Neural Mechanisms of Homeostatic Need and Reward

Author

Bowen Tan

Date of Award

2023

Document Type

Thesis

Degree Name

Doctor of Philosophy (PhD)

RU Laboratory

Friedman Laboratory

Abstract

Feeding, drinking, sleep, mating, fighting, and parenting are instinct behaviors essential for animal survival. Brain has evolved sophisticated cell types and circuits that encode or fulfill the need states for food, water, sleep, sex, aggression and parenting. These need-sensing neurons monitor both environmental cues and interoceptive conditions to elicit appropriate and specific behavioral programs towards restoration of these needs. The homeostatic control of these need states further enables animals to maintain their survival strength and adapt to the environment. In contrast,dysregulation of these need-sensing neurons causes diseases such as obesity and sleep disorders,both of which are key risk factors for a shorter lifespan across animal species including human. This thus raises two key questions in neurobiology of need and homeostasis: 1. How do need-sensing neurons maintain physiological homeostasis? How does dysregulation of need-sensing neurons cause diseases? The present thesis aims to study these two questions, exemplified by the hypothalamic control of energy homeostasis and mesolimbic processing of motivation, as well as their roles in the pathogenesis of obesity and addiction, respectively. The first study aims to elucidate the mechanism of how the feeding center develops resistance to leptin, a satiety hormone, and means to reverse it. Diet-induced obese (DIO) mice and obese humans have high circulating levels of leptin and do not respond to the exogenous leptin suggesting that they develop leptin resistance. However, the underlying cellular and molecular mechanisms that reduce leptin signaling are unknown. As part of a metabolomic screen for biomarkers of a leptin effect, we found that leptin reduced the level of leucine and methionine, mTOR ligands, only in leptin-sensitive animals, raising the possibility that mTOR activation might contribute to leptin resistance. We tested this by first treating DIO, chow-fed, ob/ob and db/db mice with rapamycin, an mTOR inhibitor. Rapamycin reduced food intake and adiposity in DIO mice but not in ob/ob or db/db animals. Whole-brain mapping revealed that the levels of phosphoS6, a marker of mTOR activity, were increased in the arcuate nucleus (ARC) and other hypothalamic nuclei in DIO mice. Subsequent multi-modal single-nucleus RNA sequencing of the ARC in DIO, chow-fed and ob/ob mice revealed that rapamycin altered gene expression exclusively in POMC neurons of DIO mice which also showed normalization of pSTAT3 levels after rapamycin treatment. Consistent with an effect on POMC neurons, rapamycin did not alter food intake or adiposity in mice after an ablation of POMC neurons or in MC4R knockout mice. In contrast, a POMC-specific deletion of Tsc1, which leads to a cell specific increase of mTOR activity, resulted in leptin resistance in chow-fed animals and reduced leptin sensitivity in these and ob/ob mice. In ob/ob-POMCtsc1-/- mice, rapamycin did not reduce food intake or adiposity in the absence of leptin. Finally, POMC-specific deletion of mTOR activators decreased the weight gain in mice fed a HFD. These data suggest that leptin resistance in DIO mice is the result of increased mTOR activity in POMC neurons and that inhibition of mTOR reduces obesity by reversing leptin resistance. Feeding and energy homeostasis are regulated by a pair of cell types: POMC and Agouti-related- peptide (AgRP) neurons in the hypothalamus. Activation of POMC neurons decreases food intake and resembles the state of food satiation, while activation of AgRP neurons drives food seeking and consumption and resembles the state of hunger. However, neuronal activity of AgRP neurons decreases upon sensory detection of food in mice. Similarly, neuronal activity of nNOS neurons in the subfornical organ, the activation of which resembles thirst, also decreases upon sensory detection of water. The similar neural dynamics observed between hunger- and thirst-coding neurons raise the question: How are the need states for food and water fulfilled by neural substrates downstream of the feeding and drinking centers? In the second study, we examined nucleus accumbens (NAc), a canonical reward center that regulates feeding and drinking, but it is not known whether these behaviors are mediated by same or different neurons. We employed two-photon calcium imaging in awake, behaving mice and found that during the appetitive phase, both hunger and thirst are sensed by a nearly identical population of individual D1 and D2 neurons in the NAc that respond monophasically to food cues in fasted animals and water cues in dehydrated animals. During the consummatory phase, we identified three distinct neuronal clusters that are temporally correlated with action initiation, consumption, and cessation shared by feeding and drinking. These dynamic clusters also show a nearly complete overlap of individual D1 neurons and extensive overlap among D2 neurons. Modulating D1 and D2 neural activities revealed analogous effects on feeding versus drinking behaviors. In aggregate, these data show that a highly overlapping set of D1 and D2 neurons in NAc detect food and water reward and elicit concordant responses to hunger and thirst. These studies establish a general role of this mesolimbic pathway in fulfilling need states and refining ongoing behaviors by controlling motivation-associated variables. Food and water are also naturally rewarding. In the third study, we set out to study the mechanism of how drugs of abuse hijack the mesolimbic circuit that directs motivation. Using whole-brain FOS mapping and in vivo single-neuron calcium imaging, we found that drugs of abuse augment ensemble activity in the NAc and disorganize overlapping ensemble responses to natural rewards in a cell-type-specific manner. Combining FOS-Seq, CRISPR-perturbations, and snRNA-seq, we identified Rheb as a shared molecular substrate that regulates cell-type-specific signal transductions in NAc while enabling drugs to suppress natural reward responses. Retrograde circuit mapping pinpointed orbitofrontal cortex which, upon activation, mirrored drug effects on innate needs. These findings characterize the dynamic, molecular, and circuit basis of a common reward pathway, wherein drug exposure suppresses fulfillment of innate needs. In summary, the current work unravels the molecular and dynamic basis of neural substrates underpinning homeostatic needs, thereby elucidating fundamental mechanisms driving the pathogenesis of obesity and addiction.

Comments

A Thesis Presented to the Faculty of The Rockefeller University in Partial Fulfillment of the Requirements for the degree of Doctor of Philosophy

Recommended Citation

Tan, Bowen, "Neural Mechanisms of Homeostatic Need and Reward" (2023). Student Theses and Dissertations. 789.
https://digitalcommons.rockefeller.edu/student_theses_and_dissertations/789