Activation of gonadotropin-releasing hormone receptors induces a long-term enhancement of excitatory postsynaptic currents mediated by ionotropic glutamate receptors in the rat hippocampus

Mechanisms linking neurological disorders with reproductive endocrine dysfunction: Insights from epilepsy research

Gonadal hormone actions in the brain can both worsen and alleviate symptoms of neurological disorders. Although neurological conditions and reproductive endocrine function are seemingly disparate, compelling evidence indicates that reciprocal interactions exist between certain disorders and hypothalamic-pituitary-gonadal (HPG) axis irregularities. Epilepsy is a neurological disorder that shows significant reproductive endocrine dysfunction (RED) in clinical populations. Seizures, particularly those arising from temporal lobe structures, can drive HPG axis alterations, and hormones produced in the HPG axis can reciprocally modulate seizure activity. Despite this relationship, mechanistic links between seizures and RED, and vice versa, are still largely unknown. Here, we review clinical evidence alongside recent investigations in preclinical animal models into the contributions of seizures to HPG axis malfunction, describe the effects of HPG axis hormonal feedback on seizure activity, and discuss how epilepsy research can offer insight into mechanisms linking neurological disorders to HPG axis dysfunction, an understudied area of neuroendocrinology.

The roles of GnRH in the human central nervous system

It is widely known that GnRH plays a role in facilitating reproductive function via the HPG axis, and this was once believed to be its only function. However, over the last several decades important neuromodulatory roles of GnRH in multiple brain functions have been elucidated. Multiple GnRH isoforms and receptors have been detected outside the HPG-axis across different species. In this review, we focus on the human CNS where GnRH I and II isoforms and a functional GnRH I receptor have been isolated. We first describe the traditional understanding of GnRH within the hypothalamus and the pituitary and current clinical use of GnRH analogues. We then review the location and function of GnRH-producing neurons and receptors located outside the HPG axis. We next review the GnRH I and II neuron location and quantity and GnRH I receptor gene expression throughout the human brain, using the Allen Brain Map Atlas. This analysis demonstrates a wide expression of GnRH throughout the brain, including prominent expression in the basal forebrain and cerebellum. Lastly, we examine the potential role of GnRH in aging and inflammation and its therapeutic potential for neurodegenerative disease and spinal cord lesions.

Functions of the GABAergic system on serum LH concentrations in pre-pubertal Nellore heifers

This study was conducted to evaluate the luteinizing hormone (LH) secretion pattern after gamma-aminobutyric acid (GABA_A) antagonist to determine the effects of the GABAergic system on LH secretion during reproductive maturation in pre-pubertal Nellore heifers. Nellore heifers (n = 10) were administered a picrotoxin injection of 0.18 mg/kg, i.v. Blood samples were collected every 15 min for 3 h at different developmental stages (8, 10, 14 and 17 mo of age). Plasma concentrations of LH were quantified using an RIA (sensitivity of 0.04 ng/mL and CV of 15 %). There was an interaction between treatment and age (P = 0.034). Picrotoxin-treated heifers had lesser (P ≤  0.05) LH mean concentrations during a 3 h period at 10 and 17 mo of age compared to control heifers (P ≤  0.05). Comparing the period before and after Picrotoxin injection in the same animals, there was a 33 % decrease in LH concentration at 8 mo of age (P = 0.0165). These results indicate the GABAergic system has a stimulatory function in inducing LH secretion in pre-pubertal Nellore heifers. These findings corroborate previous results that GABA increases GnRH/LH secretion in other species during the pre-pubertal period.

GnRH protective effects against amyloid β-induced cognitive decline: A potential role of the 17β-estradiol

INTRODUCTION

The 17β-estradiol (E2) enhances hippocampal dendritic spine synapses, facilitates learning processes, and exerts neuroprotection. Brain estrogen decline has been reported in Alzheimer's disease. The role of GnRH in modulating steroid biosynthesis convinced us to examine whether hippocampal GnRH administration could enhance the local E2 levels and overcome the development of cognition decline in amyloid β (Aβ) neurotoxicity. To explore if GnRH acts through regulating E2 synthesis, letrozole, an aromatase inhibitor, has been applied in combination with GnRH.

