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

Neural mechanisms of human temporal fear conditioning

Learning the temporal relationship between a warning cue (conditioned stimulus; CS) and aversive threat (unconditioned stimulus; UCS) is an important aspect of Pavlovian conditioning. Although prior functional magnetic resonance imaging (fMRI) research has identified brain regions that support Pavlovian conditioning, it remains unclear whether these regions support time-related processes important for this type of associative learning. Elucidating the neural substrates of temporal conditioning is important for a complete understanding of the Pavlovian conditioning process. Therefore, the present study used a temporal Pavlovian conditioning procedure to investigate brain activity that mediates the formation of temporal associations. During fMRI, twenty-three healthy volunteers completed a temporal conditioning procedure and a control task that does not support conditioning. Specifically, during the temporal conditioning procedure, the UCS was presented at fixed intervals (ITI: 20s) while in the control condition the UCS was presented at random intervals (Average ITI: 20s, ITI Range: 6-34s). We observed greater skin conductance responses and expectancy of the UCS during fixed (i.e., temporal conditioning) relative to random (i.e., control procedure) interval trials. These findings demonstrate fixed trials support temporal conditioning, while random trials do not. During fixed interval trials, greater conditioned fMRI signal responses were observed within dorsolateral prefrontal cortex, inferior parietal lobule, inferior and middle temporal cortex, hippocampus, and amygdala. The current findings suggest these brain regions constitute a neural circuit that encodes the temporal information necessary for Pavlovian fear conditioning.

Ethanol Increases Mechanical Pain Sensitivity in Rats via Activation of GABAA Receptors in Medial Prefrontal Cortex

Ethanol is widely known for its ability to cause dramatic changes in emotion, social cognition, and behavior following systemic administration in humans. Human neuroimaging studies suggest that alcohol dependence and chronic pain may share common mechanisms through amygdala-medial prefrontal cortex (mPFC) interactions. However, whether acute administration of ethanol in the mPFC can modulate pain perception is unknown. Here we showed that bilateral microinjections of ethanol into the prelimbic and infralimbic areas of the mPFC lowered the bilateral mechanical pain threshold for 48 h without influencing thermal pain sensitivity in adult rats. However, bilateral microinjections of artificial cerebrospinal fluid into the mPFC or bilateral microinjections of ethanol into the dorsolateral PFC (also termed as motor cortex area 1 in Paxinos and Watson's atlas of The Rat Brain. Elsevier Academic Press, Amsterdam, 2005) failed to do so, suggesting regional selectivity of the effects of ethanol. Moreover, bilateral microinjections of ethanol did not change the expression of either pro-apoptotic (caspase-3 and Bax) or anti-apoptotic (Bcl-2) proteins, suggesting that the dose was safe and validating the method used in the current study. To determine whether γ-aminobutyric acid A (GABAA) receptors are involved in mediating the ethanol effects, muscimol, a selective GABAA receptor agonist, or bicuculline, a selective GABAA receptor antagonist, was administered alone or co-administered with ethanol through the same route into the bilateral mPFC. The results showed that muscimol mimicked the effects of ethanol while bicuculline completely reversed the effects of ethanol and muscimol. In conclusion, ethanol increases mechanical pain sensitivity through activation of GABAA receptors in the mPFC of rats.

The persistence of long-term potentiation in the projection from ventral hippocampus to medial prefrontal cortex in awake rats

A potentially vital pathway in the processing of spatial memory is the pathway from ventral hippocampus to medial prefrontal cortex (vHPC-mPFC). To assess long-term potentiation (LTP) induction and maintenance across days in this pathway, the effects of several induction paradigms were compared in awake, freely moving rats. Two different high-frequency stimulation (HFS) protocols generated LTP lasting no longer than 1 week. However, after delivering HFS on three consecutive days, LTP lasted an average of 20 days, due mainly to the greater initial induction. Thus the pathway does not require extensive multi-day stimulation to induce LTP, as for other intra-neocortical pathways, but also it does not exhibit the extremely long-lasting and stable LTP previously observed in area CA1 and the dentate gyrus. By using bilaterally placed stimulating and recording electrodes, we found that HFS in one vHPC generated responses and LTP in the contralateral mPFC, even when the ipsilateral mPFC was inactivated by CNQX. We attribute this crossed response to a polysynaptic pathway from the vHPC to the contralateral mPFC. Finally, we found that repeated overnight exposure to an enriched environment also potentiated the vHPC-mPFC response, but this too was a transient effect lasting < 9 days, declining to baseline even before the enriched environment treatment was completed. Overall, these findings are consistent with the view that potentiation of vHPC-mPFC pathway may play a key role in promoting the hippocampus-mPFC interplay that, over days, leads to long-term storage in the frontal cortex of memories that are independent of the hippocampus.

