Rats depend on habit memory for discrimination learning and retention

Learning to learn: Single session acquisition of new rules by freely moving mice

Learning from examples and adapting to new circumstances are fundamental attributes of human cognition. However, it is unclear what conditions allow for fast and successful learning, especially in nonhuman subjects. To determine how rapidly freely moving mice can learn a new discrimination criterion (DC), we design a two-alternative forced-choice visual discrimination paradigm in which the DCs governing the task can change between sessions. We find that experienced animals can learn a new DC after being exposed to only five training and three testing trials. The propensity for single session learning improves over time and is accurately predicted based on animal experience and criterion difficulty. After establishing the procedural learning of a paradigm, mice continuously improve their performance in new circumstances. Thus, mice learn to learn.

Hippocampal damage causes retrograde amnesia for objects' visual, but not odour, properties in male rats

Damage to the hippocampus produces profound retrograde amnesia, but odour and object discrimination memories can be spared in the retrograde direction. Prior lesion studies testing retrograde amnesia for object/odour discriminations are problematic due to sparing of large parts of the hippocampus, which may support memory recall, and/or the presence of uncontrolled, distinctive odours that may support object discrimination. To address these issues, we used a simple object discrimination test to assess memory in male rats. Two visually distinct objects, paired with distinct odour cues, were presented. One object was associated with a reward. Following training, neurotoxic hippocampal lesions were made using N-methyl-D-aspartate (NMDA). The rats were then tested on the preoperatively learned object discrimination problem, with and without the availability of odour or visual cues during testing. The rats were also postoperatively trained on a new object discrimination problem. Lesion sizes ranged from 67% to 97% of the hippocampus (average of 87%). On the preoperatively learned discrimination problem, the rats with hippocampal lesions showed preserved object discrimination memory when tested in the dark (i.e., without visual cues) but not when the explicit odour cues were removed from the objects. Hippocampal lesions increased the number of trials required to reach criterion but did not prevent rats from solving the postoperatively learned discrimination problem. Our results support the idea that long-term memories for odours, unlike recall of visual properties of objects, do not depend on the hippocampus in rats, consistent with previous observations that hippocampal damage does not cause retrograde amnesia for odour memories.

The Effect of Chronic Alcohol on Cognitive Decline: Do Variations in Methodology Impact Study Outcome? An Overview of Research From the Past 5 Years

Excessive alcohol use is often associated with accelerated cognitive decline, and extensive research using animal models of human alcohol consumption has been conducted into potential mechanisms for this relationship. Within this literature there is considerable variability in the types of models used. For example, alcohol administration style (voluntary/forced), length and schedule of exposure and abstinence period are often substantially different between studies. In this review, we evaluate recent research into alcohol-induced cognitive decline according to methodology of alcohol access, as well as cognitive behavioral task employed. Our aim was to query whether the nature and severity of deficits observed may be impacted by the schedule and type of alcohol administration. We furthermore examined whether there is any apparent relationship between the amount of alcohol consumed and the severity of the deficit, as well as the potential impact of abstinence length, and other factors such as age of administration, and sex of subject. Over the past five years, researchers have overwhelmingly used non-voluntary methods of intake, however deficits are still found where intake is voluntary. Magnitude of intake and type of task seem most closely related to the likelihood of producing a deficit, however even this did not follow a consistent pattern. We highlight the importance of using systematic and clear reporting styles to facilitate consistency across the literature in this regard. We hope that this analysis will provide important insights into how experimental protocols might influence findings, and how different patterns of consumption are more or less likely to produce an addiction-vulnerable cognitive phenotype in animal models.

Methylphenidate augmentation of escitalopram to enhance adherence to antidepressant treatment: a pilot randomized controlled trial

BACKGROUND

Adherence to treatment, i.e. the extent to which a patient's therapeutic engagement coincides with the prescribed treatment, is among the most important problems in mental health care. The current study investigated the influence of pairing an acute positive reinforcing dopaminergic/noradrenergic effect (methylphenidate, MPH) with a standard antidepressant on the rates of adherence to medication treatment. The primary objective of this study was to determine whether MPH + escitalopram resulted in higher rates of medication adherence relative to placebo + escitalopram.

METHODS

Twenty participants with moderate to severe depression were 1-1 randomized to either (1) 5 mg MPH + 10 mg escitalopram or (2) placebo + 10 mg escitalopram with the possibility for a dose increase at 4 weeks. A Bayesian analysis was conducted to evaluate the outcomes.

RESULTS

First, neither percent Pill count nor Medication Electronic Monitoring System adherence showed that MPH was superior to placebo. In fact, placebo showed slightly higher adherence rates on the primary (7.82% better than MPH) and secondary (7.07% better than MPH) outcomes. There was a less than 25% chance of MPH augmentation showing at least as good or better adherence than placebo. Second, both groups showed a significant effect of treatment on the QIDS-SR with a median effect of an 8.6-point score reduction. Third, neither subjective measures of adherence attitudes nor socio-demographic covariates had a significant influence on the primary or secondary outcome variables.

CONCLUSIONS

These data do not support the use of MPH to increase adherence to antidepressant medication in individuals with moderate to severe depression. CLINICALTRIALS.

GOV IDENTIFIER

NCT03388164 , registered on 01/02/2018.

Intact Behavioral Expression of Contextual Fear, Context Discrimination, and Object Discrimination Memories Acquired in the Absence of the Hippocampus

We test the hypothesis that the stability and precision of context and visual discrimination memories depend on interactions between the hippocampus (HPC) and other memory storage networks. In four experiments we tested the properties of memories acquired in the absence of the HPC. Long-Evans male rats were exclusively used in all experiments. Experiment 1 evaluated acquisition and retention of context fear memories in rats with prior partial or complete HPC damage. Confirming an earlier report (Zelikowsky et al., 2012) a very small but statistically reliable slowing in a single session of context fear conditioning was found after HPC damage. In contrast, retention of context fear memory was normal after HPC damage up to 30 d after learning. In experiment 2, we found that discrimination between a context paired with foot shocks and a different context never paired with foot shock was retained normally for 15 d. In experiment 3, we replicated the finding of intact context discrimination for at least 15 d in rats who display a significant impairment in acquisition of place learning in the Morris water task (MWT). In final experiment using an appetitive object discrimination task, we showed normal retention of the discrimination for at least 30 d after training in rats with complete HPC damage. These finding score against the idea that non HPC memory storage requires a period of interaction with HPC to establish a stable, precise memory.SIGNIFICANCE STATEMENT Contrary to expectations from systems memory consolidation, we find that in the absence of a functional hippocampus (HPC) context and visual memories are formed rapidly and exhibit normal persistence and precision. The findings suggest that the HPC is not obligatory for these features of long-term memories.

