Administration Of Gonadal Steroids Research Articles

Effect of Exogenous Gonadal Steroids on Reproductive Functions of the Indian Pygmy Field Mouse Mus terricolor

Different thresholds of gonadal steroids exert stimulatory or inhibitory effects on GnRH and gonadotropin release. During the reproductively active phase (RAP), the concentration of endogenous gonadal steroids remains high while, during reproductively inactive phase (RIP), it remains low. During RIP the HPG axis is sensitive to gonadal steroids but the circulatory levels of testosterone and estradiol remain low. During this phase one can observe conveniently the effects of gonadal steroids on the HPG axis in male or female rodents. Therefore, the aim of the present study was to find the effects of testosterone and estradiol on the reproductive functions of male and female Indian pygmy field mouse, Mus terricolor , during RIP. The male mice were injected aquaviron (commercially available testosterone, 1mg/100g body weight) while the female mice received estradiol-benzoate (25μg/100g body weight) for 15 consecutive days during the RIP. After completion of the treatment, a significant increase in the weights of gonads and accessory sex organs was noted in both the sexes. The biochemical constituents of accessory sex organs such as epididymal sialic acid, seminal vesicular fructose and uterine protein content reflected significant elevation accompanied by increased levels of plasma testosterone, estradiol and progesterone. Histologically, the gonads and accessory sex organs exhibited increased cellular activity. However, the gonadal cholesterol was significantly decreased in both the sexes. Over all, administration of gonadal steroids to both male and female mice accelerated the gonadal recrudescence but did not inhibit the reproductive functions when administered during the RIP. Therefore, it can be inferred from the present study that during the RIP the HPG axis is sensitive to gonadal steroids and, hence, exogenous gonadal steroids induce gonadal activity.

Sex steroids and the thyroid

Thyroid function is modulated by genetic and environmental causes as well as other illnesses and medications such as gonadal or sex steroids. The latter class of drugs (sex steroids) modulates thyroid function. Gonadal steroids exert their influence on thyroid function primarily by altering the clearance of thyroxine-binding globulin (TBG). While oestrogen administration causes an increase in serum TBG concentration, androgen therapy results in a decrease in this binding protein. These effects of gonadal steroids on TBG clearance and concentration are modulated by the chemical structure of the steroid being used, its dose and the route of administration. Despite the gonadal steroids-induced changes in serum TBG concentrations, subjects with normal thyroid glands maintain clinical and biochemical euthyroidism without changes in their serum free thyroxine (T4) or thyroid-stimulating hormone (TSH) levels. In contrast, the administration of gonadal steroids to patients with thyroid diseases causes significant biochemical and clinical alterations requiring changes in the doses of thyroid medications. Similarly, gonadal steroid therapy might unmask thyroid illness in previously undiagnosed subjects. It would be prudent to assess thyroid function in subjects with thyroid disease 6-8 weeks after gonadal steroid administration or withdrawal.

The in vitro conversion of 3H androstenedione to testosterone and dihydrotestosterone in the adrenal gland of castrated male rat: influence of gonadal steroid administration.

In the present study we considered the effects of bilateral orchiectomy on the metabolism of androstenedione in the adrenal gland of the adult male rat. Adrenal glands of intact animals incubated in the presence of 3H androstenedione resulted in its conversion to testosterone and dihydrotestosterone (the two principal metabolites investigated). The value of the latter metabolite increased with the elapsing of time from castration despite the decrease of testosterone with respect to that observed in the sham operated rats of the same age. This effect is reversed in the presence of testosterone treatment with androstenedione and testosterone levels similar to those obtained in intact animals. Androstenedione adrenal metabolism is greatly lowered following the administration of dihydrotestosterone propionate, resulting in high substrate residue values with respect to untreated castrated rats. Contemporarily, testosterone metabolite value remains unchanged while the production of dihydrotestosterone is significantly lower. Finally a decrease of DHT was also encountered in the presence of estradiol benzoate. The above results clearly point to a progressive activation of the 5 alpha-reductase of the adrenal gland after castration utilizing androstenedione as substrate. The metabolic pathway leading to the production of dihydrotestosterone from androstenedione appears to be modulated differently following the administration of gonadal steroids.

