The role of uncertainty in regulating associative change

Rescorla (2000, 2001) interpreted his compound test results to show that both common and individual error terms regulate associative change such that the element of a conditioned compound with the greater prediction error undergoes greater associative change than the one with the smaller prediction error. However, it has recently been suggested that uncertainty, not prediction error, is the primary determinant of associative change in people (Spicer et al., 2020, 2022). The current experiments use the compound test in a continuous outcome allergist task to assess the role of uncertainty in associative change, using two different manipulations of uncertainty: outcome uncertainty (where participants are uncertain of the level of the outcome on a particular trial) and causal uncertainty (where participants are uncertain of the contribution of the cue to the level of the outcome). We replicate Rescorla's compound test results in the case of both associative gains (Experiment 1) and associative losses (Experiment 3) and then provide evidence for greater change to more uncertain cues in the case of associative gains (Experiments 2 and 4), but not associative losses (Experiments 3 and 5). We discuss the findings in terms of the notion of theory protection advanced by Spicer et al., and other ways of thinking about the compound test procedure, such as that proposed by Holmes et al. (2019). (PsycInfo Database Record (c) 2024 APA, all rights reserved).

Context and Time Regulate Fear Memory Consolidation and Reconsolidation in the Basolateral Amygdala Complex

It is widely accepted that fear memories are consolidated through protein synthesis-dependent changes in the basolateral amygdala complex (BLA). However, recent studies show that protein synthesis is not required to consolidate the memory of a new dangerous experience when it is similar to a prior experience. Here, we examined whether the protein synthesis requirement for consolidating the new experience varies with its spatial and temporal distance from the prior experience. Female and male rats were conditioned to fear a stimulus (S1, e.g., light) paired with shock in stage 1 and a second stimulus (S2, e.g., tone) that preceded additional S1-shock pairings (S2-S1-shock) in stage 2. The latter stage was followed by a BLA infusion of a protein synthesis inhibitor, cycloheximide, or vehicle. Subsequent testing with S2 revealed that protein synthesis in the BLA was not required to consolidate fear to S2 when the training stages occurred 48 h apart in the same context; was required when they were separated by 14 d or occurred in different contexts; but was again not required if S1 was re-presented after the delay or in the different context. Similarly, protein synthesis in the BLA was not required to reconsolidate fear to S2 when the training stages occurred 48 h apart but was required when they occurred 14 d apart. Thus, the protein synthesis requirement for consolidating/reconsolidating fear memories in the BLA is determined by similarity between present and past experiences, the time and place in which they occur, and reminders of the past experiences.

Stimulating oxytocin receptors in the basolateral amygdala enhances stimulus processing: Differential and consistent effects for stimuli paired with fear versus sucrose in extinction and reversal learning

Oxytocin (OT) influences a range of social behaviors by enhancing the salience of social cues and regulating the expression of specific social behaviors (e.g., maternal care versus defensive aggression). We previously showed that stimulating OT receptors in the basolateral amygdala of rats also enhanced the salience of fear conditioned stimuli: relative to rats given vehicle infusions, rats infused with [Thr4,Gly7]-oxytocin (TGOT), a selective OT receptor agonist, showed greater discrimination between a cue predictive of danger, and one that signaled safety. In the present series of experiments using male rats, the effects of OT receptor activation in the basolateral amygdala on stimulus processing were examined further using conditioning protocols that consist of changes in stimulus-outcome contingencies (i.e., extinction and reversal), and with stimuli paired with aversive (i.e., foot shock) and appetitive (i.e., sucrose) outcomes. It was revealed that the effects of OTR stimulation diverge for aversive and appetitive learning - enhancing the former but not the latter. However, across both types of learning, OTR stimulation enhanced the detection of conditioned stimuli. Overall, these results are consistent with an emerging view of OT's effects on stimulus salience; facilitating the detection of meaningful stimuli while reducing responding to those that are irrelevant.

What is Learned Determines How Pavlovian Conditioned Fear is Consolidated in the Brain

Activity in the basolateral amygdala complex (BLA) is needed to encode fears acquired through contact with both innate sources of danger (i.e., things that are painful) and learned sources of danger (e.g., being threatened with a gun). However, within the BLA, the molecular processes required to consolidate the two types of fear are not the same: protein synthesis is needed to consolidate the first type of fear (so-called first-order fear) but not the latter (so-called second-order fear). The present study examined why first- and second-order fears differ in this respect. Specifically, it used a range of conditioning protocols in male and female rats, and assessed the effects of a BLA infusion of the protein synthesis inhibitor, cycloheximide, on first- and second-order conditioned fear. The results revealed that the differential protein synthesis requirements for consolidation of first- and second-order fears reflect differences in what is learned in each case. Protein synthesis in the BLA is needed to consolidate fears that result from encoding of relations between stimuli in the environment (stimulus-stimulus associations, typical for first-order fear) but is not needed to consolidate fears that form when environmental stimuli associate directly with fear responses emitted by the animal (stimulus-response associations, typical for second-order fear). Thus, the substrates of Pavlovian fear conditioning in the BLA depend on the way that the environment impinges upon the animal. This is discussed with respect to theories of amygdala function in Pavlovian fear conditioning, and ways in which stimulus-response associations might be consolidated in the brain.

The effects of extinction and an explicitly unpaired treatment on the reinforcing properties of a Pavlovian conditioned stimulus

This series of experiments examined the effects of extinction and an explicitly unpaired treatment on the ability of a conditioned stimulus (CS) to function as a reinforcer. Rats were trained to lever press for food, exposed to pairings of a noise CS and food, and, finally, tested for their willingness to lever press for the CS in the absence of the food. Experiment 1 provided a demonstration of conditioned reinforcement (using controls that were only exposed to unpaired presentations of the CS and food) and showed that it was equivalent after one or four sessions of CS-food pairings. Experiments 2 and 3 showed that, after one session of CS-food pairings, repeated presentations of the CS alone reduced its reinforcing properties; but after four sessions of CS-food pairings, repeated presentations of the CS alone had no effect on these properties. Experiment 4 showed that, after four sessions of CS-food pairings, explicitly unpaired presentations of the CS and food completely undermined conditioned reinforcement. Finally, Experiment 5 provided within-experiment evidence that, after four sessions of CS-food pairings, the reinforcing properties of the CS were disrupted by explicitly unpaired presentations of the CS and food but spared by repeated presentations of the CS alone. Together, these findings indicate that the effectiveness of extinction in undermining the reinforcing properties of a CS depends on its level of conditioning; and that, where extinction fails to disrupt these properties, they are successfully undermined by an explicitly unpaired treatment. They are discussed with respect to findings in the literature on Pavlovian-to-instrumental transfer; and the Rescorla-Wagner model, which anticipates that an explicitly unpaired treatment will be more effective than extinction in reversing the effects of conditioning.

NMDA Receptors in the Basolateral Amygdala Complex Are Engaged for Pavlovian Fear Conditioning When an Animal's Predictions about Danger Are in Error

It is widely accepted that Pavlovian fear conditioning requires activation of NMDA receptors (NMDARs) in the basolateral amygdala complex (BLA). However, it was recently shown that activation of NMDAR in the BLA is only required for fear conditioning when danger occurs unexpectedly; it is not required for fear conditioning when danger occurs as expected. This study tested the hypothesis that NMDARs in the BLA are engaged for Pavlovian fear conditioning when an animal's predictions regarding danger are in error. In each experiment, rats (females in Experiment 1 and males in Experiments 2-5) were conditioned to fear one stimulus, S1, when it was paired with foot-shock (S1→shock), and 48 h later, a second stimulus, S2, when it was presented in sequence with the already-conditioned S1 and foot-shock (S2→S1→shock). Conditioning to S2 occurred under a BLA infusion of the NMDAR antagonist, D-AP5 or vehicle. The subsequent tests of freezing to S2 alone and S1 alone revealed that the antagonist had no effect on conditioning to S2 when the shock occurred exactly as predicted by the S1, but disrupted this conditioning when the shock occurred earlier/later than predicted by S1, or at a stronger/weaker intensity. These results imply that errors in the timing or intensity of a predicted foot-shock engage NMDARs in the BLA for Pavlovian fear conditioning. They are discussed in relation to theories which propose a role for prediction error in determining how experiences are organized in memory and how activation of NMDAR in the BLA might contribute to this organization.SIGNIFICANCE STATEMENT This study is significant in showing that prediction error determines how a new experience is encoded with respect to a past experience and, thereby, whether NMDA receptors (NMDARs) in the basolateral amygdala complex (BLA) encode the new experience. When prediction error is small (e.g., danger occurs as and when expected), the new experience is encoded together with a past experience as part of the same "mental model," and NMDAR activation in the BLA is not needed for this encoding. By contrast, when prediction error is large (e.g., danger occurs at an unexpected intensity or time), the new experience is encoded separately from the past experience as part of a new mental model, and NMDAR activation in the BLA is needed for this encoding.

