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.

Infant rats can acquire, but not retain contextual associations in object-in-context and contextual fear conditioning paradigms.

Context learning in postnatal day (PD) 16-18 rats has been taken by Revillo, Cotella, Paglini, and Arias (2015, Physiology & Behavior, 148, 6-21) to challenge the view that the ontogeny of contextual learning is related to the development of the hippocampal system (Rudy, 1993, Behavioral Neuroscience, 107(5), 887-891; Schiffino, Murawski, Rosen, & Stanton, 2011 Neurobiology of Learning and Memory, 95(2), 190-198). Whether context learning is "incidental" or "reinforcement-driven" may determine the ontogeny and neural systems involved (Rudy, 2009, Learning & Memory (Cold Spring Harbor, N.Y.), 16, 573-585). However, we have shown differential ontogeny of two different forms of incidental context learning, the context pre-exposure facilitation effect (CPFE; Jablonski, Schiffino, & Stanton, 2012, Developmental Psychobiology, 54(7), 714-722), which emerges between PD 17 and 21; and object-in-context recognition (OiC, Ramsaran, Westbrook, & Stanton, 2016, Developmental Psychobiology, 58(7), 883-895; Ramsaran, Sanders, & Stanton, 2016, Behavioural Brain Research, 298, 37-47), which is present on PD17. We investigated whether this task-dissociation reflects an encoding or a retention deficit, by varying the sample-to-testing intervals for both tasks. Experiment 1A found that PD17 rats were able to perform the OiC task after short (5 min) but not long (24 hr) sample-to-test intervals. Experiments 1B and 1C found that PD17 rats trained on the CPFE are able to acquire and express context-shock associations after short but not long retention intervals. These findings suggest that pre-weanling rats encode contexts but show poor consolidation or retrieval after longer retention intervals.

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.

Cholinergic rescue of neurocognitive insult following third-trimester equivalent alcohol exposure in rats.

Neonatal ethanol exposure during the third trimester equivalent of human pregnancy in the rat significantly impairs hippocampal and prefrontal neurobehavioral functioning. Postnatal day [PD] 4-9 ethanol exposure in rats disrupts long-term context memory formation, resulting in abolished post-shock and retention test freezing in a variant of contextual fear conditioning called the Context Preexposure Facilitation Effect (CPFE). This behavioral impairment is accompanied by disrupted medial prefrontal, but not dorsal hippocampal expression of the immediate early genes (IEGs) c-Fos, Arc, Egr-1, and Npas4 (Heroux, Robinson-Drummer, Kawan, Rosen, & Stanton, 2019). The current experiment examined if systemic administration of the acetylcholinesterase inhibitor physostigmine (PHY) prior to context learning would rescue prefrontal IEG expression and freezing in the CPFE. From PD4-9, Long-Evans rats received oral intubation of ethanol (EtOH; 5.25 g/kg/day) or sham-intubation (SI). Rats received a systemic injection of saline (SAL) or PHY (0.01 mg/kg) prior to all three phases (Experiment 1) or just context exposure (Experiment 2) in the CPFE from PD31-33. A subset of rats were sacrificed 30 min after context learning to assay changes in IEG expression in the medial prefrontal cortex (mPFC), dorsal hippocampus (dHPC), and ventral hippocampus (vHPC). Administration of PHY prior to all three phases or just context learning rescued both post-shock and retention test freezing in the CPFE in EtOH rats without altering performance in SI rats. EtOH-SAL rats had significantly reduced mPFC but not dHPC expression of c-Fos, Arc, Egr-1, and Npas4. EtOH-PHY treatment rescued mPFC expression of c-Fos in ethanol-exposed rats and increased Arc and Npas4 regardless of dosing condition. While there was no effect of PHY on dHPC or vHPC expression of Arc, Egr-1, or Npas4, this treatment significantly boosted hippocampal expression of c-Fos regardless of ethanol treatment. These findings implicate impaired cholinergic and prefrontal function in cognitive deficits arising from 3rd-trimester equivalent alcohol exposure.

Medial prefrontal and ventral hippocampal contributions to incidental context learning and memory in adolescent rats.

