Excitation of neurones in a restricted portion of the midbrain periaqueductal grey elicits both behavioural and cardiovascular components of the defence reaction in the unanaesthetised decerebrate cat.

Microinjections of the excitant amino acid D,L-homocysteic acid (DLH) into a restricted part of the midbrain periaqueductal grey (PAG) of unanaesthetized decerebrate cats evoked a distinctive pattern of facio-vocal and cardiovascular changes characteristic of a defence reaction, including pupillary dilatation, howling vocalization, an increase in arterial pressure and heart rate, and skeletal muscle vasoconstriction. These facio-vocal and cardiovascular responses always occurred together, and thus may arise from excitation of a common population of neurones. DLH injections within a greater extent of the PAG elicited other facio-vocal changes characteristic of defence, such as hissing or growling, but these were not accompanied by significant cardiovascular changes.