The paraventricular nucleus of the thalamus is recruited by both natural rewards and drugs of abuse: recent evidence of a pivotal role for orexin/hypocretin signaling in this thalamic nucleus in drug-seeking behavior

Sex-specific behavioral and thalamo-accumbal circuit adaptations after oxycodone abstinence

Opioid use disorder is marked by a progressive change in the motivation to administer the drug even in the presence of negative consequences. After long periods of abstinence, the urge to return to taking the drug intensifies over time, known as incubation of craving. Conditioned responses to drug-related stimuli, can acquire motivational properties and exert control over motivated behaviors leading to relapse. Although, preclinical data suggest that the behavioral expression of opioid use is similar between male and female rodents, we do not have conclusive results on sex differences on craving and relapse across abstinence periods. Here, we investigated the effects of abstinence from oxycodone self-administration on neurotransmission in the paraventricular thalamus (PVT) to nucleus accumbens shell (NAcSh) pathway in male and female rats. Using optogenetics and ex vivo electrophysiology, we assessed synaptic strength and glutamate release probability in this pathway, as well as NAcSh medium spiny neurons (MSN) intrinsic excitability, in slices from rats which were subjected to either 1 (acute) or 14 (prolonged) days of forced abstinence after self-administration. Our results revealed no sex differences in oxycodone self-administration or somatic withdrawal symptoms following acute abstinence. However, we found a sex-specific enhancement in cue-induced relapse after prolonged, but not acute, abstinence from oxycodone self-administration, with females exhibiting higher relapse rates. Notably, prolonged abstinence led to similar increases in synaptic strength at PVT-NAcSh inputs compared to saline controls in both sexes, which was not observed after acute abstinence. Thus, prolonged abstinence results in a time-dependent increase in PVT-NAcSh synaptic strength and sex-specific effects on cue-induced relapse rates. These findings suggest that prolonged abstinence leads to significant synaptic changes, contributing to heightened relapse vulnerability, highlighting the need for targeted therapeutic strategies in opioid use disorder.

The Paraventricular Thalamic Nucleus and Its Projections in Regulating Reward and Context Associations

The paraventricular thalamic nucleus (PVT) is a brain region that mediates aversive and reward-related behaviors as shown in animals exposed to fear conditioning, natural rewards, or drugs of abuse. However, it is unknown whether manipulations of the PVT, in the absence of external factors or stimuli (e.g., fear, natural rewards, or drugs of abuse), are sufficient to drive reward-related behaviors. Additionally, it is unknown whether drugs of abuse administered directly into the PVT are sufficient to drive reward-related behaviors. Here, using behavioral as well as pathway and cell-type specific approaches, we manipulate PVT activity as well as the PVT-to-nucleus accumbens shell (NAcSh) neurocircuit to explore reward phenotypes. First, we show that bath perfusion of morphine (10 µM) caused hyperpolarization of the resting membrane potential, increased rheobase, and decreased intrinsic membrane excitability in PVT neurons that project to the NAcSh. Additionally, we found that direct injections of morphine (50 ng) in the PVT of mice were sufficient to generate conditioned place preference (CPP) for the morphine-paired chamber. Mimicking the inhibitory effect of morphine, we employed a chemogenetic approach to inhibit PVT neurons that projected to the NAcSh and found that pairing the inhibition of these PVT neurons with a specific context evoked the acquisition of CPP. Lastly, using brain slice electrophysiology, we found that bath-perfused morphine (10 µM) significantly reduced PVT excitatory synaptic transmission on both dopamine D1 and D2 receptor-expressing medium spiny neurons in the NAcSh, but that inhibiting PVT afferents in the NAcSh was not sufficient to evoke CPP.

Meta-analysis of pre-clinical studies on the effects of opioid receptor ligands on food intake, motivation, and choice

The opioid receptors (OR) regulate food intake. Still, despite extensive pre-clinical research, the overall effects and individual contribution of the mu (MOR), kappa (KOR), and delta (DOR) OR subtypes to feeding behaviors and food intake remain unclear. To address this, we conducted a pre-registered systematic search and meta-analysis of rodent dose-response studies to evaluate the impact of central and peripheral administration of non-selective and selective OR ligands on intake, motivation, and choice of food. All studies had a high bias risk. Still, the meta-analysis confirmed the overall orexigenic and anorexigenic effects of OR agonists and antagonists, respectively. Our results support a larger orexigenic role for central MOR agonists among OR subtypes and that peripheral OR antagonists reduce motivation for and intake of preferred foods. In binary food choice studies, peripheral OR agonists selectively increase the intake of fat-preferred foods; in contrast, they did not increase the intake of sweet carbohydrate-preferred foods. Overall, these data support that OR regulation of intake, motivation, and choice is influenced by food macronutrient composition.

Neuroinflammatory Response in Reward-Associated Psychostimulants and Opioids: A Review

Substance abuse is one of the significant problems in social and public health worldwide. Vast numbers of evidence illustrate that motivational and reinforcing impacts of addictive drugs are primarily attributed to their ability to change dopamine signaling in the reward circuit. However, the roles of classic neurotransmitters, especially dopamine and neuromodulators, monoamines, and neuropeptides, in reinforcing characteristics of abused drugs have been extensively investigated. It has recently been revealed that central immune signaling includes cascades of chemokines and proinflammatory cytokines released by neurons and glia via downstream intracellular signaling pathways that play a crucial role in mediating rewarding behavioral effects of drugs. More interestingly, inflammatory responses in the central nervous system modulate the mesolimbic dopamine signaling and glutamate-dependent currents induced by addictive drugs. This review summarized researches in the alterations of inflammatory responses accompanied by rewarding and reinforcing properties of addictive drugs, including cocaine, methamphetamine, and opioids that were evaluated by conditioned place preference and self-administration procedures as highly common behavioral tests to investigate the motivational and reinforcing impacts of addictive drugs. The neuroinflammatory responses affect the rewarding properties of psychostimulants and opioids.

Structural and functional organization of the midline and intralaminar nuclei of the thalamus

The midline and intralaminar nuclei of the thalamus form a major part of the "limbic thalamus;" that is, thalamic structures anatomically and functionally linked with the limbic forebrain. The midline nuclei consist of the paraventricular (PV) and paratenial nuclei, dorsally and the rhomboid and nucleus reuniens (RE), ventrally. The rostral intralaminar nuclei (ILt) consist of the central medial (CM), paracentral (PC) and central lateral (CL) nuclei. We presently concentrate on RE, PV, CM and CL nuclei of the thalamus. The nucleus reuniens receives a diverse array of input from limbic-related sites, and predominantly projects to the hippocampus and to "limbic" cortices. The RE participates in various cognitive functions including spatial working memory, executive functions (attention, behavioral flexibility) and affect/fear behavior. The PV receives significant limbic-related afferents, particularly the hypothalamus, and mainly distributes to "affective" structures of the forebrain including the bed nucleus of stria terminalis, nucleus accumbens and the amygdala. Accordingly, PV serves a critical role in "motivated behaviors" such as arousal, feeding/consummatory behavior and drug addiction. The rostral ILt receives both limbic and sensorimotor-related input and distributes widely over limbic and motor regions of the frontal cortex-and throughout the dorsal striatum. The intralaminar thalamus is critical for maintaining consciousness and directly participates in various sensorimotor functions (visuospatial or reaction time tasks) and cognitive tasks involving striatal-cortical interactions. As discussed herein, while each of the midline and intralaminar nuclei are anatomically and functionally distinct, they collectively serve a vital role in several affective, cognitive and executive behaviors - as major components of a brainstem-diencephalic-thalamocortical circuitry.

Infralimbic cortex functioning across motivated behaviors: Can the differences be reconciled?

The rodent infralimbic cortex (IL) is implicated in higher order executive functions such as reward seeking and flexible decision making. However, the precise nature of its role in these processes is unclear. Early evidence indicated that the IL promotes the extinction and ongoing inhibition of fear conditioning and cocaine seeking. However, evidence spanning other behavioral domains, such as natural reward seeking and habit-based learning, suggests a more nuanced understanding of IL function. As techniques have advanced and more studies have examined IL function, identifying a unifying explanation for its behavioral function has become increasingly difficult. Here, we discuss evidence of IL function across motivated behaviors, including associative learning, drug seeking, natural reward seeking, and goal-directed versus habit-based behaviors, and emphasize how context-specific encoding and heterogeneous IL neuronal populations may underlie seemingly conflicting findings in the literature. Together, the evidence suggests that a major IL function is to facilitate the encoding and updating of contingencies between cues and behaviors to guide subsequent behaviors.

