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1.
Nitric oxide synthase (NOS)-containing neurons, termed NOergic neurons, occur in various regions of the hypothalamus, including the median eminence-arcuate region, which plays an important role in controlling the release of luteinzing hormone-releasing hormone (LHRH). We examined the effect of NO on release of gamma-aminobutyric acid (GABA) from medial basal hypothalamic (MBH) explants incubated in vitro. Sodium nitroprusside (NP) (300 microM), a spontaneous releaser of NO, doubled the release of GABA. This release was significantly reduced by incubation of the tissue with hemoglobin, a scavenger of NO, whereas hemoglobin alone had no effect on the basal release of GABA. Elevation of the potassium concentration (40 mM) in the medium increased GABA release 15-fold; this release was further augmented by NP. Hemoglobin blocked the increase in GABA release induced by NP but had no effect on potassium-induced release, suggesting that the latter is not related to NO. As in the case of hemoglobin, NG-monomethyl-L-arginine (NMMA), a competitive inhibitor of NOS, had no effect on basal release of GABA, which indicates again that NO is not significant to basal GABA release. However, NMMA markedly inhibited the release of GABA induced by high potassium, which indicates that NO plays a role in potassium-induced release of GABA. In conditions in which the release of GABA was substantially augmented, there was a reduction in GABA tissue stores as well, suggesting that synthesis of GABA in these conditions did not keep up with release of the amine. Although NO released GABA, there was no effect of the released GABA on NO production, for incubation of MBH explants with GABA had no effect on NO release as measured by [14C]citrulline production. To determine whether GABA had any effect on the release of LHRH from these MBH explants, GABA was incubated with the tissue and the effect on LHRH release was determined. GABA (10(-5) or 10(-6) M) induced a 70% decrease in the release of LHRH, indicating that in the male rat GABA inhibits the release of this hypothalamic peptide. This inhibition in LHRH release induced by GABA was blocked by NMMA (300 microM), which indicates that GABA converts the stimulatory effect of NO on LHRH release into an inhibitory one, presumably via GABA receptors, which activate chloride channels that hyperpolarize the cell. Previous results have indicated that norepinephrine stimulates release of NO from the NOergic neurons, which then stimulates the release of LHRH. The current results indicate that the NO released also induces release of GABA, which then inhibits further LHRH release. Thus, in vivo the norepinephrinergic-driven pulses of LHRH release may be terminated by GABA released from GABAergic neurons via NO.  相似文献   

2.
gamma-Aminobutyric acid (GABA) neurones are present in the zona incerta (ZI) where other systems have been shown to influence gonadotrophin release. This report investigates the effect of GABA agents in the ZI on ovulation and luteinizing hormone (LH) release. In intact females under Saffan anaesthesia, bilateral stereotactic injections into the ZI of two GABA transaminase inhibitors [amino(oxy)acetic acid and gamma-acetylene GABA] on the morning of pro-estrus or two GABA agonists on the afternoon of pro-estrus inhibited ovulation. The selective GABA B agonist baclofen was effective at 0.05 nM; muscimol, a mixed GABA A and B agonist, was 50-fold less potent, while the selective GABA A agonist isoguvacine had no effect at 500 nM. Administration of baclofen at 0.05 and 5 nM into the ZI significantly reduced plasma LH concentration in untreated ovariectomized rats and also prevented the rise in LH normally induced in ovariectomised rats primed with 5 micrograms oestradiol benzoate (OB) plus 0.5 mg progesterone. In ovariectomised rats primed with 5 micrograms OB alone, administration of the selective GABA A antagonist bicuculline (200 and 260 pg/side) had no effect on plasma LH, while the GABA B antagonist phaclofen (10 pg/side) stimulated a rise in plasma LH, 40 and 60 min after injection. These results indicate that GABA activity in the ZI exerts an inhibitory effect on LH release and ovulation and this is preferentially exerted via GABA B receptors.  相似文献   

