Single neurons in the dentate gyrus and CA1 of the hippocampus exhibit inverse patterns of encoding during trace fear conditioning

Abstinence and Fear Experienced during This Period Produce Distinct Cortical and Hippocampal Adaptations in Alcohol-Dependent Rats

Previous studies demonstrate that ethanol dependence induced by repeating cycles of chronic intermittent ethanol vapor exposure (CIE) followed by protracted abstinence produces significant gray matter damage via myelin dysfunction in the rodent medial prefrontal cortex (mPFC) and alterations in neuronal excitability in the mPFC and the dentate gyrus (DG) of the hippocampus. Specifically, abstinence-induced neuroadaptations have been associated with persistent elevated relapse to drinking. The current study evaluated the effects of forced abstinence for 1 day (d), 7 d, 21 d, and 42 d following seven weeks of CIE on synaptic plasticity proteins in the mPFC and DG. Immunoblotting revealed reduced expression of CaMKII in the mPFC and enhanced expression of GABAA and CaMKII in the DG at the 21 d time point, and the expression of the ratio of GluN2A/2B subunits did not change at any of the time points studied. Furthermore, cognitive performance via Pavlovian trace fear conditioning (TFC) was evaluated in 3 d abstinent rats, as this time point is associated with negative affect. In addition, the expression of the ratio of GluN2A/2B subunits and a 3D structural analysis of neurons in the mPFC and DG were evaluated in 3 d abstinent rats. Behavioral analysis revealed faster acquisition of fear responses and reduced retrieval of fear memories in CIE rats compared to controls. TFC produced hyperplasticity of pyramidal neurons in the mPFC under control conditions and this effect was not evident or blunted in abstinent rats. Neurons in the DG were unaltered. TFC enhanced the GluN2A/2B ratio in the mPFC and reduced the ratio in the DG and was not altered by abstinence. These findings indicate that forced abstinence from CIE produces distinct and divergent alterations in plasticity proteins in the mPFC and DG. Fear learning-induced changes in structural plasticity and proteins contributing to it were more profound in the mPFC during forced abstinence.

The hippocampus contributes to retroactive stimulus associations during trace fear conditioning

Binding events that occur at different times are essential for memory formation. In trace fear conditioning, animals associate a tone and footshock despite no temporal overlap. The hippocampus is thought to mediate this learning by maintaining a memory of the tone until shock occurrence, however, evidence for sustained hippocampal tone representations is lacking. Here, we demonstrate a retrospective role for the hippocampus in trace fear conditioning. Bulk calcium imaging revealed sustained increases in CA1 activity after footshock that were not observed after tone termination. Optogenetic silencing of CA1 immediately after footshock impaired subsequent memory. Additionally, footshock increased the number of sharp-wave ripples compared to baseline during conditioning. Therefore, post-shock hippocampal activity likely supports learning by reactivating and linking latent tone and shock representations. These findings highlight an underappreciated function of post-trial hippocampal activity in enabling retroactive temporal associations during new learning, as opposed to persistent maintenance of stimulus representations.

The hearing hippocampus

The hippocampus has a well-established role in spatial and episodic memory but a broader function has been proposed including aspects of perception and relational processing. Neural bases of sound analysis have been described in the pathway to auditory cortex, but wider networks supporting auditory cognition are still being established. We review what is known about the role of the hippocampus in processing auditory information, and how the hippocampus itself is shaped by sound. In examining imaging, recording, and lesion studies in species from rodents to humans, we uncover a hierarchy of hippocampal responses to sound including during passive exposure, active listening, and the learning of associations between sounds and other stimuli. We describe how the hippocampus' connectivity and computational architecture allow it to track and manipulate auditory information - whether in the form of speech, music, or environmental, emotional, or phantom sounds. Functional and structural correlates of auditory experience are also identified. The extent of auditory-hippocampal interactions is consistent with the view that the hippocampus makes broad contributions to perception and cognition, beyond spatial and episodic memory. More deeply understanding these interactions may unlock applications including entraining hippocampal rhythms to support cognition, and intervening in links between hearing loss and dementia.

Learning-related changes in cellular activity within mouse dentate gyrus during trace eyeblink conditioning

Because the dentate gyrus serves as the first site for information processing in the hippocampal trisynaptic circuit, it an important structure for the formation of associative memories. Previous findings in rabbit had recorded populations of cells within dentate gyrus that may bridge the temporal gap between stimuli to support memory formation during trace eyeblink conditioning, an associative learning task. However, this previous work was unable to identify the types of cells demonstrating this type of activity. To explore these changes further, we did in vivo single-neuron recording in conjunction with physiological determination of cell types to investigate the functional role of granule cells, mossy cells, and interneurons in dentate gyrus during learning. Tetrode recordings were performed in young-adult mice during training on trace eyeblink conditioning, a hippocampal-dependent temporal associative memory task. Conditioned mice were able to successfully learn the task, with male mice learning at a faster rate than female mice. In the conditioned group, granule cells tended to show an increase in firing rate during conditioned stimulus presentation while mossy cells showed a decrease in firing rate during the trace interval and the unconditioned stimulus. Interestingly, populations of interneurons demonstrated learning-related increases and decreases in activity that began at onset of the conditioned stimulus and persisted through the trace interval. The current study also found a significant increase in theta power during stimuli presentation in conditioned animals, and this change in theta decreased over time. Ultimately, these data suggest unique involvement of granule cells, mossy cells, and interneurons in dentate gyrus in the formation of a trace associative memory. This work expands our knowledge of dentate gyrus function, helping to discern how aging and disease might disrupt this process.

Prefrontal projections to the nucleus reuniens signal behavioral relevance of stimuli during associative learning

The nucleus reuniens (RE) is necessary for memories dependent on the interaction between the medial prefrontal cortex (mPFC) and hippocampus (HPC). One example is trace eyeblink conditioning, in which the mPFC exhibits differential activity to neutral conditioned stimuli (CS) depending on their contingency with an aversive unconditioned stimulus (US). To test if this relevancy signal is routed to the RE, we photometrically recorded mPFC axon terminals within the RE and tracked their changes with learning. As a comparison, we measured prefrontal terminal activity in the mediodorsal thalamus (MD), which lacks connectivity with the HPC. In naïve male rats, prefrontal terminals within the RE were not strongly activated by tone or light. As the rats associated one of the stimuli (CS+) with the US, terminals gradually increased their response to the CS+ but not the other stimulus (CS-). In contrast, stimulus-evoked responses of prefrontal terminals within the MD were strong even before conditioning. They also became augmented only to the CS+ in the first conditioning session; however, the degree of activity differentiation did not improve with learning. These findings suggest that associative learning selectively increased mPFC output to the RE, signaling the behavioral relevance of sensory stimuli.

