Inhibition of the Proestrous Surge of Prolactin in the Rat by Nicotine1

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Abstract
Under controlled lighting conditions (14 hr light and 10 hr dark) female rats were injected with nicotine tartrate on the afternoon of proestrus, and periodic measurements were made of their circulating prolactin levels by radioimmunoassay. In rats bled by heart puncture nicotine treatment at 1200, 1400 and 1600 delayed the prolactin surge by at least 90 min. An additional injection of nicotine at 1700 delayed the rise in serum prolactin by at least another 2 hr. A majority of these rats ovulated that night. Extended studies employed intra—atrial cannulas, inserted on the morning of proestrus, through which successive blood samples could be withdrawn from the same animals without anesthesia. Under these conditions peaks of similar amplitude in plasma prolactin were observed at 1800, 1930 and 2130, respectively, in controls and rats treated with 3 and 4 injections of nicotine. The plasma prolactin curves of all 3 groups of rats, destined to ovulate that night, had returned to pre—surge levels by 2300. In similarly treated groups which failed to ovulate there was a brief rise in plasma prolactin starting at 2000 and peaking at 2100 at less than half the elevation attained by the ovulating rats. The close parallelism of the effects of nicotine on the proestrous surges of prolactin and LH emphasizes that there are common elements in the neuroendocrine mechanisms controlling the release of these 2 gonadotropins. (Endocrinology92: 1334, 1973)

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