Effect of clonidine on the excitability of vasomotor loci in the cat.

1. The effect of clonidine on the direct excitability of hypothalamic, medullary and spinal vasomotor loci has been investigated in cats anaesthetized with chloralose. 2. Clonidine inhibited the excitability of these loci when it was localized to the central sites by intracerebroventricular, intravertebral arterial or intrathecal injection in very low doses (1-2 mug). 3. Topical application of clonidine (0.01 percent and 1.0 percent) to the floor of the fourth ventricle inhibited pressor responses evoked either by stimulation of medullary or hypothalamic vasomotor areas. Inhibition of the pressor responses was accompanied by hypotension and bradycardia in many experiments. 4. It appears that effects of clonidine on the vasomotor loci of the medulla oblongata and the spinal cord contribute to its hypotensive action.