Interaction between ovarian and adrenal steroids in the regulation of gonadotropin secretion

Recent work from our laboratory suggests that a complex interaction exists between ovarian and adrenal steroids in the regulation of preovulatory gonadotropin secretion. Ovarian estradiol serves to set the neutral trigger for the preovulatory gonadotropin surge, while progesterone from both the adrenal and the ovary serves to (1) initiate, (2) synchronize, (3) potentiate and (4) limit the preovulatory LH surge to a single day. Administration of RU486 or the progesterone synthesis inhibitor, trilostane, on proestrous morning attenuated the preovulatory LH surge. Adrenal progesterone appears to play a role in potentiating the LH surge since RU486 still effectively decreased the LH surge even in animals ovariectomized at 0800 h on proestrus. The administration of ACTH to estrogen-primed ovariectomized (ovx) immature rats caused a LH and FSH surge 6 h later, demonstrating that upon proper stimulation, the adrenal can induce gonadotropin surges. The effect was specific for ACTH, required estrogen priming, and was blocked by adrenalectomy or RU486, but not by ovariectomy. Certain corticosteroids, most notably deoxycorticosterone and triamcinolone acetonide, were found to possess "progestin-like" activity in the induction of LH and FSH surges in estrogen-primed ovx rats. In contrast, corticosterone and dexamethasone caused a preferential release of FSH, but not LH. Progesterone-induced surges of LH and FSH appear to require an intact N-methyl-D-aspartate (NMDA) neurotransmission line, since administration of the NMDA receptor antagonist, MK801, blocked the ability of progesterone to induce LH and FSH surges. Similarly, NMDA neurotransmission appears to be a critical component in the expression of the preovulatory gonadotropin surge since administration of MK801 during the critical period significantly diminished the LH and PRL surge in the cycling adult rat. FSH levels were lowered by MK801 treatment, but the effect was not statistically significant. The progesterone-induced gonadotropin surge appears to also involve mediation through NPY and catecholamine systems. Immediately preceding the onset of the LH and FSH surge in progesterone-treated estrogen-primed ovx. rats, there was a significant elevation of MBH and POA GnRH and NPY levels, which was followed by a significant fall at the onset of the LH surge. The effect of progesterone on inducing LH and FSH surges also appears to involve alpha 1 and alpha 2 adrenergic neuron activation since prazosin and yohimbine (alpha 1 and 2 blockers, respectively) but not propranolol (a beta-blocker) abolished the ability of progesterone to induce LH and FSH surges. Progesterone also caused a dose-dependent decrease in occupied nuclear estradiol receptors in the pituitary.(ABSTRACT TRUNCATED AT 400 WORDS)     

The stimulatory roles of catecholamines and acetylcholine in the regulation of gonadotropin release in ovariectomized estrogen-primed rats.

Injections of 2 mg of progesterone into ovariectomized estrogen-primed rats significantly increased serum LH and FSH concentrations 3, 5 and 8 hr later. Receptor blockers of noradrenaline (NA), dopamine (DA) or acetylcholine (ACH), phenoxybenzamine (20 mg/kg body weight), pimozide (1mg/kg body weight) or atropine (700 mg/kg body weight), respectively, prevented the progesterone-induced gonadotropin release. On the other hand, none of them blocked the gonadotropin release following unilateral electrochemical stimulation (100 microA for 60 sec) of the medial preoptic area which occurred 0.5 and 1.5 hr later, although pimozide or atropine reduced serum LH concentrations at 4.0 hr after stimulation. Furthermore, the sites of action of NA, DA and ACH with respect to LH release were examined by intracerebral implantation in ovariectomized estrogen-primed rats DA or ACH, when implanted unilaterally into the medial preoptic urea, induced a significant increase in serum LH 5 hr later, whereas NA decreased LH levels. Implantations of NA or ACH into the bed nucleus of the stria terminalis or the medial amygdala increased serum LH although the effect of NA into the latter was not statistically significant. Only implantations of NA among the three substances into the lateral septum induced LH release. These results suggest that all of NA, DA and ACH play stimulatory roles in the regulation of gonadotropin secretion, and that there are regional differences of their effectivenesses in releasing LH within the limbic-preoptic area.     