Effects of low level lead exposure on associative learning and memory in the rat: Influences of sex and developmental timing of exposure

Lead (Pb) exposure during development impairs a variety of cognitive, behavioral and neurochemical processes resulting in deficits in learning, memory, attention, impulsivity and executive function. Numerous studies have attempted to model this effect of Pb in rodents, with the majority of studies focusing on hippocampus-associated spatial learning and memory processes. Using a different paradigm, trace fear conditioning, a process requiring coordinated integration of both the medial prefrontal cortex and the hippocampus, we have assessed the effects of Pb exposure on associative learning and memory. The present study examined both female and male long evans rats exposed to three environmentally relevant levels of Pb (150 ppm, 375 ppm and 750 ppm) during different developmental periods: perinatal (PERI; gestation-postnatal day 21), early postnatal (EPN; postnatal days 1-21) and late postnatal (LPN; postnatal days 1-55). Testing began at postnatal day 55 and consisted of a single day of acquisition training, and three post training time points (1, 2 and 10 days) to assess memory consolidation and recall. All animals, regardless of sex, developmental window or level of Pb-exposure, successfully acquired conditioned-unconditioned stimulus association during training. However, there were significant effects of Pb-exposure on consolidation and memory recall at days 1-10 post training. In females, EPN and LPN exposure to 150 ppm Pb (but not PERI exposure) significantly impaired recall. In contrast, only PERI 150 ppm and 750 ppm-exposed males had significant recall deficits. These data suggest a complex interaction between sex, developmental window of exposure and Pb-exposure level on consolidation and recall of associative memories.

Basolateral amygdala projections to ventral hippocampus modulate the consolidation of footshock, but not contextual, learning in rats

The basolateral amygdala (BLA) modulates memory consolidation for a variety of types of learning, whereas other brain regions play more selective roles in specific kinds of learning suggesting a role for differential consolidation via distinct BLA pathways. The ventral hippocampus (VH), an efferent target of the BLA, has been suggested to selectively process emotion-related learning, yet whether the BLA → VH pathway modulates memory consolidation, and does so in a learning-specific manner, is unknown. To address this issue, the BLA of male Sprague-Dawley rats was bilaterally transduced to express either ChR2(E123A) or eArchT3.0. Fiber optic probes were implanted in the VH to provide illumination of BLA axons. Rats then underwent a modified contextual fear conditioning task permitting separation of context and footshock learning. On day 1, rats received 3 min of pre-exposure to the apparatus. On day 2, rats were placed into the apparatus, received an immediate footshock, and quickly removed. Retention was tested on day 4. Optical stimulation of the BLA → VH pathway following footshock, but not context, training using trains of 40-Hz light pulses enhanced retention. Continuous optical inhibition of this pathway for 15 min starting 25 min after footshock training impaired retention. These findings indicate that BLA → VH projections influence the consolidation for footshock, but not context, learning of a modified CFC task and provide direct evidence that BLA projections to other brain regions modulate memory consolidation selectively depending on the kind of learning involved.