Circuit and neuropeptide mechanisms of the paraventricular thalamus across stages of alcohol and drug use

The paraventricular nucleus of the thalamus (PVT) is a midline thalamic brain region that has emerged as a critical circuit node in the regulation of behaviors across domains of affect and motivation, stress responses, and alcohol- and drug-related behaviors. The influence of the PVT in this diverse array of behaviors is a function of its ability to integrate and convey information about salience and valence through its connections with cortical, hypothalamic, hindbrain, and limbic brain regions. While understudied to date, recent studies suggest that several PVT efferents play critical and complex roles in drug and alcohol-related phenotypes. The PVT is also the site of signaling for many neuropeptides released from the synaptic terminals of distal inputs and local neuropeptidergic neurons within. While there is some evidence that neuropeptides including orexin, neurotensin, substance P, and cocaine and amphetamine-related transcript (CART) signal in the PVT to regulate alcohol/drug intake and reinstatement, there remains an overall lack of understanding of the roles of neuropeptides in the PVT in addiction-related behaviors, especially in a circuit-specific context. In this review, we present the current status of preclinical research regarding PVT circuits and neuropeptide modulation of the PVT in three aspects of the addiction cycle: reward/acquisition, withdrawal, and relapse, with a focus on alcohol, opioids (particularly morphine), and psychostimulants (particularly cocaine). Given the PVT's unique position within the broader neural landscape, we further discuss the potential ways in which neuropeptides may regulate these behaviors through their actions upon PVT circuits.

Disturbed craving regulation to gaming cues in internet gaming disorder: Implications for uncontrolled gaming behaviors

BACKGROUND

The ability to control craving for games is very important to abstain from Internet gaming disorder (IGD) and abundant clinical evidence has suggested that craving dysregulation is the essential pathogenesis for IGD. However, the neural mechanism underlying this feature remains unclear.

METHODS

Subjective evaluation and fMRI data from 44 participants (IGD participants: 21; recreational Internet game users (RGUs): 23) were collected while they were performing a regulation of craving task. We analyzed and compared their brain features while they regulated cravings to gaming stimuli.

RESULTS

Compared to RGUs, IGD participants showed enhanced brain activation in the right anterior cingulate cortex, posterior cingulate cortex (PCC), orbitofrontal cortex and middle temporal gyrus and in the left dorsolateral prefrontal cortex and thalamus during the regulation of craving task. Generalized psychophysiological interaction (gPPI) analysis revealed that IGD participants showed decreased functional connectivity between the right PCC and right inferior parietal lobule compared to that in RGU participants.

CONCLUSIONS

The results suggested that deficits of craving regulation in IGD participant were associated with the imbalanced coordination between the reward network and the executive network. Enhanced game-seeking motivation and disturbed executive control are responsible for craving dysregulation in IGD participants. These findings suggest a biological mechanism for IGD that may help in finding potential interventions.

The Paraventricular Thalamus as a Critical Node of Motivated Behavior via the Hypothalamic-Thalamic-Striatal Circuit

In this review, we highlight evidence that supports a role for the paraventricular nucleus of the thalamus (PVT) in motivated behavior. We include a neuroanatomical and neurochemical overview, outlining what is known of the cellular makeup of the region and its most prominent afferent and efferent connections. We discuss how these connections and distinctions across the anterior-posterior axis correspond to the perceived function of the PVT. We then focus on the hypothalamic-thalamic-striatal circuit and the neuroanatomical and functional placement of the PVT within this circuit. In this regard, the PVT is ideally positioned to integrate information regarding internal states and the external environment and translate it into motivated actions. Based on data that has emerged in recent years, including that from our laboratory, we posit that orexinergic (OX) innervation from the lateral hypothalamus (LH) to the PVT encodes the incentive motivational value of reward cues and thereby alters the signaling of the glutamatergic neurons projecting from the PVT to the shell of the nucleus accumbens (NAcSh). The PVT-NAcSh pathway then modulates dopamine activity and resultant cue-motivated behaviors. As we and others apply novel tools and approaches to studying the PVT we will continue to refine the anatomical, cellular, and functional definitions currently ascribed to this nucleus and further elucidate its role in motivated behaviors.

The Function of Paraventricular Thalamic Circuitry in Adaptive Control of Feeding Behavior

The paraventricular nucleus of the thalamus (PVT) is a complex area that is uniquely embedded across the core feeding, reward, arousal, and stress circuits. The PVT role in the control of feeding behavior is discussed here within a framework of adaptive behavioral guidance based on the body’s energy state and competing drives. The survival of an organism depends on bodily energy resources and promotion of feeding over other behaviors is adaptive except when in danger or sated. The PVT is structurally set up to respond to homeostatic and hedonic needs to feed, and to integrate those signals with physiological and environmental stress, as well as anticipatory needs and other cognitive inputs. It can regulate both food foraging (seeking) and consumption and may balance their expression. The PVT is proposed to accomplish these functions through a network of connections with the brainstem, hypothalamic, striatal, and cortical areas. The connectivity of the PVT further indicates that it could broadcast the information about energy use/gain and behavioral choice to impact cognitive processes—learning, memory, and decision-making—through connections with the medial and lateral prefrontal cortical areas, the hippocampal formation, and the amygdala. The PVT is structurally complex and recent evidence for specific PVT pathways in different aspects of feeding behavior will be discussed.

Functional Connectivity of the Human Paraventricular Thalamic Nucleus: Insights From High Field Functional MRI

The paraventricular thalamic nucleus (PVT) is a small but highly connected nucleus of the dorsal midline thalamus. The PVT has garnered recent attention as a context-sensitive node within the thalamocortical arousal system that modulates state-dependent motivated behaviors. Once considered related to generalized arousal responses with non-specific impacts on behavior, accumulating evidence bolsters the contemporary view that discrete midline thalamic subnuclei belong to specialized corticolimbic and corticostriatal circuits related to attention, emotions, and cognition. However, the functional connectivity patterns of the human PVT have yet to be mapped. Here, we combined high-quality, high-resolution 7T and 3T resting state MRI data from 121 young adult participants from the Human Connectome Project (HCP) and thalamic subnuclei atlas masks to investigate resting state functional connectivity of the human PVT. The 7T results demonstrated extensive positive functional connectivity with the brainstem, midbrain, ventral and dorsal medial prefrontal cortex (mPFC), anterior and posterior cingulate, ventral striatum, hippocampus, and amygdala. These connections persist upon controlling for functional connectivity of the rest of the thalamus. Whole-brain contrasts provided further evidence that, compared to three nearby midline thalamic subnuclei, functional connectivity of the PVT is strong with the hippocampus, amygdala, ventral and dorsal mPFC, and middle temporal gyrus. These findings suggest that, even during rest, the human PVT is functionally coupled with many regions known to be structurally connected to rodent and non-human primate PVT. Further, cosine similarity analysis results suggested the PVT is integrated into the default mode network (DMN), an intrinsic connectivity network associated with episodic memory and self-referential thought. The current work provides a much-needed foundation for ongoing and future work examining the functional roles of the PVT in humans.

The paraventricular nucleus of the thalamus: an integrative node underlying homeostatic behavior

Early anatomical evidence suggested that the paraventricular nucleus of the thalamus (PVT) regulates arousal, as well as emotional and motivated behaviors. We discuss recent studies using modern techniques which now confirm and expand the involvement of the rodent PVT in these functions. Despite the emerging notion that the PVT is implicated in various behavioral processes, a recurrent theme is that activity in this brain region depends on internal state information arriving from the hypothalamus and brainstem, and is influenced by prior experience. We propose that the primary function of the PVT is to detect homeostatic challenges by integrating information about prior experiences, competing needs, and internal state to guide adaptive behavioral responses aimed at restoring homeostasis.

