".... recent studies have shown that if rats are given injections of adrenal steriods at levels that mimic very severe stress, there is a dramatic decrease in the amount of a certain chemical, called corticotropin-releasing factor (CRF), in the part of the hypothalmus that controls the release of the stress hormone, ACTH, from the pituitary gland. CRF is in fact the neurotransmitter that stimulates ACTH release. The decrease in CRF in this pathway reflects the negative feedback control over stress hormones by the hippocampus - once the blood level of adrenal steriods reaches a certain level, the hippocampus tells the hypothalmus to slow down the secretions. And when the steriod level reaches a critical point, the hippocampal circuits begin to falter. In stark contrast, there is a dramatic increase in CRF in the central nucleus of the amygdala under the same conditions - as blood levels of steriods increase, the amygdala may keep getting more and more active. The bottom line is that the effects of stress on the amygdala seem very different from the effects on the hippocampal-hypothalmic circuit."On the basis of these observations, ... (we) ... predicted that during intense stress the learning and memory processes mediated by the amygdala might be facilitated and we examined the effects of stress hormone overload on conditioned fear behavior. In line with the prediction, we found that the strength of learned fear was increased in the steriod-treated rats relative to other rats that didn't have the steriods. Although this result is somewhat preliminary, studies using other forms of Pavlovian conditioning have also found that stress enhances conditioned responses.
"If indeed the hippocampus is impaired and the amygdala facilitated by stress, it would suggest the possibility that stress shifts us into a mode of operation in which we react to danger rather than think about it. It's not clear whether this is a specific adaptation or whether we're just lucky that when the higher functions break down our fallback position is one in which we can let evolution do the thinking for us."