09 April 2008
Bromocriptine or other dopamine agonists are usually effective for the treatment of prolactin-secreting adenomas. Five to 18% of prolactinomas, however, do not respond to such therapy. We have shown previously that such resistance to bromocriptine correlates with reduced binding to the D<sub>2</sub> receptor subtype of dopamine, the major PRL inhibiting factor. In the present work, we demonstrated that reduced binding actually corresponds to decreased expression of the gene coding for the D<sub>2</sub> receptor in the pituitary from bromocriptine-resistant patients, as shown by 4-fold lower levels of the corresponding mRNAs compared to those coding for actin. The existence of two D<sub>2</sub> receptor isoforms, D<sub>2</sub>S and D<sub>2</sub>L generated by alternative splicing, has been described in several tissues, including the pituitary. Both are negatively coupled to adenylyl cyclase and inhibit prolactin secretion, but, in addition, the shortest one (D<sub>2</sub>S) is more efficiently coupled to phospholipase C. Consequently, we also investigated whether expression of a particular D<sub>2</sub> receptor isoform was preferentially affected in resistant adenomas. The proportion of messengers corresponding to the short receptor isoform (D<sub>2</sub>S) was lower in resistant compared to responsive adenomas: D<sub>2</sub>S/D<sub>2</sub>L = 0.74 ± 0.08 and 1.00 ± 0.07, respectively. In parallel, much lower levels of D<sub>2</sub> receptor mRNAs were found in growth hormone-secreting adenomas, with a D<sub>2</sub>S/D<sub>2</sub>L ratio comparable to those of both normal human pituitary and bromocriptine-sensitive prolactinomas (1.05 ± 0.11). Thus, resistance to bromocriptine therapy seems to involve defects in D<sub>2</sub> dopamine receptor expression and possibly in posttranscriptional splicing.