Optogenetic deconstruction of sleep-wake circuitry in the brain

Abstract

How does the brain regulate the sleep-wake cycle? What are the temporal codes of sleep and wake-promoting neural circuits? How do these circuits interact with each other across the light/dark cycle? Over the past few decades, many studies from a variety of disciplines have made substantial progress in answering these fundamental questions. For example, neurobiologists have identified multiple, redundant wake-promoting circuits in the brainstem, hypothalamus, and basal forebrain. Sleep-promoting circuits have been found in the preoptic area and hypothalamus. One of the greatest challenges in recent years has been to selectively record and manipulate these sleep-wake centers in vivo with high spatial and temporal resolution. Recent developments in microbial opsin-based neuromodulation tools, collectively referred to as "optogenetics," have provided a novel method to demonstrate causal links between neural activity and specific behaviors. Here, we propose to use optogenetics as a fundamental tool to probe the necessity, sufficiency, and connectivity of defined neural circuits in the regulation of sleep and wakefulness.

Keywords: hypocretins/orexins; hypothalamus; optogenetics; sleep; wakefulness.

Figure 1

Comparison between electrical/pharmacological activation or inhibition and optogenetic activation of Hcrt neurons in the lateral hypothalamus. 2D architecture of the Hcrt field showing the heterogeniety of cell types found in the lateral hypothalamus, including MCH, Hcrt, corticotropin-releasing factor (CRF), NPY/AgRP, POMC/CART, growth hormone-releasing hormone (GHRH), growth hormone (GH), neurotensin (NT), and substance P. Note that, in addition to these defined cell types, the hypothalamus contain glutamatergic and GABAergic neurons. For clarity, Nesfatin-1, thyrotropin-releasing hormone (TRH), vasopressin and oxytocin are not represented. (A) Limitations of traditional microstimulation or pharmacological techniques to perform loss-of-function or gain-of function studies include confounding effect on neuronal and non-neuronal cells surrounding the tarted cells. For instance, infusion of Hcrt peptide or non-peptide agonists or antagonists can spread up to 1000 μm away from the infusion site, therefore activating or inhibiting several neuronal populations in addition to the Hcrt neurons. Alternatively, temporally precise microelectrode stimulation cannot distinguish between cell types in the stimulated area, and thus may inadvertently stimulate other, unintended regions that are adjacent to the electrode. Concentric circles represent distance (in μm) from possible injection, stimulation or lesion site. (B) Optogenetic technology allow selective stimulation (using ChR2) or inhbition (using NpHR) of genetically targeted Hcrt neurons, with no confounding modulation of surrounding cells that may regulate the same brain function.