Unexpected responses of the hypothalamic gonadotropin-releasing hormone "pulse generator" to physiological estradiol inputs in the absence of the ovary

In the rhesus monkey (Macaca mulatta), the frequency of pulsatile gonadotropic hormone release is relatively constant in the face of widely varying levels of estradiol (E2) in the peripheral circulation--e.g., in the course of the follicular phase of the menstrual cycle and after ovariectomy. This suggests that modulation of the hypothalamic gonadotropin-releasing hormone (GnRH) "pulse generator" by this steroid is not of major physiological importance. Herein is described the unexpected inhibition or total blockade of the electrical activity of this pulse generator in ovariectomized monkeys by physiological levels of exogenous E2. This inhibition began 2-4.5 hr after the initiation of E2 infusion and was noted 1 to 3 weeks after subcutaneous implantation of E2-containing capsules. Pulse generator activity was also arrested during the initiation and subsequent development of estrogen-induced surges of luteinizing hormone. We propose that this inhibition of hypothalamic GnRH pulse generator activity by E2 in ovariectomized monkeys reflects the absence of an ovarian factor that normally protects this neuronal system from the inhibitory action of estrogen during the menstrual cycle.

Gas Chromatographic-Thermal Energy Analysis Method for iV-Nitrosodibutylamine in Latex Infant Pacifiers: Collaborative Study

Each of 5 collaborating laboratories determined volatile N-nitrosamines in 3 blind quadruplicate sets of latex rubber infant pacifier samples, using a gas chromatographic-thermal energy analysis (GCTEA) method. Volatile N-nitrosamines are extracted from cut-up pacifier nipples with CH2C12. The extract is concentrated and subjected to high temperature purge and trap, and the nitrosamines are eluted from the trap and determined by GC-TEA. N-Nitrosodibutylamine (NDBA) was the only nitrosamine found in sufficient concentration to allow analysis. NDBA concentrations of the 3 sets of samples were 82.6,21.0, and 7.12 ng/g rubber. The repeatability relative standard deviations ranged from 7.46 to 24.0% and the reproducibility relative standard deviations from 7.46 to 29.2%. The minimum detectable level of NDBA by this method is 3.6 ng/g rubber. The method has been adopted official first action.

Changes in sex hormone-binding globulin binding capacity and percent free estradiol during development in the female rhesus monkey (Macaca mulatta): relation to the metabolic clearance rate of estradiol

Circulating sex hormone-binding globulin (SHBG) binding capacity and percent free estradiol (% free E2) were measured in separate groups of female rhesus monkeys from 2 months of age through adulthood (greater than 4.5 yr old). The SHBG concentration alone was also measured in 11 adult and 6 sexually immature animals on the same day as the blood MCR of E2. Serum SHBG levels were the highest (range, 12-25 micrograms T/dl) and the % free E2 the lowest (0.88%) in animals 6 months old or less. After the age, serum SHBG binding capacity declined generally at an average rate of 0.11 SHBG U (microgram T/dl)/month toward a nadir in adulthood. There was no difference in the SHBG levels in the follicular or luteal phase of the menstrual cycle. The relative blood MCR E2 and circulating SHBG binding capacity were significantly greater (P less than 0.001) in the sexually immature animals [MCR E2, 48.4 +/- 5.2 (SEM) liters/day . kg body wt; SHBG, 9.8 +/- 1.0 microgram T/dl, n = 6] than in adult animals (MCR E2, 27.7 +/- 1.7 liters/day kg body wt; SHBG, 4.6 +/- 0.3 microgram T/dl, n = 11). There was no relation between the MCR E2 and circulating SHBG levels within each group of adult or immature animals. The mean % free E2 doubled (to 1.6%) between 1 and 54 months of age; there was no relation between total circulating E2 and % free E2. Although a high SHBG binding capacity and a low % free E2 in the circulation of the immature animal does not inhibit the metabolic clearance of E2; it remains possible that these factors (and others) may hinder the access of E2 to reproductive target tissues and thereby contribute to the slow acquisition of reproductive competence in this species.