Identification of Calcitonin Expression in the Chicken Ovary: Influence of Follicular Maturation and Ovarian Steroids1

Calcitonin (CALCA), a hormone primarily known for its role in calcium homeostasis, has recently been linked to reproduction, specifically as a marker for embryo implantation in the uterus. Although CALCA expression has been documented in several tissues, there has been no report of production of CALCA in the ovary of any vertebrate species. We hypothesized that the Calca gene is expressed in the chicken ovary, and its expression will be altered by follicular maturation or gonadal steroid administration. Using RT-PCR, we detected Calca mRNA and the calcitonin receptor (Calcr) mRNA in the granulosa and theca layers of preovulatory and prehierarchial follicles. Both CALCA and Calca mRNA were localized in granulosa and thecal cells by confocal microscopy. Using quantitative PCR analysis, F1 follicle granulosa layer was found to contain significantly greater Calca mRNA and Calcr mRNA levels compared with those of any other preovulatory or prehierarchial follicle. The granulosa layer contained relatively greater Calca and Calcr mRNA levels compared with the thecal layer in both prehierarchial and preovulatory follicles. Progesterone (P(4)) treatment of sexually immature chickens resulted in a significantly greater abundance of ovarian Calca mRNA, whereas estradiol (E(2)) or P(4) + E(2) treatment significantly reduced ovarian Calca mRNA quantity. Treatment of prehierarchial follicular granulosa cells in vitro with CALCA significantly decreased FSH-stimulated cellular viability. Collectively, our results indicate that follicular maturation and gonadal steroids influence Calca and Calcr gene expression in the chicken ovary. We conclude that ovarian CALCA is possibly involved in regulating follicular maturation in the chicken ovary.

Time course of induction of oxytocin messenger ribonucleic acid levels in the hypothalamic paraventricular nucleus of ovariectomized rats following gonadal steroid administration

The nonapeptide oxytocin (OT) is important for uterine contractility at parturition, milk ejection during lactation, and the induction of maternal behavior. OT messenger ribonucleic acid (mRNA) levels increase in the paraventricular and supraoptic nuclei (PVN and SON) of late pregnant and lactating rats and are modulated by the steroid milieu that accompanies these states. Specifically, sequential exposure to estradiol (E 2) and progesterone (P) followed by P withdrawal 48 hrs prior to sacrifice increases PVN, and to a lesser but significant degree, SON OT mRNA. To better define the time course of induction of OT mRNA levels following P withdrawal, ovariectomized Sprague-Dawley rats were treated with empty or steroid-filled capsules. On day 1, animals received an E 2-filled or empty capsule, followed by P-filled or empty capsules on day 3. On day 14, P-filled or empty capsules were removed and animals were sacrificed 24, 36, or 48 hrs later. The hypothalamic PVN were analyzed for OT mRNA by in situ hybridization histochemistry. Significant differences in PVN OT mRNA were found among the groups (P < 0.0001, Kruskal-Wallis). Animals in the 48 hr (P = 0.007) and 36 hr (P = 0.005), but not the 24 hr, steroid-treated groups had significantly increased OT mRNA relative to their respective sham-treated cohorts (Mann-Whitney U test). The relative abundance of PVN OT mRNA differed among the steroid-treated groups (Kruskal-Wallis, P < 0.0003), with highest levels at 48 hr. We conclude that increases in PVN OT mRNA occur by 36 hrs, and are highest at 48 hrs, after P withdrawal in the E 2-primed rat. Future studies will determine if OT-mediated changes in behavior or physiology that surround parturition are related to these changes in OT mRNA.