Danger Changes the Way the Brain Consolidates Neutral Information; and Does So by Interacting with Processes Involved in the Encoding of That Information

This study examined the effect of danger on consolidation of neutral information in two regions of the rat (male and female) medial temporal lobe: the perirhinal cortex (PRh) and basolateral amygdala complex (BLA). The neutral information was the association that forms between an auditory stimulus and a visual stimulus (labeled S2 and S1) across their pairings in sensory preconditioning. We show that, when the sensory preconditioning session is followed by a shocked context exposure, the danger shifts consolidation of the S2-S1 association from the PRh to the BLA; and does so by interacting with processes involved in encoding of the S2-S1 pairings. Specifically, we show that the initial S2-S1 pairing in sensory preconditioning is encoded in the BLA and not the PRh; whereas the later S2-S1 pairings are encoded in the PRh and not the BLA. When the sensory preconditioning session is followed by a context alone exposure, the BLA-dependent trace of the early S2-S1 pairings decays and the PRh-dependent trace of the later S2-S1 pairings is consolidated in memory. However, when the sensory preconditioning session is followed by a shocked context exposure, the PRh-dependent trace of the later S2-S1 pairings is suppressed and the BLA-dependent trace of the initial S2-S1 pairing is consolidated in memory. These findings are discussed with respect to mutually inhibitory interactions between the PRh and BLA, and the way that these regions support memory in other protocols, including recognition memory in people.SIGNIFICANCE STATEMENT The perirhinal cortex (PRh) and basolateral amygdala complex (BLA) process the pairings of neutral auditory and visual stimuli in sensory preconditioning. The involvement of each region in this processing is determined by the novelty/familiarity of the stimuli as well as events that occur immediately after the preconditioning session. Novel stimuli are represented in the BLA; however, as these stimuli are repeatedly presented without consequence, they come to be represented in the PRh. Whether the BLA- or PRh-dependent representation is consolidated in memory depends on what happens next. When nothing of significance occurs, the PRh-dependent representation is consolidated and the BLA-dependent representation decays; but when danger is encountered, the PRh-dependent representation is inhibited and the BLA-dependent representation is selected for consolidation.

Second-order fear conditioning involves formation of competing stimulus-danger and stimulus-safety associations

How do animals process experiences that provide contradictory information? The present study addressed this question using second-order fear conditioning in rats. In second-order conditioning, rats are conditioned to fear a stimulus, S1, through its pairings with foot-shock (stage 1); and some days later, a second stimulus, S2, through its pairings with the already-conditioned S1 (stage 2). However, as foot-shock is never presented during conditioning to S2, we hypothesized that S2 simultaneously encodes 2 contradictory associations: one that drives fear to S2 (S2-danger) and another that reflects the absence of the expected unconditioned stimulus and partially masks that fear (e.g. S2-safety). We tested this hypothesis by manipulating the substrates of danger and safety learning in the brain (using a chemogenetic approach) and assessing the consequences for second-order fear to S2. Critically, silencing activity in the basolateral amygdala (important for danger learning) reduced fear to S2, whereas silencing activity in the infralimbic cortex (important for safety learning) enhanced fear to S2. These bidirectional changes are consistent with our hypothesis that second-order fear conditioning involves the formation of competing S2-danger and S2-safety associations. More generally, they show that a single set of experiences can produce contradictory associations and that the brain resolves the contradiction by encoding these associations in distinct brain regions.

The Benefits of Switching to a Healthy Diet on Metabolic, Cognitive, and Gut Microbiome Parameters Are Preserved in Adult Rat Offspring of Mothers Fed a High-Fat, High-Sugar Diet

SCOPE

Maternal obesity increases the risk of health complications in children, highlighting the need for effective interventions. A rat model of maternal obesity to examine whether a diet switch intervention could reverse the adverse effects of an unhealthy postweaning diet is used.

METHODS AND RESULTS

Male and female offspring born to dams fed standard chow or a high-fat, high-sugar "cafeteria" (Caf) diet are weaned onto chow or Caf diets until 22 weeks of age, when Caf-fed groups are switched to chow for 5 weeks. Adiposity, gut microbiota composition, and place recognition memory are assessed before and after the switch. Body weight and adiposity fall in switched groups but remain significantly higher than chow-fed controls. Nonetheless, the diet switch improves a deficit in place recognition memory observed in Caf-fed groups, increases gut microbiota species richness, and alters β diversity. Modeling indicate that adiposity most strongly predicts gut microbiota composition before and after the switch.

CONCLUSION

Maternal obesity does not alter the effects of switching diet on metabolic, microbial, or cognitive measures. Thus, a healthy diet intervention lead to major shifts in body weight, adiposity, place recognition memory, and gut microbiota composition, with beneficial effects preserved in offspring born to obese dams.

Prediction Error Determines Whether NMDA Receptors in the Basolateral Amygdala Complex Are Involved in Pavlovian Fear Conditioning

It is widely accepted that activation of NMDA receptors (NMDAR) is necessary for the formation of fear memories in the basolateral amygdala complex (BLA). This acceptance is based on findings that blockade of NMDAR in the BLA disrupts Pavlovian fear conditioning in rodents when initially innocuous stimuli are paired with aversive and unexpected events (surprising foot shock). The present study challenges this acceptance by showing that the involvement of NMDAR in Pavlovian fear conditioning is determined by prediction errors in relation to aversive events. In the initial experiments, male rats received a BLA infusion of the NMDAR antagonist, D-AP5 and were then exposed to pairings of a novel target stimulus and foot shock. This infusion disrupted acquisition of fear to the target when the shock was surprising (experiments 1a, 1b, 2a, 2b, 3a, and 3b) but spared fear to the target when the shock was expected based on the context, time and other stimuli that were present (experiments 1a and 1b). Under the latter circumstances, fear to the target required activation of calcium-permeable AMPAR (CP-AMPA; experiments 4a, 4b, and 4c), which, using electrophysiology, were shown to regulate the activity of interneurons in the BLA (experiment 5). Thus, NMDAR activation is not required for fear conditioning when danger occurs as expected given the context, time and stimuli present, but is required for fear conditioning when danger occurs unexpectedly. These findings are related to current theories of NMDAR function and ways that prediction errors might influence the substrates of fear memory formation in the BLA.SIGNIFICANCE STATEMENT It is widely accepted that NMDA receptors (NMDAR) in the basolateral amygdala complex (BLA) are activated by pairings of a conditioned stimulus (CS) and an aversive unconditioned (US) stimulus, leading to the synaptic changes that underlie formation of a CS-US association. The present findings are significant in showing that this theory is incomplete. When the aversive US is unexpected, animals encode all features of the situation (context, time and stimuli present) as a new fear/threat memory, which is regulated by NMDAR in the BLA. However, when the US is expected based on the context, time and stimuli present, the new fear memory is assimilated into networks that represent those features, which occurs independently of NMDAR activation in the BLA.

The influence of maternal unhealthy diet on maturation of offspring gut microbiota in rat

Background

Despite well-known effects of diet on gut microbiota diversity, relatively little is known about how maternal diet quality shapes the longitudinal maturation of gut microbiota in offspring. To investigate, we fed female rats standard chow (Chow) or a western-style, high-choice cafeteria diet (Caf) prior to and during mating, gestation and lactation. At weaning (3 weeks), male and female offspring were either maintained on their mother’s diet (ChowChow, CafCaf groups) or switched to the other diet (ChowCaf, CafChow). Fecal microbial composition was assessed in dams and longitudinally in offspring at 3, 7 and 14 weeks of age.

