Changes in reproductive parameters during the spawning migration of pink salmon, Oncorhynchus gorbuscha (Walbaum)

The hormones 17β-estradiol, 17α-hydroxy-20β-dihydroprogesterone(17α, 20β-P), 11-ketotestosterone, testosterone, gonadotropin and also vitellogenin, were determined during the spawning migration of wild pink salmon in the Fraser and Thompson Rivers in British Columbia. This stock of pink salmon takes approximately 2 weeks to migrate the 333 km upstream to the spawning grounds. Both sexes were at an advanced stage of sexual development when they entered fresh water. In females both the 17β-estradiol and vitellogenin levels fell precipitously during the migration, to be very low at spawning, whereas the 17α,20β-P level rose rapidly, to be highest at arrival on the spawning grounds. The gonadotropin level also rose rapidly during the migration, and was highest in spent fish. Testosterone was at a high level throughout, although this level decreased steadily during migration. In many respects similar endocrine changes were observed in the male. For example, in the case of androgen levels, both testosterone and 11-ketotestosterone fell steadily during migration but were still relatively high at spawning, whereas both gonadotropin and 17α, 20β-P levels rose markedly as migration progress. However, although the qualitative changes were often similar between the sexes, the levels of 17α, 20β-P, testosterone, and gonadotropin were considerably higher throughout in females than in males. It is concluded that this stock of pink salmon is at an advanced stage of sexual development when it enters fresh water. The endocrine changes observed during this study represent those controlling the final stages of reproduction, specifically final oocyte maturation and ovulation in females, and the final stages of spermatogenesis and spermiation in males.     

Developmental patterns of FSH and LH in female rats deprived of light before puberty.

Plasma FSH and LH levels were examined in female rats reared in the dark at different ages from birth until sexual maturation to investigate whether, and to what extent, external factors such as light, influence gonadotropin levels during development. Control animals were raised in diurnal lighting consisting of 12 hours of light and 12 hours of dark. Light deprivation did not eliminate the characteristic peak of gonadotropins seen in early postnatal development but significantly increased levels of FSH and slightly decreased levels of LH (except for a transient rise at day 12). Constant darkness tended to lower whole body, ovarian and pituitary weights but to increase pineal weight. Whereas the time of eye-opening was the same in control and light-deprived animals, puberty (as judged by vaginal opening and first ovulation) was delayed in animals raised in the dark. The data suggest that environmental light has a mediating action on patterns of gonadotropin release, particularly on FSH, during prepuberal development.     

Reproductive biology and endocrine regulation of final oocyte maturation of captive white bass

The ovarian development, and plasma levels of gonadotropin II (GtH II) and sex-steroid hormones at the end of vitellogenesis were examined in captive white bass Morone chrysops. The changes in plasma hormone levels and oocyte morphology associated with gonadotropinreleasing hormone agonist (GnRHa)-induced final oocyte maturation (FOM) were studied. Although plasma 17β-oestradiol (E₂) and oocyte diameter increased, there were no changes in GtH II, testosterone (T), 17,20β-dihydroxy-4-pregnen-3-one (17,20β-P) or 17,20β,21-dihydroxy-4-pregnen-3-one (17,20β,21-P) in non-hormone-treated females, and no FOM was observed. Treatment with a sustained-release GnRHa delivery system (GnRHa implant) induced two FOM cycles separated by about 24 h, with the release of approximately equal numbers of eggs in each spawn. Plasma GtH II levels were elevated significantly throughout FOM, reaching a maximum of 9·07 ± 1·55 ng ml^?1 in ovulated fish. Both plasma E₂ and T increased soon after the GnRHa treatment, but E₂ declined in fish undergoing germinal vesicle (GV) migration. Plasma T increased further during FOM (7·55 ± 2·87 ng ml^?1), but declined precipitously at ovulation. A surge in plasma 17,20β-P and 17,20β,21-P (4·11 ± 0·97 ng ml^?1 and 3·10 ± 0·77 ng ml^?1, respectively) was observed in females undergoing GV breakdown (GVBD). Based on the involvement of different sex-steroid hormones, FOM was separated into two stages. Early FOM included lipid-droplet coalescence and GV migration, and was associated with elevations in plasma GtH II and T. Late FOM included GVBD and yolk-globule coalescence, and was associated with elevations in plasma GtH II, 17,20β-P and 17,20β,21-P. The results of this study point to the absence of a surge in plasma GtH II as the missing link in the reproductive axis responsible for the failure of captive white bass to undergo FOM at the end of vitellogenesis. Sustained elevation of plasma GtH II via treatment with a GnRHa implant induced two consecutive spawns with an overall egg production two- to eightfold higher than previously obtained from captive broodstocks, and similar to annual egg production Values reported for wild fish.     

