Excitatory amino acids: evidence for a role in the control of reproduction and anterior pituitary hormone secretion

Polymorphism and expression of GLUD1 in relation to reproductive performance in Jining Grey goats

Understanding the molecular mechanism of mammalian reproduction (puberty and prolificacy) will play a part in improving animal reproductive performance. GLUD1 (glutamate dehydrogenase 1) is important for mammalian reproduction, as shown in previous studies; however, its roles in puberty and prolificacy have rarely been reported. In this study, we designed seven pairs of primers (P1 to P7) for cloning and sequencing genomic DNA of Jining Grey goats and Liaoning Cashmere goats. Primer 8 (P8) was designed to detect single nucleotide polymorphism (SNP) of the GLUD1 in both sexually precocious and high-fecundity breeds (Jining Grey, Nanjiang Brown and Matou goats) and sexually late-maturing and low-fecundity breeds (Liaoning Cashmere, Inner Mongolia Cashmere and Taihang goats) by PCR-RFLP (restriction fragment length polymorphism). The real-time quantitative polymerase chain reaction (RT-qPCR) technique was used to detect the expression of GLUD1 in a variety of tissues. The results showed that the A197C mutation was only found in the amplification product of P6. For this SNP locus, only two genotypes (AA and AC) were detected in Nanjiang Brown goats, while three genotypes (AA, AC and CC) were detected in the other five breeds. In Jining Grey goats, the frequency of genotypes AA, AC and CC was 0.69, 0.26 and 0.05, respectively. In Jining Grey goats, AA genotype had 0.54 (P < 0.05 ) and 0.3 (P < 0.05 ) more kids than the CC and AC genotype, respectively, and no significant difference (P > 0.05 ) was found in kidding number between the AC and CC genotype. GLUD1 was expressed in five tissues of different developmental stages. The expression level of GLUD1 in the hypothalamus was higher than that in the other four tissues except during puberty of Liaoning Cashmere goats. In puberty in goats, GLUD1 expression was significantly higher in ovaries than that in the juvenile period (P < 0.01 ). RT-qPCR results showed that the expression of GLUD1 in ovaries may relate to the puberty of goats. The present study preliminarily indicated that there might be an association between the 197 locus of GLUD1 and sexual precocity in goats, and allele A of GLUD1 was a potential DNA marker for improving kidding number in Jining Grey goats.

Molecular Mechanisms of the Regulation of Liver Cytochrome P450 by Brain NMDA Receptors and via the Neuroendocrine Pathway-A Significance for New Psychotropic Therapies

Recent investigations have highlighted the potential utility of the selective antagonist of the NMDA receptor GluN2B subunit for addressing major depressive disorders. Our previous study showed that the systemic administration of the antagonist of the GluN2B subunit of the NMDA receptor, the compound CP-101,606, affected liver cytochrome P450 expression and activity. To discern between the central and peripheral mechanisms of enzyme regulation, our current study aimed to explore whether the intracerebral administration of CP-101,606 could impact cytochrome P450. The injection of CP-101,606 to brain lateral ventricles (6, 15, or 30 µg/brain) exerted dose-dependent effects on liver cytochrome P450 enzymes and hypothalamic or pituitary hormones. The lowest dose led to an increase in the activity, protein, and mRNA level of CYP2C11 compared to the control. The activities of CYP2A, CYP2B, CYP2C11, CYP2C6, CYP2D, and protein levels of CYP2B, CYP2C11 were enhanced compared to the highest dose. Moreover, CP-101,606 increased the CYP1A protein level coupled with elevated CYP1A1 and CYP1A2 mRNA levels, but not activity. The antagonist decreased the pituitary somatostatin level and increased the serum growth hormone concentration after the lowest dose, while independently decreasing the serum corticosterone concentration of the dose. The findings presented here unveil a novel physiological regulatory mechanism whereby the brain glutamatergic system, via the NMDA receptor, influences liver cytochrome P450. This regulatory process appears to involve the endocrine system. These results may have practical applications in predicting alterations in cytochrome P450 activity and endogenous metabolism, and potential metabolic drug-drug interactions elicited by drugs that cross the blood-brain barrier and affect NMDA receptors.

Glutamate Supply Reactivates Ovarian Function while Increases Serum Insulin and Triiodothyronine Concentrations in Criollo x Saanen-Alpine Yearlings' Goats during the Anestrous Season

The possible effect of glutamate supplementation upon ovarian reactivation and serum concentrations of insulin (INS) and triiodothyronine (T3) in anestrous yearling goats was evaluated. Goats (n = 32, 12 mo., 26° North, 1117 m) with a similar live weight (LW) and body condition score (BCS) were blood sampled twice per week for two weeks (2 × 1 week × 2 weeks) to confirm the anestrus status (<1 ng P4/mL; RIA). Thereafter, goats were randomly assigned to either 1) Glutamate (GLUT; n = 16, LW = 27.1 ± 1.09 kg, 3.5 ± 0.18 units, IV-supplemented with 7 mg of glutamate kg^-1 LW), or 2) Control (CONT; n = 16; LW = 29.2 ± 1.09 kg; BCS = 3.5 ± 0.18, IV saline). During the treatment period, 16 goats (eight/group) were blood sampled twice per week for six weeks. Such serum samples (2 × 1 week × 6 weeks) were quantified by their P4 content to evaluate the ovarian-luteal activity, whereas a sample subset (1 × 1 week × 6 weeks) was used to quantify their INS & T3 content to evaluate their metabolic status. Neither LW (28.19 kg; p > 0.05) nor BCS (3.51 units; p > 0.05) differed between treatments. Goats depicting ovarian reactivation favored the GLUT group (50 vs. 12.5%; p < 0.05). Neither INS (1.72 ± 0.15 ng mL^-1) nor T3 (2.32 ± 0.11 ng mL^-1) differed between treatments, yet a treatment x time interaction regarding INS & T3 concentration across time favored (p < 0.05) the GLUT group. The results unveil exogenous glutamate as an interesting modulator not only of ovarian reactivation, but of metabolic hormone synthesis.

