Adenosine receptor agonists affect taurine release from mouse brain stem slices in ischemia

The release of the inhibitory amino acid taurine is markedly enhanced under ischemic conditions in both adult and developing brain stem, together with a pronounced increase in the release of the neuromodulator adenosine. We now studied the effects of adenosine receptor agonists and antagonists on [³H]taurine release in the brain stem in normoxia and ischemia, using a superfusion system. Under standard conditions, the adenosine A₁ receptor agonist N⁶-cyclohexyladenosine (CHA) potentiated basal taurine release in adult mice, which response was blocked by the antagonist 8-cyclopentyl-1,3-dipropylxanthine (DPCPX). CHA and the A_2a receptor agonist 2-p-(2-carboxyethyl)phenylamino-5′-N-ethylcarboxaminoadenosinehydrochloride (CGS 21680) had no effect on the release in developing mice. In ischemia, CHA depressed both basal and K⁺-stimulated taurine release in developing mice in a receptor-mediated manner, blocked by DPCPX. The A_2a receptor agonist CGS 21680 was also inhibitory. Taurine and adenosine may thus not cooperate in developing mice to prevent ischemic neuronal damage. On the other hand, CGS 21680 enhanced taurine release in the adult brain stem in ischemia, both basal and K⁺-stimulated release being affected. These effects were abolished by the antagonist 3,7-dimethyl-1-propargylxanthine (DMPX), indicating a receptor-mediated process. In this case elevated levels of taurine could be beneficial, protecting against hyperexcitation and excitotoxicity.     

Modulation of taurine release in glucose-free media by glutamate receptors in hippocampal slices from developing and adult mice

Taurine has been thought to protect neural cells against cell-damaging conditions to which the hippocampus is particularly vulnerable. We studied now how the release of preloaded [³H]taurine is regulated by glutamate receptors in glucose-free media in slices prepared from the mouse hippocampus from developing (7 days old) and young adult (3 months old) mice, using a superfusion system. The lack of glucose enhanced taurine release more from slices from developing mice than from slices from adults. At both ages ionotropic glutamate agonists significantly increased the release in a receptor-mediated manner. Of the metabotropic glutamate receptors those belonging to the group III were effective. The release was enhanced in adult mice but attenuated in developing mice. Both effects were blocked by the receptor antagonists. The results show that glutamate receptors affect taurine release in the absence of glucose in which condition taurine should be neuroprotective.     

Characteristics of taurine release in slices from adult and developing mouse brain stem

Summary. Taurine has been thought to function as a regulator of neuronal activity, neuromodulator and osmoregulator. Moreover, it is essential for the development and survival of neural cells and protects them under cell-damaging conditions. Taurine is also involved in many vital functions regulated by the brain stem, including cardiovascular control and arterial blood pressure. The release of taurine has been studied both in vivo and in vitro in higher brain areas, whereas the mechanisms of release have not been systematically characterized in the brain stem. The properties of release of preloaded [³H]taurine were now characterized in slices prepared from the mouse brain stem from developing (7-day-old) and young adult (3-month-old) mice, using a superfusion system. In general, taurine release was found to be similar to that in other brain areas, consisting of both Ca²⁺-dependent and Ca²⁺-independent components. Moreover, the release was mediated by Na⁺-, Cl⁻-dependent transporters operating outwards, as both Na⁺-free and Cl⁻ -free conditions greatly enhanced it. Cl⁻ channel antagonists and a Cl⁻ transport inhibitor reduced the release at both ages, indicating that a part of the release occurs through ion channels. Protein kinases appeared not to be involved in taurine release in the brain stem, since substances affecting the activity of protein kinase C or tyrosine kinase had no significant effects. The release was modulated by cAMP second messenger systems and phospholipases at both ages. Furthermore, the metabotropic glutamate receptor agonists likewise suppressed the K⁺-stimulated release at both ages. In the immature brain stem, the ionotropic glutamate receptor agonists N-methyl-D-aspartate (NMDA) and 2-amino-3-hydroxy-5-methyl-4-isoxazolepropionate (AMPA) potentiated taurine release in a receptor-mediated manner. This could constitute an important mechanism against excitotoxicity, protecting the brain stem under cell-damaging conditions.     

