The hypothalamo–pituitary–adrenal (HPA) stress axis. The neuropeptides corticotropin-releasing hormone (CRH) and arginine vasopressin (AVP) are expressed in the parvocelluar neurons of the hypothalamic nucleus paraventricularis. The joint release of CRH and AVP into the portal blood vessels leads to potent stimulation of anterior pituitary ACTH secretion and in turn of corticosterone from the adrenal glands. The activational effects of the HPA axis are counteracted by the inhibitory effects of glucocorticoid receptors expressed in the hippocampus, hypothalamus, and anterior pituitary.