GABAA 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 GABAA 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 GABAA receptor antagonism is dependent upon the local cholinergic system. Consistent with these findings, it has been reported that GABAA receptor antagonists microdialyzed into PnO resulted in increased levels of acetylcholine. We hypothesize that GABAA receptors located on cholinergic boutons in the PnO are responsible for the REM sleep induction by GABAA 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 GABAA receptor subunit protein γ2. This finding implicates a specific subtype and location of GABAA receptors in PnO of rat in the control of REM sleep.