Characteristics of GABA release modified by glutamate receptors in mouse hippocampal slices

The major part of hippocampal innervation is glutamatergic, regulated by inhibitory GABA-releasing interneurons. The modulation of [(3)H]GABA release by ionotropic and metabotropic glutamate receptors and by nitric oxide was here characterized in superfused mouse hippocampal slices. The ionotropic glutamate receptor agonists kainate, N-methyl-D-aspartate and 2-amino-3-hydroxy-5-methyl-4-isoxazolepropionate potentiated the basal GABA release. These effects were blocked by their respective antagonists 6-nitro-7-cyanoquinoxaline-2,3-dione (CNQX), dizocilpine and 2,3-dioxo-6-nitro-1,2,3,4-tetrahydrobenzo(f)quinoxaline-7-sulfonamide (NBQX), indicating receptor-mediated mechanisms. The NO-generating compounds S-nitroso-N-acetylpenicillamine (SNAP), sodiumnitroprusside and hydroxylamine enhanced the basal GABA release. Particularly the sodiumnitroprusside-evoked release was attenuated by the NO synthase inhibitor N(G)-nitro-L-arginine (L-NNA) and the inhibitor of soluble guanylyl cyclase 1H-(1,2,4)oxadiazolo(4,3a)quinoxalin-1-one (ODQ), indicating the involvement of the NO/cGMP pathway. This inference is corroborated by the enhancing effect of zaprinast, a phosphodiesterase inhibitor, which is known to increase cGMP levels. The K(+)-stimulated hippocampal GABA release was reduced by the groups I and III agonists of metabotropic glutamate receptors (+/-)-1-aminocyclopentane-trans-1,3-dicarboxylate (t-ACPD) and L-(+)-2-amino-4-phosphonobutyrate (L-AP4), which effects were abolished by their respective antagonists (RS)-1-aminoindan-1,5-dicarboxylate (AIDA) and (RS)-2-cyclopropyl-4-phosphonophenylglycine (CPPG), again indicating modification by receptor-mediated mechanisms.     

Metabotropic glutamate receptors modulate GABA release from mouse hippocampal slices

The effects of metabotropic glutamate receptor agonists on the basal and potassium (50 mM K⁺)-stimulated release of [³H]GABA from mouse hippocampal slices were investigated using a superfusion system. The group I agonist (1±)-1-aminocyclopentane-trans-1,3-dicarboxylate enhanced the basal GABA release and reduced the K⁺-evoked release by a mechanism antagonized by (RS)-1-aminoindan-1,5-dicarboxylate in both cases. The group II agonist (2S,2R,3R)-2-(2,3-dicarboxycyclopropyl)glycine failed to have any effect on the basal release, but inhibited the stimulated release. This inhibition was not affected by the antagonist (2S)-2-ethylglutamate. The group III agonists L(+)-amino-4-phosphonobutyrate and O-phospho-L-serine inhibited the basal GABA release, which effects were blocked by the antagonist (RS)-2-cyclopropyl-4-phosphonophenylglycine. Moreover, the suppression of the K⁺-evoked release by L(+)2-amino-4-phosphonobutyrate was apparently receptor-mediated, being blocked by (RS)-2-cyclopropyl-4-phosphonophenylglycine. The results show that activation of metabotropic glutamate receptors of group I is able to potentiate the basal release of GABA, whereas activation of groups I and III receptors reduce K⁺-stimulated release in mouse hippocampal slices.     

Taurine release in mouse brain stem slices under cell-damaging conditions

Summary. Taurine has been thought to be essential for the development and survival of neural cells and to protect them under cell-damaging conditions. In the brain stem taurine regulates many vital functions, including cardiovascular control and arterial blood pressure. We have recently characterized the release of taurine in the adult and developing brain stem under normal conditions. Now we studied the properties of preloaded [³H]taurine release under various cell-damaging conditions (hypoxia, hypoglycemia, ischemia, the presence of metabolic poisons and free radicals) in slices prepared from the mouse brain stem from developing (7-day-old) and young adult (3-month-old) mice, using a superfusion system. Taurine release was greatly enhanced under these cell-damaging conditions, the only exception being the presence of free radicals in both age groups. The ischemia-induced release was characterized to consist of both Ca²⁺-dependent and -independent components. Moreover, the release was mediated by Na⁺-, Cl⁻-dependent transporters operating outwards, particularly in the immature brain stem. Cl⁻ channel antagonists reduced the release at both ages, indicating that a part of the release occurs through ion channels, and protein kinase C appeared to be involved. The release was also modulated by cyclic GMP second messenger systems, since inhibitors of soluble guanylyl cyclase and phosphodiesterases suppressed ischemic taurine release. The inhibition of phospholipases also reduced taurine release at both ages. This ischemia-induced taurine release could constitute an important mechanism against excitotoxicity, protecting the brain stem under cell-damaging conditions.     

