brevetoxin and Hypoxia

Harmful algal blooms are increasing in frequency and extent worldwide and occur nearly annually off the west coast of Florida where they affect both humans and wildlife. The dinoflagellate Karenia brevis is a key organism in Florida red tides that produces a suite of potent neurotoxins collectively referred to as the brevetoxins (PbTx). Brevetoxins bind to and open voltage gated sodium channels (VGSC), increasing cell permeability in excitable cells and depolarizing nerve and muscle tissue. Exposed animals may thus show muscular and neurological symptoms including head bobbing, muscle twitching, paralysis, and coma; large HABs can result in significant morbidity and mortality of marine life, including fish, birds, marine mammals, and sea turtles. Brevetoxicosis however is difficult to treat in endangered sea turtles as the physiological impacts have not been investigated and the magnitude and duration of brevetoxin exposure are generally unknown. In this study we used the freshwater turtle Trachemys scripta as a model organism to investigate the effects of the specific brevetoxin PbTx-3 in the turtle brain. Primary turtle neuronal cell cultures were exposed to a range of PbTx-3 concentrations to determine excitotoxicity. Agonists and antagonists of voltage-gated sodium channels and downstream targets were utilized to confirm the toxin's mode of action. We found that turtle neurons are highly resistant to PbTx-3; while cell viability decreased in a dose dependent manner across PbTx-3 concentrations of 100-2000nM, the EC

Topics: Animals; Calcium; Cell Survival; Cells, Cultured; Dizocilpine Maleate; Exocytosis; Female; Florida; Harmful Algal Bloom; Humans; Hypoxia; Marine Toxins; Neurons; Oxocins; Receptors, N-Methyl-D-Aspartate; Signal Transduction; Tetrodotoxin; Turtles; Voltage-Gated Sodium Channels; Water Pollutants

In contrast to mammalian brain, which exhibits rapid degeneration during anoxia, the brains of certain species of turtles show an extraordinary capacity to survive prolonged anoxia. The decrease in energy expenditure shown by the anoxic turtle brain is likely to be a key factor for anoxic survival. The "channel arrest" hypothesis proposes that ion channels, which regulate brain electrical activity in normoxia, may be altered during anoxia in the turtle brain as a mechanism to spare energy. Goals of present research were to test this hypothesis and to determine whether down-regulation of sodium channels is a possible explanation for spike threshold shifts seen during anoxia in isolated turtle cerebellum. We report here that anoxia induced a significant (42%) decline in voltage-gated sodium channel density as determined by studies of the binding of a sodium channel ligand, [3H]brevetoxin. This study demonstrates that sodium channel densities in brain may be regulated by tissue oxygenation or by physiological events associated with anoxia. Moreover, it also suggests that downregulation of sodium channels may be a basis for changes in action potential thresholds, the electrical depression and energy conservation that provide the unique anoxic tolerance of turtle brain.

Topics: Animals; Binding Sites; Binding, Competitive; Cerebellum; Down-Regulation; Hypoxia; Marine Toxins; Oxocins; Sodium Channels; Turtles