Regulation of Prolactin Secretion by Dopamine and Vasoactive Intestinal Peptide at the Level of the Pituitary in the Turkey

Abstract

This study investigated the capability of dopamine (DA) to prevent avian prolactin (PRL) secretion by antagonizing the PRL-releasing factor, vasoactive intestinal peptide (VIP), at the level of the pituitary. To test this hypothesis, combined intracranial and intrapituitary infusions of DA, DA agonists, and VIP, plus electrical stimulation of the medial preoptic area (ES/POM), were used to characterize the actions of DA on PRL secretion in anesthetized laying turkey hens. Infused into the third ventricle at the rate of 10 nmol/min, DA induced a 2.8-fold increase in circulating PRL levels (63.8 ± 15.1 to 181.3 ± 44.3 ng/ml, p < 0.05), similar to the 3.1-fold PRL increase induced by ES/POM (65 ± 10.3 to 199.1 ± 57.3 ng/ml, p < 0.05). Infused into the anterior pituitary at the rate of 15 ng/min, VIP induced a 2.2-fold increase in PRL (78.6 ± 22.9 to 173.6 ± 39.5 ng/ml, p < 0.05). When DA (10 nmol/min) was infused into the anterior pituitary it completely blocked both ES/POM- and VIP-induced PRL secretion. The D₂ DA receptor agonist R- (–) -Propylnorapomorphine HCl inhibited VIP-induced PRL secretion at the level of the anterior pituitary, allowing only an insignificant rise in PRL (54.8 ± 14.3 to 73.9 ± 21.6 ng/ml, p > 0.05), while the D₁ DA receptor agonist (±)-SKF-38393 HCl failed to prevent VIP-induced PRL release, allowing PRL to rise 2.5-fold (49.1 ± 10.8 to 121.0 ± 34.8 ng/ml, p < 0.05). Pituitary infusion of DA, DA agonists or vehicle alone caused no change in PRL levels. The data showed that DA prevented avian PRL secretion by blocking the action of VIP at the level of the anterior pituitary. DA effected this blockade of PRL via D₂ DA receptors residing within the anterior pituitary. The data also suggested that there were no stimulatory D₁ DA receptors related to PRL secretion in the avian anterior pituitary.

Dopaminergic regulation of avian prolactin gene transcription

“…These results are in agreement with earlier findings from our laboratory (Youngren et al 1998). D 1 DA receptors are not found on mammalian pituitary cells (Rovescalli et al 1987, Sunahara et al 1991.…”

“…The presence of both D 1 and D 2 DA receptor mRNA in the turkey brain and pituitary (Schnell et al 1999, Chaiseha et al 2003 suggests that DA may exhibit biphasic actions within the turkey hypothalamus and pituitary gland. DA appears to regulate PRL secretion centrally through stimulatory D 1 and inhibitory D 2 DA receptors, while at the pituitary level it exhibits an inhibitory effect on PRL secretion induced by VIP or electrical stimulation (Youngren et al 1998).…”

Relationship between the Production Traits and three Candidate Genes in the Prolactin’s In/Del×In/Del Population of Silkie Fowl

“…Physiologically, it has been well established that PRL is the main candidate gene for broodiness owing to its critical roles in the onset of broodiness (Sharp et al, 1988;March et al, 1994;Youngren et al, 1998). Increased plasma PRL concentrations have been associated with the occurrence of broodiness in female turkeys (Burke and Dennison, 1980).…”

Mucuna pruriens seed powder feeding influences reproductive conditions and development in Japanese quail Coturnix coturnix japonica

“…The dopaminergic D 2 receptor agonist quinpirole inhibits VIP-induced PRL secretion and PRL mRNA level when incubated with turkey anterior pituitary cells (Xu et al, 1996), suggesting the presence of inhibitory D 2 DA receptors within the pituitary. Central infusion of DA into the turkey brain at low concentration results in significant increase in plasma PRL (Youngren et al, 1998).…”

“…The dopaminergic D 2 receptor agonist quinpirole inhibits VIP-induced PRL secretion and PRL mRNA level when incubated with turkey anterior pituitary cells (Xu et al, 1996), suggesting the presence of inhibitory D 2 DA receptors within the pituitary. Central infusion of DA into the turkey brain at low concentration results in significant increase in plasma PRL (Youngren et al, 1998).Egg size represents a key, hormonally mediated, lifehistory trait, which varies markedly among species (from 0.25 g in bee hummingbirds, Mellisuga helenae, to 1500 g in the ostrich, Struthio camelus). Most of the variation in egg size within species is due to difference in the mean egg size produced by individual females, i.e.…”