Characteristics of taurine release induced by free radicals in mouse hippocampal slices

Summary. The release of the inhibitory neuromodulator taurine in the hippocampus is markedly enhanced under various neural cell-damaging conditions, including ischemia and exposure to free radicals. The properties and regulation of the release evoked by a medium containing free radicals was investigated in hippocampal slices from adult (3-month-old) and developing (7-day-old) mice, using a superfusion system. The free radical damage was induced by applying 0.01% H₂O₂. The release of [³H]taurine was in both adult and developing hippocampus partly Ca²⁺-independent, mediated by Na⁺-dependent transporters and probably resulting from disruption of cell membranes and subsequent ion imbalance. The release in developing mice appeared to be more susceptible to regulation than that in adults, the stimulation by free radicals being in the latter already maximal. The release was reduced by adenosine A₁ receptor agonist R(–)N⁶-(2-phenylisopropyl)adenosine, which effect was, however, abolished by the antagonist 8-cyclopentyl-1,3-dipropylxanthine only in the immature hippocampus, indicating a receptor-mediated process. Moreover, the evoked taurine release in developing mice was potentiated by the ionotropic glutamate receptor agonists N-methyl-D-aspartate, kainate and 2-amino-3-hydroxy-5-methyl-4-isoxazolepropionate in a receptor-mediated manner, since the effects were abolished by their respective antagonists. The metabotropic glutamate receptors are of only minor significance in the release, the agonists of group I and II receptors slightly reducing the release. Furthermore, NO may also be involved in this release, the NO-generating compounds hydroxylamine and S-nitroso-N-acetylpenicillamine being able to enhance the free-radical-evoked release. It seems that the free-radical-stimulated release, potentiated by ionotropic glutamate receptor activation and NO production, could constitute part of the neuroprotective properties of taurine, being important particularly in the developing hippocampus and hence preventing excitotoxicity.     

Involvement of metabotropic glutamate receptors in taurine release in the adult and developing mouse hippocampus

Summary The inhibitory amino acid taurine has been held to function as an osmoregulator and modulator of neural activity, being particularly important in the immature brain. lonotropic glutamate receptor agonists are known markedly to potentiate taurine release. The effects of different metabotropic glutamate receptor (mGluR) agonists and antagonists on the basal and K⁺-stimulated release of [³H]taurine from hippocampal slices from 3-month-old (adult) and 7-day-old mice were now investigated using a superfusion system. Of group I metabotropic glutamate receptor agonists, quisqualate potentiated basal taurine release in both age groups, more markedly in the immature hippocampus. This action was not antagonized by the specific antagonists of group I but by 6-cyano-7-nitroquinoxaline-2,3-dione (CNQX) and 6-nitro-7-sulphamoylbenzo[f]quinoxaline-2,3-dione (NBQX), which would suggest an involvement of ionotropic glutamate receptors. (S)-3,5-dihydroxyphenylglycine (DHPG) potentiated the basal release by a receptor-mediated mechanism in the immature hippocampus. The group II agonist (2S, 2R, 3R)-2-(2,3-dicarboxycyclopropyl)glycine (DCG IV) markedly potentiated basal taurine release at both ages. These effects were antagonized by dizocilpine, indicating again the participation of ionotropic receptors. Group III agonists slightly potentiated basal taurine release, as did several antagonists of the three metabotropic receptor groups. Potassium-stimulated (50 mM K⁺) taurine release was generally significantly reduced by mGluR agents, mainly by group I and II compounds. This may be harmful to neurons in hyperexcitatory states. On the other hand, the potentiation by mGluRs of basal taurine release, particularly in the immature hippocampus, together with the earlier demonstrated pronounced enhancement by activation of ionotropic glutamate receptors, may protect neurons against excitotoxicity.Abbreviations ACPD (1±)-1-aminocyclopentane-trans-1,3-dicarboxylate - AIDA (RS)-1-aminoindan-1,5-dicarboxylate - AMPA 2-amino-3-hydroxy05-methyl-4-isoxazolepropionate - CNQX 6-cyano-7-nitroquinoxaline-2,3-dione - CPPG (RS)-2-cyclopropyl-4-phosphonophenylglycine - DCG IV (2S,2R,3R)-2-(2,3-dicarboxycyclopropyl)glycine - DHPG (S)-3,5-dihydroxyphenylglycine - EGLU (2S)-2-ethylglutamate - L-AP3 L(+)-2-amino-3-phosphonopropionate - L-AP4 L(+)-2-amino-4-phosphonobutyrate - L-AP6 L(+)-2-amino-6-phosphonohexanoate - L-SOP O-phospho-L-serine - MPPG (RS)-2-methyl-4-phosphonophenylglycine - MSOP (RS)-2-methylserine-O-phosphate - MSOPPE (RS)-2-methylserine-O-phosphate monophenyl ester - MTPG (RS)-2-methyl-4-tetrazolylphenylglycine - NBQX 6-nitro-7-sulphamoyl[f]quinoxaline-2,3-dione - NMDA N-methyl-D-aspartate - QA quisqualate - S-3C4H-PG (S)-3-carboxy-4-hydroxyphenylglycine - S-4C-PG (S)-4-carboxyphenylglycine; - S-MCGP (S)-2-methyl-4-carboxyphenylglycine     

