Differential effect of lesioning of the central amygdala on the bradycardiac and behavioral response of the rat in relation to conditioned social and solitary stress.

The central nucleus of the amygdala (CEA) is considered to be involved in stress-dependent regulation of autonomic functions. In the present study the effects of a bilateral electrolytic lesion of the CEA on the cardiac response and on the duration of immobility behavior in two conditioned stress situations have been analysed in Wistar rats. Fear of a previously received electric footshock elicited in almost all animals a profound early bradycardia and a marked immobility behavior in comparison to non-stressed freely moving controls. Lesioning of the CEA completely abolished the bradycardiac response. Immobility behavior was slightly diminished. The conditioned fear of a dominant rat following social defeat was accompanied by a bradycardiac response in about 50% of the rats. This bradycardiac response was not accompanied by significant immobility behavior. The rest of the defeated rats--i.e. cardiac 'non-responders'--displayed an early increase in immobility behavior. The CEA lesion abolished again the bradycardiac response, but the immobility behavior remained unchanged. In the cardiac 'non-responders' CEA lesion failed to affect immobility behavior, and the heart rate also remained unchanged. These results suggest that the CEA is involved in the organization of conditioned parasympathetic responses independent of the nature of stress. In contrast, the role of this nucleus in the organization of the immobility behavior seems to depend on the nature of stress and other, probably individual characteristics of the rats' behavior.