Diverse sources of reward value signals in the basal ganglia nuclei transmitted to the lateral habenula in the monkey

Periaqueductal gray passes over disappointment and signals continuity of remaining reward expectancy

Disappointment is a vital factor in the learning and adjustment of strategies in reward-seeking behaviors. It helps them conserve energy in environments where rewards are scarce, while also increasing their chances of maximizing rewards by prompting them to escape to environments where richer rewards are anticipated (e.g., migration). However, another key factor in obtaining the reward is the ability to monitor the remaining possibilities of obtaining the outcome and to tolerate the disappointment in order to continue with subsequent actions. The periaqueductal gray (PAG) has been reported as one of the key brain regions in regulating negative emotions and escape behaviors in animals. The present study suggests that the PAG could also play a critical role in inhibiting escape behaviors and facilitating ongoing motivated behaviors to overcome disappointing events. We found that PAG activity is tonically suppressed by reward expectancy as animals engage in a task to acquire a reward outcome. This tonic suppression of PAG activity was sustained during a series of sequential task procedures as long as the expectancy of reward outcomes persisted. Notably, the tonic suppression of PAG activity showed a significant correlation with the persistence of animals' reward-seeking behavior while overcoming intermittent disappointing events. This finding highlights that the balance between distinct tonic signaling in the PAG, which signals remaining reward expectancy, and phasic signaling in the LHb, which signals disappointment, could play a crucial role in determining whether animals continue or discontinue reward-seeking behaviors when they encounter an unexpected negative event. This mechanism would be essential for animals to efficiently navigate complex environments with various reward volatilities and ultimately contributes to maximizing their reward acquisition.

The putative role of the habenula in animal migration

BACKGROUND

When an addicted animal seeks a specific substance, it is based on the perception of internal and external cues that strongly motivate to pursue the acquisition of that compound. In essence, a similar process acts out when an animal leaves its present area to begin its circannual migration. This review article examines the existence of scientific evidence for possible relatedness of migration and addiction by influencing Dorsal Diencephalic Conduction System (DDCS) including the habenula.

METHODS

For this review especially the databases of Pubmed and Embase were frequently and non-systematically searched.

RESULTS

The mechanisms of bird migration have been thoroughly investigated. Especially the mechanism of the circannual biorhythm and its associated endocrine regulation has been well elucidated. A typical behavior called "Zugunruhe" marks the moment of leaving in migratory birds. The role of magnetoreception in navigation has also been clarified in recent years. However, how bird migration is regulated at the neuronal level in the forebrain is not well understood. Among mammals, marine mammals are most similar to birds. They use terrestrial magnetic field when navigating and often bridge long distances between breeding and foraging areas. Population migration is further often seen among the large hoofed mammals in different parts of the world. Importantly, learning processes and social interactions with conspecifics play a major role in these ungulates. Considering the evolutionary development of the forebrain in vertebrates, it can be postulated that the DDCS plays a central role in regulating the readiness and intensity of essential (emotional) behaviors. There is manifold evidence that this DDCS plays an important role in relapse to abuse after prolonged periods of abstinence from addictive behavior. It is also possible that the DDCS plays a role in navigation.

CONCLUSIONS

The role of the DDCS in the neurobiological regulation of bird migration has hardly been investigated. The involvement of this system in relapse to addiction in mammals might suggest to change this. It is recommended that particularly during "Zugunruhe" the role of neuronal regulation via the DDCS will be further investigated.

Exercise therapy for chronic pain: How does exercise change the limbic brain function?

We are exposed to various external and internal threats which might hurt us. The role of taking flexible and appropriate actions against threats is played by "the limbic system" and at the heart of it there is the ventral tegmental area and nucleus accumbens (brain reward system). Pain-related fear causes excessive excitation of amygdala, which in turn causes the suppression of medial prefrontal cortex, leading to chronification of pain. Since the limbic system of chronic pain patients is functionally impaired, they are maladaptive to their situations, unable to take goal-directed behavior and are easily caught by fear-avoidance thinking. We describe the neural mechanisms how exercise activates the brain reward system and enables chronic pain patients to take goal-directed behavior and overcome fear-avoidance thinking. A key to getting out from chronic pain state is to take advantage of the behavioral switching function of the basal nucleus of amygdala. We show that exercise activates positive neurons in this nucleus which project to the nucleus accumbens and promote reward behavior. We also describe fear conditioning and extinction are affected by exercise. In chronic pain patients, the fear response to pain is enhanced and the extinction of fear memories is impaired, so it is difficult to get out of "fear-avoidance thinking". Prolonged avoidance of movement and physical inactivity exacerbate pain and have detrimental effects on the musculoskeletal and cardiovascular systems. Based on the recent findings on multiple bran networks, we propose a well-balanced exercise prescription considering the adherence and pacing of exercise practice. We conclude that therapies targeting the mesocortico-limbic system, such as exercise therapy and cognitive behavioral therapy, may become promising tools in the fight against chronic pain.

Insights into the Promising Prospect of G Protein and GPCR-Mediated Signaling in Neuropathophysiology and Its Therapeutic Regulation

G protein-coupled receptors (GPCRs) are intricately involved in the conversion of extracellular feedback to intracellular responses. These specialized receptors possess a crucial role in neurological and psychiatric disorders. Most nonsensory GPCRs are active in almost 90% of complex brain functions. At the time of receptor phosphorylation, a GPCR pathway is essentially activated through a G protein signaling mechanism via a G protein-coupled receptor kinase (GRK). Dopamine, an important neurotransmitter, is primarily involved in the pathophysiology of several CNS disorders; for instance, bipolar disorder, schizophrenia, Parkinson's disease, and ADHD. Since dopamine, acetylcholine, and glutamate are potent neuropharmacological targets, dopamine itself has potential therapeutic effects in several CNS disorders. GPCRs essentially regulate brain functions by modulating downstream signaling pathways. GPR6, GPR52, and GPR8 are termed orphan GPCRs because they colocalize with dopamine D1 and D2 receptors in neurons of the basal ganglia, either alone or with both receptors. Among the orphan GPCRs, the GPR52 is recognized for being an effective psychiatric receptor. Various antipsychotics like aripiprazole and quetiapine mainly target GPCRs to exert their actions. One of the most important parts of signal transduction is the regulation of G protein signaling (RGS). These substances inhibit the activation of the G protein that initiates GPCR signaling. Developing a combination of RGS inhibitors with GPCR agonists may prove to have promising therapeutic potential. Indeed, several recent studies have suggested that GPCRs represent potentially valuable therapeutic targets for various psychiatric disorders. Molecular biology and genetically modified animal model studies recommend that these enriched GPCRs may also act as potential therapeutic psychoreceptors. Neurotransmitter and neuropeptide GPCR malfunction in the frontal cortex and limbic-related regions, including the hippocampus, hypothalamus, and brainstem, is likely responsible for the complex clinical picture that includes cognitive, perceptual, emotional, and motor symptoms. G protein and GPCR-mediated signaling play a critical role in developing new treatment options for mental health issues, and this study is aimed at offering a thorough picture of that involvement. For patients who are resistant to current therapies, the development of new drugs that target GPCR signaling cascades remains an interesting possibility. These discoveries might serve as a fresh foundation for the creation of creative methods for pharmacologically useful modulation of GPCR function.

