Transgenic Archaerhodopsin-3 Expression in Hypocretin/Orexin Neurons Engenders Cellular Dysfunction and Features of Type 2 Narcolepsy

Abstract

Narcolepsy, characterized by excessive daytime sleepiness, is associated with dysfunction of the hypothalamic hypocretin/orexin (Hcrt) system, either due to extensive loss of Hcrt cells (Type 1, NT1) or hypothesized Hcrt signaling impairment (Type 2, NT2). Accordingly, efforts to recapitulate narcolepsy-like symptoms in mice have involved ablating these cells or interrupting Hcrt signaling. Here, we describe orexin/Arch mice, in which a modified archaerhodopsin-3 gene was inserted downstream of the prepro-orexin promoter, resulting in expression of the yellow light-sensitive Arch-3 proton pump specifically within Hcrt neurons. Histological examination along with ex vivo and in vivo electrophysiological recordings of male and female orexin/Arch mice demonstrated silencing of Hcrt neurons when these cells were photoilluminated. However, high expression of the Arch transgene affected cellular and physiological parameters independent of photoillumination. The excitability of Hcrt neurons was reduced, and both circadian and metabolic parameters were perturbed in a subset of orexin/Arch mice that exhibited high levels of Arch expression. Orexin/Arch mice also had increased REM sleep under baseline conditions but did not exhibit cataplexy, a sudden loss of muscle tone during wakefulness characteristic of NT1. These aberrations resembled some aspects of mouse models with Hcrt neuron ablation, yet the number of Hcrt neurons in orexin/Arch mice was not reduced. Thus, orexin/Arch mice may be useful to investigate Hcrt system dysfunction when these neurons are intact, as is thought to occur in narcolepsy without cataplexy (NT2). These results also demonstrate the utility of extended phenotypic screening of transgenic models when specific neural circuits have been manipulated.

The Impacts of Age and Sex in a Mouse Model of Childhood Narcolepsy

“…Young-onset and adult-onset mice spent a similar amount of time in cataplexy and had a similar number of cataplexy bouts. At 3 weeks after DOX removal, the adult-onset group spent about 2% of the dark period (∼15 min) in cataplexy which is similar to prior descriptions of adult orexin-tTA; TetO DTA mice ( Tabuchi et al, 2014 ; Williams et al, 2019 ). However, Tabuchi et al (2014) reported that at 13 weeks after DOX removal, adult-onset mice spent about 8% (∼60 min) of the dark period in cataplexy ( Tabuchi et al, 2014 ).…”

A calibrated optogenetic toolbox of stable zebrafish opsin lines

“…Data from eArch3.0-expressing neurons could not be collected due to degradation in the quality of recordings or cells becoming highly depolarised (i.e. resting membrane potential > –50 mV) by the later stages of the protocol, suggesting that repeated eArch3.0 activation may alter electrical properties of neurons (Williams et al ., 2019).…”

Dual Orexin and MCH neuron-ablated mice display severe sleep attacks and cataplexy

“…Diagnostic Classification Steering Committee., 1990;Burgess and Scammell, 2012). Optogenetic activation of orexin neurons induces wakefulness from sleep while optogenetic inhibition induces sleep from wakefulness (Adamantidis et al, 2007;Tsunematsu et al, 2011;Schone et al, 2012;Williams et al, 2019). Together, these studies indicate that orexin neurons play an important role in the maintenance of wakefulness and prevent cataplexy induced by positive emotions.…”