Factors altering the secretion of LHRH from superfused fragments of rat hypothalamus

Gonadotropin-releasing hormone-1 neuronal activity is independent of hyperpolarization-activated cyclic nucleotide-modulated channels but is sensitive to protein kinase a-dependent phosphorylation

Pulsatile release of GnRH-1 stimulates the anterior pituitary and induces secretion of gonadotropin hormones. GnRH-1 release is modulated by many neurotransmitters that act via G protein-coupled membrane receptors. cAMP is the most ubiquitous effector for these receptors. GnRH-1 neurons express hyperpolarization-activated cyclic nucleotide-modulated (HCN) channel protein in vivo. HCN channels are involved in neuronal pacemaking and can integrate cAMP signals. cAMP-dependent protein kinase (PKA) is also activated by cAMP signals, and PKA-dependent phosphorylation modulates voltage-activated channels. In this report, these two pathways were examined in GnRH-1 neurons as integrators of forskolin (FSK)-induced stimulation. The HCN3 isoform was detected in GnRH-1 neurons obtained from mouse nasal explants. ZD7288, a HCN channel blocker, significantly reduced the efficiency of FSK to stimulate GnRH-1 neurons, whereas blockade of PKA with Rp-adenosine-3',5'-cyclic monophosphorothioate triethylammonium did not attenuate the FSK-induced stimulation. To ensure that disruption of HCN channels on GnRH-1 neurons was responsible for reduction of FSK stimulation, experiments were performed removing gamma-aminobutyric acid (GABA), the major excitatory input to GnRH-1 neurons in nasal explants. Under these conditions, Rp-adenosine-3',5'-cyclic monophosphorothioate triethylammonium, but not ZD7288, altered the FSK-induced response of GnRH-1 neurons. These studies indicate that PKA-dependent phosphorylation is involved in the FSK-induced stimulation of GnRH-1 neurons rather than HCN channels, and HCN channels integrate the FSK-induced stimulation on GABAergic neurons. In addition, blockade of HCN channels did not modify basal GnRH-1 neuronal activity when GABAergic input was intact or removed, negating a role for these channels in basal GABAergic or GnRH-1 neuronal activity.

Gonadotropin-releasing hormone-1 neuronal activity is independent of cyclic nucleotide-gated channels

Pulsatile release of GnRH-1 is essential for secretion of gonadotropin hormones. The frequency of GnRH-1 pulses is regulated during the reproductive cycle by numerous neurotransmitters. Cyclic nucleotide-gated (CNG) channels have been proposed as a mechanism to integrate the cAMP signal evoked by many neurotransmitters. This study reports the expression of the CNGA2 subunit in GnRH-1 neurons obtained from mouse nasal explants and shows the ability of GnRH-1 neurons to increase their activity in response to forskolin (activator of adenylyl cyclases), or 3-isobutyl-1-methylxanthine (inhibitor of phosphodiesterases) even after removal of gamma-aminobutyric acid (A)-ergic input. Next, the endogenous activity of adenylyl cyclases was evaluated as a component of the oscillatory mechanism of GnRH-1 neurons. Inhibition of endogenous activity of adenylyl cyclases did not alter GnRH-1 activity. The potential involvement of CNGA2 subunit in basal or induced activity was tested on GnRH-1 neurons obtained from CNGA2-deficient mice. Without up-regulation of CNGA1 or CNGA3, the absence of functional CNGA2 did not alter either the endogenous GnRH-1 neuronal activity or the response to forskolin, negating CNG channels from cAMP-sensitive mechanisms leading to changes in GnRH-1 neuronal activity. In addition, the potential role of CNGA2 subunit in the synchronization of calcium oscillations previously described was evaluated in GnRH-1 neurons from CNGA2-deficient explants. Synchronized calcium oscillations persisted in CNGA2-deficient GnRH-1 neurons. Taken together, these results indicate that CNGA2 channels are not necessary for either the response of GnRH-1 neurons to cAMP increases or the basal rhythmic activity of GnRH-1 neurons.

