Delivery Of Gonadotropin-releasing Hormone Research Articles

The roles of kisspeptin and neurokinin B in GnRH pulse generation in humans, and their potential clinical application.

The delivery of gonadotropin-releasing hormone (GnRH) in a pulsatile mode to the gonadotropes has long been known to be essential for normal reproductive function. There have been numerous studies aimed at dissecting out the mechanisms underlying GnRH pulse generation. The discovery of kisspeptin as an upstream regulator of GnRH attracted the possibility that pulsatile kisspeptin governed the pulsatile secretion of GnRH. Subsequent studies have shown the importance of the neurokinin B (NKB) system in modulating kisspeptin secretion and this GnRH. A number of studies in laboratory rodents have supported this notion. By contrast, we present data from clinical studies in men and women, in a range of contexts, showing that continuous infusion of kisspeptin 10 at receptor-saturating levels gives rise to an increase in luteinizing hormone (LH) (GnRH) pulse frequency. This has been demonstrated in normal healthy and hypogonadal men, in normal women during the mid-cycle LH surge, in men and women with mutations in the genes encoding NKB or its receptor, neurokinin 3 receptor (NK3R), in women with polycystic ovary syndrome treated with NK3R antagonist, and in women treated with NK3R antagonist during the LH surge. These finds indicate that pulsatile secretion and action of kisspeptin on GnRH neurons is not required for the generation of LH (GnRH) pulses in humans. We also report that there is an absence of desensitization in humans exposed to continuous infusion of kisspeptin-10 at receptor-saturating concentrations over 22h and briefly review GnRH, kisspeptin and NKB analogs and their clinical application.

The key action of estradiol and progesterone enables GnRH delivery during gestation in the South American plains vizcacha, Lagostomus maximus

The South American plains vizcacha, Lagostomus maximus, is the only mammal described so far that shows expression of estrogen receptors (ERs) and progesterone receptors (PRs) in gonadotropin-releasing hormone (GnRH) neurons. This animal therefore constitutes an exceptional model for the study of the effect of steroid hormones on the modulation of the hypothalamic-pituitary-ovarian (HPO) axis. By using both in vivo and ex vivo approaches, we have found that pharmacological doses of progesterone (P4) and estradiol (E2) produced an inhibition in the expression of hypothalamic GnRH, while physiological doses produced a differential effect on the pulsatile release frequency or genomic expression of GnRH. Our ex vivo experiment indicates that a short-term effect of E2 modulates the frequency of GnRH release pattern that would be associated with membrane ERs. On the other hand, our in vivo approach suggests that a long-term effect of E2, acting through the classical nuclear ERs-PRs pathway, would produce the modification of GnRH mRNA expression during the GnRH pre-ovulatory surge. Particularly, P4 induced a rise in GnRH mRNA expression and protein release with a decrease in its release frequency. These results suggest different levels of action of steroid hormones on GnRH modulation. We conclude that the fine action of E2 and P4 constitute the key factor to enable the hypothalamic activity during the pregnancy of this mammal.

Local production of neurostradiol affects gonadotropin-releasing hormone (GnRH) secretion at mid-gestation in Lagostomus maximus (Rodentia, Caviomorpha).

Females of the South American plains vizcacha, Lagostomus maximus, show peculiar reproductive features such as massive polyovulation up to 800 oocytes per estrous cycle and an ovulatory process around mid‐gestation arising from the reactivation of the hypothalamic–hypophyseal–ovary (H.H.O.) axis. Estradiol (E2) regulates gonadotropin‐releasing hormone (GnRH) expression. Biosynthesis of estrogens results from the aromatization of androgens by aromatase, which mainly occurs in the gonads, but has also been described in the hypothalamus. The recently described correlation between GnRH and ER α expression patterns in the hypothalamus of the vizcacha during pregnancy, with coexpression in the same neurons of the medial preoptic area, suggests that hypothalamic synthesis of E2 may affect GnRH neurons and contribute with systemic E2 to modulate GnRH delivery during the gestation. To elucidate this hypothesis, hypothalamic expression and the action of aromatase on GnRH release were evaluated in female vizcachas throughout pregnancy. Aromatase and GnRH expression was increased significantly in mid‐pregnant and term‐pregnant vizcachas compared to early‐pregnant and nonpregnant females. In addition, aromatase and GnRH were colocalized in neurons of the medial preoptic area of the hypothalamus throughout gestation. The blockage of the negative feedback of E2 induced by the inhibition of aromatase resulted in a significant increment of GnRH‐secreted mass by hypothalamic explants. E2 produced in the same neurons as GnRH may drive intracellular E2 to higher levels than those obtained from systemic circulation alone. This may trigger for a prompt GnRH availability enabling H.H.O. activity at mid‐gestation with ovulation and formation of accessory corpora lutea with steroidogenic activity that produce the necessary progesterone to maintain gestation to term and guarantee the reproductive success.

