Comparison of the forms of the dopamine D2 receptor expressed in GH4C1 cells

Estrogen inhibits D2S receptor-regulated Gi3 and Gs protein interactions to stimulate prolactin production and cell proliferation in lactotropic cells

The neurotransmitter dopamine (DA) is known to inhibit prolactin (PRL) secretion and the proliferation of lactotropes in the pituitary gland. Dopamine-2 (D2) receptor short (D2S) isoform is expressed in a reduced level while the D2 receptor long (D2L) isoform is expressed in an elevated level during estradiol (E(2))-induced PRL production and cell proliferation in lactotropes. To evaluate the role of these D2 receptor isoforms in E(2)-regulated lactotropic cell function, we compared E(2) effects on the level of PRL, cell proliferation, and G proteins in enriched lactotropes and lactotrope-derived PR1 cells containing only D2S isoform (D2S cells), D2L isoform (D2L cells), or no D2 receptor (V cells). Additionally, we determined the effects of G protein blockade on the E(2)-induced PRL production and cell proliferation in these cells. We here show that E(2) actions on G proteins, PRL production, and cell proliferation were maximally achieved in D2S cells, oppositely or marginally achieved in D2L cells, and absent in V cells. We also show that the DA and pertussis toxin modulations of E(2) actions on PRL, G proteins, and cell proliferation were maximally achieved in D2S cells compared with in D2L or V cells. Furthermore, we provide evidence for the existence of an inhibitory action of Gi3 on Gs that is under the control of the D2S receptor and is inhibited by E(2). These results suggest that the suppression of D2S-regulated Gi3 inhibition of Gs protein may be one of the mechanisms controlling E(2)-activated PRL synthesis and cell proliferation in lactotropes.

The novel antipsychotic aripiprazole is a partial agonist at short and long isoforms of D2 receptors linked to the regulation of adenylyl cyclase activity and prolactin release

Aripiprazole is a novel antipsychotic with a unique mechanism of action, which differs from currently marketed typical and atypical antipsychotics. Aripiprazole has been shown to be a partial agonist at the D(2) family of dopamine (DA) receptors in biochemical and pharmacological studies. To demonstrate aripiprazole's action as a partial D(2) agonist in pituitary cells at the molecular level, we retrovirally transduced the short (D(2S)) and the long (D(2L)) form of the human DA D(2) receptor gene into a rat pituitary cell line, GH4C1. [(3)H]-raclopride saturation binding analyses revealed a B(max) value approximately four-fold higher at D(2S) receptor-expressing GH4C1 cells than at D(2L) receptor-expressing GH4C1 cells, while a K(d) value was similar. Aripiprazole inhibited forskolin-stimulated release of prolactin in both D(2S) and D(2L) receptor-expressing GH4C1 cells, whereas the maximal inhibition of prolactin release was less than that of DA. Similarly, aripiprazole partially inhibited forskolin-induced cAMP accumulation in both D(2) receptor-expressing cells. Aripiprazole antagonized the suppression attained by DA (10(-7) M) in both D(2) receptor-expressing cells and, at the maximal blockade of cAMP, yielded residual cAMP levels equal to those produced by aripiprazole alone. These results indicate that aripiprazole acts as a partial agonist at both D(2S) and D(2L) receptors expressed in GH4C1 cells. These data may explain, at least in part, the observations that aripiprazole shows a novel antipsychotic activity with minimal potential for adverse events including no significant increase of serum prolactin levels in clinical studies.

G protein preferences for dopamine D2 inhibition of prolactin secretion and DNA synthesis in GH4 pituitary cells

Dopamine is the primary inhibitory regulator of lactotroph proliferation and prolactin (PRL) secretion in vivo, acting via dopamine D2 receptors (short D2S and long D2L forms). In GH4C1 pituitary cells transfected with D2S or D2L receptor cDNA, dopamine inhibits PRL secretion and DNA synthesis. These actions were blocked by pertussis toxin, implicating G(i)/G(o) proteins. To address roles of specific G(i)/G(o)4 proteins in these actions a series of GH4C1 cell lines specifically depleted of individual Galpha subunits was examined. D2S-mediated inhibition of BayK8644-stimulated PRL secretion was primarily dependent on G(o) over G(i), as observed for BayK8644-induced calcium influx. By contrast, inhibitory coupling of the D2S receptor to TRH-induced PRL secretion was partially impaired by depletion of any single G protein, but especially G(i)3. Inhibitory coupling of D2L receptors to PRL secretion required G(o), but not G(i)2, muscarinic receptor coupling was resistant to depletion of any G(i)/G(o) protein, whereas the 5-HT1A and somatostatin receptors required G(i)2 or G(i)3 for coupling. The various receptors also demonstrated distinct G protein requirements for inhibition of DNA synthesis: depletion of any G(i)/G(o) subunit completely uncoupled the D2S receptor, the D2L receptor was uncoupled by depletion of G(i)2, and muscarinic and somatostatin receptors were resistant to depletion of G(i)2 only. These results demonstrate distinct receptor-G protein preferences for inhibition of TRH-induced PRL secretion and DNA synthesis.

