Differential contribution of hippocampal circuits to appetitive and consummatory behaviors during operant conditioning of behaving mice

Cortical and subcortical activities during food rewards versus social interaction in rats

Balancing food foraging with social interaction is crucial for survival and reproduction in many species of mammals. We wanted to investigate the reward preferences in adult male rats by allowing them to lever-press for both food and social rewards (interaction with another rat), while their performance and electrophysiological activities were recorded. Local field potentials (LFPs) were analyzed across five neuroanatomical regions involved in reward processing, decision-making, and social behavior. Despite ad libitum food availability, rats consistently prioritized food. LFP analysis revealed a decrease in nucleus accumbens (NAc) spectral power following social interaction, accompanied by specific alterations in delta and theta bands within the medial prefrontal cortex (mPFC). The spectral power of LFPs delta and/or theta bands were different for the five selected regions following food reward vs. social interactions. Cross-frequency coupling analysis provided further insights, demonstrating dynamic changes in theta-to-gamma coupling during both food and social rewards, with distinct roles for slow- and fast-gamma frequencies. These findings shed light on the intricate neural processes underlying reward preferences and/or decision-making choices, highlighting the NAc's potential role in social reward processing, and the mPFC's involvement in modulating theta-gamma rhythms during reward-related decision-making.

Development of a Pain Measurement Device Using 3D Printing and Electronic Air Pressure Control

This article describes the design of a wireless pain monitor system, also known as a pain meter, which can be used to diagnose people with fibromyalgia. Background/Objectives: As the test should be done while a simultaneous Magnetic Resonance Imaging (MRI) scan is being performed on the patient to observe their brain activity, the device must not have metallic components. Methods: Solid modelling and additive manufacturing were used for the manufacture of the device, an electropneumatic control has also been defined, and several prototypes were manufactured and tested. The work focuses on the validation of the designed pain meter, built by Material Extrusion (MEX) technology in different materials and with different printers. The surface finishes and manufacturing tolerances of the critical parts were tested, and their suitability for the necessary functions is verified. Conclusions: A proper mechanical pain meter device has been designed to be used in fibromyalgia diagnosis without metal components nor wires, which is therefore compatible with simultaneous MRI.

Functional states of prelimbic and related circuits during the acquisition of a GO/noGO task in rats

GO/noGO tasks enable assessing decision-making processes and the ability to suppress a specific action according to the context. Here, rats had to discriminate between 2 visual stimuli (GO or noGO) shown on an iPad screen. The execution (for GO) or nonexecution (for noGO) of the selected action (to touch or not the visual display) were reinforced with food. The main goal was to record and to analyze local field potentials collected from cortical and subcortical structures when the visual stimuli were shown on the touch screen and during the subsequent activities. Rats were implanted with recording electrodes in the prelimbic cortex, primary motor cortex, nucleus accumbens septi, basolateral amygdala, dorsolateral and dorsomedial striatum, hippocampal CA1, and mediodorsal thalamic nucleus. Spectral analyses of the collected data demonstrate that the prelimbic cortex was selectively involved in the cognitive and motivational processing of the learning task but not in the execution of reward-directed behaviors. In addition, the other recorded structures presented specific tendencies to be involved in these 2 types of brain activity in response to the presentation of GO or noGO stimuli. Spectral analyses, spectrograms, and coherence between the recorded brain areas indicate their specific involvement in GO vs. noGO tasks.

Challenges and advanced concepts for the assessment of learning and memory function in mice

The mechanisms underlying the formation and retrieval of memories are still an active area of research and discussion. Manifold models have been proposed and refined over the years, with most assuming a dichotomy between memory processes involving non-conscious and conscious mechanisms. Despite our incomplete understanding of the underlying mechanisms, tests of memory and learning count among the most performed behavioral experiments. Here, we will discuss available protocols for testing learning and memory using the example of the most prevalent animal species in research, the laboratory mouse. A wide range of protocols has been developed in mice to test, e.g., object recognition, spatial learning, procedural memory, sequential problem solving, operant- and fear conditioning, and social recognition. Those assays are carried out with individual subjects in apparatuses such as arenas and mazes, which allow for a high degree of standardization across laboratories and straightforward data interpretation but are not without caveats and limitations. In animal research, there is growing concern about the translatability of study results and animal welfare, leading to novel approaches beyond established protocols. Here, we present some of the more recent developments and more advanced concepts in learning and memory testing, such as multi-step sequential lockboxes, assays involving groups of animals, as well as home cage-based assays supported by automated tracking solutions; and weight their potential and limitations against those of established paradigms. Shifting the focus of learning tests from the classical experimental chamber to settings which are more natural for rodents comes with a new set of challenges for behavioral researchers, but also offers the opportunity to understand memory formation and retrieval in a more conclusive way than has been attainable with conventional test protocols. We predict and embrace an increase in studies relying on methods involving a higher degree of automatization, more naturalistic- and home cage-based experimental setting as well as more integrated learning tasks in the future. We are confident these trends are suited to alleviate the burden on animal subjects and improve study designs in memory research.