ERα and GnRH co-localize in the hypothalamic neurons of the South American plains vizcacha, Lagostomus maximus (Rodentia, Caviomorpha).

Gonadotropin-releasing hormone (GnRH) is the key regulator of the hypothalamic-pituitary-gonadal axis. Estradiol (E2) affects GnRH synthesis and delivery. Hypothalamic estrogen receptors (ER) modulate GnRH expression acting as transcription factors. The South American plains vizcacha, Lagostomus maximus, is able to ovulate up to 800 oocytes per reproductive cycle, and shows continuous folliculogenesis with pre-ovulatory follicle formation and an ovulatory event at mid-gestation. The aim of this work was to analyze the hypothalamic expression of ER in the vizcacha at different gestational time-points, and its relationship with GnRH expression, serum luteinizing hormone (LH) and E2. The hormonal pattern of mid-gestating vizcachas was comparable to ovulating-females with significant increases in GnRH, LH and E2. Hypothalamic protein and mRNA expression of ERα varied during pregnancy with a significant increase at mid-gestation whereas ERβ mRNA expression did not show significant variations. Hypothalamic immunolocalization of ERα was observed in neurons of the diagonal band of Brocca, medial preoptic area (mPOA), periventricular, suprachiasmatic, supraoptic (SON), ventromedial, and arcuate nuclei, and medial eminence, with a similar distribution throughout gestation. In addition, all GnRH neurons of the mPOA and SON showed ERα expression with no differences across the reproductive status. The correlation between GnRH and ERα at mid-gestation, and their co-localization in the hypothalamic neurons of the vizcacha, provides novel information compared with other mammals suggesting a direct action of estrogen as part of a differential reproductive strategy to assure GnRH synthesis during pregnancy.

Novel Hydrogels for Rhythmic Pulsatile Drug Delivery

AbstractSummary: We are investigating an autonomous glucose‐driven hydrogel/enzyme‐based device prototype for rhythmic, pulsed delivery of gonadotropin releasing hormone (GnRH). The device employs a pH‐sensitive hydrogel membrane in conjunction with the enzyme glucose oxidase. This system delivers GnRH in rhythmic pulses when exposed to a constant level of glucose. These pulses result from autonomous pH oscillations inside the device that are created by an unstable nonlinear feedback between hydrogel permeability to glucose and production of acid by glucose oxidase. Previous versions of this prototype utilized p(N‐isopropylacrylamide‐co‐methylacrylic acid) p(NIPA‐co‐MAA) hydrogels, with 10 mol% MAA incorporated. With this membrane, which undergoes a volume transition (VT) near pH 5, pH oscillations centered around pH 5 are observed. This range is too low to sustain oscillations in physiologically buffered media. To shift the operating pH of oscillations closer to physiologic pH, we have sought ways to increase the pH of the volume transition. In this study we show that increasing the side chain length of the α‐alkylacrylic acid (RAA) comonomer enhances the overall hydrophobicity of the copolymer, and shifts the VT pH closer to physiological pH values. We also demonstrate the ability of such membranes to affect an alkaline shift in the range of oscillations in the prototype oscillator device.