Influences of Stress and Sex on the Paraventricular Thalamus: Implications for Motivated Behavior

The paraventricular nucleus of the thalamus (PVT) is a critical neural hub for the regulation of a variety of motivated behaviors, integrating stress and reward information from environmental stimuli to guide discrete behaviors via several limbic projections. Neurons in the PVT are activated by acute and chronic stressors, however several roles of the PVT in behavior modulation emerge only following repeated stress exposure, pointing to a role for hypothalamic pituitary adrenal (HPA) axis modulation of PVT function. Further, there may be a reciprocal relationship between the PVT and HPA axis in which chronic stress-induced recruitment of the PVT elicits an additional role for the PVT to regulate motivated behavior by modulating HPA physiology and thus the neuroendocrine response to stress itself. This complex interaction may make the PVT and its role in influencing motivated behavior particularly susceptible to chronic stress-induced plasticity in the PVT, especially in females who display increased susceptibility to stress-induced maladaptive behaviors associated with neuropsychiatric diseases. Though literature is describing the sex-specific effects of acute and chronic stress exposure on HPA axis activation and motivated behaviors, the impact of sex on the role of the PVT in modulating the behavioral and neuroendocrine response to stress is less well established. Here, we review what is currently known regarding the acute and chronic stress-induced activation and behavioral role of the PVT in male and female rodents. We further explore stress hormone and neuropeptide signaling mechanisms by which the HPA axis and PVT interact and discuss the implications for sex-dependent effects of chronic stress on the PVT's role in motivated behaviors.

The Paraventricular Nucleus of the Thalamus as an Integrating and Relay Node in the Brain Anxiety Network

The brain anxiety network is composed of a number of interconnected cortical regions that detect threats and execute appropriate defensive responses via projections to the shell of the nucleus accumbens (NAcSh), dorsolateral region of the bed nucleus of the stria terminalis (BSTDL) and lateral region of the central nucleus of the amygdala (CeL). The paraventricular nucleus of the thalamus (PVT) is anatomically positioned to integrate threat- and arousal-related signals from cortex and hypothalamus and then relay these signals to neural circuits in the NAcSh, BSTDL, and CeL that mediate defensive responses. This review describes the anatomical connections of the PVT that support the view that the PVT may be a critical node in the brain anxiety network. Experimental findings are reviewed showing that the arousal peptides orexins (hypocretins) act at the PVT to promote avoidance of potential threats especially following exposure of rats to a single episode of footshocks. Recent anatomical and experimental findings are discussed which show that neurons in the PVT provide divergent projections to subcortical regions that mediate defensive behaviors and that the projection to the NAcSh is critical for the enhanced social avoidance displayed in rats exposed to footshocks. A theoretical model is proposed for how the PVT integrates cortical and hypothalamic signals to modulate the behavioral responses associated with anxiety and other challenging situations.

The Contribution of Thalamic Nuclei in Salience Processing

The brain continuously receives diverse information about the external environment and changes in the homeostatic state. The attribution of salience determines which stimuli capture attention and, therefore, plays an essential role in regulating emotions and guiding behaviors. Although the thalamus is included in the salience network, the neural mechanism of how the thalamus contributes to salience processing remains elusive. In this mini-review, we will focus on recent advances in understanding the specific roles of distinct thalamic nuclei in salience processing. We will summarize the functional connections between thalamus nuclei and other key nodes in the salience network. We will highlight the convergence of neural circuits involved in reward and pain processing, arousal, and attention control in thalamic structures. We will discuss how thalamic activities represent salience information in associative learning and how thalamic neurons modulate adaptive behaviors. Lastly, we will review recent studies which investigate the contribution of thalamic dysfunction to aberrant salience processing in neuropsychiatric disorders, such as drug addiction, posttraumatic stress disorder (PTSD), and schizophrenia. Based on emerging evidence from both human and rodent research, we propose that the thalamus, different from previous studies that as an information relay, has a broader role in coordinating the cognitive process and regulating emotions.

Distinct and Overlapping Patterns of Acute Ethanol-Induced C-Fos Activation in Two Inbred Replicate Lines of Mice Selected for Drinking to High Blood Ethanol Concentrations

The inbred high drinking in the dark (iHDID1 and iHDID2) strains are two replicate lines bred from the parent HS/Npt (HS) line for achieving binge levels of blood ethanol concentration (≥80 mg/dL BEC) in a four-hour period. In this work, we sought to evaluate differences in baseline and ethanol-induced c-Fos activation between the HS, iHDID1, and iHDID2 genetic lines in brain regions known to process the aversive properties of ethanol. Methods: Male and female HS, iHDID1, and iHDID2 mice underwent an IP saline 2 3 g/kg ethanol injection. Brain sections were then stained for c-Fos expression in the basolateral/central amygdala (BLA/CeA), bed nucleus of the stria terminals (BNST), A2, locus coeruleus (LC), parabrachial nucleus (PBN), lateral/medial habenula (LHb/MHb), paraventricular nucleus of the thalamus (PVT), periaqueductal gray (PAG), Edinger–Westphal nuclei (EW), and rostromedial tegmental nucleus (RMTg). Results: The iHDID1 and iHDID2 lines showed similar and distinct patterns of regional c-Fos; however, in no region did the two both significantly differ from the HS line together. Conclusions: Our findings lend further support to the hypothesis the iHDID1 and the iHDID2 lines arrive at a similar behavior phenotype through divergent genetic mechanisms.

The lateral hypothalamus and orexinergic transmission in the paraventricular thalamus promote the attribution of incentive salience to reward-associated cues

RATIONALE

Prior research suggests that the neural pathway from the lateral hypothalamic area (LHA) to the paraventricular nucleus of the thalamus (PVT) mediates the attribution of incentive salience to Pavlovian reward cues. However, a causal role for the LHA and the neurotransmitters involved have not been demonstrated in this regard.

OBJECTIVES

To examine (1) the role of LHA in the acquisition of Pavlovian conditioned approach (PavCA) behaviors, and (2) the role of PVT orexin 1 receptors (OX1r) and orexin 2 receptors (OX2r) in the expression of PavCA behaviors and conditioned reinforcement.

METHODS

Rats received excitotoxic lesions of the LHA prior to Pavlovian training. A separate cohort of rats characterized as sign-trackers (STs) or goal-trackers (GTs) received the OX1r antagonist SB-334867, or the OX2r antagonist TCS-OX2-29, into the PVT, to assess their effects on the expression of PavCA behavior and on the conditioned reinforcing properties of a Pavlovian reward cue.

RESULTS

LHA lesions attenuated the development of sign-tracking behavior. Administration of either the OX1r or OX2r antagonist into the PVT reduced sign-tracking behavior in STs. Further, OX2r antagonism reduced the conditioned reinforcing properties of a Pavlovian reward cue in STs.

CONCLUSIONS

The LHA is necessary for the development of sign-tracking behavior; and blockade of orexin signaling in the PVT attenuates the expression of sign-tracking behavior and the conditioned reinforcing properties of a Pavlovian reward cue. Together, these data suggest that LHA orexin inputs to the PVT are a key component of the circuitry that encodes the incentive motivational value of reward cues.

Assessing the Role of Corticothalamic and Thalamo-Accumbens Projections in the Augmentation of Heroin Seeking in Chronically Food-Restricted Rats

Drug addiction is a chronic disorder characterized by compulsive drug seeking, and involves repetitive cycles of compulsive drug use, abstinence, and relapse. In both human and animal models of addiction, chronic food restriction increases rates of relapse. Our laboratory has reported a robust increase in drug seeking following a period of withdrawal in chronically food-restricted rats compared with sated controls. Recently, we reported that activation of the paraventricular nucleus of the thalamus (PVT) abolished heroin seeking in chronically food-restricted rats. However, the precise inputs and outputs of the PVT that mediate this effect remain elusive. The goal of the current study was to determine the role of corticothalamic and thalamo-accumbens projections in the augmentation of heroin seeking induced by chronic food restriction. Male Long-Evans rats were trained to self-administer heroin for 10 d. Next, rats were removed from the self-administration chambers and were subjected to a 14 d withdrawal period while sated (unlimited access to food) or mildly food-restricted (FDR). On day 14, rats were returned to the self-administration context for a 3 h heroin-seeking test under extinction conditions during which corticothalamic and thalamo-accumbens neural activity was altered using chemogenetics. Surprisingly, chemogenetic activation or inhibition of corticothalamic projections did not alter heroin-seeking behavior. Chemogenetic activation of thalamo-accumbens shell, but not core, projectors attenuated heroin seeking in FDR rats. The results indicate an important role for the PVT to nucleus accumbens shell projections in the augmentation of heroin seeking induced by chronic food restriction.SIGNIFICANCE STATEMENT Relapse to heroin use is one of the major obstacles in the treatment of opiate addiction. Triggers for relapse are modulated by environmental challenges such as caloric restriction. Elucidating the brain mechanisms that underlie relapse is critical for evidence-based treatment development. Here we demonstrate a critical role for the input from the paraventricular thalamus (PVT), a hub for cortical, sensory, and limbic information, to the nucleus accumbens shell (an area known to be important for reward and motivation) in the augmentation of heroin seeking in food-restricted rats. Our findings highlight a previously unknown role for the PVT in heroin seeking following a period of abstinence.