Uncorrelated bilateral cortical input becomes timed across hippocampal subfields for long waves whereas gamma waves are largely ipsilateral

The role of interhemispheric connections along successive segments of cortico-hippocampal circuits is poorly understood. We aimed to obtain a global picture of spontaneous transfer of activity during non-theta states across several nodes of the bilateral circuit in anesthetized rats. Spatial discrimination techniques applied to bilateral laminar field potentials (FP) across the CA1/Dentate Gyrus provided simultaneous left and right readouts in five FP generators that reflect activity in specific hippocampal afferents and associative pathways. We used a battery of correlation and coherence analyses to extract complementary aspects at different time scales and frequency bands. FP generators exhibited varying bilateral correlation that was high in CA1 and low in the Dentate Gyrus. The submillisecond delays indicate coordination but not support for synaptic dependence of one side on another. The time and frequency characteristics of bilateral coupling were specific to each generator. The Schaffer generator was strongly bilaterally coherent for both sharp waves and gamma waves, although the latter maintained poor amplitude co-variation. The lacunosum-moleculare generator was composed of up to three spatially overlapping activities, and globally maintained high bilateral coherence for long but not short (gamma) waves. These two CA1 generators showed no ipsilateral relationship in any frequency band. In the Dentate Gyrus, strong bilateral coherence was observed only for input from the medial entorhinal areas, while those from the lateral entorhinal areas were largely asymmetric, for both alpha and gamma waves. Granger causality testing showed strong bidirectional relationships between all homonymous bilateral generators except the lateral entorhinal input and a local generator in the Dentate Gyrus. It also revealed few significant relationships between ipsilateral generators, most notably the anticipation of lateral entorhinal cortex toward all others. Thus, with the notable exception of the lateral entorhinal areas, there is a marked interhemispheric coherence primarily for slow envelopes of activity, but not for pulse-like gamma waves, except in the Schafer segment. The results are consistent with essentially different streams of activity entering from and returning to the cortex on each side, with slow waves reflecting times of increased activity exchange between hemispheres and fast waves generally reflecting ipsilateral processing.

Environmental enrichment improves cognitive flexibility in rainbow trout in a visual discrimination task: first insights

Research on fish cognition provides strong evidence that fish are endowed with high level cognitive skills. However, most studies on cognitive flexibility and generalization abilities, two key adaptive traits for captive animals, focused on model species, and farmed fish received too little attention. Environmental enrichment was shown to improve learning abilities in various fish species, but its influence on cognitive flexibility and generalization abilities is still unknown. We studied farmed rainbow trout (Oncorhynchus mykiss) as an aquaculture model to study how environmental enrichment impacts their cognitive abilities. Using an operant conditioning device, allowing the expression of a motivated choice, we measured fish cognitive flexibility with serial reversal learning tests, after a successful acquisition phase based on two colors discrimination (2-alternative forced choice, 2-AFC), and their ability to generalize a rewarded color to any shape. Eight fish were divided into two groups: Condition E (fish reared from fry stages under enriched conditions with plants, rocks and pipes for ~9 months); Condition B (standard barren conditions). Only one fish (condition E) failed in the habituation phase of the device and one fish (condition B) failed in the 2-AFC task. We showed that after a successful acquisition phase in which the fish correctly discriminated two colors, they all succeeded in four reversal learnings, supporting evidence for cognitive flexibility in rainbow trout. They were all successful in the generalization task. Interestingly, fish reared in an enriched environment performed better in the acquisition phase and in the reversal learning (as evidenced by fewer trials needed to reach the learning criterion), but not in the generalization task. We assume that color-based generalization may be a simpler cognitive process than discriminative learning and cognitive flexibility, and does not seem to be influenced by environmental conditions. Given the small number of individuals tested, our results may be considered as first insights into cognitive flexibility in farmed fish using an operant conditioning device, but they pave the way for future studies. We conclude that farming conditions should take into account the cognitive abilities of fish, in particular their cognitive flexibility, by allowing them to live in an enriched environment.

3D Synaptic Organization of the Rat CA1 and Alterations Induced by Cocaine Self-Administration

The hippocampus plays a key role in contextual conditioning and has been proposed as an important component of the cocaine addiction brain circuit. To gain knowledge about cocaine-induced alterations in this circuit, we used focused ion beam milling/scanning electron microscopy to reveal and quantify the three-dimensional synaptic organization of the neuropil of the stratum radiatum of the rat CA1, under normal circumstances and after cocaine-self administration (SA). Most synapses are asymmetric (excitatory), macular-shaped, and in contact with dendritic spine heads. After cocaine-SA, the size and the complexity of the shape of both asymmetric and symmetric (inhibitory) synapses increased but no changes were observed in the synaptic density. This work constitutes the first detailed report on the 3D synaptic organization in the stratum radiatum of the CA1 field of cocaine-SA rats. Our data contribute to the elucidation of the normal and altered synaptic organization of the hippocampus, which is crucial for better understanding the neurobiological mechanisms underlying cocaine addiction.

Operant conditioning deficits and modified local field potential activities in parvalbumin-deficient mice

Altered functioning of GABAergic interneurons expressing parvalbumin (PV) in the basal ganglia-thalamo-cortical circuit are likely to be involved in several human psychiatric disorders characterized by deficits in attention and sensory gating with dysfunctional decision-making behavior. However, the contribution of these interneurons in the ability to acquire demanding learning tasks remains unclear. Here, we combine an operant conditioning task with local field potentials simultaneously recorded in several nuclei involved in reward circuits of wild-type (WT) and PV-deficient (PVKO) mice, which are characterized by changes in firing activity of PV-expressing interneurons. In comparison with WT mice, PVKO animals presented significant deficits in the acquisition of the selected learning task. Recordings from prefrontal cortex, nucleus accumbens (NAc) and hippocampus showed significant decreases of the spectral power in beta and gamma bands in PVKO compared with WT mice particularly during the performance of the operant conditioning task. From the first to the last session, at all frequency bands the spectral power in NAc tended to increase in WT and to decrease in PVKO. Results indicate that PV deficiency impairs signaling necessary for instrumental learning and the recognition of natural rewards.

