Lesions of the pedunculopontine tegmental nucleus reduce paradoxical sleep (PS) propensity: evidence from a short-term PS deprivation study in rats

Cholinergic neurons in the mesopontine tegmentum are thought to play a critical role in the generation of paradoxical sleep (PS). However, no study has yet examined whether lesions of these neurons cause deficits of PS in the rat. We describe here the effects of lesions of the pedunculopontine tegmental nucleus (PPT) on spontaneous PS and on PS propensity, expressed during and after a short period of PS deprivation. Lesions were induced by bilateral injections of ibotenate. PS deprivation was performed manually by gently waking rats each time they showed polygraphic signs of PS. Two weeks after lesions, an 8-h baseline recording was performed; the following day, rats were PS deprived for 6 h and polygraphic recordings were then continued for 2 h, to examine recovery sleep. The same protocol was repeated 1 week later. Compared with controls and with rats with limited PPT lesions, rats bearing > 60% NADPH-diaphorase-positive cell loss within the PPT showed unaffected PS under baseline conditions. However, they made fewer attempts to enter PS during deprivation and they exhibited an attenuated rebound increase in PS time after deprivation. The number of PS attempts and the magnitude of PS rebound were negatively correlated with the percent loss of diaphorase-positive neurons within the PPT. Thus, PS propensity that accumulated as a result of PS deprivation was reduced after extensive PPT lesions. In summary, although spontaneous PS was found to be unaltered, the PS deprivation procedure used in this study demonstrated the dysfunctioning of PS caused by PPT lesions.