Let's focus now on the wake genes that involved in energy metabolism.
Early studies are using the RNA protection,
I say, in situ hybridization and differential display,
that already showed that indeed some mitochondria genes coded by
the mitochondrial genome are induced even just after three hours wake relative to sleep.
Here, you see, for instance,
the sub-unit 2 of NADH dehydrogenase and,
here, the sub-unit 1 of cytochrome c oxidase.
And the follow up studies show that it's not just that the amount of messenger RNA,
but even the activity,
the enzymatic activity of some of
these mitochondrial enzymes involved in the respiratory chain.
Here, you see cytochrome oxidase activity is increased
after a few hours of wake or sleep deprivation relative to sleep.
Of course, in one sense,
it's not surprising that there is upregulation of metabolic genes in
wake relative to sleep because we haven't known for
quite some time based on studies in primates,
as well as in humans,
studies using measuring glucose consumption,
as well as oxygen use, that, overall,
the energy needs of the brain are 20,
25 percent higher after wake than after non-rapid eye movement sleep or non-REM sleep,
which accounts for most 70 percent of all sleep.
And the reason why in wake,
there is increased energy demand is because an activity of
the excitatory glutamatergic cells in the brain is higher in wake than in sleep.
In the cerebral cortex, for instance,
up to 80 percent of neurons are glutamatergic excitatory neurons,
and up to 80 percent of all the oxidative metabolism of the brain
is used to meet the energy needs of those neurons.
The REMaining 10 to 20 percent are used for
glial activity and inhibitory GABAergic cells activity.
We also know that in resting conditions of quiet wake,
there is a very tight coupling between
glucose consumption in the brain and the glutamate to glutamine cycling,
which is a marker of glutamatergic activity.
And, here, you see an example of
how indeed excitatory activity in the cortex is higher during wake than during sleep.
You see here the scalp EEG,
the activated EEG during wake and these low waves of non-REM sleep.
Here, you see multi-unit activity spike activity from single neurons,
and it's obvious that these neurons are actively tonic and firing during wake,
as well as during REM sleep.
But during non-REM sleep,
they stop firing every half a second or
so during the periods of that so-called off periods.
And it is because of these periods of silence that, overall,
firing rates in the cortex are as shown here, 20,
25 percent or 30 percent lower in non-REM sleep relative to wake.
If we look at consistent with this finding,
the extracellular level of glutamate in the cortex,
and we measure how they change across the 24-hour sleep wake cycle using amperometry,
fixed potential amperometry, which is a technique with
very high temporal resolution of one second or so.
We can see that, overall,
glutamate levels are lower during the day
when the rat is mainly asleep and higher during the night.
And, also, that during each wake episode,
which is shown here in red,
glutamate levels that tend to increase and instead then they decrease
during the periods of non-REM sleep do increase again here and here,
for instance, during these green periods that correspond to the bouts of REM sleep.
By contrast, if we measured glucose levels in the extracellular space,
again, in the rat cortex,
they behave in the opposite way,
and so they tend to decline here during wake,
increase during non-REM sleep and decline again during REM sleep.
Let's now focus on the third category of genes,
which are those related to the cellular stress response.
Perhaps the most consistent finding across all species in all laboratories has been
the induction of this so-called stress response gene
BiP or HSPA 5 in wake relative to sleep.
BiP is part of the unfolded protein responses,
so it's induced in very known physiological situations, for instance,
in cases of significant calcium imbalance in the cell or glucose depletion,
and the induction of the UPR causes a slowing down of protein synthesis.
BiP is a chaperone in endoplasmic reticulum,
and the induction of these chaperone promotes in known physiological conditions,
the degradation of misfolded proteins.
However, BiP is also induced in very physiological conditions during which it
helps in the folding of newly normally synthesized proteins.
And there is evidence indeed, for instance,
in these very well characterized model of learning,
the long term and sensitization training Aplysia.
The BiP, as shown here,
is strongly induced after training in this model.
There is also recent evidence that BiP may be involved in the transfer
in the dendrite of the glutamatergic receptors that were mentioned before,
and that the entire induction of the UPR response is
helping and is promoting the surface expression of these receptors.
So, perhaps, it's not by chance that these genes are
all upregulated during waking because they might be strongly linked,
and they might all reflect a higher level
of activity and plasticity during wake relative to sleep.
Ralph Lydic, Ph.D.Professor Emeritus, Molecular and Integrative Physiology, Professor Emeritus, Anesthesiology
Helen Baghdoyan, Ph.D.Professor Emeritus, Anesthesiology, Professor Emeritus, Pharmacology, Professor of Psychiatry