Rainbow trout discriminate 2-D photographs of conspecifics from distracting stimuli using an innovative operant conditioning device

Cognitive abilities were studied in rainbow trout, the first continental fish production in Europe. Increasing public concern for the welfare of farmed-fish species highlighted the need for better knowledge of the cognitive status of fish. We trained and tested 15 rainbow trout with an operant conditioning device composed of self-feeders positioned in front of visual stimuli displayed on a screen. The device was coupled with a two-alternative forced-choice (2-AFC) paradigm to test whether rainbow trout can discriminate 2-D photographs of conspecifics (S+) from different visual stimuli (S-). The S- were applied in four stages, the last three stages representing increasing discrimination difficulty: (1) blue shapes; (2) black shape (star); (3) photograph of an object (among a pool of 60); (4) photograph of another fish species (among a pool of 60). Nine fish (out of 15) correctly managed to activate the conditioning device after 30-150 trials. The rainbow trout were able to discriminate images of conspecifics from an abstract shape (five individuals out of five) or objects (four out of five) but not from other fish species. Their ability to learn the category "fish shape" rather than distinguishing between conspecifics and heterospecifics is discussed. The successful visual discrimination task using this complex operant conditioning device is particularly remarkable and novel for this farmed-fish species, and could be exploited to develop cognitive enrichments in future farming systems. This device can also be added to the existing repertoire of testing devices suitable for investigating cognitive abilities in fish.

The Claustrum is Involved in Cognitive Processes Related to the Classical Conditioning of Eyelid Responses in Behaving Rabbits

It is assumed that the claustrum (CL) is involved in sensorimotor integration and cognitive processes. We recorded the firing activity of identified CL neurons during classical eyeblink conditioning in rabbits, using a delay paradigm in which a tone was presented as conditioned stimulus (CS), followed by a corneal air puff as unconditioned stimulus (US). Neurons were identified by their activation from motor (MC), cingulate (CC), and medial prefrontal (mPFC) cortices. CL neurons were rarely activated by single stimuli of any modality. In contrast, their firing was significantly modulated during the first sessions of paired CS/US presentations, but not in well-trained animals. Neuron firing rates did not correlate with the kinematics of conditioned responses (CRs). CL local field potentials (LFPs) changed their spectral power across learning and presented well-differentiated CL–mPFC/CL–MC network dynamics, as shown by crossfrequency spectral measurements. CL electrical stimulation did not evoke eyelid responses, even in trained animals. Silencing of synaptic transmission of CL neurons by the vINSIST method delayed the acquisition of CRs but did not affect their presentation rate. The CL plays an important role in the acquisition of associative learning, mostly in relation to the novelty of CS/US association, but not in the expression of CRs.

Chronic adult-onset of growth hormone/IGF-I hypersecretion improves cognitive functions and LTP and promotes neuronal differentiation in adult rats

AIM

Besides their metabolic and endocrine functions, the growth hormone (GH) and its mediated factor, the insulin-like growth factor I (IGF-I), have been implicated in different brain functions, including neurogenesis. Long-lasting elevated GH and IGF-I levels result in non-reversible somatic, endocrine and metabolic morbidities. However, the subcutaneous implantation of the GH-secreting (GH-S) GC cell line in rats leads to the controllable over-secretion of GH and elevated IGF-I levels, allowing the experimental study of their short-term effects on brain functions.

METHODS

Adult rats were implanted with GC cells and checked 10 weeks later, when a GH/IGF-I-secreting tumour was already formed.

RESULTS

Tumour-bearing rats acquired different operant conditioning tasks faster and better than controls and tumour-resected groups. They also presented better retentions of long-term memories in the passive avoidance test. Experimentally evoked long-term potentiation (LTP) in the hippocampus was also larger and longer lasting in the tumour bearing than in the other groups. Chronic adult-onset of GH/IGF-I hypersecretion caused an acceleration of early progenitors, facilitating a faster neural differentiation, maturation and integration in the dentate gyrus, and increased the complexity of dendritic arbours and spine density of granule neurons.

CONCLUSION

Thus, adult-onset hypersecretion of GH/IGF-I improves neurocognitive functions, long-term memories, experimental LTP and neural differentiation, migration and maturation.

The activity of the prelimbic cortex in rats is enhanced during the cooperative acquisition of an instrumental learning task

The objective of this study was to identify the functional properties of the prefrontal cortex that allow animals to work together to obtain a mutual reward. We induced pairs of male rats to develop a cooperative behavior in two adjacent Skinner boxes divided by a metallic grille. The experimental boxes allowed the two rats to see and to smell each other and to have limited physical contact through the grille. Rats were progressively trained to climb onto two separate platforms (and stay there simultaneously for >0.5 s) to get food pellets for both. This set-up was compatible with the in vivo recording of local field potentials (LFPs) at the prelimbic (PrL) cortex throughout the task. A dominant delta/theta activity appeared mostly during the period in which rats were located on the platforms. Spectral powers were larger when rats had to stay together on the platforms than when they jumped individually onto them. When paired together, rats presented significant differences in the power of delta and low theta bands depending if they were leading or following the joint activity. PrL cortex encodes neural commands related to the individual and joint acquisition of an operant conditioning task by behaving rats.