Maturity and fertility of rhesus monkey oocytes collected at different intervals after an ovulatory stimulus (human chorionic gonadotropin) in in vitro fertilization cycles

In rhesus monkeys undergoing ovarian stimulation for in vitro fertilization (IVF), a midcycle injection of human chorionic gonadotropin (hCG) substitutes for the LH surge and induces preovulatory oocyte maturation. The time interval between injection and oocyte collection, ideally, allows for the completion of oocyte maturation without ovulation, which would reduce the number of oocytes available for harvest. To evaluate the influence of this time interval on oocyte parameters following hCG administration, we conducted a series of gonadotropin treatment protocols in 51 animals in which the interval from hCG administration to follicular aspiration was systematically varied from 27 to 36 hr. Follicle number and size, evaluated prior to hCG administration by sonography, did not vary significantly or consistently with preovulatory maturation time. Oocytes were harvested by laparotomy or laparoscopy, and scored for maturity before insemination. The percentage of mature, metaphase II (MII) oocytes at recovery increased significantly with increasing preovulatory time and was inversely proportional to that of metaphase I (MI) oocytes. However, oocyte yield tended toward a progressive decrease with increasing preovulatory maturation times from a high of 27 oocytes at 27 hr to a low of 17 oocytes/animal at the 36 hr time interval. Fertilization levels declined significantly from a high of 50% at 27 hr to a low of 30% at 36 hr. Thus, although higher percentages of mature oocytes were recovered at the longer time intervals, optimal oocyte/embryo harvests were realized after the shorter time intervals (27 and 32 hr) and are most compatible with the goal of achieving high yields of fertile oocytes and embryos following gonadotropin stimulation in rhesus monkeys. © 1996 Wiley-Liss, Inc.     

Ovarian development and plasma sex steroid levels in cultured female Senegalese sole Solea senegalensis

Ovarian development was studied in cultured female Senegalese sole Solea senegalensis. Females with regressed ovaries, mainly occupied by perinucleolar oocytes, predominated throughout summer exhibiting low condition factor (K), gonadosomatic index (I(G)), and plasma 17beta-estradiol and testosterone levels. Throughout autumn and winter (ovaries at early and intermediate maturation), oocytes progressed to cortical alveoli and vitellogenic stages accompanied by increasing K, I(G), and plasma 17beta-estradiol and testosterone levels. At late winter/early spring, ovarian development reached its maximum with the predominance of females at intermediate and final maturation (the latter occupied by late vitellogenic oocytes and few early maturation oocytes) and peak values of K, I(G), and 17beta-estradiol and testosterone concentrations. Steroid levels were lower (especially testosterone) than those for naturally-spawning females, which might cause extensive atresia without final oocyte maturation (no spawning was observed). This degenerative process reduced de size of the ovary (initial and intermediate phases of regression) in association with declining K, I(G), and plasma 17beta-estradiol and testosterone levels and increasing proportions of perinucleolar oocytes. The circulating 17,20beta-dihydroxy-4-pregnen-3-one levels, the proposed maturation-inducing steroid, remained relatively constant throughout the experimental period, suggesting that oocytes were unable to respond adequately to its stimulation. We propose the inadequate seasonal thermal regime as the main cause of such dysfunction.     

