Abstract

Developmental increases in serum LH were assessed in female rhesus monkeys to test the hypotheses that (1) the final stages of puberty are characterized by a decrease in hypersensitivity to oestradiol negative feedback of LH and (2) that increases in IGF-I secretion accelerate this decrease in hypersensitivity. In order to test the first hypothesis, serum LH in the absence of oestradiol and in response to three doses of oestradiol were compared between ovariectomized adult (n=6) and adolescent female monkeys (control group; n=6). The control females were not treated with oestradiol until serum LH had risen to within the 95% confidence interval of serum LH observed in ovariectomized adults. Doses of oestradiol achieved serum levels of approximately 80 ('low'), 160 ('intermediate'), and 250 ('high') pmol/l. For control group females, treatment with the next higher dose of oestradiol was not initiated until serum LH was no longer suppressed by the lower dose. Treatment with oestradiol produced a dose-dependent suppression in serum LH in adults. In contrast, low-dose oestradiol maximally suppressed serum LH throughout the initial treatment period in the control group compared with the adult females. The low oestradiol dose effectively suppressed serum LH throughout the study period in 4/6 of the control group and became ineffective at suppressing LH after 8 months of treatment in 2/6 control group females. Initiation of the intermediate dose of oestradiol to these females again maximally suppressed LH compared with adult females.

In order to determine whether IGF-I regulates this change in hypersensitivity to oestradiol negative feedback, a second group of ovariectomized, adolescent monkeys (n=6) were treated chronically with IGF-I to elevate serum IGF-I levels above those of control group females. Using the same protocol described for the control females, developmental changes in serum LH in the absence of oestradiol and in response to oestradiol negative feedback were evaluated. Treatment with IGF-I had no effect on the initial increases in serum LH occurring in the absence of oestradiol. In contrast, the decrease in hypersensitivity to the negative feedback effects of the low oestradiol dose was significantly accelerated in IGF-I-treated females, as the interval from the initiation of treatment to the point at which serum LH was no longer suppressed was shorter in IGF-I-treated (4·4±0·7 months; mean ± s.e.m.) compared with control group females (8·4±1·9 months). Although none of the control group females escaped from the negative feedback effects of the intermediate dose of oestradiol during the course of the study, 2/7 of the IGF-I-treated females did so within 5·5±1·4 months of the initiation of the treatment.

The present data indicate that the later stages of puberty in female monkeys are characterized by a decreasing in sensitivity to oestradiol negative feedback inhibition of serum LH and that timing this decrease is regulated by circulating concentrations of IGF-I. These data confirm earlier reports that the developmental increases in the GH axis accelerate the tempo of puberty without affecting its onset.

Journal of Endocrinology (1995) 145, 121–130

Abstract

The present study examined the effects of IGF-I on serum concentrations of IGF binding protein-3 (IGFBP-3) and GH and assessed how treatment with estradiol and IGF-I would stimulate adolescent growth in monkeys with normal pituitary function. In study I, ovariectomized, juvenile female rhesus monkeys (21 months of age; n=6) received a bolus injection of IGF-I (1 mg/kg s.c.) and serum samples were collected periodically through 48 h. The consequential elevation in serum IGF-I resulted in a parallel increase in serum IGFBP-3 at 1 and 3 h after treatment with values returning to baseline by 7 h. In contrast, the elevation in serum IGF-I resulted in a significant decline in serum GH within 3 h of treatment. These data confirm that an elevation in IGF-I increases IGFBP-3 while simultaneously acting in a negative feedback capacity to inhibit GH.

In study II, ovariectomized, juvenile female rhesus monkeys served either as controls (Con, n=6) or received a constant s.c. infusion of IGF-I (300 μg/day; Igf, n=6) from 13 through 32 months of age. At approximately 26 months, females entered an estradiol-treatment protocol in which they received alternating blocks of 3 weeks of estradiol followed by 3 weeks of no estradiol. As found in study I, the elevation in serum IGF-I resulted in a significant increase in serum IGFBP-3 throughout the study in Igf compared with Con females. Estradiol administration significantly increased serum IGF-I and IGFBP-3 levels in both groups. Although the nano-molar ratio of IGF-I to IGFBP-3 was consistently higher in Igf females, the magnitude of the change in IGF-I:IGFBP-3 following estradiol treatment was similar between groups. Finally, the age-dependent increase in serum GH was dampened in Igf compared with Con females and the increase in response to estradiol was less in Igf females. Although total growth in crown-rump length was similar in both groups, Igf females grew more prior to estradiol replacement while Con females grew more once estradiol treatment was initiated. In addition, skeletal maturity was advanced more quickly in Igf females once estradiol treatment had been initiated.

