Homeostatic regulation of sleep
The basal forebrain is a wakefulness-promoting area, where three types of neurons (the cholinergic, GABAergic and glutamatrgic) send their projections to the cortex, and through the activity of these projections, increase cortical activity.
Adenosine, the core molecule of ATP, acts also as a neurotransmitter: through A1 receptors it inhibits neuronal activity. Increase in extracellular adenosine concentration in the basal forebrain decreases the activity of the cortically-projecting neurons thus promoting sleep. We have proposed that prolonged neuronal activity during prolonged wakefulness increases extracellular adenosine through excessive consumption of ATP. This would be a short negative feed-back loop from energy consumption to decreased neuronal activity.
nitric oxide (no)
Nitric oxide is a gaseous neurotransmitter, which increases adenosine in neuronal cultures. We found that without exception, nitric oxide and adenosine concentrations increased/decreased in the basal forebrain together. NO is metabolized by three different enzymes: neuronal, epithelial and inducible. With a series of experiments we showed that NO during sleep restriction is solely increased by the iducible synthase. This was rather surprising, since this metabolic pathway is normally activated only as a part of the innate immune defense.
basal forebrain (BF)
Basal forebrain turned out to be a very specific structure. Surprisigly, we were not able to measure adenosine increases in any other waking-promoting nuclei, suggesting that the BF cortically-projecting neurons would have a specific role in regulation of sleep homeostasis.
The role of the three types of cortically-projecting neurons was studied using a specific lesion of the cholinergic cells. This lesion completely prevented in crease in both adenosine and nitric oxide (and also lactate), as well as increase in sleep (measured as duration of NREM and EEG delta power) after sleep restriction.
WE also studied the hypothesis that energy depletion in the basal forebrain can induce excesssive sleep, resembling that induced by prolonged wakefulness. We showed that indeed, a local, experimentally induced energy depletion increased sleep as well as extracellular adenosine levels. These results suggested that a local energy depletion in the BF could be an essential component of sleep homeostasis regulation.
Aging is accompanied by many sleep problems. We studied the potential disturbance in regulation of sleep homeostasis and found that adenosine-, nitric oxide- and lactate levels increase less in old than in young as responste to sleep restriction .
In search for molecules that could mediate the effects of prolonged wakefulness in the brain and translate them to longer and deeper sleep, we first found adenosine. These studies were prompted by previous studies by others showing that adenosine agonists increase sleep and adenosine antagonists, including caffeine, decrease sleep. Using in vivo microdialysis, we collected samples from different parts of the brain during spontaneous sleep and prolonged wakefulness (sleep deprivation) and measured adenosine levels.
This is the first article to describe the increase in adenosine levels in the basal forebrain. It also showed that increasing adenosine pharmacologically in the BF also increases sleep and that during spontaneous wakefulness, adenosine levels are higher than during sleep. In the second article we showed that adenosine levels increase first in the basal forebrain, and later in the cortex, but no increases were found in any other brain areas. We had expected that activation in the other waking-promoting areas (including Locus coeruleus, Raphe nuclei and PPT) would induce similar adenosine increases as in the basal forebrain, but that was not the case. These results emphasize the central role of the basal forebrain in regulation of sleep homeostasis.
Publications: Porkka-Heiskanen, T., Strecker,R.E., Bjorkum, A.A., Thakkar, M., Greene,R.W., McCarley, R.W., Adenosine: A mediator of the sleep-inducing effects of prolonged wakefulness. Science 276: 1265-1268, 1997
Porkka-Heiskanen, T., Strecker, R.E., McCarley,R.W., Brain site specificity of extracellular adenosine concentration changes during sleep deprivation and spontaneous sleep: an in vivo microdialysis study.Neuroscience 99 (3): 507-517, 2000
After discovering that adenosine level increase in the basal forebrain, we searched for the source of adenosine, and as part of this project, measured also nitric oxide levels.These two articles show that nitric oxide (NO), a gaseous neuromodulator, is synthetized by inducible nitric oxide synthase in the basal forebrain during prolonged wakefulness. As iNOS is induced during inflammation, the results formed a link between prolonged wakefulness (sleep deprivation) and activation of the immune response.
Kalinchuk, A., Stenberg,D., Rosenberg, P., Porkka-Heiskanen,T., Inducible and neuronal nitric oxide synthases (NOS) have complementary roles in recovery sleep induction. Eur J Neurosci 24:1443-1456, 2006
We hypothesized that the basal forebrain adenosine increase was induced by the increased activity of the cortically projecting neurons that induce wakefulness. To study this hypothesis, we made a specific lesion using the neurotoxin saporin, targeted for the BF cholinergic cells. After the lesion, prolonged wakefulness was no more able to increase sleep, and moreover, adenosine and nitric oxide concentrations in the basal forebrain did not increase.