Lack of Neuronal Glycogen Impairs Memory Formation and Learning-Dependent Synaptic Plasticity in Mice

Since brain glycogen is stored mainly in astrocytes, the role of this polysaccharide in neurons has been largely overlooked. To study the existence and relevance of an active neuronal glycogen metabolism in vivo, we generated a mouse model lacking glycogen synthase specifically in the Camk2a-expressing postnatal forebrain pyramidal neurons (GYS1^Camk2a–KO), which include the prefrontal cortex and the CA3 and CA1 cell layers of the hippocampus. The latter are involved in memory and learning processes and participate in the hippocampal CA3-CA1 synapse, the function of which can be analyzed electrophysiologically. Long-term potentiation evoked in the hippocampal CA3-CA1 synapse was decreased in alert behaving GYS1^Camk2a–KO mice. They also showed a significant deficiency in the acquisition of an instrumental learning task – a type of associative learning involving prefrontal and hippocampal circuits. Interestingly, GYS1^Camk2a–KO animals did not show the greater susceptibility to hippocampal seizures and myoclonus observed in animals completely depleted of glycogen in the whole CNS. These results unequivocally demonstrate the presence of an active glycogen metabolism in neurons in vivo and reveal a key role of neuronal glycogen in the proper acquisition of new motor and cognitive abilities, and in the changes in synaptic strength underlying such acquisition.

When and Where Learning is Taking Place: Multisynaptic Changes in Strength During Different Behaviors Related to the Acquisition of an Operant Conditioning Task by Behaving Rats

Although it is generally assumed that brain circuits are modified by new experiences, the question of which changes in synaptic efficacy take place in cortical and subcortical circuits across the learning process remains unanswered. Rats were trained in the acquisition of an operant conditioning in a Skinner box provided with light beams to detect animals' approaches to lever and feeder. Behaviors such as pressing the lever, eating, exploring, and grooming were also recorded. Animals were chronically implanted with stimulating and recording electrodes in hippocampal, prefrontal, and subcortical sites relevant to the task. Field synaptic potentials were evoked during the performance of the above-mentioned behaviors and before, during, and after the acquisition process. Afferent pathways to the hippocampus and the intrinsic hippocampal circuit were slightly modified in synaptic strength during the performance of those behaviors. In contrast, afferent and efferent circuits of the medial prefrontal cortex were significantly modified in synaptic strength across training sessions, mostly at the moment of the largest change in the learning curve. Performance of behaviors nondirectly related to the acquisition process (exploring, grooming) also evoked changes in synaptic strength across training. This study helps to understand when and where learning is being engraved in the brain.

Cannabinoid disruption of learning mechanisms involved in reward processing

The increasing use of cannabis, its derivatives, and synthetic cannabinoids for medicinal and recreational purposes has led to burgeoning interest in understanding the addictive potential of this class of molecules. It is estimated that ∼10% of marijuana users will eventually show signs of dependence on the drug, and the diagnosis of cannabis use disorder (CUD) is increasing in the United States. The molecule that sustains the use of cannabis is Δ⁹-tetrahydrocannabinol (Δ⁹-THC), and our knowledge of its effects, and those of other cannabinoids on brain function has expanded rapidly in the past two decades. Additionally, the identification of endogenous cannabinoid (endocannabinoid) systems in brain and their roles in physiology and behavior, demonstrate extensive involvement of these lipid signaling molecules in regulating CNS function. Here, we examine roles for endogenous cannabinoids in shaping synaptic activity in cortical and subcortical brain circuits, and we discuss mechanisms in which exogenous cannabinoids, such as Δ⁹-THC, interact with endocannabinoid systems to disrupt neuronal network oscillations. We then explore how perturbation of the interaction of this activity within brain reward circuits may lead to impaired learning. Finally, we propose that disruption of cellular plasticity mechanisms by exogenous cannabinoids in cortical and subcortical circuits may explain the difficulty in establishing viable cannabinoid self-administration models in animals.

Forebrain-specific, conditional silencing of Staufen2 alters synaptic plasticity, learning, and memory in rats

Background

Dendritic messenger RNA (mRNA) localization and subsequent local translation in dendrites critically contributes to synaptic plasticity and learning and memory. Little is known, however, about the contribution of RNA-binding proteins (RBPs) to these processes in vivo.

Results

To delineate the role of the double-stranded RBP Staufen2 (Stau2), we generate a transgenic rat model, in which Stau2 expression is conditionally silenced by Cre-inducible expression of a microRNA (miRNA) targeting Stau2 mRNA in adult forebrain neurons. Known physiological mRNA targets for Stau2, such as RhoA, Complexin 1, and Rgs4 mRNAs, are found to be dysregulated in brains of Stau2-deficient rats. In vivo electrophysiological recordings reveal synaptic strengthening upon stimulation, showing a shift in the frequency-response function of hippocampal synaptic plasticity to favor long-term potentiation and impair long-term depression in Stau2-deficient rats. These observations are accompanied by deficits in hippocampal spatial working memory, spatial novelty detection, and in tasks investigating associative learning and memory.

Conclusions

Together, these experiments reveal a critical contribution of Stau2 to various forms of synaptic plasticity including spatial working memory and cognitive management of new environmental information. These findings might contribute to the development of treatments for conditions associated with learning and memory deficits.

Electronic supplementary material

The online version of this article (doi:10.1186/s13059-017-1350-8) contains supplementary material, which is available to authorized users.

