Melanin-concentrating hormone (MCH) and hypocretin/orexin (HCRT)-expressing neurons are intermingled populations in the tuberal hypothalamus that project widely throughout the brain to many of the same terminal fields. Whereas the HCRT system has been implicated in the control of wakefulness because the sleep disorder narcolepsy results when these cells degenerate, this system is also involved in energy metabolism. Conversely, the MCH system has primarily been associated with food intake and energy metabolism, but recent studies have established that MCH neurons also participate in the regulation of sleep and wakefulness. The hypothesis underlying this proposal is that the HCRT system is wake-stabilizing and REM-inhibiting whereas the MCH system is sleep-facilitating and REM-stabilizing. We will test this hypothesis by determining the phenotype of mice in which either the HCRT or MCH neurons have been partially ablated by removal of doxycycline in the diet of two conditional mouse models. We will then evaluate whether partial ablation of the HCRT neurons results in a phenotype of narcolepsy without cataplexy and whether cataplexy is exacerbated by simultaneously eliminating both neuronal populations. We will also assess whether direct connectivity exists between these cell groups using optogenetically-assisted neuroanatomical tracing and whole-cell patch- clamp electrophysiology in the presence and absence of selective HCRT and MCH receptor antagonists. To assess what occurs in the brain when the HCRT neurons degenerate as in human narcolepsy, we will use the conditional HCRT neuron ablation model to determine how the excitability of the MCH population is affected by chronic loss of HCRT input. We will also use conditional MCH neuron ablation to assess the converse effect of MCH loss on HCRT neuron excitability. Based on recordings from a limited number of cells in head-fixed animals, the HCRT and MCH neurons have been reported to have reciprocal activity across the sleep-wake cycle with HCRT neurons having their highest firing rates during active wakefulness and MCH neurons being primarily active during REM sleep. To determine the accuracy of this conclusion, we will use genetically- encoded Ca2+ indicators and microendoscopic imaging to measure the activity of hundreds of HCRT and MCH neurons across the sleep/wake cycle in unrestrained, freely-moving animals. To evaluate whether the HCRT and MCH neurons are functionally interconnected, we will pharmacogenetically activate one population while imaging Ca2+ fluorescence in the other population in the presence of selective HCRT and MCH receptor antagonists. Lastly, since these two populations project to many of the same brain regions, we will assess their relative input to brain areas known to be involved in arousal state control, specifically, the locus coeruleus, tuberomammillary nucleus, medial septum, and the amygdala. Together, these experiments should provide a more complete picture of the anatomical and functional connectivity of these two populations and the consequences of selective loss of one population or the other.
Melanin-concentrating hormone (MCH) and hypocretin/orexin (HCRT)-expressing neurons are intermingled cell populations in the hypothalamus that have been implicated in both metabolism and sleep/wake control. We will determine the anatomical and physiological interconnectivity of these cell types and the functional consequences of loss of one population on the activity of the other. Since these two cell types project to many of the same brain regions, we will also assess their relative input to brain areas known to be involved in arousal state control. These experiments will provide a more complete picture of the connectivity of these two neuronal populations and the consequences of selective loss of one population on the other, which will have implications for our understanding of the sleep disorder narcolepsy as well as for normal sleep/wake control.
| Szabo, Steven T; Thorpy, Michael J; Mayer, Geert et al. (2018) Neurobiological and immunogenetic aspects of narcolepsy: Implications for pharmacotherapy. Sleep Med Rev 43:23-36 |
| Williams, Rhîannan H; Black, Sarah W; Thomas, Alexia M et al. (2018) Excitation of Cortical nNOS/NK1R Neurons by Hypocretin 1 is Independent of Sleep Homeostasis. Cereb Cortex : |
| Peyron, Christelle; Kilduff, Thomas S (2017) Mapping the Hypocretin/Orexin Neuronal System: An Unexpectedly Productive Journey. J Neurosci 37:2268-2272 |
