Microinjection of glutamate into the pedunculopontine tegmentum induces REM sleep and wakefulness in the rat

The aim of this study was to test the hypothesis that the cells in the brain stem pedunculopontine tegmentum (PPT) are critically involved in the normal regulation of wakefulness and rapid eye movement (REM) sleep. To test this hypothesis, one of four different doses of the excitatory amino acid L-glutamate (15, 30, 60, and 90 ng) or saline (control vehicle) was microinjected unilaterally into the PPT while the effects on wakefulness and sleep were quantified in freely moving chronically instrumented rats. All microinjections were made during wakefulness and were followed by 6 h of polygraphic recording. Microinjection of 15-ng (0.08 nmol) and 30-ng (0.16 nmol) doses of L-glutamate into the PPT increased the total amount of REM sleep. Both doses of L-glutamate increased REM sleep at the expense of slow-wave sleep (SWS) but not wakefulness. Interestingly, the 60-ng (0.32 nmol) dose of L-glutamate increased both REM sleep and wakefulness. The total increase in REM sleep after the 60-ng dose of L-glutamate was significantly less than the increase from the 30-ng dose. The 90-ng (0.48 nmol) dose of L-glutamate kept animals awake for 2-3 h by eliminating both SWS and REM sleep. These results show that the L-glutamate microinjection into the PPT can increase wakefulness and/or REM sleep depending on the dosage. These findings support the hypothesis that excitation of the PPT cells is causal to the generation of wakefulness and REM sleep in the rat. In addition, the results of this study led to the identification of the PPT dosage of L-glutamate that optimally induces wakefulness and REM sleep. The knowledge of this optimal dose will be useful in future studies investigating the second messenger systems involved in the regulation of wakefulness and REM sleep.