Responses of the in vitro turtle brain to visual and auditory stimuli during severe hypoxia

Abstract

North American pond turtles (Emydidae) are renowned for their ability to survive extreme hypoxia and anoxia, which enables several species to overwinter in ice-locked, anoxic freshwater ponds and bogs for months. Centrally important for surviving these conditions is a profound metabolic suppression, which enables ATP demands to be met entirely with glycolysis. Despite this, turtles have occasionally been observed exploring their natural and laboratory environments while anoxic and can still respond to sensory stimuli. To better understand whether anoxia limits a special sensory function, we recorded evoked potentials in a reduced brain preparation, in vitro, that was perfused with severely hypoxic artificial cerebral spinal fluid (aCSF). For recordings of visual responses, an LED light was flashed onto retinal eyecups while evoked potentials were recorded from the retina or the optic tectum. For recordings of auditory responses, a piezo motor-controlled glass actuator vibrated the tympanic membrane while evoked potentials were recorded from the cochlear nuclei. We found that visual responses decreased even with moderately hypoxic perfusate (aCSF PO2 = 30-60 torr) and completely abolished under severe hypoxia. In contrast, the evoked response within the cochlear nuclei was unattenuated with severe hypoxia (aCSF PO2 < 20 torr). These data provide further support that pond turtles have a limited ability to sense visual information in their environment even while moderately hypoxic, but that auditory input may become a principal avenue of sensory perception during extreme diving in this species. Summary StatementSevere hypoxia attenuates the visual, but not auditory responses in a reduced brain preparation from a pond turtle.