Kalinchuk, A.V., McCarley, R.W., Stenberg, D., Porkka-Heiskanen, T., Basheer, R, The role of cholinergic basal forebrain neurons in adenosine-mediated homeostatic control of sleep: lessons from 192 IgG saporin lesions. Neuroscience 157: 238–253, 2008
Histamine is one of the cortex-activating neurotransmitters, and its infusion to the basal forebrain increases wakefulness and decreases sleep. The lesion of the cholinergic cells prevented increase in waking and decrease in NREM, indicating that the effect of histamine on cortical arousal is mediated via BF cholinergic cells. Later we showed the same mechanism also for noradrenaline (NREM but not REM sleep!), but not glutamate. (Lelkes Z, Porkka-Heiskanen,T, Stenberg, D. Cholinergic basal forebrain structures are involved in the mediation of the arousal effect of noradrenalin. Journal of Sleep Research 2013 Dec;22(6):721-6. doi: 10.1111/jsr.12061)
Zant JC, Rozov S, Wigren H-K, Pertti Panula P, and Porkka-Heiskanen T, Histamine release in the basal forebrain mediates cortical activation through cholinergic neurons. J Neurosci 19;32(38):13244-54, 2012
Prolonged wakefulness presumably prolongs the period of neuronal activity in many brain areas, and as a consequence, increases energy consumption. Normally, mitochondrial energy production complements the loss, but under energy depletion, the concentration of adenosine increases. Our hypothesis was that sleep homeostasis is, at least partly, connected to energy depletion induced by prolonged neuronal activation during prolonged wakefulness.
To study our hypothesis we induced a local energy depletion in the basal forebrain and measured its effects on sleep. Local energy depletion was induced using dinitrophenol (DNP), a molecule that prevents production of ATP. Infusion of DNP into the basal forebrain dose-dependently increased sleep, while infusion to other brain areas had no effect on sleep. In the BF, adenosine and NO levels increased also dose-dependently.
Kalinchuk, A., Urrila, A., Alanko, L., Heiskanen, S., Wigren, H-K., Suomela, M., Stenberg, D., Porkka-Heiskanen, T., Local energy depletion in the basal forebrain increases sleep. Eur J Neurosci, 17:863-869, 2003.
AMP-activated protein kinase (AMP-kinase) is a sensor of energy levell and is activated under energy depletion. This work shows that AMP is activated during prolonged wakefulness in the basal forebrain but not in the cortex, indicating that indeed, prolonged wakefulness induces an energy depletion condition specifically in the BF.
The question whether any cortical activation is able to induce sleep homeostasis was studied using three neuronal activators: NMDA, AMPA and glutamate. Each of them induced a prolonged cortical activation of equal duration, comparable to prolonged wakefulness, but only NMDA induced increase in high-frequency theta. Also increase in successive sleep (comparable to sleep homeostasis) was induced only by NMDA. This work shows that not only the duration of previous wakefulness, but also the quality of waking (active waking assessed as theta activity) is important in induction of sleep homeostasis.
Wigren, H-K., Schepens,M., Matto,V., Stenberg,D., Porkka-Heiskanen,T., Glutamatergic stimulation of the basal forebrain in vivo elevates extracellular adenosine concentration and increases subsequent NREM sleep. Neuroscience 147: 811-823, 2007
We studied the effects of aging on responses to neuronal activation and sleep homeostasis. Sleep after sleep restriction (recovery sleep) increased less in the older animals. Also the increases in adenosine, nitric oxide and lactate concentrations were smaller in older, showing that the core regulators of sleep homeostasis, and as consequence, the homeostatic sleep responses, are affected by aging.
During vivid neuronal activation, neurons use lactate as their energy substrate, and extracellular lactate levels increase. Using 1H MRS imaging in humans, we measured the level of neuronal activation as response to task performance and lactate levels showing that lactate levels increase in young adults, but not in older subjects. After sleep restriction, stimulation did not increase lactate levels even in the young subjects. Prolonged wakefulness induces temporarily similar physiological changes as aging.
Urrila, A.S., Hakkarainen, Heikkinen, S.,Vuori, K., Stenberg, D., A., Häkkinen, A-M., Lundbom, N., Porkka-Heiskanen, T., Metabolic imaging of human cognition: an fMRI/ 1H MRS study of brain lactate response to silent word generation. J Cereb Blood Flow and Metab 23(8): 942-948, 2003.