Homeostatic regulation of memory systems and adaptive decisions

Contemporary neurocognitive models of memory: A descriptive comparative analysis

The great complexity involved in the study of memory has given rise to numerous hypotheses and models associated with various phenomena at different levels of analysis. This has allowed us to delve deeper in our knowledge about memory but has also made it difficult to synthesize and integrate data from different lines of research. In this context, this work presents a descriptive comparative analysis of contemporary models that address the structure and function of multiple memory systems. The main goal is to outline a panoramic view of the key elements that constitute these models in order to visualize both the current state of research and possible future directions. The elements that stand out from different levels of analysis are distributed neural networks, hierarchical organization, predictive coding, homeostasis, and evolutionary perspective.

A general theory of rehabilitation: Rehabilitation catalyses and assists adaptation to illness

BACKGROUND

There is no general theory of rehabilitation, only definitions and descriptions, with the biopsychosocial model of illness as a structure.

OBJECTIVE

To develop a general theory of rehabilitation that explains how healthcare rehabilitation changes outcomes and to evaluate its validity.

NEED

A general rehabilitation theory would help research, improve services, increase understanding, modify resource allocation and explain some anomalies, such as how rehabilitation helps when no natural recovery occurs.

BUILDING BLOCKS

People adapt to change throughout their lives. Illness is a change, and people adapt to their illness. Adaptation's purpose is to maintain an equilibrium in a person's life. The balanced components are related to Maslow's five needs: basic, safety, affiliation, status and self-fulfilment. The general theory of behaviour suggests that a person's behaviours change to maintain balance, regulated by a central homeostatic mechanism.

THE THEORY

Rehabilitation aids adaptation to changes associated with illness through accurate diagnosis and formulation, catalysing adaptation, optimising the environment and assisting the person in making necessary changes by safely practising activities and teaching self-management.

IMPLICATIONS

The theory makes the person the central active agent, emphasises the importance of the environment in facilitating adaptation, explains why all conditions may benefit, including progressive and static conditions, suggests that health can be equated to someone maintaining their equilibrium and explains why a small dose may be very effective.

CONCLUSION

The general theory of rehabilitation emphasises the catalytic effects of rehabilitation in facilitating and guiding adaptation and suggests areas for research and improvement.

Forebrain E-I balance controlled in cognition through coordinated inhibition and inhibitory transcriptome mechanism

Introduction

Forebrain neural networks are vital for cognitive functioning, and their excitatory-inhibitory (E-I) balance is governed by neural homeostasis. However, the homeostatic control strategies and transcriptomic mechanisms that maintain forebrain E-I balance and optimal cognition remain unclear.

Methods

We used patch-clamp and RNA sequencing to investigate the patterns of neural network homeostasis with suppressing forebrain excitatory neural activity and spatial training.

Results

We found that inhibitory transmission and receptor transcription were reduced in tamoxifen-inducible Kir2.1 conditional knock-in mice. In contrast, spatial training increased inhibitory synaptic connections and the transcription of inhibitory receptors.

Discussion

Our study provides significant evidence that inhibitory systems play a critical role in the homeostatic control of the E-I balance in the forebrain during cognitive training and E-I rebalance, and we have provided insights into multiple gene candidates for cognition-related homeostasis in the forebrain.

Chronic nicotine increases midbrain dopamine neuron activity and biases individual strategies towards reduced exploration in mice

Long-term exposure to nicotine alters brain circuits and induces profound changes in decision-making strategies, affecting behaviors both related and unrelated to drug seeking and consumption. Using an intracranial self-stimulation reward-based foraging task, we investigated in mice the impact of chronic nicotine on midbrain dopamine neuron activity and its consequence on the trade-off between exploitation and exploration. Model-based and archetypal analysis revealed substantial inter-individual variability in decision-making strategies, with mice passively exposed to nicotine shifting toward a more exploitative profile compared to non-exposed animals. We then mimicked the effect of chronic nicotine on the tonic activity of dopamine neurons using optogenetics, and found that photo-stimulated mice adopted a behavioral phenotype similar to that of mice exposed to chronic nicotine. Our results reveal a key role of tonic midbrain dopamine in the exploration/exploitation trade-off and highlight a potential mechanism by which nicotine affects the exploration/exploitation balance and decision-making.