Vascular Endothelial Cells Promote Acute Plasticity in Ependymoglial Cells of the Neuroendocrine Brain

Glial and endothelial cells interact throughout the brain to define specific functional domains. Whether endothelial cells convey signals to glia in the mature brain is unknown but is amenable to examination in circumventricular organs. Here we report that purified endothelial cells of one of these organs, the median eminence of the hypothalamus, induce acute actin cytoskeleton remodeling in isolated ependymoglial cells and show that this plasticity is mediated by nitric oxide (NO), a diffusible factor. We found that both soluble guanylyl cyclase and cyclooxygenase products are involved in this endothelial-mediated control of ependymoglia cytoarchitecture. We also demonstrate by electron microscopy that activation of endogenous NO release in the median eminence induces rapid structural changes, allowing a direct access of neurosecretory axons containing gonadotropin-releasing hormone (GnRH) (the neuropeptide controlling reproductive function) to the portal vasculature. Local in vivo inhibition of NO synthesis disrupts reproductive cyclicity, a process that requires a pulsatile, coordinated delivery of GnRH into the hypothalamic-adenohypophyseal portal system. Our results identify a previously unknown function for endothelial cells in inducing neuroglial plasticity and raise the intriguing possibility that endothelial cells throughout the brain may use a similar signaling mechanism to regulate glial-neuronal interactions.

Peptidases that degrade gonadotropin-releasing hormone: influence on LH secretion in the ewe.

Several peptidases have been postulated to degrade the hypothalamic peptide gonadotropin-releasing hormone (GnRH), but it is not known if such enzymes contribute significantly to the delivery of GnRH to the pituitary in vivo. Furthermore, the activity of GnRH-inactivating peptidases may vary in different reproductive states, such as across the estrous cycle. In the present study, specific fluorescent substrates were used to measure the activity of the two major GnRH-degrading enzymes, prolyl endopeptidase (PEP) and endopeptidase 3.4.24.15 (EP 24.15), in soluble extracts of the median eminentes (ME) of ewes during different phases of the estrous cycle. Levels of EP 24.15 and PEP activity in the ME did not vary significantly across the cycle, although PEP activity was lowest at the time of the preovulatory luteinizing hormone (LH) surge. However, a statistically significant decline in PEP activity (18%, P = 0.02) was observed in the ME of OVX ewes in which a surge was induced by estrogen when compared to oil-treated OVX controls, suggesting a possible negative regulation of PEP activity by this steroid. The effect of intracerebroventricular (i.c.v.) infusion of several peptidase inhibitors on the pulsatile release of LH in the conscious OVX ewe was also examined. No consistent changes in the pattern of LH release were observed with i.c.v. infusion of the EP 24.15 inhibitor N-[1(R,S)-carboxy-3-phenylpropyl]-Ala-Ala-Tyr-p-aminobenzoate (cFP-AAY-pAB) or the angiotensin-converting enzyme (ACE) inhibitor captopril. Similarly, administration of the prolyl endopeptidase inhibitor bacitracin, or a more specific inhibitor of this enzyme, Z-Proprolinal (ZPP), did not alter LH release patterns. The results did not demonstrate a major role for changes in the activity of EP 24.15, PEP, or ACE in altering the pattern of GnRH secretion, but a minor reduction in PEP levels may occur at the time of the estrogen-induced LH surge.

Needleless Implant Delivery of Gonadotropin-Releasing Hormone Enhances the Calving Rate of Beef Cows Synchronized with Norgestomet and Estradiol Valerate

One-hundred and six beef cows were included in a study to determine if gonadot-ropin-releasing hormone (GnRH) administered via needleless implants would enhance calving rate like conventional GnRH administration. All cows were synchronized with the norgestomet and estradiol valerate estrus synchronization procedure and then randomly assigned to one of three groups: no GnRH, GnRH via conventional implants, and GnRH via needleless implants. GnRH was administered 30 hr after norgestomet implant removal. Although needleless implants were administered while cows were restrained, they may be administered remotely. GnRH administered by both methods equally enhanced (p < 0.05) calving rate and the needleless implant caused minimal response by the cows. Therefore, remote administration of GnRH may accomplish therapeutic efficacy and reduce the time, labor, stress, and risk of injury associated with providing conventional animal therapy.

Concentrations of gonadotropin-releasing hormone (GnRH) receptor messenger ribonucleic acid in pituitary tissue of orchidectomized sheep: effect of estradiol and GnRH.