Learning as a Functional State of the Brain: Studies in Wild-Type and Transgenic Animals

Contemporary neuroscientists are paying increasing attention to subcellular, molecular, and electrophysiological mechanisms underlying learning and memory processes. Recent studies have examined the development of transgenic mice affected at different stages of the learning process, or have emulated in animals various human pathological conditions involving cognition and motor learning. However, a parallel effort is needed to develop stimulating and recording techniques suitable for use in behaving mice in order to understand activity-dependent synaptic changes taking place during the very moment of the learning process. The in vivo models should incorporate information collected from different molecular and in vitro approaches. Long-term potentiation (LTP) has been proposed as the neural mechanism underlying synaptic plasticity, and NMDA receptors have been proposed as the molecular substrate of LTP. It now seems necessary to study the relationship of both LTP and NMDA receptors to functional changes in synaptic efficiency taking place during actual learning in selected cerebral cortical structures. Here, we review data collected in our laboratory during the past 10 years on the involvement of different hippocampal synapses in the acquisition of the classically conditioned eyelid responses in behaving mice. Overall the results indicate a specific contribution of each cortical synapse to the acquisition and storage of new motor and cognitive abilities. Available data show that LTP, evoked by high-frequency stimulation of Schaffer collaterals, disturbs both the acquisition of conditioned eyelid responses and the physiological changes that occur at hippocampal synapses during learning. Moreover, the administration of NMDA-receptor antagonists is able not only to prevent LTP induction in vivo, but also to hinder both the formation of conditioned eyelid responses and functional changes in the strength of the CA3-CA1 synapse. Nevertheless, many other neurotransmitter receptors, intracellular mediators, and transcription factors are also involved in learning and memory processes. In summary, it can be proposed that learning and memory in behaving mammals are the result of the activation of complex and distributed functional states involving many different cerebral cortical synapses, with the participation also of various neurotransmitter systems.

Functional states of rat cortical circuits during the unpredictable availability of a reward-related cue

Proper performance of acquired abilities can be disturbed by the unexpected occurrence of external changes. Rats trained with an operant conditioning task (to press a lever in order to obtain a food pellet) using a fixed-ratio (1:1) schedule were subsequently placed in a Skinner box in which the lever could be removed randomly. Field postsynaptic potentials (fPSPs) were chronically evoked in perforant pathway-hippocampal CA1 (PP-CA1), CA1-subiculum (CA1-SUB), CA1-medial prefrontal cortex (CA1-mPFC), mPFC-nucleus accumbens (mPFC-NAc), and mPFC-basolateral amygdala (mPFC-BLA) synapses during lever IN and lever OUT situations. While lever presses were accompanied by a significant increase in fPSP slopes at the five synapses, the unpredictable absence of the lever were accompanied by decreased fPSP slopes in all, except PP-CA1 synapses. Spectral analysis of local field potentials (LFPs) recorded when the animal approached the corresponding area in the lever OUT situation presented lower spectral powers than during lever IN occasions for all recording sites, apart from CA1. Thus, the unpredictable availability of a reward-related cue modified the activity of cortical and subcortical areas related with the acquisition of operant learning tasks, suggesting an immediate functional reorganization of these neural circuits to address the changed situation and to modify ongoing behaviors accordingly.

The Outward Spiral: A vicious cycle model of obesity and cognitive dysfunction

Chronic failure to suppress intake during states of positive energy balance leads to weight gain and obesity. The ability to use context - including interoceptive satiety states - to inhibit responding to previously rewarded cues appears to depend on the functional integrity of the hippocampus. Recent evidence implicates energy dense Western diets in several types of hippocampal dysfunction, including reduced expression of neurotrophins and nutrient transporters, increased inflammation, microglial activation, and blood brain barrier permeability. The functional consequences of such insults include impairments in an animal's ability to modulate responding to a previously reinforced cues. We propose that such deficits promote overeating, which can further exacerbate hippocampal dysfunction and thus initiate a vicious cycle of both obesity and progressive cognitive decline.

Systematic variation of acquisition rate in delay eyelid conditioning

Averaging artifacts inherent in group acquisition curves can mask behavioral phenomena that are potentially revealing in terms of underlying neural mechanisms. To address this, we implemented a behavioral analysis of 106 rabbits trained over 4 sessions using delay eyelid conditioning. Group results showed the typical monotonic increase in conditioned responses (CRs). For most subjects CRs first appeared (as indexed by the criterion of 8 CRs in 9 trials) during the first 18 trials of the second training session. Subdividing subjects according to the training block at which they met criterion revealed systematic differences in the subsequent rate that CR amplitudes increased, but not in asymptotic CR amplitudes. Subjects meeting criterion early in sessions showed more rapid increases in CR amplitude than those meeting criterion later in sessions. This effect was solely dependent on how early within a session criterion was met, as subjects meeting criterion at the beginning of the third and fourth sessions showed more rapid increases in CR amplitude than those meeting criterion after the first 18 trials of the second session. The exceptions were the 7% of the subjects that met criterion late in the first session. Their CR amplitudes increased at a rate similar to subjects meeting criterion early in sessions. These results suggest an interplay between consolidation processes and a previously reported short-term plasticity process that makes CR acquisition a nonmonotonic and complex function of the point during training sessions at which CRs first appear. (PsycINFO Database Record

Presynaptic GABAB Receptors Regulate Hippocampal Synapses during Associative Learning in Behaving Mice

GABAB receptors are the G-protein-coupled receptors for GABA, the main inhibitory neurotransmitter in the central nervous system. Pharmacological activation of GABAB receptors regulates neurotransmission and neuronal excitability at pre- and postsynaptic sites. Electrophysiological activation of GABAB receptors in brain slices generally requires strong stimulus intensities. This raises the question as to whether behavioral stimuli are strong enough to activate GABAB receptors. Here we show that GABAB1a-/- mice, which constitutively lack presynaptic GABAB receptors at glutamatergic synapses, are impaired in their ability to acquire an operant learning task. In vivo recordings during the operant conditioning reveal a deficit in learning-dependent increases in synaptic strength at CA3-CA1 synapses. Moreover, GABAB1a-/- mice fail to synchronize neuronal activity in the CA1 area during the acquisition process. Our results support that activation of presynaptic hippocampal GABAB receptors is important for acquisition of a learning task and for learning-associated synaptic changes and network dynamics.

