Prolactin-releasing hormone, actions

The role of cyclic AMP as modulator of prolactin secretion was first suggested by the finding of a stimulatory effect of cyclic AMP derivatives (17-22) and inhibitors of cyclic nucleotide phosphodiesterase activity such as theophylline and IBMX (22-26) on the secretion of this hormone. More convincing evidence supporting a role of cyclic AMP in the action of dopamine on prolactin secretion had to be obtained, however, by measurement of adenohypophysial adenylate cyclase activity or cyclic AMP accumulation under the influence of the catecholamine. As illustrated in Fig. 1, addition of 100 nM dopamine to male rat hemipituitaries led to a rapid inhibition of cyclic AMP accumulation, a maximal effect (30% inhibition) being already obtained 5 min after addition of the catecholamine. Thus, while dopamine is well known to stimulate adenylate cyclase activity in the striatum (27, 28), its effect at the adenohypophysial level in intact cells is inhibitory. Dopamine has also been found to exert parallel inhibitory effects on cyclic AMP levels and prolactin release in ovine adenohypophysial cells in culture (29) and purified rat mammotrophs (30). Using paired hemipituitaries obtained from female rats, Ray and Wallis (22) have found a rapid inhibitory effect of dopamine on cyclic AMP accumulation to approximately 75% of control. 

Of course there are other chemicals in the body s Action/Reaction Factor closet that positively regulate prolactin. The major ones are GnRH, TRH (thyroid Releasing Hormone) and VIP (Vasoactive Intestinal Polypeptide). By the way, hyper-stimulation of the nipples may have a stimulatory effect upon prolactin release as well. But that is one we will leave alone. 

Nickel ions have been shown to depress the in vivo and in vitro release of prolactin [336], while the release of growth hormone was stimulated, and only at relatively high ion concentrations. Hyperglycemia occurs in rats following intraperitoneal or intratracheal injections of NiCl2 [265, 337, 338], The mechanism of action of nickel appears to be inhibition of insulin release this inhibition could be related to the extremely high concentration of nickel found in the pituitary and the effect on the secretion of the pituitary hormones (growth hormone and adrenocorticotropic hormone). 

Rettori V, Wenger T, Snyder G, Dalterio S, McCann SM. (1988). Hypothalamic action ofdelta-9-tetrahydrocannabinol to inhibit the release of prolactin and growth hormone in the rat. Neuroendocrinology. 47(6) 498-503. 

All antipsychotics except clozapine and perhaps olanzapine produce hyperprolactinemia by removing the inhibitory actions of dopamine on prolactin secretion. This results in amenorrhea, galactorrhea, and infertility in women and in loss of libido and impotence in men. Inhibition of the release of follicle-stimulating and luteinizing hormones may also play a role. In addition, weight gain is common, and food intake must be monitored. 

It is a peptide containing 14 amino acids and inhibits the release of growth hormone, TSH and prolactin from the pituitary and insulin and glucagon in pancreas. It has a very short plasma half-life. Because of its shorter duration of action and lack of specificity in inhibiting only GH secretion, its use in the treatment of acromegaly is limited. 

However, it is now known to exist in various nerve tracts and neuroendocrine tissues and it has general inhibitor actions. It can also inhibit release of other pituitary hormones (including thyroid-stimulating hormone (TSH) and prolactin). other endocrine hormones including pancreatic hormones (insulin and glucagon), peptide hormones from a variety of neuroendocrine tumours (e.g. VIPomas and glucagonomas) and also the release of most intestinal hormones. It is produced in the gut, the pancreas and in some peripheral nerves (see hypothalamic hormones PITUITARY hormones). Somatostatin is a cyclic peptide of 14 residues (SRIF-14) but is formed from a precursor of 28 residues (SRIF-28). 

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