Solubilization of stereospecific and quisqualate-sensitive activity of [3H]glutamate binding in the pituitary of the rat

GluR5-mediated glutamate signaling regulates hypothalamo-pituitary-adrenocortical stress responses at the paraventricular nucleus and median eminence

Glutamate is the main excitatory neurotransmitter in the central nervous system, and plays an excitatory role in generation of hypothalamic-pituitary-adrenocortical (HPA) axis responses to stress. The current study assesses the role of kainate-preferring receptors in glutamatergic excitation of the HPA axis. In situ hybridization and immunohistochemical analyses confirmed the existence of the GluR5 kainate subunit in the paraventricular nucleus of the hypothalamus (PVN). Importantly, GluR5 immunoreactivity was enriched in the external lamina of the median eminence, where it is co-localized with corticotropin releasing hormone (CRH). Intra-PVN infusion of LY382884 increased plasma adrenocorticotropin (ACTH), corticosterone and PVN c-Fos immunoreactivity. Infusions of LY382884 into the median eminence region, on the other hand, reduced restraint induced ACTH release without altering c-Fos expression. Together, these findings provide evidence for glutamate-mediated signaling in control of CRH release at the PVN and median eminence, mediated by way of kainate-preferring receptor complexes.

Glutamate signaling in peripheral tissues

The hypothesis that l-glutamate (Glu) is an excitatory amino acid neurotransmitter in the mammalian central nervous system is now gaining more support after the successful cloning of a number of genes coding for the signaling machinery required for this neurocrine at synapses in the brain. These include Glu receptors (signal detection), Glu transporters (signal termination) and vesicular Glu transporters (signal output through exocytotic release). Relatively little attention has been paid to the functional expression of these molecules required for Glu signaling in peripheral neuronal and non-neuronal tissues; however, recent molecular biological analyses show a novel function for Glu as an extracellular signal mediator in the autocrine and/or paracrine system. Emerging evidence suggests that Glu could play a dual role in mechanisms underlying the maintenance of cellular homeostasis - as an excitatory neurotransmitter in the central neurocrine system and an extracellular signal mediator in peripheral autocrine and/or paracrine tissues. In this review, the possible Glu signaling methods are outlined in specific peripheral tissues including bone, testis, pancreas, and the adrenal, pituitary and pineal glands.

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.

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.

Consolidation of transient ionotropic glutamate signals through nuclear transcription factors in the brain

Long-lasting alterations of neuronal functions could involve mechanisms associated with consolidation of transient extracellular signals through modulation of de novo synthesis of particular functional proteins in the brain. In eukaryotes, protein de novo synthesis is mainly under the control at the level of gene transcription by transcription factors in the cell nucleus. Transcription factors are nuclear proteins with an ability to recognize particular core nucleotides at the upstream and/or downstream of target genes, and thereby to modulate the activity of RNA polymerase II that is responsible for the formation of mRNA from double stranded DNA. Gel retardation electrophoresis is widely employed for conventional detection of DNA binding activities of a variety of transcription factors with different protein motifs. Extracellular ionotropic glutamate (Glu) signals lead to rapid and selective potentiation of DNA binding of the nuclear transcription factor activator protein-1 (AP1) that is a homo- and heterodimeric complex between Jun and Fos family members, in addition to inducing expression of the corresponding proteins, in a manner unique to each Glu signal in murine hippocampus. Therefore, extracellular Glu signals may be differentially transduced into the nucleus to express AP1 with different assemblies between Jun and Fos family members, and thereby to modulate de novo synthesis of the individual target proteins at the level of gene transcription in the hippocampus. Such mechanisms may be operative on synaptic plasticity as well as delayed neuronal death through consolidation of alterations of a variety of cellular functions induced by transient extracellular signals in the brain.

