Modality-specific retrograde amnesia of fear

A genetic test of the effects of mutations in PKA on mossy fiber LTP and its relation to spatial and contextual learning

Using a genetic approach, we assessed the effects of mutations in protein kinase A (PKA) on long-term potentiation (LTP) in the mossy fiber pathway and its relationship to spatial and contextual learning. Ablation by gene targeting of the C beta 1 or the RI beta isoform of PKA produces a selective defect in mossy fiber LTP, providing genetic evidence for the role of these isoforms in the mossy fiber pathway. Despite the elimination of mossy fiber LTP, the behavioral responses to novelty, spatial learning, and conditioning to context are unaffected. Thus, contrary to current theories about hippocampal function, mossy fiber LTP does not appear to be required for spatial or contextual learning. In the absence of mossy fiber LTP, adequate spatial and contextual information might reach the CA1 region via other pathways from the entorhinal cortex.

What does the hippocampus really do?

Much of the evidence used to implicate the hippocampus in learning and memory has been obtained from clinical cases and/or experimental studies with animals where the damage is extensive and includes more than just the hippocampus. When the damage is limited to the cells that comprise the hippocampus (CA1-CA3 pyramidal cells, hilar and granule cells in the dentate gyrus) the effect on behavior in the rat is more limited than what is usually reported. Selective, axon-sparing ibotenic acid lesions of the hippocampus were used in the experiments that are reviewed to study the effects of removing the hippocampus on: (1) the acquisition of spatial and non-spatial information; (2) complex, non-spatial representational learning; and (3) acquisition and utilization of contextual information. The results indicated that rats with the hippocampus removed were impaired on those tasks that require the utilization of spatial and contextual information but performed like controls in learning about and handling (even complex) non-spatial information. Future research utilizing selective lesions of the hippocampus and sensitive behavioral testing techniques should help clarify the extent to which the impairments in the acquisition of spatial information and the ability to utilize contextual, background cues can be reduced to a single, underlying learning process.

Fear-potentiated startle in posttraumatic stress disorder

Exaggerated startle is reputed to be one of the cardinal symptoms of posttraumatic stress disorder (PTSD); however, objective studies have given conflicting results as to whether or not startle is increased in PTSD. The present study investigated startle in PTSD during the threat of shock (fear-potentiated startle). The eyeblink component of the startle reflex was measured at various times preceding and following the anticipation of unpleasant electric shocks in 9 PTSD subjects and 10 age-matched, healthy controls. Startle amplitude was significantly greater during baseline and during shock anticipation in the PTSD subjects, compared to the controls. Habituation of the startle reflex was normal. Because other studies in the literature, as well as in our own laboratory, have failed to find exaggerated startle at baseline (i.e., absence of stress) in PTSD patients, it is unlikely that the present results reflect a chronic elevation of startle in this group. Instead, the higher levels of startle in the PTSD group probably resulted from a greater conditioned emotional response in this group, triggered by anticipation of electric shocks that generalized to the unfamiliar experimental context in which testing occurred. Hence, emotionally charged test procedures may be especially informative in distinguishing PTSD patients from other psychiatric diagnostic groups.

Double dissociation of conditioning and declarative knowledge relative to the amygdala and hippocampus in humans

A patient with selective bilateral damage to the amygdala did not acquire conditioned autonomic responses to visual or auditory stimuli but did acquire the declarative facts about which visual or auditory stimuli were paired with the unconditioned stimulus. By contrast, a patient with selective bilateral damage to the hippocampus failed to acquire the facts but did acquire the conditioning. Finally, a patient with bilateral damage to both amygdala and hippocampal formation acquired neither the conditioning nor the facts. These findings demonstrate a double dissociation of conditioning and declarative knowledge relative to the human amygdala and hippocampus.

An integrative analysis to sleep functions

Various studies suggest that some sleep functions, especially some slow wave sleep functions, are indispensable in mammals and related to brain regulation. It has been proposed that two of these functions are the adjustment of emotional balance and the processing of acquired emotional memories. During waking, the gradual accumulation of various randomly learned emotional memories in the limbic structures would inevitably imbalance and disorganize emotional behaviors. Although the emotional balance can be restored during waking by the ascending NA, DA, ACh and 5-HT systems, their roles in memory retention and emotional regulation may sometimes be dissociated and their adjustment of the emotional balance can only be a transient effect. On the other hand, the function of slow wave sleep for emotional adjustment can be long-lasting and is in agreement with its function on the processing of emotional memories. As a result, these sleep functions become indispensable in preventing the emotional imbalance inevitably caused by the accumulation of emotional memories. The effects of rapid eye movement sleep on memory and emotional regulation are just opposite to those of slow wave sleep. Low vigilance is required as premise for sleep to accomplish these indispensable functions.