These data suggest that, in female monkeys with normal pituitary function, IGF-I administration inhibits endogenous GH secretion but is capable of stimulating crownrump growth. Although IGF-I increased serum levels of IGFBP-3, the increase was not proportional to the increase in serum IGF-I achieved by the treatment. These data would suggest that IGF-I may regulate the release of this binding protein but that GH may be required to maintain equi-molar proportions of IGF-I to IGFBP-3. In addition, the observation that serum concentrations of IGF-I were increased further in IGF-I-treated females by the administration of estradiol without a change in serum GH, suggests that estradiol has a direct effect on IGF-I synthesis and release independent of GH.

Journal of Endocrinology (1997) 153, 327–335

ABSTRACT

The duration of lactational infertility is prolonged significantly in adolescent, primiparous rhesus monkey (Macaca mulatta) mothers compared with adult, multiparous mothers. The present study examined the hypothesis that this parity/age difference in lactational infertility is due to a difference in the physiological responsiveness to nursing behaviour between adolescent and fully adult mothers and is not a consequence of differences in nursing behaviour, per se. At 22 weeks postpartum, mother–infant pairs were randomly assigned to one of four conditions: primiparous, nursing restricted (PR; n=9); primiparous, nursing unrestricted (PU; n= 11); multiparous, nursing restricted (MR; n= 12); and multiparous, nursing unrestricted (MU; n=8). Nursing was restricted for a 2-week period by mothers wearing a primate vest which prevented suckling behaviour but allowed infants to interact with their mothers. Nursing restriction resulted in a significant increase in serum oestradiol concentrations in both PR and MR mothers. Although nursing bout frequencies and durations were similar between PU and MU mothers, serum oestradiol also rose in MU mothers but remained suppressed in PU mothers. Once the nursing manipulation period ended and all mothers were allowed to nurse ad libitum, serum oestradiol concentrations continued to rise in all but the PU mothers. This brief interruption of nursing at 22 weeks postpartum advanced the subsequent timing of the first postpartum ovulation in MR and PR mothers relative to that of PU mothers. Again, despite similarities in nursing behaviour, the occurrence of first ovulation was also advanced in MU mothers compared with PU mothers.

Just prior to the first postpartum ovulation, females were randomly assigned to one of four treatment groups to determine the effects of nursing behaviour on the hormonal parameters of the luteal phase: primiparous, nursing restricted (PR_L; n = 9); primiparous, nursing unrestricted (PU_L; n = 11); multiparous, nursing restricted (MR_L; n =10); and multiparous, nursing unrestricted (MU_L; n=10). Nursing restriction significantly elevated serum progesterone concentrations in PR_L females compared with other mothers. Serum concentrations of oestradiol were higher in PR_L, MR_L and MU_L mothers relative to PU_L females. Again, this difference in oestradiol between PU_L and MU_L mothers occurred despite similarities in nursing behaviour.

These data suggest that parity/age differences in the period of lactational infertility are not due to differences in nursing behaviour but rather to an increased sensitivity to the inhibitory aspects of the suckling stimulus in adolescent primiparous mothers. Furthermore, the lack of a difference in the reproductive parameters measured between nursing restricted and unrestricted multiparous mothers suggests that these females may become refractory to the inhibitory aspects of the suckling stimulus as lactation progresses.