Lateral Habenula Responses During Eye Contact in a Reward Conditioning Task

For many animals, social interaction may have intrinsic reward value over and above its utility as a means to the desired end. Eye contact is the starting point of interactions in many social animals, including primates, and abnormal patterns of eye contact are present in many mental disorders. Whereas abundant previous studies have shown that negative emotions such as fear strongly affect eye contact behavior, modulation of eye contact by reward has received scant attention. Here we recorded eye movement patterns and neural activity in lateral habenula while monkeys viewed faces in the context of Pavlovian and instrumental conditioning tasks. Faces associated with larger rewards spontaneously elicited longer periods of eye contact from the monkeys, even though this behavior was not required or advantaged in the task. Concurrently, lateral habenula neurons were suppressed by faces signaling high value and excited by faces signaling low value. These results suggest that the reward signaling of lateral habenula may contribute to social behavior and disorders, presumably through its connections with the basal ganglia.

Lateral Habenula Beyond Avoidance: Roles in Stress, Memory, and Decision-Making With Implications for Psychiatric Disorders

In this Perspective review, we highlight some of the less explored aspects of lateral habenula (LHb) function in contextual memory, sleep, and behavioral flexibility. We provide evidence that LHb is well-situated to integrate different internal state and multimodal sensory information from memory-, stress-, motivational-, and reward-related circuits essential for both survival and decision making. We further discuss the impact of early life stress (ELS) on LHb function as an example of stress-induced hyperactivity and dysregulation of neuromodulatory systems within the LHb that promote anhedonia and motivational deficits following ELS. We acknowledge that recent technological advancements in manipulation and recording of neural circuits in simplified and well-controlled behavioral paradigms have been invaluable in our understanding of the critical role of LHb in motivation and emotional regulation as well as the involvement of LHb dysfunction in stress-induced psychopathology. However, we also argue that the use of ethologically-relevant behaviors with consideration of complex aspects of decision-making is warranted for future studies of LHb contributions in a wide range of psychiatric illnesses. We conclude this Perspective with some of the outstanding issues for the field to consider where a multi-systems approach is needed to investigate the complex nature of LHb circuitry interactions with environmental stimuli that predisposes psychiatric disorders.

Distinct Roles of the Human Subthalamic Nucleus and Dorsal Pallidum in Parkinson's Disease Impulsivity

BACKGROUND

Impulsivity and impulse control disorders are common in Parkinson's disease and lead to increased morbidity and reduced quality of life. Impulsivity is thought to arise from aberrant reward processing and inhibitory control, but it is unclear why deep brain stimulation of either the subthalamic nucleus (STN) or globus pallidus internus (GPi) affects levels of impulsivity. Our aim was to assess the role of the STN and GPi in impulsivity using invasive local field potential (LFP) recordings from deep brain stimulation electrodes.

METHODS

We measured LFPs during a simple rewarding Go/NoGo paradigm in 39 female and male human patients with Parkinson's disease manifesting variable amounts of impulsivity who were undergoing unilateral deep brain stimulation of either the STN (18 nuclei) or GPi (28 nuclei). We identified reward-specific LFP event-related potentials and correlated them to impulsivity severity.

RESULTS

LFPs in both structures modulated during reward-specific Go and NoGo stimulus evaluation, reward feedback, and loss feedback. Motor and limbic functions were anatomically separable in the GPi but not in the STN. Across participants, LFP reward processing responses in the STN and GPi uniquely depended on the severity of impulsivity.

CONCLUSIONS

This study establishes LFP correlates of impulsivity within the STN and GPi regions. We propose a model for basal ganglia reward processing that includes the bottom-up role of the GPi in reward salience and the top-down role of the STN in cognitive control.

Networks of habenula-projecting cortical neurons regulate cocaine seeking

How neurons in the medial prefrontal cortex broadcast stress-relevant information to subcortical brain sites to regulate cocaine relapse remains unclear. The lateral habenula (LHb) serves as a “hub” to filter and propagate stress- and aversion-relevant information in the brain. Here, we show that chemogenetic inhibition of cortical inputs to LHb attenuates relapse-like reinstatement of extinguished cocaine seeking in mice. Using an RNA sequencing–based brain mapping procedure with single-cell resolution, we identify networks of cortical neurons that project to LHb and then preferentially innervate different downstream brain sites, including the ventral tegmental area, median raphe nucleus, and locus coeruleus (LC). By using an intersectional chemogenetics approach, we show that inhibition of cortico-habenular neurons that project to LC, but not to other sites, blocks reinstatement of cocaine seeking. These findings highlight the remarkable complexity of descending cortical inputs to the habenula and identify a cortico-habenulo-hindbrain circuit that regulates cocaine seeking.

