Cholecystokinin innervation of the ventral striatum: a morphological and radioimmunological study

Compartmental neuronal degeneration in the ventral striatum induced by status epilepticus in young rats' brain in comparison with adults

According to experimental and clinical studies, status epilepticus (SE) causes neurodegenerative morphological changes not only in the hippocampus and other limbic structures, it also affects the thalamus and the neocortex. In addition, several studies reported atrophy, metabolic changes, and neuronal degeneration in the dorsal striatum. The literature lacks studies investigating potential neuronal damage in the ventral component of the striatopallidal complex (ventral striatum [VS] and ventral pallidum) in SE experimentations. To better understand the development of neuronal damage in the striatopallidal complex associated with SE, the detected neuronal degeneration in the compartments of the VS, namely, the nucleus accumbens (NAc) and the olfactory tubercle (OT), was analyzed. The experiments were performed on Wistar rats at age of 25-day-old pups and 3-month-old adult animals. Lithium-pilocarpine model of SE was used. Lithium chloride (3 mmol/kg, ip) was injected 24 h before administering pilocarpine (40 mg/kg, ip). This presented study demonstrates the variability of post SE neuronal damage in 25-day-old pups in comparison with 3-month-old adult rats. The NAc exhibited small to moderate number of Fluoro-Jade B (FJB)-positive neurons detected 4 and 8 h post SE intervals. The number of degenerated neurons in the shell subdivision of the NAc significantly increased at survival interval of 12 h after the SE. FJB-positive neurons were evidently more prominent occupying the whole anteroposterior and mediolateral extent of the nucleus at longer survival intervals of 24 and 48 h after the SE. This was also the case in the bordering vicinity between the shell and the core compartments but with clusters of degenerating cells. The severity of damage of the shell subdivision of the NAc reached its peak at an interval of 24 h post SE. Isolated FJB-positive neurons were detected in the ventral peripheral part of the core compartment. Degenerated neurons persisted in the shell subdivision of the NAc 1 week after SE. However, the quantity of cell damage had significantly reduced in comparison with the aforementioned shorter intervals. The third layer of the OT exhibited more degenerated neurons than the second layer. The FJB-positive cells in the young animals were higher than in the adult animals. The morphology of those cells was identical in the two age groups except in the OT.

Application of the wholebrain calculation interactive framework to map whole-brain neural connectivity networks

The aim of this work was to develop a simple and feasible method of mapping the neural network topology of the mouse brain. Wild-type C57BL/6 J mice (n = 10) aged 8-10 weeks were injected with the cholera toxin subunit B (CTB) tracer in the anterior (NAcCA) and posterior (NAcCP) parts of the nucleus accumbens (NAc) core and in the medial (NAcSM) and lateral (NAcSL) parts of the NAc shell. The labeled neurons were reconstructed using the WholeBrain Calculation Interactive Framework. The NAcCA receives neuronal projections from the olfactory areas (OLF) and isocortex; the thalamus and isocortex project more fibers to the NAcSL, and the hypothalamus send more fiber projections to the NAcSM. Cell resolution can be automatically annotated, analyzed, and visualized using the WholeBrain Calculation Interactive Framework, making large-scale mapping of mouse brains at cellular and subcellular resolutions easier and more accurate.

Aging in nucleus accumbens and its impact on alcohol use disorders

Ethanol is one of the most widely consumed drugs in the world and prolonged excessive ethanol intake might lead to alcohol use disorders (AUDs), which are characterized by neuroadaptations in different brain regions, such as in the reward circuitry. In addition, the global population is aging, and it appears that they are increasing their ethanol consumption. Although research involving the effects of alcohol in aging subjects is limited, differential effects have been described. For example, studies in human subjects show that older adults perform worse in tests assessing working memory, attention, and cognition as compared to younger adults. Interestingly, in the field of the neurobiological basis of ethanol actions, there is a significant dichotomy between what we know about the effects of ethanol on neurochemical targets in young animals and how it might affect them in the aging brain. To be able to understand the distinct effects of ethanol in the aging brain, the following questions need to be answered: (1) How does physiological aging impact the function of an ethanol-relevant region (e.g., the nucleus accumbens)? and (2) How does ethanol affect these neurobiological systems in the aged brain? This review discusses the available data to try to understand how aging affects the nucleus accumbens (nAc) and its neurochemical response to alcohol. The data show that there is little information on the effects of ethanol in aged mice and rats, and that many studies had considered 2-3-month-old mice as adults, which needs to be reconsidered since more recent literature defines 6 months as young adults and >18 months as an older mouse. Considering the actual relevance of an aged worldwide population and that this segment is drinking more frequently, it appears at least reasonable to explore how ethanol affects the brain in adult and aged models.

Prolonged Longitudinal Transcutaneous Auricular Vagus Nerve Stimulation Effect on Striatal Functional Connectivity in Patients with Major Depressive Disorder

BACKGROUND

Transcutaneous auricular vagus nerve stimulation (taVNS) is effective for treating major depressive disorder (MDD). We aimed to explore the modulating effect of prolonged longitudinal taVNS on the striatal subregions' functional connectivity (FC) in MDD patients.

METHODS

Sixteen MDD patients were enrolled and treated with taVNS for 8 weeks. Sixteen healthy control subjects (HCs) were recruited without intervention. The resting-state FC (rsFC) based on striatal subregion seed points and the Hamilton Depression Scale (HAMD) were evaluated in the MDD patients and HCs at baseline and after 8 weeks. A two-way ANCOVA test was performed on each rsFC metric to obtain the (group-by-time) interactions.

RESULTS

The rsFC values between the left ventral caudate (vCa) and right ventral prefrontal cortex (vPFC), and between the right nucleus accumbens (NAc) and right dorsal medial prefrontal cortex (dmPFC) and ventrolateral prefrontal cortex (vlPFC) are lower in the MDD patients compared to the HCs at baseline, and increase following taVNS; the rsFC values between the left vCa and right, superior occipital gyrus (SOG), and between the left dorsal caudate (dCa) and right cuneus are higher in MDD patients and decrease following taVNS.

CONCLUSIONS

Prolonged longitudinal taVNS can modulate the striatum rsFC with the prefrontal cortex, occipital cortex, temporal cortex, and intra-striatum, and these changes partly underlie any symptomatic improvements. The results indicate that prolonged longitudinal taVNS may produce beneficial treatment effects by modulating the cortical striatum circuitry in patients with MDD.

Vigilance, arousal, and acetylcholine: Optimal control of attention in a simple detection task

Paying attention to particular aspects of the world or being more vigilant in general can be interpreted as forms of ‘internal’ action. Such arousal-related choices come with the benefit of increasing the quality and situational appropriateness of information acquisition and processing, but incur potentially expensive energetic and opportunity costs. One implementational route for these choices is widespread ascending neuromodulation, including by acetylcholine (ACh). The key computational question that elective attention poses for sensory processing is when it is worthwhile paying these costs, and this includes consideration of whether sufficient information has yet been collected to justify the higher signal-to-noise ratio afforded by greater attention and, particularly if a change in attentional state is more expensive than its maintenance, when states of heightened attention ought to persist. We offer a partially observable Markov decision-process treatment of optional attention in a detection task, and use it to provide a qualitative model of the results of studies using modern techniques to measure and manipulate ACh in rodents performing a similar task.

