A search for brain stem cell groups integrating the defence reaction in the rat

The defence areas of the rat brain stem have been extensively explored using electrical and chemical stimulation in an attempt to locate the regions containing the perikarya of neurones which may initiate or integrate the visceral and behavioural components of the defence reaction. In rats anaesthetized with alphaxalone-alphadolone, a cannula electrode was used to compare the responses to electrical stimuli with those evoked by microinjection of the excitatory amino acid D,L-homocysteic acid (DLH), at the same site. A total of 128 sites throughout the brain stem was studied in 75 rats. The pattern of visceral and somatic changes characteristic of the defence reaction, viz. increases in arterial blood pressure and heart rate, vasodilatation in hind-limb muscles and vasoconstriction in the kidney, hyperpnoea and tachypnoea, exophthalmos, mydriasis, twitching of the vibrissae and retroflexion of the tail, was evoked by electrical stimulation within well-defined regions of the brain stem, from the anterior hypothalamus to the pons. Microinjection of DLH into the same regions could evoke the full defence reaction, as defined above, but only from the dorsomedial periaqueductal grey matter. Three other regions were defined from which almost all the autonomic components of the defence reaction were evoked, except that blood pressure fell. These were located: immediately dorsal to the optic chiasma, in the medial tuberal region of the hypothalamus and in the lateral pontine tegmentum. In conscious rats with implanted guide cannulae, darting and flight responses were evoked by microinjections of DLH into the periaqueductal grey matter but not from the hypothalamus or tegmentum. Brisk locomotion followed injections of DLH into the region overlying the optic chiasma. It is concluded that the brain stem neurones involved in integrating the somatic and visceral components of the defence reaction are concentrated within the four regions defined above. Whereas neurones in the periaqueductal grey matter can initiate the fully integrated defence reaction, those concentrated in the three other areas cannot be shown to do so. Of these three cell groups, the suprachiasmatic neurones seem to be closer in function to the periaqueductal group than are the neurones in the tuberal hypothalamus and pontine tegmentum.