3.
P M Conn  D C Rogers  S G Seay  D Staley 《Endocrinology》1984,115(5):1913-1917
In the present work we examined the effect of cationic polymers on the pituitary gonadotrope. Such polymers are widely used to anchor gonadotropes and other cell types to culture dishes and other substrata to which they are not normally adherent. Homopolymers of Lys (eight size classes from 4,000-700,000 daltons) stimulate Ca+2-dependent LH release from pituitary cell cultures. In contrast, release does not occur in response to the epsilon-CBZ or succinyl derivatives (which have no internal charge) or in response to polymers of L-Glu, D-Glu, or Gly. The observation that polymers of D-Lys, L-Lys, and L-Arg all stimulate LH release with similar efficacy and potency indicates that simple charge interactions, rather than interaction with specific polymer-binding sites, are the cause of LH release. Since monomeric Lys neither stimulates LH release nor competitively inhibits release in response to Lys polymers, it appears that multiple charge coordination by Lys polymers is responsible for activation of the release mechanism. Putrescine, spermine, and spermidine (which have more closely spaced charges) do not stimulate LH release, suggesting that a certain minimal distance of charge separation must occur to obtain efficacy. The reduced potency of heteropolymers of Lys (spaced with Ala or Tyr) suggests that a maximal effective distance also exists. Consecutive and concomitant incubation studies indicate that LH released in response to poly-L-Lys or GnRH comes from the same pool as that released by GnRH. The time courses of release are similar for the two compounds.  相似文献   

4.
The juvenile-peripubertal transition period in the female rat is associated with an ovarian-independent afternoon increase in the amplitude of plasma luteinizing hormone (LH) pulses. To determine if the immature pituitary could be activated to cause precocious puberty juvenile female rats were subjected for 4 days to a microprocessor-driven pulsatile intravenous administration of LH-releasing hormone (LHRH) at a dose that produced a peripubertal pattern of LH release. To determine if the LHRH neurons themselves could be prematurely activated to induce such a pattern of plasma LH, and hence lead to precocious puberty, the neuroexcitatory amino acid analog N-methyl-DL-aspartic acid (NMA) was similarly administered. The time of puberty (vaginal opening and first ovulation) was advanced by both the LHRH and NMA treatments, by 5 and 7 days, respectively. Ovarian weight and incidence of corpora lutea at first diestrus were similar in all animals regardless of treatment, but a juvenile body weight was retained by the animals that underwent precocious puberty. Therefore, just as the adenohypophysis can be driven by exogenous LHRH to initiate puberty, the LHRH neuronal system can be precociously activated by the episodic administration of an excitatory amino acid analog that is known to interact with specific brain receptors. It is likely, therefore, that sexual maturation is limited by factors that lie further upstream in the hypothalamo-pituitary axis (e.g., the neuronal circuits that impinge upon LHRH-producing neurons).  相似文献   

5.
We studied the actions of arachidonic acid (AA) on luteinizing hormone (LH) release versus synthesis (translation or glycosylation) by cultured rat anterior pituitary cells. Monolayer cells were incubated for 3-4 h with secretagogues (AA, melittin, gonadotropin-releasing hormone; GnRH) which either increase endogenous AA levels or release AA. LH translation and glycosylation were monitored by measuring the incorporation of [14C]alanine and [3H]glucosamine, respectively, into total (cell plus medium) immunoprecipitable LH. Immunoreactive (IR) LH was measured by radioimmunoassay. Nonlytic doses of AA increased (p less than 0.01) IR-LH release without increasing total [3H]glucosamine-LH, [14C]alanine-LH, or [3H]glucosamine-protein. AA either had no effect or slightly increased (p less than 0.05) [3H]glucosamine uptake, but decreased (p less than 0.01) [14C]alanine uptake and incorporation into total [14C]alanine protein. AA at 250 microM lysed cells, thus increasing medium IR-LH and decreasing (p less than 0.01) [3H]glucosamine and [14C]alanine uptake and incorporation into total LH and protein. Melittin, which releases AA by activating phospholipase A2 (245 and 490 nM), increased medium IR-LH (p less than 0.01) without affecting any other parameter measured. GnRH at 1 nM enhanced (p less than 0.01) both LH (IR-LH, [3H]glucosamine-LH and [14C]alanine-LH) release and total [3H]glucosamine-LH, but had no effect on total [14C]alanine-LH. In summary, AA and melittin at doses which stimulated LH release did not stimulate either LH glycosylation or translation. This suggests that increased LH release by nonspecific secretagogues is not necessarily accompanied by increased LH glycosylation or translation.  相似文献   

