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Biomedical subjects

S R Ojeda

Publications and source records attributed to S R Ojeda.

At least 127 records · Page 7Linked to original sources
PubMed1984

Effect of selective removal of the adrenal medulla on female sexual development.

The ovary and adenohypophysis of the rat contain beta-adrenergic receptors and respond to beta-adrenergic stimulation with hormone release. To determine the importance of the adrenal medulla as a source of adrenergic influences regulating prepubertal ovarian and pituitary function, a technique was developed to remove most of the adrenal medulla without compromising adrenocortical function. Medullectomy (MED) of 24-day-old female rats depressed both spontaneous diurnal changes in plasma epinephrine (EPI), and the EPI and norepinephrine (NE) response to decapitation, without affecting corticosterone (B) levels. Vaginal opening and first ovulation were delayed in MED rats. Serum luteinizing hormone (LH) and follicle-stimulating hormone (FSH) levels were normal in MED rats, but those of growth hormone (GH) and prolactin (Prl) were depressed. MED reduced the ovarian weight response to pregnant mare's serum gonadotropin (PMSG) and the ovarian steroidal response to human chorionic gonadotropin (hCG) in vitro, but it did not affect ovarian beta-adrenergic receptors. Cultured granulosa cells, harvested from juvenile ovaries and primed in vitro with FSH, responded to nanomolar concentrations of EPI with progesterone (P) secretion. EPI also augmented hCG- and FSH-induced P secretion. The EPI effect was reproduced by Zinterol, a beta 2-adrenergic agonist and was prevented by propranolol, a beta-adrenergic antagonist. Blockade of alpha-adrenergic receptors with phentolamine was ineffective. It is suggested that EPI of adrenomedullary origin supports female prepubertal development by a) stimulating ovarian P secretion, b) favoring Prl and GH release and c) amplifying the stimulatory effect of low gonadotropin levels on ovarian steroidogenesis. The effects of EPI on ovarian function appear to be mediated by beta-adrenergic receptors of the beta 2 type.

Adrenal Medulla↗
PubMed1984

An automated system for the study of pulsatile hormone secretion in the immature rat.

A procedure is described for prolonged sampling of blood from unrestrained immature rats which overcomes some of the problems associated with contemporary techniques. 1 or 2 days before sampling, the animal is fitted with two indwelling catheters, one inserted into the right jugular vein and the other into a femoral vein. On the day of the experiment blood is continuously withdrawn from the jugular vein (30 microliter/min) using a peristaltic pump and is dispensed into sample tubes every 5 min by means of an automatic fraction collector. A second channel of the same pump continuously infuses a blood replacement mixture (at 37 degrees C) into the femoral vein thereby keeping the animal's blood volume constant. In the present study this technique was used to sample blood from both ovariectomized and immature, intact female rats for as long as 5-7 h. The procedure did not appear to disturb the normal locomotion, feeding, drinking and sleeping behavior of the animals. Plasma luteinizing hormone (LH) profiles obtained from the ovariectomized rats showed a normal pulsatile pattern of LH secretion. The frequency and amplitude of the pulses was not affected, even when the animals were bled on 2 consecutive days. Moreover, the procedure permitted accurate detection and characterization of pulsatile LH release in intact, immature rats although the episodes of LH secretion were more variable and much less pronounced. It is believed that this novel sampling technique could prove to be a valuable tool in the study of pulsatile hormone secretion in small laboratory rodents.

Animals↗
PubMed1984

Effect of experimental alterations in serum levels of 5 alpha-androstane-3 beta,17 beta-diol on the timing of puberty in the female rat.

