Flight and immobility evoked by excitatory amino acid microinjection within distinct parts of the subtentorial midbrain periaqueductal gray of the cat

Unilateral microinjections of the excitatory amino acid, d,l-homocysteic acid (DLH) made in the lateral and ventrolateral parts of the subtentorial (A 1.0-P 1.5) midbrain periaqueductal gray (PAG) of the freely moving cat evoked two distinct patterns of coordinated somatic changes. When DLH injection (80 nmol) was made within the lateral part of the subtentorial PAG it evoked a flight reaction, characterized by strong locomotion (running) and multiple jumps. This flight reaction was quite distinct from the defensive threat display previously described following DLH microinjection in the lateral part of the pretentorial PAG. When DLH injection (80 nmol) was made in the subtentorial PAG region, ventrolateral to the aqueduct, it elicited a cessation of both spontaneous locomotion and general movements (e.g. licking, scratching, grooming, head and limb movements), a reaction termed immobility. The subtentorial PAG regions from which flight and immobility were evoked are seemingly identical to the lateral and ventrolateral subtentorial PAG regions in which hypertensive and hypotensive reactions have been evoked previously by DLH microinjection. The present results together with our previous studies suggest that: (1) the lateral PAG of the cat contains at least two, topographically separable neuronal pools, which mediate different types of defense reactions (i.e. threat di display - lateral part of the pretentorial PAG; flight reaction - lateral part of the subtentorial PAG); and (2) excitation of neurons in the ventrolateral PAG alters autonomic and somatic functions, but in a direction opposite to that of lateral PAG neurons, namely decreased somatomotor activity and hypotension.