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.