Characterization of L-glutamate and kainate binding sites in the brain of a freshwater fish, Telapilia monsanbica

Convergent Energy State-Dependent Antagonistic Signaling by Cocaine- and Amphetamine-Regulated Transcript (CART) and Neuropeptide Y (NPY) Modulates the Plasticity of Forebrain Neurons to Regulate Feeding in Zebrafish

Dynamic reconfiguration of circuit function subserves the flexibility of innate behaviors tuned to physiological states. Internal energy stores adaptively regulate feeding-associated behaviors and integrate opposing hunger and satiety signals at the level of neural circuits. Across vertebrate lineages, the neuropeptides cocaine- and amphetamine-regulated transcript (CART) and neuropeptide Y (NPY) have potent anorexic and orexic functions, respectively, and show energy-state-dependent expression in interoceptive neurons. However, how the antagonistic activities of these peptides modulate circuit plasticity remains unclear. Using behavioral, neuroanatomical, and activity analysis in adult zebrafish of both sexes, along with pharmacological interventions, we show that CART and NPY activities converge on a population of neurons in the dorsomedial telencephalon (Dm). Although CART facilitates glutamatergic neurotransmission at the Dm, NPY dampens the response to glutamate. In energy-rich states, CART enhances NMDA receptor (NMDAR) function by protein kinase A/protein kinase C (PKA/PKC)-mediated phosphorylation of the NR1 subunit of the NMDAR complex. Conversely, starvation triggers NPY-mediated reduction in phosphorylated NR1 via calcineurin activation and inhibition of cAMP production leading to reduced responsiveness to glutamate. Our data identify convergent integration of CART and NPY inputs by the Dm neurons to generate nutritional state-dependent circuit plasticity that is correlated with the behavioral switch induced by the opposing actions of satiety and hunger signals.SIGNIFICANCE STATEMENT Internal energy needs reconfigure neuronal circuits to adaptively regulate feeding behavior. Energy-state-dependent neuropeptide release can signal energy status to feeding-associated circuits and modulate circuit function. CART and NPY are major anorexic and orexic factors, respectively, but the intracellular signaling pathways used by these peptides to alter circuit function remain uncharacterized. We show that CART and NPY-expressing neurons from energy-state interoceptive areas project to a novel telencephalic region, Dm, in adult zebrafish. CART increases the excitability of Dm neurons, whereas NPY opposes CART activity. Antagonistic signaling by CART and NPY converge onto NMDA-receptor function to modulate glutamatergic neurotransmission. Thus, opposing activities of anorexic CART and orexic NPY reconfigure circuit function to generate flexibility in feeding behavior.

Ammonia toxicity induces glutamine accumulation, oxidative stress and immunosuppression in juvenile yellow catfish Pelteobagrus fulvidraco

A study was carried to test the response of yellow catfish for 28 days under two ammonia concentrations. Weight gain of fish exposure to high and low ammonia abruptly increased at day 3. There were no significant changes in fish physiological indexes and immune responses at different times during 28-day exposure to low ammonia. Fish physiological indexes and immune responses in the treatment of high ammonia were lower than those of fish in the treatment of low ammonia. When fish were exposed to high ammonia, the ammonia concentration in the brain increased by 19-fold on day 1. By comparison, liver ammonia concentration reached its highest level much earlier at hour 12. In spite of a significant increase in brain and liver glutamine concentration, there was no significant change in glutamate level throughout the 28-day period. The total superoxide dismutase (SOD), glutathione peroxidase (GPX) and glutathione reductase (GR) activities in the brain gradually decreased from hour 0 to day 28. Liver SOD, GPX and GR activities reached the highest levels at hour 12, and then gradually decreased. Thiobarbituric acid reactive substance brain and liver content gradually increased throughout the 28-day period. Lysozyme, acid phosphatase and alkaline phosphatase activities in the liver reached exceptionally low levels after day 14. This study indicated that glutamine accumulation in the brain was not the major cause of ammonia poisoning, the toxic reactive oxygen species is not fully counter acted by the antioxidant enzymes and immunosuppression is a process of gradual accumulation of immunosuppressive factors.