Journal of Endocrinology (1992) 134, 493–503

ABSTRACT

Lactational infertility in rhesus monkeys is significantly prolonged in adolescent primiparous compared with adult multiparous mothers. In order to determine if this longer period of infertility for young mothers is the result of a greater sensitivity to nursing-induced inhibition of LH release either by enhanced oestradiol negative feedback or a direct non-gonadally mediated suppression, the effects of periodic administration of oestradiol on serum LH concentrations in nursing ovariectomized adolescent primiparous (Prp; n = 5) and adult multiparous (Mlt; n = 7) mothers was assessed. Females were treated every 5–6 weeks with a 21-day time-release capsule of oestradiol which produced serum concentrations of approximately 250, 90 and 45 pmol/l by +6, +13 and +20 days after treatment. Thus, the design permitted assessment of LH and prolactin concentrations under a regime of 21 days of decreasing oestradiol levels followed by 2–3 weeks of no oestradiol treatment. Females were studied from week 3 to week 42 post partum and oestradiol treatment occurred during weeks 5, 11, 21, 26, 31, 36 and 41.

Behavioural observations indicated that the amount of time mothers nursed their offspring decreased in a similar fashion throughout the lactational period for both Prp and Mlt females. LH concentrations under the 'no oestradiol' conditions progressively increased throughout lactation reaching maximum levels by week 36 post partum in a similar manner for both Prp and Mlt mothers. These data suggest that differences in fertility between adolescent and adult nursing mothers observed previously cannot be attributed to a difference in a direct non-gonadally-mediated affect on LH.

With respect to oestradiol negative feedback inhibition of LH, oestradiol treatment effectively suppressed serum LH concentrations at all points during lactation up to week 31, at which time LH concentrations were maximally suppressed in both Prp and Mlt mothers at + 6 days after treatment but by day + 13 LH values were significantly higher in Mlt females. After oestradiol treatment during week 36, LH values were again maximally suppressed on day +6 in Prp females but not in Mlt females. However, the response of serum LH after oestradiol treatment during week 41 was similar between Prp and Mlt females at all sampling points. Thus, the effectiveness with which oestradiol suppressed LH concentrations diminished at an earlier point in lactation in Mlt compared with Prp mothers. Furthermore, this parity/age difference in oestradiol negative feedback inhibition of LH was not associated with differences in nursing behaviour. These data suggest that the prolonged period of lactational infertility in adolescent primiparous monkey mothers is the result of a hypersensitivity to oestradiol negative feedback inhibition of LH which is still functional at a time when the nursing stimulus has diminished.

Journal of Endocrinology (1993) 136, 127–136

ABSTRACT

Female rhesus monkeys (n = 5), having normal pituitary function, were treated for 50 months with recombinant human growth hormone (rhGH; 250 μg/kg) 3 days/week (Monday, Wednesday and Friday) and rates of growth were compared with a group of age-matched untreated females (n = 6). Treatment was initiated at 20 months of age, approximately 10 months before the expected age of menarche. Long-term treatment with rhGH accelerated bone maturation and increased the velocity of increase in crown–rump length, tibia length and body weight. The period of acceleration occurred coincident with the occurrence of spontaneous puberty. Body measurements remained larger in the treated females until growth ceased. Long-term rhGH treatment increased final adult crown–rump length by some 3%, with a slight increase in tibia length and body weight, without having any untoward effects on reproductive capacity or health. One treated animal exhibited higher estimates of antibodies to rhGH throughout the study period, and this female also had a smaller increment in crown–rump length than the other treated females. These data suggest that long-term treatment of normal-pituitary females with rhGH augments crown–rump growth without any untoward effects of health.

Journal of Endocrinology (1991) 130, 435–441

SUMMARY

Marked pubic separation, comparable to that in the guinea-pig, mouse and deermouse, was observed to occur in the Skomer bank vole towards the end of gestation when the pubic gap measured 4–5 mm. Ovariectomized Skomer voles treated with oestradiol showed a pubic separation of 0·8–0·9 mm., while treatment with oestradiol and relaxin caused a separation of 2·5–4·6 mm. Relaxin alone had no effect.

