Which cue to "want?" Central amygdala opioid activation enhances and focuses incentive salience on a prepotent reward cue

A 40-year analysis of central neuroanatomical and neurochemical circuits mediating homeostatic intake and hedonic intake and preferences in rodents

This perspective review was written in response to the celebration of the 60th anniversary of the journal, Brain Research, and covers the evolving focus of my laboratory's work over 40 years in the neurobiological substrates of ingestive behavior in rodents. Following our initial work examining the effects of systemic and ventricular administration of general and selective opioid receptor agonists and antagonists on food intake under spontaneous, deprivation, glucoprivic and hedonic conditions, my laboratory in close collaboration with Drs. Gavril Pasternak and Ying-Xian Pan utilized an antisense oligodoxynucleotide knock-down technique affecting MOR-1, DOR-1, KOR-1 and ORL-1 genes as well as against G-protein subunits to study receptor mediation of opioid receptor agonist-induced feeding as well as feeding following regulatory challenges. Our laboratory employed intracerebral microinjection techniques to map limbic nucleus accumbens and ventral tegmental area central brain circuits mediating homeostatic and hedonic feeding responses through the use of selective mu, delta1, delta2 and kappa opioid receptor subtype agonists in combination with general and selective opioid, dopamineric, glutamatergic and GABAergic antagonists administered into the same site or the reciprocal site, allowing for the identification of a distributed brain network mediating these ingestive effects. Our laboratory in close collaboration with Dr. Anthony Sclafani then focused on the pharmacological, neuroanatomical and learning mechanisms related to the development of sugar- (sucrose, glucose and fructose) and fat- (corn oil) conditioned flavor preferences (CFP) in rats, and on murine genetic variance in food intake, preferences and the process of appetition.

Genomic factors associated with substance use disorder relapse: A critical review

Several genetic and epigenetic factors contribute to the elevated substance use disorder (SUD) relapse vulnerability, yet a comprehensive investigation into these factors is lacking. This review aims to delve into current literature to highlight key genomic factors associated with SUD relapse. Focusing on genetic predisposition and epigenetic modifications the review synthesized research findings of several genetic polymorphisms, histone modifications and DNA methylation patterns contributing to the initiation of SUD and the elevated relapse susceptibility. Notably, specific gene polymorphisms, such as Dopamine Receptor D2 gene (DRD2), Gamma-Aminobutyric Acid Receptor Alpha gene (GABRA2), Catechol-O-methyltransferase (COMT) gene, Dopamine Transporter (DAT1) gene and others were identified to be connected to various patterns of SUD relapse. Furthermore, SUD initiation and relapse has been shown to be influenced by epigenetics. Specifically, CpG hypermethylation has been associated with severe alcohol use disorder in the 5' untranslated region of the Bladder Cancer Associated Protein gene (BLCAP) and the upstream region of the Active BCR Related gene (ABR). Co-users of cannabis and tobacco showed notable variations in CpG site methylation, especially at the Aryl Hydrocarbon Receptor Repressor (AHRR), and factor II receptor-like 3 gene sites (F2RL3). In conclusion, there is good evidence of certain associations between genomic factors and relapse to SUD. However, further research is needed to ascertain causality effects of these factors and develop novel interventions for effective treatment and relapse prevention.

Leveraging individual differences in cue-reward learning to investigate the psychological and neural basis of shared psychiatric symptomatology: The sign-tracker/goal-tracker model

In our modern environment, we are bombarded with stimuli or cues that exert significant influence over our actions. The extent to which such cues attain control over or disrupt goal-directed behavior is dependent on several factors, including one's inherent tendencies. Using a rodent model, we have shown that individuals vary in the value they place on stimuli associated with reward. Some individuals, termed "goal-trackers," primarily attribute predictive value to reward cues, whereas others, termed "sign-trackers," attribute predictive and incentive value. Thus, for sign-trackers, the reward cue is transformed into an incentive stimulus that is capable of eliciting maladaptive behaviors. The sign-tracker/goal-tracker animal model has allowed us to refine our understanding of behavioral and computational theories related to reward learning and to parse the underlying neural processes. Further, the neurobehavioral profile of sign-trackers is relevant to several psychiatric disorders, including substance use disorder, impulse control disorders, obsessive-compulsive disorder, attention-deficit/hyperactivity disorder, and posttraumatic stress disorder. This model, therefore, can advance our understanding of the psychological and neurobiological mechanisms that contribute to individual differences in vulnerability to psychopathology. Notably, initial attempts at translation-capturing individual variability in the propensity to sign-track in humans-have been promising and in line with what we have learned from the animal model. In this review, we highlight the pivotal role played by the sign-tracker/goal-tracker animal model in enriching our understanding of the psychological and neural basis of motivated behavior and psychiatric symptomatology. (PsycInfo Database Record (c) 2024 APA, all rights reserved).

Encoding and context-dependent control of reward consumption within the central nucleus of the amygdala

Dysregulation of the central amygdala is thought to underlie aberrant choice in alcohol use disorder, but the role of central amygdala neural activity during reward choice and consumption is unclear. We recorded central amygdala neurons in male rats as they consumed alcohol or sucrose. We observed activity changes at the time of reward approach, as well as lick-entrained activity during ongoing consumption of both rewards. In choice scenarios where rats could drink sucrose, alcohol, or quinine-adulterated alcohol with or without central amygdala optogenetic stimulation, rats drank more of stimulation-paired options when the two bottles contained identical options. Given a choice among different options, central amygdala stimulation usually enhanced consumption of stimulation-paired rewards. However, optogenetic stimulation during consumption of the less-preferred option, alcohol, was unable to enhance alcohol intake while sucrose was available. These findings indicate that the central amygdala contributes to refining motivated pursuit toward the preferred available option.

Inhibition of Dopamine Neurons Prevents Incentive Value Encoding of a Reward Cue: With Revelations from Deep Phenotyping

The survival of an organism is dependent on its ability to respond to cues in the environment. Such cues can attain control over behavior as a function of the value ascribed to them. Some individuals have an inherent tendency to attribute reward-paired cues with incentive motivational value, or incentive salience. For these individuals, termed sign-trackers, a discrete cue that precedes reward delivery becomes attractive and desirable in its own right. Prior work suggests that the behavior of sign-trackers is dopamine-dependent, and cue-elicited dopamine in the NAc is believed to encode the incentive value of reward cues. Here we exploited the temporal resolution of optogenetics to determine whether selective inhibition of ventral tegmental area (VTA) dopamine neurons during cue presentation attenuates the propensity to sign-track. Using male tyrosine hydroxylase (TH)-Cre Long Evans rats, it was found that, under baseline conditions, ∼84% of TH-Cre rats tend to sign-track. Laser-induced inhibition of VTA dopamine neurons during cue presentation prevented the development of sign-tracking behavior, without affecting goal-tracking behavior. When laser inhibition was terminated, these same rats developed a sign-tracking response. Video analysis using DeepLabCutTM revealed that, relative to rats that received laser inhibition, rats in the control group spent more time near the location of the reward cue even when it was not present and were more likely to orient toward and approach the cue during its presentation. These findings demonstrate that cue-elicited dopamine release is critical for the attribution of incentive salience to reward cues.SIGNIFICANCE STATEMENT Activity of dopamine neurons in the ventral tegmental area (VTA) during cue presentation is necessary for the development of a sign-tracking, but not a goal-tracking, conditioned response in a Pavlovian task. We capitalized on the temporal precision of optogenetics to pair cue presentation with inhibition of VTA dopamine neurons. A detailed behavioral analysis with DeepLabCutTM revealed that cue-directed behaviors do not emerge without dopamine neuron activity in the VTA. Importantly, however, when optogenetic inhibition is lifted, cue-directed behaviors increase, and a sign-tracking response develops. These findings confirm the necessity of dopamine neuron activity in the VTA during cue presentation to encode the incentive value of reward cues.

Separating desire from prediction of outcome value

Individuals typically want what they expect to like, often based on memories of previous positive experiences. However, in some situations desire can decouple completely from memories and from learned predictions of outcome value. The potential for desire to separate from prediction arises from independent operating rules that control motivational incentive salience. Incentive salience, or 'wanting', is a type of mesolimbic desire that evolved for adaptive goals, but can also generate maladaptive addictions. Two proof-of-principle examples are presented here to show how motivational 'wanting' can soar above memory-based predictions of outcome value: (i) 'wanting what is remembered to be disgusting', and (ii) 'wanting what is predicted to hurt'. Consequently, even outcomes remembered and predicted to be negatively aversive can become positively 'wanted'. Similarly, in human addictions, people may experience powerful cue-triggered cravings for outcomes that are not predicted to be enjoyable.