Ventral hippocampal shock encoding modulates the expression of trace cued fear

The hippocampus is crucial for associative fear learning when the anticipation of threat requires temporal or contextual binding of predictive stimuli as in trace and contextual fear conditioning. Compared with the dorsal hippocampus, far less is known about the contribution of the ventral hippocampus to fear learning. The ventral hippocampus, which is highly interconnected with defensive and emotional networks, has a prominent role in both innate and learned affective behaviors including anxiety, fear, and reward. Lesions or temporary inactivation of the ventral hippocampus impair both cued and contextual fear learning, but whether the ventral hippocampal role in learning is driven by affective processing, associative encoding, or both is not clear. Here, we used trace fear conditioning in mixed sex cohorts to assess the contribution of shock-encoding to the acquisition of cued and contextual fear memories. Trace conditioning requires subjects to associate an auditory conditional stimulus (CS) with a shock unconditional stimulus (UCS) that are separated in time by a 20-s trace interval. We first recorded neuronal activity in the ventral hippocampus during trace fear conditioning and found that ventral CA1 predominantly encoded the shock reinforcer. Potentiated firing to the CS was evident at testing, but no encoding of the trace interval was observed. We then tested the necessity of shock encoding for conditional fear acquisition by optogenetically silencing ventral hippocampal activity during the UCS on each trial of training. Contrary to our predictions, preventing hippocampal shock-evoked firing did not impair associative fear. Instead, it led to a more prolonged expression of CS freezing across test trials, an effect observed in males, but not females. Contextual fear learning was largely intact, although a subset of animals in each sex were differentially affected by shock-silencing. Taken together, the results show that shock encoding in the ventral hippocampus modulates the expression of learned fear in a sex-specific manner.

Isoxazole-9 reduces enhanced fear responses and retrieval in ethanol-dependent male rats

Plasticity in the dentate gyrus (DG) is strongly influenced by ethanol, and ethanol experience alters long-term memory consolidation dependent on the DG. However, it is unclear if DG plasticity plays a role in dysregulation of long-term memory consolidation during abstinence from chronic ethanol experience. Outbred male Wistar rats experienced 7 weeks of chronic intermittent ethanol vapor exposure (CIE). Seventy-two hours after CIE cessation, CIE and age-matched ethanol-naïve Air controls experienced auditory trace fear conditioning (TFC). Rats were tested for cue-mediated retrieval in the fear context either twenty-four hours (24 hr), ten days (10 days), or twenty-one days (21 days) later. CIE rats showed enhanced freezing behavior during TFC acquisition compared to Air rats. Air rats showed significant fear retrieval, and this behavior did not differ at the three time points. In CIE rats, fear retrieval increased over time during abstinence, indicating an incubation in fear responses. Enhanced retrieval at 21 days was associated with reduced structural and functional plasticity of ventral granule cell neurons (GCNs) and reduced expression of synaptic proteins important for neuronal plasticity. Systemic treatment with the drug Isoxazole-9 (Isx-9; small molecule that stimulates DG plasticity) during the last week and a half of CIE blocked altered acquisition and retrieval of fear memories in CIE rats during abstinence. Concurrently, Isx-9 modulated the structural and functional plasticity of ventral GCNs and the expression of synaptic proteins in the ventral DG. These findings identify that abstinence-induced disruption of fear memory consolidation occurs via altered plasticity within the ventral DG, and that Isx-9 prevented these effects.

Fluoxetine and Ketamine Reverse the Depressive but Not Anxiety Behavior Induced by Lesion of Cholinergic Neurons in the Horizontal Limb of the Diagonal Band of Broca in Male Rat

The basal forebrain cholinergic system is involved in cognitive processes, but the role of the basal forebrain cholinergic system in depression is unknown. We investigated whether a lesion of cholinergic neurons in the horizontal limb of the diagonal band of Broca (HDB) produces depressive-like behavior and whether fluoxetine or ketamine inhibits such depressive-like behaviors. Here, in rats, we used 192 IgG-saporin to eliminate the cholinergic neurons of the HDB and evaluated depressive-like behaviors using a preference test for sucrose solution and the forced swimming test. Fourteen days after the injection of 192 IgG-saporin into the HDB, the rats exhibited a significantly fewer number of choline acetyltransferase positive cell density in HDB, accompanied with neuronal loss in the entire hippocampus. Meanwhile, these rats significantly reduced preference for sucrose solution, increased immobility time in the forced swimming test, reduced locomotor activity, decreased context dependent memory in fear conditioning and the time spent in the open arms of the plus-maze. A single dose of ketamine (10 mg/kg) increased the sucrose solution consumption, reduced the immobility time in the forced swim test (FST), and increased locomotor activity compared to vehicle-treated rats. Moreover, in rats that were continuously treated with fluoxetine (10 mg/kg/day for 11 days), the sucrose solution consumption increased, the immobility time in the FST decreased, and locomotor activity increased compared to vehicle-treated rats. The present results demonstrate that a lesion of HDB cholinergic neurons results in depressive-like and anxiety-like behaviors and that antidepressants such as fluoxetine or ketamine, can reverse these depressive-like behaviors but not anxiety-like behaviors, and suggest that a lesion of HDB cholinergic neurons and followed hippocampus damage may be involved in the pathogenesis of depression.

Dynamics of a hippocampal neuronal ensemble encoding trace fear memory revealed by in vivo Ca2+ imaging

Although the biochemical signaling events in area CA1 of the hippocampus underlying memory acquisition, consolidation, retrieval, and extinction have been extensively studied, little is known about the activity dynamics of hippocampal neurons in CA1 during Pavlovian fear conditioning. Here, we use fiber-optic confocal microscopy coupled with the calcium indicator GCaMP6m to monitor neuron activity in freely moving mice during trace fear conditioning. We show that the activity of a group of CA1 neurons increases not only after the stimulus presentations, but also during the stimulus-free trace period when the conditioned mice exhibit a high level of freezing behavior. Therefore, we designate these cells “trace cells”. Interestingly, the activity of the trace cells increases in response to the conditioned stimuli during memory retrieval but diminishes during memory extinction. Importantly, the dynamics of neuron activity exhibit a high degree of correlation with the freezing behavior of the mice, suggesting that a neuronal ensemble responsible for encoding the trace fear memory is repeatedly reactivated during memory retrieval and later extinguished during memory extinction.

Behavioral Experimental Paradigms for the Evaluation of Drug’s Influence on Cognitive Functions: Interpretation of Associative, Spatial/Nonspatial and Working Memory

Background:

Currently, a large number of people throughout the world are affected by neurodegenerative disorders such as Alzheimer’s disease, Parkinson’s disease and Huntington’s disease which appear with a lapse in recall, attention and altered cognitive functions. Learning and memory, the fundamental indices defining cognitive functions, are the complex psychological processes governing acquisition, consolidation, and retrieval of stored information. These processes are synchronized by the coordination of various parts of the brain including hippocampus, striatum and amygdala.

