Functional glutamate receptors in a subpopulation of anterior pituitary cells

Amyloid Beta Oligomers-Induced Ca2+ Entry Pathways: Role of Neuronal Networks, NMDA Receptors and Amyloid Channel Formation

The molecular basis of amyloid toxicity in Alzheimer’s disease (AD) remains controversial. Amyloid β (Aβ) oligomers promote Ca2+ influx, mitochondrial Ca2+ overload and apoptosis in hippocampal neurons in vivo and in vitro, but the primary Ca2+ entry pathways are unclear. We studied Ca2+ entry pathways induced by Aβ oligomers in rat hippocampal and cerebellar neurons. Aβ oligomers induce Ca2+ entry in neurons. Ca2+ responses to Aβ oligomers are large after synaptic networking and prevented by blockers of synaptic transmission. In contrast, in neurons devoid of synaptic connections, Ca2+ responses to Aβ oligomers are small and prevented only by blockers of amyloid channels (NA7) and NMDA receptors (MK801). A combination of NA7 and MK801 nearly abolished Ca2+ responses. Non-neuronal cells bearing NMDA receptors showed Ca2+ responses to oligomers, whereas cells without NMDA receptors did not exhibit Ca2+ responses. The expression of subunits of the NMDA receptor NR1/ NR2A and NR1/NR2B in HEK293 cells lacking endogenous NMDA receptors restored Ca2+ responses to NMDA but not to Aβ oligomers. We conclude that Aβ oligomers promote Ca2+ entry via amyloid channels and NMDA receptors. This may recruit distant neurons intertwisted by synaptic connections, spreading excitation and recruiting further NMDA receptors and voltage-gated Ca2+ channels, leading to excitotoxicity and neuron degeneration in AD.

The Selective NMDA Receptor GluN2B Subunit Antagonist CP-101,606 with Antidepressant Properties Modulates Cytochrome P450 Expression in the Liver

Recent research indicates that selective NMDA receptor GluN2B subunit antagonists may become useful for the treatment of major depressive disorders. We aimed to examine in parallel the effect of the selective NMDA receptor GluN2B subunit antagonist CP-101,606 on the pituitary/serum hormone levels and on the regulation of cytochrome P450 in rat liver. CP-101,606 (20 mg/kg ip. for 5 days) decreased the activity of CYP1A, CYP2A, CYP2B, CYP2C11 and CYP3A, but not that of CYP2C6. The alterations in enzymatic activity were accompanied by changes in the CYP protein and mRNA levels. In parallel, a decrease in the pituitary growth hormone-releasing hormone, and in serum growth hormone and corticosterone (but not T₃ and T₄) concentration was observed. After a 3-week administration period of CP-101,606 less changes were found. A decrease in the CYP3A enzyme activity and protein level was still maintained, though no change in the mRNA level was found. A slight decrease in the serum concentration of corticosterone was also maintained, while GH level returned to the control value. The obtained results imply engagement of the glutamatergic system in the neuroendocrine regulation of cytochrome P450 and potential involvement of drugs acting on NMDA receptors in metabolic drug–drug interactions.

Relationships Between 24-hour LH and Testosterone Concentrations and With Other Pituitary Hormones in Healthy Older Men

Objective

To investigate the relationship between LH and testosterone (T), which characteristics associate with the strength of this relationship, and their interrelationships with GH, TSH, cortisol, and ACTH.

Design

Hormones were measured in serum samples collected every 10 minutes during 24 hours from 20 healthy men, comprising 10 offspring of long-lived families and 10 control subjects, with a mean (SD) age of 65.6 (5.3) years. We performed cross-correlation analyses to assess the relative strength between 2 timeseries for all possible time shifts.

Results

Mean (95% CI) maximal correlation was 0.21 (0.10-0.31) at lag time of 60 minutes between LH and total T concentrations. Results were comparable for calculated free, bioavailable, or secretion rates of T. Men with strong LH-T cross-correlations had, compared with men with no cross-correlation, lower fat mass (18.5 [14.9-19.7] vs. 22.3 [18.4-29.4] kg), waist circumference (93.6 [5.7] vs. 103.1 [12.0] cm), high-sensitivity C-reactive protein (0.7 [0.4-1.3] vs. 1.8 [0.8-12.3] mg/L), IL-6 (0.8 [0.6-1.0] vs. 1.2 [0.9-3.0] pg/mL), and 24-hour mean LH (4.3 [2.0] vs. 6.1 [1.5] U/L), and stronger LH-T feedforward synchrony (1.5 [0.3] vs. 1.9 [0.2]). Furthermore, T was positively cross-correlated with TSH (0.32 [0.21-0.43]), cortisol (0.26 [0.19-0.33]), and ACTH (0.26 [0.19-0.32]).

Conclusions

LH is followed by T with a delay of 60 minutes in healthy older men. Men with a strong LH-T relationship had more favorable body composition, inflammatory markers, LH levels, and LH-T feedforward synchrony. We observed positive correlations between T and TSH, cortisol, and ACTH.

Integrated mRNA and miRNA transcriptome analysis provides novel insights into the molecular mechanisms underlying goose pituitary development during the embryo-to-hatchling transition

It is well established that the endocrine system plays a pivotal role in preparing the avian embryos for the abrupt switch from chorioallantoic to pulmonary respiration during the critical embryo-to-hatchling transition. However, as the master gland of the endocrine system, there has been little research focusing on the molecular mechanisms controlling the development and function of the pituitary gland during the peri-hatch period in birds. In the present study, we aimed to determine the genome-wide mRNA and miRNA transcriptome profiles of the pituitary during the embryo-to-hatchling transition period from embryonic day 22 (E22) to post-hatching day 6 (P6) in the goose (Anser cygnoides). Of note, expression of Anser_cygnoides_newGene_32456 and LOC106031011 were significantly different among these 4 stages (i.e., E22, E26, P2, and P6). Meanwhile, the neuroactive ligand-receptor interaction pathway was significantly enriched by the DEGs commonly identified among three pairwise comparisons. At the miRNA transcriptome level, there were not commonly identified DE miRNAs among these 4 stages, while the 418 of their predicted target genes were mutually shared. Both the target genes of DE miRNAs in each comparison and these 418 shared target genes were significantly enriched in the ECM-receptor interaction and focal adhesion pathways. In the predicted miRNA-mRNA interaction networks of these 2 pathways, novel_miRNA_467, novel_miRNA_154, and novel_miRNA_340 were the hub miRNAs. In addition, multiple DE miRNAs also showed predicted target relationships with the DEGs associated with extracellular matrix (ECM) components. Among them, expression of novel_miR_120, tgu-miR-92-3p, and novel_miR_398 was significantly negatively correlated with that of LAMC3 (laminin subunit gamma3), suggesting that these miRNAs may regulate pituitary tissue remodeling and functional changes through targeting LAMC3 during development. These identified DE mRNAs and miRNAs as well as their predicted interaction networks involved in regulation of tissue remodeling and cellular functions were most likely to play critical roles in facilitating the embryo-to-hatchling transition. These results provide novel insights into the early developmental process of avian pituitary gland and will help better understand the underlying molecular mechanisms.

Interrelationships Between Pituitary Hormones as Assessed From 24-hour Serum Concentrations in Healthy Older Subjects

CONTEXT

Hormones of the hypothalamic-pituitary-target gland axes are mostly investigated separately, whereas the interplay between hormones might be as important as each separate hormonal axis.

OBJECTIVE

Our aim is to determine the interrelationships between GH, TSH, ACTH, and cortisol in healthy older individuals.

DESIGN

We made use of 24-hour hormone serum concentrations assessed with intervals of 10 minutes from 38 healthy older individuals with a mean age (SD) of 65.1 (5.1) years from the Leiden Longevity Study. Cross-correlation analyses were performed to assess the relative strength between 2 24-hour hormone serum concentration series for all possible time shifts. Cross-approximate entropy was used to assess pattern synchronicity between 2 24-hour hormone serum concentration series.

RESULTS

Within an interlinked hormonal axis, ACTH and cortisol were positively correlated with a mean (95% confidence interval) correlation coefficient of 0.78 (0.74-0.81) with cortisol following ACTH concentrations with a delay of 10 minutes. Between different hormonal axes, we observed a negative correlation coefficient between cortisol and TSH of -0.30 (-0.36 to -0.25) with TSH following cortisol concentrations with a delay of 170 minutes. Furthermore, a positive mean (95% confidence interval) correlation coefficient of 0.29 (0.22-0.37) was found between TSH and GH concentrations without any delay. Moreover, cross-approximate entropy analyses showed that GH and cortisol exhibit synchronous serum concentration patterns.

CONCLUSIONS

This study demonstrates that interrelations between hormones from interlinked as well as different hypothalamic-pituitary-target gland axes are observed in healthy older individuals. More research is needed to determine the biological meaning and clinical consequences of these observations.