It has been suggested that in the rat, 5 alpha-androstane-3 alpha,17 beta-diol (3 alpha-diol) is physiologically involved in restraining the onset of female puberty. To test this hypothesis several experiments were performed. In normal rats, serum levels of 3 alpha-diol decline slightly during the initial phases of puberty and then sharply several hours before the afternoon preovulatory LH surge on the day of first proestrus. Inhibition of 5 alpha-reductase activity with a highly specific inhibitor, 17 beta-N,N-diethylcarbamoyl-4-methyl-4-aza-5 alpha-androstane-3-one (4-MA) strikingly depressed both ovarian content and serum levels of 3 alpha-diol, but failed to advance vaginal opening or first ovulation. Administration of different doses of 3 alpha-diol to juvenile rats via Silastic capsules produced a dose-related increase in serum 3 alpha-diol levels. Titers attained ranged from values similar to those of untreated juvenile rats to levels more than 10-fold higher. None of the concentrations, however, inhibited the LH surge and ovulation induced by pregnant mare serum gonadotropin (PMSG). When a similar treatment was administered to normally maturing rats, only the high dose of 3 alpha-diol delayed the age of vaginal opening and of first ovulation. Serum 3 alpha-diol levels attained with this dose were markedly higher than those of untreated juvenile rats (1,086 +/- 267 vs. 124 +/- 14 pg/ml, respectively).(ABSTRACT TRUNCATED AT 250 WORDS)

5-alpha Reductase Inhibitors↗
PubMed1984

Release of prostaglandin E2 from the hypothalamus depends on extracellular Ca2+ availability: relation to LHRH release.

The present experiments were performed to investigate the role of extracellular Ca2+ in the process of prostaglandin E2 (PGE2) release from the median eminence (ME) of the hypothalamus. Changes in the release of LHRH were also evaluated. Incubation of ME fragments with different concentrations of K+ induced a dose-related increase in PGE2 and LHRH release. The effect of K+ depended upon the Ca2+ concentration in the incubation medium. A Ca2+ concentration of 0.1 mM was sufficient to permit a significant response in both PGE2 and LHRH release to K+. The effect of a maximal K+ concentration (56 mM) was almost completely obliterated by omitting Ca2+ from the incubation medium and by chelating the remaining Ca2+ with EDTA or EGTA. The Ca2+ ionophore A23187, tested at different concentrations (1-50 microM) significantly increased the release of both PGE2 and LHRH from the ME in a Ca2+-dependent manner. A Ca2+ concentration of 0.25 mM allowed maximal LHRH response to the ionophore, but permitted only a partial (50%) PGE2 response. Blockade of Ca2+ channels with Verapamil, a Ca2+ entry blocker, prevented the effect of K+ on both PGE2 and LHRH release in a dose-dependent manner. The results demonstrate that release of PGE2 from ME nerve terminals depends upon the concentration of extracellular Ca2+ and suggest that PGE2 release is initiated by an increase in Ca2+ influx to the terminals. In addition, the data provide further evidence that release of LHRH is, to a significant extent, a function of Ca2+ influx through Ca2+ voltage-dependent channels.

Animals↗
PubMed1984

Prepubertal ovarian function is finely regulated by direct adrenergic influences. Role of noradrenergic innervation.

In the rat, the main source of adrenergic fibers innervating nonvascular ovarian tissue is the superior ovarian nerve (SON). To determine the influence of the SON on prepubertal ovarian function and, hence, on the time of puberty, several experiments were conducted. Transection of the SON in early juvenile rats (day 24) led to more than 60% depletion in ovarian norepinephrine (NE) content, but affected neither the timing of vaginal opening nor that of first ovulation. SON section, however, produced a 2-fold increase in beta-adrenergic receptor density, with no change in affinity, suggesting that hypersensitivity by denervation had developed. Removal of all adrenergic influences from ovarian granulosa cells by culturing the isolated cells in vitro produced a striking increase in beta-adrenergic receptor number, which was maximal at 72 h. In spite of this elevated receptor content, the beta-adrenergic agonist Zinterol did not induce progesterone release unless the cells were previously exposed for 48 h to a physiological concentration of FSH. Both NE and epinephrine down-regulated the receptors in a dose-related manner, without affecting receptor affinity. To test the hypothesis that denervation of the ovary results in greater steroidal sensitivity to adrenergic stimulation, a model was constructed in which cells were simultaneously exposed to FSH and NE or epinephrine (innervated cells) or exposed only to FSH (denervated cells) for 48 h. The response to adrenergic stimulation was subsequently measured by incubating the cells with Zinterol for 24 h. Zinterol was highly effective in eliciting progesterone release from the denervated cells, but failed to do so in innervated cells. Additionally, the agonist increased cAMP in the former cells, but it was ineffective in the latter, indicating that prior exposure to catecholamines desensitized the adenyl cyclase enzyme coupled to beta-adrenergic receptor sites. The results suggest that during prepubertal maturation, noradrenergic fibers reaching the ovary via the SON may modify ovarian steridogenic sensitivity to catecholamines, by regulating the number of functional beta-adrenergic receptors in the gland. They also indicate that the apparent failure of SON denervation to alter the maturation of ovarian steroidogenesis and the time of puberty may be due to development of compensatory hypersensitivity after the denervation.