Impairment of spatial but not contextual memory in CaMKII mutant mice with a selective loss of hippocampal LTP in the range of the theta frequency

We assessed hippocampal-dependent memory in mice with a Ca(2+)-independent form of CaMKII generated by the introduction of an aspartate at amino acid 286. The CaMKII-Asp-286 mice show normal LTP at high frequency stimulation, but in the 5-10 Hz range, they show a shift in the frequency-response curve favoring LTD. This range of frequencies is similar to the theta rhythm, which is associated with exploration in rodents. Using the Barnes maze to assess spatial memory, we found the transgenic mice could not learn to navigate to a specific location using spatial cues. In contrast, one line of transgenic mice performed normally in contextual fear conditioning, a task that is also hippocampal dependent. This dissociation between spatial and contextual memory suggests that even though both require the hippocampus, they may be mediated by different synaptic mechanisms.

Concussion-induced retrograde amnesia in rats

Three experiments were conducted to investigate the characteristics of retrograde amnesia (RA) induced by concussion in rats. In Experiment 1, rats receiving experimental concussion shortly after training in a single punishment trial exhibited severe forgetting on a retention test 48 h later. In the second experiment, rats receiving a concussion within 6 h after training showed severe RA, while those receiving concussion one day to five days after training exhibited progressively weaker amnesia. In Experiment 3, amnesic animals in one group received pretest noncontingent foot shock as a reminder treatment. This pretest cue significantly increased the cross-through latency, thus indicating a reduction in the memory deficit resulting from concussion. These results suggest that experimental concussion can be an effective method to induce retrograde memory loss in rats; that the RA caused by concussion is time-dependent; and that concussion-induced RA can be alleviated by a pretest cue indicating that the underlying mechanism of concussion-induced RA is more likely to be a retrieval deficit than a consolidation failure.

The effects of AMPA-induced lesions of the medial septum and vertical limb nucleus of the diagonal band of Broca on spatial delayed non-matching to sample and spatial learning in the water maze

These experiments investigated in the rat the impact on spatial delayed non-matching to sample and on acquisition of the Morris water maze of (i) AMPA-induced lesions of the medial septal nucleus, which produced a marked reduction of hippocampal choline acetyltransferase activity and acetylcholine levels (measured using in vivo dialysis) together with lesser reductions in cholinergic markers in the cingulate cortex and (ii) similar AMPA-induced lesions of the vertical limb nucleus of the diagonal band of Broca (vDB), which produced more marked reductions in cholinergic markers in the cingulate cortex than in the hippocampus. Medial septal lesions produced a delay-dependent deficit in spatial working memory, while lesions of the vDB resulted in a delay-independent performance deficit. In addition, rats with vDB lesions adopted biased response strategies during the imposition of long delays. Neither lesion significantly affected the acquisition of a spatial reference memory task, the Morris water maze. The results are discussed in terms of cholinergic- and GABAergic-dependent functions of the hippocampal formation and cingulate cortex in spatial short-term and reference memory.

Retrograde amnesia and memory consolidation: a neurobiological perspective

The fact that information acquired before the onset of amnesia can be lost (retrograde amnesia) has fascinated psychologists, biologists, and clinicians for over 100 years. Studies of retrograde amnesia have led to the concept of memory consolidation, whereby medial temporal lobe structures direct the gradual establishment of memory representations in neocortex. Recent theoretical accounts have inspired a simple neural network model that produces behavior consistent with experimental data and makes these ideas about memory consolidation more concrete. Recent physiological and anatomical findings provide important information about how memory consolidation might actually occur.