Heme oxygenase in the rat anterior pituitary: immunohistochemical localization and possible role in gonadotropin and prolactin secretion

The objectives of this study were to determine if heme oxygenase (HO), which catalyzes the degradation of heme and the formation of carbon monoxide (CO), is localized in the rat anterior pituitary and, if so, to determine if hemin (a substrate for HO) or chromium mesoporphyrin (CrMP) (an inhibitor of HO), alter pituitary gonadotropin and prolactin secretion. For localization of HO, sections of anterior pituitaries obtained from mature Holtzman Sprague-Dawley rats in different stages of the estrous cycle were immunostained for two of the HO isoforms, HO-1 and HO-2. The immunostaining for the inducible HO isoform (HO-1) was limited to discrete populations of pituitary cells, whereas the constitutive isoform (HO-2) had a more widespread distribution. The afternoon surge of leutinizing hormone (LH) in the plasma of ovariectomized, estradiol-treated rats was advanced by 2 hr after 7 days of treatment with CrMP (4 micro M/kg), and this effect was reversed when hemin (30 micro M/kg) was co-administered with CrMP. The afternoon follicle-stimulating hormone (FSH) surge was not affected by either treatment. In contrast, the afternoon prolactin (PRL) surge was completely blocked or delayed by CrMP treatment, and this effect was not reversed by hemin. In vitro perifusion of pituitary explants with CrMP also significantly reduced PRL release compared with secretion from untreated explants. In vitro gonadotropin-releasing hormone (GnRH)-stimulated FSH secretion was significantly increased from pituitary explants of ovariectomized, estradiol-treated rats treated in vivo with hemin but was unaffected by CrMP treatment, whereas GnRH-stimulated LH release was not affected by hemin but was increased by CrMP treatment. In conclusion, this study demonstrates that HO exists in the rat anterior pituitary gland, and that a substrate and an inhibitor of this enzyme alter the secretion of gonadotropins and PRL.     

Studies on the mechanism of secretion of rat adrenal steroids in vitro

The possibility that in the rat adrenal cortex synthesis and secretion of steroid hormones may be distinct. processes has been studied using in vitro methods.The uptake and subsequent release of radioactive corticosterone and 18-OH-DOC added to incubation media showed little difference between the two steroids. Both were taken up rapidly by the tissue, and after exchange of medium, 50% was released within 15 min. The properties of muscle and adrenal tissue were qualitatively similar in these respects, and were consistent with steroid movement by diffusion.Different results were obtained when steroids formed from the tissues' endogenous precursor supply were studied. Following incubation for 2h tissue and medium steroid cohtent were measured, and tissue/medium ratios of steroid were used as an index of secretion. With varying volumes of incubation medium (1.4–10 ml), secretion of corticosterone was related to volume, although synthesis was constant. With 18-hydroxydeoxycorticosterone (18-OH-DOC) on the other hand neither synthesis nor secretion was affected by medium volume.The effects of ouabain (10⁻⁴ M) on synthesis and secretion were also studied, using a 5 ml volume of medium. Under these conditions, tissue levels of corticosterone were very low, while 18-OH-DOC was extractable from tissues in amounts up to 30% of the total synthesised. In capsules, steroid synthesis was decreased by ouabain, or by a lower potassium medium (3.6 mM) or both together. Secretion of corticosterone was not affected by these treatements, whereas secretion of 18-OH-DOC and aldosterone was greatly inhibited: maximally 50% of the synthesised 18-oxygenated steroid was retained within the tissue. These treatments also affected inner zone function, ouabain inhibited steroid synthesis, but the low potassium medium did not. Secretion of corticosterone was unaffected, whereas secretion of 18-OH-DOC was again inhibited both by low potassium medium and by ouabain. Maximally as in the capsules, 50% of the synthesised 18-OH-DOC was retained within the tissue.The results suggest that the secretion of 18-OH-DOC formed from endogenous precursors by rat adrenal tissue is controlled by a mechanism involving a ouabain sensitive process in the cell membrane. Corticosterone is released by a different mechanism, quite possibly passive diffusion.     

