Modulation of glutamate receptor functions by glutathione

In addition to its well-known antioxidant effects, glutathione apparently has an additional double role in the central nervous system as a neurotransmitter and neuromodulator. A number of recent neurochemical, neuropharmacological and electrophysiological studies have yielded evidence on both functions. As an excitatory neurotransmitter, glutathione depolarizes neurons by acting as ionotropic receptors of its own which are different from any other excitatory amino acid receptors. As a neuromodulator, it displaces ionotropic glutamate receptor ligands from their binding sites and regulates calcium influx through N-methyl-D-aspartate receptor-governed ionophores. In brain slices glutathione has been shown to regulate the release of other transmitters, e.g., gamma-aminobutyrate and dopamine, mediated by N-methyl-D-aspartate receptors. In the present article, we review recent findings on the neuromodulatory actions of glutathione and discuss possible physiological and pathophysiological consequences.     

Intrinsic and extrinsic neuromodulation of motor circuits

Neuromodulation of motor circuits by extrinsic inputs provides enormous flexibility in the production of behavior. Recent work has shown that neurons intrinsic to central pattern-generating circuits can evoke neuromodulatory effects in addition to their neurotransmitting actions. Modulatory neurons often elicit a multitude of different effects attributable to actions at different receptors and/or through the release of co-transmitters. Differences in neuromodulation between species can account for differences in behavior. Modulation of neuromodulation may provide an additional level of flexibility to motor circuits.     

Effects of dorsal root section and occlusion of dorsal spinal artery on the neurotransmitter candidates in rat spinal cord

In order to obtain further evidence of putative neurotransmitters in primary sensory neurons and interneurons in the dorsal spinal cord, we have studied the effects of unilateral section of dorsal roots and unilateral occlusion of the dorsal spinal artery on cholinergic enzyme activity and on selected amino acid levels in the spinal cord. One week after sectioning dorsal roots from caudal cervical (C₇) to cranial thoracic (T₂) levels, the specific activity of choline acetyltransferase (ChAT) was significantly decreased and acetylcholinesterase (AChE) showed a tendency to decrease in the dorsal quadrant on the operated side of the spinal cord. Dorsal root sectioning had little effect on the levels of free glutamic acid or other amino acids in the dorsal spinal cord. These results suggest that primary sensory neurons may include some cholinergic axons, and that levels of putative amino acid transmitters are not regulated by materials supplied by axonal transport from the dorsal root ganglia. By contrast, one week following unilateral occlusion of the dorsal spinal artery, the activities of ChAT and AChE were unchanged in the operated quadrant of the spinal cord, while decreases of Asp, Glu, and GABA, and an increase in Tau were detected. These findings are consistent with the proposals that such amino acids, but not ACh, may function as neurotransmitter candidates in interneurons of the dorsal spinal cord.Abbreviation used ACh acetylcholine - AChE acetylcholinesterase - Asp aspartic acid - ChAT choline acetyltransferase - GABA -aminobutyric acid - Glu glutamic acid - Gly glycine - SP substance P - Tau taurine     

Amino acids as central synaptic transmitters or modulators in mammalian thermoregulation

Of the amino acids that affect the activity of central neurons, aspartate and glutamate (which exert generally excitatory influences) and glycine, taurine, and gamma-aminobutyric acid (GABA) (which generally exert inhibitory influences) are the strongest neurotransmitter candidates. As with other putative transmitter substances, their effects on body temperature when injected into the cerebral ventricles or the preoptic hypothalamus tend to vary within and between species. These effects are uninterpretable without accompanying information regarding effector activity changes and the influences of dose and ambient temperature. Observations necessary for analysis of apparent action have been made in studies of the effects of intracerebroventricular injections of these amino acids into sheep. Aspartate and glutamate have similar excitatory effects on the neural pathways that activate both heat production and heat loss effectors. Glycine appears to be without effect.     

Amino acid neurotransmitters: separation approaches and diagnostic value

Amino acids in the central nervous system can be divided into non-neurotransmitter or neurotransmitter depending on their function. The measurement of these small molecules in brain tissue and extracellular fluid has been used to develop effective treatment strategies for neuropsychiatric and neurodegenerative diseases and for the diagnosis of such pathologies. Here we describe the separation and detection techniques that have been used for the measurement of amino acids at trace levels in brain tissue and dialysates. An overview of the function of amino acid transmitters in the brain is given. In addition, the type of sampling techniques that are used for the determination of amino acid levels in the brain is described.     

