Intranasal administration of olanzapine has beneficial outcome in a rat activity-based anorexia model.

The atypical antipsychotic drug olanzapine is prescribed despite clinical studies on olanzapine treatment showing mixed results on treatment efficacy in anorexia nervosa. We investigated the effect of systemic and intranasal administration of olanzapine in the activity-based anorexia (ABA) model. Rats were habituated to a running wheel and exposed to the ABA model while treated with olanzapine. During ABA rats had 1.5 h of daily access to food and ad libitum access to a running wheel for seven consecutive days. Olanzapine was administered via an osmotic minipump (1, 2.75, and 7.5 mg/kg) or intranasally 2 h before dark onset (1 and 2.75 mg/kg). We monitored body weight, food intake, wheel revolutions, body temperature, and adipose tissue. We found 2.75 and 7.5 mg/kg systemic olanzapine decreased wheel revolutions during ABA. Relative adipose tissue mass was increased in the 7.5 mg/kg olanzapine-treated group while body weight, food intake, and body temperature were unaltered by the systemic olanzapine. 1 and 2.75 mg/kg intranasal olanzapine diminished wheel revolutions and body temperature during the first 2 h after administration. The intranasal olanzapine-treated rats had a higher body weight at the end of ABA. We find that olanzapine has beneficial outcomes in the ABA via two administration routes by acting mainly on running wheel activity. Intranasal olanzapine showed a rapid effect in the first hours after administration in reducing locomotor activity. We recommend further exploring intranasal administration of olanzapine in anorectic patients to assist them in coping with restlessness.

Inhibition of ventral tegmental area projections to the nucleus accumbens shell increases premature responding in the five-choice serial reaction time task in rats.

Exaggerated impulsivity and attentional impairments are hallmarks of certain disorders of behavioural control such as attention-deficit/hyperactivity disorder (ADHD), schizophrenia and addiction. Pharmacological studies have implicated elevated dopamine (DA) levels in the nucleus accumbens shell (NAcbS) in impulsive actions. The NAcbS receives its DA input from the ventral tegmental area (VTA), and we have previously shown that optogenetic activation of VTA-NAcbS projections impaired impulse control and attention in the five-choice serial reaction time task (5-CSRTT) in rats. To better understand the role of VTA-NAcbS projections in impulsivity and attention, the present study sought to inhibit this projection using optogenetics. We demonstrate that inhibiting VTA-NAcbS efferents during the last seconds of the inter-trial interval (i.e. immediately before presentation of the instructive cue) induces exaggerated impulsive action, in the absence of changes in attentional or motivational parameters in the 5-CSRTT. Together with our earlier observations, this suggests that impulse control in the 5-CSRTT is tightly controlled by VTA-NAcbS activity, with deviations in both directions resulting in increased impulsivity.

Stress-driven potentiation of lateral hypothalamic synapses onto ventral tegmental area dopamine neurons causes increased consumption of palatable food.

Stress can cause overconsumption of palatable high caloric food. Despite the important role of stress eating in obesity and (binge) eating disorders, its underlying neural mechanisms remain unclear. Here we demonstrate in mice that stress alters lateral hypothalamic area (LHA) control over the ventral tegmental area (VTA), thereby promoting overconsumption of palatable food. Specifically, we show that glutamatergic LHA neurons projecting to the VTA are activated by social stress, after which their synapses onto dopamine neurons are potentiated via AMPA receptor subunit alterations. We find that stress-driven strengthening of these specific synapses increases LHA control over dopamine output in key target areas like the prefrontal cortex. Finally, we demonstrate that while inducing LHA-VTA glutamatergic potentiation increases palatable fat intake, reducing stress-driven potentiation of this connection prevents such stress eating. Overall, this study provides insights in the neural circuit adaptations caused by stress that drive overconsumption of palatable food.

Increased elasticity of sucrose demand during hyperdopaminergic states in rats.