The effects of estradiol (E2) and E2 withdrawal on tissue concentrations of GnRH receptor mRNA were assessed in orchidectomized sheep (wethers). Wethers received E2 (in 20% ethanol-saline [vehicle]) by continuous infusion at rates of 0.31, 1.25, 5, or 20 micrograms/h (n = 12 animals per group). Control animals received vehicle alone. Anterior pituitary tissue was collected at the end of the 48-h infusion period. Steady-state levels of GnRH receptor mRNA were quantified by solution hybridization. Tissue levels of mRNA were significantly increased above control levels (0.5 +/- 0.1 pg/microgram RNA) by E2 infusion at 1.25 micrograms/h (1.4 +/- 0.2 pg/microgram RNA). Maximal tissue concentrations of GnRH receptor mRNA were induced by E2 delivery at 5 micrograms/h (2.4 +/- 0.3 pg/micrograms RNA). In a second study, wethers (n = 6 animals per group) received 5 micrograms E2/h for 24 h. Pituitary tissue was collected at the end of the infusion period or at 12, 24, or 48 h after cessation of E2 delivery. Infusion of E2 induced a 6-fold increase in GnRH receptor mRNA (0.4 +/- 0.1 and 2.9 +/- 0.6 pg/micrograms RNA in control and E2-treated groups, respectively). Cessation of E2 delivery resulted in rapid reduction in steady-state levels of GnRH receptor mRNA. Tissue concentrations of receptor mRNA returned to pretreatment levels within 12 h of E2 withdrawal (0.6 +/- 0.1 pg/microgram RNA). In a third study, the concentration of GnRH receptor mRNA was determined in pituitary tissue collected during preovulatory surge-like secretion induced in E2-infused wethers by episodic delivery of high-amplitude GnRH (1600 ng GnRH per hourly pulse). Estradiol (5 micrograms/ml in 10% ethanol-saline) was delivered as a continuous infusion. Episodic delivery of GnRH was initiated 24 h after E2 infusion was begun, and concurrent administration of E2 and episodic GnRH was continued to slaughter. Anterior pituitary tissue was collected at 0, 3, 6, 12, or 24 h after beginning circhoral delivery of GnRH (n = 6 wethers per group). As noted above, continuous infusion of E2 for 24 h significantly increased tissue concentrations of GnRH receptor mRNA. However, steady-state levels of GnRH receptor mRNA were returned to pretreatment levels after 3 h of circhoral delivery of GnRH (0.4 +/- 0.1 pg/microgram RNA). Taken together, these data demonstrate that physiological concentrations of E2 increase steady-state levels of GnRH receptor mRNA in a dose-dependent manner. In addition, continued estrogenic support is required to maintain enhanced tissue concentrations of GnRH receptor mRNA. Finally, high-amplitude GnRH stimulation induces down-regulation of tissue levels of GnRH receptor mRNA. These data suggest that the dynamic changes in tissue concentrations of GnRH receptor mRNA during the periovulatory period may be due to the inductive effects of gonadal steroids from the developing follicle and to the combined suppressive effects of the increased GnRH stimulation and E2 withdrawal that are associated with the gonadotropin surge.

The effect of constant delivery of GnRH on the episodic secretion of equine LH

Abstract Based on the results of two experiments, constant delivery of gonadotropin releasing hormone (GnRH) via osmotic pumps to progestin-pretreated anovulatory mares elevated circulating concentrations of luteinizing hormone (LH) to levels observed in estrous mares. Furthermore, the episodic nature of LH concentrations were still detected for the six days that mares were continuously administered GnRH. However, there was an apparent 22% decrease in the frequency of LH peaks per hour while mean peak lengths were 27% longer for mares administered a constant delivery of GnRH than for untreated mares.

Hypophyseal actions of pulsatile gonadotropin-releasing hormone in the ewe: development and application of a new experimental model.

Ovariectomy of ewes during seasonal anestrus and immediate replacement with subcutaneous Silastic progesterone implants which maintained a midluteal-phase level of circulating progesterone obliterated pulsatile luteinizing hormone (LH) secretion for up to 2 weeks without preventing a normal response of the pituitary to exogenous pulses of gonadotropin-releasing hormone (GnRH). Consideration was given to the possibility that such 'progesterone-suppressed ewes' would be useful as an animal model for isolating the pituitary from pulsatile GnRH secretion, and for testing the hypophyseotropic actions of exogenous GnRH. Two experiments were conducted using this progesterone-suppressed ewe as an animal model. In the first, the amplitude of LH pulses elicited by episodic delivery of GnRH was found to depend upon the frequency of exogenous GnRH pulses. Hourly frequency produced larger LH pulses than a 30-min frequency of GnRH. In the second experiment, LH surges were induced in progesterone-suppressed ewes by a combined treatment of estradiol and GnRH in patterns designed to approximate those secreted in the follicular phase of the estrous cycle. Our findings suggest that the progesterone-suppressed ewe is a suitable animal model for studying the hypophyseotropic actions of GnRH. Further, they are consistent with two hypotheses concerning the regulation of the tonic and surge modes of LH secretion. (1) The inverse relationship between LH pulse frequency and amplitude observed in a number of situations can be accounted for, at least in part, by a differential response of the pituitary to GnRH. (2) Progesterone can block the LH surge by an action on the brain and an inhibition of pulsatile GnRH release.