Glial Ensheathment of GnRH Neurons in Pubertal Female Rhesus Macaques

During the period of development, prior to full sexual maturity, gonadotropin hormone-releasing hormone (GnRH) neurons are fully capable of synthesizing and processing the GnRH decapeptide. Nonetheless, the secretion of the hormone is not adequate to stimulate adult patterns of gonadotropin release. The present study was undertaken to examine ultrastructural characteristics of the GnRH neuron and its relationship to its environment in early-midpubertal female rhesus monkey. The neurons bore all the ultrastructural immunocytochemical characteristics of those in mature animals, but quantitative morphometrics revealed that they were extensively apposed by glial processes. Such ensheathment was described earlier in ovariectomized adult animals and was found to be reversible by administration of gonadal steroids. The density of synaptic input to GnRH neurons in the pubertal animals did not differ significantly from that of adult intact or ovariectomized animals from a previous study. Chemical identification will be required to determine whether there are age or hormonal differences in the innervation of these neurons. These results provide anatomical evidence in support of indications from other studies that the ovarian steroidal milieu affects GnRH-glial relationships. Further testing will be required to determine whether the attainment of sexual maturity in the female rhesus macaque is dependent upon a reduction in glial ensheathment of GnRH neurons.

The Development of Astrocytes Immunoreactive for Glial Fibrillary Acidic Protein in the Mediobasal Hypothalamus of Hypogonadal Mice

Numbers of astrocytes immunoreactive for glial fibrillary acidic protein (GFAP) are markedly increased in the mediobasal hypothalamus (MBH) of adult hypogonadal (hpg) mice. The astrocytosis cannot be reversed by administration of gonadal steroids. To investigate whether the glial changes are established in the perinatal period, when crucial developments occur in rodent brain, we determined the distribution of GFAP-IR astrocytes in normal and hpg mouse brain from birth to adulthood. The period up to 3 weeks of age was characterized by the gradual disappearance of radial glia and the increase in mature astrocytes in some brain regions, for example hippocampus. However, there were no apparent differences in GFAP-IR elements between normal and hpg brains and very few astrocytes in the MBH. From age 1 month, increased numbers of GFAP-IR astrocytes were apparent in the hypothalamus in hpg mice and this difference was more obvious at 2 months. By 4 months of age the characteristic astrocytosis in the MBH had been attained and this did not change in older hpg mice. These observations provide no evidence for upregulation of the GFAP gene during the first 2 postnatal weeks when its transcription is highest and astrocytes are proliferating most rapidly. It is more likely that the astrocyte response in the MBH in hpg mice reflects permanent differences in steroid-induced neuronal connectivity, caused by the hypogonadism.

The Characteristics of Medial Septal Neurons Antidromically Identified as Projecting to the Median Eminence and their Response to Gonadal Steroids

Abstract We have obtained extracellular single unit recordings from 129 neurons in the medial septum of the male rat that projected to the median eminence. These cells were identified on the basis of their antidromic responses to electrical stimulation in the median eminence. The antidromically-evoked spike was always followed by a period of inhibition (-100 ms) which often (80%) persisted at stimulus intensities below threshold for antidromic activation. The identified neurons displayed one of three patterns of activity, the relative proportions of which were dependent upon the anesthetic. In rats anesthetized with urethane, 15% of the antidromically identified septal neurons displayed an oscillating, 25% a continuous, and 61% a low irregular pattern of spontaneous activity or no activity. In contrast, in barbiturate-anesthetized animals only 6% exhibited an oscillating and 3% a continuous pattern of activity. The remaining neurons (91%) displayed either a low firing rate or were silent. We then assessed the responses of these identified neurons to systemic administration of gonadal steroids. Systemic administration of 17beta-estradiol elicited a rapid decrease in the firing rate of 9 of 29 neurons tested. The inhibitory response was specific to those cells displaying an oscillating (6/9) or continuous (3/6) firing pattern of which 60% were inhibited. In contrast, systemic administration of testosterone failed to elicit such changes in the activity of the identified septal neurons (n = 12) including one neuron which was subsequently shown to be inhibited by estradiol. As such identified septal neurons may contain gonadotropin-releasing hormone, these results suggest a role for estradiol but not testosterone in the short-term regulation of the secretion of this releasing hormone in the male rat.

