The hypothalamic and brainstem areas from which the cardiovascular and behavioural components of the defence reaction are elicited in the rat.

Electrical stimulation has been employed to map areas of the rat's brain from which the cardiovascular and behavioural components of the defence reaction are elicited and hence to identify the defence areas in this species. In the anaesthetized rat, the cardiovascular pattern of response includes increases in arterial blood pressure and heart-rate, an atropine-resistant vasodilatation in the hind-limb skeletal muscle, with renal and splanchnic vasoconstriction. This was elicited from comparatively well localized areas, not confined to any particular nuclei. Responses were evoked from the rostro-caudal extent of the hypothalamus but most consistently from a region ventral to the fornix. In the midbrain, responsive sites were localized to the dorsal half of the central grey matter, the tegmentum ventro-lateral to it and a ventro-medial region which continued into the pons. Stimulation using implanted electrodes in conscious rats, within the hypothalamic and midbrain areas described above, elicited typical 'flight' and 'escape' behaviour: thus, the localized regions from which the visceral alerting response is elicited contain neurones or nerve fibres integrating the whole defence-alerting response in the rat, as in other species.