The gonadotropin-releasing hormone containing ventral hypothalamic tract in the fetal rhesus monkey (Macaca mulatta).

A well-defined, gonadotropin-releasing hormone (GnRH)-containing fiber pathway, the ventral hypothalamic tract (VHT), is described by immunostaining in fetal rhesus macaques (109-156 days gestation). The VHT arises above the lateral aspects of the optic chiasm near the supraoptic nucleus, and courses ventromedially close to the ventral hypothalamic surface to terminate in the infundibulum and zona externa of the median eminence. It is formed by the confluence of GnRH-immunopositive (GnRH+) axons from local neurons, from a few GnRH+ cells in the inferior thalamic peduncle, and probably from more anterior neurons in the septum and preoptic area. Bipolar GnRH+ neurons contributing directly to the VHT are grouped at its origin dorsolateral to the optic chiasm, dorsal and medial to the optic tracts, at the infundibular lip, and within the pathway between. At the infundibular lip, GnRH+ perikarya are generally lateral or ventral to the infundibular (arcuate) nucleus, and are rarely within the nucleus itself. Cell bodies here are sometimes tripolar, but GnRH+ intercellular contacts are seldom seen. A few VHT fibers extend to the ventral surface of the brain just beneath the pia mater. Abundant capillaries in the subarachnoid space suggest a possible route for delivery of GnRH to the adenohypophysis in early gestation, before maturation of the hypophysial portal system occurs. Posterior to the infundibulum, a few VHT fibers are joined by descending periventricular fibers forming a dense fiber band beneath the premammillary recess of the third ventricle. Totals of GnRH+ cell bodies in the prosencephalon of the fetal rhesus macaque are estimated to be 5,600 in females (n = 2) and 2,600 in males (n = 3). More than 60% of VHT neurons are located in the medial basal hypothalamus, and the majority of basal hypothalamic GnRH+ neurons (86%) are associated with the VHT. Furthermore, reports of the autonomy of the medial basal hypothalamic-hypophysial unit in control of gonadotropin secretion suggest that the VHT may be the most important GnRH system involved in primate reproduction. It is clear that fetal material may offer the best model to study the GnRH neuronal system in primates.

Gonadotropin-releasing hormone (GnRH) receptor dynamics and gonadotrope responsiveness during and after continuous GnRH stimulation.

Gonadotrope responsiveness, serum and tissue levels of luteinizing hormone (LH), and tissue concentration of gonadotropin-releasing hormone (GnRH) receptors in ovariectomized rats were determined during and after continuous GnRH stimulation. Intraperitoneal placement of GnRH-containing osmotic minipumps for 96 h established a rate of GnRH delivery (1 microgram/h) that resulted in stable serum levels of GnRH (500-700 pg/ml). Secretion of LH increased 8-fold within 6 h; however, serum LH returned to pretreatment levels by 24 h, even with continued GnRH stimulation. Tissue concentration of LH was depressed within 48 h of initiation of treatment but levels were restored by 96 h. Tissue levels of GnRH receptor remained elevated during the first 6 h of treatment but were reduced by 60% within 24 h and remained depressed for the duration of treatment. Gonadotrope responsiveness 48 h and 96 h after initiation of treatment was reduced by 50% and 90%, respectively. Removal of the GnRH delivery vehicle resulted in rapid disappearance of GnRH from serum. Dramatic reduction (75%) in circulating levels of LH, and a 2-fold increase in tissue levels of LH and in GnRH receptor concentration were noted within 6 h of minipump removal. Although tissue concentration of GnRH receptor returned to pretreatment levels within 48 h of minipump removal, both basal LH secretion and gonadotrope responsiveness remained depressed even 96 h after cessation of continuous GnRH stimulation. These data indicate that GnRH can "down regulate" its receptor, gonadotrope responsiveness is not obligatorily linked to receptor concentration, and desensitization that follows hyperstimulation represents effects directed at post-receptor loci.

