Sleep homeostasis in lizards and the role of cortex

Although their phenotypes are diverse, slow-wave sleep (SWS) and rapid eye movement sleep (REMS) are the two primary components of electrophysiological sleep (e-sleep) in mammals and birds. Slow waves in the cortex not only characterize SWS but are also used as biological markers for sleep homeostasis, given their rebound after sleep deprivation (SD). Recently, it has been reported that the Australian dragon Pogona vitticeps exhibits two-stage sleep pattern in the dorsal ventricular ridge (DVR), which includes a homologue of the mammalian claustrum (CLA). It remains unclear whether reptilian e-sleep, which has been characterized by activity outside the cortex, compensates for sleep loss, as observed in mammals. We here report a significant rebound in the local field potential (LFP) after 7 hours of SD, during both SWS and REMS. Meanwhile, the cycle and mean bout length of SWS/REMS remained unaffected. We further investigated a possible role of the cortex in e-sleep regulation and homeostasis in Pogona and found that, although a corticotomy had no obvious effect on the LFP features investigated, it abolished LFP power rebound in the CLA/DVR after SD. These findings suggest that e-sleep homeostasis is a common feature in amniotes, and that cortex is involved in regulating activity rebounds in reptiles and mammals.