title: Cyanocobalamin, vitamin B12, depresses glutamate release through inhibition of voltage-dependent Ca2+ influx in rat cerebrocortical nerve terminals (synaptosomes).
abstract: The effect of cyanocobalamin, vitamin B12, on glutamate release in isolated nerve terminals (synaptosomes) prepared from rat prefrontal cortex was examined. Cyanocobalamin inhibited the release of glutamate evoked by 4-aminopyridine in a concentration-dependent manner. The inhibitory action of cyanocobalamin was blocked by the vesicular transporter inhibitor bafilomycin A1, not by the glutamate transporter inhibitor L-transpyrrolidine-2,4-dicarboxylic acid or the nontransportable glutamate inhibitor DL-threo-beta-benzyloxyaspartate, indicating that this release inhibition results from a reduction of vesicular exocytosis and not from an inhibition of Ca(2+)-independent efflux via glutamate transporter. Examination of the effect of cyanocobalamin on cytosolic free Ca(2+) concentration revealed that the inhibition of glutamate release could be attributed to a reduction in voltage-dependent Ca(2+) influx. Consistent with this, the N- and P/Q-type Ca(2+) channel blocker omega-conotoxin MVIIC, largely attenuated the inhibitory effect of cyanocobalamin on 4-aminopyridine-evoked glutamate release, but the Ca(2+) release inhibitor dantrolene had no effect. Cyanocobalamin did not alter the resting synaptosomal membrane potential or 4-aminopyridine-mediated depolarization; thus, the inhibition of 4-aminopyridine-evoked Ca(2+) influx and glutamate release produced by cyanocobalamin was not due to its decreasing synaptosomal excitability. In addition, cyanocobalamin-mediated inhibition of 4-aminopyridine-evoked Ca(2+) influx and glutamate release was significantly attenuated by protein kinase C inhibitors GF109203X and Ro318220. Furthermore, 4-aminopyridine-induced phosphorylation of protein kinase C was significantly reduced by cyanocobalamin. These results suggest that cyanocobalamin effects a decrease in protein kinase C activation, which subsequently reduces the Ca(2+) entry through voltage-dependent N- and P/Q-type Ca(2+) channels to cause a decrease in evoked glutamate release.
authors.affiliation: School of Medicine, Fu Jen Catholic University, Hsin-Chuang, Taipei Hsien, Taiwan.
authors.forename: Kun-Long
authors.initials: KL
authors.lastname: Hung
authors.forename: Chia-Chuan
authors.initials: CC
authors.lastname: Wang
authors.forename: Chia-Yu
authors.initials: CY
authors.lastname: Huang
authors.forename: Su-Jane
authors.initials: SJ
authors.lastname: Wang
mesh_terms: D015761:4-Aminopyridine / Q000494:pharmacology; D027322:Amino Acid Transport System X-AG / Q000037:antagonists & inhibitors / Q000378:metabolism; D000818:Animals; D002118:Calcium / Q000378:metabolism*; D002121:Calcium Channel Blockers / Q000494:pharmacology*; D015220:Calcium Channels / Q000378:metabolism*; D020864:Calcium Channels, N-Type / Q000378:metabolism; D002614:Chelating Agents / Q000378:metabolism; D018698:Glutamic Acid / Q000378:metabolism*; D042541:Intracellular Space / Q000187:drug effects / Q000378:metabolism; D008297:Male; D008564:Membrane Potentials / Q000187:drug effects; D018696:Neuroprotective Agents / Q000494:pharmacology; D011493:Protein Kinase C / Q000378:metabolism; D051381:Rats; D017207:Rats, Sprague-Dawley; D015398:Signal Transduction / Q000187:drug effects; D013574:Synaptosomes / Q000187:drug effects* / Q000378:metabolism*; D050496:Vesicular Glutamate Transport Proteins / Q000037:antagonists & inhibitors / Q000378:metabolism; D014805:Vitamin B 12 / Q000494:pharmacology*
chemical_list: D027322:Amino Acid Transport System X-AG; D002121:Calcium Channel Blockers; D015220:Calcium Channels; D020864:Calcium Channels, N-Type; D002614:Chelating Agents; D018696:Neuroprotective Agents; D050496:Vesicular Glutamate Transport Proteins; D018698:Glutamic Acid; D015761:4-Aminopyridine; D011493:Protein Kinase C; D014805:Vitamin B 12; D002118:Calcium