Relationship between the ventromedullary clonidine-sensitive area and the posterior hypothalamus

The connections between the areas 'S' which have been previously described as the ventromedullary sites of the action of clonidine and the posterior hypothalamus have been investigated. Superficial electrocoagulation of the left area 'S' suppresses the pressor response to electrical stimulation of the homolateral part of the posterior hypothalamus. Although such medullary lesions cause a significant reduction of the mean arterial pressure, the contralateral hypothalamic stimulation can still increase blood pressure. Clonidine it self applied topically (8 micrograms/kg) to the ventral face of the brain stem decreases the blood pressure response to liminal hypothalamic stimulation. It is concluded that efferent pathways, which are involved in vasomotor regulation, originate in the posterior hypothalamus and run through the ventrolateral part of the brain stem. The mechanism of the blocking effect of clonidine on these pathways is discussed.