Nitric oxide is involved in taurine release in the mouse brain stem under normal and ischemic conditions

Summary. Nitric oxide (NO) has been shown to regulate neurotransmitter release in the brain; both inhibitory and excitatory effects have been seen. Taurine is essential for the development and survival of neural cells and protects them under cell-damaging conditions. In the brain stem, it regulates many vital functions such as cardiovascular control and arterial blood pressure. Now we studied the effects of the NO-generating compounds hydroxylamine (HA), S-nitroso-N-acetylpenicillamine (SNAP) and sodium nitroprusside (SNP) on the release of preloaded [³H]taurine under normal and ischemic conditions in slices prepared from the mouse brain stem from developing (7-day-old) to young adult (3-month-old) mice. In general, the effects of NO on the release were somewhat complex and difficult to explain, as expected from the multifunctional role of NO in the central nervous system. The basal initial release under normal conditions was enhanced by the NO donors 5 mM HA and 1.0 mM SNAP at both ages, but SNP was inhibitory in developing mice. The release was markedly enhanced by K⁺ stimulation. The effects of HA, SNAP and SNP on the basal release were not antagonized by the NO synthase inhibitor N^G-nitro-L-arginine (L-NNA, 1.0 mM), demonstrating that mechanisms other than NO synthesis are involved. Taurine release in developing mice in the presence of SNP was reduced by the inhibitor of soluble guanylate cyclase, 1H-(1,2,3)oxadiazolo(4,3-a)quinoxalin-1-one (ODQ), indicating the possible involvement of cGMP. In normoxia, N-methyl-D-aspartate (NMDA, 1.0 mM) enhanced the SNAP- and HA-evoked taurine release in developing mice and the HA-evoked release in adults. In ischemia, both K⁺ stimulation and NMDA potentiated the NO-induced release, particularly in the immature mice, probably without the involvement of the NO synthase or cGMP. The substantial release of taurine in the developing brain stem evoked by NO donors together with NMDA might represent signs of important mechanisms against excitotoxicity which protect the brain stem under cell-damaging conditions. Authors’ address: Prof. Pirjo Saransaari, Brain Research Center, Medical School University of Tampere, Tampere, FIN-3 3014, Finland     

Modulation of GABA-augmented norepinephrine release in female rat brain slices by opioids and adenosine

GABA_A receptor activation augments electrically-stimulated release of norepinephrine (NE) from rat brain slices. Because this effect is not observed in synaptoneurosomes, GABA probably acts on inhibitory interneurons to disinhibit NE release. To determine whether opioids or adenosine influence GABA-augmented NE release, hypothalamic and cortical slices from female rats were superfused with GABA or vehicle in the presence and absence of 10 M morphine or 100 M adenosine. GABA augments [³H]NE release in the cortex and hypothalamus. Morphine alone has no effect on [³H]NE release, but attenuates GABA augmentation of [³H]NE release in both brain regions. Adenosine alone modestly inhibits [³H]NE release in the cortex, but not in the hypothalamus. Adenosine inhibits GABA-augmented [³H]NE release in both brain regions. The general protein kinase inhibitor H-7, augments [³H]NE release in both brain regions and may have additive effects with GABA in cortical slices. These results implicate opioid and adenosine interneurons and possibly protein kinases in regulating GABAergic influences on NE transmission.     