Unified Classification of Molecular, Network, and Endocrine Features of Hypothalamic Neurons

Peripheral endocrine output relies on either direct or feed-forward multi-order command from the hypothalamus. Efficient coding of endocrine responses is made possible by the many neuronal cell types that coexist in intercalated hypothalamic nuclei and communicate through extensive synaptic connectivity. Although general anatomical and neurochemical features of hypothalamic neurons were described during the past decades, they have yet to be reconciled with recently discovered molecular classifiers and neurogenetic function determination. By interrogating magnocellular as well as parvocellular dopamine, GABA, glutamate, and phenotypically mixed neurons, we integrate available information at the molecular, cellular, network, and endocrine output levels to propose a framework for the comprehensive classification of hypothalamic neurons. Simultaneously, we single out putative neuronal subclasses for which future research can fill in existing gaps of knowledge to rationalize cellular diversity through function-determinant molecular marks in the hypothalamus.

AMPA receptor expression in mouse testis and spermatogonial GC-1 cells: A study on its regulation by excitatory amino acids

Excitatory amino acids (EAAs) are found present in the nervous and reproductive systems of animals. Numerous studies have demonstrated a regulatory role for Glutamate (Glu), d-aspartate ( d-Asp) and N-methyl- d-aspartate (NMDA) in the control of spermatogenesis. EAAs are able to stimulate the Glutamate receptors, including the α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid receptor (AMPAR). Here in, we assess expression of the main AMPAR subunits, GluA1 and GluA2/3, in the mouse testis and in spermatogonial GC-1 cells. The results showed that both GluA1 and GluA2/3 were localized in mouse testis prevalently in spermatogonia. The subunit GluA2/3 was more highly expressed compared with GluA1 in both the testis and the GC-1 cells. Subsequently, GC-1 cells were incubated with medium containing l-Glu, d-Glu, d-Asp or NMDA to determine GluA1 and GluA2/3 expressions. At 30 minutes and 2 hours of incubation, EAA-treated GC-1 cells showed significantly higher expression levels of both GluA1 and GluA2/3. Furthermore, p-extracellular signal-regulated kinase (ERK), p-Akt, proliferating cell nuclear antigen (PCNA), and Aurora B expressions were assayed in l-Glu-, d-Glu-, and NMDA-treated GC-1 cells. At 30 minutes and 2 hours of incubation, treated GC-1 cells showed significantly higher expression levels of p-ERK and p-Akt. A consequent increase of PCNA and Aurora B expressions was induced by l-Glu and NMDA, but not by d-Glu. Our study demonstrates a direct effect of the EAAs on spermatogonial activity. In addition, the increased protein expression levels of GluA1 and GluA2/3 in EAA-treated GC-1 cells suggest that EAAs could activate ERK and Akt pathways through the AMPAR. Finally, the increased PCNA and Aurora B levels may imply an enhanced proliferative activity.

Neurotransmitter receptors as signaling platforms in anterior pituitary cells

The functions of anterior pituitary cells are controlled by two major groups of hypothalamic and intrapituitary ligands: one exclusively acts on G protein-coupled receptors and the other activates both G protein-coupled receptors and ligand-gated receptor channels. The second group of ligands operates as neurotransmitters in neuronal cells and their receptors are termed as neurotransmitter receptors. Most information about pituitary neurotransmitter receptors was obtained from secretory studies, RT-PCR analyses of mRNA expression and immunohistochemical and biochemical analyses, all of which were performed using a mixed population of pituitary cells. However, recent electrophysiological and imaging experiments have characterized γ-aminobutyric acid-, acetylcholine-, and ATP-activated receptors and channels in single pituitary cell types, expanding this picture and revealing surprising differences in their expression between subtypes of secretory cells and between native and immortalized pituitary cells. The main focus of this review is on the electrophysiological and pharmacological properties of these receptors and their roles in calcium signaling and calcium-controlled hormone secretion.

Single-Cell Phenotypic Characterization of Human Pituitary GHomas and Non-Functioning Adenomas Based on Hormone Content and Calcium Responses to Hypothalamic Releasing Hormones

Human pituitary tumors are generally benign adenomas causing considerable morbidity due to excess hormone secretion, hypopituitarism, and other tumor mass effects. Pituitary tumors are highly heterogeneous and difficult to type, often containing mixed cell phenotypes. We have used calcium imaging followed by multiple immunocytochemistry to type growth hormone secreting (GHomas) and non-functioning pituitary adenomas (NFPAs). Individual cells were typed for stored hormones and calcium responses to classic hypothalamic releasing hormones (HRHs). We found that GHomas contained growth hormone cells either lacking responses to HRHs or responding to all four HRHs. However, most GHoma cells were polyhormonal cells responsive to both thyrotropin-releasing hormone (TRH) and GH-releasing hormone. NFPAs were also highly heterogeneous. Some of them contained ACTH cells lacking responses to HRHs or polyhormonal gonadotropes responsive to LHRH and TRH. However, most NFPAs were made of cells storing no hormone and responded only to TRH. These results may provide new insights on the ontogeny of GHomas and NFPAs.

Purinergic signaling pathways in endocrine system

Adenosine-5'-triphosphate is released by neuroendocrine, endocrine, and other cell types and acts as an extracellular agonist for ligand-gated P2X cationic channels and G protein-coupled P2Y receptors in numerous organs and tissues, including the endocrine system. The breakdown of ATP by ectonucleotidases not only terminates its extracellular messenger functions, but also provides a pathway for the generation of two additional agonists: adenosine 5'-diphosphate, acting via some P2Y receptors, and adenosine, a native agonist for G protein-coupled adenosine receptors, also expressed in the endocrine system. This article provides a review of purinergic signaling pathways in the hypothalamic magnocellular neurosecretory cells and neurohypophysis, hypothalamic parvocellular neuroendocrine system, adenohypophysis, and effector glands organized in five axes: hypothalamic-pituitary-gonadal, hypothalamic-pituitary-thyroid, hypothalamic-pituitary-adrenal, hypothalamic-pituitary-growth hormone, and hypothalamic-pituitary-prolactin. We attempted to summarize current knowledge of purinergic receptor subtypes expressed in the endocrine system, including their roles in intracellular signaling, hormone secretion, and other cell functions. We also briefly review the release mechanism for adenosine-5'-triphosphate by neuroendocrine, endocrine and surrounding cells, the enzymes involved in adenosine-5'-triphosphate hydrolysis to adenosine-5'-diphosphate and adenosine, and the relevance of this pathway for sequential activation of receptors and termination of signaling.

Calcium homoeostasis modulator 1 (CALHM1) reduces the calcium content of the endoplasmic reticulum (ER) and triggers ER stress

CALHM1 (calcium homoeostasis modulator 1), a membrane protein with similarity to NMDA (N-methyl-D-aspartate) receptor channels that localizes in the plasma membrane and the ER (endoplasmic reticulum) of neurons, has been shown to generate a plasma-membrane Ca2+ conductance and has been proposed to influence Alzheimer's disease risk. In the present study we have investigated the effects of CALHM1 on intracellular Ca2+ handling in HEK-293T [HEK (human embryonic kidney)-293 cells expressing the large T-antigen of SV40 (simian virus 40)] cells by using targeted aequorins for selective monitorization of Ca2+ transport by organelles. We find that CALHM1 increases Ca2+ leak from the ER and, more importantly, reduces ER Ca2+ uptake by decreasing both the transport capacity and the Ca2+ affinity of SERCA (sarcoplasmic/endoplasmic reticulum Ca2+-ATPase). As a result, the Ca2+ content of the ER is drastically decreased. This reduction in the Ca2+ content of the ER triggered the UPR (unfolded protein response) with induction of several ER stress markers, such as CHOP [C/EBP (CCAAT/enhancer-binding protein)-homologous protein], ERdj4, GRP78 (glucose-regulated protein of 78 kDa) and XBP1 (X-box-binding protein 1). Thus CALHM1 might provide a relevant link between Ca2+ homoeostasis disruption, ER stress and cell damage in the pathogenesis of neurodegenerative diseases

Calcium entry-calcium refilling (CECR) coupling between store-operated Ca(2+) entry and sarco/endoplasmic reticulum Ca(2+)-ATPase

Cross-talk between subcellular organelles is essential for cellular Ca(2+) homeostasis. We have studied the effects of knocking down STIM1, the Ca(2+) sensor of the endoplasmic reticulum (ER), on several homeostatic Ca(2+)-handling mechanisms, including plasma membrane Ca(2+) entry and transport by ER, mitochondria and nucleus. We have used targeted aequorins to selectively measure calcium fluxes in different organelles. Actions of STIM1 were extremely selective, restricted to store operated Ca(2+) channels (SOC) and Ca(2+) uptake by the ER. No interactions with uptake or release of Ca(2+) by mitochondria or nucleus were detected. Ca(2+) exit from the ER, including passive leak, release via inositol 1,4,5-trisphosphate and ryanodine receptors, was unaffected. STIM1 knock-down inhibited ER Ca(2+) uptake in intact but not in permeabilized cells, suggesting a privileged calcium entry-calcium refilling (CECR) coupling between plasma membrane SOC and ER calcium pump in the intact cell. As a result a large part of the entering Ca(2+) is taken up into the ER without reaching the bulk cytosol. The tightness of CECR, as measured by the slope of the stimulus-signal strength function, was comparable to classic excitation-response coupling mechanisms, such as excitation-contraction, excitation-secretion or excitation-transcription coupling.