Results

The effect of maternal diet on maturation of offspring gut microbiota was assessed by α- and β-diversities, Deseq2/LEfSe, and SourceTracker analyses. Weanling gut microbiota composition was characterised by reduced α- and β-diversity profiles that clustered away from dams and older siblings. After weaning, offspring gut microbiota came to resemble an adult-like gut microbiota, with increased α-diversity and reduced dissimilarity of β-diversity. Similarly, Deseq2/LEfSe analyses found fewer numbers of altered operational taxonomic units (OTUs) between groups from weaning to adulthood. SourceTracker analyses indicated a greater overall contribution of Caf mothers’ microbial community (up to 20%) to that of their offspring than the contribution of Chow mothers (up to 8%). Groups maintained on the maternal diet (ChowChow, CafCaf), versus those switched to the other diet (ChowCaf, CafChow) post-weaning significantly differed from each other at 14 weeks (Permutational Multivariate Analysis of Variance), indicating interactive effects of maternal and post-weaning diet on offspring gut microbiota maturation. Nevertheless, this developmental trajectory was unaffected by sex and appeared consistent between ChowChow, CafCaf, ChowCaf and CafChow groups.

Conclusions

Introducing solid food at weaning triggered the maturation of offspring gut microbiota to an adult-like profile in rats, in line with previous human studies. Postweaning Caf diet exposure had the largest impact on offspring gut microbiota, but this was modulated by maternal diet history. An unhealthy maternal Caf diet did not alter the developmental trajectory of offspring gut microbiota towards an adult-like profile, insofar as it did not prevent the age-associated increase in α-diversity and reduction in β-diversity dissimilarity.

Supplementary Information

The online version contains supplementary material available at 10.1186/s42523-022-00185-w.

Affective Valence Regulates Associative Competition in Pavlovian Conditioning

Evidence suggests that, in Pavlovian conditioning, associations form between conditioned stimuli and multiple components of the unconditioned stimulus (US). It is common, for example, to regard USs as composed of sensory and affective components, the latter being either appetitive (e.g., food or water) or aversive (e.g., shock or illness) and, therefore, to suppose different USs of the same affective class activate a common affective system. Furthermore, evidence is growing for the suggestion that, in competitive learning situations, competition between predictive stimuli is primarily for association with the affective system activated by the US. Thus, a conditioned stimulus (CS) previously paired with one US will block conditioning to another CS when both are presented together and paired with a different US of the same affective class, a phenomenon called transreinforcer blocking. Importantly, similar effects have been reported when steps are taken to turn the pretrained CS into a conditioned inhibitor, which activates the opposing affective state to the excitor that it inhibits. Thus, an appetitive inhibitor can block conditioning to a second CS when they are presented together and paired with foot shock. Here we show that the same is true of an aversive inhibitor. In two experiments conducted in rats, we found evidence that an aversive inhibitor blocked conditioning to a second CS when presented in a compound and paired with food. Such findings demonstrate that affective processes and their opponency organize appetitive-aversive interactions and establish the valences on which they are based, consistent with incentive theories of Pavlovian conditioning.

Male Rat Offspring Are More Impacted by Maternal Obesity Induced by Cafeteria Diet than Females-Additive Effect of Postweaning Diet

Maternal obesity increases the risk of health complications in offspring, but whether these effects are exacerbated by offspring exposure to unhealthy diets warrants further investigation. Female Sprague-Dawley rats were fed either standard chow (n = 15) or ‘cafeteria’ (Caf, n = 21) diets across pre-pregnancy, gestation, and lactation. Male and female offspring were weaned onto chow or Caf diet (2–3/sex/litter), forming four groups; behavioural and metabolic parameters were assessed. At weaning, offspring from Caf dams were smaller and lighter, but had more retroperitoneal (RP) fat, with a larger effect in males. Maternal Caf diet significantly increased relative expression of ACACA and Fasn in male and female weanling liver, but not CPT-1, SREBP and PGC1; PPARα was increased in males from Caf dams. Maternal obesity enhanced the impact of postweaning Caf exposure on adult body weight, RP fat, liver mass, and plasma leptin in males but not females. Offspring from Caf dams appeared to exhibit reduced anxiety-like behaviour on the elevated plus maze. Hepatic CPT-1 expression was reduced only in adult males from Caf fed dams. Post weaning Caf diet consumption did not alter liver gene expression in the adult offspring. Maternal obesity exacerbated the obesogenic phenotype produced by postweaning Caf diet in male, but not female offspring. Thus, the impact of maternal obesity on adiposity and liver gene expression appeared more marked in males. Our data underline the sex-specific detrimental effects of maternal obesity on offspring.

The Opioid Receptor Antagonist Naloxone Enhances First-Order Fear Conditioning, Second-Order Fear Conditioning and Sensory Preconditioning in Rats

The opioid receptor antagonist naloxone enhances Pavlovian fear conditioning when rats are exposed to pairings of an initially neutral stimulus, such as a tone, and a painful foot shock unconditioned stimulus (US; so-called first-order fear conditioning; Pavlov, 1927). The present series of experiments examined whether naloxone has the same effect when conditioning occurs in the absence of US exposure. In Experiments 1a and 1b, rats were exposed to tone-shock pairings in stage 1 (one trial per day for 4 days) and then to pairings of an initially neutral light with the already conditioned tone in stage 2 (one trial per day for 4 days). Experiment 1a confirmed that this training results in second-order fear of the light; and Experiment 1b showed that naloxone enhances this conditioning: rats injected with naloxone in stage 2 froze more than vehicle-injected controls when tested with the light alone (drug-free). In Experiments 2a and 2b, rats were exposed to light-tone pairings in stage 1 (one trial per day for 4 days) and then to tone-shock pairings in stage 2 (one trial per day for 2 days). Experiment 2a confirmed that this training results in sensory preconditioned fear of the light; and Experiment 2b showed that naloxone enhances sensory preconditioning when injected prior to each of the light-tone pairings: rats injected with naloxone in stage 1 froze more than vehicle-injected controls when tested with the light alone (drug-free). These results were taken to mean that naloxone enhances fear conditioning independently of its effect on US processing; and more generally, that opioids regulate the error-correction mechanisms that underlie associative formation.

Tamoxifen offers long-term neuroprotection after hippocampal silent infarct in male rats

Silent infarcts (SI) are a cerebral small vessel disease characterized by small subcortical infarcts. These occur in the absence of typical ischemia symptoms but are linked to cognitive decline and dementia. While there are no approved treatments for SI, recent results from our laboratory suggest that tamoxifen, a selective estrogen receptor modulator, is a viable candidate. In the present study, we induced SI in the dorsal hippocampal CA1 region of rats and assessed the effects of systemic administration of tamoxifen (5 mg/kg, twice) 21 days after injury on cognitive and pathophysiological measures, including cell loss, apoptosis, gliosis and estrogen receptors (ERs). We found that tamoxifen protected against the SI-induced cognitive dysfunction on the hippocampal-dependent, place recognition task, cell and ER loss, and increased apoptosis and gliosis in the CA1. Exploratory data analyses using a scatterplot matrix and principal component analysis indicated that SI-tamoxifen rats were indistinguishable from sham controls while they differed from SI rats, who were characterized by enhanced cell loss, apoptosis and gliosis, lower ERs, and recognition memory deficit. Supervised machine learning using support vector machine (SVM) determined predictors of progression from the early ischemic state to the dementia-like state. It showed that caspase-3 and ERα in the CA1 and exploration proportion were reliable and accurate predictors of this progression. Importantly, tamoxifen ameliorated SI-induced effects on all three of these variables, providing further evidence for its viability as a candidate treatment for SI and prevention of associated dementia.

