Involvement of pentylenetetrazole in synapsin I phosphorylation associated with calcium influx in synaptosomes from rat cerebral cortex.

To determine precisely how pentylenetetrazole (PTZ) is involved in the biochemical processes at the presynaptic nerve terminal, the effect of PTZ, under various conditions, on the phosphorylation of synapsin I (previously called protein I) was investigated, using 32Pi in synaptosomes from rat cerebral cortex. PTZ markedly stimulated the incorporation of 32P into this protein as determined by sodium dodecyl sulfate (SDS)-polyacrylamide gel electrophoresis and autoradiography, but it failed to stimulate protein phosphorylation in Ca2+-free medium containing ethylene glycol bis-(beta-aminoethylether)-N',N'-tetraacetic acid (EGTA). Moreover, the PTZ-stimulated synapsin I phosphorylation was reversed by addition of EGTA sufficient to chelate all external free Ca2+. PTZ also stimulated synaptosomal accumulation of Ca2+. The PTZ-stimulatory effects of both synapsin I phosphorylation and synaptosomal accumulation of Ca2+ were inhibited markedly by tetrodotoxin as well as by cobalt chloride and lanthanum chloride. The calmodulin antagonists N-(6-aminohexyl)-5-chloro-1-naphthalenesulfonamide (W-7, strongly) and N-(6-aminohexyl)-1-naphthalenesulfonamide (W-5, weakly) reduced the PTZ-stimulatory effect on synapsin I phosphorylation by about 75 and 15%, respectively, whereas these antagonists had essentially no effect on PTZ-stimulated synaptosomal accumulation of Ca2+. These results suggest that PTZ causes the influx of Ca2+ into the presynaptic nerve terminal secondary to the elevated Na+ and is consequently involved in the synapsin I phosphorylation step, facilitating the Ca2+/calmodulin-mediated presynaptic event leading to seizure discharge.