Ion channels and signaling in the pituitary gland

Endocrine pituitary cells are neuronlike; they express numerous voltage-gated sodium, calcium, potassium, and chloride channels and fire action potentials spontaneously, accompanied by a rise in intracellular calcium. In some cells, spontaneous electrical activity is sufficient to drive the intracellular calcium concentration above the threshold for stimulus-secretion and stimulus-transcription coupling. In others, the function of these action potentials is to maintain the cells in a responsive state with cytosolic calcium near, but below, the threshold level. Some pituitary cells also express gap junction channels, which could be used for intercellular Ca(2+) signaling in these cells. Endocrine cells also express extracellular ligand-gated ion channels, and their activation by hypothalamic and intrapituitary hormones leads to amplification of the pacemaking activity and facilitation of calcium influx and hormone release. These cells also express numerous G protein-coupled receptors, which can stimulate or silence electrical activity and action potential-dependent calcium influx and hormone release. Other members of this receptor family can activate calcium channels in the endoplasmic reticulum, leading to a cell type-specific modulation of electrical activity. This review summarizes recent findings in this field and our current understanding of the complex relationship between voltage-gated ion channels, ligand-gated ion channels, gap junction channels, and G protein-coupled receptors in pituitary cells.

The endoplasmic reticulum of dorsal root ganglion neurons contains functional TRPV1 channels

Transient receptor potential vanilloid type 1 (TRPV1) is a plasma membrane Ca(2+) channel involved in transduction of painful stimuli. Dorsal root ganglion (DRG) neurons express ectopic but functional TRPV1 channels in the endoplasmic reticulum (ER) (TRPV1(ER)). We have studied the properties of TRPV1(ER) in DRG neurons and HEK293T cells expressing TRPV1. Activation of TRPV1(ER) with capsaicin or other vanilloids produced an increase of cytosolic Ca(2+) due to Ca(2+) release from the ER. The decrease of [Ca(2+)](ER) was directly revealed by an ER-targeted aequorin Ca(2+) probe, expressed in DRG neurons using a herpes amplicon virus. The sensitivity of TRPV1(ER) to capsaicin was smaller than the sensitivity of the plasma membrane TRPV1 channels. The low affinity of TRPV1(ER) was not related to protein kinase A- or C-mediated phosphorylations, but it was due to inactivation by cytosolic Ca(2+) because the sensitivity to capsaicin was increased by loading the cells with the Ca(2+) chelator BAPTA. Decreasing [Ca(2+)](ER) did not affect the sensitivity of TRPV1(ER) to capsaicin. Disruption of the TRPV1 calmodulin-binding domains at either the C terminus (Delta35AA) or the N terminus (K155A) increased 10-fold the affinity of TRPV1(ER) for capsaicin, suggesting that calmodulin is involved in the inactivation. The lack of TRPV1 sensitizers, such as phosphatidylinositol 4,5-bisphosphate, in the ER could contribute to decrease the affinity for capsaicin. The low sensitivity of TRPV1(ER) to agonists may be critical for neuron health, because otherwise Ca(2+) depletion of ER could lead to ER stress, unfolding protein response, and cell death.

Role of metabotropic glutamate receptors in the control of neuroendocrine function

Glutamate exerts its effects through binding and activation of two classes of specific receptors: ionotropic (iGluRs) and metabotropic (mGluRs). Group I mGluR includes mGluR1 and mGluR5 subtypes, group II includes mGluR2 and mGluR3 subtypes and group III includes the subtypes mGluR 4, 6, 7 and 8. Glutamate and its receptors are found in all key hypothalamic areas critically involved in reproduction and neuroendocrine function. To date, considerable data support an important role for iGluRs in the control of neuroendocrine function; however, the role of mGluRs as regulators of hypothalamic-pituitary function has not been clearly elucidated. mGluRs could be exerting a fine tune on the release of hypothalamic factors that regulate hormone release such as Substance P, GABA, alpha-MSH and CRH. Group II mGluR exert a direct inhibitory effect on anterior pituitary prolactin and GH secretion. Moreover, some group II mGluR agonists, like LY 354,740 and LY 379,268, can modulate PRL secretion from the anterior pituitary through their actions as dopamine receptor agonists. Evidence suggests a role for group III mGluR subtypes in stress-related behavioral disorders. Several reports indicate that selective ligands for mGluR subtypes have potential for the treatment of a wide variety of neurological and psychiatric disorders, including depression, anxiety disorders, schizophrenia, epilepsy and Alzheimer's disease among others. Since converging lines of evidence suggest a role for mGluRs subtypes in neuroendocrine regulation of hormone secretion, mGluRs neuroendocrine actions must be taken in consideration to insure proper treatment of these diseases. Moreover, discovery of selective agonists provides an opportunity to investigate the physiological role of mGluR subtypes and to directly test the neuroendocrine actions of mGluRs. Finally, mGluRs selective agonists may have an impact in the treatment of conditions involving chronic stress, such as depression and anxiety disorders, since they regulate neuroendocrine stress circuits involving the HPA axis and stress-sensitive hormones such as oxytocin and prolactin. This review aims to provide a survey of our current understanding of the effects of mGluR activation on neuroendocrine function.

Rapid changes in anterior pituitary cell phenotypes in male and female mice after acute cold stress

The anterior pituitary (AP) is made of five different cell types. The relative abundance and phenotype of AP cells may change in different physiological situations as an expression of pituitary plasticity. Here, we analyze in detail the phenotype of mouse corticotropes and the effects of acute cold stress on AP cell populations. The hormone content and the expression of hypothalamic-releasing hormone (HRH) receptors in all the five AP cell types were studied in the male and female mice at rest and after a 30-min cold stress. Expression of HRH receptors was evidenced by imaging the single-cell cytosolic Ca(2+) responses in fura-2-loaded cells. Hormone contents were studied by multiple, simultaneous immunofluorescence of all the five hormones. Corticotropes displayed a striking sexual dimorphism, even in the resting condition. Male corticotropes showed the orthodox phenotype. They were monohormonal, storing only ACTH, and monoreceptorial, responding only to CRH. In contrast, female corticotropes were made of about equal parts of orthodox cells and multifunctional cells, which co-stored additional AP hormones and expressed additional HRH receptors. Cold stress did not modify the number of ACTH containing cells, but, according to immunostaining, it increased the relative abundance of other AP cell types at the expense of the pool of cells storing no hormones. Cold stress also modified the response to CRH and other HRHs. Most of these phenotypical changes presented a strong sexual dimorphism. These results indicate that pituitary plasticity is even larger than previously thought.

No genetic association between polymorphisms in the kainate-type glutamate receptor gene, GRIK4, and schizophrenia in the Chinese population

BACKGROUND

Schizophrenia is a chronic psychiatric disorder with a strong genetic component. Several studies have suggested that dysfunctions in the glutamatergic transmission are linked to the pathogenesis of schizophrenia, and that the kainate ionotropic glutamate receptors are involved in this mechanism. A recent study provides cytogenetic and genetic evidence to support a role for the kainate-type glutamate receptor gene (GRIK4), in schizophrenia. A systematic case-control association study of GRIK4 involving a Scottish population found that three SNPs, rs4935752, rs6589846 and rs4430518, were associated with schizophrenia.

METHODS

Here, we investigated rs4935752, rs6589846, rs4430518 and other 2 SNPs within the GRIK4 gene in an association study of the Chinese population. Our sample consisted of 288 schizophrenia and 288 control subjects. All recruits were Han Chinese drawn from the city of Shanghai.

RESULTS

No individual SNP nor any haplotype was associated with schizophrenia in our study.

CONCLUSION

These results suggest that the five SNPs within the GRIK4 gene are unlikely to play a major role in the susceptibility to schizophrenia in the Chinese population.

