Differential activation of accumbens shell and core dopamine by sucrose reinforcement with nose poking and with lever pressing

Mild forced exercise in young mice prevents anergia induced by dopamine depletion in late adulthood: Relation to CDNF and DARPP-32 phosphorylation patterns in nucleus accumbens

Mesolimbic dopamine (DA) plays a critical role in behavioral activation and exertion of effort in motivated behaviors. DA antagonism and depletion in nucleus accumbens (Nacb) induces anergia in effort-based decision-making tasks. Exercise improves motor function in Parkinson's disease (PD). However, the beneficial effects of physical exercise on anergia, a symptom present in many psychiatric and neurological pathologies needs to be studied. During 9 weeks, young CD1 male mice were trained to run at a moderate speed in automatically turning running wheels (RW) (forced exercise group) or locked in static RWs (control group) in 1 h daily sessions. Both groups were tested in a 3-choice-T-maze task developed for the assessment of preference between active (RW) vs. sedentary reinforcers, and vulnerability to DA depletion-induced anergia was studied after tetrabenazine administration (TBZ; VMAT-2 blocker). Exercise did not change spontaneous preferences, did not affect body weight, plasma corticosterone levels or measures of anxiety, but it increased the cerebral DA neurotrophic factor (CDNF) in Nacb core, suggesting a neuroprotective effect in this nucleus. After TBZ administration, only the non-trained group showed a shift in relative preferences from active to sedentary options, reducing time running but increasing consumption of pellets, thus showing a typical anergic but not anhedonic effect. Moreover, only in the non-trained group, phosphorylation of DARPP-32(Thr34) increased after TBZ administration. These results are the first to show that mild forced exercise carried out from a young age to adulthood could act on Nacb-related functions, and prevent the anergia-inducing effects of DA depletion.

Thyrotropin-Releasing Hormone and Food Intake in Mammals: An Update

Thyrotropin-releasing hormone (TRH; pGlu-His-Pro-NH2) is an intercellular signal produced mainly by neurons. Among the multiple pharmacological effects of TRH, that on food intake is not well understood. We review studies demonstrating that peripheral injection of TRH generally produces a transient anorexic effect, discuss the pathways that might initiate this effect, and explain its short half-life. In addition, central administration of TRH can produce anorexic or orexigenic effects, depending on the site of injection, that are likely due to interaction with TRH receptor 1. Anorexic effects are most notable when TRH is injected into the hypothalamus and the nucleus accumbens, while the orexigenic effect has only been detected by injection into the brain stem. Functional evidence points to TRH neurons that are prime candidate vectors for TRH action on food intake. These include the caudal raphe nuclei projecting to the dorsal motor nucleus of the vagus, and possibly TRH neurons from the tuberal lateral hypothalamus projecting to the tuberomammillary nuclei. For other TRH neurons, the anatomical or physiological context and impact of TRH in each synaptic domain are still poorly understood. The manipulation of TRH expression in well-defined neuron types will facilitate the discovery of its role in food intake control in each anatomical scene.

Chronic Red Bull Consumption during Adolescence: Effect on Mesocortical and Mesolimbic Dopamine Transmission and Cardiovascular System in Adult Rats

Energy drinks are very popular nonalcoholic beverages among adolescents and young adults for their stimulant effects. Our study aimed to investigate the effect of repeated intraoral Red Bull (RB) infusion on dopamine transmission in the nucleus accumbens shell and core and in the medial prefrontal cortex and on cardiac contractility in adult rats exposed to chronic RB consumption. Rats were subjected to 4 weeks of RB voluntary consumption from adolescence to adulthood. Monitoring of in vivo dopamine was carried out by brain microdialysis. In vitro cardiac contractility was studied on biomechanical properties of isolated left-ventricular papillary muscle. The main finding of the study was that, in treated animals, RB increased shell dopamine via a nonadaptive mechanism, a pattern similar to that of drugs of abuse. No changes in isometric and isotonic mechanical parameters were associated with chronic RB consumption. However, a prolonged time to peak tension and half-time of relaxation and a slower peak rate of tension fall were observed in RB-treated rats. It is likely that RB treatment affects left-ventricular papillary muscle contraction. The neurochemical results here obtained can explain the addictive properties of RB, while the cardiovascular investigation findings suggest a hidden papillary contractility impairment.

