Abstract

For the past years there have been not a few reports that suggested the amygdala is responsible for conditioned fear as well as for unconditioned fear. In recent years the attempts have been made to divide the amygdala into several sub-structures and to specify each of them in view of its function. And it is generally accepted that among the sub-divisions the central amygdala takes part in the expression of fear because it projects its fibers most expensively to the brainstem areas which control a variety of fear responses. But with respect to the acquisition of fear, the role of the central amygdala is of much controversy. The present study is therefore to investigate what effects the reversible lesions of the central amygdaloid nucleus have on the acquisition and the expression of fear, using the fear-potentiated startle paradigm as an animal model for measuring fear or anxiety, and accordingly to identify the role of the centra amygdala in fear conditioning. To separate conditioning trial from test trial, the local anesthetic lidocaine was used, and so the experiment consisted of four groups on the basis of lidocaine micro-injected right before the conditioning trial or just before the test trial: lidocaine-saline group, saline-lidocaine group, saline-saline group, and lidocaine-lidocaine group. As a result, the animals which had been lidocaine-injected either before the conditioning trial or before the test trial, showed little increase in the startle responses. But the result cannot be due to the state dependent retrieval failure since the lidocaine-lidocaine group showed no fear-potentiated startle responses either. Thus, the result of the study strongly suggests that the central amygdala is involved not only in the expression of fear but also in the acquisition of fear.