Paying attention to a stimulus is costly, both in terms of energy and the lost opportunity to pay attention to something else. It is also beneficial, providing more information about its target. Thus, whether and when we pay more or less attention may best be considered as a choice of internal action that responds to this trade-off. Furthermore, measurements and manipulation of the neuromodulator acetylcholine have suggested that it is one of the instruments of attention, providing us with a window onto this choice. Here, we build an abstract model of a task in which an animal must look out for a brief visual stimulus that may or may not occur on each trial. We show that optimal attentional choices in the model depend on many factors, including how likely a signal is to occur across time, the balance between the improvement in information possible by paying greater attention and its increased cost, and whether there are also costs associated with switching between different attentional states. We also show that our model can qualitatively match results from experiments involving acetylcholine.

Neural circuit mechanisms of the cholecystokinin (CCK) neuropeptide system in addiction

Given historical focus on the roles for cholecystokinin (CCK) as a peripheral hormone controlling gastrointestinal processes and a brainstem peptide regulating food intake, the study of CCK as a limbic neuromodulator coordinating reward-seeking and emotional behavior remains underappreciated. Furthermore, localization of CCK to specialized interneurons throughout the hippocampus and cortex relegated CCK to being examined primarily as a static cell type marker rather than a dynamic functional neuromodulator. Yet, over three decades of literature have been generated by efforts to delineate the central mechanisms of addiction-related behaviors mediated by the CCK system across the striatum, amygdala, hypothalamus, and midbrain. Here, we cover fundamental findings that implicate CCK neuron activity and CCK receptor signaling in modulating drug intake and drug-seeking (focusing on psychostimulants, opioids, and alcohol). In doing so, we highlight the few studies that indicate sex differences in CCK expression and corresponding drug effects, emphasizing the importance of examining hormonal influences and sex as a biological variable in translating basic science discoveries to effective treatments for substance use disorders in human patients. Finally, we point toward understudied subcortical sources of endogenous CCK and describe how continued neurotechnology advancements can be leveraged to modernize understanding of the neural circuit mechanisms underlying CCK release and signaling in addiction-relevant behaviors.

Role of the striatal dopamine, GABA and opioid systems in mediating feeding and fat intake

Food intake, which is a highly reinforcing behavior, provides nutrients required for survival in all animals. However, when fat and sugar consumption goes beyond the daily needs, it can favor obesity. The prevalence and severity of this health problem has been increasing with time. Besides covering nutrient and energy needs, food and in particular its highly palatable components, such as fats, also induce feelings of joy and pleasure. Experimental evidence supports a role of the striatal complex and of the mesolimbic dopamine system in both feeding and food-related reward processing, with the nucleus accumbens as a key target for reward or reinforcing-associated signaling during food intake behavior. In this review, we provide insights concerning the impact of feeding, including fat intake, on different types of receptors and neurotransmitters present in the striatal complex. Reciprocally, we also cover the evidence for a modulation of palatable food intake by different neurochemical systems in the striatal complex and in particular the nucleus accumbens, with a focus on dopamine, GABA and the opioid system.

Endocannabinoid Modulation of Nucleus Accumbens Microcircuitry and Terminal Dopamine Release

The nucleus accumbens (NAc) is located in the ventromedial portion of the striatum and is vital to valence-based predictions and motivated action. The neural architecture of the NAc allows for complex interactions between various cell types that filter incoming and outgoing information. Dopamine (DA) input serves a crucial role in modulating NAc function, but the mechanisms that control terminal DA release and its effect on NAc neurons continues to be elucidated. The endocannabinoid (eCB) system has emerged as an important filter of neural circuitry within the NAc that locally shapes terminal DA release through various cell type- and site-specific actions. Here, we will discuss how eCB signaling modulates terminal DA release by shaping the activity patterns of NAc neurons and their afferent inputs. We then discuss recent technological advancements that are capable of dissecting how distinct cell types, their afferent projections, and local neuromodulators influence valence-based actions.

The Role of Neuropeptide Y in the Nucleus Accumbens

Neuropeptide Y (NPY), an abundant peptide in the central nervous system, is expressed in neurons of various regions throughout the brain. The physiological and behavioral effects of NPY are mainly mediated through Y1, Y2, and Y5 receptor subtypes, which are expressed in regions regulating food intake, fear and anxiety, learning and memory, depression, and posttraumatic stress. In particular, the nucleus accumbens (NAc) has one of the highest NPY concentrations in the brain. In this review, we summarize the role of NPY in the NAc. NPY is expressed principally in medium-sized aspiny neurons, and numerous NPY immunoreactive fibers are observed in the NAc. Alterations in NPY expression under certain conditions through intra-NAc injections of NPY or receptor agonists/antagonists revealed NPY to be involved in the characteristic functions of the NAc, such as alcohol intake and drug addiction. In addition, control of mesolimbic dopaminergic release via NPY receptors may take part in these functions. NPY in the NAc also participates in fat intake and emotional behavior. Accumbal NPY neurons and fibers may exert physiological and pathophysiological actions partly through neuroendocrine mechanisms and the autonomic nervous system.

Circuit selectivity in drug versus natural reward seeking behaviors

Substance use disorder (SUD) is characterized, in part by behavior biased toward drug use and away from natural sources of reward (e.g., social interaction, food, sex). The neurobiological underpinnings of SUDs reveal distinct brain regions where neuronal activity is necessary for the manifestation of SUD-characteristic behaviors. Studies that specifically examine how these regions are involved in behaviors motivated by drug versus natural reward allow determinations of which regions are necessary for regulating seeking of both reward types, and appraisals of novel SUD therapies for off-target effects on behaviors motivated by natural reward. Here, we evaluate studies directly comparing regulatory roles for specific brain regions in drug versus natural reward. While it is clear that many regions drive behaviors motivated by all reward types, based on the literature reviewed we propose a set of interconnected regions that become necessary for behaviors motivated by drug, but not natural rewards. The circuitry is selectively necessary for drug seeking includes an Action/Reward subcircuit, comprising nucleus accumbens, ventral pallidum, and ventral tegmental area, a Prefrontal subcircuit comprising prelimbic, infralimbic, and insular cortices, a Stress subcircuit comprising the central nucleus of the amygdala and the bed nucleus of the stria terminalis, and a Diencephalon circuit including lateral hypothalamus. Evidence was mixed for nucleus accumbens shell, insular cortex, and ventral pallidum. Studies for all other brain nuclei reviewed supported a necessary role in regulating both drug and natural reward seeking. Finally, we discuss emerging strategies to further disambiguate the necessity of brain regions in drug- versus natural reward-associated behaviors.