Encoding and context-dependent control of reward consumption within the central nucleus of the amygdala

The ability to evaluate and select a preferred option among a variety of available offers is an essential aspect of goal-directed behavior. Dysregulation of this valuation process is characteristic of alcohol use disorder, with the central amygdala being implicated in persistent alcohol pursuit. However, the mechanism by which the central amygdala encodes and promotes the motivation to seek and consume alcohol remains unclear. We recorded single-unit activity in male Long-Evans rats as they consumed 10% ethanol or 14.2% sucrose. We observed significant activity at the time of approach to alcohol or sucrose, as well as lick-entrained activity during the ongoing consumption of both alcohol and sucrose. We then evaluated the ability of central amygdala optogenetic manipulation time-locked to consumption to alter ongoing intake of alcohol or sucrose, a preferred non-drug reward. In closed two-choice scenarios where rats could drink only sucrose, alcohol, or quinine-adulterated alcohol with or without central amygdala stimulation, rats drank more of stimulation-paired options. Microstructural analysis of licking patterns suggests these effects were mediated by changes in motivation, not palatability. Given a choice among different options, central amygdala stimulation enhanced consumption if the stimulation was associated with the preferred reward while closed-loop inhibition only decreased consumption if the options were equally valued. However, optogenetic stimulation during consumption of the less-preferred option, alcohol, was unable to enhance overall alcohol intake while sucrose was available. Collectively, these findings indicate that the central amygdala processes the motivational value of available offers to promote pursuit of the most preferred available option.

Critical tests of fuzzy trace theory in brain and behavior: uncertainty across time, probability, and development

Uncertainty permeates decisions from the trivial to the profound. Integrating brain and behavioral evidence, we discuss how probabilistic (varied outcomes) and temporal (delayed outcomes) uncertainty differ across age and individuals; how critical tests adjudicate between theories of uncertainty (prospect theory and fuzzy-trace theory); and how these mechanisms might be represented in the brain. The same categorical gist representations of gains and losses account for choices and eye-tracking data in both value-allocation (add money to gambles) and risky-choice tasks, disconfirming prospect theory and confirming predictions of fuzzy-trace theory. The analysis is extended to delay discounting and disambiguated choices, explaining hidden-zero effects that similarly turn on categorical distinctions between some gain and no gain, certain gain and uncertain gain, gain and loss, and now and later. Bold activation implicates dorsolateral prefrontal and posterior parietal cortices in gist strategies that are not just one tool in a grab-bag of cognitive options but rather are general strategies that systematically predict behaviors across many different tasks involving probabilistic and temporal uncertainty. High valuation (e.g., ventral striatum; ventromedial prefrontal cortex) and low executive control (e.g., lateral prefrontal cortex) contribute to risky and impatient choices, especially in youth. However, valuation in ventral striatum supports reward-maximizing and gist strategies in adulthood. Indeed, processing becomes less "rational" in the sense of maximizing gains and more noncompensatory (eye movements indicate fewer tradeoffs) as development progresses from adolescence to adulthood, as predicted. Implications for theoretically predicted "public-health paradoxes" are discussed, including gist versus verbatim thinking in drug experimentation and addiction.

Endocannabinoid administration affects taste preference and the expression of cannabinoid and opioid receptors in the amygdala of early lactating cows

The aim of the study was to investigate the influence of intraperitoneal N-arachidonoylethanolamide (AEA) on taste preference for feed and water, tongue taste receptor signalling (TAS1R2, GNAT3), and endocannabinoid (CNR1, CNR2, GPR55) and opioid (OPRD1, OPRK1, OPRM1, OPRL1) receptors in the amygdala and nucleus accumbens in periparturient cows. We conducted taste preference tests using unaltered, umami-tasting, and sweet-tasting water and feed, before and after calving. After calving, eight cows received AEA injections (3 µg/(kg bodyweight × day), 25 days), whereas eight control (CON) cows received saline injections. Tissue was sampled 30 days after calving. Before calving, both cow groups preferred sweet-tasting feed and umami-tasting water. After calving, only the AEA-treated group preferred sweet-tasting feed, whereas the CON group showed no clear taste preference. In the amygdala, the mRNA expression of CNR1, OPRD1 (left hemisphere) and OPRK1 (right hemisphere) was lower in AEA animals than in CON animals, whereas no differences were found in the nucleus accumbens and tongue taste receptor expression. In conclusion, AEA administration enhanced existing taste preferences and reduced the expression of specific endocannabinoid and opioid receptors in the amygdala. The results support endocannabinoid-opioid interactions in the control of taste-dependent feed preference in early lactating cows.

Neural ensembles in the murine medial prefrontal cortex process distinct information during visual perceptual learning

BACKGROUND

Perceptual learning refers to an augmentation of an organism's ability to respond to external stimuli, which has been described in most sensory modalities. Visual perceptual learning (VPL) is a manifestation of plasticity in visual information processing that occurs in the adult brain, and can be used to ameliorate the ability of patients with visual defects mainly based on an improvement of detection or discrimination of features in visual tasks. While some brain regions such as the primary visual cortex have been described to participate in VPL, the way more general high-level cognitive brain areas are involved in this process remains unclear. Here, we showed that the medial prefrontal cortex (mPFC) was essential for both the training and maintenance processes of VPL in mouse models.

RESULTS

We built a new VPL model in a custom-designed training chamber to enable the utilization of miniScopes when mice freely executed the VPL task. We found that pyramidal neurons in the mPFC participate in both the training process and maintenance of VPL. By recording the calcium activity of mPFC pyramidal neurons while mice freely executed the task, distinct ON and OFF neural ensembles tuned to different behaviors were identified, which might encode different cognitive information. Decoding analysis showed that mouse behaviors could be well predicted using the activity of each ON ensemble. Furthermore, VPL recruited more reward-related components in the mPFC.

CONCLUSION

We revealed the neural mechanism underlying vision improvement following VPL and identify distinct ON and OFF neural ensembles in the mPFC that tuned to different information during visual perceptual training. These results uncover an important role of the mPFC in VPL, with more reward-related components being also involved, and pave the way for future clarification of the reward signal coding rules in VPL.

A motivation model of sex addiction - Relevance to the controversy over the concept

An integrative model of sexual addiction is presented, involving a combination of models based upon (i) incentive motivation theory and (ii) the dual organisation of the control of behavior. The model is related to ongoing arguments about the validity of the notion of addiction when applied to sexual behavior. It is suggested that the evidence strongly favors the viability of an addiction model of sex. Strong similarities to the classical addiction to hard drugs are observed and features can be better understood with the help of the model. These include tolerance, escalation and withdrawal symptoms. It is argued that other candidates for accounting for the phenomena, such as obsessive-compulsive behavior, faulty impulse control, high drive and hypersexuality do not fit the evidence. The role of dopamine is central to the model. The model's relevance to stress, abuse, development, psychopathy, fantasy, sex differences, evolutionary psychology and the interaction with drug-taking is shown.

Enduring disruption of reward and stress circuit activities by early-life adversity in male rats

In humans, early-life adversity (ELA) such as trauma, poverty, and chaotic environment is linked to increased risk of later-life emotional disorders including depression and substance abuse. These disorders involve underlying disruption of reward circuits and likely vary by sex. Accordingly, we previously found that ELA leads to anhedonia for natural rewards and cocaine in male rodents, whereas in females ELA instead increases vulnerability to addiction-like use of opioid drugs and palatable food. While these findings suggest that ELA-induced disruption of reward circuitry may differ between the sexes, the specific circuit nodes that are influenced by ELA in either sex remain poorly understood. Here, in adult male Sprague-Dawley rats, we ask how ELA impacts opioid addiction-relevant behaviors that we previously tested after ELA in females. We probe potential circuit mechanisms in males by assessing opioid-associated neuronal activation in stress and reward circuit nodes including nucleus accumbens (NAc), amygdala, medial prefrontal cortex (mPFC), and paraventricular thalamus. We find that ELA diminishes opioid-seeking behaviors in males, and alters heroin-induced activation of NAc, PFC, and amygdala, suggesting a potential circuit-based mechanism. These studies demonstrate that ELA leads to behavioral and neurobiological disruptions consistent with anhedonia in male rodents, unlike the increased opioid seeking we previously saw in females. Our findings, taken together with our prior work, suggest that men and women could face qualitatively different mental health consequences of ELA, which may be essential for individually tailoring future intervention strategies.