The selective estrogen receptor modulator tamoxifen protects against subtle cognitive decline and early markers of injury 24 h after hippocampal silent infarct in male Sprague-Dawley rats

Silent infarcts (SI) are subcortical cerebral infarcts occurring in the absence of typical ischemia symptoms and are linked to cognitive decline and dementia development. There are no approved treatments for SI. One potential treatment is tamoxifen, a selective estrogen receptor modulator. It is critical to establish whether treatments effectively target the early consequences of SI to avoid progression to complete injury. We induced SI in the dorsal hippocampal CA1 of rats and assessed whether tamoxifen is protective 24 h later against cognitive deficits and injury responses including gliosis, apoptosis, inflammation and changes in estrogen receptors (ERs). SI led to subtle cognitive impairment on the object place task, an effect ameliorated by tamoxifen administration. SI did not lead to detectable hippocampal cell loss but increased apoptosis, astrogliosis, microgliosis and inflammation. Tamoxifen protected against the effects of SI on all measures except microgliosis. SI increased ERα and decreased ERβ in the hippocampus, which were mitigated by tamoxifen. Exploratory data analyses using scatterplot matrices and principal component analysis indicated that SI rats given tamoxifen were indistinguishable from controls. Further, SI rats were significantly different from all other groups, an effect associated with low levels of ERα and increased apoptosis, gliosis, inflammation, ERβ, and time spent with the unmoved object. The results demonstrate that tamoxifen is protective against the early cellular and cognitive consequences of hippocampal SI 24 h after injury. Tamoxifen mitigates apoptosis, gliosis, and inflammation and normalization of ER levels in the CA1, leading to improved cognitive outcomes after hippocampal SI.

Acquisition and extinction of second-order context conditioned fear: Role of the amygdala

Second-order fear conditioning has been demonstrated in protocols using discrete and simple stimuli, and much is now known about its behavioral and neural characteristics. In contrast, the mechanisms of second-order conditioning to more complex stimuli, such as contexts, are unknown. To address this gap in our knowledge, we conducted a series of experiments to investigate the neural and behavioral characteristics of second-order context fear conditioning in rats. We found that rats acquire fear to a context in which a first-order conditioned stimulus is presented (Experiment 1); neuronal activity in the basolateral amygdala (BLA) is required for the acquisition (Experiment 2) and extinction (Experiment 3) of second-order context fear; second-order context fear can be reduced by extinction of its first-order conditioned stimulus associate (Experiment 4); and that second-order fear reduced in this way is restored when fear of the first-order conditioned stimulus spontaneously recovers or is reconditioned (Experiment 5). Thus, second-order context fear requires neuronal activity in the BLA, and once established, tracks the level of fear to its first-order conditioned stimulus-associate. These results are discussed with respect to the substrates of second-order fear conditioning in other protocols, and the role of the amygdala in different forms of conditioning.

The separate and combined effects of a dangerous context and an epinephrine injection on sensory preconditioning in rats

Four experiments examined the effects of a dangerous context and a systemic epinephrine injection on sensory preconditioning in rats. In each experiment, rats were exposed to presentations of a tone and light in stage 1, light-shock pairings in stage 2, and test presentations of the tone alone and light alone in stage 3. Presentations of the tone and light in stage 1 occurred in either a safe or a previously shocked context, and/or under a systemic injection of epinephrine. Experiment 1 showed that a trace interval of 20 sec between presentations of the tone and light produced sensory preconditioning of the tone in a previously shocked context but not in a safe context, while experiment 2 provided evidence that this trace preconditioning was associative, due to the formation of a tone-light association. Experiment 3 showed that, in a safe context, exposure to the trace protocol under the influence of an epinephrine injection also produced sensory preconditioning of the tone, while experiment 4 provided evidence that a shocked context and an epinephrine injection have additive effects on trace preconditioning. These findings are discussed in relation to theories of trace conditioning. They suggest that the release of epinephrine by danger enhances attention and/or working memory processes, and thereby associative formation across a trace interval.

Latent inhibition is facilitated when a target stimulus is preexposed in compound with a nontarget stimulus, but only when the two stimuli coterminate: A test of the Hall-Rodriguez theory

The Hall-Rodriguez (Hall & Rodriguez, 2010) theory predicts that latent inhibition can be facilitated when a target stimulus is preexposed in compound with a second, nontarget stimulus: specifically, latent inhibition will be facilitated when the target coterminates with the second stimulus in preexposure, but facilitation will fail to occur when the two stimuli do not coterminate. The present study tested these predictions. In each experiment, rats were preexposed to a 30 s target stimulus alone or in compound with a second stimulus across its final 10 s, or they were preexposed to the context. All rats were then exposed to pairings of the target stimulus and foot shock, and finally, tested for freezing to the target. Experiment 1 demonstrated standard latent inhibition. Experiment 2 provided evidence that preexposure to a 30 s auditory target stimulus, in compound with a visual stimulus across its final 10 s, produced facilitated latent inhibition. Experiment 3 demonstrated that the latent inhibition was also facilitated when each 30 s presentation of a target visual stimulus was compounded with an auditory stimulus across its final 10 s. Experiment 4 showed that facilitation did not occur when each 30 s presentation of the target was compounded with a second stimulus across its initial 10 s, while Experiment 5 found that latent inhibition of the target was impaired when each of its 30 s presentations terminated in the onset of a second (10 s) stimulus. These findings are consistent with the predictions of the Hall-Rodriguez theory. They confirm that the facilitation of latent inhibition depends on coterminations of target and nontarget stimuli in preexposure and, more generally, that the impact of a second stimulus on latent inhibition to a target depends on their temporal relation in preexposure. (PsycInfo Database Record (c) 2021 APA, all rights reserved).

Hippocampal silent infarct leads to subtle cognitive decline that is associated with inflammation and gliosis at twenty-four hours after injury in a rat model

Silent infarcts (SI) are subcortical cerebral infarcts that occur in the absence of clinical symptoms commonly associated with ischemia and are linked to dementia development. Little is known about the pathophysiology underlying the cognitive dysfunction associated with SI, and few studies have examined the early cellular responses and neurobiological underpinnings. We induced SI in adult male Sprague-Dawley rats using an infusion of endothelin-1 in the CA1 dorsal hippocampus. Twenty-four hours later, we assessed cognition using the hippocampal-dependent object place recognition task. We also examined whether the resulting cognitive effects were associated with common markers of ischemia, specifically cell and synapse loss, gliosis, and inflammation, using histology and immunohistochemistry. Hippocampal SI led to subtle cognitive impairment on the object place recognition task 24 -hs post-injury. This was characterized by a significant difference in exploration proportion relative to a pre-injury baseline and a positive association between time spent with both the moved and unmoved objects. SI did not result in any detectable cell or synaptophysin loss, but did increase apoptosis, gliosis and inflammation in the CA1. Principal component analysis indicated the main variables associated with hippocampal SI included increased time spent with the unmoved object, gliosis, apoptosis and inflammation as well as decreased exploration proportion and CA1 cells. Our data demonstrate that hippocampal SI can lead to cognitive dysfunction 24 -hs after injury. Further, this appears to be driven by early degenerative processes including apoptosis, gliosis and inflammation, suggesting that these may be targets for early interventions treating hippocampal SI and its cognitive consequences.

Treadmill exercise has minimal impact on obesogenic diet-related gut microbiome changes but alters adipose and hypothalamic gene expression in rats

Background

Exercise has been extensively utilised as an effective therapy for overweight- and obesity-associated changes that are linked to health complications. Several preclinical rodent studies have shown that treadmill exercise alongside an unhealthy diet improves metabolic health and microbiome composition. Furthermore, chronic exercise has been shown to alter hypothalamic and adipose tissue gene expression in diet-induced obesity. However, limited work has investigated whether treadmill exercise commenced following exposure to an obesogenic diet is sufficient to alter microbiome composition and metabolic health.

Methods

To address this gap in the literature, we fed rats a high-fat/high-sugar western-style cafeteria diet and assessed the effects of 4 weeks of treadmill exercise on adiposity, diet-induced gut dysbiosis, as well as hypothalamic and retroperitoneal white adipose tissue gene expression. Forty-eight male Sprague-Dawley rats were allocated to either regular chow or cafeteria diet and after 3 weeks half the rats on each diet were exposed to moderate treadmill exercise for 4 weeks while the remainder were exposed to a stationary treadmill.

Results

Microbial species diversity was uniquely reduced in exercising chow-fed rats, while microbiome composition was only changed by cafeteria diet. Despite limited effects of exercise on overall microbiome composition, exercise increased inferred microbial functions involved in metabolism, reduced fat mass, and altered adipose and hypothalamic gene expression. After controlling for diet and exercise, adipose Il6 expression and liver triglyceride concentrations were significantly associated with global microbiome composition.

