Biomedical subjects

H Jarry

Publications and source records attributed to H Jarry.

At least 55 records · Page 3Linked to original sources

PubMed1998

Ontogeny of the GNRH-, glutaminase- and glutamate decarboxylase-gene expression in the hypothalamus of female rats.

Amino acid neurotransmitters like gamma-aminobutyric acid (GABA) and glutamate (GLU) are involved in the regulation of hypothalamic gonadotropin releasing hormone (GnRH) release. We investigated, whether there are changes of gene expression in the rat hypothalamus for GnRH, GnRH receptor, as well as glutaminase and glutamate decarboxylase, two enzymes regulating neurotransmitter concentrations of GLU and GABA in the brain during the ontogeny. After reverse transcription-polymerase chain reaction (RT-PCR) we used an ELISA method to quantify PCR products. In 15-day old animals high plasma luteinizing hormone (LH) levels with pronounced variations were found. In 25-day old animals LH values were low, whereas in 35-day old rats LH levels increased significantly indicating the reactivation of the GnRH-pulse generator at the beginning of puberty. In parallel to these changes, the mRNA levels of the GnRH receptor in the mediobasal hypothalamus were high at day 15, significantly lower at day 25 and again high at day 35 after birth (ELISA O.D. GnRH-R day 15: 0.46+/-0.07, day 25: 0.16+/-0.04, day 35: 0.36+/-0.04; p<0.01), but no changes of GnRH receptor gene expression were found in the preoptic area. The mRNA of GnRH in the preoptic area as well as mRNA levels of glutaminase and glutamate decarboxylase in the mediobasal hypothalamus and the preoptic area did not change during ontogeny. We conclude that hypothalamic GnRH receptors are involved in the characteristic changes of LH secretion patterns during sexual maturation. Major changes of GnRH receptor gene expression occurred in the mediobasal hypothalamus and correlated well with plasma LH levels, whereas hypothalamic mRNA levels of GnRH, glutaminase and glutamate decarboxylase did not change within the different age groups. Thus the activity of the GABA- and glutamatergic system during ontogeny may be regulated at the receptor or postreceptor level.

Aging↗

PubMed1998

Changes in mediobasal hypothalamic dopamine and GABA release: a possible mechanism underlying taurine-induced prolactin secretion.

Taurine (Tau), a putative inhibitory amino acid neurotransmitter, has been shown to stimulate prolactin (PRL) release. Using ovariectomized, estrogen-replaced adult rats we investigated initially the effect of this amino acid, injected by different routes, on PRL secretion in vivo. Tau (100-500 mg/kg) had no effect on PRL release when given i.p.; 15 min after i.c.v. injection of Tau (3 mumoles), a significant increase in serum PRL levels was observed (78 +/- 9 ng/ml over basal levels, p < 0.01 vs. controls). In vitro (cultured anterior pituitary cells) PRL release was not affected by a 5 h incubation with Tau (10(-3)-10(-8) M). Basal dopamine (DA) or gamma-aminobutyric acid (GABA) output from superfused mediobasal hypothalamic fragments (MBH) was not affected by Tau (10(-3) M or 10(-5) M). However, during stimulation with KCl (50 mM), Tau (10(-3) M) significantly lowered DA release, and increased GABA output. It is concluded that Tau acts at a central level to increase PRL secretion, most probably by modulating the hypothalamic release of neurotransmitters controlling lactotroph function.

Animals↗

PubMed1998

Immortalized gonadotropin-releasing hormone neurons secrete gamma-aminobutyric acid-evidence for an autocrine regulation.