Organisation of cingulum bundle fibres connecting the anterior thalamic nuclei with the rodent anterior cingulate and retrosplenial cortices

Despite considerable interest in the properties of the cingulum bundle, descriptions of the composition of this major pathway in the rodent brain have not kept pace with advances in tract tracing. Using complementary approaches in rats and mice, this study examined the dense, reciprocal connections the anterior thalamic nuclei have with the cingulate and retrosplenial cortices, connections thought to be major contributors to the rodent cingulum bundle. The rat data came from a mixture of fluorescent and viral tracers, some injected directly into the bundle. The mouse data were collated from the Allen Mouse Brain Atlas. The projections from the three major anterior thalamic nuclei occupied much of the external medullary stratum of the cingulum bundle, where they were concentrated in its more medial portions. These anterior thalamic projections formed a rostral-reaching basket of efferents prior to joining the cingulum bundle, with anteromedial efferents taking the most rostral routes, often reaching the genu of the corpus callosum, while anterodorsal efferents took the least rostral route. In contrast, the return cortico-anterior thalamic projections frequently crossed directly through the bundle or briefly joined the internal stratum of the cingulum bundle, often entering the internal capsule before reaching the anterior thalamus. These analyses confirm that anterior thalamic connections comprise an important component of the rodent cingulum bundle, while also demonstrating the very different routes used by thalamo-cortical and cortico-thalamic projections. This information reveals how the composition of the cingulum bundle alters along its length.

Habits, Quick and Easy: Perceived Complexity Moderates the Associations of Contextual Stability and Rewards With Behavioral Automaticity

BACKGROUND

Habits have been proposed to develop as a function of the extent to which a behavior is rewarded, performed frequently, and executed in a stable context. The present study examines how each of these factors are associated with behavioral automaticity across a broad variety of behaviors drawn from previous habits research. This study further assesses how perceived complexity of the behavior influences the associations of rewards, frequency, and contextual stability with automaticity.

METHODS

Participants (N = 459) completed an online survey assessing their experiences and engagement with 25 different behaviors, including exercise, handwashing, smoking, and medication adherence, among others. Exploratory factor analysis validated a short, relatively novel scale of perceived behavioral complexity, and multilevel analyses grouped by participant were used to examine the factors that contribute to automaticity.

RESULTS

Across behaviors, frequency, contextual stability, and perceived rewards were positively associated with automaticity. Perceived complexity was negatively associated with automaticity and moderated the influence of contextual stability and rewards, but not frequency, on automaticity. Both contextual stability and rewards were stronger predictors of automaticity when behavioral complexity was high rather than low, as predicted; in addition, when contextual stability was high, more complex behaviors showed greater automaticity than simpler behaviors.

CONCLUSION

The results of this study confirm that behavioral frequency, rewards, and contextual stability are each independently associated with automaticity across a spectrum of behaviors. This study further demonstrates that perceived complexity of a behavior moderates the extent to which contextual stability and rewards are associated with automaticity. The results affirm a need to further understand the components of habits and how they differ across varying behaviors.

Exploratory study of the impact of perceived reward on habit formation

Background

Habits (learned automatic responses to contextual cues) are considered important in sustaining health behaviour change. While habit formation is promoted by repeating behaviour in a stable context, little is known about what other variables may contribute, and whether there are variables which may accelerate the habit formation process. The aim of this study was to explore variables relating to the perceived reward value of behaviour – pleasure, perceived utility, perceived benefits, and intrinsic motivation. The paper tests whether reward has an impact on habit formation which is mediated by behavioural repetition, and whether reward moderates the relationship between repetition and habit formation.

Methods

Habit formation for flossing and vitamin C tablet adherence was investigated in the general public following an intervention, using a longitudinal, single-group design. Of a total sample of 118 participants, 80 received an online vitamin C intervention at baseline, and all 118 received a face-to-face flossing intervention four weeks later. Behaviour, habit, intention, context stability (whether the behaviour was conducted in the same place and point in routine every time), and reward variables were self-reported every four weeks, for sixteen weeks. Structured equation modelling was used to model reward-related variables as predictors of intention, repetition, and habit, and as moderators of the repetition-habit relationship.

Results

Habit strength and behaviour increased for both target behaviours. Intrinsic motivation and pleasure moderated the relationship between behavioural repetition and habit. Neither perceived utility nor perceived benefits predicted behaviour nor interacted with repetition. Limited support was obtained for the mediation hypothesis. Strong intentions unexpectedly weakened the repetition-habit relationship. Context stability mediated and for vitamin C, also moderated the repetition-habit relationship.

Conclusions

Pleasure and intrinsic motivation can aid habit formation through promoting greater increase in habit strength per behaviour repetition. Perceived reward can therefore reinforce habits, beyond the impact of reward upon repetition. Habit-formation interventions may be most successful where target behaviours are pleasurable or intrinsically valued.

Electronic supplementary material

The online version of this article (10.1186/s40359-018-0270-z) contains supplementary material, which is available to authorized users.

Three Scientific Controversies to Engage Students in Reading Primary Literature

In this review, three sets of papers are presented. Each of the sets presents a historical or active controversy in neuroscience ranging from cell biology and cell signaling, to developmental neuroscience, to cognitive neuroscience. The first set captures a historical controversy about whether the beta/gamma subunit of G-proteins can be active in opening ion channels. The second set represents a modern instantiation of the oldest debate in neuroscience: are our minds and brains the product of innate factors or environmental influences. This debate plays out in a series of papers on the development of the visual system. The third set contrasts the view that the hippocampus and surrounding structures primarily function to represent our location in space (a position for which the 2014 Nobel Prize in Physiology or Medicine was awarded to three investigators) with the perspective that the hippocampus is a general-purpose structure for declarative memories, spatial or non-spatial. The first and third controversies feature publications of virtually identical experiments that show opposing results. All three controversies are discussed in regards to the individual scientists who did the experiments and debated directly with each other. The first (beta/gamma subunits) emphasizes the value of reproducibility in scientific research, the second (visual cortex development) emphasizes the value of new techniques and updating scientific models, and the third (hippocampus) exposes students to an ongoing, albeit under discussed, debate.