Glial strategy for metabolic shuttling and neuronal function

Glial cells serve a variety of functions in nervous systems, some of which are activated by neurotransmitters released from neurons. Glial cells respond to these neurotransmitters via receptors, but also take up some of the transmitters to help terminate the synaptic process. Among these, glutamate uptake by glial cells is pivotal to avoid transmitter-mediated excitotoxicity. Here, a new model is proposed in which glutamate uptake via the excitatory amino acid transporter (EAAT) is functionally coupled to other glial transporters, in particular the sodium-bicarbonate cotransporter (NBC) and the monocarboxylate transporter (MCT), as well as other glial functions, such as calcium signalling, a high potassium conductance and CO(2) consumption. The central issue of this hypothesis is that the shuttling of sodium ions and acid/base equivalents, which drive the metabolite transport across the glial membrane, co-operate with each other, and hence save energy. As a result, glutamate removal from synaptic domains and lactate secretion serving the energy supply to neurons would be facilitated and could operate with greater capacity.     

Weighing the role of hypothalamic feeding neurotransmitters

Growing health problems related to obesity have focused considerable attention on a number of neurotransmitters, particularly hypothalamic neuropeptides, involved in regulating energy homeostasis and food intake. As the fast-acting transmitters GABA and glutamate underlie the majority of fast synaptic activity in the hypothalamus, understanding neuropeptide modulation of amino acid transmitter actions may be key to a full appreciation of how the brain controls caloric balances.     

Histochemistry and functional significance of putative neocortical transmitter substances: a review

1. Intrinsic neuronal chains of the neocortex communicate most probably with amino acid transmitters. These involve both excitatory (glutamate, aspartate--Nadler et al. 1976) both inhibitory (GABA--Ribak 1978) amino acids, and ensure fast, ionotropic postsynaptic actions (Eccles, McGeer 1979). 2. Some interneurons of the neocortex seemingly operate with the peptide transmitter VIP (Lorén et al. 1979). Presumably, this is a metabotropic, slowly acting substance (Dodd, Kelly and Said 1979). 3. The existence of intrinsic cholinergic neurons in the neocortex is a matter of question (Krnjevic and Silver 1965). It is worth to mention that in the periphery, cholinergic terminals also contain and release VIP (H?kfelt et al. 1980). It is not known, whether this transmitter dualism can be found in neocortex, too. An ascending cholinergic system projecting from the basal forebrain to the neocortex exists and exerts profound influence on cortical function (Shute and Lewis 1967). 4. Diffusely terminating, ascending monoamine axons innervate the neocortex and modulate interneuronal transmission (Thiery et al. 1977; Morrison et al. 1981, Lidov et al. 1981). 5. The neuropeptide SP excites cortical neurons (Phillis and Limacher 1974), and its presence in thin axons can be demonstrated immunohistochemically (H?kfelt et al. 1976). 6. Neocortical efferents to the thalamus and striatum seemingly use glutamate or aspartate (Fonnum et al. 1981). The transmitters of other corticofugal projections are not known. 7. The transmitters of specific thalamic afferents and those of callosal and association projections are unknown, too. 8. The main task of future histochemistry is to explore the synaptology of neocortical neurons and afferent systems with identified or evidenced transmitters, viz. to explore the neurochemical subsystems of cortical organization. The tool for it could be the immunohistochemistry, and future development depends mainly on the synthesis and purification of suitable antigens. The knowledge on the synaptology of identified neurochemical units of the cortex would be the basis of the understanding at least partly of the pharmacological effects exerted by the putative neocortical transmitters.     

Steroid Hormone Actions on the Brain: When Is the Genome Involved?