Neuronal damage and decrease of central acetylcholine level following permanent occlusion of bilateral common carotid arteries in rat

The neuronal damages and the changes in central acetylcholine (ACh) and choline (Ch) contents following permanent occlusion of bilateral common carotid arteries (2VO) of rats were investigated 1 and 4 months after the operation. Two types of neuronal damages were observed in the rats with permanent 2VO. The first type was the infarctions observed in the cerebral cortex and striatum. The infarction in the cortex and striatum was observed in 28.6 and 42.9% of the animals examined 1 month after permanent 2VO, respectively. These ratios did not change even when examined 4 months after permanent 2VO, suggesting that this type of neuronal damage is due to acute ischemic attacks. The second type was progressive neuronal damages observed in the hippocampus and white matter: the neuronal loss in the CA1 subfield appeared 4 months but not 1 month after permanent 2VO and the rarefaction of white matter which was observed 1 months after permanent 2VO and markedly increased 4 months after the operation. Moreover, ACh level significantly decreased in the striatum but not in the cortex, hippocampus or hypothalamus 1 month after permanent 2VO, while the ACh levels in the cortex, striatum and hypothalamus, and Ch levels in all the regions tested significantly decreased when tested 4 months after the operation. These changes did not accompany necrosis. These results suggest that the progressive neuronal degeneration and cholinergic dysfunction following the permanent 2VO are in part involved in chronic cerebral hypoperfusion-induced long-lasting cognition deficits in rats.

Behavioral conditioning of lipopolysaccharide-induced anorexia

One manifestation of the acute phase response, sickness behavior, is now considered an important response in the organism's overall attempt to reinstate homeostasis. This report aimed to determine whether the sickness behavior of anorexia was conditionable using the conditioned taste aversion paradigm. To investigate this phenomenon, lipopolysaccharide (LPS) (100 micrograms/kg) was used as the unconditioned stimulus, and was paired with a novel 1% saccharin solution (conditioned stimulus). Upon conditioned stimulus representation, the anorectic effects of LPS were observed. These data are consistent with recent literature showing acute phase events to be conditionable.

The effects of AMPA-induced lesions of the septo-hippocampal cholinergic projection on aversive conditioning to explicit and contextual cues and spatial learning in the water maze

The environmental context of an animal both subsumes and is associated with the explicit cues that guide its behavioural responses. Recent work in this laboratory suggests that learning about the relationship between the cues which comprise a context depends on the hippocampus. In the present study the role of the cholinergic input to the hippocampus in contextual learning was assessed in rats using a conditioned stimulus/context conditioning paradigm and spatial learning in the Morris water maze. In the former, a place preference apparatus provided the context. The subject was confined in the black chamber and a 'clicker' conditioned stimulus was presented five times in a 20 min period. A trace interval of 5 or 30 s, depending on the group, was interposed between the end of the clicker and a footshock. Theory predicts that animals in the 5 s condition will learn more about the clicker as a predictor of shock and become strongly conditioned, while those in the 30 s condition learn relatively more about the context. Conditioning to the clicker (conditioned stimulus) was measured in a separate lick suppression chamber--presentation of the clicker suppresses drinking, and contextual learning was determined by recording the time spent on the black side of the place preference apparatus when both the black and a familiar white chamber were accessible. Lesions of the medial septum/diagonal band induced by RS-alpha-amino-3-hydroxy-5-methyl-4-isoxazole propionic acid (AMPA) enhanced contextual learning in this paradigm but disrupted conditioned stimulus conditioning in the 30 s condition. Acquisition of the Morris water maze was largely unimpaired. The results are suggested to reflect a shift towards the use of hippocampal-dependent contextual learning strategies in lesioned animals.

Selective damage to the hippocampal region blocks long-term retention of a natural and nonspatial stimulus-stimulus association

Normal rats rapidly acquire and remember associations between nonspatial stimuli as expressed in the social transmission of food preferences. In the present study, rats with selective neurotoxic lesions including all subdivisions of the hippocampal region (hippocampus proper, dentate gyrus, and subiculum) normally acquired and briefly retained the food odor association as demonstrated by intact memory immediately after social training. However, long-term memory in these animals was severely impaired in contrast to strong 24-h retention by intact rats. More selective lesions to the hippocampus proper plus dentate gyrus alone, or the subiculum alone had no effect on memory at either test interval. These findings indicate that the hippocampal region is required for long-term retention of a nonspatial form of natural memory.