Eating driven by the gustatory insula: contrasting regulation by infralimbic vs. prelimbic cortices

Subregions within insular cortex and medial prefrontal cortex (mPFC) have been implicated in eating disorders; however, the way these brain regions interact to produce dysfunctional eating is poorly understood. The present study explored how two mPFC subregions, the infralimbic (IL) and prelimbic (PRL) cortices, regulate sucrose hyperphagia elicited specifically by a neurochemical manipulation of the agranular/dysgranular region of gustatory insula (AI/DI). Using intra-AI/DI infusion of the mu-opioid receptor (µ-OR) agonist, DAMGO (1 µg), sucrose hyperphagia was generated in ad-libitum-maintained rats, while in the same rat, either the IL or prelimbic (PRL) subregion of mPFC was inactivated bilaterally with muscimol (30 ng). Intra-IL muscimol markedly potentiated AI/DI DAMGO-induced sucrose hyperphagia by increasing eating bout duration and food consumption per bout. In contrast, PRL attenuated intra-AI/DI DAMGO-driven sucrose intake and feeding duration and eliminated the small DAMGO-induced increase in feeding bout initiation. Intra-IL or -PRL muscimol alone (i.e., without intra-AI/DI DAMGO) did not alter feeding behavior, but slightly reduced exploratory-like rearing in both mPFC subregions. These results reveal anatomical heterogeneity in mPFC regulation of the intense feeding-motivational state engendered by µ-OR signaling in the gustatory insula: IL significantly curtails consummatory activity, while PRL modestly contributes to feeding initiation. Results are discussed with regard to potential circuit-based mechanisms that may underlie the observed results.

Engaging endogenous opioid circuits in pain affective processes

The pervasive use of opioid compounds for pain relief is rooted in their utility as one of the most effective therapeutic strategies for providing analgesia. While the detrimental side effects of these compounds have significantly contributed to the current opioid epidemic, opioids still provide millions of patients with reprieve from the relentless and agonizing experience of pain. The human experience of pain has long recognized the perceived unpleasantness entangled with a unique sensation that is immediate and identifiable from the first-person subjective vantage point as "painful." From this phenomenological perspective, how is it that opioids interfere with pain perception? Evidence from human lesion, neuroimaging, and preclinical functional neuroanatomy approaches is sculpting the view that opioids predominately alleviate the affective or inferential appraisal of nociceptive neural information. Thus, opioids weaken pain-associated unpleasantness rather than modulate perceived sensory qualities. Here, we discuss the historical theories of pain to demonstrate how modern neuroscience is revisiting these ideas to deconstruct the brain mechanisms driving the emergence of aversive pain perceptions. We further detail how targeting opioidergic signaling within affective or emotional brain circuits remains a strong avenue for developing targeted pharmacological and gene-therapy analgesic treatments that might reduce the dependence on current clinical opioid options.

Nicotine Enhances Goal-Tracking in Ethanol and Food Pavlovian Conditioned Approach Paradigms

Rationale

Nicotine promotes alcohol intake through pharmacological and behavioral interactions. As an example of the latter, nicotine can facilitate approach toward food- and alcohol-associated stimuli ("sign-tracking") in lever-Pavlovian conditioned approach (PavCA) paradigms. However, we recently reported that nicotine can also enhance approach toward locations of reward delivery ("goal-tracking") triggered by ethanol-predictive stimuli when the location of ethanol delivery is non-static (i.e., a retractable sipper bottle).

Objective

To determine whether the non-static nature of the reward location could have biased the development of goal-tracking in our previous study (Loney et al., 2019); we assessed the effect of nicotine in a lever-PavCA paradigm wherein the location of ethanol delivery was static (i.e., a stationary liquid receptacle). Then, to determine whether nicotine's enhancement of goal-tracking is unique to ethanol-predictive stimuli, we assessed the effect of systemic nicotine on approach triggered by food-predictive stimuli in a lever-PavCA paradigm.

Methods

Long-Evans rats were used in two PavCA experiments wherein a lever predicted the receipt of ethanol (15% vol/vol; experiment 1) or food (experiment 2) into a stationary receptacle. Prior to testing, rats were administered nicotine (0.4 mg/kg subcutaneously) or saline systemically.

Results

In both experiments, nicotine increased measures of goal-tracking, but not sign-tracking.

Conclusion

Nicotine can facilitate approach to reward locations without facilitating approach to reward-predictive stimuli. As such, conceptualization of the mechanisms by which nicotine affects behavior must be expanded to explain an enhancement of goal-tracking by nicotine.

Incentive motivation: 'wanting' roles of central amygdala circuitry

The central nucleus of amygdala (CeA) mediates positively-valenced reward motivation as well as negatively-valenced fear. Optogenetic or neurochemical stimulation of CeA circuitry can generate intense incentive motivation to pursue and consume a paired natural food, sex, or addictive drug reward, and even create maladaptive 'wanting what hurts' such as attraction to a shock rod. Evidence indicates CeA stimulations selectively amplify incentive motivation ('wanting') but not hedonic impact ('liking') of the same reward. Further, valence flips can occur for CeA contributions to motivational salience. That is, CeA stimulation can promote either incentive motivation or fearful motivation, even in the same individual, depending on situation. These findings may carry implications for understanding CeA roles in neuropsychiatric disorders involving aberrant motivational salience, ranging from addiction to paranoia and anxiety disorders.

Central amygdala circuits in valence and salience processing

Behavioral responses to environmental stimuli are dictated by the affective valence of the stimulus, good (positive valence) or bad (negative valence). These stimuli can innately elicit an affective response that promotes approach or avoidance behavior. In addition to innately valenced stimuli, valence can also be assigned to initially neutral stimuli through associative learning. A stimulus of a given valence can vary in salience depending on the strength of the stimulus, the underlying state of the animal, and the context of the stimulus presentation. Salience endows the stimulus with the ability to direct attention and elicit preparatory responses to mount an incentive-based motivated behavior. The central nucleus of the amygdala (CeA) has emerged as an early integration point for valence and salience detection to engage preparatory autonomic responses and behavioral posturing in response to both aversive and appetitive stimuli. There are numerous cell types in the CeA that are involved in valence and salience processing through a variety of connections, and we will review the recent progress that has been made in identifying these circuit elements and their roles in these processes.

The Developmental Origins of Opioid Use Disorder and Its Comorbidities

Opioid use disorder (OUD) rarely presents as a unitary psychiatric condition, and the comorbid symptoms likely depend upon the diverse risk factors and mechanisms by which OUD can arise. These factors are heterogeneous and include genetic predisposition, exposure to prescription opioids, and environmental risks. Crucially, one key environmental risk factor for OUD is early life adversity (ELA). OUD and other substance use disorders are widely considered to derive in part from abnormal reward circuit function, which is likely also implicated in comorbid mental illnesses such as depression, bipolar disorder, and schizophrenia. ELA may disrupt reward circuit development and function in a manner predisposing to these disorders. Here, we describe new findings addressing the effects of ELA on reward circuitry that lead to OUD and comorbid disorders, potentially via shared neural mechanisms. We discuss some of these OUD-related problems in both humans and animals. We also highlight the increasingly apparent, crucial contribution of biological sex in mediating the range of ELA-induced disruptions of reward circuitry which may confer risk for the development of OUD and comorbid neuropsychiatric disorders.

Behavioral determinants in the expression of the Kamin blocking effect: Implications for associative learning theory

Associative learning makes important contributions to our behavior and decisions. The Kamin blocking effect is an associative learning phenomenon that plays a central role in understanding of the psychological principles underlying associative learning. However, several recent failures to replicate the blocking effect suggest that the conditions necessary for blocking are poorly understood. To understand the conditions necessary for blocking, here we review studies into the expression of blocking in subjects that either approach and interact with the conditioned cue (sign trackers) or approach and interact with the reward location (goal trackers) during appetitive classical conditioning. Psychological theory and the neurophysiological correlates of appetitive classical conditioning make opposing predictions regarding the expression of blocking in sign and goal trackers. We reconcile these opposing predictions in a qualitative model using two parallel learning processes. Such models offer a better framework for understanding the psychological associative structures acquired during learning, their interactions contributing to the conditioned response, and how they affect subsequent learning and the expression of the Kamin blocking effect.