METHODS

Female rats received an intracerebroventricular injection of Aβ. The GnRH and, or letrozole were injected into the CA1 for 14 consecutive days. Working memory, novel object recognition memory, and anxiety-like behavior were evaluated. Serum and hippocampal E2 levels were measured. Hippocampal mRNA expression of GnRH (GnRH-R) and E2 (ERα and ERβ) receptors was assessed. GnRH effect on the excitability of pyramidal cells was studied by in vivo single-unit recording.

RESULTS

GnRH increased hippocampal E2 levels, evoked an increase in the spontaneous firing of pyramidal neurons, and caused mRNA overexpression of hippocampal GnRH receptors. GnRH prevented the adverse effects of Aβ on working memory, NOR index, and anxiogenic behavior. Letrozole did not reverse GnRH modulatory effects on hippocampal E2 levels and neuroprotection.

CONCLUSION

GnRH prevented the Aβ-induced memory deficit, which may be mediated through hippocampal E2 levels enhancement. The electrophysiological analysis revealed the enhanced neuronal excitability in the CA1 region. All these data suggest that GnRH might be a promising candidate that reduces anxiety and improves memory indices in the context of Aβ neurotoxicity.

Synaptic plasticity modulation by circulating peptides and metaplasticity: Involvement in Alzheimer's disease

Synaptic plasticity is a cellular process involved in learning and memory whose alteration in its two main forms (Long Term Depression (LTD) and Long Term Potentiation (LTP)), is observed in most brain pathologies, including neurodegenerative disorders such as Alzheimer's disease (AD). In humans, AD is associated at the cellular level with neuropathological lesions composed of extracellular deposits of β-amyloid (Aβ) protein aggregates and intracellular neurofibrillary tangles, cellular loss, neuroinflammation and a general brain homeostasis dysregulation. Thus, a dramatic synaptic environment perturbation is observed in AD patients, involving changes in brain neuropeptides, cytokines, growth factors or chemokines concentration and diffusion. Studies performed in animal models demonstrate that these circulating peptides strongly affect synaptic functions and in particular synaptic plasticity. Besides this neuromodulatory action of circulating peptides, other synaptic plasticity regulation mechanisms such as metaplasticity are altered in AD animal models. Here, we will review new insights into the study of synaptic plasticity regulatory/modulatory mechanisms which could influence the process of synaptic plasticity in the context of AD with a particular attention to the role of metaplasticity and peptide dependent neuromodulation.

Growth Hormone (GH) and Gonadotropin-Releasing Hormone (GnRH) in the Central Nervous System: A Potential Neurological Combinatory Therapy?

This brief review of the neurological effects of growth hormone (GH) and gonadotropin-releasing hormone (GnRH) in the brain, particularly in the cerebral cortex, hypothalamus, hippocampus, cerebellum, spinal cord, neural retina, and brain tumors, summarizes recent information about their therapeutic potential as treatments for different neuropathologies and neurodegenerative processes. The effect of GH and GnRH (by independent administration) has been associated with beneficial impacts in patients with brain trauma and spinal cord injuries. Both GH and GnRH have demonstrated potent neurotrophic, neuroprotective, and neuroregenerative action. Positive behavioral and cognitive effects are also associated with GH and GnRH administration. Increasing evidence suggests the possibility of a multifactorial therapy that includes both GH and GnRH.

Histopathological and immunohistochemical study of the protective effect of triptorelin on the neurocytes of the hippocampus and the cerebral cortex of male albino rats after short-term exposure to cyclophosphamide