Configural association theory and the hippocampal formation: an appraisal and reconfiguration

Sutherland and Rudy ([1989] Psychobiology 17:129-144) proposed that the hippocampal system is critical to normal learning and memory because of its function as the central part of a configural association system. This system constructs a unique representation of the joint occurrence of the independent elements of a compound. There is evidence consistent with the theory's predictions, however, there also are data that unambiguously demonstrate that, under some conditions, animals lacking an intact hippocampal system acquire configural associations. Thus, Sutherland and Rudy's fundamental assumption cannot be correct. To integrate the supporting and contradictory data, we propose two simple modifications of our position: 1) The critical neural system for configural associations is in cortical circuitry outside the hippocampus, and 2) the output from the hippocampal formation contributes to configural processing by selectively enhancing, thereby making more salient, cortical units representing stimulus conjunctions. This enhancement has two important effects: 1) It decreases the similarity between the configural units representing the co-occurrence of cues and the units representing the cues, and 2) It increases the rate at which the configural units can acquire associative strength. The modified theory explains why damage to the hippocampal formation only impairs learning on a subset of nonlinear discrimination problems. It also integrates recent data on the effects of hippocampal formation damage on conditioning involving context cues and makes novel predictions about performance on nonlinear discrimination problems and place learning.

Functional neuroanatomy of visually elicited simple phobic fear: additional data and theoretical analysis

We investigated central nervous system correlates of simple phobic fear. Regional cerebral blood flow (rCBF) was measured using positron emission tomography (PET) in eight volunteers with symptomatic spider phobia that were exposed to visual phobogenic and neutral stimuli. Diazepam (0.1 mg/kg body weight i.v.) or placebo was administered under double-blind conditions after initial PET scans. The PET scans were then repeated. The presence of fear was confirmed by rating procedures and increased number of nonspecific electrodermal fluctuations and by higher heart rate during phobic than during neutral stimulation. Phobic as compared to neutral stimulation elevated the regional to whole brain (relative) CBF in the secondary visual cortex but reduced relative rCBF in the hippocampus, prefrontal, orbitofrontal, temporopolar, and posterior cingulate cortex. Diazepam treatment did not affect the relative rCBF or the subjective or physiological fear indices. The observed rCBF pattern replicates our previous findings in snake phobics (M. Fredrikson et al. [1993] Psychophysiology, 30, 127-131; G. Wik et al. [1993] Psychiatry Research (Neuroimaging), 50, 15-24) and indicates that fear and anxiety affect cortical areas outside the classic limbic system areas.

Sex differences in hippocampal long-term potentiation (LTP) and Pavlovian fear conditioning in rats: positive correlation between LTP and contextual learning

Three experiments investigated sex differences in hippocampal long-term potentiation (LTP) and Pavlovian fear conditioning in rats. Experiment 1 revealed a robust sex difference in the magnitude of LTP induced at perforant path synapses in the dentate gyrus of pentobarbital-anesthetized rats. This sex difference in LTP was evident in rats of 35 and 60 days of age and was not the result of pre-LTP sex differences in perforant path synaptic transmission; 20-day-old rats did not show LTP. An analysis of field potentials evoked during LTP induction revealed a sex difference in the magnitude of N-methyl-D-aspartate (NMDA) receptor activation that was highly correlated with the magnitude of LTP. Experiment 2 showed that males condition more fear, measured as freezing, to the contextual conditional stimuli (CSs) of a conditioning chamber compared to their female counterparts. This sex difference in conditional freezing was apparent with both low and high unconditional stimulus (US, footshock) intensities. Experiment 3 revealed that the enhanced fear conditioning in males was specific to contextual CSs, and consisted of a more rapid rate of conditioning. Together, these experiments reveal a positive correlation between the magnitude of hippocampal LTP and a form of learning that depends on the hippocampus. Furthermore, they suggest a neural basis for sex differences in hippocampus-dependent learning tasks.

Deficient long-term memory in mice with a targeted mutation of the cAMP-responsive element-binding protein

The cAMP-responsive element-binding protein (CREB) has been implicated in the activation of protein synthesis required for long-term facilitation, a cellular model of memory in Aplysia. Our studies with fear conditioning and with the water maze show that mice with a targeted disruption of the alpha and delta isoforms of CREB are profoundly deficient in long-term memory. In contrast, short-term memory, lasting between 30 and 60 min, is normal. Consistent with models claiming a role for long-term potentiation (LTP) in memory, LTP in hippocampal slices from CREB mutants decayed to baseline 90 min after tetanic stimulation. However, paired-pulse facilitation and posttetanic potentiation are normal. These results implicate CREB-dependent transcription in mammalian long-term memory.

