Endogenous GABA Receptor Ligands in Hypophysial Portal Blood

Because ⌬-9-tetrahydrocannabinol (THC) inhibited luteinizing hormone-releasing hormone (LHRH) in male rats, we hypothesized that the endocannabinoid, anandamide (AEA), would act similarly. AEA microinjected intracerebroventricularly (i.c.v.) decreased plasma luteinizing hormone (LH) at 30 min in comparison to values in controls (P < 0.001). The cannabinoid receptor 1 (CB1-r)-specific antagonist, [N-(piperidin-1-yl)-1-(2,4-dichlorophenyl)-5-(4-chlorophenyl)-4-methyl-1H-pyrazole-3-carboxamide] (AM251), produced a significant elevation in plasma LH (P < 0.01). AEA (10 ؊9 M) decreased LHRH release from medial basal hypothalami incubated in vitro. These results support the concept that endogenous AEA inhibits LHRH followed by decreased LH release in male rats. In ovariectomized (OVX) female rats, AEA i.c.v. also inhibited LH release, but in this case AM251 had an even greater inhibitory effect than AEA. In vitro, AEA had no effect on LHRH in OVX rats. It seems that endogenous AEA inhibits LHRH followed by decreased LH release in OVX rats but that AM251 has an inhibitory action in this case. In striking contrast, in OVX, estrogen-primed (OVX-E) rats, AEA i.c.v. instead of decreasing LH, increased its release. This effect was completely blocked by previous injection of AM251. When medial basal hypothalami of OVX-E rats were incubated, AEA increased LHRH release. The synthesized AEA was higher in OVX-E rats than in OVX and males, indicating that estrogen modifies endocannabinoid levels and effects. The results are interpreted to mean that sex steroids have profound effects to modify the response to AEA. It inhibits LHRH and consequently diminishes LH release in males and OVX females, but stimulates LHRH followed by increased LH release in OVX-E-primed rats.AM251 ͉ CB1 receptor ͉ medial basal hypothalamus I t is well known that ⌬-9-tetrahydrocannabinol (THC), the active principle isolated from Cannabis sativa, alters many reproductive parameters in both male and female laboratory animals and humans (1). It has been reported that THC decreases secretion of hormones that control reproduction in male rats (2) but on the other hand stimulates sexual behavior in female rats (3). Also, a marked depression within 1 h in luteinizing hormone (LH) secretion after THC administration has been reported in male rats (2, 4). The molecular target for the action of this plant-derived cannabinol is the endocannabinoid system. This endogenous system consists of two types of GTP-binding protein-coupled receptors, cannabinoid receptors type 1 (CB1-r) and cannabinoid receptors type 2, and their endogenous ligands named anandamide (AEA) and 2-arachidonyl glycerol (5). This endocannabinoid system plays modulatory roles in many processes of the brain, such as the regulation of appetite, temperature, memory, and sexual and motor behavior (6). The activation of the endocannabinoid system acts in series with changes in the activity of several neurotransmitters including ␥-aminobutyric acid (GABA), dopamine, and glutamate (7).We had previously demon...

Section: Methodsmentioning

confidence: 99%

The inhibitory effect of anandamide on luteinizing hormone-releasing hormone secretion is reversed by estrogen

Scorticati

Fernández‐Solari

Laurentiis

et al. 2004

“…The supernatant was frozen at Ϫ70°C until determination of GABA by the [ 3 H]muscimol radioreceptor assay as described by Bernasconi et al (16) (sensitivity range 12.5-200 pmol). This radioreceptor method measures the concentration of endogenous GABA and other GABA receptor ligands metabolically related to GABA (17). Aliquots of homogenates of tissue were used in determining protein concentration by method of Lowry et al (18).…”

Section: Methodsmentioning

confidence: 99%

β-Endorphin blocks luteinizing hormone-releasing hormone release by inhibiting the nitricoxidergic pathway controlling its release

