2 major determinants of sleep: Homeostatic component- long sleep compensates for prolonged wakefulness Circadian component - alertness varies with time of day
Sleep homeostasis: adenosine ATP ADP AMP Adenosine Dependent on glucose, glycogen, and O 2 Brain glycogen falls with sleep deprivation Adenosine concentration rises during wake and falls during sleep Caffeine blocks adenosine receptors Other somnogens: PGD 2, TNF ...
Amines (locus coeruleus, dorsal raphe, tuberomammillary nucleus) Acetylcholine (LDT/PPT, basal forebr.) Orexin/Hypocretin GABA (ventrolateral preoptic nucleus) Wake Non-REM REM O O O O O Activity of state-regulatory nuclei
Sleep disorders are clinically important 15% of adults have chronic insomnia 24% of adults have chronic sleepiness 25% of motor vehicle accidents with loss of consciousness are due to falling asleep 60% of fatal truck accidents are due to sleepiness
A 23 year old woman is referred for excessive sleepiness after having fallen asleep while driving. She reports that her sleepiness has been present since high school, and she often struggles to remain awake. She occasionally feels weak in the knees when laughing. Once, she fell to the ground while laughing during a party and could not get up for 1-2 minutes. If she is sleepy while driving, she may imagine seeing an animal in the road. Once she was terrified to find herself unable to move for a minute after awakening.
Narcolepsy Daytime sleepiness Disrupted nighttime sleep Fragments of REM sleep Cataplexy - sudden, brief episodes of muscular weakness Hypnagogic hallucinations - vivid, dream-like hallucinations at the beginning or end of sleep Sleep paralysis - inability to move upon awakening
Polysomnograms in control and untreated narcolepsy patient Adapted from Rogers et al. Sleep. 1994;17:590. Time of day Control Untreated narcolepsy Time of day 200024000400080012001600 W REM 1 2 3/4 200024000400080012001600 W REM 1 2 3/4 MT
Loss of orexin in human narcolepsy Crocker, et al, 05
Impaired orexin signaling and narcolepsy Daytime sleepiness Fragmented sleep Cataplexy Sleep paralysis Hypnagogic hallucinations Loss of orexin neurons HumansMice/Rats/Dogs Lack of orexin Loss of orexin neurons Lack of orexin receptors Narcolepsy
Probable mechanisms of narcolepsy LDT/PPT REM-on cells motor neurons LC raphe no orexin
What causes narcolepsy? Most narcoleptics do not have mutations in the genes coding for orexin or its receptors Only 1/3 of monozygotic twins will both develop narcolepsy 85% of narcoleptics with cataplexy have HLA DQB1* 0602 compared to only about 25% of the general population
What causes narcolepsy? Narcoleptics may have gliosis (scarring) in the orexin neuron region HLA DQB1* 0602 and other genes may confer a susceptibility for some individuals to develop narcolepsy, possibly through an autoimmune attack on the orexin neurons
Orexin neurons respond to metabolic factors Yamanaka, et al, 03
Hunger-induced wake requires the orexin neurons Yamanaka, et al, 03
Narcolepsy and Metabolism People with narcolepsy are mildly obese (BMI=28) but eat less than normal Thus, metabolic rate may be reduced in narcolepsy Drugs: tricyclic antidepressants Modafinil trazedone Amphetamines Gamma hydroxy butyrate
Orexin/ataxin-3 mice are overweight but eat less than normal decreased metabolic rate and locomotor activity? less motivation to eat?
Orexin and drug addiction Addiction to amphetamines appears to be quite rare in people with narcolepsy Orexin neurons activate brain regions implicated in drug-seeking (ventral tegmental area, nucleus accumbens) and makes VTA neurons more excitable Mice lacking orexin have almost no conditioned place preference to morphine …Maybe orexin provides the impetus to seek rewarding stimuli like food and drugs
Waking is due to the coordinated action of neurons producing amines, acetylcholine, and orexin Pontine pathways regulate REM sleep, and preoptic nuclei promote non-REM sleep Orexin deficiency produces narcolepsy Orexin may promote many aspects of arousal
Orexin KO run less but the diurnal pattern is normal
Orexin KO spend less time running Still, orexin KO mice initiate wheel running as often and run as fast as WT mice
KO fall asleep or have cataplexy soon after running 28% of running bouts are soon followed by cataplexy
Why do orexin KO mice run less? Sleepiness Imminent cataplexy Less motivated to keep running (perhaps running is less rewarding)