Increased fear reactions in cats after carbachol administered intraventricularly

The effects upon normal defensive behavior of carbachol administered intraventricularly have been studied in the cat. The dose of carbachol (7 – 9 μg) elicited no observable behavioral response. The defense reaction in the animal receiving carbachol was evoked by the presence of a second cat which was impelled to emit threatening behavior by concurrent hypothalamic electrical stimulation. The intensity of withdrawal, backward flattening of the ears, and hissing induced by threat, increased significantly as a consequence of otherwise non-behaviorally active doses of intraventricular carbachol. Rage or attack behavior were not observed at these doses of carbachol. The conclusion drawn from these experiments is that doses of carbachol which are below the threshold for elicitation of overt behavior increase a form of behavior which is thought to be motivated or accompanied by fright or fear.