Chronic and acute ammonia toxicity in mudskippers, Periophthalmodon schlosseri and Boleophthalmus boddaerti: brain ammonia and glutamine contents, and effects of methionine sulfoximine and MK801

The objective of this study was to elucidate if chronic and acute ammonia intoxication in mudskippers, Periophthalmodon schlosseri and Boleophthalmus boddaerti, were associated with high levels of ammonia and/or glutamine in their brains, and if acute ammonia intoxication could be prevented by the administration of methionine sulfoximine [MSO; an inhibitor of glutamine synthetase (GS)] or MK801 [an antagonist of N-methyl D-aspartate type glutamate (NMDA) receptors]. For P. schlosseri and B. boddaerti exposed to sublethal concentrations (100 and 8 mmol l(-1) NH4Cl, respectively, at pH 7.0) of environmental ammonia for 4 days, brain ammonia contents increased drastically during the first 24 h, and they reached 18 and 14.5 micromol g(-1), respectively, at hour 96. Simultaneously, there were increases in brain glutamine contents, but brain glutamate contents were unchanged. Because glutamine accumulated to exceptionally high levels in brains of P. schlosseri (29.8 micromol g(-1)) and B. boddaerti (12.1 micromol g(-1)) without causing death, it can be concluded that these two mudskippers could ameliorate those problems associated with glutamine synthesis and accumulation as observed in patients suffering from hyperammonemia. P. schlosseri and B. boddaerti could tolerate high doses of ammonium acetate (CH3COONH4) injected into their peritoneal cavities, with 24 h LC50 of 15.6 and 12.3 micromol g(-1) fish, respectively. After the injection with a sublethal dose of CH3COONH4 (8 micromol g(-1) fish), there were significant increases in ammonia (5.11 and 8.36 micromol g(-1), respectively) and glutamine (4.22 and 3.54 micromol g(-1), respectively) levels in their brains at hour 0.5, but these levels returned to normal at hour 24. By contrast, for P. schlosseri and B. boddaerti that succumbed within 15-50 min to a dose of CH3COONH4 (15 and 12 micromol g(-1) fish, respectively) close to the LC50 values, the ammonia contents in the brains reached much higher levels (12.8 and 14.9 micromol g(-1), respectively), while the glutamine level remained relatively low (3.93 and 2.67 micromol g(-1), respectively). Thus, glutamine synthesis and accumulation in the brain was not the major cause of death in these two mudskippers confronted with acute ammonia toxicity. Indeed, MSO, at a dosage (100 microg g(-1) fish) protective for rats, did not protect B. boddaerti against acute ammonia toxicity, although it was an inhibitor of GS activities from the brains of both mudskippers. In the case of P. schlosseri, MSO only prolonged the time to death but did not reduce the mortality rate (100%). In addition, MK801 (2 microg g(-1) fish) had no protective effect on P. schlosseri and B. boddaerti injected with a lethal dose of CH3COONH4, indicating that activation of NMDA receptors was not the major cause of death during acute ammonia intoxication. Thus, it can be concluded that there are major differences in mechanisms of chronic and acute ammonia toxicity between brains of these two mudskippers and mammalian brains.

Evidence of histamine receptors in fish brain using an in vivo [14C]2-deoxyglucose autoradiographic method and an in vitro receptor-binding autoradiographic method

It was hypothesized that fish possess functioning H1 histamine receptors that have the ability to bind agonists and antagonists specific to the H1 histamine receptor subtype. For these experiments, a combination of a novel, in vivo 2-deoxyglucose method and a standard in vitro autoradiography procedure was utilized. A regional, statistically significant dose response in neurological functioning was observed when fish were exposed to histaminergic agents (i.e., H1 agonists and antagonists), which created the first neurological profile for the H1 histamine receptor in fish brain. The H1 histamine receptor was chosen as a characterization receptor in fish because histamine has been linked to a variety of neurological functions such as the control of arousal, attention, sensory processing, and cognition. Histamine also plays a role in pituitary hormone secretion, appetite control, and, potentially, regulation of vestigular reactivity. In addition, the fish brain is well characterized structurally, and the existence of an H3-like receptor has been documented recently in zebrafish. However, to date there is little detailed information about specific localization and functioning of the H1 histamine receptor in fish.

Glutamine synthetase in brain: effect of ammonia

Glutamine synthetase (GS) in brain is located mainly in astrocytes. One of the primary roles of astrocytes is to protect neurons against excitotoxicity by taking up excess ammonia and glutamate and converting it into glutamine via the enzyme GS. Changes in GS expression may reflect changes in astroglial function, which can affect neuronal functions. Hyperammonemia is an important factor responsible of hepatic encephalopathy (HE) and causes astroglial swelling. Hyperammonemia can be experimentally induced and an adaptive astroglial response to high levels of ammonia and glutamate seems to occur in long-term studies. In hyperammonemic states, astroglial cells can experience morphological changes that may alter different astrocyte functions, such as protein synthesis or neurotransmitters uptake. One of the observed changes is the increase in the GS expression in astrocytes located in glutamatergic areas. The induction of GS expression in these specific areas would balance the increased ammonia and glutamate uptake and protect against neuronal degeneration, whereas, decrease of GS expression in non-glutamatergic areas could disrupt the neuron-glial metabolic interactions as a consequence of hyperammonemia. Induction of GS has been described in astrocytes in response to the action of glutamate on active glutamate receptors. The over-stimulation of glutamate receptors may also favour nitric oxide (NO) formation by activation of NO synthase (NOS), and NO has been implicated in the pathogenesis of several CNS diseases. Hyperammonemia could induce the formation of inducible NOS in astroglial cells, with the consequent NO formation, deactivation of GS and dawn-regulation of glutamate uptake. However, in glutamatergic areas, the distribution of both glial glutamate receptors and glial glutamate transporters parallels the GS location, suggesting a functional coupling between glutamate uptake and degradation by glutamate transporters and GS to attenuate brain injury in these areas. In hyperammonemia, the astroglial cells located in proximity to blood-vessels in glutamatergic areas show increased GS protein content in their perivascular processes. Since ammonia freely crosses the blood-brain barrier (BBB) and astrocytes are responsible for maintaining the BBB, the presence of GS in the perivascular processes could produce a rapid glutamine synthesis to be released into blood. It could, therefore, prevent the entry of high amounts of ammonia from circulation to attenuate neurotoxicity. The changes in the distribution of this critical enzyme suggests that the glutamate-glutamine cycle may be differentially impaired in hyperammonemic states.

