SLEEP PRESSURE AND ADENOSINE

SLEEP MEDICINE

SLEEP PRESSURE AND ADENOSINE

by Regina Patrick RPSGT

denosine is a sleep-promoting molecule, and the brain’s level of adenosine normally decreases during sleep and increases during periods of wakefulness. It progressively increases sleep pressure, the desire to go to sleep, as wakefulness is prolonged.

Sleep pressure normally fluctuates and helps to maintain sleep homeostasis, or stable sleep/wake cycles. Sleep deprivation worsens sleep pressure and, as a result, a person not only feels a great need to sleep but will sleep for a prolonged period of time when allowed to sleep.

Regulation of

adenosine could

lead to new

treatments for

sleep disorders

A recent animal study indicates that altering the brain levels of adenosine can reduce the degree of sleep pressure after a period of sleep deprivation. This finding could potentially lead to new treatments for sleep disorders involving excessive sleepiness or insomnia.

In the brain, one type of cell that produces adenosine is the astrocyte, a star-shaped non-neuronal cell. Scientists previously believed that astrocytes did not transmit signals and that they only provided a supportive role to neurons such as the transport of glucose molecules to neurons in the brain. However, in recent years, researchers have learned that astrocytes can play a role in the propagation of signals in the brain.

What are astrocytes?

Three forms of astrocytes exist in the central nervous system: fibrous, protoplasmic and plasmatofibrous. The fibrous astrocytes are mainly located in the white matter and have a few long, thin, unbranched cellular processes extending from the cell body. The protoplasmic astrocytes are found in gray matter and have several branching short, thick processes.

The plasmatofibrous astrocytes have the combined features of the other two types and are found at the junction of the gray and white matter in the brain. The processes of a plasmatofibrous astrocyte that extend into the white matter are fibrous while the processes extending into the gray matter are like those of a protoplasmatic astrocyte.

Processes extending from an astrocyte can enwrap several nerve cell bodies or synapse with hundreds of neuronal dendrites.The juxtaposition of an astrocyte’s processes with several nerve cell bodies or with the dendrites of several neurons gives the astrocyte the potential to modulate the activity of neurons.

Astrocytes release chemical transmitters called gliotransmitters. Scientists are not fully sure how astrocytes transmit their gliotransmitters. One possibility may be through a process called exocytosis in which the gliotransmitters, which are enclosed within an astrocyte’s vesicles, are actively discharged from the cell.

In exocytosis, a vesicle first fuses its membrane with the astrocyte’s membrane. A pore then forms at the site of the membrane fusion. This allows the gliotransmitter to escape out of the vesicle, through the membrane of the astrocyte and outside the astrocyte. The gliotransmitter may then travel to and bind with receptors on a nearby neuron.

Once a gliotransmitter binds to the neuronal receptor, the gliotransmitter can alter the activity of the neuron. In this way, astrocytes can modulate the activity of neurons. The membrane fusion process in astrocytic exocytosis occurs when sufficient levels of intracellular calcium are present in the astrocyte. Without sufficient levels of calcium or if some other factor interferes with the fusion of the membranes, a gliotransmitter can not be released.

An example of a gliotransmitter is adenosine triphosphate (ATP, an adenosine molecule bound to three phosphate [PO4^–] ions). Once ATP is released outside of an astrocyte, the molecule progressively loses its three phosphate ions as it is metabolized, thereby becoming adenosine. Increased levels of adenosine outside the astrocyte (i.e., extracellular adenosine) bind to receptors on neurons and result in the slowing of nerve impulses. This may set the stage for sleep pressure and for neuronal slowing in sleep.

Some research indicates that extracellular adenosine modulates slow-wave activity. Slow-wave activity (consisting of waves 0.5 to 4.0 cycles per second) is present during wake and sleep but is most pronounced on an electroencephalogram (EEG) during slow wave sleep.

Modulation of slow wave activity may be the result of activation of the adenosine A1 receptors located on neurons. Studies show that activation of the adenosine A1 receptors by adenosine reduces the neuronal release

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of excitatory transmitters (e.g., glutamate) into the synapse and instead increases hyperpolarization of the neurons, thereby facilitating slow wave oscillations in the neurons.

