SWS: Neurobiological mechanisms for the regulation of mammalian sleep-wake behavior: Reinterpretation of historical evidence and inclusion of contemporary cellular and molecular evidence

In paper citation (Datta & MacLean, 2007)

There are two major warrants to the activity dependent homeostatic theory of sleep initiation:

1. "The waking state requires a critical level of brain activity, which is maintained by a steady flow of ascending impulses arising in the brainstem reticular formation"
2. "A reduction of tonic activity of the ascending reticular system (ARAS) is responsible for physiological sleep."

Basically, the initiation of sleep is a passive process caused by the withdrawl of wakefulness. But what causes this withdrawl. Here's what the theory says:

• During wakefulness, the increased rate of metabolite synthesis is higher than the rate of clearance. Individual cells begin to demand a lower metabolic state and cease firing. Eventually, this effects behavior at the systemic level, and the body initiates sleep. 
• This theory predicts that duration of sleep periods are inversely correlated with the rate of metabolite clearance.
• Factors only involved in sleep induction are not metabolic byproducts. These are synthesized after SWS has started.
• Adenosine
• Cells use ATP by catabolizing it into adenosine and ADP. Adenosine moves along its concentration gradient and can therefore get built up by periods of large use. 
• Adenosine accumulates in the BF and cortex during forced sleep deprivation.
• Direct administration of adenosine increases sleep duration and enhances SWS activity in the rat.
• Blocking adenosine synthesis eliminates SWS and increases wakefulness.
• Adenosine also inhibits the activity of Hcrt neurons in the LH.
• Inhibitory amino acids (GABA)
• GABA - everywhere
• glycine - spinal cord & brainstem
• Glutamate is decarboxylated to form GABA by GAD. 
• Increased neuronal activity results in a local increase in GABA synthesis.
• Glycine synthesized from the degradation of serine. 
• Choline can metabolize into glycine by stepping through betaine and losing  its methyl groups.
• A global increase in GABA via ventricular infusion promoted the physiological signs of NREM sleep. 
• Prostaglandin
• Prostaglandins are a naturally occurring unsaturated fatty acid group (loosely) made from arachidonic acid.
• PG is observed most often in the CSF between the arachnoid membrane and the pia mater.
• Primarily synthesized in the leptomeninges, the epithelial cells of the choroid plexus, and the oligodendrocytes.
• Sleep deprivation raises the typical PG concentration in the CSF.
• Injection of PGD2 into the Preoptic area, or into the lateral vesicles increases NREM sleep.
• Cytokines
• Cytokines stimulate subtle changes in cellular metabolism.
• Cytokines affect the input-output relationships within their neural circuit of origination.
• Interleukine-1beta is highest when the demand for NREM sleep is highest in the rat (beginning of the day), taper off for awakening
• In humans, IL-1B is highest at initiation of sleep, and lowest at awakening.
• Injecting IL-1B into the brain directly increases amount of NREM sleep, but only if the the subject is already in NREM sleep. (NOT causative)
• Substances that inhibit IL-1B decrease spontaneous sleep. 
• TNF-alpha is also 10X higher at the initiation of sleep than during its minimal waking values.
• TNF-alpha in the POA enhances NREM sleep in rats.
• TNF inhibitors also inhibit NREM sleep and decrease spontaneous sleep. 

Mechanisms for the generation and maintenance of SWS:

• After sleep initiation, GABA and galanin synthesizing cells in the anterior hypothalamus/ POA become active and project to the major wake promoting areas, inhibiting them. 
• GABA hyperpolarizes the thalamus to raise the threshold for sensory information getting relayed to the cortex.
• POA critical for SWS generation. Lesions in the POA can prevent SWS in mammals.
• FMRI data shows that mPOA is more active than other parts of the hypothalamus and basal forebrain during SWS. 
• POA lesions will knockout SWS, but only for a matter of days depending on the extent of the damage. 
• Paradoxically, NE and 5HT in the POA increase POA activity and can induce wakefulness.
• GHRH (a sleep inducing factor) may be needed to help GABA from the POA work. 
• GHRH made in the arcuate nucleus (largest #), ventromedial nucleus (VMN), and paraventricular nucleus (PVN). These cells project to the anterior pituitary (from the arcN) and the POA.
• GHRH  is high around the initiation of sleep, and highest immediately after sleep.
• Highest GHRH synthesis occurs at the period of deepest SWS. 
• 2/3 of all growth hormone (GH) secreted in young males occurs during SWS. 
• GHRH injections systemically and ventricularly increase SWS in rats.
• GH injection decreases SWS by providing negative feedback to the GHRH production system.
• GHRH  is released into the POA and binds to GHRH recptors to activate POA GABAergic cells.

Remaining questions: 

• Does TNF-alpha injection only increase NREM sleep during NREM periods, or can it initiate NREM?
• What are the sleep induction factors that get synthesized during SWS?