ABSTRACT

Diurnal concentrations of serum melatonin were determined longitudinally in female rhesus monkeys throughout sexual maturation to ascertain how levels varied with advancing age and reproductive onset. Females were housed either in outdoor enclosures (n = 8) exposed to ambient environmental conditions, or indoors (n = 4) under a photoperiod of 12 h light: 12 h darkness and fixed temperature of 20–23 °C. Animals were studied from immaturity (15 months) through first ovulation and were additionally compared with fully adult female rhesus monkeys (n = 5) studied during the annual breeding season. The diurnal melatonin pattern was described for the developing females in the summer, autumn and winter in 3 successive years from samples collected at 10.00, 18.00, 22.00, 02.00, 06.00 and 10.00 h.

Nocturnal levels of melatonin declined significantly during development in both indoor- and outdoor-housed females with a progressive decrease up to 33 months of age. Daytime values were consistently low but exhibited a slight decline also with age. Nocturnal values in all months sampled fell significantly with greater decreases occurring at the earliest ages. Furthermore, superimposed upon this developmental change, animals housed outdoors responded to seasonal changes in photoperiod with diurnal increases in melatonin occurring after sunset. The females in the present study exhibited first ovulation at two distinct ages: 32–37 months ('early', n = 6) and 41–45 months ('later', n=5). One female did not ovulate within the study period. Although nocturnal levels of serum melatonin were similar between the two groups up to 29 months of age, a post-hoc analysis revealed that concentrations were significantly lower by 34 months of age for the early group, a time coincident with first ovulation. Nocturnal levels of melatonin remained high, relative to the early group, in the later ovulating females until 43 months of age, coincident with first ovulation for these animals. The diurnal pattern of serum melatonin at first ovulation, regardless of chronological age, was similar to that observed during the ovulatory season for adult female rhesus monkeys.

These data suggest that nocturnal melatonin concentrations decline with advancing chronological age in prepubertal female rhesus monkeys. Furthermore, the timing of sexual maturation was inversely related to nocturnal melatonin. Whether this change in melatonin is causally related to reproductive onset or, rather, is a consequence of other factors regulating the occurrence of first ovulation remains to be determined. Furthermore, the observation that the melatonin rhythm in outdoor-housed females follows the prevailing photoperiod permits the hypothesis that this rhythm may mediate any photoperiodic effect on the seasonal occurrence of first ovulation characteristic of rhesus monkeys housed outdoors.

Journal of Endocrinology (1989) 121, 553–562

ABSTRACT

The effects of oestradiol (OE₂) on adolescent growth in female rhesus monkeys were evaluated by testing the hypothesis that, upon removal of the ovary, the increase in growth normally seen at the time of puberty would be abolished and that treatment with OE₂ would restore it. Juvenile monkeys (n= 12) were ovariectomized and were given either an OE₂-bearing silicone elastomer capsule implanted subcutaneously to simulate mid-pubertal concentrations ('treated = ', n = 8) or no steroid treatment ('control = ', n = 4). Females were studied from 18 to 42 months of age which, in intact females, typically encompasses the prepubertal period to the occurrence of first ovulation. Over the whole period, growth in body weight, crown–rump (CR) length and tibia length for control females were less than the 95% confidence limits of females treated with OE₂. However, significant spurts of growth in both CR and tibia length occurred in the control as well as treated animals, although the peak velocities were somewhat lower for non-OE₂-treated animals. Peak growth velocities occurred at an earlier chronological age in treated females, although at the same degree of skeletal maturity as found in control females. Skeletal maturity was significantly advanced in treated females from 27 months onward. Serum concentrations of nocturnal GH increased significantly with advancing age in both groups, with greater increases observed in treated females. Serum concentrations of IGF-I were higher in treated females until some 30 months of age, at which point concentrations increased in a similar fashion in both groups. IGF-I concentrations were elevated in the months preceding and following the peak CR growth velocity in treated females whereas concentrations of IGF-I rose coincidently with peak CR growth in control females. These data indicate that, in the absence of OE₂, growth in female monkeys has periods of acceleration and that OE₂ may enhance this pre-set pattern. Furthermore, the growth-promoting effects of OE₂ are determined by the degree of skeletal maturity. Once a certain degree of skeletal maturity is attained, bones may be primed for maximal growth, but, as skeletal maturity continues to advance, the effect of OE₂ on growth diminishes.

