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

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