Somatic and autonomic integration in the midbrain of the unanesthetized decerebrate cat: a distinctive pattern evoked by excitation of neurones in the subtentorial portion of the midbrain periaqueductal grey.

Microinjections of the excitant amino acid D,L-homocysteic acid (DLH) made in a restricted part of the subtentorial (P0.2-P0.9) midbrain periaqueductal grey (PAG) of the unanesthetized decerebrate cat evoked a distinctive pattern of coordinated somatic and autonomic changes which was characterized by strenuous hindlimb movement and a concomitant vasodilation in the hindlimb vascular bed. The vasodilation was not secondary to movement as it could still be evoked in the paralyzed preparation. The autonomic changes also included pupillary dilation, increases in arterial pressure and heart rate, and vasoconstriction in renal and mesenteric vascular beds. This evoked response is quite different from that elicited by DLH microinjections made in a restricted part of the pretentorial PAG of the unanesthetized cat (Carrive et al., Neurosci. Lett., 81 (1987) 273-278). This latter response is characterized by a threat display which includes strong facial and vocal changes, but no strenuous hindlimb movement, and skeletal muscle vasoconstriction. The present results together with our previous research suggest that two distinct sets of neurons located in different midbrain PAG regions mediate coordinated patterns of somatic and autonomic change characteristics of different aspects of defensive behavior.