Lecture 11: Orexins and Narcolepsy Flashcards
what is narcolepsy?
Sleep disorder incidence of 1 in 2000, appears between ages of 15 and 30 years, with 4 main symptoms:
Excessive daytime sleepiness with irresistible sleep attacks during the day.
Cataplexy (brief episodes of muscle weakness/paralysis precipitated by strong emotions such as laughter or surprise).
Sleep paralysis–abnormal episode of REM sleep atonia.
Hypnagogic hallucinations or dream-like images that occur at sleep onset.
what are orexins/hypocretins?
MOA?
Areas innervated by orexin/hypocretin Ns?
Excitatory or inhibitory?
Types and origin thereof?
Two carboxy-terminally amidated neuropeptides of related sequence.
Synthesized by a few 1000 cells in the brain.
Function via two G-protein-coupled receptors.
Differentially distributed throughout the brain.
Fibres containing these neuropeptides innervate many brain areas including those implicated in the control of cardiovascular function, ingestive behaviour, and sleep-wake cycles.
Peptides primarily excitatory and central injection stimulates food intake, blood pressure, locomotor activity, increases wakefulness, and suppressing REM sleep.
2 types,
Hcrt1, Hcrt2 or OX1R OX2R both from same precursor.
Evolution of orexins and similarity to other pepties
preprohypocretin (precursor) in puffer fish and various frog species–> gene arose early in chordate lineage.
Sequence similarities with various members of incretin family. Secretin gene, indicate that Hcrt gene formed from secretin gene by three genetic arrangements.
3-D structure determined by NMR show similarities between secretins and hypocretins.
Hypocretins/orexins and the hypothalamus
Hypothalamus specific peptides with neuroexcitatory activity.
Particularly in the lateral hypothalamus.
Hypocretins found in both cell bodies and terminal vesicles
Hypocretin increases the frequency of postsynaptic events in most cultured hypothalamic neurons.
Orexin orexin-R discovery
Family of hypothalamic neuropeptides and GPCR that regulate feeding behaviour
Lateral hypothalamus again
(rats)
Injection of orexin into the brain stimulates food intake, both OX1/2R. even in full satisfied rats.
fasted rat has high orexin –> reduced post meal (stimulates appetite)
Orexin expression and projection
Only lateral hypothalamus, small number but extensive and diffuse projections
(hypo, basal forebrain, cortex, hippocampus, spinal cord)
Orexin at the synapse
Excitatory, stimulates postsynaptic cell.
Orexin-ir (immunoreactive) vesicles found
in presynaptic terminals
forming presumed
excitatory synapses.
Orexin neurons with green fluorescent protein (GFP) mice.
GFP expressed by the cells with orexin gene.
Low concentrations of glucose activate orexin neurons (high inhibit… response to metabolic challenge)
Leptin and ghrelin on orexin
Leptin inhibits orexin neurons (indicates fat reserves)
Ghrelin (metabolically compromised, peripheral hormone gets in brain)
activates orexin neurons.
How do mice lacking orexin neurons respond to fasting?
Super weirdly !!!!
Orexin KO mice + sleep
WT: transitions smoothly between wake/SWS/REM.
KO: rapid transitions to REM from wake… behaviourally unstable
Have more transitions between all behavioural states, both light/dark compared to WT, also have cataplexy which WT don’t.
mice with Ox1/2R KO
OX1R KO:
wake/REM & non-rem/REM transitions.
cataplexy, paralysis, SOREM periods.
OX2R KO:
wake/non-rem transitions.
non-REM sleep attacks, excessive sleepiness (+OX1R symptoms)
both contribute to narcolepsy-cataplexy syndrome but O2XR more important
Other narcoleptic models
3
Doberman Pinschers:
Narcolepsy inherited as autosomal-recessive fully penetrant phenotype; defective canine narcolepsy gene encodes Hctr2/Ox2R receptor; truncated receptor protein which does not localize to membrane and does not bind ligands ie orexin.
Labradors: their Hcrtr2/Ox2R gene has a distinct mutation skipping exon 6 leading to truncated receptor protein.
Dachshunds: point mutation in Hcrtr2/Ox2R gene results in receptor protein that reaches membrane but cannot bind the hypocretin.
humans narcoleptic vs controls
Orexin mRNA in narcoleptics absent in lateral hypothalamus.
MCH mRNA (cocalised w OX) same in narcoleptic and controls.
Thus selective loss of orexin.
Progression of narcolepsy
Onset is usually 15-30 –> OX-Ns lost over time
Maybe autoimmune
Don’t show symptoms until lost a certain amount of orexin/dopamine
(similar to PD)
Orexin wrt activity during REM and waking
triggers what?
Orexin neurons silent during REM (PS) along with muscle atonia. They become active at end of REM sleep and precede return to waking and muscle tone.
Trigger return into wakefulness.
Orexin/Hypocretin Neurons Active in Exploratory behaviour, Grooming, Eating, and Quiet Waking. Inactive in Slow Wave Sleep
Using optogenetics to selective activate orexin neurons with light
(LH injection with lentiviral vector!!)
channelrhodopsin (blue-R)-
blue light during REM sleep, rat wakes up.
Can show that blue light activates channelrhodopsin and orexin have increased AP in vitro.
Mice in vivo, fibre optic into brain –>no transition between wake/REM.
Halorhodopsin inhibition of OX-Ns (hyperpol) reduces daytime wakefulness
orexin receptor antagonists
Almorexant: blocks both types of OX-Rs: reduces time spent in awake states and facilitates maintenance of sleep state in many animals including humans. With people with insomnia, almorexant improved sleep efficiency (time spent asleep
when confined to a bed for 8h). Also decreased latency to sleep onset and decreased number of wake bouts. Also improved subjective reporting of sleep quality.
In rat, recently developed Ox2R antagonists more effective at reducing latency to sleep and increased NREM more potently
than almorexant. So, targeting Ox2R could be the route to follow.
