Painted turtle cortex is resistant to an in vitro mimic of the ischemic mammalian penumbra.

Anoxia or ischemia causes hyperexcitability and cell death in mammalian neurons. Conversely, in painted turtle brain anoxia increases γ-amino butyric acid (GABA)ergic suppression of spontaneous electrical activity, and cell death is prevented. To examine ischemia tolerance in turtle neurons, we treated cortical sheets with an in vitro mimic of the penumbral region of stroke-afflicted mammalian brain (ischemic solution, IS).

Neuronal membrane potential is mildly depolarized in the anoxic turtle cortex.

Neuronal membrane potential (E(m)) regulates the activity of excitatory voltage-sensitive channels. Anoxic insults lead to a severe loss of E(m) and excitotoxic cell death (ECD) in mammalian neurons. Conversely, anoxia-tolerant freshwater turtle neurons depress energy usage during anoxia by altering ionic conductance to reduce neuronal excitability and ECD is avoided.

Endogenous reductions in N-methyl-d-aspartate receptor activity inhibit nitric oxide production in the anoxic freshwater turtle cortex.

Increased nitric oxide (NO) production from hypoxic mammalian neurons increases cerebral blood flow (CBF) but also glutamatergic excitotoxicity and DNA fragmentation. Anoxia-tolerant freshwater turtles have evolved NO-independent mechanisms to increase CBF; however, the mechanism(s) of NO regulation are not understood. In turtle cortex, anoxia or NMDAR blockade depressed NO production by 27+/-3% and 41+/-5%, respectively.

Adenosine A1 receptor activation mediates NMDA receptor activity in a pertussis toxin-sensitive manner during normoxia but not anoxia in turtle cortical neurons.

Adenosine is a defensive metabolite that is critical to anoxic neuronal survival in the freshwater turtle. Channel arrest of the N-methyl-d-aspartate receptor (NMDAR) is a hallmark of the turtle's remarkable anoxia tolerance and adenosine A1 receptor (A1R)-mediated depression of normoxic NMDAR activity is well documented. However, experiments examining the role of A1Rs in regulating NMDAR activity during anoxia have yielded inconsistent results.

Mitochondrial ATP-sensitive K+ channels regulate NMDAR activity in the cortex of the anoxic western painted turtle.

Hypoxic mammalian neurons undergo excitotoxic cell death, whereas painted turtle neurons survive prolonged anoxia without apparent injury. Anoxic survival is possibly mediated by a decrease in N-methyl-d-aspartate receptor (NMDAR) activity and maintenance of cellular calcium concentrations ([Ca(2+)](c)) within a narrow range during anoxia. In mammalian ischaemic models, activation of mitochondrial ATP-sensitive K(+) (mK(ATP)) channels partially uncouples mitochondria resulting in a moderate increase in [Ca(2+)](c) and neuroprotection.

AMPA receptors undergo channel arrest in the anoxic turtle cortex.

Without oxygen, all mammals suffer neuronal injury and excitotoxic cell death mediated by overactivation of the glutamatergic N-methyl-D-aspartate receptor (NMDAR). The western painted turtle can survive anoxia for months, and downregulation of NMDAR activity is thought to be neuroprotective during anoxia. NMDAR activity is related to the activity of another glutamate receptor, the alpha-amino-3-hydroxy-5-methylisoxazole-4-propionic acid receptor (AMPAR).

Anoxia-induced changes in reactive oxygen species and cyclic nucleotides in the painted turtle.

The Western painted turtle survives months without oxygen. A key adaptation is a coordinated reduction of cellular ATP production and utilization that may be signaled by changes in the concentrations of reactive oxygen species (ROS) and cyclic nucleotides (cAMP and cGMP). Little is known about the involvement of cyclic nucleotides in the turtle's metabolic arrest and ROS have not been previously measured in any facultative anaerobes.

Calcium and protein phosphatase 1/2A attenuate N-methyl-D-aspartate receptor activity in the anoxic turtle cortex.

Excitotoxic cell death (ECD) is characteristic of mammalian brain following min of anoxia, but is not observed in the western painted turtle following days to months without oxygen. A key event in ECD is a massive increase in intracellular Ca(2+) by over-stimulation of N-methyl-d-aspartate receptors (NMDARs). The turtle's anoxia tolerance may involve the prevention of ECD by attenuating NMDAR-induced Ca(2+) influx.