RATIONALE: Deficits in cost-benefit decision-making are a core feature of several psychiatric disorders, including substance addiction, eating disorders and bipolar disorder. Mesocorticolimbic dopamine signalling has been implicated in various processes related to cognition and reward, but its precise role in reward valuation and cost-benefit trade-off decisions remains incompletely understood. OBJECTIVES: We assessed the role of mesocorticolimbic dopamine signalling in the relationship between price and consumption of sucrose, to better understand its role in cost-benefit decisions. METHODS: Dopamine neurons in the ventral tegmental area (VTA) were chemogenetically activated in rats, and a behavioural economics approach was used to quantify the relationship between price and consumption of sucrose. Motivation for sucrose was also assessed under a progressive ratio (PR) schedule of reinforcement. To further gauge the role of dopamine in cost-benefit trade-offs for sucrose, the effects of treatment with D-amphetamine and the dopamine receptor antagonist alpha-flupentixol were assessed. RESULTS: Chemogenetic activation of VTA dopamine neurons increased demand elasticity, while responding for sucrose under a PR schedule of reinforcement was augmented upon stimulation of VTA dopamine neurons. Treatment with amphetamine partially replicated the effects of chemogenetic dopamine neuron activation, whereas treatment with alpha-flupentixol reduced free consumption of sucrose and had mixed effects on demand elasticity. CONCLUSIONS: Stimulation of mesocorticolimbic dopaminergic neurotransmission altered cost-benefit trade-offs in a complex manner. It reduced the essential value of palatable food, increased incentive motivation and left free consumption unaltered. Together, these findings imply that mesocorticolimbic dopamine signalling differentially influences distinct components of cost expenditure processes aimed at obtaining rewards.

Optimization of whole-brain rabies virus tracing technology for small cell populations.

The lateral hypothalamus (LH) is critically involved in the regulation of homeostatic energy balance. Some neurons in the LH express receptors for leptin (LepRb), a hormone known to increase energy expenditure and decrease energy intake. However, the neuroanatomical inputs to LepRb-expressing LH neurons remain unknown. We used rabies virus tracing technology to map these inputs, but encountered non-specific tracing. To optimize this technology for a minor cell population (LepRb is not ubiquitously expressed in LH), we used LepRb-Cre mice and assessed how different titers of the avian tumor virus receptor A (TVA) helper virus affected rabies tracing efficiency and specificity. We found that rabies expression is dependent on TVA receptor expression, and that leakiness of TVA receptors is dependent on the titer of TVA virus used. We concluded that a titer of 1.0-3.0 × 107 genomic copies per µl of the TVA virus is optimal for rabies tracing. Next, we successfully applied modified rabies virus tracing technology to map inputs to LepRb-expressing LH neurons. We discovered that other neurons in the LH itself, the periventricular hypothalamic nucleus (Pe), the posterior hypothalamic nucleus (PH), the bed nucleus of the stria terminalis (BNST), and the paraventricular hypothalamic nucleus (PVN) are the most prominent input areas to LepRb-expressing LH neurons.

Temporally Specific Roles of Ventral Tegmental Area Projections to the Nucleus Accumbens and Prefrontal Cortex in Attention and Impulse Control.

Deficits in impulse control and attention are prominent in the symptomatology of mental disorders such as attention deficit hyperactivity disorder (ADHD), substance addiction, schizophrenia, and bipolar disorder, yet the underlying mechanisms are incompletely understood. Frontostriatal structures, such as the nucleus accumbens (NAcb), the medial prefrontal cortex (mPFC), and their dopaminergic innervation from the ventral tegmental area (VTA) have been implicated in impulse control and attention. What remains unclear is how the temporal pattern of activity of these VTA projections contributes to these processes. Here, we optogenetically stimulated VTA dopamine (DA) cells, as well as VTA projections to the NAcb core (NAcbC), NAcb shell (NAcbS), and the mPFC in male rats performing the 5-choice serial reaction time task (5-CSRTT). Our data show that stimulation of VTA DA neurons, and VTA projections to the NAcbC and the mPFC immediately before presentation of the stimulus cue, impaired attention but spared impulse control. Importantly, in addition to reducing attention, activation of VTA-NAcbS also increased impulsivity when tested under a longer intertrial interval (ITI), to provoke impulsive behavior. Optogenetic stimulation at the beginning of the ITI only partially replicated these effects. In sum, our data show how attention and impulsivity are modulated by neuronal activity in distinct ascending output pathways from the VTA in a temporally specific manner. These findings increase our understanding of the intricate mechanisms by which mesocorticolimbic circuits contribute to cognition.SIGNIFICANCE STATEMENT Deficits in impulse control and attention are prominent in the symptomatology of several mental disorders, yet the brain mechanisms involved are incompletely understood. Since frontostriatal circuits have been implicated in impulse control and attention, we here examined the role of ascending projections from the midbrain ventral tegmental area (VTA) to the nucleus accumbens (NAcb) and prefrontal cortex (PFC). Using optogenetics to individually stimulate these projections with time-locked precision, we distinguished the role that each of these projections plays, in both impulse control and attention. As such, our study enhances our understanding of the neuronal circuitry that drives impulsive and attentive behavior.