Hindbrain 5'-Adenosine Monophosphate-activated Protein Kinase Mediates Short-term Food Deprivation Inhibition of the Gonadotropin-releasing Hormone-Luteinizing Hormone Axis: Role of Nitric Oxide

Hindbrain-derived stimuli restrain the gonadotropin-releasing hormone (GnRH)-pituitary luteinizing hormone (LH) reproductive neuroendocrine axis during energy insufficiency. Interruption of food intake, planned or unplanned, is emblematic of modern life. This study investigated the premise that the hindbrain energy sensor 5'-adenosine monophosphate-activated protein kinase (AMPK) inhibits reproductive neuroendocrine function in short term, e.g. 18-h food-deprived (FD) estradiol (E)-implanted ovariectomized female rats. Intra-caudal fourth ventricular administration of the AMPK inhibitor Compound C (Cc) reversed FD-induced inhibition of rostral preoptic (rPO) GnRH protein expression and LH release in animals given E to replicate proestrus (high-E dose-, but not metestrus (low-E dose)-stage plasma steroid levels. FD caused Cc-reversible augmentation or diminution of preoptic norepinephrine (NE) activity in high- versus low-E rats, respectively, and AMPK-independent reductions in hypothalamic NE accumulation in the latter. Nitric oxide (NO) and kisspeptin are key stimulatory signals for the preovulatory LH surge. Here, FD inhibited rPO neuronal nitric oxide synthase protein expression in high-, but not low-E-dosed animals. Lateral ventricular delivery of the NO donor 3-morpholinosydnonimine (SIN-1) reversed inhibitory GnRH and LH responses to FD in high-E rats, and normalized rPO Vglut2, anteroventral periventricular KiSS1, and dorsomedial hypothalamic RFRP-3 mRNA and/or protein profiles. Data show that FD curtails reproductive neuroendocrine outflow by hindbrain AMPK-dependent mechanisms in the presence of peak estrous cycle E levels. Results indicate that neural networks linking this sensor to GnRH neurons likely involve NO signaling, which may function upstream of one or more neurotransmitters identified here by SIN-1-reversible inhibitory responses to FD.

Dynamics of GnRH Neuron Ionotropic GABA and Glutamate Synaptic Receptors Are Unchanged during Estrogen Positive and Negative Feedback in Female Mice

Inputs from GABAergic and glutamatergic neurons are suspected to play an important role in regulating the activity of the gonadotropin-releasing hormone (GnRH) neurons. The GnRH neurons exhibit marked plasticity to control the ovarian cycle with circulating estradiol concentrations having profound "feedback" effects on their activity. This includes "negative feedback" responsible for suppressing GnRH neuron activity and "positive feedback" that occurs at mid-cycle to activate the GnRH neurons to generate the preovulatory luteinizing hormone surge. In the present study, we employed brain slice electrophysiology to question whether synaptic ionotropic GABA and glutamate receptor signaling at the GnRH neuron changed at times of negative and positive feedback. We used a well characterized estradiol (E)-treated ovariectomized (OVX) mouse model to replicate negative and positive feedback. Miniature and spontaneous postsynaptic currents (mPSCs and sPSCs) attributable to GABA_A and glutamatergic receptor signaling were recorded from GnRH neurons obtained from intact diestrous, OVX, OVX + E (negative feedback), and OVX + E+E (positive feedback) female mice. Approximately 90% of GnRH neurons exhibited spontaneous GABA_A-mPSCs in all groups but no significant differences in the frequency or kinetics of mPSCs were found at the times of negative or positive feedback. Approximately 50% of GnRH neurons exhibited spontaneous glutamate mPSCs but again no differences were detected. The same was true for spontaneous PSCs in all cases. These observations indicate that the kinetics of ionotropic GABA and glutamate receptor synaptic transmission to GnRH neurons remain stable across the different estrogen feedback states.

Adolescent alcohol exposure: Are there separable vulnerable periods within adolescence?

There are two key alcohol use patterns among human adolescents that confer increased vulnerability for later alcohol abuse/dependence, along with neurocognitive alterations: (a) early initiation of use during adolescence, and (b) high rates of binge drinking that are particularly prevalent late in adolescence. The central thesis of this review is that lasting neurobehavioral outcomes of these two adolescent exposure patterns may differ. Although it is difficult to disentangle consequences of early use from later binge drinking in human studies given the substantial overlap between groups, these two types of problematic adolescent use are differentially heritable and hence separable to some extent. Although few studies using animal models have manipulated alcohol exposure age, those studies that have have typically observed timing-specific exposure effects, with more marked (or at least different patterns of) lasting consequences evident after exposures during early-mid adolescence than late-adolescence/emerging adulthood, and effects often restricted to male rats in those few instances where sex differences have been explored. As one example, adult male rats exposed to ethanol during early-mid adolescence (postnatal days [P] 25-45) were found to be socially anxious and to retain adolescent-typical ethanol-induced social facilitation into adulthood, effects that were not evident after exposure during late-adolescence/emerging adulthood (P45-65); exposure at the later interval, however, induced lasting tolerance to ethanol's social inhibitory effects that was not evident after exposure early in adolescence. Females, in contrast, were little influenced by ethanol exposure at either interval. Exposure timing effects have likewise been reported following social isolation as well as after repeated exposure to other drugs such as nicotine (and cannabinoids), with effects often, although not always, more pronounced in males where studied. Consistent with these timing-specific exposure effects, notable maturational changes in brain have been observed from early to late adolescence that could provide differential neural substrates for exposure timing-related consequences, with for instance exposure during early adolescence perhaps more likely to impact later self-administration and social/affective behaviors, whereas exposures later in adolescence may be more likely to influence cognitive tasks whose neural substrates (such as the prefrontal cortex [PFC]) are still undergoing maturation at that time. More work is needed, however to characterize timing-specific effects of adolescent ethanol exposures and their sex dependency, determine their neural substrates, and assess their comparability to and interactions with adolescent exposure to other drugs and stressors. Such information could prove critical for informing intervention/prevention strategies regarding the potential efficacy of efforts directed toward delaying onset of alcohol use versus toward reducing high levels of use and risks associated with that use later in adolescence.