The immortalized hypothalamic neuronal cell lines GT1-1 and GT1-7 represent unique model systems to investigate the physiological control of gonadotropin-releasing hormone (GnRH) secretion. Using immunofluorescence microscopy, key proteins of regulated exocytosis, e.g. synaptotagmin, synaptobrevin and SNAP-25 (synaptosomal associated protein of 25 kDa) were found in GT1 neurons. In addition, GT1 neurons contained synaptophysin, a marker protein for small synaptic vesicles (SSVs) which are responsible for the storage of neurotransmitters such as gamma-aminobutyric acid (GABA). Upon subcellular fractionation, a lighter vesicle population characterized by synaptophysin separated from a denser vesicle population containing GnRH. Both vesicle populations contained synaptobrevin and synaptotagmin. Besides GnRH, GT1 neurons expressed glutamic acid decarboxylase at the mRNA-level and synthesized GABA. More importantly, GT1 neurons took up and stored 3H-GABA. The stored GABA was released after stimulation with increasing K+ concentrations and by alpha-latrotoxin. Reducing the extracellular Ca2+-concentration abolished stimulated secretion, indicating that GABA was released by regulated exocytosis. Hormone secretion from GT1 neurons is controlled by GABA via GABA(A) and GABA(B) receptors reflecting the situation in vivo. Our data provide the first evidence that GT1 neurons possess a second regulated secretory pathway sustained by SSVs storing and releasing GABA. The released GABA influences GnRH secretion by an auto- or paracrine loop.

Animals↗

PubMed1998

Synergistic effects of prostaglandin F2alpha and tumor necrosis factor to induce luteolysis in the pig.

There is ample evidence that prostaglandin F2alpha (PGF2alpha) is a luteolytic substance in sows, however, there is also some evidence that it may stimulate progesterone (P4) secretion in young corpora lutea (CL). In vitro studies also suggested that tumor necrosis factor alpha (TNF) is inhibitory to luteal cell P4 and estradiol-17beta (E2) release. Since E2 is a strong luteotropic substance in porcine CL, we studied the effects of intraluteal application of PGF2alpha and TNF alone and in combination on the secretion of P4 and E2 in freely moving sows. Furthermore, the effects of intraluteal infusion of E2 and its stereoisomer, estradiol-17alpha, on luteal function, were also determined. Microdialysis systems were implanted into CL at Day 10 of the estrous cycle. After a 24-h recovery period, PGF2alpha (10(-6) M) or E2 (10(-6) M) was applied daily for 6 h into the CL. PGF2alpha caused a stimulation of E2 and P4, and E2 also stimulated P4 secretion at Days 11 and 12, but the stimulatory effect of both substances diminished as the CL approached luteolysis. Intraluteal TNF application resulted in a transient increase of P4 secretion, which was followed by a dramatic reduction of P4 release. When TNF-pretreated CL were exposed to PGF2alpha at Day 11 of the estrous cycle, the prostaglandin was no longer able to stimulate but rather inhibited E2 and P4 secretion. Intraluteal application of estradiol-17alpha had no effect on P4 secretion. These results are suggestive that the PGF2alpha-induced E2 secretion in young and middle-aged CL is stimulatory to P4 secretion. Under the influence of macrophage-derived TNF production, E2 secretion is inhibited, and thereby PGF2alpha and TNF cause functional luteolysis.

Animals↗

PubMed1997

Evidence for autocrine inhibition of gonadotropin-releasing hormone (GnRH) gene transcription by GnRH in hypothalamic GT1-1 neuronal cells.

To examine whether an ultrashort feedback mechanism of gonadotropin-releasing hormone (GnRH) operates at the level of gene transcription, we studied the effects of GnRH analogs on GnRH promoter activity and GnRH mRNA level in hypothalamic GT1-1 neuronal cells. Treatment of GT1-1 cells with buserelin, a GnRH agonist, or native GnRH for 24 h significantly decreased GnRH promoter activity and its mRNA level, whereas that with GnRH antagonists, antide or [D-Phe2,D-Ala6]-GnRH, showed no effect. The inhibitory effects of buserelin on GnRH gene transcription and GnRH mRNA level were dose-related, and a significant inhibition was observed in cells treated with buserelin at concentrations higher than 0.1 microM. Time-course experiments showed that significant decreases in GnRH promoter-driven luciferase activity and GnRH mRNA level were observed within 12 h and sustained up to 48 h. Moreover, treatment with GnRH agonist for 12 h significantly decreased the transcription rate of the mouse GnRH gene, as revealed by nuclear run-on transcription assay. The promoter analysis with the 5'-deletional constructs demonstrated that cis-acting elements important for GnRH autoregulation by GnRH agonist reside within -854 bp upstream from the transcription start site. These data clearly demonstrate that GnRH can exert autocrine regulation at the level of GnRH gene transcription.