Good taste or gut feeling? A new method in rats shows oro-sensory stimulation and gastric distention generate distinct and overlapping brain activation patterns.

Satiation is influenced by a variety of signals including gastric distention and oro-sensory stimulation. Here we developed a high-field (9.4 T) functional magnetic resonance imaging (fMRI) protocol to test how oro-sensory stimulation and gastric distention, as induced with a block-design paradigm, affect brain activation under different states of energy balance in rats. Repeated tasting of sucrose induced positive and negative fMRI responses in the ventral tegmental area and septum, respectively, and gradual neural activation in the anterior insula and the brain stem nucleus of the solitary tract (NTS), as revealed using a two-level generalized linear model-based analysis. These unique findings align with comparable human experiments, and are now for the first time identified in rats, thereby allowing for comparison between species. Gastric distention induced more extensive brain activation, involving the insular cortex and NTS. Our findings are largely in line with human studies that have shown that the NTS is involved in processing both visceral information and taste, and anterior insula in processing sweet taste oro-sensory signals. Gastric distention and sucrose tasting induced responses in mesolimbic areas, to our knowledge not previously detected in humans, which may reflect the rewarding effects of a full stomach and sweet taste, thereby giving more insight into the processing of sensory signals leading to satiation. The similarities of these data to human neuroimaging data demonstrate the translational value of the approach and offer a new avenue to deepen our understanding of the process of satiation in healthy people and those with eating disorders.

Zona incerta neurons projecting to the ventral tegmental area promote action initiation towards feeding.

KEY POINTS: The zona incerta (ZI) and ventral tegmental area (VTA) are brain areas that are both implicated in feeding behaviour. The ZI projects to the VTA, although it has not yet been investigated whether this projection regulates feeding. We experimentally (in)activated the ZI to VTA projection by using dual viral vector technology, and studied the effects on feeding microstructure, the willingness to work for food, general activity and body temperature. Activity of the ZI to VTA projection promotes feeding by facilitating action initiation towards food, as reflected in meal frequency and the willingness to work for food reward, without affecting general activity or directly modulating body temperature. We show for the first time that activity of the ZI to VTA projection promotes feeding, which improves the understanding of the neurobiology of feeding behaviour and body weight regulation. ABSTRACT: Both the zona incerta (ZI) and the ventral tegmental area (VTA) have been implicated in feeding behaviour. The ZI provides prominent input to the VTA, although it has not yet been investigated whether this projection regulates feeding. Therefore, we investigated the role of ZI to VTA projection neurons in the regulation of several aspects of feeding behaviour. We determined the effects of (in)activation of ZI to VTA projection neurons on feeding microstructure, food-motivated behaviour under a progressive ratio schedule of reinforcement, locomotor activity and core body temperature. To activate or inactivate ZI neurons projecting to the VTA, we used a combination of canine adenovirus-2 in the VTA, as well as Cre-dependent designer receptors exclusively activated by designer drugs (DREADD) or tetanus toxin (TetTox) light chain in the ZI. TetTox-mediated inactivation of ZI to VTA projection neurons reduced food-motivated behaviour and feeding by reducing meal frequency. Conversely, DREADD-mediated chemogenetic activation of ZI to VTA projection neurons promoted food-motivated behaviour and feeding. (In)activation of ZI to VTA projection neurons did not affect locomotor activity or directly regulate core body temperature. Taken together, ZI neurons projecting to the VTA exert bidirectional control overfeeding behaviour. More specifically, activity of ZI to VTA projection neurons facilitate action initiation towards feeding, as reflected in both food-motivated behaviour and meal initiation, without affecting general activity.

Dopaminergic contributions to behavioral control under threat of punishment in rats.