It has become customary to distinguish between so-called "genomic" actions of steroid hormones involving intracellular receptors and "non-genomic" effects of steroids that involve putative cell surface receptors. Whereas there is no doubt that this distinction has considerable validity, it does not go far enough in addressing the variety of mechanisms that steroid hormones use to produce their effects on cells. This is because cell surface receptors may signal changes in gene expression, while genomic actions sometimes affect neuronal excitability, often doing so quite rapidly. Moreover, steroid hormones and neurotransmitters may operate together to produce effects, and sometimes these effects involve collaborations between groups of neurons. As illustrations. evidence is reviewed in this article that a number of steroid actions in the hippocampus involves the co-participation of excitatory amino acids. These interactions are evident for the regulation of synaptogenesis by estradiol in the CA1 pyramidal neurons or hippocampus and for the induction of dendritic atrophy of CA3 neurons by repeated stress as well as by glucocorticoid injections. In addition, neurogenesis in the adult and developing dentate gyrus is "contained" by adrenal steroids as well as by excitatory amino acids. In each of these three examples, NMDA receptors are involved. These results not only point to a high degree of interdependency between certain neurotransmitters and the actions of steroid hormones but also emphasize the degree to which structural plasticity is an important aspect of steroid hormone action in the adult as well as developing nervous system.     

Chemokines: a new class of neuromodulator?

Chemokines are not only found in the immune system or expressed in inflammatory conditions: they are constitutively present in the brain in both glial cells and neurons. Recently, the possibility has been raised that they might act as neurotransmitters or neuromodulators. Although the evidence is incomplete, emerging data show that chemokines have several of the characteristics that define neurotransmitters. Moreover, their physiological actions resemble those of neuromodulators in the sense that chemokines usually have few effects by themselves in basal conditions, but modify the induced release of neurotransmitters or neuropeptides. These findings, together with the pharmacological development of agonists and antagonists that are selective for chemokine receptors and can cross the blood-brain barrier, open a new era of research in neuroscience.     

The Role of Steroid Modulation of Amino Acid Transmitters in the Regulation of Female Reproduction

SYNOPSIS. The amino acid transmitters can be placed in two generalcategories, excitatory and inhibitory. This discussion focuseson the role of the inhibitory transmitter GAB A and the excitatoryamino acids aspartate and glutamate in the control of gonadotropinsecretion and reproductive behavior. GABAergic neurotransmissionin the preoptic area inhibits gonadotropin secretion via directsynaptic contact with LHRH neurons and possibly through presynapticinhibition of noradrenergic fibers that stimulate LH release.In the arcuate-median eminence, GABA acting at GABAA receptorsincreases gonadotropin release by inhibiting a currently unidentifiedinhibitory interneuron. In regard to reproductive behavior,GABA acting in the preoptic area inhibits female sexual receptivitywhereas GABA in the mediobasal hypothalamus and the midbraincentral gray facilitates this behavior. The effects of GABAon reproductive behavior do not appear to be secondary to actionson defensive or locomotor behavior. Gonadal steroids modulateactivity at the GABAA receptor in a highly complex manner andthese effects may be involved in the role GABA plays in controllinggonadotropin secretions as well as behavior. The excitatory amino acids also affect gonadotropin secretion,exerting a stimulatory effect both in the preoptic area andat the level of the median eminence. When a specific antagonistfor one of the excitatory amino acid receptors is infused intothe preoptic area or when an excitatory amino acid receptoragonist is infused into the mediobasal hypothalamus, femalesexual behavior is inhibited. There have only been limited reportsof steroid modulation of excitatory amino acid neurotransmission.     

Relationship of glutamate and aspartate to the periaqueductal gray-raphe magnus projection: analysis using immunocytochemistry and microdialysis.

This study tested the hypothesis that the excitatory amino acid transmitters glutamate and/or aspartate are associated with the periaqueductal gray (PAG)-raphe magnus (NRM) projection. Retrograde neuroanatomical tracing procedures utilizing the tracers WGA-HRP or D-[3H]-aspartate were combined with immunocytochemical localization of glutamate or aspartate to determine if glutamate and/or aspartate immunostained neurons projected to the NRM. Both glutamate- and aspartate-immunoreactive cells in the PAG were found to project to the NRM. Double labeling immunocytochemichemical procedures indicated that glutamate and aspartate are co-localized in many PAG neurons, suggesting the following possibilities: (a) one of these two amino acids may serve as a precursor to the other; (b) both amino acids may be co-released from the same PAG neuron; or (c) both amino acids are present in high levels in the perikarya for metabolic purposes. At the EM level, both glutamate- and aspartate-immunoreactive terminals were identified in the NRM, strengthening the concept that both amino acids participate in synaptic transmission in this medullary nucleus. To determine if glutamate and aspartate are in fact released from PAG-NRM axons, the PAG was stimulated chemically with homocysteic acid (HCA) and amino acids were collected from the NRM using a microdialysis probe. Microinjection of HCA, but not vehicle, into the PAG resulted in the release of both glutamate and aspartate in the nucleus raphe magnus. These data suggest that both glutamate and aspartate are released from PAG fibers terminating in the NRM and provide strong support for the hypothesis that excitatory amino acids play a neurotransmitter role in the PAG-NRM pathway.     

