Tamoxifen Stimulates Melatonin Secretion After Exposure to a Mammary Carcinogen, the Dimethyl Benz(a)Anthracene, in Sprague Dawley Female Rat

A single intragastric administration of 7,12-dimethylbenz(a)anthracene (DMBA) has been shown to induce mammary tumors in young cycling female Sprague-Dawley rats. The appearance of the tumors is preceded, during the la- tency phase, by a series of neuroendocrine disturbances, including attenuation of the preovulatory Luteinizing Hormone surge and Gonadotropin-Releasing Hormone release and amplification of the preovulatory 17� -Estradiol (E2) surge. Also, E2 treatment leads to a complete blunting of the Isoproterenol-induced stimulation of Melatonin secretion.In this study, we examined the hypothesis that Tamoxifen, an antagonist of E2, would stimulate the Isoproterenol-induced Mela- tonin (MT) secretion from the pineal gland, during the latency phase. Sprague-Dawley rats, 55-60 days of age, received, on the Estrous day of the Estrous cycle, a single dose of 15 mg DMBA delivered by intragastric intubation. In order to avoid possible interactions with endogenous steroids or mammary tumor- derived compounds, they were ovariectomized 5 days later and, one month later, sacrificed by decapitation at 10 a.m. Then, pineal glands were removed and placed in perifusion chambers containing Hanks 199 medium. The medium was satured with O2/CO2 (95 %/5 %) and its pH was 7.4. Ten independent chambers were immersed in a water bath at 37°C. Each pineal gland received medium (flow rate : 0.16 ml/min) through a system of input lines. The fractions were collected every 10 min, and immediately frozen at -20°C until Melatonin RIA. Experiments were repeated to obtain up to five ex- perimental points for each treatment. Tamoxifen (10 -9 to 10 -7 M) was applied during the entire perifusion period (7 hours). Isoproterenol (10 -6 M) was applied for 20 min after 3 hours in perifusion. Melatonin concentrations and Areas Under the Curves were compared using two-factor ANOVA as well as parametric or nonparametric two-sample methods after test- ing sample normality. In vehicle treated rats, Tamoxifen treatment, at the concentration of 10 -9 M, leads to a non significant amplification of the Isoproterenol-induced stimulation of Melatonin secretion. In DMBA-treated rats, Tamoxifen treatment leads,starting from 10 -9 M to a dose- dependent increase (up to 400% in- crease) of the Isoproterenol-induced stimulation of Melatonin . The results suggest that in addition to the well documented beneficial effects of Tamoxifen at the mammary gland level, this E2 antagonist may also have, after DMBA treatment, an additional beneficial effect at the pineal gland level through- out the stimulation of Melatonin, which exerts an inhibitory action on the induction and on the growth of breast cancers.