Chemotherapy treats many types of cancer effectively but it often causes side effects. Chemotherapy works on active cells, such as cancer cells, and some healthy cells. Side effects happen when chemotherapy damages these healthy cells. Today, many more drugs are available to treat side effects than in the past. Triptorelin (Decapeptyl) is a gonadotropin-releasing hormone agonist that is reported to have many therapeutic effects besides being an anti-cancer agent. In the current study, intraperitoneal cyclophosphamide (65 mg/kg/day) was administered for 4 weeks to induce marked dystrophic changes in the cerebral cortex and hippocampus of male albino rats. After 4 weeks, we observed significant degeneration of neurocytes with dystrophic changes. Subcutaneous triptorelin (0.05 mg/kg/day) for 4 weeks significantly improved histological signs of degeneration and apoptosis. Anti-Bcl2 staining of sections of the cerebral cortex and hippocampus showed that the apoptotic index was increased. This finding was confirmed by the anti-p53 staining, which showed a significant decrease in the apoptotic index. Ultimately, such improvements were accompanied by significant restoration of normal brain histology, as revealed by hematoxylin and eosin. In conclusion, triptorelin can reverse the apoptotic changes induced by cyclophosphamide therapy, which is more marked in the hippocampus than cerebral cortex.

Kisspeptin-13 enhances memory and mitigates memory impairment induced by Aβ1-42 in mice novel object and object location recognition tasks

Kisspeptin (KP), the endogenous ligand of GPR54, is a recently discovered neuropeptide shown to be involved in regulating reproductive system, anxiety-related behavior, locomotion, food intake, and suppression of metastasis across a range of cancers. KP is transcribed within the hippocampus, and GPR54 has been found in the amygdala and hippocampus, suggesting that KP might be involved in mediating learning and memory. However, the role of KP in cognition was largely unclear. Here, we investigated the role of KP-13, one of the endogenous active isoforms, in memory processes, and determined whether KP-13 could mitigate memory impairment induced by Aβ1-42 in mice, using novel object recognition (NOR) and object location recognition (OLR) tasks. Intracerebroventricular (i.c.v.) infusion of KP-13 (2μg) immediately after training not only facilitated memory formation, but also prolonged memory retention in both tasks. The memory-improving effects of KP-13 could be blocked by the GPR54 receptor antagonist, kisspeptin-234 (234), and GnRH receptors antagonist, Cetrorelix, suggesting pharmacological specificity. Then the memory-enhancing effects were also presented after infusion of KP-13 into the hippocampus. Moreover, we found that i.c.v. injection of KP-13 was able to reverse the memory impairment induced by Aβ1-42, which was inhibited by 234. To sum up, the results of our work indicate that KP-13 could facilitate memory formation and prolong memory retention through activation of the GPR54 and GnRH receptors, and suppress memory-impairing effect of Aβ1-42 through activation of the GPR54, suggesting that KP-13 may be a potential drug for enhancing memory and treating Alzheimer's disease.

Neuropeptide RFRP inhibits the pacemaker activity of terminal nerve GnRH neurons

The terminal nerve gonadotropin-releasing hormone (TN-GnRH) neurons show spontaneous pacemaker activity whose firing frequency is suggested to regulate the release of GnRH peptides and control motivation for reproductive behaviors. Previous studies of the electrophysiological properties of TN-GnRH neurons reported excitatory modulation of pacemaker activity by auto/paracrine and synaptic modulations, but inhibition of pacemaker activity has not been reported to date. Our recent study suggests that neuropeptide FF, a type of Arg-Phe-amide (RFamide) peptide expressed in TN-GnRH neurons themselves, inhibits the pacemaker activity of TN-GnRH neurons in an auto- and paracrine manner. In the present study, we examined whether RFamide-related peptides (RFRPs), which are produced in the hypothalamus, modulate the pacemaker activity of TN-GnRH neurons as candidate inhibitory synaptic modulators. Bath application of RFRP2, among the three teleost RFRPs, decreased the frequency of firing of TN-GnRH neurons. This inhibition was diminished by RF9, a potent antagonist of GPR147/74, which are candidate RFRP receptors. RFRP2 changed the conductances for Na(+) and K(+). The reversal potential for RFRP2-induced current was altered by inhibitors of the transient receptor potential canonical (TRPC) channel (La(3+) and 2-aminoethoxydiphenyl borate) and by a less selective blocker of voltage-independent K(+) channels (Ba(2+)). By comparing the current-voltage relationship in artificial cerebrospinal fluid with that under each drug, the RFRP2-induced current was suggested to consist of TRPC channel-like current and voltage-independent K(+) current. Therefore, synaptic release of RFRP2 from hypothalamic neurons is suggested to inhibit the pacemaker activity of TN-GnRH neurons by closing TRPC channels and opening voltage-independent K(+) channels. This novel pathway may negatively regulate reproductive behaviors.