Regulation of ovarian 17β-hydroxysteroid dehydrogenase activity by gonadotropin

Serum testosterone levels are elevated prior to the lutropin surge, and decline abruptly following the release of endogenous lutropin. To investigate this phenomenon, the activity of 17β-hydroxysteroid dehydrogenase, the enzyme directly related to testosterone production form androstenedione, was measured. This was done in immature rats in which follicular maturation and ovulation were induced by pregnant mare serum gonadotropin administration. It appears that the effect of the gonadotropin on the enzyme activity is sharply divided into two phases that match with the follicular and the luteal phases. One day following gonadotropin administration, there was already a 7.67-fold increase in the original activity which further increased 48 h following hormone administration. At the peak of the lutropin surge, when follicular development is at its maximum, a 18.44-fold increase was measured. The activity fell abruptly 10 h following ovulation, at a time when fresh corpora lutea are already present in the ovary.It seems that the evaluation of serum testosterone followed by its abrupt decline, is directly related to the increased and decreased ovarian 17β-hydroxysteroid dehydrogenase activity. The possible importance of the observed changes to the mechanisms of the onset of puberty are discussed.     

Correlation between immunoreactive vasotocin in optic tectum and seasonal changes in reproductive behaviors of male rough-skinned newts

Previous studies have shown that intracranial administration of the amphibian neuropeptide arginine vasotocin (AVT) to male rough-skinned newts (Taricha granulosa) can stimulate masculine sexual behaviors. The present study was conducted to determine whether concentrations of endogenous AVT in specific brain areas are correlated with seasonal changes in sexual behavior. Immunoreactive (ir) AVT was measured in the dorsal preoptic area and in the optic tectum throughout the course of the reproductive cycle. In the optic tectum, irAVT levels were low throughout the summer, fall, and winter, but rose over fivefold in the spring when sexual behaviors are most prevalent in male newts. In contrast, irAVT levels in the dorsal preoptic area were high in the summer, but declined in the fall and fluctuated widely during the winter and spring. Thus, there are site-specific changes in irAVT concentrations in the brain of male newts during the seasonal reproductive cycle. The finding that irAVT concentrations in the optic tectum are seasonally correlated with newt sexual behaviors provides further evidence that AVT in this brain area may be important in the expression of sexual behavior.     

Steroid concentrations in follicular fluid aspirated repeatedly from transitional and cyclic mares

The objectives of the present study were to determine follicular progesterone (P4) and estradiol-17beta (E2) in transitional mares and to compare follicular steroid concentrations between transitional and cyclic mares. Follicles > 8 mm were aspirated under transvaginal ultrasound-guidance 4 times at 3 to 4 day intervals (T1-T4) in Norwegian pony mares during vernal transition. During the breeding season, follicular aspirations were conducted in each mare on Day 6, Day 14 and Day 18 after ovulation of 3 separate estrous cycles (Day of ovulation = Day 0). Plasma and follicular fluids were analyzed for P4 and E2 with ELISA and RIA, respectively. Plasma P4 concentrations remained below 1 ng/mL throughout T1-T4, while the follicular P4 concentrations increased significantly to cyclic levels after the first transitional aspiration. Plasma E2 concentrations similarly remained at low levels during the course of the transitional aspirations, while the follicular E2 concentrations increased gradually over the 4 aspirations to cyclic concentrations. The mares ovulated on average 9.8 +/- 1.6 (mean +/- SEM) days after the last transitional aspiration, and 16.6 +/- 0.2, 11.3 +/- 1.5 and 23.2 +/- 4.4 days after aspirations conducted on Day 6, 14 and 18, respectively. The present study demonstrates that in the transitional mare newly developing follicles exhibit biosynthesis of P4 and E2. Furthermore, an increase in follicular steroid concentrations is not necessarily reflected in the peripheral steroid concentrations.     

