Neurochemical bases of defensive behavior in animals.

This paper presents a literature review and the author's own investigations devoted to identifying the morphological organization of different subcortical structures and neurochemical processes involved in the (regulation of defensive behavior. The effects of intrahypothalamic injections of two cholinomimetic substances, carbachol and d-tubocurarine, on rage and fear reactions were explored. Also the investigations were carried out with intrahypothalamic injections of anticholinergic substances, atropine and betamon or hexamethonium, which block selectively muscarinic and nicotinic receptors. The results indicate that carbachol injected into the hypothalamus after blocking the muscarinic receptors with atropine fails to elicit any defensive response; whereas carbachol injection administrated after nicotinic receptors had been blocked by betamon or hexamethonium induces full aggressive behavior. This indicated that carbachol acts, on the hypothalamic level, through the muscarinic receptors. The mechanism of d-tubocurarine action seems to be more complex because this compound induces fear response after either the muscarinic or the nicotinic receptors have been blocked. A possible mechanism based on chemically differentiated neuronal circuits at the hypothalamic level for two different defensive drives, fear and rage, is presented.