GABA_A receptors are located in cholinergic terminals in the nucleus pontis oralis of the rat: Implications for REM sleep control

The oral pontine reticular formation (PnO) of rat is one region identified in the brainstem as a rapid eye movement (REM) sleep induction zone. Microinjection of GABA_A receptor antagonists into PnO induces a long lasting increase in REM sleep, which is similar to that produced by cholinergic agonists. We previously showed that this REM sleep-induction can be completely blocked by a muscarinic antagonist, indicating that the REM sleep-inducing effect of GABA_A receptor antagonism is dependent upon the local cholinergic system. Consistent with these findings, it has been reported that GABA_A receptor antagonists microdialyzed into PnO resulted in increased levels of acetylcholine. We hypothesize that GABA_A receptors located on cholinergic boutons in the PnO are responsible for the REM sleep induction by GABA_A receptor antagonists through blocking GABA inhibition of acetylcholine release. Cholinergic, varicose axon fibers were studied in the PnO by immunofluorescence and confocal, laser scanning microscopy. Immunoreactive cholinergic boutons were found to be colocalized with GABA_A receptor subunit protein γ2. This finding implicates a specific subtype and location of GABA_A receptors in PnO of rat in the control of REM sleep.