Differential effect of treadmill exercise on histone deacetylase activity in rat striatum at different stages of development

The study described herein aimed to evaluate the impact of exercise on histone acetylation markers in striatum from Wistar rats at different stages of development. Male Wistar rats were submitted to two different exercise protocols: a single session of treadmill (running 20 min) or a moderate daily exercise protocol (running 20 min for 2 weeks). Striata of rats aged 39 days postnatal (adolescents), 3 months (young adults), and 20 months (aged) were used. The single exercise session induced persistent effects on global HDAC activity only in the adolescent group, given that exercised rats showed decreased HDAC activity 1 and 18 h after training, without effect on histone H4 acetylation levels. However, the moderate daily exercise did not alter any histone acetylation marker in adolescent and mature groups in any time point evaluated after training. In sum, our data suggest that exercise impacts striatal HDAC activity in an age- and protocol-dependent manner. Specifically, this response seems to be more evident during the adolescent period and might suffer a molecular adaptation in response to chronic training.

Kai Xin San aqueous extract improves Aβ1-40-induced cognitive deficits on adaptive behavior learning by enhancing memory-related molecules expression in the hippocampus

ETHNOPHARMACOLOGICAL RELEVANCE

Kai Xin San (KXS), a traditional formula of Chinese medicine, has been used to treat dementia.

AIM OF THE STUDY

The present study aimed to investigate its ameliorating effects on Aβ_1-40-induced cognitive impairment in rats using a series of novel reward-directed instrumental learning tasks, and to determine its possible mechanism of action.

MATERIALS AND METHODS

Rats were pretreated with KXS aqueous extract (0.72 and 1.44g/kg, p.o.) for 10 days, and were trained to gain reward reinforcement by lever pressing at the meantime. Thereafter, rats received a bilateral microinjection of Aβ_1-40 in CA1 regions of the hippocampus. Cognitive performance was evaluated with the goal directed (higher response ratio) and habit (visual signal discrimination and extinction) learning tasks, as well as on the levels of memory-related biochemical parameters and molecules.

RESULTS

Our findings first demonstrated that KXS can improve Aβ_1-40-induced amnesia in RDIL via enhancing the comprehension of action-outcome association and the utilization of cue information to guide behavior. Then, its ameliorating effects should be attributed to the modulation of memory-related molecules in the hippocampus.

CONCLUSION

In conclusion, KXS has the potential to prevent and/or delay the deterioration of cognitive impairment in AD.

Distinct cortico-striatal connections with subthalamic nucleus underlie facets of compulsivity

The capacity to flexibly respond to contextual changes is crucial to adapting to a dynamic environment. Compulsivity, or behavioural inflexibility, consists of heterogeneous subtypes with overlapping yet discrete neural substrates. The subthalamic nucleus (STN) mediates the switch from automatic to controlled processing to slow, break or stop behaviour when necessary. Rodent STN lesions or inactivation are linked with perseveration or repetitive, compulsive responding. However, there are few studies examining the role of latent STN-centric neural networks and compulsive behaviour in healthy individuals. We therefore aimed to characterize the relationship between measures of compulsivity (goal-directed and habit learning, perseveration, and self-reported obsessive - compulsive symptoms) and the intrinsic resting state network of the STN. We scanned 77 healthy controls using a multi-echo resting state functional MRI sequence analyzed using independent components analysis (ME-ICA) with enhanced signal-to-noise ratio to examine small subcortical structures. Goal directed model-based behaviour was associated with higher connectivity of STN with medial orbitofrontal cortex (mOFC) and ventral striatum (VS) and more habitual model-free learning was associated with STN connectivity with hippocampus and dorsal anterior cingulate cortex (ACC). Perseveration was associated with reduced connectivity between STN and premotor cortex and finally, higher obsessive -compulsive inventory scores were associated with reduced STN connectivity with dorsolateral prefrontal cortex (PF). We highlight unique contributions of diffuse cortico-striatal functional connections with STN in dissociable measures of compulsivity. These findings are relevant to the development of potential biomarkers of treatment response in neurosurgical procedures targeting the STN for neurological and psychiatric disorders.

Playful activity post-learning improves training performance in Labrador Retriever dogs (Canis lupus familiaris)

Situations that are emotional and arousing have an effect on cognitive performance. It is thought that beta adrenergic activation and the release of stress hormones enhance memory consolidation and lead to an increase in memorability of emotional events. This beneficial effect has been shown in humans, non-human primates and rodents. Techniques which could enhance memory for learning specific tasks would be highly valuable, especially in dogs, which are extensively trained to aid humans. A pseudo-randomized, counterbalanced, between subject study designs was utilised and 16 Labrador Retrievers ranging from 1 to 9years of age were trained in a 2-choice discrimination paradigm. After task acquisition, either a playful activity intervention (N=8) or a resting period (N=8) took place, lasting for 30min. A range of factors including age, sex, training experience and trials to criterion on each day was subjected to a multiple factor/covariate General Linear Model analysis. The results show that playful activity post-learning improved training performance evidenced by fewer trials needed to re-learn the task 24h after initial acquisition (playful activity group: mean number of trials 26, SD 6; resting group: mean number of trials 43, SD 19, effect size 1.2). Average heart rate, as a measure of arousal, during the intervention was significantly higher in the playful activity group (143beats/min, SD 16) versus the resting group (86beats/min, SD 19, P<0.001). Salivary cortisol did not significantly differ between groups during training, however a significant decrease (T: -4.1 P<0.01) was seen after the playful activity. To our knowledge this is the first evidence that posttraining activity may influence training performance in dogs.