Conclusions

Moderate treadmill exercise induced subtle microbiome composition changes in chow-fed rats but did not overcome the microbiome changes induced by prolonged exposure to cafeteria diet. Predicted metabolic function of the gut microbiome was increased by exercise. The effects of exercise on the microbiome may be modulated by obesity severity. Future work should investigate whether exercise in combination with microbiome-modifying interventions can synergistically reduce diet- and obesity-associated comorbidities.

An application of Wagner's Standard Operating Procedures or Sometimes Opponent Processes (SOP) model to experimental extinction

The present study used simulations to examine whether Wagner's Standard Operating Procedures or Sometimes Opponent Processes (SOP) model explains various extinction phenomena. These included the so-called signature characteristics of extinction-renewal, reinstatement, and spontaneous recovery-as well as the effects on extinction of manipulations such as preexposure, the interval between extinction trials, the rate at which reinforcement ceases, and the presence of other stimuli. The simulations showed that SOP accounts for the effects of each of these manipulations. It does so for 2 reasons. First, the form of stimulus representation and rules for generating associative change mean that SOP can explain conditioning phenomena by appeal to changes in processing of both conditioned (CS) and unconditioned (US) stimuli, in contrast to other theories which confine changes in processing to either the CS (e.g., attentional theories) or the US (e.g., the Rescorla-Wagner model). Second, the processes that generate associative change in SOP are at least partially independent of those that generate performance. Hence, stimuli that differ in associative strength can extinguish at the same rate, and stimuli with equal associative strength can undergo different amounts of renewal, reinstatement or recovery. (PsycInfo Database Record (c) 2020 APA, all rights reserved).

Intermittent cafeteria diet identifies fecal microbiome changes as a predictor of spatial recognition memory impairment in female rats

Excessive consumption of diets high in saturated fat and sugar impairs short-term spatial recognition memory in both humans and rodents. Several studies have identified associations between the observed behavioral phenotype and diet-induced changes in adiposity, hippocampal gene expression of inflammatory and blood-brain barrier-related markers, and gut microbiome composition. However, the causal role of such variables in producing cognitive impairments remains unclear. As intermittent cafeteria diet access produces an intermediate phenotype, we contrasted continuous and intermittent diet access to identify specific changes in hippocampal gene expression and microbial species that underlie the cognitive impairment observed in rats fed continuous cafeteria diet. Female adult rats were fed either regular chow, continuous cafeteria diet, or intermittent cafeteria diet cycles (4 days regular chow and 3 days cafeteria) for 7 weeks (12 rats per group). Any cafeteria diet exposure affected metabolic health, hippocampal gene expression, and gut microbiota, but only continuous access impaired short-term spatial recognition memory. Multiple regression identified an operational taxonomic unit, from species Muribaculum intestinale, as a significant predictor of performance in the novel place recognition task. Thus, contrasting intermittent and continuous cafeteria diet exposure allowed us to identify specific changes in microbial species abundance and growth as potential underlying mechanisms relevant to diet-induced cognitive impairment.

The role of hippocampal estradiol in synaptic plasticity and memory: A systematic review

The consolidation of long-term memory is influenced by various neuromodulators. One of these is estradiol, a steroid hormone that is synthesized both in peripheral endocrine tissue and in the brain, including the hippocampus. Here, we examine the evidence regarding the role of estradiol in the hippocampus, specifically, in memory formation and its effects on the molecular mechanisms underlying synaptic plasticity. We conclude that estradiol improves memory consolidation and, thereby, long-term memory. Previous studies have shown that it does this in three, interconnected ways: (1) via functional changes in excitatory activity, (2) signaling changes in calcium dynamics, protein phosphorylation and protein expression, and (3) structural changes to synaptic morphology. Through a functional network analysis of proteins affected by estradiol, we identify potential protein-protein interactions that further support a role for estradiol in modulating synaptic plasticity as well as highlight signaling pathways that may be involved in these changes within the hippocampus.

The Conditions under Which Consolidation of Serial-Order Conditioned Fear Requires De Novo Protein Synthesis in the Basolateral Amygdala Complex

Consolidation of conditioned fear to a stimulus (S1) paired with shock requires de novo protein synthesis in the basolateral amygdala complex (BLA), whereas consolidation of conditioned fear to a stimulus (S2) paired with the fear-eliciting S1 requires DNA methylation but not de novo protein synthesis in the BLA. The present experiments merged these protocols by exposing rats to pairings of a serial S2-S1 compound and shock to examine if/when protein synthesis in the BLA is required to consolidate fear to S2. Rats received a BLA infusion of the protein synthesis inhibitor, cycloheximide, immediately after the S2-S1-shock session and were subsequently tested with S2. The infusion disrupted consolidation of fear to S2 when there had been no prior training of S1 (Experiment 1), the prior training had consisted of unpaired presentations of S1 and shock (Experiment 4), or in pairings of S1 and sucrose (Experiment 5). Consolidation of fear to S2 was unaffected by the infusion of cycloheximide but was disrupted by the DNA methyltransferase inhibitor, 5-AZA, when S1 had been previously fear-conditioned (Experiments 2a, 2b, and 3). These findings imply that what has already been learned about S1 determines the BLA processes that consolidate fear to S2. The already-fear-conditioned S1 blocks the S2-shock association that otherwise forms (and whose consolidation requires de novo protein synthesis in the BLA) while simultaneously acting as a learned source of danger for its S2 associate (whose consolidation requires DNA methylation but not de novo protein synthesis in the BLA).SIGNIFICANCE STATEMENT Protein synthesis is widely thought to be crucial for consolidating new learning into stable memories, including the consolidation of conditioned fear memories in the basolateral amygdala complex (BLA). However, our data provide clear evidence that the requirement for protein synthesis to consolidate conditioned fear in the BLA depends on an animal's previous training history, and the type of learning that is consolidated. Further, within the BLA, our data show that DNA methylation, and not protein synthesis, is necessary to consolidate higher-order conditioned fear, indicating that epigenetic mechanisms may provide a more fundamental mnemonic substrate.

The conditions that regulate formation of a false fear memory in rats

People and animals sometimes associate events that never occurred together. These false memories can have disastrous consequences, yet little is known about the conditions under which they form. In four experiments, we investigated how rats learn to fear a context in which they have never experienced danger (i.e., how they form a false context fear memory). In each experiment, rats were pre-exposed to a context on day 1, shocked in a similar-but-different context on day 2, and tested in the pre-exposed or explicitly-conditioned context on day 3. The results revealed that: (1) the true memory of the explicitly-conditioned context and false memory of the pre-exposed context develop simultaneously and independently; and (2) the conditions of pre-exposure on day 1 and time of shock exposure on day 2 interact to determine the strength of the false memory. These findings are anticipated by a recent computational model, the Bayesian Context Fear Algorithm/Automaton (BACON; Krasne, Cushman, & Fanselow, 2015). They are discussed in relation to this model and more general theories of context learning.

Oxytocin receptor activation in the basolateral complex of the amygdala enhances discrimination between discrete cues and promotes configural processing of cues

Oxytocin (OT) is a neuropeptide which influences the expression of social behavior and regulates its distribution according to the social context - OT is associated with increased pro-social effects in the absence of social threat and defensive aggression when threats are present. The present experiments investigated the effects of OT beyond that of social behavior by using a discriminative Pavlovian fear conditioning protocol with rats. In Experiment 1, an OT receptor agonist (TGOT) microinjected into the basolateral amygdala facilitated the discrimination between an auditory cue that signaled shock and another auditory cue that signaled the absence of shock. This TGOT-facilitated discrimination was replicated in a second experiment where the shocked and non-shocked auditory cues were accompanied by a common visual cue. Conditioned responding on probe trials of the auditory and visual elements indicated that TGOT administration produced a qualitative shift in the learning mechanisms underlying the discrimination between the two compounds. This was confirmed by comparisons between the present results and simulated predictions of elemental and configural associative learning models. Overall, the present findings demonstrate that the neuromodulatory effects of OT influence behavior outside of the social domain.

