Action Potentials and Na+ voltage-gated ion channels in Placozoa

Placozoa are small disc-shaped animals, representing the simplest known, possibly ancestral, organization of free-living animals. With only six morphological distinct cell types, without any recognized neurons or muscle, placozoans exhibit fast effector reactions and complex behaviors. However, little is known about electrogenic mechanisms in these animals. Here, we showed the presence of rapid action potentials in four species of placozoans (Trichoplax adhaerens [H1 haplotype], Trichoplax sp.[H2], Hoilungia hongkongensis [H13], and Hoilungia sp. [H4]). These action potentials are sodium-dependent and can be inducible. The molecular analysis suggests the presence of 5-7 different types of voltage-gated sodium channels, which showed substantial evolutionary radiation compared to many other metazoans. Such unexpected diversity of sodium channels in early-branched animal lineages reflect both duplication events and parallel evolution of unique behavioral integration in these nerveless animals. HighlightsO_LIPlacozoans are the simplest known animals without recognized neurons and muscles C_LIO_LIWith only six morphological cell types, placozoans showed complex & rapid behaviors C_LIO_LISodium-dependent action potentials have been discovered in intact animals C_LIO_LIVoltage-gated sodium channels (Nav) in Placozoa support a rapid behavioral integration C_LIO_LIPlacozoans have more Nav channels that any studied invertebrate animal so far C_LIO_LIDiversification of Nav-channels highlight the unique evolution of these nerveless animals C_LI O_FIG O_LINKSMALLFIG WIDTH=200 HEIGHT=157 SRC="FIGDIR/small/243113v2_ufig1.gif" ALT="Figure 1"> View larger version (52K): org.highwire.dtl.DTLVardef@4d9c5forg.highwire.dtl.DTLVardef@15539bforg.highwire.dtl.DTLVardef@4246faorg.highwire.dtl.DTLVardef@141e5fe_HPS_FORMAT_FIGEXP M_FIG C_FIG