Inhibition Of Pulsatile Luteinizing Hormone Release Research Articles

Long-Term Neonatal Estrogen Exposure Causes Irreversible Inhibition of LH Pulses by Suppressing Arcuate Kisspeptin Expression via Estrogen Receptors α and β in Female Rodents.

Exposure to estrogen during the developmental period causes reproductive dysfunction in mammals, because the developing brain is highly sensitive to estrogens. In the present study, we report that long-term exposure to supraphysiological doses of estrogen during the neonatal critical period causes irreversible suppression of Kiss1/kisspeptin expression in the arcuate nucleus (ARC) via estrogen receptor-alpha (ERα) and ERβ, resulting in reproductive dysfunction in female rats. Daily estradiol-benzoate (EB) administration from days 0 to 10 postpartum caused persistent vaginal diestrus in female rats. The female rats showed profound suppression of pulsatile luteinizing hormone (LH) release and ARC Kiss1/kisspeptin expression even after ovariectomy at adulthood. In contrast, female rats treated with a single EB injection at day 5 postpartum exhibited persistent vaginal estrus and showed comparable LH pulses and numbers of ARC Kiss1-expressing cells to vehicle-treated controls after ovariectomy at adulthood. Because the LH secretory response to exogenous kisspeptin was spared in female rats with neonatal long-term estrogen exposure, the LH pulse suppression was most probably due to ARC kisspeptin deficiency. Furthermore, neonatal estrogen might act through both ERα and ERβ, because EB exposure significantly reduced the number of ARC Kiss1-expressing cells in wild-type mice but not in ERα or ERβ knockout mice. Taken together, long-term exposure to supraphysiological doses of estrogen in the developing brain might cause defects in ARC kisspeptin neurons via ERα and ERβ, resulting in inhibition of pulsatile LH release and lack of estrous cyclicity.

Atrazine Inhibits Pulsatile Gonadotropin-Releasing Hormone (GnRH) Release Without Altering GnRH Messenger RNA or Protein Levels in the Female Rat1

Atrazine (ATR) is a commonly used pre-emergence/early postemergence herbicide. Previous work has shown that exposure to high doses of ATR in rats results in blunting of the hormone-induced luteinizing hormone (LH) surge and inhibition of pulsatile LH release without significantly reducing pituitary sensitivity to a gonadotropin-releasing hormone (GnRH) agonist. Accompanying the reduction in the LH surge was an attenuation of GnRH neuronal activation. These findings suggest that ATR exposure may be acting to inhibit GnRH release. In this study, we examined GnRH directly to determine the effect of high doses of ATR on GnRH pulsatile release, gene expression, and peptide levels in the female rat. Ovariectomized adult female Wistar rats were treated with ATR (200 mg/kg) or vehicle for 4 days via gavage. Following the final treatment, GnRH release was measured from ex vivo hypothalamic explants for 3 h. In another experiment, animals were administered either vehicle or ATR (50, 100, or 200 mg/kg) daily for 4 days. Following treatment, in situ hybridization was performed to examine total GnRH mRNA and the primary GnRH heterogeneous nuclear RNA transcript. Finally, GnRH immunoreactivity and total peptide levels were measured in hypothalamic tissue of treated animals. ATR treatment resulted in no changes to GnRH gene expression, peptide levels, or immunoreactivity but a reduction in GnRH pulse frequency and an increased pulse amplitude. These findings suggest that ATR acts to inhibit the secretory dynamics of GnRH pulses without interfering with GnRH mRNA and protein synthesis.

Photoperiod modulates access of 2,2′,4,4′,5,5′-hexachlorobiphenyl (PCB153) to the brain and its effect on gonadotropin and thyroid hormones in adult ewes

The effects of photoperiod on the cerebrospinal fluid (CSF) concentration of six ortho-substituted polychlorinated biphenyls (PCBs: PCB28, PCB52, PCB101, PCB138, PCB153, and PCB180), the effects of an orally administered low dose of PCB153 (0.3mg/kg, three times a week for three weeks) on PCBs and thyroid hormones (THs) concentrations in the CSF and plasma, and the release of luteinizing hormone (LH) were determined in ovariectomized, estradiol-implanted ewes (2.5 years old) maintained indoors under artificial long day (LD, 16L: 8D) and short day (SD, 8L: 16D) conditions. Concentrations of two PCBs (PCB28 and PCB153) in the plasma and four PCBs in the CSF (PCB101, PCB138, PCB153, and PCB180) were significantly higher during LD than SD. Following PCB153 treatment, its concentration in the plasma was higher in SD (1.2±0.3ng/ml) than LD (0.2±0.05ng/ml), but similar in the CSF (10.2±3.7pg/ml vs. 13±0.7pg/ml) under both photoperiods. During SD, the concentration of PCB153 in the CSF was higher in treated animals than controls, while no differences were noted under LD. These findings indicate that in ewes, exposure of the brain to more highly chlorinated, ortho-substituted PCBs may be modulated by photoperiod. PCB153 treatment had no effect on plasma THs, but reduced total triiodothyronine concentration during LD and free thyroxine during SD in the CSF. Under both photoperiods, PCB153 reduced basal plasma LH and reinforced the inhibition of pulsatile LH release during LD. As PCB153 reduced LH and THs (which are involved in the seasonal control of reproduction in ewes), it may have a braking effect on seasonal transitions between active and inactive phases of reproduction.

