Kubin Lab - Overview of Current Research

 


 

uring rapide eye movement (REM) sleep, the brain exhibits electrocortical activity typical of a highly aroused state (desynchronized cortical EEG, hippocampal theta rhythm, rapid eye movements) while motor activity is suppressed and one is unaware of, and relatively unresponsive to, the external environment. Wake-active neuronal groups (serotonergic, noradrenergic and histaminergic) are silent, while many brainstem cholinergic neurons have high levels of activity. Dreams occur during this stage of sleep. Neuronal networks in the pons, medulla and hypothalamus control the generation of REM sleep, but neither the mechanisms of triggering and termination of the state, nor its function(s) have been elucidated. Under the normal sleep-wake conditions, the activity of wake-related neurons is greatly reduced during slow-wave sleep and this facilitates the subsequent occurrence of REM sleep.  A distinct group of hypothalamic, wake-active cells containing the excitatory peptides orexins (also called hypocretins) also plays an important role in suppressing the occurrence of REM sleep because humans, dogs and mice with disrupted synthesis of orexins or their receptors have symptoms of narcolepsy/cataplexy, a disorder characterized by an "out of context" (during wakefulness) generation of REM sleep or some of its phenomena.  When forebrain influences are suppressed or eliminated, the neuronal network of the brainstem can relatively freely generate multiple REM sleep-like episodes.

n individuals with anatomically predisposed upper airways (small size, often associated with excessive fat deposits around the airway walls), hypoventilations and obstructive apneas occur repeatedly during sleep. They are caused by the characteristic of sleep hypotonia of upper airway muscles.  In severe obstructive sleep apnea (OSA) cases, obstructive episodes are longest during REM sleep. The REM sleep-related decrements in the activity of upper airway muscles are caused by distinct neurochemical changes within, and upstream from, upper airway motoneurons. Our research indicates that these changes are mainly caused by a withdrawal of  excitation mediated by norepinephrine and serotonin.  Although active inhibitory processes specific for REM sleep are frequently cited as the cause of motor atonia of REM sleep, current evidence indicates that, while some inhibitory synaptic events occur in motoneurons during REM sleep, acitve inhibition alone cannot explain the REM sleep-related suppression of activity in upper airway motoneurons.

ue to the complex nature of REM sleep, most functional (electrophysiological) studies need to be performed in vivo. For this we use reduced, pharmacological models of REM sleep, as well as chronically instrumented rats in which we can monitor, and pharmacologically interfere with, the natural sleep-wake behavior. In anesthetized rats, we pharmacologically activate distinct pathways responsible for the generation and maintenance of REM sleep and investigate REM sleep-related changes in the control of breathing and various groups of central neurons. The generation of REM sleep and occurrence of REM sleep-related changes in the control of breathing depend on many neurotransmitters and peptides (acetylcholine, serotonin, norepinephrine, glutamate, GABA, glycine, orexin) and are mediated by multiple neurotransmitter receptors. In order to identify those receptors and determine their relative roles, we microinject various neurotransmitter receptor agonists and antagonists into relevant brain regions and observe changes in hypoglossal nerve activity and sleep-wake behavior. In complementary studies, we use reverse transcription-polymerase chain reaction to determine which neurotransmitter receptor mRNAs are present in central neurons of different phenotypes and functions (single-cell RT-PCR). We also use neurotransmitter receptor immunohistochemistry to determine the distribution of neurotransmitter receptor proteins in upper airway motoneurons.
  e use immunohistochemistry in combination with neuroanatomical tract tracing to study the connections between pontine and medullary neurons that generate REM sleep and/or transmitt the effects of the state to upper airway motoneurons.
  n early days of sleep research, two important discoveries were made regarding the role of the hypothalamus in the regulation of sleep. First, based on the analysis of brain lesions in patients with the encephalitis lethargica, von Economo concluded that the anterior hypothalamus contains neurons that actively induce sleep, whereas cells important for the maitenance of wakefulness are located in the posterior hypothalamus [von Economo, J. Nerv. Ment. Dis., 71: 249-259, 1930]. Second, Sallanon et al. noted that stimulation of GABAA receptors in the posterior hypothalamus in cats that had been earlier made insomniac by anterior hypothalamic lesions induced intense sleep [Sallanon, Denoyer, Kitahama, Aubert, Gay and Jouvet, Neuroscience, 32: 669-683, 1989]. These findings suggested to us that the sensitivity, or availability, of GABAA receptors in certain wake-related cells of the posterior hypothalamus may gradually increase with the duration of wakefulness, and then decrease during sleep. Such a state-, or activity-, dependent plasticity of posterior hypothalamic GABAA receptors could represent an important neurochemical substrate of the brain's "sleepiness signal" and could contribute to the homeostatic regulation of sleep. Our mRNA studies show that hypothalamic GABAA receptor mRNA levels decrease in a regionally-selective manner with the duration of sleep (see abstract), as well as following short-term exposure to GABA in vitro (PDF).
  n obstructive sleep apnea (OSA) patients, there is a statistically significant association between the severity of the disorder and insulin resistance, suggesting that certain aspects of OSA predispose, or aggravete, type 2 diabetes. This association appears to be independent of other cardiovascular and metabolic conditions frequently present in OSA patients (high body mass index and hypertension). Sleep deprivation, sleep fragmentation, recurrent nocturnal episodes of mechanical occlusion of the upper airway and recturent episodes of hypoxemia may contribute to metabolic disregulation in OSA patients. We investigate whether normal adult rodents exposed for many days to cyclic variations of ambient oxygen in a manner that mimics recurrent hypoxic episodes in OSA patients (chronic-intermittent hypoxia, or IH) have altered central and/or peripheral mechanisms relevant for the regulation of glucose metablism.

 

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