Effects of Pituitary Adenylate Cyclase‐Activating Polypeptide (PACAP) and Vasoactive Intestinal Polypeptide (VIP) on Prolactin, Luteinizing Hormone and Growth Hormone Secretion in the Ewe

Abstract

This study was undertaken to investigate the roles of PACAP and VIP in the control of pituitary hormone secretion in the ewe. The first experiment was designed to identify any direct effects at the level of the pituitary and was conducted during the luteal phase of a prostaglandin-synchronized oestrous cycle. PACAP (0.008, 0.04, 0.2 and 1.0 nmol/min) or VIP (0.06, 0.2, 0.6 and 1.8 nmol/min) was infused into the carotid artery over a 10 min period. Blood samples were taken before and after the infusions so that plasma PRL, LH and GH concentrations could be measured. Blood pressure was also monitored to determine if the doses used were biologically active. In no case was an effect on hormone secretion observed. In contrast, the highest dose of each peptide induced an increase in heart rate to almost three-fold the resting value. Although both peptides are active in vivo, this result suggests that neither peptide has a direct effect on hormone release from the pituitary of prostaglandin-synchronized ewes. In a second experiment, we investigated whether the peptides had central effects on hormone secretion. Intracerebroventricular (ICV) injection of PACAP or VIP at the dose 10 nmol was tested in ovariectomized ewes. After injection, PACAP suppressed PRL and GH secretion so that plasma hormone concentrations from 1-3 h after injection were significantly different from the control (P < 0.05 for PRL, P < 0.01 for GH). In addition, PACAP significantly reduced mean LH concentration (P < 0.05) and LH pulse frequency (P < 0.01).(ABSTRACT TRUNCATED AT 250 WORDS)

Interactions of ovarian steroids with pituitary adenylate cyclase-activating polypeptide and GnRH in anterior pituitary cells

“…In accordance with this possibility, we have previously described that estradiol and progesterone modulate LH secretion from gonadotrophs in response to other agonists that activate phospholipase C such as endothelin (36). The fact that gonadal steroids exert stimulatory and inhibitory actions on PACAP-stimulated LH release might explain the controversial data of in vivo studies on the ability of PACAP to induce gonadotropin secretion (19,(37)(38)(39).…”

Prolactin-releasing peptide in the ewe: cDNA cloning, mRNA distribution and effects on prolactin secretion in vitro and in vivo

“…Prolactin concentrations in culture medium or plasma and LH concentrations in plasma were determined by radioimmunoassay as previously described (Sawangjaroen & Curlewis 1994). Prolactin assay sensitivity was 0·8 ng/ml and the intra-and interassay coefficients of variation were 7·2 and 16·3%.…”

“…During experiments, blood samples were collected from a jugular cannula that was inserted on the day preceding each experiment. Intravenous treatments were administered via the jugular cannula and intracerebroventricular injections were performed as previously described (Sawangjaroen & Curlewis 1994). Procedures performed in this study were approved by the Animal Experimentation and Ethics Committee of The University of Queensland.…”

“…At the time of ovariectomy each ewe received a subcutaneous oestradiol implant (1·0 cm) (Anderson et al 1996) that was removed 2 weeks before the 3-week period during which the experiments were conducted. The lateral cerebral ventricle was chronically cannulated under anaesthesia as previously described (Sawangjaroen & Curlewis 1994). During experiments, blood samples were collected from a jugular cannula that was inserted on the day preceding each experiment.…”

A Role of Pituitary Adenylate Cyclase Activating Polypeptide(PACAP) as a Regulator of Paracrine Interactions between Folliculo-Stellate Cells and Gonadotropes through the Control of Activin-Follistatin Interactions.

“…For example, PACAP appears to modulate pulsatile GnRH release from the hypothalamus, which in turn affects the secretion and synthesis of pituitary gonadotropins [31,38]. Cross-talk between the actions of PACAP and GnRH at different intracellular levels in gonadotropes is also suggested [5,25,32].…”