Factors governing single-trial contextual fear conditioning in the weanling rat

Retrieval and savings of contextual fear memories across an extended retention interval in juvenile and adult male and female rats

Adult rodents exhibit an exceptional ability to retrieve context fear memories across lengthy retention intervals. In contrast, these memories established in younger rodents are susceptible to significant forgetting. The present study aimed to examine the persistence of contextual fear memories established in juvenile and adult Long-Evans male and female rats. Testing 1-day after conditioning, adult males exhibited evidence for greater conditioning than juvenile males, while in females, conditioning did not differ between juvenile and adult rats. In adults, males displayed greater conditioning than females, while in juveniles, males and females reached similar conditioning levels. At the 60-day retention interval, adult sex differences were maintained; however, juvenile rats failed to retrieve this remote contextual fear memory. Next, we examined whether a savings test procedure could recover these remotely established juvenile memories. Following a 60-day retention test, the now adult rats were presented with an additional context-shock pairing to assess the level of savings. While this procedure produced greater conditioning in males than females, the relative savings of this early life memory were similar in males and females. The results of these experiments indicate that adult sex differences in contextual fear memory are maintained across an extended retention interval, while in juveniles, there were no significant sex differences. A novel finding in the present study was that both male and female rats failed to retrieve an initial juvenile memory following an extended retention interval. However, these memories were recovered with a single reminder of the original juvenile experience. (PsycInfo Database Record (c) 2023 APA, all rights reserved).

Social buffering reduces fear expression in Wistar rats when tested in pairs, but not when retested alone

Social buffering is a phenomenon in which the stress response of an individual can be reduced by the presence of another individual. However, little is known about the effect of social buffering on aversive after memory extinction, especially when animals are tested alone afterwards. The aim of this study was to verify the social buffering effect in rats during the extinction session of the contextual fear conditioning model and the fear response when animals are tested alone in the following day. Animals were divided into subjects and associates, with the subjects undergoing the fear conditioning protocol and the associates paired with the subjects during the fear extinction session. Across five different experiments, we tested moderate and high intensity contextual fear conditioning protocols, as well four variations of pairs: (i) two conditioned subjects, (ii) a conditioned subject and a non-conditioned associate, (iii) a conditioned subject and an associate who observed the conditioning of the partner and (iv) two conditioned subjects, with one treated with diazepam. The social buffering effect was found efficient to reduce the fear memory expression during the fear extinction session. In the moderate intensity protocol, the reduction in freezing time occurred only in subjects accompanied by non-conditioned associates and observer associates. In the high intensity protocol, the social buffering effect occurred in subjects accompanied by either conditioned or non-conditioned associates, although the effect was more evident in the presence of non-conditioned subjects. Treatment of the conditioned associates with diazepam did not improve the social buffering effect. Moreover, social buffering effects were not correlated with self-grooming or prosocial behaviors, which indicates that the presence of another animal might decrease freezing by promotion of exploratory activity. Finally, the social buffering effect was not observed in the extinction test, either because the extinction was too effective in the moderate intensity protocol or because the extinction was equally ineffective in the high intensity protocol. Our results suggest that social buffering does not improve fear extinction consolidation.

The effects of early-life immune activation on microglia-mediated neuronal remodeling and the associated ontogeny of hippocampal-dependent learning in juvenile rats

Many neurodevelopmental disorders and associated learning deficits have been linked to early-life immune activation or ongoing immune dysregulation (Laskaris et al., 2016; O'Connor et al., 2014; Frick et al., 2013). Neuroscientists have begun to understand how the maturation of neural circuits allows for the emergence of cognitive and learning behaviors; yet we know very little about how these developing neural circuits are perturbed by certain events, including risk-factors such as early-life immune activation and immune dysregulation. To answer these questions, we examined the impact of early-life immune activation on the emergence of hippocampal-dependent learning in juvenile male and female rats using a well-characterized hippocampal-dependent learning task and we investigated the corresponding, dynamic multicellular interactions in the hippocampus that may contribute to these learning deficits. We found that even low levels of immune activation can result in hippocampal-depedent learning deficits days later, but only when this activation occurs during a sensitive period of development. The initial immune response and associated cytokine production in the hippocampus resolved within 24 h, several days prior to the observed learning deficit, but notably the initial immune response was followed by altered microglial-neuronal communication and synapse remodeling that changed the structure of hippocampal neurons during this period of juvenile brain development. We conclude that immune activation or dysregulation during a sensitive period of hippocampal development can precipitate the emergence of learning deficits via a multi-cellular process that may be initiated by, but not the direct result of the initial cytokine response. SIGNIFICANCE STATEMENT: Many neurodevelopmental disorders have been linked to early-life immune activation or immune dysregulation; however, very little is known about how dynamic changes in neuroimmune cells mediate the transition from normal brain function to the early stages of cognitive disorders, or how changes in immune signaling are subsequently integrated into developing neuronal networks. The current experiments examined the consequences of immune activation on the cellular and molecular changes that accompany the emergence of learning deficits during a sensitive period of hippocampal development. These findings have the potential to significantly advance our understanding of how early-life immune activation or dysregulation can result in the emergence of cognitive and learning deficits that are the largest source of years lived with disability in humans.