The Role of the Lateral Habenula in Inhibitory Learning from Reward Omission

The lateral habenula (LHb) is a phylogenetically primitive brain structure that plays a key role in learning to inhibit distinct responses to specific stimuli. This structure is activated by primary aversive stimuli, cues predicting an imminent aversive event, unexpected reward omissions, and cues associated with the omission of an expected reward. The most widely described physiological effect of LHb activation is acutely suppressing midbrain dopaminergic signaling. However, recent studies have identified multiple means by which the LHb promotes this effect as well as other mechanisms of action. These findings reveal the complex nature of LHb circuitry. The present paper reviews the role of this structure in learning from reward omission. We approach this topic from the perspective of computational models of behavioral change that account for inhibitory learning to frame key findings. Such findings are drawn from recent behavioral neuroscience studies that use novel brain imaging, stimulation, ablation, and reversible inactivation techniques. Further research and conceptual work are needed to clarify the nature of the mechanisms related to updating motivated behavior in which the LHb is involved. As yet, there is little understanding of whether such mechanisms are parallel or complementary to the well-known modulatory function of the more recently evolved prefrontal cortex.

The habenula-targeting neurons in the mouse entopeduncular nucleus contain not only somatostatin-positive neurons but also nitric oxide synthase-positive neurons

The entopeduncular nucleus (EPN) in rodents is one of the two major output nuclei of the basal ganglia and corresponds to the internal segment of the globus pallidus in primates. Previous studies have shown that the EPN contains three types of neurons that project to different targets, namely, parvalbumin (PV)-, somatostatin (SOM)-, and choline acetyltransferase-positive neurons. However, we have recently reported that neurons lacking immunoreactivities for these substances are present in the EPN. Here, we demonstrate that 27.7% of all EPN neurons showed immunoreactivity for nitric oxide synthase (NOS). Among them, NOS-only positive and NOS/SOM double-positive neurons accounted for 20.1% and 6.8%, respectively, whereas NOS/PV double-positive neurons were rarely observed. NOS-containing neurons were distributed in a shell region surrounding the thalamus-targeting, PV-rich core region of the EPN, especially in the ventromedial part of the shell. The retrograde tracer fluoro-gold (FG) was injected into several target regions of EPN neurons. Among FG-labeled EPN neurons after injection into the lateral habenula (LHb), NOS-only positive, NOS/SOM double-positive, and SOM-only positive neurons accounted for 25.7%, 15.2%, and 59.1%, respectively. We conclude that NOS-positive neurons are the second major population of LHb-targeting EPN neurons, suggesting their possible involvement in behaviors in response to aversive stimuli.

The Neural Correlates of Cued Reward Omission

Compared to our understanding of positive prediction error signals occurring due to unexpected reward outcomes, less is known about the neural circuitry in humans that drives negative prediction errors during omission of expected rewards. While classical learning theories such as Rescorla-Wagner or temporal difference learning suggest that both types of prediction errors result from a simple subtraction, there has been recent evidence suggesting that different brain regions provide input to dopamine neurons which contributes to specific components of this prediction error computation. Here, we focus on the brain regions responding to negative prediction error signals, which has been well-established in animal studies to involve a distinct pathway through the lateral habenula. We examine the activity of this pathway in humans, using a conditioned inhibition paradigm with high-resolution functional MRI. First, participants learned to associate a sensory stimulus with reward delivery. Then, reward delivery was omitted whenever this stimulus was presented simultaneously with a different sensory stimulus, the conditioned inhibitor (CI). Both reward presentation and the reward-predictive cue activated midbrain dopamine regions, insula and orbitofrontal cortex. While we found significant activity at an uncorrected threshold for the CI in the habenula, consistent with our predictions, it did not survive correction for multiple comparisons and awaits further replication. Additionally, the pallidum and putamen regions of the basal ganglia showed modulations of activity for the inhibitor that did not survive the corrected threshold.

A systems-neuroscience model of phasic dopamine

We describe a neurobiologically informed computational model of phasic dopamine signaling to account for a wide range of findings, including many considered inconsistent with the simple reward prediction error (RPE) formalism. The central feature of this PVLV framework is a distinction between a primary value (PV) system for anticipating primary rewards (Unconditioned Stimuli [USs]), and a learned value (LV) system for learning about stimuli associated with such rewards (CSs). The LV system represents the amygdala, which drives phasic bursting in midbrain dopamine areas, while the PV system represents the ventral striatum, which drives shunting inhibition of dopamine for expected USs (via direct inhibitory projections) and phasic pausing for expected USs (via the lateral habenula). Our model accounts for data supporting the separability of these systems, including individual differences in CS-based (sign-tracking) versus US-based learning (goal-tracking). Both systems use competing opponent-processing pathways representing evidence for and against specific USs, which can explain data dissociating the processes involved in acquisition versus extinction conditioning. Further, opponent processing proved critical in accounting for the full range of conditioned inhibition phenomena, and the closely related paradigm of second-order conditioning. Finally, we show how additional separable pathways representing aversive USs, largely mirroring those for appetitive USs, also have important differences from the positive valence case, allowing the model to account for several important phenomena in aversive conditioning. Overall, accounting for all of these phenomena strongly constrains the model, thus providing a well-validated framework for understanding phasic dopamine signaling. (PsycInfo Database Record (c) 2020 APA, all rights reserved).

A quantitative reward prediction error signal in the ventral pallidum

The nervous system is hypothesized to compute reward prediction errors (RPEs) to promote adaptive behavior. Correlates of RPEs have been observed in the midbrain dopamine system, but the extent to which RPE signals exist in other reward-processing regions is less well understood. In the present study, we quantified outcome history-based RPE signals in the ventral pallidum (VP), a basal ganglia region functionally linked to reward-seeking behavior. We trained rats to respond to reward-predicting cues, and we fit computational models to predict the firing rates of individual neurons at the time of reward delivery. We found that a subset of VP neurons encoded RPEs and did so more robustly than the nucleus accumbens, an input to the VP. VP RPEs predicted changes in task engagement, and optogenetic manipulation of the VP during reward delivery bidirectionally altered rats' subsequent reward-seeking behavior. Our data suggest a pivotal role for the VP in computing teaching signals that influence adaptive reward seeking.

Compartmental function and modulation of the striatum

The striatum plays a central role in guiding numerous complex behaviors, ranging from motor control to action selection and reward learning. The diverse responsibilities of the striatum are reflected by the complexity of its organization. In this review, we will summarize what is currently known about the compartmental layout of the striatum, an organizational principle that is crucial for allowing the striatum to guide such a diverse array of behaviors. We will focus on the anatomical and functional properties of striosome (patch) and matrix compartments of the striatum, and how the engagement of these compartments is uniquely controlled by their afferents, intrinsic properties, and neuromodulation. We will give examples of how advances in technology have opened the door to functionally dissecting the striatum's compartmental design, and close by offering thoughts on the future and relevance for human disease.