Animals↗

PubMed1997

Gonadotropin releasing hormone modulates gamma-aminobutyric acid-evoked intracellular calcium increase in immortalized hypothalamic gonadotropin releasing hormone neurons.

To examine the functional role of calcium signaling in the interactive modulation of gonadotropin releasing hormone (GnRH) neurons by gamma-aminobutyric acid (GABA) and GnRH itself, we analyzed the intracellular calcium level ([Ca2+]i), using fura-2AM fluorescent dye in immortalized hypothalamic GT1-1 cells. GT1-1 cells showed spontaneous [Ca2+]i oscillations, which were dependent on extracellular Ca2+ level, L-type Ca2+ channel and SK-type K+ channel. When GABA or a specific GABAA type receptor agonist, muscimol was applied to the media, [Ca2+]i rapidly increased through L-type Ca2+ channel in a dose-dependent manner, and subsequently decreased below the basal level without any oscillation. However, a specific GABAB type receptor agonist, baclofen showed no effect. On the other hand, application of GnRH or its potent agonist buserelin, rapidly abolished the spontaneous [Ca2+]i oscillations. Interestingly, a prior treatment with buserelin abolished GABA-evoked increase in [Ca2+]i in a noncompetitive manner. Since buserelin also blocked K(+)-evoked increase in [Ca2+]i, we suggest that GnRH may block spontaneous [Ca2+]i oscillation through modulating the L-type [Ca2+]i channel activity. These results show that GABAergic agents may exert both stimulatory and inhibitory controls over the GnRH neuronal activity, and GnRH can block the stimulatory effect of GABA, implicating the possible existence of an ultrashort feedback circuit.

Animals↗

PubMed1997

Short stature and failure of pubertal development in thalassaemia major: evidence for hypothalamic neurosecretory dysfunction of growth hormone secretion and defective pituitary gonadotropin secretion.

UNLABELLED: In patients with beta-thalassaemia major, frequent blood transfusions combined with desferrioxamine chelation therapy lead to an improved rate of survival. Endocrine disorders related to secondary haemosiderosis such as short stature, delayed puberty and hypogonadism are major problems in both adolescent and adult patients. A total of 32 patients with beta-thalassaemia major undergoing treatment at the Children's Hospital, University of Göttingen were examined. Fourteen of these were short in stature. Growth hormone (GH) secretion was investigated in 13 patients exhibiting either a short stature or reduced growth rate. The stimulated GH secretion of 10 patients in this subgroup lay within the normal range. Studies of their spontaneous GH secretion during the night revealed that these patients had a markedly reduced mean GH and reduced amplitudes in their GH peaks. Low insulin-like growth factor (IGF)-I levels were seen in the growth-retarded thalassaemic patients. Eight were subjected to an IGF generation test and showed a strong increase in both IGF-I and insulin-like growth factor binding protein (IGFBP)-3 levels indicating intact IGF-I generation by the liver. Hypogonadotropic hypogonadism was found to be present in both the male and female patients with impaired sexual development. After priming with LH-releasing hormone (GnRH) per pump in 2 female and 5 male patients, no change in either their serum oestradiol or testosterone levels or in LH/FSH response to GnRH was observed suggesting that they were suffering from a severe pituitary gonadotropin insufficiency. Three male patients at the age of puberty but exhibiting short stature. low GH, low IGF-I and hypogonadism received low dose long-acting testosterone. After 3 12 months of therapy there was a marked growth spurt, higher nocturnal GH levels and an increase in both IGF-I and IGFBP-3. CONCLUSION: Reduced GH secretion and low IGF-I in thalassaemic patients are related to a neurosecretory dysfunction due to iron overload rather than to liver damage. Hypogonadotropic hypogonadism is caused by the selective loss of pituitary gonadotropin function. In patients with both GH deficiency and hypogonadism, low dose sexual steroid treatment should be considered either as an alternative or an additional treatment before starting GH therapy.