The metabolic clearance rate and the production rate of estradiol in sexually immature and adult female rhesus monkeys

The blood MCR of estradiol (MCRE2) was measured in 34 experiments with 10 adult (4.7-8.2 kg) and 13 prepubertal (1.8-3.0 kg; 13-23 months old) female rhesus monkeys using the constant infusion technique. Twenty-six of the studies were performed using an adult and an immature animal simultaneously. Twenty-four of the studies were performed in pentobarbital-anesthetized animals, while the remainder used conscious animals restrained in primate chairs. The blood MCRE2 in the adult female was 167.5 +/- 9.5 liters/day (mean +/- SE; n = 14) or 27.5 +/- 1.4 liters/day x kg BW, and was not altered by anesthesia, stage of the menstrual cycle, amenorrhea of more than 60 days duration, or the site of origin of the blood used to calculate the MCR (radial artery, femoral artery, femoral vein, or saphenous vein). While the absolute MCRE2 in the immature animal (either anesthetized or conscious) was less than that in the adult, when corrected for body weight, the relative MCRE2 (in liters per day/kg BW) of the conscious immature animal was double that seen in the adult [48.4 +/- 5.2 (n = 6) vs. 27.5 +/- 1.4 (n = 1.4)]. Anesthesia caused a profound depression of the MCRE2 in the immature animal, which could be prevented if the body temperature of the animal was maintained at 37 C during the prolonged period of anesthesia. The production rate of estradiol (PRE2) was calculated as the product of the serum estradiol concentration (in micrograms per liter; measured by RIA techniques) and the plasma MCRE2 (blood MCRE2 x 1 - hematocrit). In the adult animals, the PRE2 ranged from 1.9 - 35.5 micrograms/day, and was lowest in the amenorrheic animals and highest during the late follicular phase. The PRE2 in the immature animals ranged from unmeasurable to 1.7 micrograms/day, averaging 0.7 +/- 0.2 micrograms/day (n = 12) in those animals where it could be measured. These data support the hypothesis that the low circulating estradiol levels in the immature animal are the consequence of a low PRE2 coupled with a high MCRE2.

Relation between levels of circulating ovarian steroids and pituitary gonadotropin content during the menstrual cycle of the rhesus monkey

Anterior pituitary glands were removed from 27 intact cycling rhesus monkeys sacrificed in the early (Day 2), mid (Days 6--9) and late (Days 11--12) follicular phase, and in the early and late luteal phase (3--5 and 10--15 days after the midcycle luteinizing hormone (LH) surge). Assignment of cycle stage was confirmed by the pattern of circulating steroid and gonadotropin levels seen in the blood samples taken daily throughout the cycle. The anterior pituitary glands were weighed, stored at -30 degrees C and assayed for LH and follicle-stimulating hormone (FSH) content by specific radioimmunoassays. Serum estradiol levels and pituitary LH and FSH contents rose simultaneously during the follicular phase. After the preovulatory gonadotropin surge, pituitary LH content was low and invariant. Pituitary FSH content reached a nadir in the early luteal phase and tended to rise in the late luteal phase. Multiple correlation analyses revealed that there is a positive correlation between rising levels of estradiol in the circulation and pituitary LH (p = 0.003) and FSH (p = 0.017) content, and that there is a significant negative correlation between circulating progesterone levels and pituitary FSH content (p = 0.002). Pituitary LH content is less strongly related to circulating progesterone levels. There was no significant difference in the wet weights of the anterior pituitary glands during the five phases of the menstrual cycle studied.

Monkey pituitary oestrogen receptors and the biphasic action of oestradiol on gonadotropin secretion

Evidence has been advanced in favour of the hypothesis that, in the rhesus monkey, the negative feedback as well as the stimulatory or so-called "positive feedback" action of oestrogen on gonadotropin secretion is at the level of the pituitary gland. The cellular basis for this biphasic action of oestrogen on the gonadotroph is not understood. In oestrogen target tissues the elicitation of physiological responses is generally associated with the binding of oestrogen to cytoplasmic receptor proteins which are subsequently translocated to the nucleus. Such receptors have been characterized in the anterior pituitary of the rat. In the ovariectomized rhesus monkey refused with 3H-oestradiol, autoradiography and parallel cell fractionation have demonstrated the accumulation of 3H-oestradiol by the nuclei of a significant number of pituitary cells. We describe here some of the properties of these putative oestrogen receptors in the anterior pituitary of the monkey and observe that their distribution between cytoplasmic and nuclear compartments is constant in response to sustained elevations in serum oestrogen concentration which produce a biphasic pattern of circulating gonadotropins.