Chronic fluoxetine treatment impairs motivation and reward learning by affecting neuronal plasticity in the central amygdala

BACKGROUND AND PURPOSE

The therapeutic effects of fluoxetine are believed to be due to increasing neuronal plasticity and reversing some learning deficits. Nevertheless, a growing amount of evidence shows adverse effects of this drug on cognition and some forms of neuronal plasticity.

EXPERIMENTAL APPROACH

To study the effects of chronic fluoxetine treatment, we combine an automated assessment of motivation and learning in mice with an investigation of neuronal plasticity in the central amygdala and basolateral amygdala. We use immunohistochemistry to visualize neuronal types and perineuronal nets, along with DI staining to assess dendritic spine morphology. Gel zymography is used to test fluoxetine's impact on matrix metalloproteinase-9, an enzyme involved in synaptic plasticity.

KEY RESULTS

We show that chronic fluoxetine treatment in non-stressed mice increases perineuronal nets-dependent plasticity in the basolateral amygdala, while impairing MMP-9-dependent plasticity in the central amygdala. Further, we illustrate how the latter contributes to anhedonia and deficits of reward learning. Behavioural impairments are accompanied by alterations in morphology of dendritic spines in the central amygdala towards an immature state, most likely reflecting animals' inability to adapt. We strengthen the link between the adverse effects of fluoxetine and its influence on MMP-9 by showing that behaviour of MMP-9 knockout animals remains unaffected by the drug.

CONCLUSION AND IMPLICATIONS

Chronic fluoxetine treatment differentially affects various forms of neuronal plasticity, possibly explaining its opposing effects on brain and behaviour. These findings are of immediate clinical relevance since reported side effects of fluoxetine pose a potential threat to patients.

'Liking' and 'wanting' in eating and food reward: Brain mechanisms and clinical implications

It is becoming clearer how neurobiological mechanisms generate 'liking' and 'wanting' components of food reward. Mesocorticolimbic mechanisms that enhance 'liking' include brain hedonic hotspots, which are specialized subregions that are uniquely able to causally amplify the hedonic impact of palatable tastes. Hedonic hotspots are found in nucleus accumbens medial shell, ventral pallidum, orbitofrontal cortex, insula cortex, and brainstem. In turn, a much larger mesocorticolimbic circuitry generates 'wanting' or incentive motivation to obtain and consume food rewards. Hedonic and motivational circuitry interact together and with hypothalamic homeostatic circuitry, allowing relevant physiological hunger and satiety states to modulate 'liking' and 'wanting' for food rewards. In some conditions such as drug addiction, 'wanting' is known to dramatically detach from 'liking' for the same reward, and this may also occur in over-eating disorders. Via incentive sensitization, 'wanting' selectively becomes higher, especially when triggered by reward cues when encountered in vulnerable states of stress, etc. Emerging evidence suggests that some cases of obesity and binge eating disorders may reflect an incentive-sensitization brain signature of cue hyper-reactivity, causing excessive 'wanting' to eat. Future findings on the neurobiological bases of 'liking' and 'wanting' can continue to improve understanding of both normal food reward and causes of clinical eating disorders.

Suppression of the Swallowing Reflex during Rhythmic Jaw Movements Induced by Repetitive Electrical Stimulation of the Dorsomedial Part of the Central Amygdaloid Nucleus in Rats

A previous study indicated that the swallowing reflex is inhibited during rhythmic jaw movements induced by electrical stimulation of the anterior cortical masticatory area. Rhythmic jaw movements were induced by electrical stimulation of the central amygdaloid nucleus (CeA). The swallowing central pattern generator is the nucleus of the solitary tract (NTS) and the lateral reticular formation in the medulla. Morphological studies have reported that the CeA projects to the NTS and the lateral reticular formation. It is therefore likely that the CeA is related to the control of the swallowing reflex. The purpose of this study was to determine if rhythmic jaw movements driven by CeA had inhibitory roles in the swallowing reflex induced by electrical stimulation of the superior laryngeal nerve (SLN). Rats were anesthetised with urethane. The SLN was solely stimulated for 10 s, and the swallowing reflex was recorded (SLN stimulation before SLN + CeA stimulation). Next, the SLN and the CeA were electrically stimulated at the same time for 10 s, and the swallowing reflex was recorded during rhythmic jaw movements (SLN + CeA stimulation). Finally, the SLN was solely stimulated (SLN stimulation following SLN + CeA stimulation). The number of swallows was reduced during rhythmic jaw movements. The onset latency of the first swallow was significantly longer in the SLN + CeA stimulation than in the SLN stimulation before SLN + CeA stimulation and SLN stimulation following SLN + CeA stimulation. These results support the idea that the coordination of swallowing reflex with rhythmic jaw movements could be regulated by the CeA.

Sex Differences in Demand for Highly Palatable Foods: Role of the Orexin System

BACKGROUND

The prevalence of eating disorders, including binge eating disorder, is significantly higher in women. These findings are mirrored by preclinical studies, which indicate that female rats have a higher preference for palatable food and show greater binge-like eating compared with male rats.

METHODS

Here, we describe a novel within-session behavioral-economic paradigm that allows for the simultaneous measurement of the intake at null cost (Q0) and normalized demand elasticity (α) of 3 types of palatable food (low fat, high fat, and chocolate sucrose pellets) via demand curve analysis. In light of evidence that the orexin (hypocretin) system is critically involved in reward and feeding behaviors, we also examined the role of orexin function in sex differences of economic demand for palatable foods.

RESULTS

The novel within-session behavioral-economic approach revealed that female rats have higher intake (demand) than males for all palatable foods at low cost (normalized to body weight) but no difference in intake at higher prices, indicating sex-dependent differences in the hedonic, but not motivational, aspects of palatable food. Immediately following behavioral-economic testing, we observed more orexin-expressing neurons and Fos expression (measure of recent neural activation) in these neurons in female rats compared with male rats. Moreover, the orexin-1 receptor antagonist SB334867 reduced both low- and high-cost intake for palatable food in both male and female rats.

CONCLUSIONS

These findings provide evidence of higher demand at low prices for palatable food in females and indicate that these behavioral differences may be associated with sexual dimorphism in orexin system function.

The central amygdala recruits mesocorticolimbic circuitry for pursuit of reward or pain

How do brain mechanisms create maladaptive attractions? Here intense maladaptive attractions are created in laboratory rats by pairing optogenetic channelrhodopsin (ChR2) stimulation of central nucleus of amygdala (CeA) in rats with encountering either sucrose, cocaine, or a painful shock-delivering object. We find that pairings make the respective rats pursue either sucrose exclusively, or cocaine exclusively, or repeatedly self-inflict shocks. CeA-induced maladaptive attractions, even to the painful shock-rod, recruit mesocorticolimbic incentive-related circuitry. Shock-associated cues also gain positive incentive value and are pursued. Yet the motivational effects of paired CeA stimulation can be reversed to negative valence in a Pavlovian fear learning situation, where CeA ChR2 pairing increases defensive reactions. Finally, CeA ChR2 valence can be switched to neutral by pairing with innocuous stimuli. These results reveal valence plasticity and multiple modes for motivation via mesocorticolimbic circuitry under the control of CeA activation.

Manipulations of Central Amygdala Neurotensin Neurons Alter the Consumption of Ethanol and Sweet Fluids in Mice

The central nucleus of the amygdala plays a significant role in alcohol use and other affective disorders; however, the genetically-defined neuronal subtypes and projections that govern these behaviors are not well known. Here we show that neurotensin neurons in the central nucleus of the amygdala of male mice are activated by in vivo ethanol consumption and that genetic ablation of these neurons decreases ethanol consumption and preference in non-ethanol-dependent animals. This ablation did not impact preference for sucrose, saccharin, or quinine. We found that the most robust projection of the central amygdala neurotensin neurons was to the parabrachial nucleus, a brain region known to be important in feeding behaviors, conditioned taste aversion, and alarm. Optogenetic stimulation of projections from these neurons to the parabrachial nucleus is reinforcing, and increases ethanol drinking as well as consumption of sucrose and saccharin solutions. These data suggest that this central amygdala to parabrachial nucleus projection influences the expression of reward-related phenotypes and is a novel circuit promoting consumption of ethanol and palatable fluids.SIGNIFICANCE STATEMENT Alcohol use disorder (AUD) is a major health burden worldwide. Although ethanol consumption is required for the development of AUD, much remains unknown regarding the underlying neural circuits that govern initial ethanol intake. Here we show that ablation of a population of neurotensin-expressing neurons in the central amygdala decreases intake of and preference for ethanol in non-dependent animals, whereas the projection of these neurons to the parabrachial nucleus promotes consumption of ethanol as well as other palatable fluids.