RATIONALE: Excessive intake of rewards, such as food and drugs, often has explicit negative consequences, including the development of obesity and addiction, respectively. Thus, choosing not to pursue reward is the result of a cost/benefit decision, proper execution of which requires inhibition of behavior. An extensive body of preclinical and clinical evidence implicates dopamine in certain forms of inhibition of behavior, but it is not fully known how it contributes to behavioral inhibition under threat of explicit punishment. OBJECTIVES: To assess the involvement of midbrain dopamine neurons and their corticostriatal output regions, the ventral striatum and prefrontal cortex, in control over behavior under threat of explicit (foot shock) punishment in rats. METHODS: We used a recently developed behavioral inhibition task, which assesses the ability of rats to exert behavioral restraint at the mere sight of food reward, under threat of foot shock punishment. Using in vivo fiber photometry, chemogenetics, c-Fos immunohistochemistry, and behavioral pharmacology, we investigated how dopamine neurons in the ventral tegmental area, as well as its output areas, the ventral striatum and prefrontal cortex, contribute to behavior in this task. RESULTS: Using this multidisciplinary approach, we found little evidence for a direct involvement of ascending midbrain dopamine neurons in inhibitory control over behavior under threat of punishment. For example, photometry recordings suggested that VTA DA neurons do not directly govern control over behavior in the task, as no differences were observed in neuronal population activity during successful versus unsuccessful behavioral control. In addition, chemogenetic and pharmacological manipulations of the mesocorticolimbic DA system had little or no effect on the animals' ability to exert inhibitory control over behavior. Rather, the dopamine system appeared to have a role in the motivational components of reward pursuit. CONCLUSIONS: Together, our data provide insight into the mesocorticolimbic mechanisms behind motivated behaviors by showing a modulatory role of dopamine in the expression of cost/benefit decisions. In contrast to our expectations, dopamine did not appear to directly mediate the type of behavioral control that is tested in our task.

Rats that are predisposed to excessive obesity show reduced (leptin-induced) thermoregulation even in the preobese state.

Both feeding behavior and thermogenesis are regulated by leptin. The sensitivity to leptin's anorexigenic effects on chow diet was previously shown to predict the development of diet-induced obesity. In this study, we determined whether the sensitivity to leptin's anorexigenic effects correlates with leptin's thermogenic response, and if this response is exerted at the level of the dorsomedial hypothalamus (DMH), a brain area that plays an important role in thermoregulation. Based on the feeding response to injected leptin on a chow diet, rats were divided into leptin-sensitive (LS) and leptin-resistant (LR) groups. The effects of leptin on core body, brown adipose tissue (BAT) and tail temperature were compared after intravenous versus intra-DMH leptin administration. After intravenous leptin injection, LS rats increased their BAT thermogenesis and reduced heat loss via the tail, resulting in a modest increase in core body temperature. The induction of these thermoregulatory mechanisms with intra-DMH leptin was smaller, but in the same direction as with intravenous leptin administration. In contrast, LR rats did not show any thermogenic response to either intravenous or intra-DMH leptin. These differences in the thermogenic response to leptin were associated with a 1°C lower BAT temperature and reduced UCP1 expression in LR rats under ad libitum feeding. The preexisting sensitivity to the anorexigenic effects of leptin, a predictor for obesity, correlates with the sensitivity to the thermoregulatory effects of leptin, which appears to be exerted, at least in part, at the level of the DMH.

An Intersectional Approach to Target Neural Circuits With Cell- and Projection-Type Specificity: Validation in the Mesolimbic Dopamine System.

Development of tools to manipulate activity of specific neurons is important for dissecting the function of neural circuits. Viral vectors and conditional transgenic animal lines that target recombinases to specific cells facilitate the successful manipulation and recording of specific subsets of neurons. So far, it has been possible to target neuronal subtypes within a certain brain region based on transcriptional control regions from a gene selectively expressed in those cells or based upon its projections. Nevertheless, there are only a few tools available that combine this and target a neuronal subtype within a projection. We tested a viral vector system, consisting of a canine adenovirus type 2 expressing a Cre-dependent Flp recombinase (CavFlexFlp) and an adeno-associated viral (AAV) vector expressing a Flp-dependent cDNA, which targets neurons in a subtype- and projection-specific manner. As proof of principle we targeted expression of a Designer Receptor Exclusively Activated by Designer Drugs (DREADD) to the dopamine neurons of the mesolimbic projection, which allows the transient activation of neurons by the ligand Clozapine-N-Oxide (CNO). We validated that the system specifically targets dopamine neurons and that chemogenetic activation of these neurons induces an increase in locomotor activity. We thus validated a valuable tool that allows in vivo neuronal activation in a projection- and subtype-specific manner.