Biologically active substances-enriched diet regulates gonadotrope cell activation pathway in liver of adult and old rats

According to the Hippocrates’ theorem “Let food be your medicine and medicine be your food”, dietary interventions may induce changes in the metabolic and inflammatory state by modulating the expression of important genes involved in the chronic disorders. The aim of the present study was to evaluate the influence of long-term (14 months) use of biologically active substances-enriched diet (BASE-diet) on transcriptomic profile of rats’ liver. The experiment was conducted on 36 Sprague–Dawley rats divided into two experimental groups (fed with control or BASE-diet, both n = 18). Control diet was a semi-synthetic diet formulated according to the nutritional requirements for laboratory animals. The BASE-diet was enriched with a mixture of polyphenolic compounds, β-carotene, probiotics, and n-3 and n-6 polyunsaturated fatty acids. In total, n = 3,017 differentially expressed (DE) genes were identified, including n = 218 DE genes between control and BASE groups after 3 months of feeding and n = 1,262 after 14 months. BASE-diet influenced the expression of genes involved particularly in the gonadotrope cell activation pathway and guanylate cyclase pathway, as well as in mast cell activation, gap junction regulation, melanogenesis and apoptosis. Especially genes involved in regulation of GnRH were strongly affected by BASE-diet. This effect was stronger with the age of animals and the length of diet use. It may suggest a link between the diet, reproductive system function and aging.

Kisspeptin, neurokinin B, and dynorphin act in the arcuate nucleus to control activity of the GnRH pulse generator in ewes

Recent work has led to the hypothesis that kisspeptin/neurokinin B/dynorphin (KNDy) neurons in the arcuate nucleus play a key role in GnRH pulse generation, with kisspeptin driving GnRH release and neurokinin B (NKB) and dynorphin acting as start and stop signals, respectively. In this study, we tested this hypothesis by determining the actions, if any, of four neurotransmitters found in KNDy neurons (kisspeptin, NKB, dynorphin, and glutamate) on episodic LH secretion using local administration of agonists and antagonists to receptors for these transmitters in ovariectomized ewes. We also obtained evidence that GnRH-containing afferents contact KNDy neurons, so we tested the role of two components of these afferents: GnRH and orphanin-FQ. Microimplants of a Kiss1r antagonist briefly inhibited LH pulses and microinjections of 2 nmol of this antagonist produced a modest transitory decrease in LH pulse frequency. An antagonist to the NKB receptor also decreased LH pulse frequency, whereas NKB and an antagonist to the receptor for dynorphin both increased pulse frequency. In contrast, antagonists to GnRH receptors, orphanin-FQ receptors, and the N-methyl-D-aspartate glutamate receptor had no effect on episodic LH secretion. We thus conclude that the KNDy neuropeptides act in the arcuate nucleus to control episodic GnRH secretion in the ewe, but afferent input from GnRH neurons to this area does not. These data support the proposed roles for NKB and dynorphin within the KNDy neural network and raise the possibility that kisspeptin contributes to the control of GnRH pulse frequency in addition to its established role as an output signal from KNDy neurons that drives GnRH pulses.

Acute shift in glutamate concentrations following experimentally induced panic with cholecystokinin tetrapeptide--a 3T-MRS study in healthy subjects

According to preclinical studies, glutamate has been implicated in the pathogenesis of anxiety. In order to elucidate the role of glutamate in anxiety and panic in humans, brain glutamate+glutamine (Glx) levels were measured during cholecystokinin-tetrapeptide (CCK-4)-induced panic using magnetic resonance spectroscopy (MRS). Eighteen healthy subjects underwent a CCK-4 challenge. MR spectra were obtained from the anterior cingulate cortex (ACC) using a single voxel point-resolved spectroscopy method and analyzed using LCModel. A combined fitting of Glx was performed. Panic was assessed using the Acute Panic Inventory (API) and Panic Symptom Scale (PSS) scores. Moreover, hypothalamic-pituitary-adrenal axis stimulation was monitored throughout the challenge. There was a significant panic response following CCK-4 as revealed by a marked increase in both the panic scores (API: F(1,17)=149.41; p<0.0001; PSS: F(1,17)=88.03; p<0.0001) and heart rate (HR: F(1,17)=72.79; p<0.0001). MRS measures showed a significant increase of brain Glx/creatine (Glx/Cr) levels peaking at 2-10 min after challenge (F(1,17)=15.94; p=0.001). There was also a significant increase in CCK-4-related cortisol release (F(6,11)=8.68; p=0.002). Finally, significant positive correlations were found between baseline Glx/Cr and both APImax (r=0.598; p=0.009) and maximum heart rate (HR(max)) during challenge (r=0.519; p=0.027). Our results suggest that CCK-4-induced panic is accompanied by a significant glutamate increase in the bilateral ACC. The results add to the hypothesis of a disturbance of the inhibitory-excitatory equilibrium and suggest that apart from static alterations rapid and dynamic neurochemical changes might also be relevant for the neural control of panic attacks.

