Vasopressin as a neuroendocrine regulator of anterior pituitary hormone secretion

Secretion of the anterior pituitary hormones adrenocorticotropin (ACTH), β-endorphin and prolactin (PRL) is complex and involves a variety of factors. This review focuses on the involvement of arginine-vasopressin (AVP) in neuroendocrine regulation of these anterior pituitary hormones with special reference to receptor involvement, mode of action and origin of AVP. Arginine-vasopressin may act via at least two types of receptors: V 1 − and V 2 −receptors, where the pituitary V 1 −receptor is designated V 1b . The mode of action of AVP may be mediating, i.e. anterior pituitary hormone secretion is transmitted via release of AVP, or the mode of action may be permissive, i.e. the presence of AVP at a low and constant level is required for anterior pituitary hormones to be stimulated. Under in vivo conditions, the AVP-induced release of ACTH and β-endorphin is mainly mediated via activation of hypothalamic V 1 − receptors, which subsequently leads to the release of corticotropin-releasing hormone. Under in vitro conditions, the AVP-stimulated release of ACTH and β-endorphin is mediated via pituitary V 1b − receptors. The mode of action of AVP in the ACTH and β-endorphin response to stress and to histamine, which is involved in stress-induced secretion of anterior pituitary hormones, is mediating (utilizing V 1 − receptors) as well as permissive (utilizing mainly V 1 − but also V 2 −receptors). The AVP-induced release of PRL under in vivo conditions is conveyed mainly via activation of V 1 −receptors but V 2 −receptors and probably additional receptor(s) may also play a role. In stress- and histamine induced PRL secretion the role of AVP is both mediating (utilizing V 1 −receptors) and permissive (utilizing both V 1 − and V 2 − receptors). Arginine-vasopressin may be a candidate for the PRL-releasing factor recently identified in the posterior pituitary gland. Arginine-vasopressin of both magno- and parvocellular origin may be involved in the regulation of anterior pituitary hormone secretion and may reach the corticotrophs and the lactotrophs via three main routes: the peripheral circulation, the long pituitary portal vessels or the short pituitary portal vessels.