Time-dependent changes in hippocampal and striatal glycogen long after maze training in male rats

Long-lasting biological changes reflecting past experience have been studied in and typically attributed to neurons in the brain. Astrocytes, which are also present in large number in the brain, have recently been found to contribute critically to learning and memory processing. In the brain, glycogen is primarily found in astrocytes and is metabolized to lactate, which can be released from astrocytes. Here we report that astrocytes themselves have intrinsic neurochemical plasticity that alters the availability and provision of metabolic substrates long after an experience. Rats were trained to find food on one of two versions of a 4-arm maze: a hippocampus-sensitive place task and a striatum-sensitive response task. Remarkably, hippocampal glycogen content increased while striatal levels decreased during the 30 days after rats were trained to find food in the place version, but not the response version, of the maze tasks. A long-term consequence of the durable changes in glycogen stores was seen in task-by-site differences in extracellular lactate responses activated by testing on a working memory task administered 30 days after initial training, the time when differences in glycogen content were most robust. These results suggest that astrocytic plasticity initiated by a single experience may augment future availability of energy reserves, perhaps priming brain areas to process learning of subsequent experiences more effectively.

Extracellular levels of glucose in the hippocampus and striatum during maze training for food or water reward in male rats

Glucose potently enhances cognitive functions whether given systemically or directly to the brain. The present experiments examined changes in brain extracellular glucose levels while rats were trained to solve hippocampus-sensitive place or striatum-sensitive response learning tasks for food or water reward. Because there were no task-related differences in glucose responses, the glucose results were pooled across tasks to form combined trained groups. During the first 1-3 min of training for food reward, glucose levels in extracellular fluid (ECF) declined significantly in the hippocampus and striatum; the declines were not seen in untrained, rewarded rats. When trained for water reward, similar decreases were observed in both brain areas, but these findings were less consistent than those seen with food rewards. After the initial declines in ECF glucose levels, glucose increased in most groups, approaching asymptotic levels ∼15-30 min into training. Compared to untrained food controls, training with food reward resulted in significant glucose increases in the hippocampus but not striatum; striatal glucose levels exhibited large increases to food intake in both trained and untrained groups. In rats trained to find water, glucose levels increased significantly above the values seen in untrained rats in both hippocampus and striatum. The decreases in glucose early in training might reflect an increase in brain glucose consumption, perhaps triggering increased brain uptake of glucose from blood, as evident in the increases in glucose later in training. The increased brain uptake of glucose may provide additional neuronal metabolic substrate for metabolism or provide astrocytic substrate for production of glycogen and lactate.

Using a memory systems lens to view the effects of estrogens on cognition: Implications for human health

Understanding the organizing and activating effects of gonadal steroids on adult physiology can guide insight into sex differences in and hormonal influences on health and disease, ranging from diabetes and other metabolic disorders, emotion and stress regulation, substance abuse, pain perception, immune function and inflammation, to cognitive function and dysfunction accompanying neurological disorders. Because the brain is highly sensitive to many forms of estrogens, it is not surprising that many adult behaviors, including cognitive function, are modulated by estrogens. Estrogens are known for their facilitating effects on learning and memory, but it is becoming increasingly clear that they also can impair learning and memory of some classes of tasks and may do so through direct actions on specific neural systems. This review takes a multiple memory systems approach to understanding how estrogens can at the same time enhance hippocampus-sensitive place learning and impair striatum-sensitive response learning by exploring the role estrogen receptor signaling may play in the opposing cognitive effects of estrogens. Accumulating evidence suggests that neither receptor subtype nor the timing of treatment, i.e. rapid vs slow, explain the bidirectional effects of estrogens on different types of learning. New findings pointing to neural metabolism and the provision of energy substrates by astrocytes as a candidate mechanism for cognitive enhancement and impairment are discussed.

Multi-system state shifts and cognitive deficits induced by chronic morphine during abstinence

