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The object of this study was to examine ovarian regulation of pulsatile luteinizing hormone (LH) secretion during early gestation. This was done primarily by analyzing pulsatile LH release in rats that were either sham ovariectomized (OVX) on Day 7 of pregnancy, implanted with empty Silastic capsules, and bled on Day 8, or OVX on Day 7, immediately implanted with Silastic capsules producing plasma levels of estradiol and/or progesterone characteristic of Day 7 to 8 of pregnancy, and bled on Day 8. In addition, the role of progesterone in regulating pulsatile LH secretion was also examined by administration of the progesterone receptor antagonist, RU486, on Day 7 and examining pulsatile LH release on Day 8 of pregnancy. OVX caused a marked increase in LH pulse amplitude and frequency within 24 h. Replacement with physiological plasma levels of estradiol or progesterone alone had no suppressive effect on this OVX-induced increase in pulsatile LH secretion. Restoration of physiological plasma levels of both estradiol and progesterone returned LH pulse amplitude to values seen in sham OVX controls, and prevented the OVX-induced increase in LH pulse frequency. The group mean LH pulse frequency tended to be less in estradiol + progesterone-treated rats than in sham OVX controls, but this difference was not statistically significant. RU486 blocked uterine progesterone receptors as evidenced by endometrial hemorrhaging. In agreement with the OVX + steroid replacement data, RU486 administration also resulted in increases in LH pulse amplitude and frequency. These data demonstrate that the frequency and amplitude of LH pulses on Day 8 of gestation are held in check by negative feedback signals coming from the ovary. Neither steroid alone exerts any suppressive influence over pulsatile LH secretion during early gestation, but both steroids acting together exert a prominent negative feedback regulation on the pulsatile LH release process.

The object of this study was to examine the influence of both estradiol (E(2)) and progesterone (P) alone or in combination on luteinizing hormone (LH) pulse amplitude and frequency during the interval between Days 21 and 22 of gestation. This was done by analyzing pulsatile LH release in rats bled on Days 21 and 22 of gestation, and in animals ovariectomized (OVX) on Day 21, implanted with silastic capsules producing plasma levels of E(2) and/or P characteristic of the Day 21 to 22 interval, and bled on Day 22 Pulsatile LH release increased between Days 21 and 22 due to an increase in pulse frequency and a small elevation in pulse amplitude. OVX produced no further increase in pulse frequency but markedly enhanced the small change in pulse amplitude. Preventing either the decline in plasma P that normally occurs between Days 21 and 22, or just the small additional decrease in plasma P levels produced by OVX, had no suppressive effect on pulse amplitude or frequency, although Day 22 levels of P alone augmented the normal increase in pulse frequency occurring between Days 21 and 22. Restoration of physiological plasma E(2) levels had no effect on the normal increase in pulse frequency, but partially attenuated the OVX-induced increase in pulse amplitude. Replacement of physiological Day 22 levels of both E(2) and P also decreased LH pulse amplitude, although amplitude was not significantly different from that seen following E(2) replacement alone, and was still greater than the normal Day 22 value. In contrast, restoration of physiological plasma levels of E(2)+ P caused a suppression of LH pulse frequency below that normally seen on Day 22. While E(2)+ P did not completely prevent the OVX-induced increase in pulse amplitude, administration of charcoal-extracted porcine follicular fluid to rats OVX on Day 21, and in which physiological plasma levels of E(2)+ P were restored, caused a further reduction in pulse amplitude. These data demonstrate that 1) marked increases in LH pulse amplitude are prevented from occurring between Days 21 and 22 of gestation by ovarian steroids, notably E(2), and that this suppression is enhanced by a non-steroidal factor present in porcine follicular fluid, 2) neither E(2) or P alone suppresses LH pulse frequency on Day 22 of gestation; LH pulse frequency increases on Day 22 because the plasma level of one of these steroids, P, markedly declines, and 3) restoration of physiological plasma levels of both steroids in the absence of the ovary produces an unphysiological suppression of pulse frequency, i.e. results in a lower pulse frequency than normally occurs in the presence of these same plasma steroid levels in animals with their ovaries intact. One hypothesis consistent with the latter observation is that at the end of gestation in the rat the ovary may produce a factor which 'protects' the frequency of the LH pulse generator from the negative feedback action of ovarian steroids. This allows an increase in LH pulse frequency and mean blood LH levels, and thereby facilitates ovarian follicular development and the normal progress of the first postpartum estrous cycle.