Variants of contextual fear conditioning induce differential patterns of Egr-1 activity within the young adult prefrontal cortex

Contextual fear conditioning is a form of associative learning where animals must experience a context before they can associate it with an aversive stimulus. Single-trial contextual fear conditioning (sCFC) and the context preexposure facilitation effect (CPFE) are two variants of CFC where learning about the context is temporally contiguous (sCFC) with or separated (CPFE) from receiving a footshock in that context. Neural activity within CA1 of the dorsal hippocampus (CA1), amygdala (LA), and prefrontal cortex (PFC) may play a critical role when animals learn to associate a context with a footshock (i.e., training). Previous studies from our lab have found that early-growth-response gene 1 (Egr-1), an immediate early gene, exhibits unique patterns of activity within regions of the PFC following training in sCFC and the CPFE of juvenile rats. In the present study, we extended our studies by examining Egr-1 expression in young adult rats to determine (1) if our previous work reflected changes unique to development or extend into adulthood and (2) to contrast expression profiles between sCFC and the CPFE. Rats that learned context fear with sCFC showed increased Egr-1 in the anterior cingulate, orbitofrontal and infralimbic cortices relative to non-associative controls following training, but expression in prelimbic cortex did not differ between fear conditioned and non-associative controls. In contrast, rats trained in the CPFE also showed increased Egr-1 in all the prefrontal cortex regions, including prelimbic cortex. These findings replicate our previous findings in juveniles and suggest that Egr-1 in specific PFC subregions may be uniquely involved in learning context-fear in the CPFE compared to sCFC.

Increased tone-offset response in the lateral nucleus of the amygdala underlies trace fear conditioning

Accumulating evidence suggests that the lateral nucleus of the amygdala (LA) stores associative memory in the form of enhanced neural response to the sensory input following classical fear conditioning in which the conditioned stimulus (CS) and the unconditioned stimulus (US) are presented in a temporally continuous manner. However, little is known about the role of the LA in trace fear conditioning where the CS and the US are separated by a temporal gap. Single-unit recordings of LA neurons before and after trace fear conditioning revealed that the short-latency activity to the CS offset, but not that to the onset, increased significantly and accompanied the conditioned fear response. The increased short-latency activity was evident in two aspects: the number of offset-responsive neurons was increased and the latency of the neuronal response to the CS offset was shortened. On the contrary, changes in the firing rate to either the onset or the offset were negligible following unpaired presentations of the CS and US. In sum, our results suggest that increased synaptic efficacy in the CS offset pathway in the LA might underlie the association between temporally distant stimuli in trace fear conditioning.

The retrosplenial cortex is involved in the formation of memory for context and trace fear conditioning

The retrosplenial cortex (RSC) is known to play a role in the retrieval of context memory, but its involvement in memory formation and consolidation is unclear. To better characterize the role of the RSC, we tested its involvement in the formation and retrieval of memory for trace fear conditioning, a task that requires the association of two cues separated by an empty period of time. We have previously shown that trace fear extinction requires the RSC (Kwapis, Jarome, Lee, Gilmartin, & Helmstetter, 2014) and have hypothesized that trace memory may be stored in a distributed cortical network that includes prelimbic and retrosplenial cortices (Kwapis, Jarome, & Helmstetter, 2015). Whether the RSC participates in acquiring and storing cued trace fear, however, is currently unknown. Here, we demonstrate that blocking protein synthesis in the RSC before, but not after acquisition impairs rats' memory for trace CS and context fear without affecting memory for the CS in standard delay fear conditioning. We also show that NMDA receptor blockade in the RSC transiently impairs memory retrieval for trace, but not delay memory. The RSC therefore appears to critically contribute to formation of trace and context fear memory in addition to its previously recognized role in context memory retrieval.

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.

Glutaminase1 heterozygous mice show enhanced trace fear conditioning and Arc/Arg3.1 expression in hippocampus and cingulate cortex

