Activation of the central nucleus of the amygdala following anxiolytic and anxiogenic drugs: The neural circuits mediating expression of early growth response gene-1

This dissertation was aimed at elucidating the role of the central nucleus of the amygdala (CeA) following administration of systemic and psychogenic stimuli. Psychogenic stimuli include those involved in fear conditioning, restraint, and foot-shock, and are transmitted to the CeA via sensory and somatosensory paths. In comparison, systemic stimuli include cardiovascular, osmotic, and immune challenges, and are transmitted to the CeA via visceral efferent pathways.; Studies measuring changes in expression of immediate-early genes (IEG) have demonstrated increases in the lateral nucleus of the amygdala, but not in the CeA, following fear conditioning. Furthermore, IEG activity decreases in the lateral but increases in the CeA when fear conditioning is blocked using anxiolytic drugs, suggesting that the CeA may be a passive participant in fear conditioning but active when fear is inhibited. However, the CeA is also part of the autonomic system involved in processing and directing visceral information and output, and the effects of anxiolytics in the CeA may be due to visceral stimulation by the drugs. Two sets of studies were conducted to investigate IEG expression in the CeA. The first study looked at IEG expression following anxiolytic and anxiogenic drugs and data revealed that both types of drugs induced IEG activity in the CeA, suggesting that IEG induction in the CeA is not specific to anxiolytic compounds. To determine if the psychogenic properties of diazepam (DZ), conveyed via the BLA complex, were responsible for activation of the CeA, lesions of the BLA complex were performed. In addition, lesions of the nucleus tractus solitarius (NTS) were performed to determine if activation of the CeA was due to visceral stimulation by DZ. These studies revealed that BLA lesions did not reduce diazepam induction of IEG in the CeA, but lesions of the NTS blocked DZ induction in the CeA. These studies clarify the role of the CeA in psychogenic vs. systemic information processing, and suggest an additional role for the CeA in addition to its role during fear conditioning. This data suggests a prominent role of the CeA in processing visceral sensory efferent information and in the maintenance of physiological homeostasis.