Cardiovascular effects of stimulation of neurones within the 'defence area' of the hypothalamus and midbrain of the rabbit.

In anesthetized rabbits, electrical stimulation applied at many sites within the medial hypothalamus and dorsal midbrain elicited a skeletal muscle vasodilatation. Vasodilator responses were only elicited by microinjections of glutamate ions (which excite only cell bodies) into a much more restricted region: the periaqueductal grey (PAG) in the midbrain, and the ventromedial nucleus in the hypothalamus. The results suggest that cell groups within the PAG and ventromedial nucleus are capable of generating skeletal muscle vasodilatation, which is characteristic of the 'defence reaction'. At other sites within the 'defence area', the cardiovascular response to electrical stimulation arises from axons of passage.