Detecting loci under recent positive selection in dairy and beef cattle by combining different genome-wide scan methods

As the methodologies available for the detection of positive selection from genomic data vary in terms of assumptions and execution, weak correlations are expected among them. However, if there is any given signal that is consistently supported across different methodologies, it is strong evidence that the locus has been under past selection. In this paper, a straightforward frequentist approach based on the Stouffer Method to combine P-values across different tests for evidence of recent positive selection in common variations, as well as strategies for extracting biological information from the detected signals, were described and applied to high density single nucleotide polymorphism (SNP) data generated from dairy and beef cattle (taurine and indicine). The ancestral Bovinae allele state of over 440,000 SNP is also reported. Using this combination of methods, highly significant (P<3.17×10⁻⁷) population-specific sweeps pointing out to candidate genes and pathways that may be involved in beef and dairy production were identified. The most significant signal was found in the Cornichon homolog 3 gene (CNIH3) in Brown Swiss (P = 3.82×10⁻¹²), and may be involved in the regulation of pre-ovulatory luteinizing hormone surge. Other putative pathways under selection are the glucolysis/gluconeogenesis, transcription machinery and chemokine/cytokine activity in Angus; calpain-calpastatin system and ribosome biogenesis in Brown Swiss; and gangliosides deposition in milk fat globules in Gyr. The composite method, combined with the strategies applied to retrieve functional information, may be a useful tool for surveying genome-wide selective sweeps and providing insights in to the source of selection.

Potential metabolic mechanism of girls' central precocious puberty: a network analysis on urine metabonomics data

BACKGROUND

Central precocious puberty (CPP) is a common pediatric endocrine disease caused by early activation of hypothalamic-putuitary-gonadal (HPG) axis, yet the exact mechanism was poorly understood. Although there were some proofs that an altered metabolic profile was involved in CPP, interpreting the biological implications at a systematic level is still in pressing need. To gain a systematic understanding of the biological implications, this paper analyzed the CPP differential urine metabolites from a network point of view.

RESULTS

In this study, differential urine metabolites between CPP girls and age-matched normal ones were identified by LC-MS. Their basic topological parameters were calculated in the background network. The network decomposition suggested that CPP differential urine metabolites were most relevant to amino acid metabolism. Further proximity analysis of CPP differential urine metabolites and neuro-endocrine metabolites showed a close relationship between CPP metabolism and neuro-endocrine system. Then the core metabolic network of CPP was successfully constructed among all these differential urine metabolites. As can be demonstrated in the core network, abnormal aromatic amino acid metabolism might influence the activity of HPG and hypothalamic pituitary adrenal (HPA) axis. Several adjustments to the early activation of puberty in CPP girls could also be revealed by urine metabonomics.

CONCLUSIONS

The present article demonstrated the ability of urine metabonomics to provide several potential metabolic clues for CPP's mechanism. It was revealed that abnormal metabolism of amino acid, especially aromatic amino acid, might have a close correlation with CPP's pathogenesis by activating HPG axis and suppressing HPA axis. Such a method of network-based analysis could also be applied to other metabonomics analysis to provide an overall perspective at a systematic level.

Gonadotropin-releasing hormone plasticity: a comparative perspective

Gonadotropin-releasing hormone 1 (GnRH1) is a key regulator of the reproductive neuroendocrine system in vertebrates. Recent developments have suggested that GnRH1 neurons exhibit far greater plasticity at the cellular and molecular levels than previously thought. Furthermore, there is growing evidence that sub-populations of GnRH1 neurons in the preoptic area are highly responsive to specific environmental and hormonal conditions. In this paper we discuss findings that reveal large variation in GnRH1 mRNA and protein expression that are regulated by social cues, photoperiod, and hormonal feedback. We draw upon studies using histochemistry and immediate early genes (e.g., c-FOS/ZENK) to illustrate that specific groups of GnRH1 neurons are topographically organized. Based on data from diverse vertebrate species, we suggest that GnRH1 expression within individuals is temporally dynamic and this plasticity may be evolutionarily conserved. We suggest that the plasticity observed in other neuropeptide systems (i.e. kisspeptin) may have evolved in a similar manner.

Hirsutism, virilism, polycystic ovarian disease, and the steroid-gonadotropin-feedback system: a career retrospective

This career retrospective describes how the initial work on the mechanism of hormone action provided the tools for the study of hirsutism, virilism, and polycystic ovarian disease. After excessive ovarian and or adrenal androgen secretion in polycystic ovarian disease had been established, the question whether the disease was genetic or acquired, methods to manage hirsutism and methods for the induction of ovulation were addressed. Recognizing that steroid gonadotropin feedback was an important regulatory factor, initial studies were done on the secretion of LH and FSH in the ovulatory cycle. This was followed by the study of basic mechanisms of steroid-gonadotropin feedback system, using castration and steroid replacement and the events surrounding the natural onset of puberty. Studies in ovariectomized rats showed that progesterone was a pivotal enhancer of estrogen-induced gonadotropin release, thus accounting for the preovulatory gonadotropin surge. The effects of progesterone were manifested by depletion of the occupied estrogen receptors of the anterior pituitary, release of hypothalamic LHRH, and inhibition of enzymes that degrade LHRH. Progesterone also promoted the synthesis of FSH in the pituitary. The 3α,5α-reduced metabolite of progesterone brought about selective LH release and acted using the GABA(A) receptor system. The 5α-reduced metabolite of progesterone brought about selective FSH release; the ability of progesterone to bring about FSH release was dependent on its 5α-reduction. The GnRH neuron does not have steroid receptors; the steroid effect was shown to be mediated through the excitatory amino acid glutamate, which in turn stimulated nitric oxide. These observations led to the replacement of the long-accepted belief that ovarian steroids acted directly on the GnRH neuron by the novel concept that the steroid feedback effect was exerted at the glutamatergic neuron, which in turn regulated the GnRH neuron. The neuroprotective effects of estrogens on brain neurons are of considerable interest.