Prenatal dietary choline supplementation modulates long-term memory development in rat offspring

Background: Previous studies on preclinical models have shown that giving supplemental choline during the embryonic period improves performance on memory tasks during adulthood. However, the effects of an early intervention on the development of cognitive functions in the immature brain have not been widely studied. In addition, it has been well established that short-term memory in rats emerges at an earlier stage than long-term memory.Objective: The aim of this work was to examine the effect of prenatal dietary choline supplementation on long-term memory development in rats.Methods: In order to assess long-term memory, we used an object-recognition task, which evaluates the ability to recall a previously presented stimulus. Pregnant rats were fed with the diets AIN 76-A standard (1.1 g choline/Kg food) or supplemented (5 g choline/Kg food) between embryonic days (E) 12 and E18. On the first post-natal day (PN 0), male offspring of the rats fed with the supplemented and standard diet were cross-fostered to rat dams fed a standard diet during pregnancy and tested at the age of PN21-22 or PN29-31 applying 24-hour retention tests.Results: The supplemented animals spent less time exploring the familiar object after a 24-hour retention interval, an effect that was observed in both the group tested at PN21-22 days of age and that tested at PN29-31 days. The non-supplemented rats only showed this effect in the group tested at PN29-31 days.Conclusions: These results suggest that prenatal supplementation with choline accelerates the development of long-term memory in rats.

Mechanisms of context conditioning in the developing rat

The article reviews our studies of contextual fear conditioning (CFC) in rats during a period of development---Postnatal Day (PND) 17-33---that represents the late-infant, juvenile, and early-adolescent stages. These studies seek to acquire 'systems level' knowledge of brain and memory development and apply it to a rodent model of Fetal Alcohol Spectrum Disorder (FASD). This rodent model focuses on alcohol exposure from PND4-9, a period of brain development equivalent to the human third trimester, when neocortex, hippocampus, and cerebellum are especially vulnerable to adverse effects of alcohol. Our research emphasizes a variant of CFC, termed the Context Preexposure Facilitation Effect (CPFE, Fanselow, 1990), in which context representations incidentally learned on one occasion are retrieved and associated with immediate shock on a subsequent occasion. These representations can be encoded at the earliest developmental stage but seem not to be retained or retrieved until the juvenile period. This is associated with developmental differences in context-elicited expression, in prefrontal cortex, hippocampus, and amygdala, of immediate early genes (IEGs) that are implicated in long-term memory. Loss-of-function studies establish a functional role for these regions as soon as the CPFE emerges during ontogeny. In our rodent model of FASD, the CPFE is much more sensitive to alcohol dose than other commonly used cognitive tasks. This impairment can be reversed by acute administration during behavioral testing of drugs that enhance cholinergic function. This effect is associated with normalized IEG expression in prefrontal cortex during incidental context learning. In summary, our findings suggest that long-term memory of incidentally-learned context representations depends on prefrontal-hippocampal circuitry that is important both for the normative development of context conditioning and for its disruption by developmental alcohol exposure.

Sensorimotor developmental factors influencing the performance of laboratory rodents on learning and memory

Behavioral studies in animal models have advanced our knowledge of brain function and the neural mechanisms of human diseases. Commonly used laboratory rodents, such as mice and rats, provide a useful tool for studying the behaviors and mechanisms associated with learning and memory processes which are cooperatively regulated by multiple underlying factors, including sensory and motor performance and emotional/defense innate components. Each of these factors shows unique ontogeny and governs the sustainment of behavioral performance in learning tasks, and thus, understanding the integrative processes of behavioral development are crucial in the accurate interpretation of the functional meaning of learning and memory behaviors expressed in commonly employed behavioral test paradigms. In this review, we will summarize the major findings in the developmental processes of rodent behavior on the basis of the emergence of fundamental components for sustaining learning and memory behaviors. Briefly, most sensory modalities (except for vision) and motor abilities are functional at the juvenile stage, in which several defensive components, including active and passive defensive strategies and risk assessment behavior, emerge. Sex differences are detectable from the juvenile stage through adulthood and are considerable factors that influence behavioral tests. The test paradigms addressed in this review include associative learning (with an emphasis on fear conditioning), spatial learning, and recognition. This basic background information will aid in accurately performing behavioral studies in laboratory rodents and will therefore contribute to reducing inappropriate interpretations of behavioral data and further advance research on learning and memory in rodent models.