Dissociated paradoxical sleep deprivation effects on inhibitory avoidance and conditioned fear

Rats were submitted to paradoxical sleep deprivation (PSD) for 24, 72, or 96 h and were trained on a double aversively motivated task, encompassing a step-through inhibitory avoidance and a classical conditioning of fear to a brief tone serving as conditional stimulus. Retention test of the inhibitory avoidance was performed at the same apparatus of training (without tone presentation). Retention of conditioned fear was assessed in an open field apparatus, where the freezing reaction to the tone was measured. PSD for 24 and 72 h preceding the training session had no effect on either task. However, PSD during the 96 h preceding the training session impaired acquisition of inhibitory avoidance, but had no effect on classically conditioned fear. It is concluded that PSD had differential effects on the two tasks, both aversively motivated and trained at the same time and conditions.

Intrahippocampal administration of both the D- and the L-isomers of AP5 disrupt spontaneous alternation behavior and evoked potentials

We previously reported that systemically administered N-methyl-D-aspartate (NMDA) antagonists significantly impair spontaneous alternation behavior. Others have reported that the restricted blockade of hippocampal NMDA receptors disrupts performance on different tests of spatial learning and have suggested that the resulting impairments are attributable to a disruption of endogenous NMDA-dependent long-term potentiation (LTP). In the present study, we determined whether spontaneous alternation performance was disrupted by circumscribed blockade of hippocampal NMDA receptors as well as by a second class of compounds which disrupt LTP, protein kinase inhibitors. The effect of hippocampal NMDA blockade on inhibitory avoidance was also examined insofar as this behavior too is disrupted by systemically administered NMDA antagonists. When injected into the hippocampus 15 min prior to spontaneous alternation testing, the NMDA antagonists CPP and D,L-AP5 each decreased alternation rates. The specific protein kinase C (PKC) inhibitor, NPC 15437, also disrupted spontaneous alternation, whereas the more general kinase inhibitor, PMXB, did not. When injected 15 min prior to inhibitory avoidance training, CPP also impaired inhibitory avoidance learning as assessed during a subsequent test session, 48 h later. Interpretation of these data was complicated by the additional findings that intrahippocampal infusion of L-AP5 (which is inactive with respect to NMDA receptors) also disrupted alternation performance, and that both the D- and the L-isomers of AP5 as well as each kinase inhibitor dramatically disrupted evoked responses (i.e., population spike amplitude, spike latency, and EPSP slope), as recorded in the dentate gyrus and evoked by perforant path stimulation. These data indicate that behaviorally effective doses of AP5 may have effects which extend beyond NMDA blockade. Moreover, the effects of these compounds on hippocampal transmission, in general, suggest that attribution of the amnestic consequences of their administration to impaired LTP may be unwarranted.

Cholesterol treatment facilitates spatial learning performance in DBA/2Ibg mice

DBA/2Ibg mice were treated with cholesterol pellets for 11 days. On the seventh day after treatment, animals began 5 consecutive days of training on the spatial form of Morris water task, followed on the third and fourth days by a probe trial, and random platform training on the fifth day. DBA mice with cholesterol pellets exhibited enhanced performance compared to DBA mice that underwent a sham surgery. Our results suggest that subchronic treatment with the steroid hormone precursor, cholesterol, enhances spatial learning performance in DBA mice.

A deficit in one-trial context fear conditioning is not due to opioid analgesia

Rats given a foot shock immediately after placement in a box subsequently freeze (immobility) much less in that box than rats given the same shock 2 min after placement. A possible explanation of this result is that these two procedures might induce different levels of opioid analgesia at the time of shock. Opioids might be present immediately after handling, transporting, and exposure to a new situation, but absent 2 min later. Two experiments examined this possibility by giving the opioid antagonist naloxone before conditioning (Experiment 1) or before conditioning and testing (Experiment 2). There was no effect of naloxone relative to saline controls. The results do not support the analgesia hypothesis. Experiment 2 precludes a stage-dependent learning account of the results.

