CALCIUM-ION IMPEDES TRANSLATION INITIATION AT THE SYNAPSE

Stimulation of synaptoneurosome suspensions by the neurotransmitter gl utamate gives rise to rapid loading of ribosomes onto mRNA and increas ed incorporation of amino acids into trichloroacetic acid-precipitable polypeptides. Metabotropic glutamate receptors (mGluRs) are responsib le for this effect, Although simultaneous Ca2+ entry and mGluR stimula tion do not change the response, entry of Ca2+ 30 s or 3 min before mG luR stimulation markedly depresses the polyribosomal loading. Either N MDA or ionophore (A23187) produces the depression. A calmodulin antago nist, W7, alleviates the effect, suggesting that inactivation of phosp holipase A(2) by calcium-calmodulin-dependent kinase II is partially r esponsible for the phenomenon. Thus, interaction between different cla sses of glutamate receptors affects the control of protein translation at the synapse. This effect may partially explain recent observations of negative interactions between receptor classes in induction of lon gterm potentiation.