Regional Changes in Brain Oxytocin Receptors Post‐Partum: Time‐Course and Relationship to Maternal Behaviour

Abstract In a previous report, receptors for oxytocin in rat brain were reported to increase during the early post-partum period. The current study set out to replicate this finding using the novel, highly selective oxytocin receptor ligand, [(125) l]d(CH(2))(5)[Tyr(Me)(2), Thr(4), Tyr-NH(2) (9)]OVT ([(125) I]-OTA). Binding was measured using in vitro receptor autoradiography in rat brain on Day 15 of pregnancy, on Days 1 and 6 post-partum, and at least 6 days following the end of lactation. Relative to pregnancy, oxytocin receptor binding was increased on Day 1 (but not Day 6) post-partum in the bed nucleus of the stria terminalis and the ventromedial nucleus of the hypothalamus. In several other regions, including the anterior olfactory nucleus, the central nucleus of the amygdala, and the ventral subiculum binding was equivalent across the groups. Most surprising, binding in the bed nucleus of the stria terminalis was lowest in the post-lactating group (post-partum Day 1 group was 87% higher than post-lactating group). Saturation studies suggested that binding differences reflected changes in number and not affinity of sites. These findings are consistent with earlier studies of changes in brain oxytocin receptors following exogenous gonadal steroid administration. To determine if maternal behaviour elicited by nonhormonal means (i.e. concaveation) was also associated with increased oxytocin receptors, virgin females with extensive pup exposure were studied. No change in oxytocin receptor binding in the bed nucleus of the stria terminalis was noted in virgin, maternal females-demonstrating that the increase observed post-partum was not essential for the onset of maternal behaviour.

Effect of Estradiol and Progesterone Administration upon the Circadian Rhythm of Oxytocin in the Cerebrospinal Fluid of Rhesus Monkeys

The effect of administration of estradiol (E2) alone or combined with progesterone upon the circadian rhythm of oxytocin concentrations in cerebrospinal fluid (CSF) was examined in adult ovariectomized rhesus monkeys bearing temporary subarachnoid catheters and maintained in a constant photoperiod (lights on 06.00-18.00 h). Animals were subcutaneously implanted with silastic capsules containing 17 beta-E2 for 6 days and progesterone for the last 3 days of E2 administration. Hourly samples of CSF were collected before, during and after gonadal steroid administration and measured for oxytocin by RIA and reverse-phase high-performance liquid chromatography (RP-HPLC). A significant increase in the serum concentration of E2 and the plasma concentration of oxytocin neurophysin, but not the plasma concentration of oxytocin, was found during gonadal steroid administration. Each animal displayed a dirunal pattern of secretion of oxytocin in CSF with peak and trough levels during light and dark hours, respectively. No significant differences were found across experimental conditions in the following CSF oxytocin parameters: mean level of oxytocin in CSF during the light, dark, or light and dark hours combined; the daily phase or amplitude of the CSF oxytocin rhythm; the peak or nadir concentration of oxytocin in CSF; or the total amount of oxytocin secreted into the CSF as expressed as the area under the curve (multivariate repeated measures ANOVA). The CSF oxytocin parameters in the animals that were restudied using empty silastic implants were not significantly different across time (multivariate repeated measures ANOVA).(ABSTRACT TRUNCATED AT 250 WORDS)

Gonadal regulation of corticotropin-releasing factor immunoreactivity in hypothalamus

