SODIUM-CHANNEL DISTRIBUTION IN A SPIDER MECHANOSENSORY ORGAN

A site-directed antibody was used immunocytochemically to measure the distribution of sodium channels in the tissues of a spider mechanorece ptor organ. The VS-3 slit sense organ contains 7-8 pairs of bipolar se nsory neurons; these neurons are representative of a wide range of art hropod mechanoreceptors. Sensory transduction is thought to occur at t he tips of the dendrites and to cause action potentials that are regen eratively conducted to the cell bodies, although it has not been possi ble to confirm this by direct intracellular recordings from the dendri tes. Wholemount preparations were labelled by immunofluorescence and t hin sections were immunogold labelled, using an antibody to the highly conserved SP19 sequence of the voltage-activated sodium channel. Labe lling for sodium channels was found in the neurons and in their surrou nding glial cells. Both cytoplasm and membranes were labelled, but imm unogold particles were clearly aligned along cell membranes, indicatin g that the majority of labelling represented membrane-bound sodium cha nnels. Channel density in the dendrites was similar to the axons and h igher than in the cell bodies, supporting the idea of active conductio n in the sensory dendrites. Labelling in glial cell membranes was indi stinguishable from the neighboring neurons, suggesting a significant r ole for sodium channels in the functions of these supporting cells.