Stimulatory and inhibitory effects of progesterone on FSH secretion by the anterior pituitary.

The purpose of this study was to investigate whether progesterone exerted progesterone receptor mediated direct effects on the anterior pituitary in the secretion of FSH and whether such effects were mediated through the 5 alpha-reduction of progesterone. Treatment of anterior pituitary dispersed cells for 48 h with 0.5 nM estradiol reduced the ED50 for gonadotropin releasing hormone (GnRH)-stimulated FSH release from 0.58 to 0.36 ng/ml and the ED50 for GnRH-induced LH release from 0.54 to 0.19 ng/ml. When dispersed pituitary cells were treated with 0.5 nM estradiol and exposed to various doses of progesterone for 1 to 6 h, the most consistent rise in basal and GnRH-stimulated FSH release was observed with the 50 nM dose of progesterone with a 3-h exposure period. All three doses of progesterone elevated basal LH and GnRH-stimulated LH was increased by the 50 and 100 nM doses of progesterone during the 3-h period of treatment. Using the 50 nM dose of progesterone, basal and GnRH-stimulated LH was increased after 2, 3 and 6 h of progesterone treatment. When the period of exposure of progesterone was extended to 12, 36 or 48 h, there was a significant inhibition of GnRH-stimulated FSH release. GnRH-stimulated LH release was inhibited at 36 and 48 but not 12 h after progesterone treatment. These studies showed that the effect of progesterone administered for periods of 1 to 6 h enhanced the secretion of LH and FSH whereas progesterone administered for periods beyond 12 h inhibited FSH and LH release by dispersed pituitary cells in culture. These results are similar to those observed in vivo after progesterone treatment. Furthermore estrogen priming of the dispersed pituitary cells was necessary to observe the effects of progesterone. The progesterone antagonist RU486 prevented the progesterone-induced rise in GnRH-stimulated FSH release. Furthermore the 5 alpha-reductase inhibitor N,N-diethyl-4-methyl-3-oxo-4-aza-5 alpha-androstane- 17 beta-carboxamide also prevented the progesterone-induced rise in GnRH-stimulated FSH release in estrogen-treated dispersed pituitary cells. These results indicate that the anterior pituitary is a major site of action of progesterone in the release of FSH and that 5 alpha-reduction of progesterone plays an important role in FSH release.     

Novel 8 alpha-ergolines with inhibitory and stimulatory effects on prolactin secretion in rats

Four derivatives of the ergot dopamine (DA) agonist lisuride (LIS), namely 6-n-propyl-lisuride (6-n-propyl-LIS), transdihydrolisuride (TDHL), 6-n-propyl-transdi-hydrolisuride (6-n-propyl-TDHL) and 2-bromolisuride (2-Br-LIS) were investigated in female rats with regard to their influence on hyperprolactinaemia induced by pretreatment with reserpine (2 mg/kg i.p., 24 h) at various intervals following their subcutaneous or oral administration (0.05 mg/kg). Two hours after administration, LIS, 6-n-propyl-LIS, and 6-n-propyl-TDHL caused a statistically significant inhibition of reserpine-induced hyperprolactinaemia of about the same extent. Eight hours after administration 6-n-propyl-LIS and 6-n-propyl-TDHL were as active as after 2 h in inhibiting prolactin (PRL) secretion whereas LIS was almost ineffective in this respect. TDHL caused a statistically significant inhibition of PRL secretion at 2 and 8 h after oral administration; this effect was less pronounced after s.c. administration. In contrast to the aforementioned derivatives 2-Br-LIS further increased the reserpine-induced hyperprolactinaemia. In normal male rats pretreatment with 2-Br-LIS (0.025-6.25 mg/kg s.c., 2 h) dose-dependently stimulated PRL secretion. The present data support the assumption of the longlasting DA agonistic action of 6-n-propyl-LIS and 6-n-propyl-TDHL and of the antidopaminergic properties of 2-Br-LIS recently derived from behavioural studies.     