The effect of gonadectomy and subsequent gonadal steroid administration on hypothalamic corticotropin-releasing factor immunoreactivity (CRF-ir) in male and female rats was determined using a radioimmunoassay specific for rat CRF. When compared to appropriate controls, gonadectomy resulted in significantly decreased CRF-ir in both males and females ( p<0.025). Short-term replacement with testosterone or estradiol did not significantly alter hypothalamic CRF-ir. Acute IC administration of gonadotropin-releasing hormone to male rats significantly decreased CRF-ir in the median eminence 15 minutes later. Chronic administration of estradiol for 3 weeks significantly reduced CRF-ir in ovariectomized rats ( p<0.005). This decrease was shown not to correlate to the marked increase in anterior pituitary weight ( r 2=0.063, not significant). Chronic administration of estradiol to hypophysectomized female rats significantly decreased hypothalamic CRF-ir ( p<0.01), an effect localized to the median eminence ( p<0.005). These data imply that the gonadectomy-induced reduction in hypothalamic CRF-ir was not directly mediated by the loss of gonadal steroids. Chronic estradiol treatment decreased hypothalamic CRF-ir, by a non-pituitary mechanism. These data show significant effects of the gonadal axis in the regulation of CRF in the hypothalamus in both sexes. Corticotropin-releasing factor Hypothalamus Median eminence Ovariectomy Orchidectomy

Ovine luteinizing hormone. II. Effects of castration and steroid administration on the levels of uncombined subunits within the pituitary.

Pituitaries were removed from rams, wethers, and wethers that received Silastic implants containing 5 alpha-dihydrotestosterone (DHT), 17 beta-estradiol (E2) or DHT + E2. After homogenization and centrifugation (100,000 X g), aliquots of the supernatants were subjected to analytical gel filtration on Sephadex G-100 Superfine to separate native ovine luteinizing hormone (oLH) from its uncombined subunits. Immunoreactive oLH and oLH subunits were quantified in the elution profiles to examine the effects of castration and gonadal steroid administration on the intracellular levels of uncombined oLH subunits. Pituitaries from rams contained 1.41 +/- 0.26, 0.191 +/- 0.024, and 0.0246 +/- 0.0043 micrograms oLH, oLH alpha and oLH beta per mg tissue, respectively, which translated to oLH alpha/oLH and oLH beta/oLH molar ratios of approximately equal to 0.29 and approximately equal to 0.04. Castration decreased the concentrations of oLH and its subunits by approximately 50%, but did not significantly alter the oLH alpha/oLH and oLH beta/oLH molar ratios. All three steroid treatments further decreased the concentrations of oLH and oLH beta. Pituitaries from DHT-implanted wethers exhibited similar oLH alpha/oLH and oLH beta/oLH molar ratios to rams and unimplanted wethers. However, in E2- or DHT + E2-implanted wethers, there was a greater reduction in the concentration of native oLH than in the uncombined subunits. Thus, both the oLH alpha/oLH and oLH beta/oLH molar ratios were significantly higher in E2- or DHT + E2-implanted wethers than in the other groups. The apparent molecular sizes of oLH or its subunits were not significantly altered by castration or steroid administration. These results suggest that DHT and E2 decrease the concentrations of uncombined oLH beta as well as native oLH in the pituitary, but do not appear to alter the apparent molecular size of either oLH or its uncombined subunits However, because the levels of uncombined subunits were not decreased to the same degree as oLH in E2-implanted wethers, estrogens may affect the process of oLH subunit combination or may result in the production of molecular forms of oLH that are easier to dissociate.

Ovine Luteinizing Hormone. I. Effects of Castration and Steroid Administration on the Charge Heterogeneity of Pituitary Luteinizing Hormone1