The potential usefulness of taurine on diabetes mellitus and its complications

Taurine (2-aminoethanesulfonic acid) is a free amino acid found ubiquitously in millimolar concentrations in all mammalian tissues. Taurine exerts a variety of biological actions, including antioxidation, modulation of ion movement, osmoregulation, modulation of neurotransmitters, and conjugation of bile acids, which may maintain physiological homeostasis. Recently, data is accumulating that show the effectiveness of taurine against diabetes mellitus, insulin resistance and its complications, including retinopathy, nephropathy, neuropathy, atherosclerosis and cardiomyopathy, independent of hypoglycemic effect in several animal models. The useful effects appear due to the multiple actions of taurine on cellular functions. This review summarizes the beneficial effects of taurine supplementation on diabetes mellitus and the molecular mechanisms underlying its effectiveness.     

Effects of hemorrhagic hypotension on tyrosine concentrations in rat spinal cord and plasma

Tyrosine is the precursor for catecholamine neurotransmitters. When catecholamine-containing neurons are physiologically active (as sympathoadrenal cells are in hypotension), tyrosine administration increases catecholamine synthesis and release. Since hypotension can alter plasma amino acid composition, we examined the effects of an acute hypotensive insult on tyrosine concentrations in plasma and spinal cord. Rats were cannulated and bled until the systolic blood pressure was 50 mmHg, or were kept normotensive for 1 h. Tyrosine and other large neutral amino acids (LNAA) known to compete with tyrosine for brain uptake were assayed in plasma and spinal cord. The rate at which intra-arterial [3H]tyrosine disappeared from the plasma was also estimated in hemorrhaged and control rats. In plasma of hemorrhaged animals, both the tyrosine concentration and the tyrosine/LNAA ratio was elevated; moreover, the disappearance of [3H]tyrosine was slowed. Tyrosine concentrations also increased in spinal cords of hemorrhaged-hypotensive rats when compared to normotensive controls. Changes in plasma amino acid patterns may thus influence spinal cord concentrations of amino acid precursors for neurotransmitters during the stress of hemorrhagic shock.     

Neurotransmitters: past, present, and future directions

As originally conceived, central neurotransmitters operated uniformly, exciting or inhibiting postsynaptic targets by receptors that activated passive ionic conductances. As the list of transmitter substances and their actions expanded, concepts of transmitter actions have broadened and grown more complex to include a variety of intramembranous and intracytoplasmic second messengers that can regulate both active and passive ionic conductances. Present-day research directions center on further expansion of the lists of identified transmitter candidates, and on the more precise characterization of their sites and mechanisms of receptor regulation and transduction. Current research is also illuminating the means by which neurotransmitters act in a coordinated fashion to regulate common synaptic targets. Future directions will likely include new forms of interneuronal, intraneuronal, and glial signals, including lipids, steroids, and as-yet-undiscovered superfamilies of peptides and receptors. Although recent advances in understanding specific transmitters have been achieved largely through in vitro electrophysiological analyses, it is hoped that future research will recast these events in the context of the intact functioning brain. Neurotransmitters are likely to remain a productive focus of future research.     

Scope and limitations of in vivo brain dialysis: a comparison of its application to various neurotransmitter systems

Brain dialysis is rapidly becoming a routine research method with a wide range of applications. Since 1982 this sampling technique is frequently used as a method to study the in vivo release of endogenous neurotransmitters such as dopamine, noradrenaline, serotonin, acetylcholine and certain amino acids. In this review most of the studies that have appeared in this field, are evaluated. Special attention was given to the question whether the neurotransmitter content in the dialysate is related to neurotransmission. Criteria such as the presence of a high tissue/dialysate concentration ratio, the sensitivity of the transmitters to membrane active compounds and the occurrence of receptor-mediated effects, are discussed. It is concluded that dopamine, noradrenaline and acetylcholine found in the dialysate are directly derived from neurotransmission, whereas the overflow of excitatory amino acid neurotransmitters is related to neurogenic as well as to metabolic events.     