Direct evidence for the co-expression of URP and GnRH in a sub-population of rat hypothalamic neurones: anatomical and functional correlation

Urotensin-II-related peptide (URP) is an eight amino-acid neuropeptide recently isolated from rat brain and considered as the endogenous ligand for the GPR14 receptor. Using single and double immunohistochemical labelling, in situ hybridization and ultrastructural immunocytochemistry, we explored the cellular and subcellular localization of URP in the male rat brain. URP peptide was detected in numerous varicose fibres of the median eminence (ME) and organum vasculosum laminae terminalis (OVLT) as well as in neuronal cell bodies of the medial septal nucleus and diagonal band of Broca where corresponding mRNA were also detected. Combining in situ hybridization with immunohistochemistry, we showed that cell bodies of the rat anterior hypothalamus contained both URP mRNA and GnRH peptide. In addition, double ultrastructural immunodetection of URP and GnRH peptides clearly revealed, in the median eminence, the co-localization of both peptides in the same neuronal processes in the vicinity of fenestrated portal vessels. This remarkable cellular and subcellular distribution led us to test the effect of URP on the GnRH-induced gonadotrophins release in the anterior pituitary, and to discuss its putative role at the level of the median eminence.

Prostaglandin E2 release from astrocytes triggers gonadotropin-releasing hormone (GnRH) neuron firing via EP2 receptor activation

Astrocytes in the hypothalamus release prostaglandin E(2) (PGE(2)) in response to cell-cell signaling initiated by neurons and glial cells. Upon release, PGE(2) stimulates the secretion of gonadotropin-releasing hormone (GnRH), the neuropeptide that controls reproduction, from hypothalamic neuroendocrine neurons. Whether this effect on GnRH secretion is accompanied by changes in the firing behavior of these neurons is unknown. Using patch-clamp recording we demonstrate that PGE(2) exerts a dose-dependent postsynaptic excitatory effect on GnRH neurons. These effects are mimicked by an EP2 receptor agonist and attenuated by protein kinase A (PKA) inhibitors. The acute blockade of prostaglandin synthesis by indomethacin (INDO) or the selective inhibition of astrocyte metabolism by fluoroacetate (FA) suppresses the spontaneous firing activity of GnRH neurons in brain slices. Similarly, GnRH neuronal activity is reduced in mice with impaired astrocytic PGE(2) release due to defective erbB signaling in astrocytes. These results indicate that astrocyte-to-neuron communication in the hypothalamus is essential for the activity of GnRH neurons and suggest that PGE(2) acts as a gliotransmitter within the GnRH neurosecretory system.

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.

Ionotropic glutamate receptor AMPA 1 is associated with ovulation rate

Ionotropic glutamate receptors mediate most excitatory neurotransmission in the central nervous system by opening ion channels upon the binding of glutamate. Despite the essential roles of glutamate in the control of reproduction and anterior pituitary hormone secretion, there is a limited understanding of how glutamate receptors control ovulation. Here we reveal the function of the ionotropic glutamate receptor AMPA-1 (GRIA1) in ovulation. Based on a genome-wide association study in Bos taurus, we found that ovulation rate is influenced by a variation in the N-terminal leucine/isoleucine/valine-binding protein (LIVBP) domain of GRIA1, in which serine is replaced by asparagine. GRIA1^Asn has a weaker affinity to glutamate than GRIA1^Ser, both in Xenopus oocytes and in the membrane fraction of bovine brain. This single amino acid substitution leads to the decreased release of gonadotropin-releasing hormone (GnRH) in immortalized hypothalamic GT1-7 cells. Cows with GRIA1^Asn have a slower luteinizing hormone (LH) surge than cows with GRIA1^Ser. In addition, cows with GRIA1^Asn possess fewer immature ovarian follicles before superovulation and have a lower response to hormone treatment than cows with GRIA1^Ser. Our work identified that GRIA1 is a critical mediator of ovulation and that GRIA1 might be a useful target for reproductive therapy.

Enhanced glutamatergic and decreased GABAergic synaptic appositions to GnRH neurons on proestrus in the rat: modulatory effect of aging

Background

Previous work by our lab and others has implicated glutamate as a major excitatory signal to gonadotropin hormone releasing hormone (GnRH) neurons, with gamma amino butyric acid (GABA) serving as a potential major inhibitory signal. However, it is unknown whether GABAergic and/or glutamatergic synaptic appositions to GnRH neurons changes on the day of the proestrous LH surge or is affected by aging.

Methodology/Principal Findings

To examine this question, synaptic terminal appositions on GnRH neurons for VGAT (vesicular GABA transporter) and VGLUT2 (vesicular glutamate transporter-2), markers of GABAergic and glutamatergic synaptic terminals, respectively, was examined by immunohistochemistry and confocal microscopic analysis in young and middle-aged diestrous and proestrous rats. The results show that in young proestrous rats at the time of LH surge, we observed reciprocal changes in the VGAT and VGLUT2 positive terminals apposing GnRH neurons, where VGAT terminal appositions were decreased and VGLUT2 terminal appositions were significantly increased, as compared to young diestrus control animals. Interestingly, in middle-aged cycling animals this divergent modulation of VGAT and VGLUT2 terminal apposition was greatly impaired, as no significant differences were observed between VGAT and VGLUT2 terminals apposing GnRH neurons at proestrous. However, the density of VGAT and VGLUT2 terminals apposing GnRH neurons were both significantly increased in the middle-aged animals.

Conclusions/Significance

In conclusion, there is an increase in glutamatergic and decrease in GABAergic synaptic terminal appositions on GnRH neurons on proestrus in young animals, which may serve to facilitate activation of GnRH neurons. In contrast, middle-aged diestrous and proestrous animals show a significant increase in both VGAT and VGLUT synaptic terminal appositions on GnRH neurons as compared to young animals, and the cycle-related change in these appositions between diestrus and proestrus that is observed in young animals is lost.