Midbrain and Lateral Nucleus Accumbens Dopamine Depletion Affects Free-choice High-fat high-sugar Diet Preference in Male Rats

Dopamine influences food intake behavior. Reciprocally, food intake, especially of palatable dietary items, can modulate dopamine-related brain circuitries. Among these reciprocal impacts, it has been observed that an increased intake of dietary fat results in blunted dopamine signaling and, to compensate this lowered dopamine function, caloric intake may subsequently increase. To determine how dopamine regulates food preference we performed 6-hydroxydopamine (6-OHDA) lesions, depleting dopamine in specific brain regions in male Sprague Dawley rats. Food preference was assessed by providing the rats with free choice access to control diet, fat, 20% sucrose and tap water. Rats with midbrain lesions targeting the substantia nigra (which is also a model of Parkinson's disease) consumed fewer calories, as reflected by a decrease in control diet intake, but they surprisingly displayed an increase in fat intake, without change in the sucrose solution intake compared to sham animals. To determine which of the midbrain dopamine projections may contribute to this effect, we next compared the impact of 6-OHDA lesions of terminal fields, targeting the dorsal striatum, the lateral nucleus accumbens and the medial nucleus accumbens. We found that 6-OHDA lesion of the lateral nucleus accumbens, but not of the dorsal striatum or the medial nucleus accumbens, led to increased fat intake. These findings indicate a role for lateral nucleus accumbens dopamine in regulating food preference, in particular the intake of fat.

Nucleus Accumbens Tac1-Expressing Neurons Mediate Stress-Induced Anhedonia-like Behavior in Mice

Major depressive disorder (MDD) presents with two primary symptoms: depressed mood and anhedonia, which suggests that distinct neuronal circuits may regulate MDD. However, the underlying circuits of these individual symptoms linked to depression remain elusive. Herein, we identify a discrete circuit of tachykinin precursor 1 (Tac1)-expressing neurons in the nucleus accumbens (NAc) lateral shell, which project to ventral pallidum and contribute to stress-induced anhedonia-like behavior. Selective inhibition and activation of Tac1^NAc neurons bidirectionally modulate stress susceptibility, revealing that Tac1 neurons in the NAc are critical for regulating anhedonia-like behaviors. We find that a subpopulation of VP neurons receives inhibitory inputs from Tac1^NAc neurons and exhibits decreased excitability in susceptible mice. Furthermore, the inhibition of the neurokinin 1 receptor promotes susceptibility to social stress. Overall, our study reveals a discrete circuit regulating anhedonia-like behavior in mice.

Melanin-concentrating hormone and food intake control: Sites of action, peptide interactions, and appetition

Given the increased prevalence of obesity and its associated comorbidities, understanding the mechanisms through which the brain regulates energy balance is of critical importance. The neuropeptide melanin-concentrating hormone (MCH) is produced in the lateral hypothalamic area and the adjacent incerto-hypothalamic area and promotes both food intake and energy conservation, overall contributing to body weight gain. Decades of research into this system has provided insight into the neural pathways and mechanisms (behavioral and neurobiological) through which MCH stimulates food intake. Recent technological advancements that allow for selective manipulation of MCH neuron activity have elucidated novel mechanisms of action for the hyperphagic effects of MCH, implicating neural "volume" transmission in the cerebrospinal fluid and sex-specific effects of MCH on food intake control as understudied areas for future investigation. Highlighted here are historical and recent findings that illuminate the neurobiological mechanisms through which MCH promotes food intake, including the identification of various specific neural signaling pathways and interactions with other peptide systems. We conclude with a framework that the hyperphagic effects of MCH signaling are predominantly mediated through enhancement of an "appetition" process in which early postoral prandial signals promote further caloric consumption.

Distinct regions of the striatum underlying effort, movement initiation and effort discounting

The ventral striatum is believed to encode the subjective value of cost-benefit options; however, this effect has notably been absent during choices that involve physical effort. Previous work in freely moving animals has revealed opposing striatal signals, with greater response to increasing effort demands and reduced responses to rewards requiring effort. Yet, the relationship between these conflicting signals remains unknown. Using functional magnetic resonance imaging with a naturalistic maze-navigation paradigm, we identified functionally segregated regions within the ventral striatum that separately encoded effort activation, movement initiation and effort discounting of rewards. In addition, activity in regions associated with effort activation and discounting oppositely predicted striatal encoding of effort during effort-based decision-making. Our results suggest that the dorsomedial region hitherto associated with action may instead represent the cost of effort and raise fundamental questions regarding the interpretation of striatal 'reward' signals in the context of effort demands. This has implications for uncovering the neural architecture underlying motivated behaviour.

Brain-Wide Mapping of Afferent Inputs to Accumbens Nucleus Core Subdomains and Accumbens Nucleus Subnuclei

The nucleus accumbens (NAc) is the ventral part of the striatum and the interface between cognition, emotion, and action. It is composed of three major subnuclei: i.e., NAc core (NAcC), lateral shell (NAcLS), and medial shell (NAcMS), which exhibit functional heterogeneity. Thus, determining the synaptic inputs of the subregions of the NAc is important for understanding the circuit mechanisms involved in regulating different functions. Here, we simultaneously labeled subregions of the NAc with cholera toxin subunit B conjugated with multicolor Alexa Fluor, then imaged serial sections of the whole brain with a fully automated slide scanning system. Using the interactive WholeBrain framework, we characterized brain-wide inputs to the NAcC subdomains, including the rostral, caudal, dorsal, and ventral subdomains (i.e., rNAcC, cNAcC, dNAcC, and vNAcC, respectively) and the NAc subnuclei. We found diverse brain regions, distributed from the cerebrum to brain stem, projecting to the NAc. Of the 57 brain regions projecting to the NAcC, the anterior olfactory nucleus (AON) exhibited the greatest inputs. The input neurons of rNAcC and cNAcC are two distinct populations but share similar distribution over the same upstream brain regions, whereas the input neurons of dNAcC and vNAcC exhibit slightly different distributions over the same upstream regions. Of the 55 brain regions projecting to the NAcLS, the piriform area contributed most of the inputs. Of the 72 brain regions projecting to the NAcMS, the lateral septal nucleus contributed most of the inputs. The input neurons of NAcC and NAcLS share similar distributions, whereas the NAcMS exhibited brain-wide distinct distribution. Thus, the NAcC subdomains appeared to share the same upstream brain regions, although with distinct input neuron populations and slight differences in the input proportions, whereas the NAcMS subnuclei received distinct inputs from multiple upstream brain regions. These results lay an anatomical foundation for understanding the different functions of NAcC subdomains and NAc subnuclei.

Neurochemical organization of the ventral striatum's olfactory tubercle

The ventral striatum is a collection of brain structures, including the nucleus accumbens, ventral pallidum and the olfactory tubercle (OT). While much attention has been devoted to the nucleus accumbens, a comprehensive understanding of the ventral striatum and its contributions to neurological diseases requires an appreciation for the complex neurochemical makeup of the ventral striatum's other components. This review summarizes the rich neurochemical composition of the OT, including the neurotransmitters, neuromodulators and hormones present. We also address the receptors and transporters involved in each system as well as their putative functional roles. Finally, we end with briefly reviewing select literature regarding neurochemical changes in the OT in the context of neurological disorders, specifically neurodegenerative disorders. By overviewing the vast literature on the neurochemical composition of the OT, this review will serve to aid future research into the neurobiology of the ventral striatum.