6.
Evidence exists that a norepinephrine/prostaglandin E2 (PGE2)/cAMP pathway is involved in the regulation of luteinizing hormone-releasing hormone (LHRH) secretion. The aim of the present experiments was to determine if release of LHRH from the immature rat hypothalamus could also be stimulated by activation of protein kinase C. Median eminences from 28-day-old female rats were incubated in vitro with either dioctanoylglycerol (a synthetic diacylglycerol that selectively activates protein kinase C in intact cells) or 4 beta-phorbol 12 beta-myristate 13 alpha-acetate (another protein kinase C activator). Both agents increased LHRH release, the response to dioctanoylglycerol being more pronounced than that to the phorbol ester. This direct activation of protein kinase C was not accompanied by changes in PGE2 formation. Activation of the PGE2/cAMP pathway by either norepinephrine, PGE2, or forskolin (a stimulator of adenylate cyclase) increased LHRH release. Dioctanoylglycerol or phorbol ester in conjunction with either norepinephrine, PGE2 or forskolin resulted in an additive effect on LHRH release suggesting coexistence of both pathways. Phospholipase C, which activates protein kinase C via formation of diacylglycerol, increased the release of both LHRH and PGE2. This suggests that an increase in endogenous phospholipase C activity caused by neurotransmitter inputs may lead to both activation of protein kinase C and PGE2 formation. Blockade of cyclooxygenase activity by indomethacin obliterated phospholipase C-induced PGE2 release. The same treatment reduced the LHRH response by only 50% indicating that protein kinase C activation can cause LHRH release in the absence of PGE2 synthesis. It is suggested that the median eminence of the rat possesses a protein kinase C-dependent pathway that is coupled positively to LHRH release and complements PGE2/cAMP-dependent mechanisms. Norepinephrine, however, does not appear to be the neurotransmitter responsible for activating the protein kinase C pathway. Simultaneous activation of both pathways may provide a mechanism by which a large increase in LHRH secretion occurs, such as in the afternoon of first proestrus.  相似文献   