Faletti

Mastronardi

Lomniczi

et al. 1999

ABSTRACT␤-Endorphin blocks release of luteinizing hormone (LH)-releasing hormone (LHRH) into the hypophyseal portal vessels by stimulating -opiate receptors, thereby inhibiting secretion of LH. LHRH release is controlled by release of nitric oxide from nitricoxidergic (NOergic) neurons in the basal tuberal hypothalamus. To determine whether ␤-endorphin exerts its inhibitory action on this NOergic pathway, medial basal hypothalami (MBH) from male rats were incubated with ␤-endorphin (10 ؊8 M). ␤-Endorphin decreased basal secretion of LHRH, and significantly inhibited the release of prostaglandin E 2 (PGE 2 ), a known stimulant of LHRH release. Incubation of MBH with ␤-endorphin at various concentrations (10 ؊9 -10 ؊6 M) in vitro decreased the activity of NO synthase (NOS) (measured by the conversion of [ 14 C]arginine to labeled citrulline). Conversely, the activity of NOS was increased by the -receptor antagonist, naltrexone (10 ؊8 M). Not only was the inhibitory action of ␤-endorphin on LHRH and PGE 2 release blocked by naltrexone (10 ؊8 M), but it increased NOS activity and LHRH and PGE 2 release. ␤-Endorphin also stimulated ␥-aminobutyric acid (GABA) release. Because GABA inhibits both nitroprusside (NPinduced PGE 2 and LHRH release by blocking the activation of cyclooxygenase by NO, this is another mechanism by which ␤-endorphin inhibits NP-induced PGE 2 and LHRH release. The results indicate that ␤-endorphin stimulates -opioid receptors on NOergic neurons to inhibit the activation and consequent synthesis of NOS in the MBH. ␤-Endorphin also blocks the action of NO on PGE 2 release and, consequently, on LHRH release, by stimulating GABAergic inhibitory input to LHRH terminals that blocks NO-induced activation of cyclooxygenase and consequent PGE 2 secretion.The endogenous opioid peptide, ␤-endorphin, plays an important role in inhibiting the release of luteinizing hormone (LH)-releasing hormone (LHRH) (1-3). Changes in release of ␤-endorphin within the hypothalamus are important in controlling the cyclic release of LHRH that induces the preovulatory surge of LH (3-5). Inhibitory opioid tone mediated by ␤-endorphin is removed in response to the increased plasma estrogen concentrations resulting from estrogen secretion by the preovulatory follicles, thereby facilitating the preovulatory release of LHRH. This tone maintains LHRH secretion at low levels during the rest of the estrous cycle of the rat and the menstrual cycle of monkeys (4). ␤-Endorphin acts mainly by activating -opioid receptors because -opioid receptor blockers such as naltrexone prevent its action (5).The mechanism by which ␤-endorphin inhibits LHRH release is not well understood. Recent evidence indicates that LHRH release is controlled primarily by nitricoxidergic (NOergic) neurons in the arcuate-median eminence region (6-9). The NO released from these neurons diffuses into the terminals of the LHRH neurons and activates not only guanylyl cyclase, leading to the release of cGMP, but also cyclooxygenase, leading to release of prostaglandins, i...

“…The supernatants were stored at -70°C until the time of GABA determination by the [3H]muscimol radioreceptor assay as described by Bernasconi et al (14) (sensitivity range, 12.5-200 pmol). This radioreceptor method measures the concentration of endogenous GABA and other GABA receptor ligands metabolically related to GABA (15).…”

Section: Methodsmentioning

confidence: 99%

Nitric oxide inhibits hypothalamic luteinizing hormone-releasing hormone release by releasing gamma-aminobutyric acid.

Seilicovich

Duvilanski

Pisera

et al. 1995

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 luteinizing hormone-releasing hormone (LHRH). We examined the effect of NO on release of y-aminobutyric acid (GABA) from medial basal hypothalamic (MBH) explants incubated in vitro. Sodium nitroprusside (NP) (300 ,uM), 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 potassiuminduced 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%o 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 ,uM), 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.Constitutive nitric oxide synthase (NOS) occurs in hypothalamic neurons (1, 2). These neurons, which convert L-arginine to citrulline and NO, have been termed NOergic neurons (2,The publication costs of this article were defrayed in part by page charge payment. This article must therefore be hereby ma...