Excitatory projections from hypothalamic and midbrain defense regions to nucleus paragigantocellularis lateralis in the rat.

Electrical stimulation of the hypothalamic and midbrain defense regions evoked convergent excitatory responses in neurons in nucleus paragigantocellularis lateralis (PGL) in rats anesthetized with urethane. Stimulation in PGL activated neurons in the periaqueductal gray matter (PAG) and in the hypothalamus antidromically. In the PAG the projecting neurons were localized in the defense region but in the hypothalamus they were more widely distributed lateral and dorsal to it, mainly in the lateral hypothalamic region and the zona incerta. We conclude that there are independent excitatory pathways from the midbrain and hypothalamic defense regions which relay on neurons in PGL. Stimulation in the midbrain defense region may activate the monosynaptic projection to PGL as well as polysynaptic pathways. In contrast, stimulation of the hypothalamic defense region probably excites mainly fibers of passage: the integrated pattern of autonomic changes produced by electrical stimulation may be due to activation of the efferent outflow from widely dispersed cell bodies whose axons run through the defense region.