Locus ceruleus lesions block pulsatile LH release in ovariectomized rats

Luteinizing hormone (LH) secretion during the reproductive cycle and in ovariectomized (OVX) rats is pulsatile and this pattern of secretion is determined by intermittent discharges of LH-releasing hormone (LHRH) into the hypophysial portal vessels. LHRH secretion is probably controlled by prior pulsatile norepinephrine (NE) release. The locus ceruleus (LC) is an important source of NE to the LHRH neurons. We have shown previously that LC lesions block the preovulatory LH surge and ovulation and also cause a decrease in plasma LH concentrations in OVX rats. The possible role of the LC in regulating pulsatile LH release has not been explored. Therefore, the aim of this work was to investigate, in OVX rats, the effects of LC lesions on pulsatile LH secretion. LC lesions were produced in adult female rats three weeks after OVX. On the next morning, the jugular vein was catheterized and, on the afternoon of the same day, blood samples (0.3 ml) were withdrawn every 5 min, during 90 min, from conscious freely moving rats. Plasma LH was measured by radioimmunoassay. LC lesions greatly suppressed pulsatile LH secretion by decreasing both LH pulse frequency and amplitude. The basal as well as total secretion of LH were also decreased. This inhibitory effect of the lesions was observed only when at least 50% of the nucleus was destroyed. Data from sham-operated animals as well as those with less than 50% destruction of the LC did not differ from those of the control rats without brain lesions. Since LC lesions induce a decrease in NE content in the preoptic area and median eminence, the inhibition of pulsatile LH release in ovariectomized rats with LC lesions occurs presumably as result of decreased pulsatile NE release into these areas of the brain that decreases both the frequency and the amount of LHRH released per pulse.

Inhibition of hypothalamic GnRH secretion in the ewe by antigonadotropic decapeptide during the estrous cycle and nonbreeding season.

Previous experiments from our laboratory and others have shown that the peptide antigonadotropic decapeptide (AGD) has marked inhibitory effects on luteinizing hormone (LH) secretion in rats and ewes. The first objective of this study was to determine whether AGD inhibits LH secretion by regulating hypothalamic release of gonadotropin hormone (GnRH). AGD (200 microg in 200 microL of 0.3% bovine serum albumin [BSA] saline) or vehicle was infused into the lateral ventricle of ovariectomized (OVX) ewes with hypophyseal-portal cannulae, and GnRH secretion was monitored. The frequency of GnRH and LH pulses in AGD-treated ewes was significantly decreased (p < 0.05) but did not change in the control ewes. The second objective of this investigation was to evaluate changes in hypothalamic sensitivity to AGD in the ewe during the estrous cycle and nonbreeding season. During the estrous cycle, the effects of AGD on LH secretion were assessed following ovariectomy, during the metestrous, diestrous, and proestrous phases of the estrous cycle. The response to AGD during the estrous cycle was compared to its effect during the anestrous season. LH, cortisol, and prolactin (PRL) concentrations were assayed in peripheral blood samples obtained at 10-min intervals over a 6-h period prior to injection of either vehicle (200 microL of 0.3% BSA in 0.9% saline) or AGD (200 microg in 200 microL of vehicle), and for an additional 10 h following treatment. LH pulse frequency decreased after treatment with AGD (p < 0.05) at all times in OVX and intact ewes compared to vehicle-treated controls. During the anestrous season, AGD treatment was more effective in inhibiting LH pulse frequency than during the breeding season (p < 0.05). Furthermore, there was a significant increase (p < 0.05) in mean cortisol concentrations after AGD infusion in all AGD-treated groups compared to controls independent of season or reproductive status. PRL concentrations were also increased (p < 0.05) following treatment with AGD. These results suggest that inhibition of pulsatile LH release induced by AGD is modulated by alterations in frequency of hypothalamic discharges of GnRH. Furthermore, changes in the inhibitory actions of AGD may contribute to the seasonal regulation of hypothalamic GnRH secretion in the ewe.

Effect of Hypothalamic Deafferentation on the Pulsatile Secretion of Luteinizing Hormone in Ovariectomized Lactating Rats

Abstract The pulsatile luteinizing hormone (LH) secretion in ovariectomized lactating rats bearing complete (CD), anterior (AD), anterolateral (ALD), posterior (PD), or roof (RD) deafferentation of the hypothalamus was determined. All lactating rats were ovariectomized on Day 2 of lactation (Day 0, day of parturition). The deafferentation of nerve fibres to the mediobasal hypothalamus was performed on Day 6 or 7 of lactation. Twenty-four h after the surgery, blood samples were taken through the indwelling atrial catheter every 6 min for 3 h. Plasma concentrations of LH and prolactin (PRL) were measured by radioimmunoassay. The loss of LH pulses associated with lactation was still apparent following AD, PD and sham-deafferentation (SD); pulsatile LH secretion was, however, present in rats with CD, ALD and RD despite continued suckling. The only significant difference in plasma PRL concentrations among the various groups was a reduction in the PRL level in rats with RD in comparison to those with SD. We conclude that the neural signal responsible for the inhibition of pulsatile LH release by suckling is conveyed through the dorsal part of the hypothalamus and PRL does not mediate the suppression of LH pulses in mid-lactation.

