Complex sentence: Increased expression of hippocampal glucocorticoid receptors causes the hippocampus to release less ACTH-releasing hormone, which acts on the pituitary gland to release less ACTH, which in turn acts on the adrenal glands to release less cortisol.
Split into simpler sentences: Increased expression of hippocampal glucocorticoid receptors causes the hippocampus to release less ACTH-releasing hormone, which acts on the pituitary gland to release less ACTH. In turn, ACTH acts on the adrenal glands to release less cortisol.