Nuclear calcium signaling by inositol trisphosphate in GH3 pituitary cells

It has been proposed that nuclear and cytosolic Ca(2+) ([Ca(2+)](N) and [Ca(2+)](C)) may be regulated independently. We address here the issue of whether inositol trisphosphate (IP(3)) can, bypassing changes of [Ca(2+)](C), produce direct release of Ca(2+) into the nucleoplasm. We have used targeted aequorins to selectively measure and compare the changes in [Ca(2+)](C) and [Ca(2+)](N) induced by IP(3) in GH(3) pituitary cells. Heparin, an IP(3) inhibitor that does not permeate the nuclear pores, abolished the [Ca(2+)](C) peaks but inhibited only partly the [Ca(2+)](N) peaks. The permeant inhibitor 2-aminoethoxy-diphenyl-borate (2-APB) blocked both responses. Removal of ATP also inhibited more strongly the [Ca(2+)](C) than [Ca(2+)](N) peak. The [Ca(2+)](N) and [Ca(2+)](C) responses differed also in their sensitivity to IP(3), the nuclear response showing higher affinity. Among IP(3) receptors, type 2 (IP(3)R2) has a higher affinity for IP(3) and is not inactivated by ATP removal. We find that IP(3)R2 immunoreactivity is present inside the nucleus whereas the other IP(3)R subtypes are detected only in the cytoplasm. The nuclear envelope (NE) of GH(3) cells showed deep invaginations into the nucleoplasm, with cytosol and cytoplasmic organella inside. These results indicate that GH(3) pituitary cells possess mechanisms able to produce selective increases of [Ca(2+)](N).

Ionotropic glutamate receptor activation increases intracellular calcium in prolactin-releasing cells of the adenohypophysis

Endocrine cells of the anterior pituitary are controlled by the central nervous system through hormonal interactions and are not believed to receive direct synaptic connections from the brain. Studies suggest that some pituitary cells may be modulated by the neurotransmitter glutamate. We investigated prolactin (PRL)-releasing cells of the anterior pituitary of a euryhaline fish, the tilapia (Oreochromis mossambicus), for the presence of possible glutamate receptors (GluRs). Fura-2 imaging addressed the ability of glutamate to increase intracellular calcium. We observed a dose-dependent increase in intracellular calcium with transient perfusion (1-2 min) of glutamate (10 nM to 1 mM) in two-thirds of imaged cells. This increase was attenuated by the ionotropic GluR antagonist kynurenic acid (0.5-1.0 mM). The increase was also blocked or attenuated by antagonists of L-type voltage-gated calcium channels. The GluR agonist alpha-amino-3-hydroxy-5-methylisoxazole propionic acid (AMPA; 100 microM) produced intracellular calcium increases that were reversibly blocked by the selective AMPA antagonist 6-cyano-7-nitroquinoxaline-2,3-dione (CNQX). In contrast, the selective agonist N-methyl-D-aspartate (NMDA; 100 microM to 1 mM in magnesium-free solution with 10 microM glycine) had no effect on intracellular calcium. Radioimmunoassays demonstrated that glutamate stimulated PRL release. CNQX but not the NMDA receptor antagonist 2-amino-5-phosphonovaleric acid blocked this release. Antibodies for mammalian AMPA- and NMDA-type GluR produced a similar punctate immunoreactivity in the periphery of PRL cells. However, the NMDA antibody recognized a protein of a different molecular mass in PRL cells compared with brain cells. These results clearly indicate the presence of GluRs on tilapia PRL cells that can stimulate PRL release.

Changes in expression of hypothalamic releasing hormone receptors in individual rat anterior pituitary cells during maturation, puberty and senescence

Anterior pituitary (AP) is formed by five different cell types, each one producing a different AP hormone whose secretion is regulated by a specific hypothalamic-releasing hormone (HRH). On the other hand, a significant number of AP cells express multiple HRH receptors (multiresponsive cells). Plastic changes in expression of HRH receptors in individual AP cells are involved in critical endocrine events. Here we have characterized the changes in functional responses to CRH, LHRH, TRH, and GHRH in individual AP cells throughout the whole life span of the rat. To this end, calcium responses to the HRHs were followed by single-cell imaging in freshly dispersed AP cells prepared from rats of different ages (0-540 postnatal days). Three different cell pools were identified: 1) monoresponsive cells, holding a single class of HRH receptor; 2) multiresponsive cells; and 3) nonresponsive cells. The relative abundance of each pool changed with age. Nonresponsive cells were abundant at birth, multiresponsive cells were abundant at puberty, and monoresponsive cells dominated at senescence. The relative abundance of each HRH receptor changed largely with age but not gender. In addition, the contribution of monoresponsive and multiresponsive cells to responses to each HRH changed very much with age. Thus, the anterior pituitary shows large changes in cell populations typed by functional responses to HRHs during maturation, puberty, and senescence.

Glutamate agonists activate the hypothalamic-pituitary-adrenal axis through hypothalamic paraventricular nucleus but not through vasopressinerg neurons

The hypothalamic-pituitary-adrenal (HPA) axis plays a crucial role in the stress processes. The nucleus paraventricularis hypothalami (PVN) with corticotropin-releasing hormone (CRH)-containing and arginine vasopressin (AVP)-containing neurons is the main hypothalamic component of the HPA. The glutamate, a well-known excitatory neurotransmitter, can activate the HPA inducing adrenocorticotropin hormone (ACTH) elevation. The aim of our study was to examine the involvement of PVN and especially AVP in glutamate-induced HPA activation using agonists of the N-methyl-d-aspartate (NMDA) and kainate receptors. Two approaches were used: in male Wistar rats the PVN was lesioned, and AVP-deficient homozygous Brattleboro rats were also studied. Blood samples were taken through indwelling cannula and ACTH, and corticosterone (CS) levels were measured by radioimmunoassay. The i.v. administered NMDA (5 mg/kg) or kainate (2.5 mg/kg) elevated the ACTH and CS levels already at 5 min in control (sham-operated Wistar or heterozygous Brattleboro) rats. The PVN lesion had no influence on basal ACTH and CS secretion but blocked the NMDA- or kainate-induced ACTH and CS elevations. The lack of AVP in the Brattleboro animals had no significant influence on the basal or glutamate-agonists-induced ACTH and CS elevations. Our results suggest that NMDA and kainate may activate the HPA axis at central (PVN) level and not at the level of pituitary or adrenal gland and that AVP has minor role in glutamate-HPA axis interaction. The time course of the ACTH secretion was different with NMDA or kainate. If we compared the two curves, the results were not coherent with the general view that NMDA activation requires previous kainate activation. Although it has to be mentioned that the conclusion which can be drawn is limited, the bioavailability of the compounds could be different as well.

Phenotypic characterization of multi-functional somatotropes, mammotropes and gonadotropes of the mouse anterior pituitary

The existence of bihormonal anterior pituitary (AP) cells co-storing growth hormone and either prolactin (mammosomatotrope) or gonadotropins (somatogonadotrope) has been described. These cells have been proposed to be involved in "paradoxical" secretion [secretion of an AP hormone induced by a non-related hypothalamic releasing factor (HRH) and transdifferentiation (a phenotypic switch between different cell types without cell division]. Here we combine calcium imaging (to assess HRH responsiveness) and multiple sequential immunoassay of the six AP hormones to perform a single-cell phenotypic study of multifunctional somatotropes, mammotropes and gonadotropes in the normal male and female mouse pituitaries. AP cell phenotypes differed from the classic view, showing multiple HRH-receptor expression and/or hormone storage. Mammosomatotropes represented only 5-6% of somatotropes and were poorly responsive to HRHs, suggesting that their contribution to paradoxical secretion should be very limited. Somatogonadotropes were present only in females and contained adrenocorticotropic hormone. They responded to growth hormone-releasing hormone but failed to respond to gonadotropin-releasing hormone (LHRH). Other polyhormonal cells identified include (1) gonadocorticotropes, restricted to females, where they make up more than 50% of all the gonadotropes and contain other AP hormones; (2) gonadomammotropes, which are present preferentially in female cells and respond to LHRH; and (3) gonadothyrotropes, which are present similarly in male and female pituitaries.

Anterior pituitary thyrotropes are multifunctional cells

Anterior pituitary (AP) contains some unorthodox multifunctional cells that store and secrete two different AP hormones (polyhormonal cells) and/or respond to several hypothalamic-releasing hormones (HRHs; multiresponsive cells). Multifunctional cells may be involved in paradoxical secretion (secretion of a given AP hormone evoked by a noncorresponding HRH) and transdifferentiation (phenotypic switch between different mature cell types without cell division). Here we combine calcium imaging (to assess responses to the four HRHs) and multiple sequential immunoassay of the six AP hormones to perform a single-cell phenotypic study of thyrotropes in normal male and female mice. Surprisingly, most of the thyrotropes were polyhormonal, containing, in addition to thyrotropin (TSH), luteinizing hormone (40-42%) and prolactin (19-21%). Thyrotropes costoring growth hormone and/or ACTH were found only in females (24% of each type). These results suggest that costorage of the different hormones does not happen at random and that gender favors certain hormone combinations. Our results indicate that thyrotropes are a mosaic of cell phenotypes rather than a single cell type. The striking promiscuity of TSH storage should originate considerable mix-up of AP hormone secretions on stimulation of thyrotropes. However, response to thyrotropin-releasing hormone was much weaker in the polyhormonal thyrotropes than in the monohormonal ones. This would limit the appearance of paradoxical secretion under physiological conditions and suggests that timing of hormone and HRH receptor expression during the transdifferentiation process is finely and differentially regulated.