Prolactin: structure, function, and regulation of secretion

Prolactin is a protein hormone of the anterior pituitary gland that was originally named for its ability to promote lactation in response to the suckling stimulus of hungry young mammals. We now know that prolactin is not as simple as originally described. Indeed, chemically, prolactin appears in a multiplicity of posttranslational forms ranging from size variants to chemical modifications such as phosphorylation or glycosylation. It is not only synthesized in the pituitary gland, as originally described, but also within the central nervous system, the immune system, the uterus and its associated tissues of conception, and even the mammary gland itself. Moreover, its biological actions are not limited solely to reproduction because it has been shown to control a variety of behaviors and even play a role in homeostasis. Prolactin-releasing stimuli not only include the nursing stimulus, but light, audition, olfaction, and stress can serve a stimulatory role. Finally, although it is well known that dopamine of hypothalamic origin provides inhibitory control over the secretion of prolactin, other factors within the brain, pituitary gland, and peripheral organs have been shown to inhibit or stimulate prolactin secretion as well. It is the purpose of this review to provide a comprehensive survey of our current understanding of prolactin's function and its regulation and to expose some of the controversies still existing.

Relationships between dopamine-induced changes in cytosolic free calcium concentration ([Ca2+]i) and rate of prolactin secretion. Elevated [Ca2+]i does not indicate prolactin release

This study was undertaken to investigate the relationship between dopamine (DA) induced changes in the cytosolic calcium concentration ([Ca2+]i) and the rate of prolactin secretion using GH4ZR7, a rat pituitary cell line, which express only one subtype of D2 receptor. GH4ZR7 cells were loaded with Fluo-3, a fluorescent Ca2+ indicator, and then perifused with two different doses of DA (10(-7) mol/L and 5 x 10(-4) mol/L). We monitored changes in [Ca2+]i and rate of prolactin release simultaneously by attaching a spectrofluorometer to a dynamic perifusion system. DA has stimulatory and inhibitory effect on prolactin secretion in GH4ZR7 cells; 10(-7) mol/LDA slightly increased [Ca2+]i and stimulated prolactin release, whereas 5 x 10(-4) mol/LDA decreased [Ca2+]i and inhibited prolactin secretion. When the cells were pretreated with pertussis toxin (PTX), 10(-7) mol/L DA had no significant change in [Ca2+]i while stimulating prolactin release, and 5 x 10(-4) mol/L DA reduced [Ca2+]i without having any significant effect on the rate of prolactin secretion. The results of this study demonstrate that changes in [Ca2+]i do not always correlate with the rate of prolactin release from lactotrophs. The dissociation between [Ca2+]i and prolactin release is somewhat expected considering the diverse role of [Ca2+]i and post-[Ca2+]i events, which can change the rate of prolactin release.

Dopamine D2 receptor mediates both inhibitory and stimulatory actions on prolactin release

Dopamine is considered to be the major physiological tonic inhibitor of prolactin release, yet there is increasing evidence showing that it can also stimulate prolactin release from lactotrophs. In primary cultured lactotrophs, the major dopamine receptors responsible for inhibiting prolactin release are dopamine D2 receptors. A dopamine receptor subtype may be responsible for the stimulatory action, yet one cannot exclude the possibility that a dopamine D2 receptor can play dual roles. This study was therefore undertaken to investigate if dopamine both stimulates and inhibits prolactin secretion through activation of the same dopamine D2 receptor. GH4ZR7 cells, which have only one type of dopamine receptors--D2s, were perifused with different concentrations of dopamine, and the perifusate was assayed for prolactin; 10(-7) mol/L dopamine stimulated prolactin release (p < 0.05; n = 5), whereas 5 x 10(-4) mol/L dopamine inhibited prolactin secretion (p < 0.05; n = 5). In the pertussis toxin-treated cells, 10(-7) mol/L dopamine stimulated prolactin release (p < 0.05; n = 5), and 5 x 10(-4) mol/L dopamine did not significantly change the rate of prolactin release. These results indicate that both the stimulatory and inhibitory actions of dopamine are likely mediated by the same D2 receptor subtype, since GH4ZR7 cells express only D2s receptors. They also confirm that the inhibitory action of dopamine is mediated through a Gi protein; and the stimulatory action of dopamine is mediated through a PTX-insensitive pathway. These findings suggest that D2 receptors are coupled to both Gi and Gs proteins.

