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Concept: Mesolimbic pathway

241

Sleep control is ascribed to a two-process model, a widely accepted concept that posits homoeostatic drive and a circadian process as the major sleep-regulating factors. Cognitive and emotional factors also influence sleep-wake behaviour; however, the precise circuit mechanisms underlying their effects on sleep control are unknown. Previous studies suggest that adenosine has a role affecting behavioural arousal in the nucleus accumbens (NAc), a brain area critical for reinforcement and reward. Here, we show that chemogenetic or optogenetic activation of excitatory adenosine A2A receptor-expressing indirect pathway neurons in the core region of the NAc strongly induces slow-wave sleep. Chemogenetic inhibition of the NAc indirect pathway neurons prevents the sleep induction, but does not affect the homoeostatic sleep rebound. In addition, motivational stimuli inhibit the activity of ventral pallidum-projecting NAc indirect pathway neurons and suppress sleep. Our findings reveal a prominent contribution of this indirect pathway to sleep control associated with motivation.In addition to circadian and homoeostatic drives, motivational levels influence sleep-wake cycles. Here the authors demonstrate that adenosine receptor-expressing neurons in the nucleus accumbens core that project to the ventral pallidum are inhibited by motivational stimuli and are causally involved in the control of slow-wave sleep.

Concepts: Ventral tegmental area, Mesolimbic pathway, Substantia nigra, Nucleus accumbens, Striatum, Basal ganglia, Globus pallidus, Dopamine

207

The firing of mesolimbic dopamine neurons is important for drug-induced reinforcement, although underlying genetic factors remain poorly understood. In a recent genome-wide association metaanalysis of alcohol intake, we identified a suggestive association of SNP rs26907 in the ras-specific guanine-nucleotide releasing factor 2 (RASGRF2) gene, encoding a protein that mediates Ca(2+)-dependent activation of the ERK pathway. We performed functional characterization of this gene in relation to alcohol-related phenotypes and mesolimbic dopamine function in both mice and adolescent humans. Ethanol intake and preference were decreased in Rasgrf2(-/-) mice relative to WT controls. Accordingly, ethanol-induced dopamine release in the ventral striatum was blunted in Rasgrf2(-/-) mice. Recording of dopamine neurons in the ventral tegmental area revealed reduced excitability in the absence of Ras-GRF2, likely because of lack of inhibition of the I(A) potassium current by ERK. This deficit provided an explanation for the altered dopamine release, presumably linked to impaired activation of dopamine neurons firing. Functional neuroimaging analysis of a monetary incentive-delay task in 663 adolescent boys revealed significant association of ventral striatal activity during reward anticipation with a RASGRF2 haplotype containing rs26907, the SNP associated with alcohol intake in our previous metaanalysis. This finding suggests a link between the RASGRF2 haplotype and reward sensitivity, a known risk factor for alcohol and drug addiction. Indeed, follow-up of these same boys at age 16 y revealed an association between this haplotype and number of drinking episodes. Together, these combined animal and human data indicate a role for RASGRF2 in the regulation of mesolimbic dopamine neuron activity, reward response, and alcohol use and abuse.

Concepts: Striatum, Mesolimbic pathway, Nucleus accumbens, Drug addiction, Addiction, Ventral tegmental area, Reward system, Dopamine

192

Dopamine has a central role in motivation and reward. Dopaminergic neurons in the ventral tegmental area (VTA) signal the discrepancy between expected and actual rewards (that is, reward prediction error), but how they compute such signals is unknown. We recorded the activity of VTA neurons while mice associated different odour cues with appetitive and aversive outcomes. We found three types of neuron based on responses to odours and outcomes: approximately half of the neurons (type I, 52%) showed phasic excitation after reward-predicting odours and rewards in a manner consistent with reward prediction error coding; the other half of neurons showed persistent activity during the delay between odour and outcome that was modulated positively (type II, 31%) or negatively (type III, 18%) by the value of outcomes. Whereas the activity of type I neurons was sensitive to actual outcomes (that is, when the reward was delivered as expected compared to when it was unexpectedly omitted), the activity of type II and type III neurons was determined predominantly by reward-predicting odours. We ‘tagged’ dopaminergic and GABAergic neurons with the light-sensitive protein channelrhodopsin-2 and identified them based on their responses to optical stimulation while recording. All identified dopaminergic neurons were of type I and all GABAergic neurons were of type II. These results show that VTA GABAergic neurons signal expected reward, a key variable for dopaminergic neurons to calculate reward prediction error.