Objective:

The present review is centered on different behavioral paradigms in rodents interpreting learning and memory both explicitly and implicitly. Furthermore, it is also emphasizing on the interaction of various brain structures during different stages of associative, spatial and non-spatial memory.

Methods:

We embarked on an objective review of literature relevant to screening methods for evaluation of drug’s influence on a wide range of cognitive functions (learning and memory) as well as the underlying mechanism responsible for modulation of these functions.

Results:

Our review highlighted the behavioral paradigms based on associative, spatial/nonspatial and working memory. The cited research acknowledged the hippocampal and striatal control on learning and memory.

Conclusion:

Since the neurodegenerative disorders and dementia have continuously been increasing, a wide range of therapeutic targets have been developed at the cellular and molecular level. This arises the necessity of screening of these targets in different cognitive behavioral paradigms which reflect their memory enhancing potential. The understanding of behavioral models and the involvement of brain structures in cognitive functions highlighted in the present review might be helpful to advance therapeutic interventions.

Differential responsivity of neurons in perirhinal cortex, lateral entorhinal cortex, and dentate gyrus during time-bridging learning

Many studies have focused on the function of hippocampal region CA1 as a critical site for associative memory, but much less is known about changes in the afferents to CA1. Here we report the activity of multiple single neurons from perirhinal and entorhinal cortex and from dentate gyrus during trace eyeblink conditioning as well as consolidated recall, and in pseudo-conditioned control rabbits. We also report an analysis of theta activity filtered from the local field potential (LFP). Our results show early associative changes in single-neuron firing rate as well as theta oscillations in lateral entorhinal cortex (EC) and dentate gyrus (DG), and increases in the number of responsive neurons in perirhinal cortex. In both EC and DG, a subset of neurons from conditioned animals exhibited an elevated baseline firing rate and large responses to the conditioned stimulus and trace period. A similar population of cells has been seen in DG and in medial, but not lateral, EC during spatial tasks, suggesting that lateral EC contains cells responsive to a temporal associative task. In contrast to recent studies in our laboratory that found significant CA1 contributions to long-term memory, the activity profiles of neurons within EC and DG were similar for conditioned and pseudoconditioned rabbits during post-consolidation sessions. Collectively these results demonstrate that individual subregions of medial temporal lobe differentially support new and remotely acquired memories. Neuron firing profiles were similar on training trials when conditioned responses were and were not exhibited, demonstrating that these temporal lobe regions represent the CS-US association and do not control the behavioral response. The analysis of theta activity revealed that theta power was modulated by the conditioning stimuli in both the conditioned and pseudoconditioned groups and that although both groups exhibited a resetting of phase to the corneal airpuff, only the conditioned group exhibited a resetting of phase to the whisker conditioned stimulus.

Hippocampal neural progenitor cells play a distinct role in fear memory retrieval in male and female CIE rats

Adult male and female GFAP-TK transgenic rats experienced six weeks of chronic intermittent ethanol vapor inhalation (CIE). During the last week of CIE, a subset of male and female TK rats were fed with Valcyte to ablate neural progenitor cells (NPCs). Seventy-two hours after CIE cessation, all CIE and age-matched ethanol naïve controls experienced auditory trace fear conditioning (TFC). Twenty-four hours later all animals were tested for cue-mediated retrieval in the fear context. Adult male CIE rats showed a significant burst in NPCs paralleled by reduction in fear retrieval compared to naïve controls and Valcyte treated CIE rats. Adult female CIE rats did not show a burst in NPCs and showed similar fear retrieval compared to naïve controls and Valcyte treated CIE rats, indicating that CIE-mediated impairment in fear memory and its regulation by NPCs was sex dependent. Valcyte significantly reduced Ki-67 and NeuroD labeled cells in the dentate gyrus (DG) in both sexes, demonstrating a role for NPCs in reduced fear retrieval in males. Valcyte prevented adaptations in GluN2A receptor expression and synaptoporin density in the DG in males, indicating that NPCs contributed to alterations in plasticity-related proteins and mossy fiber projections that were associated with reduced fear retrieval. These data suggest that DG NPCs born during withdrawal and early abstinence from CIE are aberrant, and could play a role in weakening long-term memory consolidation dependent on the hippocampus.

eEF2K/eEF2 Pathway Controls the Excitation/Inhibition Balance and Susceptibility to Epileptic Seizures

Alterations in the balance of inhibitory and excitatory synaptic transmission have been implicated in the pathogenesis of neurological disorders such as epilepsy. Eukaryotic elongation factor 2 kinase (eEF2K) is a highly regulated, ubiquitous kinase involved in the control of protein translation. Here, we show that eEF2K activity negatively regulates GABAergic synaptic transmission. Indeed, loss of eEF2K increases GABAergic synaptic transmission by upregulating the presynaptic protein Synapsin 2b and α5-containing GABAA receptors and thus interferes with the excitation/inhibition balance. This cellular phenotype is accompanied by an increased resistance to epilepsy and an impairment of only a specific hippocampal-dependent fear conditioning. From a clinical perspective, our results identify eEF2K as a potential novel target for antiepileptic drugs, since pharmacological and genetic inhibition of eEF2K can revert the epileptic phenotype in a mouse model of human epilepsy.

The role of working memory and declarative memory in trace conditioning

Translational assays of cognition that are similarly implemented in both lower and higher-order species, such as rodents and primates, provide a means to reconcile preclinical modeling of psychiatric neuropathology and clinical research. To this end, Pavlovian conditioning has provided a useful tool for investigating cognitive processes in both lab animal models and humans. This review focuses on trace conditioning, a form of Pavlovian conditioning typified by the insertion of a temporal gap (i.e., trace interval) between presentations of a conditioned stimulus (CS) and an unconditioned stimulus (US). This review aims to discuss pre-clinical and clinical work investigating the mnemonic processes recruited for trace conditioning. Much work suggests that trace conditioning involves unique neurocognitive mechanisms to facilitate formation of trace memories in contrast to standard Pavlovian conditioning. For example, the hippocampus and prefrontal cortex (PFC) appear to play critical roles in trace conditioning. Moreover, cognitive mechanistic accounts in human studies suggest that working memory and declarative memory processes are engaged to facilitate formation of trace memories. The aim of this review is to integrate cognitive and neurobiological accounts of trace conditioning from preclinical and clinical studies to examine involvement of working and declarative memory.