NMDA receptors and the ontogeny of post-shock and retention freezing during contextual fear conditioning

The ontogeny and NMDA-receptor (NMDAR) mechanisms of context conditioning were examined during standard contextual fear conditioning (sCFC) - involving context and context-shock learning in the same trial - as a comparison with our previous reports on the Context Preexposure Facilitation Effect (CPFE), which separates these two types of learning by 24 hr. In Experiment 1, systemic administration of the NMDAR antagonist, MK-801, prior to conditioning disrupted retention but not post-shock freezing during sCFC in PD31 rats. Experiment 2 replicated and extended this effect to PD17 versus PD31 rats. Consistent with Experiment 1, pre-training MK-801 spared post-shock freezing but impaired retention freezing in PD31 rats. In contrast, pre-training MK-801 disrupted post-shock freezing in PD17 rats, which showed no retention freezing regardless of drug. These results reveal developmental differences in the role of NMDAR activity in the acquisition versus retention of a context-shock association during sCFC in pre-weanling and adolescent rats.

Age and sex differences in the innate defensive behaviors of C57BL/6 mice exhibited in a fear conditioning paradigm and upon exposure to a predatory odor

Defensive responses of mice include an array of specific behaviors that involve adaptive components based on the assessment of the threat of confrontation. The freezing response is represented by a motionless moment without any specific posture or behavioral sequence, and it is widely used in the fear conditioning paradigms and other relevant defensive situations. However, freezing measurements include fragmental components of several defensive behaviors that are exhibited during the session, such as behavioral inhibition, crouching, and a moment of risk assessment behavior. From an ethological view, behavioral analyses of C57BL/6 mice of both sexes and three different ages (postnatal days (P) 25, 35, and 65) revealed a rich variety of defensive behaviors during a fear conditioning session and in response to predatory odor exposure as a nonconditioned behavior. P-25 and 35 mice exhibited more behavioral inhibition than P-65 adult mice, and P-65 mice exhibited a crouching posture more often than younger mice. This age difference was more pronounced in males. The stretch-attend posture (SAP) increased with age, except in P-25 males, which exhibited robust SAP in response to a conditioned cue; this response indicates that P-25 males are defensive in a situation-nonmatching manner. Situation-dependent defense strategies were revealed in P-35 and 65 mice: Fear conditioning paradigm induced more robust defensive responses than predatory odor exposure, to which mice primarily exhibited SAP. A sex-based difference was revealed in adult mice. Males tended to show more passive defensive responses, such as crouching and withdrawal, and females exhibited more active responses, such as SAP. These age- and sex-based differences may stem from the ethological demands and illustrate adolescent ontogenetic processes of defense behavior.

Impairment of the context preexposure facilitation effect in juvenile rats by neonatal alcohol exposure is associated with decreased Egr-1 mRNA expression in the prefrontal cortex

The context preexposure facilitation effect (CPFE) is a variant of contextual fear conditioning in which learning about the context (preexposure) and associating the context with a shock (training) occur on separate occasions. The CPFE is sensitive to a range of neonatal alcohol doses (Murawski & Stanton, 2011). The current study examined the impact of neonatal alcohol on Egr-1 mRNA expression in the infralimbic (IL) and prelimbic (PL) subregions of the mPFC, the CA1 of dorsal hippocampus (dHPC), and the lateral nucleus of the amygdala (LA), following the preexposure and training phases of the CPFE. Rat pups were exposed to a 5.25 g/kg/day single binge-like dose of alcohol (Group EtOH) or were sham intubated (SI; Group SI) over postnatal days (PD) 7-9. In behaviorally tested rats, alcohol administration disrupted freezing. Following context preexposure, Egr-1 mRNA was elevated in both EtOH and SI groups compared with baseline control animals in all regions analyzed. Following both preexposure and training, Group EtOH displayed a significant decrease in mPFC Egr-1 mRNA expression compared with Group SI. However, this decrease was greatest after training. Training day decreases in Egr-1 expression were not found in LA or CA1 in Group EtOH compared with Group SI. A second experiment confirmed that the EtOH-induced training-day deficits in mPFC Egr-1 mRNA expression were specific to groups which learned contextual fear (vs. nonassociative controls). Thus, memory processes that engage the mPFC during the context-shock association may be most susceptible to the teratogenic effects of neonatal alcohol. (PsycINFO Database Record (c) 2018 APA, all rights reserved).

