ADRENERGIC CONTROL OF THE SECRETION OF ANTERIOR-PITUITARY HORMONES

The hypothalamic hypophysiotrophic neurons are densely innervation by adrenergic and noradrenergic nerve terminals. Activation of α1-adrenoceptors located in the brain stimulates the secretion of ACTH, prolactin and TSH. The effects of the α1-adrenoceptors seem to be exerted on hypothalamic neurones that secrete vasopressin, CRH-41 and TRH. These mechanisms are important in the physiological control of the secretion of ACTH and TSH in humans. α2-Adrenoceptors are not involved in the control of secretion of these hormones under basal conditions in humans. However, α2-adrenoceptors exert an inhibitory effect that acts as a negative feedback mechanism, limiting excessive secretion of these hormones. There is no convincing evidence for the involvement of β-adrenoceptors in the control of the secretion of these three hormones in humans. Studies on cultured anterior pituitary cells suggested that adrenaline and noradrenaline may influence the secretion of ACTH, prolactin and TSH directly at the level of the pituitary. However, these effects are not demonstrable in humans, and are likely to be due to alterations in the pituitary adrenoceptors during culture. In the case of growth hormone, activation of α2-adrenoceptors located in the brain stimulates secretion of this hormone both by increasing the secretion of GHRH and by inhibiting the secretion of somatostatin. Activation of β-adrenoceptors inhibits the secretion of growth hormone via an increase in the secretion of somatostatin. The effects of the central α2- and β-adrenoceptors are important in the physiological control of growth hormone secretion in humans. A considerable amount of evidence implicates brain α1-adrenoceptors in the control of secretion of the gonadotrophins in experimental animals, but, despite intensive study, no convincing evidence has been found in humans of reproductive age. © 1993 Baillière Tindall.