Modulation of cellular differentiation by N-methyl-D-aspartate receptors in osteoblasts

N-methyl-D-aspartate (NMDA) receptors for the central neurotransmitter l-glutamate (Glu) have been shown to be present in both osteoblasts and osteoclasts. Sustained exposure to the NMDA channel antagonist dizocilpine (MK-801) significantly prevented increases in both alkaline phosphatase activity and Ca2+ accumulation in a concentration-dependent manner in osteoblasts cultured for 7-28 days in vitro (DIV), without significantly affecting cell survivability. Osteocalcin expression was markedly reduced in the presence of MK-801 in osteoblasts cultured for 28 DIV. Both an NMDA domain antagonist and a glycine domain antagonist similarly prevented Ca2+ accumulation in osteoblasts exposed for 28 consecutive DIV. MK-801 was effective in significantly inhibiting Ca2+ accumulation determined at 28 DIV in osteoblasts exposed before 7 DIV but was ineffective in cells exposed after 11-21 DIV. Sustained exposure to MK-801 significantly inhibited DNA binding activity and expression of core binding factor alpha-1 (CBFA1) in osteoblasts exposed after 7 DIV up to 28 DIV, but not in those exposed before 7 DIV. These results suggest that heteromeric NMDA receptor channels may be functionally expressed to regulate mechanisms underlying cellular differentiation rather than proliferation and/or maturation through modulation of expression of CBFA1 in cultured rat calvarial osteoblasts.

Multifunctional cells of mouse anterior pituitary reveal a striking sexual dimorphism

The existence of cells storing and secreting two different anterior pituitary (AP) hormones (polyhormonal cells) or responding to several hypothalamic releasing hormones (HRHs) (multiresponsive cells) has been reported previously. These multifunctional cells could be involved in paradoxical secretion (AP hormone secretion evoked by a non-corresponding HRH) and transdifferentiation (phenotypic switch between mature cell types without cell division). Despite their putative physiological relevance, a comprehensive characterization of multifunctional AP cells is lacking. Here we combine calcium imaging (to assess responses to the four HRHs) and multiple sequential immunoassay of the six AP hormones in the same individual cells to perform a complete phenotypic characterization of mouse AP cells. Polyhormonal and multiresponsive cells were identified within all five AP cell types. They were scarce in the more abundant cell types, somatotropes and lactotropes, but quite frequent in corticotropes and gonadotropes. Cells with mixed phenotypes were the rule rather than the exception in thyrotropes, where 56-83 % of the cells stored two to five different hormones. Multifunctional AP cells were much more abundant in females than in males, indicating that the hormonal changes associated with the sexual cycle may promote transdifferentiation. As the phenotypic analysis was performed here after stimulation with HRHs, the fraction of polyhormonal cells might have been underestimated. With this limitation, the polyhormonal cells detected here responded to the HRHs less than the monohormonal ones, suggesting that they might contribute less than expected a priori to paradoxical secretion. Overall, our results reveal a striking sexual dimorphism, the female pituitary being much more plastic than the male pituitary.

Stimulation of luteinizing hormone secretion by N-methyl-D,L-aspartic acid in the adult male guinea-pig: incomplete blockade by gonadotropin-releasing hormone receptor antagonism

Stimulation of luteinizing hormone (LH) secretion by N-methyl-D,L-aspartic acid (NMA), reported for several mammalian species, is generally accepted to be mediated through stimulation of hypothalamic gonadotropin-releasing hormone (GnRH) release. In view of a previously reported unexpected inhibitory action of NMA on GnRH release from hypothalamic explant of intact male guinea-pigs, the aim of the present study was to assess the in vivo effects of NMA in the adult male guinea-pig. In the gonadally intact male, NMA (5 mg/animal) elicited a robust LH secretion, which was blocked by the N-methyl-D-asparte-receptor antagonist DL-2-amino-5-phosphonovaleric acid (AP-5,12 mg/animal). In the castrated male, NMA elicited only a marginal and inconsistent LH secretion. Cetrorelix (CET), a GnRH receptor antagonist, administered intracardiacally 1 min or 45 min preceding bolus injection of NMA significantly reduced the LH response to NMA in the intact male. Surprisingly, following GnRH receptor blockade with CET, there still was a substantial residual serum LH response to NMA, while CET completely abolished the serum LH response to high dose (1 microg or 10 microg) guinea-pig GnRH (gpGnRH). These results indicate that NMA stimulates LH secretion in the gonadally intact male guinea-pig in vivo and that this effect is mediated in part through gpGnRH-independent mechanisms.

Calcium influx through receptor-operated channel induces mitochondria-triggered paraptotic cell death

We address the specific role of cytoplasmic Ca(2+) overload as a cell death trigger by expressing a receptor-operated specific Ca(2+) channel, vanilloid receptor subtype 1 (VR1), in Jurkat cells. Ca(2+) uptake through the VR1 channel, but not capacitative Ca(2+) influx stimulated by the muscarinic type 1 receptor, induced sustained intracellular [Ca(2+)] rises, exposure of phosphatidylserine, and cell death. Ca(2+) influx was necessary and sufficient to induce mitochondrial damage, as assessed by opening of the permeability transition pore and collapse of the mitochondrial membrane potential. Ca(2+)-induced cell death was inhibited by ruthenium red, protonophore carbonyl cyanide m-chlorophenylhydrazone, or cyclosporin A treatment, as well as by Bcl-2 expression, indicating that this process requires mitochondrial calcium uptake and permeability transition pore opening. Cell death occurred without caspase activation, oligonucleosomal/50-kilobase pair DNA cleavage, or release of cytochrome c or apoptosis inducer factor from mitochondria, but it required oxidative/nitrative stress. Thus, Ca(2+) influx triggers a distinct program of mitochondrial dysfunction leading to paraptotic cell death, which does not fulfill the criteria for either apoptosis or necrosis.

Prolactin secretion and intracellular Ca(2+) change in rat lactotroph subpopulations stimulated by thyrotropin-releasing hormone

Thyrotropin-releasing hormone (TRH) may stimulate lactotrophs to increase intracellular Ca(2+) and to secrete prolactin (PRL). In this study, PRL contents in lactotrophs were determined by the sequential cell immunoblot assay (SCIBA) and their changes in intracellular Ca(2+) was analyzed by confocal microscopy. Significant correlations were found in the corresponding parameters between TRH treatments with a recovery interval of 2 h. Measuring the PRL contents after the first TRH treatment and then determining the intracellular Ca(2+) changes after the second TRH treatment revealed four lactotroph subpopulations. Type I cells (51%) showed significant responses of both PRL secretion and intracellular Ca(2+) concentration. Type II cells (22%) increased in PRL secretion, but without changes in intracellular Ca(2+). Type III cells (17%) have increased in intracellular Ca(2+), but without changes in PRL secretion. Type IV cells (10%) did not show changes in PRL secretion and intracellular Ca(2+).

Dampening of cytosolic Ca2+ oscillations on propagation to nucleus

Ca(2+) signals may regulate gene expression. The increase of the cytosolic Ca(2+) concentration ([Ca(2+)](c)) promotes activation and/or nuclear import of some transcription factors, but others require the increase of the nuclear Ca(2+) concentration ([Ca(2+)](N)) for activation. Whether the nuclear envelope may act as a diffusion barrier for propagation of [Ca(2+)](c) signals remains controversial. We have studied the spreading of Ca(2+) from the cytosol to the nucleus by comparing the cytosolic and the nuclear Ca(2+) signals reported by targeted aequorins in adrenal chromaffin, PC12, and GH(3) pituitary cells. Strong stimulation of either Ca(2+) entry (by depolarization with high K(+) or acethylcholine) or Ca(2+) release from the intracellular Ca(2+) stores (by stimulation with caffeine, UTP, bradykinin, or thyrotropin-releasing hormone (TRH)) produced similar Ca(2+) signals in cytosol and nucleus. In contrast, both spontaneous and TRH-stimulated oscillations of cytosolic Ca(2+) in single GH(3) cells were considerably dampened during propagation to the nucleus. These results are consistent with the existence of a kinetic barrier that filters high frequency physiological [Ca(2+)](c) oscillations without disturbing sustained [Ca(2+)](c) increases. Thus, encoding of the Ca(2+) signal may allow differential control of Ca(2+)-dependent mechanisms located at either the cytosol or the nucleus.

