Illuminating the Function of Hypothalamic Feeding Circuit
Gero Miesenböck, M.D.
Yale University School of Medicine, New Haven, CT
David Mahoney Neuroimaging Program
June 2005, for 3 years
Using Imaging to Understand how the Brain’s “Feeding Circuit” Controls Weight
Yale researchers will use cellular optical imaging techniques in mice to study how the “feeding circuit,” located in the brain’s hypothalamus, balances appetite and satiety, and how this balance can be altered and lead to obesity.
Research suggests that a feeding circuit exists in the hypothalamus. This circuit involves neurons that signal hunger and neurons that signal satiety and control appetite and body weight. A feedback loop involving synapses between these two types of neurons, the investigators hypothesize, governs the set-point, and weight gain or loss results from synaptic plasticity in this feeding circuit. Using optical imaging techniques in tissue culture cells taken from mice, the researchers will manipulate the cells’ activities, first increasing and then decreasing the “hunger” cells’ activity to see if weight gain and weight loss occur, respectively. Next, the researchers will image laboratory tissue cultures to see if the satiety cells can inhibit the hunger cells. Thereafter, the scientists will induce metabolic changes in live mice to see if these changes affect the relative strength of the synaptic connections between the hunger and satiety neurons. If so, such synaptic alterations might be experimentally explored to see if they reduce obesity.
Significance: This study of the hypothalamic feeding circuit could lead to a better understanding of the brain’s control of obesity and could suggest potential methods for interfering with this feedback circuit to prevent obesity. Additionally, the approach of coupling optical imaging with cellular activation methods developed here could be applied to understanding more complex brain circuits involved in normal functioning and in disease.
Illuminating the Function of Hypothalamic Feeding Circuits
Obesity, one of the major causes of preventable illness and premature death in Westernized societies, is a disorder of the nervous system. Appetite, energy expenditure, and body weight are regulated by a small neuronal circuit in the hypothalamus. Previous studies have suggested that the feeding circuit incorporates two principal neuronal elements and computes the balance of their opposing influences. One of these elements, termed NPY cells, would signal hunger, increase appetite, decrease energy expenditure, and increase fat deposition. The antagonistic neuronal element in this scheme of opposing forces, termed POMC cells, would signal satiety, reduce appetite, increase energy expenditure, and decrease fat deposition.
The proposed work will seek to answer three distinct but related questions about the function of the feeding circuit in mice, which serve as an experimentally tractable model of metabolic regulation in humans. The first question will test the central predictions of the simple "Yin-Yang" scheme of opposing NPY and POMC forces. Do food intake and body weight increase if the activity of NPY cells is artificially increased over the activity of POMC cells? Conversely, do food intake and body weight decrease if the activity of POMC cells is artificially increased over the activity of NPY cells? Novel optical methods for remote-controlling the activities of defined groups of neurons in the intact brain will be deployed to perform the necessary manipulations of NPY and POMC cell activity.
The second question asks how the feeding circuit is wired to compute a balance between NPY and POMC cell activities. It is known that the hunger-promoting NPY cells can inhibit the satiety-promoting POMC cells, but it is unclear if there is reciprocal influence from POMC to NPY cells. The absence of such a link would suggest that the feeding circuit is poised by default to promote consumption. Such hardwired insatiability may have been advantageous in the distant past, when resources were scarce, but contribute to the obesity epidemic of today. To elucidate the wiring diagram of the feeding circuit, optical point stimulation of NPY or POMC cells in hypothalamic slices will be combined with optical imaging of synaptic outflow with the help of synapto-pHluorin and/or electrophysiological analyses of synaptic inputs to individually recorded target cells.
The third question combines elements of the first and second. If the feeding circuit functions as a homeostat that computes a balance between antagonistic NPY and POMC signals, it must operate with a desired metabolic setpoint. How is this setpoint encoded? Adjustments of the strengths of synaptic connections onto or between the two antagonistic cell types could provide one potential mechanism. To test whether and in which way experimentally induced metabolic changes alter the wiring diagram of the circuit, synapses formed by NPY and POMC cells will be imaged in hypothalamic slices from animals that have been subjected to artificial stimulation of one or the other cell type in vivo.
The combination of innovative methods for optical sensing and actuation of nerve cell activity promises to shed light on the fundamental operational principles of a simple, yet clinically important, circuit in the brain. If successful, the approaches pioneered in this project can be generalized immediately to other, more complex, problems in circuitry analysis.
Appetite and body mass are regulated by a hypothalamic feedback circuit involving antagonistic neuronal elements, the so-called NPY and POMC cells. Changes in feeding behavior and body mass result from synaptic plasticity in this circuit.
Connectivity, function, and plasticity of the hypothalamic feeding circuit have been difficult to study because NPY and POMC cells are diffusely interspersed throughout the arcuate nucleus and anatomically indistinguishable. Our intent is to express genetically encoded optical sensors and actuators of neuronal function selectively in the NPY and POMC cells of genetically modified mice. The ability to observe and control NPY and POMC cell activity will allow us to clarify the roles of these neurons in feeding behavior, elucidate the connectivity of the circuit and its activity-driven modification, and examine the behavioral consequences of synaptic plasticity.
Keys to the proposed experiments are three methodological elements: genetically encoded optical sensors, termed synapto-pHluorins, that report synaptic transmission; genetically encoded phototriggers that cause neurons to fire action potentials upon illumination; and genetic control elements that permit the cell-type specific expression of the genetically encoded sensors and actuators in the hypothalamic neurons of interest.
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