A plant-based meal affects thalamus perfusion differently than an energy- and macronutrient-matched conventional meal in men with type 2 diabetes, overweight/obese, and healthy men: A three-group randomized crossover study

BACKGROUND & AIMS

Reward circuitry in the brain plays a key role in weight regulation. We tested the effects of a plant-based meal on these brain regions.

METHODS

A randomized crossover design was used to test the effects of two energy- and macronutrient-matched meals: a vegan (V-meal) and a conventional meat (M-meal) on brain activity, gastrointestinal hormones, and satiety in participants with type 2 diabetes (T2D; n = 20), overweight/obese participants (O; n = 20), and healthy controls (H; n = 20). Brain perfusion was measured, using arterial spin labeling functional brain imaging; satiety was assessed using a visual analogue scale; and plasma concentrations of gut hormones were determined at 0 and 180 min. Repeated-measures ANOVA was used for statistical analysis. Bonferroni correction for multiple comparisons was applied. The Hedge's g statistic was used to measure the effect size for means of paired difference between the times (180-0 min) and meal types (M-V meal) for each group.

RESULTS

Thalamus perfusion was the highest in patients with T2D and the lowest in overweight/obese individuals (p = 0.001). Thalamus perfusion decreased significantly after ingestion of the M-meal in men with T2D (p = 0.04) and overweight/obese men (p = 0.004), and it decreased significantly after ingestion of the V-meal in healthy controls (p < 0.001; Group x Meal x Time: F = 3.4; p = 0.035). The effect size was -0.41 (95% CI, -1.14 to 0.31; p = 0.26) for men with diabetes; -0.72 (95% CI, -1.48 to 0.01; p = 0.05) for overweight/obese men; and 0.82 (95% CI, 0.09 to 1.59; p = 0.03) for healthy men. Postprandial secretion of active GLP-1 increased after the V-meal compared with the M-meal by 42% (95% CI 25-62%; p = 0.003) in men with T2D and by 41% (95% CI 24-61%; p = 0.002) in healthy controls. Changes in thalamus perfusion after ingestion of both test meals correlated with changes in satiety (r = +0.68; p < 0.01), fasting plasma insulin (r = +0.40; p < 0.01), C-peptide (r = +0.48; p < 0.01) and amylin (r = +0.55; p < 0.01), and insulin secretion at 5 mmol/l (r = +0.77; p < 0.05).

CONCLUSIONS

The higher postprandial GLP-1 secretion after the V-meal in men with T2D, with concomitant greater satiety and changes in thalamus perfusion, suggest a potential use of plant-based meals in addressing the key pathophysiologic mechanisms of food intake regulation. Trial registration ClinicalTrials.gov number, NCT02474147.

Morphine-mediated release of miR-138 in astrocyte-derived extracellular vesicles promotes microglial activation

Opioids, such as morphine, are the mainstay for the management of postsurgical pain. Over the last decade there has been a dramatic increase in deaths related to opioid overdose. While opioid abuse has been shown to result in increased neuroinflammation, mechanism(s) underlying this process, remain less understood. In recent years, microRNAs have emerged as key mediators of gene expression regulating both paracrine signaling and cellular crosstalk. MiRNAs constitute the extracellular vesicle (EV) cargo and can shuttle from the donor to the recipient cells. Exposure of human primary astrocytes to morphine resulted in induction and release of miR-138 in the EVs isolated from conditioned media of cultured astrocytes. Released EVs were, in turn, taken up by the microglia, leading to activation of these latter cells. Interestingly, activation of microglia involved binding of the GUUGUGU motif of miR138 to the endosomal toll like receptor (TLR)7, leading, in turn, to cellular activation. These findings were further corroborated in vivo in wildtype mice wherein morphine administration resulted in increased microglial activation in the thalamus. In TLR7-/- mice on the other hand, morphine failed to induce microglial activation. These findings have ramifications for the development of EV-loaded anti-miRNAs as therapeutics for alleviating neuroinflammation in opioids abusers.

Cannabidiol modulates the expression of neuroinflammatory factors in stress- and drug-induced reinstatement of methamphetamine in extinguished rats

Methamphetamine (METH) is a highly potent and addictive psychostimulant that is frequently abused worldwide. Although the biggest challenge to the efficient treatment of drug dependence is relapse, its mechanism is completely unclear. Plenty of evidence suggests that inflammation contributes to drug-induced reward especially in brain regions that are involved in the reward system, but there is no document about relapse. Cannabidiol (CBD) is a nonpsychoactive cannabinoid that has powerful anti-inflammatory and immunosuppressive properties. A previous research in our laboratory has demonstrated that CBD prevents reinstatement of METH even in 24-hour rapid eye movement (REM) sleep-deprived (RSD) rats. The aim of this study was to assess whether CBD prevents reinstatement of METH through change of gene expression of cytokines such as interleukin-1β, interleukin-6, interleukin-10, and tumor necrosis factor α (TNF-α) in extinguished rats. Real-time polymerase chain reaction (PCR) was used in this research to assay gene expression of cytokines. We found that stress- and drug-induced reinstatement of METH enhanced mRNA expression of cytokines in the prefrontal cortex (PFC) and hippocampus. Furthermore, CBD treatment significantly reduced the mRNA expression of cytokines in the PFC and hippocampus, but CBD treatment in RSD rats increased expression of cytokines in the hippocampus. It seems that enhancement of cytokines leads to change in neurotransmission and so triggers reinstatement of METH.

Timing of Morphine Administration Differentially Alters Paraventricular Thalamic Neuron Activity

The paraventricular thalamic nucleus (PVT) is a brain region involved in regulating arousal, goal-oriented behaviors, and drug seeking, all key factors playing a role in substance use disorder. Given this, we investigated the temporal effects of administering morphine, an opioid with strongly addictive properties, on PVT neuronal function in mice using acute brain slices. Here, we show that morphine administration and electrophysiological recordings that occur during periods of animal inactivity (light cycle) elicit increases in PVT neuronal function during a 24-h abstinence time point. Furthermore, we show that morphine-induced increases in PVT neuronal activity at 24-h abstinence are occluded when morphine administration and recordings are performed during an animals' active state (dark cycle). Based on our electrophysiological results combined with previous findings demonstrating that PVT neuronal activity regulates drug-seeking behaviors, we investigated whether timing morphine administration with periods of vigilance (dark cycle) would decrease drug-seeking behaviors in an animal model of substance use disorder. We found that context-induced morphine-seeking behaviors were intact regardless of the time morphine was administered (e.g., light cycle or dark cycle). Our electrophysiological results suggest that timing morphine with various states of arousal may impact the firing of PVT neurons during abstinence. Although, this may not impact context-induced drug-seeking behaviors.

