Thyrotrophin‐Releasing Hormone, Vasoactive Intestinal Peptide, Prolactin‐Releasing Peptide and Dopamine Regulation of Prolactin Secretion By Different Lactotroph Morphological Subtypes in the Rat

Abstract

In the male rat anterior pituitary, three morphological subtypes of cells secreting primarily prolactin (PRL) (lactotrophs) have been described. Type I contain predominantly large irregularly shaped granules, whereas type II and type III lactotrophs contain smaller spherical granules. We have previously shown that oestradiol and testosterone exert a rapid stimulatory effect selectively on type II lactotrophs but it is not known how the lactotroph subtypes respond to peptide secretagogues. We have therefore examined which cell subtype(s) release PRL in response to vasoactive intestinal peptide (VIP), thyrotrophin-releasing hormone (TRH) and prolactin-releasing peptide (PrRP-31). Pituitary segments were incubated in medium containing tannic acid (to capture exocytosis of secretory granules), either alone or with secretagogue peptide. VIP (1-10 nM), TRH (10 nM) and PrRP-31 (10 nM) all caused a significant increase (P< 0.05) in the amount of PRL granule exocytosis from type II and III lactotrophs, but had no effect on PRL exocytosis from type I. Dopamine (100 nM) inhibited basal exocytosis of immunoreactive (ir)-PRL from type I, II and III lactotrophs and PrRP-31-stimulated ir-PRL granule exocytosis from II and III lactotrophs. Treatment of lactating female rats with the dopamine D(2) receptor antagonist sulpiride (40 microg/kg) produced a significant increase (P < 0.05) in PRL granule exocytosis from type I and type III lactotrophs and a significant increase (P < 0.05) in the proportion of type I and II cells undergoing exocytosis of PRL. In conclusion, VIP, TRH and PrRP-31 selectively stimulate exocytosis from type II and III lactotrophs in the male rat, whereas all three lactotroph types are sensitive to dopamine inhibition of exocytosis in male and female rats.

Prolactin and the physiological regulation of its secretion. A review

“…In rats the PrRPs-31 stimulates exocytosis of prolactin selectively from type II and type III lactotrophs but not from type I PRL-producing cells (Christian et al, 2007). These results indicate that respective receptors and signalling cascades required inducing PRL release.…”

“…TRH receptors have been identified on type II and type III of lactotrophs but not any labelling was evident on type I. Other studies have revealed that the stimulatory influence of PrRPs-31 on PRL release in rats is rather low compared to TRH or VIP (Samson et al, 1998;Curlewis et al, 2002;Swinnen et al, 2005;Christian et al, 2007).…”

“…These results indicate that respective receptors and signalling cascades required inducing PRL release. However, there are no still published results regarding the PrRPs-31 receptor expression in lactotroph subtypes (Christian et al, 2007). On the other hand, it is well documented, that not all pituitary lactotrophs in rats express TRH receptors, suggesting the functional heterogeneity of these cells (Christian et al, 2007).…”

“…However, there are no still published results regarding the PrRPs-31 receptor expression in lactotroph subtypes (Christian et al, 2007). On the other hand, it is well documented, that not all pituitary lactotrophs in rats express TRH receptors, suggesting the functional heterogeneity of these cells (Christian et al, 2007). TRH receptors have been identified on type II and type III of lactotrophs but not any labelling was evident on type I.…”

“…In rodents, the three morphological types of lactotrophs have been defined by electron microscopy (Christian et al, 2007). Type I 'classical lactotrophs' contain large irregular shaped electro-dense secretory granules (diameter 300-700 nm), type II cells contain numerous sized spherical electron dense granules (diameter 200-250 nm), and type III cells, which have small (100-200 nm) spherical granules.…”