Further pointing to the role of adenosine A1 receptors in sleep, studies demonstrate that chemically blocking (antagonizing) adenosine A1 receptors in mammals results in increased wakefulness. Despite these findings, scientists remain unsure of the extent of the role that astrocytes may play in sleep. Recently, a team of scientists, headed by Michael Halassa investigated this. Halassa and colleagues bred mice that had a gene mutation (dnSNARE) that, under specific conditions, suppresses the production of adenosine. The explemental condition in this study was the addition of the antibiotic doxycycline in the mice’s diet.

When dnSNARE mice are administered doxycycline in their diet, the biochemical interactions between the antibiotic and the mutated gene prevent the production of extracellular adenosine. Therefore, extracellular amounts of adenosine were lower in the mice with the mutation. (When doxycycline was later removed from the diet for a few weeks, the extracellular levels of adenosine in the dnSNARE mice increased significantly.)

The researchers noted that, while the dnSNARE mice were administered doxycycline in their diet, there was no significant difference between dnSNARE and wild-type (normal) mice in the amount of rapid eye movement sleep (REM), non-REM (NREM) sleep and wakefulness. However, when the researchers deprived both groups of mice of sleep for short periods, they found that the dnSNARE mice had less sleep pressure after sleep deprivation and they did not require as much compensatory sleep to recover, compared with the control mice.

The researchers measured sleep pressure by the amount of low-frequency (0.5 to 1.5 cycles/second), slow-wave activity the mice had during sleep - the greater the amount of low-frequency slow wave activity during sleep, the greater is the amount of sleep pressure. By contrast, the wild-type mice had increased amounts and prolonged periods of slow wave activity after sleep deprivation.

In an in vitro aspect of their study, the researchers chemically blocked the production of adenosine with the A1 receptor antagonist 8-cyclopentyl-1,3-dimethylxanthine (CPT) in brain slices from the dnSNARE and the wild-type mice. The brain slices were from areas in the brain known to play a role in sleep (e.g., hippocampus, cortex). They then measured the synaptic activity of the neurons in the brain slices. They expected the activity to be reduced in the dnSNARE mice and found this to be the case. By contrast, the synaptic transmission of the wild-type mice increased with antagonism of the A1 receptors.

To see how CPT would impact sleep in in vivo conditions, the chemical was directly administered into the brains of the mice. The researchers found that dnSNARE mutation blocked the wake-promoting effects of CPT in the dnSNARE mice. But the administration of CPT in the wild-type mice resulted in reduced sleep pressure after a period of sleep deprivation. From these results Halassa, et al concluded that astrocytes play a role in sleep, and that the A1 receptor plays a role in sleep homeostasis.

The role of astrocytes in sleep is a relatively recent finding that may offer new avenues in treating excessive sleepiness or insomnia. For example, therapeutic drugs could target the adenosine A1 receptor or target enzymes or other chemicals involved in adenosine metabolism. Genetic approaches could potentially offer more individualized and, therefore, improved treatment.

Scientists currently know of more than 30 variations of the gene that produces adenosine deaminase, an enzyme that metabolizes adenosine. People who have one variation (22G3A, which involves the substitution of one amino acid) have 20 to 30 percent lower enzymatic activity and, as a result, increased adenosine levels. As a consequence, people with this variant gene have deeper sleep and longer slow wave sleep periods.

Swiss researchers recently found that people who have a certain mutation in the adenosine A2 receptor may be more susceptible to the effects of caffeine (which also antagonizes this receptor). In cases such as these, the treatment for people who have a certain genetic makeup could differ from the treatment for people without the variant gene or genes. This distinction in treatment could potentially enhance treatment outcomes by avoiding drugs that would not be beneficial.

Caffeine (an adenosine A1 receptor antagonist) is commonly used to increase wakefulness at desired times. However, the resulting wakefulness is often short-lived. As an alternative, many people use prescription stimulant drugs. However, these can have drawbacks such as addiction, nervousness, and insomnia.

New treatment approaches that could result in a sustained reduction of sleepiness at desired times, while avoiding drug-induced adverse effects, could bring welcome relief to people who struggle with insomnia, sleepiness, or who may need to avoid the consequences of jet lag.

Regina Patrick is a Sleep Technologist at St. Vincent Mercy Sleep Center in Toledo, OH and appears regularly in Focus Journal. She can be reached at rpsgtwritr@aol.com.

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