An extracellular sulfhydryl group modulates background Na(+) conductance and cytosolic Ca(2+) in pituitary cells

Treatment of GH(3) pituitary cells with p-chloromercurybenzenesulfonate (PCMBS) increased the cytosolic Ca(2+) concentration ([Ca(2+)](i)). This effect was reversed by dithiothreitol and blocked by L-type Ca(2+) channel antagonists or Na(+) removal. PCMBS increased membrane conductance and depolarized the plasma membrane. Apart from minor effects on K(+) and Ca(2+) channels, PCMBS increased (6 times at -80 mV) an inward Na(+) current whose properties were similar to those of a background Na(+) conductance (BNC) described previously, necessary for generation of spontaneous electrical activity. In rat lactotropes and somatotropes in primary culture, PCMBS also produced a Na(+)-dependent [Ca(2+)](i) increase, whereas little or no effect was observed in thyrotropes, corticotropes, and gonadotropes. The Na(+) conductance elicited by PCMBS in somatotropes seemed to be the same as that stimulated by the hypothalamic growth hormone (GH)-releasing hormone, which regulates membrane excitability and GH secretion. The BNC studied here could play a physiological role, regulating excitability and spontaneous activity, and explains satisfactorily the [Ca(2+)](i)-increasing actions of the mercurials reported previously in several excitable tissues.

Constitutive expression of heterologous N-methyl-D-aspartate receptor subunits in rat adrenal medulla

An RT-PCR analysis revealed constitutive expression of mRNA for N-methyl-D-aspartate (NMDA) receptor (NR)-1, NR-2C, and NR-2D subunits in rat adrenal and pituitary glands, in addition to brain and retina. Constitutive expression of mRNA was detected for the NR-2A subunit in pituitary but not adrenal gland. Although on Western blotting assays adrenal medulla exhibited expression of NR-1 subunit protein without expression of NR-2C and NR-2D subunit proteins, an immunohistochemical investigation clearly showed selective localization of proteins for NR-1, NR-2C, and NR-2D subunits in adrenal medulla but not in adrenal cortex. Prior treatment with different glycosidases invariably resulted in a marked increase in immunoreactivity to the anti-NR-1 antibody in both hippocampus and adrenal medulla. An intraperitoneal injection of a blocker of NMDA receptor channel, but not NMDA itself, led to marked potentiation of DNA binding activity of the transcription factor activator protein-1 in adrenal nuclear extracts 2 hr after administration. These results suggest that heteromeric NMDA receptor channels may be constitutively and functionally expressed with glycosylation of NR-1 subunit under the influence of tonic stimulation by circulating agonists such as L-glutamate in rat adrenal medulla.

Insect NMDA receptors mediate juvenile hormone biosynthesis

In vertebrates, the N-methyl-D-aspartate subtype of glutamate receptors (NMDAR) appears to play a role in neuronal development, synaptic plasticity, memory formation, and pituitary activity. However, functional NMDAR have not yet been characterized in insects. We have now demonstrated immunohistochemically glutamatergic nerve terminals in the corpora allata of an adult female cockroach, Diploptera punctata. Cockroach corpus allatum (CA) cells, exposed to NMDA in vitro, exhibited elevated cytosolic [Ca(2+)], but not in culture medium nominally free of calcium or containing NMDAR-specific channel blockers: MK-801 and Mg(2+). Sensitivity of cockroach corpora allata to NMDA changed cyclically during the ovarian cycle. Highly active glands of 4-day-old mated females, exposed to 3 microM NMDA, produced 70% more juvenile hormone (JH) in vitro, but the relatively inactive glands of 8-day-old mated females showed little response to the agonist. The stimulatory effect of NMDA was eliminated by augmenting the culture medium with MK-801, conantokin, or high Mg(2+). Having obtained substantive evidence of functioning NMDAR in insect corpora allata, we used reverse transcription PCR to demonstrate two mRNA transcripts, DNMDAR1 and DNMDAR2, in the ring gland and brain of last-instar Drosophila melanogaster. Immunohistochemical labeling, using mouse monoclonal antibody against rat NMDAR1, showed that only one of the three types of endocrine cells in the ring gland, CA cells, expressed rat NMDAR1-like immunoreactive protein. This antibody also labeled two brain neurons in the lateral protocerebrum, one neuron per brain hemisphere. Finally, we used the same primers for DNMDAR1 to demonstrate a fragment of putative NMDA receptor in the corpora allata of Diploptera punctata. Our results suggest that the NMDAR has a role in regulating JH synthesis and that ionotropic-subtype glutamate receptors became specialized early in animal evolution.

Mitochondrial [Ca(2+)] oscillations driven by local high [Ca(2+)] domains generated by spontaneous electric activity

Mitochondria take up calcium during cell activation thus shaping Ca(2+) signaling and exocytosis. In turn, Ca(2+) uptake by mitochondria increases respiration and ATP synthesis. Targeted aequorins are excellent Ca(2+) probes for subcellular analysis, but single-cell imaging has proven difficult. Here we combine virus-based expression of targeted aequorins with photon-counting imaging to resolve dynamics of the cytosolic, mitochondrial, and nuclear Ca(2+) signals at the single-cell level in anterior pituitary cells. These cells exhibit spontaneous electric activity and cytosolic Ca(2+) oscillations that are responsible for basal secretion of pituitary hormones and are modulated by hypophysiotrophic factors. Aequorin reported spontaneous [Ca(2+)] oscillations in all the three compartments, bulk cytosol, nucleus, and mitochondria. Interestingly, a fraction of mitochondria underwent much larger [Ca(2+)] oscillations, which were driven by local high [Ca(2+)] domains generated by the spontaneous electric activity. These oscillations were large enough to stimulate respiration, providing the basis for local tune-up of mitochondrial function by the Ca(2+) signal.

Expression of GluR6/7 subunits of kainate receptors in rat adenohypophysis

We have previously demonstrated the presence of unidentified [3H]glutamate (Glu) binding sites with stereo-selectivity, high affinity and saturability in rat peripheral excitable tissues such as the pituitary (Yoneda, Y., Ogita, K., 1986a. [3H]Glutamate binding sites in the rat pituitary. Neurosci. Res. 3, 430--435) and adrenal (Yoneda, Y., Ogita, K., 1986b. Localization of [3H]glutamate binding sites in rat adrenal medulla. Brain Res. 383, 387--391, 1986). In this study, peripheral binding sites were further evaluated for the ionotropic Glu receptor subtype insensitive to N-methyl-D-aspartate by using reverse transcription polymerase chain reaction (RT-PCR) and Western blotting, in addition to receptor binding using radiolabeled ligands other than [3H]Glu. Binding of [3H]kainate (KA) and [3H]DL-alpha-amino-3-hydroxy-5-methylisoxazole-4-propionate was detected in membrane preparations obtained from the rat pituitary and adrenal irrespective of prior treatment with Triton X-100. An RT-PCR analysis revealed constitutive expression of mRNA for GluR1, GluR3, GluR5, KA1 and KA2 subunits in the rat adrenal and pituitary, as well as the brain and retina. The pituitary also expressed mRNA for GluR2, GluR4, GluR6 and GluR7 subunits in contrast to the adrenal. Under our experimental conditions employed, however, Western blotting assays failed to confirm the expression of receptor proteins for GluR1, GluR2/3 and GluR4 subunits in the adrenal cortex, adrenal medulla, adenohypophysis and neurohypophysis. Immunoreactive GluR6/7 subunits were only detectable in the adenohypophysis, but not in the adrenal cortex, adrenal medulla and neurohypophysis. An intraperitoneal injection of KA doubled DNA binding activity of the nuclear transcription factor activator protein-1 in the rat pituitary, with concomitant more potent potentiation of that in the hippocampus. These results suggest that GluR6/7 subunits of KA receptors may be constitutively expressed with responsiveness to the systemic administration of an agonist at least in the rat adenohypophysis.

Glutamate receptors in peripheral tissues: current knowledge, future research, and implications for toxicology

We illustrate the specific cellular distribution of different subtypes of glutamate receptors (GluRs) in peripheral neural and non-neural tissues. Some of the noteworthy locations are the heart, kidney, lungs, ovary, testis and endocrine cells. In these tissues the GluRs may be important in mediating cardiorespiratory, endocrine and reproductive functions which include hormone regulation, heart rhythm, blood pressure, circulation and reproduction. Since excitotoxicity of excitatory amino acids (EAAs) in the CNS is intimately associated with the GluRs, the toxic effects may be more generalized than initially assumed. Currently there is not enough evidence to suggest the reassessment of the regulated safety levels for these products in food since little is known on how these receptors work in each of these organs. More research is required to assess the extent that these receptors participate in normal functions and/or in the development of diseases and how they mediate the toxic effects of EAAs. Non-neural GluRs may be involved in normal cellular functions such as excitability and cell to cell communication. This is supported by the wide distribution in plants and animals from invertebrates to primates. The important tasks for the future will be to clarify the multiple biological roles of the GluRs in neural and non-neural tissues and identify the conditions under in which these are up- or down-regulated. Then this could provide new therapeutic strategies to target GluRs outside the CNS.