Integration of the effects of estradiol and progesterone in the modulation of gonadotropin secretion

Estradiol secreted by the maturing follicle is the primary trigger for the surge of gonadotropins leading to ovulation. Progesterone has stimulatory or inhibitory actions on this estrogen-induced gonadotropin surge depending upon the time and dose of administration. The administration of progesterone to immature ovariectomized rats primed with a low dose of estradiol induced a well-defined LH surge and prolonged FSH release, a pattern similar to the proestrus surge of gonadotropins. A physiological role of progesterone is indicated in the normal ovulatory process because a single injection of the progesterone antagonist RU 486 on the day of proestrus in the adult cycling rat and on the day of the gonadotropin surge in the pregnant mare's serum gonadotropin stimulated immature rat resulted in an attenuated gonadotropin surge and reduced the number of ova per ovulating rat. Progesterone administration brought about a rapid LHRH release and an decrease in nuclear accumulation of estrogen receptors in the anterior pituitary but not the hypothalamus. The progesterone effect was demonstrated in vitro in the uterus and anterior pituitary and appears to be confined to occupied estradiol nuclear receptors. In in vivo experiments the progesterone effect on estradiol nuclear receptors appeared to be of approximately 2-h duration, which coincided with the time period of progesterone nuclear receptor accumulation after a single injection of progesterone. During the period of progesterone effects on nuclear estrogen receptors, the ability of estrogens to induce progesterone receptors was impaired. Based on the above results, a model is proposed for the stimulatory and inhibitory effects of progesterone on gonadotropin secretion.     

A possible role for progesterone in the preovulatory gonadotropin surge through modulation of LHRH degrading activity

The preovulatory surge of gonadotropins is triggered by estradiol and enhanced to its full magnitude by progesterone. Progesterone may exert this effect through several mechanisms. One of the mechanisms is through the ability of progesterone to induce an increase in the hypothalamic content and release of LHRH. The purpose of this study was to determine if progesterone might not act through yet another mechanism and facilitate LHRH release of the proestrous gonadotropin surge through modulation of luteinizing hormone releasing hormone (LHRH) degrading activity. Sixty-day-old Sprague-Dawley rats were ovariectomized; 14 days later, the estradiol-progesterone milieu of proestrous was mimicked in these animals through the use of estradiol containing silastic implants and subcutaneous progesterone injections. The LHRH degrading activity of the hypothalamus, pituitary and serum were monitored subsequently at preselected time points. In the hypothalamus, estradiol alone was capable of inducing significant increase in degrading activity; progesterone alone had no effect; however, progesterone subsequent to estradiol priming suppressed the increase induced by estradiol alone. In the pituitary, neither estradiol alone nor progesterone alone nor progesterone subsequent to estradiol priming had any significant effect on degrading activity. In the serum, estradiol induced a rapid and significant increase in activity; progesterone alone suppressed activity; progesterone subsequent to estradiol priming induced a similar but more rapid suppression. Therefore, the overall tendency was for estradiol to stimulate and progesterone to suppress LHRH degrading activity in the tissues studied. The results of this study indicate that progesterone has the capacity to suppress LHRH degrading activity and may be one of the mechanisms capable of increasing the availability of LHRH to the anterior pituitary gland thereby facilitating the preovulatory gonadotropin surges.     

Different effects of aging on the opioid mechanisms controlling gonadotropin and cortisol secretion in man