Predominant Striatal Input to the Lateral Habenula in Macaques Comes from Striosomes

Striosomes, neurochemically specialized modules in the striatum, are thought to be nodes in circuits extending, via basal ganglia pathways, from mood-related neocortical regions to dopamine-containing neurons of the substantia nigra. Yet striosomes have remained beyond the reach of electrophysiological methods to identify them, especially in non-human primates. Such work is needed for translational as well as for basic science. Here we introduce a method to identify striosomes on-line in awake, behaving macaques. We combined electrical microstimulation of the striatum with simultaneous electrophysiological recording in the lateral habenula (LHb) followed by immunohistochemistry. We demonstrate that striosomes provide the predominant striatal input to the macaque pallido-habenular circuit, which is known to function in relation to reinforcement signaling. Further, our experiments suggest that striosomes from different striatal regions may convergently influence the lateral habenula. This work now opens the way to defining the functions of striosomes in behaving primates in relation to mood, motivation, and action.

The evolutionary old forebrain as site of action to develop new psychotropic drugs

BACKGROUND

Previously, the authors have developed a model of how reward-seeking and distress- avoiding behaviour is regulated by the human brain. The forebrain's evolution in vertebrates was taken as a starting point.

AIMS

The authors want to inspire colleagues to study in particular the pharmacological effects on the described ancient forebrain structures in order to modify specific symptoms of mental disorders.

METHODS

Compilation of data and ideas of previous articles, with examples to illustrate.

RESULTS

A primary (lamprey-like), secondary (frog-like) and tertiary (mammal-like) forebrain can be distinguished, organized according to a Russian doll model. The first constituent is primarily involved in producing the emotional response, while the last is principally concerned with constructing conscious cognitive behaviour (including verbal and written communication). Mental disorders comprise (partly related and partly unrelated) biological and rational phenomena. The secondary system regulates the intensity of reward-seeking and distress-avoiding behaviour. An essential component of the primary forebrain evaluates the results of behavioural actions: the lateral habenula-projecting pallidum. These neurons regulate the activity of ascending dopaminergic pathways. The authors suggest that these habenula-projecting pallidum neurons are targeted by subanaesthetic dosages of ketamine. The medial habenula is enriched with nicotinergic acetylcholine receptors and regulates the activity of ascending adrenergic and serotonergic neurons. This may link varenicline-induced hostility to selective serotonin reuptake inhibitor-induced aggression.

CONCLUSIONS

Studying the effects of new compounds on the primary and secondary brains in lampreys and frogs may yield interesting new treatments of mental disorders.

Non-motor Characterization of the Basal Ganglia: Evidence From Human and Non-human Primate Electrophysiology

Although the basal ganglia have been implicated in a growing list of human behaviors, they include some of the least understood nuclei in the brain. For several decades studies have employed numerous methodologies to uncover evidence pointing to the basal ganglia as a hub of both motor and non-motor function. Recently, new electrophysiological characterization of the basal ganglia in humans has become possible through direct access to these deep structures as part of routine neurosurgery. Electrophysiological approaches for identifying non-motor function have the potential to unlock a deeper understanding of pathways that may inform clinical interventions and particularly neuromodulation. Various electrophysiological modalities can also be combined to reveal functional connections between the basal ganglia and traditional structures throughout the neocortex that have been linked to non-motor behavior. Several reviews have previously summarized evidence for non-motor function in the basal ganglia stemming from behavioral, clinical, computational, imaging, and non-primate animal studies; in this review, instead we turn to electrophysiological studies of non-human primates and humans. We begin by introducing common electrophysiological methodologies for basal ganglia investigation, and then we discuss studies across numerous non-motor domains–emotion, response inhibition, conflict, decision-making, error-detection and surprise, reward processing, language, and time processing. We discuss the limitations of current approaches and highlight the current state of the information.

Circuits regulating pleasure and happiness: evolution and role in mental disorders

Taking the evolutionary development of the forebrain as a starting point, the authors developed a biological framework for the subcortical regulation of human emotional behaviour which may offer an explanation for the pathogenesis of the principle symptoms of mental disorders. Appetitive-searching (reward-seeking) and distress-avoiding (misery-fleeing) behaviour are essential for all free-moving animals to stay alive and to have offspring. Even the oldest ocean-dwelling animal creatures, living about 560 million years ago and human ancestors, must therefore have been capable of generating these behaviours. Our earliest vertebrate ancestors, with a brain comparable with the modern lamprey, had a sophisticated extrapyramidal system generating and controlling all motions as well as a circuit including the habenula for the evaluation of the benefits of their actions. Almost the complete endbrain of the first land animals with a brain comparable with that of amphibians became assimilated into the human amygdaloid and hippocampal complex, whereas only a small part of the dorsal pallium and striatum developed into the ventral extrapyramidal circuits and the later insular cortex. The entire neocortex covering the hemispheres is of recent evolutionary origin, appearing first in early mammals. During the entire evolution of vertebrates, the habenular system was well conserved and maintained its function in regulating the intensity of reward-seeking (pleasure-related) and misery-fleeing (happiness-related) behaviour. The authors propose that the same is true in humans. Symptomatology of human mental disorders can be considered to result from maladaptation within a similar amygdalo/hippocampal-habenular-mesencephalic-ventral striatal system.

The lateral habenula interacts with the hypothalamo-pituitary adrenal axis response upon stressful cognitive demand in rats

The lateral habenula (LHb) is involved in emotional and cognitive behaviors. Recently, we have shown in rats that blockade of excitatory inputs to the LHb not only induced deficits of memory retrieval in the water maze, but also altered swim strategies (i.e., induced excessive thigmotaxis). The latter observation, although consistent with the occurrence of memory deficits, could also possibly be the consequence of an excessive level of stress, further suggesting a role for the LHb in the stress response in our behavioral paradigm. To test this hypothesis we performed in rats intra-LHb infusion of 6-cyano-7-nitroquinoxaline-2,3-dione (CNQX, 267 ng/side in 0.3 μL), or vehicle, and assessed the responsiveness of the hypothalamo-pituitary adrenal (HPA) axis to environmental stressful or non-stressful situations. We have measured plasma corticosterone (CORT) concentrations at different time points before and following intra-LHb infusion of CNQX - or of the same volume of vehicle - in three conditions: during the probe test of a water maze experiment; in an anxiety test, the elevated plus maze; and in a home cage condition. Whereas there were no differences in the home cage condition and in the elevated plus maze, in the water maze experiment we observed that CNQX-treated rats presented, along with memory deficits, a higher level of blood CORT than vehicle-treated rats. These results suggest that perturbations of the modulation of the HPA axis are consecutive to the alteration of LHb function, whether it is the result of a defective direct control of the LHb over the HPA axis, or the consequence of memory deficits.