Effects of temperature and mode of delivery on responses to gonadotropin-releasing hormone by superfused frog pituitaries

An in vitro superfusion system was used to examine the effects of temperature on the responsiveness of frog, Rana pipiens, hemipituitaries to chronic superfusion with gonadotropin-releasing hormone (GnRH). The effects of pulsatile as opposed to continuous delivery of GnRH on luteinizing hormone (LH) and follicle-stimulating hormone (FSH) secretion were also studied at 30°. Secretion of both gonadotropins (FSH and LH) increased significantly in a dose-dependent manner in response to a brief exposure to 0.1–1000 ng/ml (n M) GnRH at both 18 and 32°, but the magnitude of response at each dose was temperature dependent. Moreover, the temporal pattern of the chronic response to a standard dose of GnRH (100 ng/ml) also varied markedly between 10 and 32°: Hemipituitaries superfused at 10 or 18° showed greatly attenuated responsiveness to GnRH after the first hour of GnRH treatment, whereas responsiveness was maintained or even continued to increase for many hours at higher temperatures. These temperature effects may be related to seasonal changes in reproductive activity. Pulsatile administration of GnRH was more effective in inducing elevated gonadotropin (GtH) secretion during the first few hours of superfusion, but continuous superfusion with GnRH was more effective at maintaining elevated GtH levels over long periods. The results support and extend previous studies by indicating that, unlike the situation in many mammals and birds, the frog pituitary is highly resistant to desensitization by even high doses of GnRH, and in fact, self-priming and maintained responsiveness to GnRH is better supported by chronic treatment than by pulsatile delivery of GnRH.

Luteinizing hormone and follicle-stimulating hormone responses to intransal gonadotropin-releasing hormone

For determination of the dose-response relationships of plasma luteinizing hormone (LH) and follicle-stimulating hormone (FSH) to the intranasal administration of gonadotropin-releasing hormone (GnRH), normal adult men were administered doses of 100, 200, 400, and 800 micrograms of GnRH on separate days, and plasma LH and FSH were measured before and after nasal insufflation of GnRH. Plasma LH was increased after a minimum dose of 200 micrograms GnRH. Median peak plasma LH levels occurred 30 minutes after intranasal GnRH and followed a log-dose relationship. When compared with intravenous GnRH, the biopotency of intranasal GnRH at the 200-, 400-, and 800-microgram doses was 1.1%, 2.3%, and 6.2%, respectively. Plasma FSH levels rose significantly only after the highest (800-micrograms) intranasal GnRH dose. From these data, we conclude that in eugonadal adult men the minimal effective dose of intranasal GnRH to elicit a significant pituitary (LH) response is 200 micrograms and that the relative efficacy of intranasal GnRH increases with the dose. In spite of the apparently low biopotency for intranasal GnRH, this route of administration may be considered as an alternative to the parenteral mode of GnRH delivery, and the lower biopotency can be partly overcome by increasing the dose.

Delivery of gonadotropin releasing hormone from polymer solutions - influence of viscosity