Oestradiol Stimulates the Release of AVP and GnRH from the Ewe Hypothalamus In Vitro

Oestradiol (E(2)) sensitizes the stress and reproductive axes in vivo. Our current aim is to investigate whether E(2) directly influences hypothalamic AVP and GnRH release in vitro. Within 10 min of ewe killing, saggital midline hypothalamic slices (from the anterior preoptic area to mediobasal hypothalamus, 2 mm thick, two per sheep) were dissected, placed in oxygenated MEM-alpha at 4 degrees C and within next 2 h were singly perifused at 37 degrees C with oxygenated MEM-alpha (pH 7.4; flow rate 150 microl/min) alone (vehicle; n = 15), with low (6 pg/ml; n = 14) or high E(2) (24 pg/ml; n = 13). After 5 h equilibration, 10 min fractions were collected for 3 h with exposure to 100 mm KCl for 10 min within the last hour. Concentrations of AVP and GnRH were measured by RIA. Baselines for AVP and GnRH were 7.0 +/- 1.1 and 17.4 +/- 0.8 pg/ml respectively. Basal values with low E(2) were similar to vehicle for AVP (7.5 +/- 1.2 pg/ml) and GnRH (17.5 +/- 1.1 pg/ml). However, high E(2) increased basal AVP (11.7 +/- 1.4 pg/ml; p < 0.05) and GnRH (23.7 +/- 1.4 pg/ml; p < 0.05). After KCl, AVP and GnRH respectively, increased (p < 0.05) to 25.6 +/- 7.5 and 38.2 +/- 5.6 (vehicle), 26.3 +/- 7.5 and 23.6 +/- 2.1 (low E(2)) and 24.1 +/- 5.4 and 41.3 +/- 6.6 pg/ml (high E(2)). After KCl, maximum values of AVP occurred at 20 and GnRH at 30 min. In conclusion, high E(2) concentration augments AVP and GnRH release by direct action on the ewe hypothalamus.

Essential role of G protein-gated inwardly rectifying potassium channels in gonadotropin-induced regulation of GnRH neuronal firing and pulsatile neurosecretion

Activation of the luteinizing hormone/human chorionic gonadotropin (LH/hCG) receptor (LHR) in cultured hypothalamic cells and immortalized GnRH (gonadotropin-releasing hormone) neurons (GT1-7 cells) transiently stimulates and subsequently inhibits cAMP production and pulsatile GnRH release. The marked and delayed impairment of cAMP signaling and episodic GnRH release in GT1-7 cells is prevented by pertussis toxin (PTX). This, and the LH-induced release of membrane-bound Galpha(s) and Galpha(i3) subunits, are indicative of differential G protein coupling to the LHR. Action potential (AP) firing in identified GnRH neurons also initially increased and then progressively decreased during LH treatment. The inhibitory action of LH on AP firing was also prevented by PTX. Reverse transcriptase-PCR analysis of GT1-7 neurons revealed the expression of G protein-gated inwardly rectifying potassium (GIRK) channels in these cells. The LH-induced currents were inhibited by PTX and were identified as GIRK currents. These responses indicate that agonist stimulation of endogenous LHR expressed in GnRH neurons activates GIRK channels, leading to suppression of membrane excitability and inhibition of AP firing. These findings demonstrate that regulation of GIRK channel function is a dominant factor in gonadotropin-induced abolition of pulsatile GnRH release. Furthermore, this mechanism could contribute to the suppression of pituitary function during pregnancy.

Regulation of gonadotropin-releasing hormone secretion: insights from GT1 immortal GnRH neurons

The study of the mammalian GnRH system has been greatly advanced by the development of immortalized cell lines. Of particular relevance are the so-called GT1 cells. Not only do they exhibit many of the known physiologic characteristics of GnRH neurons in situ, but in approximately one decade have yielded new insights regarding the intrinsic physiology of individual cells and networks of GnRH neurons, as well as the nature of central and peripheral signals that directly modulate their function. For instance, valuable information has been generated concerning intrinsic properties of the system such as the inherent pulsatile pattern of secretion displayed by networks of GT1 cells. Concepts regarding feedback regulation and autocrine feedback of GnRH neurons have been dramatically expanded. Likewise, the nature of the receptors and of the proximal and distal signal transduction mechanisms involved in the actions of multiple afferent signals has been identified. Understanding this neuronal system allows a better comprehension of the hypothalamic-pituitary-gonadal axis and of the regulatory influences that ultimately control reproductive competence.

