Inhibition of noradrenergic locus coeruleus neurons by C1 adrenergic cells in the rostral ventral medulla.

Recent anatomical and physiological experiments indicate that the nucleus locus coeruleus receives a predominant excitatory amino acid input, as well as a substantial inhibitory input, from the nucleus paragigantocellularis in the ventrolateral medulla. To determine whether C1 adrenergic neurons are involved in the inhibitory projection, the effects of the alpha-2 adrenoceptor antagonist, idazoxan, on inhibitory responses of locus coeruleus neurons to paragigantocellularis stimulation were characterized in the rat. Intravenous administration of idazoxan (0.2-1 mg/kg) attenuated paragigantocellularis-evoked inhibition, and often revealed an underlying weak excitation. Intraventricular administration of kynurenate, an excitatory amino acid antagonist, eliminated excitation from paragigantocellularis and disclosed an underlying inhibitory response in many locus coeruleus neurons that were previously excited by paragigantocellularis stimulation. These results revealed that about 90% of locus coeruleus neurons receive inhibition from the paragigantocellularis. Intravenous idazoxan significantly reduced such paragigantocellularis-evoked inhibition, completely blocking this response in three of eight locus coeruleus cells tested. Idazoxan was much more potent when locally infused into the locus coeruleus. Local infusion of idazoxan (0.1-2.5 ng) into locus coeruleus produced a dose-dependent decrease of paragigantocellularis-evoked inhibition and completely blocked the inhibition in 10/33 locus coeruleus neurons, indicating that the site of idazoxan action was in the locus coeruleus. These results extend our previous anatomical studies of adrenergic input to locus coeruleus, and indicate that C1 adrenergic neurons in the paragigantocellularis provide a direct inhibitory input to the great majority of locus coeruleus noradrenergic neurons. In addition, this is the first report of a neuronal response to activation of C1 adrenergic cells indicating that these neurons are strongly inhibitory when acting at alpha-2 receptors in vivo.