Fast cerebellar reflex circuitry requires synaptic vesicle priming by munc13-3

Munc13-3 is a member of the Munc13 family of synaptic vesicle priming proteins and mainly expressed in cerebellar neurons. Munc13-3 null mutant (Munc13-3^−/−) mice show decreased synaptic release probability at parallel fiber to Purkinje cell, granule cell to Golgi cell, and granule cell to basket cell synapses and exhibit a motor learning deficit at highest rotarod speeds. Since we detected Munc13-3 immunoreactivity in the dentate gyrus, as reported here for the first time, and current studies indicated a crucial role for the cerebellum in hippocampus-dependent spatial memory, we systematically investigated Munc13-3^−/− mice versus wild-type littermates of both genders with respect to hippocampus-related cognition and a range of basic behaviors, including tests for anxiety, sensory functions, motor performance and balance, sensorimotor gating, social interaction and competence, and repetitive and compulsive behaviors. Neither basic behavior nor hippocampus-dependent cognitive performance, evaluated by Morris water maze, hole board working and reference memory, IntelliCage-based place learning including multiple reversals, and fear conditioning, showed any difference between genotypes. However, consistent with a disturbed cerebellar reflex circuitry, a reliable reduction in the acoustic startle response in both male and female Munc13-3^−/− mice was found. To conclude, complete deletion of Munc13-3 leads to a robust decrease in the acoustic startle response. This readout of a fast cerebellar reflex circuitry obviously requires synaptic vesicle priming by Munc13-3 for full functionality, in contrast to other behavioral or cognitive features, where a nearly perfect compensation of Munc13-3 deficiency by related synaptic proteins has to be assumed.

The amygdala is critical for trace, delay, and contextual fear conditioning

Numerous investigations have definitively shown amygdalar involvement in delay and contextual fear conditioning. However, much less is known about amygdala contributions to trace fear conditioning, and what little evidence exists is conflicting as noted in previous studies. This discrepancy may result from selective targeting of individual nuclei within the amygdala. The present experiments further examine the contributions of amygdalar subnuclei to trace, delay, and contextual fear conditioning. Rats were trained using a 10-trial trace, delay, or unpaired fear conditioning procedure. Pretraining lesions targeting the entire basolateral amygdala (BLA) resulted in a deficit in trace, delay, and contextual fear conditioning. Immediate post-training infusions of the protein synthesis inhibitor, cycloheximide, targeting the basal nucleus of the amygdala (BA) attenuated trace and contextual fear memory expression, but had no effect on delay fear conditioning. However, infusions targeting the lateral nucleus of the amygdala (LA) immediately following conditioning attenuated contextual fear memory expression, but had no effect on delay or trace fear conditioning. In follow-up experiments, rats were trained using a three-trial delay conditioning procedure. Immediate post-training infusions targeting the LA produced deficits in both delay tone and context fear, while infusions targeting the BA produced deficits in context but not delay tone fear. These data fully support a role for the BLA in trace, delay, and contextual fear memories. Specifically, these data suggest that the BA may be more critical for trace fear conditioning, whereas the LA may be more critical for delay fear memories.

Prefrontal cortical regulation of fear learning

The prefrontal cortex regulates the expression of fear based on previously learned information. Recently, this brain area has emerged as being crucial in the initial formation of fear memories, providing new avenues to study the neurobiology underlying aberrant learning in anxiety disorders. Here we review the circumstances under which the prefrontal cortex is recruited in the formation of memory, highlighting relevant work in laboratory animals and human subjects. We propose that the prefrontal cortex facilitates fear memory through the integration of sensory and emotional signals and through the coordination of memory storage in an amygdala-based network.

Protein degradation and protein synthesis in long-term memory formation

Long-term memory (LTM) formation requires transient changes in the activity of intracellular signaling cascades that are thought to regulate new gene transcription and de novo protein synthesis in the brain. Consistent with this, protein synthesis inhibitors impair LTM for a variety of behavioral tasks when infused into the brain around the time of training or following memory retrieval, suggesting that protein synthesis is a critical step in LTM storage in the brain. However, evidence suggests that protein degradation mediated by the ubiquitin-proteasome system (UPS) may also be a critical regulator of LTM formation and stability following retrieval. This requirement for increased protein degradation has been shown in the same brain regions in which protein synthesis is required for LTM storage. Additionally, increases in the phosphorylation of proteins involved in translational control parallel increases in protein polyubiquitination and the increased demand for protein degradation is regulated by intracellular signaling molecules thought to regulate protein synthesis during LTM formation. In some cases inhibiting proteasome activity can rescue memory impairments that result from pharmacological blockade of protein synthesis, suggesting that protein degradation may control the requirement for protein synthesis during the memory storage process. Results such as these suggest that protein degradation and synthesis are both critical for LTM formation and may interact to properly “consolidate” and store memories in the brain. Here, we review the evidence implicating protein synthesis and degradation in LTM storage and highlight the areas of overlap between these two opposing processes. We also discuss evidence suggesting these two processes may interact to properly form and store memories. LTM storage likely requires a coordinated regulation between protein degradation and synthesis at multiple sites in the mammalian brain.

Rapid amygdala responses during trace fear conditioning without awareness

The role of consciousness in learning has been debated for nearly 50 years. Recent studies suggest that conscious awareness is needed to bridge the gap when learning about two events that are separated in time, as is true for trace fear conditioning. This has been repeatedly shown and seems to apply to other forms of classical conditioning as well. In contrast to these findings, we show that individuals can learn to associate a face with the later occurrence of a shock, even if they are unable to perceive the face. We used a novel application of magnetoencephalography (MEG) to non-invasively record neural activity from the amygdala, which is known to be important for fear learning. We demonstrate rapid (∼170–200 ms) amygdala responses during the stimulus free period between the face and the shock. These results suggest that unperceived faces can serve as signals for impending threat, and that rapid, automatic activation of the amygdala contributes to this process. In addition, we describe a methodology that can be applied in the future to study neural activity with MEG in other subcortical structures.

Bridging the interval: theory and neurobiology of trace conditioning

An early finding in the behavioral analysis of learning was that conditioned responding weakens as the conditioned stimulus (CS) and unconditioned stimulus (US) are separated in time. This "trace" conditioning effect has been the focus of years of research in associative learning. Theoretical accounts of trace conditioning have focused on mechanisms that allow associative learning to occur across long intervals between the CS and US. These accounts have emphasized degraded contingency effects, timing mechanisms, and inhibitory learning. More recently, study of the neurobiology of trace conditioning has shown that even a short interval between the CS and US alters the circuitry recruited for learning. Here, we review some of the theoretical and neurobiological mechanisms underlying trace conditioning with an emphasis on recent studies of trace fear conditioning. Findings across many studies have implications not just for how we think about time and conditioning, but also for how we conceptualize fear conditioning in general, suggesting that circuitry beyond the usual suspects needs to be incorporated into current thinking about fear, learning, and anxiety.