Luteinizing hormone-releasing hormone (LHRH) neurons: mechanism of pulsatile LHRH release

Many types of neurons and glia exhibit oscillatory changes in membrane potentials and cytoplasmic Ca2+ concentrations. In neurons and neuroendocrine cells an elevation of intracellular Ca2+ concentration is associated with neurosecretion. Since both oscillatory membrane potentials and intracellular Ca2+ oscillations have been described in primary LHRH neurons and in GT1 cells, it is evident that an endogenous pulse-generator/oscillator is present in the LHRH neuron in vitro. The hourly rhythms of LHRH neurosecretion appear to be the synchronization of a population of LHRH neurons. How a network of LHRH neurons synchronizes their activity, i.e., whether by the result of synaptic mechanisms or electrical coupling through gap junctions or through a diffusible substance(s), remains to be clarified. Even though LHRH neurons themselves possess an endogenous pulse-generating mechanism, they may be controlled by other neuronal and nonneuronal elements in vivo. NE, NPY, glutamate, and GABA are neurotransmitters possibly controlling pulsatile LHRH release, and NO, cAMP, and ATP may be diffusible substances involved in pulsatile LHRH release without synaptic input. Although synaptic inputs to the perikarya of LHRH neurons could control the activity of LHRH neurons, a line of evidence suggests that direct neuronal and nonneuronal inputs, especially those from astrocytes to LHRH neuroterminals, appear to be more important for pusatile LHRH release.

Autocrine regulation of calcium influx and gonadotropin-releasing hormone secretion in hypothalamic neurons

Gonadotropin-releasing hormone (GnRH) receptors are expressed in hypothalamic tissues from adult rats, cultured fetal hypothalamic cells, and immortalized GnRH-secreting neurons (GT1 cells). Their activation by GnRH agonists leads to an overall increase in the extracellular Ca2+-dependent pulsatile release of GnRH. Electrophysiological studies showed that GT1 cells exhibit spontaneous, extracellular Ca2+-dependent action potentials, and that their inward currents include Na+, T-type and L-type Ca2+ components. Several types of potassium channels, including apamin-sensitive Ca2+-controlled potassium (SK) channels, are also expressed in GT1 cells. Activation of GnRH receptors leads to biphasic changes in intracellular Ca2+ concentration ([Ca2+]i), with an early and extracellular Ca2+-independent peak and a sustained and extracellular Ca2+-dependent plateau phase. During the peak [Ca2+]i response, electrical activity is abolished due to transient hyperpolarization that is mediated by SK channels. This is followed by sustained depolarization and resumption of firing with increased spike frequency and duration. The agonist-induced depolarization and increased firing are independent of [Ca2+]i and are not mediated by inhibition of K+ currents, but by facilitation of a voltage-insensitive and store depletion-activated Ca2+-conducting inward current. The dual control of pacemaker activity by SK and store depletion-activated Ca2+ channels facilitates voltage-gated Ca2+ influx at elevated [Ca2+]i levels, but also protects cells from Ca2+ overload. This process accounts for the autoregulatory action of GnRH on its release from hypothalamic neurons.

Ascorbic acid acts as an inhibitory transmitter in the hypothalamus to inhibit stimulated luteinizing hormone-releasing hormone release by scavenging nitric oxide

Because ascorbic acid (AA) is concentrated in synaptic vesicles containing glutamic acid, we hypothesized that AA might act as a neurotransmitter. Because AA is an antioxidant, it might therefore inhibit nitric oxidergic (NOergic) activation of luteinizing hormone-releasing hormone (LH-RH) release from medial basal hypothalamic explants by chemically reducing NO. Cell membrane depolarization induced by increased potassium concentration [K(+)] increased medium concentrations of both AA and LH-RH. An inhibitor of NO synthase (NOS), N(G)-monomethyl-l-arginine (NMMA), prevented the increase in medium concentrations of AA and LH-RH induced by high [K(+)], suggesting that NO mediates release of both AA and LH-RH. Calcium-free medium blocked not only the increase in AA in the medium but also the release of LH-RH. Sodium nitroprusside, which releases NO, stimulated LH-RH release and decreased the concentration of AA in the incubation medium, presumably because the NO released oxidized AA to dehydro-AA. AA (10(-5) to 10(-3) M) had no effect on basal LH-RH release but completely blocked high [K(+)]- and nitroprusside-induced LH-RH release. N-Methyl-d-aspartic acid (NMDA), which mimics the action of the excitatory amino acid neurotransmitter glutamic acid, releases LH-RH by releasing NO. AA (10(-5) to 10(-3) M) inhibited the LH-RH-releasing action of NMDA. AA may be an inhibitory neurotransmitter that blocks NOergic stimulation of LH-RH release by chemically reducing the NO released by the NOergic neurons.