Die Makromolekulare ChemieVolume 9, Issue S19851 p. 163-168 Article Delivery of gonadotropin releasing hormone from polymer solutions - influence of viscosity D. Lorenz, D. Lorenz Academy of Sciences of the GDR, Institute of Drug Research, Alfred-Kowalke-Str. 4, DDR-1136 Berlin, GDRSearch for more papers by this authorK. Fechner, K. Fechner Academy of Sciences of the GDR, Institute of Drug Research, Alfred-Kowalke-Str. 4, DDR-1136 Berlin, GDRSearch for more papers by this authorN. Heinrich, N. Heinrich Academy of Sciences of the GDR, Institute of Drug Research, Alfred-Kowalke-Str. 4, DDR-1136 Berlin, GDRSearch for more papers by this authorH. Berger, H. Berger Academy of Sciences of the GDR, Institute of Drug Research, Alfred-Kowalke-Str. 4, DDR-1136 Berlin, GDRSearch for more papers by this authorB. Mehlis, B. Mehlis Academy of Sciences of the GDR, Institute of Drug Research, Alfred-Kowalke-Str. 4, DDR-1136 Berlin, GDRSearch for more papers by this author D. Lorenz, D. Lorenz Academy of Sciences of the GDR, Institute of Drug Research, Alfred-Kowalke-Str. 4, DDR-1136 Berlin, GDRSearch for more papers by this authorK. Fechner, K. Fechner Academy of Sciences of the GDR, Institute of Drug Research, Alfred-Kowalke-Str. 4, DDR-1136 Berlin, GDRSearch for more papers by this authorN. Heinrich, N. Heinrich Academy of Sciences of the GDR, Institute of Drug Research, Alfred-Kowalke-Str. 4, DDR-1136 Berlin, GDRSearch for more papers by this authorH. Berger, H. Berger Academy of Sciences of the GDR, Institute of Drug Research, Alfred-Kowalke-Str. 4, DDR-1136 Berlin, GDRSearch for more papers by this authorB. Mehlis, B. Mehlis Academy of Sciences of the GDR, Institute of Drug Research, Alfred-Kowalke-Str. 4, DDR-1136 Berlin, GDRSearch for more papers by this author First published: January 1985 https://doi.org/10.1002/macp.1985.020091985123Citations: 1AboutPDF ToolsRequest permissionExport citationAdd to favoritesTrack citation ShareShare Give accessShare full text accessShare full-text accessPlease review our Terms and Conditions of Use and check box below to share full-text version of article.I have read and accept the Wiley Online Library Terms and Conditions of UseShareable LinkUse the link below to share a full-text version of this article with your friends and colleagues. Learn more.Copy URL Share a linkShare onFacebookTwitterLinkedInRedditWechat Citing Literature Volume9, IssueS19851Supplement: Polymers in Medicine and BiologyJanuary 1985Pages 163-168 RelatedInformation

Desensitization to Gonadotropin-Releasing Hormone Observed in Superfused Pituitary Cells on Cytodex Beads*

Chronic exposure to gonadotropin-releasing hormone (GnRH) in vivo eventually results in a failure to maintain maximal gonadotropin secretion, suggesting a loss of pituitary responsiveness. To examine changes in pituitary sensitivity in vitro, we have developed a superfusion technique in which dissociated rat anterior pituitary cells are attached to Cytodex beads. In this system, continuous administration of 30 nM GnRH caused an initial 20-fold increase in LH and FSH secretion rates. However, hormone secretion declined rapidly, reaching basal levels within 12 h. Both the calcium ionophore A23187 and the cocarcinogen phorbol myristate acetate were effective in releasing additional LH and FSH from cells made refractory to 30 nM GnRH, whereas higher doses of GnRH and 8-bromo-cAMP were minimally active. A rapid loss of response was also seen with a superactive GnRH agonist but was not observed with a GnRH antagonist. Desensitization to prolonged superfusion with phorbol myristate acetate was also observed, although these cells remained sensitive to GnRH. In contrast to the development of tolerance produced by continuous exposure to GnRH, repeated delivery of GnRH in a pulsatile fashion maintained pituitary responsiveness for more than 24 h. A priming effect was observed for both LH and FSH. Such results demonstrate that continuous, but not intermittent, exposure to GnRH is capable of producing desensitization in superfused anterior pituitary cells.

Hypophysial responses to continuous and intermittent delivery of hypopthalamic gonadotropin-releasing hormone.

In rhesus monkeys with hypothalamic lesions that abolish gonadotropic hormone release by the pituitary gland, the constant infusion of exogenous gonadotropin-releasing hormone (GnRH) fails to restore sustained gonadotropin secretion. In marked contrast, intermittent administration of the synthetic decapeptide once per hour, the physiological frequency of gonadotropin release in the monkeys, reestablishes pituitary gonadotropin secretion. This phenomenon is attributable to the pattern of GnRH delivery rather than to the amounts of this hormone to which the cells of the pituitary are exposed. Moreover, the initiation of continuous GnRH administration in animals with lesions and in which gonadotropin secretion is reestablished by intermittent GnRH replacement can result in a "desensitization" or "down regulation" of the processes responsible for gonadotropin release.