Extinguishing trace fear engages the retrosplenial cortex rather than the amygdala

Extinction learning underlies the treatment for a variety of anxiety disorders. Most of what is known about the neurobiology of extinction is based on standard "delay" fear conditioning, in which awareness is not required for learning. Little is known about how complex, explicit associations extinguish, however. "Trace" conditioning is considered to be a rodent model of explicit fear because it relies on both the cortex and hippocampus and requires explicit contingency awareness in humans. Here, we explore the neural circuit supporting trace fear extinction in order to better understand how complex memories extinguish. We first show that the amygdala is selectively involved in delay fear extinction; blocking intra-amygdala glutamate receptors disrupted delay, but not trace extinction. Further, ERK phosphorylation was increased in the amygdala after delay, but not trace extinction. We then identify the retrosplenial cortex (RSC) as a key structure supporting trace extinction. ERK phosphorylation was selectively increased in the RSC following trace extinction and blocking intra-RSC NMDA receptors impaired trace, but not delay extinction. These findings indicate that delay and trace extinction require different neural circuits; delay extinction requires plasticity in the amygdala whereas trace extinction requires the RSC. Anxiety disorders linked to explicit memory may therefore depend on cortical processes that have not been traditionally targeted by extinction studies based on delay fear.

Prefrontal activity links nonoverlapping events in memory

The medial prefrontal cortex (mPFC) plays an important role in memory. By maintaining a working memory buffer, neurons in prelimbic (PL) mPFC may selectively contribute to learning associations between stimuli that are separated in time, as in trace fear conditioning (TFC). Until now, evidence for this bridging role was largely descriptive. Here we used optogenetics to silence neurons in the PL mPFC of rats during learning in TFC. Memory formation was prevented when mPFC was silenced specifically during the interval separating the cue and shock. Our results provide support for a working memory function for these cells and indicate that associating two noncontiguous stimuli requires bridging activity in PL mPFC.

Enhanced sensitivity to learning fearful associations during adolescence

The majority of anxiety disorders emerge during adolescence, yet there is a paucity of research examining factors that contribute to the "storm and stress" of this period. Understanding how juvenile (P23), adolescent (P35), and adult (P90) rats differ on basic fear conditioning tasks may shed light on this issue. In Experiment 1, P23, P35, and P90 rats were given 6 CS-US presentations. There were four training conditions: Delay (i.e., CS co-terminating with the US), Trace 20 and Trace 40 (i.e., an interval of 20s and 40s between the CS and US, respectively), and Unpaired (i.e., explicitly Unpaired presentations of the CS and US). Twenty-four hours after conditioning, freezing was measured to assess fear of the CS in a novel context. At test, there were no age differences in CS-elicited freezing in group Delay, and this condition exhibited significantly higher levels of freezing compared to group Unpaired. However, the adolescent rats were the only age group to exhibit higher levels of freezing following training with the 20s and 40s trace intervals, compared to Unpaired controls. Experiment 2 replicated the finding that adolescent but not adult rats exhibit fear following conditioning with a 20s trace interval, while also demonstrating that both age groups display learning with a shorter trace interval of 5s. Experiment 3 showed that exposure to corticosterone (200 μg/ml) in the drinking water for 1 week prior to conditioning selectively disrupts Trace 20 but not Delay conditioning during adolescence. Lastly, in Experiment 4 the test procedures were changed such that freezing was measured both during the CS and during a stimulus free trace interval. Once again, P35 but not P90 rats exhibited fear following training with a 20s trace interval. Taken together, these findings demonstrate that adolescent rats show a heightened propensity to learn fearful associations, and that this is disrupted following exposure to corticosterone.

Defective synaptic transmission and structure in the dentate gyrus and selective fear memory impairment in the Rsk2 mutant mouse model of Coffin-Lowry syndrome

The Coffin-Lowry syndrome (CLS) is a syndromic form of intellectual disability caused by loss-of-function of the RSK2 serine/threonine kinase encoded by the rsk2 gene. Rsk2 knockout mice, a murine model of CLS, exhibit spatial learning and memory impairments, yet the underlying neural mechanisms are unknown. In the current study, we examined the performance of Rsk2 knockout mice in cued, trace and contextual fear memory paradigms and identified selective deficits in the consolidation and reconsolidation of hippocampal-dependent fear memories as task difficulty and hippocampal demand increase. Electrophysiological, biochemical and electron microscopy analyses were carried out in the dentate gyrus of the hippocampus to explore potential alterations in neuronal functions and structure. In vivo and in vitro electrophysiology revealed impaired synaptic transmission, decreased network excitability and reduced AMPA and NMDA conductance in Rsk2 knockout mice. In the absence of RSK2, standard measures of short-term and long-term potentiation (LTP) were normal, however LTP-induced CREB phosphorylation and expression of the transcription factors EGR1/ZIF268 were reduced and that of the scaffolding protein SHANK3 was blocked, indicating impaired activity-dependent gene regulation. At the structural level, the density of perforated and non-perforated synapses and of multiple spine boutons was not altered, however, a clear enlargement of spine neck width and post-synaptic densities indicates altered synapse ultrastructure. These findings show that RSK2 loss-of-function is associated in the dentate gyrus with multi-level alterations that encompass modifications of glutamate receptor channel properties, synaptic transmission, plasticity-associated gene expression and spine morphology, providing novel insights into the mechanisms contributing to cognitive impairments in CLS.

NR2A- and NR2B-containing NMDA receptors in the prelimbic medial prefrontal cortex differentially mediate trace, delay, and contextual fear conditioning

Activation of N-methyl-D-aspartate receptors (NMDAR) in the prelimbic medial prefrontal cortex (PL mPFC) is necessary for the acquisition of both trace and contextual fear memories, but it is not known how specific NR2 subunits support each association. The NR2B subunit confers unique properties to the NMDAR and may differentially regulate these two fear memories. Here we show that NR2A-containing NMDARs mediate trace, delay, and contextual fear memories, but NR2B-containing NMDARs are required only for trace conditioning, consistent with a role for PL mPFC in working memory.

Disruption of the perineuronal net in the hippocampus or medial prefrontal cortex impairs fear conditioning

The perineuronal net (PNN) surrounds neurons in the central nervous system and is thought to regulate developmental plasticity. A few studies have shown an involvement of the PNN in hippocampal plasticity and memory storage in adult animals. In addition to the hippocampus, plasticity in the medial prefrontal cortex (mPFC) has been demonstrated to be critical for the storage of long-term memory, particularly memories for temporally separated events. In the present study, we examined the role of PNN in the acquisition and retention of memories for trace (in which the conditioned and unconditioned stimuli are temporally separated) and delayed (in which the conditioned and unconditioned stimuli overlap) fear conditioning in both the hippocampus and the mPFC. Consistent with a role for the hippocampus in memory storage in both delayed and trace fear conditioning, removal of hippocampal PNN disrupted contextual and trace fear memory. Disruption of the PNN in the mPFC impaired long-term trace and conditioned stimulus (CS)-elicited fear memory in the trace fear conditioning task. Interestingly, CS-elicited fear memory was also impaired when a delayed fear conditioning paradigm was used. These findings further support a role for the PNN in neural plasticity and implicate PNN-regulated plasticity in neocortical memory storage.

