EVIDENCE THAT STIMULATORY DOPAMINE-RECEPTORS MAY BE INVOLVED IN THE REGULATION OF PROLACTIN SECRETION

It is well established that dopamine (DA) effectively inhibits PRL sec retion from anterior pituitary mammotropes via D-2-DA receptors. Parad oxically, it is reported that the monoamine can actually increase PRL release under appropriate experimental conditions. Although the mechan ism underlying this stimulatory effect remains undefined, the ability of D-1- and D-5-DA receptors to activate adenylyl cyclase raises the p ossibility that a similar receptor subtype is present in the anterior pituitary and mediates the stimulatory effects of DA on PRL release. T he purpose of the present study was to explore this possibility. First , we tested whether D-1 and D-5 receptors could couple to and stimulat e PRL secretion. Subclones of GH(4)C(1) cells (which secrete PRL, but do not express DA receptors) stably expressing human D-1 or D-5 recept ors were treated with DA (10(-16)-10(-6) M), and the medium PRL conten t was measured by RIA. Subclones transfected with short or long forms of the human D-2 receptor were also tested. As expected, DA (10(-6) M) inhibited PRL release from cells expressing either short or long D-2 receptors by 41% and 39%, respectively (P < 0.01; n = 4 separate exper iments). In contrast, comparable concentrations of DA (10(-8) and 10(- 6) M) increased PRL release from cells expressing D-1 or D-5 receptors by 76% and 122%, respectively (P < 0.01; n = 4). Thus, both D-1 and D -5 receptors were fully capable of stimulating PRL release from transf ected GH(4)C(1) cells. We next sought to determine whether the gene fo r at least one of these structurally similar receptors was expressed i n rat anterior pituitary tissue. First strand cDNA was synthesized, us ing a rat D-5-specific oligonucleotide primer and reverse transcriptas e, from total RNA extracted from the anterior pituitary glands of five lactating female rats. The specifically primed cDNA then served as a template for 35 cycles of polymerase chain reaction amplification in w hich nested primers specific for the rat D-5 receptor were used. Elect rophoresis of the DNA resolved a 696-basepair band corresponding to a fragment of the D-5 receptor in each of five anterior pituitary sample s (verified by digestion with three different restriction endonuclease s). Taken together, these results demonstrate that both D-1 and D-5 re ceptors are capable of mediating the stimulatory effects of DA on PRL release and that the mRNA for DA D-5 receptors is present in rat anter ior pituitary glands. Our findings support the view that PRL release i n vivo may be modulated via one or more stimulatory DA receptors.