Differential expression of the immediate early genes c-Fos, Arc, Egr-1, and Npas4 during long-term memory formation in the context preexposure facilitation effect (CPFE)

The context preexposure facilitation effect (CPFE) is a contextual fear conditioning paradigm in which learning about the context, acquiring the context-shock association, and retrieving/expressing contextual fear are temporally dissociated into three distinct phases (context preexposure, immediate-shock training, and retention). The current study examined changes in the expression of plasticity-associated immediate early genes (IEGs) during context and contextual fear memory formation on the preexposure and training days of the CPFE, respectively. Using adolescent Long-Evans rats, preexposure and training day expression of the IEGs c-Fos, Arc, Egr-1, and Npas4 in the medial prefrontal cortex (mPFC), dorsal hippocampus (dHPC), and basolateral amygdala (BLA) was analyzed using qPCR as an extension of previous studies from our lab examining Egr-1 via in situ hybridization (Asok, Schreiber, Jablonski, Rosen, & Stanton, 2013; Schreiber, Asok, Jablonski, Rosen, & Stanton, 2014). In Expt. 1, context preexposure induced expression of c-Fos, Arc, Egr-1 and Npas4 significantly above that of home-cage (HC) controls in all three regions. In Expt. 2, immediate-shock was followed by a post-shock freezing test, resulting in increased mPFC c-Fos expression in a group preexposed to the training context but not a control group preexposed to an alternate context, indicating expression related to associative learning. This was not seen with other IEGs in mPFC or with any IEG in dHPC or BLA. Finally, when the post-shock freezing test was omitted in Expt. 3, training-related increases were observed in prefrontal c-Fos, Arc, Egr-1, and Npas4, hippocampal c-Fos, and amygdalar Egr-1 expression. These results indicate that context exposure in a post-shock freezing test re-engages IEG expression that may obscure associatively-induced expression during contextual fear conditioning. Additionally, these studies suggest a key role for long-term synaptic plasticity in the mPFC in supporting the CPFE.

Neonatal infection produces significant changes in immune function with no associated learning deficits in juvenile rats

The current experiments examined the impact of early-life immune activation and a subsequent mild immune challenge with lipopolysaccharide (LPS; 25µg/kg) on hippocampal-dependent learning, proinflammatory cytokine expression in the brain, and peripheral immune function in juvenile male and female rats at P24, an age when hippocampal-dependent learning and memory first emerges. Our results indicate that neonatal infection did not produce learning deficits in the hippocampal-dependent context pre-exposure facilitation effect paradigm in juvenile males and females, contrary to what has been observed in adults. Neonatal infection produced an increase in baseline IL-1β expression in the hippocampus (HP) and medial prefrontal cortex (mPFC) of juvenile rats. Furthermore, neonatally infected rats showed exaggerated IL-1β expression in the HP following LPS treatment as juveniles; and juvenile females, but not males, showed exaggerated IL-1β expression in the mPFC following LPS treatment. Neonatal infection attenuated the production of IL-6 expression following LPS treatment in both the brain and the spleen, and neonatal infection decreased the numbers of circulating white blood cells in juvenile males and females, an effect that was further exacerbated by subsequent LPS treatment. Together, our data indicate that the consequences of neonatal infection are detectable even early in juvenile development, though we found no concomitant hippocampal-dependent learning deficits at this young age. These findings underscore the need to consider age and associated on-going neurodevelopmental processes as important factors contributing to the emergence of cognitive and behavioral disorders linked to early-life immune activation. © 2017 Wiley Periodicals, Inc. Develop Neurobiol 77: 1221-1236, 2017.

Differential involvement of the medial prefrontal cortex across variants of contextual fear conditioning

The context preexposure facilitation effect (CPFE) is a contextual fear conditioning paradigm in which learning about the context, acquiring the context-shock association, and retrieving/expressing contextual fear are temporally dissociated into three distinct phases. In contrast, learning about the context and the context-shock association happens concurrently in standard contextual fear conditioning (sCFC). By infusing the GABA_A receptor agonist muscimol into medial prefrontal cortex (mPFC) in adolescent Long-Evans rats, the current set of experiments examined the functional role of the mPFC in each phase of the CPFE and sCFC. In the CPFE, the mPFC is necessary for the following: acquisition and/or consolidation of context memory (Experiment 1), reconsolidation of a context memory to include shock (Experiment 2), and expression of contextual fear memory during a retention test (Experiment 3). In contrast to the CPFE, inactivation of the mPFC prior to conditioning in sCFC has no effect on acquisition, consolidation, or retention of a contextual fear memory (Experiment 4). Interestingly, the mPFC is not required for acquiring a context-shock association (measured by post-shock freezing) in the CPFE or sCFC (Experiment 2b and 4). Taken together, these results indicate that the mPFC is differentially recruited across stages of learning and variants of contextual fear conditioning (CPFE versus sCFC). More specifically, separating out learning about the context and the context-shock association necessitates activation of the medial prefrontal cortex during early learning and/or consolidation.

