Intravenous injections of nicotine decrease the pulsatile secretion of LH by inhibiting the gonadotropin-releasing hormone (GnRH) pulse generator activity in female rats

Whether nicotine inhibits the electrical activity of the gonadotropin-relea sing hormone (GnRH) pulse generator to suppress pulsatile LH secretion, and whether this suppression of LH secretion by nicotine is mediated by opioid neurons, were studied in ovariectomized rats by examining changes in LH se cretion and the multiunit activity (MUA) of the medial basal hypothalamus. Intravenous (IV) injection of nicotine (nicotine bitartrate, 100 mu g) sign ificantly increased the interval between characteristic increases (volleys) in MUA and LH pulses. This inhibitory effect of nicotine on the GnRH pulse generator activity was not blocked by the prior injection of an opiate rec eptor antagonist naloxone (naloxone hydrochlolide, 7 mg/kg bw), which was e ffective in significantly decreasing the interval between MUA volleys. The results suggest that nicotine alters the activity of the GnRH pulse generat or, and that cholinergic neurons appear to be directly involved in suppress ing pulsatile secretion of LH. (C) 1999 Elsevier Science Ltd. All rights re served.