Concepts: Neuron, Mesolimbic pathway, American films, Game theory, Ventral tegmental area, Reward system, Nervous system, Dopamine

154

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.

Concepts: Ventral tegmental area, Muscarinic acetylcholine receptor, Nicotine, Neurotransmitter, Mesolimbic pathway, Acetylcholine, Dopamine, Nicotinic acetylcholine receptor

145

Reward hypersensitization is a common feature of neuropsychiatric disorders, manifesting as impulsivity for anticipated incentives. Temporally specific changes in activity within the nucleus accumbens (NAc), which occur during anticipatory periods preceding consummatory behavior, represent a critical opportunity for intervention. However, no available therapy is capable of automatically sensing and therapeutically responding to this vulnerable moment in time when anticipation-related neural signals may be present. To identify translatable biomarkers for an off-the-shelf responsive neurostimulation system, we record local field potentials from the NAc of mice and a human anticipating conventional rewards. We find increased power in 1- to 4-Hz oscillations predominate during reward anticipation, which can effectively trigger neurostimulation that reduces consummatory behavior in mice sensitized to highly palatable food. Similar oscillations are present in human NAc during reward anticipation, highlighting the translational potential of our findings in the development of a treatment for a major unmet need.

Concepts: Reward, Nucleus accumbens, Psychology, Human behavior, Ventral tegmental area, Mesolimbic pathway, Dopamine, Motivation

137

The number of highly caffeinated products has increased dramatically in the past few years. Among these products, highly caffeinated energy drinks are the most heavily advertised and purchased, which has resulted in increased incidences of co-consumption of energy drinks with alcohol. Despite the growing number of adolescents and young adults reporting caffeine-mixed alcohol use, knowledge of the potential consequences associated with co-consumption has been limited to survey-based results and in-laboratory human behavioral testing. Here, we investigate the effect of repeated adolescent (post-natal days P35-61) exposure to caffeine-mixed alcohol in C57BL/6 mice on common drug-related behaviors such as locomotor sensitivity, drug reward and cross-sensitivity, and natural reward. To determine changes in neurological activity resulting from adolescent exposure, we monitored changes in expression of the transcription factor ΔFosB in the dopaminergic reward pathway as a sign of long-term increases in neuronal activity. Repeated adolescent exposure to caffeine-mixed alcohol exposure induced significant locomotor sensitization, desensitized cocaine conditioned place preference, decreased cocaine locomotor cross-sensitivity, and increased natural reward consumption. We also observed increased accumulation of ΔFosB in the nucleus accumbens following repeated adolescent caffeine-mixed alcohol exposure compared to alcohol or caffeine alone. Using our exposure model, we found that repeated exposure to caffeine-mixed alcohol during adolescence causes unique behavioral and neurochemical effects not observed in mice exposed to caffeine or alcohol alone. Based on similar findings for different substances of abuse, it is possible that repeated exposure to caffeine-mixed alcohol during adolescence could potentially alter or escalate future substance abuse as means to compensate for these behavioral and neurochemical alterations.

Concepts: Coffee, Adolescence, Addiction, Caffeine, Coca-Cola, Mesolimbic pathway, Drug addiction, Dopamine