Journal of Endocrinology (1993) 137, 519–527

ABSTRACT

Nocturnal concentrations of melatonin in serum decline significantly with advancing pubertal development in both children and non-human primates and elevated levels may be associated with anovulation in adults. Three studies, using female rhesus monkeys, were performed to determine whether (1) the decline in nocturnal melatonin concentrations in adolescents was due to maturational increases in serum oestradiol, (2) the experimental elevation in nocturnal melatonin would delay the normal progression of puberty in post-menarchial monkeys, and (3) the experimental elevation in nocturnal melatonin would disrupt normal ovulatory function in adults. In experiment 1, juvenile female rhesus monkeys, housed indoors in a fixed photoperiod (12 h light: 12 h darkness), were assigned randomly to one of two treatment groups: ovariectomized with no replacement therapy (control; n= 4) or ovariectomized with oestradiol replacement therapy maintaining oestradiol at ∼ 90 pmol/l (treated; n= 8). Twenty-four hour as well as daytime serum samples were collected from 19 to 35 months of age. Nocturnal melatonin concentrations declined significantly in all females with advancing chronological age and this change was unaffected by oestradiol treatment. The decline in nocturnal melatonin concentrations occurred, on average, 2·0 ±0·2 months after the initial rise in serum LH in control females and 6·0 ±0·8 months in treated females. Furthermore, this decline in night-time melatonin was not related to significant developmental changes in body weight.

In experiment 2, control (n = 6) and melatonintreated (treated; n =6) adolescent female monkeys were studied from −30 to +105 days from menarche. Beginning at 45 days following menarche, treated females received 30 days of nocturnal melatonin infusion to elevate levels to prepubertal values. Developmental changes in perineal swelling and coloration as well as serum oestradiol and insulin-like growth factor-I (IGF-I) were compared with values observed during the 45-day pretreatment and 30-day post-treatment conditions as well as with those observed in control females. Despite a significant elevation in nightly melatonin levels for the 30-day period in treated females, developmental changes in oestradiol, IGF-I, and perineal coloration and swelling were not different compared with the control females.

In experiment 3, adult females were given melatonin nightly beginning on the first day of menses following an ovulatory cycle and treatment was continued for 45 days or until the next menstruation occurred. Melatonin was elevated to supraphysiological levels every night throughout the treatment period. Despite this elevation, an ovulation, inferred from serum progesterone levels, occurred in every female and serum oestradiol, LH or progesterone were not affected compared with the values obtained during the untreated cycle.

These data indicate that the decline in nocturnal melatonin concentrations is not related to a developmental increase in oestradiol secretion. Furthermore, experimentally elevated concentrations of nocturnal melatonin did not delay the normal progression of puberty following menarche nor did it disrupt ovulatory function in adults. These data suggest that the enhanced nocturnal melatonin concentrations are not causally linked to either puberty onset or anovulatory conditions in adults.

Journal of Endocrinology (1993) 137, 299–309

ABSTRACT

The influence of gonadal status on the pattern of changes in body weight was examined in three groups of outdoor-housed premenarchial rhesus monkeys (Macaca mulatta) from 1 to 2 years of age. Gonadally intact (n=8) and ovariectomized oestradiol-treated females (n=5) gained significantly more weight than ovariectomized untreated females (n=5). The overall larger weight gains for the intact and ovariectomized oestradiol-treated groups were associated with significantly higher fasting levels of serum insulin. All subjects exhibited two periods of significant weight gain from 12 to 16 months of age between May and September and from 21 to 24 months of age between February and May. Serum GH and somatomedin-C levels were also raised during these periods of accelerated growth. Although the pattern of serum GH was similar among groups, serum somatomedin-C levels were significantly higher in both intact and ovariectomized oestradiol-treated subjects from 21 to 24 months of age, suggesting physiological levels of oestradiol may facilitate somatomedin-C production during periods of increased GH secretion. Thus, absolute changes in body weight in outdoor-housed premenarchial rhesus monkeys are influenced by gonadal status, possibly mediated through increases in serum insulin. Furthermore, dynamic changes in body weight occur during prepubertal development, with the largest increments associated with increased somatomedin-C levels at specific ages synchronized with natural changes in the environment.

J. Endocr. (1984) 102, 311–317