Baclofen acts in the central amygdala to reduce synaptic transmission and impair context fear conditioning

The two main sub-divisions of the Central amygdala (CeA), the lateral-capsular (CeA-LC) and the medial (CeA-M), contain extensive networks of inhibitory interneurons. We have previously shown that activation of GABA_B-receptors reduces excitatory transmission between axons of the pontine parabrachial nucleus and neurons of the CeA-LC by inhibiting glutamate release from presynaptic terminals¹³. Here we have characterised GABA_B-receptor activation on other excitatory and inhibitory projections within the CeA. Using whole-cell, patch-clamp recordings, we found that the GABA_B-receptor agonist baclofen significantly reduced excitatory and inhibitory transmission from all tested inputs into the CeA-LC and CeA-M. In all but one of the inputs, reductions in transmission were accompanied by an increase in paired pulse ratio, indicating that presynaptic GABA_B-receptors acted to reduce the release probability of synaptic vesicles. To examine the impact of GABA_B-receptors in the CeA on contextual fear-conditioning, we infused baclofen into the CeA immediately prior to training. Compared to vehicle-infused rats, baclofen-infused rats displayed significantly less freezing both during the final stages of the training period and at test 24 hours later. The results of this study demonstrate that, by suppressing excitatory and inhibitory transmission, activation of presynaptic GABA_B-receptors in the CeA inhibits the development of context conditioned fear.

Commonalities and Differences in the Substrates Underlying Consolidation of First- and Second-Order Conditioned Fear

Consolidation of newly formed fear memories requires a series of molecular events within the basolateral complex of the amygdala (BLA). Once consolidated, new information can be assimilated into these established associative networks to form higher-order associations. Much is known about the molecular events involved in consolidating newly acquired fear memories but little is known about the events that consolidate a secondary fear memory. Here, we show that, within the male rat BLA, DNA methylation and gene transcription are crucial for consolidating both the primary and secondary fear memories. We also show that consolidation of the primary, but not the secondary, fear memory requires de novo protein synthesis in the BLA. These findings show that consolidation of a fear memory and its updating to incorporate new information recruit distinct processes in the BLA, and suggest that DNA methylation in the BLA is fundamental to consolidation of both types of conditioned fear.SIGNIFICANCE STATEMENT Our data provide clear evidence that a different set of mechanisms mediate consolidation of learning about cues that signal learned sources of danger (i.e., second-order conditioned fear) compared with those involved in consolidation of learning about cues that signal innate sources of danger (i.e., first-order conditioned fear). These findings carry important implications because second-order learning could underlie aberrant fear-related behaviors (e.g., in anxiety disorders) as a consequence of neutral secondary cues being integrated into associative fear networks established through first-order pairings, and thereby becoming potent conditioned reinforcers and predictors of fear. Therefore, our data suggest that targeting such second-order conditioned triggers of fear may require pharmacological intervention different to that typically used for first-order conditioned cues.

α2-adrenoceptor-mediated inhibition in the central amygdala blocks fear-conditioning

The central amygdala is critical for the acquisition and expression of fear memories. This region receives a dense innervation from brainstem noradrenergic cell groups and has a high level of α₂-adrenoceptor expression. Using whole-cell electrophysiological recordings from rat brain slices, we characterise the role of pre-synaptic α₂-adrenoceptor in modulating discrete inhibitory and excitatory connections within both the lateral and medial division of the central amygdala. The selective α₂-adrenoceptor agonist clonidine blocked the excitatory input from the pontine parabrachial neurons onto neurons of the lateral central amygdala. In addition, clonidine blocked inhibitory connections from the medial paracapsular intercalated cell mass onto both lateral and medial central amygdala neurons. To examine the behavioural consequence of α₂-adrenoceptor-mediated inhibition of these inputs, we infused clonidine into the central amygdala prior to contextual fear-conditioning. In contrast to vehicle-infused rats, clonidine-infused animals displayed reduced levels of freezing 24 hours after training, despite showing no difference in freezing during the training session. These results reveal a role for α₂-adrenoceptors within the central amygdala in the modulation of synaptic transmission and the formation of fear-memories. In addition, they provide further evidence for a role of the central amygdala in fear-memory formation.

A dangerous context changes the way that rats learn about and discriminate between innocuous events in sensory preconditioning

Four experiments used a sensory preconditioning protocol to examine how a dangerous context influences learning about innocuous events. In Experiments 1, 2, and 3, rats were exposed to presentations of a tone followed immediately or 20-sec later by presentations of a light. These tone-light pairings occurred in a context that was either familiar and safe, or equally familiar but dangerous, that is, it was a context in which rats had been exposed to footshock. Rats were next exposed to parings of the light and shock and then tested with the tone (and light). The experiments showed that a dangerous context permits formation of a tone-light association under circumstances that preclude formation of that same association in a safe context (Experiments 1 and 2), and that this facilitative effect on associative formation depends on the content being currently dangerous rather than having been dangerous in the past (Experiment 3). Experiment 4 examined whether a dangerous context facilitates discrimination between two innocuous events. In a safe or dangerous context, rats were exposed to a tone that signaled the light and then to a white noise presented alone. Subsequent to conditioning of the light, the tests revealed that rats that had been exposed to these tone-light and white noise alone presentations in a dangerous context froze to the tone but not to the noise, whereas those exposed in a safe context froze to both the tone and the white noise. The results were related to previous evidence that the amygdala is critical for processing information about innocuous stimuli in a dangerous but not a safe context. They were attributed to an amygdala-based enhancement of arousal and/or attention in a dangerous context, hence the facilitation of associative formation and enhanced discriminability in this context.

Incentive contrast effects regulate responding to a flavor presented in compound with a saccharin unconditioned stimulus in rats

A flavor conditioned stimulus (conditional stimulus; CS) presented in simultaneous compound with a sweet-tasting unconditioned stimulus (US) acquires a certain sweetness and/or hedonic value. The present study examined whether responding to the flavor CS is influenced by postconditioning changes in the strength of the sweet US representation. In each experiment, rats were exposed to presentations of each of 2 flavors, A and B, in simultaneous compound with a 0.4% saccharin solution, and then tested with presentations of CS A in water. Experiments 1 and 2 showed that responding to CS A depended on its pairing with saccharin and increased with the training-to-test interval. Experiment 3 showed that a progressive reduction in the saccharin concentration of the trained compounds led to an increase in responding to CS A when tested in the absence of saccharin. Experiments 4 and 5 showed that, after a 7-day training-to-test delay, responding to CS A decreased following pretest exposure to a strong saccharin solution, and increased following pretest exposure to a very weak saccharin solution. These findings are taken to imply that incentive contrast effects regulate responding to a flavor CS. Hence, responding to the flavor CS increases with the training-to-test interval as the representation of the sweet US decays; decreases following pretest exposure to very sweet solutions as these reinstate the decayed sweet US representation (negative contrast); and increases following pretest exposure to weakly sweet solutions as these are perceived as less attractive than the CS itself (positive contrast). (PsycINFO Database Record

Daily Exposure to Sucrose Impairs Subsequent Learning About Food Cues: A Role for Alterations in Ghrelin Signaling and Dopamine D2 Receptors

The prevalence of hedonic foods and associated advertising slogans has contributed to the rise of the obesity epidemic in the modern world. Research has shown that intake of these foods disrupt dopaminergic systems. It may be that a disruption of these circuits produces aberrant learning about food-cue relationships. We found that rodents given 28 days of intermittent access to sucrose exhibited a deficit in the ability to block learning about a stimulus when it is paired in compound with food and another stimulus that has already been established as predictive of the food outcome. This deficit was characterized by an approach to a cue signaling food delivery that is usually blocked by prior learning, an effect dependent on dopaminergic prediction-error signaling in the midbrain. Administering the D2 agonist quinpirole during learning restored blocking in animals with a prior history of sucrose exposure. Further, repeated central infusions of ghrelin produced a deficit in blocking in the same manner as sucrose exposure. We argue that changes in dopaminergic systems resulting from sucrose exposure are mediated by a disruption of ghrelin signaling as rodents come to anticipate delivery of the highly palatable sucrose outside of normal feeding schedules. This suggestion is supported by our finding that both sucrose and ghrelin treatments resulted in increases in amphetamine-induced locomotor responding. Thus, for the first time, we have provided evidence of a potential link between alterations in D2 receptors caused by the intake of hedonic foods and aberrant learning about cue-food relationships capable of promoting inappropriate feeding habits. In addition, we have found preliminary evidence to suggest that this is mediated by changes in ghrelin signaling, a finding that should stimulate further research into modulation of ghrelin activity to treat obesity.