Concurrent and Delayed Behavioral and Monoamine Alterations by Excessive Sucrose Intake in Juvenile Mice

Our daily diet in the modern society has substantially changed from that in the ancient past. Consequently, new disorders associated with such dietary changes have emerged. For instance, excessive intake of compounds, such as sucrose (SUC), has recently been reported to induce pathological neuronal changes in adults, such as food addiction. It is still largely unclear whether and how excessive intake of such nutrients affects neurodevelopment. We investigated changes in behavior and monoamine signaling caused by excessive, semi-chronic intake of SUC and the non-caloric sweetener saccharin (SAC) in juvenile mice, using a battery of behavioral tests and high-performance liquid chromatography. Both SUC and SAC intake induced behavioral alterations such as altered amphetamine responses, sucrose preference, stress response, and anxiety, but did not affect social behavior and cognitive function such as attention in juvenile and adult mice. Moreover, SUC and SAC also altered dopamine and serotonin transmission in mesocorticolimbic regions. Some of these behavioral and neural alterations were triggered by SAC and SUC but others were distinct between the treatments. Moreover, alterations induced in juvenile mice were also different from those observed in adult mice. These results suggest that excessive SUC and SAC intake during the juvenile period may cause concurrent and delayed behavioral and monoamine signaling alterations in juvenile and adult mice, respectively.

Maternal Diet Influences the Reinstatement of Cocaine-Seeking Behavior and the Expression of Melanocortin-4 Receptors in Female Offspring of Rats

Recent studies have emphasized the role of the maternal diet in the development of mental disorders in offspring. Substance use disorder is a major global health and economic burden. Therefore, the search for predisposing factors for the development of this disease can contribute to reducing the health and social damage associated with addiction. In this study, we focused on the impact of the maternal diet on changes in melanocortin-4 (MC-4) receptors as well as on behavioral changes related to cocaine addiction. Rat dams consumed a high-fat diet (HFD), high-sugar diet (HSD, rich in sucrose), or mixed diet (MD) during pregnancy and lactation. Using an intravenous cocaine self-administration model, the susceptibility of female offspring to cocaine reward and cocaine-seeking propensities was evaluated. In addition, the level of MC-4 receptors in the rat brain structures related to cocaine reward and relapse was assessed. Modified maternal diets did not affect cocaine self-administration in offspring. However, the maternal HSD enhanced cocaine-seeking behavior in female offspring. In addition, we observed that the maternal HSD and MD led to increased expression of MC-4 receptors in the nucleus accumbens, while increased MC-4 receptor levels in the dorsal striatum were observed after exposure to the maternal HSD and HFD. Taken together, it can be concluded that a maternal HSD is an important factor that triggers cocaine-seeking behavior in female offspring and the expression of MC-4 receptors.

Maternal high-sugar diet changes offspring vulnerability to reinstatement of cocaine-seeking behavior: Role of melanocortin-4 receptors

Maternal diet significantly influences the proper development of offspring in utero. Modifications of diet composition may lead to metabolic and mental disorders that may predispose offspring to a substance use disorder. We assessed the impact of a maternal high-sugar diet (HSD, rich in sucrose) consumed during pregnancy and lactation on the offspring phenotype in the context of the rewarding and motivational effects of cocaine and changes within the central melanocortin (MC) system. Using an intravenous cocaine self-administration model, we showed that maternal HSD leads to increased relapse of cocaine-seeking behavior in male offspring. In addition, we demonstrated that cocaine induces changes in the level of MC-4 receptors in the offspring brain, and these changes depend on maternal diet. These studies also reveal that an MC-4 receptor antagonist reduces the reinstatement of cocaine-seeking behavior, and offspring exposed to maternal HSD are more sensitive to its effects than offspring exposed to the maternal control diet. Taken together, the results suggest that a maternal HSD and MC-4 receptors play an important role in cocaine relapse.