Characteristic localization of non-NMDA type glutamate receptor subunits in the rat pituitary gland

While the involvement of the glutamate receptors in the hypothalamo-hypophyseal system has been clarified at the hypothalamic level, the existence of glutamate receptors in the pituitary gland has remained obscure. We investigated the localization of the glutamate receptors, the non-NMDA type receptor subunits (GluR1-4) in particular, by immunocytochemistry using specific antibodies. The antibodies specific to GluR1, GluR2/3 and GluR4 exhibited the characteristic localization of the receptor molecules in each lobe of the pituitary gland. GluR1- and GluR2/3-positive cells were identified in the anterior and intermediate lobe, and intense terminals of GluR4 and weak terminals of GluR2/3 were observed in the posterior lobe. Such immunoreactivity appeared to be at the axonal terminal of the neurosecretory magnocellular cells. This was confirmed by in situ hybridization histochemistry using specific oligodeoxynucleotide probes and by immunocytochemistry in the neurosecretory magnocellular neurons. The GluR4 mRNA-positive signal and GluR4 immunoreactivity were abundantly observed in magnocells of paraventricular and supraoptic nuclei. In addition to the positive fibers, some pituicytes in the posterior lobe exhibited GluR2/3 immunoreactivity. This suggests that pituicytes have non-NMDA type glutamate receptors. Thus, present study suggests that some anterior pituitary cells and pituicytes in the neural lobe are regulated by the glutamate.

Characterization of a [3H]glutamate binding site in rat pineal gland: enhanced affinity following superior cervical ganglionectomy

Glutamate, an excitatory neurotransmitter/neuromodulator involved in cell-to-cell communication within the central nervous system, is now believed to play a role in neuroendocrine function. In this study we describe a single, saturable, stereospecific, and temperature-, time-, and pH-dependent binding site for glutamate in the pineal gland of the rat (Kd = 612 +/- 23 nM, Bmax = 3.17 +/- 0.33 pmol/mg protein). After removal of the sympathetic innervation to the pineal gland, [3H]glutamate binding displayed a higher apparent affinity (Kd = 412 +/- 28 nM) (P < 0.05) without a change in binding site number (Bmax = 3.60 +/- 0.24 pmol/mg protein). No difference in [3H]glutamate binding site number was observed in pineal glands obtained from animals sacrificed during the middle of the light and dark periods. These data suggest a possible modulatory role for a glutamate binding site in pineal gland function.

Biochemical, autoradiographic and functional studies on a unique glutamate binding site in adrenal gland

L-Glutamate is known to function as a major excitatory neurotransmitter in the mammalian central nervous system, and recent reports suggest the existence of receptors for glutamate in several peripheral tissues. In the present study, the characteristics of the binding of [3H]L-glutamate to sections of bovine adrenal gland were studied, and the localisation of this binding was investigated in adrenal glands from cow, dog, rat and guinea pig. In addition, the effects of glutamate on catecholamine release from the perfused isolated bovine adrenal gland were investigated. Binding of [3H]L-glutamate to slide-mounted sections of bovine adrenal gland was of high affinity (Kd 0.4 microM), rapid, saturable, reversible, stereospecific and to a single population of sites. The pharmacological profile of this binding site appeared to be unique, and did not correspond to any of the central receptor subtypes for glutamate so far identified. In the adrenal gland of the cow, rat and guinea pig, the binding density of [3H]L-glutamate was higher in cortex than medulla, while this pattern was reversed in the canine adrenal gland. Glutamate had no effect on the basal secretion of noradrenaline or adrenaline from the perfused isolated bovine adrenal gland, and neither glutamate nor the glutamate receptor antagonist kynurenate altered the nicotine-stimulated release of these catecholamines. These results suggest the existence of a novel peripheral binding site for glutamate in the adrenal gland. The differential autoradiographic localisation of this binding site in the adrenal glands of the various species studied may reflect different functional properties of glutamate in these species, and suggests possible roles for glutamate in the modulation of adrenal function.

Excitatory amino acid receptors in the mammalian periphery

Aspartate and glutamate occur ubiquitously in free and chemically bound forms and have been considered primarily as substances of metabolic relevance. This focus has changed with the more recent discovery of their specific role as excitatory synaptic transmitters in the mammalian CNS. Enthusiasm for this concept has overshadowed the possibility that glutamate and aspartate may also have specific, receptor-mediated functions in the periphery. In this review, Sándor Erdö summarizes the current knowledge of excitatory amino acid (EAA) receptors outside the CNS, through which EAAs may modulate various functions in peripheral organs and tissues.