Counterconditioned fear responses exhibit greater renewal than extinguished fear responses

This series of experiments used rats to compare counterconditioning and extinction of conditioned fear responses (freezing) with respect to the effects of a context shift. In each experiment, a stimulus was paired with shock in context A, extinguished or counterconditioned through pairings with sucrose in context B, and then tested for renewal outside of context B. Counterconditioned fear responses exhibited greater ABA renewal than extinguished fear responses. This result was observed using a between-subjects design (Experiment 1) and a within-subject design in which counterconditioned and extinguished stimuli were equated in all respects other than their signaling of sucrose (Experiment 2). Counterconditioned fear responses also exhibited greater ABC renewal than extinguished fear responses (Experiment 3). This result was observed using a within-subject design in which context C was identical to context B in terms of its associative history, and when counterconditioned and extinguished CSs were tested in compounds matched for their association with both shock and sucrose (Experiment 4). These results are consistent with models which hold that context regulates expression of associations formed in counterconditioning and extinction, and allow the level of regulation to be greater following counterconditioning than extinction, as noted in previous studies.

An appetitive conditioned stimulus enhances fear acquisition and impairs fear extinction

Four experiments used between- and within-subject designs to examine appetitive–aversive interactions in rats. Experiments 1 and 2 examined the effect of an excitatory appetitive conditioned stimulus (CS) on acquisition and extinction of conditioned fear. In Experiment 1, a CS shocked in a compound with an appetitive excitor (i.e., a stimulus previously paired with sucrose) underwent greater fear conditioning than a CS shocked in a compound with a neutral stimulus. Conversely, in Experiment 2, a CS extinguished in a compound with an appetitive excitor underwent less extinction than a CS extinguished in a compound with a neutral stimulus. Experiments 3 and 4 compared the amount of fear conditioning to an appetitive excitor and a familiar but neutral target CS when the compound of these stimuli was paired with shock. In each experiment, more fear accrued to the appetitive excitor than to the neutral CS. These results show that an appetitive excitor influences acquisition and extinction of conditioned fear to a neutral CS and itself undergoes a greater associative change than the neutral CS across compound conditioning. They are discussed with respect to the role of motivational information in regulating an associative change in appetitive–aversive interactions.

Rats Fed a Diet Rich in Fats and Sugars Are Impaired in the Use of Spatial Geometry

A diet rich in fats and sugars is associated with cognitive deficits in people, and rodent models have shown that such a diet produces deficits on tasks assessing spatial learning and memory. Spatial navigation is guided by two distinct types of information: geometrical, such as distance and direction, and featural, such as luminance and pattern. To clarify the nature of diet-induced spatial impairments, we provided rats with standard chow supplemented with sugar water and a range of energy-rich foods eaten by people, and then we assessed their place- and object-recognition memory. Rats exposed to this diet performed comparably with control rats fed only chow on object recognition but worse on place recognition. This impairment on the place-recognition task was present after only a few days on the diet and persisted across tests. Critically, this spatial impairment was specific to the processing of distance and direction.

Integration of reward signalling and appetite regulating peptide systems in the control of food-cue responses

Understanding the neurobiological substrates that encode learning about food-associated cues and how those signals are modulated is of great clinical importance especially in light of the worldwide obesity problem. Inappropriate or maladaptive responses to food-associated cues can promote over-consumption, leading to excessive energy intake and weight gain. Chronic exposure to foods rich in fat and sugar alters the reinforcing value of foods and weakens inhibitory neural control, triggering learned, but maladaptive, associations between environmental cues and food rewards. Thus, responses to food-associated cues can promote cravings and food-seeking by activating mesocorticolimbic dopamine neurocircuitry, and exert physiological effects including salivation. These responses may be analogous to the cravings experienced by abstaining drug addicts that can trigger relapse into drug self-administration. Preventing cue-triggered eating may therefore reduce the over-consumption seen in obesity and binge-eating disorder. In this review we discuss recent research examining how cues associated with palatable foods can promote reward-based feeding behaviours and the potential involvement of appetite-regulating peptides including leptin, ghrelin, orexin and melanin concentrating hormone. These peptide signals interface with mesolimbic dopaminergic regions including the ventral tegmental area to modulate reactivity to cues associated with palatable foods. Thus, a novel target for anti-obesity therapeutics is to reduce non-homeostatic, reward driven eating behaviour, which can be triggered by environmental cues associated with highly palatable, fat and sugar rich foods.

False context fear memory in rats

Four experiments used rats to study false context fear memories. In Experiment 1, rats were pre-exposed to a distinctive chamber (context A) or to a control environment (context C), shocked after a delay in a second chamber (context B) and tested either in B or A. Rats pre-exposed to A froze just as much as control rats in B but more than control rats in A. In Experiment 2, rats were pre-exposed to A or C, subjected to an immediate shock in B and tested in B or A. Rats pre-exposed to A froze when tested in A but did not freeze when tested in B and control rats did not freeze in either A or B. The false fear memory to the pre-exposed A was contingent on its similarity with the shocked B. In Experiment 3, rats pre-exposed to A and subjected to immediate shock in B froze when tested in A but did not freeze when tested in C and rats pre-exposed to C did not freeze when tested either in A or C. In Experiment 4, rats pre-exposed to A and subjected to immediate shock in B froze more when tested in A than rats whose pre-exposure to A began with an immediate shock. The results were discussed in terms of a dual systems explanation of context fear conditioning: a hippocampal-dependent process that forms a unitary representation of context and an amygdala-based process which associates this representation with shock.

Oxytocin signaling in basolateral and central amygdala nuclei differentially regulates the acquisition, expression, and extinction of context-conditioned fear in rats

The present study investigated how oxytocin (OT) signaling in the central (CeA) and basolateral (BLA) amygdala affects acquisition, expression, and extinction of context-conditioned fear (freezing) in rats. In the first set of experiments, acquisition of fear to a shocked context was impaired by a preconditioning infusion of synthetic OT into the CeA (Experiment 1) or BLA (Experiment 2). In the second set of experiments, expression of context fear was enhanced by a pre- or post-extinction CeA infusion of synthetic OT (Experiments 3–6) or a selective OT receptor agonist, TGOT (Experiment 4). This enhancement of fear was blocked by coadministration of an OT receptor antagonist, OTA (Experiment 5) and context fear was suppressed by administration of the antagonist alone (Experiment 6). In the third set of experiments, expression of context fear was suppressed, not enhanced, by a preextinction BLA infusion of synthetic OT or a selective OT receptor agonist, TGOT (Experiments 7 and 8). This suppression of fear was blocked by coadministration of the OT receptor antagonist, OTA (Experiment 8). Taken together, these findings show that the involvement of the CeA and BLA in expression and extinction of context-conditioned fear is dissociable, and imply a critical role for oxytocin signaling in amygdala-based regulation of aversive learning.

Emotion perception after moderate-severe traumatic brain injury: The valence effect and the role of working memory, processing speed, and nonverbal reasoning

OBJECTIVE

Traumatic brain injury (TBI) impairs emotion perception. Perception of negative emotions (sadness, disgust, fear, and anger) is reportedly affected more than positive (happiness and surprise) ones. It has been argued that this reflects a specialized neural network underpinning negative emotions that is vulnerable to brain injury. However, studies typically do not equate for differential difficulty between emotions. We aimed to examine whether emotion recognition deficits in people with TBI were specific to negative emotions, while equating task difficulty, and to determine whether perception deficits might be accounted for by other cognitive processes.

METHOD

Twenty-seven people with TBI and 28 matched control participants identified 6 basic emotions at 2 levels of intensity (a) the conventional 100% intensity and (b) "equated intensity"-that is, an intensity that yielded comparable accuracy rates across emotions in controls.