Timing of sexual maturity in female rhesus monkeys (Macaca mulatta) housed outdoors

A comparison of the age and season at first parturition was made for spring-born female rhesus monkeys and for females born in the fall to mothers who had been laboratory-housed before being transferred outdoors. Females (N = 9) born during the fall had first parturition during the spring and summer, as did all spring-born females (N = 68), and not during the fall as would be predicted if age were the determining factor. A separate analysis of post-menarchial, spring-born females (N = 5) beginning in September at 29 months of age revealed that the ensuing 12 months were characterized by low serum levels of oestradiol (less than 50 pg/ml), progesterone (less than 1.0 ng/ml), LH (less than 7.0 ng/ml), and FSH (less than 5.50 micrograms/ml). First ovulation subsequently occurred in the fall in all subjects at a mean age of 41.9 +/- 0.1 months, and was preceded by significant elevations in basal LH and FSH, coincident in time with the transition of summer to fall (September). Female copulatory behaviour was restricted to the period surrounding first ovulation, beginning some 2 weeks before and ceasing within 3 days after the oestradiol peak. The most rapid gain in weight occurred during the summer months before first ovulation, and was associated with significant elevations in serum GH and prolactin. These data suggest that season may influence the timing of sexual maturation in rhesus monkeys kept outside in such a way that the occurrence of first ovulation is restricted to the fall and winter months.     

Testosterone treatment augments LHRH‐a induced ovulation vin greenback flounder

Greenback flounder were treated with either an injection of des Gly¹⁰ [D‐Ala⁶] LHRH ethylamide (LHRH‐a), a silastic pellet implant of testosterone (T) or LHRH‐a+T. LHRH‐a and LHRH‐a+T induced more ovulatory events than in control fish or fish treated with T alone. T treatment did not significantly increase the number of ovulatory events above control levels. Treatment with LHRH‐a+T stimulated more ovulatory events than treatment with LHRH‐a alone, suggesting T increases pituitary responsiveness to LHRH‐a. Plasma levels of 17β‐estradiol (E ₂) were significantly higher in fish treated with LHRH‐a and LHRH‐a+T than in control fish or fish treated with T. Plasma levels of 17,20β‐dihydroxy‐4‐pregnen‐3‐one were not elevated above control levels. Daily ovulations, elevated E ₂ and T levels and presumably elevated GtH‐II levels, persisted well after the expected clearance time of exogenous LHRH‐a. Positive steroid feedback may have contributed to the processes of repeated final oocyte maturation and ovulation.     

Calcium metabolism and osmoregulation during sexual maturation of river running Atlantic salmon

As Atlantic salmon return from the ocean to undertake the anadromous spawning migration up the river of origin, profound changes in calcium metabolism and osmoregulation take place. Using tartrate resistant acid phosphatase as a marker, scale osteoclast activity was found to increase throughout sexual maturation and spawning migration. Thus, the participation of osteoclasts in the elevated scale resorption observed during this phase is established. As calcium was simultaneously accumulated in the female gonads, it is proposed that the scales are resorbed in order to provide calcium for the growing ovaries. Plasma oestradiol-17 β levels were elevated in females during sexual maturation, and had decreased at the time of spawning. Plasma testosterone levels were similar in males and females during the first part of the upriver migration, but had increased in males and decreased in females at spawning. In addition to the role of these sex steroids in the gonadal growth, their possible involvement in the increased scale resorption during this phase is discussed. Plasma growth hormone and thyroxine levels were elevated in both sexes at spawning, with the triiodothyronine/thyroxine (T₃/T₄) ratio declining sharply, indicating possible roles for these hormones in the maturational process. The relatively low gill Na⁺, K⁺-ATPase activity of salmon caught in the estuary implies that the fish had already adapted to a hypoosmotic environment. During the upriver migration, the gill Na⁺, K⁺-ATPase activity decreased further, indicating that the hypoosmoregulatory ability was suppressed further during sexual maturation and spawning migration.     