Endogenous opioid modulation of food intake and body weight: Implications for opioid influences upon motivation and addiction

This review is part of a special issue dedicated to Opioid addiction, and examines the influential role of opioid peptides, opioid receptors and opiate drugs in mediating food intake and body weight control in rodents. This review postulates that opioid mediation of food intake was an example of "positive addictive" properties that provide motivational drives to maintain opioid-seeking behavior and that are not subject to the "negative addictive" properties associated with tolerance, dependence and withdrawal. Data demonstrate that opiate and opioid peptide agonists stimulate food intake through homeostatic activation of sensory, metabolic and energy-related In contrast, general, and particularly mu-selective, opioid receptor antagonists typically block these homeostatically-driven ingestive behaviors. Intake of palatable and hedonic food stimuli is inhibited by general, and particularly mu-selective, opioid receptor antagonists. The selectivity of specific opioid agonists to elicit food intake was confirmed through the use of opioid receptor antagonists and molecular knockdown (antisense) techniques incapacitating specific exons of opioid receptor genes. Further extensive evidence demonstrated that homeostatic and hedonic ingestive situations correspondingly altered the levels and expression of opioid peptides and opioid receptors. Opioid mediation of food intake was controlled by a distributed brain network intimately related to both the appetitive-consummatory sites implicated in food intake as well as sites intimately involved in reward and reinforcement. This emergent system appears to sustain the "positive addictive" properties providing motivational drives to maintain opioid-seeking behavior.

A Motivational and Neuropeptidergic Hub: Anatomical and Functional Diversity within the Nucleus Accumbens Shell

The mesocorticolimbic pathway is canonically known as the "reward pathway." Embedded within the center of this circuit is the striatum, a massive and complex network hub that synthesizes motivation, affect, learning, cognition, stress, and sensorimotor information. Although striatal subregions collectively share many anatomical and functional similarities, it has become increasingly clear that it is an extraordinarily heterogeneous region. In particular, the nucleus accumbens (NAc) medial shell has repeatedly demonstrated that the rules dictated by more dorsal aspects of the striatum do not apply or are even reversed in functional logic. These discrepancies are perhaps most easily captured when isolating the functions of various neuromodulatory peptide systems within the striatum. Endogenous peptides are thought to play a critical role in modulating striatal signals to either amplify or dampen evoked behaviors. Here we describe the anatomical-functional backdrop upon which several neuropeptides act within the NAc to modulate behavior, with a specific emphasis on nucleus accumbens medial shell and stress responsivity. Additionally, we propose that, as the field continues to dissect fast neurotransmitter systems within the NAc, we must also provide considerable contextual weight to the roles local peptides play in modulating these circuits to more comprehensively understand how this important subregion gates motivated behaviors.

Central Amygdala Prepronociceptin-Expressing Neurons Mediate Palatable Food Consumption and Reward

Food palatability is one of many factors that drives food consumption, and the hedonic drive to feed is a key contributor to obesity and binge eating. In this study, we identified a population of prepronociceptin-expressing cells in the central amygdala (Pnoc^CeA) that are activated by palatable food consumption. Ablation or chemogenetic inhibition of these cells reduces palatable food consumption. Additionally, ablation of Pnoc^CeA cells reduces high-fat-diet-driven increases in bodyweight and adiposity. Pnoc^CeA neurons project to the ventral bed nucleus of the stria terminalis (vBNST), parabrachial nucleus (PBN), and nucleus of the solitary tract (NTS), and activation of cell bodies in the central amygdala (CeA) or axons in the vBNST, PBN, and NTS produces reward behavior but did not promote feeding of palatable food. These data suggest that the Pnoc^CeA network is necessary for promoting the reinforcing and rewarding properties of palatable food, but activation of this network itself is not sufficient to promote feeding.

Early life stress alters opioid receptor mRNA levels within the nucleus accumbens in a sex-dependent manner

Early life stress (ELS) strongly impacts mental health, but little is known about its interaction with biological sex and postnatal development to influence risk and resilience to psychopathologies. A number of psychiatric disorders, such as social anhedonia and drug addiction, involve dysfunctional opioid signaling; moreover, there is evidence for differential central opioid function in males vs. females. The present study examined opioid receptor gene expression in the nucleus accumbens (NAc) and amygdala of male and female rats subjected to a neonatal predator odor exposure (POE) paradigm to model ELS. Brain tissue was collected at two developmental time points: neonatal and juvenile. Results showed that, following the neonatal POE experience, opioid receptor mRNA levels in the NAc were differentially regulated at the neonatal and juvenile time points. POE downregulated neonatal mu- and kappa-opioid receptor mRNA levels in neonatal females, but upregulated mu- and delta-opioid receptor mRNA levels in juvenile females. Intriguingly, POE had no significant effect on NAc opioid receptor mRNA levels in males at either time point, indicating that the impact of POE on opioid system development is sex-dependent. Finally, POE failed to alter amygdalar opioid receptor gene expression in either sex at either time-point. The spatiotemporally- and sex-specific impact of ELS within the developing brain may confer differential risk or resilience for males and females to develop atypical opioid-regulated behaviors associated with conditions such as depression and addiction.

Chemogenetic Manipulations of Ventral Tegmental Area Dopamine Neurons Reveal Multifaceted Roles in Cocaine Abuse

Ventral tegmental area (VTA) dopamine (DA) neurons perform diverse functions in motivation and cognition, but their precise roles in addiction-related behaviors are still debated. Here, we targeted VTA DA neurons for bidirectional chemogenetic modulation during specific tests of cocaine reinforcement, demand, and relapse-related behaviors in male rats, querying the roles of DA neuron inhibitory and excitatory G-protein signaling in these processes. Designer receptor stimulation of G_q signaling, but not G_s signaling, in DA neurons enhanced cocaine seeking via functionally distinct projections to forebrain limbic regions. In contrast, engaging inhibitory G_i/o signaling in DA neurons blunted the reinforcing and priming effects of cocaine, reduced stress-potentiated reinstatement, and altered behavioral strategies for cocaine seeking and taking. Results demonstrate that DA neurons play several distinct roles in cocaine seeking, depending on behavioral context, G-protein-signaling cascades, and DA neuron efferent targets, highlighting their multifaceted roles in addiction.SIGNIFICANCE STATEMENT G-protein-coupled receptors are crucial modulators of ventral tegmental area (VTA) dopamine neuron activity, but how this metabotropic signaling impacts the complex roles of dopamine in reward and addiction is poorly understood. Here, we bidirectionally modulate dopamine neuron G-protein signaling with DREADDs (designer receptors exclusively activated by designer drugs) during a variety of cocaine-seeking behaviors, revealing nuanced, pathway-specific roles in cocaine reward, effortful seeking, and relapse-like behaviors. G_q and G_s stimulation activated dopamine neurons, but only G_q stimulation robustly enhanced cocaine seeking. G_i/o inhibitory signaling reduced some, but not all, types of cocaine seeking. Results show that VTA dopamine neurons modulate numerous distinct aspects of cocaine addiction- and relapse-related behaviors, and point to potential new approaches for intervening in these processes to treat addiction.

Updates on the neurobiology of food reward and their relation to the obesogenic environment

PURPOSE OF REVIEW

To summarize recent findings about the neurobiological control of food reward and discuss their relevance for hedonic food intake and obesity in our current obesogenic environment.

RECENT FINDINGS

Recent data show new roles for circuits involving neuronal subpopulations within the central amyglada (CeA) and lateral hypothalamus in the regulation of feeding and reward in rodents under free and operant conditions and also in restrain from reward consumption. Recent work also shows that the orbitofrontal cortex (OFC) codes for subjective perception of food features during reward assessment of individual foods and that activity in the nucleus accumbens (NAc) codes for anticipation for reward, which can be blocked by time-locked neurostimulation of NAc.