Differential Effects of Dorsal and Ventral Medial Prefrontal Cortex Inactivation during Natural Reward Seeking, Extinction, and Cue-Induced Reinstatement

Rodent dorsal medial prefrontal cortex (mPFC), typically prelimbic cortex, is often described as promoting actions such as reward seeking, whereas ventral mPFC, typically infralimbic cortex, is thought to promote response inhibition. However, both dorsal and ventral mPFC are necessary for both expression and suppression of different behaviors, and each region may contribute to different functions depending on the specifics of the behavior tested. To better understand the roles of dorsal and ventral mPFC in motivated behavior we pharmacologically inactivated each area during operant fixed ratio 1 (FR1) seeking for a natural reward (sucrose), extinction, cue-induced reinstatement, and progressive ratio (PR) sucrose seeking in male Long–Evans rats. Bilateral inactivation of dorsal mPFC, but not ventral mPFC increased reward seeking during FR1. Inactivation of both dorsal and ventral mPFC decreased seeking during extinction. Bilateral inactivation of ventral mPFC, but not dorsal mPFC decreased reward seeking during cue-induced reinstatement. No effect of inactivation was found during PR. Our data contrast sharply with observations seen during drug seeking and fear conditioning, indicating that previously established roles of dorsal mPFC = going versus ventral mPFC = stopping are not applicable to all motivated behaviors and/or outcomes. Our results indicate that dichotomous functions of dorsal versus ventral mPFC, if they exist, may align better with other models, or may require the development of a new framework in which these multifaceted brain areas play different roles in action control depending on the behavioral context in which they are engaged.

Dopaminergic cellular and circuit contributions to kappa opioid receptor mediated aversion

Neural circuits that enable an organism to protect itself by promoting escape from immediate threat and avoidance of future injury are conceptualized to carry an "aversive" signal. One of the key molecular elements of these circuits is the kappa opioid receptor (KOR) and its endogenous peptide agonist, dynorphin. In many cases, the aversive response to an experimental manipulation can be eliminated by selective blockade of KOR function, indicating its necessity in transmitting this signal. The dopamine system, through its contributions to reinforcement learning, is also involved in processing of aversive stimuli, and KOR control of dopamine in the context of aversive behavioral states has been intensely studied. In this review, we have discussed the multiple ways in which the KORs regulate dopamine dynamics with a central focus on dopamine neurons and projections from the ventral tegmental area. At the neuronal level, KOR agonists inhibit dopamine neurons both in the somatodendritic region as well as at terminal release sites, through various signaling pathways and ion channels, and these effects are specific to different synaptic sites. While the dominant hypotheses are that aversive states are driven by decreases in dopamine and increases in dynorphin, reported exceptions to these patterns indicate these ideas require refinement. This is critical given that KOR is being considered as a target for development of new therapeutics for anxiety, depression, pain, and other psychiatric disorders.

The Modulating Role of Sex and Anabolic-Androgenic Steroid Hormones in Cannabinoid Sensitivity

Cannabis is the most commonly used illicit drug worldwide. Although its use is associated with multiple adverse health effects, including the risk of developing addiction, recreational and medical cannabis use is being increasing legalized. In addition, use of synthetic cannabinoid drugs is gaining considerable popularity and is associated with mass poisonings and occasional deaths. Delineating factors involved in cannabis use and addiction therefore becomes increasingly important. Similarly to other drugs of abuse, the prevalence of cannabis use and addiction differs remarkably between males and females, suggesting that sex plays a role in regulating cannabinoid sensitivity. Although it remains unclear how sex may affect the initiation and maintenance of cannabis use in humans, animal studies strongly suggest that endogenous sex hormones modulate cannabinoid sensitivity. In addition, synthetic anabolic-androgenic steroids alter substance use and further support the importance of sex steroids in controlling drug sensitivity. The recent discovery that pregnenolone, the precursor of all steroid hormones, controls cannabinoid receptor activation corroborates the link between steroid hormones and the endocannabinoid system. This article reviews the literature regarding the influence of endogenous and synthetic steroid hormones on the endocannabinoid system and cannabinoid action.

