Mechanisms of Dopamine Action on the Lactotroph. In: Horseman ND, editor. Prolactin. Boston: Springer US; 2001. pp. 45-61. [DOI: 10.1007/978-1-4615-1683-5_3]

The dopaminergic control of Cushing's syndrome

Cushing's Syndrome (CS), or chronic endogenous hypercortisolism, is a rare and serious disease due to corticotroph pituitary (Cushing's disease, CD) and extra-pituitary (ectopic CS) tumours overproducing ACTH, or cortisol-secreting adrenal tumours or lesions (adrenal CS). The first-line treatment for CS is represented by the surgical removal of the responsible tumour, but surgery might be unfeasible or ineffective and medical treatment can be required in a relevant percentage of patients with CS, especially CD and ectopic CS. Corticotroph pituitary and extra-pituitary tumours, as well as adrenal tumours and lesions responsible for CS express dopamine receptors (DRs), which have been found to mediate inhibition of hormone secretion and/or cell proliferation in experimental setting, suggesting that dopaminergic system, particularly DRs, might represent a target for the treatment of CS. Dopamine agonists (DAs), particularly cabergoline (CAB), are currently used as off-label treatment for CD, the most common form of CS, demonstrating efficacy in controlling hormone secretion and tumour growth in a relevant number of cases, with the improvement of clinical picture, and displaying good safety profile. Therefore, CAB may be considered a reasonable alternative treatment for persistent or recurrent CD after pituitary surgery failure, but occasionally also before pituitary surgery, as adjuvant treatment, or even instead of pituitary surgery as first-line treatment in case of surgery contraindications or refusal. A certain beneficial effect of CAB has been also reported in ectopic CS. However, the role of DAs in the clinical management of the different types of CS requires further evaluations.

Decipher the inhibitory potential of phytocompounds from Leptadenia reticulata on dopamine D2 receptor to enhance prolactin secretion

Dopamine is secreted by the hypothalamus, which inhibits the proliferation and effectiveness of lactotroph cells that release prolactin via dopamine D2 receptor (D2R). D2R activation inhibits lactotroph cell prolactin synthesis and regulates prolactin gene expression. Although, commercial medications are available for hypogalactia and agalactia, various plant sources significantly alleviate these problems. Leptadenia reticulata (Jivanti) is one of the important medicinal plants often consumed by nursing mothers to improve breast milk production. However, mechanism and chemical constituents involved in the inhibition of D2R by Jivanti is unclear. Therefore, in this study the phytocompounds reported from Jivanti were used for in-silico analysis to predict D2R inhibitory potential. The binding affinity value of campesterol and β-sitosterol (- 10.1 and -10.0 kcal/mol) with D2R has high revealed by molecular docking and stable interaction reveled by molecular dynamics simulation. Thus, these lead compounds could exert more D2R inhibitory activity resulting into prolactin release, which may lead to an increase in breast milk production. Although all selected compounds had fine permeation, non-toxic, and non-carcinogenic characteristics predicted by ADMET, campesterol had good solubility, absorption characteristics compared to other. Therefore, Jivanti, which is traditionally known medicinal plant, could be explored as a medication candidate to boost breast milk production.

Estrogen regulation of the dopamine-activated GIRK channel in pituitary lactotrophs: implications for regulation of prolactin release during the estrous cycle

Prolactin (PRL), synthesized and secreted from lactotrophs of the anterior pituitary gland, is tonically inhibited by hypothalamic dopamine (DA) throughout the female reproductive (estrous) cycle. Our laboratory has shown that DA hyperpolarizes these cells by activating G protein-coupled inwardly rectifying K(+) (GIRK) channels; however, this response is only observed on proestrus. While the cellular mechanisms that allow for functional expression of this unique DA-signaling pathway are unclear, we hypothesized that activation of the DA-GIRK effector pathway is due to the rise in circulating estrogen (E₂) during the preceding day of diestrus. Thus, we examined the effects of E₂ on primary lactotrophs isolated from female rats. Treatment with a physiological concentration of E₂ (40-80 pg/ml, in vivo or in vitro) induced a proestrous phenotype in diestrous lactotrophs. These cells exhibited a DA-induced membrane hyperpolarization, as well as a secretory rebound of PRL following DA withdrawal (characteristic of proestrous cells). Internal dialysis of GTPγS demonstrated that E₂ exposure enabled functional expression of GIRK channels, and this regulation by E₂ did not involve the D₂R. The effect of E₂ was blocked by the receptor antagonist, ICI 182,780, and by the protein synthesis inhibitor, cycloheximide. Single-cell analysis revealed increased mRNA expression of GIRK channel subunits in E₂-treated lactotrophs. While E₂ is known to have multiple actions on the lactotroph, the present findings illuminate a novel action of E₂ in lactotrophs-regulation of the expression of a DA effector, the GIRK channel.

Implication of pituitary vasoactive intestinal peptide in dopaminergic inhibition of estrogen-induced pituitary hyperplasia and vascular endothelial growth factor expression

We have shown that pituitary vasoactive intestinal peptide (VIP) mediates the effects of estrogen on lactotrope hyperplasia, angiogenesis and hyperprolactinemia, and reduces the pituitary content of transforming growth factor beta beta1 (TGF-beta1, an inhibitor of lactotrope proliferation). Dopamine agonists reverse lactotrope hyperplasia and hyperprolactinemia and also reduce the pituitary VIP content in hyperestrogenized rats. To elucidate the interaction of bromocriptine (BC) and pituitary VIP, a VIP receptor antagonist (VA), BC, or both drugs were administered for 5 days to F344 rats treated with diethylstilbestrol (DES). Both BC and VA similarly blocked the effects of DES on pituitary weight and pituitary content of prolactin (PRL), proliferating cell nuclear antigen, and vascular endothelial growth factor, without evidence of synergism. The estrogen effect on pituitary TGF-beta1 was completely inhibited by VA, but only partially by BC. On the contrary, serum PRL was close to the normal levels in the BC group 2 h after the first dose, while VA only reduced serum PRL after 5 days. DES increased VIP and VIP mRNA levels specifically at the pituitary, this effect being partially blocked by BC. These data suggest that the dopamine agonists inhibit lactotrope proliferation and angiogenesis by blocking the autocrine/paracrine action of VIP. On the other hand, the dopamine agonists inhibit the estrogen-induced hyperprolactinemia by acting through different pathways than those implicated in the proliferative process.