The Context Preexposure Facilitation Effect (CPFE) is a contextual fear conditioning (CFC) paradigm in which context learning, context-shock learning, and retrieval of contextual fear occur in three distinct phases. The medial prefrontal cortex (mPFC), dorsal hippocampus (dHPC), and ventral hippocampus (vHPC) are required for the acquisition and/or consolidation of a context representation during incidental context exposure (Heroux et al., 2017; Robinson-Drummer et al., 2016; Rudy & Matus-Amat, 2006). This exposure also induces the expression of the immediate early genes (IEGs) c-Fos, Arc, Egr-1, and Npas4 in these regions (Heroux et al., 2018, 2019). Despite these studies, it is still unclear how mPFC and vHPC contribute to incidental context learning and memory. The current study examined whether prefrontal or ventral hippocampal inactivation during context preexposure interferes with long-term context memory and IEG activity in the mPFC, vHPC, dHPC and the ventral midline thalamus (VMT, a region connected to both the mPFC and HPC). Adolescent Long-Evans rats were given intra-mPFC (Experiment 1) or intra-vHPC (Experiment 2) infusions of the GABAA receptor agonist muscimol or PBS prior to context preexposure, and then were sacrificed 30 min later and whole mPFC, dHPC, vHPC, and VMT were collected and assayed for IEG mRNA expression via qPCR. Prefrontal or ventral hippocampal inactivation during context exposure abolished subsequent post-shock and retention test freezing in behaviorally-tested littermates of the sacrificed groups. In Experiment 1, prefrontal inactivation reduced expression of c-Fos, Arc, Egr-1, and Npas4 in the mPFC, c-Fos, Arc, and Npas4 in the vHPC, and c-Fos in the VMT, to the level of behaviorally-naïve home-cage controls. Prefrontal inactivation did not alter IEG expression in the dHPC during context exposure. In Experiment 2, ventral hippocampal inactivation impaired expression of all IEGs in the mPFC, dHPC, and vHPC, with no effect in the VMT. Taken together, these results suggest that context memory processes on the preexposure day of the CPFE may depend on mPFC-vHPC circuitry not typically emphasized in studies of incidental or configural learning and memory.

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.

Efficacy of Maternal Choline Supplementation During Pregnancy in Mitigating Adverse Effects of Prenatal Alcohol Exposure on Growth and Cognitive Function: A Randomized, Double-Blind, Placebo-Controlled Clinical Trial.

BACKGROUND: We recently demonstrated the acceptability and feasibility of a randomized, double-blind choline supplementation intervention for heavy drinking women during pregnancy. In this study, we report our results relating to the efficacy of this intervention in mitigating adverse effects of prenatal alcohol exposure (PAE) on infant growth and cognitive function. METHODS: Sixty-nine Cape Coloured (mixed ancestry) heavy drinkers in Cape Town, South Africa, recruited in mid-pregnancy, were randomly assigned to receive a daily oral dose of either 2 g of choline or placebo from time of enrollment until delivery. Each dose consisted of an individually wrapped packet of powder that, when mixed with water, produced a sweet tasting grape-flavored drink. The primary outcome, eyeblink conditioning (EBC), was assessed at 6.5 months. Somatic growth was measured at birth, 6.5, and 12 months, recognition memory and processing speed on the Fagan Test of Infant Intelligence, at 6.5 and 12 months. RESULTS: Infants born to choline-treated mothers were more likely to meet criterion for conditioning on EBC than the placebo group. Moreover, within the choline arm, degree of maternal adherence to the supplementation protocol strongly predicted EBC performance. Both groups were small at birth, but choline-treated infants showed considerable catch-up growth in weight and head circumference at 6.5 and 12 months. At 12 months, the infants in the choline treatment arm had higher novelty preference scores, indicating better visual recognition memory. CONCLUSIONS: This exploratory study is the first to provide evidence that a high dose of choline administered early in pregnancy can mitigate adverse effects of heavy PAE on EBC, postnatal growth, and cognition in human infants. These findings are consistent with studies of alcohol-exposed animals that have demonstrated beneficial effects of choline supplementation on classical conditioning, learning, and memory.

Functional MRI of Human Eyeblink Classical Conditioning in Children with Fetal Alcohol Spectrum Disorders.

Prenatal alcohol exposure has been linked to a broad range of developmental deficits, with eyeblink classical conditioning (EBC) among the most sensitive endpoints. This fMRI study compared EBC-related brain activity in 47 children with fetal alcohol syndrome (FAS), partial FAS (PFAS), heavily exposed (HE) non-syndromal children, and healthy controls. All of the children had previously participated in two EBC studies conducted as part of our longitudinal study of fetal alcohol spectrum disorders. Although learning-related behavioral differences were seen in all groups during the scans, controls showed more conditioned responses (CR) than the alcohol-exposed groups. Despite lower conditioning levels relative to controls, the exposed groups exhibited extensive cerebellar activations. Specifically, children with FAS/PFAS showed increased activation of cerebellar lobule VI in session 2, while HE children showed increased activation in session 1. Continuous measures of prenatal alcohol use correlated with learning-related activations in cerebellum and frontal cortices. Only controls showed significant cerebellar activation-CR correlations in the deep nuclei and lateral lobule VI, suggesting that these key regions supporting EBC may be functionally disorganized in alcohol-exposed children. These findings are the first to characterize abnormalities in brain function associated with the behavioral conditioning deficits seen in children with prenatal alcohol exposure.

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 GABAA 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.

