Second messengers prolactin

The G protein-GTP complexes related to receptors for these hormones activate adenylyl cyclase, which synthesizes the second messenger cAMP. Cyclic AMP activates protein kinases, which phosphorylate certain intracellular proteins (eg, enzymes), thus producing the hormonal effect. Conversely, dopamine binding to lactotroph receptors causes conformational changes in its G protein that reduce the activity of adenylyl cyclase and inhibit the secretion of prolactin. 

The nature of the mechanisms coupling binding of prolactin to its receptor at the plasma membrane to intracellular responses is poorly understood. The effects of the hormone are seen within the cell, including the nucleus, and a second messenger mechanism or its equivalent is therefore expected. Considerable effort has been devoted to investigating possible second messengers for prolactin [26,68,79,80], but there is little clear-cut evidence in favour of any one. 

There has been some work on the nature of the second messengers that mediate the mitogenic actions of prolactin on the crop sac. A role for cyclic GMP has been suggested [100]. Injection of this nucleotide with a small quantity of prolactin into the skin above the crop potentiated the actions of the hormone, although when injected alone the cyclic nucleotide had no effect. 

The nature of the second messenger(s) involved in the action of prolactin on Nb2 cells has been studied [108]. Levels of intracellular cyclic AMP do not change when the cells are treated with prolactin, and cyclic AMP analogues and phosphodiesterase inhibitors do not induce proliferation. Similarly, no clear evidence could be obtained for a clear-cut involvement of prostaglandins in Nb2 cell proliferation. 

---