An embodied biologically constrained model of foraging: from classical and operant conditioning to adaptive real-world behavior in DAC-X

Animals successfully forage within new environments by learning, simulating and adapting to their surroundings. The functions behind such goal-oriented behavior can be decomposed into 5 top-level objectives: 'how', 'why', 'what', 'where', 'when' (H4W). The paradigms of classical and operant conditioning describe some of the behavioral aspects found in foraging. However, it remains unclear how the organization of their underlying neural principles account for these complex behaviors. We address this problem from the perspective of the Distributed Adaptive Control theory of mind and brain (DAC) that interprets these two paradigms as expressing properties of core functional subsystems of a layered architecture. In particular, we propose DAC-X, a novel cognitive architecture that unifies the theoretical principles of DAC with biologically constrained computational models of several areas of the mammalian brain. DAC-X supports complex foraging strategies through the progressive acquisition, retention and expression of task-dependent information and associated shaping of action, from exploration to goal-oriented deliberation. We benchmark DAC-X using a robot-based hoarding task including the main perceptual and cognitive aspects of animal foraging. We show that efficient goal-oriented behavior results from the interaction of parallel learning mechanisms accounting for motor adaptation, spatial encoding and decision-making. Together, our results suggest that the H4W problem can be solved by DAC-X building on the insights from the study of classical and operant conditioning. Finally, we discuss the advantages and limitations of the proposed biologically constrained and embodied approach towards the study of cognition and the relation of DAC-X to other cognitive architectures.

Theta and beta oscillatory dynamics in the dentate gyrus reveal a shift in network processing state during cue encounters

The hippocampus is an important structure for learning and memory processes, and has strong rhythmic activity. Although a large amount of research has been dedicated toward understanding the rhythmic activity in the hippocampus during exploratory behaviors, specifically in the theta (5–10 Hz) frequency range, few studies have examined the temporal interplay of theta and other frequencies during the presentation of meaningful cues. We obtained in vivo electrophysiological recordings of local field potentials (LFP) in the dentate gyrus (DG) of the hippocampus as rats performed three different associative learning tasks. In each task, cue presentations elicited pronounced decrements in theta amplitude in conjunction with increases in beta (15–30 Hz) amplitude. These changes were often transient but were sustained from the onset of cue encounters until the occurrence of a reward outcome. This oscillatory profile shifted in time to precede cue encounters over the course of the session, and was not present during similar behaviors in the absence of task relevant stimuli. The observed decreases in theta amplitude and increases in beta amplitude in the DG may thus reflect a shift in processing state that occurs when encountering meaningful cues.

Functional basis of associative learning and its relationships with long-term potentiation evoked in the involved neural circuits: Lessons from studies in behaving mammals

While contemporary neuroscience is paying increasing attention to subcellular and molecular events and other intracellular phenomena underlying the acquisition, storage, and retrieval of newly acquired motor and cognitive abilities, parallel attention should be paid to the study of the electrophysiological phenomena taking place at selected cortical and subcortical neuronal and synaptic sites during the precise moment of learning acquisition, extinction, and recall. These in vivo approaches to the study of learning and memory processes will allow the proper integration of the important information collected from in vitro and delayed molecular studies. Here, we summarize studies in behaving mammals carried out in our laboratory during the past ten years on the relationships between experimentally evoked long-term potentiation (LTP) and activity-dependent changes in synaptic strength taking place in hippocampal, prefrontal and related cortical and subcortical circuits during the acquisition of classical eyeblink conditioning or operant learning tasks. These studies suggest that different hippocampal synapses are selectively modified in strength during the acquisition of classical, but not instrumental, learning tasks. In contrast, selected prefrontal and striatum synapses are more directly modified by operant conditioning. These studies also show that besides N-methyl-D-aspartate (NMDA) receptors, many other neurotransmitter, intracellular mediating, and transcription factors participate in these two types of associative learning. Although experimentally evoked LTP seems to prevent the acquisition of classical eyeblink conditioning when induced at selected hippocampal synapses, it proved to be ineffective in preventing the acquisition of operant conditioned tasks when induced at numerous hippocampal, prefrontal, and striatal sites. The differential roles of these cortical structures during these two types of associative learning are discussed, and a diagrammatic representation of their respective functions is presented.

