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Am J Physiol. 1992 Sep;263(3 Pt 2):R544-52.

Hypothalamic and cortical sympathetic responses relay in the medulla of the rat.

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John P. Robarts Research Institute, University of Western Ontario, London, Canada.


Previously, investigations have indicated that the efferent pathway for sympathetic responses originating in the insular cortex (IC) must initially synapse in the lateral hypothalamic area (LHA). The LHA projects to both the ventrolateral medulla (VLM) and directly to the thoracic spinal cord. To determine the role of the VLM in mediating sympathetic responses from the IC and the LHA, in alpha-chloralose-anesthetized rats, renal nerve responses were recorded following electrical stimulation of these two forebrain sites before and after bilateral injection (300 nl) of cobalt (a synaptic blocking agent) into the VLM. The results demonstrated that a complete block of the increase in sympathetic nerve activity following stimulation of the IC or the LHA could be obtained with cobalt injections into the VLM. The most effective injection sites were located in the rostral ventrolateral medulla at the rostrocaudal middle and posterior regions of the C1 group of neurons. Chemical stimulation with DL-homocysteic acid, in the LHA, to activate cell bodies only evoked a decrease in arterial blood pressure and sympathetic nerve activity. These responses were also blocked by cobalt injection into the VLM. Injections of cobalt into the nucleus of the solitary tract did not block sympathetic responses elicited from the LHA. These results indicate that the efferent pathway for sympathetic responses from the IC through the LHA goes to the preganglionic region of the spinal cord via a mandatory synapse in the VLM.

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