Facilitation of glutamate release by ionotropic glutamate receptors in osteoblasts

Constitutive expression of mRNA was seen for the vesicular glutamate transporter brain-specific Na(+)-dependent inorganic phosphate cotransporter (BNPI), but not differentiation-associated Na(+)-dependent inorganic phosphate cotransporter, in rat calvarial osteoblasts cultured for 7 and 21 days in vitro (DIV). Three different agonists for ionotropic glutamate receptors (iGluR) at 1mM, as well as 50mM KCl, significantly increased the release of endogenous L-glutamate from osteoblasts cultured for 7DIV when determined 5 min after the addition by using a high performance liquid chromatograph. The inhibitor of desensitization of DL-alpha-amino-3-hydroxy-5-methylisoxasole-4-propionate (AMPA) receptors cyclothiazide significantly potentiated and prolonged the release of endogenous L-glutamate evoked by AMPA in a dose-dependent manner. The release evoked by AMPA was significantly prevented by the addition of an AMPA receptor antagonist as well as by the removal of Ca(2+) ions. These results suggest that endogenous L-glutamate could be released from intracellular vesicular constituents associated with BNPI through activation of particular iGluR subtypes expressed in cultured rat calvarial osteoblasts.     

Methotrexate induces seizure and decreases glutamate uptake in brain slices: prevention by ionotropic glutamate receptors antagonists and adenosine

Methotrexate (MTX)-induced neurotoxicity may occur after intrathecal or systemic administration at low, intermediate and high doses for the treatment of malignant or inflammatory diseases. The mechanisms of MTX neurotoxicity are not totally understood, and appear to be multifactorial. In this study we characterized a model of MTX-induced seizures in mice to evaluate the convulsive and toxic MTX properties. Additionally, the effect of MTX-induced seizures on the activity of glutamate transporters, as well as the anticonvulsant role of MK-801, DNQX and adenosine on glutamate uptake in brain slices was investigated . MTX induced tonic-clonic seizures in approximately 95% of animals and pre-treatment with MK-801, DNQX and adenosine prevented seizure in 80%, 62% and 50% of animals, respectively. Moreover, MTX leads 59% of mice to death, which was prevented in 100% and 94% when animals received MK-801 and DNQX, respectively. Glutamate uptake decreased by 20% to 30% in cortical slices after MTX-induced seizures. Interestingly, when seizures were prevented by MK-801, DNQX or adenosine, glutamate uptake activity remained at the same level as the control group. Thus, our results demonstrate the involvement of the glutamatergic system in MTX-induced seizures.     

Effects of cations on taurine,hypotaurine, and GABA uptake in mouse brain slices

The effects of cations on taurine, hypotaurine and GABA uptake were studied in mouse brain slices under identical experimental conditions. The uptakes were all strictly sodium-dependent. The omission or excess of K⁺ inhibited similarly taurine, hypotaurine and GABA uptake. The effects of omission of Ca²⁺ or Mg²⁺ were less pronounced. In both normal-sodium and low-sodium media all uptakes were saturable, consisting of both low-and high-affinity transport components. TheK _m constants for both low-and high-affinity transport components of hypotaurine and GABA increased in low-sodium medium, suggesting that sodium ions are necessary for their attachment to possible carrier sites in plasma membranes. In the case of taurine, however, the translation rate rather than the affinity of carrier sites was affected in Na⁺-free media. More than two sodium ions may be involved in the transport of one hypotaurine and one GABA molecule, whereas the coupling ratio between sodium and taurine was at least three. In its cation dependence hypotaurine uptake thus resembled more GABA uptake than taurine uptake.     