Dopamine Encodes Retrospective Temporal Information in a Context-Independent Manner

The dopamine system responds to reward-predictive cues to reflect a prospective estimation of reward value, although its role in encoding retrospective reward-related information is unclear. We report that cue-evoked dopamine release in the nucleus accumbens core encodes the time elapsed since the previous reward or rather the wait time. Specifically, a cue that always follows the preceding reward with a short wait time elicits a greater dopamine response relative to a distinct cue that always follows the preceding reward with a long wait time. Differences in the dopamine response between short wait and long wait cues were evident even when these cues were never experienced together within the same context. Conditioned responding updated accordingly with a change in cue-evoked dopamine release but was unrelated to a difference in the dopamine response between cues. Collectively, these findings illustrate that the cue-evoked dopamine response conveys a subjective estimation of the relative reward rate.

Systemic Administration of Orexin a Loaded Liposomes Potentiates Nucleus Accumbens Shell Dopamine Release by Sucrose Feeding

Orexin neurons originate in the lateral and dorsomedial hypothalamus and perifornical area and produce two different neuropeptides: orexin A (OxA) and orexin B (OxB), which activate OxR1 and OxR2 receptors. In the lateral hypothalamus (LH) orexin neurons are involved in behavior motivated by natural rewards such as palatable food (sugar, high-fat food) and it has been demonstrated similarly that the orexin signaling in the ventral tegmental area (VTA) is implicated in the intake of high-fat food. The VTA is an important area involved in reward processing. Given the involvement of nucleus accumbens (NAc) shell dopamine (DA) in motivation for food, we intended to investigate the effect of OxA on the basal and feeding-activated DA transmission in the NAc shell. OxA is a large peptide and does not cross the blood-brain barrier and for this reason was loaded on two kinds of liposomes: anti-transferrin-monoclonal antibodies (OX26-mAb) and lactoferrin-modified stealth liposomes. The effect of IV administration of both OxA liposomes on NAc shell DA was studied by microdialysis in freely moving rats. OxA, administered using both kinds of liposomes, produced a delayed and transitory increase in dialysate DA in the NAc shell, strongly and lastingly potentiated the increase in dialysate DA elicited by sucrose pellet consumption and increased the number of eaten pellets. These effects of OxA on DA transmission and feeding were prevented by the OxR1 antagonist SB 334867. Hence, OxA acting on VTA OxR1 can facilitate sucrose-stimulated NAc shell DA transmission directly by increasing the basal activity of VTA DA neurons that send their projections to the NAc shell.

Nucleus Accumbens Shell Dopamine Preferentially Tracks Information Related to Outcome Value of Reward

Effective decision-making requires organisms to predict reward values and bias behavior toward the best available option. The mesolimbic dopamine system, including the nucleus accumbens (NAc) shell and core, is involved in this process. Although studies support a role of the shell and core in specific aspects of decision-making (e.g., risk, effort, delay), no studies have directly compared dopamine release dynamics in these subregions to cues exclusively signaling the availability of different reward magnitudes. Here, fast-scan cyclic voltammetry was used to compare rapid dopamine release dynamics in the NAc subregions during a magnitude-based decision-making task. Rats learned that distinct cues signaled the availability of either a small or large reward (one or two sugar pellets), and then were given an opportunity to choose their preferred option. We found that peak dopamine release tracked the more preferred (higher-magnitude) option in both core and shell subregions. Critically, however, overall (i.e., global) dopamine release was significantly higher and longer lasting in the shell and tracked the preferred magnitude during the entire cue period. Further, in the shell (not core), dopamine signaling significantly declined immediately at the lever press for reward but increased during the period of reward consumption. Collectively, the results indicate that although dopamine release in both the core and shell are activated by cues signaling the opportunity to respond for rewards of different magnitudes, dopamine release in the shell plays a differential and unique role in tracking information related to the outcome value of reward.

