Anorectic and aversive effects of GLP-1 receptor agonism are mediated by brainstem cholecystokinin neurons, and modulated by GIP receptor activation.

OBJECTIVE: Glucagon-like peptide-1 receptor agonists (GLP-1RAs) are effective medications to reduce appetite and body weight. These actions are centrally mediated; however, the neuronal substrates involved are poorly understood. METHODS: We employed a combination of neuroanatomical, genetic, and behavioral approaches in the mouse to investigate the involvement of caudal brainstem cholecystokinin-expressing neurons in the effect of the GLP-1RA exendin-4. We further confirmed key neuroanatomical findings in the non-human primate brain. RESULTS: We found that cholecystokinin-expressing neurons in the caudal brainstem are required for the anorectic and body weight-lowering effects of GLP-1RAs and for the induction of GLP-1RA-induced conditioned taste avoidance. We further show that, while cholecystokinin-expressing neurons are not a direct target for glucose-dependent insulinotropic peptide (GIP), GIP receptor activation results in a reduced recruitment of these GLP-1RA-responsive neurons and a selective reduction of conditioned taste avoidance. CONCLUSIONS: In addition to disclosing a neuronal population required for the full appetite- and body weight-lowering effect of GLP-1RAs, our data also provide a novel framework for understanding and ameliorating GLP-1RA-induced nausea - a major factor for withdrawal from treatment.

Lorcaserin improves glycemic control via a melanocortin neurocircuit.

OBJECTIVE: The increasing prevalence of type 2 diabetes (T2D) and associated morbidity and mortality emphasizes the need for a more complete understanding of the mechanisms mediating glucose homeostasis to accelerate the identification of new medications. Recent reports indicate that the obesity medication lorcaserin, a 5-hydroxytryptamine (5-HT, serotonin) 2C receptor (5-HT2CR) agonist, improves glycemic control in association with weight loss in obese patients with T2D. Here we evaluate whether lorcaserin has an effect on glycemia without body weight loss and how this effect is achieved. METHODS: Murine models of common and genetic T2D were utilized to probe the direct effect of lorcaserin on glycemic control. RESULTS: Lorcaserin dose-dependently improves glycemic control in mouse models of T2D in the absence of reductions in food intake or body weight. Examining the mechanism of this effect, we reveal a necessary and sufficient neurochemical mediator of lorcaserin's glucoregulatory effects, brain pro-opiomelanocortin (POMC) peptides. To clarify further lorcaserin's therapeutic brain circuit, we examined the receptor target of POMC peptides. We demonstrate that lorcaserin requires functional melanocortin4 receptors on cholinergic preganglionic neurons (MC4RChAT) to exert its effects on glucose homeostasis. In contrast, MC4RChAT signaling did not impact lorcaserin's effects on feeding, indicating a divergence in the neurocircuitry underpinning lorcaserin's therapeutic glycemic and anorectic effects. Hyperinsulinemic-euglycemic clamp studies reveal that lorcaserin reduces hepatic glucose production, increases glucose disposal and improves insulin sensitivity. CONCLUSIONS: These data suggest that lorcaserin's action within the brain represents a mechanistically novel treatment for T2D: findings of significance to a prevalent global disease.

Activation of Serotonin 2C Receptors in Dopamine Neurons Inhibits Binge-like Eating in Mice.

BACKGROUND: Neural networks that regulate binge eating remain to be identified, and effective treatments for binge eating are limited. METHODS: We combined neuroanatomic, pharmacologic, electrophysiological, Cre-lox, and chemogenetic approaches to investigate the functions of 5-hydroxytryptamine (5-HT) 2C receptor (5-HT2CR) expressed by dopamine (DA) neurons in the regulation of binge-like eating behavior in mice. RESULTS: We showed that 5-HT stimulates DA neural activity through a 5-HT2CR-mediated mechanism, and activation of this midbrain 5-HT→DA neural circuit effectively inhibits binge-like eating behavior in mice. Notably, 5-HT medications, including fluoxetine, d-fenfluramine, and lorcaserin (a selective 5-HT2CR agonist), act on 5-HT2CRs expressed by DA neurons to inhibit binge-like eating in mice. CONCLUSIONS: We identified the 5-HT2CR population in DA neurons as one potential target for antibinge therapies, and provided preclinical evidence that 5-HT2CR agonists could be used to treat binge eating.

