Antagonist of pituitary adenylate cyclase activating polypeptide suppresses prolactin secretion without changing the activity of dopamine neurons in lactating rats.
Pituitary adenylate cyclase activating polypeptide (PACAP) is a relatively new neuropeptide, and it has a potent stimulatory effect on adenylate cyclase activity in rat pituitary cells. However, the role of PACAP in the physiological control of prolactin (PRL) secretion is still unclear. In the present study, we investigated the physiological significance of endogenous PACAP on PRL secretion in lactating rats. On lactation days 7-8, pups were separated from their mother rats for 5 h before the onset of suckling and PACAP6-38 (16 microg), a receptor antagonist, was injected through the lateral ventricle cannula just after the removal of pups. The effects of PACAP6-38 on PRL and oxytocin secretion, and on the activity of tyrosine hydroxylase (TH), were examined after the onset of suckling. Administration of PACAP6-38 inhibited PRL levels in response to suckling, but it did not affect the activity of TH, as measured by DOPA accumulation at 15 min after administration of NSD 1015 (25.0 mg/kg), an L-aromatic amino acid decarboxylase inhibitor, or the plasma concentrations of oxytocin in lactating rats. Injection of alpha-methyl-p-tyrosine (alpha-MT; 50 mg/kg), an inhibitor of dopamine synthesis, increased PRL levels, and suckling caused a further increase in the plasma concentrations of PRL. An injection of PACAP6-38 (i.c.v.) also inhibited the PRL response to suckling under dopamine depletion. These results suggest that endogenous PACAP acts as a neurotransmitter or neuromodulator within the hypothalamus and plays an important role for PRL secretion in lactating rats. Endogenous PACAP may regulate PRL secretion, possibly mediated by PRL-releasing factors such as vasoactive intestinal polypeptide or vasopressin.
Tohei
A
A
Department of Pharmacology, Tokyo University of Pharmacy and Life Science, Hachioji, Tokyo, Japan. tohei@ps.toyaku.ac.jp
Matsuzaki
M
M
Kogo
H
H
D000818:Animals; D000833:Animals, Suckling; D004298:Dopamine / Q000096:biosynthesis / Q000378:metabolism*; D004791:Enzyme Inhibitors / Q000494:pharmacology; D005260:Female; D007276:Injections, Intraventricular; D007774:Lactation / Q000502:physiology*; D008297:Male; D008473:Median Eminence / Q000166:cytology / Q000201:enzymology / Q000378:metabolism; D009474:Neurons / Q000187:drug effects / Q000378:metabolism*; D009479:Neuropeptides / Q000037:antagonists & inhibitors* / Q000494:pharmacology; D010121:Oxytocin / Q000378:metabolism; D010446:Peptide Fragments / Q000494:pharmacology; D051219:Pituitary Adenylate Cyclase-Activating Polypeptide; D011388:Prolactin / Q000378:metabolism*; D051381:Rats; D017208:Rats, Wistar; D014446:Tyrosine 3-Monooxygenase / Q000037:antagonists & inhibitors / Q000378:metabolism; D019805:alpha-Methyltyrosine / Q000494:pharmacology
C495403:Adcyap1 protein, rat; D004791:Enzyme Inhibitors; D009479:Neuropeptides; D010446:Peptide Fragments; D051219:Pituitary Adenylate Cyclase-Activating Polypeptide; C075647:pituitary adenylate-cyclase-activating-peptide (6-38); D010121:Oxytocin; D019805:alpha-Methyltyrosine; D011388:Prolactin; D014446:Tyrosine 3-Monooxygenase; D004298:Dopamine