The paraventricular thalamic nucleus: A key hub of neural circuits underlying drug addiction

Drug addiction is a chronic relapsing brain disease characterized by compulsive, out-of-control drug use and the appearance of negative somatic and emotional consequences when drug access is prevented. The limited efficacy of treatment urges researchers toward a deeper understanding of the neural mechanism of drug addiction. Brain circuits that regulate reward and motivation are considered to be the neural substrate of drug addiction. An increasing body of literature indicates that the paraventricular thalamic nucleus (PVT) could serve as a key node in the neurocircuits that control goal-directed behaviors. In this review, we summarize the anatomical and functional evidence that the PVT regulates drug-related behaviors. The PVT receives extensive inputs from the brainstem and hypothalamus, and is reciprocally connected with the limbic system. Neurons in the PVT are recruited by drug exposure as well as cues and context associated with drug taking. Pathway-specific perturbation studies have begun to decipher the precise role of PVT circuits in drug-related behaviors. We also highlight recent findings about the involvement of neural plasticity of the PVT pathways in drug addiction and provide perspectives on future studies.

The thalamus in drug addiction: from rodents to humans

Impairments in response inhibition and salience attribution (iRISA) have been proposed to underlie the clinical symptoms of drug addiction as mediated by cortico-striatal-thalamo-cortical networks. The bulk of evidence supporting the iRISA model comes from neuroimaging research that has focused on cortical and striatal influences with less emphasis on the role of the thalamus. Here, we highlight the importance of the thalamus in drug addiction, focusing on animal literature findings on thalamic nuclei in the context of drug-seeking, structural and functional changes of the thalamus as measured by imaging studies in human drug addiction, particularly during drug cue and non-drug reward processing, and response inhibition tasks. Findings from the animal literature suggest that the paraventricular nucleus of the thalamus, the lateral habenula and the mediodorsal nucleus may be involved in the reinstatement, extinction and expression of drug-seeking behaviours. In support of the iRISA model, the human addiction imaging literature demonstrates enhanced thalamus activation when reacting to drug cues and reduced thalamus activation during response inhibition. This pattern of response was further associated with the severity of, and relapse in, drug addiction. Future animal studies could widen their field of focus by investigating the specific role(s) of different thalamic nuclei in different phases of the addiction cycle. Similarly, future human imaging studies should aim to specifically delineate the structure and function of different thalamic nuclei, for example, through the application of advanced imaging protocols at higher magnetic fields (7 Tesla).This article is part of a discussion meeting issue 'Of mice and mental health: facilitating dialogue between basic and clinical neuroscientists'.

Sleep-Wake Cycling and Energy Conservation: Role of Hypocretin and the Lateral Hypothalamus in Dynamic State-Dependent Resource Optimization

The hypocretin (Hcrt) system has been implicated in a wide range of physiological functions from sleep-wake regulation to cardiovascular, behavioral, metabolic, and thermoregulagtory control. These wide-ranging physiological effects have challenged the identification of a parsimonious function for Hcrt. A compelling hypothesis suggests that Hcrt plays a role in the integration of sleep-wake neurophysiology with energy metabolism. For example, Hcrt neurons promote waking and feeding, but are also sensors of energy balance. Loss of Hcrt function leads to an increase in REM sleep propensity, but a potential role for Hcrt linking energy balance with REM sleep expression has not been addressed. Here we examine a potential role for Hcrt and the lateral hypothalamus (LH) in state-dependent resource allocation as a means of optimizing resource utilization and, as a result, energy conservation. We review the energy allocation hypothesis of sleep and how state-dependent metabolic partitioning may contribute toward energy conservation, but with additional examination of how the loss of thermoregulatory function during REM sleep may impact resource optimization. Optimization of energy expenditures at the whole organism level necessitates a top-down network responsible for coordinating metabolic operations in a state-dependent manner across organ systems. In this context, we then specifically examine the potential role of the LH in regulating this output control, including the contribution from both Hcrt and melanin concentrating hormone (MCH) neurons among a diverse LH cell population. We propose that this hypothalamic integration system is responsible for global shifts in state-dependent resource allocations, ultimately promoting resource optimization and an energy conservation function of sleep-wake cycling.

Application of Research Domain Criteria to childhood and adolescent impulsive and addictive disorders: Implications for treatment

The Research Domain Criteria (RDoC) initiative provides a large-scale, dimensional framework for the integration of research findings across traditional diagnoses, with the long-term aim of improving existing psychiatric treatments. A neurodevelopmental perspective is essential to this endeavor. However, few papers synthesizing research findings across childhood and adolescent disorders exist. Here, we discuss how the RDoC framework may be applied to the study of childhood and adolescent impulsive and addictive disorders in order to improve neurodevelopmental understanding and to enhance treatment development. Given the large scope of RDoC, we focus on a single construct highly relevant to addictive and impulsive disorders - initial responsiveness to reward attainment. Findings from genetic, molecular, neuroimaging and other translational research methodologies are highlighted.

The hypocretin/orexin system as a target for excessive motivation in alcohol use disorders

The hypocretin/orexin (ORX) system has been repeatedly demonstrated to regulate motivation for drugs of abuse, including alcohol. In particular, ORX seems to be critically involved in highly motivated behaviors, as is observed in high-seeking individuals in a population, in the seeking of highly palatable substances, and in models of dependence. It seems logical that this system could be considered as a potential target for treatment for addiction, particularly alcohol addiction, as ORX pharmacological manipulations significantly reduce drinking. However, the ORX system also plays a role in a wide range of other behaviors, emotions, and physiological functions and is disrupted in a number of non-dependence-associated disorders. It is therefore important to consider how the ORX system might be optimally targeted for potential treatment for alcohol use disorders either in combination with or separate from its role in other functions or diseases. This review will focus on the role of ORX in alcohol-associated behaviors and whether and how this system could be targeted to treat alcohol use disorders while avoiding impacts on other ORX-relevant functions. A brief overview of the ORX system will be followed by a discussion of some of the factors that makes it particularly intriguing as a target for alcohol addiction treatment, a consideration of some potential challenges associated with targeting this system and, finally, some future directions to optimize new treatments.

Dynorphin Counteracts Orexin in the Paraventricular Nucleus of the Thalamus: Cellular and Behavioral Evidence

The orexin (Orx) system plays a critical role in drug addiction and reward-related behaviors. The dynorphin (Dyn) system promotes depressive-like behavior and plays a key role in the aversive effects of stress. Orx and Dyn are co-released and have opposing functions in reward and motivation in the ventral tegmental area (VTA). Previous studies suggested that OrxA transmission in the posterior paraventricular nucleus of the thalamus (pPVT) participates in cocaine-seeking behavior. This study determined whether Orx and Dyn interact in the pPVT. Using the brain slice preparation for cellular recordings, superfusion of DynA onto pPVT neurons decreased the frequency of spontaneous and miniature excitatory postsynaptic currents (s/mEPSCs). OrxA increased the frequency of sEPSCs but had no effect on mEPSCs, suggesting a network-driven effect of OrxA. The amplitudes of s/mEPSCs were unaffected by the peptides, indicating a presynaptic action on glutamate release. Augmentation of OrxA-induced glutamate release was reversed by DynA. Utilizing a behavioral approach, separate groups of male Wistar rats were trained to self-administer cocaine or sweetened condensed milk (SCM). After extinction, rats received intra-pPVT administration of OrxA±DynA±the κ-opioid receptor antagonist nor-binaltorphimine (NorBNI) under extinction conditions. OrxA reinstated cocaine- and SCM-seeking behavior, with a greater effect in cocaine animals. DynA blocked OrxA-induced cocaine seeking but not SCM seeking. NorBNI did not induce or potentiate cocaine-seeking behavior induced by OrxA but reversed DynA effect. This indicates that the κ-opioid system in the pPVT counteracts OrxA-induced cocaine seeking, suggesting a novel therapeutic target to prevent cocaine relapse.