Adolescent↗

PubMed1997

Involvement of gamma amino butyric acid (GABA) in the postnatal function of the GnRH pulse generator as determined on the basis of GnRH and GnRH-receptor gene expression in the hypothalamus and the pituitary.

In many species the GnRH pulse generator functions early postnatally to become arrested during infancy. In rats highly variable LH levels in 15-day-old animals are suggestive that LH is being released by the pituitary in pulses whereas between day 20 after birth and puberty LH levels are low indicating that the GnRH pulse generator is arrested. In the present study we show on the basis of consecutively withdrawn blood samples in 15-day-old animals that LH pulses are indeed present at that age. The proper function of GnRH receptors in the pituitary is crucially dependent on pulsatile GnRH release from the hypothalamus. In addition, GnRH receptors have been demonstrated in the medial preoptic area and in the mediobasal hypothalamus of adult rats. In 15-day-old animals the functional GnRH pulse generator results in upregulated GnRH receptor gene expression as demonstrated by quantitative RT-PCR. It is not known what neural mechanisms are involved in turning the GnRH pulse generator off during infancy and a GABAergic brake has been discussed. Indeed, when 30-day-old animals were injected with the GABA-A receptor blocking drug bicuculline, this resulted in increased serum LH levels indicating that a tonic GABAergic inhibition is indeed operative at this age.

Aging↗

PubMed1997

3,5-diiodo-L-thyronine stimulates type 1 5'deiodinase activity in rat anterior pituitaries in vivo and in reaggregate cultures and GH3 cells in vitro.

Local deiodination of L-thyroxine (T4) to the active thyroid hormone T3 via two 5'deiodinase isoenzymes (5'DI and 5'DII) plays an important role for various T3-dependent functions of the anterior pituitary (AP). Recently, it was reported that 3,5-T2, the 5'deiodination product of T3, acts as a specific agonist in the feedback mechanism on TSH secretion at the pituitary level. We now examined the effects of 3,5-T2 on pituitary 5'deiodinase activities in vivo in male, adult rats and in vitro using rat AP reaggregate cultures and the somatomammotroph cell line GH3. 5'DI activity in the AP was transiently increased after a single injection of 3,5-T2. Serum TSH levels declined, and 24 h after 3,5-T2 application, betaTSH steady-state mRNA levels in the APs were markedly lower. In reaggregate cultures of the AP, 3,5-T2 stimulated 5'DI activity 24 h after application, dose-dependently. Compared with 5'DI activities, those of 5'DII were an order of magnitude lower, in vivo as well as in vitro, and were rapidly and transiently decreased by the higher dose of 3,5-T2. GH3 cells responded to 3,5-T2 and T3 by an 1.7-fold stimulation of 5'DI activity. Stimulation of DNA-binding was demonstrated in electrophoretic mobility shift assays for a specific RXR-containing protein complex with a DR+4 thyroid hormone response element of the human type 1 5'DI promoter using nuclear extracts from GH3 cells treated with 3,5-T2. In summary, 3,5-T2 and T3 exert direct thyromimetic effects on 5'DI activity and TSHbeta expression at the pituitary level. 5'DI is regulated by its substrate(s) and/or products and may serve an important function within the modulation of thyroid hormone-dependent gene expression in the AP.

Analysis of Variance↗

PubMed1997

Immune-endocrine interactions affecting luteal function in pigs.

