NREM sleep with low-voltage EEG in the rat.

NREM sleep in the rat has traditionally been defined by electroencephalographic (EEG) amplitudes above those of wakefulness (W) and paradoxical sleep (PS); we refer to this high-amplitude NREM sleep as "HS." We have found that approximately 5% of total time is occupied by episodes in which EEG amplitude is low, distinguishing it from HS; theta amplitude is low, distinguishing it from PS; and electromyographic (EMG) amplitude is low, distinguishing it from W. We have called these low-EEG, low-theta, low-EMG episodes "low-amplitude sleep" (LS). Three studies are done to elucidate additional characteristics of LS. Polygraphically scored 30-s epochs were matched with independent classifications of rat behavior as W, NREM, or PS; 87% of polygraphically scored LS epochs were matched with NREM sleep behavior. Response thresholds to noxious stimuli were lowest in W, intermediate and similar in LS and HS, and highest in PS. The incidence of PGO-type (ponto-geniculo-occipital) waves in W, HS, and LS were all very low in comparison with rates in PS. Thus, LS and HS exhibited similarly quiescent spontaneous behavior, similar intermediate response thresholds, and similar low rates of PGO-type activity. Accordingly, we have proposed that LS, along with HS, is an NREM sleep stage.