Density of sodium channels in insect synaptic nerve endings

Specific binding of [11-(3)H]saxitoxin (STX) and activity of ouabain sensitive adenosine-triphosphatase (Na(+), K(+)-ATPase) were determined in neuronal membrane fractions using a subcellular preparation from the central nervous system of the cockroach Periplaneta americana. The nerve ending fractions (synaptosomes) contained 90-95% of the total specific activity of ouabain sensitive Na(+), K(+)-ATPase, and 60-70% of specific STX binding of the crude nerve homogenate. Sodium influx induced by veratridine in synaptosomes was inhibited by saxitoxin at a half-maximal concentration of 4 nM, and kinetics were consistent with reversible binding of one molecule of saxitoxin to each sodium channel receptor site, with an equilibrium dissociation constant (K(D)) of approx. 3 nM. The density of saturable binding sites was 2 pmol/mg protein which was estimated to correspond to about 95 binding sites per ?m(2) of synaptic membrane. The results of transport and binding data show that insect synaptosomes possess the capability to conduct inward sodium currents at least comparable to those found in other neuronal membranes, and thus provide a physiologically viable preparation to assess the effect of cation fluxes on the synaptic transmitter release process.