Neuromodulatory Effect of GnRH on the Synaptic Transmission of the Olfactory Bulbar Neural Circuit in Goldfish, Carassius auratus

Gonadotropin-releasing hormone (GnRH) is well known as a hypophysiotropic hormone that is produced in the hypothalamus and facilitates the release of gonadotropins from the pituitary gonadotropes. On the other hand, the functions of extrahypothalamic GnRH systems still remain elusive. Here we examined whether the activity of the olfactory bulbar neural circuits is modulated by GnRH that originates mainly from the terminal nerve (TN) GnRH system in goldfish ( Carassius auratus). As the morphological basis, we first observed that goldfish TNs mainly express salmon GnRH (sGnRH) mRNA and that sGnRH-immunoreactive fibers are distributed in both the mitral and the granule cell layers. We then examined by extracellular recordings the effect of GnRH on the electrically evoked in vitro field potentials that arise from synaptic activities from mitral to granule cells. We found that GnRH enhances the amplitude of the field potentials. Furthermore, these effects were observed in both cases when the field potentials were evoked by stimulating either the lateral or the medial olfactory tract, conveying functionally different sensory information, separately, and suggesting that GnRH may modulate the responsiveness to wide categories of odorants in the olfactory bulb. Because GnRH also changed the paired-pulse ratio, it is suggested that the increased amplitude of the field potential results from changes in the presynaptic glutamate release of mitral cells rather than the increase in the glutamate receptor sensitivity of granule cells. These results suggest that TN regulates the olfactory responsiveness of animals appropriately by releasing sGnRH peptides in the olfactory bulbar neural circuits.

Gonadotropin-releasing hormone receptor system: modulatory role in aging and neurodegeneration

Receptors for hormones of the hypothalamic-pituitary-gonadal axis are expressed throughout the brain. Age-related decline in gonadal reproductive hormones cause imbalances of this axis and many hormones in this axis have been functionally linked to neurodegenerative pathophysiology. Gonadotropin-releasing hormone (GnRH) plays a vital role in both central and peripheral reproductive regulation. GnRH has historically been known as a pituitary hormone; however, in the past few years, interest has been raised in GnRH actions at non-pituitary peripheral targets. GnRH ligands and receptors are found throughout the brain where they may act to control multiple higher functions such as learning and memory function and feeding behavior. The actions of GnRH in mammals are mediated by the activation of a unique rhodopsin-like G protein-coupled receptor that does not possess a cytoplasmic carboxyl terminal sequence. Activation of this receptor appears to mediate a wide variety of signaling mechanisms that show diversity in different tissues. Epidemiological support for a role of GnRH in central functions is evidenced by a reduction in neurodegenerative disease after GnRH agonist therapy. It has previously been considered that these effects were not via direct GnRH action in the brain, however recent data has pointed to a direct central action of these ligands outside the pituitary. We have therefore summarized the evidence supporting a central direct role of GnRH ligands and receptors in controlling central nervous physiology and pathophysiology.

Electrophysiological analysis of the inhibitory effects of FMRFamide-like peptides on the pacemaker activity of gonadotropin-releasing hormone neurons