Impulsivity in Parkinson's disease is associated with altered subthalamic but not globus pallidus internus activity

BACKGROUND

A significant subset of patients with Parkinson's disease (PD) suffer from impulse control disorders (ICDs). A hallmark feature of many ICDs is the pursuit of rewarding behaviours despite negative consequences. Recent evidence implicates the subthalamic nucleus (STN) and globus pallidus internus (GPi) in reward and punishment processing, and deep brain stimulation (DBS) of these structures has been associated with changes in ICD symptoms.

METHODS

We tested the hypothesis that in patients with PD diagnosed with ICD, neurons in the STN and GPi would be more responsive to reward-related stimuli and less responsive to loss-related stimuli. We studied a cohort of 43 patients with PD (12 with an ICD and 31 without) undergoing DBS electrode placement surgery. Patients performed a behavioural task in which their action choices were motivated by the potential for either a monetary reward or a monetary loss. During task performance, the activity of individual neurons was recorded in either the STN (n=100) or the GPi (n=100).

RESULTS

The presence of an ICD was associated with significantly greater proportions of reward responsive neurons (p<0.01) and significantly lower proportions of loss responsive neurons (p<0.05) in the STN, but not in the GPi.

CONCLUSIONS

These findings provide further evidence of STN involvement in impulsive behaviour in the PD population.

Review of the cytology and connections of the lateral habenula, an avatar of adaptive behaving

The cytology and connections of the lateral habenula (LHb) are reviewed. The habenula is first introduced, after which the cytology of the LHb is discussed mainly with reference to cell types, general topography and descriptions of subnuclei. An overview of LHb afferent connections is given followed by some details about the projections to LHb from a number of structures. An overview of lateral habenula efferent connections is given followed by some details about the projections from LHb to a number of structures. In considering the afferent and efferent connections of the LHb some attention is given to the relative validity of regarding it as a bi-partite structure featuring 'limbic' and 'pallidal' parts. The paper ends with some concluding remarks about the relative place of the LHb in adaptive behaving.

Control of behavioral flexibility by the lateral habenula

The ability to rapidly switch behaviors in dynamic environments is fundamental to survival across species. Recognizing when an ongoing behavioral strategy should be replaced by an alternative one requires the integration of a diverse number of cues both internal and external to the organism including hunger, stress, or the presence of reward predictive cues. Increasingly sophisticated behavioral paradigms coupled with state of the art electrophysiological and pharmacological approaches have delineated a brain circuit involved in behavioral flexibility. However, how diverse contextual cues are integrated to influence strategy selection on a trial by trial basis remains largely unknown. One promising candidate for integration of internal and external cues to determine whether an ongoing behavioral strategy is appropriate is the lateral habenula (LHb). The LHb receives input from many brain areas that signal both internal and external environmental contexts and in turn projects to areas involved in behavioral monitoring and plasticity. This review examines how these connections, combined with recent pharmacological and electrophysiological results reveal a critical role for the LHb in behavioral flexibility in dynamic environments. This proposed role extends the known contributions of the LHb to motivated behaviors and suggests that the fundamental role of the LHb in these behaviors goes beyond signaling rewards and punishments to dopaminergic systems.

The Lateral Habenula Circuitry: Reward Processing and Cognitive Control

There has been a growing interest in understanding the role of the lateral habenula (LHb) in reward processing, affect regulation, and goal-directed behaviors. The LHb gets major inputs from the habenula-projecting globus pallidus and the mPFC, sending its efferents to the dopaminergic VTA and SNc, serotonergic dorsal raphe nuclei, and the GABAergic rostromedial tegmental nucleus. Recent studies have made advances in our understanding of the LHb circuit organization, yet the precise mechanisms of its involvement in complex behaviors are largely unknown. To begin to address this unresolved question, we present here emerging cross-species perspectives with a goal to provide a more refined understanding of the role of the LHb circuits in reward and cognition. We begin by highlighting recent findings from rodent experiments using optogenetics, electrophysiology, molecular, pharmacology, and tracing techniques that reveal diverse neural phenotypes in the LHb circuits that may underlie previously undescribed behavioral functions. We then discuss results from electrophysiological studies in macaques that suggest that the LHb cooperates with the anterior cingulate cortex to monitor action outcomes and signal behavioral adjustment. Finally, we provide an integrated summary of cross-species findings and discuss how further research on the connectivity, neural signaling, and physiology of the LHb circuits can deepen our understanding of the role of the LHb in normal and maladaptive behaviors associated with mental illnesses and drug abuse.

Role of the lateral habenula in memory through online processing of information

Our memory abilities, whether they involve short-term working memory or long-term episodic or procedural memories, are essential for our well-being, our capacity to adapt to constraints of our environment and survival. Therefore, several key brain regions and neurotransmitter systems are engaged in the processing of sensory information to either maintain such information in working memory so that it will quickly be used, and/or participate in the elaboration and storage of enduring traces useful for longer periods of time. Animal research has recently attracted attention on the lateral habenula which, as shown in rodents and non-human primates, seems to process information stemming in the main regions involved in memory processing, e.g., the medial prefrontal cortex, the hippocampus, the amygdala, the septal region, the basal ganglia, and participates in the control of key memory-related neurotransmitters systems, i.e., dopamine, serotonin, acetylcholine. Recently, the lateral habenula has been involved in working and spatial reference memories, in rodents, likely by participating in online processing of contextual information. In addition, several behavioral studies strongly suggest that it is also involved in the processing of the emotional valance of incoming information in order to adapt to particularly stressful situations. Therefore, the lateral habenula appears like a key region at the interface between cognition and emotion to participate in the selection of appropriate behaviors.

