Tonic catecholaminergic inhibition of the spinal somato-sympathetic reflexes originating in the ventrolateral medulla oblongata.

In chloralose-anesthetized cats activity of the spinal and supraspinal components of the somato-sympathetic reflex were evoked in the white ramus at T3 by stimulation of the corresponding intercostal nerve. A blockade of all spinal pathways by means of a reversible cold blockade of the spinal cord at C2-C3 produced the following effects: (1) mean arterial blood pressure fell to 30-50 mm Hg and the tonic background activity in the white ramus was markedly reduced; (2) the amplitude of the spinal reflex was significantly increased and the supraspinal reflex was completely abolished; (3) localized cold block of the dorsolateral funiculus produced the same effect as cold block of the whole spinal cord; (4) neither baroreceptor denervation nor midcollicular decerebration altered these effects; and (5) the alpha-adrenoceptor agonist clonidine reduced the increased amplitude of the spinal reflex during cold blockade; this effect was reversed by the alpha-adrenoceptor antagonist yohimbine. Bilateral cold blockade of areas on the ventrolateral surface of the brain stem between the rootlets of the hypoglossal nerve and the trapezoid body caused the same effect on background and reflex activity in the white ramus as did spinal cord blockade. A mapping of the catecholaminergic (CA) neurons in the lower brain stem of the cat by means of the fluorescence method showed CA neurons in the ventrolateral medulla at two levels: (1) one group of neurons in the caudal medulla, which lies ventral and dorsal to the lateral reticular nucleus (corresponding to area A1 in the rat); and (2) a second group found more cranially and located ventrally to the facial nucleus (corresponding to area A5 in the rat). CA nerve terminals in the spinal cord mainly innervate the intermediolateral cell column. From these findings it is concluded that in the anesthetized cat the spinal component of the somato-sympathetic reflex is modulated by a descending tonic inhibition. This inhibition is independent of baroreceptor input. The pathways descend in the dorsolateral funiculus of the spinal cord, and it is suggested that they originate either in the cranial part of area A1 and/or area A5.