Changes in Dopamine Transmission in the Nucleus Accumbens Shell and Core during Ethanol and Sucrose Self-Administration

Ethanol, like other substances of abuse, preferentially increases dopamine (DA) transmission in the rat nucleus accumbens (NAc) following passive administration. It remains unclear, however, whether ethanol also increases NAc DA transmission following operant oral self-administration (SA). The NAc is made-up of a ventro-medial compartment, the shell and a dorso-lateral one, the core, where DA transmission responds differentially following exposure to drugs of abuse. Previous studies from our laboratory investigated changes in dialysate DA in the NAc shell and core of rats responding for sucrose pellets and for drugs of abuse. As a follow up to these studies, we recently investigated the changes in NAc shell and core DA transmission associated to oral SA of a 10% ethanol solution. For the purpose of comparison with literature studies utilizing sucrose + ethanol solutions, we also investigated the changes in dialysate DA associated to SA of 20% sucrose and 10% ethanol + 20% sucrose solutions. Rats were trained to acquire oral SA of the solutions under a Fixed Ratio 1 (FR1) schedule of nose-poking. After training, rats were monitored by microdialysis on three consecutive days under response contingent (active), reward omission (extinction trial) and response non-contingent (passive) presentation of ethanol, sucrose or ethanol + sucrose solutions. Active and passive ethanol administration produced a similar increase in dialysate DA in the two NAc subdivisions, while under extinction trial DA increased preferentially in the shell compared to the core. Conversely, under sucrose SA and extinction DA increased exclusively in the shell. These observations provide unequivocal evidence that oral SA of 10% ethanol increases dialysate DA in the NAc, and also suggest that stimuli conditioned to ethanol exposure contribute to the increase of dialysate DA observed in the NAc following ethanol SA. Comparison between the pattern of DA changes detected in the NAc subdivisions under sucrose and ethanol SA likewise suggests that the NAc shell and core DA play different roles in sucrose as compared to ethanol reinforcement.

Behavioral sensitization of the reinforcing value of food: What food and drugs have in common

Sensitization is a basic property of the nervous system whereby repeated exposure to a stimulus results in an increase in responding to that stimulus. This increase in responding contributes to difficulty with treatment of drug abuse, as stimuli associated with substance use become signals or triggers for drug craving and relapse. Our work over the past decade has applied the theoretical framework of incentive sensitization to overeating. We have shown, in several studies, that lean adults do not commonly demonstrate behavioral sensitization after repeated exposure to snack food, but a subset of obese adults reliably does. This review will discuss this change in behavioral response to repeated consumption of snack food in obese individuals and apply the theoretical framework of incentive sensitization to drugs of abuse to high fat/high sugar snack foods. We will also show data that suggest that behavioral sensitization to repeated administration of snack food is predictive of weight gain, which may enhance its utility as a diagnostic tool for identifying at-risk individuals for obesity. Finally, we will discuss the future directions of this line of research, including studying the phenomenon in children and adolescents and determining if similar principles can be used to increase motivation to eat healthier food. A combination of reductions in unhealthy food intake and increases and healthy food intake is necessary to reduce obesity rates and improve health.

Neuronal Nicotinic Acetylcholine Receptor Modulators Reduce Sugar Intake

Excess sugar consumption has been shown to contribute directly to weight gain, thus contributing to the growing worldwide obesity epidemic. Interestingly, increased sugar consumption has been shown to repeatedly elevate dopamine levels in the nucleus accumbens (NAc), in the mesolimbic reward pathway of the brain similar to many drugs of abuse. We report that varenicline, an FDA-approved nicotinic acetylcholine receptor (nAChR) partial agonist that modulates dopamine in the mesolimbic reward pathway of the brain, significantly reduces sucrose consumption, especially in a long-term consumption paradigm. Similar results were observed with other nAChR drugs, namely mecamylamine and cytisine. Furthermore, we show that long-term sucrose consumption increases α4β2 * and decreases α6β2* nAChRs in the nucleus accumbens, a key brain region associated with reward. Taken together, our results suggest that nAChR drugs such as varenicline may represent a novel treatment strategy for reducing sugar consumption.