Periparturitional serum concentrations of prolactin, the gonadotropins, and the gonadal hormones in the rhesus monkey

The time courses of serum concentrations of prolactin, estradiol, estrone, progesterone, LH, and FSH were studied in seven pregnant rhesus monkeys from 1 month prior to delivery until 1 month after parturition. All animals nursed their young. Circulating levels of estradiol and estrone increased during the last few days of pregnancy, reaching peak values of 700 pg/ml and 350 pg/ml, respectively, on the day prior to delivery, fell precipitously to about 25 pg/ml within 1 day after parturition, and remained at this level for at least 30 days. Serum prolactin concentrations also increased during the week preceding parturition, rose abruptly at delivery, and then declined gradually. Serum progesterone levels ranged between 2 and 3 ng/ml during the last month of pregnancy, rose slightly a few days prior to parturition, decreased sharply at delivery to 50% of prepartum levels and declined gradually thereafter. Serum LH and FSH levels were not detectable during the entire sampling period. The administration of estradiol benzoate to two pregnant monkeys at midgestation, in a manner which replicated the normal prepartum increase in serum estradiol concentrations, failed to elicit an elevation in circulating prolactin levels or to induce premature delivery of the fetus.

Functional luteolysis in the rhesus monkey: ovarian estrogen and progesterone during the luteal phase of the menstrual cycle

In a continuing effort to elucidate the processes underlying in primates, the concentrations of estradiol, estrone and progesterone were measured in ovarian tissues and in ovarian vein plasma through the luteal phase of the menstrual cycle of the rhesus monkey. The concentration of progesterone in corpora lutea collected 4-6 days after the preovulatory LH peak (early luteal phase) was more than twice that found in those collected 8-13 days after the LH surge (late luteal phase) while estradiol and estrone concentrations in the latter had increased 4-fold. These changes in luteal steroid concentrations were paralleled by a striking increase in the concentration of estrone in the ipsilateral ovarian vein. Estrone predominated in the venous effluent of the ovary beaing the corpus luteum while estradiol concentrations were similar in both ovarian veins suggesting that estrone is the principal estrogen released by the corpus luteum. The results of this study are consonant with the hypothesis that estrogen produced by the corpus luteum is the physiologic luteolsin in the rhesus monkey.

Surgical disconnection of the medial basal hypothalamus and pituitary function in the rhesus monkey. II. GH and cortisol secretion

Eighteen female rhesus monkeys subjected to complete or anterior disconnection of the medial basal hypothalamus (MBH) were studied to assess the effects of these deafferentation procedures on GH and cortisol secretion. Basal serum levels of GH were not disturbed or were slightly elevated following complete or anterior MBH disconnection. GH secretion in response to vasopressin administration or insulin hypoglycemia, however, was abolished by complete isolation of the MBH. In contrast, the elevations in serum cortisol concentrations observed in response to these noxious stimuli were not noticeably affected. The normal diurnal rhythm in cortisol secretion remained fully evident following anterior deafferentation, but was severely attenuated or abolished when all neuronal inputs to the MBH were transected. Such observations suggest that the central components of the neuroendocrine systems which regulate basal GH secretion and which subserve stress-induced elevations in cortisol secretion are resident within the MBH-hypophysial unit. In addition, these data indicate that the mechanisms underlying the diurnal rhythm in cortisol secretion, as well as those mediating the discharges of GH in response to vasopressin administration and insulin hypoglycemia, are dependent on the integrity of neuronal connections between the MBH and other hypothalamic and/or extrahypothalamic areas.