Mechanism for Hypocretin-mediated sleep-to-wake transitions

Abstract

Current models of sleep/wake regulation posit that Hypocretin (Hcrt)-expressing neurons in the lateral hypothalamus promote and stabilize wakefulness by projecting to subcortical arousal centers. However, the critical downstream effectors of Hcrt neurons are unknown. Here we use optogenetic, pharmacological, and computational tools to investigate the functional connectivity between Hcrt neurons and downstream noradrenergic neurons in the locus coeruleus (LC) during nonrapid eye movement (NREM) sleep. We found that photoinhibiting LC neurons during Hcrt stimulation blocked Hcrt-mediated sleep-to-wake transitions. In contrast, when LC neurons were optically stimulated to increase membrane excitability, concomitant photostimulation of Hcrt neurons significantly increased the probability of sleep-to-wake transitions compared with Hcrt stimulation alone. We also built a conductance-based computational model of Hcrt-LC circuitry that recapitulates our behavioral results using LC neurons as the main effectors of Hcrt signaling. These results establish the Hcrt-LC connection as a critical integrator-effector circuit that regulates NREM sleep/wake behavior during the inactive period. This coupling of distinct neuronal systems can be generalized to other hypothalamic integrator nuclei with downstream effector/output populations in the brain.ChR2 | norepinephrine | step function opsin T he neural basis of wakefulness and arousal is thought to depend on subcortical populations of "arousal-promoting" nuclei located in the hypothalamus and brainstem (1, 2). Neurons in the lateral hypothalamus that express hypocretins (Hcrts-also called "orexins"), a pair of neuropeptides produced from the same genetic precursor (3, 4), have been proposed to play a key role in stabilizing wake states by directly projecting throughout the brain to other arousal populations (1,5,6). Electrophysiological recordings of Hcrt neurons show that they are relatively silent during sleep compared with wakefulness, with phasic bursts of activity preceding transitions to wakefulness (7,8). Loss-of-function perturbation of the Hcrts or their receptors causes a narcolepsy phenotype (9-11). When centrally administered, the Hcrts increase the time spent awake and decrease nonrapid eye movement (NREM) and rapid eye movement (REM) sleep (12, 13).Although it is evident that Hcrts promote wakefulness and arousal, it is unknown which downstream structures are necessary and/or sufficient to mediate their effects. Hcrt neurons project diffusely throughout the brain (14), and it is possible that their effects on wakefulness are due to widespread, global postsynaptic targets. Alternatively, Hcrts may affect arousal primarily by projecting to key downstream arousal centers. An intriguing possibility is that Hcrt neurons affect wakefulness by projecting to the locus coeruleus (LC), a noradrenergic structure in the brainstem known to promote wakefulness and arousal (15, 16). Indeed, the LC receives the densest afferent projections from Hcrt neurons (14, 17). Application o...

The Dual Hypocretin Receptor Antagonist Almorexant is Permissive for Activation of Wake-Promoting Systems

“…Hcrt neurons activate these brain regions (Carter et al, 2012;Eggermann et al, 2001;Eriksson et al, 2001;Liu et al, 2002;Schone et al, 2012) through the co-release of Hcrt, glutamate (GLU) (Schone et al, 2012), and dynorphin (Eriksson et al, 2004;Li et al, 2014;Muschamp et al, 2014). Hcrt neurons are active primarily during wakefulness (Lee et al, 2005), and extracellular Hcrt levels are highest during awakening and periods of heightened emotionality (Blouin et al, 2013), consistent with a role in the regulation of arousal.…”

Orexin/Hypocretin and Organizing Principles for a Diversity of Wake-Promoting Neurons in the Brain

“…This also suggests that orexin type 2 receptorindependent actions of orexin/hypocretin cells, such as those mediated by their other transmitters (glutamate, dynorphin, Narp) or by orexin/hypocretin via type 1 receptors, are insufficient to achieve proper arousal control without orexin/hypocretin action of type 2 orexin/hypocretin receptors. The latter point may seem surprising, considering that some arousal-promoting neurons, such as noradrenaline neurons of the LC which are important for orexin/hypocretin-induced wakefulness [25], are excited by orexin/hypocretin via type 1 rather than type 2 OX receptors [26]. In contrast, other arousal-promoting neurons, such as histamine cells of the tuberomammillary hypothalamus, are excited by orexin/hypocretin via type 2 receptors [26,27].…”

“…Therefore, we briefly comment on some of them here (for more in-depth discussions of current literature on this topic, see [2,115]). Carter et al examined the mechanism of orexin-mediated wakefulness by optogenetically stimulating orexin/hypocretin neurons while concurrently optogenetically silencing one of their downstream effectors, the orexin type 1 receptor expressing noradrenaline neurons of the locus coeruleus (LC) [25]. Note that this does not address the question of which orexin/ hypocretin targets are critical for orexin/hypocretin-dependent wakefulness stability.…”

Timing control by redundant inhibitory neuronal circuits

“…This model is capable of reproducing the main observed electrophysiological characteristics of a neuron from the hypothalamus network introduced in Ref. 10. This type of neuron has been implicated in neuroendocrine control, energy regulation and various aspects of autonomic function as well as in the sleep/wake cycle.…”

“…A well known mechanism of excitability in the hypothalamus is the effect of neuropeptide receptors like the hypocretin receptor. 10,18 These receptors change the intrinsic excitability of individual neurons. Thus, we used V t as a parameter to describe excitation in the neurons.…”