7.
The mechanism by which gamma-aminobutyric acid (GABA) stimulates the release of LH was analyzed in cultured female rat pituitary cells. In 3-h incubations, GABA (1-100 microM) caused a dose-dependent increase in LH release, with the maximal response about 16% of that evoked by 10 nM GnRH. GABA action was independent of the GnRH receptor, since 1 microM GnRH antagonist [( N-acetyl-D-p-Cl-Phe1,2,D-Trp3,D-Lys6,D-Ala10] GnRH), which completely inhibits GnRH action, did not affect the response to GABA. In studies on the effects of GABA receptor agonists and antagonists, 4,5,6,7-tetrahydoisoxazolo-[5,4-c]pyridin-3(2H)-one (THIP) and muscimol (GABAA agonists) gave similar response patterns, with the same maximal stimulation as GABA but much higher potencies. In contrast, the GABAB receptor agonist baclofen did not stimulate LH release. The GABAA receptor antagonist SR95531 caused dose-dependent inhibition of the LH-releasing effects of GABA and muscimol (10 microM), with complete blockade at 10 microM SR95531. T-Butylbicyclophosphorothionate, an inhibitor of the GABAA receptor-associated chloride channel, also dose-dependently reduced the releasing effect of 100 microM GABA. These results indicate that GABA action is mediated by the chloride channel-associated GABAA receptor. However, the other GABAA receptor antagonists, including bicuculline, picrotoxin, and strychnine, did not attenuate the LH-releasing effect of 100 microM GABA in concentrations up to 100 microM, suggesting that GABA action is mediated by nonclassical GABAA receptors. Incubation in the presence of nifedipine (1 microM) or in calcium-free medium inhibited the LH-releasing action of GABA, indicating that calcium influx through voltage-sensitive calcium channels (VSCC) is required for GABA-induced LH release. Such entry of Ca2+ would result from activation of VSCC by depolarization due to the increased Cl- conductance caused by GABAA receptor activation. In cell perfusion studies, the actions of GABA and muscimol were attenuated or abolished after repetitive stimulation, consistent with desensitization of the GABA receptors. These findings have demonstrated that the stimulation of LH release by GABA is independent of GnRH action, occurs via binding to nonclassical GABAA receptors, which rapidly desensitize, and is mediated by the activation of VSCC.  相似文献   

8.
Earlier work established that neural secretion of luteinizing hormone-releasing hormone (LH-RH) and the resultant release of pituitary gonadotropins could be facilitated by activating alpha-receptors of a central noradrenergic (NA) system. The present study emphasizes that central NA mechanisms may also inhibit LH release largely through activation of beta-adrenergic receptors.  相似文献   

9.
The push-pull perfusion technique was used in combination with a sequential bleeding schedule to estimate simultaneously the release patterns of LHRH and LH in unanesthetized ovariectomized sheep and to determine the temporal relationship between the release of these two hormones. Ovariectomized (greater than 30 days) ewes received unilateral push-pull cannula (PPC) implants (od, 0.85 mm) into the median eminence. After at least 6 days of recovery, each ewe was fitted with an indwelling jugular catheter. For push-pull perfusion, a stylette was removed from the outer PPC, and an inner cannula assembly (od, 0.40 mm) was inserted. Artificial cerebrospinal fluid was pushed through the inner cannula and pulled up between the cannulae at 20 microliters/min. Continuous 10-min perfusate fractions were collected, acidified, and stored at -20 C for LHRH RIA. Blood samples were obtained every 10 min via the jugular catheter, each being drawn 5 min after the start of a perfusate collection interval. Plasma LH levels were determined by RIA. The duration of the sampling was 3-7 h. LHRH output was distinctly pulsatile, occurring at a frequency of approximately one pulse every 30-40 min (n = 5 sheep). LHRH pulse amplitude and frequency remained relatively constant throughout each perfusion. Plasma LH values also were pulsatile, and all LH peaks occurred either during the same interval or during the interval after a LHRH pulse. LH pulses always were accompanied or directly preceded by LHRH pulses, but LHRH pulses were not always followed by LH pulses. The amplitudes of LH pulses and corresponding LHRH pulses were highly correlated (r = 0.81; P less than 0.01). Histological examination revealed that detection of LHRH in perfusates depended upon the placement of the PPC tip into either the zona externa of the central median eminence or adjacent areas. These experiments demonstrate that 1) hypothalamic LHRH release in the Ovx ewe occurs in discrete pulses, with a mean interpulse interval of 38.7 +/- 1.5 min, 2) LH pulses invariably are preceded or accompanied by LHRH pulses, and 3) LH pulse amplitude is highly correlated with LHRH pulse amplitude.  相似文献   