White matter deficits mediate effects of prenatal alcohol exposure on cognitive development in childhood.

Fetal alcohol spectrum disorders comprise the spectrum of cognitive, behavioral, and neurological impairments caused by prenatal alcohol exposure (PAE). Diffusion tensor imaging (DTI) was performed on 54 children (age 10.1 ± 1.0 years) from the Cape Town Longitudinal Cohort, for whom detailed drinking histories obtained during pregnancy are available: 26 with full fetal alcohol syndrome (FAS) or partial FAS (PFAS), 15 nonsyndromal heavily exposed (HE), and 13 controls. Using voxelwise analyses, children with FAS/PFAS showed significantly lower fractional anisotropy (FA) in four white matter (WM) regions and higher mean diffusivity (MD) in seven; three regions of FA and MD differences (left inferior longitudinal fasciculus (ILF), splenium, and isthmus) overlapped, and the fourth FA cluster was located in the same WM bundle (right ILF) as an MD cluster. HE children showed lower FA and higher MD in a subset of these regions. Significant correlations were observed between three continuous alcohol measures and DTI values at cluster peaks, indicating that WM damage in several regions is dose dependent. Lower FA in the regions of interest was attributable primarily to increased radial diffusivity rather than decreased axonal diffusivity, suggesting poorer axon packing density and/or myelination. Multiple regression models indicated that this cortical WM impairment partially mediated adverse effects of PAE on information processing speed and eyeblink conditioning. Hum Brain Mapp 37:2943-2958, 2016. © 2016 Wiley Periodicals, Inc.

Determinants of object-in-context and object-place-context recognition in the developing rat.

Since the seminal report on novel object recognition in the rat (Ennaceur & Delacour, 1988), novelty recognition paradigms have become increasingly prevalent in learning and memory research. Novelty recognition tasks do not require extensive training or complex behaviors, and thus are especially suitable for studying the ontogeny of various forms of memory (e.g., object, spatial, and contextual memory). However, relatively little is known about the determinants of recognition memory during development. The present study extends our recent research on the development of recognition memory by further characterizing the ontogeny of contextual recognition (Ramsaran, Westbrook, & Stanton, 2016). We report that long-term retention of object-in-context (OiC) memory emerges during early development in the rat (Experiment 1), and that performance of object-place-context (OPC), a spatial variant of the OiC task, also displays protracted development until early adolescence (Experiment 2). In addition, we examined the role of NMDA receptors (NMDARs) in contextual recognition and found that OiC memory is not dependent on NMDAR-mediated plasticity whereas performance of spatial task variants including the distal cue OiC (Ramsaran et al., 2016) and OPC tasks are NMDAR-dependent (Experiments 3 and 4). The ontogeny of contextual recognition is influenced by memory retention and spatial processing demands, which may also determine the neurobiological mechanisms supporting task performance.

White matter integrity of the cerebellar peduncles as a mediator of effects of prenatal alcohol exposure on eyeblink conditioning.

Fetal alcohol spectrum disorders (FASD) are characterized by a range of neurodevelopmental deficits that result from prenatal exposure to alcohol. These can include cognitive, behavioural, and neurological impairment, as well as structural and functional brain damage. Eyeblink conditioning (EBC) is among the most sensitive endpoints affected in FASD. The cerebellar peduncles, large bundles of myelinated nerve fibers that connect the cerebellum to the brainstem, constitute the principal white matter element of the EBC circuit. Diffusion tensor imaging (DTI) is used to assess white matter integrity in fibre pathways linking brain regions. DTI scans of 54 children with FASD and 23 healthy controls, mean age 10.1 ± 1.0 years, from the Cape Town Longitudinal Cohort were processed using voxelwise group comparisons. Prenatal alcohol exposure was related to lower fractional anisotropy (FA) bilaterally in the superior cerebellar peduncles and higher mean diffusivity (MD) in the left middle peduncle, effects that remained significant after controlling for potential confounding variables. Lower FA and higher MD in these regions were associated with poorer EBC performance. Moreover, effects of alcohol exposure on EBC decreased significantly after inclusion of these DTI measures in regression models, suggesting that these white matter deficits partially mediate the relation of prenatal alcohol exposure to EBC. The associations of greater alcohol consumption with these DTI measures are largely attributable to greater radial diffusivity, possibly indicating poorer myelination. Thus, these data suggest that fetal alcohol-related deficits in EBC are attributable, in part, to poorer myelination in key regions of the cerebellar peduncles.

Functional MRI of cerebellar activity during eyeblink classical conditioning in children and adults.