Progressive cognitive impairment following chronic cerebral hypoperfusion induced by permanent occlusion of bilateral carotid arteries in rats

The effects of chronic cerebral hypoperfusion induced by permanent occlusion of bilateral common carotid arteries (2VO) on learning and memory performance were examined in rats using an eight-arm radial maze task. The learning of the task was severely impaired in the permanent 2VO rats that had not been pretrained, while the retention was slightly impaired and soon recovered in the permanent 2VO rats that had been fully pretrained when tested within 1 month after the 2VO operation. The performance, however, was impaired in the pretrained rats when a 3-min delay was interposed between the fourth and fifth choices. Moreover, when retrained in the radial maze 4 months after the permanent 2VO, these same rats showed a performance impairment. Some loss of the hippocampal pyramidal neurons was observed 1 month after the permanent 2VO, although the decrease was not significant. However, significant loss of the cells was observed in the hippocampus CA1 subfield 4 months after the operation. We concluded that: (1) In the early stage (1 month after permanent 2VO), a learning deficit was observed in the non-pretrained rats. In the pretrained rats, working memory was not impaired, whereas longer term memory was compromised; (2) in the late stage (4 months after permanent 2VO), working memory may have also been impaired in the pretrained rats; and (3) this progressive cognitive deficit seemed to parallel the progress of neuronal damage.

Reactivation of hippocampal ensemble memories during sleep

Simultaneous recordings were made from large ensembles of hippocampal "place cells" in three rats during spatial behavioral tasks and in slow-wave sleep preceding and following these behaviors. Cells that fired together when the animal occupied particular locations in the environment exhibited an increased tendency to fire together during subsequent sleep, in comparison to sleep episodes preceding the behavioral tasks. Cells that were inactive during behavior, or that were active but had non-overlapping spatial firing, did not show this increase. This effect, which declined gradually during each post-behavior sleep session, may result from synaptic modification during waking experience. Information acquired during active behavior is thus re-expressed in hippocampal circuits during sleep, as postulated by some theories of memory consolidation.

Memory consolidation and the medial temporal lobe: a simple network model

Some forms of memory have been shown to depend on a system of medial temporal lobe structures that includes the hippocampus and the adjacent cortical areas (entorhinal, perirhinal, and parahippocampal cortex). The role of this system is only temporary, however, as indicated by the fact that, after damage to the medial temporal lobe, recent memories are impaired but very remote memories are intact. Here we review the evidence that the medial temporal lobe memory system is involved in a process of consolidation: memories are initially dependent on this system but gradually become established in other areas of the brain. We then review some of the ideas that have been proposed about the phenomenon of consolidation and suggest a synthesis of these views. Finally, we describe a simple neural network model that captures some key features of consolidation.

The animal model of human amnesia: long-term memory impaired and short-term memory intact

Normal monkeys and monkeys with lesions of the hippocampal formation and adjacent cortex (the H+ lesion) were trained on the delayed nonmatching to sample (DNMS) task with a delay of 0.5 s between the sample and the choice. The animals with H+ lesions learned the task normally at this short delay and also exhibited the same pattern of response latencies as normal monkeys. This finding contrasts with previous observations that initial learning of the DNMS task with delays of 8-10 s is impaired after H+ lesions. The absence of an impairment at a delay of 0.5 s indicates that the H+ lesion does not affect short-term memory. In contrast, when monkeys with H+ lesions were tested at longer delays (> 30 s), an impairment was observed. This selective impairment occurred when the delays were presented sequentially (from 0.5 s to 10 min) and also when delays were presented in a mixed order (1 s, 1 min, and 10 min). The data indicate that the H+ lesion produces a selective impairment in long-term memory, in the absence of a detectable deficit in short-term memory or perception. Accordingly, the findings confirm the long-standing idea, based primarily on studies of humans, that short-term memory is independent of medial temporal lobe function. The findings thereby establish an important parallel between memory impairment in monkeys and humans and provide additional support for the validity of the animal model of human amnesia in the monkey.

Plastic genes are in!

Even though the synthesis of new proteins is thought to be essential for long-term changes in synaptic plasticity, as well as for long-term memory, little is known about the identity of the required proteins. The hunt for these molecules is under way, however, and in the past year several groups of researchers have entered this fascinating search by introducing new approaches that have lead to the identification of several potential candidates, amongst which are trophic factors, kinases, ion channels, and proteases. The results will have much to say not only about the nature of memory, but also about the mechanisms of learning.