Chromatofocusing was used to separate and characterize the isohormones of ovine luteinizing hormone (oLH) in the pituitaries of rams, wethers, and wethers receiving Silastic implants containing 5 alpha-dihydrotestosterone (DHT), 17 beta-estradiol (E2), or DHT plus E2. Extracts of anterior pituitaries were prepared by homogenization and centrifugation at 100,000 X g. Castration reduced the amount of oLH in the pituitary, even though peripheral levels were elevated. Pituitary oLH concentrations in wethers were further reduced by all three steroid treatments. When subjected to chromatofocusing on pH 10.5 to 7.0 gradients, pituitary extracts yielded eight peaks of immunoreactive oLH, which eluted with apparent isoelectric points of greater than 9.8, 9.26, 9.14, 9.07, 8.98, 8.91, and less than or equal to 7.0. These isohormones were designated A-G and Z, respectively. In rams, isohormones F and G were the predominant species, representing approximately equal to 57% of the immunoreactive oLH recovered from the column. Castration resulted in a subtle shift toward more basic isohormones. DHT administration caused an increase in the relative amount of isohormone A, whereas E2 treatment resulted in an increase in isohormone Z. DHT and E2 in combination produced increases in the relative amounts of both isohormones A and Z. All eight oLH isohormones were active in an in vitro LH bioassay and exhibited biological-to-immunological-assay (B/I) ratios in the ram ranging from 0.4 to 2.8. Isohormone F exhibited the highest B/I ratio in all the treatment groups. Similarly, isohormone F was clearly the predominant biologically active form of oLH in all groups. These results demonstrate that at least eight immunoreactive and biologically active forms of pituitary oLH can be separated by using chromatofocusing; the pattern of oLH isohormones is markedly different from that in the rat; castration has a minimal effect on the pattern of oLH isohormones in pituitary extracts; and exogenous gonadal steroid administration reduces the amount of oLH in the pituitary and changes the pattern of oLH isohormones, resulting in a higher percentage of less biologically active forms.

Administration of gonadal steroids to neonatal rats affects beta-endorphin levels in the adult

Administration of gonadal steroids to neonatal rats has a profound effect on the function of the neuroendocrine system in the adult animal. Considering that gonadal steroids modulate hypothalamic and pituitary levels of beta-endorphin (BE) in adult male and female rats, the effects of neonatal gonadal steroid treatment on BE levels in the adult animal were investigated. Neonatal male rats were administered testosterone and neonatal female rats were treated with estrogen. Matched control littermates received vehicle. All animals were sacrificed at 90 days of age. Neonatal gonadal steroid treatment did not affect the level of immunoreactive beta-endorphin (IR-BE) in the anterior pituitary (AP) of male rats but did result in a significant increase in IR-BE in the AP of female rats. Neonatal administration of gonadal steroids produced a significant decrease in IR-BE in the neurointermediate lobe of the pituitary (NIL) of both male and female rats, with the magnitude of the decrease being greater in the NIL of the female rats. IR-BE levels in the hypothalamus of male or female rats were not altered by the treatments. Column chromatography indicated that the increase in IR-BE in the AP represented a proportional increase in BE and beta-lipotropin, while the reduction in IR-BE in the NIL of the treated rats represented a reduction in BE. These findings suggest that gonadal steroids may influence the development of the neurotransmitter systems which regulate BE levels in the adult pituitary, the development of the biosynthetic mechanisms of the adult pituitary, or both.

The Effects of Gonadal Steroids on the Content of Substance P in the Rat Anterior Pituitary*

The effects of gonadectomy and of the administration of gonadal steroids on the content of substance P in the anterior pituitary (AP-SP) were studied in adult rats. The effect of gonadal status on the AP-SP content of thyroidectomized (TX) rats was also studied. We have confirmed that the AP-SP content in adult males is higher than that in adult females. Ovariectomy (OVX) caused an increase in AP-SP content which was apparent 6 days after surgery. Estradiol (E2; 2 micrograms/rat daily) administered for 13 days beginning the day after OVX prevented the increase in AP-SP content induced by OVX. Orchiectomy of adult rats had no effect on AP-SP content 14 and 45 days after surgery. E2 administered to adult female rats for 13 days caused a reduction in the AP-SP content, whereas dihydrotestosterone (0.2 mg/rat daily for 13 days) caused an increase that was even more pronounced in TX rats. E2 administration to TX adult female rats caused a significant decrease in the AP-SP content both when treatment was begun on the day after surgery or 2 weeks later. Administration of T4 (1.5 and 25 micrograms/100 g BW daily for 7 days) to rats made hypothyroid by thyroidectomy 2 weeks earlier abolished the increase in AP-SP content seen in TX animals. Neither E2 nor dihydrotestosterone had an effect on the substance P content of any of the brain regions examined. The AP-SP content of pregnant or lactating rats was not different from that of age-matched controls. The content of substance P in the AP and median eminence did not vary significantly throughout the estrous cycle. The data indicate that AP-SP content is dependent on the gonadal status of the animal and that gonadal steroids interact with thyroid hormones in the regulation of substance P turnover in the AP.