Gonadotropin-releasing hormone (GnRH) neurons in the terminal nerve (TN) show endogenous pacemaker activity, which is suggested to be dependent on the physiological conditions of the animal. The TN-GnRH neurons have been suggested to function as a neuromodulatory neuron that regulates long-lasting changes in the animal behavior. It has been reported that the TN-GnRH neurons are immunoreactive to FMRFamide. Here, we find that the pacemaker activity of TN-GnRH neuron is inhibited by FMRFamide: bath application of FMRFamide decreased the frequency of pacemaker activity of TN-GnRH neurons in a dose-dependent manner. This decrease was suppressed by a blockage of G protein-coupled receptor pathway by GDP-β-S. In addition, FMRFamide induced an increase in the membrane conductance, and the reversal potential for the FMRFamide-induced current changed according to the changes in [K(+)](out) as predicted from the Nernst equation for K(+). We performed cloning and sequence analysis of the PQRFamide (NPFF/NPAF) gene in the dwarf gourami and found evidence to suggest that FMRFamide-like peptide in TN-GnRH neurons of the dwarf gourami is NPFF. NPFF actually inhibited the pacemaker activity of TN-GnRH neurons, and this inhibition was blocked by RF9, a potent and selective antagonist for mammalian NPFF receptors. These results suggest that the activation of K(+) conductance by FMRFamide-like peptide (≈NPFF) released from TN-GnRH neurons themselves causes the hyperpolarization and then inhibition of pacemaker activity in TN-GnRH neurons. Because TN-GnRH neurons make tight cell clusters in the brain, it is possible that FMRFamide-like peptides released from TN-GnRH neurons negatively regulates the activities of their own (autocrine) and/or neighboring neurons (paracrine).

Gonadotropin-releasing hormone receptor in spinal cord neurons of embryos and adult rats

Mammalian gonadotropin-releasing hormone (GnRH) and its receptor have been found in the neuroendocrine reproductive axis. However, they can be localized in other extra-pituitary tissues as well including the central nervous system. The present study reports the expression of GnRH receptor and its mRNA in spinal cord neurons of rat embryos and adult rats, using immunohistochemistry and reverse transcriptase polymerase chain reaction (RT-PCR). Immunohistochemistry showed that the spinal cord neurons of rat embryos and adult rats expressed the GnRH receptor. The study of GnRH receptor mRNAs revealed that both cultured spinal cord neurons of rat embryos and adult rats expressed the GnRH receptor mRNA. Additional in vitro experiments showed that the expression of GnRH receptor mRNA was less in the spinal cord neurons exposed to GnRH compared to unexposed ones. These results raise the possibility that GnRH may play other roles independently from its participation in reproductive function.

A biological role for the gonadotrophin-releasing hormone (GnRH) metabolite, GnRH-(1-5)

Gonadotrophin-releasing hormone (GnRH) was first isolated in the mammal and shown to be the primary regulator of the reproductive system through its initiation of pituitary gonadotrophin release. Subsequent to its discovery, this form of GnRH has been shown to be one of many structural variants found in the brain and peripheral tissues. Accordingly, the original form first discovered and cloned in the mammal is commonly referred to as GnRH-I. In addition to the complex regulation of GnRH-I synthesis, release and function, further evidence suggests that the processing of GnRH-I produces yet another layer of complexity in its activity. GnRH-I is processed by a zinc metalloendopeptidase EC 3.4.24.15 (EP24.15), which cleaves the hormone at the covalent bond between the fifth and sixth residue of the decapeptide (Tyr(5)-Gly(6)) to form GnRH-(1-5). It was previously thought that the cleavage of GnRH-I by EP24.15 represents the initiation of its degradation. Here, we review the evidence for the involvement of GnRH-(1-5), the metabolite of GnRH-I, in the regulation of GnRH-I synthesis, secretion and facilitation of reproductive behaviour.

Effects of gonadotrophin-releasing hormone outside the hypothalamic-pituitary-reproductive axis

Gonadotrophin-releasing hormone (GnRH) is a hypothalamic decapeptide with an undisputed role as a primary regulator of gonadal function. It exerts this regulation by controlling the release of gonadotrophins. However, it is becoming apparent that GnRH may have a variety of other vital roles in normal physiology. A reconsideration of the potential widespread action that this traditional reproductive hormone exerts may lead to the generation of novel therapies and provide insight into seemingly incongruent outcomes from current treatments using GnRH analogues to combat diseases such as prostate cancer.

Immunoreactive GnRH type I receptors in the mouse and sheep brain

Gonadotropin Releasing Hormone-I (GnRH) has been implicated in an array of functions outside the neuroendocrine reproductive axis. Previous investigations have reported extensive GnRH binding in numerous sites and this has been supported by in situ hybridization studies reporting GnRH receptor mRNA distribution. The present study on mice and sheep supports and extends these earlier investigations by revealing the distribution of cells immunoreactive for the GnRH receptor. In addition to sites previously shown to express GnRH receptors such as the hippocampus, amygdala and the arcuate nucleus, the improved resolution afforded by immunocytochemistry detected cells in the mitral cell lay of the olfactory bulb as well as the central grey of the mesencephalon. In addition, GnRH receptor immunoreactive neurons in the hippocampus and mesencephalon of the sheep were shown to colocalize with estrogen receptor beta. Although GnRH may act at some of these sites to regulate reproductive processes, evidence is accumulating to support an extra-reproductive role for this hypothalamic decapeptide.

