Since the initial studies by Irving and Scholander in the 1940s, the focus of research on diving in marine animals has been on the classical `dive response'. This reflex serves to conserve oxygen by lowering heart rate and decreasing peripheral blood flow to preserve stored O2 for the brain, heart, and lungs, the tissues considered most vulnerable to oxygen deprivation (hypoxia). Low blood flow also cools tissues and thus decreases energy demand, which coupled with typically high blood and tissue oxygen stores was thought to explain how diving marine mammals and birds can remain submerged for (in some cases) over an hour. But Lars Folkow and his colleagues at the University of Tromsø, Norway, still wondered how seals not only survive extended dives but also remain alert when blood oxygen levels drop so low that they would cause unconsciousness and neural damage in non-divers. They hypothesized that, like neonate mammals and anoxia-tolerant turtles, the brains of diving marine mammals are intrinsically resistant to low oxygen levels. Falkow knew they would have to measure brain electrical activity to find out how the tissue copes with low oxygen levels, but how could they set about doing electrophysiology studies on diving seals?
The team gained access to hooded seal brains while on a research cruise to the Greenland Sea and were able to investigate how the tissue responded to low oxygen levels, like those the seal brain experiences during a dive. Slicing the brains into thin sections and maintaining them under oxygenated warm artificial cerebrospinal fluid, the team then exposed the brain samples to severe hypoxia by bubbling 95% nitrogen through the fluid for 1 h. This rapidly reduced the O2 tension to less than 2% at the surface of the cortical slices, and to undetectable levels midway through the slice. They also tried similar studies (on land!) on adult and neonatal mice. Adult mice die rapidly when exposed to severe hypoxia, and thus served as controls for`typical' brain responses. Neonate mammals, by contrast, are relatively resistant to low oxygen, and these thus served as controls demonstrating hypoxia tolerance.
The investigators found that while the neuronal resting membrane potential was similar in all three animals, hypoxia caused a slight depolarization(about 13 mV in seals) and a depolarization of 21 to 26 mV in neonate mice during a 5 to 10 min hypoxic exposure. Thus the seals and newborn mice brains were relatively unaffected by the oxygen depletion, although the seals were better protected than the baby mice. By contrast, adult mouse neurons completely depolarized (65 mV) over 10 min hypoxia, and ceased to discharge at all within 5 min, showing that hypoxia is indeed fatal for the adult mouse brain tissue. In hooded seals and neonate mice, though, at least some neurons were still firing after 60 min hypoxia.
The investigators conclude that the neurons of the hooded seal, at least,are indeed highly hypoxia-tolerant. This tolerance is presumably due to the existence of similar energy-conserving pathways to those that have been well studied in the anoxia-tolerant turtle, such as reductions in ion flux, though the group points out that, unlike hibernating turtles, diving seals must remain alert and thus cannot completely suppress brain activity. The authors suggest that the seals perhaps employ selective hypometabolism, shutting down some sections of the brain while others remain alert. Simultaneous work and sleep, what a great idea!