The formation, normal function and destruction of corpora lutea are essential features of normal reproduction. Although the formation of corpora lutea from follicles is largely dependent on pituitary gonadotrophins, the process of luteolysis is locally regulated and poorly understood. The corpus luteum consists of several steroidogenic and nonsteroidogenic cell types that interact with each other in a paracrine manner. Under cell culture conditions, large luteal cells that stem from follicular granulosa cells can be identified easily under the microscope and collected individually for single cell RT-PCR. As each of the 120 large luteal cells express the gene encoding 3 beta-hydroxysteroid dehydrogenase, it appears that all large luteal cells are steroidogenic. Large luteal cells also express the oestrogen receptor gene and as they are known to produce oestradiol, it can be concluded that the steroid acts in an auto- or intracrine manner in large luteal cells. Since we showed previously that oestradiol stimulates progesterone release under in vitro and in vivo conditions, it can be concluded that the steroid is an important intraluteally acting luteotrophic signal. At the time of luteal regression, macrophages invade the corpora lutea and their cytokine products, particularly tumour necrosis factor alpha (TNF alpha), appear to be involved in reduced steroid secretion. Indeed, TNF alpha inhibits production of progesterone and oestradiol from cultivated luteal cells. In sows, oestradiol is a strong luteotrophic factor and the production of oestradiol and of its receptor is downregulated by TNF alpha. Thereby, TNF alpha not only exerts direct luteolytic effects but also prevents the luteotrophic effects of oestradiol. Hence, it has an anti-luteotrophic action. In most species, functional luteolysis is accompanied by morphological regression of the corpus luteum. This structural luteolysis also appears to involve TNF alpha, as we have shown in pigs that expression of TNF alpha gene is high during luteolysis. Furthermore, TNF alpha stimulates programmed cell death (apoptosis) in luteal cells kept under culture conditions.

Animals↗

PubMed1996

Evidence to suggest that gonadotropin-releasing hormone inhibits its own secretion by affecting hypothalamic amino acid neurotransmitter release.

The mediobasal hypothalamus of rats contains gonadotropin-releasing hormone (GnRH) receptors. These hypothalamic neurons also express the GnRH corresponding gene. Under these circumstances, the possibility exists that these GnRH receptors could be localized in other neurons, which are GnRH-receptive, unknowing the neurotransmitter quality. Therefore, we studied the in vitro effects of the GnRH agonist buserelin on GnRH, glutamate, gamma-amino-butyric acid (GABA) and taurine release from explanted superfused hypothalami of untreated and buserelin-pretreated (down-regulated) male rats. When buserelin was added to the superfusion medium it inhibited promptly the release of GnRH and the excitatory amino acid neurotransmitter glutamate, but stimulated the release of the inhibitory neurotransmitters, GABA and taurine. Hypothalamic release of GnRH from hypothalami collected from buserelin-treated (30 micrograms/100 g b.w. twice daily for 4 days) male rats released significantly less GnRH, glutamate and more GABA and taurine. The inhibitory effect of buserelin was maintained when the superfusion medium continuously contained the GnRH analog. When superfusion of hypothalami from buserelin-pretreated animals was performed in the absence of buserelin, GnRH and glutamate release increased significantly within 45-60 min, whereas GABA and taurine release decreased at this time point. When buserelin was added to the superfusion medium 2 h after buserelin-free superfusion, GnRH and glutamate release decreased whereas GABA and taurine release increased instantaneously. Buserelin-treated rats showed significantly low values of LH and testosterone than the untreated rats. These results suggest that GnRH receptors may not only be present in GnRH axon terminals in the median eminence, but also on glutamatergic, GABAergic and taurinergic neurons by which GnRH may exert an autoinhibitory ultrashort loop feedback on its own secretion. This effect appears to be connected with glutamatergic, GABAergic and taurinergic neurons.

Amino Acids↗

PubMed1996

The GABAergic control of gonadotropin-releasing hormone secretion in male rats during sexual maturation involves effects on hypothalamic excitatory and inhibitory amino acid systems.