SUMMARY

New data illustrates that different aspects of hedonic intake and food reward are coded in a distributed brain network. In particular, as our obesogenic environment facilitates access to palatable food and promotes cue-induced feeding, neuronal circuits related to control of impulsivity, food valuation and duration of hedonic intake episodes might have a significant role in our ability to control food intake and development of obesity by excess intake.

Opioid-Independent and Opioid-Mediated Modes of Pain Modulation

Pain is regulated endogenously through both opioid and non-opioid mechanisms. We hypothesized that two novel pain modulation tasks, one drawing on context/expectations and one using voluntary reappraisal, would show differing levels of opioid dependence. Specifically, we expected that naloxone would block context-related analgesia, whereas mental imagery-based pain reappraisal would be opioid-independent.A double-blind, placebo-controlled intravenous naloxone versus saline crossover design was used. Twenty healthy volunteers completed the two modulation tasks with acute heat stimuli calibrated to induce moderate pain. In the mental imagery task, participants imagined either a "pleasant" or a "comparison" scenario during painful heat. In the relative relief task, moderate heat stimuli coincided with visual cues eliciting relief from the expectation of intense pain, and were compared with moderate heat stimuli delivered under the expectation of non-painful warmth. Both "pleasant imagery" and "relative relief" conditions significantly improved ratings of pain intensity and pleasantness during saline treatment. Indeed, the target stimuli in both tasks, which had been calibrated to induce moderate pain, were rated as mildly pleasant. Furthermore, consistently with the main hypothesis, blocking endogenous opioid signaling with naloxone did not significantly affect imagery-induced regulation of pain intensity or pleasantness. In contrast, the relative relief-induced pain regulation (i.e., context/expectation) was blocked by naloxone. We conclude that endogenous opioid signaling is necessary for expectation-related relative relief analgesia, but not for pain reappraisal through mental imagery. These results support mental imagery as a powerful and clinically relevant strategy for regulating pain affect also in patients where endogenous opioid mechanisms might be compromised.SIGNIFICANCE STATEMENT Neurotransmitter systems in the human brain can be probed through antagonist drugs. Studies using the opioid antagonist naloxone have demonstrated that the brain relies on both opioid and non-opioid mechanisms to downregulate pain. This holds clinical relevance given altered endogenous opioid processes in many chronic pain conditions. The present study used a double-blinded, placebo-controlled naloxone blockage of endogenous opioids in healthy humans to show differential opioid involvement in two pain modulation tasks. Context/expectation-driven (relative relief-related) analgesia was blocked by naloxone. In contrast, pain reappraisal through mental imagery was intact despite opioid receptor blockade, suggesting opioid independence. These results support mental imagery as a powerful, clinically relevant strategy for regulating pain as it does not rely on a functioning opioidergic system.

Dopamine and opioids inhibit synaptic outputs of the main island of the intercalated neurons of the amygdala

Neural circuits in the amygdala are important for associating the positive experience of drug taking with the coincident environmental cues. During abstinence, cue re-exposure activates the amygdala, increases dopamine release in the amygdala and stimulates relapse to drug use in an opioid dependent manner. Neural circuits in the amygdala and the learning that underlies these behaviours are inhibited by GABAergic synaptic inhibition. A specialised subtype of GABAergic neurons in the amygdala are the clusters of intercalated cells. We focussed on the main-island of intercalated cells because these neurons, located ventromedial to the basolateral amygdala, express very high levels of dopamine D1-receptor and μ-opioid receptor, release enkephalin and are densely innervated by the ventral tegmental area. However, where these neurons project to was not fully described and their regulation by opioids and dopamine was incomplete. To address this issue we electrically stimulated in the main-island of the intercalated cells in rat brain slices and made patch-clamp recordings of GABAergic synaptics from amygdala neurons. We found that main-island neurons had a strong GABAergic inhibitory output to pyramidal neurons of the basolateral nucleus and the medial central nucleus, the major output zones of the amygdala. Opioids inhibited both these synaptic outputs of the intercalated neurons and thus would disinhibit these target zones. Additionally, dopamine acting at D1-receptors inhibited main-island neuron synapses onto other main-island neurons. This data indicates that the inhibitory projections from the main-island neurons could influence multiple aspects of addiction and emotional processing in an opioid and dopamine dependent manner.

Manipulating the revision of reward value during the intertrial interval increases sign tracking and dopamine release

Recent computational models of sign tracking (ST) and goal tracking (GT) have accounted for observations that dopamine (DA) is not necessary for all forms of learning and have provided a set of predictions to further their validity. Among these, a central prediction is that manipulating the intertrial interval (ITI) during autoshaping should change the relative ST-GT proportion as well as DA phasic responses. Here, we tested these predictions and found that lengthening the ITI increased ST, i.e., behavioral engagement with conditioned stimuli (CS) and cue-induced phasic DA release. Importantly, DA release was also present at the time of reward delivery, even after learning, and DA release was correlated with time spent in the food cup during the ITI. During conditioning with shorter ITIs, GT was prominent (i.e., engagement with food cup), and DA release responded to the CS while being absent at the time of reward delivery after learning. Hence, shorter ITIs restored the classical DA reward prediction error (RPE) pattern. These results validate the computational hypotheses, opening new perspectives on the understanding of individual differences in Pavlovian conditioning and DA signaling.

The Virtual Personalities Neural Network Model: Neurobiological Underpinnings

The Virtual Personalities Model is a motive-based neural network model that provides both a psychological model and a computational implementation that explicates the dynamics and often large within-person variability in behavior that arises over time. At the same time the same model can produce -- across many virtual personalities - between subject variability in behavior that when factor analyzed yields familiar personality structure (e.g., the Big-5). First, we describe our personality model and its implementation as a neural network model. Second, we focus on detailing the neurobiological underpinnings of this model. Third, we examine the learning mechanisms, and their biological substrates, as ways that the model gets "wired up", discussing Pavlovian and instrumental conditioning, Pavlovian to instrumental transfer (PIT), and habits. Finally, we describe the dynamics of how initial differences in propensities (e.g., dopamine functioning), wiring differences due to experience, and other factors could operate together to develop and change personality over time, and how this might be empirically examined. Thus, our goal is to contribute to the rising chorus of voices seeking a more precise neurobiologically-based science of the complex dynamics underlying personality.

Optogenetic activation of the central amygdala generates addiction-like preference for reward

Drug and behavioural addictions are characterized by an intense and focused pursuit of a single reward above all others. Pursuit of the addictive reward is often compulsively sought despite adverse consequences and better alternative outcomes. Here, we explored the ability of the central amygdala (CeA) to powerfully bias choice, causing specific rewards to be almost compulsively preferred. Rats were trained on an operant choice task in which they could choose to respond on either of the two levers to receive a sucrose reward, one of which was paired with optogenetic stimulation of the CeA using channelrhodopsin-2 (ChR2). Rats developed an almost exclusive preference for the laser-paired reward over the otherwise equal unpaired reward. We found that this preference for stimulation-paired reward persists even when a much larger sucrose reward is offered as an alternative (contingency management) or when this preferred reward is paired with adverse consequences such as progressively larger electric foot shock, time delays or effort requirements. We also report that when challenged with foot shock, a small proportion of these animals (≈20%) retained an exclusive laser-paired reward preference, whereas others began to seek the alternate reward when the shock reached high levels. Lastly, we confirmed that optogenetic CeA stimulation was not independently rewarding if delivered in the absence of a paired sucrose reward. These results suggest a role for the CeA in focusing motivation and desire to excessive levels, generating addiction-like behaviour that persists in the face of more rewarding alternatives and adverse consequences.

Neurofeedback of visual food cue reactivity: a potential avenue to alter incentive sensitization and craving

FMRI-based neurofeedback transforms functional brain activation in real-time into sensory stimuli that participants can use to self-regulate brain responses, which can aid the modification of mental states and behavior. Emerging evidence supports the clinical utility of neurofeedback-guided up-regulation of hypoactive networks. In contrast, down-regulation of hyperactive neural circuits appears more difficult to achieve. There are conditions though, in which down-regulation would be clinically useful, including dysfunctional motivational states elicited by salient reward cues, such as food or drug craving. In this proof-of-concept study, 10 healthy females (mean age = 21.40 years, mean BMI = 23.53) who had fasted for 4 h underwent a novel 'motivational neurofeedback' training in which they learned to down-regulate brain activation during exposure to appetitive food pictures. FMRI feedback was given from individually determined target areas and through decreases/increases in food picture size, thus providing salient motivational consequences in terms of cue approach/avoidance. Our preliminary findings suggest that motivational neurofeedback is associated with functionally specific activation decreases in diverse cortical/subcortical regions, including key motivational areas. There was also preliminary evidence for a reduction of hunger after neurofeedback and an association between down-regulation success and the degree of hunger reduction. Decreasing neural cue responses by motivational neurofeedback may provide a useful extension of existing behavioral methods that aim to modulate cue reactivity. Our pilot findings indicate that reduction of neural cue reactivity is not achieved by top-down regulation but arises in a bottom-up manner, possibly through implicit operant shaping of target area activity.