Mice heterozygous for a mutation in the glutaminase (GLS1) gene (GLS1 HZ mice), with reduced glutamate recycling and release, display reduced hippocampal function as well as memory of contextual cues in a delay fear conditioning (FC) paradigm. Here, we asked whether this deficit reflects an inability to process contextual information or a selective alteration in salience attribution. In addition, we asked whether baseline and activity-induced hippocampal activity were diminished in GLS1 HZ mice. For this purpose, we manipulated the relative salience of the conditioned stimulus (CS) and contextual cues in FC tasks, and examined gene expression of the immediate early gene Arc (Arc/Arg3.1) in hippocampus and anterior cingulate cortex (ACC) following trace FC (tFC). The results indicate that GLS1 HZ mice succeed in processing contextual information when the salient CS is absent or less predictive. In addition, in the hippocampus-dependent tFC paradigm GLS1 HZ mice display enhanced CS learning. Furthermore, while baseline arc activation was reduced in GLS1 HZ mice in the hippocampus, in line with previous fMRI findings, it was enhanced in the hippocampus and anterior cingulate cortex following tFC. These findings suggest that GLS1 HZ mice have a pro-cognitive profile in the tFC paradigm, and this phenotype involves activation of both hippocampus and ACC. Taken together with previous work on the GLS1 HZ mouse, this study sheds light on the importance of glutamate transmission to memory processes that require the allocation of attentional resources, and extends our understanding of the underpinnings of attention deficits in SZ.

Dopamine modulation of GABAergic function enables network stability and input selectivity for sustaining working memory in a computational model of the prefrontal cortex

Dopamine modulation of GABAergic transmission in the prefrontal cortex (PFC) is thought to be critical for sustaining cognitive processes such as working memory and decision-making. Here, we developed a neurocomputational model of the PFC that includes physiological features of the facilitatory action of dopamine on fast-spiking interneurons to assess how a GABAergic dysregulation impacts on the prefrontal network stability and working memory. We found that a particular non-linear relationship between dopamine transmission and GABA function is required to enable input selectivity in the PFC for the formation and retention of working memory. Either degradation of the dopamine signal or the GABAergic function is sufficient to elicit hyperexcitability in pyramidal neurons and working memory impairments. The simulations also revealed an inverted U-shape relationship between working memory and dopamine, a function that is maintained even at high levels of GABA degradation. In fact, the working memory deficits resulting from reduced GABAergic transmission can be rescued by increasing dopamine tone and vice versa. We also examined the role of this dopamine-GABA interaction for the termination of working memory and found that the extent of GABAergic excitation needed to reset the PFC network begins to occur when the activity of fast-spiking interneurons surpasses 40 Hz. Together, these results indicate that the capability of the PFC to sustain working memory and network stability depends on a robust interplay of compensatory mechanisms between dopamine tone and the activity of local GABAergic interneurons.

Modeling fragile X syndrome in the Fmr1 knockout mouse

Fragile X Syndrome (FXS) is a commonly inherited form of intellectual disability and one of the leading genetic causes for autism spectrum disorder. Clinical symptoms of FXS can include impaired cognition, anxiety, hyperactivity, social phobia, and repetitive behaviors. FXS is caused by a CGG repeat mutation which expands a region on the X chromosome containing the FMR1 gene. In FXS, a full mutation (> 200 repeats) leads to hypermethylation of FMR1, an epigenetic mechanism that effectively silences FMR1 gene expression and reduces levels of the FMR1 gene product, fragile X mental retardation protein (FMRP). FMRP is an RNA-binding protein that is important for the regulation of protein expression. In an effort to further understand how loss of FMR1 and FMRP contribute to FXS symptomology, several FXS animal models have been created. The most well characterized rodent model is the Fmr1 knockout (KO) mouse, which lacks FMRP protein due to a disruption in its Fmr1 gene. Here, we review the behavioral phenotyping of the Fmr1 KO mouse to date, and discuss the clinical relevance of this mouse model to the human FXS condition. While much remains to be learned about FXS, the Fmr1 KO mouse is a valuable tool for understanding the repercussions of functional loss of FMRP and assessing the efficacy of pharmacological compounds in ameliorating the molecular and behavioral phenotypes relevant to FXS.