Molecular mechanisms underlying forskolin-mediated up-regulation of human dopamine D2L receptors

1. Human dopamine (DA) D2long (hD2L) receptors, expressed by Ltk- cells, can be up-regulated by treating the cells with forskolin for 16 hr (Johansson and Westlind-Danielsson, 1994). We have examined some of the molecular mechanisms underlying this forskolin-mediated up-regulation. 2. Forskolin (100 microM, 16 hr), but not 1,9-dideoxyforskolin, a forskolin analogue that is unable to activate adenylyl cyclase and raise intracellular cAMP concentrations, up-regulates the hD2L receptor population by 43%. The implication of a cAMP-dependent increase in the receptor up-regulation was further substantiated by treating the cells with 8-bromo-cAMP or prostaglandin E1 (PGE1). The forskolin-mediated rise in receptor number was blocked by cycloheximide or an antisense phosphorothioate oligodeoxynucleotide (ODN) directed toward the hD2L mRNA. KT5720, a specific protein kinase A (PKA) inhibitor, completely blocked the receptor rise, whereas pertussis toxin (PTX) attenuated the increase considerably. Forskolin also produced an increase in the level of the DA hD2short (hD2S) receptor expressed by Ltk- cells. This increase was 2.5-fold higher than that found for the hD2L receptor. 3. The forskolin-mediated hD2L receptor rise is dependent on de novo protein synthesis, a rise in cAMP levels, PKA activation, and, at least partially, PTX-sensitive G proteins. 4. Long-term increases in intracellular cAMP levels may change the sensitivity of a DA receptor expressing cell to DA by increasing D2 receptor density through enhanced cAMP-dependent transcription.

Functional expression of olfactory-adrenergic receptor chimeras and intracellular retention of heterologously expressed olfactory receptors

Replacing the G-protein-coupling domains of the beta 2-adrenergic receptor with homologous domains of putative olfactory receptors produced chimeric receptors which were able to stimulate pigment dispersion in Xenopus melanophores, a G-protein-mediated pathway. A multiple replacement chimera containing the second, third and C-terminal cytoplasmic domains of receptor OR5 elevated cyclic adenosine 3':5'-monophosphate (cAMP) and suppressed production of inositol phosphates. Co-expression of G alpha olf did not alter the strength of response of this chimera. A novel rat olfactory receptor cDNA (U131) was isolated and sequenced. Expression of U131 and OR5 constructs containing an N-terminal epitope-tag or C-terminal fusion to green fluorescent protein occurred in an intracellular network but not in the plasma membrane of heterologous cells. Similarly treated beta 2-adrenergic receptors were functional and were observed in the plasma membrane and the intracellular network. These results demonstrate that the putative cytoplasmic domains of olfactory receptors are capable of functional interaction with heterologous G-proteins of the G alpha s subtype. Instead, the absence of these receptors from the plasma membrane of heterologous cells appears to explain our inability to determine if odorants can activate the olfactory receptor clones. We hypothesize that the olfactory receptors have requirements for maturation and targeting to the plasma membrane that are different from most other G-protein-coupled receptors.

Effects of the estrous cycle stage on the prolactin secretory response to dopamine in vitro

Dopamine (DA) will both stimulate and inhibit prolactin (PRL) secretion from the anterior pituitary gland in vitro and in vivo. The present study was designed to determine if there are selected times during the estrous cycle of the rat when one function is favored over the other. Anterior pituitary glands collected on diestrus-1 (D1), diestrus-2 (D2), the morning of proestrus (Pro-AM), the afternoon of proestrus (Pro-PM), and estrus (E) were enzymatically dissociated and placed in monolayer culture. On the fourth day in culture, cells were challenged for 10, 20, 30, 60, 120, 180, or 240 min with media alone or media containing either 100 pM or 1 μM DA. The concentration of PRL in the media was determined by radioimmunoassay. Regression analysis revealed that in the absence of DA, PRL secretion from cultured cells differed significantly depending on the stage of the estrous cycle during which they were obtained. Cells obtained during the morning of diestrus-2 secreted PRL at the greatest rate compared to other stages of the cycle. When all stages were compared, the rates of PRL secretion were: D2>E>D1>Pro-AM>Pro-PM (each significantly different from the others,P<0.01). By 20-30 min of exposure to 100 pM DA, the rate of PRL secretion from cells obtained during each stage of the cycle was significantly enhanced. This enhanced secretion persisted in cells obtained during D2 and Pro-PM but was short-lived in cells obtained during other stages. No inhibition of PRL secretion was induced by this dose of DA. PRL secretion was inhibited when treated with 1 μM DA in cells obtained at all stages of the estrous cycle. Inhibition was more prolonged in cells obtained on D1, D2, and Pro-AM. DA was least effective as an inhibitor of PRL secretion in cells obtained during Pro-PM and E. Prior to inhibiting PRL secretion in cells obtained during Pro-PM, 1 μM DA rapidly stimulated PRL secretion. This effect persisted for 60 min. These data suggest that in the absence of DA, the dynamics of PRL secretion from anterior pituitary cells in vitro differ depending on the stage of the estrous cycle during which the cells were obtained. Moreover, the in vivo environment of the cell determines the direction and magnitude of the PRL-secretory response to DA.