Gonadotropin-releasing hormone regulates spine density via its regulatory role in hippocampal estrogen synthesis

Spine density in the hippocampus changes during the estrus cycle and is dependent on the activity of local aromatase, the final enzyme in estrogen synthesis. In view of the abundant gonadotropin-releasing hormone receptor (GnRH-R) messenger RNA expression in the hippocampus and the direct effect of GnRH on estradiol (E2) synthesis in gonadal cells, we asked whether GnRH serves as a regulator of hippocampal E2 synthesis. In hippocampal cultures, E2 synthesis, spine synapse density, and immunoreactivity of spinophilin, a reliable spine marker, are consistently up-regulated in a dose-dependent manner at low doses of GnRH but decrease at higher doses. GnRH is ineffective in the presence of GnRH antagonists or aromatase inhibitors. Conversely, GnRH-R expression increases after inhibition of hippocampal aromatase. As we found estrus cyclicity of spine density in the hippocampus but not in the neocortex and GnRH-R expression to be fivefold higher in the hippocampus compared with the neocortex, our data strongly suggest that estrus cycle–dependent synaptogenesis in the female hippocampus results from cyclic release of GnRH.

LHRH-(1-5): a bioactive peptide regulating reproduction

Luteinizing hormone-releasing hormone-I (LHRH-I) was isolated from the mammalian hypothalamus and shown to be the primary regulator of reproduction through its initiation of pituitary gonadotropin release. Subsequently, it has also been shown to have non-pituitary actions. Although the regulation of LHRH-I synthesis and release has been extensively studied, there is additional evidence to suggest that processing of the peptide represents another layer of regulation. The focus of this review will be on evidence for the action of LHRH-(1-5), the pentapeptide metabolite of LHRH-I, in regulating LHRH-I synthesis, secretion and reproductive behavior. The involvement of LHRH-(1-5) in the control of aspects of reproduction might represent yet another level of regulatory complexity through neuropeptide processing.

Studying sex and gender differences in pain and analgesia: a consensus report

In September 2006, members of the Sex, Gender and Pain Special Interest Group of the International Association for the Study of Pain met to discuss the following: (1) what is known about sex and gender differences in pain and analgesia; (2) what are the "best practice" guidelines for pain research with respect to sex and gender; and (3) what are the crucial questions to address in the near future? The resulting consensus presented herein includes input from basic science, clinical and psychosocial pain researchers, as well as from recognized experts in sexual differentiation and reproductive endocrinology. We intend this document to serve as a utilitarian and thought-provoking guide for future research on sex and gender differences in pain and analgesia, both for those currently working in this field as well as those still wondering, "Do I really need to study females?"

The influence of gonadal hormones on neuronal excitability, seizures, and epilepsy in the female

It is clear from both clinical observations of women, and research in laboratory animals, that gonadal hormones exert a profound influence on neuronal excitability, seizures, and epilepsy. These studies have led to a focus on two of the primary ovarian steroid hormones, estrogen and progesterone, to clarify how gonadal hormones influence seizures in women with epilepsy. The prevailing view is that estrogen is proconvulsant, whereas progesterone is anticonvulsant. However, estrogen and progesterone may not be the only reproductive hormones to consider in evaluating excitability, seizures, or epilepsy in the female. It seems unlikely that estrogen and progesterone would exert single, uniform actions given our current understanding of their complex pharmacological and physiological relationships. Their modulatory effects are likely to depend on endocrine state, relative concentration, metabolism, and many other factors. Despite the challenges these issues raise to future research, some recent advances have helped clarify past confusion in the literature. In addition, testable hypotheses have developed for complex clinical problems such as "catamenial epilepsy." Clinical and animal research, designed with the relevant endocrinological and neurobiological issues in mind, will help advance this field in the future.