Early-life adversity facilitates acquisition of cocaine self-administration and induces persistent anhedonia

Early-life adversity increases the risk for emotional disorders such as depression and schizophrenia. Anhedonia, thought to be a core feature of these disorders, is provoked by our naturalistic rodent model of childhood adversity (i.e., rearing pups for one week in cages with limited bedding and nesting, LBN). Drug use and addiction are highly comorbid with psychiatric disorders featuring anhedonia, yet effects of LBN on drug-seeking behavior and the reward and stress-related circuits that underlie it remain unknown. Here we examined the effects of LBN on cocaine intake and seeking, using a battery of behavioral tests measuring distinct aspects of cocaine reward, and for comparison, chocolate intake. We also examined activation of neurons within the pleasure/reward and stress circuits following cocaine in LBN and control rats. Early-life adversity reduced spontaneous intake of palatable chocolate, extending prior reports of sucrose and social-play anhedonia. In a within-session cocaine behavioral economic test, LBN rats self-administered lower dosages of cocaine under low-effort conditions, consistent with a reduced hedonic set-point for cocaine, and potentially anhedonia. In contrast, cocaine demand elasticity was not consistently affected, indicating no major changes in motivation to maintain preferred cocaine blood levels. These changes were selective, as LBN did not cause an overt anxiety-like phenotype, nor did it affect sensitivity to self-administered cocaine dose, responding for cocaine under extinction conditions, cocaine- or cue-induced reinstatement of cocaine seeking, or locomotor response to acute cocaine. However, high Fos expression was seen after cocaine in both reward- and stress-related brain regions of LBN rats, including nucleus accumbens core, central amygdala, and lateral habenula. In contrast, hypothalamic orexin neuron activation after cocaine was significantly attenuated in LBN rats. Together, these findings demonstrate enduring effects of early-life adversity on both reward- and fear/anxiety-related neural circuits, as well as anhedonia-like reductions in consumption of natural and drug rewards.

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Adversity during a sensitive developmental period provokes persistent anhedonia.

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This adversity reduces cocaine hedonic set point, but not motivation.

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Cocaine-associated Fos is altered in reward- and anxiety/fear circuits.

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Cocaine-dose sensitivity, reinstatement, and locomotion are unchanged.

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Effects are selective, as anxiety-related behaviors were unaltered.

Adolescent cannabinoid exposure effects on natural reward seeking and learning in rats

RATIONALE

Adolescence is characterized by endocannabinoid (ECB)-dependent refinement of neural circuits underlying emotion, learning, and motivation. As a result, adolescent cannabinoid receptor stimulation (ACRS) with phytocannabinoids or synthetic agonists like "Spice" cause robust and persistent changes in both behavior and circuit architecture in rodents, including in reward-related regions like medial prefrontal cortex and nucleus accumbens (NAc).

OBJECTIVES AND METHODS

Here, we examine persistent effects of ACRS with the cannabinoid receptor 1/2 specific agonist WIN55-212,2 (WIN; 1.2 mg/kg/day, postnatal day (PD) 30-43), on natural reward-seeking behaviors and ECB system function in adult male Long Evans rats (PD 60+).

RESULTS

WIN ACRS increased palatable food intake, and altered attribution of incentive salience to food cues in a sign-/goal-tracking paradigm. ACRS also blunted hunger-induced sucrose intake, and resulted in increased anandamide and oleoylethanolamide levels in NAc after acute food restriction not seen in controls. ACRS did not affect food neophobia or locomotor response to a novel environment, but did increase preference for exploring a novel environment.

CONCLUSIONS

These results demonstrate that ACRS causes long-term increases in natural reward-seeking behaviors and ECB system function that persist into adulthood, potentially increasing liability to excessive natural reward seeking later in life.

Neural and Molecular Mechanisms Involved in Controlling the Quality of Feeding Behavior: Diet Selection and Feeding Patterns

We are what we eat. There are three aspects of feeding: what, when, and how much. These aspects represent the quantity (how much) and quality (what and when) of feeding. The quantitative aspect of feeding has been studied extensively, because weight is primarily determined by the balance between caloric intake and expenditure. In contrast, less is known about the mechanisms that regulate the qualitative aspects of feeding, although they also significantly impact the control of weight and health. However, two aspects of feeding quality relevant to weight loss and weight regain are discussed in this review: macronutrient-based diet selection (what) and feeding pattern (when). This review covers the importance of these two factors in controlling weight and health, and the central mechanisms that regulate them. The relatively limited and fragmented knowledge on these topics indicates that we lack an integrated understanding of the qualitative aspects of feeding behavior. To promote better understanding of weight control, research efforts must focus more on the mechanisms that control the quality and quantity of feeding behavior. This understanding will contribute to improving dietary interventions for achieving weight control and for preventing weight regain following weight loss.

Optogenetic Central Amygdala Stimulation Intensifies and Narrows Motivation for Cocaine

Addiction is often characterized by intense motivation for a drug, which may be narrowly focused at the expense of other rewards. Here, we examined the role of amygdala-related circuitry in the amplification and narrowing of motivation focus for intravenous cocaine. We paired optogenetic channelrhodopsin (ChR2) stimulation in either central nucleus of amygdala (CeA) or basolateral amygdala (BLA) of female rats with one particular nose-poke porthole option for earning cocaine infusions (0.3 mg/kg, i.v.). A second alternative porthole earned identical cocaine but without ChR2 stimulation. Consequently, CeA rats quickly came to pursue their CeA ChR2-paired cocaine option intensely and exclusively, elevating cocaine intake while ignoring their alternative cocaine alone option. By comparison, BLA ChR2 pairing failed to enhance cocaine motivation. CeA rats also emitted consummatory bites toward their laser-paired porthole, suggesting that higher incentive salience made that cue more attractive. A separate progressive ratio test of incentive motivation confirmed that CeA ChR2 amplified rats' motivation, raising their breakpoint effort price for cocaine by 10-fold. However, CeA ChR2 laser on its own lacked any reinforcement value: laser by itself was never self-stimulated, not even by the same rats in which it amplified motivation for cocaine. Conversely, CeA inhibition by muscimol/baclofen microinjections prevented acquisition of cocaine self-administration and laser preference, whereas CeA inhibition by optogenetic halorhodopsin suppressed cocaine intake, indicating that CeA circuitry is needed for ordinary cocaine motivation. We conclude that CeA ChR2 excitation paired with a cocaine option specifically focuses and amplifies motivation to produce intense pursuit and consumption focused on that single target.SIGNIFICANCE STATEMENT In addiction, intense incentive motivation often becomes narrowly focused on a particular drug of abuse. Here we show that pairing central nucleus of amygdala (CeA) optogenetic stimulation with one option for earning intravenous cocaine makes that option almost the exclusive focus of intense pursuit and consumption. CeA stimulation also elevated the effort cost rats were willing to pay for cocaine and made associated cues become intensely attractive. However, we also show that CeA laser had no reinforcing properties at all when given alone for the same rats. Therefore, CeA laser pairing makes its associated cocaine option and cues become powerfully attractive in a nearly addictive fashion.

Central amygdala circuits modulate food consumption through a positive-valence mechanism

The complex behaviors underlying reward seeking and consumption are integral to organism survival. The hypothalamus and mesolimbic dopamine system are key mediators of these behaviors, yet regulation of appetitive and consummatory behaviors outside of these regions is poorly understood. The central nucleus of the amygdala (CeA) has been implicated in feeding and reward, but the neurons and circuit mechanisms that positively regulate these behaviors remain unclear. Here, we defined the neuronal mechanisms by which CeA neurons promote food consumption. Using in vivo activity manipulations and Ca²⁺ imaging in mice, we found that GABAergic serotonin receptor 2a (Htr2a)-expressing CeA neurons modulate food consumption, promote positive reinforcement and are active in vivo during eating. We demonstrated electrophysiologically, anatomically and behaviorally that intra-CeA and long-range circuit mechanisms underlie these behaviors. Finally, we showed that CeA^Htr2a neurons receive inputs from feeding-relevant brain regions. Our results illustrate how defined CeA neural circuits positively regulate food consumption.