Changes in medium radioactivity and composition accompany high-affinity uptake of glutamate and aspartate by mouse brain slices

In measurements of high affinity transport in tissue slices, the incubation medium is often treated as an infinitely large pool. External substrate concentrations, even at the micromolar level, are assumed to be constant and metabolic interactions between tissue and medium are neglected. In the present report we describe experiments in which glutamic and aspartic acid uptake by mouse brain slices were studied using techniques that could test these assumptions. Cerebral hemispheres were cut into 0.1 mm sections and about 90 mg of tissue incubated in 10 ml of oxygenated medium. After 45 minutes of equilibration, radioactive substrates were added and the concentrations and specific activities of the amino acids and their metabolites in the medium were determined. During the first 10 min following substrate addition, rapid decreases in glutamic and aspartic acid concentrations in the medium were accompanied by large decreases in specific activity caused by the continuous release of these amino acids from the tissue. In addition, extensive conversion of both substrates to glutamine and the preferential accumulation of this metabolite, in the medium, was found. These results demonstrate that metabolism and release occur simultaneously with uptake during transport experiments in vitro and that these processes can take place in specific tissue compartments. It is therefore necessary to measure the tissue and medium concentration levels of amino acids along with their radioactivity in such experiments, since all three processes (transport, metabolism, and compartmentation) are interrelated in the clearance of amino acids from the incubation medium and probably from the extracellular spaces in vivo as well.     

Metabotropic glutamate receptors inhibit microglial glutamate release

Pro-inflammatory stimuli evoke an export of glutamate from microglia that is sufficient to contribute to excitotoxicity in neighbouring neurons. Since microglia also express various glutamate receptors themselves, we were interested in the potential feedback of glutamate on this system. Several agonists of mGluRs (metabotropic glutamate receptors) were applied to primary rat microglia, and the export of glutamate into their culture medium was evoked by LPS (lipopolysaccharide). Agonists of group-II and -III mGluR ACPD [(1S,3R)-1-aminocyclopentane-1,3-dicarboxylic acid] and L-AP4 [L-(+)-2-amino-4-phosphonobutyric acid] were both capable of completely blocking the glutamate export without interfering with the production of NO (nitric oxide); the group-I agonist tADA (trans-azetidine-2,4-dicarboxylic acid) was ineffective. Consistent with the possibility of feedback, inhibition of mGluR by MSPG [(R,S)-α-2-methyl-4sulfonophenylglycine] potentiated glutamate export. As the group-II and -III mGluR are coupled to Gα_i-containing G-proteins and the inhibition of adenylate cyclase, we explored the role of cAMP in this effect. Inhibition of cAMP-dependent protein kinase [also known as protein kinase A (PKA)] by H89 mimicked the effect of ACPD, and the mGluR agonist had its actions reversed by artificially sustaining cAMP through the PDE (phosphodiesterase) inhibitor IBMX (isobutylmethylxanthine) or the cAMP mimetic dbcAMP (dibutyryl cAMP). These data indicate that mGluR activation attenuates a potentially neurotoxic export of glutamate from activated microglia and implicate cAMP as a contributor to this aspect of microglial action.     

Detection of gamma-aminobutyric acid-induced glutamate release in acute mouse hippocampal slices with a patch sensor

gamma-Aminobutyric acid (GABA)-stimulated release of L-glutamate from various neuronal regions of acute mouse hippocampal slices was detected with a patch sensor that responds to L-glutamate at the sub-micromolar level. The response of the patch sensor to L-glutamate was evaluated in terms of an integrated current. The integrated current increased with the concentration of L-glutamate ranging from 0.50 to 5.0 microM. By using the patch sensor, GABA-induced L-glutamate release from acute mouse hippocampal slices was detected. The effect of antagonists for GABA(A) and GABA(B) receptors on the L-glutamate release was also investigated. The GABA (25 microM) stimulation induced the release of L-glutamate via GABA(A) receptor in the CA1 region, but GABA did not induce L-glutamate release in the CA3 region. However, in the presence of the GABA(B) receptor antagonist (3-aminopropyl)(diethoxymethyl)phosphinic acid (CGP-35348), release of L-glutamate in the CA3 region was evoked by GABA stimulation. The glutamate release was completely suppressed when both GABA(A) and GABA(B) receptor were inhibited. The current results show that the glutamate release in the CA3 region occurs via a GABA(A) pathway when GABA(B) receptors are inhibited.     