Animals↗
PubMed1984

Ovarian adrenergic nerves play a role in maintaining preovulatory steroid secretion.

Little is known about the role of ovarian nerves in the control of steroid secretion. We have examined the effect of sectioning the adrenergic superior ovarian nerve (SON) on the secretion of progesterone (P), and estradiol (E2) from the ovary. The steroids were measured in blood samples collected every 4 min from the ovarian vein in the proestrous and estrous phases of the rat estrous cycle. Upon section of the nerve at 1100 h of proestrus, secretion of both steroids dropped within 4 min and remained at about 50-60% of presection values for the rest of the sampling period. At 1600 h of proestrus, systemic plasma LH was elevated and P output was markedly increased. SON section produced a transient (8-min) decrease in P levels, but E2 concentrations remained depressed. Section of the SON during estrus failed to alter the output of either steroid. SON section did not alter blood flow. It is suggested that neural impulses, possibly adrenergic, which reach the ovary via the SON contribute to the maintenance of gonadotropin-supported secretion of ovarian steroids on the day of proestrus. The results are consistent with the view that, in addition to its hormonal regulation, the ovary is directly controlled by the central nervous system (CNS) through specific neural pathways.

Animals↗
PubMed1984

Maternal modulation of infantile ovarian development and available ovarian luteinizing hormone-releasing hormone (LHRH) receptors via milk LHRH.

In the rat, ovarian LHRH receptor content, measured by the binding of [125I]D-Ala6-Pro9-LHRH ([125I]A-LHRH) to ovarian membranes, declines during the days preceding the first preovulatory LH surge. The present results show that LHRH receptor content is low during neonatal-infantile days (days 5-15) and increases markedly thereafter to a maximum by day 25. When ovarian membranes from 10-day-old rats were pretreated with MgCl2 to dissociate endogenously bound hormone, LHRH-binding capacity increased to levels similar to those in untreated ovaries from 25-day-old rats. No significant increase was observed in ovaries from the latter group after MgCl2 exposure. Equilibrium association constants were similar (6-8 X 10(9) M-1) in MgCl2-treated and control ovaries, indicating that the increase in binding was not due to a change in receptor affinity. Pups prevented from suckling for various time intervals (1-24 h) exhibited an increase in available ovarian LHRH-binding sites which was maximal by 4 h. When pups were allowed to suckle subsequent to a 6-h fast, available ovarian LHRH receptors decreased rapidly (1 h). This decline was not due to a local increase in LHRH triggered by a neural reflex in response to suckling and/or stomach distension. Intragastric administration of milk reproduced the decline in available receptors induced by suckling, whereas stomach distension produced by saline was ineffective. Concomitant with the decline in receptors, suckling produced a significant increase in both stomach content and plasma levels of an LHRH-like material. Fractionation of acid milk extracts in Sephadex G-25 yielded two peaks, one in a position similar to that of synthetic LHRH. Milk LHRH purified in a C-18 Sep-Pak column displaced binding of [125I]A-LHRH to ovarian membranes and yielded a parallel binding curve in the LHRH RIA. This milk LHRH also released LH and FSH from pituitaries in vitro and inhibited FSH-induced estradiol and progesterone secretion from granulosa cells in culture in a dose-related manner, comparable to A-LHRH. The suckling-induced decrease in available LHRH receptors was prevented by iv administration of a specific LHRH antiserum directed against all 10 amino acids of native LHRH, suggesting that milk LHRH reaches the ovary via the bloodstream. The results indicate that an LHRH-like substance of milk origin binds to LHRH receptors in the pup ovary, thereby controlling receptor availability.(ABSTRACT TRUNCATED AT 400 WORDS)