This study examined the importance of pulsatile luteinizing hormone (LH) release on diestrus 1 (D1; metestrus) in the rat estrous cycle to ovarian follicular development and estradiol (E2) secretion. Single injections of a luteinizing hormone-releasing hormone (LHRH) antagonist given at -7.5 h prior to the onset of a 3-h blood sampling period on D1 reduced mean blood LH levels by decreasing LH pulse amplitude, while frequency was not altered. Sequential injections at -7.5 and -3.5 h completely eliminated pulsatile LH secretion. Neither treatment altered the total number of follicles/ovary greater than 150 mu in diameter, the number of follicles in any size group between 150 and 551 mu, or plasma E2, progesterone, or follicle-stimulating hormone (FSH) levels. However, both treatments with LHRH antagonist significantly increased the percentage of atretic follicles in the ovary. These data indicate that: 1) pulsatile LH release is an important factor in determining the rate at which follicles undergo atresia on D1; 2) reductions in LH pulse amplitude alone are sufficient to increase the rate of follicular atresia on D1; 3) an absence of pulsatile LH release for a period of up to 10 h on D1 is not sufficient to produce a decline in ovarian E2 secretion, most likely because the atretic process was in its early stages and had not yet affected a sufficient number of E2-secreting granulosa cells to reduce the follicle's capacity to secrete E2; and 4) suppression or elimination of pulsatile LH release on D1 is not associated with diminished FSH secretion.

The object of this study was to further define the roles of both norepinephrine (NE) and epinephrine (EPIN) in regulating pulsatile luteinizing hormone (LH) release in 4-day ovariectomized rats, in particular to examine the effect of decreasing NE synthesis on pulsatile LH secretion in animals with already greatly depleted levels of brain EPIN. Rats were injected ip with vehicle or drug at -27, -20, -5 and - 3 h relative to the onset of a 3-h blood sampling period. Hypothalamic-preoptic area (HPOA) levels of NE and EPIN were determined by high-performance liquid chromatography. Compared to controls, FLA-63 (25 mg/kg, a dopamine-ss- hydroxylase inhibitor), given at - 3 h, produced 50% and 22% declines in HPOA-NE and EPIN, respectively, and reductions in pulse amplitude and frequency. LY134046 (50 mg/kg, a phenylethanolamine N-methyltransferase inhibitor), given at - 27, - 20 and - 5 h, or -27, -20, -5 and -3 h, produced no change in NE, 88% and 86% declines in EPIN, respectively, and reductions in pulse frequency only. Each LY134046 treatment protocol produced the same decline in EPIN and pulse frequency. Thus, EPIN levels were maximally decreased by three LY134046 injections. When rats were given LY134046 at -27, -20 and -5 h, and FLA-63 at -3 h, compared to rats treated with LY134046 alone, there was no further decrease in HPOA-EPIN (82% decline), a 46% decline in NE, a further reduction in pulse frequency and a reduction in pulse amplitude. This further suppression of LH release must be due to a reduction in HPOA-NE levels since no further decrease in EPIN levels occurred. These data demonstrate within the same animal that NE and EPIN are both stimulatory to pulsatile LH release. NE stimulates the amplitude and frequency, and EPIN stimulates the frequency of pulsatile LH secretion.

This study examined the importance of pulsatile luteinizing hormone (LH) release on diestrus 1 (D1; metestrus) in the rat estrous cycle to ovarian follicular development and estradiol (E2) secretion. Single injections of a luteinizing hormone-releasing hormone (LHRH) antagonist given at -7.5 h prior to the onset of a 3-h blood sampling period on D1 reduced mean blood LH levels by decreasing LH pulse amplitude, while frequency was not altered. Sequential injections at -7.5 and -3.5 h completely eliminated pulsatile LH secretion. Neither treatment altered the total number of follicles/ovary greater than 150 mu in diameter, the number of follicles in any size group between 150 and 551 mu, or plasma E2, progesterone, or follicle-stimulating hormone (FSH) levels. However, both treatments with LHRH antagonist significantly increased the percentage of atretic follicles in the ovary. These data indicate that: 1) pulsatile LH release is an important factor in determining the rate at which follicles undergo atresia on D1; 2) reductions in LH pulse amplitude alone are sufficient to increase the rate of follicular atresia on D1; 3) an absence of pulsatile LH release for a period of up to 10 h on D1 is not sufficient to produce a decline in ovarian E2 secretion, most likely because the atretic process was in its early stages and had not yet affected a sufficient number of E2-secreting granulosa cells to reduce the follicle's capacity to secrete E2; and 4) suppression or elimination of pulsatile LH release on D1 is not associated with diminished FSH secretion.