Differential sensitivity of four central sympathetic pathways to depression by clonidine

The dose-response effects of clonidine HCl (2.5-200 micrograms/kg i.v.) on transmission through somatospinal reflex, viscerospinal reflex, intraspinal, and spinal-bulbospinal reflex pathways were determined in spinal or chloralose-anesthetized cats to assess principle sites of drug action. Evoked sympathetic discharges were recorded from upper thoracic preganglionic rami. Clonidine rapidly produced parallel, dose-dependent depression of transmission through each pathway which was rapidly antagonized by tolazoline or yohimbine. The two spinal reflex pathways were least sensitive to depression which was identical and was limited to 60%. In contrast, both descending pathways could be depressed completely. Although the spinal-bulbospinal reflex pathway was more sensitive to depression than its efferent, descending intraspinal pathway alone, analysis of the relative depression of transmission at spinal and at brainstem levels indicates that the spinal site is more sensitive to clonidine that it is generally considered to be.