Chronic morphine administration induces neural plasticity followed by withdraw. And clinic observation indicates that obvious cognitive deficits are found during withdrawal. However, current neural substrates that regulate dysfunction in withdrawal are unknown. In our studies, chronic morphine administration was used to induce the spontaneous withdrawal model in rats. A series of cognitive abilities was tested to explore brain function. To further evaluate the neural substrates of dysfunction, Manganese-enhanced MRI(MEMRI) was used to map the dysfunctional regions in vivo.We observed that chronic morphine administration could induce obvious withdrawal behaviors in abstinence followed by cognitive impairments, such as impairments in working memory, reward, interaction and enhancement of anxiety. Our in-vivo MEMRI data using the voxel-wise comparisons showed that the manganese-enhanced signal intensity (VMI) within morphine withdrawal groups was increased in cingulate cortex (Cg), secondary motor cortex (M2), CA3 subfield of hippocampus, dorsal striatum (D-striatum), retrosplenial cortex (RS), shell subregion of NAc (AcbSh), core subregion of NAc (AcbC), central nucleus of amygdala (CeC), basolateral amygdaloid nucleus (BLA), central amygdaloid nucleus (CeM), anterior hypothalamic area, central (AHC), ventral tegmental area (VTA) and scaphoid thalamic nucleus (SC).However, decreasing of VMI was found in the ventrolateral striatum (V-striatum) and lateral posterior thalamic nucleus (LP) compared to the control group. These brain regions were beleived to be components of the memory, executive, limbic and regulatory systems. Therefore, our present studies indicate that withdrawal induced by chronic morphine adiministration could disturb brain function leading to multi-systems state shifts and cognitive deficits in abstinence.

Training-induced elevations in extracellular lactate in hippocampus and striatum: Dissociations by cognitive strategy and type of reward

Recent evidence suggests that astrocytes convert glucose to lactate, which is released from the astrocytes and supports learning and memory. This report takes a multiple memory perspective to test the role of astrocytes in cognition using real-time lactate measurements during learning and memory. Extracellular lactate levels in the hippocampus or striatum were determined with lactate biosensors while rats were learning place (hippocampus-sensitive) or response (striatum-sensitive) versions of T-mazes. In the first experiment, rats were trained on the place and response tasks to locate a food reward. Extracellular lactate levels in the hippocampus increased beyond those of feeding controls during place training but not during response training. However, striatal lactate levels did not increase beyond those of controls when rats were trained on either the place or the response version of the maze. Because food ingestion itself increased blood glucose and brain lactate levels, the contribution of feeding may have confounded the brain lactate measures. Therefore, we conducted a second similar experiment using water as the reward. A very different pattern of lactate responses to training emerged when water was used as the task reward. First, provision of water itself did not result in large increases in either brain or blood lactate levels. Moreover, extracellular lactate levels increased in the striatum during response but not place learning, whereas extracellular lactate levels in the hippocampus did not differ across tasks. The findings from the two experiments suggest that the relative engagement of the hippocampus and striatum dissociates not only by task but also by reward type. The divergent lactate responses of the hippocampus and striatum in place and response tasks under different reward conditions may reflect ethological constraints tied to foraging for food and water.

D4 Receptor Activation Differentially Modulates Hippocampal Basal and Apical Dendritic Synapses in Freely Moving Mice

Activation of D4 receptors (D4Rs) has been shown to improve cognitive performance, potentially affecting synaptic strength. We investigated the D4R agonist PD 168077 (PD) in hippocampal CA1 of freely moving mice. We electrically stimulated in stratum oriens (OR) or radiatum (RAD) and evoked local field potentials (LFPs). Intraperitoneally injected PD dose-dependently and reversibly attenuated LFPs for longer time in basal (OR) than apical (RAD) dendrites. High-frequency stimulation induced LTP that was stronger and more stable in OR than RAD. LTP lasted at least 4 h during which the paired-pulse ratio remained reduced. A PD concentration not affecting synaptic transmission was sufficient to reduce LTP in OR but not in RAD. A PD concentration reducing synaptic transmission reduced the early phase LTP in OR additionally and the late phase LTP in RAD exclusively. Furthermore, cell type-specific expression of mCherry in DATCre mice generated fluorescence in dorsal CA1 that was highest in lacunosum moleculare and similar in OR/RAD, indicating that midbrain dopaminergic fibers distribute evenly in OR/RAD. Together, the D4R-mediated modulation of hippocampal synaptic transmission and plasticity is stronger in OR than RAD. This could affect information processing in CA1 neurons, since signals arriving via basal and apical afferents are distinct.

Altered subjective reward valuation among drug-deprived heavy marijuana users: Aversion to uncertainty

Marijuana is the most commonly used illicit drug in the United States and its use is rising. Nonetheless, scientific efforts to clarify the risk for addiction and other harm associated with marijuana use have been lacking. Maladaptive decision-making is a cardinal feature of addiction that is likely to emerge in heavy users. In particular, distorted subjective reward valuation related to homeostatic or allostatic processes has been implicated for many drugs of abuse. Selective changes in responses to uncertainty have been observed in response to intoxication and deprivation from various drugs of abuse. To assess for these potential neuroadaptive changes in reward valuation associated with marijuana deprivation, we examined the subjective value of uncertain and certain rewards among deprived and nondeprived heavy marijuana users in a behavioral economics decision-making task. Deprived users displayed reduced valuation of uncertain rewards, particularly when these rewards were more objectively valuable. This uncertainty aversion increased with increasing quantity of marijuana use. These results suggest comparable decision-making vulnerability from marijuana use as other drugs of abuse, and highlights targets for intervention.