Predominant expression of group-II metabotropic glutamate receptors in the goldfish brain

Group-II metabotropic glutamate (mGlu) receptors (mGlu2/3 receptors) were highly expressed in various regions (telencephalon, optic tectum, and cerebellum, but not vagal lobe) of the goldfish brain. In the goldfish telencephalon, expression of mGlu2/3 receptors was even higher than in the rat cerebral cortex. In contrast, mGlu5 receptors showed low levels of expression in all goldfish brain regions, whereas mGlu1a receptors were only expressed in the goldfish cerebellum. Pharmacological activation of group-II mGlu receptors with the selective agonists, 2R,4R-4-aminopyrrolidine-2, 4-dicarboxylic acid and (2S,2'R,3'R)-2-(2,3-dicarboxycyclopropyl) glycine, reduced the evoked release of glutamate from goldfish brain synaptosomes, whereas agonists of group-I and -III mGlu receptors (3, 5-dihydroxyphenylglycine and L-2-amino-4-phosphonobutanoate) were inactive. The predominance of group-II over group-I mGlu receptors in the goldfish brain may provide a natural defense against excitotoxic neuronal death and contribute to the unusually high resistance of goldfish against hypoxic brain damage.

Effects of gonadal steroids on the GABA and glutamate contents of the early developing tilapia brain

The effects of gonadal steroids on the gamma-aminobutyric acid (GABA) and glutamate (Glu) contents of the early developing brain were investigated. Seven-day-old (7 days post-hatch) tilapia were divided into three groups which were continuously treated with 100 mg/kg diet 17beta-estradiol (E2), 100 mg/kg diet methyltestosterone (MT), and a normal diet, respectively. Until 10, 20, and 30 days old, the GABA and Glu contents of the brains were detected by HPLC-ECD. The brain GABA and Glu contents, before 30 days old, significantly increased with age. These results demonstrate that before 30 days old is a developing period of both GABA and Glu systems in the tilapia brain. During this period, both E2 and MT have a facilitative effect on the GABAergic and Gluergic system during a restricted effective period.

Molecular and electrophysiological characterizations of fGluR3 alpha, an ionotropic glutamate receptor subunit of a teleost fish

Here we report the cloning and functional analysis of a cDNA encoding a functional glutamate receptor subunit of Oreochromis sp., a freshwater teleost fish. The deduced amino acid sequence of this cDNA clone, fGluR3 alpha, displays the highest sequence identity to that of the mammalian GluR3 subunit. Results of quantitative reverse-transcriptase polymerase chain reaction (RT-PCR) analysis indicated that the expression level of fGluR3 alpha in the cerebellum was much less than that in the telencephalon and optical lobe. Similar to its mammalian counterpart, variants of fGluR3 alpha were created by alternative splicing and RNA editing at the R/G site. The channel properties of homomeric fGluR3 alpha expressed in Xenopus oocytes were similar to those of the mammalian alpha-amino-3-hydroxy-5-methyl-4-isoxazole propionic acid (AMPA)-preferring receptors. The rank order of agonist potency of the expressed fGluR3 alpha is AMPA > or = glutamate > or = quisqualate > domoate > or = kainate. This is the first functional glutamate receptor of teleost fish being demonstrated to be sensitive to AMPA. Furthermore, this study suggested a strong functional conservation of AMPA-preferring receptors in vertebrates.