In order to evaluate the possible participation of the hypothalamic excitatory and inhibitory amino acid neurotransmitter systems in the GnRH release response to GABAergic drugs, hypothalami (preoptic and mediobasal area) of immature (26 days of age) and adult male rats were perifused with GABA-A and -B agonists and antagonists. GnRH and amino acid neurotransmitter concentrations (glutamate, taurine, GABA) were measured in perfusate samples collected every 15 min during 150 min. In immature rats, muscimol and baclofen (GABA-A and GABA-B agonists, respectively) increased GnRH, glutamate and GABA release and decreased taurine output, while in adults these agonists showed opposite effects on GnRH and glutamate release, and increased GABA and taurine output. On the other hand, in immature rats bicuculline and phaclofen (GABA-A and GABA-B antagonists, respectively) decreased GnRH, glutamate and GABA release, increasing taurine outflow. In adult animals, these antagonists enhanced GnRH and glutamate release, decreasing taurine and GABA outflow. These results indicate that GABA stimulates GnRH release in immature male rats and confirm the inhibitory role of this amino acid neurotransmitter in adult animals. This effect might be associated, at least partially, with the modifications observed in the excitatory and inhibitory amino acid release. On the other hand, in immature rats, stimulation of GABA-A and GABA-B receptors increased GABA release. Although ultrastructural studies have not produced any evidence of GABA-GABA neurointeractions, our results suggest the existence of a positive feedback mechanism of GABA autoregulation active during the prepubertal stage. Participation of this mechanism in the onset of puberty cannot be discarded.

Aging↗

PubMed1996

The neurochemistry of the GnRH pulse generator.

We review the crucial role of the two neurotransmitters norepinephrine (NE) and GABA in eliciting GnRH pulse. NE acts via an alpha l-receptor mechanism and also GABA acts at the alpha-subtype of the GABA receptor. The function of NE appears to be induction of phasic activation of GnRH neurons and GABA inhibits GnRH neurons tonically until they are all ready for phasic activation. By an unknown mechanism preoptic GABA release in dramatically reduced which causes simultaneous desinhibition of the GnRH neurons. Hence they release their product into the portal vessels simultaneously which is the appropriate signal for the pituitary ganodotrophs. The action of norepinephrine and GABA is most likely exerted at the perikarya level of the GnRH neurons since the alpha l-adreno receptor blocker doxazosin and GABA inhibit GnRH secretion only when applied into the medial preoptic/anterior hypothalamic area (where in the rat brain the GnRH perikarya are located). Utilizing a quantitative reverse transcription polymerase chain reaction, we demonstrate furthermore that GnRH receptors are present in the mediobasal hypothalamus as well as in the preoptic area of rats. Their function appears to serve autoinhibitory purposes since Buserelin added to medium significantly decreased GnRH release. Simultaneously, the release of GABA was increased and that of glutamate decreased. We conclude from these experiments that GABAergic and glutamatergic neurons in the hypothalamus may also be GnRH-receptive.

Animals↗

PubMed1995

Enhanced cAMP production mediates the stimulatory action of pituitary adenylate cyclase activating polypeptide (PACAP) on in vitro catecholamine secretion from bovine adrenal chromaffin cells.