Molecular Plasticity of the Nucleus Accumbens Revisited-Astrocytic Waves Shall Rise

Part of the ventral striatal division, the nucleus accumbens (NAc) drives the circuit activity of an entire macrosystem about reward like a "flagship," signaling and leading diverse conducts. Accordingly, NAc neurons feature complex inhibitory phenotypes that assemble to process circuit inputs and generate outputs by exploiting specific arrays of opposite and/or parallel neurotransmitters, neuromodulatory peptides. The resulting complex combinations enable versatile yet specific forms of accumbal circuit plasticity, including maladaptive behaviors. Although reward signaling and behavior are elaborately linked to neuronal circuit activities, it is plausible to propose whether these neuronal ensembles and synaptic islands can be directly controlled by astrocytes, a powerful modulator of neuronal activity. Pioneering studies showed that astrocytes in the NAc sense citrate cycle metabolites and/or ATP and may induce recurrent activation. We argue that the astrocytic calcium, GABA, and Glu signaling and altered sodium and chloride dynamics fundamentally shape metaplasticity by providing active regulatory roles in the synapse- and network-level flexibility of the NAc.

Circuit directionality for motivation: Lateral accumbens-pallidum, but not pallidum-accumbens, connections regulate motivational attraction to reward cues

Sign-tracking behavior, in which animals interact with a cue that predicts reward, provides an example of how incentive salience can be attributed to cues and elicit motivation. The nucleus accumbens (NAc) and ventral pallidum (VP) are two regions involved in cue-driven motivation. The VP, and NAc subregions including the medial shell and core, are critical for sign-tracking. Further, connections between the medial shell and VP are known to participate in sign-tracking and other motivated behaviors. The NAc lateral shell (NAcLSh) is a distinct and understudied subdivision of the NAc, and its contribution to the process by which reward cues acquire value remains unclear. The NAcLSh has been implicated in reward-directed behavior, and has reciprocal connections with the VP, suggesting that NAcLSh and VP interactions could be important mechanisms for incentive salience. Here, we use DREADDs (Designer Receptors Exclusively Activated by Designer Drugs) and an intersectional viral delivery strategy to produce a biased inhibition of NAcLSh neurons projecting to the VP, and vice versa. We find that disruption of connections from NAcLSh to VP reduces sign-tracking behavior while not affecting consumption of food rewards. In contrast, VP to NAcLSh disruption affected neither sign-tracking nor reward consumption, but did produce a greater shift in animals' behavior more towards the reward source when it was available. These findings indicate that the NAcLSh → VP pathway plays an important role in guiding animals towards reward cues, while VP → NAcLSh back-projections may not and may instead bias motivated behavior towards rewards.

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

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

Reward-representing D1-type neurons in the medial shell of the accumbens nucleus regulate palatable food intake

BACKGROUND/OBJECTIVES

Dysfunction in reward-related aspects of feeding, and consequent overeating in humans, is a major contributor to obesity. Intrauterine undernutrition and overnutrition are among the predisposing factors, but the exact mechanism of how overeating develops is still unclear. Consummatory behavior is regulated by the medial shell (mSh) of the accumbens nucleus (Nac) through direct connections with the rostral part of the lateral hypothalamic area (LHA). Our aim was to investigate whether an altered Nac-LHA circuit may underlie hyperphagic behavior.

SUBJECTS/METHODS

Intrauterine protein-restricted (PR) male Wistar rats were used as models for hyperphagia. The experiments were performed using young adult control (normally nourished) and PR animals. Sweet condensed milk (SCM) served as a reward to test consumption and subsequent activation (Fos+) of Nac and LHA neurons. Expression levels of type 1 and 2 dopamine receptors (D1R, D2R) in the Nac, as well as tyrosine hydroxylase (TH) levels in the ventral tegmental area, were determined. The D1R agonist SKF82958 was injected into the mSh-Nac of control rats to test the effect of D1R signaling on SCM intake and neuronal cell activation in the LHA.

RESULTS

A group of food reward-representing D1R+ neurons was identified in the mSh-Nac. Activation (Fos+) of these neurons was highly proportional to the consumed palatable food. D1R agonist treatment attenuated SCM intake and diminished the number of SCM-activated cells in the LHA. Hyperphagic PR rats showed increased intake of SCM, reduced D1R expression, and an impaired response to SCM-evoked neuronal activation in the mSh-Nac, accompanied by an elevated number of Fos+ neurons in the LHA compared to controls.

CONCLUSIONS

Sensitivity of food reward-representing neurons in the mSh-Nac determines the level of satisfaction that governs cessation of consumption, probably through connections with the LHA. D1R signaling is a key element in this function, and is impaired in obesity-prone rats.

Anatomic Review of the Ventral Capsule/Ventral Striatum and the Nucleus Accumbens to Guide Target Selection for Deep Brain Stimulation for Obsessive-Compulsive Disorder

BACKGROUND

Disturbances in the reward network of the brain underlie addiction, depression, and obsessive-compulsive disorder. The ventral capsule/ventral striatum and nucleus accumbens (NAc) region is a clinically approved target for deep brain stimulation for obsessive-compulsive disorder.

METHODS

We performed a comprehensive literature review to define clinically relevant anatomy and connectivity of the ventral capsule/ventral striatum and NAc region to guide target selection for deep brain stimulation.

RESULTS

Architecturally and functionally, the NAc is divided into the core and the shell, with each area having different connections. The shell primarily receives limbic information, and the core typically receives information from the motor system. In general, afferents from the prefrontal cortex, hippocampus, and amygdala are excitatory. The dopaminergic projections to the NAc from the ventral tegmental area modulate the balance of these excitatory inputs. Several important inputs to the NAc converge at the junction of the internal capsule (IC) and the anterior commissure (AC): the ventral amygdalofugal pathways that run parallel to and underneath the AC, the precommissural fornical fibers that run anterior to the AC, axons from the ventral prefrontal cortex and medial orbitofrontal cortex that occupy the most ventral part of the IC and embedding within the NAc and AC, and the superolateral branch of the medial forebrain bundle located parallel to the anterior thalamic radiation in the IC.

CONCLUSIONS

The caudal part of the NAc passing through the IC-AC junction may be an effective target for deep brain stimulation to improve behavioral symptoms associated with obsessive-compulsive disorder.

Region-specific changes in markers of neuroplasticity revealed in HIV-1 transgenic rats by low-dose methamphetamine

Methamphetamine abuse co-occurring with HIV infection presents neuropathology in brain regions that mediate reward and motivation. A neuronal signaling cascade altered acutely by meth and some HIV-1 proteins is the mitogen-activated protein kinase (MAPK) pathway. It remains unknown if chronic co-exposure to meth and HIV-1 proteins converge on MAPK in vivo. To make this determination, we studied young adult Fischer 344 HIV-1 transgenic (Tg) and non-Tg rats that self-administered meth (0.02-0.04 mg/kg/0.05 ml iv infusion, 2 h/day for 21 days) and their saline-yoked controls. One day following the operant task, rats were killed. Brain regions involved in reward-motivation [i.e., nucleus accumbens (NA) and ventral pallidum (VP)], were assayed for a MAPK cascade protein, extracellular signal-regulated kinase (ERK), and a downstream transcription factor, ΔFosB. In the NA, activated (phosphorylated; p) ERK-to-ERK ratio (pERK/ERK) was increased in meth-exposed Tg rats versus saline Tg controls, and versus meth non-Tg rats. ΔFosB was increased in meth Tg rats versus saline and meth non-Tg rats. Assessment of two targets of ΔFosB-regulated transcription revealed (1) increased dopamine D1 receptor (D1R) immunoreactivity in the NA shell of Tg-meth rats versus saline Tg controls, but (2) no changes in the AMPA receptor subunit, GluA2. No changes related to genotype or meth occurred for ERK, ΔFosB or D1R protein in the VP. Results reveal a region-specific activation of ERK, and increases in ΔFosB and D1R expression induced by HIV-1 proteins and meth. Such effects may contribute to the neuronal and behavioral pathology associated with meth/HIV comorbidity.

