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Synaptic vesicles contain a protein (agglutinin) which binds cell plasma membranes. The protein activity is titrated measuring the agglutination of rabbit trypsinized-fixed red blood cells induced by serial dilutions of purified vesicles. The protein is highly concentrated in vesicles: the specific activity is 150-fold higher in vesicles compared to the brain homogenate, and 20-fold higher compared to the crude synaptosomal membrane fraction. The binding is specifically inhibited by the four major brain sialoglycosphingolipids, with a higher affinity for polysialo- rather than for monosialoganglioside (GM1) IC50 are 3.9 x 10(-5) M (GT1b), 8.9 x 10(-5) M (GD1a), 2.8 x 10(-4) M (GD1b) and 4.9 x 10(-4) M (GM1). Cells which are not agglutinated by the vesicles (human red blood cells) can be activated by incubation with gangliosides: insertion of the glycolipids in the plasma membranes makes them sensitive to the vesicle haemagglutinin.

CaM kinase II, a regulator of synaptic plasticity, is implicated in pathophysiology and pharmacology of psychiatric disorders. Chronic treatment with antidepressants desipramine and reboxetine up-regulated CaM kinase II in neuronal cell bodies of hippocampus. mRNA/protein expression for αCaM kinase II was unchanged, whereas Thr 286 phosphorylation was increased in pyramidal/granular cell bodies, suggesting that increased phosphorylation is responsible for kinase activation. Short-term treatment of neuronal cultures with reboxetine reduced kinase activation in a concentration-dependent manner. The short-term inhibitory effect of reboxetine suggests that kinase up-regulation during antidepressant drug treatment is an adaptive response compensating for initial functional down-regulation.

Purified synaptic vesicles are highly enriched with a protein which binds cell plasma membranes. The binding is selective for acidic phospholipids and sialoglycosphingolipids. In partition chromatography of vesicle proteins, the binding activity was co-eluted with a limited set of proteins. Among them the most abundant species were two vesicle-specific proteins: p65 and synaptophysin. In affinity chromatography of vesicle proteins, only p65 bound to a column of immobilized lysoganglioside. The same protein, purified by preparative electrophoresis, retained the binding activity and fully reproduced the hemagglutinating property and the selectivity for acidic lipids of whole vesicles. The results suggest that the (hemagglutinating) lipid binding properties of the vesicles are mainly if not exclusively due to p65.