The lateral hypothalamus to lateral habenula projection, but not the ventral pallidum to lateral habenula projection, regulates voluntary ethanol consumption

The lateral habenula (LHb) is an epithalamic brain region implicated in aversive processing via negative modulation of midbrain dopamine (DA) and serotonin (5-HT) systems. Given the role of the LHb in inhibiting DA and 5-HT systems, it is thought to be involved in various psychiatric pathologies, including drug addiction. In support, it has been shown that LHb plays a critical role in cocaine- and ethanol-related behaviors, most likely by mediating drug-induced aversive conditioning. In our previous work, we showed that LHb lesions increased voluntary ethanol consumption and operant ethanol self-administration and blocked yohimbine-induced reinstatement of ethanol self-administration. LHb lesions also attenuated ethanol-induced conditioned taste aversion suggesting that a mechanism for the increased intake of ethanol may be reduced aversion learning. However, whether afferents to the LHb are required for mediating effects of the LHb on these behaviors remained to be investigated. Our present results show that lesioning the fiber bundle carrying afferent inputs to the LHb, the stria medullaris (SM), increases voluntary ethanol consumption, suggesting that afferent structures projecting to the LHb are important for mediating ethanol-directed behaviors. We then chose two afferent structures as the focus of our investigation. We specifically studied the role of the inputs from the lateral hypothalamus (LH) and ventral pallidum (VP) to the LHb in ethanol-directed behaviors. Our results show that the LH-LHb projection is necessary for regulating voluntary ethanol consumption. These results are an important first step towards understanding the functional role of afferents to LHb with regard to ethanol consumption.

The roles of the orbitofrontal cortex via the habenula in non-reward and depression, and in the responses of serotonin and dopamine neurons

Cortical regions such as the orbitofrontal cortex involved in reward and in non-reward and which are implicated in depression, and the amygdala, are connected to the habenula via the striatum and pallidum, and via subcortical limbic structures. The habenula in turn projects to the raphe nuclei, the source of the serotonin-containing neurons that project to the forebrain. It is proposed that this provides a route for cortical signals related to reward, and to not obtaining expected rewards, to influence the serotonin-containing neuronal system that is influenced by many antidepressant treatments. This helps to provide a more circuit-based understanding of the brain mechanisms related to depression, and how some treatments influence this system. The habenula also projects via the rostromedial tegmental nucleus to the dopamine-containing neurons, and this, it is proposed, provides a route for reward prediction error signals and other reward- and punishment-related signals of cortical and striatal origin to influence the dopamine system.

The human subthalamic nucleus and globus pallidus internus differentially encode reward during action control

The subthalamic nucleus (STN) and globus pallidus internus (GPi) have recently been shown to encode reward, but few studies have been performed in humans. We investigated STN and GPi encoding of reward and loss (i.e., valence) in humans with Parkinson's disease. To test the hypothesis that STN and GPi neurons would change their firing rate in response to reward- and loss-related stimuli, we recorded the activity of individual neurons while participants performed a behavioral task. In the task, action choices were associated with potential rewarding, punitive, or neutral outcomes. We found that STN and GPi neurons encode valence-related information during action control, but the proportion of valence-responsive neurons was greater in the STN compared to the GPi. In the STN, reward-related stimuli mobilized a greater proportion of neurons than loss-related stimuli. We also found surprising limbic overlap with the sensorimotor regions in both the STN and GPi, and this overlap was greater than has been previously reported. These findings may help to explain alterations in limbic function that have been observed following deep brain stimulation therapy of the STN and GPi. Hum Brain Mapp 38:1952-1964, 2017. © 2017 Wiley Periodicals, Inc.

Circuits Regulating Pleasure and Happiness: The Evolution of the Amygdalar-Hippocampal-Habenular Connectivity in Vertebrates

Appetitive-searching (reward-seeking) and distress-avoiding (misery-fleeing) behavior are essential for all free moving animals to stay alive and to have offspring. Therefore, even the oldest ocean-dwelling animal creatures, living about 560 million years ago and human ancestors, must have been capable of generating these behaviors. The current article describes the evolution of the forebrain with special reference to the development of the misery-fleeing system. Although, the earliest vertebrate ancestor already possessed a dorsal pallium, which corresponds to the human neocortex, the structure and function of the neocortex was acquired quite recently within the mammalian evolutionary line. Up to, and including, amphibians, the dorsal pallium can be considered to be an extension of the medial pallium, which later develops into the hippocampus. The ventral and lateral pallium largely go up into the corticoid part of the amygdala. The striatopallidum of these early vertebrates becomes extended amygdala, consisting of centromedial amygdala (striatum) connected with the bed nucleus of the stria terminalis (pallidum). This amygdaloid system gives output to hypothalamus and brainstem, but also a connection with the cerebral cortex exists, which in part was created after the development of the more recent cerebral neocortex. Apart from bidirectional connectivity with the hippocampal complex, this route can also be considered to be an output channel as the fornix connects the hippocampus with the medial septum, which is the most important input structure of the medial habenula. The medial habenula regulates the activity of midbrain structures adjusting the intensity of the misery-fleeing response. Within the bed nucleus of the stria terminalis the human homolog of the ancient lateral habenula-projecting globus pallidus may exist; this structure is important for the evaluation of efficacy of the reward-seeking response. The described organization offers a framework for the regulation of the stress response, including the medial habenula and the subgenual cingulate cortex, in which dysfunction may explain the major symptoms of mood and anxiety disorders.

