Dopamine agonists both stimulate and inhibit prolactin release in GH4ZR7 cells

Abstract

Prolactin secretion from the anterior pituitary gland is regulated by multiple factors including prolactin-release inhibiting factors (PIFs) and prolactin releasing factors. PIFs, however, usually dominate to exert a tonic inhibition in the biological system, and the physiological PIF is believed to be dopamine. However, there is accumulating evidence that dopamine can not only inhibit but also stimulate prolactin release. Many investigators believe that this is achieved by activating inhibitory and stimulatory subtypes of dopamine receptors. We tried to demonstrate that one subtype of dopamine receptors is capable of both inhibiting or stimulating prolactin release using GH 4 ZR 7 cells. GH 4 ZR 7 cells express only a short form of dopamine D 2 receptors (D 2s ). Low concentrations of three well-established D 2 receptor agonists (dopamine, apomorphine and bromocriptine) stimulated prolactin release from GH 4 ZR 7 cells while high concentrations inhibited the release. Haloperidol, a D 2 receptor antagonist, blocked the inhibitory action, but was unable to block the dopamine-induced stimulatory action. Pretreatment of cells with phenoxybenzamine, a receptor alkylating agent, abolished both the dopamine-induced stimulatory and inhibitory actions. Our results support the thesis that the stimulation of prolactin release induced by dopamine is mediated through dopamine D 2s receptors since the GH 4 ZR 7 cells have only D 2s receptors among dopamine receptors. We have concluded that the D 2s receptor is capable of both stimulating and inhibiting prolactin release, probably via the activation of a G s protein by low concentrations and a G i protein by high concentrations of dopaminergic agents.

Mucuna pruriens improves male fertility by its action on the hypothalamus–pituitary–gonadal axis

“…Seminal vesicles and vas deferens, including testis and epididymides, are also rich sources of adrenaline and noradrenaline in human semen (32). These catecholamines have been shown to enhance sperm motility, transport, capacitation, and acrosome reaction (33), and they are also involved in contraction of the seminal vesicles and inhibition of lipid peroxidation in spermatozoa (34).…”

Suppression of prolactin and reduction of milk secretion by effect of cabergoline in lactating dairy ewes

“…Given that cabergoline half-life in cows is 19 h (EMA, 2015), and due to its high affinity for dopamine D 2 -like receptors (Kvernmo et al, 2006), the occurrence of the PRL rebound on d 14 after injection may be related to a midterm feedback of pituitary's lactotrophs, decreasing the release of natural dopamine, which will result in a rise of PRL. Mechanism of PRL rebound in rats after dopamine withdrawal was explained by Chen et al (1993) and Chang and Shin (1999), who demonstrated that dopamine acts on D 2 receptors both to inhibit and to stimulate PRL release. We hypothesize that this may be related to the decrease of the activity of tyrosine hydroxylase (TH), the rate-limiting key enzyme in the biosynthesis and availability of catecholamines (i.e., dopamine, noradrenaline, and adrenaline) during the cabergoline treatment.…”

Low dose of dopamine may stimulate prolactin secretion by increasing fast potassium currents

“…Although this effect has been observed in several laboratories, there is contradictory evidence regarding the pathways involved in the stimulation of secretion. Some laboratories have shown that a D2 receptor subtype is involved (Burris et al, 1991;Chang & Shin, 1999), while others have proposed that D5 receptors may play a role in the stimulatory response (Porter et al, 1994). Similarly, a pertussis toxin sensitive G-protein pathway was suspected in one study (Tagawa et al, 1992) but had no effect in other studies (Burris et al, 1992;Chang et al, 1997).…”