The role of NPY in learning and memory

High levels of NPY expression in brain regions important for learning and memory together with its neuromodulatory and neurotrophic effects suggest a regulatory role for NPY in memory processes. Therefore it is not surprising that an increasing number of studies have provided evidence for NPY acting as a modulator of neuroplasticity, neurotransmission, and memory. Here these results are presented in relation to the types of memory affected by NPY and its receptors. NPY can exert both inhibitory and stimulatory effects on memory, depending on memory type and phase, dose applied, brain region, and NPY receptor subtypes. Thus NPY act as a resilience factor by impairing associative implicit memory after stressful and aversive events, as evident in models of fear conditioning, presumably via Y1 receptors in the amygdala and prefrontal cortex. In addition, NPY impairs acquisition but enhances consolidation and retention in models depending on spatial and discriminative types of associative explicit memory, presumably involving Y2 receptor-mediated regulations of hippocampal excitatory transmission. Moreover, spatial memory training leads to increased hippocampal NPY gene expression that together with Y1 receptor-mediated neurogenesis could constitute necessary steps in consolidation and long-term retention of spatial memory. Altogether, NPY-induced effects on learning and memory seem to be biphasic, anatomically and temporally differential, and in support of a modulatory role of NPY at keeping the system in balance. Obtaining further insight into memory-related effects of NPY could inspire the engineering of new therapeutics targeting diseases where impaired learning and memory are central elements.

Dorsolateral striatum implicated in the acquisition, but not expression, of immediate response learning in rodent submerged T-maze

Animals can use multiple strategies when learning about, and navigating within, their environment. Typically, in the frequently-studied food-rewarded T-maze, rats initially adopt a flexible, hippocampal-dependent place strategy. However, as learning progresses, rats switch to an automatic, striatal-dependent response strategy (Packard & McGaugh, 1996). Interestingly, in a similar but aversively motivating water-submerged T-maze, rats exhibit the opposite behavioral pattern, initially adopting a response strategy but switching to a place strategy with extended training (Asem & Holland, 2013). Here, we examined the effects of transient lidocaine inactivation of the dorsolateral striatum (DLS) on rats' acquisition and expression of place and response strategies in the submerged T-maze. DLS inactivation prior to probe tests had no effect on rats' initial expression of a response strategy nor on their transition to the use of a place strategy with further training. Nevertheless, in a second experiment using the same rats, identical inactivation parameters significantly affected performance in an appetitively motivating positive control task, which required a response strategy. Furthermore, in a third experiment, DLS inactivation prior to early learning trials interfered with the acquisition of the response strategy in the submerged T-maze. These differences in DLS inactivation effects across appetitive and aversive tasks support the view that task motivation plays crucial roles in guiding learning, memory, and behavior. Additionally, differences in DLS inactivation effects between tests of acquisition and expression suggest that the DLS is required during early acquisition but not expression of the response learning strategy.

Complex assessment of distinct cognitive impairments following ouabain injection into the rat dorsoloateral striatum

A stroke in humans may induce focal injury to the brain tissue resulting in various disabilities. Although motor deficits are the most discernible, cognitive impairments seem to be crucial for patients mental well-being. The current lack of effective treatments encourages scientists and clinicians to develop novel approaches. Before applying them in clinic, testing for safety and effectiveness in non-human models is necessary. Such animal model should include significant cognitive impairments resulting from brain lesion. We used ouabain stereotactic injection into the right dorsolateral striatum of male Wistar rats, and enriched environment housing. To confirm the brain injury before cognitive testing, rats were given a beam-walking task to evaluate the level of sensorimotor deficits. To determine the cognitive impairment after focal brain damage, rats underwent a set of selected tasks over an observation period of 30 days. Brain injury induced by ouabain significantly impaired the acquisition of the T-maze habit learning task, where 'win-stay' strategy rules were applied. The injured rats also showed significant deficits in the performance of the T-maze switching task, which involved shifting from multiple clues previously relevant to the only one important clue. Focal brain injury also significantly changed 'what--where' memory, tested in the object exploration task, in which a novel object consecutively appeared in the same place while the location of a familiar item was continuously changed. In conclusion, we developed an animal model of distinct cognitive impairments after focal brain injury that provides a convenient method to test the effectiveness of restorative therapies.

Tong Luo Jiu Nao ameliorates Aβ1-40-induced cognitive impairment on adaptive behavior learning by modulating ERK/CaMKII/CREB signaling in the hippocampus

BACKGROUND

Tong Luo Jiu Nao (TLJN), a modern formula of Chinese medicine extracts on the basis of Traditional Chinese Medicine theory, has been used to treat dementia. The present study aimed to investigate its ameliorating effects on Aβ1-40-induced cognitive impairment in rats using a series of novel reward-directed instrumental learning (RDIL) tasks, and to determine its possible mechanism of action.

METHODS

Rats were pretreated with TLJN extract (0.9 and 1.8 g/kg, p.o.) for 10 daysbefore surgery, and were trained to gain reward reinforcement by lever pressing at the meantime. Thereafter, rats received a bilateral microinjection of Aβ1-40 in CA1 regions of the hippocampus. Cognitive performance was evaluated with the goal directed (higher response ratio) and habit (visual signal discrimination and extinction) learning tasks, as well as on the levels of biochemical parameters and molecules.

RESULTS

Our findings first demonstrated that TLJN can improve Aβ1-40-induced amnesia in RDIL via enhancing the comprehension of action-outcome association and the utilization of cue information to guide behavior. Then, its ameliorating effects should attribute to the modulation of ERK/CaMKII/CREB signaling in the hippocampus.

CONCLUSION

TLJN can markedly enhance cognitions of Aβ1-40 microinjection animal model in adaptive behavioral tasks. It has the potential, possibly as complementary and alternative therapy, to prevent and/or delay the deterioration of cognitive impairment in AD.

Distinct neural representation in the dorsolateral, dorsomedial, and ventral parts of the striatum during fixed- and free-choice tasks

The striatum is a major input site of the basal ganglia, which play an essential role in decision making. Previous studies have suggested that subareas of the striatum have distinct roles: the dorsolateral striatum (DLS) functions in habitual action, the dorsomedial striatum (DMS) in goal-directed actions, and the ventral striatum (VS) in motivation. To elucidate distinctive functions of subregions of the striatum in decision making, we systematically investigated information represented by phasically active neurons in DLS, DMS, and VS. Rats performed two types of choice tasks: fixed- and free-choice tasks. In both tasks, rats were required to perform nose poking to either the left or right hole after cue-tone presentation. A food pellet was delivered probabilistically depending on the presented cue and the selected action. The reward probability was fixed in fixed-choice task and varied in a block-wise manner in free-choice task. We found the following: (1) when rats began the tasks, a majority of VS neurons increased their firing rates and information regarding task type and state value was most strongly represented in VS; (2) during action selection, information of action and action values was most strongly represented in DMS; (3) action-command information (action representation before action selection) was stronger in the fixed-choice task than in the free-choice task in both DLS and DMS; and (4) action-command information was strongest in DLS, particularly when the same choice was repeated. We propose a hypothesis of hierarchical reinforcement learning in the basal ganglia to coherently explain these results.