The functional logic of corticostriatal connections

Unidirectional connections from the cortex to the matrix of the corpus striatum initiate the cortico-basal ganglia (BG)-thalamocortical loop, thought to be important in momentary action selection and in longer-term fine tuning of behavioural repertoire; a discrete set of striatal compartments, striosomes, has the complementary role of registering or anticipating reward that shapes corticostriatal plasticity. Re-entrant signals traversing the cortico-BG loop impact predominantly frontal cortices, conveyed through topographically ordered output channels; by contrast, striatal input signals originate from a far broader span of cortex, and are far more divergent in their termination. The term 'disclosed loop' is introduced to describe this organisation: a closed circuit that is open to outside influence at the initial stage of cortical input. The closed circuit component of corticostriatal afferents is newly dubbed 'operative', as it is proposed to establish the bid for action selection on the part of an incipient cortical action plan; the broader set of converging corticostriatal afferents is described as contextual. A corollary of this proposal is that every unit of the striatal volume, including the long, C-shaped tail of the caudate nucleus, should receive a mandatory component of operative input, and hence include at least one area of BG-recipient cortex amongst the sources of its corticostriatal afferents. Individual operative afferents contact twin classes of GABAergic striatal projection neuron (SPN), distinguished by their neurochemical character, and onward circuitry. This is the basis of the classic direct and indirect pathway model of the cortico-BG loop. Each pathway utilises a serial chain of inhibition, with two such links, or three, providing positive and negative feedback, respectively. Operative co-activation of direct and indirect SPNs is, therefore, pictured to simultaneously promote action, and to restrain it. The balance of this rival activity is determined by the contextual inputs, which summarise the external and internal sensory environment, and the state of ongoing behavioural priorities. Notably, the distributed sources of contextual convergence upon a striatal locus mirror the transcortical network harnessed by the origin of the operative input to that locus, thereby capturing a similar set of contingencies relevant to determining action. The disclosed loop formulation of corticostriatal and subsequent BG loop circuitry, as advanced here, refines the operating rationale of the classic model and allows the integration of more recent anatomical and physiological data, some of which can appear at variance with the classic model. Equally, it provides a lucid functional context for continuing cellular studies of SPN biophysics and mechanisms of synaptic plasticity.

Inhibiting Lateral Habenula Improves L-DOPA-Induced Dyskinesia

BACKGROUND

A systematic search of brain nuclei putatively involved in L-3,4-dihydroxyphenylalanine (L-DOPA)-induced dyskinesia (LID) in Parkinson's disease shed light, notably, upon the lateral habenula (LHb), which displayed an overexpression of the ∆FosB, ARC, and Zif268 immediate-early genes only in rats experiencing abnormal involuntary movements (AIMs). We thus hypothesized that LHb might play a role in LID.

METHODS

∆FosB immunoreactivity, 2-deoxyglucose uptake, and firing activity of LHb were studied in experimental models of Parkinson's disease and LID. ΔFosB-expressing LHb neurons were then targeted using the Daun02-inactivation method. A total of 18 monkeys and 55 rats were used.

RESULTS

LHb was found to be metabolically modified in dyskinetic monkeys and its neuronal firing frequency significantly increased in ON L-DOPA dyskinetic 6-hydroxydopamine-lesioned rats, suggesting that increased LHb neuronal activity in response to L-DOPA is related to AIM manifestation. Therefore, to mechanistically test if LHb neuronal activity might affect AIM severity, following induction of AIMs, 6-hydroxydopamine rats were injected with Daun02 in the LHb previously transfected with ß-galactosidase under control of the FosB promoter. Three days after Daun02 administration, animals were tested daily with L-DOPA to assess LID and L-DOPA-induced rotations. Inactivation of ∆FosB-expressing neurons significantly reduced AIM severity and also increased rotations. Interestingly, the dopaminergic D1 receptor was overexpressed only on the lesioned side of dyskinetic rats in LHb and co-localized with ΔFosB, suggesting a D1 receptor-mediated mechanism supporting the LHb involvement in AIMs.

CONCLUSIONS

This study highlights the role of LHb in LID, offering a new target to innovative treatments of LID.

Sources of input to the rostromedial tegmental nucleus, ventral tegmental area, and lateral habenula compared: A study in rat

Profound inhibitory control exerted on midbrain dopaminergic neurons by the lateral habenula (LHb), which has mainly excitatory outputs, is mediated by the GABAergic rostromedial tegmental nucleus (RMTg), which strongly innervates dopaminergic neurons in the ventral midbrain. Early reports indicated that the afferent connections of the RMTg, excepting its very strong LHb inputs, do not differ appreciably from those of the ventral tegmental area (VTA). Presumably, however, the RMTg contributes more to behavioral synthesis than to simply invert the valence of the excitatory signal coming from the LHb. Therefore, the present study was done to directly compare the inputs to the RMTg and VTA and, in deference to its substantial involvement with this circuitry, the LHb was also included in the comparison. Data indicated that, while the afferents of the RMTg, VTA, and LHb do originate within the same large pool of central nervous system (CNS) structures, each is also related to structures that project more strongly to it than to the others. The VTA gets robust input from ventral striatopallidum and extended amygdala, whereas RMTg biased inputs arise in structures with a more direct impact on motor function, such as deep layers of the contralateral superior colliculus, deep cerebellar and several brainstem nuclei, and, via a relay in the LHb, the entopeduncular nucleus. Input from the ventral pallidal-lateral preoptic-lateral hypothalamus continuum is strong in the RMTg and VTA and dominant in the LHb. Axon collateralization was also investigated, providing additional insights into the organization of the circuitry of this important triad of structures.

A Corticostriatal Path Targeting Striosomes Controls Decision-Making under Conflict

A striking neurochemical form of compartmentalization has been found in the striatum of humans and other species, dividing it into striosomes and matrix. The function of this organization has been unclear, but the anatomical connections of striosomes indicate their relation to emotion-related brain regions, including the medial prefrontal cortex. We capitalized on this fact by combining pathway-specific optogenetics and electrophysiology in behaving rats to search for selective functions of striosomes. We demonstrate that a medial prefronto-striosomal circuit is selectively active in and causally necessary for cost-benefit decision-making under approach-avoidance conflict conditions known to evoke anxiety in humans. We show that this circuit has unique dynamic properties likely reflecting striatal interneuron function. These findings demonstrate that cognitive and emotion-related functions are, like sensory-motor processing, subject to encoding within compartmentally organized representations in the forebrain and suggest that striosome-targeting corticostriatal circuits can underlie neural processing of decisions fundamental for survival.

Basal ganglia circuit loops, dopamine and motivation: A review and enquiry

Dopamine neurons located in the midbrain play a role in motivation that regulates approach behavior (approach motivation). In addition, activation and inactivation of dopamine neurons regulate mood and induce reward and aversion, respectively. Accumulating evidence suggests that such motivational role of dopamine neurons is not limited to those located in the ventral tegmental area, but also in the substantia nigra. The present paper reviews previous rodent work concerning dopamine's role in approach motivation and the connectivity of dopamine neurons, and proposes two working models: One concerns the relationship between extracellular dopamine concentration and approach motivation. High, moderate and low concentrations of extracellular dopamine induce euphoric, seeking and aversive states, respectively. The other concerns circuit loops involving the cerebral cortex, basal ganglia, thalamus, epithalamus, and midbrain through which dopaminergic activity alters approach motivation. These models should help to generate hypothesis-driven research and provide insights for understanding altered states associated with drugs of abuse and affective disorders.