Aging↗
PubMed1983

Hyperprolactinemia enhances ovarian estrogen responsiveness to gonadotropins in prepubertal rats: antagonistic effect of adrenalectomy.

Hyperprolactinemia (HP) induced in female rats by dopaminergic receptor blockers enhanced ovarian estradiol (E2) release in response to human chorionic gonadotropin (hCG) or human follicle-stimulating hormone (hFSH) in vitro. Uterine weight and ovarian aromatase activity were also increased. In contrast, ovarian androgen (A) release in response to hCG was reduced. Injections of ovine prolactin (oPrl) also enhanced E2 response to hCG in vitro. The increased E2 response was not due to a direct effect of Prl on ovarian aromatase activity since administration of oPrl to hypophysectomized rats failed to enhance the formation of E2 from testosterone (T) in vitro, and inhibited the increase in the enzyme activity induced by FSH. Adrenalectomy (ADRX) of intact rats, which did not affect mean serum gonadotropin levels, blunted the effect of HP on the E2 response to hCG. The suppression was partially reversed by corticosterone (B). Serum progesterone (P) and T were similar in controls and HP-ovariectomized (OVX) rats with intact adrenals. Likewise, serum androstenedione (delta 4) and dehydroepiandrosterone (DHA) were not altered in intact, HP rats as compared with controls. Thus, an increase in adrenal secretion of these steroids does not appear to mediate the effect of Prl on ovarian E2 response to gonadotropins. Ovaries of HP rats showed more large follicles than controls. In contrast, ovaries of HP-ADRX rats had a decreased number of large follicles. It is suggested that: a) in intact prepubertal rats Prl increases the E2 response of the ovary to gonadotropins by facilitating follicular development rather than by a direct action on aromatase activity, and b) when follicular development is stimulated by FSH, an inhibitory effect of Prl on aromatase activity becomes apparent. The effect of Prl on the E2 response of the ovary to gonadotropins is not mediated by the adrenal cortex. Rather, it appears that while Prl facilitates the development of large, E2-producing follicles by promoting the growth of small- and medium-sized follicles, an adrenal component influences follicular growth at a step subsequent to Prl.

Adrenalectomy↗
PubMed1983

Peripubertal decline in ovarian LHRH receptor content: characterization and distribution.

In the rat, ovarian LHRH receptor content, as measured by the binding of 125I-D-Ala6, Pro9-LHRH (A-LHRH) to ovarian membranes, decreases markedly during the days preceding the first preovulatory gonadotropin surge. The present study shows that the decrease in LHRH receptor number occurs exclusively in granulosa cells, and that the greatest rate of decline takes place between the juvenile period and the early proestrous phase of puberty. In contrast to granulosa cells, membranes prepared from ovarian tissues after removal of these cells demonstrated no significant change in LHRH binding capacity throughout puberty. Scatchard analysis of competition curves, constructed using membranes from whole ovaries, revealed that receptor affinity remains unchanged (Ka = 9 X 10(9) M-1) as the receptor number decreases. Kinetic binding studies demonstrated that LHRH binding to its receptor reaches equilibrium within 40 min and can be displaced by addition of unlabeled hormone. A kinetic estimate of receptor affinity provided values similar to those obtained by Scatchard analysis. Treatment of ovarian membranes with MgCl2 to uncover occupied binding sites results in more than 70% dissociation of bound labeled ligand. Dissociation of endogenous LHRH using this procedure resulted in some increase in available binding sites only in juvenile rats, but not in first proestrous or first estrous animals in which receptor content was low. The results demonstrate that the decline in ovarian LHRH receptors observed during peripubertal days is confined to granulosa cells, is not due to changes in receptor affinity, and represents a true loss of binding sites.