Effects of nucleus accumbens oxytocin and its antagonist on social approach behavior

Severe impairment of social interaction is a core symptom of numerous psychiatric disorders. Oxytocin (OT) has been shown to be involved in various aspects of social behavior related to reproduction, but little is known about its effects on nonreproductive social interaction between adults or the neuroanatomical location where OT exerts its action. Here, we examined the nucleus accumbens, a region of the brain containing high levels of the oxytocin receptor (OTR) and comprising an important node in the neural circuitry possibly related to social interaction. Behavioral effects of a local microinfusion of OT (0.1, 1, and 10 ng/side) and an oxytocin receptor antagonist (OTR-A) (1, 10, and 100 ng/side) were evaluated in naturally high social and low social female and male monogamous mandarin voles (Microtus mandarinus) using the social preference paradigm and open-field tests. The results showed that administration of 1 ng/side OT increased social preference; however, this effect was not apparent at lower or higher doses. OT did not alter anxiety-like behavior or total locomotion. Microinfusions of a selective OTR-A at 10 and 100 ng doses reduced social approach behavior; a dose of 1 ng had no effect. In conclusion, our results suggest that accumbal OT and OTR-A regulate social preferences in voles in a dose-dependent manner.

Distinctive Modulation of Dopamine Release in the Nucleus Accumbens Shell Mediated by Dopamine and Acetylcholine Receptors

Nucleus accumbens (NAc) shell shows unique dopamine (DA) signals in vivo and plays a unique role in DA-dependent behaviors such as reward-motivated learning and the response to drugs of abuse. A disynaptic mechanism for DA release was reported and shown to require synchronized firing of cholinergic interneurons (CINs) and activation of nicotinic acetylcholine (ACh) receptors (nAChRs) in DA neuron (DAN) axons. The properties of this disynaptic mechanism of DA transmission are not well understood in the NAc shell. In this study, in vitro fast-scan cyclic voltammetry was used to examine the modulation of DA transmission evoked by CINs firing in the shell of mice and compared with other striatal regions. We found that DA signals in the shell displayed significant degree of summation in response to train stimulation of CINs, contrary to core and dorsal striatum. The summation was amplified by a D2-like receptor antagonist and experiments with mice with targeted deletion of D2 receptors to DANs or CINs revealed that D2 receptors in CINs mediate a fast inhibition observed within 100 ms of the first pulse, whereas D2 autoreceptors in DAN terminals are engaged in a slower inhibition that peaks at ∼500 ms. ACh also contributes to the use-dependent inhibition of DA release through muscarinic receptors only in the shell, where higher activity of acetylcholinesterase minimizes nAChR desensitization and promotes summation. These findings show that DA signals are modulated differentially by endogenous DA and ACh in the shell, which may underlie the unique features of shell DA signals in vivoSIGNIFICANCE STATEMENT The present study reports that dopamine (DA) release evoked by activation of cholinergic interneurons displays a high degree of summation in the shell and shows unique modulation by endogenous DA and acetylcholine. Desensitization of nicotinic receptors, which is a prevailing mechanism for use-dependent inhibition in the nucleus accumbens core and dorsal striatum, is also minimal in the shell in part due to elevated acetylcholinesterase activity. This distinctive modulation of DA transmission in the shell may have functional implications in the acquisition of reward-motivated behaviors and reward seeking.

Effects of Ventral Striatum Lesions on Stimulus-Based versus Action-Based Reinforcement Learning

Learning the values of actions versus stimuli may depend on separable neural circuits. In the current study, we evaluated the performance of rhesus macaques with ventral striatum (VS) lesions on a two-arm bandit task that had randomly interleaved blocks of stimulus-based and action-based reinforcement learning (RL). Compared with controls, monkeys with VS lesions had deficits in learning to select rewarding images but not rewarding actions. We used a RL model to quantify learning and choice consistency and found that, in stimulus-based RL, the VS lesion monkeys were more influenced by negative feedback and had lower choice consistency than controls. Using a Bayesian model to parse the groups' learning strategies, we also found that VS lesion monkeys defaulted to an action-based choice strategy. Therefore, the VS is involved specifically in learning the value of stimuli, not actions.SIGNIFICANCE STATEMENT Reinforcement learning models of the ventral striatum (VS) often assume that it maintains an estimate of state value. This suggests that it plays a general role in learning whether rewards are assigned based on a chosen action or stimulus. In the present experiment, we examined the effects of VS lesions on monkeys' ability to learn that choosing a particular action or stimulus was more likely to lead to reward. We found that VS lesions caused a specific deficit in the monkeys' ability to discriminate between images with different values, whereas their ability to discriminate between actions with different values remained intact. Our results therefore suggest that the VS plays a specific role in learning to select rewarded stimuli.

Multimodal connectivity-based parcellation reveals a shell-core dichotomy of the human nucleus accumbens

The subdifferentiation of the nucleus accumbens (NAc) has been extensively studied using neuroanatomy and histochemistry, yielding a well-accepted dichotomic shell/core architecture that reflects dissociable roles, such as in reward and aversion, respectively. However, in vivo parcellation of these structures in humans has been rare, potentially impairing future research into the structural and functional characteristics and alterations of putative NAc subregions. Here, we used three complementary parcellation schemes based on tractography, task-independent functional connectivity, and task-dependent co-activation to investigate the regional differentiation within the NAc. We found that a 2-cluster solution with shell-like and core-like subdivisions provided the best description of the data and was consistent with the earlier anatomical shell/core architecture. The consensus clusters from this optimal solution, which was based on the three schemes, were used as the final parcels for the subsequent connection analyses. The resulting connectivity patterns presented inter-hemispheric symmetry, convergence and divergence across the modalities, and, most importantly, clearly distinct patterns between the two subregions. This convergent connectivity patterns also confirmed the connections in animal models, supporting views that the two subregions could have antagonistic roles in some circumstances. Finally, the identified parcels should be helpful in further neuroimaging studies of the NAc. Hum Brain Mapp 38:3878-3898, 2017. © 2017 Wiley Periodicals, Inc.