Norepinephrine stimulates mitogen-activated protein kinase activity in GT1-1 gonadotropin-releasing hormone neuronal cell lines

The GT1-1 GnRH neuronal cell lines exhibit highly differentiated properties of GnRH neurons. We have used GT1-1 cells to study the roles of norepinephrine (NE), membrane depolarization, calcium influx, and phorbol esters in the regulation of mitogen-activated protein (MAP) kinase. NE, which is known to stimulate the release of GnRH, induced MAP kinase activity, the tyrosine phosphorylation of MAP kinase, and MAP kinase kinase activity. Forskolin led to activation of MAP kinase comparable with that induced by NE, and a selective inhibitor of cAMP-dependent protein kinase, H8, attenuated the NE-induced activation of MAP kinase. On the other hand, elimination of extracellular calcium by EGTA completely blocked NE-induced tyrosine phosphorylation of MAP kinase, and a selective inhibitor of calcium/calmodulin-dependent protein kinase, KN-62, attenuated the NE-induced activation of MAP kinase. Furthermore, depolarization of GT1-1 cells with 75 mM KCl, 10 microM BayK 8644, or 1 microM calcium ionophore (A23187) induced rapid tyrosine phosphorylation of MAP kinase. The omission of calcium from the extracellular medium completely abolished these effects of tyrosine phosphorylation of MAP kinase. Phorbol 12-myristate 13-acetate (PMA) also induced MAP kinase activity, but pretreatment of the cultured cells with PMA to down-regulate protein kinase C did not abolish the activation of MAP kinase by NE. In addition, although phosphorylation of Raf-1 kinase was stimulated by PMA, this phosphorylation was not induced by either NE or A23187. These results demonstrate that NE activates MAP kinase directly in GT1-1 cells, and that the effect of NE is mediated by increase in the cAMP level and by calcium influx, but not by PMA-sensitive protein kinase C or Raf-1 kinase.

Gonadotropin-releasing hormone neurons: intrinsic pulsatility and receptor-mediated regulation

The pulsatile pattern of gonadotropin-releasing hormone (GnRH) release from the hypothalamus is driven by a functionally interconnected and synchronized network of GnRH neurons termed the GnRH pulse generator. Several recent observations have revealed that immortalized GnRH neurons can generate an episodic pattern of GnRH release when cultured in the absence of other cell types. The in vitro operation of the pulse generator depends on the development of synaptic contacts among GnRH neurons, the electrical properties of individual GnRH neurons, and the GnRH-induced modulation of its secretory mechanism. The expression o f several other receptors by GnRH neurons provides the means for integrated regulation of pulse generator activity from without the network by agonists including glutamate, GABA, endothelin, and catecholamines.

Calcium signaling and episodic secretion of gonadotropin-releasing hormone in hypothalamic neurons

Gonadotropin-releasing hormone (GnRH) is released episodically into the pituitary portal vessels and from hypothalamic tissue of male and female rats in vitro. Perifused primary cultures of rat hypothalamic neurons, as well as the GT1-1 GnRH neuronal cell line, spontaneously exhibited episodic GnRH secretion of comparable frequency to that observed with perifused hypothalami. Such pulsatile GnRH release from GT1 cells indicates that GnRH neurons generate rhythmic secretory activity in the absence of input from other cell types. In primary hypothalamic cultures, the frequency of GnRH pulses increased with the duration of culture. The spontaneous pulsatility in GnRH release was abolished in Ca(2+)-deficient medium and was markedly attenuated in the presence of nifedipine, an antagonist of voltage-sensitive Ca2+ channels. The basal intracellular Ca2+ level of perifused GT1-1 cells cultured on coverslips was also dose-dependently reduced by nifedipine. Conversely, depolarization with high K+ increased intracellular Ca2+ and GnRH release in an extracellular Ca(2+)-dependent and nifedipine-sensitive manner. The dihydropyridine Ca2+ channel agonist Bay K 8644 increased basal and K(+)-induced elevations of intracellular Ca2+ concentration and GnRH secretion. These findings demonstrate that pulsatile neuropeptide secretion is an intrinsic property of GnRH neuronal networks and is dependent on voltage-sensitive Ca2+ influx for its maintenance.