The loading of neurotransmitters into synaptic vesicles

Classical (non-peptide) transmitters are stored into secretory vesicles by a secondary active transporter driven by a V-type H(+)-ATPase. Five vesicular neurotransmitter uptake activities have been characterized in vitro and, for three of them, the transporters involved have been identified at the molecular level using cDNA cloning and/or Caenorhabditis elegans genetics. These transporters belong to two protein families, which are both unrelated to the Na(+)-coupled neurotransmitter transporters operating at the plasma membrane. The two isoforms of the mammalian vesicular monoamine transporter, VMAT1 and VMAT2, are related to the vesicular acetylcholine transporter (VACHT), while a novel, unrelated vesicular inhibitory amino acid transporter (VIAAT), also designated vesicular GABA transporter (VGAT), is responsible for the storage of GABA, glycine or, at some synapses, both amino acids into synaptic vesicles. The observed effects of experimentally altered levels of VACHT or VMAT2 on synaptic transmission and behavior, as well as the recent awareness that GABAergic or glutamatergic receptors are not always saturated at central synapses, suggest a potential role of vesicular loading in synaptic plasticity.     

A putative vesicular transporter expressed in Drosophila mushroom bodies that mediates sexual behavior may define a neurotransmitter system

Vesicular transporters are required for the storage of?all classical and amino acid neurotransmitters in synaptic vesicles. Some neurons lack known vesicular transporters, suggesting additional neurotransmitter systems remain unidentified. Insect mushroom bodies (MBs) are critical for several behaviors, including learning, but the neurotransmitters released by the intrinsic Kenyon cells (KCs) remain unknown. Likewise, KCs do not express a known vesicular transporter. We report the identification of a novel Drosophila gene portabella (prt) that is structurally similar to known vesicular transporters. Both larval and adult brains express PRT in the KCs of the MBs. Additional PRT cells project to the central complex and optic ganglia. prt mutation causes an olfactory learning deficit and an unusual defect in the male's position during copulation that is rescued by expression in KCs. Because prt is expressed in neurons that lack other known vesicular transporters or neurotransmitters, it may define a previously unknown neurotransmitter system responsible for sexual behavior and a component of olfactory learning.     

Studies on a possible pituitary effect of monoamines on luteinizing hormone release in ovariectomized rats

Incubation of anterior pituitaries from ovariectomized rats with LHRH and various concentrations of dopamine, norepinephrine or serotonin indicated that none of these neurotransmitters could decrease pituitary LH secretion in response to the releasing hormone. This indicated that the inhibitions of pulsatile LH release previously observed in our laboratory in ovariectomized rats in response to intraventricularly administered catecholamines or stimulation of brain serotoninergic neurons are due to central rather than pituitary effects of these transmitters.     

Neuromuscular synaptic transmission in Limulus polyphemus--II. Release of amino acid putative transmitters from the neuromuscular preparation

High-performance liquid chromatography with fluorescence detection was used to assay the release of putative amino acid transmitters from the Limulus neuromuscular preparation. Motor axon stimulation increased the concentrations of aspartate, glutamate and eight other amino acids in fluid bathing the neuromuscular preparation. Pentobarbital, which attenuates the excitatory postsynaptic potential of Limulus muscle, was used to block both synaptic activation of muscle fibers and any amino acid release that may have resulted from this activation. Stimulus-induced release of glutamate and five other amino acids was blocked by pentobarbital, while release of aspartate and three other amino acids was unaffected; a result which suggests that the latter group of amino acids was released presynaptically. Aspartate is the only physiologically active compound in this group. Consideration is given both to the difficulties involved in interpreting sites of amino acid release and to the problem of using pentobarbital as a presumed postsynaptic antagonist. The evidence concerning the relative merits of either aspartate or glutamate as the natural excitatory transmitter at the Limulus neuromuscular junction is discussed.