PARTIAL DOPAMINE D(2)-RECEPTOR AGONISTS ANTAGONIZE PROLACTIN-REGULATING D(2)-RECEPTORS IN A TRANSFECTED CLONAL CELL-LINE (GH4ZR7)

(-)-(3-Hydroxyphenyl)-N-n-propylpiperidine ((-)-3-PPP) and transdihydrolisuride (terguride, TDHL) are partial dopamine D2 receptor agonists displaying low intrinsic activity at normosensitive postsynaptic dopamine D2 receptors in brain while activating prolactin-regulating dopamine D2 receptors in male rats with an efficacy close to that of full dopamine D2 receptor agonists. In the present study we examined the effects of these partial dopamine D2 receptor agonists on prolactin release in vitro. For this purpose prolactin-producing tumour cells which have been transfected with the dopamine D2 receptor cDNA and hence which express a dopamine D2 receptor (short isoform) that is functionally active with respect to inhibition of adenylate cyclase and prolactin release (GH4ZR7; Albert et al., J. Biol. Chem. (1990) 265, 2098) were used. While the full dopamine D2 receptor agonists, quinpirole, (+)-(3-hydroxyphenyl)-N-n-propylpiperidine ((+)-3-PPP) and dopamine induced a dose-dependent suppression of vasoactive intestinal peptide (VIP)-induced prolactin release from GH4ZR7, both (-)-3-PPP and terguride were inactive. Furthermore, the prolactin-suppressive effects of dopamine and quinpirole were significantly antagonized by pretreatment with (-)-3-PPP and terguride. It is concluded that partial dopamine D2 receptor agonists, which activate male lactotroph dopamine D2 receptors in vivo, may antagonize dopamine D2 receptors on GH4ZR7 cells. There were similar results from an experiment using monolayers of anterior pituitary cells from male rats. Thus, in this in vitro preparation (+)-3-PPP suppressed spontaneous prolactin release while (-)-3-PPP again was inactive. Possible explanations, are offered for the ineffectiveness of partial dopamine D2 receptor agonists with respect to the suppression of prolactin release in vitro.