The glutamate agonist NMDA blocks gonadal regression and enhances antibody response to an immune challenge in Siberian hamsters (Phodopus sungorus)

Seasonal variation in behavior and physiology, including changes in immune function, are common. This variability is elicited by changes in photoperiod and often covaries with fluctuations in both energy reserves and reproductive state. It is unclear, however, whether changes in either variable alone drive seasonal changes in immunity. We investigated the relative contributions of reproduction and energy balance to changes in immune function. To accomplish this, we uncoupled seasonal changes in reproduction from those related to energy balance via daily injections of N-methyl-D: -aspartate (NMDA) in Siberian hamsters (Phodopus sungorus). NMDA is a glutamatergic agonist that blocks short day-induced gonadal regression, while leaving short-day declines in body mass unaffected. In Experiment 1, we examined the effect of differing doses of NMDA on testosterone production as a proxy for NMDA effects on reproduction; a dose-dependent rise in testosterone was observed. In Experiment 2, animals were maintained on long or short days and received daily injections of NMDA. After 8 weeks, all animals underwent a humoral immune challenge. Short-day animals receiving daily injections of NMDA maintained long day-like gonads; however, contrary to our predictions, no trade-off between reproduction or energy balance and immune function was observed. Unexpectedly, NMDA treatment increased immunoglobulin levels in all groups, suggesting that NMDA may provide an immunomodulatory signal, presumably through actions on peripheral glutamate receptors. These results support a previous finding that NMDA blocks reproductive regression. In addition, these findings demonstrate a general immunoenhancing effect of NMDA that appears independent of changes in reproductive or energetic state of the animal.

Hormonal modulation of amino acid neurotransmitter metabolism in the arcuate nucleus of the adult female rat: a novel action of estradiol

Morphological plasticity in response to estradiol is a hallmark of astrocytes in the arcuate nucleus. The functional consequences of these morphological changes have remained relatively unexplored. Here we report that in the arcuate nucleus estradiol significantly increased the protein levels of the two enzymes in the glutamate-glutamine cycle, glutamine synthetase and glutaminase. We further demonstrate that these estradiol-mediated changes in the enzyme protein levels may underlie functional changes in neurotransmitter availability as: 1) total glutamate concentration in the arcuate nucleus was significantly increased and 2) microdialysis revealed a significant increase in extracellular glutamate levels after a synaptic challenge in the presence of estradiol. These data implicate the glutamate-glutamine cycle in the generation and/or maintenance of glutamate and suggest that the difference in extracellular glutamate between estradiol- and oil-treated animals may be related to an increased efficiency of the cycle enzymes. In vivo enzyme activity assays revealed that the estradiol mediated increase in glutamate-glutamine cycle enzymes in the arcuate nucleus led to an increase in gamma-aminobutyric acid and is likely not related to the increase in extracellular glutamate. Thus, we have observed two-independent effects of estradiol on amino acid neurotransmission in the arcuate nucleus. These data suggest a possible functional consequence of the well-established changes in glial morphology that occur in the arcuate nucleus in the presence of estradiol and suggest the importance of neuronal-glial cooperation in the regulation of hypothalamic functions such as food intake and body weight.

FXYD1, a modulator of Na,K-ATPase activity, facilitates female sexual development by maintaining gonadotrophin-releasing hormone neuronal excitability

The excitatory tone to gonadotrophin-releasing hormone (GnRH) neurones is a critical component underlying the pubertal increase in GnRH secretion. However, the homeostatic mechanisms modulating the response of GnRH neurones to excitatory inputs remain poorly understood. A basic mechanism of neuronal homeostasis is the Na(+),K(+)-ATPase-dependent restoration of Na(+) and K(+) transmembrane gradients after neuronal excitation. This activity is reduced in a mouse model of Rett syndrome (RTT), a neurodevelopmental disorder in which expression of FXYD1, a modulator of Na(+),K(+)-ATPase activity, is increased. We now report that the initiation, but not the completion of puberty, is advanced in girls with RTT, and that, in rodents, FXYD1 may contribute to the neuroendocrine regulation of female puberty by modulating GnRH neuronal excitability. Fxyd1 mRNA abundance reaches maximal levels in the female rat hypothalamus by the fourth postnatal week of life (i.e., around the time when the mode of GnRH secretion acquires an adult pattern of release). Although Fxyd1 mRNA expression is low in the hypothalamus, approximately 50% of GnRH neurones contain Fxyd1 transcripts. Whole-cell patch recording of GnRH-EGFP neurones revealed that the neurones of Fxyd1-null female mice respond to somatic current injections with a lower number of action potentials than wild-type cells. Both the age at vaginal opening and at first oestrous were delayed in Fxyd1(-/-) mice, but adult reproductive capacity was normal. These results suggest that FXYD1 contributes to facilitating the advent of puberty by maintaining GnRH neuronal excitability to incoming transsynaptic stimulatory inputs.

Kinesin superfamily-associated protein 3 is preferentially expressed in glutamatergic neurons and contributes to the excitatory control of female puberty

It was earlier shown that expression of kinesin superfamily-associated protein 3 (KAP3), involved in the neuronal anterograde, microtubule-dependent transport of membrane organelles, increases in the hypothalamus of female rats during the juvenile phase of sexual development. KAP3 mRNA is abundant in the hypothalamus, suggesting that it might be expressed in broadly disseminated neuronal systems controlling neuroendocrine function. The present study identifies one of these systems and provides evidence for an involvement of KAP3 in the excitatory control of female puberty. In situ hybridization and immunohistofluorescence studies revealed that the KAP3 gene is expressed in glutamatergic neurons but not in GABAergic or GnRH neurons. Hypothalamic KAP3 mRNA levels increase during the juvenile period of female prepubertal development, remaining elevated throughout puberty. These changes appear to be, at least in part, estradiol dependent because ovariectomy decreases and estradiol increases KAP3 mRNA abundance. Lowering hypothalamic KAP3 protein levels via intraventricular administration of an antisense oligodeoxynucleotide resulted in reduced release of both glutamate and GnRH from the median eminence and delayed the onset of puberty. The median eminence content of vesicular glutamate transporter 2, a glutamate neuron-selective synaptic protein, and synaptophysin, a synaptic vesicle marker, were also reduced, suggesting that the loss of KAP3 diminishes the anterograde transport of these proteins. Altogether, these results support the view that decreased KAP3 synthesis diminishes GnRH output and delays female sexual development by compromising hypothalamic release of glutamate.