Reversal of hippocampal sexual dimorphism by gonadal steroid manipulation

Hippocampal slices display a sexually dimorphic pattern of physiological neuromodulation to gonadal steroid administration. To determine if this is the result of organizational or activational hormone action, plasma steroid levels of adult male rats were manipulated. The results support an activational mechanism since steroid levels can be manipulated to abolish, reverse, or maintain the functional dimorphism. The results suggest a role for gonadal steroid action in rapid neuromodulation of brain function.

Gonadal steroid modulation of naloxone-induced LH secretion in the rat

The effect of acute administration of the opioid receptor antagonist naloxone hydrochloride (5 mg/kg, s.c.) on plasma LH levels was evaluated in female and male rats 24, 36 and 48 h and 1, 3 and 5 weeks after gonadectomy and in 5-week gonadectomized rats after acute or chronic (2 weeks) administration of oestradiol benzoate (OB, 10 micrograms/rat per day, s.c.), testosterone propionate (TP, 150 micrograms/rat, s.c.) or dihydrotestosterone propionate (DHT, 150 micrograms/rat, s.c.) respectively. Concurrent evaluation of plasma LH after administration of LH releasing hormone (LHRH, 1 microgram/kg, i.p.) was performed in the same experimental groups. In rats of both sexes, a significant rise in plasma LH after naloxone was observed in sham-operated and recently gonadectomized rats (24-48 h); in female rats 36 and 48 h after gonadectomy the rise was higher than in controls. One, 3 and 5 weeks after gonadectomy, naloxone failed to stimulate LH release in both female and male rats. In gonadectomized rats undergoing steroid replacement therapy, OB administered 72 h before testing, TP (16 and 72 h) and DHT (16 h) were the most effective in reinstituting the LH response to naloxone. Chronic administration of gonadal steroids did not restore normal LH responsiveness to naloxone. In most experimental groups, LH responses after naloxone were clearly unrelated to pituitary LH responsiveness to LHRH, which indicates that the opioid antagonist was acting via the central nervous system.(ABSTRACT TRUNCATED AT 250 WORDS)

Cycle initiation in amenorrhoea; the effect of progesterone and oestrogen administration on the pulsatile release of gonadotrophins.

The modulation of pulsatile gonadotrophin release by endogenous ovarian steroids during the normal menstrual cycle may be involved in the initiation of the following menstrual cycle. The absence of this cyclical variation may, in some cases, be the cause of, or contribute to the cause of, amenorrhoea. To assess this the modulatory effect of gonadal steroid administration on the pulsatile release of gonadotrophins was studied in fourteen amenorrhoeic and four oligomenorrhoeic women. Pulsatility was assessed by samples collected at 10 min intervals during a 4 h morning period before and after treatment with either progesterone or micronized oestradiol or a sequential combination of both. Ten patients with intact positive oestrogen-gonadotrophin feedback responded to progesterone treatment by both a significant reduction in LH pulse frequency, from a mean of 4.1 to a mean of 2.1 pulses within the 4 h study period and an increase in pulse amplitude. Progesterone therapy did not affect mean LH concentrations but there was a significant reduction in mean FSH concentrations. In the eight patients with absence of positive feedback, none of the treatment regimes elicited significant changes in LH release. No definable FSH pulses were detected before or after treatment in either group. Both the changes in LH and FSH concentrations and their release observed in this study, support the concept that cycle initiation may be related to a reduced pituitary exposure to LHRH associated with elevated progesterone concentration in the luteal phase of the cycle. This selectively induces FSH synthesis and storage. Release of this stored FSH may occur as a result of failure of the corpus luteum and falling progesterone concentrations.

