Evidence for inhibition of sympathetic preganglionic neurons by bulbospinal epinephrine pathways

Two selective inhibitors of central epinephrine synthesis, LY 134046 and SKF 64139 (20 mg/kg, i.v.), gradually but markedly enhanced descending intraspinal transmission to sympathetic preganglionic neurons in spinal cats. Enhancement increased linearly to maximum values of 200% and 250%, respectively, at 4.5-5.5 h. Spinal sympathetic reflexes were not enhanced by either drug. The results support the proposal that bulbospinal epinephrine pathways depress the excitability of sympathetic preganglionic neurons by activating postsynaptic alpha 2-adrenergic receptors.