Activation of Ventral Tegmental Area 5-HT2C Receptors Reduces Incentive Motivation.

Obesity is primarily due to food intake in excess of the body's energetic requirements, intake that is not only associated with hunger but also the incentive value of food. The 5-hydroxytryptamine 2C receptor (5-HT2CR) is a target for the treatment of human obesity. Mechanistically, 5-HT2CRs are positioned to influence both homeostatic feeding circuits within the hypothalamus and reward circuits within the ventral tegmental area (VTA). Here we investigated the role of 5-HT2CRs in incentive motivation using a mathematical model of progressive ratio (PR) responding in mice. We found that the 5-HT2CR agonist lorcaserin significantly reduced both ad libitum chow intake and PR responding for chocolate pellets and increased c-fos expression in VTA 5-HT2CR expressing γ-aminobutyric acid (GABA) neurons, but not 5-HT2CR expressing dopamine (DA) neurons. We next adopted a chemogenetic approach using a 5-HT2CRCRE line to clarify the function of subset of 5-HT2C receptor expressing VTA neurons in the modulation of appetite and food-motivated behavior. Activation of VTA 5-HT2C receptor expressing neurons significantly reduced ad libitum chow intake, operant responding for chocolate pellets, and the incentive value of food. In contrast, chemogenetic inhibition of VTA 5-HT2C receptor expressing neurons had no effect on the feeding behavior. These results indicate that activation of the subpopulation of 5-HT2CR neurons within the VTA is sufficient to significantly reduce homeostatic feeding and effort-based intake of palatable food, and that this subset has an inhibitory role in motivational processes. These findings are relevant to the treatment of obesity.

Sex difference in physical activity, energy expenditure and obesity driven by a subpopulation of hypothalamic POMC neurons.

OBJECTIVE: Obesity is one of the primary healthcare challenges of the 21st century. Signals relaying information regarding energy needs are integrated within the brain to influence body weight. Central among these integration nodes are the brain pro-opiomelanocortin (POMC) peptides, perturbations of which disrupt energy balance and promote severe obesity. However, POMC neurons are neurochemically diverse and the crucial source of POMC peptides that regulate energy homeostasis and body weight remains to be fully clarified. METHODS: Given that a 5-hydroxytryptamine 2c receptor (5-HT2CR) agonist is a current obesity medication and 5-HT2CR agonist's effects on appetite are primarily mediated via POMC neurons, we hypothesized that a critical source of POMC regulating food intake and body weight is specifically synthesized in cells containing 5-HT2CRs. To exclusively manipulate Pomc synthesis only within 5-HT2CR containing cells, we generated a novel 5-HT 2C R (CRE) mouse line and intercrossed it with Cre recombinase-dependent and hypothalamic specific reactivatable Pomc (NEO) mice to restrict Pomc synthesis to the subset of hypothalamic cells containing 5-HT2CRs. This provided a means to clarify the specific contribution of a defined subgroup of POMC peptides in energy balance and body weight. RESULTS: Here we transform genetically programed obese and hyperinsulinemic male mice lacking hypothalamic Pomc with increased appetite, reduced physical activity and compromised brown adipose tissue (BAT) into lean, healthy mice via targeted restoration of Pomc function only within 5-HT2CR expressing cells. Remarkably, the same metabolic transformation does not occur in females, who despite corrected feeding behavior and normalized insulin levels remain physically inactive, have lower energy expenditure, compromised BAT and develop obesity. CONCLUSIONS: These data provide support for the functional heterogeneity of hypothalamic POMC neurons, revealing that Pomc expression within 5-HT2CR expressing neurons is sufficient to regulate energy intake and insulin sensitivity in male and female mice. However, an unexpected sex difference in the function of this subset of POMC neurons was identified with regard to energy expenditure. We reveal that a large sex difference in physical activity, energy expenditure and the development of obesity is driven by this subpopulation, which constitutes approximately 40% of all POMC neurons in the hypothalamic arcuate nucleus. This may have broad implications for strategies utilized to combat obesity, which at present largely ignore the sex of the obese individual.