长臀（鱼危）脑垂体和血清中促性腺激素的生殖周期变化

鲇形目鱼类在世界养殖鱼类中占有重要的位置.     

Sex steroids, growth and condition of Arctic charr broodstock during an annual cycle

Changes in plasma concentrations of sex steroids, growth rate and condition of repeat spawning (3+) male and female Arctic charr were studied throughout an annual reproductive cycle. Individually marked fish (mean weight approx. 500 g) were held under conditions of liberal food supply, constant temperature (4° C) and simulated natural photoperiod (Tromsø, 70° N). Once each month fish were weighed, measured and blood samples taken for steroid analysis. Plasma concentrations of testosterone (T), 11-ketotestosterone (11-KT) and oestradiol-17β (E₂) were determined using radioimmunoassay (RIA). Both male and female fish displayed distinct seasonal changes in plasma concentrations of sex steroids, growth rate and condition. From February (minimal concentrations) to March all sex steroids increased slightly and these elevated concentrations were maintained until May. Thereafter, there was a second, and far more pronounced, increase in plasma steroid concentrations which culminated in peak steroid concentrations in September–October. There was then a rapid decline during the spawning period. In winter, growth rate and condition were generally low, then increased during the spring, reached a peak during the summer, and then declined with the onset of autumn. During spring (March–May), the frequency distributions of plasma testosterone concentrations in both male and female fish were bimodal. The fish of the upper modal group of the distribution had significantly higher growth rates and condition than those in the lower modal group. In summer and early autumn (June–September) the association between T and growth rate changed. Significant negative correlations between T and growth rates were observed in females. There was an increase in endocrine activity, indicated by elevated plasma sex steroid concentrations in March, 7–8 months prior to maturation. It is suggested that this may be one factor influencing the onset of spring growth and energy deposition among maturing charr.     

Plasma levels of gonadotropin and 17α,20β-dihydroxy-4-pregnen-3-one in relation to spawning behaviour of rainbow trout, Oncorhynchus mykiss(Walbaum)

Manipulation of the opportunity to spawn was used to investigate the relationship between endocrine events, egg viability and spawning behaviour in female rainbow trout. Females were prevented from spawning by isolating them from males and gravel for up to 21 days after ovula- tion. Blood samples were taken before pairing with a male, at the onset of nesting activity, and at the completion of spawning. Plasma hormone levels of gonadotropin (GtH) and 17α,20β-dihydroxy-4-pregnen-3-one (17,2OP) were measured by specific radioimmunoassays. There were no qualitative or quantitative differences in the spawning behaviour of females paired on the day of ovulation or 7. 14, or 21 days after ovulation. There was a general decrease in the viability of eggs with increasing retention times. In females paired on the day of ovulation, or after 7 or 14 days, GtH levels increased with the onset of nesting behaviour and declined as fish reached the post-spawning condition. By day 21, GtH levels before pairing were significantly higher than prepairing levels in the other three treatment groups, and did not increase at the onset of nesting, or decrease in post-spawning fish. Plasma 17,20P remained high in prepairing and nesting samples of all four groups and declined to low levels in fish in post-spawning condition. In females paired on the day of ovulation there was a significant increase in 17,20P from the prepairing to the nesting stage. These results suggest that 17,20P plays a key role in the synchronization of behavioural and maturational events at the time of spawning.     

Effect of infusion of gonadotropin releasing hormone upon plasma concentrations of sex hormones in prepubertal and pubertal males.