Animals↗
PubMed1983

Neuroendocrine mechanisms controlling the onset of female puberty: the rat as a model.

The advent of female puberty represents the culmination of a diversity of developmental processes which affect all components of the reproductive axis. Development of neuroendocrine reproductive functions proceeds in a harmonious and interrelated manner. No unique 'trigger' of puberty can be discerned, but rather puberty represents the climax of a cascade of events, finely interconnected throughout the continuum of sexual maturation. A resetting of the hypothalamic 'gonadostat' to steroid negative feedback appears to be a phenomenon associated with puberty, but not its cause. Although the central nervous system plays a pivotal role in the development of the ovary, it is the acquisition of ovarian ovulatory capacity which finally determines the timing of the first preovulatory surge of gonadotropins. In contrast to primates, development of the central component of estradiol positive feedback is an early event in the female rat. However, in most species--including the rat--amplification (or initiation) of a particular, synchronous pattern of LHRH release appears essential for the initiation of puberty. The mechanisms underlying this functional change of the LHRH secreting system are not clearly understood. In the rat, ovarian development proceeds under the influence of gonadotropins, and the somatomammotropins PRL and GH. More intriguingly, evidence is now emerging that the central nervous system may convey direct information to the ovary via the ovarian nerves, thus providing a hormone-independent fine tuning for its control. Upon reaching adequate development, the ovary through its secretory products, acts on an already competent hypothalamic-pituitary axis to activate the central component of estradiol positive feedback.

Animals↗
PubMed1983

Novel hypothalamic arachidonate products stimulate somatostatin release from the median eminence.

Arachidonic acid (AA) stimulates the in vitro release of somatostatin (SRIF) from the hypothalamic median eminence (ME). This effect is inhibited by 5, 8, 11, 14-eicosatetraynoic acid (ETYA) but not by indomethacin (ID). Microsomal fractions from the rat hypothalamus catalyze an NADPH-dependent metabolism of AA to form several oxygenated products of which the major metabolites are 5, 6-epoxyeicosatrienoic acid (5, 6-EET) and its hydration product, 5, 6-dihydroxyeicosatrienoic acid (5, 6-DHET). Both novel arachidonate metabolites, particularly 5, 6-EET, are potent in vitro stimuli for the release of SRIF. To a lesser extent, 5, 6-EET is also capable of evoking luteinizing hormone-releasing hormone (LHRH) release. The results suggest that these "epoxygenase" metabolites of AA may be physiologically involved in the control of SRIF release.

5,8,11,14-Eicosatetraynoic Acid↗
PubMed1982

Studies on the central effect of prolactin in inducing precocious puberty in the female rat.

Implantation of prolactin (PRL) into the median eminence (PRL-ME implants) of 23 day old female rats markedly advanced the onset of puberty, as measured by the age at vaginal opening and at first ovulation. Precocious puberty was preceded by steroidogenic activation of the ovary, as reflected by increases in uterine weight and an enhanced in vitro steroidal responsiveness of the ovary to hCG. The stimulatory effect of PRL-ME implants could not be attributed to alterations in the release of LH, FSH, GH or TSH from the anterior pituitary. Likewise, the PRL effect was neither exerted through the adrenal gland nor involved activation of a direct neural, vagal-mediated influence on the ovary. Furthermore, the effect of PRL-ME implants was not due to a decrease in pituitary secretion of opioid substances, which appear to restrain chronically gonadotropin release during female prepubertal development. These latter experiments also showed that administration of the opioid agonist, morphine, can delay the onset of puberty in the female rat. Although local exposure of the medial basal hypothalamus to high PRL levels is extremely effective in accelerating puberty, the mechanisms by which this effect is exerted remains to be elucidated.

Animals↗
PubMed1982

Evidence for involvement of alpha-adrenergic receptors in norepinephrine-induced prostaglandin E2 and luteinizing hormone-releasing hormone release from the median eminence.

