Cadmium rapidly and irreversibly blocks presynaptic phospholipase C-linked metabotropic glutamate receptors.
Calcium ions (Cd2+) inhibit inositol phosphate (IP) formation elicited by glutamate (GLU) or K+ ions, without affecting carbachol (Carb)-induced IP response in 8-day-old rat forebrain synaptoneurosomes and synaptosomes. On the contrary, Cd2+ was almost ineffective in blocking GLU- and K(+)-responses in hippocampal neurones in culture. The mechanism of Cd2+ inhibition was thus examined in synaptoneurosomes. Extensive washing of synaptoneurosomes pretreated for 1, 5, 15, or 30 min by 100 microM Cd2+ did not modify the inhibitory effect of Cd2+ on GLU-, K(+)- and A23187-evoked IP formation or its lack of effect on Carb response. The later addition of a high affinity Cd2+ chelator (100 microM), N,N,N',N'-tetrakis(2-pyridylmethyl)ethylenediamine (TPEN) also did not reverse the inhibitory effect. TPEN, however, penetrates into synaptoneurosomes and efficiently displaces Cd2+ from the Fura-2-Cd2+ complex as shown by Fura-2 fluorescence recordings. TPEN is not easily removed from the intracellular space, as demonstrated by its ability to still block Cd(2+)-induced Fura-2 fluorescence increase after extensive washing. Pretreatment of synaptoneurosomes by this chelator did not prevent Cd2+ inhibition of GLU-induced IP formation. These data indicate that Cd2+ ions rapidly, irreversibly and extracellularly inhibit GLU-elicited IP formation in synaptoneurosomes or synaptosomes, but not in hippocampal neurones in culture. It is speculated that Cd2+ ions could allow one to distinguish the activity of presynaptic metabotropic glutamate receptors (mGLURs) linked to phosphoinositide metabolism from that of mGLURs located postsynaptically.
Vignes
M
M
INSERM U254, Hôpital St Charles, Montpellier, France.
Blanc
E
E
Davos
F
F
Guiramand
J
J
Récasens
M
M
D000818:Animals; D001921:Brain / Q000166:cytology / Q000378:metabolism; D002104:Cadmium / Q000494:pharmacology*; D000001:Calcimycin / Q000494:pharmacology; D018698:Glutamic Acid / Q000494:pharmacology; D006624:Hippocampus / Q000166:cytology / Q000378:metabolism; D011188:Potassium / Q000494:pharmacology; D017729:Presynaptic Terminals / Q000378:metabolism*; D016318:Quisqualic Acid / Q000494:pharmacology; D051381:Rats; D017470:Receptors, Glutamate / Q000378:metabolism*; D013574:Synaptosomes / Q000378:metabolism; D010738:Type C Phospholipases / Q000378:metabolism*
D017470:Receptors, Glutamate; D002104:Cadmium; D000001:Calcimycin; D018698:Glutamic Acid; D016318:Quisqualic Acid; D010738:Type C Phospholipases; D011188:Potassium