The 38 amino acid peptide pituitary adenylate cyclase activating polypeptide (PACAP) induced a dose dependent increase of catecholamine secretion in cultures of bovine chromaffin cells. This secretagogue activity of PACAP was strictly dependent on the presence of calcium in the culture medium. If calcium was omitted from the medium no effect of PACAP on catecholamine secretion could be detected during an incubation of 20 min. Preincubation of cells with 1 nM PACAP for 5 min facilitated the subsequent nicotine stimulated catecholamine secretion during a 20 min incubation without addition of the peptide. PACAP induced catecholamine secretion was clearly accompanied by a dose dependent increase of intracellular cAMP concentrations. The percentage of cells responding to PACAP with increased catecholamine secretion was assessed by immunocytochemistry of the transient appearance of dopamine-beta-hydroxylase, associated with the membranes of the chromaffin granules on the cell surface during the secretory process. About 70% of adrenal medullary cells responded to 100 nM PACAP with enhanced secretory activity. Though PACAP stimulated catecholamine secretion, we did not observed major effects on intracellular free calcium concentrations ([Ca2+]i) as determined with fura-2 by single cell fluorescence microscopy. In maximally 20% of the cells a rise in [Ca2+]i in response to a challenge with 500 nM PACAP was observed. Lower concentrations of PACAP were without an effect on [Ca2+]i. These data indicate that the stimulatory action of PACAP on in vitro catecholamine secretion from bovine chromaffin cells is linked to a rise of intracellular cAMP.

Adrenal Medulla↗

PubMed1995

The inhibitory effect of beta-endorphin on LH release in ovariectomized rats does not involve the preoptic GABAergic system.

In rats, beta-endorphin (beta-END) and gamma-aminobutyric acid (GABA) suppress LH secretion by hypothalamic mechanisms involving the preoptic area (POA). Systemic injection of naloxone (NAL) increases LH secretion in male rats, an effect which can be prevented by coadministration of GABA agonists. Application of NAL into the POA of ovariectomized (ovx), progesterone substituted sheep modulates preoptic GABA release. These findings have been interpreted such that the endogenous opioids act via the preoptic GABAergic system to regulate LH release. To evaluate this hypothesis we implanted ovx rats with push pull cannula into the POA and measured GABA secretion prior to and during the preoptic application of either NAL or beta-END. Blood samples were collected to assess the effects of the drugs on LH secretion. In addition, ovx rats were substituted with estradiol (E2) to induce a negative feedback effect on LH release. Intrapreoptic application of beta-END caused a rapid decline of LH release in ovx rats which was completely reversible after termination of beta-END perfusion. Though LH levels were clearly suppressed, no change of GABA release in the POA was observed. During preoptic NAL perfusion both LH secretion and GABA release remained unaffected. Likewise, during beta-END perfusion into the POA of E2 treated rats neither LH nor GABA secretion changed. In contrast, NAL perfusion rapidly increased LH release but again this action of the opioidergic drug was not accompanied by alterations of GABA release. We conclude from these data: 1) Intrapreoptically applied beta-END inhibits LH release only in the absence of steroids. In turn, blockade of opioid receptors is effective only in the presence of steroids. Both findings indicate that in the POA opioidergic activity is low in ovx rats, but high during negative feedback of E2. 2) No changes of GABA secretion were observed during manipulations of the opioidergic tonus in the POA suggesting that both beta-END and GABA do not interact to regulate LH release. Thus, beta-END may directly inhibit the activity of GnRH neurons located in the POA or acts via a neurotransmitter other than GABA.

Animals↗

PubMed1995

Paracrine cell to cell interactions determine the effects of pituitary adenylate cyclase activating polypeptide (PACAP) on in vitro prolactin release from rat pituitary cells.

In static cultures of dispersed rat pituitary cells and in the reverse hemolytic plaque assay PACAP inhibits prolactin (Prl) secretion, while in vivo application of PACAP stimulates Prl release in rats. To elucidate the mechanism of this contradictory action, we compared the in vitro effects of PACAP on Prl secretion in cultures of dispersed or reaggregated cells and in pituitary fragments. While in monolayer cultures Prl release was inhibited by PACAP, in cultures of aggregated cells and in pituitary fragments Prl release was stimulated. Dopamine (DA) inhibited Prl release in either type of culture. PACAP also stimulated interleukin 6 (IL6) release under each of the experimental conditions. We conclude that PACAP has a direct inhibitory action on lactotropes. In addition, PACAP may induce the release of a paracrine acting factor within the pituitary which stimulates Prl release and which may be IL6. In the intact pituitary tissue and in reaggregated cells this paracrine factor stimulates Prl release more potently than PACAP directly inhibits Prl secretion resulting in a net effect of enhanced hormone release. In monolayer cultures, however, the direct inhibition is dominant, because the stimulatory paracrine factor is diluted in the culture medium. Therefore we suggest that paracrine cell to cell communication is crucial for the action of PACAP on Prl release.