Understanding the contributions of visual stimuli to contextual fear conditioning: A proof-of-concept study using LCD screens

The precise contribution of visual information to contextual fear learning and discrimination has remained elusive. To better understand this contribution, we coupled the context pre-exposure facilitation effect (CPFE) fear conditioning paradigm with presentations of distinct visual scenes displayed on 4 LCD screens surrounding a conditioning chamber. Adult male Long-Evans rats received non-reinforced context pre-exposure on Day 1, an immediate 1.5mA foot shock on Day 2, and a non-reinforced context test on Day 3. Rats were pre-exposed to either digital Context (dCtx) A, dCtx B, a distinct Ctx C, or no context on Day 1. Digital context A and B were identical except for the visual image displayed on the LCD screens. Immediate shock and retention testing occurred in dCtx A. Rats pre-exposed dCtx A showed the CPFE with significantly higher levels of freezing compared to controls. Rats pre-exposed to Context B failed to show the CPFE, with freezing that did not highly differ from controls. These results suggest that visual information contributes to contextual fear learning and that visual components of the context can be manipulated via LCD screens. Our approach offers a simple modification to contextual fear conditioning paradigms whereby the visual features of a context can be manipulated to better understand the factors that contribute to contextual fear discrimination and generalization.

Cholinergic mechanisms of the context preexposure facilitation effect in adolescent rats

The context preexposure facilitation effect (CPFE) is a variant of contextual fear conditioning in which context learning, context-shock association, and expression of context conditioning occur in 3 separate phases-preexposure, training, and testing. During the preexposure phase, the CPFE is disrupted by hippocampal NMDA receptor blockade in juvenile rats (Schiffino et al., 2011), and a similar deficit is seen with a subcutaneous injection of the muscarinic receptor antagonist, scopolamine, in adult mice (Brown, Kennard, Sherer, Comalli, & Woodruff-Pak, 2011). As a foundation for further developmental research, the present study examined the role of cholinergic function in the CPFE in adolescent rats during each phase of the CPFE protocol. In Experiment 1, an i.p injection of either 0.5 or 1.0 mg/kg dose of scopolamine administered prior to all 3 phases of the CPFE protocol impaired the CPFE. Experiment 2 further showed that a 0.5 mg/kg injection prior to just 1 of the 3 phases of the CPFE also disrupted contextual fear conditioning. We further showed that the CPFE is impaired by localized scopolamine infusions into dorsal hippocampus on the preexposure day (Experiment 3a), training day (Experiment 3b), and test day (Experiment 3c). These findings demonstrate a role of cholinergic signaling in hippocampus during each of the 3 phases of the CPFE in adolescent rats. Implications for the development and neural basis of the CPFE are discussed. (PsycINFO Database Record

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.

NMDA receptor antagonism disrupts acquisition and retention of the context preexposure facilitation effect in adolescent rats

The context preexposure facilitation effect (CPFE) is a contextual fear conditioning paradigm in which learning about the context, acquiring the context-shock association, and retrieving/expressing contextual fear are temporally dissociated. The current study investigated the involvement of NMDA receptors in contextual fear acquisition, retention, and expression across all phases of the CPFE in adolescent rats. In Experiment 1 systemic injections of 0.1mg/kg MK-801, a non-competitive NMDA receptor antagonist, given before multiple context preexposure disrupted the acquisition of a context representation. In Experiment 2, pre-training MK-801 disrupted both immediate acquisition of contextual fear measured by postshock freezing, as well as retention test freezing 24h later. Experiment 3 showed that expression of contextual fear via a 24h retention freezing test does not depend on NMDA receptors, indicating that MK-801 disrupts learning rather than performance of freezing behavior. In Experiment 4, consolidation of contextual information was partially disrupted by post-preexposure MK-801 whereas consolidation of contextual fear was not disrupted by post-training MK-801. Finally, Experiment 5 employed a dose-response design and found that a pre-training dose of 0.1mg/kg MK-801 disrupted both postshock and retention test freezing while lower pre-training doses of MK-801 (0.025 or 0.05mg/kg) only disrupted retention freezing. This is the first study to distinguish the role of NMDA receptors in acquisition (post-shock freezing), retention, expression, and consolidation of context vs. context-shock learning using the CPFE paradigm in adolescent rats. The findings provide a foundation for similar developmental studies examining these effects from early ontogeny through adulthood.

Differences in stress-induced changes in extinction and prefrontal plasticity in postweanling and adult animals

BACKGROUND

Postweaning is a critical developmental stage during which the medial prefrontal cortex (mPFC) undergoes major changes and the brain is vulnerable to the effects of stress. Surprisingly, the engagement of the mPFC in extinction of fear was reported to be identical in postweanling (PW) and adult animals. Here, we examined whether the effect of stress on extinction and mPFC plasticity would be similar in PW and adult animals.

METHODS

PW and adult animals were fear conditioned and exposed to the elevated platform stress paradigm, and extinction and long-term potentiation were examined. The dependency of stress-induced modulation of extinction and plasticity on N-methyl-D-aspartate receptors was examined as well.

RESULTS

We show that exposure to stress is associated with reduction of fear and enhanced induction of long-term potentiation (LTP) in PW pups, in contrast to its effects in adult animals. Furthermore, we report opposite effects in the occlusion of LTP following the enhanced or impaired extinction in the two age groups and that the reversal of the effects of stress is independent of N-methyl-D-aspartate receptor activation in PW animals.