Subcellular distribution of 3 alpha-hydroxysteroid dehydrogenase and antiestrogen action on androgen-metabolizing enzymes in rat pituitary gland.

Gonadectomy of male rats led to a threefold increase of 3 alpha-hydroxysteroid dehydrogenase (3 alpha-HSDH) activity in pituitary homogenates that could be completely reversed by chronic administration of estradiol or 5 alpha-dihydrotestosterone (DHT). 3 alpha-HSDH was found to be distributed mainly between the 10,000 g and 100,000 g sediments from whole homogenates. The microsomal enzyme activity showed a substantial specificity for NADH whereas the cytosolic enzyme (100,000 g supernatant) demonstrated a slight preference for NADPH. The changes in Vmax found in homogenates following gonadectomy and gonadal steroid administration reflected changes in NADH-linked activity of the microsomal, but not the cytosolic enzyme. Estradiol-induced suppression of NADH-linked 3 alpha-HSDH activity in pituitary homogenates from gonadectomized rats of either sex was accompanied by a similar suppression of NADPH-linked 5 alpha-reductase activity and a marked decrease of luteinizing hormone (LH) and follicle-stimulating hormone (FSH) release. In the ovariectomized rat chronic administration of nonsteroidal antiestrogens had strong estrogenic effects on 3 alpha-HSDH activity and LH release, but not on 5 alpha-reductase activity and FSH release. In the gonadectomized male rat, which was much less sensitive to intrinsic estrogenicity of the antiestrogens tested, nafoxidine completely blocked estradiol-induced suppression of 5 alpha-reductase activity and FSH release and partially antagonized suppression of LH release. The trans-isomeric, substituted triphenylethylenes, tamoxifen, and enclomiphene, as well as nitromifene (mixture of trans and cis isomers) were able partially to counteract estradiol-induced suppression of 5 alpha-reductase, but not 3 alpha-HSDH activity. It is concluded that estradiol action on pituitary 5 alpha-reductase, but not 3 alpha-HSDH activity, involves an estrogen receptor mechanism.

Role of somatostatin in the modulation of hypophysial growth hormone production by gonadal steroids.

AbstractGrowth hormone‐release‐inhibiting hormone (somatostatin) has been localized in several central nervous system sites, but the contribution of somatostatin in these various loci to the control of growth hormone (GH) production is unclear. In the present study, the efficacy of castration, alone, or with subsequent gonadal steroid administration, in modulating hypophysial GH production was examined, and the response of somatostatin‐containing nerve endings in the median eminence and the organum vasculosum of the lamina terminalis (OVLT) was evaluated by means of immunocytochemistry. Orchidectomy reduced the size and number of immunoreactive GH‐cells, and testosterone administration prevented this change. Ovariectomy appeared to stimulate the somatotropes, but this effect was neither as dramatic nor as consistent as the effect of castration in the male. Administration of estradiol, however, resulted in a marked reduction in the number, size, and GH content of somatotropes in all animals examined. Somatostatin was localized in the external lamina of the median eminence and in nerve endings surrounding the core of capillaries in the OVLT. Only the median eminence responded to steroid manipulations with a visible change in the immunoreactive pool of somatostatin, with a decrease following orchidectomy, which was reversible by testosterone treatment, and an increase following ovariectomy, which was reversible by estradiol treatment. However, as in the GH responses to steroid manipulations, the somatostatin responses were more variable in females than in males.These results suggest that: (1) testosterone promotes and estradiol, at least at high doses, inhibits storage of GH in the anterior pituitary; (2) these changes in GH production may be regulated, in part, by altered somatostatin release from the median eminence into the hypophysial portal blood; and (3) somatostatin in the OVLT may not be involved in the steroid‐induced modulation of GH production.