Animals↗

PubMed1995

Activation of central GABAA-but not of GABAB-receptors rapidly reduces pituitary LH release and GnRH gene expression in the preoptic/anterior hypothalamic area of ovariectomized rats.

gamma-Aminobutyric acid (GABA) exerts an inhibitory action on gonadotropin-releasing hormone (GnRH) release from the hypothalamus. In vivo, this inhibitory action appears to be mediated via the GABAA receptor since in ovariectomized (ovx) rats and sheep direct application of muscimol (MUS), a GABAA agonist, into the preoptic area (POA), the site were the GnRH cell bodies are located, caused an immediate reduction of LH release. This effect may be the result of an inhibition of GnRH release but also GnRH biosynthesis may be affected. Using competitive reverse transcription-polymerase chain reaction (RT-PCR) we now addressed the question, whether an acute inhibition of the GnRH pulse generator in ovx rats by GABA involves reduction of GnRH biosynthesis as determined by GnRH mRNA levels in micropunches of the POA. To activate either the GABAA or GABAB receptor, we injected intraventricularly (icv) MUS or baclofen (BAC). Intracerebroventricular injection of 10 nmol MUS caused a rapid and lasting inhibition of LH release from about 7.5 ng/ml (pretreatment value) to approximately 1.5 ng/ml. Neither application of BAC or saline (control injections) affected LH secretion. Two hours after icv injections, rats were decapitated and GnRH mRNA levels were determined. MUS induced a pronounced decrease of GnRH levels in the POA (control rats: 2.26 pg GnRH mRNA; MUS-treated rats: 0.85 pg, n = 10/group). BAC was without any effect on GnRH mRNA levels. Thus, we confirm the inhibitory action of GABA on LH release in vivo which is exerted via the A-subtype of the receptor.(ABSTRACT TRUNCATED AT 250 WORDS)

Animals↗

PubMed1995

Preoptic rather than mediobasal hypothalamic amino acid neurotransmitter release regulates GnRH secretion during the estrogen-induced LH surge in the ovariectomized rat.

Inhibitory and excitatory amino acid neurotransmitters have been suggested to participate in the feedback actions of estradiol (E2) on LH secretion. In the rat estrogen-receptive neurons have been demonstrated in the preoptic/anterior hypothalamic area (POA) and mediobasal hypothalamus/median eminence (MBH) and many of these neurons utilize gamma-aminobutyric acid (GABA) as neurotransmitter. The actions of excitatory amino acids (EAA) differ in ovariectomized (ovx) and ovx E2-substituted rats indicating that EAAs also participate in the positive feedback action of E2 on LH release. However, little information is available as to whether in vivo these transmitters exert their effects in the POA, where most of the GnRH perikarya are located, or in the MBH, i.e. at the nerve terminals. Therefore we conducted push pull cannula perfusions to compare the release rates of GABA, aspartate (ASP) and glutamate (GLU) in the MBH and POA. A subcutaneous implant of a silastic tube containing E2 resulted in LH surges in the afternoon of all treated animals. Prior to and during this LH surge the MBH release rates of neither GABA nor ASP nor GLU were significantly altered. In contrast, a conspicuous drop in preoptic GABA release occurred prior to and during the time of estrogen-induced LH surges and this was accompanied by enhanced preoptic secretion of ASP and GLU. In conclusion, we present the first data about amino acid release in the MBH during the E2-induced LH surge. Since only in the POA the LH surge is associated with changes in amino acid release, it appears that both inhibitory and excitatory amino acids act at the level of the GnRH cell bodies and/or dendrites and not on GnRH nerve terminals to mediate the feedback mechanism of E2 on LH release.

Animals↗