The involvement of adrenergic, serotonergic and cholinergic mechanisms in the diurnal surge of plasma prolactin (Prl) secretion has been examined using ovariectomized, polyestradiol phosphate-treated (PEP) rats bearing aortic catheters. An afternoon surge in plasma Prl was observed to peak at 15.00 h and 17.00 h followed by declining levels at 19.00 and 21.00 h. This pattern was observed between 5 and 21 days after PEP administration. The α-adrenergic blocker, phenoxybenzamine (Phenox), completely prevented the Prl surge. The β-blocker, propranolol (Propra), appeared to delay the onset and intensity of the diurnal Prl surge so that maximum levels were observed at 19.00 and 21.00 h. The serotonergic blocker, methysergide (MES), delayed the maximum diurnal Prl level until 21.00 h, while cyproheptadine (Cypro), another serotonergic blocker, significantly inhibited the surge. The muscarinic cholinergic agonist, arecoline (Arec), when administered at 12.00 h, delayed the surge, while the repeated administration of Arec completely blocked the surge. Atropine (Atro) (10 m/kg at 12.00 h and 5 mg/kg every 2 h thereafter) did not have any effect on the Prl surge, but when administered simultaneously with Arec, prevented the inhibitory effect of Arec on Prl release. The data suggested that the adrenergic and serotonergic systems have a positive input in the occurrence and magnitude of the surge and that the cholinergic system does not appear to have a physiologic role in tonically inhibiting Prl release, but may function under certain special conditions.

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