Neurochemical aspects of the N-methyl-D-aspartate receptor complex

The N-methyl-D-aspartic acid (NMDA)-sensitive subclass of brain excitatory amino acid receptors is supposed to be a receptor-ionophore complex consisting of at least 3 different major domains including an NMDA recognition site, glycine (Gly) recognition site and ion channel site. Biochemical labeling of the NMDA domain using [3H]L-glutamic acid (Glu) as a radioactive ligand often meets with several critical methodological pitfalls and artifacts that cause a serious misinterpretation of the results. Treatment of brain synaptic membranes with a low concentration of Triton X-100 induces a marked disclosure of [3H]Glu binding sensitive to displacement by NMDA with a concomitant removal of other several membranous constituents with relatively high affinity for the neuroactive amino acid. The NMDA site is also radiolabeled by the competitive antagonist (+/-)-3-(2-carboxypiperazin-4-yl)propyl-1-phosphonic acid that reveals possible heterogeneity of the site. The Gly domain is sensitive to D-serine and D-alanine but insensitive to strychnine, and this domain seems to be absolutely required for an opening of the NMDA channels by agonists. The ionophore domain is radiolabeled by a non-competitive type of NMDA antagonist that is only able to bind to the open but not closed channels. The binding of these allosteric antagonists is markedly potentiated by NMDA agonists in a manner sensitive to antagonism by isosteric antagonists in brain synaptic membranes and additionally enhanced by further inclusion of Gly agonists through the Gly domain. Furthermore, physiological and biochemical responses mediated by the NMDA receptor complex are invariably potentiated by several endogenous polyamines, suggesting a novel polyamine site within the complex. At any rate, activation of the NMDA receptor complex results in a marked influx of Ca2+ as well as Na+ ions, which subsequently induces numerous intracellular metabolic alterations that could be associated with neuronal plasticity or excitotoxicity. Therefore, any isosteric and allosteric antagonists would be of great benefit for the therapy and treatment of neurodegenerative disorders with a risk of impairing the acquisition and formation process of memories.

Profiles of [3H]N-[1-(2-thienyl)cyclohexyl]piperidine binding in brain synaptic membranes treated with Triton X-100

Binding of [3H]N-[1-(2-thienyl)cyclohexyl]piperidine (TCP) was examined using rat brain synaptic membranes treated with a low concentration of Triton X-100. This compound is assumed to be a non-competitive antagonist for the N-methyl-D-aspartate(NMDA)-sensitive subclass of central excitatory amino acid receptors. Binding was quite low but detectable in Triton-treated membranes irrespective of the incubation temperature, and the temperature-dependent portion of the binding was greatly reduced in these Triton-treated membranes. However, binding was drastically potentiated by the inclusion of L-glutamate and its analogous amino acids in a concentration-dependent manner at a concentration range of 10 nM to 0.1 mM. Agonists for the NMDA-sensitive subclass also potentiated binding, with agonists for the other subclasses being ineffective. Glycine at a concentration above 10 nM was not only effective as a stimulant of potentiated binding by glutamate, but was also active in enhancing binding in the absence of added glutamate. Glycine increased both the association and dissociation rates without significantly affecting the dissociation constant. Pharmacological profiles of binding in Triton-treated membranes were not significantly different from those in untreated membranes, except for that of haloperidol. Haloperidol is proposed to be highly selective for brain sigma-receptors on the basis of a potent inhibition of sigma-receptor binding. The inhibitory potency of this sigma-ligand was markedly attenuated in the presence of both glutamate and glycine in Triton-treated membranes, as compared with that in untreated membranes. These results suggest that [3H]TCP binding in Triton-treated membranes is a useful biochemical tool to evaluate predominantly the activated state of ion channels associated with the NMDA-sensitive receptors in terms of freedom from the confounding effects of endogenous amino acids.