Immunohistochemical study on the neuronal diversity and three-dimensional organization of the mouse entopeduncular nucleus

The entopeduncular nucleus (EPN) is one of the major output nuclei of the basal ganglia in rodents. Previous studies have divided it into rostral and caudal halves, with the former containing somatostatin (SOM)-immunoreactive neurons and the latter dominated by parvalbumin (PV)-containing neurons, respectively. However, it is unclear whether this simple rostrocaudal segmentation is appropriate, and the possibility of the existence of other neuronal populations remains to be investigated. In this study the cytoarchitecture of the mouse EPN was analyzed immunohistochemically. Substance P (SP)-immunoreactivity determined the extent of the EPN, which was 800 μm-long along the rostrocaudal axis. PV-positive neurons were concentrated in the caudal two-thirds of this range. PV-negative neurons were abundant in the rostral half but were further located caudally around the PV neuron-rich core. PV(+)/SOM(-) and PV(-)/SOM(+) neurons constituted 28.6% and 45.7% of EPN neurons, respectively, whereas the remaining population (25.7%) exhibited neither immunoreactivity. Eleven percent of EPN neurons lacked immunoreactivity for glutamic acid decarboxylase, indicating their non-GABAergic nature. Three-dimensional reconstruction revealed that PV-rich/SP-poor core was surrounded by PV-poor/SP-rich shell region. Therefore, presumptive thalamus-targeting PV neurons are outnumbered by other populations, and the regional heterogeneity shown here might be related to functionally distinct pathways through the basal ganglia.

Lateral habenula as a link between dopaminergic and serotonergic systems contributes to depressive symptoms in Parkinson's disease

Degeneration of substantia nigra dopaminergic neurons is a key pathological change of Parkinson's disease (PD), and its motor consequences have been widely recognized. Recently, mood disorders associated with PD have begun to attract a great deal of interest, however, their pathogenesis remains unclear. PD is associated with not only degenerative changes in dopaminergic neurons in the substantia nigra but also changes in serotonergic neurons in the raphe nuclei. The abnormalities in central 5-hydroxytryptamine (5-HT) neurotransmission are thought to play a key role in the pathogenesis of depression. The lateral habenula (LHb) is closely related to the substantia nigra and raphe nuclei, and its hyperactivity is closely related to the pathogenesis of depression. In this study, we screened rats with depressive-like behaviors from PD model animals and found that cytochrome c oxidase activity in the LHb of these rats was twice that seen in the control rats. In the forced swim test, LHb lesions caused a decrease in depressive-like behavior of PD rats as indexed by decreased immobility times and increased climbing times. Additionally, LHb lesions caused an enhance in 5-HT levels in the raphe nuclei. These results suggest that LHb lesions may improve depressive-like behavior in PD rats by increasing 5-HT levels in the raphe nuclei. Thus, LHb contributes to the depressive-like behavior in PD rats via mediating the effects of dopaminergic neurons in the substantia nigra on serotonergic neurons in the raphe nuclei.

An update on the connections of the ventral mesencephalic dopaminergic complex

This review covers the intrinsic organization and afferent and efferent connections of the midbrain dopaminergic complex, comprising the substantia nigra, ventral tegmental area and retrorubral field, which house, respectively, the A9, A10 and A8 groups of nigrostriatal, mesolimbic and mesocortical dopaminergic neurons. In addition, A10dc (dorsal, caudal) and A10rv (rostroventral) extensions into, respectively, the ventrolateral periaqueductal gray and supramammillary nucleus are discussed. Associated intrinsic and extrinsic connections of the midbrain dopaminergic complex that utilize gamma-aminobutyric acid (GABA), glutamate and neuropeptides and various co-expressed combinations of these compounds are considered in conjunction with the dopamine-containing systems. A framework is provided for understanding the organization of massive afferent systems descending and ascending to the midbrain dopaminergic complex from the telencephalon and brainstem, respectively. Within the context of this framework, the basal ganglia direct and indirect output pathways are treated in some detail. Findings from rodent brain are briefly compared with those from primates, including humans. Recent literature is emphasized, including traditional experimental neuroanatomical and modern gene transfer and optogenetic studies. An attempt was made to provide sufficient background and cite a representative sampling of earlier primary papers and reviews so that people new to the field may find this to be a relatively comprehensive treatment of the subject.

Dopamine system: manager of neural pathways

There are a growing number of roles that midbrain dopamine (DA) neurons assume, such as, reward, aversion, alerting and vigor. Here I propose a theory that may be able to explain why the suggested functions of DA came about. It has been suggested that largely parallel cortico-basal ganglia-thalamo-cortico loops exist to control different aspects of behavior. I propose that (1) the midbrain DA system is organized in a similar manner, with different groups of DA neurons corresponding to these parallel neural pathways (NPs). The DA system can be viewed as the "manager" of these parallel NPs in that it recruits and activates only the task-relevant NPs when they are needed. It is likely that the functions of those NPs that have been consistently activated by the corresponding DA groups are facilitated. I also propose that (2) there are two levels of DA roles: the How and What roles. The How role is encoded in tonic and phasic DA neuron firing patterns and gives a directive to its target NP: how vigorously its function needs to be carried out. The tonic DA firing is to provide the needed level of DA in the target NPs to support their expected behavioral and mental functions; it is only when a sudden unexpected boost or suppression of activity is required by the relevant target NP that DA neurons in the corresponding NP act in a phasic manner. The What role is the implementational aspect of the role of DA in the target NP, such as binding to D1 receptors to boost working memory. This What aspect of DA explains why DA seems to assume different functions depending on the region of the brain in which it is involved. In terms of the role of the lateral habenula (LHb), the LHb is expected to suppress maladaptive behaviors and mental processes by controlling the DA system. The demand-based smart management by the DA system may have given animals an edge in evolution with adaptive behaviors and a better survival rate in resource-scarce situations.