Wake-active brainstem GABA neurons signal sleep pressure by upregulating AMPA receptors to drive recovery sleep, 2026, Ba et al.

[SNT Gatchaman]

Wake-active brainstem GABA neurons signal sleep pressure by upregulating AMPA receptors to drive recovery sleep

Spoiler: Authors

Wei Ba; Edward C Harding; Mathieu Nollet; Kyoko Tossell; Li-Li Li; Sara Wong; Berta Anuncibay Soto; Raquel Yustos; Li Li; Jagoda Ostaszewska; Hanns Ulrich Zeilhofer; Alexei L Vyssotski; Michael J Courtney; William Wisden; Nicholas P Franks

How the brain compensates for sleep deprivation (SD) by generating recovery sleep (RS) is not understood. Using Ca²⁺ photometry, we identified a WAKE/rapid eye movement sleep (REMS)-active somatostatin/parvalbumin GABAergic population in the mouse brainstem oral pontine reticular nucleus (PnOVgat).

Following SD, PnOVgat cells transiently switched for the first hour to higher activity during non-REMS (NREMS), promoting RS. Chemogenetic activation of PnOVgat neurons prolonged NREMS, whereas ablation blunted electroencephalogram (EEG) delta power rebound and slowed RS accumulation. During RS, the selective switch of PnOVgat cells to having higher Ca2+ levels in NREMS correlated with elevated levels of synaptic proteins PSD95, activated calmodulin-dependent kinase II CaMKII (pCaMKII T286), activated PKA (pPKA T197), and GluA1-containing AMPA receptor subunits with enhanced serine phosphorylation. All increases started during SD and persisted after the first hour of RS. Patch-clamp recordings demonstrated increased postsynaptic AMPA/sleep homeostasis (NMDA) receptor ratios in PnOVgat cells 1 h after RS, indicating increased excitability and greater capacity to drive RS.

In contrast, an intermingled population of GABA/glycinergic neurons did not respond to SD, despite having similar baseline WAKE/REMS activities and an ability to promote NREMS. The PnO also contained an intermingled population of excitatory PnOVglut2 WAKE/REMS-active neurons; lesioning them caused hypoactivity, but sleep or WAKE amounts were unaffected. The synaptic homeostasis hypothesis (SHY) proposes that as wakefulness progresses, synaptic AMPA receptor activity is enhanced, and subsequently downregulated during NREMS to rebalance circuit function.

We suggest that a variation of SHY implements catching up on lost sleep, with glutamate receptor plasticity in the PnO tracking time awake and adjusting NREMS amounts accordingly.

HIGHLIGHTS
• PnO GABA neurons switch from WAKE/REMS to NREMS after sleep loss

• Activating PnO GABA neurons prolongs NREMS and enhances rebound sleep

• Sleep deprivation increases GluA1 AMPA receptors in PnO GABA neurons

• Enhanced AMPA drive onto PnO GABA neurons implements sleep homeostasis

Web | DOI | PDF | Current Biology | Open Access

 

[SNT Gatchaman]

Both stages of mammalian sleep, non-rapid eye movement sleep (NREMS) and rapid eye movement sleep (REMS), are regulated by homeostatic mechanisms—the longer wakefulness persists, the stronger the urge to sleep becomes, and there is usually then some catching up on lost sleep.

How the brain detects sleep loss and then repays this sleep debt remains a central and unsolved question, especially because the need to catch up on lost sleep suggests there is an important function for sleep. Understanding what is being measured or tracked with wakefulness may reveal this function.

During WAKE and sleep periods, some synapses physically change size, increasing and decreasing, change their composition, change their strength, or even disappear depending on synapse type and brain area, and one function of NREMS could be to rebalance synaptic strength. This is known as the synaptic homeostasis hypothesis (SHY). The essence of SHY is that during wakefulness, neocortical and hippocampal neurons acquire higher levels of excitatory ionotropic glutamate/AMPA receptors, as measured by more GluA1 AMPA receptor subunits during wakefulness than sleep, and more PKA- and calmodulindependent kinase II (CaMKII)-dependent phosphorylation on critical serines in the intracellular domains of GluA1, which produces more excitability. These levels are reset during just a few hours of NREMS.

An intriguing idea is that sleep pressure itself arises from synaptic plasticity. In this model, a longer time awake leads to more strengthening of synapses between sleep pressure-sensing cells and sleep-promoting cells.

Here, we investigated circuitry governing sleep-wake and, in particular, NREMS homeostasis circuitry in the brainstem, in a small region known as the oral pontine reticular nucleus (PnO), a likely component of the ascending reticular activating system.

To our knowledge, this study is the first to report a defined neuronal population altering its spontaneous sleep wake activity profile in response to extended wakefulness. We find that the mechanism for how the initial part of this process is likely implemented, with an upregulation of synaptic scaffold proteins, kinases, and GluA1 AMPA receptors, highly resembles that proposed for SHY.