Limbic control over the homeostatic need for sodium.

The homeostatic need for sodium is one of the strongest motivational drives known in animals. Although the brain regions involved in the sensory detection of sodium levels have been mapped relatively well, data about the neural basis of the motivational properties of salt appetite, including a role for midbrain dopamine cells, have been inconclusive. Here, we employed a combination of fiber photometry, behavioral pharmacology and c-Fos immunohistochemistry to study the involvement of the mesocorticolimbic dopamine system in salt appetite in rats. We observed that sodium deficiency affected the responses of dopaminergic midbrain neurons to salt tasting, suggesting that these neurons encode appetitive properties of sodium. We further observed a significant reduction in the consumption of salt after pharmacological inactivation of the nucleus accumbens (but not the medial prefrontal cortex), and microstructure analysis of licking behavior suggested that this was due to decreased motivation for, but not appreciation of salt. However, this was not dependent on dopaminergic neurotransmission in that area, as infusion of a dopamine receptor antagonist into the nucleus accumbens did not alter salt appetite. We conclude that the nucleus accumbens, but not medial prefrontal cortex, is important for the behavioral expression of salt appetite by mediating its motivational component, but that the switch in salt appreciation after sodium depletion, although detected by midbrain dopamine neurons, must arise from other areas.

Anatomical projections of the dorsomedial hypothalamus to the periaqueductal grey and their role in thermoregulation: a cautionary note.

The DMH is known to regulate brown adipose tissue (BAT) thermogenesis via projections to sympathetic premotor neurons in the raphe pallidus, but there is evidence that the periaqueductal gray (PAG) is also an important relay in the descending pathways regulating thermogenesis. The anatomical projections from the DMH to the PAG subdivisions and their function are largely elusive, and may differ per anterior-posterior level from bregma. We here aimed to investigate the anatomical projections from the DMH to the PAG along the entire anterior-posterior axis of the PAG, and to study the role of these projections in thermogenesis in Wistar rats. Anterograde channel rhodopsin viral tracing showed that the DMH projects especially to the dorsal and lateral PAG. Retrograde rabies viral tracing confirmed this, but also indicated that the PAG receives a diffuse input from the DMH and adjacent hypothalamic subregions. We aimed to study the role of the identified DMH to PAG projections in thermogenesis in conscious rats by specifically activating them using a combination of canine adenovirus-2 (CAV2Cre) and Cre-dependent designer receptor exclusively activated by designer drugs (DREADD) technology. Chemogenetic activation of DMH to PAG projections increased BAT temperature and core body temperature, but we cannot exclude the possibility that at least some thermogenic effects were mediated by adjacent hypothalamic subregions due to difficulties in specifically targeting the DMH and distinct subdivisions of the PAG because of diffuse virus expression. To conclude, our study shows the complexity of the anatomical and functional connection between the hypothalamus and the PAG, and some technical challenges in studying their connection.

Is leptin resistance the cause or the consequence of diet-induced obesity?

BACKGROUND/OBJECTIVES: Obesity is strongly associated with leptin resistance. It is unclear whether leptin resistance results from the (over)consumption of energy-dense diets or if reduced leptin sensitivity is also a pre-existing factor in rodent models of diet-induced obesity (DIO). We here tested whether leptin sensitivity on a chow diet predicts subsequent weight gain and leptin sensitivity on a free choice high-fat high-sucrose (fcHFHS) diet. METHODS: Based upon individual leptin sensitivity on chow diet, rats were grouped in leptin sensitive (LS, n = 22) and leptin resistant (LR, n = 19) rats (P = 0.000), and the development of DIO on a fcHFHS diet was compared. The time-course of leptin sensitivity was measured over weeks in individual rats. RESULTS: Both on a chow and a fcHFHS diet, high variability in leptin sensitivity was observed between rats, but not over time per individual rat. Exposure to the fcHFHS diet revealed that LR rats were more prone to develop DIO (P = 0.013), which was independent of caloric intake (p ≥ 0.320) and the development of diet-induced leptin resistance (P = 0.769). Reduced leptin sensitivity in LR compared with LS rats before fcHFHS diet exposure, was associated with reduced leptin-induced phosphorylated signal transducer and activator of transcription 3 (pSTAT3) levels in the dorsomedial and ventromedial hypothalamus (P ≤ 0.049), but not the arcuate nucleus (P = 0.558). CONCLUSIONS: A pre-existing reduction in leptin sensitivity determines the susceptibility to develop excessive DIO after fcHFHS diet exposure. Rats with a pre-existing reduction in leptin sensitivity develop excessive DIO without eating more calories or altering their leptin sensitivity.