The Mesoaccumbens Pathway: A Retrograde Labeling and Single-Cell Axon Tracing Analysis in the Mouse

Neurons in the ventral tegmental area (VTA) that innervate the nucleus accumbens (Acb) constitute the so-called mesoaccumbens system. Increased activity by these neurons is correlated with the expectation and achievement of reward. The mesoaccumbens projection neurons are regarded as a central node in the brain networks that regulate drive and hedonic experience, and their dysregulation is a common pathophysiological step in addictive behaviors as well as major depression. Despite previous anatomical studies that have analyzed the origin of the mesoaccumbens axons within the VTA, regarded as a unit, the exact contributions of the various cytoarchitectural subdivisions of the VTA to this innervation is still unexplored; understanding these contributions would help further our understanding of their precise anatomical organization. With the aim of deciphering the contribution of the various VTA subdivisions to accumbal innervation, the present study has used retrograde tracer microinjections in the Acb to map the location within the various VTA subdivisions of neurons targeting either the shell or core compartments of the Acb in mice. Furthermore, the dopaminergic nature of these projections has also been analyzed using tyrosine-hydroxylase immunohistochemistry. We demonstrate here that small territories of the Acb core and shell are innervated simultaneously by many VTA subdivisions, contributing dopaminergic as well as non-dopaminergic axons to the accumbal innervation. In fact, single VTA subdivisions harbor both dopaminergic and non-dopaminergic neurons that project to the same accumbal territory. The most medial VTA subnuclei, like the caudal linear nucleus, project abundantly to medial aspects of the Acb core, whereas more lateral territories of the Acb are preferentially targeted by neurons located in the parabrachial pigmented and paranigral nuclei. Overall, about half of the mesoaccumbens neurons are putatively dopaminergic in mice. Anterograde single-cell labeling (Sindbis-pal-eGFP vector) of a limited sample of neurons revealed that mesoaccumbens neurons form profuse terminal arborizations to cover large volumes of either the Acb core or shell, and, unlike other VTA projection neuron populations, they do not branch to other striatal or extrastriatal structures. These anatomical observations are consistent with reports of an intense response in many Acb neurons after stimulation of very few VTA cells.

Corticostriatal circuitry

Corticostriatal connections play a central role in developing appropriate goal-directed behaviors, including the motivation and cognition to develop appropriate actions to obtain a specific outcome. The cortex projects to the striatum topographically. Thus, different regions of the striatum have been associated with these different functions: the ventral striatum with reward; the caudate nucleus with cognition; and the putamen with motor control. However, corticostriatal connections are more complex, and interactions between functional territories are extensive. These interactions occur in specific regions in which convergence of terminal fields from different functional cortical regions are found. This article provides an overview of the connections of the cortex to the striatum and their role in integrating information across reward, cognitive, and motor functions. Emphasis is placed on the interface between functional domains within the striatum.

The Retrograde Connections and Anatomical Segregation of the Göttingen Minipig Nucleus Accumbens

Nucleus accumbens (NAcc) has been implicated in several psychiatric disorders such as treatment resistant depression (TRD), and obsessive-compulsive disorder (OCD), and has been an ongoing experimental target for deep brain stimulation (DBS) in both rats and humans. In order to translate basic scientific results from rodents to the human setting a large animal model is needed to thoroughly study the effect of such therapeutic interventions. The aim of the study was, accordingly, to describe the basic anatomy of the Göttingen minipig NAcc and its retrograde connections. Tracing was carried out by MRI-guided stereotactic unilateral fluorogold injections in the NAcc of Göttingen minipigs. After 2 weeks the brains were sectioned and subsequently stained with Nissl-, autometallographic (AMG) development of myelin, and DARPP-32 and calbindin immunohistochemistry. The minipig NAcc was divided in a central core and an outer medial, ventral and lateral shell. We confirmed the NAcc to be a large and well-segregated structure toward its medial, ventral and lateral borders. The fluorogold tracing revealed inputs to NAcc from the medial parts of the prefrontal cortex, BA 25 (subgenual cortex), insula bilaterally, amygdala, the CA1-region of hippocampus, entorhinal cortex, subiculum, paraventricular and anterior parts of thalamus, dorsomedial parts of hypothalamus, substantia nigra, ventral tegmental area (VTA), the retrorubral field and the dorsal and median raphe nuclei. In conclusion the Göttingen minipig NAcc is a large ventral striatal structure that can be divided into a core and shell with prominent afferent connections from several subrhinal and infra-/prelimbic brain areas.

Morphology of Human Nucleus Accumbens Neurons Based on the Immunohistochemical Expression of Gad67

The nucleus accumbens is a part of the ventral striatum along with the caudate nucleus and putamen. The role of the human nucleus accumbens in drug addiction and other psychiatric disorders is of great importance. The aim of this study was to characterize medium spiny neurons in the nucleus accumbens according to the immunohistochemical expression of GAD67.

This study was conducted on twenty human brains of both sexes between the ages of 20 and 75. The expression of GAD67 was assessed immunohistochemically, and the characterization of the neurons was based on the shape and size of the soma and the number of impregnated primary dendrites.

We showed that neurons of the human nucleus accumbens expressed GAD67 in the neuron soma and in the primary dendrites. An analysis of the cell body morphology revealed the following four different types of neurons: fusiform neurons, fusiform neurons with lateral dendrites, pyramidal neurons and multipolar neurons.

An immunohistochemical analysis showed a strong GAD67 expression in GABAergic medium spiny neurons, which could be classifi ed into four different types, and these neurons morphologically correlated with those described by the Golgi study.

Circuit-Based Corticostriatal Homologies Between Rat and Primate

BACKGROUND

Understanding the neural mechanisms of psychiatric disorders requires the use of rodent models; however, frontal-striatal homologies between rodents and primates are unclear. In contrast, within the striatum, the shell of the nucleus accumbens, the hippocampal projection zone, and the amygdala projection zone (referred to as the striatal emotion processing network [EPN]) are conserved across species. We used the relationship between the EPN and projections from the anterior cingulate cortex (ACC) and orbitofrontal cortex (OFC) to assess network similarities across rats and monkeys.

METHODS

We first compared the location and extent of each major component of the EPN in rats and macaques. Next, we used anatomic cases with anterograde injections in ACC/OFC to determine the extent to which corticostriatal terminal fields overlapped with these components and with each other.

RESULTS

The location and size of each component of the EPN were similar across species, containing projections primarily from infralimbic cortex in rats and area 25 in monkeys. Other ACC/OFC terminals overlapped extensively with infralimbic cortex/area 25 projections, supporting cross-species similarities in OFC topography. However, dorsal ACC had different connectivity profiles across species. These results were used to segment the monkey and rat striata according to ACC/OFC inputs.

CONCLUSIONS

Based on connectivity with the EPN, and consistent with prior literature, the infralimbic cortex and area 25 are likely homologues. We also see evidence of OFC homologies. Along with segmenting the striatum and identifying striatal hubs of overlapping inputs, these results help to translate findings between rodent models and human pathology.

Neurochemical compartmentalization within the pigeon basal ganglia

The goals of this study were to use multiple informative markers to define and characterize the neurochemically distinct compartments of the pigeon basal ganglia, especially striatum and accumbens. To this end, we used antibodies against 12 different neuropeptides, calcium-binding proteins or neurotransmitter-related enzymes that are enriched in the basal ganglia. Our results clarify boundaries between previously described basal ganglia subdivisions in birds, and reveal considerable novel heterogeneity within these previously described subdivisions. Sixteen regions were identified that each displayed a unique neurochemical organization. Four compartments were identified within the dorsal striatal region. The neurochemical characteristics support previous comparisons to part of the central extended amygdala, somatomotor striatum, and associational striatum of mammals, respectively. The medialmost part of the medial striatum, however, has several unique features, including prominent pallidal-like woolly fibers and thus may be a region unique to birds. Four neurochemically distinct regions were identified within the pigeon ventral striatum: the accumbens, paratubercular striatum, ventrocaudal striatum, and the ventral area of the lateral part of the medial striatum that is located adjacent to these regions. The pigeon accumbens is neurochemically similar to the mammalian rostral accumbens. The pigeon paratubercular and ventrocaudal striatal regions are similar to the mammalian accumbens shell. The ventral portions of the medial and lateral parts of the medial striatum, which are located adjacent to accumbens shell-like areas, have neurochemical characteristics as well as previously reported limbic connections that are comparable to the accumbens core. Comparisons to neurochemically identified compartments in reptiles, mammals, and amphibians indicate that, although most of the basic compartments of the basal ganglia were highly conserved during tetrapod evolution, uniquely avian compartments may exist as well.