Harnessing the power of theta: natural manipulations of cognitive performance during hippocampal theta-contingent eyeblink conditioning

Neurobiological oscillations are regarded as essential to normal information processing, including coordination and timing of cells and assemblies within structures as well as in long feedback loops of distributed neural systems. The hippocampal theta rhythm is a 3–12 Hz oscillatory potential observed during cognitive processes ranging from spatial navigation to associative learning. The lower range, 3–7 Hz, can occur during immobility and depends upon the integrity of cholinergic forebrain systems. Several studies have shown that the amount of pre-training theta in the rabbit strongly predicts the acquisition rate of classical eyeblink conditioning and that impairment of this system substantially slows the rate of learning. Our lab has used a brain-computer interface (BCI) that delivers eyeblink conditioning trials contingent upon the explicit presence or absence of hippocampal theta. A behavioral benefit of theta-contingent training has been demonstrated in both delay and trace forms of the paradigm with a two- to four-fold increase in learning speed. This behavioral effect is accompanied by enhanced amplitude and synchrony of hippocampal local field potential (LFP)s, multi-unit excitation, and single-unit response patterns that depend on theta state. Additionally, training in the presence of hippocampal theta has led to increases in the salience of tone-induced unit firing patterns in the medial prefrontal cortex, followed by persistent multi-unit activity during the trace interval. In cerebellum, rhythmicity and precise synchrony of stimulus time-locked LFPs with those of hippocampus occur preferentially under the theta condition. Here we review these findings, integrate them into current models of hippocampal-dependent learning and suggest how improvement in our understanding of neurobiological oscillations is critical for theories of medial temporal lobe processes underlying intact and pathological learning.

Involvement of the GABAergic septo-hippocampal pathway in brain stimulation reward

The hippocampus is a structure related to several cognitive processes, but not very much is known about its putative involvement in positive reinforcement. In its turn, the septum has been related to instrumental brain stimulation reward (BSR) by its electrical stimulation with trains of pulses. Although the anatomical relationships of the septo-hippocampal pathway are well established, the functional relationship between these structures during rewarding behaviors remains poorly understood. To explore hippocampal mechanisms involved in BSR, CA3-evoked field excitatory and inhibitory postsynaptic potentials (fEPSPs, fIPSPs) were recorded in the CA1 area during BSR in alert behaving mice. The synaptic efficiency was determined from changes in fEPSP and fIPSP amplitudes across the learning of a BSR task. The successive BSR sessions evoked a progressive increase of the performance in inverse relationship with a decrease in the amplitude of fEPSPs, but not of fIPSPs. Additionally, we evaluated CA1 local field potentials (LFPs) during a preference task, comparing 8-, 20-, and 100-Hz trains of septal BSR. We corroborate a clear preference for BSR at 100 Hz (in comparison with BSR at 20 Hz or 8 Hz), in parallel with an increase in the spectral power of the low theta band, and a decrease in the gamma. These results were replicated by intrahippocampal injections of a GABAB antagonist. Thus, the GABAergic septo-hippocampal pathway seems to carry information involved in the encoding of reward properties, where GABAB receptors seem to play a key role. With regard to the dorsal hippocampus, fEPSPs evoked at the CA3-CA1 synapse seem to reflect the BSR learning process, while hippocampal rhythmic activities are more related to reward properties.

Differential contributions of microglial and neuronal IKKβ to synaptic plasticity and associative learning in alert behaving mice

Microglia are CNS resident immune cells and a rich source of neuroactive mediators, but their contribution to physiological brain processes such as synaptic plasticity, learning, and memory is not fully understood. In this study, we used mice with partial depletion of IκB kinase β, the main activating kinase in the inducible NF-κB pathway, selectively in myeloid lineage cells (mIKKβKO) or excitatory neurons (nIKKβKO) to measure synaptic strength at hippocampal Schaffer collaterals during long-term potentiation (LTP) and instrumental conditioning in alert behaving individuals. Resting microglial cells in mIKKβKO mice showed less Iba1-immunoreactivity, and brain IL-1β mRNA levels were selectively reduced compared with controls. Measurement of field excitatory postsynaptic potentials (fEPSPs) evoked by stimulation of the CA3-CA1 synapse in mIKKβKO mice showed higher facilitation in response to paired pulses and enhanced LTP following high frequency stimulation. In contrast, nIKKβKO mice showed normal basic synaptic transmission and LTP induction but impairments in late LTP. To understand the consequences of such impairments in synaptic plasticity for learning and memory, we measured CA1 fEPSPs in behaving mice during instrumental conditioning. IKKβ was not necessary in either microglia or neurons for mice to learn lever-pressing (appetitive behavior) to obtain food (consummatory behavior) but was required in both for modification of their hippocampus-dependent appetitive, not consummatory behavior. Our results show that microglia, through IKKβ and therefore NF-κB activity, regulate hippocampal synaptic plasticity and that both microglia and neurons, through IKKβ, are necessary for animals to modify hippocampus-driven behavior during associative learning.

Relative contributions of CA3 and medial entorhinal cortex to memory in rats

The hippocampal CA1 field processes spatial information, but the relative importance of intra- vs. extra-hippocampal sources of input into CA1 for spatial behavior is unclear. To characterize the relative roles of these two sources of input, originating in the hippocampal field CA3 and in the medial entorhinal cortex (MEC), we studied effects of discrete neurotoxic lesions of CA3 or MEC on concurrent spatial and nonspatial navigation tasks, and on synaptic transmission in afferents to CA1. Lesions in CA3 or MEC regions that abolished CA3-CA1, or reduced MEC-CA1 synaptic transmission, respectively, impaired spatial navigation and unexpectedly interfered with cue response, suggesting that in certain conditions of training regimen, hippocampal activity may influence behavior otherwise supported by nonhippocampal neural networks. MEC lesions had milder and temporary behavioral effects, but also markedly amplified transmission in the CA3-CA1 pathway. Extensive behavioral training had a similar, but more modest effect on CA3-CA1 transmission. Thus, cortical input to the hippocampus modulates CA1 activity both directly and indirectly, through heterosynaptic interaction, to control information flow in the hippocampal loop. Following damage to hippocampal cortical input, the functional coupling of separate intra- and extra-hippocampal inputs to CA1 involved in normal learning may initiate processes that support recovery of behavioral function. Such a process may explain how CA3 lesions, which do not significantly modify the basic features of CA1 neural activity, nonetheless impair spatial recall, whereas lesions of EC input to CA1, which reduce the spatial selectivity of CA1 firing in foraging rats, have only mild effects on spatial navigation.