Early adolescent MK-801 exposure impairs the maturation of ventral hippocampal control of basolateral amygdala drive in the adult prefrontal cortex

The adolescent susceptibility to the onset of psychiatric disorders is only beginning to be understood when factoring in the development of the prefrontal cortex (PFC). The functional maturation of the PFC is dependent upon proper integration of glutamatergic inputs from the ventral hippocampus (vHipp) and the basolateral amygdala (BLA). Here we assessed how transient NMDAR blockade during adolescence alters the functional interaction of vHipp-BLA inputs in regulating PFC plasticity. Local field potential recordings were used to determine changes in long-term depression (LTD) and long-term potentiation (LTP) of PFC responses resulting from vHipp and BLA high-frequency stimulation in adult rats that received repeated injections of saline or the NMDAR antagonist MK-801 from postnatal day 35 (P35) to P40. We found that early adolescent MK-801 exposure elicited an age- and input-specific dysregulation of vHipp-PFC plasticity, characterized by a shift from LTD to LTP without altering the BLA-induced LTP. Data also showed that the vHipp normally resets the LTP state of BLA transmission; however, this inhibitory regulation is absent following early adolescent MK-801 treatment. This deficit was reminiscent of PFC responses seen in drug-naive juveniles. Notably, local prefrontal upregulation of GABAAα1 function completely restored vHipp functionality and its regulation of BLA plasticity in MK-801-treated rats. Thus, NMDAR signaling is critical for the periadolescent acquisition of a GABA-dependent hippocampal control of PFC plasticity, which enables the inhibitory control of the prefrontal output by the vHipp. A dysregulation of this pathway can alter PFC processing of other converging afferents such as those from the BLA.

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.

Trace fear conditioning in mice

In this experiment we present a technique to measure learning and memory. In the trace fear conditioning protocol presented here there are five pairings between a neutral stimulus and an unconditioned stimulus. There is a 20 sec trace period that separates each conditioning trial. On the following day freezing is measured during presentation of the conditioned stimulus (CS) and trace period. On the third day there is an 8 min test to measure contextual memory. The representative results are from mice that were presented with the aversive unconditioned stimulus (shock) compared to mice that received the tone presentations without the unconditioned stimulus. Trace fear conditioning has been successfully used to detect subtle learning and memory deficits and enhancements in mice that are not found with other fear conditioning methods. This type of fear conditioning is believed to be dependent upon connections between the medial prefrontal cortex and the hippocampus. One current controversy is whether this method is believed to be amygdala-independent. Therefore, other fear conditioning testing is needed to examine amygdala-dependent learning and memory effects, such as through the delay fear conditioning.

The role of the medial prefrontal cortex in trace fear extinction

The extinction of delay fear conditioning relies on a neural circuit that has received much attention and is relatively well defined. Whether this established circuit also supports the extinction of more complex associations, however, is unclear. Trace fear conditioning is a better model of complex relational learning, yet the circuit that supports extinction of this memory has received very little attention. Recent research has indicated that trace fear extinction requires a different neural circuit than delay extinction; trace extinction requires the participation of the retrosplenial cortex, but not the amygdala, as noted in a previous study. Here, we tested the roles of the prelimbic and infralimbic regions of the medial prefrontal cortex in trace and delay fear extinction by blocking NMDA receptors during extinction learning. We found that the prelimbic cortex is necessary for trace, but not for delay fear extinction, whereas the infralimbic cortex is involved in both types of extinction. These results are consistent with the idea that trace fear associations require plasticity in multiple cortical areas for successful extinction. Further, the infralimbic cortex appears to play a role in extinction regardless of whether the animal was initially trained in trace or delay conditioning. Together, our results provide new information about how the neural circuits supporting trace and delay fear extinction differ.

The prelimbic cortex and subthalamic nucleus contribute to cue-guided behavioral switching

Frontal cortex-basal ganglia circuitry supports behavioral switching when a change in outcome information is used to adapt response patterns. Less is known about whether specific frontal cortex-basal ganglia circuitry supports behavioral switching when cues signal that a change in response patterns should occur. The present experiments investigated whether the prelimbic cortex and subthalamic nucleus in male Long-Evans rats supports cue-guided switching in a conditional discrimination test. Rats learned in a cross-maze that a start arm cue (black or white) signaled which of two maze arms to enter for a food reward. The cue was switched every 3-6 trials. Baclofen and muscimol infused into the prelimbic cortex significantly impaired performance by increasing switch trial errors, as well as trials immediately following a switch trial (perseveration) and after initially making a correct switch (maintenance error). NMDA receptor blockade in the subthalamic nucleus significantly impaired performance by increasing switch errors and perseveration. Contralateral disconnection of these areas significantly reduced conditional discrimination performance by increasing switch and perseverative errors. These findings suggest that the prelimbic area and subthalamic nucleus support the use of cue information to facilitate an initial switch away from a previously relevant response pattern.