Trace and contextual fear conditioning are impaired following unilateral microinjection of muscimol in the ventral hippocampus or amygdala, but not the medial prefrontal cortex

Trace fear conditioning, in which a brief empty "trace interval" occurs between presentation of the CS and UCS, differs from standard delay conditioning in that contributions from both the hippocampus and prelimbic medial prefrontal cortex (PL mPFC) are required to form a normal long term memory. Little is currently known about how the PL interacts with various temporal lobe structures to support learning across this temporal gap between stimuli. We temporarily inactivated PL along with either ventral hippocampus or amygdala in a disconnection design to determine if these structures functionally interact to acquire trace fear conditioning. Disconnection (contralateral injections) of the PL with either the ventral hippocampus or amygdala impaired trace fear conditioning; however, ipsilateral control rats were also impaired. Follow-up experiments examined the effects of unilateral inactivation of the PL, ventral hippocampus, or amygdala during conditioning. The results of this study demonstrate that unilateral inactivation of the ventral hippocampus or amygdala impairs memory, while bilateral inactivation of the PL is required to produce a deficit. Memory deficits after unilateral inactivation of the ventral hippocampus or amygdala prevent us from determining whether the mPFC functionally interacts with the medial temporal lobe using a disconnection approach. Nonetheless, our findings suggest that the trace fear network is more integrated than previously thought.

Dorsal hippocampal lesions disrupt Pavlovian delay conditioning and conditioned-response timing

The involvement of the rat dorsal hippocampus (dhpc) in Pavlovian conditioning and timing of conditioned responding was examined in an appetitive preparation in which presentation of a relatively long, 40-s auditory conditioned stimulus (CS) was followed immediately by food delivery. Dorsal hippocampal lesions impaired Pavlovian conditioning in this task. They also produced a deficit in interval timing, replicating previous findings with short CSs. The conditioning and timing deficits observed are consistent with the findings from single-unit recording studies in other species, and suggest that the involvement of the dhpc in Pavlovian processes could be more general than is assumed by many of the current theories of hippocampal function.

Time determines the neural circuit underlying associative fear learning

Ultimately associative learning is a function of the temporal features and relationships between experienced stimuli. Nevertheless how time affects the neural circuit underlying this form of learning remains largely unknown. To address this issue, we used single-trial auditory trace fear conditioning and varied the length of the interval between tone and foot-shock. Through temporary inactivation of the amygdala, medial prefrontal-cortex (mPFC), and dorsal-hippocampus in rats, we tested the hypothesis that different temporal intervals between the tone and the shock influence the neuronal structures necessary for learning. With this study we provide the first experimental evidence showing that temporarily inactivating the amygdala before training impairs auditory fear learning when there is a temporal gap between the tone and the shock. Moreover, imposing a short interval (5 s) between the two stimuli also relies on the mPFC, while learning the association across a longer interval (40 s) becomes additionally dependent on a third structure, the dorsal-hippocampus. Thus, our results suggest that increasing the interval length between tone and shock leads to the involvement of an increasing number of brain areas in order for the association between the two stimuli to be acquired normally. These findings demonstrate that the temporal relationship between events is a key factor in determining the neuronal mechanisms underlying associative fear learning.

Memory consolidation in both trace and delay fear conditioning is disrupted by intra-amygdala infusion of the protein synthesis inhibitor anisomycin

Memory for delay fear conditioning requires the synthesis of new mRNA and protein in the basolateral amygdala. It is currently unknown whether similar molecular processes in the amygdala are required for the formation of trace fear memory, in which a stimulus-free interval is inserted between the conditional stimulus (CS) and unconditional stimulus (UCS). Here, we show that infusion of the protein synthesis inhibitor anisomycin into the basolateral amygdala disrupts consolidation of both trace and delay fear conditioning. This is the first evidence that protein synthesis in the amygdala is necessary for the formation of both trace and delay fear memory.

Learning increases the survival of newborn neurons provided that learning is difficult to achieve and successful

Learning increases neurogenesis by increasing the survival of new cells generated in the adult hippocampal formation [Shors, T. J. Saving new brain cells. Scientific American, 300, 46-52, 2009]. However, only some types of learning are effective. Recent studies demonstrate that animals that learn the conditioned response (CR) but require more trials to do so retain more new neurons than animals that quickly acquire the CR or that fail to acquire the CR. In these studies, task parameters were altered to modify the number of trials required to learn a CR. Here, we asked whether pharmacological manipulations that prevent or facilitate learning would decrease or increase, respectively, the number of cells that remain in the hippocampus after training. To answer this question, we first prevented learning with the competitive N-methyl-D-aspartate (NMDA) receptor antagonist (RS)-3-(2-carboxypiperazin-4-yl) propyl-1-phosphonic acid. As a consequence, training did not increase cell survival. Second, we facilitated learning with the cognitive enhancer D-cycloserine, which increases NMDA receptor activity via its actions at the glycine binding site. Administration of D-cycloserine each day before training increased the number of learned responses and the number of cells that survived. All animals that learned the CR retained more of the new cells, but those that learned very quickly retained fewer than those that required more training trials to learn. Together, these results demonstrate that NMDA receptor activation modifies learning and as a consequence alters the number of surviving cells in the adult hippocampus.

The role of nicotinic acetylcholine receptors in the medial prefrontal cortex and hippocampus in trace fear conditioning

Acute nicotine enhances multiple types of learning including trace fear conditioning but the underlying neural substrates of these effects are not well understood. Trace fear conditioning critically involves the medial prefrontal cortex and hippocampus, which both express nicotinic acetylcholine receptors (nAChRs). Therefore, nicotine could act in either or both areas to enhance trace fear conditioning. To identify the underlying neural areas and nAChR subtypes, we examined the effects of infusion of nicotine, or nicotinic antagonists dihydro-beta-erythroidine (DHβE: high-affinity nAChRs) or methyllycaconitine (MLA: low-affinity nAChRs) into the dorsal hippocampus, ventral hippocampus, and medial prefrontal cortex (mPFC) on trace and contextual fear conditioning. We found that the effects of nicotine on trace and contextual fear conditioning vary by brain region and nAChR subtype. The dorsal hippocampus was involved in the effects of nicotine on both trace and contextual fear conditioning but each task was sensitive to different doses of nicotine. Additionally, dorsal hippocampal infusion of the antagonist DHβE produced deficits in trace but not contextual fear conditioning. Nicotine infusion into the ventral hippocampus produced deficits in both trace and contextual fear conditioning. In the mPFC, nicotine enhanced trace but not contextual fear conditioning. Interestingly, infusion of the antagonists MLA or DHβE in the mPFC also enhanced trace fear conditioning. These findings suggest that nicotine acts on different substrates to enhance trace versus contextual fear conditioning, and that nicotine-induced desensitization of nAChRs in the mPFC may contribute to the effects of nicotine on trace fear conditioning.