Cognitive decline due to excess synaptic Zn(2+) signaling in the hippocampus

Zinc is an essential component of physiological brain function. Vesicular zinc is released from glutamatergic (zincergic) neuron terminals and serves as a signal factor (Zn²⁺ signal) in both the intracellular (cytosol) compartment and the extracellular compartment. Synaptic Zn²⁺ signaling is dynamically linked to neurotransmission and is involved in processes of synaptic plasticity such as long-term potentiation and cognitive activity. On the other hand, the activity of the hypothalamic–pituitary–adrenal (HPA) axis, i.e., glucocorticoid secretion, which can potentiate glutamatergic neuron activity, is linked to cognitive function. HPA axis activity modifies synaptic Zn²⁺ dynamics at zincergic synapses. An increase in HPA axis activity, which occurs after exposure to stress, may induce excess intracellular Zn²⁺ signaling in the hippocampus, followed by hippocampus-dependent memory deficit. Excessive excitation of zincergic neurons in the hippocampus can contribute to cognitive decline under stressful and/or pathological conditions. This paper provides an overview of the ``Hypothesis and Theory'' of Zn²⁺-mediated modification of cognitive activity.

Novel space alters theta and gamma synchrony across the longitudinal axis of the hippocampus

Hippocampal theta (6–10 Hz) and gamma (25–50 Hz and 65–100 Hz) local field potentials (LFPs) reflect the dynamic synchronization evoked by inputs impinging upon hippocampal neurons. Novel experience is known to engage hippocampal physiology and promote successful encoding. Does novelty synchronize or desynchronize theta and/or gamma frequency inputs across the septotemporal (long) axis of the hippocampus (HPC)? The present study tested the hypothesis that a novel spatial environment would alter theta power and coherence across the long axis. We compared theta and gamma LFP signals at individual (power) and millimeter distant electrode pairs (coherence) within the dentate gyrus (DG) and CA1 region while rats navigated a runway (1) in a familiar environment, (2) with a modified path in the same environment and (3) in a novel space. Locomotion in novel space was related to increases in theta and gamma power at most CA1 and DG sites. The increase in theta and gamma power was concurrent with an increase in theta and gamma coherence across the long axis of CA1; however, there was a significant decrease in theta coherence across the long axis of the DG. These findings illustrate significant shifts in the synchrony of entorhinal, CA3 and/or neuromodulatory afferents conveying novel spatial information to the dendritic fields of CA1 and DG targets across the long axis of the HPC. This shift suggests that the entire theta/gamma-related input to the CA1 network, and likely output, receives and conveys a more coherent message in response to novel sensory experience. Such may contribute to the successful encoding of novel sensory experience.

The GABAergic septohippocampal pathway is directly involved in internal processes related to operant reward learning

We studied the role of γ-aminobutyric acid (GABA)ergic septohippocampal projections in medial septum (MS) self-stimulation of behaving mice. Self-stimulation was evoked in wild-type (WT) mice using instrumental conditioning procedures and in J20 mutant mice, a type of mouse with a significant deficit in GABAergic septohippocampal projections. J20 mice showed a significant modification in hippocampal activities, including a different response for input/output curves and the paired-pulse test, a larger long-term potentiation (LTP), and a delayed acquisition and lower performance in the MS self-stimulation task. LTP evoked at the CA3-CA1 synapse further decreased self-stimulation performance in J20, but not in WT, mice. MS self-stimulation evoked a decrease in the amplitude of field excitatory postsynaptic potentials (fEPSPs) at the CA3-CA1 synapse in WT, but not in J20, mice. This self-stimulation-dependent decrease in the amplitude of fEPSPs was also observed in the presence of another positive reinforcer (food collected during an operant task) and was canceled by the local administration of an antibody-inhibiting glutamate decarboxylase 65 (GAD65). LTP evoked in the GAD65Ab-treated group was also larger than in controls. The hippocampus has a different susceptibility to septal GABAergic inputs depending on ongoing cognitive processes, and the GABAergic septohippocampal pathway is involved in consummatory processes related to operant rewards.

Synaptic plasticity studies and their applicability in mouse models of neurodegenerative diseases

During the past few years, there have been many important contributions to the better understanding of the different types of memory and the putative neural structures that generate them. Moreover, various studies on neurodegenerative diseases in human beings have added useful information about learning and memory formation, and about their loss in patients with these disabling diseases. The development of sophisticated pharmacogenetic tools applied to mouse models, reproducing different types of neurodegenerative disease or, at least, some of their main symptoms, has turned out to be extremely useful for the further development of contemporary neuroscience. In addition, ingenious behavioral and electrophysiological approaches have been developed to study the activity-dependent changes in synaptic strength during learning and memory processes in the best possible way — that is, in the alert behaving animal. Collected data from these numerous studies have enabled us to know more about the role of many different molecular components integrating the synaptic cleft, and to draw some conclusions about the concordance between in vitro and in vivo recorded data, and the generalization of the results to other types of learning and/or brain-related structures. As described here, changes in synaptic strength studied in key neural synapses during learning and memory processes in genetically manipulated mice can represent an interesting and powerful approach to the better understanding of neural processes underlying the acquisition of new motor and cognitive abilities and how they are affected by different human diseases involving the neural tissue.