Sleep and Sleep Disorders 2

What causes canine narcolepsy

-positional cloning project led to isolation of

- canine narcolepsy gene in 1999

canine narcolepsy gene codes for hypocretin receptor 2 (Hcrtr2)

Discovery of hypocretin signalling system

Hypocretin signalling system consists of: two peptides:

- Hypocretin-1 (Hcrt1; also known as Orexin-A)

- Hypocretin-2 (Hcrt2; also known as Orexin-B)

Hypocretin receptors

two receptors:

- Hypocretin-1 receptor (Hcrtr1; also known as Orexin 1 receptor, OX1R)

-Hypocretin-2 receptor (Hcrtr2; also known as Orexin 2 receptor, OX2R)

Hypocretin receptor signalling

- Hcrtr1/OX1R is coupled to the Gq signalling pathway and has excitatory functions

- Hcrtr2/OX2R is coupled to either Gq or Gi/o and has both excitatory and inhibitory functions

What causes canine narcolepsy

various Hcrtr2 mutations have been shown to be linked to narcolepsy in dogs

Evidence for link between hypocretin signalling and narcolepsy

- Hcrtr1 knockout mice only show mild sleep fragmentation

- Hcrtr2 knockout mice also show cataplexy

- double receptor knock-outs show full narcolepsy-like phenotype

-hypocretin knockout mice also show narcolepsy-like phenotype

Human narcolepsy and hypocretin signalling

screening of narcolepsy patients for mutations of hypocretin gene, Hcrtr1 and Hcrtr2 only identified a single mutation in one unusual case:

- atypical onset at 6 month of age

- mutation is a T to G transversion causingleucine to arginine change in hydrophobiccore of signal peptide, peptide accumulates in ER

- NO known mutations of hypocretin receptors

Human hypocretin signalling

- 90% of patients have undetectable levels of hypocretin-1 in CSF

-postmortem study in ten patients showed 80-100% reduction in hypocretin-containing cells in hypothalamus

Human narcolepsy aetiology

humans narcolepsy symptoms usually start to develop during adolescence; mainly sporadic, but familial clustering has been observed

some genetic predisposition:

- 20-40x increased risk in 1° relatives of narcoleptic patient

- monozygotic twins: 25-30% concordant for disorder

Associated with immune system

tightly associated with specific immune system phenotypes:

- The human leucocyte antigen (HLA) gene complex located on chromosome 6 encodes for the major histocompatibility complex (MHC)

- over 90% of narcoleptics with cataplexy carry DQB1*06:02 allele (highest known disease-HLA linkage)

Further evidence

Further evidence for link to immune system:

- 2009 H1N1 influenza pandemic caused 3-fold increase in narcolepsy in China

- a H1N1 vaccine (Pandemrix®) caused 6-9 fold increase in narcolepsy cases in Sweden/Finland

Hypocretin system as central integrator

Hypocretin-positive neurons integrate signals from various sources

- SCN (biological clock)

- Sleep-promoting areas:

- VLPO (ventrolateral preoptic area; 'sleep centre')

- Wake-promoting areas:tuberomammiliary nucleus, dorsal raphe nuclei, locus coeruleus, laterodorsal tegmental nuclei, pedunculopontine tegmental nuclei ('arousal system')

- Metabolic cues

- Hypocretin activates wake/arousal-promoting monoaminergic systems

Circadian changes in hypocretin system activity

- more activity during awake state than during non-REM sleep

- hypocretin levels in CSF increase during the second half of active period

- hypocretin-1 levels are increased during sleep deprivation

(long-term sleep deprivation causes hyperphagia, hyperthermia and increased metabolic expenditure)

Role of hypocretins in normal sleep/wake regulation

Sleep, hypocretin and narcolepsy - a model

1. AWAKE

2. SLEEP

3. NARCOLEPSY

Awake

hypocretin neurons are activated

maintain activity in monoaminergic neurons

Sleep

activity in hypocretin neurons is reduced

sleep-active neurons in ventrolateral preoptic area are active➔ inhibit monoaminergic neurons

Narcolepsy

hypocretin neurons are absent

VLPO and monoaminergic neurons form 'flip-flop' switch➔ rapid transition from awake to sleep and vice versa

Treatment of narcolepsy - current

Behavioural adjustment (napping, etc. ⇒ usually insufficient on its own)

Fine line therapeutics

- Sodium oxybate (sodium salt of gamma-hydroxybutyrate, GHB)

- consolidates nocturnal sleep, decreases cataplexy and daytime sleepiness - short-half life, small safety margin, drug abuse - Mechanisms unclear, probably via (indirect?) interaction with GABAB receptors and dopamine transmission

Drugs

Modafinil/armodafinil

- Increased wakefulness, but not effective for cataplexy

- Less abuse potential than amphetamines

- Mechanisms not fully understood, but probably via dopamine re-uptake inhibition

Recently developed drugs

Recently developed drugs

- Pitolisant

- Selective histamine H3-receptorinverse agonist/antagonist

- Targets excessive daytime sleepiness

Solriamfetol (JZP-110)

- Selective dopamine and noradrenaline-reuptake inhibitor

- High-potency wake-promoting agent

Second/Third line therapeutics

- methylphenidate (Ritalin) and amphetamine

- increase release of dopamine/reduced dopamine re-uptake

- side-effects: vasoconstriction, cardiac stimulation

Antidepressants

- Off-label use

- Tri-cyclic anti-depressants and selective monoaminergic re-uptake inhibitors have been reported to reduce cataplexy

- Used in combination with modafinil

- Example: venlafaxine (SNRI)

Treatment of narcolepsy - future perspectives

Treatment of insomnia - recent developments

Summary

Narcolepsy is a form of hypersomnia, characterised by fragmentation of wake/sleep cycle

- Most narcoleptic patients show reduced levels of hypocretin synthesis, but no mutations in the hypocretin receptors

- Hypocretin system acts to stabilise transitions between wake and sleep states and promotes the wake state

- Hypocretin system provides promising target for future treatment of narcolepsy and insomnia