Role of the amygdaloid nuclei in the modulation of tonic immobility in guinea pigs

Tonic immobility (TI) is thought to represent the terminal reaction in the chain of antipredatory responses involved in maintaining survival. TI is an inhibitory behavioral response in which the animal presents a significant decrease in body activity and responsiveness to the environment induced by some form of physical restraint. This response is induced in the laboratory by inversion of the animal and brief postural contention of its movements. Tn nature, the TI response may be triggered by some threatening pr predato ry stimulus, indicating the physical contact between response occurs when t here is physical contact between prey and predator. In this case, the physi cal inactivity of the prey may prevent the continuation of the attack. The neural substrate of this response is not well known, and the objective of t he present study was to investigate the effect of cholinergic stimulation o f amygdala regions on TI modulation in guinea pigs. Microinjection of carba chol (0.5 mu g/0.2 mu L) into the central (CEA), basolateral (BLA), and lat eral posterior (LPA) nuclei of the amygdala promoted a reduction in the dur ation of TI episodes. Pretreatment with atropine (0.5 mu g/0.2 mu L) showed that the action of carbachol is mediated by muscarinic receptors. (C) 1999 Elsevier Science Inc.