Facilitatory effect of glutamate exocytosis from rat cerebrocortical nerve terminals by alpha-tocopherol, a major vitamin E component.
The effect of alpha-tocopherol, the major vitamin E component, on the release of endogenous glutamate has been investigated using rat cerebrocortical nerve terminals. Results showed that alpha-tocopherol facilitated the Ca2+-dependent but not the Ca2+-independent glutamate release evoked by 4-aminopyridine (4AP). This release facilitation was insensitive to glutamate transporter inhibitor L-trans-PDC or DL-TBOA, and blocked by the exocytotic neurotransmitter release inhibitor tetanus neurotoxin, indicating that alpha-tocopherol affects specifically the physiological exocytotic vesicular release without affecting the non-vesicular release. Facilitation of glutamate exocytosis by alpha-tocopherol was not due to its increasing synaptosomal excitability, because alpha-tocopherol did not alter the 4AP-evoked depolarization of the synaptosomal plasma membrane potential. Rather, examination of the effect of alpha-tocopherol on cytoplasmic free Ca2+ concentration revealed that the facilitation of glutamate release could be attributed to an increase in voltage-dependent Ca2+ influx. Consistent with this, the alpha-tocopherol-mediated facilitation of glutamate release was significantly reduced in synaptosomes pretreated with omega-CgTX MVIIC, a wide spectrum blocker of N- and P/Q-type Ca2+ channels. In addition, alpha-tocopherol modulation of glutamate release appeared to involve a protein kinase C (PKC) signalling cascade, insofar as pretreatment of synaptosomes with the PKC inhibitor GF109203X effectively suppressed the facilitatory effect of alpha-tocopherol on 4AP- or ionomycin-evoked glutamate release. Furthermore, alpha-tocopherol increased the phosphorylation of MARCKS, the major presynapic substrate for PKC, and this effect was also significantly attenuated by PKC inhibition. Together, these results suggest that alpha-tocopherol exerts an increase in PKC activation, which subsequently enhances voltage-dependent Ca2+ influx and vesicular release machinery to cause an increase in evoked glutamate release from rat cerebrocortical glutamatergic terminals. This finding might provide important information regarding to the action of vitamin E in the central nervous system.
Yang
Tsung-Tsair
TT
School of Medicine, Fu Jen Catholic University, 510, Chung-Cheng Road, Hsin-Chuang, Taipei, Hsien 24205, Taiwan.
Wang
Su-Jane
SJ
D015761:4-Aminopyridine / Q000494:pharmacology; D000818:Animals; D015220:Calcium Channels / Q000187:drug effects / Q000378:metabolism; D020013:Calcium Signaling / Q000187:drug effects / Q000502:physiology; D002540:Cerebral Cortex / Q000187:drug effects / Q000378:metabolism*; D004789:Enzyme Activation / Q000187:drug effects / Q000502:physiology; D005089:Exocytosis / Q000187:drug effects / Q000502:physiology*; D018698:Glutamic Acid / Q000378:metabolism*; D047908:Intracellular Signaling Peptides and Proteins / Q000187:drug effects / Q000378:metabolism; D008297:Male; D008565:Membrane Proteins / Q000187:drug effects / Q000378:metabolism; D008666:Metalloendopeptidases / Q000494:pharmacology; D000076250:Myristoylated Alanine-Rich C Kinase Substrate; D026902:Potassium Channel Blockers / Q000494:pharmacology; D017729:Presynaptic Terminals / Q000187:drug effects / Q000378:metabolism*; D011493:Protein Kinase C / Q000187:drug effects / Q000378:metabolism; D051381:Rats; D017207:Rats, Sprague-Dawley; D013570:Synaptic Membranes / Q000187:drug effects / Q000378:metabolism; D009435:Synaptic Transmission / Q000187:drug effects / Q000502:physiology; D013574:Synaptosomes; D013744:Tetanus Toxin / Q000494:pharmacology; D050496:Vesicular Glutamate Transport Proteins / Q000037:antagonists & inhibitors / Q000378:metabolism; D033921:Vesicular Transport Proteins / Q000187:drug effects / Q000378:metabolism; D024502:alpha-Tocopherol / Q000378:metabolism* / Q000494:pharmacology
D015220:Calcium Channels; D047908:Intracellular Signaling Peptides and Proteins; C000619712:Marcks protein, rat; D008565:Membrane Proteins; D026902:Potassium Channel Blockers; D013744:Tetanus Toxin; D050496:Vesicular Glutamate Transport Proteins; D033921:Vesicular Transport Proteins; C035979:tetanospasmin; D000076250:Myristoylated Alanine-Rich C Kinase Substrate; D018698:Glutamic Acid; D015761:4-Aminopyridine; D011493:Protein Kinase C; D008666:Metalloendopeptidases; D024502:alpha-Tocopherol