Hyperexcitable arousal neurons drive sleep instability in old mice, study finds

For many older adults, a good night’s rest is elusive. The implications of chronically poor sleep can be far-reaching and include a decline in cognitive functioning and detrimental effects on health and general well-being.

Fortunately, relief may be in sight. A new study led by investigators at the Stanford University School of Medicine shows that neurons in the lateral hypothalamus, a brain region, play a pivotal role in sleep loss in old mice. More specifically, the arousal-promoting hypocretin neurons become hyperexcitable, driving sleep interruptions. 

The finding could pave the way to new drug treatments for age-related sleep problems in humans, said the study’s senior author, Luis de Lecea, PhD, a professor of psychiatry and behavioral sciences.

“Together with nutrition, exercise and relaxation, sleep is the fourth pillar essential for healthy living, and sleep disruption is highly intertwined with neuropsychiatric disorders,” de Lecea said. 

Shi-Bin Li, PhD, a basic life research scientist in the de Lecea lab, is the lead author of the study, which was published online Feb. 24 in Science. “Our study shows a mechanism underlying fragmented sleep with age and provides a potential strategy for improving sleep,” Li said. 

The Stanford researchers compared the sleeping and waking patterns of young (3- to 5-month-old) and old (18- to 22-month-old) mice and found a significant loss of hypocretin neurons in the old mice, whose sleep patterns were fragmented. 

Mice in both groups were implanted with electrodes that measured brain and muscle activity to monitor their wake and sleep states, including non-rapid eye movement, or NREM, sleep — a time of deep relaxation; and REM sleep, when vivid dreams occur. The scientists determined that the mean length of NREM sleep is significantly shorter in old mice. Like humans, mice experience more disrupted sleep as they age.

Hyperexcitable neurons in old mice

To understand why the old mice got less rest, the neuroscientists assessed hypocretin neurons, which produce neuropeptides that promote and stabilize arousal and wakefulness. With fiber photometry, a technique for monitoring the neurons’ activity, the researchers found that hypocretin neurons in the brains of older mice were more active, or hyperexcitable, disrupting sleep continuity.

Analyzing brain tissue from the young and old cohorts, the researchers found that old mice had nearly 38% fewer hypocretin neurons than young mice.

Sleep disruption is highly intertwined with neuropsychiatric disorders.

Using a technology developed at Stanford called optogenetics, which uses light to activate specific neurons, the scientists discovered that the remaining hypocretin neurons in older mice more easily switched on to induce and maintain a wakeful state.

“There is something wrong with the brake system of the older hypocretin neurons, making it easier for them to fire more frequently and disrupt sleep,” Li said, explaining that the older hypocretin neurons show an impaired repolarizing current. “This current, mediated by channels known as KCNQ2/3, regulates neuron activity and contributes to improved sleep stability.”

The neuroscientists next turned to molecular methods and determined that selective disruption of the KCNQ2/3 channels can destabilize sleep in young mice. They also found that flupirtine, a drug that activates the channels, prevents hypocretin neurons from becoming overly active, thereby enabling the mice to sleep restfully.   

The ability to regulate sleep patterns through this targeted pharmacology could lead to the development of new drug treatments for people with age-related sleep disorders, de Lecea said. The potential therapies would also provide an alternative to hypnotics — drugs that are traditionally used to treat insomnia. 

The results of the study support the hypothesis that the arousal circuitry, and particularly the hypocretin system in older people, becomes more easily activated with age. About half of men and women ages 65-74 experience bouts of wakefulness at night. Meanwhile, it is estimated that 40%-70% of older adults have chronic sleep problems. Healthy older adults who lose out on a good night’s slumber have higher risks of mortality even after controlling for age, gender and other health issues, research shows.

Effects of sleep deprivation

Given that sleep deprivation affects people’s ability to concentrate and retain and retrieve memories, understanding its mechanisms is a critical step toward improving the health and well-being of older people. Studies have found that sleep disruption contributes to cognition decline, more accidents from falls, daytime drowsiness, Alzheimer’s disease and a host of nervous-system conditions and cardiovascular diseases. According to research, disrupted sleep and related health issues are also a leading cause of institutionalization among the elderly. 

There is something wrong with the brake system of the older hypocretin neurons.

“Currently, sleep problems in older individuals are treated with drugs that make them drowsy, and their effects may generate even more problems,” de Lecea said. “Our results suggest that flupirtine, a non-opioid painkiller, could be optimized and repurposed to treat insomnia and sleep fragmentation in the elderly.”

Other Stanford co-authors of the study are postdoctoral scholars Valentina Martinez Damonte, PhD, Justus M. Kebschull, PhD, Hiroshi Yamaguchi, PhD, Wen-Jie Bian, PhD, and Carolin Purmann, PhD; Reenal Pattni; Gordon X. Wang, PhD, clinical assistant professor of psychiatry and behavioral sciences;  Alexander Eckehart Urban, PhD, associate professor of psychiatry and behavioral sciences; Philippe Mourrain, PhD, associate professor of psychiatry and behavioral sciences; and Julie Kauer, PhD, professor of psychiatry and behavioral sciences.

Researchers at the University of North Carolina-Chapel Hill also contributed to the study. 

The research was supported by the National Institutes of Health (grants R01AG047671, R01MH116470, R01NS104950, R01DA011289, R01NS106301, K01AG061230, P30EY026877), the Sleep Research Society Foundation, a Scully Family Seed Grant from the Stanford Alzheimer’s Disease Research Center-, and the New York Stem Cell Foundation.