Luteinizing Hormone Requirements for Ovulation in the Pentobarbital-Treated Proestrous Rat11

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Abstract
The LH [luteinizing hormone] requirements for ovulation in the pentobarbital-blocked proestrous CD rat were studied by increasing serum gonadotropin levels through electrical stimulation of the brain and subsequently comparing the effects of timed hypophysectomy on ovulation and serum LH concentrations. The arcuate nucleus (ARC) or the medial preoptic area (POA) was stimulated unilaterally for 45 min with matched pairs of biphasic rectangular pulses through a coaxial Pt electrode. Serum LH was significantly elevated above basal values at the end of stimulation but not in sham-stimulated controls. The results of hormone measurement and hypophysectomy showed that the pituitary continued to release LH after extrinsic stimulation of the hypothalamus had ceased. Animals did not ovulate if they had been hypophysectomized at the end of the 45 min stimulation, whereas nearly all ovulated if hypophysectomy was delayed for an additional 20 min. Some evidence suggested that the pituitary could be removed earlier without affecting ovulation if the rate of LH release was increased. The minimum peak LH concentration measured in rats that subsequently ovulated fully was 187 ng/ml, substantially lower than concentrations ordinarily attained in the spontaneous proestrous surge. When serum LH was insufficiently high to cause follicle rupture, meiosis and luteinization of the large ovarian follicles resumed. Attempts were made to restore ovulability in animals presumed to have released a subovulatory quota of gonadotropin. Ovulation was obtained when such animals, prepared by hypophysectomy after the 45 min stimulation, had been bilaterally nephrectomized prior to stimulation. Multiple injections of progesterone after hypophysectomy were without effect. The results are discussed in relation to variables that affect minimum requirements of LH for ovulation.