Control Of LHRH Release Research Articles

Allopregnanolone induces LHRH and glutamate release through NMDA receptor modulation

LHRH release from hypothalamus is influenced by the neurotransmitter glutamate that acts, among others, on NMDA receptors present in LHRH neurons. On the other hand, the neurosteroid allopregnanolone can modulate the activity of specific neurotransmitter receptors and affect neurotransmitter release. We examined the role of allopregnanolone on in vitro LHRH and glutamate release from mediobasal hypothalamus and anterior preoptic area of ovariectomized rats with estrogen and progesterone replacement. Moreover, we evaluated whether the neurosteroid might act through modulation of NMDA receptors. Allopregnanolone induced an increase in LHRH release. This effect was reversed when the NMDA receptors were blocked by the NMDA antagonist 2-amino-7-phosphonoheptanoic acid (AP-7) indicating that this neurosteroid would interact with NMDA receptors. Moreover allopregnanolone induced an augment in K(+) evoked [(3)H]-glutamate release from mediobasal hypothalamus-anterior preoptic area explants and this effect was also reversed when NMDA receptors were blocked with AP-7. These results suggest an important physiologic function of allopregnanolone on the regulation of neuroendocrine function in female adult rats. Not only appears to be involved in enhancing LHRH release through modulation of NMDA receptors but also in the release of glutamate which is critical in the control of LHRH release.

LHRF, LHRH, GnRH: what controls the secretion of this hormone?

In the present study, the possible direct effects of opioids on the release of LHRH have been studied utilizing the GT1-1 cell model. It has been shown that only opioid agonists which bind to delta receptors bring about a significant inhibition of the release of LHRH, when this is stimulated by forskolin- or PGE2. The effects of delta opioid agonists are dose-dependent, and are reversed by the delta specific antagonist naltrindole. If one assumes that the GT1-1 cells represent a good model for the study of LHRH-secreting neurons, the obvious conclusion from these data is that endogenous opioid peptides may influence LHRH secretion also by acting directly on LHRH-secreting neurons. In another study it has been shown that GT1-1 cells possess high affinity-low capacity binding sites for estradiol and androgens. Moreover, a two-fold induction of androgen-binding sites has been observed after treatment of GT1-1 cells with estradiol, indicating that estrogen receptors are functional. The present observations suggest that gonadal steroids might influence LHRH release acting directly on the cells which manufacture and release the hypothalamic hormone. In order to evaluate the possible existence of a humoral communication between glial cells and LHRH-secreting neurons, two different approaches have been used: (a) GT1-1 cells were co-incubated with purified cultures of type 1 astrocytes; (b) GT1-1 cells were exposed to the conditioned medium (CM, untreated or submitted to different experimental procedures) where type 1 astrocytes were grown for 24 h. In both sets of experiments LHRH was measured by RIA in the incubation media of GT1-1 cells. The data show that short periods of either co-culture or exposure to previously frozen or heated CM significantly increase the release of LHRH from the GT1-1 cells. The stimulatory effect on LHRH release appears to be specific for type 1 astrocytes. Surprisingly, fresh CM proved to be inactive; this suggested that the factor(s) involved needed to be 'activated'. Since it is known that TGF beta may be liberated from its supporting proteins by freezing or heating, the effects of TGF beta and of a TGF beta-neutralizing antibody were analyzed. The stimulatory effect exerted by the frozen CM was completely abolished by the antibody, while TGF beta 1 proved able to significantly increase LHRH release from GT1-1 cells. The present data unequivocally show that type 1 astrocytes in culture release some principle(s), probably TGF beta, able to stimulate LHRH release from the LHRH-producing GT1-1 cell line. To the authors' knowledge, the present data provide the first clear-cut demonstration that the glia may directly intervene in the control of LHRH release via the secretion of humoral factors.

