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.