Effects of early life stress on amygdala and striatal development

Species-expected caregiving early in life is critical for the normative development and regulation of emotional behavior, the ability to effectively evaluate affective stimuli in the environment, and the ability to sustain social relationships. Severe psychosocial stressors early in life (early life stress; ELS) in the form of the absence of species expected caregiving (i.e., caregiver deprivation), can drastically impact one's social and emotional success, leading to the onset of internalizing illness later in life. Development of the amygdala and striatum, two key regions supporting affective valuation and learning, is significantly affected by ELS, and their altered developmental trajectories have important implications for cognitive, behavioral and socioemotional development. However, an understanding of the impact of ELS on the development of functional interactions between these regions and subsequent behavioral effects is lacking. In this review, we highlight the roles of the amygdala and striatum in affective valuation and learning in maturity and across development. We discuss their function separately as well as their interaction. We highlight evidence across species characterizing how ELS induced changes in the development of the amygdala and striatum mediate subsequent behavioral changes associated with internalizing illness, positing a particular import of the effect of ELS on their interaction.

Uncovering the role of the nucleus accumbens in schizophrenia: A postmortem analysis of tyrosine hydroxylase and vesicular glutamate transporters

The nucleus accumbens (NAcc) is often implicated in schizophrenia (SZ) pathology, but with little evidence to support its role. This study examined postmortem human tissue to determine if abnormalities are present in the dopaminergic or glutamatergic systems in the NAcc in SZ. We compared the protein levels of tyrosine hydroxylase (TH) and vesicular glutamate transporters vGLUT1 and vGLUT2 in control (n=7) and schizophrenia (n=13) subjects using Western blot analysis. The SZ subjects were further divided by treatment status: SZ on-drug (SZ-ON, n=6) and SZ off-drug (SZ-OFF, n=7), to assess the effects of antipsychotic treatment. TH protein levels were similar between control and SZ subjects, and there was no difference between SZ-ON and SZ-OFF subjects. Protein levels of vGLUT1 were similar in control and SZ subjects, and there was no difference in vGLUT1 protein levels between SZ-ON and SZ-OFF subjects. In contrast, vGLUT2 protein levels were significantly elevated in the SZ group (25% increase). Protein levels of vGLUT2 did not differ between SZ-ON and SZ-OFF subjects. Similar levels of TH suggest the presynaptic DA pathway may be normal in the NAcc in SZ. The elevated vGLUT2 protein levels, but not vGLUT1, suggest the NAcc receives increased glutamatergic input in SZ, possibly from thalamic or other subcortical origins. The similarity between SZ-ON and SZ-OFF subjects suggests that the results are not caused by APD treatment. These findings provide further insight into the role of the NAcc in SZ.

Dopaminergic reward sensitivity can promote adolescent health: A new perspective on the mechanism of ventral striatum activation

The prevailing view in the field of adolescent brain development is that heightened activity in the mesolimbic dopaminergic reward system serves as a liability, orienting adolescents toward risky behaviors, increasing their sensitivity to social evaluation and loss, and resulting in compromised well-being. Several findings inconsistent with this deficit view challenge the perspective that adolescent reward sensitivity largely serves as a liability and highlights the potential adaptive function that heightened striatal reactivity can serve. The goal of this review is to refine our understanding of dopaminergic reward sensitivity in adolescence. I review several studies showing that ventral striatum activation serves an adaptive function for adolescents’ health and well being relating to declines in both risk taking and depression and increases in cognitive persistence and achievement.

Assessing contributions of nucleus accumbens shell subregions to reward-seeking behavior

BACKGROUND

The nucleus accumbens (NAc) plays a key role in brain reward processes including drug seeking and reinstatement. Several anatomical, behavioral, and neurochemical studies discriminate between the limbic-associated shell and the motor-associated core regions. Less studied is the fact that the shell can be further subdivided into a dorsomedial shell (NAcDMS) and an intermediate zone (NAcINT) based on differential expression of transient c-Fos and long-acting immediate-early gene ΔFosB upon cocaine sensitization. These disparate expression patterns suggest that NAc shell subregions may play distinct roles in reward-seeking behavior. In this study, we examined potential differences in the contributions of the NAcDMS and the NAcINT to reinstatement of reward-seeking behavior after extinction.

METHODS

Rats were trained to intravenously self-administer cocaine, extinguished, and subjected to a reinstatement test session consisting of an intracranial microinfusion of either amphetamine or vehicle targeted to the NAcDMS or the NAcINT.

RESULTS

Small amphetamine microinfusions targeted to the NAcDMS resulted in statistically significant reinstatement of lever pressing, whereas no significant difference was observed for microinfusions targeted to the NAcINT. No significant difference was found for vehicle microinfusions in either case.

CONCLUSION

These results suggest heterogeneity in the behavioral relevance of NAc shell subregions, a possibility that can be tested in specific neuronal populations in the future with recently developed techniques including optogenetics.

Dopaminergic Regulation of Striatal Interneurons in Reward and Addiction: Focus on Alcohol

Corticobasal ganglia networks coursing through the striatum are key structures for reward-guided behaviors. The ventral striatum (nucleus accumbens (nAc)) and its reciprocal connection with the ventral tegmental area (VTA) represent a primary component of the reward system, but reward-guided learning also involves the dorsal striatum and dopaminergic inputs from the substantia nigra. The majority of neurons in the striatum (>90%) are GABAergic medium spiny neurons (MSNs), but both the input to and the output from these neurons are dynamically controlled by striatal interneurons. Dopamine is a key neurotransmitter in reward and reward-guided learning, and the physiological activity of GABAergic and cholinergic interneurons is regulated by dopaminergic transmission in a complex manner. Here we review the role of striatal interneurons in modulating striatal output during drug reward, with special emphasis on alcohol.

Varenicline decreases ethanol intake and increases dopamine release via neuronal nicotinic acetylcholine receptors in the nucleus accumbens

BACKGROUND AND PURPOSE

Varenicline, a neuronal nicotinic acetylcholine receptor (nAChR) modulator, decreases ethanol consumption in rodents and humans. The proposed mechanism of action for varenicline to reduce ethanol consumption has been through modulation of dopamine (DA) release in the nucleus accumbens (NAc) via α4*-containing nAChRs in the ventral tegmental area (VTA). However, presynaptic nAChRs on dopaminergic terminals in the NAc have been shown to directly modulate dopaminergic signalling independently of neuronal activity from the VTA. In this study, we determined whether nAChRs in the NAc play a role in varenicline's effects on ethanol consumption.

EXPERIMENTAL APPROACH

Rats were trained to consume ethanol using the intermittent-access two-bottle choice protocol for 10 weeks. Ethanol intake was measured after varenicline or vehicle was microinfused into the NAc (core, shell or core-shell border) or the VTA (anterior or posterior). The effect of varenicline treatment on DA release in the NAc was measured using both in vivo microdialysis and in vitro fast-scan cyclic voltammetry (FSCV).

