Rat cortical synaptosomes have more than one mechanism for Ca2+ entry linked to rapid glutamate release: studies using the Phoneutria nigriventer toxin PhTX2 and potassium depolarization.
PhTX2, one of the components of the venom of the South American spider Phoneutria nigriventer, inhibits the closure of voltage-sensitive Na+ channels. Incubation of cerebral-cortical synaptosomes with PhTX2 causes a rapid increase in the intrasynaptosomal free Ca2+ concentration and a dose-dependent release of glutamate. This release is made up of a slow component, which appears to be due to reversal of Na(+)-dependent glutamate uptake, and more rapid component that is dependent on the entry of extrasynaptosomal Ca2+. It has previously been shown that membrane depolarization using KCl can cause rapid Ca(2+)-dependent release of glutamate from synaptosomes. This requires Ca2+ entry through a specific type of Ca2+ channel that is sensitive to Aga-GI, a toxic component of the venom of the spider Agelenopsis aperta. We have compared the effects of PhTX2 and KCl on elevation of intrasynaptosomal free Ca2+ and glutamate release, and a number of differences have emerged. Firstly, PhTX2-mediated Ca2+ influx and glutamate release, but not those caused by KCl, are inhibited by tetrodotoxin. Secondly, KCl produces a clear additional increase in Ca2+ and glutamate release following those elicited by PhTX2. Finally, 500 microM MnCl2 abolishes PhTX2-mediated, but not KCl-mediated, glutamate release. These findings suggest that more than one mechanism of Ca2+ entry may be coupled to glutamate release from nerve endings.
Romano-Silva
M A
MA
Department of Biochemistry and Immunology, ICB, Universidade Federal de Minas Gerais, Belo Horizonte, Brazil.
Ribeiro-Santos
R
R
Ribeiro
A M
AM
Gomez
M V
MV
Diniz
C R
CR
Cordeiro
M N
MN
Brammer
M J
MJ
D000818:Animals; D002540:Cerebral Cortex / Q000378:metabolism*; D002712:Chlorides / Q000494:pharmacology; D005971:Glutamates / Q000378:metabolism*; D018698:Glutamic Acid; D006096:Gramicidin / Q000494:pharmacology; D007700:Kinetics; D008297:Male; D017895:Manganese Compounds / Q000494:pharmacology; D009479:Neuropeptides / Q000633:toxicity*; D009498:Neurotoxins / Q000633:toxicity*; D011189:Potassium Chloride / Q000494:pharmacology*; D051381:Rats; D017208:Rats, Wistar; D012964:Sodium / Q000378:metabolism; D015222:Sodium Channels / Q000187:drug effects / Q000502:physiology; D013111:Spider Venoms / Q000633:toxicity; D013574:Synaptosomes / Q000187:drug effects / Q000378:metabolism*; D013779:Tetrodotoxin / Q000633:toxicity
D002712:Chlorides; D005971:Glutamates; D017895:Manganese Compounds; D009479:Neuropeptides; D009498:Neurotoxins; D015222:Sodium Channels; D013111:Spider Venoms; D006096:Gramicidin; C076902:Tx2 neurotoxin; D018698:Glutamic Acid; D013779:Tetrodotoxin; D011189:Potassium Chloride; D012964:Sodium; C025340:manganese chloride