Specificity of Arecoline and Apomorphine and the Site of Action of Arecoline in Inhibiting the Diurnal Prolactin Surge 1

Summary

Ovariectomized, polyestradiol phosphate (PEP)-injected, catheterized rats exhibiting a daily diurnal plasma prolactin surge were used to investigate the specificity of cholinergic receptor blocker drugs and to demonstrate the presence of cholinergic receptors outside the blood brain barrier. Atropine methylnitrate (AMN) blocked the inhibitory effect of arecoline on the diurnal surge of plasma prolactin and this blockade was more dramatic for the later time periods. The tremorogenic effect of arecoline (a CNS action) was not blocked, whereas the salivation effect of arecoline (a peripheral action) was blocked by AMN. The prolactin and behavioral responses were taken to indicate that AMN does not pass the blood brain barrier and that there are cholinergic receptors inhibiting prolactin release outside this barrier, perhaps directly on the pituitary. Mecamylamine, a nicotine blocker, did not have any effect either on the arecoline-induced blockade of the prolactin surge or on the arecoline-induced behavioral responses. Atropine sulfate (AS), a muscarinic blocker, did not influence either the apomorphine-induced inhibition of the prolactin surge or the apomorphine-induced behavioral responses.

We would like to express our appreciation to Mrs. Cynthia Van De Walle for her expert technical assistance in prolactin radioimmunoassay and excellent art work. We also appreciate receiving as a gift the rat prolactin used for iodination and standards from NIAMDD Rat Pituitary Hormone Distribution Program.