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Exposure to stress and the stress hormone corticosterone has been shown to decrease the expression of BDNF in rats, and leads to an eventual atrophy of the hippocampus if exposure is persistent. Similar atrophy has been shown to take place in humans suffering from chronic depression. In addition, rats bred to be heterozygous for BDNF, therefore reducing its expression, have been observed to exhibit similar hippocampal atrophy, suggesting that an etiological link between the development of depressive illness and regulation of BDNF exists. On the other hand, the excitatory neurotransmitter glutamate , voluntary exercise, caloric restriction, intellectual stimulation, and various treatments for depression (such as antidepressants and electroconvulsive therapy) strongly increase expression of BDNF in the brain, and have been shown to protect against this atrophy.