Hypothalamic–pituitary–adrenal axis the midbrain that mediate the general adaptation syndrome (GAS). While steroid hormones are produced mainly in vertebrates, the physiological role of the HPA axis and corticosteroids in stress response is so fundamental that analogous systems can be found in invertebrates and monocellular organisms as well. The HPA axis, HPG axis, HPT axis, and the hypothalamic–neurohypophyseal system are the four major neuroendocrine systems through which the hypothalamus and pituitary direct neuroendocrine function. The key elements of the HPA axis are: CRH and vasopressin are released from neurosecretory nerve terminals at the median eminence. CRH is transported to the anterior pituitary through
Pituitary adenylate cyclase-activating peptide a maladaptive psychological response to traumatic, i.e. existence-threatening, events. Ressler et al. identified an association of a SNP in the gene coding for pituitary adenylate cyclase-activating polypeptide (PACAP), implicating this peptide and its receptor (PAC1) in PTSD. Pituitary adenylate cyclase-activating peptide has been shown to interact with secretin receptor. Pituitary adenylate cyclase-activating peptide Pituitary adenylate cyclase-activating polypeptide also known as PACAP is a protein that in humans is encoded by the "ADCYAP1" gene. PACAP is similar to vasoactive intestinal peptide. One of its effects is to stimulate enterochromaffin-like cells. It binds to vasoactive intestinal peptide receptor and to the PACAP