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.

Unilateral lateral entorhinal inactivation impairs memory expression in trace eyeblink conditioning

Memory in trace eyeblink conditioning is mediated by an inter-connected network that involves the hippocampus (HPC), several neocortical regions, and the cerebellum. This network reorganizes after learning as the center of the network shifts from the HPC to the medial prefrontal cortex (mPFC). Despite the network reorganization, the lateral entorhinal cortex (LEC) plays a stable role in expressing recently acquired HPC-dependent memory as well as remotely acquired mPFC-dependent memory. Entorhinal involvement in recent memory expression may be attributed to its previously proposed interactions with the HPC. In contrast, it remains unknown how the LEC participates in memory expression after the network disengages from the HPC. The present study tested the possibility that the LEC and mPFC functionally interact during remote memory expression by examining the impact of pharmacological inactivation of the LEC in one hemisphere and the mPFC in the contralateral hemisphere on memory expression in rats. Memory expression one day and one month after learning was significantly impaired after LEC-mPFC inactivation; however, the degree of impairment was comparable to that after unilateral LEC inactivation. Unilateral mPFC inactivation had no effect on recent or remote memory expression. These results suggest that the integrity of the LEC in both hemispheres is necessary for memory expression. Functional interactions between the LEC and mPFC should therefore be tested with an alternative design.

Mapping and deciphering neural codes of NMDA receptor-dependent fear memory engrams in the hippocampus

Mapping and decoding brain activity patterns underlying learning and memory represents both great interest and immense challenge. At present, very little is known regarding many of the very basic questions regarding the neural codes of memory: are fear memories retrieved during the freezing state or non-freezing state of the animals? How do individual memory traces give arise to a holistic, real-time associative memory engram? How are memory codes regulated by synaptic plasticity? Here, by applying high-density electrode arrays and dimensionality-reduction decoding algorithms, we investigate hippocampal CA1 activity patterns of trace fear conditioning memory code in inducible NMDA receptor knockout mice and their control littermates. Our analyses showed that the conditioned tone (CS) and unconditioned foot-shock (US) can evoke hippocampal ensemble responses in control and mutant mice. Yet, temporal formats and contents of CA1 fear memory engrams differ significantly between the genotypes. The mutant mice with disabled NMDA receptor plasticity failed to generate CS-to-US or US-to-CS associative memory traces. Moreover, the mutant CA1 region lacked memory traces for “what at when” information that predicts the timing relationship between the conditioned tone and the foot shock. The degraded associative fear memory engram is further manifested in its lack of intertwined and alternating temporal association between CS and US memory traces that are characteristic to the holistic memory recall in the wild-type animals. Therefore, our study has decoded real-time memory contents, timing relationship between CS and US, and temporal organizing patterns of fear memory engrams and demonstrated how hippocampal memory codes are regulated by NMDA receptor synaptic plasticity.

Reversal learning and associative memory impairments in a BACHD rat model for Huntington disease