Regulation of neurotransmitter release by metabotropic glutamate receptors

The G protein-coupled metabotropic glutamate (mGlu) receptors are differentially localized at various synapses throughout the brain. Depending on the receptor subtype, they appear to be localized at presynaptic and/or postsynaptic sites, including glial as well as neuronal elements. The heterogeneous distribution of these receptors on glutamate and nonglutamate neurons/cells thus allows modulation of synaptic transmission by a number of different mechanisms. Electrophysiological studies have demonstrated that the activation of mGlu receptors can modulate the activity of Ca(2+) or K(+) channels, or interfere with release processes downstream of Ca(2+) entry, and consequently regulate neuronal synaptic activity. Such changes evoked by mGlu receptors can ultimately regulate transmitter release at both glutamatergic and nonglutamatergic synapses. Increasing neurochemical evidence has emerged, obtained from in vitro and in vivo studies, showing modulation of the release of a variety of transmitters by mGlu receptors. This review addresses the neurochemical evidence for mGlu receptor-mediated regulation of neurotransmitters, such as excitatory and inhibitory amino acids, monoamines, and neuropeptides.     

Influence of cooling rate on activity of ionotropic glutamate receptors in brain slices at hypothermia

Hypothermia is a known approach in the treatment of neurological pathologies. Mild hypothermia enhances the therapeutic window for application of medicines, while deep hypothermia is often accompanied by complications, including problems in the recovery of brain functions. The purpose of present study was to investigate the functioning of glutamate ionotropic receptors in brain slices cooled with different rates during mild, moderate and deep hypothermia. Using a system of gradual cooling combined with electrophysiological recordings in slices, we have shown that synaptic activity mediated by the alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid and N-methyl-D-aspartate receptors in rat olfactory cortex was strongly dependent on the rate of lowering the temperature. High cooling rate caused a progressive decrease in glutamate receptor activity in brain slices during gradual cooling from mild to deep hypothermia. On the contrary, low cooling rate slightly changed the synaptic responses in deep hypothermia. The short-term potentiation may be induced in slices by electric tetanization at 16  °C in this case. Hence, low cooling rate promoted preservation of neuronal activity and plasticity in the brain tissue.     

Nitric oxide-evoked glutamate release and cGMP production in cerebellar slices: control by presynaptic 5-HT1D receptors

We previously reported that pre- and postsynaptic 5-hydroxytryptamine (5-HT) receptors effectively control glutamatergic transmission in adult rat cerebellum. To investigate where 5-HT acts in the glutamate ionotropic receptors/nitric oxide/guanosine 3',5'-cyclic monophosphate (cGMP) pathway, in the present study 5-HT modulation of the cGMP response to the nitric oxide donor S-nitroso-penicillamine (SNAP) was studied in adult rat cerebellar slices. While cGMP elevation produced by high-micromolar SNAP was insensitive to 5-HT, 1 microM SNAP, expected to release nitric oxide in the low-nanomolar concentration range, elicited cGMP production and endogenous glutamate release both of which could be prevented by activating presynaptic 5-HT1D receptors. Released nitric oxide appeared responsible for cGMP production and glutamate release evoked by 1 microM SNAP, as both the effects were mimicked by the structurally unrelated nitric oxide donor 2-(N,N-diethylamino)-diazenolate-2-oxide (0.1 microM). Dependency of the 1 microM SNAP-evoked release of glutamate on external Ca2+, sensitivity to presynaptic release-regulating receptors and dependency on ionotropic glutamate receptor functioning, suggest that nitric oxide stimulates exocytotic-like, activity-dependent glutamate release. Activation of ionotropic glutamate receptors/nitric oxide synthase/guanylyl cyclase pathway by endogenously released glutamate was involved in the cGMP response to 1 microM SNAP, as blockade of NMDA/non-NMDA receptors, nitric oxide synthase or guanylyl cyclase, abolished the cGMP response. To conclude, in adult rat cerebellar slices low-nanomolar exogenous nitric oxide could facilitate glutamate exocytotic-like release possibly from parallel fibers that subsequently activated the glutamate ionotropic receptors/nitric oxide/cGMP pathway. Presynaptic 5-HT1D receptors could regulate the nitric oxide-evoked release of glutamate and subsequent cGMP production.