Hypothalamic Gonadotropin-Releasing Hormone Secretion: Methodological Aspects of in vitro Systems

Abstract The regulation of gonadotropin-releasing hormone (GnRH) secretion from the medial basal hypothalamus (MBH) of adult male rats has been studied using two types of in vitro methods, the dynamic superfusion and static incubation systems. We compared both systems for methodological features including stability of baseline GnRH release and technical limitations or artifacts as well as for response to norepinephrine and naloxone, an opiate antagonist, which represent the two major physiological neuromodulators regulating the GnRH neuron. Baseline GnRH release rate per MBH was similar in both systems for at least 6 h, although owing to dilution of effluent samples by flow-through medium, the dynamic superfusion system required six MBH per chamber compared with the static incubation system which required only one MBH per chamber. Procedural losses of GnRH were low with either system without the need for proteolytic enzyme inhibitors and were mainly due to adhesion to plastic surfaces of chambers and tubings in the dynamic superfusion system. Mechanical agitation of chambers by either manual swinging or continuous shaking increased GnRH release rate. Addition of hypertonic KCl (final 50 mM) or equivalent NaCl or mannitol solution in a minimal volume directly into the tissue chambers, induced a non-specific GnRH release due to osmotic effects in the dynamic superfusion system. In contrast, the static incubation system, where hypothalamic tissue is exposed to the exact final concentration of solute by complete change of media, was more resistant to GnRH release by hypertonic stimulus. Thus, only in the static incubation system did 50 mM KCl concentrations cause GnRH release through depolarization alone. Using systems optimized to avoid technical artifacts, stimulation with naloxone demonstrated dose-dependent GnRH release in both systems whereas norepinephrine gave a clear dose-dependent GnRH release only in the static system. Thus both incubation systems produce stable and similar baseline GnRH release for at least 6 h. The static incubation system proved superior requiring fewer hypothalami, a shorter preincubation stabilization period, allowing more accurate concentration of additives and was less prone to mechanical and osmotic artifacts while preserving responsiveness to physiological stimuli. The static incubation system, being technically simpler, more robust and accurate, provided a more valid approach to the in vitro study of GnRH release and its regulation by neurochemicals.

Evidence for synergism between copper and prostaglandin E2 in stimulating the release of gonadotropin-releasing hormone from median eminence explants: Na+/Cl- requirements

Copper (Cu) and PGE2 are known to stimulate LHRH release from explants of the median eminence area (MEA) by two mechanisms distinguishable by their Ca2+ dependence. Moreover, exposure to Cu and PGE2 results in an amplified release of LHRH which is partially Ca2+ dependent, thus, resembling the release process stimulated by PGE2 alone. We have shown that LHRH release stimulated by Cu alone is Na+/Cl- dependent. By defining the Na+/Cl- dependence of PGE2- and Cu/PGE2-stimulated release of LHRH, we wished to ascertain if there is synergism between Cu and PGE2 actions. MEA of adult male rats were incubated for 5 min with 150 microM Cu and then for 15 min with 10 microM PGE2 (Cu/PGE2). Controls were incubated with Cu or PGE2. LHRH release into the medium was evaluated by RIA. Substituting Cl- in the incubation buffer with the non-permeant anion, isethionate, did not alter PGE2 stimulation of LHRH release, but it drastically inhibited Cu/PGE2 stimulation of LHRH release, indicating that this process requires a permeant monovalent anion. PGE2 and Cu/PGE2 stimulation of LHRH release were both inhibited when Na+ was substituted with Li+, or when 0.5 mM ouabain was included in the Na+-containing buffer; neither 10 microM tetrodotoxin (TTX) nor 100 microM amiloride were inhibitory. To ascertain if Na+ is required for Cu uptake, we evaluated the uptake of 67Cu by MEA explants and found that neither ouabain nor Li+ inhibited uptake, indicating that the extracellular Na+ and the activity of Na+/K+ ATPase are required for the process of LHRH release.(ABSTRACT TRUNCATED AT 250 WORDS)

