6-methyl-2-(phenylethynyl)pyridine and (alpha-carboxycyclopropyl)glycine

Hyperammonemia impairs signal transduction associated to glutamate receptors and phosphorylation of some neuronal proteins including microtubule-associated protein 2 (MAP-2). The aim of this work was to analyze the effects of hyperammonemia on modulation of MAP-2 phosphorylation by metabotropic glutamate receptors (mGluRs) in rat cerebellar neurons in culture. Hyperammonemia increased basal phosphorylation of MAP-2 (180%). Activation of mGluRs 1 and 5 with (S)-3,5-dihydroxyphenylglycine (DHPG) increased MAP-2 phosphorylation (170%) in control neurons but not in neurons exposed to ammonia. Activation of mGluRs 2 and 3 with (2S,3S,4S)-CCG/(2S, 1'S,2'S)-2-(carboxycyclopropyl)glycine increased slightly (25%) MAP-2 phosphorylation in neurons exposed to ammonia or not. Activation of mGluR5 with (+/-)-trans-azetidine-2,4-dicarboxylic acid increased MAP-2 phosphorylation (24%) in control neurons but decreased it by 56% in neurons exposed to ammonia. Activation of mGluR1 using 2-methyl-6-(phenylethynyl)pyridine and DHPG increased MAP-2 phosphorylation 183% in control neurons but only 89% in neurons exposed to ammonia. In control neurons mGluR1 activation greatly increases phosphorylation of MAP-2, while activation of mGluRs 5, 2 or 3 increased it slightly. Taken together, hyperammonemia reduces the increase in MAP-2 phosphorylation induced by mGluR1activation. Moreover, in neurons exposed to ammonia activation of mGluR5 reduces MAP-2 phosphorylation. These effects reflect significant alterations in signal transduction associated to mGluR1 and mGluR5 in hyperammonemia that may contribute to altered glutamatergic neurotransmission and to the neurological alterations in hyperammonemia and hepatic encephalopathy.

Topics: Amino Acids, Dicarboxylic; Ammonia; Animals; Azetidinecarboxylic Acid; Cells, Cultured; Cerebellum; Excitatory Amino Acid Agonists; Immunoprecipitation; Methoxyhydroxyphenylglycol; Microtubule-Associated Proteins; Neurons; Phosphorylation; Pyridines; Rats; Rats, Wistar; Receptor, Metabotropic Glutamate 5; Receptors, Metabotropic Glutamate

We have previously shown that the release of central neurotransmitters can be modulated by the activation of Group I and Group II subtypes of G-protein-linked metabotropic glutamate (mGlu) receptors. To date, however, very little is known about the regulation of serotonergic neurotransmission by these receptor subtypes. In the present study, we have utilized in vivo intracerebral microdialysis to elucidate the roles of Group I and Group II mGlu receptors in the regulation of neuronal 5-hydroxytryptamine (5-HT) release in the frontal cortex of conscious, freely moving rats. Dialysate 5-HT was of neuronal origin with basal release showing strong calcium dependency and tetrodotoxin sensitivity and marked elevation following K(+)-induced depolarization. The broad-spectrum mGlu receptor agonist (1S,3R)-1-aminocyclopentane-1,3-dicarboxylic acid [(1S,3R)-ACPD; 1-3 mM] did not significantly modify basal cerebrocortical 5-HT release. Similarly, the Group I mGlu receptor-specific agonist (RS)-3,5-dihydroxyphenylglycine [(RS)-3,5-DHPG; 1-3 mM] showed no marked effect on cortical dialysate 5-HT levels. To eliminate the possibility that these findings were the result of receptor desensitization, the effects of lower concentrations of (RS)-DHPG (100-300 microM) and shorter ligand exposure time (15 min) were also evaluated. Dialysate 5-HT levels remained unmodified by these manipulations. In comparison, the Group II mGlu receptor agonist, (2S,1'S,2'S)-2-(carboxycyclopropyl)glycine (L-CCG-1; 500 microM), evoked a marked facilitation of release (approximately 150% of basal) which was fully reversed by the Group I/II antagonist, (S)-alpha-methyl-4-carboxyphenylglycine [(S)-MCPG; 3 mM]. The modulatory action of L-CCG-1 showed a bell-shaped concentration-response relationship. (S)-MCPG (3 mM) and the potent and selective mGlu(5) receptor antagonist, 2-methyl-6-(phenylethynyl)pyridine (MPEP; 100 microM), when given alone, did not significantly modify 5-HT levels.The current data provide strong evidence to suggest that while the release of neuronal 5-HT in the rat frontal cortex is not subject to regulation by facilitatory Group I mGlu receptors, it may be positively modulated by activation of Group II mGlu receptors. Taken together with data from other studies, the present investigation lends emphasis to the notion that neuromodulation by mGlu receptors is a region-specific phenomenon and also proposes that the heterogeneous distribution of these receptors is neurone-specific

Topics: Amino Acids, Dicarboxylic; Analysis of Variance; Anesthetics, Local; Animals; Benzoates; Calcium; Cerebral Cortex; Cycloleucine; Dose-Response Relationship, Drug; Excitatory Amino Acid Agonists; Excitatory Amino Acid Antagonists; Glycine; Ligands; Male; Microdialysis; Potassium Chloride; Pyridines; Rats; Rats, Wistar; Receptors, Metabotropic Glutamate; Serotonin; Synaptic Transmission; Tetrodotoxin