Chorea and psychiatric symptoms are hallmarks of Huntington disease (HD), a neurodegenerative disorder, genetically characterized by the presence of expanded CAG repeats (>35) in the Huntingtin (HTT) gene. HD patients present psychiatric symptoms prior to the onset of motor symptoms and we recently found a similar emergence of non motor and motor deficits in BACHD rats carrying the human full length mutated HTT (97 CAG-CAA repeats). We evaluated cognitive performance in reversal learning and associative memory tests in different age cohorts of BACHD rats. Male wild type (WT) and transgenic (TG) rats between 2 and 12 months of age were tested. Learning and strategy shifting were assessed in a cross-maze test. Associative memory was evaluated in different fear conditioning paradigms (context, delay and trace). The possible confound of a fear conditioning phenotype by altered sensitivity to a 'painful' stimulus was assessed in a flinch-jump test. In the cross maze, 6 months old TG rats showed a mild impairment in reversal learning. In the fear conditioning tasks, 4, 6 and 12 months old TG rats showed a marked reduction in contextual fear conditioning. In addition, TG rats showed impaired delay conditioning (9 months) and trace fear conditioning (3 months). This phenotype was unlikely to be affected by a change in 'pain' sensitivity as WT and TG rats showed no difference in their threshold response in the flinch-jump test. Our results suggest that BACHD rats have a profound associative memory deficit and, possibly, a deficit in reversal learning as assessed in a cross maze task. The time course for the emergence of these symptoms (i.e., before the occurrence of motor symptoms) in this rat model for HD appears similar to the time course in patients. These data suggest that BACHD rats may be a useful model for preclinical drug discovery.

Memory formation for trace fear conditioning requires ubiquitin-proteasome mediated protein degradation in the prefrontal cortex

The cellular mechanisms supporting plasticity during memory consolidation have been a subject of considerable interest. De novo protein and mRNA synthesis in several brain areas are critical, and more recently protein degradation, mediated by the ubiquitin-proteasome system (UPS), has been shown to be important. Previous work clearly establishes a relationship between protein synthesis and protein degradation in the amygdala, but it is unclear whether cortical mechanisms of memory consolidation are similar to those in the amygdala. Recent work demonstrating a critical role for prefrontal cortex (PFC) in the acquisition and consolidation of fear memory allows us to address this question. Here we use a PFC-dependent fear conditioning protocol to determine whether UPS mediated protein degradation is necessary for memory consolidation in PFC. Groups of rats were trained with auditory delay or trace fear conditioning and sacrificed 60 min after training. PFC tissue was then analyzed to quantify the amount of polyubiquibated protein. Other animals were trained with similar procedures but were infused with either a proteasome inhibitor (clasto-lactacystin β-lactone) or a translation inhibitor (anisomycin) in the PFC immediately after training. Our results show increased UPS-mediated protein degradation in the PFC following trace but not delay fear conditioning. Additionally, post-training proteasome or translation inhibition significantly impaired trace but not delay fear memory when tested the next day. Our results further support the idea that the PFC is critical for trace but not delay fear conditioning and highlight the role of UPS-mediated degradation as critical for synaptic plasticity.

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.

Cellular, molecular, and genetic substrates underlying the impact of nicotine on learning

Addiction is a chronic disorder marked by long-lasting maladaptive changes in behavior and in reward system function. However, the factors that contribute to the behavioral and biological changes that occur with addiction are complex and go beyond reward. Addiction involves changes in cognitive control and the development of disruptive drug-stimuli associations that can drive behavior. A reason for the strong influence drugs of abuse can exert on cognition may be the striking overlap between the neurobiological substrates of addiction and of learning and memory, especially areas involved in declarative memory. Declarative memories are critically involved in the formation of autobiographical memories, and the ability of drugs of abuse to alter these memories could be particularly detrimental. A key structure in this memory system is the hippocampus, which is critically involved in binding multimodal stimuli together to form complex long-term memories. While all drugs of abuse can alter hippocampal function, this review focuses on nicotine. Addiction to tobacco products is insidious, with the majority of smokers wanting to quit; yet the majority of those that attempt to quit fail. Nicotine addiction is associated with the presence of drug-context and drug-cue associations that trigger drug seeking behavior and altered cognition during periods of abstinence, which contributes to relapse. This suggests that understanding the effects of nicotine on learning and memory will advance understanding and potentially facilitate treating nicotine addiction. The following sections examine: (1) how the effects of nicotine on hippocampus-dependent learning change as nicotine administration transitions from acute to chronic and then to withdrawal from chronic treatment and the potential impact of these changes on addiction, (2) how nicotine usurps the cellular mechanisms of synaptic plasticity, (3) the physiological changes in the hippocampus that may contribute to nicotine withdrawal deficits in learning, and (4) the role of genetics and developmental stage (i.e., adolescence) in these effects.