Touchscreen tasks in mice to demonstrate differences between hippocampal and striatal functions

In mammals, hippocampal and striatal regions are engaged in separable cognitive processes usually assessed through species-specific paradigms. To reconcile cognitive testing among species, translational advantages of the touchscreen-based automated method have been recently promoted. However, it remains undetermined whether similar neural substrates would be involved in such behavioral tasks both in humans and rodents. To address this question, the effects of hippocampal or dorso-striatal fiber-sparing lesions were first assessed in mice through a battery of tasks (experiment A) comprising the acquisition of two touchscreen paradigms, the Paired Associates Learning (dPAL) and Visuo-Motor Conditional Learning (VMCL) tasks, and a more classical T-maze alternation task. Additionally, we sought to determine whether post-acquisition hippocampal lesions would alter memory retrieval in the dPAL task (experiment B). Pre-training lesions of dorsal striatum caused major impairments in all paradigms. In contrast, pre-training hippocampal lesions disrupted the performance of animals trained in the T-maze assay, but spared the acquisition in touchscreen tasks. Nonetheless, post-training hippocampal lesions severely impacted the recall of the previously learned dPAL task. Altogether, our data show that, after having demonstrated their potential in genetically modified mice, touchscreens also reveal perfectly adapted to taxing functional implications of brain structures in mice by means of lesion approaches. Unlike its human counterpart requiring an intact hippocampus, the acquisition of the dPAL task requires the integrity of the dorsal striatum in mice. The hippocampus only later intervenes, when acquired information needs to be retrieved. Touchscreen assays may therefore be suited to study striatal- or hippocampal-dependent forms of learnings in mice.

Hippocampus, perirhinal cortex, and complex visual discriminations in rats and humans

Structures in the medial temporal lobe, including the hippocampus and perirhinal cortex, are known to be essential for the formation of long-term memory. Recent animal and human studies have investigated whether perirhinal cortex might also be important for visual perception. In our study, using a simultaneous oddity discrimination task, rats with perirhinal lesions were impaired and did not exhibit the normal preference for exploring the odd object. Notably, rats with hippocampal lesions exhibited the same impairment. Thus, the deficit is unlikely to illuminate functions attributed specifically to perirhinal cortex. Both lesion groups were able to acquire visual discriminations involving the same objects used in the oddity task. Patients with hippocampal damage or larger medial temporal lobe lesions were intact in a similar oddity task that allowed participants to explore objects quickly using eye movements. We suggest that humans were able to rely on an intact working memory capacity to perform this task, whereas rats (who moved slowly among the objects) needed to rely on long-term memory.

Executive function

Components of human executive function, like rule generation and selection in response to stimuli (attention set-shifting) or overcoming a habit (reversal learning), can be reliably modelled in rodents. The rodent paradigms are based upon tasks that assess cognitive flexibility in clinical populations and have been effective in distinguishing the neurobiological substrates and the underlying neurotransmitter systems relevant to executive function. A review of the literature on the attentional set-shifting task highlights a prominent role for the medial region of the prefrontal cortex in the ability to adapt to a new rule (extradimensional shift) while the orbitofrontal cortex has been associated with the reversal learning component of the task. In other paradigms specifically developed to examine reversal learning in rodents, the orbitofrontal cortex also plays a prominent role. Modulation of dopamine, serotonin, and glutamatergic receptors can disrupt executive function, a feature commonly exploited to develop concepts underlying psychiatric disorders. While these paradigms do have excellent translational construct validity, they have been less effective as predictive preclinical models for cognitive enhancers, especially for cognition in health subjects. Accordingly, a more diverse battery of tasks may be necessary to model normal human executive function in the rodent for drug development.

Re-evaluating the PCP challenge as a pre-clinical model of impaired cognitive flexibility in schizophrenia

NMDA-R antagonists are a popular translational pharmacological challenge to induce cognitive deficits associated with schizophrenia. Amongst their many cognitive and non-cognitive effects is an ability to impair cognitive flexibility in general, and reversal learning in particular. Here, we test the hypothesis that the NMDA-R antagonist phencyclidine when given acutely selectively effects reversal learning by simultaneously measuring reversal learning and baseline responding, or acquisition and baseline responding, under identical conditions. Animals were trained to simultaneously perform two different visual discriminations in a touch-screen equipped operant box. Accordingly the reward contingencies associated with one pair could be altered, while the second pair acted as an experimental control. As such, the effect of a manipulation on reversal learning, stimuli acquisition, or baseline responding can be more accurately evaluated through the use of a double visual discrimination. A similar approach was also used to investigate the influence of sub-chronic phencyclidine administration on cognitive flexibility. Phencyclidine (1mg/kg) given before testing caused a slowing in acquisition and reversal learning, while having a minimal effect on secondary measures. Sub-chronic phencyclidine administration had no significant effect on any of the measures used within this study. While acute phencyclidine impairs reversal learning, it is clear from these results that other aspects of cognition (learning/relearning) are also impaired, potentially questioning the specificity of acute phencyclidine in conjunction with reversal learning paradigms as a model of impaired cognitive flexibility.

Social discrimination of cage-mates and non-cage-mates by rats

The ability to discriminate between group-mates and non-group-mates likely underpins the occurrence of affiliative and aggressive behaviour towards 'in-group' and 'out-group' individuals. Here we present two experiments aimed at testing the ability of rats (Rattus norvegicus) to discriminate between cage-mate (CM: animals residing in the subject's home cage) and non-cage-mate (NCM) conspecifics. In experiment 1, rats were trained to discriminate between different exemplars of CM and NCM using a lever pressing task employing symmetrical reinforcement. Subjects did not reach performance criterion, but they did show some evidence of discrimination between the two types of stimuli. In experiment 2, we employed a digging task to determine if rats can discriminate between odour cues from CM and NCM presented simultaneously on two sand-filled bowls. Subjects reached performance criterion on the first pair of odours, and on three more different pairs of CM and NCM odours. The results of a reversal task, using a fifth pair of odours, indicate that the rats were using a common factor to discriminate between social cues from CM and NCM conspecifics, rather than learning each pair independently. Possible candidates include a group-specific odour cue, or the development of a CM/NCM category concept.