Molecular analysis of cDNA molecules encoding glutamate receptor subunits, fGluR1 alpha and fGluR1 beta, of Oreochromis sp

In this study, we isolated two cDNA molecules encoding putative glutamate receptor subunits, fGluR1 alpha and fGluR1 beta, from an Oreochromis sp. brain cDNA library by hybridizing with the glutamate receptor cDNA, fGluR2 beta, of the same fish. The deduced amino acid sequence of the fGluR1 alpha consists of 908 residues with an 18-residue signal peptide and displays a sequence identity of 74% to the amino acid sequence of rat GluR1 subunit. Northern blotting indicates that the expression level of fGluR1 alpha in telencephalon is higher than that in optic tectum and cerebellum in adult fish brain. Reverse-transcriptase polymerase chain reaction and genomic analyses reveal the presence of variants created by alternative splicing at the flip-flop module and the carboxyl terminus of fGluR1 alpha transcripts. The amino acid sequence of fGluR1 alpha is unique in that it contains a glutamine-rich sequence inserted at the loop 1 (L1) between transmembrane domains 1 and 2. A second incomplete cDNA clone, designated fGluR1 beta, coding for a polypeptide showing sequence identity to the rat GluR1 and fGluR1 alpha was isolated from the same library. Insertion of a serine- and glutamine-rich sequence at the L1 was also detected in the translated sequence of fGluR1 beta.

Characterization of two fish glutamate receptor cDNA molecules: absence of RNA editing at the Q/R site

Two cDNA clones encoding putative ionotropic glutamate receptor subunits were isolated from a brain cDNA library of a freshwater fish, Oreochromis sp. The deduced amino acid sequences of these two cDNAs, fGluR2 alpha and fGluR2 beta, display the highest sequence identity (85%) to that of the rat GluR2 (AMPA receptor subunit) and they contain an arginine codon at the Q/R editing site of the TM2 segment. Genomic sequence analysis of the exons encoding the TM2 reveals the presence of an arginine codon at the Q/R site, suggesting that the RNA editing mechanism acting in the mammalian GluR2 does not operate at the homologous site in these two fish genes. In contrast to the absence of RNA editing at the Q/R site, transcripts of fGluR2 alpha and fGluR2 beta are subjected to RNA editing at a second site, the R/G site. A splicing variant of fGluR2 alpha, fGluR2 alpha-c, with a shorter C-terminal sequence was found; however, no C-terminal splicing variant of fGluR2 beta was detected in the mature fish. Similar to the mammalian AMPA receptor, variants created by the alternate choice of flip and flop modules were found among transcripts of fGluR2 alpha-c and fGluR2 beta. The amino acid sequences of flip and flop modules of fGluR2 beta are identical to that of the rat GluR2, whereas the amino acid sequences of the flip and flop modules of fGluR2 alpha-c differ from the invariant consensus sequences of the rat AMPA receptor subunits.

An endogenous ligand for the kainate-type binding sites from rat brain

Extracts from the rat brain were screened to identify a putative endogenous ligand for the binding sites of the neuroexcitant kainic acid (KA). The extracted substances were separated by chromatographic techniques and tested for their ability to inhibit KA binding to fish synaptosomes and to membranes from rat brain. A substance isolated in this way (rat kainate-binding inhibitor, RKBI) display a competitive interaction with KA for the low-affinity binding sites in rat brain membranes. According to the separation behavior in the purification step, RKBI is distinct from an inhibitor formerly isolated from fish nervous tissue (KBI). The substance exhibits positive co-operativity with KA for a very-low-affinity site population, particularly concentrated in the cerebellum, and could play a physiological role in this area.

High-affinity kainate binding sites in living slices of rat neocortex: characterization and regulation

We have characterized a high-affinity kainate binding site in in vitro living rat neocortical slices using [3H]kainate. [3H]Kainate labelled at least two binding sites, the higher affinity site with a Kd of 7.1 nM and a Bmax of 71.2 fmol/mg protein. This high-affinity binding site showed a pharmacology consistent with a kainate receptor with competition by kainate and domoic acid, as well as the (RS)-alpha-amino-3-hydroxy-5-methyl-isoxazole-4-propionate antagonist 6-cyano-2,3-dihydroxy-7-nitroquinoxaline. Increases in cellular depolarization induced by 2-h preincubations in veratridine and glutamate led to a significant 55% average decrease in [3H]kainate binding in adult cortex. Similarly, preincubation in kainate led to a significant average 26% decrease in binding. In both instances, Eadie-Hofstee analysis of saturation binding data revealed that the decreased binding reflected changes in receptor number. At different postnatal ages, increases in cellular depolarization significantly decreased binding (< 20 days postnatal age, -86%; > 60 days, -48%). Kainate treatment also significantly decreased binding at all ages (-64% at < 20 days; > 60 days, -18%), with significant differences noted between ages. These age-dependent effects are unlike those previously described for either N-methyl-D-aspartate [Lanius and Shaw (1992) Anat. Rec. 232, 54(A)] or (RS)-alpha-amino-3-hydroxy-5-methyl-isoxazole-4-propionate high affinity receptors [Shaw and Lanius (1992) Devl Brain Res. 68, 225-233].(ABSTRACT TRUNCATED AT 250 WORDS)