Partial ablation of the orexin field induces a sub-narcoleptic phenotype in a conditional mouse model of orexin neurodegeneration

Abstract

Narcolepsy type 1 (Na-1) and 2 (Na-2) are characterized by an inability to sustain wakefulness and are likely caused by degeneration of orexin neurons. Near complete orexin neurodegeneration depletes orexin-A from the cerebrospinal fluid and produces Na-1. The pathophysiology of Na-2 is less understood but has been hypothesized to be due to less extensive loss of orexin neurotransmission. The orexin-tTA; TetO diphtheria toxin A mouse allows conditional control over the extent and timing of orexin neurodegeneration. To evaluate partial ablation of the orexin field as a model of Na-2, orexin-A positive cell counts and sleep/wake phenotypes (determined by piezoelectric monitoring) were correlated within individual mice after different protocols of diet-controlled neurodegeneration. Partial ablations that began during the first 8 days of study were 14% larger than partial ablations induced during the last 8 days of study, 6 weeks later and prior to sacrifice of all mice, suggesting orexin-A positive cell death continued despite the resumption of conditions intended to keep orexin neurons intact. Sleep/wake of mice with 71.0% orexin-A positive cell loss, initiated at the beginning of study, resembled that of orexin-intact controls more than mice with near complete neurodegeneration. Conversely, mice with 56.6% orexin-A positive cell loss, created at the end of study, had sleep/wake phenotypes that were similar to those of mice with near complete orexin-A positive cell loss. Collectively, these results suggest that compensatory wake-promotion develops in mice that have some critical portion of their orexinergic system remaining after partial ablation.

An anatomically distinct subpopulation of orexin neurons project from the lateral hypothalamus to the olfactory bulb

“…An average of 225 orexin‐A‐expressing neurons were present in the ipsilateral hypothalamus in each preparation following quantification of alternate sections separated by 160 μm. Our counts are consistent with previous reports of ∼1000 orexin‐A neurons in each hypothalamus hemisphere when considering our counting frequency (every fourth section) in the anterior–posterior axis (Berteotti et al., 2021; Black et al., 2018; González & Prehn, 2018; Obukuro et al., 2010). This suggests that our estimate of the relative proportion of orexin‐A‐expressing neurons that innervate the olfactory bulb is based on a reasonable estimate of the total population of orexin‐A neurons.…”

“…It remains unclear whether the hypothalamic pathway to the main olfactory bulb is exclusively orexinergic, and if not, what proportion of the pathway orexin neurons comprise. Although the total number of orexin‐expressing neurons is relatively small (Berteotti et al., 2021; Black et al., 2018; González & Prehn, 2018; Obukuro et al., 2010), a current model proposes that they are a heterogeneous population (Sagi et al., 2021). It remains unknown whether the orexinergic input to the main olfactory bulb reflects a distinct subpopulation of the overall orexin population.…”

Validation of PiezoSleep Scoring Against EEG/EMG Sleep Scoring in Rats

“…The system was originally validated in mice, [5][6][7][8][9][10][11] and has been used extensively since validation. 5,[11][12][13][14][15][16][17][18][19][20][21][22][23][24] However, only one study employed the PiezoSleep system in rats. 25…”

“…[5][6][7][8][9][10][11] PiezoSleep uses a piezoelectric film transducer that detects the rodent's movements and respiration with high sensitivity and uses an algorithm to automatically score sleep. Since validation, PiezoSleep has been employed in studies of sleep, Alzheimer's, 12,13 traumatic brain injury, [14][15][16][17] narcolepsy, 5,11,18 inflammation, 19 mood disorders, 20,21 aging, 22 and genomics. 23,24 All studies except for one 25 have been completed in mice.…”

International Union of Basic and Clinical Pharmacology CXIV: Orexin Receptor Function, Nomenclature and Pharmacology

“…Orexin neurons are found in the perifornical, dorsal and lateral hypothalamic areas, as determined in several mammalian species using antibodies against PPO, detection of PPO mRNA, labeling of the orexin neurons with the EGFP gene under the Hcrt promoter (Li et al, 2002) and tracing of the orexin neurons from target areas (more recent studies: Ciriello et al, 2013;Ciriello and Caverson, 2014;Garcia-Garcia et al, 2013;Yokota et al, 2016; reviewed in Sakurai, 2007;Kukkonen, 2013;Sakurai et al, 2021) In humans, the number of orexin neurons is estimated at 80 000 at most and just a few thousand or hundreds in rats and mice (Peyron et al, 1998;Harrison et al, 1999;Fronczek et al, 2005;Thannickal et al, 2000;Fronczek et al, 2007;Thannickal et al, 2009;Obukuro et al, 2010;Black et al, 2018;Gonzalez and Prehn, 2018;Berteotti et al, 2021) (see also IV.C.1. and IV.D.…”