A cholinergic-dependent role for the entorhinal cortex in trace fear conditioning

Trace conditioning is considered a model of higher cognitive involvement in simple associative tasks. Studies of trace conditioning have shown that cortical areas and the hippocampal formation are required to associate events that occur at different times. However, the mechanisms that bridge the trace interval during the acquisition of trace conditioning remain unknown. In four experiments with fear conditioning in rats, we explored the involvement of the entorhinal cortex (EC) in the acquisition of fear under a trace-30 s protocol. We first determined that pretraining neurotoxic lesions of the EC selectively impaired trace-, but not delay-conditioned fear as evaluated by freezing behavior. A local cholinergic deafferentation of the EC using 192-IgG-saporin did not replicate this deficit, presumably because cholinergic interneurons were spared by the toxin. However, pretraining local blockade of EC muscarinic receptors with the M1 antagonist pirenzepine yielded a specific and dose-dependent deficit in trace-conditioned responses. The same microinjections performed after conditioning were without effect on trace fear responses. These effects of blocking M1 receptors are consistent with the notion that conditioned stimulus (CS)-elicited, acetylcholine-dependent persistent activities in the EC are needed to maintain a representation of a tone CS across the trace interval during the acquisition of trace conditioning. This function of the EC is consistent with recent views of this region as a short-term stimulus buffer.

Female rats learn trace memories better than male rats and consequently retain a greater proportion of new neurons in their hippocampi

Learning increases the survival of new cells that are generated in the hippocampal formation before the training experience, especially if the animal learns to associate stimuli across time [Gould E, Beylin A, Tanapat P, Reeves A, Shors TJ (1999) Nat Neurosci 2:260-265]. All relevant studies have been conducted on male rats, despite evidence for sex differences in this type of learning. In the present study, we asked whether sex differences in learning influence the survival of neurons generated in the adult hippocampus. Male and female adult rats were injected with one dose of bromodeoxyuridine (BrdU; 200 mg/kg), to label one population of dividing cells. One week later, half of the animals were trained with a temporal learning task of trace eyeblink conditioning, while the other half were not trained. Animals were killed 1 day after training (12 days after the BrdU injection). Hippocampal tissue was stained for BrdU and a marker of immature neurons, doublecortin. Both sexes learned to emit the conditioned eyeblink response during the trace interval. As a consequence, more new neurons remained in their hippocampi than in sex-matched controls. In individual animals, the number of surviving cells correlated positively with asymptotic performance; those that expressed more learned responses retained more new neurons. However, animals that learned very well retained even more new cells if they required many trials to do so. Because females emitted more learned responses than males did, they retained nearly twice as many new cells per unit volume of tissue. This effect was most evident in the ventral region of the hippocampal formation. Thus, sex differences in learning alter the anatomical structure of the hippocampus. As a result, male and female brains continue to differentiate in adulthood.

Neurogenesis, learning and associative strength

Though the role of the hippocampus in processes of learning and memory is well established, the role of new neurons generated there is less understood. Training on some associative learning tasks increases the likelihood that new cells in the subgranular zone of the dentate gyrus will survive. In the rat, an effective training procedure is trace eyeblink conditioning, in which a conditioned stimulus (CS) is paired with an aversive stimulation to the eyelid (unconditioned stimulus; US), but the stimuli are separated by a temporal gap. Here, we manipulated the asymptote or rate of acquisition during trace conditioning, and examined survival of cells generated 1 week before training. Acquisition was disrupted by decreasing associative strength by insertion of unpredicted USs or slowed with latent inhibition. The number of cells was increased in animals that were trained with trace conditioning, irrespective of the decrease in associative strength or slowed acquisition. Disrupting acquisition with unsignaled USs still increased cell numbers, suggesting that the learning effect on cell survival is not dependent on reliable expression of the conditioned response. Further, animals in the latent inhibition conditions that learned but required more trials also retained more of the new cells than animals requiring fewer trials. The number of cells that survived after the effective training procedures was similar to the number of cells that were available for rescue at the beginning of training. Thus, learning can rescue the majority of cells expressed at the beginning of training, and does so most effectively when acquisition requires many trials.

Hippocampal NMDA receptors are necessary for auditory trace fear conditioning measured with conditioned hypoalgesia in rats

We tested whether N-methyl-D-aspartate (NMDA) receptors in the dorsal hippocampus (DH) are critical for the acquisition of trace fear conditioning using conditioned hypoalgesia (CHA), decrease in pain reactivity, as the conditioned response (CR) instead of commonly used freezing. Infusions of the NMDA receptor antagonist, DL-2-amino-5-phosphonovaleric acid (APV) into DH prior to conditioning resulted in impaired CHA, measured with the radiant heat tail flick test, only in the trace-conditioning group when they were tested during the trace interval. The same infusion had no effect on CHA in the delay-conditioned animals. The results support that NMDA receptors in DH are critically involved in associating the CS with the US across a temporal gap. In addition, temporal specificity of the CR was revealed as CHA was induced only in the temporal vicinity of the US used for the training.

Neurogenesis and learning: acquisition and asymptotic performance predict how many new cells survive in the hippocampus

Previous research has shown that some associative learning tasks prevent the death of new neurons in the adult hippocampus. However, it is unclear whether it is mere exposure to the training stimuli that rescues neurons or whether successful learning of the task is required for enhanced neuronal survival. If learning is the important variable, then animals that learn better given the same amount of training should retain more of the new cells after learning than animals that do not learn as well. Here, we examined the effects of training versus learning on cell survival in the adult hippocampus. Animals were injected with BrdU to label a population of cells and trained one week later on one of two trace conditioning tasks, one of which depends on the hippocampus and one that does not. Increases in cell number occurred only in animals that acquired the learned response, irrespective of the task. There were significant correlations between acquisition and cell number, as well as between asymptotic performance and cell number. These data support the idea that learning and not simply training increases the survival of the new cells in the hippocampus.

A link between the hippocampal and the striatal memory systems of the brain

Two major memory systems have been recognized over the years (Squire 1987): the declarative memory system, which is under the control of the hippocampus and related temporal lobe structures, and the procedural or habit memory system, which is under the control of the striatum and its connections. Most if not all learning tasks studied in animals, however, involve either the performance or the suppression of movement; this, if learned well, may be viewed as having become a habit. It is agreed that memory rules change from their first association to those that take place when the task is mastered. Does this change of rules involve a switch from one memory system to another? Here we will comment on: 1) reversal learning in the Morris water maze (MWM), in which the declarative or spatial component of a task is changed but the procedural component (to swim to safety) persists and needs to be re-linked with a different set of spatial cues; and 2) a series of observations on an inhibitory avoidance task that indicate that the brain systems involved change with further learning.