A neuronal mechanism underlying decision-making deficits during hyperdopaminergic states.

Hyperdopaminergic states in mental disorders are associated with disruptive deficits in decision making. However, the precise contribution of topographically distinct mesencephalic dopamine pathways to decision-making processes remains elusive. Here we show, using a multidisciplinary approach, how hyperactivity of ascending projections from the ventral tegmental area (VTA) contributes to impaired flexible decision making in rats. Activation of the VTA-nucleus accumbens pathway leads to insensitivity to loss and punishment due to impaired processing of negative reward prediction errors. In contrast, activation of the VTA-prefrontal cortex pathway promotes risky decision making without affecting the ability to choose the economically most beneficial option. Together, these findings show how malfunction of ascending VTA projections affects value-based decision making, suggesting a potential mechanism through which increased forebrain dopamine signaling leads to aberrant behavior, as is seen in substance abuse, mania, and after dopamine replacement therapy in Parkinson's disease.

Enhancing excitability of dopamine neurons promotes motivational behaviour through increased action initiation.

Motivational deficits are a key symptom in multiple psychiatric disorders, including major depressive disorder, schizophrenia and addiction. A likely neural substrate for these motivational deficits is the brain dopamine (DA) system. In particular, DA signalling in the nucleus accumbens, which originates from DA neurons in the ventral tegmental area (VTA), has been identified as a crucial substrate for effort-related and activational aspects of motivation. Unravelling how VTA DA neuronal activity relates to motivational behaviours is required to understand how motivational deficits in psychiatry can be specifically targeted. In this study, we therefore used designer receptors exclusively activated by designer drugs (DREADD) in TH:Cre rats, in order to determine the effects of chemogenetic DA neuron activation on different aspects of motivational behaviour. We found that chemogenetic activation of DA neurons in the VTA, but not substantia nigra, significantly increased responding for sucrose under a progressive ratio schedule of reinforcement. More specifically, high effort exertion was characterized by increased initiations of reward-seeking actions. This effect was dependent on effort requirements and instrumental contingencies, but was not affected by sucrose pre-feeding. Together, these findings indicate that VTA DA neuronal activation drives motivational behaviour by facilitating action initiation. With this study, we show that enhancing excitability of VTA DA neurons is a viable strategy to improve motivational behaviour.

A novel approach to map induced activation of neuronal networks using chemogenetics and functional neuroimaging in rats: A proof-of-concept study on the mesocorticolimbic system.

Linking neural circuit activation at whole-brain level to neuronal activity at cellular level remains one of the major challenges in neuroscience research. We set up a novel functional neuroimaging approach to map global effects of locally induced activation of specific midbrain projection neurons using chemogenetics (Designer Receptors Exclusively Activated by Designer Drugs (DREADD)-technology) combined with pharmacological magnetic resonance imaging (phMRI) in the rat mesocorticolimbic system. Chemogenetic activation of DREADD-targeted mesolimbic or mesocortical pathways, i.e. projections from the ventral tegmental area (VTA) to the nucleus accumbens (NAcc) or medial prefrontal cortex (mPFC), respectively, induced significant blood oxygenation level-dependent (BOLD) responses in areas with DREADD expression, but also in remote defined neural circuitry without DREADD expression. The time-course of brain activation corresponded with the behavioral output measure, i.e. locomotor (hyper)activity, in the mesolimbic pathway-targeted group. Chemogenetic activation specifically increased neuronal activity, whereas functional connectivity assessed with resting state functional MRI (rs-fMRI) remained stable. Positive and negative BOLD responses distinctively reflected simultaneous ventral pallidum activation and substantia nigra pars reticulata deactivation, respectively, demonstrating the concept of mesocorticolimbic network activity with concurrent activation of the direct and indirect pathways following stimulation of specific midbrain projection neurons. The presented methodology provides straightforward and widely applicable opportunities to elucidate relationships between local neuronal activity and global network activity in a controllable manner, which will increase our understanding of the functioning and dysfunctioning of large-scale neuronal networks in health and disease.