Plasma testosterone (T), dihydrotestosterone (DHT), 17-hydroxyprogesterone (17OHP), androstenedione (A), estradiol (E2), and dehydroepiandrosterone sulfate (DHAS) were measured by radioimmunoassay after celite chromatography prior to and after a 3-hour infusion of the synthetic gonadotropin releasing factor, GnRH, in normal prepubertal and pubertal boys. Plasma T levels rose (p less than 0.001) in the pubertal but not prepubertal boys. 17OHP concentrations increased in those boys who had an increment of T. A, DHT, E2 or DHAS levels did not increase after GnRH. Basal levels of T, DHT, A and DHAS correlated with the peak and mean serum LH levels attained during the GnRH infusion. These data confirm the greater Leydig cell responsivity to transient rises of endogenous gonadotropin in pubertal males and also suggest that there may be a relationship between adrenal androgen production and maturation of the hypothalamic-pituitary-gonadal system.     

Intrafollicular concentrations of steroids and steroidogenic enzymes in relation to follicular development in the mare

The objective of the present study was to determine the changes in follicular fluid steroid concentrations and in granulosa cell steroidogenic enzyme expression during the follicular phase, in relation to follicular size and physiological status in the mare. Follicular fluid and follicular cells were recovered by ultrasound-guided follicular punctures either around the time of emergence of the dominant follicle, at the end of the dominant follicle growth, or at the preovulatory stage, after injection of gonadotropin to induce ovulation. Cellular relative amounts of steroidogenic acute regulatory protein (StAR), P450-side chain cleavage (P450(scc)), 3beta-hydroxysteroid dehydrogenase (3betaHSD), 17alpha-hydroxylase, and aromatase were assessed by semiquantitative Western blot and densitometry. Follicular fluid was assayed for cholesterol concentrations by colorimetric assay and for progesterone, testosterone, and estradiol-17beta concentrations by RIA. Intrafollicular concentrations of progesterone and estradiol-17beta significantly increased in the dominant follicle during growth. After injection of gonadotropin, follicular maturation was characterized by a decrease in estradiol-17beta concentrations and a further increase in progesterone concentrations. Granulosa cells from dominant follicles had increased levels of StAR, P450(scc), 3betaHSD, and aromatase during growth, but decreased levels during maturation. Levels of StAR, P450(scc), 3betaHSD, and aromatase, as well as progesterone and estradiol-17beta, were lower in granulosa cells from subordinate than from dominant follicles. We did not observe a relationship between the steroidogenic activity of follicles and the capacity of their enclosed oocytes to complete meiosis in vitro.     

Progestagen,androgen and oestrogen levels in plasma and ovarian follicular fluid during the oestrous cycle of the mare

The pattern of steroid hormone concentrations in the blood plasma of five mares was determined throughout eight oestrous cycles by radioimmunoassay. In three other mares the steroid hormone concentrations in the follicular fluid of 16 isolated follicles (⪖ 1 cm diameter) from both ovaries were analyzed on the first and third day of behavioural oestrus.The plasma levels of pregnenolone and progesterone as well as their 17α-hydroxylated metabolites showed similar ranges of concentration throughout the oestrous cycle. Luteolysis occurred 6 days prior to ovulation and was accompanied by a drop of all progestagens. Throughout the oestrous period (5 days prior to and including the day of ovulation) mean plasma concentrations of progestagens were <0.5 ng/ml and increased significantly one day after ovulation. Maximum plateau values were reached on day 6 after ovulation. A distinct (but not statistically significant) rise of androstenedione and testosterone plasma levels occurred during oestrus whereas dehydroepiandrosterone values increased significantly 6 days prior to ovulation and reached a maximum mean value of 1.14 ng/ml one day before ovulation. Levels then declined significantly on the day of ovulation. Oestrone and oestradiol-17β plasma concentrations increased significantly 4 and 3 days prior to the day of ovulation, respectively, and both remained elevated until one day before ovulation.A significant positive correlation could be detected between increasing follicle diameters and androstenedione as well as oestradiol-17β concentrations in the follicular fluid, whereas pregnenolone values showed a negative correlation with follicular diameter. Oestradiol-17β could be determined in 9 of the 16 follicular fluid samples. In 8 of these 9, oestradiol-17β predominated over all other steroid hormones.In view of the low concentrations of dehydroepiandrosterone detected in the follicular fluid, it is suggested that the increase in peripheral plasma values during oestrus is caused by an extra-follicular source(s).     