Hippocampal and cerebellar single-unit activity during delay and trace eyeblink conditioning in the rat

In delay eyeblink conditioning, the CS overlaps with the US and only a brainstem-cerebellar circuit is necessary for learning. In trace eyeblink conditioning, the CS ends before the US is delivered and several forebrain structures, including the hippocampus, are required for learning, in addition to a brainstem-cerebellar circuit. The interstimulus interval (ISI) between CS onset and US onset is perhaps the most important factor in classical conditioning, but studies comparing delay and trace conditioning have typically not matched these procedures in this crucial factor, so it is often difficult to determine whether results are due to differences between delay and trace or to differences in ISI. In the current study, we employed a 580-ms CS-US interval for both delay and trace conditioning and compared hippocampal CA1 activity and cerebellar interpositus nucleus activity in order to determine whether a unique signature of trace conditioning exists in patterns of single-unit activity in either structure. Long-Evans rats were chronically implanted in either CA1 or interpositus with microwire electrodes and underwent either delay eyeblink conditioning, or trace eyeblink conditioning with a 300-ms trace period between CS offset and US onset. On trials with a CR in delay conditioning, CA1 pyramidal cells showed increases in activation (relative to a pre-CS baseline) during the CS-US period in sessions 1-4 that was attenuated by sessions 5-6. In contrast, on trials with a CR in trace conditioning, CA1 pyramidal cells did not show increases in activation during the CS-US period until sessions 5-6. In sessions 5-6, increases in activation were present only to the CS and not during the trace period. For rats with interpositus electrodes, activation of interpositus neurons on CR trials was present in all sessions in both delay and trace conditioning. However, activation was greater in trace compared to delay conditioning in the first half of the CS-US interval (during the trace CS) during early sessions of conditioning and, in later sessions of conditioning, activation was greater in the second half of the CS-US interval (during the trace interval). These results suggest that the pattern of hippocampal activation that differentiates trace from delay eyeblink conditioning is a slow buildup of activation to the CS, possibly representing encoding of CS duration or discrimination of the CS from the background context. Interpositus nucleus neurons show strong modeling of the eyeblink CR regardless of paradigm but show a changing pattern across conditioning that may be due to the necessary contributions of forebrain processing to trace conditioning.

The connection between the hippocampal and the striatal memory systems of the brain: A review of recent findings

Two major memory systems have been recognized over the years (Squire, in Memory and Brain, 1987): the declarative memory system, which is under the control of the hippocampus and related temporal lobe structures, and the procedural or habit memory system, which is under the control of the striatum and its connections (Mishkin et al., in Neurobiology of Learning by G Lynch et al., 1984; Knowlton et al., Science 273:1399, 1996). Most if not all learning tasks studied in animals, however, involve either the performance or the suppression of movement. Animals acquire connections between environmental or discrete sensory cues (conditioned stimuli, CSs) and emotionally or otherwise significant stimuli (unconditioned stimuli, USs). As a result, they learn to perform or to inhibit the performance of certain motor responses to the CS which, when learned well, become what can only be called habits (Mishkin et al., 1984): to regularly walk or swim to a place or away from a place, or to inhibit one or several forms of movement. These responses can be viewed as conditioned responses (CRs) and may sometimes be very complex. This is of course also seen in humans: people learn how to play on a keyboard in response to a mental or written script and perform the piano or write a text; with practice, the performance improves and eventually reaches a high criterion and becomes a habit, performed almost if not completely without awareness. Commuting to school in a big city in the shortest possible time and eschewing the dangers is a complex learning that children acquire to the point of near-perfection. It is agreed that the rules that connect the perception of the CS and the expression of the CR change from their first association to those that take place when the task is mastered. Does this change of rules involve a switch from one memory system to another? Are different brain systems used the first time one plays a sonata or goes to school as compared with the 100th time? Here we will comment on: 1) reversal learning in the Morris water maze (MWM), in which the declarative or spatial component of a task is changed but the procedural component (to swim) persists and needs to be re-linked with a different set of spatial cues; and 2) a series of observations on an inhibitory avoidance task that indicate that the brain systems involved change with further learning.

Single neurons in the medial prefrontal cortex of the rat exhibit tonic and phasic coding during trace fear conditioning

Trace fear conditioning is a learning task that requires the association of an auditory conditioned stimulus (CS) and a shock unconditioned stimulus (US) that are separated by a 20-s trace interval. Single neuron activity was recorded from the prelimbic and infralimbic areas of the medial prefrontal cortex in rats during trace fear conditioning or nonassociative unpaired training. Prelimbic neurons showed learning-related increases in activity to the CS and US, whereas infralimbic neurons showed learning-related decreases in activity to these stimuli. A subset of prelimbic neurons exhibited sustained increases in activity during the trace interval. These sustained prelimbic responses may provide a bridging code that allows for overlapping representations of CS and US information within the trace fear conditioning circuit.

Midazolam inhibits neophobia-induced Fos expression in the rat hippocampus

The effect of midazolam on expression of c-Fos protein was examined in the rat hippocampus, following the open field test of neophobia. It was found that pretreatment of rats with midazolam, at the dose of 0.5 mg/kg, enhanced rat exploratory behavior, and inhibited neophobia related stimulation of c-Fos in the CA-1 and CA-3 areas of the hippocampus. The presented results provide new immunocytochemical data on the involvement of hippocampus in emotional processes related to neophobia, and indicate a possible site of action of benzodiazepines.

Temporal processing of information: The role of the medial prefrontal cortex and hippocampus: Theoretical comment on gilmartin and mcechron (2005)

M. R. Gilmartin and M. D. McEchron (2005) reported that single cells recorded in the prelimbic cortex of rats during the acquisition of trace fear conditioning display multiple patterns of neuronal firing during the trace. These finding are discussed in the context of the role of the prelimbic cortex in processing temporal information during trace conditioning and delayed matching- or nonmatching-to-sample paradigms based on both electrophysiology and lesion evidence. In addition, evidence is provided for a role of the hippocampus in supporting temporal processing of information and its potential interaction with the prelimbic cortex.

The bed nucleus of the stria terminalis is critically involved in enhancing associative learning after stressful experience

Exposure to an acute stressful event enhances trace eyeblink conditioning in male rats, even when rats begin training days after the stressor (Shors, 2001). The authors examined whether the bed nucleus of the stria terminalis (BNST), an area involved in stress and anxiety, is critically involved in this effect and, if so, when. The authors found that excitotoxic lesions to the BNST prevented the enhanced conditioning after stressor exposure. In addition, temporary inactivation of the BNST during the stressor did not alter enhanced responding, whereas inactivation during training prevented the enhancement. These data indicate that stressful experience induces persistent changes in the BNST that are necessary for enhancing learning well after the stressful event has ceased.