ant Na channel that is resistant to TTX, thus allowing these snakes to
safely eat the TTX-containing newts.
Other poisons besides TTX bind with voltage-gated sodium
channels and block the passage of Na through the pore in the chan-
nel protein. Saxitoxin (STX), a molecule found in several species of
dinoflagellates, also blocks Na* channels in a similar way to TTX and
similarly interferes with nervous system function. Dinoflagellates are
single-celled marine organisms, part of a diverse group referred to as
plankton. Dinoflagellates are consumed by shellfish (such as clams,
mussels, and scallops) and humans can be exposed to this poison
if they eat shellfish that have accumulated STX. This is especially
a problem during dinoflagellate blooms—population explosions of
dinoflagellates. (Because these dinoflagellates may have red pigment,
the blooms are sometimes called red tides.) Coastal areas are occasion-
ally closed to the collection of shellfish during dinoflagellate blooms
to reduce the risk of human exposure to this poison.
HN N2a o =. OH
OH
Saxitoxin
The medical condition resulting from STX poisoning has been
called paralytic shellfish poisoning (PSP), because in humans it would
most often result from consuming shellfish containing STX. The
symptoms of PSP are like those of TTX poisoning: numbness, muscle
weakness, and paralysis, with death, if it occurs, from respiratory