Role of Anterior Intralaminar Nuclei of Thalamus Projections to Dorsomedial Striatum in Incubation of Methamphetamine Craving

Relapse to methamphetamine (Meth) seeking progressively increases after withdrawal from drug self-administration (incubation of Meth craving). We previously demonstrated a role of dorsomedial striatum (DMS) dopamine D1 receptors (D1Rs) in this incubation. Here, we studied the role of afferent glutamatergic projections into the DMS and local D1R-glutamate interaction in this incubation in male rats. We first measured projection-specific activation on day 30 relapse test by using cholera toxin b (retrograde tracer) + Fos (activity marker) double-labeling in projection areas. Next, we determined the effect of pharmacological reversible inactivation of lateral or medial anterior intralaminar nuclei of thalamus (AIT-L or AIT-M) on incubated Meth seeking on withdrawal day 30. We then used an anatomical asymmetrical disconnection procedure to determine whether an interaction between AIT-L→DMS glutamatergic projections and postsynaptic DMS D1Rs contributes to incubated Meth seeking. We also determined the effect of unilateral inactivation of AIT-L and D1R blockade of DMS on incubated Meth seeking, and the effect of contralateral disconnection of AIT-L→DMS projections on nonincubated Meth seeking on withdrawal day 1. Incubated Meth seeking was associated with selective activation of AIT→DMS projections; other glutamatergic projections to DMS were not activated. AIT-L (but not AIT-M) inactivation or anatomical disconnection of AIT-L→DMS projections decreased incubated Meth seeking. Unilateral inactivation of AIT-L or D1R blockade of the DMS had no effect on incubated Meth craving, and contralateral disconnection of AIT-L→DMS projections had no effect on nonincubated Meth seeking. Our results identify a novel role of AIT-L and AIT-L→DMS glutamatergic projections in incubation of drug craving and drug seeking.SIGNIFICANCE STATEMENT Methamphetamine seeking progressively increases after withdrawal from drug self-administration, a phenomenon termed incubation of methamphetamine craving. We previously found that D1R-mediated dopamine transmission in the dorsomedial striatum plays a critical role in this incubation phenomenon. Here, we used neuroanatomical and neuropharmacological methods in rats to demonstrate that an interaction between the glutamatergic projection from the lateral anterior intralaminar nuclei of the thalamus to the dorsomedial striatum and local dopamine D1 receptors plays a critical role in relapse to methamphetamine seeking after prolonged withdrawal. Our study identified a novel motivation-related thalamostriatal projection critical to relapse to drug seeking.

Contribution of Dynorphin and Orexin Neuropeptide Systems to the Motivational Effects of Alcohol

Understanding the neural systems that drive alcohol motivation and are disrupted in alcohol use disorders is of critical importance in developing novel treatments. The dynorphin and orexin/hypocretin neuropeptide systems are particularly relevant with respect to alcohol use and misuse. Both systems are strongly associated with alcohol-seeking behaviors, particularly in cases of high levels of alcohol use as seen in dependence. Furthermore, both systems also play a role in stress and anxiety, indicating that disruption of these systems may underlie long-term homeostatic dysregulation seen in alcohol use disorders. These systems are also closely interrelated with one another - dynorphin/kappa opioid receptors and orexin/hypocretin receptors are found in similar regions and hypocretin/orexin neurons also express dynorphin - suggesting that these two systems may work together in the regulation of alcohol seeking and may be mutually disrupted in alcohol use disorders. This chapter reviews studies demonstrating a role for each of these systems in motivated behavior, with a focus on their roles in regulating alcohol-seeking and self-administration behaviors. Consideration is also given to evidence indicating that these neuropeptide systems may be viable targets for the development of potential treatments for alcohol use disorders.

CART neurons in the lateral hypothalamus communicate with the nucleus accumbens shell via glutamatergic neurons in paraventricular thalamic nucleus to modulate reward behavior

Paraventricular thalamic nucleus (PVT) serves as a transit node processing food and drug-associated reward information, but its afferents and efferents have not been fully defined. We test the hypothesis that the CART neurons in the lateral hypothalamus (LH) project to the PVT neurons, which in turn communicate via the glutamatergic fibers with the nucleus accumbens shell (AcbSh), the canonical site for reward. Rats conditioned to self-stimulate via an electrode in the right LH-medial forebrain bundle were used. Intra-PVT administration of CART (55-102) dose-dependently (10-50 ng/rat) lowered intracranial self-stimulation (ICSS) threshold and increased lever press activity, suggesting reward-promoting action of the peptide. However, treatment with CART antibody (intra-PVT) or MK-801 (NMDA antagonist, intra-AcbSh) produced opposite effects. A combination of sub-effective dose of MK-801 (0.01 µg/rat, intra-AcbSh) and effective dose of CART (25 ng/rat, intra-PVT) attenuated CART's rewarding action. Further, we screened the LH-PVT-AcbSh circuit for neuroadaptive changes induced by conditioning experience. A more than twofold increase was noticed in the CART mRNA expression in the LH on the side ipsilateral to the implanted electrode for ICSS. In addition, the PVT of conditioned rats showed a distinct increase in the (a) c-Fos expressing cells and CART fiber terminals, and (b) CART and vesicular glutamate transporter 2 immunostained elements. Concomitantly, the AcbSh showed a striking increase in expression of NMDA receptor subunit NR1. We suggest that CART in LH-PVT and glutamate in PVT-AcbSh circuit might support food-seeking behavior under natural conditions and also store reward memory.

Microstructures in striato-thalamo-orbitofrontal circuit in methamphetamine users

Background Striato-thalamo-orbitofrontal (STO) circuit plays a key role in the development of drug addiction. Few studies have investigated its microstructural abnormalities in methamphetamine (MA) users. Purpose To evaluate the microstructural changes and relevant clinical relevance of the STO circuit in MA users using diffusion tensor imaging (DTI). Material and Methods Twenty-eight MA users and 28 age-matched normal volunteers were enrolled. 3T magnetic resonance imaging (MRI) was employed to obtain structural T1-weighted (T1W) imaging and diffusion-tensor imaging (DTI) data. Freesurfer software was used for automated segmentation of the bilateral nucleus accumbens (NAc), thalami, and orbitofrontal cortex (OFC). Four DTI measures maps, fractional anisotropy (FA), mean diffusivity (MD), axial diffusion (AD), and radial diffusion (RD) were generated and non-linearly co-registered to structural space. Comparisons of DTI measures of the STO circuit were carried out between MA and controls using repeated measures analysis of variance. Correlation analyses were performed between STO circuit DTI measures and clinical characteristics. Results The MA group had significant FA reduction in the bilateral NAc, OFC, and right thalamus ( P < 0.05). Lower left OFC FA and right NAc FA/AD were associated with longer duration of MA use. Lower right OFC FA was associated with younger age at first MA use. Higher FA and lower MD/RD in the thalamus, as well as higher left OFC RD, were associated with increased psychiatric symptoms. Conclusion The STO circuit has reduced microstructural integrity in MA users. Microstructural changes in the thalamus may compensate for dysfunction in functionally connected cortices, which needs further investigation.

Monosynaptic retrograde tracing of neurons expressing the G-protein coupled receptor Gpr151 in the mouse brain

GPR151 is a G‐protein coupled receptor for which the endogenous ligand remains unknown. In the nervous system of vertebrates, its expression is enriched in specific diencephalic structures, where the highest levels are observed in the habenular area. The habenula has been implicated in a range of different functions including behavioral flexibility, decision making, inhibitory control, and pain processing, which makes it a promising target for treating psychiatric and neurological disease. This study aimed to further characterize neurons expressing the Gpr151 gene, by tracing the afferent connectivity of this diencephalic cell population. Using pseudotyped rabies virus in a transgenic Gpr151‐Cre mouse line, monosynaptic afferents of habenular and thalamic Gpr151‐expressing neuronal populations could be visualized. The habenular and thalamic Gpr151 systems displayed both shared and distinct connectivity patterns. The habenular neurons primarily received input from basal forebrain structures, the bed nucleus of stria terminalis, the lateral preoptic area, the entopeduncular nucleus, and the lateral hypothalamic area. The Gpr151‐expressing neurons in the paraventricular nucleus of the thalamus was primarily contacted by medial hypothalamic areas as well as the zona incerta and projected to specific forebrain areas such as the prelimbic cortex and the accumbens nucleus. Gpr151 mRNA was also detected at low levels in the lateral posterior thalamic nucleus which received input from areas associated with visual processing, including the superior colliculus, zona incerta, and the visual and retrosplenial cortices. Knowledge about the connectivity of Gpr151‐expressing neurons will facilitate the interpretation of future functional studies of this receptor.

