Differential Effects of Pituitary Stalk-Section on Posterior Pituitary and Hypothalamic Contents of Prolactin-Releasing Factor, Oxytocin, Dopamine and Beta-Endorphin

Abstract

We have recently shown that the posterior pituitary (neurointermediate lobe) contains a potent prolactin (PRL)-releasing factor (PRF) which is distinct from known PRL secretagogues. Posterior pituitary PRF appears to be a small peptide of an unknown cellular origin. Using pituitary stalk-sectioned (SS) male rats, the objectives of this study were: (1) to determine if PRF is transported from the hypothalamus or is synthesized within the pituitary gland, and (2) to compare changes in PRF activity with alterations in the posterior pituitary content of β-endorphin (β-END), oxytocin (OXY), and dopamine (DA). One or two weeks following pituitary SS or sham surgery (SHAM), acid extracts of the posterior pituitary and medial basal hypothalamus (MBH) were analyzed for their hormone content. PRF activity was assessed by determining the stimulation of PRL secretion from perifused anterior pituitary cells, DA was measured by HPLC, and OXY and β-END levels were determined by RIAs. OXY and DA concentrations in the posterior pituitary were reduced more than 95% at both 1 and 2 weeks after SS. PRF activity in the posterior pituitary was significantly reduced by 75 and 90%, 1 and 2 weeks after SS, respectively. In contrast, β-END levels in the posterior pituitary at these times increased 20 and 60%, as compared to SHAM rats. Unlike the posterior pituitary, OXY levels in the MBH increased 123% 1 week following SS, and 1,260% at 2 weeks. These increases may reflect the accumulation of OXY-containing secretory vesicles in the severed nerves. DA concentrations in the MBH showed a biphasic pattern. DA levels were initially decreased by 70%, and then increased, but remained 30% below SHAM levels. The reason for these alterations in DA levels is not clear. It is concluded that: (1) similar to OXY and DA, but unlike β-END, PRF is likely transported from the hypothalamus to the posterior pituitary, and (2) the low levels of PRF activity in the MBH suggest that PRF may be synthesized as a biologically inactive precursor, the amount of PRF in the MBH is too low to detect, or inhibitors of PRL secretion mask the PRF activity.

Effects of corticosterone on the secretion of corticotrophin-releasing factor, arginine vasopressin and oxytocin into hypophysial portal blood in long-term hypophysectomized rats

“…Similar results have been described previously, showing a greater than ten¬ fold increase in the oxytocin content of the MBH 2 weeks after pituitary stalk section (Hyde, Murai & Ben-Jonathan, 1988). Over a period of several weeks after hypophysectomy the proximal end of the cut pituitary stalk becomes reorganized into tissue which resembles neural lobe and which is capable of storing and releasing oxytocin and AVP into systemic blood and may secrete these peptides directly into the portal blood vessels (Billenstien & Leveque, 1955;Raisman, 1973).…”

Neuroendocrine Regulation of Prolactin Secretion During Late Pregnancy: Easing the Transition into Lactation

“…There is no argument that the rat NIL contains potent prolactin‐releasing ability; NIL extracts stimulated prolactin release in a rapid, hormone‐specific and dose‐dependent manner either in vivo (52) or in vitro (53). Furthermore, the prolactin‐releasing factor in the NIL appears to be produced in the hypothalamus and transported to the NIL in a similar fashion to oxytocin and dopamine (54) but, despite this, NIL cells can maintain prolactin‐releasing factor activity for over 1 week in culture. Upon dissection of the NIL, prolactin‐releasing factor activity was almost exclusively localised in the intermediate lobe (55).…”

Does pituitary stalk compression cause hyperprolactinemia?

“…It is present in pituitary corticotrophs and may exert a paracrine stimulatory effect on lactotrophs to account for the hyperprolactinemia frequently found in association with Cushing's disease (4), although it may also cause hypothalamic dopamine depletion. Posterior pituitary betaendorphin rises after stalk section and appears to be inhibited by dopamine (5). Its role in prolactin secretion is unclear, especially as the prolactin-releasing activity of posterior pituitary extracts following stalk section diminishes by up to 90% (5).…”

“…Posterior pituitary betaendorphin rises after stalk section and appears to be inhibited by dopamine (5). Its role in prolactin secretion is unclear, especially as the prolactin-releasing activity of posterior pituitary extracts following stalk section diminishes by up to 90% (5). Another small peptide with potent prolactin-releasing properties was found in the posterior pituitary, but its structure has not yet been elucidated (5).…”

“…Its role in prolactin secretion is unclear, especially as the prolactin-releasing activity of posterior pituitary extracts following stalk section diminishes by up to 90% (5). Another small peptide with potent prolactin-releasing properties was found in the posterior pituitary, but its structure has not yet been elucidated (5). Arginine vasopressin, oxytocin, and neurotensin cause protactin release provided only that the hypothalamus-pituitary flow is intact.…”