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Science: Can't sleep well at an older age? The culprit may be Hcrt neurons being activated

Adequate and continuous sleep not only enables us to start the day refreshed, but also contributes to all aspects of health. Conversely, poor sleep quality may lead to a range of problems such as decreased immunity, cognitive decline, obesity, and cardiovascular disease.

The reasoning is simple, but for many people, a good night's sleep is out of reach. Especially as we age, many people will suffer from sleep problems. One common problem is sleep fragmentation - the inability to sleep through the night, but instead waking up briefly many times during the night.

We note that sleep fragmentation during aging has long been observed clinically and that this phenomenon has been reported in other species such as rodents and fruit flies, but the literature has not been reviewed for a clear mechanism behind this phenomenon," describes Dr. Shibin Li of Stanford University.

In a recent paper published in Science (Dr. Shibin Li is the first author), a team of researchers led by Professor Luis de Lecea of Stanford University uncovered a neural mechanism of sleep fragmentation and suggested potential treatment ideas.

Science: Can't sleep well at an older age? The culprit may be Hcrt neurons being activated

This study focused on hypothalamic secretin (Hcrt, also known as appetite hormone) neurons. Previous studies have found that Hcrt neurons located in the lateral hypothalamus are closely associated with wakefulness: when these neurons are active, they facilitate the transition from sleep to wakefulness and maintain the waking state. In contrast, severe loss of Hcrt neurons may lead to the rare type I episodic sleep disorder: patients may suddenly pass out uncontrollably into a sleep-like state when emotionally stimulated, even when awake.

Based on the role of Hcrt neurons in facilitating the sleep-wake transition and maintaining wakefulness, the team speculated that sleep fragmentation during aging is likely to be associated with altered activity of such neurons.

To test this conjecture, the team first compared the sleep of mice of different ages. They noticed that older mice showed sleep fragmentation compared to younger mice, suggesting that sleep fragmentation may have similar conserved neural mechanisms across species. And during further studies of the mice, two seemingly contradictory clues emerged.

Science: Can't sleep well at an older age? The culprit may be Hcrt neurons being activated

Hcrt neurons play an important role in the transition between sleep and wake states (image credit: Ref. [1])

On the one hand, in vivo calcium imaging results show that Hcrt neurons are more frequently active in older mice, that they are more likely to enter the waking state from sleep, and that the average duration of sleep is shorter. On the other hand, in vivo optogenetic studies show that although older mice have fewer Hcrt neurons expressing photosensitive channels (ChR2), Hcrt neurons are activated in older mice and have a longer duration of wakefulness than in younger mice.

In addition, quantitative analysis of Hcrt neurons in whole brain section experiments in mice showed that up to 38% of Hcrt neurons were lost in the older group.

Given that the role of Hcrt neurons is to facilitate awakening from sleep, it would seem that as the number of neurons decreases, the sleep state should be more sustained and stable. However, the opposite was actually observed. What is the mechanism by which older mice use fewer Hcrt neurons, resulting in more awakenings?

With the help of ex vivo electrophysiological experiments, the team found the answer at the cellular level. Hcrt neurons in older mice have higher resting potentials, or are closer to the threshold for firing. Therefore, with a slight stimulus, these neurons enter a firing state; in contrast, the Hcrt neurons of young mice require a stronger stimulus before they enter an excited state and start firing.

Thus, during aging, Hcrt neurons shift to a hyperexcitable state, and it is this change that causes older mice to be more likely to awaken from sleep. So, what is it that causes the hyperexcitability of aged Hcrt neurons again?

Science: Can't sleep well at an older age? The culprit may be Hcrt neurons being activated

▲Hcrt neurons in aged mice are in a hyperexcitable state (Image source: Ref. [1])

Voltage-gated potassium channels (KCNQs) play a key role in the repolarization of neuronal action potentials and in maintaining the resting state of neurons. These channels pump positively charged potassium ions from the intracellular to the extracellular compartment, thereby increasing the potential difference between the inside and outside of the cell membrane and making it easier for neurons to maintain a resting state.

So we're thinking that perhaps during aging, due to oxidation or other factors, the expression of potassium channels decreases, which leads to depolarization of the membrane potential in older Hcrt neurons, and therefore easier activation," said Dr. Shibin Li.

With this in mind, the research team analyzed such channels. In older mice, KCNQ2/3-mediated currents were weaker; also, high-resolution imaging showed a lower density of KCNQ2 expression in their Hcrt neurons. These results suggest that KCNQ2/3 channel-mediated downregulation of repolarizing currents leads to hyperexcitability of aged Hcrt neurons, which in turn causes sleep fragmentation.

Science: Can't sleep well at an older age? The culprit may be Hcrt neurons being activated

▲Downregulation of KCNQ2/3 channel expression leads to changes in Hcrt neurons in aged mice (Image credit: Reference [1])

The team then used gene editing tools to verify the results: they disrupted the Kcnq2/3 gene in young mice, at which point they developed sleep fragmentation similar to that of older mice.

Based on these neural mechanisms, the researchers used a pharmacological approach to enhance KCNQ2/3-mediated repolarizing currents, and the agonist of KCNQ2/3, flupirtine, resulted in more polarized resting potentials and less frequent action potentials in older Hcrt neurons, as well as a more stable sleep state in older mice. This experiment suggests that pharmacological enhancement of KCNQ2/3-mediated repolarizing currents may lead to new therapeutic options for those who suffer from low sleep quality.

It is worth noting that flupirtine was once approved by the European Union as a non-opiate analgesic, and one of the side effects of this drug is drowsiness. The marketing authorization for flupirtine has since been withdrawn due to the discovery of hepatotoxicity. If new KCNQ2/3 agonists with less toxic side effects can be found, we can expect to develop new drugs along this line to improve sleep quality in the future.

Science: Can't sleep well at an older age? The culprit may be Hcrt neurons being activated

▲This study reveals an important mechanism of sleep fragmentation during aging (image credit: Ref. [1])

Commenting on the study, Dr. Shibin Li said, "The most important point of this study is that we have identified a mechanism by which sleep fragmentation occurs during the aging process. Of course, this may not be the whole mechanism, but given this clear mechanism, it may be possible to develop relevant therapies in a symptomatic and targeted manner."

Currently, physicians tend to use sleep-promoting drugs indiscriminately for people with fragmented sleep - whether they are young or old patients. However, in reality, the mechanisms that lead to sleep problems may be different for different populations. Based on this study, the future treatment of sleep disorders in the elderly is expected to usher in a whole new way of thinking.

Reference.

[1] Shi-Bin Li等人,超兴奋的唤醒电路驱动衰老过程中的睡眠不稳定。科学》(2022)https://doi.org/10.1126/science.abh3021