Studies were conducted to determine if the stimulatory effect that norepinephrine (NE) exerts on the release of prostaglandin E2 (PGE2) and LHRH from the median eminence is mediated by alpha- or beta-adrenergic receptors. Incubation of median eminence fragments from adult male rats with different concentrations of phentolamine, an alpha-receptor antagonist, resulted in a dose-related inhibition of the release of both PGE2 and LHRH induced by NE, with an IC50 of 3.5 X 10(-7) and 0.9 X 10(-7) M for PGE2 and LHRH, respectively. Complete suppression of the NE effects was observed with a phentolamine concentration of 5 X 10(-6) M. This dose also reduced basal PGE2 and LHRH release. In contrast to phentolamine, the beta-receptor antagonist propranolol, tested at a concentration of 5 X 10(-6) M, was completely ineffective in altering the stimulatory effect of NE on either PGE2 or LHRH release. Moreover (and as previously observed for LHRH), blockade of dopaminergic receptors with Pimozide (10(-6) M) failed to inhibit the release of PGE2 induced by NE, thus indicating that the stimulatory effect of NE is not mediated by a dopaminergic mechanism. It is concluded that NE stimulates the release of PGE2 and LHRH from nerve terminals of the median eminence by first interacting with an alpha-adrenergic receptor.

Animals↗
PubMed1982

Pituitary LHRH receptors during puberty in the female rat: is the proestrous decline in receptor content due to occupancy?

As during the adult estrous cycle, the number of pituitary luteinizing hormone-releasing hormone (LHRH) receptors was found to increase prior to the first preovulatory surge of LH. Receptor content, as measured by the binding of the analog 125I-D-Ala6-Pro9-LHRH to pituitary membranes, declined markedly at the time of the first LH surge, remaining low during the first estrus and first diestrus. When pituitary membranes from animals undergoing an LH surge were incubated with MgCl2 to dissociate endogenously bound hormone, available binding sites were restored to pre-LH surge values. The increase in LHRH binding capacity was not related to an increase in receptor affinity, and it was not demonstrable in other pubertal phases. MgCl2 may have unmasked receptors made cryptic at the time of the LHRH-induced LH surge. Nevertheless, the results are compatible with the view that the proestrous decline in LHRH receptor content is, at least in part, due to occupancy of binding sites by endogenous hormone.

Animals↗
PubMed1982

Changes in aromatase activity in the rat brain during embryonic, neonatal, and infantile development.

We assessed the activity of the aromatase enzyme complex in slices of brain from rats by measuring the release of 3H2O from [1 beta-3H]testosterone. In hypothalami from 12-day-old rats, the rate of aromatase activity was linear with time and amount of tissue. The reaction was saturated at a substrate concentration of 0.1 microM, and the apparent Km of the reaction was 27 nM. The production of 3H2O was inhibited by 4-hydroxyandrostenedione, with an apparent Ki of 20 nM. Aromatase activity was first detected in the diencephalon of 16-day-old fetuses and reached maximum rates in hypothalamic tissue between days 18 and 20 of gestation. The highest rate of activity per mg protein (approximately 4.8 pmol h-1 mg protein-1) was observed in the preoptic area (POA) on the 20th day of embryonic development. However, when expressed as a rate per tissue fragment, aromatase activity was as high in the medial basal hypothalamus as in the POA. After day 20 of gestation aromatase activity rapidly decreased in the POA and medial basal hypothalamus of both males and females. The lowest levels were observed between postnatal days 16 and 20. Aromatase activity was not detectable in cerebral cortex and cerebellum at any age studied. Since serum testosterone was higher in males than females during the first 4 days of postnatal life, and since aromatase activity is elevated in the hypothalamus at this time, our results support the current concept that local formation of estrogen mediates testosterone-induced masculinization of the brain during the neonatal period. However, our results also indicate that failure of the rat brain to undergo complete sexual differentiation before birth cannot be due to an inability of the fetal hypothalamus to aromatize androgens, since aromatase activity was higher in the hypothalamus than in any other fetal tissue.

Androstenedione↗