Exercise enhances memory consolidation in the aging brain

Exercise has been shown to reduce age-related losses in cognitive function including learning and memory, but the mechanisms underlying this effect remain poorly understood. Memory formation occurs in stages that include an initial acquisition phase, an intermediate labile phase, and then a process of consolidation which leads to long-term memory formation. An effective way to examine the mechanism by which exercise improves memory is to introduce the intervention (exercise), post-acquisition, making it possible to selectively examine memory storage and consolidation. Accordingly we evaluated the effects of post-trial exercise (10 min on a treadmill) on memory consolidation in aged canines both right after, an hour after, and 24 h after acute exercise training in concurrent discrimination, object location memory (OLM), and novel object recognition tasks. Our study shows that post-trial exercise facilitates memory function by improving memory consolidation in aged animals in a time-dependent manner. The improvements were significant at 24 h post-exercise and not right after or 1 h after exercise. Aged animals were also tested following chronic exercise (10 min/day for 14 consecutive days) on OLM or till criterion were reached (for reversal learning task). We found improvements from a chronic exercise design in both the object location and reversal learning tasks. Our studies suggest that mechanisms to improve overall consolidation and cognitive function remain accessible even with progressing age and can be re-engaged by both acute and chronic exercise.

Chronic scopolamine-injection-induced cognitive deficit on reward-directed instrumental learning in rat is associated with CREB signaling activity in the cerebral cortex and dorsal hippocampus

RATIONALE

Scopolamine, a nonselective muscarinic receptor antagonist, has been used in experimental animal models of dementia. It has been demonstrated to disrupt performances in a battery of behavioral tests. However, no attempt has been made to determine how scopolamine-treated animals would respond to a series of reward-directed instrumental learning (RDIL) tasks.

OBJECTIVES

The present study was designed to investigate the effects of chronic intraperitoneal injection of scopolamine in Wistar rats on RDIL, as well as on the expression of memory-related molecules in the dorsal hippocampus (DH) and cerebral cortex (CCx).

METHODS

The effects of the pretraining injection of scopolamine on the acquisition of instrumental response (experiment 1) were first investigated. Then, the effects of post-training manipulation on the maintenance of instrumental response and the responses to changes in contingency degradation and signal discrimination were assessed (experiment 2). Finally, the expression of cyclic AMP response element-binding protein (CREB), phosphorylated CREB, and brain-derived neurotrophic factor in the DH and CCx were examined using Western blotting and enzyme-linked immunosorbent assay.

RESULTS

The acquisition of instrumental conditioning is more vulnerable than its maintenance. The 3.0-mg/kg dose of scopolamine rendered rats unable to make adaptive changes in facing contingency degradation and correct responses in signal discrimination tasks. Furthermore, CREB signaling was inactivated by pretraining scopolamine treatment in both the DH and CCx. Nevertheless, this pathway was selectively suppressed by post-training treatment only in the CCx during memory reconsolidation.

CONCLUSIONS

The results suggest that scopolamine-induced cognitive deficits on RDIL are related to the distinguishing alteration of CREB signaling in the DH and CCx.

Factors affecting graded and ungraded memory loss following hippocampal lesions

This review evaluates three current theories--Standard Consolidation (Squire & Wixted, 2011), Overshadowing (Sutherland, Sparks, & Lehmann, 2010), and Multiple Trace-Transformation (Winocur, Moscovitch, & Bontempi, 2010)--in terms of their ability to account for the role of the hippocampus in recent and remote memory in animals. Evidence, based on consistent findings from tests of spatial memory and memory for acquired food preferences, favours the transformation account, but this conclusion is undermined by inconsistent results from studies that measured contextual fear memory, probably the most commonly used test of hippocampal involvement in anterograde and retrograde memory. Resolution of this issue may depend on exercising greater control over critical factors (e.g., contextual environment, amount of pre-exposure to the conditioning chamber, the number and distribution of foot-shocks) that can affect the representation of the memory shortly after learning and over the long-term. Research strategies aimed at characterizing the neural basis of long-term consolidation/transformation, as well as other outstanding issues are discussed.

Enhanced odor discrimination learning in aged Bax-KO mice

Throughout life, new neurons are continuously generated from subventricular zone and added to the olfactory bulb (OB). Because a subset of mature OB neurons undergoes spontaneous cell death, adult OB neurogenesis serves for the replacement of this cell loss. Spontaneous cell turnover should alter the neuronal circuits, but the significance of cell turnover on olfactory learning is yet poorly understood. In this study, we explored the olfactory learning behaviors of model mice showing (1) absence of cell death and cell addition (aged Bax-KO mice); (2) absence of cell death but presence of cell addition (young Bax-KO mice); or (3) presence cell death but absence of cell addition (surgical lesion of rostral migratory stream of neuroblasts). Interestingly, aged Bax-KO mice with no cell replacement acquired the ability to discriminate odor differences faster than WT littermates, whereas other model mice exhibited virtually normal learning ability. These results suggest that the cell replacement is necessary for the normal olfactory learning behavior, and the chronic perturbation of cell replacement may result in the imbalance of neural circuits driving unexpected enhancement of olfactory learning ability.

The dorsolateral periaqueductal gray and its role in mediating fear learning to life threatening events

The dorsolateral column of the periaqueductal gray (dlPAG) integrates aversive emotional experiences and represents an important site responding to life threatening situations, such as hypoxia, cardiac pain and predator threats. Previous studies have shown that the dorsal PAG also supports fear learning; and we have currently explored how the dlPAG influences associative learning. We have first shown that N-methyl-D-aspartate (NMDA) 100 pmol injection in the dlPAG works as a valuable unconditioned stimulus (US) for the acquisition of olfactory fear conditioning (OFC) using amyl acetate odor as conditioned stimulus (CS). Next, we revisited the ascending projections of the dlPAG to the thalamus and hypothalamus to reveal potential paths that could mediate associative learning during OFC. Accordingly, the most important ascending target of the dlPAG is the hypothalamic defensive circuit, and we were able to show that pharmacological inactivation using β-adrenoceptor blockade of the dorsal premammillary nucleus, the main exit way for the hypothalamic defensive circuit to thalamo-cortical circuits involved in fear learning, impaired the acquisition of the OFC promoted by NMDA stimulation of the dlPAG. Moreover, our tracing study revealed multiple parallel paths from the dlPAG to several thalamic targets linked to cortical-hippocampal-amygdalar circuits involved in fear learning. Overall, the results point to a major role of the dlPAG in the mediation of aversive associative learning via ascending projections to the medial hypothalamic defensive circuit, and perhaps, to other thalamic targets, as well. These results provide interesting perspectives to understand how life threatening events impact on fear learning, and should be useful to understand pathological fear memory encoding in anxiety disorders.

