Long-term changes in sleep and electroencephalographic activity by chronic vagus nerve stimulation in cats

We previously reported the effect of vagus nerve electrical stimulation (VNS) on sleep and behavior in cats. The aim of the present study is to analyze the long-term effects of VNS on the electroencephalographic (EEG) power spectrum and on the different stages of the sleep-wakefulness cycle in the freely moving cat. To achieve this, six male cats were implanted with electrodes on the left vagal nerve and submitted to 15 rounds of 23 h continuous sleep recordings in three categories: baseline (BL), VNS and post-stimulus recording (PSR). The following parameters were analyzed: EEG power spectrum, total time and number of sleep phases, ponto-geniculo-occipital (PGO) wave density of the rapid eye movement (REM) sleep, and the number of times the narcoleptic reflex was present (sudden transition from wakefulness to REM sleep). Significant changes were detected, such as an enhancement of slow-wave sleep (SWS) stage II; a power increase in the bands corresponding to sleep spindles (8-14 Hz) and delta waves (1-4 Hz) with VNS and PSR; an increase in the total time, number of stages, and density of PGO wave in REM sleep with VNS; a decrease of wakefulness in PSR, and the eventual appearance of the narcoleptic reflex with VNS. The results show that the effect of the VNS changes during different stages of the sleep-wakefulness cycle. In REM sleep, the effect was present only during VNS, while the SWS II was affected beyond VNS periods. This suggests that ponto-medullar and thalamic mechanisms of slow EEG activity may be due to plastic changes elicited by vagal stimulation.