Naloxone administration to female hamsters advances puberty by enhancing luteinizing hormone release

In the female hamster, a daily rhythm of gonadotropin release begins almost 3 weeks prior to the initiation of 4-day estrous cycles. A temporal relationship exists between the onset of this cyclic release of gonadotropin and age at puberty. We hypothesized that since opiate agonists depress circulating gonadotropins and antagonists increase them in both adult and immature rodents, endogenous opiates may influence the mechanism controlling cyclical gonadotropin release in the prepubertal female hamster and thus affect rate of sexual maturation and hence the age at puberty. This proposal was tested by chronic administration of naloxone (NAL), an opiate receptor antagonist. We predicted that NAL might induce the early initiation of daily surges of luteinizing hormone (LH) if endogenous opiates inhibit sexual maturation. Naloxone was injected daily (50 mg/kg body wt) at about 1300 hr from Days 1 through 17 of age. The NAL injections increased serum LH and significantly advanced the age at which first estrus vaginal discharge was observed (32 vs 38 days for saline-injected controls in Experiment I and 31 vs 37 days in Experiment II). However, the NAL injections did not correspondingly advance the age of initiation of endogenously generated daily cycles of circulating LH. We conclude that blockade of opiate receptors accelerates sexual maturation by directly inducing the release of LH and not by advancing the age of initiation of endogenous gonadotropin surges.     

Upregulation of the maturation-inducing steroid membrane receptor in spotted seatrout ovaries by gonadotropin during oocyte maturation and its physiological significance

Changes in ovarian maturation-inducing steroid (MIS; 17,20 beta, 21-trihydroxy-4-pregnen-3-one [20 beta-S]) membrane receptor concentrations during the reproductive cycle were investigated in spotted seatrout (Cynoscion nebulosus) captured at their spawning grounds. Ovarian receptor concentrations increased gradually during ovarian recrudescence and subsequently increased rapidly during oocyte maturation, reaching 3.5-fold the prematuration values by the beginning of ovulation. The significant elevation of receptor concentrations by the germinal vesicle migration stage of oocyte maturation was accompanied by increases in circulating levels of gonadotropin (LH, GTH II) and MIS (20 beta-S). The regulation and physiological significance of the increase in ovarian MIS membrane receptor concentrations were investigated in a double in vitro incubation system. Incubation of fully grown, follicle-enclosed oocytes with hCG (10 IU/ml) for 6 h caused a two- to fourfold increase in oocyte and ovarian MIS receptor concentrations and the development of oocyte maturational competence (OMC; ability to complete oocyte maturation in vitro in response to exogenous 20 beta-S in a second incubation). Both upregulation of the MIS receptor and development of OMC in response to gonadotropin were blocked by coincubation with actinomycin D or cycloheximide, which are inhibitors of mRNA and protein synthesis, respectively, but not by cyanoketone, which is an inhibitor of 3 beta-hydroxysteroid dehydrogenase-dependent steroid synthesis. Incubation with a variety of steroids, including 20 beta-S, failed to increase receptor concentrations or to induce OMC, further supporting a steroid-independent mechanism of gonadotropin action. In contrast, insulin-like growth factor I (IGF-I) mimicked the actions of gonadotropin, which suggests IGF-I may be a component of the hormone signaling pathway. A close correlation was found between the relative increase in MIS receptor concentrations and the percentage of oocytes that became maturationally competent after treatment with different concentrations of gonadotropins and drugs that elevate cAMP levels. The finding that upregulation of the MIS receptor in response to gonadotropin and other treatments is invariably associated with the development of OMC indicates that these two processes are intimately related, and it suggests that the increase in MIS receptor concentrations is a critical regulatory step in the hormonal control of oocyte maturation.