KEY RESULTS

Microinfusion of varenicline into the NAc core and core-shell border, but not into the NAc shell or VTA, reduced ethanol intake following long-term ethanol consumption. During microdialysis, a significant enhancement in accumbal DA release occurred following systemic administration of varenicline and FSCV showed that varenicline also altered the evoked release of DA in the NAc.

CONCLUSION AND IMPLICATIONS

Following long-term ethanol consumption, varenicline in the NAc reduces ethanol intake, suggesting that presynaptic nAChRs in the NAc are important for mediating varenicline's effects on ethanol consumption.

Frontostriatal systems comprising connections between ventral medial prefrontal cortex and nucleus accumbens subregions differentially regulate motor impulse control in rats

RATIONALE

Deficits in impulse control are prevalent in several neuropsychiatric disorders that are based on impaired frontostriatal communication. The ventral medial prefrontal cortex (vmPFC) and the nucleus accumbens (NAc) are key substrates of impulse control in rats. The NAc core and shell are considered to be differentially involved suggesting a functional distinction between the connections of the vmPFC and particular NAc subregions concerning impulse control.

OBJECTIVES/METHODS

In the present study, simultaneous inactivation of the rats' vmPFC and NAc core or shell by contralateral microinfusion of the GABAA receptor agonist muscimol was used to investigate their relevance for impulse control in the five-choice serial reaction time task (5-CSRTT).

RESULTS

Disconnection of the vmPFC and NAc shell produced specific impairments in inhibitory control, indicated by significantly increased premature responding and an enhanced number of time-out responses, closely resembling the effects of bilateral inactivation of either the vmPFC or NAc shell previously reported using the same task. In contrast, disconnection of the vmPFC and NAc core only slightly increased the rate of omissions and latency of reward collection indicating attentional and motivational deficits.

CONCLUSIONS

Our results extend previous findings indicating the functional specialisation of frontostriatal networks and show a differential contribution of specific vmPFC-NAc connections to behavioural control depending on the NAc subregion. We conclude that the regulation of impulse control in rats requires an intact connection between the vmPFC and the NAc shell, while the vmPFC-NAc core projection seems to be of minor importance.

Protective effects of cholecystokinin-8 on methamphetamine-induced behavioral changes and dopaminergic neurodegeneration in mice

We investigated whether pretreatment with the neuropeptide cholecystokinin-8 affected methamphetamine (METH)-induced behavioral changes and dopaminergic neurodegeneration in male C57/BL6 mice. CCK-8 pretreatment alone had no effect on locomotion and stereotypic behavior and could not induce behavioral sensitization; however, it attenuated, in a dose-dependent manner, hyperlocomotion and behavioral sensitization induced by a low dose of METH (1mg/kg). CCK-8 attenuated METH-induced stereotypic behavior at a dose of 3mg/kg but not at 10mg/kg. CCK-8 pretreatment attenuated METH (10mg/kg)-induced hyperthermia, the decrease of tyrosine hydroxylase (TH) and dopamine transporter (DAT) in the striatum, and TH in the substantia nigra. CCK-8 alone had no effect on rectal temperature, TH and DAT expression in the nigrostriatal region. In conclusion, our study demonstrated that pretreatment with CCK-8 inhibited changes typically induced by repeated exposure to METH, such as hyperlocomotion, behavioral sensitization, stereotypic behavior, and dopaminergic neurotoxicity. These findings make CCK-8 a potential therapeutic agent for the treatment of multiple symptoms associated with METH abuse.

Melanin-concentrating hormone inputs to the nucleus accumbens originate from distinct hypothalamic sources and are apposed to GABAergic and cholinergic cells in the Long-Evans rat brain

Melanin-concentrating hormone [MCH] is a neuropeptide that modulates several behaviors, such as feeding and reward. Because the hedonic and rewarding features of a food also influence feeding behavior, the nucleus accumbens [Acb] has been highlighted as a key area integrating these roles. Functional data confirm that MCH acts on a subdivision of the Acb; however, considering the importance of finding anatomical and neurochemical data that correlate the previously demonstrated function of MCH, we delineated this investigation based on the following points: (1) Is there a pattern of innervation by MCH fibers regarding the subregions within the Acb? (2) Specifically, which hypothalamic nuclei synthesize MCH and innervate the Acb? (3) Finally, what are the neurochemical identities of the accumbal neurons innervated by MCH inputs? We examined the MCH immunoreactivity [MCH-ir] in the Acb in rat brains using the peroxidase technique. Additionally, after injecting retrograde neuronal tracer [Fluoro-Gold® - FG®] into subdivisions of the Acb [shell or core], we mapped single- or double-labeled cells. Moreover, using a double immunoperoxidase protocol, we investigated the MCH-ir fibers for gamma-aminobutyric acid [GABA]-ir and choline acetyltransferase [ChAT]-ir cells in the shell subdivision of the Acb [AcbSh]. We found that the MCH-ir fibers preferentially innervate the medial AcbSh, particularly the septal pole. This innervation originated from the incerto-hypothalamic area [IHy], internuclear area, lateral hypothalamic area, perifornical area, periventricular nucleus and posterior hypothalamus. Moreover, the IHy has the highest relationship between double/single retrogradely labeled cells [n=5.33±0.66/16±0.93, i.e. 33.33%] in the whole hypothalamus. Furthermore, our data suggest that MCH-ir fibers are in apposition to GABAergic and cholinergic cells in the AcbSh. Therefore, we provide anatomical support to the ongoing functional studies investigating the relation among the hypothalamus, MCH transmission into the Acb and the involvement of known neuronal phenotypes within the AcbSh.

Nucleus accumbens core and shell inactivation differentially affects impulsive behaviours in rats

Impulsivity is a multifactorial phenomenon, determined by deficits in decision-making (impulsive choice) and impulse control (impulsive action). Recent findings indicate that impulsive behaviour is not only top-down controlled by cortical areas, but also modulated at subcortical level. The nucleus accumbens (NAc) might be a key substrate in cortico-limbic-striatal circuits involved in impulsive behaviour. Dissociable effects of the NAc subregions in various behavioural paradigms point to a potential functional distinction between NAc core and shell concerning different types of impulsivity. The present study used reversible inactivation of the rats' NAc core and shell via bilateral microinfusion of the GABAA receptor agonist muscimol (0.05μg/0.3μl) and fluorophore-conjugated muscimol (FCM, 0.27μg/0.3μl) in order to study their contribution to different aspects of impulse control in a 5-choice serial reaction time task (5-CSRTT) and impulsive choice in a delay-based decision-making T-maze task. Acute inactivation of NAc core as well as shell by muscimol increased impulsive choice, with higher impairments of the rats' waiting capacity in the T-maze following core injections compared to shell. Intra-NAc shell infusion of muscimol also induced specific impulse control deficits in the 5-CSRTT, while deactivation of the core caused severe general impairments in task performance. FCM did not affect animal behaviour. Our findings reveal clear involvement of NAc shell in both forms of impulsivity. Both subareas play a key role in the regulation of impulsive decision-making, but show functional dichotomy regarding impulse control with the core being more implicated in motivational and motor aspects.