This study characterized human cerebellar activity during eyeblink classical conditioning (EBC) in children and adults using functional magnetic resonance imaging (fMRI). During fMRI, participants were administered delay conditioning trials, in which the conditioned stimulus (a tone) precedes, overlaps, and coterminates with the unconditioned stimulus (a corneal airpuff). Behavioral eyeblink responses and brain activation were measured concurrently during two phases: pseudoconditioning, involving presentations of tone alone and airpuff alone, and conditioning, during which the tone and airpuff were paired. Although all participants demonstrated significant conditioning, the adults produced more conditioned responses (CRs) than the children. When brain activations during pseudoconditioning were subtracted from those elicited during conditioning, significant activity was distributed throughout the cerebellar cortex (Crus I-II, lateral lobules IV-IX, and vermis IV-VI) in all participants, suggesting multiple sites of associative learning-related plasticity. Despite their less optimal behavioral performance, the children showed greater responding in the pons, lateral lobules VIII, IX, and Crus I, and vermis VI, suggesting that they may require greater activation and/or the recruitment of supplementary structures to achieve successful conditioning. Correlation analyses relating brain activations to behavioral CRs showed a positive association of activity in cerebellar deep nuclei (including dentate, fastigial, and interposed nuclei) and vermis VI with CRs in the children. This is the first study to compare cerebellar cortical and deep nuclei activations in children versus adults during EBC.

An in vivo 1H magnetic resonance spectroscopy study of the deep cerebellar nuclei in children with fetal alcohol spectrum disorders.

BACKGROUND: Prenatal alcohol exposure has been linked to impairment in cerebellar structure and function, including eyeblink conditioning. The deep cerebellar nuclei, which play a critical role in cerebellar-mediated learning, receive extensive inputs from brain stem and cerebellar cortex and provide the point of origin for most of the output fibers to other regions of the brain. We used in vivo (1) H magnetic resonance spectroscopy (MRS) to examine effects of prenatal alcohol exposure on neurochemistry in this important cerebellar region. METHODS: MRS data from the deep cerebellar nuclei were acquired from 37 children with heavy prenatal alcohol exposure and 17 non- or minimally exposed controls from the Cape Coloured (mixed ancestry) community in Cape Town, South Africa. RESULTS: Increased maternal alcohol consumption around time of conception was associated with lower N-Acetylaspartate (NAA) levels in the deep nuclei (r = -0.33, p < 0.05). Higher levels of alcohol consumption during pregnancy were related to lower levels of the choline-containing metabolites (r = -0.37, p < 0.01), glycerophosphocholine plus phosphocholine (Cho). Alcohol consumption levels both at conception (r = 0.35, p < 0.01) and during pregnancy (r = 0.38, p < 0.01) were related to higher levels of glutamate plus glutamine (Glx). All these effects continued to be significant after controlling for potential confounders. CONCLUSIONS: The lower NAA levels seen in relation to prenatal alcohol exposure may reflect impaired neuronal integrity in the deep cerebellar nuclei. Our finding of lower Cho points to disrupted Cho metabolism of membrane phospholipids, reflecting altered neuropil development with potentially reduced content of dendrites and synapses. The alcohol-related alterations in Glx may suggest a disruption of the glutamate-glutamine cycling involved in glutamatergic excitatory neurotransmission.

Ontogeny of object versus location recognition in the rat: acquisition and retention effects.

Novel object and location recognition tasks harness the rat's natural tendency to explore novelty (Berlyne, 1950) to study incidental learning. The present study examined the ontogenetic profile of these two tasks and retention of spatial learning between postnatal day (PD) 17 and 31. Experiment 1 showed that rats ages PD17, 21, and 26 recognize novel objects, but only PD21 and PD26 rats recognize a novel location of a familiar object. These results suggest that novel object recognition develops before PD17, while object location recognition emerges between PD17 and PD21. Experiment 2 studied the ontogenetic profile of object location memory retention in PD21, 26, and 31 rats. PD26 and PD31 rats retained the object location memory for both 10-min and 24-hr delays. PD21 rats failed to retain the object location memory for the 24-hr delay, suggesting differential development of short- versus long-term memory in the ontogeny of object location memory.

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.

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.

T-maze learning in weanling lambs.

A major advantage of sheep models in experimental studies of neurodevelopmental disorders (e.g., with prenatal neurotoxicant exposure) is that the equivalent of all three trimesters of human brain development occurs in sheep entirely in utero. However, studies of learning and memory in sheep are limited. The goal of this study was to extend the analysis of spatial learning and memory in adolescent sheep using several traditional T-maze tasks. Both 9- and 14-week-old lambs acquired a delayed nonmatching-to-place task, but the older lambs learned the task significantly faster. In contrast, acquisition of a matching-to-place task was significantly more difficult. Lambs, like rodents, appear to have a predisposition toward learning "win-shift" spatial problems in a T-maze under appetitive motivation. Lambs also rapidly acquired a position habit and showed typical reversal learning curves. These findings support the use of T-maze tasks to assess behavioral outcomes in various sheep models.