Genes expressed in the mouse pituitary corticotrope AtT-20/D-16v tumor cell line

The pituitary corticotrope AtT-20 stable cell line has been used as a model system to study peptide secretion, glucocorticoid regulation, and several other processes. In order to better understand this model cell line, a phage cDNA library was generated from AtT-20/D-16v cell mRNA and cDNA sequences were obtained for 317 clones representing 203 known genes and 48 novel cDNAs. The sequencing results revealed the prevalence of the mouse leukemia virus in this cell line and also identified a number of putatively secreted molecules that were not previously recognized as being secreted from AtT-20/D-16v cells or pituitary corticotropes. Nine completely novel cDNAs and 39 cDNAs homologous to known ESTs were also identified. A listing of other genes known to be expressed in AtT-20/D-16v cells is also provided.

d-Aspartate in a prolactin-secreting clonal strain of rat pituitary tumor cells (GH(3))

d-Aspartate (d-Asp) is found in prolactin (PRL)-containing cells of the rat anterior pituitary gland [Lee et al., Brain Res. 838, 193-199, 1999]. In order to determine whether d-Asp is actually produced by the anterior pituitary gland and whether it plays a physiological role in PRL function, a PRL-secreting clonal strain of rat pituitary tumor cells (GH(3)) was employed in this study. HPLC analysis and immunocytochemical staining detected the presence and synthesis of d-Asp in the cytoplasm of these cells. In addition, thyrotropin-releasing hormone-stimulated PRL secretion was increased in a dose-dependent fashion by d-Asp from these cells. These results suggest that the anterior pituitary gland synthesizes d-Asp and that d-Asp acts as a messenger in this gland.

Distribution of the glutamate transporters GLAST and GLT-1 in rat circumventricular organs, meninges, and dorsal root ganglia

The glial glutamate transporters GLAST and GLT-1 are primarily responsible for the removal of glutamate from brain extracellular fluid. This study compares the distribution of GLAST and GLT-1 expression in the circumventricular organs of the brain, in the meninges, and in the dorsal root ganglion. By using a highly sensitive nonisotopic in situ hybridization method and immunostaining, we demonstrate marked differences in the expression patterns for the two transporters. In the three sensory circumventricular organs that contain neuronal elements, i.e., the subfornical organ, the vascular organ of the lamina terminalis, and the area postrema, GLAST is strongly expressed, whereas GLT-1 is faintly expressed or absent. Both transporters are absent from the choroid plexus, and only GLAST mRNA is found in the subcommisural organ. In the pineal gland, GLAST is expressed by astrocytic cells near the pineal stalk, whereas GLT-1 is expressed by pinealocytes throughout the gland. In the pituitary gland, GLAST is likely expressed by folliculo-stellate cells in the anterior lobe, by a group of astrocyte-like cells and by marginal cells in the intermediate lobe, and by pituicytes in the posterior lobe, whereas GLT-1 is expressed only by the astrocyte-like cells in the intermediate lobe. Finally, GLAST, but not GLT-1, is expressed by specific layers of the meninges, and by satellite cells in the dorsal root ganglion. These results show that GLAST is the primary glutamate transporter in the circumventricular organs. The data provide further evidence that these two glutamate transporters fulfill markedly different functions in the nervous system.

Potential target sites in peripheral tissues for excitatory neurotransmission and excitotoxicity

Glutamate receptors (GluRs) are ubiquitously present in the central nervous system (CNS) as the major mediators of excitatory neurotransmission and excitotoxicity. Neural injury associated with trauma, stroke, epilepsy, and many neurodegenerative diseases such as Alzheimer's, Huntington's, and Parkinson's diseases and amyotrophic lateral sclerosis may be mediated by excessive activation of GluRs. Neurotoxicity associated with excitatory amino acids encountered in food, such as domoic acid and monosodium glutamate, has also been linked to GluRs. Less is known about GluRs outside the CNS. Recent observations suggest that several subtypes of GluRs are widely distributed in peripheral tissues. Using immunochemical and molecular techniques, the presence of GluR subtypes was demonstrated in the rat and monkey heart, with preferential distribution within the conducting system, nerve terminals, and cardiac ganglia. GluR subtypes NMDAR 1, GluR 2/3, and mGluR 2/3 are also present in kidney, liver, lung, spleen, and testis. Further investigations are needed to assess the role of these receptors in peripheral tissues and their importance in the toxicity of excitatory compounds. Therefore, food safety assessment and neurobiotechnology focusing on drugs designed to interact with GluRs should consider these tissues as potential target/effector sites.

Glutamate-induced rise in cytosolic calcium concentration stimulates in vitro rates of juvenile hormone biosynthesis in corpus allatum of Diploptera punctata

We show that in a cockroach, Diploptera punctata, endocrine function of the corpus allatum may be modulated by L-glutamate, a major fast excitatory neurotransmitter in the central nervous system of vertebrates and invertebrates. The widely accepted concept that synthesis of juvenile hormone (JH) depends upon intracellular calcium concentration, is extended by the finding that 60 and 100 microM L-glutamate induces both an increase in calcium concentration in the cytosol of corpus allatum cells, and stimulates JH synthesis in vitro. We show that L-glutamate stimulates JH synthesis by inducing calcium influx since in calcium-free medium the stimulatory effect is not observed. Furthermore, the non-specific glutamate-receptor antagonist, 100 microM kynurenate, and 1.8 mM magnesium, inhibit the stimulatory effect of L-glutamate on JH synthesis in vitro. These results suggest that functional ionotropic glutamate receptors are present on the surface of the cells in corpus allatum, and that rates of JH are at least in part regulated via these receptors.

Choline uptake across the ventricular membrane of neonate rat choroid plexus

The uptake of [3H]choline from the cerebrospinal fluid (CSF) side of the rat neonatal choroid plexus was characterized in primary cultures of the choroidal epithelium grown on solid supports. Cell-to-medium concentration ratios were approximately 5 at 1 min and as high as 70 at 30 min. Apical choline uptake was facilitated; the Km was approximately 50 microM. Several organic cations (e.g., hemicholinium-3 and N1-methylnicotinamide) inhibited uptake. The reduction or removal of external Na+ or the addition of 5 mM LiCl had no effect on uptake. However, increasing external K+ concentration from 3 to 30 mM depolarized ventricular membrane potential (-70 to -15 mV) and reduced uptake to 45% of that for the control. Treatment with 1 mM ouabain or 2 mM BaCl2 reduced uptake 45%, and intracellular acidification reduced uptake to approximately 90% of that for controls. These data indicate that the uptake of choline from CSF across the ventricular membrane of the neonatal choroidal epithelium is not directly coupled to Na+ influx but is sensitive to plasma membrane electrical potential.

Glucocorticoids induce glutamine synthetase in folliculostellate cells of rat pituitary glands in vivo and in vitro

Glutamine synthetase (GS) is a glucocorticoid-inducible enzyme that has a key role for glutamate metabolism in the central and peripheral nervous system. In this study GS activity was measured and the amount of immunoreactive GS (ir-GS) cells in the rat anterior pituitary gland was quantified as a function of age. In addition, the effects of GS inhibitors, glucocorticoid administration, and adrenalectomy on GS activity were examined. Some of the ir-GS cells were also immunoreactive for S100 protein (ir-S100) which is a known marker for folliculostellate cells (FS) in the anterior pituitary. FS cells expressing GS were first detected in 3-d-old rats, and this cell population, expressed as the immunostained cell area divided by a standard unit area, increased as a function of age. The percentages of FS cells also expressing GS were 0.2, 6.4, 25 and 74% at 3 d, 30 d, 60 d and 2 y of age, respectively. GS enzyme activity also increased in parallel with the increase of ir-GS cell population maturation. The subcutaneous injection of methionine sulphoximine, a GS and gamma-glutamylcysteine synthetase inhibitor, reduced pituitary GS activity by 83%, but increased the population of ir-GS cells 3.5-fold in 30-d-old rats. Buthionine sulphoximine, a specific inhibitor of y-glutamylcysteine synthetase, had little effect on GS activity or the ir-GS cell population. Neither methionine sulphoximine nor buthionine sulphoximine changed the population of ir-S100 protein cells (FS cells). Dexamethasone and hydrocortisone increased the population of ir-GS cells by 3.1 and 4.2-fold, respectively, within 12 h after administration. A significant increase of GS activity due to the injection of glucocorticoids was observed in the anterior pituitary, but not in the brain, retina or liver of immature rats. Adrenalectomy did not cause decrease of pituitary GS activity, and dexamethasone administration increased GS activity in both adrenalectomised and intact rats. In the monolayer culture of anterior pituitary cells, glucocorticoids increased GS activity by x 1.5, and methionine sulphoximine reduced the activity by over 94%. These results demonstrate that GS in folliculostellate cells is a glucocorticoid-inducible enzyme in vivo and in vitro, and that the age-dependent increase of GS activity is independent of endogenous adrenal glucocorticoids.

