Influence of pulsatile administration of LRH on the development of the augmentative (positive) effect of oestradiol on the pituitary response to LRH

Abstract

Abstract. Ovariectomized (OVX) rats were treated for 24 h with pulses of LRH (1.25 ng/100 g b.w. every 20 min) or of saline. Moreover, the animals received 3 sc injections of oestradiol benzoate (EB) (50 μg) or oil during this period. At the end of this period all animals received phenobarbital (80 mg/kg ip) and the animals were given pulses of LRH (once every 20 min) containing 1.25, 2.5, 5.0 or 10 ng for 3 h. Pulsatile administration of 1.25 ng pulses for 24 h did not lead to pituitary insensitivity. Treatment with EB caused the development of an increased response to LRH (positive effect). When during the 24–27 h period 5.0 or 10.0 ng pulses of LRH were given, the positive response was the same whether LRH had been administered during the 0–24 h period or not. However, the response to 2.5 ng pulses of LRH was halved when the animals had been pre-treated with LRH. It is concluded that the presence of LRH can inhibit partly the development of the positive pituitary response to LRH as caused by EB. When the dose of LRH was kept at 1.25 ng/pulse no clear positive effect developed. Chronic administration of phenobarbital for 24 h caused pituitary insensitivity, presumably through a direct toxic effect.