Figure 3. Optical deconstruction of Hypothalamus local circuits.

Schematic drawing of a coronal section through the rat showing hypothalamic neuronal populations involved in sleep and metabolism. Hcrt neurons are the targets of projections from multiple brain areas, including NPY/AgRP and POMC/CART neurons of the arcuate nucleus. Although the interplay between hypothalamic nuclei in regulating energy homeostasis remains unclear, NPY/AgRP neurons inhibit Hcrt cells, which in turn activate NPY/AgRP cells and inhibit POMC/CART neurons. Complex hypothalamic circuits can be functionally deconstructed with high temporal and spatial resolution using optogenetics. Genetic targeting of ChR2 or NpHR into defined classes of hypothalamic neurons (e.g. Hcrt as shown in the figure) allow bimodal manipulation of specific circuit activity without inadvertent activation/inhibition of neighbouring cells (e.g. grey neuronal populations as shown in the figure). Combination of optogenetics with imaging of fluorescent calcium sensors (Ca²⁺) or voltage sensitive dyes (VSD) of identified neuronal populations (e.g. NPY/AgRP as shown in the figure) in brain slices will reveal synaptic function and plasticity associated with metabolism and arousal. In addition, bath application of metabolic factors (leptin, ghrelin, glucose), variation of environmental parameters (temperature, pH, CO₂) and the use of animal models for neurological disorders (narcoleptic mice, ob/ob obese mice, etc.) may model pathophysiological conditions.