Neuroanatomical Relationships between Orexin/Hypocretin-Containing Neurons/Nerve Fibers and Nicotine-Induced c-Fos-Activated Cells of the Reward-Addiction Neurocircuitry

Orexin/hypocretin-containing neurons in lateral hypothalamus (LH) are implicated in the neurobiology of nicotine addiction. However, the neuroanatomical relationships between orexin-neurons/nerve fibers and nicotine-activated cells within the reward-addiction neurocircuitry is not known. In the present study in mice, we first used c-Fos immunohistochemistry to identify CNS cells stimulated by an acute single injection of nicotine (NIC, 2 mg/kg, IP). Sequential double-labelling was then performed to identify the location of orexin-containing neurons and nerve fibers with respect to NIC-induced c-Fos activated cells and/or tyrosine hydroxylase (TH) immunoreactive (IR) cells of the mesocorticolimbic reward-addiction pathways. Orexin-IR nerve fibers and terminals were detected at multiple sites of the NIC reward-addiction circuitry in close apposition to, and intermingled with, NIC-induced c-Fos-IR cells of locus coeruleus (LC), ventral tegmental area (VTA), nucleus accumbens (Acb), LH and paraventricular thalamic nucleus (PVT). Double-labelling of orexin with TH showed frequent contact between orexin-IR nerve fibers and noradrenergic cells of LC. However, there was infrequent contact between the orexinergic fibers and the TH-expressing dopaminergic cells of VTA, dorsal raphe nucleus (DR), posterior hypothalamus (DA11), arcuate hypothalamic nucleus (DA12) and periventricular areas (DA14). The close anatomical contact between orexinergic nerve fibers and NIC-activated cells at multiple sites of the reward-addiction pathways suggests that orexinergic projections from LH are likely to be involved in modulating activity of the neurons that are directly impacted by acute administration of nicotine.

Individual differences in orexin-I receptor modulation of motivation for the opioid remifentanil

Orexin-1 receptors (Ox1Rs) have been implicated in the motivation for drugs of abuse. Here, we utilized a within-session behavioral-economics threshold procedure to screen for individual differences in economic demand for the ultra-short-acting opioid remifentanil and to test whether antagonism of Ox1Rs reduces remifentanil demand. The behavioral-economics procedure revealed robust individual differences in free consumption of remifentanil (Q₀ parameter; hedonic set point). Rats with low baseline Q₀ (low takers) displayed high demand elasticity (α parameter; reduced responding as drug price increased indicating low motivation for drug), whereas subjects with a higher Q₀ (high takers) exhibit low demand elasticity (low α) by continuing to self-administer remifentanil despite increased cost (reflecting higher motivation for drug). In a punished responding paradigm utilizing footshock, subjects that were classified as high takers at baseline withstood twice as much shock as low takers to continue self-administering remifentanil. Interestingly, Ox1R antagonism with SB-334867 reduced Q₀ and increased α in low takers but not in high takers. Similarly, the Ox1R antagonist attenuated cue-induced, but not drug-induced, reinstatement of remifentanil seeking in low takers but had no significant effect on reinstatement of drug seeking in high takers. Together, these data reveal a novel role of orexins in demand for remifentanil: Ox1Rs modulate demand in low takers but not in individuals that exhibit addictive-like behaviors (high takers). Finally, the behavioral assays in this study can serve as a novel laboratory model for studying individual differences in opioid use disorders.

Substance P in the anterior thalamic paraventricular nucleus: promotion of ethanol drinking in response to orexin from the hypothalamus

The paraventricular nucleus of the thalamus (PVT) appears to participate in drug addiction. Recent evidence in rats shows that ethanol drinking is increased by orexin/hypocretin (OX) afferents from the hypothalamus, acting specifically in the anterior (aPVT) rather than posterior (pPVT) PVT subregion. The present study sought to identify neuropeptides transcribed within the PVT, which themselves might contribute to ethanol drinking and possibly mediate the actions of OX. We discovered that substance P (SP) in the aPVT can stimulate intermittent-access ethanol drinking, similar to OX, and that SP receptor [neurokinin 1 receptor/tachykinin receptor 1 (NK1R)] antagonists in this subregion reduce ethanol drinking. As with OX, this effect is site specific, with SP in the pPVT or dorsal third ventricle having no effect on ethanol drinking, and it is behaviorally specific, with SP in the aPVT reducing the drinking of sucrose and stimulating it in the pPVT. A close relationship between SP and OX was demonstrated by a stimulatory effect of local OX injection on SP mRNA and peptide levels, specifically in the aPVT but not pPVT, and a stimulatory effect of OX on SP expression in isolated thalamic neurons, reflecting postsynaptic actions. A functional relationship between OX and SP in the aPVT is suggested by our additional finding that ethanol drinking induced by OX is blocked by a local NK1R antagonist administered at a sub-threshold dose. These results, suggesting that SP in the aPVT mediates the stimulatory effect of OX on ethanol drinking, identify a new role for SP in the control of this behavior.

Regulation of motivation for food by neuromedin U in the paraventricular nucleus and the dorsal raphe nucleus

BACKGROUND

Motivation for high-fat food is thought to contribute to excess caloric intake in obese individuals. A novel regulator of motivation for food may be neuromedin U (NMU), a highly-conserved neuropeptide that influences food intake. Although these effects of NMU have primarily been attributed to signaling in the paraventricular nucleus of the hypothalamus (PVN), NMU has also been found in other brain regions involved in both feeding behavior and motivation. We investigate the effects of NMU on motivation for food and food intake, and identify the brain regions mediating these effects.

METHODS

The motivational state for a particular reinforcer (e.g., high-fat food) can be assessed using a progressive-ratio schedule of reinforcement under which an increasing number of lever presses are required to obtain subsequent reinforcers. Here, we have used a progressive-ratio operant responding paradigm in combination with an assessment of cumulative food intake to evaluate the effects of NMU administration in rats, and identify the brain regions mediating these effects.

RESULTS

We found that peripheral administration of NMU decreases operant responding for high-fat food in rats. Evaluation of Fos-like immunoreactivity in response to peripheral NMU indicated the PVN and dorsal raphe nucleus (DRN) as sites of action for NMU. NMU infusion into either region mimics the effects of peripheral NMU on food intake and operant responding for food. NMU-containing projections from the lateral hypothalamus (LH) to the PVN and DRN were identified as an endogenous source of NMU.

CONCLUSIONS

These results identify the DRN as a site of action for NMU, demonstrate that the LH provides endogenous NMU to the PVN and DRN and implicate NMU signaling in the PVN and DRN as a novel regulator of motivation for high-fat foods.

Heantos-4, a natural plant extract used in the treatment of drug addiction, modulates T-type calcium channels and thalamocortical burst-firing

Heantos-4 is a refined combination of plant extracts currently approved to treat opiate addiction in Vietnam. In addition to its beneficial effects on withdrawal and prevention of relapse, reports of sedation during clinical treatment suggest that arousal networks in the brain may be recruited during Heantos administration. T-type calcium channels are implicated in the generation of sleep rhythms and in this study we examined whether a Heantos-4 extraction modulates T-type calcium channel currents generated by the Cav3.1, Cav3.2 and Ca3.3 subtypes. Utilizing whole-cell voltage clamp on exogenously expressed T-type calcium channels we find that Heantos inhibits Cav3.1 and Cav3.3 currents, while selectively potentiating Cav3.2 currents. We further examined the effects of Heantos-4 extract on low-threshold burst-firing in thalamic neurons which contribute to sleep oscillations. Using whole-cell current clamp in acute thalamic brain slices Heantos-4 suppressed rebound burst-firing in ventrobasal thalamocortical neurons, which express primarily Cav3.1 channels. Conversely, Heantos-4 had no significant effect on the burst-firing properties of thalamic reticular neurons, which express a mixed population of Cav3.2 and Cav3.3 channels. Examining Heantos-4 effects following oral administration in a model of absence epilepsy revealed the potential to exacerbate seizure activity. Together, the findings indicate that Heantos-4 has selective effects both on specific T-type calcium channel isoforms and distinct populations of thalamic neurons providing a putative mechanism underlying its effects on sedation and on the thalamocortical network.

