Optogenetic Probing of Hypocretins’ Regulation of Wakefulness

Abstract

In mammals, sleep is commonly defined as “a rapidly reversible state of (behavioral) immobility and greatly reduced sensory responsiveness to environmental stimuli” [1]. Sleep and wake states have been strongly conserved during evolution, and “sleep-like” states exist in most organisms, including worms, flies, and fish [2], suggesting common underlying neural circuits and endocrine systems. During the last decades, neural circuits that modulate the sleep–wake cycle have been identified using a combination of lesion, histological, pharmacological, genetic, and in vitro and in vivo electrophysiology techniques. Collectively, they support the “reciprocal interaction” and other computational models which describe the sleep–wake cycle as a complex, yet partially defined balance between subcortical excitatory and inhibitory neural circuits in the brain [3]. However, limitations of current techniques have hampered our understanding of their dynamics and functional connectivity. In this chapter, we summarize key experiments that led to the key hypothesis that the hypocretin (Hcrt; also known as orexin) system sets the arousal threshold. We discuss our implementation of in vivo optogenetic techniques to overcome previous techniques’ limitations and establish causal links between Hcrt neuron activation and behavioral state transitions. Finally, we propose to use optogenetics as a tool to probe the necessity, sufficiency, and connectivity of defined neural circuits in the regulation of sleep and wakefulness.