Effect of Nicotine on the Proestrous Ovulatory Surge of LH in the Rat1

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Abstract
Female rats on a lighting schedule with lights on daily from 0500–1900 hr were injected with nicotine tartrate at 1200, 1400, 1600 and 1700 on the afternoon of proestrus. Practically all ovulated that night. However, radioimmunoassay of blood collected by heart puncture from these rats at 1800, 1830 and 1930 revealed low levels of circulating luteinizing hormone (LH). Furthermore, in these animals pentobarbital administered as late as 1800 was effective in blocking ovulation. Insertion of an intra-atrial cannula on the morning of proestrus permitted easy successive sampling of blood throughout the evening of proestrus without anesthesia. Rats injected with nicotine at 1200, 1400 and 1600 showed approximately a two-hr delay in the LH peak, which was also smaller in amplitude than that seen in salineinjected animals. A fourth injection of nicotine at 1700 further reduced and delayed the LH peak an additional 2 hr. Administration of nicotine tartrate (5 mg/kg) every 30 min from 1400–1900 failed to prevent ovulation, but with two additional injections at 1930 and 2000 ovulation was blocked. Thus, nicotine appears to have the capacity of delaying, for periods up to 5 hr, the proestrous ovulatory surge of LH from the adenohypophysis. The results indicate that central neural mechanisms responsible for the LH surge must be inhibited for approximately 6 hr to prevent ovulation. (Endocrinology91: 1253, 1972)

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