CONCLUSIONS

Our results show that qualitatively different mechanisms control the modulatory effects of stress on extinction and plasticity in postweanling pups compared with adult rats. Our results point to significant differences between young and adult brains, which may have potential implications for the treatment of anxiety and stress disorders across development.

Using the context preexposure facilitation effect to study long-term context memory in preweanling, juvenile, adolescent, and adult rats

The present study used the context preexposure facilitation effect (CPFE) to examine long-term retention of incidental context learning in periweanling, adolescent and adult rats. The CPFE is a variant of contextual fear conditioning in which encoding the context representation, associating this representation with shock, and expressing the context-shock association each occur on separate occasions. Experiment 1 manipulated the retention interval-1d, 8d, 15d, or 22d-between context preexposure and training with immediate shock to determine how long the encoded context could be remembered (testing always occurred 24h following training). The other factors were age-postnatal day (PND) 24 vs 31-and training group-Preexposed to the training context (Pre) vs. an alternate context (Alt-Pre). At both ages, significantly more freezing was evident in the Pre vs. Alt Pre Groups at the 24h, 8d and 15d retention intervals but not at the 22d interval, indicating that juvenile-adolescent rats remember the context for up to 15d. In contrast, context memory persists for 22days in adult rats (Experiment 2); and is not evident after 24h, 8d, or 15d retention intervals in PND 17 rats (Experiment 3). The present study illustrates the value of the CPFE paradigm for investigations of long-term context memory in developing rats. Implications for the neurobiology of infantile amnesia are discussed.

Contextual and auditory fear conditioning continue to emerge during the periweaning period in rats

Anxiety disorders often emerge during childhood. Rodent models using classical fear conditioning have shown that different types of fear depend upon different neural structures and may emerge at different stages of development. For example, some work has suggested that contextual fear conditioning generally emerges later in development (postnatal day 23–24) than explicitly cued fear conditioning (postnatal day 15–17) in rats. This has been attributed to an inability of younger subjects to form a representation of the context due to an immature hippocampus. However, evidence that contextual fear can be observed in postnatal day 17 subjects and that cued fear conditioning continues to emerge past this age raises questions about the nature of this deficit. The current studies examine this question using both the context pre-exposure facilitation effect for immediate single-shock contextual fear conditioning and traditional cued fear conditioning using Sprague-Dawley rats. The data suggest that both cued and contextual fear conditioning are continuing to develop between PD 17 and 24, consistent with development occurring the in essential fear conditioning circuit.

Egr-1 increases in the prefrontal cortex following training in the context preexposure facilitation effect (CPFE) paradigm

The context pre-exposure facilitation effect (CPFE) is a modified form of standard contextual fear conditioning that dissociates learning about the context during a preexposure phase from learning the context-shock association during an immediate shock training phase conducted on separate days. Fear conditioning in the CPFE is an associative process in which only animals that are preexposed to the same context they are later given an immediate shock in demonstrate freezing when tested for conditioned fear memory. Previous research has shown that the hippocampus and amygdala are necessary for different phases of the CPFE, but whether other brain regions are also involved is unknown. The present study examined expression of the immediate-early gene early growth response gene 1 (Egr-1; also called Zif268, Ngfi-a, Krox-24) in the dorsal hippocampus, lateral nucleus of the amygdala, retrosplenial cortex, and several prefrontal cortex regions (infralimbic and prelimbic medial prefrontal cortex, anterior cingulate, and orbitofrontal cortex) following each phase of the CPFE in juvenile rats. Animals preexposed to the conditioning context displayed fear conditioned freezing during a retention test whereas rats preexposed to an alternate context did not. Following context preexposure, Egr-1 mRNA was elevated in context and alternate context exposed animals compared to home-cage control rats in almost all regions analyzed. Following the context-shock training phase, fear conditioned rats displayed significantly more Egr-1 mRNA expression in the infralimbic, prelimbic, and orbitofrontal cortices compared to the alternate context preexposed control rats. These differences in Egr-1 expression were not found in amygdala between the preexposed context and alternate context rats. No sex differences were observed following preexposure or training in any regions analyzed. The findings suggest that increased expression of Egr-1 within the prefrontal cortex is associated with contextual fear conditioning in the CPFE paradigm.

Determinants of novel object and location recognition during development

In the novel object recognition (OR) paradigm, rats are placed in an arena where they encounter two sample objects during a familiarization phase. A few minutes later, they are returned to the same arena and are presented with a familiar object and a novel object. The object location recognition (OL) variant involves the same familiarization procedure but during testing one of the familiar objects is placed in a novel location. Normal adult rats are able to perform both the OR and OL tasks, as indicated by enhanced exploration of the novel vs. the familiar test item. Rats with hippocampal lesions perform the OR but not OL task indicating a role of spatial memory in OL. Recently, these tasks have been used to study the ontogeny of spatial memory but the literature has yielded conflicting results. The current experiments add to this literature by: (1) behaviorally characterizing these paradigms in postnatal day (PD) 21, 26 and 31-day-old rats; (2) examining the role of NMDA systems in OR vs. OL; and (3) investigating the effects of neonatal alcohol exposure on both tasks. Results indicate that normal-developing rats are able to perform OR and OL by PD21, with greater novelty exploration in the OR task at each age. Second, memory acquisition in the OL but not OR task requires NMDA receptor function in juvenile rats [corrected]. Lastly, neonatal alcohol exposure does not disrupt performance in either task. Implications for the ontogeny of incidental spatial learning and its disruption by developmental alcohol exposure are discussed.