Regulation of growth hormone secretion by alpha-amino-3-hydroxy-5-methylisoxazole-4-propionic acid receptors in infantile, prepubertal, and adult male rats

Excitatory amino acids, such as glutamate, constitute a major transmitter system in the control of hypothalamic-pituitary function. Different subtypes of glutamate receptors, such as N-methyl-D-aspartic acid and kainate receptors, have been involved in the control of GH secretion. Other excitatory amino acid receptor subtypes, as alpha-amino-3-hydroxy-5-methylisoxazole-4-propionic acid (AMPA), amino-4-phosphobutyric acid, and metabotropic receptors, have been identified, yet their role in the control of neuroendocrine function remains to be completely characterized. The purpose of this study was to assess the potential involvement of AMPA receptors in the control of GH secretion. In a first set of experiments, neonatal (5 and 10 days) and prepubertal (23 days) male rats were injected with AMPA (1, 2.5, or 5 mg/kg) or the antagonist of AMPA receptors, 1,2,3,4-tetrahydro-6-nitro-2,3-dioxo-benzo(f)quinoxaline-7-sulfonamide (NBQX; 0.25 or 0.50 mg/kg). Serum GH concentrations significantly increased 15 min after i.p. administration of AMPA in both neonatal and prepubertal male rats. In addition, serum GH concentrations decreased after NBQX treatment. The stimulatory effect of AMPA was abolished by pretreatment with the blocker of nitric oxide synthase, nitro(w)-arginine-methyl ester (40 mg/kg), and was partially counteracted by the simultaneous administration of GH-releasing hormone (500 microg/kg). Moreover, AMPA was unable to elicit in vitro GH secretion by hemipituitaries from prepubertal males, pointing out that the hypothalamus is probably the site of action for the reported stimulatory action of AMPA on GH release. In a second set of experiments, the effects of AMPA and NBQX were tested in adult male rats. As in prepubertal animals, AMPA significantly increased GH secretion in adult males, whereas NBQX (20 or 40 nmol), administered through intracerebroventricular injection, induced a significant decrease in the amplitude of GH pulses. In conclusion, our data indicate that AMPA receptors have a physiological stimulatory role in the control of GH secretion in male rats throughout the life span. This effect depends on appropriate nitric oxide synthesis during the prepubertal age. In addition, AMPA receptors appear to modulate pulsatile GH secretion in adulthood.

Excitatory amino acid regulation of gonadotropin secretion in prepubertal heifer calves

The mechanisms controlling the pulsatile release of gonadotropins in prepubertal heifers are not completely understood. We examined the role of excitatory amino acid neurotransmitters, via activation of N-methyl-D-aspartate (NMDA) receptors, in the control of pulsatile LH and FSH release during prepubertal development in heifers. Hereford heifer calves received 4.7 mg/kg of N-methyl-D,L-aspartic acid (NMA), a potent NMDA receptor agonist (n = 5, i.v.), or saline (n = 5, i.v.), as single doses, at 4, 8, 12, 24, 36, and 48 wk of age. Blood samples were collected every 15 min, for 1 h before and 9 h after injection, on the days of treatment. Injection of NMA resulted in an acute release of LH (p < 0.001) in 0, 3, 3, 4, 5, and 5 calves (p < 0.01) and of FSH (p < 0.001) in 0, 1, 2, 4, 3, and 2 calves at 4, 8, 12, 24, 36, and 48 wk of age, respectively. The peak response of LH and FSH release to NMA was at 15 min posttreatment, and these peak responses were highest at 36 wk of age (p < 0.05). We suggest that neuroexcitatory amino acids, through NMDA receptors, are involved in prepubertal development of LH and FSH secretion in heifer calves.

Functional measurements of [Ca2+] in the endoplasmic reticulum using a herpes virus to deliver targeted aequorin

Changes in the free calcium concentration of the endoplasmic reticulum ([Ca2+]er) play a central role controlling cellular functions like contraction, secretion or neuronal signaling. We recently reported that recombinant aequorin targeted to the endoplasmic reticulum (ER) [Montero M., Brini M., Marsault R. et al. Monitoring dynamic changes in free Ca2+ concentration in the endoplasmic reticulum of intact cells. EMBO J 1995; 14: 5467-5475, Montero M., Barrero M.J., Alvarez J. [Ca2+] microdomains control agonist-induced Ca2+ release in intact cells. FASEB J 1997; 11: 881-886] can be used to monitor selectively [Ca2+]er in intact HeLa cells. Here we have used a herpes simplex virus type 1 (HSV-1) based system to deliver targeted aequorin into a number of different cell types including both postmitotic primary cells (anterior pituitary cells, chromaffin cells and cerebellar neurons) and cell lines (HeLa, NIH3T3, GH3 and PC12 cells). Functional studies showed that the steady state lumenal [Ca2+]er ranged from around 300 microM in granule cells to 800 microM in GH3 cells. InsP3-coupled receptor stimulation with agonists like histamine (in HeLa, NIH3T3 and chromaffin cells), UTP and bradykinin (in PC12 cells) or thyrotropin-releasing hormone (TRH, in GH3 cells) produced a very rapid decrease in lumenal [Ca2+]er. Caffeine caused a rapid Ca2+ depletion of the ER in chromaffin cells, but not in the other cell types. Depolarization by high K+ produced an immediate and reversible increase of [Ca2+]er in all the excitable cells (anterior pituitary, GH3, chromaffin cells and granule neurons). We conclude that delivery of recombinant aequorin to the ER using HSV amplicon provides the first direct quantitative and dynamic measurements of [Ca2+]er in several primary non-dividing cells.

Molecular and immunochemical characterization of the ionotropic glutamate receptors in the rat heart

Excitatory amino acids (EAA) and glutamate receptors (GluRs) play a fundamental role in the central nervous system (CNS). Ionotropic glutamate receptors (iGluRs) are coupled to ion channels, which are classified according to their most selective agonists. These ligand-gated channels are permeable to Na+, K+, and Ca+. Interaction of EAA receptor is linked to Ca+2/Na+ influx. Influx changes lead to an action potential, which in the heart is transmitted along the cardiocyte membrane. Furthermore, the heart has a rich innervation and specialized conduction system for rapid conduction and regulation of cardiac rhythmicity. Availability of EAA receptors in the heart might be important for cardiac function. The following GluRs were cloned by isoform-specific RT-PCR from rat heart ribonucleic acid (RNA): GluR 1, GluR 3, GluR 4, GIuR 7, Ka 1, and Ka 2. Expression in cardiac tissue was confirmed by western (for anti-GluR 2/3) and northern blots (for GluR 3, NMDAR 1, and Ka 2). The anatomical distribution was investigated by immunohistochemistry. Antibodies to GluR 2/3, GluR 5/6/7, Ka 2, and NMDAR 1 showed the strongest signals. These signals were specifically localized to cardiac nerve terminals, ganglia, conducting fibers, and some to myocardiocytes particularly in the atrium. Each antibody had a specific pattern of distribution. This anatomical localization suggests that they might play a role in cardiac electrophysiology and pathology.

Further insights into the anti-aggregating activity of NMDA in human platelets

1. In the present study the effect of N-methyl-D-aspartate (NMDA) on thromboxane B2 synthesis and on [Ca2+]i was studied in human platelets. 2. NMDA (10(-7) M) completely inhibited the synthesis of thromboxane B2 from exogenous arachidonic acid (AA), while it did not interfere with the aggregating effect of the thromboxane A2 receptor agonist U-46619. 3. NMDA (0.1 microM - 10 microM) dose-dependently increased intracellular calcium in washed platelets preloaded with fura 2 AM, and this effect was not additive with that of AA. 4. NMDA shifted the dose-response curve of AA to the right. At the highest AA concentrations platelet aggregation was not inhibited. 5. The antiaggregating effect of NMDA was not antagonized by N(G)-monomethyl-L-arginine (L-NMMA), a nitric oxide synthase (NOS) inhibitor. 6. Finally, NMDA (0.01 nM - 100 nM) associated with either aspirin or indomethacin significantly potentiated the antiaggregating activity of both cyclo-oxygenase inhibitors. 7. It was concluded that NMDA is a potent inhibitor of platelet aggregation and thromboxane B2 synthesis in human platelet rich plasma (PRP).