| Summary: | We thank Grace (1) for the opportunity to discuss the role of cholinergic neurons in rapid eye movement (REM) sleep further. Grace suggests that optogenetic activation of a population of neurons does not necessarily demonstrate their role in the endogenous system when interrogating complex neural circuitry. We agree that we do not prove necessity of cholinergic neurons in REM sleep generation, as we point out in our discussion, "Future studies that selectively inhibit cholinergic neurons in the PPT [pedunculopontine tegmentum] and LDT [laterodorsal tegmentum] of nonhypercholinergic mice are needed to determine if cholinergic neurons are necessary for REM sleep generation" (2). However, in our report we do demonstrate the sufficiency of PPT/LDT neurons to influence REM sleep initiation but not influence REM sleep duration, thus distinguishing the role of cholinergic neurons on these properties of REM sleep. In addition, activation of cholinergic PPT neurons during non-REM sleep induced REM sleep versus wakefulness. Our data are consistent with the role of cholinergic neurons in generating an activated brain state and many studies pointing to the role of cholinergic neurons in REM sleep regulation (reviewed in ref. 3).
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