Prostaglandin E2 releases luteinizing hormone-releasing hormone from the female juvenile hypothalamus through a Ca2+-dependent, calmodulin-independent mechanism

Prostaglandin E2 (PGE2) has been implicated as a mediator of norepinephrine-induced luteinizing hormone-releasing hormone (LHRH) release from the hypothalamus. The present experiments were undertaken to examine the hypothesis that mobilization of Ca2+ from intracellular stores and cyclic AMP (cAMP) formation are involved in this effect. Incubation of median eminence (ME) nerve terminals from juvenile rats in Ca2+-free Krebs-Ringer bicarbonate buffer reduced, but failed to prevent the stimulatory effect of PGE2 on LHRH release. None of 5 calmodulin antagonists or a blocker of calmodulin-dependent kinase affected the LHRH response to PGE2. In contrast, inhibition of intracellular Ca2+ mobilization with TMB-8 or dantrolene in Ca2+-free medium prevented the LHRH releasing effect of PGE2. Similarly to PGE2, the stimulatory effect of the Ca2+ ionophore A 23187 on LHRH release was not affected by inhibition of calmodulin activity. This, however, blocked the increase in PGE2 formation induced by the ionophore. PGE2 evoked a dose-related increase in cAMP accumulation in Ca2+-containing medium and this effect was inhibited both by blockers of intracellular Ca2+ mobilization and by calmodulin antagonists. Surprisingly, removal of extracellular Ca2+ increased basal cAMP levels in the incubation medium without affecting LHRH release; PGE2 induced a further increase in cAMP which was prevented by inhibition of intracellular Ca2+ translocation. Stimulation of adenylate cyclase activity with forskolin (F) resulted in similar increases in cAMP levels both in the presence and absence of extracellular Ca2+. However, F failed to evoke LHRH release in Ca2+-free medium. The results indicate that: (a) the stimulatory effect of PGE2 on LHRH release involves mobilization of intracellular Ca2+ but not the participation of calmodulin; (b) the formation of PGE2 itself is calmodulin-dependent; (c) PGE2 stimulates cAMP formation through a calmodulin-dependent mechanism that requires translocation of intracellular (membrane?) Ca2+; (d) the cAMP system of a population of nerve terminals different from that responsive to PGE2 is normally subjected to a Ca2+-dependent inhibitory control; and (e) although PGE2 is a potent stimulator of cAMP formation in the ME and endogenously produced cAMP can induce LHRH release, in the absence of extracellular Ca2+ PGE2 stimulates LHRH release in a cAMP-independent manner.

Ascorbate-induced release of LHRH: noradrenergic and opioid modulation

Ascorbic acid caused a calcium-dependent release of luteinizing hormone-releasing hormone (LHRH) from the rat mediobasal hypothalamus in vitro. The ascorbate-induced release of LHRH was effectively blocked by the adrenergic alpha-receptor blocker phentolamine, but not by the beta-receptor blocker propranolol. The effect of ascorbate was also suppressed by the mu receptor selective opiate agonist sufentanil. These results suggest that the effects of ascorbic acid on the in vitro release of LHRH are mediated by endogenous norepinephrine. Moreover, our results are consistent with the hypothesis that the release of LHRH in the rat is regulated by the hypothalamic noradrenergic system, which is in turn modulated in some, as yet undetermined, fashion by opioid peptides.

Copper amplification of prostaglandin E2 stimulation of the release of luteinizing hormone-releasing hormone is a postreceptor event