Effects of steroids on the brain opioid system

The experiments reported here add further evidence in support of the view that sex steroids may influence the binding characteristics of brain opioid receptors. In particular, it has been shown that: (a) the number of mu-opioid receptors varies in the hypothalamus of regularly cycling female rats according to the different phases of the estrous cycle, which are characterized by fluctuations of circulating levels of sex steroids; (b) the number of mu-opioid receptors decreases in the hypothalamus and in the corpus striatum when ovariectomized rats are submitted to treatments with estradiol and progesterone able to induce a "positive" feedback effect on LH release. A treatment with estrogen alone able to induce a "negative" feedback effect on LH release brings about an increase of the number of mu-opioid receptors in the thalamus and in the hippocampus; (c) in addition to the mu-receptors, receptors of the delta type may also be involved in the control of gonadotropin secretion; recent results here presented indicate that a line of immortalized hypothalamic cells (GT1 cells), which synthesize and secrete LHRH, present delta opioid receptors on their membranes; these are apparently involved in the control of LHRH release from these cells.

Evidence for the presence of noradrenergic neurons and their inhibitory action on luteinizing hormone-releasing hormone release in cultured fetal rat hypothalamic cells

The control of LHRH release by catecholamine systems during fetal life (embryonic stages) was studied using hypothalamic neurons in primary cell cultures and an attempt was made to characterize the receptor type involved. Phenylephrine and clonidine, respectively α 1 an α 2 adrenoreceptor agonists, both inhibited LHRH release. These agonist inhibitory induced-effects were antagonized by the respective α 1 and α 2 adrenoreceptor antagonists (prazosin and rauwolscine). Both prazosin and rauwolscine applied alone induced a marked increase in LHRH release. Similarly, inhibition of catecholamine synthesis obtained by α-methyl-para-tyrosine (α-MT) led to a significant increase in LHRH release. The stimulatory effects induced by α 1 and α 2 adrenoreceptor antagonists or by α-MT on LHRH release suggest the presence of noradrenergic and/or adrenergic cells in fetal hypothalamic cultures. Therefore, catecholamine contents were measured in fetal hypothalamic cells in culture. Measurable amounts of norepinephrine and dopamine were found in cells, although epinephrine was undetectable. These results show: 1 - noradrenergic cells are present in primary culture of fetal hypothalamic cells. 2 - This intrinsic hypothalamic noradrenergic system exerts an inhibitory control on LHRH release at an early stage of development through α 1 and α 2 adrenoreceptors.

Different γ-Aminobutyric Acid Receptor Subtypes are Involved in the Regulation of Opiate-Dependent and Independent Luteinizing Hormone-Releasing Hormone Secretion

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.

Evidence for a cholinergic component in the neuroendocrine control of luteinizing hormone (LH) secretion.

The possible participation of cholinergic mechanisms in the control of LH and LH-RH secretion has been studied using an in vitro procedure. Halved anterior pituitaries (AP) of normal adult male rats were incubated for 6 h either alone or in the presence of hypothalamic fragments (HF). At the end of the incubation period, LH was evaluated in the media using a biological assay. AP halves, when incubated alone, released small amounts of LH. The addition of HF to the media containing AP tissue did not change LH release. Acetylcholine (Ach) added to the incubation flasks containing only AP tissue did not increase LH output of AP incubated without Ach. On the contraty, Ach significantly enhanced LH release when added to the incubation media of flasks simultaneously containing AP tissue and HF. Atropine had no effect on the release of LH from AP incubated alone. However the addition of atropine to the incubation media containing the halved AP, HF and Ach totally prevented the effects of Ach on LH release. Prostigmine enhanced the release of LH in the media containing AP plus HF. The LH-releasing effect of Ach in the AP plus HF coincubates was not influenced by the alpha-adrenergic blocking agent, phentolamine. These data indicate: (1) that from HFs incubated in vitro, Ach is able to release a factor (most probably LH-RH) which increases the secretion of LH from AP tissue; (2) that this effect of Ach follows the general rules of cholinergic systems (blockade by atropine, potentiation by prostigmine, etc.); and (3) that the LH-RH releasing activity of Ach is not linked to the liberation of catecholamines (dopamine, norepinephrine, etc.). These results suggest that cholinergic mechanisms play a role in the control of LH-RH release.