The present study was undertaken in order to assess the influence of aging on the endogenous opioid control of gonadotropin and adrenocorticotropin/cortisol secretion in man. For this purpose, the capability of the opioid antagonist naloxone to increase circulating levels of luteinizing hormone (LH), follicle-stimulating hormone (FSH) and cortisol was tested in male subjects of different ages. Thirty normal men were randomly chosen and divided into 3 groups by age: group I = 22-40 years (n = 10); group II = 41-59 years (n = 10); group III = 62-80 years (n = 10). Since the men of group III showed higher basal serum gonadotropin concentrations than the subjects of group I and group II, we selected from a large population a fourth group of elderly men with normal basal LH and FSH levels: group IV = 61-82 years (n = 7). All subjects were tested for 120 min during the intravenous administration of naloxone (4 mg given in an intravenous bolus at time 0, plus 10 mg infused for 2 h). Control tests with normal saline instead of naloxone were performed in all groups. All subjects had similar blood testosterone and cortisol levels, whereas LH and FSH concentrations were significantly higher in group III than in groups I, II and IV. Naloxone increased plasma cortisol concentrations by 50% in all groups. The cortisol secretory response followed a similar pattern regardless of age.(ABSTRACT TRUNCATED AT 250 WORDS)     

Expression of arachidonate 12-lipoxygenase in rat tissues: a possible role in glucagon secretion.

There are three isoforms of arachidonate 12-lipoxygenase in mammals: platelet, leukocyte, and epidermal types. We found in this study that the leukocyte-type enzyme was present in rat pineal gland, lung, spleen, aorta, adrenal gland, spinal cord, and pancreas, as assessed by RT-PCR. Immunohistochemical analysis showed that the enzyme was localized in macrophages in lung and spleen, alpha-cells of pancreatic islet, zona glomerulosa cells of adrenal cortex, and neuronal cells of spinal cord and superior cervical ganglion. The presence of the 12-lipoxygenase in pancreatic alpha-cells was confirmed by glucagon staining in a consecutive section. We overexpressed the leukocyte-type 12-lipoxygenase cDNA in a glucagon-secreting alphaTC clone 6 cell line that had been established from a transgenic mouse. Glucagon secretion was stimulated by approximately twofold in the 12-lipoxygenase-expressing cells compared to the mock-transfected and original cells. The results suggest that the 12-lipoxygenase of the leukocyte type augments glucagon secretion from pancreatic islets.     

Temporal changes occur in the neuroendocrine control of gonadotropin secretion in aging female rats: role of progesterone

The present study examined the gonadotropin surge-inducing actions of estradiol (E(2)), both alone and with progesterone (P(4)), in middle-aged, early persistent-estrous (PE) female rats that had become PE within 35 days. In addition, we also assessed the effect of P(4) on the mating-induced gonadotropin surges in these acyclic animals. Early PE rats were ovariectomized and received E(2) implants (Day 0). On Day 4, an s.c. injection of P(4) (0.5 mg/ 100 g body weight) at 1200 h markedly increased plasma P(4) and elicited both LH and FSH surges, whereas vehicle-treated controls displayed no rise in P(4) or gonadotropins. This observation confirms that at middle age, female rats no longer respond to the positive-feedback stimulation of E(2) on gonadotropin surges whenever the estrous cyclicity ceases. As PE continued, such a surge-inducing action of E(2) plus P(4) became diminished after 75 days of PE and disappeared thereafter. When caged with males, vehicle-treated early PE rats display a mating-induced increase in P(4) from the adrenal along with small gonadotropin surges. The amplitude of these mating-induced gonadotropin surges was enhanced by supplementation with exogenous P(4) in early PE rats. Our findings indicate that during the early phase of PE, the surge-inducing action of E(2) and P(4) remains intact but deteriorates as PE continues. Thus, a deficiency in P(4) secretion during aging may contribute to the diminished gonadotropin surge response in the hypothalamic-pituitary axis and the subsequent cessation of estrous cyclicity.     

Acute and chronic effects of the fornix section on cyclic gonadotropin secretion and ovulation in the rat.