We have shown that copper amplifies prostaglandin E2 (PGE2) stimulation of luteinizing hormone-releasing hormone (LH-RH) from explants of the median eminence area (MEA) and that this process is calcium-dependent. Since a Ca-cAMP pathway has been implicated in PGE2 action on the LH-RH neuron, in this study we wished to ascertain if copper exerts its effect on the PGE2 receptor or on a postreceptor component involved in PGE2 action. MEA of adult male rats were incubated for 5 min with 200 microM Cu/histidine (CuCl2 mixed with L-histidine at an equimolar ratio) and then incubated for 15 min either with 10 microM PGE2 (Cu/PGE2), 100 microM forskolin (Cu/forskolin), or 1 mM 8-bromoadenosine 3',5'-cyclic monophosphate (Cu/cAMP). Controls were incubated without Cu/histidine or with Cu/histidine alone. Basal release of LH-RH was 4.6 +/- 0.45 pg/15 min per MEA (mean +/- SEM). Net stimulated release during the 15-min exposure to PGE2, forskolin, or 8-bromoadenosine 3',5'-cyclic monophosphate was 3.6 +/- 0.52, 3.1 +/- 0.39, and 1.6 +/- 0.42 pg/15 min per MEA, respectively. Net stimulated release after exposure to Cu/PGE2, Cu/forskolin, or Cu/cAMP was 12.7 +/- 2.2, 9.9 +/- 1.46, and 1.4 +/- 1.9 pg/15 min per MEA, respectively, indicating that copper amplifies the action of PGE2 and forskolin but not cAMP action. When MEA were exposed to a mixture of PGE2 and forskolin for 15 min, the effects of these two secretagogues on LH-RH release were not additive, regardless of whether the MEA were pretreated with Cu/histidine. In contrast to PGE2 and forskolin, copper did not amplify K+ stimulation of LH-RH release and, moreover, when Cu/histidine-treated MEA were exposed to a mixture of PGE2 and 30 mM K+, the effects of these two secretagogues were additive. These results are supportive of the proposition that PGE2 stimulation of LH-RH release is mediated by the Ca-cAMP pathway and that copper amplification of PGE2 action is a postreceptor event.

Ascorbic acid enhances the release of luteinizing hormone-releasing hormone from the mediobasal hypothalamus in vitro

Ascorbic acid is frequently used in in vitro studies of neurotransmitter-evoked release of luteinizing hormone-releasing hormone (LHRH) from hypothalamic fragments. Although it is assumed that ascorbate merely prevents the oxidative degradation of catecholamines, we have discovered that ascorbic acid itself produces significant increases in the release of LHRH. Our studies showed that ascorbic acid, at concentrations below 1 mM, produced a dose-dependent release of LHRH from incubated rat mediobasal hypothalamus (MBH). The magnitude of the ascorbate-induced release was in the range of 100-200% above controls; significant amounts of LHRH were released only if the MBH were incubated with ascorbate for time periods longer than 30 minutes. We also found that ascorbate-induced increases in LHRH were equivalent to those produced by another LHRH secretagogue, naloxone, and that the combined effects of the two substances were additive in nature. Although the mechanisms underlying this effect are not fully understood, nonspecific chemical reduction is probably not a factor since sodium metabisulfite did not induce the release of LHRH. It seems probable that ascorbate may enhance the activity of endogenous norepinephrine in the MBH and, thereby, lead to increased release of LHRH.

Effects of prostaglandin E2, forskolin and cholera toxin on cAMP production and in vitro LH-RH release from the rat hypothalamus

The present study attempts to elucidate the possible role of adenosine 3',5'-monophosphate (cAMP) and prostaglandin E2 (PGE2) in the function of the neural luteinizing hormone-releasing hormone (LH-RH) apparatus. To this end, in vitro LH-RH release from superfused hypothalamic fragments and cAMP production by hypothalamic P2 membrane fractions were measured. Immature female rats (day 28) were ovariectomized and implanted with Silastic capsules containing estradiol (235 micrograms/ml). Two days later, animals were sacrificed and the mediobasal hypothalamic preoptic area (hypothalamic units or fragments) were removed. To examine in vitro LH-RH release from superfused hypothalamic fragments, effluents were collected into tubes on ice at 10-min intervals and LH-RH concentration was determined by radioimmunoassay (RIA). Following a 50-min control period, a step-wise increment in several doses of PGE2 (each dose for a 50-min interval) evoked a dose-related increase in LH-RH release. PGE2 induced significant (P less than 0.01) increments in LH-RH release at doses of 5.68 X 10(-7), 5.68 X 10(-6), and 5.68 X 10(-5) M, respectively. When adenylate cyclase activators, such as forskolin and cholera toxin were infused in a step-wise manner (each dose for a 50-min interval) following a 50-min control period, a dose-related increase in LH-RH release was also obtained; forskolin and cholera toxin significantly (P less than 0.01) stimulated LH-RH release at doses of 1 X 10(-4) and 5.4 X 10(-10) M, respectively. These two substances were ineffective in stimulating LH-RH release when hypothalamic fragments were superfused in calcium-free plus EGTA (10 mM) containing medium.(ABSTRACT TRUNCATED AT 250 WORDS)