Evidence that gamma-aminobutyric acid is part of the neural circuit mediating estradiol negative feedback in anestrous ewes

Seasonal anestrus in ewes is driven by an increase in response to estradiol (E2) negative feedback. Compelling evidence indicates that inhibitory A15 dopaminergic (DA) neurons mediate the increased inhibitory actions of E2 in anestrus, but these neurons do not contain estrogen receptors. Therefore, we have proposed that estrogen-responsive afferents to A15 neurons are part of the neural circuit mediating E2 negative feedback in anestrus. This study examined the possible role of afferents containing gamma-aminobutyric acid (GABA) and nitric oxide (NO) in modulating the activity of A15 neurons. Local administration of NO synthase inhibitors to the A15 had no effect on LH, but GABA receptor ligands produced dramatic changes. Administration of either a GABA A or GABA B receptor agonist to the A15 increased LH secretion in ovary-intact ewes, suggesting that GABA inhibits A15 neural activity. In ovariectomized anestrous ewes, the same doses of GABA receptor agonist had no effect, but combined administration of a GABA A and GABA B receptor antagonist to the A15 inhibited LH secretion. These data are consistent with the hypothesis that endogenous GABA release within the A15 is low in ovary-intact anestrous ewes and elevated after ovariectomy. Using dual immunocytochemistry, we observed that GABAergic varicosities make close contacts on to A15 neurons and that A15 neurons contain both the GABA A-alpha1 and the GABA B-R1 receptor subunits. Based on these data, we propose that in anestrous ewes, E2 inhibits release of GABA from afferents to A15 DA neurons, increasing the activity of these DA neurons and thus suppressing episodic secretion of GnRH and LH.

Endogenous activation of metabotropic glutamate receptors modulates GABAergic transmission to gonadotropin-releasing hormone neurons and alters their firing rate: a possible local feedback circuit

Gonadotropin-releasing hormone (GnRH) neurons are the primary central regulators of fertility, receiving input from GABAergic afferents via GABA(A) receptors. We tested whether metabotropic glutamate receptors (mGluRs) regulate GABA transmission to GnRH neurons and GnRH neuronal firing pattern. Whole-cell recordings were performed under conditions isolating ionotropic GABA postsynaptic currents (PSCs) in brain slices. The broad-spectrum mGluR agonist 1-aminocyclopentane-1,3-dicarboxylic acid (ACPD) decreased the frequency of GABA(A)-mediated spontaneous PSCs in a reversible manner. Amplitude and kinetics were not altered, suggesting that afferent GABA neurons are the primary targets. TTX eliminated the effects of ACPD in most tested neurons. Group II [2-(2,3-dicarboxycyclopropyl) glycine] and III (L-AP-4) mGluR agonists mediated this response; a group I agonist (3,5-dihydroxyphenylglycine) was not effective. The broad-spectrum antagonist alpha-methyl-4-carboxyphenylglycine (MCPG) and/or (RS)-alpha-cyclopropyl-4-phosphonophenylglycine (CPPG) (group III antagonist) enhanced spontaneous PSC frequency, particularly when initial frequency was low, suggesting that endogenous activation of mGluRs regulates GABA transmission to GnRH neurons. Extracellular recordings were used to evaluate GnRH neuron firing rate within the network. ACPD reduced firing rate, and MCPG plus CPPG had an opposite effect, indicating that mGluRs help control excitability of the GnRH network. GnRH neurons express vesicular glutamate transporters, suggesting they may corelease this transmitter. Simulation of firing activity in a GnRH neuron decreased PSC frequency in that cell, an effect blocked by antagonism of mGluRs but not GnRH receptors. These results demonstrate an inhibition of GABAergic inputs to GnRH neurons by mGluRs via a presynaptic mechanism. Through this mechanism, local glutamate milieu, possibly contributed by GnRH neurons themselves, plays an important role in modulating GnRH release and the central regulation of fertility.

Control of prolactin secretion by excitatory amino acids

This article provides an overview of the increasing number of observations indicating that excitatory amino acids are involved in the control of prolactin secretion. The information available suggests that these amino acids exert a stimulatory action on hypophysial prolactin. Administration of a glutamate receptor agonist induces significant increase in prolactin release in rats, monkeys, and rams. In contrast, noncompetitive antagonists of N-methyl-D-aspartate receptors decrease plasma levels and attenuate the preovulatory surge of prolactin. It appears that the endogenous glutamatergic system participates not only in the regulation of basal secretion of prolactin, but also in the control of physiological prolactin responses induced by the suckling stimulus or by stress. Recent findings suggest that the glutamatergic innervation of the hypothalamic paraventricular nucleus is involved in the mediation of the neural signal of the suckling stimulus-induced prolactin release as well as in the mediation of the stress-induced release of prolactin.

Episodic bursting activity and response to excitatory amino acids in acutely dissociated gonadotropin-releasing hormone neurons genetically targeted with green fluorescent protein

The gonadotropin-releasing hormone (GnRH) system, considered to be the final common pathway for the control of reproduction, has been difficult to study because of a lack of distinguishing characteristics and the scattered distribution of neurons. The development of a transgenic mouse in which the GnRH promoter drives expression of enhanced green fluorescent protein (EGFP) has provided the opportunity to perform electrophysiological studies of GnRH neurons. In this study, neurons were dissociated from brain slices prepared from prepubertal female GnRH-EGFP mice. Both current- and voltage-clamp recordings were obtained from acutely dissociated GnRH neurons identified on the basis of EGFP expression. Most isolated GnRH-EGFP neurons fired spontaneous action potentials (recorded in cell-attached or whole-cell mode) that typically consisted of brief bursts (2-20 Hz) separated by 1-10 sec. At more negative resting potentials, GnRH-EGFP neurons exhibited oscillations in membrane potential, which could lead to bursting episodes lasting from seconds to minutes. These bursting episodes were often separated by minutes of inactivity. Rapid application of glutamate or NMDA increased firing activity in all neurons and usually generated small inward currents (<15 pA), although larger currents were evoked in the remaining neurons. Both AMPA and NMDA receptors mediated the glutamate-evoked inward currents. These results suggest that isolated GnRH-EGFP neurons from juvenile mice can generate episodes of repetitive burst discharges that may underlie the pulsatile secretion of GnRH, and glutamatergic inputs may contribute to the activation of endogenous bursts.