Chemogenetic activation of dopamine neurons in the ventral tegmental area, but not substantia nigra, induces hyperactivity in rats.

Hyperactivity is a core symptom in various psychiatric disorders, including attention-deficit/hyperactivity disorder, schizophrenia, bipolar disorders, and anorexia nervosa. Although hyperactivity has been linked to dopaminergic signalling, the causal relationship between midbrain dopamine neuronal activity and locomotor hyperactivity remains unknown. In this study, we test whether increased dopamine neuronal activity is sufficient to induce locomotor hyperactivity. To do so, we used designer receptors exclusively activated by designer drugs (DREADD) to chemogenetically enhance neuronal activity in two main midbrain dopamine neuron populations, i.e. the ventral tegmental area (VTA) and substantia nigra pars compacta (SN), in TH:Cre rats. We found that activation of VTA dopamine neurons induced a pronounced and long-lasting hyperactive phenotype, whilst SN dopamine neuron activation only modestly increased home cage locomotion. Furthermore, this hyperactive phenotype was replicated by selective activation of the neuronal pathway from VTA to the nucleus accumbens (NAC). These results show a clear functional difference between neuronal subpopulations in the VTA and SN with regards to inducing locomotor hyperactivity, and suggest that the dopaminergic pathway from VTA to NAC may be a promising target for the treatment of hyperactivity disorders.

Melanocortin 3 Receptor Signaling in Midbrain Dopamine Neurons Increases the Motivation for Food Reward.

The central melanocortin (MC) system mediates its effects on food intake via MC3 (MC3R) and MC4 receptors (MC4R). Although the role of MC4R in meal size determination, satiation, food preference, and motivation is well established, the involvement of MC3R in the modulation of food intake has been less explored. Here, we investigated the role of MC3R on the incentive motivation for food, which is a crucial component of feeding behavior. Dopaminergic neurons within the ventral tegmental area (VTA) have a crucial role in the motivation for food. We here report that MC3Rs are expressed on VTA dopaminergic neurons and that pro-opiomelanocortinergic (POMC) neurons in the arcuate nucleus of the hypothalamus (Arc) innervate these VTA dopaminergic neurons. Our findings show that intracerebroventricular or intra-VTA infusion of the selective MC3R agonist γMSH increases responding for sucrose under a progressive ratio schedule of reinforcement, but not free sucrose consumption in rats. Furthermore, ex vivo electrophysiological recordings show increased VTA dopaminergic neuronal activity upon γMSH application. Consistent with a dopamine-mediated effect of γMSH, the increased motivation for sucrose after intra-VTA infusion of γMSH was blocked by pretreatment with the dopamine receptor antagonist α-flupenthixol. Taken together, we demonstrate an Arc POMC projection onto VTA dopaminergic neurons that modulates motivation for palatable food via activation of MC3R signaling.

Reducing Ventral Tegmental Dopamine D2 Receptor Expression Selectively Boosts Incentive Motivation.

Altered mesolimbic dopamine signaling has been widely implicated in addictive behavior. For the most part, this work has focused on dopamine within the striatum, but there is emerging evidence for a role of the auto-inhibitory, somatodendritic dopamine D2 receptor (D2R) in the ventral tegmental area (VTA) in addiction. Thus, decreased midbrain D2R expression has been implicated in addiction in humans. Moreover, knockout of the gene encoding the D2R receptor (Drd2) in dopamine neurons has been shown to enhance the locomotor response to cocaine in mice. Therefore, we here tested the hypothesis that decreasing D2R expression in the VTA of adult rats, using shRNA knockdown, promotes addiction-like behavior in rats responding for cocaine or palatable food. Rats with decreased VTA D2R expression showed markedly increased motivation for both sucrose and cocaine under a progressive ratio schedule of reinforcement, but the acquisition or maintenance of cocaine self-administration were not affected. They also displayed enhanced cocaine-induced locomotor activity, but no change in basal locomotion. This robust increase in incentive motivation was behaviorally specific, as we did not observe any differences in fixed ratio responding, extinction responding, reinstatement or conditioned suppression of cocaine, and sucrose seeking. We conclude that VTA D2R knockdown results in increased incentive motivation, but does not directly promote other aspects of addiction-like behavior.