the medial temporal lobe memory system, are reciprocally connected with the hippocampus both directly and via the entorhinal cortex. Damage to PER or POR before or shortly after training on a contextual fear conditioning task causes deficits in the subsequent expression of contextual fear, implicating these regions in the acquisition or expression of contextual memory. Here, we examined the contribution of PER and POR to the processing of remotely learned contextual information. Male Long-Evans rats were trained in an unsignaled contextual fear conditioning paradigm. After training, rats received bilateral neurotoxic lesions to PER or POR or sham control surgeries at three different training-to-lesion intervals: 1, 28, or 100 d after training. Two weeks after surgery, lesioned and control rats were returned to the training context to assess contextual fear as measured by freezing. Rats with PER or POR damage froze significantly less in the training context than control rats but were not different from each other. The severity of the deficit did not differ across training-to-lesion intervals for any group. This pattern of deficits differs from that of posttraining hippocampal lesions, for which longer training-to-lesion intervals produce significantly more fear-conditioned contextual freezing than shorter training-to-lesion intervals. In the absence of such a retrograde gradient in the present study, our interpretation is that PER and POR have an ongoing role in the storage or retrieval of representations for context. Alternatively, these regions may be involved in a more extended consolidation process that becomes apparent beyond 100 d after learning.

Bright light suppresses hyperactivity induced by excitotoxic dorsal hippocampus lesions in the rat

The hippocampus has been implicated in anxiety, novelty detection, spatial- contextual processing, and hyperactivity. Accordingly, the authors contrasted the role of the dorsal hippocampus (DH) and the basolateral amygdala complex (BLA) in an open field task that presents the onset and termination of a bright light gradient. In the dark, DH rats demonstrated impaired habituation of locomotion behavior and hyperactivity, whereas in bright light their behaviors were normal. DH rats responded differentially to the onset and termination of the light stimulus, which indicates they have normal novelty detection. BLA lesion rats responded normally to bright light. These results demonstrate that a mild fear stimulus, such as bright light, can suppress DH lesion-evoked hyperactivity, and this hyperactivity results from impaired contextual processing.

Discrete and contextual cue alterations eliminate the instrumental appetitive-to-aversive transfer impairment in phenytoin-treated rats

We have shown previously that the antiepileptic phenytoin impairs transfer in an instrumental learning task (Banks et al., 1999). The present study examined the effects of contextual alterations on appetitive-to-aversive transfer performance of rats treated with either phenytoin or tang. Adult rats were tested in tone-signaled appetitive and aversive instrumental tasks, where the animal bar-pressed to obtain a food reward (sugar pellet) or to avoid shock. Rats were trained on the appetitive task for 31 days. Beginning on the twenty-first day, rats were gavaged with either phenytoin or tang twice daily. Animals were then transferred to aversive training, with the phenytoin or tang treatment continuing throughout the 25 testing days. For some animals, contextual changes were introduced as they shifted from appetitive to aversive training, while for other animals these changes were not made. Phenytoin-treated rats that were presented with changes in context as they transferred from the appetitive to the aversive task learned the avoidance response to levels substantially higher than drug-treated rats not presented with the contextual changes. These results indicate that phenytoin impairs avoidance learning following transfer from the appetitive task, and that this impairment can be eliminated by introducing changes in context at the point of transfer. In the tang-treated control subjects, on the other hand, there was no improvement in transfer learning performance associated with the changes in contextual cues. This pattern of results suggests that contextual encoding processes in rats being trained in an instrumental appetitive-to-aversive paradigm are dramatically affected by phenytoin.

Regional dissociations within the hippocampus--memory and anxiety

The amnestic effects of hippocampal lesions are well documented, leading to numerous memory-based theories of hippocampal function. It is debatable, however, whether any one of these theories can satisfactorily account for all the consequences of hippocampal damage: Hippocampal lesions also result in behavioural disinhibition and reduced anxiety. A growing number of studies now suggest that these diverse behavioural effects may be associated with different hippocampal subregions. There is evidence for at least two distinct functional domains, although recent neuroanatomical studies suggest this may be an underestimate. Selective lesion studies show that the hippocampus is functionally subdivided along the septotemporal axis into dorsal and ventral regions, each associated with a distinct set of behaviours. Dorsal hippocampus has a preferential role in certain forms of learning and memory, notably spatial learning, but ventral hippocampus may have a preferential role in brain processes associated with anxiety-related behaviours. The latter's role in emotional processing is also distinct from that of the amygdala, which is associated specifically with fear. Gray and McNaughton's theory can in principle incorporate these apparently distinct hippocampal functions, and provides a plausible unitary account for the multiple facets of hippocampal function.