Cardiovascular components of the defence reaction evoked by excitation of neuronal cell bodies in the midbrain periaqueductal grey of the cat

In the anaesthetized and paralyzed cat the cardiovascular pattern characteristic of the defence reaction, i.e. skeletal muscle vasodilatation, tachycardia and increased arterial pressure, was elicited by microinjections (0.40 μl) of the excitatory amino acid, dl-homocysteic acid (DLH), into the midbrain periaqueductal grey (PAG) matter. In contrast, injections of DLH into the midbrain tegmentum failed to elicit the cardiovascular components of the defence reaction. As injections of DLH depolarize cell bodies but not axons, the results indicate that the midbrain PAG, but not the adjacent tegmentum, contains a population of neurones whose excitation elicits these reactions. Further, the data suggested that such neurones may be localized preferentially within the caudal half of the midbrain PAG.