Homeostasis Meets Motivation in the Battle to Control Food Intake

Signals of energy homeostasis interact closely with neural circuits of motivation to control food intake. An emerging hypothesis is that the transition to maladaptive feeding behavior seen in eating disorders or obesity may arise from dysregulation of these interactions. Focusing on key brain regions involved in the control of food intake (ventral tegmental area, striatum, hypothalamus, and thalamus), we describe how activity of specific cell types embedded within these regions can influence distinct components of motivated feeding behavior. We review how signals of energy homeostasis interact with these regions to influence motivated behavioral output and present evidence that experience-dependent neural adaptations in key feeding circuits may represent cellular correlates of impaired food intake control. Future research into mechanisms that restore the balance of control between signals of homeostasis and motivated feeding behavior may inspire new treatment options for eating disorders and obesity.

Automated Neuroanatomical Relation Extraction: A Linguistically Motivated Approach with a PVT Connectivity Graph Case Study

Identifying the relations among different regions of the brain is vital for a better understanding of how the brain functions. While a large number of studies have investigated the neuroanatomical and neurochemical connections among brain structures, their specific findings are found in publications scattered over a large number of years and different types of publications. Text mining techniques have provided the means to extract specific types of information from a large number of publications with the aim of presenting a larger, if not necessarily an exhaustive picture. By using natural language processing techniques, the present paper aims to identify connectivity relations among brain regions in general and relations relevant to the paraventricular nucleus of the thalamus (PVT) in particular. We introduce a linguistically motivated approach based on patterns defined over the constituency and dependency parse trees of sentences. Besides the presence of a relation between a pair of brain regions, the proposed method also identifies the directionality of the relation, which enables the creation and analysis of a directional brain region connectivity graph. The approach is evaluated over the manually annotated data sets of the WhiteText Project. In addition, as a case study, the method is applied to extract and analyze the connectivity graph of PVT, which is an important brain region that is considered to influence many functions ranging from arousal, motivation, and drug-seeking behavior to attention. The results of the PVT connectivity graph show that PVT may be a new target of research in mood assessment.

Orexin-1 receptor blockade suppresses compulsive-like alcohol drinking in mice

Addiction is promoted by pathological motivation for addictive substances, and, despite extensive efforts, alcohol use disorders (AUDs) continue to extract a very high social, physical, and economic toll. Compulsive drinking of alcohol, where consumption persists even when alcohol is paired with negative consequences, is considered a particular obstacle for treating AUDs. Aversion-resistant alcohol intake in rodents, e.g. where rodents drink even when alcohol is paired with the bitter tastant quinine, has been considered to model some compulsive aspects of human alcohol consumption. However, the critical mechanisms that drive compulsive-like drinking are only beginning to be identified. The neuropeptide orexin has been linked to high motivation for cocaine, preferred foods, and alcohol. Thus, we investigated the role of orexin receptors in compulsive-like alcohol drinking, where C57BL/6 mice had 2-hr daily access to 15% alcohol with or without quinine (100 μM). We found that systemic administration of the widely used selective orexin-1 receptor (OX1R) blocker, SB-334867 (SB), significantly reduced compulsive-like consumption at doses lower than those reported to reduce quinine-free alcohol intake. The dose of 3-mg/kg SB, in particular, suppressed only compulsive-like drinking. Furthermore, SB did not reduce concurrent water intake during the alcohol drinking sessions, and did not alter saccharin + quinine consumption. In addition, the OX2R antagonist TCS-OX2-29 (3 or 10 mg/kg) did not alter intake of alcohol with or without quinine. Together, our results suggest that OX1R signaling is particularly important for promoting compulsive-like alcohol drinking, and that OX1Rs might represent a novel therapy to counteract compulsive aspects of human AUDs.

Retrieving fear memories, as time goes by…

Research in fear conditioning has provided a comprehensive picture of the neuronal circuit underlying the formation of fear memories. In contrast, our understanding of the retrieval of fear memories is much more limited. This disparity may stem from the fact that fear memories are not rigid, but reorganize over time. To bring some clarity and raise awareness about the time-dependent dynamics of retrieval circuits, we review current evidence on the neuronal circuitry participating in fear memory retrieval at both early and late time points following auditory fear conditioning. We focus on the temporal recruitment of the paraventricular nucleus of the thalamus (PVT) for the retrieval and maintenance of fear memories. Finally, we speculate as to why retrieval circuits change with time, and consider the functional strategy of recruiting structures not previously considered as part of the retrieval circuit.

Cocaine-Induced Synaptic Alterations in Thalamus to Nucleus Accumbens Projection

Exposure to cocaine induces addiction-associated behaviors partially through remodeling neurocircuits in the nucleus accumbens (NAc). The paraventricular nucleus of thalamus (PVT), which projects to the NAc monosynaptically, is activated by cocaine exposure and has been implicated in several cocaine-induced emotional and motivational states. Here we show that disrupting synaptic transmission of select PVT neurons with tetanus toxin activated via retrograde trans-synaptic transport of cre from NAc efferents decreased cocaine self-administration in rats. This projection underwent complex adaptations after self-administration of cocaine (0.75 mg/kg/infusion; 2 h/d × 5 d, 1d overnight training). Specifically, 1d after cocaine self-administration, we observed increased levels of AMPA receptor (AMPAR)-silent glutamatergic synapses in this projection, accompanied by a decreased ratio of AMPAR-to-NMDA receptor (NMDAR)-mediated EPSCs. Furthermore, the decay kinetics of NMDAR EPSCs was significantly prolonged, suggesting insertion of new GluN2B-containing NMDARs to PVT-to-NAc synapses. After 45-d withdrawal, silent synapses within this projection returned to the basal levels, accompanied by a return of the AMPAR/NMDAR ratio and NMDAR decay kinetics to the basal levels. In amygdala and infralimbic prefrontal cortical projections to the NAc, a portion of cocaine-generated silent synapses becomes unsilenced by recruiting calcium-permeable AMPARs (CP-AMPARs) after drug withdrawal. However, the sensitivity of PVT-to-NAc synapses to CP-AMPAR-selective antagonists was not changed after withdrawal, suggesting that CP-AMPAR trafficking is not involved in the evolution of cocaine-generated silent synapses within this projection. Meanwhile, the release probability of PVT-to-NAc synapses was increased after short- and long-term cocaine withdrawal. These results reveal complex and profound alterations at PVT-to-NAc synapses after cocaine exposure and withdrawal.

Endocannabinoid 2-AG and intracellular cannabinoid receptors modulate a low-threshold calcium spike-induced slow depolarizing afterpotential in rat thalamic paraventricular nucleus neurons

In rat paraventricular thalamic nucleus (PVT) neurons, activation of low-threshold calcium (Ca(2+)) channels triggers a low-threshold spike (LTS) which may be followed by slow afterpotentials that can dramatically influence action potential patterning. Using gluconate-based internal recording solutions, we investigated the properties of a LTS-induced slow afterdepolarization (sADP) observed in a subpopulation of PVT neurons recorded in brain slice preparations. This LTS-induced sADP required T-type Ca(2+) channel opening, exhibited variable magnitudes between neurons and a voltage dependency with a maximum near -50 mV. The area under the sADP remained stable during control monitoring, but displayed gradual suppression in media where strontium replaced Ca(2+). The sADP was suppressed following bath application of 2-APB or ML204, suggesting engagement of transient receptor potential canonical (TRPC)-like channels. Further investigation revealed a reversible suppression during bath applications of membrane permeable cannabinoid receptor (CBR) blockers rimonabant, AM630 or SR144528 suggesting the presence of both CB1Rs and CB2Rs. Similar results were achieved by intracellular, but not bath application of the membrane impermeant CB1R blocker hemopressin, suggesting an intracellular localization of CB1Rs. Data from pharmacologic manipulation of endocannabinoid biosynthetic pathways suggested 2-arachidonlyglycerol (2-AG) as the endogenous cannabinoid ligand, derived via hydrolysis of diacylglycerol (DAG), with the latter formed from the pathway involving phosphatidylcholine-specific phospholipase D and phosphatic acid phosphohydrolase. The sADP suppression observed during recordings with pipettes containing LY294002, a PI3-kinase inhibitor, suggested a role for PI3kinase in the translocation of these TRPC-like channels to the plasma membrane. Drug-induced attenuation of the availability of 2-AG influences the number of action potentials that surmount the LTS evoked in PVT neurons, implying an ongoing intracellular CBR modulation of neuronal excitability during LTS-induced bursting behavior.