Alpha-1 Adrenergic Receptors in the Medial Preoptic Area are Involved in the Induction of Sleep

This paper reviews the recent studies that led to the conclusion that the noradrenergic neurons projecting to the medial preoptic area (mPOA) are hypnogenic and that they mediate this action through α1 adrenergic receptors. Microinjection of noradrenaline (NA) into the mPOA induced arousal. Studies using α2 adrenergic drugs showed that the arousal induced by intrapreoptic injection of NA was due to its action on presynaptic α2 adrenergic receptors. A combination of lesion and chemical stimulation techniques demonstrated that when NA acted on the postsynaptic α1 receptors in the mPOA, it induced sleep. Intrapreoptic injection of α1 agonist, methoxamine could induce sleep, when the hypothermia, which was simultaneously produced, was behaviorally compensated for by the animal. Increased arousal produced by the destruction of noradrenergic fibers in the mPOA further confirmed the hypnogenic role of these fibers.