Regulation of Prolactin Gene Expression by Vasoactive Intestinal Peptide and Dopamine in the Turkey: Role of Ca2+ Signalling

Our recent work has demonstrated that dopamine, acting through D2 dopamine receptors on pituitary cells, inhibits the stimulatory effects of vasoactive intestinal peptide (VIP) on prolactin release and prolactin gene transcription. It is hypothesised that the stimulatory and inhibitory roles of VIP and dopamine, respectively, on prolactin synthesis and release are mediated by their opposite effects on intracellular Ca2+ concentration ([Ca2+]i) in lactotrophs. The present study aimed: (i) to investigate the effect of VIP and dopamine on [Ca2+]i of cultured turkey anterior pituitary cells and (ii) to examine the role of Ca2+ signalling in mediating the regulatory effects of VIP and dopamine on prolactin mRNA levels and prolactin release. Changes in [Ca2+]i were measured spectrofluorometrically using Fura‐2/AM as a fluorescent Ca2+ indicator. Semi‐quantitative reverse transcription‐polymerase chain reaction and radioimmunoassay were used to determine prolactin mRNA levels and prolactin release, respectively. VIP or the L‐type Ca2+ channel activator, Bay K8644 (Bay) increased [Ca2+]i in a concentration‐ and time‐dependent fashion, an effect abolished by preincubating the cells with R(–)‐propylnorapomorphine HCl, a D2 dopamine receptor agonist (D2AG) or Verapamil (VR), a specific L‐type Ca2+ channel blocker. Similarly, either VR or D2Ag diminished the VIP/Bay stimulatory effect on prolactin expression and release. On the other hand, pretreatment of pituitary cells with thapsigargin (TG) or neomycin (NEO), to deplete the intracellular Ca2+ stores, showed no effect on basal or VIP‐stimulated prolactin mRNA levels; although VIP‐ induced prolactin release was partially inhibited by NEO but not TG. These results suggest that intracellular Ca2+ represents a common signal transduction pathway through which VIP and dopamine can exert antagonistic control on prolactin synthesis and release in avian lactotrophs.