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.

Hippocampus and medial prefrontal cortex contributions to trace and contextual fear memory expression over time

Previous work has shown that damage to the dorsal hippocampus (DH) occurring at recent, but not remote, timepoints following acquisition produces a deficit in trace conditioned fear memory expression. The opposite pattern has been observed with lesions to the medial prefrontal cortex (mPFC). The present studies address: (1) whether these lesion effects are observable within 30 d of training; (2) whether lesions of the ventral hippocampus (VH) produce temporally graded retrograde amnesia similar to DH lesions; and (3) whether the lesion-to-test interval critically contributes to these lesion deficits. In Experiment 1, excitotoxic lesions of the DH, VH, or mPFC were made at 1 or 30 d following trace fear conditioning. DH and VH lesioned animals showed a deficit in freezing to the tone at the recent, but not remote, timepoint. Medial PFC lesioned animals showed the opposite pattern. In Experiment 2, lesions to DH, VH, or mPFC were made 1 d following training, while testing occurred 30 d later. There were no deficits in freezing to the tone in any lesion condition compared to controls. These results suggest that systems consolidation of trace fear memory occurs within 30 d of acquisition, but does not depend on hippocampus-mPFC interactions during this period.

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.

Amygdala's involvement in facilitating associative learning-induced plasticity: a promiscuous role for the amygdala in memory acquisition

It is widely accepted that the amygdala plays a critical role in acquisition and consolidation of fear-related memories. Some of the more widely employed behavioral paradigms that have assisted in solidifying the amygdala's role in fear-related memories are associative learning paradigms. With most associative learning tasks, a neutral conditioned stimulus (CS) is paired with a salient unconditioned stimulus (US) that elicits an unconditioned response (UR). After multiple CS-US pairings, the subject learns that the CS predicts the onset or delivery of the US, and thus elicits a learned conditioned response (CR). Most fear-related associative paradigms have suggested that an aspect of the fear association is stored in the amygdala; however, some fear-motivated associative paradigms suggest that the amygdala is not a site of storage, but rather facilitates consolidation in other brain regions. Based upon various learning theories, one of the most likely sites for storage of long-term memories is the neocortex. In support of these theories, findings from our laboratory, and others, have demonstrated that trace-conditioning, an associative paradigm where there is a separation in time between the CS and US, induces learning-specific neocortical plasticity. The following review will discuss the amygdala's involvement, either as a site of storage or facilitating storage in other brain regions such as the neocortex, in fear- and non-fear-motivated associative paradigms. In this review, we will discuss recent findings suggesting a broader role for the amygdala in increasing the saliency of behaviorally relevant information, thus facilitating acquisition for all forms of memory, both fear- and non-fear-related. This proposed promiscuous role of the amygdala in facilitating acquisition for all memories further suggests a potential role of the amygdala in general learning disabilities.

Muscarinic receptors in amygdala control trace fear conditioning

Intelligent behavior requires transient memory, which entails the ability to retain information over short time periods. A newly-emerging hypothesis posits that endogenous persistent firing (EPF) is the neurophysiological foundation for aspects or types of transient memory. EPF is enabled by the activation of muscarinic acetylcholine receptors (mAChRs) and is triggered by suprathreshold stimulation. EPF occurs in several brain regions, including the lateral amygdala (LA). The present study examined the role of amygdalar mAChRs in trace fear conditioning, a paradigm that requires transient memory. If mAChR-dependent EPF selectively supports transient memory, then blocking amygdalar mAChRs should impair trace conditioning, while sparing delay and context conditioning, which presumably do not rely upon transient memory. To test the EPF hypothesis, LA was bilaterally infused, prior to trace or delay conditioning, with either a mAChR antagonist (scopolamine) or saline. Computerized video analysis quantified the amount of freezing elicited by the cue and by the training context. Scopolamine infusion profoundly reduced freezing in the trace conditioning group but had no significant effect on delay or context conditioning. This pattern of results was uniquely anticipated by the EPF hypothesis. The present findings are discussed in terms of a systems-level theory of how EPF in LA and several other brain regions might help support trace fear conditioning.