RESULTS

(a) At 100% intensity, the TBI group was impaired in recognizing anger, fear, and disgust but not happiness, surprise, or sadness and performed worse on negative than positive emotions. (b) At equated intensity, the TBI group was poorer than controls overall but not differentially poorer in recognizing negative emotions. Although processing speed and nonverbal reasoning were associated with emotion accuracy, injury severity by itself was a unique predictor.

CONCLUSIONS

When task difficulty is taken into account, individuals with TBI show impairment in recognizing all facial emotions. There was no evidence for a specific impairment for negative emotions or any particular emotion. Impairment was accounted for by injury severity rather than being a secondary effect of reduced neuropsychological functioning.

Dietary-induced obesity disrupts trace fear conditioning and decreases hippocampal reelin expression

Both obesity and over-consumption of palatable high fat/high sugar "cafeteria" diets in rats has been shown to induce cognitive deficits in executive function, attention and spatial memory. Adult male Sprague-Dawley rats were fed a diet that supplemented standard lab chow with a range of palatable foods eaten by people for 8 weeks, or regular lab chow. Memory was assessed using a trace fear conditioning procedure, whereby a conditioned stimulus (CS) is presented for 10s and then 30s after its termination a foot shock (US) is delivered. We assessed freezing to the CS (flashing light) in a neutral context, and freezing in the context associated with footshock. A dissociation was observed between levels of freezing in the context and to the CS associated with footshock. Cafeteria diet fed rats froze less than control chow fed rats in the context associated with footshock (P<0.01), indicating that encoding of a hippocampus-dependent context representation was impaired in these rats. Conversely, cafeteria diet fed rats froze more (P<0.05) to the CS than chow fed rats, suggesting that when hippocampal function was compromised the cue was the best predictor of footshock, as contextual information was not encoded. Dorsal hippocampal mRNA expression of inflammatory and neuroplasticity markers was analysed at the end of the experiment, 10 weeks of diet. Of these, mRNA expression of reelin, which is known to be important in long term potentiation and neuronal plasticity, was significantly reduced in cafeteria diet fed rats (P=0.003). This implicates reductions in hippocampal plasticity in the contextual fear memory deficits seen in the cafeteria diet fed rats.

Appetitive context conditioning proactively, but transiently, interferes with expression of counterconditioned context fear

Four experiments used rats to study appetitive-aversive transfer. Rats trained to eat a palatable food in a distinctive context and shocked in that context ate and did not freeze when tested 1 d later but froze and did not eat when tested 14 d later. These results were associatively mediated (Experiments 1 and 2), observed when rats were or were not food deprived (Experiments 1 and 2), and were not due to latent inhibition (Experiment 3). In contrast, rats trained to eat in the context and shocked there 13 d later froze and did not eat when tested 1 d after the shocked exposure. However, rats that received an additional eating session in the context 1 d before the shocked exposure ate and did not freeze when tested 1 d after the shocked exposure (Experiment 4). The results show that appetitive conditioning transiently interferes with aversive conditioning. They are discussed in terms of a weak context-shock association becoming stronger with the lapse of time (so-called fear incubation) or of the interference by the context-food association becoming weaker with the lapse of time.

Cafeteria diet impairs expression of sensory-specific satiety and stimulus-outcome learning

A range of animal and human data demonstrates that excessive consumption of palatable food leads to neuroadaptive responses in brain circuits underlying reward. Unrestrained consumption of palatable food has been shown to increase the reinforcing value of food and weaken inhibitory control; however, whether it impacts upon the sensory representations of palatable solutions has not been formally tested. These experiments sought to determine whether exposure to a cafeteria diet consisting of palatable high fat foods impacts upon the ability of rats to learn about food-associated cues and the sensory properties of ingested foods. We found that rats fed a cafeteria diet for 2 weeks were impaired in the control of Pavlovian responding in accordance to the incentive value of palatable outcomes associated with auditory cues following devaluation by sensory-specific satiety. Sensory-specific satiety is one mechanism by which a diet containing different foods increases ingestion relative to one lacking variety. Hence, choosing to consume greater quantities of a range of foods may contribute to the current prevalence of obesity. We observed that rats fed a cafeteria diet for 2 weeks showed impaired sensory-specific satiety following consumption of a high calorie solution. The deficit in expression of sensory-specific satiety was also present 1 week following the withdrawal of cafeteria foods. Thus, exposure to obesogenic diets may impact upon neurocircuitry involved in motivated control of behavior.

Rats eat a cafeteria-style diet to excess but eat smaller amounts and less frequently when tested with chow

Background

Obesity is associated with excessive consumption of palatable, energy dense foods. The present study used an animal model to examine feeding patterns during exposure to and withdrawal from these foods.

Methods

Male Sprague Dawley rats were exposed to standard lab chow only (Chow rats) or a range of cafeteria-style foods eaten by people (Caf rats). After 1, 4, 7 and 10 weeks of diet in their home cage, rats were subjected to 24-hour test sessions in a Comprehensive Lab Animal Monitoring System (CLAMS). In the first two test sessions, Chow rats were exposed to standard lab chow only while Caf rats were exposed to a biscuit and high-fat chow diet. In the final two test sessions, half the rats in each group were switched to the opposing diet. In each session we recorded numbers of bouts, energy consumed per bout, and intervals between bouts across the entire 24 hours.

Results

Relative to Chow rats, Caf rats initiated fewer bouts but consumed more energy per bout; however, their motivation to feed in the CLAMS declined over time, which was attributed to reduced variety of foods relative to their home cage diet. This decline in motivation was especially pronounced among Caf rats switched from the palatable CLAMS diet to standard lab chow only: the reduced energy intake in this group was due to a modest decline in bout frequency and a dramatic decline in bout size.

Conclusions

Exposure to a cafeteria-diet, rich in variety, altered feeding patterns, reduced rats' motivation to consume palatable foods in the absence of variety, and further diminished motivation to feed when palatable foods were withdrawn and replaced with chow. Hence, variety is a key factor in driving excessive consumption of energy dense foods, and therefore, excessive weight gain.

Psychological and neural mechanisms of experimental extinction: a selective review

The present review examines key psychological concepts in the study of experimental extinction and implications these have for an understanding of the underlying neurobiology of extinction learning. We suggest that many of the signature characteristics of extinction learning (spontaneous recovery, renewal, reinstatement, rapid reacquisition) can be accommodated by the standard associative learning theory assumption that extinction results in partial erasure of the original learning together with new inhibitory learning. Moreover, we consider recent behavioral and neural evidence that supports the partial erasure view of extinction, but also note shortcomings in our understanding of extinction circuits as these relate to the negative prediction error concept. Recent work suggests that common prediction error and stimulus-specific prediction error terms both may be required to explain neural plasticity both in acquisition and extinction learning. In addition, we suggest that many issues in the content of extinction learning have not been fully addressed in current research, but that neurobiological approaches should be especially helpful in addressing such issues. These include questions about the nature of extinction learning (excitatory CS-No US, inhibitory CS-US learning, occasion setting processes), especially as this relates to studies of the micro-circuitry of extinction, as well as its representational content (sensory, motivational, response). An additional understudied problem in extinction research is the role played by attention processes and their underlying neural networks, although some research and theory converge on the idea that extinction is accompanied by attention decrements (i.e., habituation-like processes).

A bout of voluntary running enhances context conditioned fear, its extinction, and its reconsolidation

Three experiments used rats to examine the effect of a single bout of voluntary activity (wheel running) on the acquisition, extinction, and reconsolidation of context conditioned fear. In Experiment 1, rats provided with access to a wheel for 3 h immediately before or after a shocked exposure to a context froze more when tested in that context than rats provided with access to the wheels 6 h after the shocked exposure or rats not provided with access to the wheels. In Experiment 2, rats provided with access to the wheels immediately before or after a nonshocked exposure to the conditioned context froze less when tested in that context than rats provided with access to the wheels 6 h after the nonshocked exposure or rats not provided with access to the wheels. In Experiment 3, rats provided with access to wheels immediately after an extended nonshocked exposure to the conditioned context again froze less, whereas rats provided with access to the wheels after a brief nonshocked exposure froze more on the subsequent test than sedentary controls. These results show that a single bout of running can enhance acquisition, extinction, and reconsolidation of context conditioned fear.