Circadian Rhythm of Luteinizing Hormone Secretion in the Ovariectomized Rat Implanted With Oestradiol

Abstract

Implantation of a solid source of oestradiol into ovariectomized rats produced constant plasma concentrations of the hormone over a long period of time. Under these conditions, LH is released in a circadian pattern with a very marked peak in the afternoon. This circadian rhythm is synchronized to the light--darkness cycle, since it follows exactly a shift in the nycthemeral cycle. The first peak appeared on day 3 after placement of the oestrogen implant; its amplitude was constant from days 3 to 9 after implantation, and decreased gradually during prolonged implantation. The afternoon peak was not correlated with changes in the pituitary sensitivity to exogenous LH releasing hormone (LH-RH), since the LH response to increasing doses of the peptide could be superimposed in the morning and in the afternoon. However, the decreased amplitude of the rhythm observed after more than 9 days of implantation seemed to depend upon a progressive desensitization of the pituitary gland to LH-RH. Pituitary LH content also decreased as a function of implantation time. It is concluded that, under conditions of constant plasma oestradiol concentrations and of constant pituitary sensitivity to LH-RH, a daily activation of the neural trigger releasing pituitary gonadotrophins occurs.

Effect of Steroid Treatment on Tonic and Surge Secretion of LH and FSH in the Female Golden Hamster: Effect of Photoperiod

“…The eventual extinction of the stimulatory effect of es trogen on LH surges has previously been demonstrated in the long-day hamster [19,20] and in the rat [8,13,23]; the present study demonstrates that once the short-day hamster is treated with estrogen, its LH surges also disappear. In studies where blood samples were taken more frequently around the time of the surge, it was demonstrated that the time of peak gonadotropin levels did not shift over 10 days of estrogen treatment, but the amplitude and duration of the surges decreased [4,20].…”

“…Consequently, it is improbable that estrogen treatment in this experiment phase shifted the surge to an extent that the surge was outside our afternoon sample time. While the pituitary content of gonadotropins and the responsiveness of the gland to gonadotro pin-releasing hormone (GnRH) decline with time in estro gen-treated rats and hamsters [8,19], it seems unlikely that this is the complete explanation of the disappearance of the afternoon LH surge. It is probable that there is a loss of daily GnRH release.…”

Suppression of Tonic Luteinizing Hormone Secretion and Norepinephrine Release near the GnRH Neurons by Estradiol in Ovariectomized Rats

“…It is well established that a neural signal is essential for the circadian timing of the LH surge in rodents [40, 45, 46, 47, 48, 49]. However, during the estrous cycle, an LH surge only occurs once every 4 or 5 days.…”

Age Factors Potentiating Drug Toxicity in the Reproductive Axis

“…The studies show that prolonged administration of estrogen causes cycle instability and ultimately cycle failure (20,21). These effects are presumably mediated by an impact of the steroid on the brain, where altered metabolism of hypothalamic monoamines and degeneration of neurons in the arcuate nucleus follow administration of estrogen (21,22).…”

“…The studies show that prolonged administration of estrogen causes cycle instability and ultimately cycle failure (20,21). These effects are presumably mediated by an impact of the steroid on the brain, where altered metabolism of hypothalamic monoamines and degeneration of neurons in the arcuate nucleus follow administration of estrogen (21,22). Pharmacodynamic changes include alterations in the metabolism of serotonin (22), whose intermittent metabolism and secretion has been linked to phasic secretion of LH (23,24).…”