A Prefrontal-Hippocampal Comparator for Goal-Directed Behavior: The Intentional Self and Episodic Memory

The hypothesis of this article is that the interactions between the prefrontal cortex and the hippocampus play a critical role in the modulation of goal-directed self-action and the strengthening of episodic memories. We describe various theories that model a comparator function for the hippocampus, and then elaborate the empirical evidence that supports these theories. One theory which describes a prefrontal-hippocampal comparator for voluntary action is emphasized. Action plans are essential for successful goal-directed behavior, and are elaborated by the prefrontal cortex. When an action plan is initiated, the prefrontal cortex transmits an efference copy (or corollary discharge) to the hippocampus where it is stored as a working memory for the action plan (which includes the expected outcomes of the action plan). The hippocampus then serves as a response intention-response outcome working memory comparator. Hippocampal comparator function is enabled by the hippocampal theta rhythm allowing the hippocampus to compare expected action outcomes to actual action outcomes. If the expected and actual outcomes match, the hippocampus transmits a signal to prefrontal cortex which strengthens or consolidates the action plan. If a mismatch occurs, the hippocampus transmits an error signal to the prefrontal cortex which facilitates a reformulation of the action plan, fostering behavioral flexibility and memory updating. The corollary discharge provides the self-referential component to the episodic memory, affording the personal and subjective experience of what behavior was carried out, when it was carried out, and in what context (where) it occurred. Such a perspective can be applied to episodic memory in humans, and episodic-like memory in non-human animal species.

Integrative hippocampal and decision-making neurocircuitry during goal-relevant predictions and encoding

It has become clear that the hippocampus plays a critical role in the identification of new contexts and for the detection of changes in familiar contexts. The hippocampus accomplishes these goals through a continual process of comparing predicted features of a context or situation to those actually experienced. A mismatch between expected and experienced context expectations is thought to lead to the generation of a context prediction error (Mizumori, 2013) that functionally alerts connected brain areas to alter subsequent decision making and response selection. Little is understood about how hippocampal context analyses impact downstream decision processes. This issue is evaluated here first by comparing the nature of the information represented in hippocampus and decision-related midbrain-striatal structures, while rats perform a hippocampal-dependent spatial memory task in which rewards of different value are found at different locations. In contrast to place-specific and egocentric neural representations, neural representations of goal information are broadly distributed in hippocampal and decision neural circuitry, but they appear in different forms for different brain structures. It is suggested that further researching on how goal information processing occurs in hippocampus and decision neural circuitry may reveal insights into the nature of the interaction between memory and decision systems. The second part of this review describes neural pathways by which hippocampal context information might arrive within the decision circuit. The third section presents a hypothesis that the nature of the interactions between hippocampal and midbrain-striatal circuitry is regulated by the prefrontal cortex.

Erythrocyte DHA level as a biomarker of DHA status in specific brain regions of n-3 long-chain PUFA-supplemented aged rats

n-3 Long-chain PUFA (n-3 LC-PUFA), particularly EPA and DHA, play a key role in the maintenance of brain functions such as learning and memory that are impaired during ageing. Ageing is also associated with changes in the DHA content of brain membranes that could contribute to memory impairment. Limited studies have investigated the effects of ageing and n-3 LC-PUFA supplementation on both blood and brain fatty acid compositions. Therefore, we assessed the relationship between fatty acid contents in plasma and erythrocyte membranes and those in the hippocampus, striatum and cerebral cortex during ageing, and after a 5-month period of EPA/DHA supplementation in rats. In the blood, ageing was associated with an increase in plasma DHA content, whereas the DHA content remained stable in erythrocyte membranes. In the brain, ageing was associated with a decrease in DHA content, which was both region-specific and phospholipid class-specific. In EPA/DHA-supplemented aged rats, DHA contents were increased both in the blood and brain compared with the control rats. The present results demonstrated that n-3 LC-PUFA level in the plasma was not an accurate biomarker of brain DHA status during ageing. Moreover, we highlighted a positive relationship between the DHA levels in erythrocyte phosphatidylethanolamine (PE) and those in the hippocampus and prefrontal cortex in EPA/DHA-supplemented aged rats. Within the framework of preventive dietary supplementation to delay brain ageing, these results suggest the possibility of using erythrocyte PE DHA content as a reliable biomarker of DHA status in specific brain regions.