Gonadotropins: potential targets for preventive and therapeutic interventions in Alzheimer’s disease

Increased prevalence of Alzheimer’s disease (AD) in women has led to an interest in the role of hormonal changes in the neurodegenerative process. In particular, research has been directed towards investigating the effect of changes in sex hormone levels following reproductive senescence. Clinical trials of hormone-replacement therapy for the prevention of AD are proving contentious, and considerably more research is necessary before the benefit of the hormone replacement strategy can be ascertained. However, evidence is now emerging to support the notion that increased gonadotropin levels may confer an increased risk of AD. Gonadotropins have been implicated in the metabolism of β-amyloid, a key protein that is central to the pathogenesis of AD. Gonadotropin reduction represents a promising new target for therapeutic intervention in AD and, potentially, dementia in general. In this review, the authors discuss the therapeutic and preventive potential of gonadotropin-reducing agents in the management of AD.

Luteinizing hormone-releasing hormone and oxytocin response to hyperosmotic stimulation: in vitro study

It was shown previously that luteinizing hormone-releasing hormone (LHRH) affects the neurohypophysial oxytocin release in water-deprived rats. However, the detailed mechanisms by which LHRH modifies the oxytocin response to hyperosmotic stimulation have not been explained so far. Using the isolated hypothalamo-neurohypophysial explants obtained from euhydrated rats, the effect of LHRH on the oxytocin secretion was studied under conditions of direct osmotic (i.e., Na(+)- evoked) as well as nonosmotic (i.e., K(+)-evoked) stimulation. Additionally, the oxytocin response to LHRH was investigated using the explants obtained from animals drinking 2% saline for eight days (systemic, i. e., both direct and indirect, osmotic stimulation). LHRH significantly enhanced Na(+)- and K(+)-evoked oxytocin release from explants taken from rats drinking tap water, indicating that LHRH could affect the Na(+)/K(+)-dependent depolarization of perikarya of oxytocin neurones. In contrast, LHRH significantly diminished the K(+)-stimulated hormone release when the neurohypophysial complex was obtained from previously salt-loaded rats, suggesting that peripheral osmotic stimulation somehow modifies the sensitivity of oxytocinergic neurones to LHRH (possible mechanisms are discussed). It is concluded that LHRH may participate in the regulation of oxytocin secretion via both direct and indirect impact on magnocellular oxytocinergic neurones depending on the current functional status of the hypothalamo-neurohypophysial complex.

Ceramide-induced sustained depression of synaptic currents mediated by ionotropic glutamate receptors in the hippocampus: an essential role of postsynaptic protein phosphatases

Ceramide, a sphingomyelin-derived second messenger, mediates cellular signals of cytokines such as tumor necrosis factor-alpha that are rapidly produced in the brain in response to vigorous neuronal activity and tissue injury. Using whole-cell patch-clamp recordings, the present study examined whether ceramide modulated excitatory postsynaptic currents mediated by ionotropic glutamate receptors in CA1 pyramidal neurons of rat hippocampal slices. Application of N-acetyl-D-sphingosine, a synthetic cell-permeable ceramide analog, promptly produced a slight increase of excitatory postsynaptic current amplitude lasting for about 3 min. However, this transient enhancement was followed by a profoundly delayed-onset, sustained depression of synaptic excitatory postsynaptic currents in a concentration-dependent fashion (1-30 microM). This ceramide-induced sustained depression was not associated with changes in paired-pulse facilitation, a phenomenon resulting from an alteration of presynaptic transmitter release. Dihydro-N-acetyl-D-erythro-sphingosine (10 microM), an inactive analog of N-acetyl-D-sphingosine, did not affect synaptic excitatory postsynaptic currents, indicating the specificity of N-acetyl-D-sphingosine's action. The induction of ceramide-induced sustained depression was primarily dependent on the activation of postsynaptic protein phosphatases, being considerably blocked by loading 30 nM okadaic acid (a potent inhibitor of protein phosphatases 1 and 2A) into neurons. In addition, following a stable establishment of ceramide-induced sustained depression, a protocol for inducing long-term depression caused no additional decreases in excitatory postsynaptic current amplitude, and vice versa. The study suggests that ceramide induces a long-term depressed modulation on synaptic transmission mediated by ionotropic glutamate receptors in the hippocampus, possibly through the activation of postsynaptic protein phosphatases 1 and 2A. In addition, ceramide-induced sustained depression seems to share some common mechanisms with long-term depression, such as the cascades of events resulting from the activation of protein phosphatases. Collectively, the long-term depressed modulation of ceramide on ionotropic glutamate receptor-mediated functions may be particularly important in various physiological and/or pathological conditions, in which the ceramide signaling pathway is activated in the mammalian brain.