Episodic luteinizing-hormone release in the ovariectomized female guinea pig: rapid inhibition by estrogen.

Negative feedback of estrogen was investigated in ovariectomized female guinea pigs. Two weeks after ovariectomy, indwelling catheters were inserted into the jugular vein, and 3 days later, blood samples were taken every 10 min to determine the pattern of luteinizing hormone (LH) secretion. LH secretion in these guinea pigs was episodic, with a mean pulse period of 32 min. The mean pulse amplitude was 2.1 ng/ml, with mean plasma LH levels of 1.8 ng/ml. Twenty-five micrograms 17 beta-estradiol (E2), given i.v., caused a pronounced inhibition of pulsatile LH release. Twenty-five microliters of 100% ethanol (vehicle) had no effect on plasma LH values. In a second set of experiments, ovariectomized female guinea pigs were given two injections of luteinizing hormone-releasing hormone (LHRH) (1 microgram/kg BW, i.v.) separated by 30 min. Sharp rises in serum LH values were detected after each injection. A third injection of LHRH was administered after an injection of either 25 micrograms E2 or 25 microliters vehicle. In the presence of E2, the LH response was significantly (p less than 0.005) diminished, whereas the vehicle did not change the LH response to LHRH. These rapid effects of E2 on LH secretion and the pituitary responsiveness to LHRH infusion indicate that in the ovariectomized guinea pig E2 can directly block gonadotropin secretion. These findings are consistent with the hypothesis that negative feedback actions of E2 are directly on the membrane of the gonadotrope.

Ontogeny of ovarian inhibition of pulsatile luteinizing hormone secretion in postnatal holstein heifers

The hypotheses that pulsatile secretion of luteinizing hormone (LH) increases in heifers between 3 and 9 weeks of age and that ovarian inhibition of pulsatile LH release is first established between 3 and 9 weeks of age were tested in Holstein heifers. Heifers were bilaterally ovariectomized at 3 (n=5), 6 (n=4) or 9 (n=4) weeks of age. Blood samples were collected via indwelling jugular cannulae at 10-min intervals for 8 hr 1 week before and 1 and 4 weeks after ovariectomy. During the preovariectomy sampling period, no LH pulses were detected in the 3- or 6-week-old heifers, while one LH pulse occurred in three of four heifers in the 9 week age group. Both mean plasma LH concentration and number of LH pulses increased (P<0.01) by 1 week after ovariectomy only in the 6 and 9 week age groups. Between 1 and 4 weeks following ovariectomy, mean LH concentration increased (P<0.01) in all age groups, while number of LH pulses increased (P<0.01) only in the 3 week group. When the data from the three age groups were combined, amplitude of LH pulses increased (P<0.01) between 1 and 4 weeks following ovariectomy. Mean plasma levels of corticoids did not differ among age groups within sampling periods. We conclude that ovarian inhibition of pulsatile LH secretion is established by 6 weeks of age in Holstein heifers and that the initial increase in mean LH concentration following ovariectomy is mainly due to an increased frequency of LH pulses and the subsequent increase is due to a greater amplitude of LH pulses. Furthermore, the delayed development of pulsatile LH secretion following ovariectomy in the 3-week-old heifers provides a basis for further studies to determine the integrity of the hypothalamic-pituitary axis in postnatal heifers.

Effect of Lesions in the Suprachiasmatic Nucleus-Retrochiasmatic Area on the Inhibition of Pulsatile LH Release Induced by Electrical Stimulation of the Midbrain Dorsal Raphe Nucleus

The present studies were designed to determine if the inhibition of pulsatile luteinizing hormone (LH) secretion reported to occur during midbrain dorsal raphe nucleus (DRN) stimulation involved some portion of the suprachiasmatic nucleus(SCN)-retrochiasmatic area (RCA). In animals having lesions that completely destroyed the SCN but did not extend into the RCA, dorsal raphe-induced inhibition of pulsatile LH release was still present. If, however, a significant portion (more than 40%) or all of the RCA was encompassed by the lesion (with variable or no SCN damage), the inhibition induced by DRN stimulation was prevented. This indicates that the SCN is not necessary for the inhibitory effect of DRN stimulation on pulsatile LH release to occur. However, the pathway responsible for this inhibition does appear to project to or through the RCA. Furthermore, pulsatile LH release was reduced, but present, during the control period prior to stimulation in rats with SCN lesions (and no damage to the RCA), and mean blood LH levels for this group were significantly decreased during the control bleeding period. Therefore, the neuronal pathway activating episodic LH release involves, but is not restricted solely to, the SCN.