Role of age, post-training consolidation, and conjunctive associations in the ontogeny of the context preexposure facilitation effect

The context preexposure facilitation effect (CPFE) is a variant of contextual fear conditioning in which context learning and context-shock associations occur on separate occasions. The CPFE with an immediate shock emerges between Postnatal Day (PND) 17 and 24 in the rat and depends on hippocampal NMDA-receptor function in PND 24 rats (Schiffino et al. [2011] Neurobiology of Learning and Memory 95(2):190-198). This study investigated this ontogenetic effect further and reports three findings: First, the CPFE is absent on PND 19 but emerges modestly in rats given exposure on PND 21. Second, the absence of the CPFE on PND 17 does not reflect inability to consolidate the context-shock association established on the training day. Lastly, the CPFE on PND 24 requires exposure to the combined features of the context. These results are the first to show that the early development of contextual fear conditioning depends on conjunctive representations and that processes underlying the CPFE begin to emerge around PND 21 in the rat.

Neonatal alcohol exposure disrupts hippocampal neurogenesis and contextual fear conditioning in adult rats

Developmental alcohol exposure can permanently alter brain structures and produce functional impairments in many aspects of behavior, including learning and memory. This study evaluates the effect of neonatal alcohol exposure on adult neurogenesis in the dentate gyrus of the hippocampus and the implications of such exposure for hippocampus-dependent contextual fear conditioning. Alcohol-exposed rats (AE) received 5.25g/kg/day of alcohol on postnatal days (PD) 4-9 (third trimester in humans), in a binge-like manner. Two control groups were included: sham-intubated (SI) and suckle-control (SC). Animals were housed in social cages (3/cage) after weaning. On PD80, animals were injected with 200mg/kg BrdU. Half of the animals were sacrificed 2h later. The remainder were sacrificed on PD114 to evaluate cell survival; separate AE, SI, and SC rats not injected with BrdU were tested for the context preexposure facilitation effect (CPFE; ~PD117). There was no difference in the number of BrdU+ cells in AE, SI and SC groups on PD80. On PD114, cell survival was significantly decreased in AE rats, demonstrating that developmental alcohol exposure damages new cells' ability to incorporate into the network and survive. Behaviorally tested SC and SI groups preexposed to the training context 24h prior to receiving a 1.5mA 2s footshock froze significantly more during the context test than their counterparts preexposed to an alternate context. AE rats failed to show the CPFE. The current study shows the detrimental, long-lasting effects of developmental alcohol exposure on hippocampal adult neurogenesis and contextual fear conditioning.

Effects of dose and period of neonatal alcohol exposure on the context preexposure facilitation effect

BACKGROUND

Alcohol exposure in the rat on postnatal days (PD) 4 to 9 is known to partially damage the hippocampus and to impair hippocampus-dependent behavioral tasks. We previously reported that PD4 to 9 alcohol exposure eliminated the context preexposure facilitation effect (CPFE) in juvenile rats, a hippocampus-dependent variant of contextual fear conditioning. In the CPFE, context exposure and immediate shock occur on successive occasions and this produces conditioned freezing relative to a control group that is not preexposed to the training context. Here, we extend our earlier findings by examining the effects of neonatal alcohol administered at multiple doses or over different neonatal exposure periods.

METHOD

Rat pups (male and female) were exposed to a single binge dose of alcohol at one of 3 doses (2.75, 4.00, or 5.25 g/kg/d) over PD4 to 9 (Experiment 1) or to 5.25 g over PD4 to 6 or PD7 to 9 (Experiment 2). Sham-intubated (SI) and undisturbed (UD) rats served as controls. On PD31, rats were preexposed to either the training context (Pre) or an alternate context (No-Pre). On PD32, rats received an immediate unsignaled footshock (1.5 mA, 2 seconds) in the training context. Finally, on PD33, all rats were returned to the training context and tested for contextual freezing over a 5-minute period.

RESULTS

Undisturbed- and SI-Pre rats showed the CPFE, i.e., context preexposure facilitated contextual conditioning, relative to their No-Pre counterparts. The immediate shock deficit was present in all No-Pre groups, regardless of previous alcohol exposure. In Experiment 1, blood alcohol level was negatively correlated with contextual freezing. Group 2.75 g-Pre did not differ from controls. Group 4.00 g-Pre froze significantly less than Groups UD- and SI-Pre but more than Group 5.25-Pre, which showed the immediate shock deficit. In Experiment 2, alcohol exposure limited to PD7 to 9, but not PD4 to 6, disrupted the CPFE.