A role for the lateral dorsal tegmentum in memory and decision neural circuitry

A role for the hippocampus in memory is clear, although the mechanism for its contribution remains a matter of debate. Converging evidence suggests that hippocampus evaluates the extent to which context-defining features of events occur as expected. The consequence of mismatches, or prediction error, signals from hippocampus is discussed in terms of its impact on neural circuitry that evaluates the significance of prediction errors: Ventral tegmental area (VTA) dopamine cells burst fire to rewards or cues that predict rewards (Schultz, Dayan, & Montague, 1997). Although the lateral dorsal tegmentum (LDTg) importantly controls dopamine cell burst firing (Lodge & Grace, 2006) the behavioral significance of the LDTg control is not known. Therefore, we evaluated LDTg functional activity as rats performed a spatial memory task that generates task-dependent reward codes in VTA (Jo, Lee, & Mizumori, 2013; Puryear, Kim, & Mizumori, 2010) and another VTA afferent, the pedunculopontine nucleus (PPTg, Norton, Jo, Clark, Taylor, & Mizumori, 2011). Reversible inactivation of the LDTg significantly impaired choice accuracy. LDTg neurons coded primarily egocentric information in the form of movement velocity, turning behaviors, and behaviors leading up to expected reward locations. A subset of the velocity-tuned LDTg cells also showed high frequency bursts shortly before or after reward encounters, after which they showed tonic elevated firing during consumption of small, but not large, rewards. Cells that fired before reward encounters showed stronger correlations with velocity as rats moved toward, rather than away from, rewarded sites. LDTg neural activity was more strongly regulated by egocentric behaviors than that observed for PPTg or VTA cells that were recorded by Puryear et al. and Norton et al. While PPTg activity was uniquely sensitive to ongoing sensory input, all three regions encoded reward magnitude (although in different ways), reward expectation, and reward encounters. Only VTA encoded reward prediction errors. LDTg may inform VTA about learned goal-directed movement that reflects the current motivational state, and this in turn may guide VTA determination of expected subjective goal values. When combined it is clear the LDTg and PPTg provide only a portion of the information that dopamine cells need to assess the value of prediction errors, a process that is essential to future adaptive decisions and switches of cognitive (i.e. memorial) strategies and behavioral responses.

Forgetfulness during aging: an integrated biology

Age-related impairments in memory are often attributed to failures, at either systems or molecular levels, of memory storage processes. A major characteristic of changes in memory with increasing age is the advent of forgetfulness in old vs. young animals. This review examines the contribution of a dysfunction of the mechanisms responsible for modulating the maintenance of memory in aged rats. A memory-modulating system that includes epinephrine, acting through release of glucose from liver glycogen stores, potently enhances memory in young rats. In old rats, epinephrine loses its ability to release glucose and loses its efficacy in enhancing memory. Brain measures of extracellular levels of glucose in the hippocampus during memory testing show decreases in glucose in both young and old rats, but the decreases are markedly greater in extent and duration in old rats. Importantly, the old rats do not have the ability to increase blood glucose levels in response to arousal-related epinephrine release, which is retained and even increased in aged rats. Glucose appears to be able to reverse fully the increased rate of forgetting seen in old rats. This set of findings suggests that physiological mechanisms outside of the brain, i.e. changes in neuroendocrine functions, may contribute substantially to the onset of rapid forgetting in aged animals.

Context prediction analysis and episodic memory

Events that happen at a particular place and time come to define our episodic memories. Extensive experimental and clinical research illustrate that the hippocampus is central to the processing of episodic memories, and this is in large part due to its analysis of context information according to spatial and temporal references. In this way, hippocampus defines ones expectations for a given context as well as detects errors in predicted contextual features. The detection of context prediction errors is hypothesized to distinguished events into meaningful epochs that come to be recalled as separate episodic memories. The nature of the spatial and temporal context information processed by hippocampus is described, as is a hypothesis that the apparently self-regulatory nature of hippocampal context processing may ultimately be mediated by natural homeostatic operations and plasticity. Context prediction errors by hippocampus are suggested to be valued by the midbrain dopamine system, the output of which is ultimately fed back to hippocampus to update memory-driven context expectations for future events. Thus, multiple network functions (both within and outside hippocampus) combine to result in adaptive episodic memories.