The fornix was sectioned in the frontal plane by means of a razor blade knife, and acute and chronic effects of this section on gonadotropin secretion were estimated. The 5-day cyclic rat which received the section of fornix under either anesthesia at 12:00 on the day of diestrus II showed advancement of the proestrous and estrous vaginal smears and as well as ovulatory gonadotropin release by one day. It was revealed that the primary effect was the inducement of FSH release on the day of section. The 4-day cyclic rat bearing the fornix section chronically resumed vaginal cyclicity after elapsing the diestrous period for 18 to 25 days. The rat ovulated normally and mean number of ova inoviducts was not different from that in the intact rat. However, the sectioned rat hadan higher concentrations of pituitary and serum FSH on the day of diestrus II than thatin the intact rat, and had an higher concentration of serum LH on the day of estrus. These results indicate that the hippocampus exerts the inhibitory influence on LH and FSH release and if this is eliminated the facilitatory influence dominates the brain mechanism controlling gonadotropin release, resulting in the advancement of estrous cycle (the acute effect) or the increase of gonadotropin release (the chronic effect).     

Anesthetic dependence of the inhibitory effect of neurotensin on pentagastrin-stimulated acid secretion in rats. A possible role for somatostatin.

The existence of possible local mediators of the inhibitory effect of neurotensin on gastric acid secretion has not been determined. We perfused rats intragastrically with warmed saline and stimulated acid secretion with intravenous pentagastrin, 32 micrograms/kg/hr, and found that anesthesia with pentobarbital resulted in marked inhibition of acid secretion by intravenous neurotensin; however, anesthesia with urethane prevented this inhibitory effect of neurotensin from occurring. In addition, we found a significant increase in somatostatin-like immunoreactivity in portal venous blood during neurotensin infusion in pentobarbital-anesthetized rats. Neither neurotensin nor pentagastrin infusion modified gastric luminal somatostatin-like immunoreactivity after either pentobarbital or urethane, and rats anesthetized with urethane did not show an increase of somatostatin-like immunoreactivity in portal venous blood during neurotensin infusion. These results suggested that somatostatin-like immunoreactivity, released into the portal circulation, was necessary for exogenous neurotensin to inhibit pentagastrin-stimulated gastric acid secretion under these conditions in anesthetized rats.     

The effects of hexachlorobenzene on circulating levels of adrenal steroids in the ovariectomized rat

Hexachlorobenzene (HCB), is a global pollutant that resists degradation and possesses a propensity to bioaccumulate. However, the effect of HCB on adrenal function remains largely unknown. Thus, circulating levels of adrenal steroids in HCB-exposed (0.0, 1.0, 10.0, or 100.0 mg/kg/day—for 30 days by gavage) adult ovariectomized Sprague–Dawley rats (N = 32) were investigated. A terminal blood sample was collected for HCB residue analysis, and levels of circulating progesterone (P₄), corticosterone (CS), and aldosterone (ALD) were quantified. Mean serum CS levels were significantly (P = 0.02) reduced by HCB exposure, starting with the lowest dose group (1.0 mg/kg/day for 30 days), whereas no differences in mean serum P₄ and ALD levels were observed. Since it has been argued that the rodent possesses the ability to produce small amounts of cortisol and that levels of this glucocorticoid are altered in pathological states, serum cortisol (C) levels were also measured. Circulating levels of C were significantly lower (P < 0.05) in the highest dose group compared with controls. The presence of C in serum was confirmed by reverse-phase HPLC. These data suggest that even at the lowest dose studied, HCB exposure induces alterations in steroidogenesis of cells of the adrenal cortex inner zone.     

Cryo-immunology: A review of the literature and proposed mechanisms for stimulatory versus suppressive immune responses

The use of cryosurgery to ablate tumors is expanding, primarily due to its technical ease and minimal morbidity. A potential secondary advantage to the in situ freezing of malignant disease is the cryo-immunologic response, the generation of an anti-tumor immune response triggered by the natural absorption of the malignant tissue. While initially proposed based on clinical observations of distant disease regressing after cryoablation of a primary tumor, results from preclinical studies have been mixed and the existence of a cryo-immunologic response has been controversial. Recent studies have shed light on the potential mechanism by which cryoablation may modulate the immune system, also reveals that both immunostimulatory and immunosuppressive responses may be triggered. This article reviews the existing evidence regarding tumor cryo-immunology and puts forward hypotheses regarding patient, tumor and technical factors that may influence the resultant immune response and warrant further investigation.