Novel odour recognition memory is independent of the hippocampus in rats

We examined the effects of hippocampal (HPC) damage on odour recognition memory, using a novel odour recognition task that was adapted from the more common novel object recognition task. Three separate experiments were conducted. In Experiment 1, we tested rats in novel odour recognition across different retention intervals (i.e. 15 min, 24 h, 1 week, 5 weeks). Given a single acquisition session, rats' performance deteriorated after 24 h, but given multiple acquisition sessions (i.e. four sessions over 2 days), rats were able to perform well after retention intervals up to 5 weeks. In Experiment 2, we examined the possible anterograde amnesic effects of HPC damage on novel odour recognition, finding that pre-training damage to the entire HPC failed to cause amnesia for retention delays extending up to 5 weeks. In Experiment 3, we examined whether post-training HPC damage would cause retrograde amnesia, but failed to find any evidence of an impairment. The combined results suggest that the neural network supporting odour recognition is independent of the HPC.

The involvement of Type II Neuregulin-1 in rat visuospatial learning and memory

The cognitive deficits observed in schizophrenia are considered a core feature of the disease. Neuregulin-1 is a risk gene for schizophrenia that is involved in many neurodevelopmental and synaptic plasticity-related processes relevant to schizophrenia. Here, we have utilized a rat model (Nrg1(Tn)), which is hypomorphic for the neuregulin-1 (Nrg1) gene, to test whether reduced Type II NRG1 in the rat brain leads to cognitive deficits relevant to schizophrenia. Wild-type and homozygous Nrg1(Tn) male rats were tested in memory tasks that evaluated spatial memory (Morris water maze) and visuospatial working and reference memory (Can Test). Nrg1(Tn) rats were not impaired on the Morris water maze, but did show a deficit in the appetitive visuospatial discrimination test. Nrg1(Tn) rats committed more reference and working memory errors in this test. These results indicate that decreased Type II NRG1 in the brain may lead to deficits in visuospatial learning and memory.

The pharmacological sensitivity of a touchscreen-based visual discrimination task in the rat using simple and perceptually challenging stimuli

RATIONALE

Cognitive testing with touchscreen-equipped operant boxes ('touchscreens') is becoming increasingly popular. Tasks, such as paired associate learning or reversal learning of visual stimuli, have the discrimination of visual stimuli as a fundamental component. However, the effect of drugs commonly used in the study of cognitive mechanisms has yet to be described in a visual discrimination.

OBJECTIVE

The objective of the study was to profile a range of psychoactive agents (glutamatergic, dopaminergic, and cholinergic agonists and antagonists) known to be important in cognitive processing on visual discrimination performance using a touch sensitive computer monitor.

METHODS

Male Lister Hooded rats were trained to a stable level of performance in a simple visual discrimination. In Experiment 1, the effect of MK-801, phencyclidine, memantine, dextroamphetamine sulphate (D-amphetamine) and scopolamine was assessed. In Experiment 2, the stimuli were blended together resulting in a perceptually more demanding discrimination and a reduction in accuracy. The rats used in Experiment 1 were then retested with these 'morphed' stimuli under the influence of the above compounds.

RESULTS

MK-801, PCP, and D-amphetamine induced selective deficits in accuracy in both versions of the task. In contrast, scopolamine and memantine produced non-selective deficits in accuracy. Morphing the stimuli reduced accuracy, but did not alter the observed behavioural profile after compound administration.

CONCLUSION

These data improve our understanding of the basic neuropharmacology of a visual discrimination in cognitive tests employing touchscreens and will aid in the interpretation of pharmacological studies with more cognitively demanding methodologies.

Functional interaction between the dorsal hippocampus and the striatum in visual discrimination learning

The hippocampus and the striatum have traditionally been considered as part of different and independent memory systems. However, there is evidence that supports a functional interaction between the hippocampus and the dorsal striatum at least in particular learning tasks. Here, we evaluated the functional contribution of both brain regions in a visual discrimination learning task using cytochrome c oxidase (CO) quantitative histochemistry. Compared with other brain metabolic mapping techniques, CO activity reflects steady-state neuronal energy demand. Rats were trained for 6 days in a water T-maze to find a hidden escape platform associated with an intramaze visual cue. A control group of animals swam for an equivalent amount of time compared as the trained group but without any escape platform available. After finishing the behavioral task, CO activity was measured in subdivisions of the dorsal hippocampus and the dorsal striatum in both groups. Results show significantly higher CO activity in the CA1 area and the dentate gyrus of the dorsal hippocampus in the trained rats compared with the control group. In addition, a significant negative functional cross-correlation between area CA1 of the dorsal hippocampus and the anterodorsal striatum was found. Our results support current theories on competitive interaction of different memory systems during visual discrimination learning.

Parallel associative processing in the dorsal striatum: segregation of stimulus-response and cognitive control subregions

Although evidence suggests that the dorsal striatum contributes to multiple learning and memory functions, there nevertheless remains considerable disagreement on the specific associative roles of different neuroanatomical subregions. We review evidence indicating that the dorsolateral striatum (DLS) is a substrate for stimulus-response habit formation - incremental strengthening of simple S-R bonds - via input from sensorimotor neocortex while the dorsomedial striatum (DMS) contributes to behavioral flexibility - the cognitive control of behavior - via prefrontal and limbic circuits engaged in relational and spatial information processing. The parallel circuits through dorsal striatum interact with incentive/affective motivational processing in the ventral striatum and portions of the prefrontal cortex leading to overt responding under specific testing conditions. Converging evidence obtained through a detailed task analysis and neurobehavioral assessment is beginning to illuminate striatal subregional interactions and relations to the rest of the mammalian brain.