Sustained enhancement of AMPA receptor- and NMDA receptor-mediated currents induced by dopamine D1/D5 receptor activation in the hippocampus: an essential role of postsynaptic Ca2+

The dopaminergic system in the limbic system, particularly the D1/D5 receptor (D1/D5r), is important for certain forms of learning and memory, such as reinforcement learning, as well as the acute development of behavioral sensitization to psychostimulants such as cocaine and methamphetamine. Here, whole-cell patch-clamp recordings of evoked excitatory postsynaptic currents (EPSCs) mediated by ionotropic glutamate receptors were made from pyramidal neurons in the CA1 area of rat hippocampal slices. Activation of the D1/D5r by a selective D1/D5r agonist (+/-)-6-chloro-PB hydrobromide (SKF 81297; 3-100 microM) concentration-dependently induced a delayed-onset, sustained enhancement of EPSCs. The D1/D5r-induced effect was blocked by a selective D1/D5r antagonist (+)SCH 23390 hydrochloride (5 microM). Furthermore, the D1/D5r-induced effect involved a sustained enhancement of both the pharmacologically isolated alpha-amino-3-hydroxy-5-methyl-4-isoxazoleproprionate receptor-(AMPAr-) and N-methyl-D-asparate receptor- (NMDAr-) mediated EPSCs, respectively. Such persistently enhanced AMPAr- and NMDAr-mediated EPSCs were also associated with significant increases in the normalized amplitudes of the decay times of the isolated EPSCs. Blockade of NMDAr activation failed to prevent the induction of D1/D5r-induced sustained enhancement, suggesting the independence of the D1/D5r-induced effect on NMDAr activation. A rise of postsynaptic calcium was required for the induction of D1/D5r-induced sustained enhancement, being abolished by loading cells with 1,2-bis(2-aminophenoxy)ethane-N,N,N',N '-tetracetic acid (BAPTA; 10 mM). Studies indicated that the D1/D5r activation induced a sustained enhancement of both the AMPAr-and NMDAr-mediated EPSCs in the hippocampus. Moreover, a rise in postsynaptic Ca2+ was necessary for triggering the D1/D5r-induced effect in the hippocampus, although the NMDAr-dependent mechanisms, such as the calcium entry via the NMDA channels, were not.

Presynaptic involvement of nitric oxide in dopamine D1/D5 receptor-induced sustained enhancement of synaptic currents mediated by ionotropic glutamate receptors in the rat hippocampus

Using whole-cell patch-clamp recordings, the study tested the possibility whether nitric oxide (NO) was involved in dopamine D1/D5 receptor (D1/D5r)-induced sustained enhancement of excitatory postsynaptic currents (EPSCs) mediated by ionotropic glutamate receptors in rat hippocampal slices. Activation of D1/D5r by a selective agonist, 50 microM (+/-)-6-chloro-PB hydrobromide, elicited not only a sustained enhancement but also an increased frequency of spontaneous EPSC. The D1/D5r-induced effect was associated with decreases in paired-pulse facilitation (PPF). A selective inhibitor of neuronal NO synthase (nNOS), 100 microM 7-nitroindazole monosodium salt, substantially attenuated both the magnitudes of D1/D5r-induced enhancement and PPF decrease. These results suggest that presynaptic effects mediated by NO, possibly synthesized by nNOS, are involved in D1/D5-induced sustained enhancement of synaptic currents mediated by ionotropic glutamate receptors in the hippocampus.