Thalamic Regulation of Sucrose Seeking during Unexpected Reward Omission

The paraventricular nucleus of the thalamus (PVT) is thought to regulate behavioral responses under emotionally arousing conditions. Reward-associated cues activate PVT neurons; however, the specific PVT efferents regulating reward seeking remain elusive. Using a cued sucrose-seeking task, we manipulated PVT activity under two emotionally distinct conditions: (1) when reward was available during the cue as expected or (2) when reward was unexpectedly omitted during the cue. Pharmacological inactivation of the anterior PVT (aPVT), but not the posterior PVT, increased sucrose seeking only when reward was omitted. Consistent with this, photoactivation of aPVT neurons abolished sucrose seeking, and the firing of aPVT neurons differentiated reward availability. Photoinhibition of aPVT projections to the nucleus accumbens or to the amygdala increased or decreased, respectively, sucrose seeking only when reward was omitted. Our findings suggest that PVT bidirectionally modulates sucrose seeking under the negative (frustrative) conditions of reward omission.

A food-predictive cue attributed with incentive salience engages subcortical afferents and efferents of the paraventricular nucleus of the thalamus

The paraventricular nucleus of the thalamus (PVT) has been implicated in behavioral responses to reward-associated cues. However, the precise role of the PVT in these behaviors has been difficult to ascertain since Pavlovian-conditioned cues can act as both predictive and incentive stimuli. The "sign-tracker/goal-tracker" rat model has allowed us to further elucidate the role of the PVT in cue-motivated behaviors, identifying this structure as a critical component of the neural circuitry underlying individual variation in the propensity to attribute incentive salience to reward cues. The current study assessed differences in the engagement of specific PVT afferents and efferents in response to presentation of a food-cue that had been attributed with only predictive value or with both predictive and incentive value. The retrograde tracer fluorogold (FG) was injected into the PVT or the nucleus accumbens (NAc) of rats, and cue-induced c-Fos in FG-labeled cells was quantified. Presentation of a predictive stimulus that had been attributed with incentive value elicited c-Fos in PVT afferents from the lateral hypothalamus, medial amygdala (MeA), and the prelimbic cortex (PrL), as well as posterior PVT efferents to the NAc. PVT afferents from the PrL also showed elevated c-Fos levels following presentation of a predictive stimulus alone. Thus, presentation of an incentive stimulus results in engagement of subcortical brain regions; supporting a role for the hypothalamic-thalamic-striatal axis, as well as the MeA, in mediating responses to incentive stimuli; whereas activity in the PrL to PVT pathway appears to play a role in processing the predictive qualities of reward-paired stimuli.

Endogenous central amygdala mu-opioid receptor signaling promotes sodium appetite in mice

Due to the importance of dietary sodium and its paucity within many inland environments, terrestrial animals have evolved an instinctive sodium appetite that is commensurate with sodium deficiency. Despite a well-established role for central opioid signaling in sodium appetite, the endogenous influence of specific opioid receptor subtypes within distinct brain regions remains to be elucidated. Using selective pharmacological antagonists of opioid receptor subtypes, we reveal that endogenous mu-opioid receptor (MOR) signaling strongly drives sodium appetite in sodium-depleted mice, whereas a role for kappa (KOR) and delta (DOR) opioid receptor signaling was not detected, at least in sodium-depleted mice. Fos immunohistochemistry revealed discrete regions of the mouse brain displaying an increased number of activated neurons during sodium gratification: the rostral portion of the nucleus of the solitary tract (rNTS), the lateral parabrachial nucleus (LPB), and the central amygdala (CeA). The CeA was subsequently targeted with bilateral infusions of the MOR antagonist naloxonazine, which significantly reduced sodium appetite in mice. The CeA is therefore identified as a key node in the circuit that contributes to sodium appetite. Moreover, endogenous opioids, acting via MOR, within the CeA promote this form of appetitive behavior.

Dopamine D2 receptors gate generalization of conditioned threat responses through mTORC1 signaling in the extended amygdala

Overgeneralization of conditioned threat responses is a robust clinical marker of anxiety disorders. In overgeneralization, responses that are appropriate to threat-predicting cues are evoked by perceptually similar safety-predicting cues. Inappropriate learning of conditioned threat responses may thus form an etiological basis for anxiety disorders. The role of dopamine (DA) in memory encoding is well established. Indeed by signaling salience and valence, DA is thought to facilitate discriminative learning between stimuli representing safety or threat. However, the neuroanatomical and biochemical substrates through which DA modulates overgeneralization of threat responses remain poorly understood. Here we report that the modulation of DA D2 receptor (D2R) signaling bidirectionally regulates the consolidation of fear responses. While the blockade of D2R induces generalized threat responses, its stimulation facilitates discriminative learning between stimuli representing safety or threat. Moreover, we show that controlled threat generalization requires the coordinated activation of D2R in the bed nucleus of the stria terminalis and the central amygdala. Finally, we identify the mTORC1 cascade activation as an important molecular event by which D2R mediates its effects. These data reveal that D2R signaling in the extended amygdala constitutes an important checkpoint through which DA participates in the control of threat processing and the emergence of overgeneralized threat responses.

The sensory features of a food cue influence its ability to act as an incentive stimulus and evoke dopamine release in the nucleus accumbens core

The sensory properties of a reward-paired cue (a conditioned stimulus; CS) may impact the motivational value attributed to the cue, and in turn influence the form of the conditioned response (CR) that develops. A cue with multiple sensory qualities, such as a moving lever-CS, may activate numerous neural pathways that process auditory and visual information, resulting in CRs that vary both within and between individuals. For example, CRs include approach to the lever-CS itself (rats that “sign-track”; ST), approach to the location of reward delivery (rats that “goal-track”; GT), or an “intermediate” combination of these behaviors. We found that the multimodal sensory features of the lever-CS were important to the development and expression of sign-tracking. When the lever-CS was covered, and thus could only be heard moving, STs not only continued to approach the lever location but also started to approach the food cup during the CS period. While still predictive of reward, the auditory component of the lever-CS was a much weaker conditioned reinforcer than the visible lever-CS. This plasticity in behavioral responding observed in STs closely resembled behaviors normally seen in rats classified as “intermediates.” Furthermore, the ability of both the lever-CS and the reward-delivery to evoke dopamine release in the nucleus accumbens was also altered by covering the lever—dopamine signaling in STs resembled neurotransmission observed in rats that normally only GT. These data suggest that while the visible lever-CS was attractive, wanted, and had incentive value for STs, when presented in isolation, the auditory component of the cue was simply predictive of reward, lacking incentive salience. Therefore, the specific sensory features of cues may differentially contribute to responding and ensure behavioral flexibility.

Deep brain stimulation in the central nucleus of the amygdala decreases 'wanting' and 'liking' of food rewards

We investigated the potential of deep brain stimulation (DBS) in the central nucleus of the amygdala (CeA) in rats to modulate functional reward mechanisms. The CeA is the major output of the amygdala with direct connections to the hypothalamus and gustatory brainstem, and indirect connections with the nucleus accumbens. Further, the CeA has been shown to be involved in learning, emotional integration, reward processing, and regulation of feeding. We hypothesized that DBS, which is used to treat movement disorders and other brain dysfunctions, might block reward motivation. In rats performing a lever-pressing task to obtain sugar pellet rewards, we stimulated the CeA and control structures, and compared stimulation parameters. During CeA stimulation, animals stopped working for rewards and rejected freely available rewards. Taste reactivity testing during DBS exposed aversive reactions to normally liked sucrose tastes and even more aversive taste reactions to normally disliked quinine tastes. Interestingly, given the opportunity, animals implanted in the CeA would self-stimulate with 500 ms trains of stimulation at the same frequency and current parameters as continuous stimulation that would stop reward acquisition. Neural recordings during DBS showed that CeA neurons were still active and uncovered inhibitory-excitatory patterns after each stimulus pulse indicating possible entrainment of the neural firing with DBS. In summary, DBS modulation of CeA may effectively usurp normal neural activity patterns to create an 'information lesion' that not only decreased motivational 'wanting' of food rewards, but also blocked 'liking' of rewards.