10.
The neurotransmitter gamma-aminobutyric acid (GABA) appears to be involved in the control of gonadotropin secretion. These studies were conducted 1) to evaluate the effect of GABAergic drugs on in vitro LHRH secretion and 2) to characterize the role of different types of GABA receptors (the GABA-A and GABA-B subtypes) in these actions. Arcuate nuclei-median eminence fragments were incubated in vitro, and the release of LHRH, prostaglandin E2 (PGE2), arginine vasopressin, and oxytocin was measured by RIA. Both GABA and muscimol at different concentrations induced an increase in LHRH release, but did not affect the release of arginine vasopressin and oxytocin. This stimulatory effect was blocked by the specific GABA antagonist bicuculline, suggesting the involvement of GABA-A type receptors. Muscimol-stimulated LHRH release was not affected by the presence of phentolamine, suggesting that the stimulatory effect of GABA-A receptors on LHRH release is not mediated by interactions with the noradrenergic system. PGE2 has been shown to be a potent secretagogue of LHRH from the median eminence in vitro, and in this model the stimulatory effect of PGE2 was enhanced by muscimol. Baclofen, a specific GABA-B type receptor agonist, had no effect on basal LHRH release, but completely suppressed naloxone-stimulated LHRH and PGE2 secretion. The inhibitory effect of baclofen was blocked by the presence of 5-aminovalerate, a drug that has been shown to block the inhibitory effect of baclofen on NE release from noradrenergic terminals. This suggests the possibility that GABA-B receptors interacting with noradrenergic terminals may be responsible for the inhibitory effect of baclofen on naloxone stimulation. This study uncovered both stimulatory and inhibitory effects of GABA on LHRH release after activation of GABA-A or GABA-B receptors, respectively. Further, the data show possible relationships among the GABAergic, endogenous opiate peptide, and noradrenergic systems in the control of LHRH release from the hypothalamus.  相似文献   

11.
12.
The luteinizing hormone (LH) releasing activities of luteinizing hormone-releasing hormone (LH-RH) and four related analogues were compared using isolated chicken anterior pituitary cells. The analogues, des-Gly10-LH-RH and Phe5-LH-RH, exhibited a greater potency than LH-RH (150 and 237%, respectively), whereas LH-RH(OH) was much less active (1.1%). The potency of Phe5-LH-RH was reduced to 0.9% by the insertion of a tyrosine molecule at position 11, indicating that chain length is a significant feature of the biological activity of the molecule. des-Gly10-LH-RH and Phe5-LH-RH were more active in the present system, than is indicated by available information for the rat.  相似文献   

13.
14.
15.
Immunization against luteinizing hormone releasing hormone (LH-RH) in adult male rats produced a progressive decline in LH and FSH in the circulation to low or non-detectable levels. D-Serine-tertiary-butyl6,des-glycine-NH210 LH-RH ethylamide is an analogue of LH-RH having highly active LH-RH properties in the normal rat. Because it is also immunologically different from LH-RH it can stimulate gonadotrophin release from the anterior pituitary gland of rats immunized against LH-RH without interference from the antibody. The analogue stimulated LH and FSH release in rats 15 weeks after immunization against LH-RH when antibody titre was highest, and after long-term (35 weeks) immunization against LH-RH. D-Serine-tertiary-butyl6,des-glycine-NH210 LH-RH ethylamide and related analogues are therefore potentially useful for reversing the effects of immunization against LH-RH.  相似文献   