Interactions between GABAergic and aminoacidergic pathways in the control of gonadotropin and GH secretion in pre-pubertal female rats

Present experiments were carried out in 23-day-old female rats to analyze the interaction between excitatory amino acids (EAAs) and gamma-aminobutyric acid (GABA) in the control of gonadotropin and GH secretion. For this purpose, serum concentrations of LH, FSH and GH were measured after injection of different agonists of EAA receptor subtypes [N-methyl-D-aspartate (NMDA); kainic acid (KA), (+/-)-alpha-amino-3-hydroxy-5-methylisoxazole-4-propionic acid (AMPA)], antagonists of GABA receptors (bicuculline, phaclofen) or the combined administration of both types of drugs. The results obtained indicated that: 1) GABA has a minor physiological role in the control of LH and GH secretion, since neither LH nor GH serum concentrations changed after administration of bicuculline (antagonist of GABA(A) receptors) or phaclofen (antagonist of GABA(B) receptors); 2) GABA has a sex-specific physiological role in the control of FSH secretion in female rats, in which FSH secretion increases after phaclofen administration; 3) GH secretion was enhanced after administration of NMDA, KA and AMPA, while LH increased only after activation of NMDA receptors; 4) the stimulatory effect of NMDA on LH secretion was counteracted by administration of phaclofen; and 5) bicuculline and phaclofen reduced the ability of NMDA and AMPA to stimulate GH secretion. In conclusion, present experiments evidenced a physiological role of GABA, mediated by GABA(B) receptors, in the control of FSH secretion and a cross-talk between excitatory and inhibitory amino acids in the control of anterior pituitary secretion.

Interactions between serotoninergic and aminoacidergic pathways in the control of PRL secretion in prepubertal male rats

Prolactin secretion is controlled by the hypothalamus through different neurotransmitters which interact with multiple receptor subtypes. The discovery of different families of receptors for serotonin (5-HT1-5-HT7) and excitatory aminoacids (NMDA, KA, AMPA and metabotropic receptors) ilustrates the complexity of this regulation. Moreover, in the rat the role of different neurotransmitters changes during pubertal development. Present experiments were carried out to analyse the interactions between AMPA and serotoninergic receptors in the control of prolactin secretion in prepubertal male rats. For this purpose, 16 and 23-day old male rats were treated with 5-hydroxytryptophan (5-HTP, precursor of serotonin synthesis) plus fluoxetine (blocker of serotonin reuptake), 8-OH-DPAT (agonist of 5-HT1A receptors), DOI and alpha-Me-5-HT (agonists of 5-HT2 receptors), 1-phenylbiguanide (agonist of 5-HT3 receptors) alone or in combination with AMPA (agonist of AMPA receptors). The results obtained indicate that: (a) activation of 5-HT1A receptors stimulated PRL secretion on day 16 and inhibited it on day 23; activation of 5-HT2 receptors stimulated PRL secretion on days 16 and 23, whereas activation of 5-HT3 receptors inhibited PRL release only on day 23; (b) activation of AMPA receptors inhibited PRL secretion on day 23, but not on day 16 and (c) a cross-talk is apparent between 5-HT2 and AMPA receptors in the regulation of PRL secretion, the stimulatory effect of DOI being blocked by AMPA.

Pre- and postsynaptic actions of opioid and orphan opioid agonists in the rat arcuate nucleus and ventromedial hypothalamus in vitro

1. Using whole-cell patch clamp recording from neurones in an in vitro slice preparation, we have examined opioid- and orphanin FQ (OFQ)-mediated modulation of synaptic transmission in the rat arcuate nucleus and ventromedial hypothalamus (VMH). 2. Application of OFQ activated a Ba2+-sensitive and inwardly rectifying K+ conductance in approximately 50 % of arcuate nucleus neurones and approximately 95 % of VMH neurones. The OFQ-activated current was blocked by the nociceptin antagonist [Phe1Psi(CH2NH)Gly2]-nociceptin(1-13) NH2 (NCA), a peptide that on its own exhibited only weak agonist activity at high concentrations (> 1 microM). Similar current activation was observed with the mu agonist DAMGO but not delta (DPDPE) or kappa (U69593) agonists. 3. In arcuate nucleus neurones, DAMGO (1 microM), U69593 (1 microM) and OFQ (100 nM to 1 microM) but not DPDPE (1 microM) were found to depress the amplitude of electrically evoked glutamatergic postsynaptic currents (EPSCs) and decrease the magnitude of paired-pulse depression, indicating that opioid receptors were located presynaptically. 4. In VMH neurones, DAMGO strongly depressed the EPSC amplitude in all cells examined. DAMGO decreased the magnitude of paired-pulse depression, indicating that mu receptors were located presynaptically. U69593 weakly depressed the EPSC while OFQ and DPDPE had no effect. 5. In VMH neurones, DAMGO depressed the frequency of miniature EPSCs (-58 %) in the presence of tetrodotoxin and Cd2+ (100 microM), suggesting that the actions of mu receptors could be mediated by an inhibition of the synaptic vesicle release process downstream of Ca2+ entry. 6. The data presented show that presynaptic modulation of excitatory neurotransmission in the arcuate nucleus occurs through mu, kappa and the orphan opioid ORL-1 receptors while in the VMH presynaptic modulation only occurs through mu opioid receptors. Additionally, postsynaptic mu and ORL-1 receptors in both the arcuate nucleus and VMH modulate neuronal excitability through activation of a K+ conductance.