Lesions of the dorsal hippocampal formation interfere with background but not foreground contextual fear conditioning

The effects of hippocampal lesions on the conditioning of fear responses (freezing responses) to contextual stimuli (static, continuously present stimuli) were examined in three conditioning paradigms: forward pairing of a phasic tone conditioned stimulus (CS) with a footshock unconditioned stimulus (US), unpaired presentations of the CS and US, or presentations of the US alone. All three procedures resulted in the acquisition of conditioned freezing to contextual stimuli. Lesions of the dorsal hippocampus prevented the acquisition of contextual conditioning in the Paired procedure, as reported previously, but not in the Unpaired or US Alone procedures. In the Paired procedure, static contextual cues occur in the background, with the phasic tone CS being the primary stimulus that enters into the association with the US. However, in the other two procedures, where there is no phasic CS, the primary associations with the US involve static contextual stimuli, which are therefore in the foreground. We refer to these types of contextual conditioning as background and foreground contextual conditioning, respectively, and argue that the hippocampus is only involved in background contextual conditioning. These results have implications for understanding both fear conditioning and hippocampal function.

Baseline and fear-potentiated startle in panic disorder patients

The present study investigated whether patients with panic disorder had an increase in the startle response and whether this effect, if present, was specific to anticipatory anxiety. The eyeblink component of the acoustic startle reflex was measured in a paradigm involving the anticipation of electric shocks (fear-potentiated startle) in 34 patients with panic disorder and 49 healthy controls. Startle was also recorded in the absence of specific threat at the beginning and at the end of the testing. The testing consisted of three phases: adaptation, fear-potentiated startle, and recovery. In the adaptation and recovery phases, startle stimuli were delivered in the absence of threat. In the fear-potentiated startle phase, startle stimuli were delivered in threat conditions, when subjects anticipated shocks, and in safe conditions that predicted the absence of shocks. Startle was larger in the younger patients (age < 40 years old) compared to the younger controls throughout the testing. The difference reached significance only during the fear-potentiated startle phase, however. Startle was nonsignificantly reduced in the older patients (age > or = 39 years old), compared to the older controls. The results are discussed in terms of the contextual effects of the experimental setting.

PKC gamma mutant mice exhibit mild deficits in spatial and contextual learning

We are undertaking a genetic approach to investigate the role that synaptic modulation in the mammalian central nervous system plays in learning and memory and to identify relevant molecular components. We have generated mice deficient in the gamma isoform of protein kinase C (PKC gamma), an enzyme that has previously been implicated in both long-term potentiation (LTP) and learning and memory. These mice have a modified LTP of synaptic transmission in the hippocampus. We demonstrate that PKC gamma-mutant mice can learn to carry out hippocampus-dependent tasks, although mild deficits are evident. Thus, hippocampal CA1 LTP induced by the conventional tetanic stimulation is not essential for the mice to exhibit spatial and contextual learning. Furthermore, the modification of hippocampal synaptic plasticity correlates with the learning deficits we observe.

Emotional memory systems in the brain

The neural mechanisms of emotion and memory have long been thought to reside side by side, if not in overlapping structures, of the limbic system. However, the limbic system concept is no longer acceptable as an account of the neural basis of memory or emotion and is being replaced with specific circuit accounts of specific emotional and memory processes. Emotional memory, a special category of memory involving the implicit (probably unconscious) learning and storage of information about the emotional significance of events, is modeled in rodent experiments using aversive classical conditioning techniques. The neural system underlying emotional memory critically involves the amygdala and structures with which it is connected. Afferent inputs from sensory processing areas of the thalamus and cortex mediate emotional learning in situations involving specific sensory cues, whereas learning about the emotional significance of more general, contextual cues involves projections to the amygdala from the hippocampal formation. Within the amygdala, the lateral nucleus (AL) is the sensory interface and the central nucleus the linkage with motor systems involved in the control of species-typical emotional behaviors and autonomic responses. Studies of cellular mechanisms in these pathways have focused on the direct relay to the lateral amygdala from the auditory thalamus. These studies show that single cells in AL respond to both conditioned stimulus and unconditioned stimulus inputs, leading to the notion that AL might be a critical site of sensory-sensory integration in emotional learning. The thalamo-amygdala pathway also exhibits long-term potentiation, a form of synaptic plasticity that might underlie the emotional learning functions of the circuit. The thalamo-amygdala pathway contains and uses the amino acid glutamate in synaptic transmission, suggesting the possibility that an amino-acid mediated form of synaptic plasticity is involved in the emotional learning functions of the pathway. We are thus well on the way to a systems level and a cellular understanding of at least one form of emotional learning and memory.