16.
The present study was aimed at localizing gamma-aminobutyric acid (GABA) and its enzyme of synthesis, glutamic acid decarboxylase (GAD), in the mouse pancreas by immunocytochemical methods. The influence of GABA on hormone release was also studied with normal mouse and rat islets and the isolated perfused rat pancreas. Particular attention was paid to glucagon release to test a recent hypothesis suggesting that GABA mediates the still unexplained glucose-induced inhibition of glucagon release. GABA and GAD were identified only in islet cells and never in the exocrine tissue. Exogenous GABA, baclofen (agonist of GABAB receptors), muscimol (agonist of GABAA receptors), or bicuculline (antagonist of GABAA receptors) did not affect insulin and somatostatin release by isolated mouse or rat islets. GABA was also without effect on glucose-induced electrical activity in mouse B-cells. Glucagon secretion by mouse islets was only slightly inhibited (approximately 20%) by GABA. Since muscimol had a similar effect, and baclofen was ineffective, the inhibition by GABA probably involves GABAA receptor activation. Bicuculline, however, did not antagonize the inhibitory effects of GABA and muscimol, probably because the antagonist alone also decreased glucagon secretion. In contrast to GABA, low (3 mM) and high (20 mM) concentrations of glucose strongly inhibited (approximately 50-65%) glucagon release; this inhibition was not prevented by bicuculline. Similar results were obtained with the perfused rat pancreas; muscimol slightly inhibited glucagon release under various conditions, and bicuculline did not reverse the strong inhibition produced by 16.7 mM glucose. In conclusion, GABA does not affect insulin and somatostatin secretion, but inhibits A-cells, probably by acting on GABAA receptors. It is unlikely, however, that this small inhibitory effect can account for the inhibition of glucagon release produced by glucose.  相似文献   

17.
18.
Epidermal growth factor (EGF) directly enhanced luteinizing hormone (LH) release from dispersed rat pituitary cells in monolayer cultures as well as in superfusion columns. This 2.3-fold stimulatory effect was dose and time dependent and was also reconfirmed in a superfusion system. Retinal, a protein kinase C inhibitor, counteracted the EGF effect only partially. Further experiments were therefore carried out to investigate alternate EGF mechanisms. Nordihydroguaiaretic acid and chloroquine suppressed the stimulatory effect of EGF in a dose-dependent manner. Moreover, EGF (10(-7) M) stimulated [3H]arachidonate release from pre-labelled rat pituitary cells. This indicates that phospholipase A2 and arachidonic acid may be involved in EGF action on LH release from rat pituicytes.  相似文献   

19.
The effects of the gamma-aminobutyric acid receptor agonists muscimol and baclofen were investigated on the secretion of GH, LH, ACTH and TSH from the anterior pituitary in vitro using a rapid superfusion system. A bicuculline-sensitive stimulatory effect of muscimol was demonstrated on the secretion of GH, LH and ACTH but not TSH. Baclofen had no effect on the basal secretion of any of the hormones, but inhibited LH-releasing hormone-stimulated release of LH and K+- and Ba2+-stimulated release of ACTH. The benzodiazepine Roll-6896 and the barbiturate secobarbital were found to potentiate the effect of muscimol on GH secretion. These results demonstrate the presence of GABAA receptors on somatotrophs, gonadotrophs and corticotrophs, and the presence of GABAB receptors on gonadotrophs and corticotrophs. Thyrotrophs appear devoid of GABA receptors.  相似文献   

20.
Preincubation of cultured pituitary cells with GnRH caused a marked decrease in subsequent LH release. The rate of desensitization increased when the preincubating concentration of GnRH and the preincubation time were increased. Pituitary cells obtained from male rats were not as sensitive to GnRH as cells obtained from female rats and the extent of desensitization was also smaller in cells from male rats. Densensitization was found to be a long-lasting effects, without any change in the viability of the cells. A superactive analogue of GnRH (D-Phe6-GnRH) caused almost complete desensitization of LH secretion, while a competitive inhibitory analogue of GnRH caused a much smaller decrease in LH response which could be overcome by increasing the concentration of GnRH used for reincubation. These data suggest that the desensitization is closely related to the biological activity of GnRH and does not correlate with receptor binding. High concentrations of potassium also induced desensitization, although to a lower extent than GnRH. Since K+ induces LH release by a different mechanism than GnRH, our data suggest that the desensitization phenomenon cannot be explained only at the receptor level. The time curve of desensitization supports the idea that GnRH action has two-phases: an acute effect which cannot be desensitized, and a secondary phase which can be densensitized.  相似文献   

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