Hippocampal damage produces retrograde but not anterograde amnesia for a cued location in a spontaneous exploratory task in rats

Performance in several memory tasks is known to be unaffected by hippocampal damage sustained before learning, but is severely disrupted if the same damage occurs after learning. Memories for preferred locations, or home bases, in exploratory tasks can be formed by rats with hippocampal damage, but it is unknown if the memory for a home base survives hippocampal damage. To examine this question, for 30 min each day for five consecutive days, rats explored a circular open field containing one local cue. By Day 5 the rats preferentially went directly to that location, spent the majority of their time at that location, made rapid direct trips to that location when returning from an excursion and so demonstrated that the location was a home base. Memory for the cued location was examined after a 24 h or 14-day interval with the cue removed. In Experiments 1 and 2, control rats and rats with prior N-methyl-D-aspartic acid hippocampal lesions demonstrated memory of the home base location by making direct trips to that location. In Experiment 3, rats that had first explored the open field and cue and then received hippocampal lesions showed no memory for the cued location. The absence of anterograde impairment vs. the presence of retrograde impairment for memory of a spatial home base confirms a role for the hippocampus in the retention of spatial memory acquired during exploration.

The representational-hierarchical view of amnesia: translation from animal to human

Animal research has, in our opinion, made an invaluable contribution to our understanding of human amnesia. In this article we summarise our and others' work in this area, focusing on a new view of amnesia we refer to as the representational-hierarchical view. According to this view-and in contrast to the prevailing paradigm in the field-the brain is best understood as a hierarchically organized continuum of representations, each of which is useful for a variety of cognitive functions. We focus our review on four visual discrimination paradigms that have been successfully translated into the human arena: configural concurrent discriminations, pair-wise "morph" discriminations, oddity discriminations, and configural oddity discriminations. The data from the animal studies are first reviewed, followed by illustrations of how the tasks have been utilized in human research. We then turn to the canonical impairment in animal models of amnesia, object recognition, and show how impairments in object recognition can be understood within the representational-hierarchical framework. This is followed by a discussion of predictions of the view related to classic issues in amnesia research, namely whether amnesia is due to a deficit of encoding, storage or retrieval, and the related issue of the role of interference in amnesia. Finally, we provide evidence from animal and human studies that even the hippocampus-almost universally regarded as a module for memory-may be better understood within the representational-hierarchical paradigm.

Hippocampus and retrograde amnesia in the rat model: a modest proposal for the situation of systems consolidation

The properties of retrograde amnesia after damage to the hippocampus have been explicated with some success using a rat model of human medial temporal lobe amnesia. We review the results of this experimental work with rats focusing on several areas of consensus in this growing literature. We evaluate the theoretically significant hypothesis that hippocampal retrograde amnesia normally exhibits a temporal gradient, affecting recent, but sparing remote memories. Surprisingly, the evidence does not provide much support for the idea that there is a lengthy process of systems consolidation following a learning episode. Instead, recent and remote memories tend to be equally affected. The extent of damage to the hippocampus is a significant factor in this work since it is likely that spared hippocampal tissue can support at least partial memory retrieval. With extensive hippocampal damage gradients are flat or, in the case of memory tasks with flavour/odour retrieval cues, the retrograde amnesia covers a period of about 1-3 days. There is consistent evidence that at the time of learning the hippocampus interferes with or overshadows memory acquisition by other systems. This contributes to the breadth and severity of retrograde amnesia relative to anterograde amnesia in the rat. The fact that multiple, distributed learning episodes can overcome this overshadowing is consistent with a parallel dual-store theory or a Distributed Reinstatement Theory in which each learning episode triggers a short period of memory replay that provides a brief hippocampal-dependent systems consolidation.

Object recognition memory and the rodent hippocampus

In rodents, the novel object recognition task (NOR) has become a benchmark task for assessing recognition memory. Yet, despite its widespread use, a consensus has not developed about which brain structures are important for task performance. We assessed both the anterograde and retrograde effects of hippocampal lesions on performance in the NOR task. Rats received 12 5-min exposures to two identical objects and then received either bilateral lesions of the hippocampus or sham surgery 1 d, 4 wk, or 8 wk after the final exposure. On a retention test 2 wk after surgery, the 1-d and 4-wk hippocampal lesion groups exhibited impaired object recognition memory. In contrast, the 8-wk hippocampal lesion group performed similarly to controls, and both groups exhibited a preference for the novel object. These same rats were then given four postoperative tests using unique object pairs and a 3-h delay between the exposure phase and the test phase. Hippocampal lesions produced moderate and reliable memory impairment. The results suggest that the hippocampus is important for object recognition memory.

Blockade of GABA(B) receptors completely reverses age-related learning impairment

Impaired cognitive functions are well-described in the aging process. GABA(B) antagonists can facilitate learning and memory in young subjects, but these agents have not been well-characterized in aging. Here we show a complete reversal of olfactory discrimination learning deficits in cognitively-impaired aged Fischer 344 rats using the GABA(B) antagonist CGP55845, such that drug treatment restored performance to that on par with young and cognitively-unimpaired aged subjects. There was no evidence that this improved learning was due to enhanced olfactory detection abilities produced by the drug. These results highlight the potential of targeting GABA(B) receptors to ameliorate age-related cognitive deficits and demonstrate the utility of olfactory discrimination learning as a preclinical model for testing novel therapies to improve cognitive functions in aging.

Caudate nucleus is critically involved in trace eyeblink conditioning

The basal ganglia are a collection of brain regions involved with motor planning and initiation. The major site of cortical and thalamic input into the basal ganglia network is the striatum, which includes a differentiated caudate nucleus (CN) and the putamen in rabbits. Trace eyeblink conditioning (EBC) is a forebrain-dependent associative learning task in which a stimulus-free time interval separates the presentation of a behaviorally neutral conditioned stimulus (CS) and a behaviorally salient unconditioned stimulus. We investigated whether the CN is essential for acquisition of trace EBC and whether learning-related changes in neuronal activity occur in the caudate nucleus during trace EBC. Bilateral lesions of the CN in rabbits prevent acquisition of trace EBC. In separate cohorts of rabbits, single-unit recordings showed that medium spiny neurons from regions shown to be critical by lesions display strong responses to the CS, especially in the initial days of training before acquisition. Cholinergic interneurons, or tonically active neurons, become responsive to the CS and show dramatic firing rate changes during the trace interval after learning criterion has been met. These data demonstrate that the CN is required for and involved in trace EBC.