Amplification of prostaglandin E2 stimulation of luteinizing hormone-releasing hormone release from median eminence explants: a metal(II)-specific effect of chelated copper

We have previously demonstrated that extracellular copper amplifies prostaglandin (PG) E2 stimulation of the release of luteinizing hormone-releasing hormone (LH-RH) from explants of the median eminence area (MEA). Two questions were addressed: what is the active form of copper and the metal(II) specificity for copper action? MEA explants were incubated for 5 min in the presence of CuCl2 (ionic) or copper complexed to histidine (CuHis) at a concentration of 200 microM each and then for 15 min in the presence of 10 microM PGE2. It was found that chelated copper but not ionic amplified PGE2 action, and that the magnitude of PGE2 stimulation of LH-RH release was 3-4-fold in copper-treated than untreated tissue. Moreover, PGE2-stimulated release was directly related to the dose of CuHis. To test the metal specificity, MEA explants were incubated for 5 min with one of the following metal(II) complexes: CuHis, NiHis, FeHis, ZnHis, CdHis, MnHis, or BaHis (200 microM each) and then for 15 min with 10 microM PGE2. Controls were incubated with metal(II) complex or PGE2. Of these complexes, only CuHis and to a lesser extent NiHis stimulated LH-RH release. However, CuHis was the only complex that amplified PGE2 stimulation of LH-RH release. Thus, amplification is specific for copper. The finding that chelated copper but not ionic copper amplifies PGE2 is suggestive that the copper-interactive sites on the LH-RH neurons are not exposed to the extracellular space but that they are either embedded in the plasma membrane, facing the intracellular space, or in the cytoplasm.(ABSTRACT TRUNCATED AT 250 WORDS)

Combined electrophysiological, immunocytochemical and peptide release measurements in the hypothalamic slice

Sagittal hypothalamic slices were prepared from cycling female guinea pigs and incubated for 10-12 h during which time intracellular recordings were made from 274 arcuate and cell-poor zone (ARC-CPZ) neurons. Many of the cells exhibited spontaneous activity and a small percentage could be driven antidromically via median eminence stimulation and orthodromically via stria terminalis stimulation. During the long-term recording, the effluent medium was collected in 30 of the experiments and radioimmunoassayed using a high-titer, conformational antisera for the peptide luteinizing hormone-releasing hormone (LH-RH). The findings revealed that LH-RH is released in detectable amounts from slices prepared from a single hypothalamus, that the amount of LH-RH released is dependent on the stage of the cycle, and that it can be stimulated by known secretagogues like prostaglandin E2. Following the electrophysiological experiments, the slices were fixed and processed for immunocytochemistry using a specific antisera (EL-14) for LH-RH. LH-RH neurons were stained throughout the ARC-CPZ area with intense fiber staining in the median eminence. These findings demonstrate that the hypothalamic slice preparation can be utilized for combined electrophysiological, immunocytochemical and peptide release measurements.

Progesterone administration in vivo stimulates release of luteinizing hormone-releasing hormone in vitro

The release of luteinizing hormone-releasing hormone (LHRH) from tissue from the mediobasal hypothalamic-anterior hypothalamic-preoptic area of prepuberal female rats was measured in a perfusion system. Measurements were also made of the concentrations of LHRH in these tissue fragments and of luteinizing hormone in serum obtained when the rats were killed. Four groups of immature rats were studied: intact, ovariectomized, ovariectomized and implanted with estradiol-containing capsules, and ovariectomized rats primed with estradiol and injected with progesterone. The release of LHRH from the tissue of ovariectomized animals was significantly less than that of intact females and was not modified when the ovariectomized rats received estradiol. However, there was a four- to fivefold increase in LHRH release from tissue of ovariectomized rats primed with estradiol when they were killed 6 hours after they received an injection of progesterone. The concentrations of LHRH in tissue and of luteinizing hormone in serum varied among groups and with the time of day that the animals were killed. The interactions among luteinizing hormone, gonadal steroids, and the photoperiod seem to set the appropriate conditions for neural processes triggering a complete and normal release of luteinizing hormone.