CONCLUSIONS

This is the first demonstration of dose-related impairment on a hippocampus-dependent task produced by neonatal alcohol exposure in the rat. Exposure period effects support previous studies of alcohol and spatial learning. The CPFE is a more sensitive behavioral task that can be used to elucidate developmental alcohol-induced deficits over a range of alcohol doses that are more relevant to human exposure levels.

Ontogeny and neural substrates of the context preexposure facilitation effect

Contextual fear conditioning emerges around post-natal day (PD) 23 in the rat. This is thought to reflect hippocampus-dependent conjunctive learning, which binds the individual features of the context into a unified representation (Rudy, 1993). However, context conditioning can also be supported by hippocampus-independent, feature-based simple associations (Rudy, 2009) and these may operate at PD 23-24 (Pugh & Rudy, 1996). To address this issue, we studied the ontogeny of a variant of contextual fear conditioning, termed the context-preexposure-facilitation-effect (CPFE), in which exposure to context and (immediate) foot shock occur on successive occasions. This variant requires conjunctive as opposed to feature-based simple associations (Rudy, 2009). We tested PD 17, 24, and 31 rats on the CPFE vs. conventional context conditioning (Exp. 1) and on the CPFE with stronger reinforcement (Exp. 2). The CPFE emerged on PD 24 regardless of reinforcer strength and in parallel with context conditioning. Infusions of the NMDA-receptor antagonist, MK-801, into the dorsal hippocampus just before pre-exposure on PD 24 eliminated the CPFE, whereas infusions occurring after pre-exposure had no effect (Exp. 3). These findings demonstrate a role of hippocampal NMDA receptors in the CPFE as early as PD 24 and implicate conjunctive learning mechanisms in the ontogeny of contextual fear conditioning.

Variants of contextual fear conditioning are differentially impaired in the juvenile rat by binge ethanol exposure on postnatal days 4-9

Neonatal ethanol exposure in the rat is known to partially damage the hippocampus, but such exposure causes only modest or inconsistent deficits on hippocampus-dependent behavioral tasks. This may reflect variable sensitivity of these tasks or residual function following partial hippocampal injury. The context preexposure facilitation effect (CPFE) is a variant of context conditioning in which context exposure and immediate shock occur on successive occasions. During testing, preexposed rats freeze more than non-preexposed controls. The CPFE is more sensitive to anterograde hippocampal injury than standard contextual fear conditioning (e.g., Rudy JW, O'Reilly RC. Conjunctive representations, the hippocampus, and contextual fear conditioning. Cogn Affect Behav Neurosci 2001;1:66-82). We report that rats exposed to a high binge dose of ethanol (5.25g/kg/day) over postnatal days [PD] 4-9 failed to demonstrate the CPFE when preexposed to the conditioning context on PD31, relative to sham-intubated and undisturbed controls (Experiment 1). Neonatal alcohol disrupted conditioned freezing to a much lesser extent relative to controls when context preexposure was followed by a standard context conditioning trial rather than immediate shock (Experiment 2). Fear conditioning to a discrete auditory cue (tone) was unaffected by neonatal alcohol exposure ruling out possible performance effects (Experiment 3). These findings suggest that the CPFE is an especially sensitive task for detecting hippocampal injury produced by neonatal alcohol. Mixed results with other tasks may reflect residual hippocampal function and/or the use of alternate neurobehavioral systems or "strategies" following alcohol-induced brain damage.

Evidence for hippocampus-dependent contextual learning at postnatal day 17 in the rat

Long-term memory for fear of an environment (contextual fear conditioning) emerges later in development (postnatal day; PD 23) than long-term memory for fear of discrete stimuli (PD 17). As contextual, but not explicit cue, fear conditioning relies on the hippocampus; this has been interpreted as evidence that the hippocampus is not fully developed until PD 23. Alternatively, the hippocampus may be functional prior to PD 23, but unable to cooperate with the amygdala for fearful learning. The current experiments investigate this by separating the phases of conditioning across developmental stages. Rats were allowed to learn about the context on one day and to form the fearful association on another. Rats exposed to the context on PD 17 exhibited significant fear only when trained and tested a week later (PD 23, 24), but not on consecutive days (PD 18, 19), demonstrating that rats can learn about a context as early as PD 17. Further experiments clarify that it is associative mechanisms that are developing between PD 18 and 23. Finally, the hippocampus was lesioned prior to training to ensure the task is being solved in a hippocampus-dependent manner. These data provide compelling evidence that the hippocampus is functional for contextual learning as early as PD 17, however, its connection to the amygdala or other relevant brain structures may not yet be fully developed.