68

Addiction to cocaine is commonly preceded by experiences with legal or decriminalized drugs, such as alcohol, nicotine, and marijuana. The biological mechanisms by which these gateway drugs contribute to cocaine addiction are only beginning to be understood. We report that in the rat, prior alcohol consumption results in enhanced addiction-like behavior to cocaine, including continued cocaine use despite aversive consequences. Conversely, prior cocaine use has no effect on alcohol preference. Long-term, but not short-term, alcohol consumption promotes proteasome-mediated degradation of the nuclear histone deacetylases HDAC4 and HDAC5 in the nucleus accumbens, a brain region critical for reward-based memory. Decreased nuclear HDAC activity results in global H3 acetylation, creating a permissive environment for cocaine-induced gene expression. We also find that selective degradation of HDAC4 and HDAC5, facilitated by the class II-specific HDAC inhibitor MC1568, enhances compulsive cocaine self-administration. These results parallel our previously reported findings that the gateway drug nicotine enhances the behavioral effects of cocaine via HDAC inhibition. Together, our findings suggest a shared mechanism of action for the gateway drugs alcohol and nicotine, and reveal a novel mechanism by which environmental factors may alter the epigenetic landscape of the reward system to increase vulnerability to cocaine addiction.

Concepts: Mesolimbic pathway, Psychoactive drug, Ventral tegmental area, Histone, Cocaine, Histone deacetylase, Dopamine, Drug addiction

57

The biological mechanisms underlying long-term partner bonds in humans are unclear. The evolutionarily conserved neuropeptide oxytocin (OXT) is associated with the formation of partner bonds in some species via interactions with brain dopamine reward systems. However, whether it plays a similar role in humans has as yet not been established. Here, we report the results of a discovery and a replication study, each involving a double-blind, placebo-controlled, within-subject, pharmaco-functional MRI experiment with 20 heterosexual pair-bonded male volunteers. In both experiments, intranasal OXT treatment (24 IU) made subjects perceive their female partner’s face as more attractive compared with unfamiliar women but had no effect on the attractiveness of other familiar women. This enhanced positive partner bias was paralleled by an increased response to partner stimuli compared with unfamiliar women in brain reward regions including the ventral tegmental area and the nucleus accumbens (NAcc). In the left NAcc, OXT even augmented the neural response to the partner compared with a familiar woman, indicating that this finding is partner-bond specific rather than due to familiarity. Taken together, our results suggest that OXT could contribute to romantic bonds in men by enhancing their partner’s attractiveness and reward value compared with other women.

Concepts: Female, Nucleus accumbens, Basal ganglia, Amphetamine, Reward system, Mesolimbic pathway, Ventral tegmental area, Dopamine

51

Glucagon-like peptide-1 (GLP-1) and its analogs act as appetite suppressants and have been proven to be clinically efficacious in reducing body weight in obese individuals. Central GLP-1 is expressed in a small population of brainstem cells located in the nucleus tractus solitarius (NTS), which project to a wide range of brain areas. However, it remains unclear how endogenous GLP-1 released in the brain contributes to appetite regulation. Using chemogenetic tools, we discovered that central GLP-1 acts on the midbrain ventral tegmental area (VTA) and suppresses high-fat food intake. We used integrated pathway tracing and synaptic physiology to further demonstrate that activation of GLP-1 receptors specifically reduces the excitatory synaptic strength of dopamine (DA) neurons within the VTA that project to the nucleus accumbens (NAc) medial shell. These data suggest that GLP-1 released from NTS neurons can reduce highly palatable food intake by suppressing mesolimbic DA signaling.

Concepts: Appetite, Nucleus accumbens, Mesolimbic pathway, Basal ganglia, Nervous system, Neuroanatomy, Ventral tegmental area, Dopamine

50

Humans devote 30-40% of speech output solely to informing others of their own subjective experiences. What drives this propensity for disclosure? Here, we test recent theories that individuals place high subjective value on opportunities to communicate their thoughts and feelings to others and that doing so engages neural and cognitive mechanisms associated with reward. Five studies provided support for this hypothesis. Self-disclosure was strongly associated with increased activation in brain regions that form the mesolimbic dopamine system, including the nucleus accumbens and ventral tegmental area. Moreover, individuals were willing to forgo money to disclose about the self. Two additional studies demonstrated that these effects stemmed from the independent value that individuals placed on self-referential thought and on simply sharing information with others. Together, these findings suggest that the human tendency to convey information about personal experience may arise from the intrinsic value associated with self-disclosure.

Concepts: Striatum, Amphetamine, Psychology, Basal ganglia, Nucleus accumbens, Mesolimbic pathway, Ventral tegmental area, Dopamine