PRESYNAPTIC MUSCARINIC-NICOTINIC MODULATION of GABAergic INPUTS to NEURONS in the REM SLEEP EXECUTIVE AREA of the ROSTRAL PONS.

A large body of evidence indicates that cholinergic neurons in the brainstem tegmental nuclei play a pivotal role in controlling neurons of the reticular nucleus pontis oralis (PnO), an area considered executive for rapid eye movement (REM) sleep onset and maintenance. More recent data indicate that the PnO is also under GABAergic control -which gates REM sleep- thereby highlighting the importance of interactions between cholinergic and GABAergic processes as a key mechanism in REM sleep regulation. Here we employed a rat mesopontine slice preparation to investigate, in vitro, the modulation of GABAergic inputs to PnO neurons by cholinergic agents. Carbachol, a mixed muscarinic-nicotinic cholinergic agonist, provoked either depression or facilitation of single monosynaptic GABAergic IPSCs evoked by extracellular stimulation of the region of brainstem tegmental nuclei. Both effects were presynaptic in origin and are likely attributable to the activation of distinct presynaptic cholinergic receptors as depression was replicated by muscarine (presynaptic inhibition) and facilitation by nicotine (presynaptic facilitation). Furthermore, IPSCs evoked by stimulation patterns mimicking physiological activity (short trains at 15 Hz) were also affected by cholinergic agonists. Muscarine caused presynaptic inhibition accompanied by frequency facilitation and nicotine promoted the opposite effect. Notably, both agonists reduced the total inhibitory charge transferred to postsynaptic neurons during the stimulus train. These findings suggest an additional mechanism by which cholinergic modulation can relieve GABAergic inhibition of PnO neurons under physiological conditions, serving as a local component of the reciprocal inhibitory interactions between sleep-regulating networks in REM sleep brainstem executive areas. NEW & NOTEWORTHYDespite decades of research into brainstem sleep mechanisms, the GABAergic-cholinergic interactions involved in gating and ultradian regulation of REM sleep remain poorly understood. We provide evidence for the functional expression of presynaptic muscarinic and nicotinic cholinergic receptors at GABAergic terminals within the pontine reticular nucleus (PnO), a region central to REM sleep onset and maintenance. Our results indicate that cholinergic signaling might relieve GABAergic inhibition of PnO neurons. As a component of a mutual inhibitory mechanism between REM sleep controlling systems, this interaction may sharpen transitions into REM sleep.