Effect of streptozotocin-induced diabetes on performance on a progressive ratio schedule.

RATIONALE: It has been suggested that streptozotocin (STZ)-induced diabetes causes a motivational deficit in rodents. However, some of the evidence adduced in support of this suggestion may be interpreted in terms of a motor impairment rather than a motivational deficit. OBJECTIVE: This experiment examined the effect of STZ-induced diabetes on performance on a progressive ratio schedule. The data were analysed using a new model derived from Killeen's (Behav Brain Sci 17:105-172, 1994) Mathematical Principles of Reinforcement model which enables the effects of interventions on motivation or incentive value to be separated from effects on motor function. METHOD: Animals were trained under a progressive ratio schedule using food-pellet reinforcement. Then they received a single intraperitoneal injection of 50 mg/kg of STZ or the vehicle. Training continued for 30 sessions after treatment. Running and overall response rates in successive ratios were analysed using the new model, and estimates of the model's parameters were compared between groups. RESULTS: The parameter expressing incentive value was reduced in the group treated with STZ, whereas the parameters expressing motor capacity and post-reinforcement pausing were not affected by the treatment. Blood glucose concentration was significantly elevated in the STZ-treated group compared to the vehicle-treated group. CONCLUSIONS: The results are consistent with the suggestion that STZ-induced diabetes is associated with a reduction of the incentive value of food.

Sensitivity to delay is affected by magnitude of reinforcement in rats.

Previous research has provided discrepant results about how reinforcement delay and magnitude are combined to determine the value of the alternatives in concurrent-chains schedules. In the present experiment, we analyzed a possible interaction between these characteristics of reinforcement, employing a two component concurrent-chains schedule, with rats as experimental subjects. Non-independent VI schedules were presented in the initial links of each component. In the terminal links, the following pairs of delays to reinforcement were presented in 4 conditions: 2-28, 6-24, 24-6, 28-2s (fixed time schedules for a group, fixed interval schedules for the other). Magnitude of reinforcement was maintained constant within components: one pellet for one component, and four pellets for the other. The results indicated that in both groups, the sensitivity to delay - calculated according to the generalized matching law - was higher in the component with the larger reinforcer. This result is in contrast with those reported in the literature of temporal discounting with human participants.

DRL performance of spontaneously hypertensive rats: dissociation of timing and inhibition of responses.

In this experiment, we used a differential reinforcement of low rates (DRL) schedule to evaluate the performance of spontaneously hypertensive rats (SHR), Wistar Kyoto (WKY) and Wistar (WIS) rats, with the goal of dissociating the processes of timing and inhibition of responses through the use of two quantitative models: the peak deviation analysis and the temporal regulation model. The subjects were divided in two groups; the first group was exposed to 70 sessions under a DRL 10s schedule. SHR rats showed an apparent temporary deficit in the inhibition of responses process; however, no differences among strains were observed in terms of the timing process. The second group of rats was exposed to 30 sessions in DRL 10s schedule, before receiving three doses (2 mg/kg, 4 mg/kg and 8 mg/kg) of methylphenidate. The results obtained through both models were consistent and indicated that at higher drug doses, the performance of all three strains of rats deteriorated. The impulsivity exhibited by SHR during acquisition supports the idea of these rats as an adequate animal model of ADHD. In contrast, evidence against this relies on the normal temporal processing found and in the worsening effect that methylphenidate produced in the process of inhibition of responses. These mixed results suggest the necessity of exploring timing behavior of other animal models in order to find a reliable animal model of ADHD.