Cardiac arrest/cardiopulmonary resuscitation (CA/CPR) causes ischemia, neuronal excitotoxicity and cognitive decline. Despite intensive efforts, outcome remains poor. Excitotoxicity results from increased glutamate neurotransmission, and the consequent excessive Ca2+ influx through NMDA-type glutamate receptors (NMDAr). Hippocampal CA1 neurons are important to learning and memory and are acutely sensitive to excitotoxicity. We have shown that small conductance Ca2+-activated K+ channels, type 2 (SK2 channels) are expressed together with NMDAr in the spines on hippocampal CA1 neurons where they act to attenuate Ca2+ influx through NMDAr. In addition, SK2 channels are removed from synapses following patterned activity, either normally as for the induction of long term potentiation (LTP), or abnormally after CA/CPR. The loss of synaptic SK2 channels removes the SK channel 'brake'on Ca2+ influx through NMDAr and is due to protein kinase A phosphorylation of the SK2 channels. Our results further show that increasing SK2 channel activity substantially improves neuronal survival after CA/CPR. Therefore, we will use an integrated technical repertoire to test these specific hypotheses: 1. Genetic or pharmacologic enhancement of SK2 channel activity protects CA1 neurons and improves cognitive outcome. We will use genetic mouse models and SK enhancing drugs to determine the i) survival of CA1 neurons and, ii) cognitive performance. 2. CA/CPR-induced ischemia causes a delayed and prolonged loss of synaptic SK2 channels in CA1 neurons, increasing the NMDAr-dependent Ca2+ transient that causes excitotoxicity. Preserving synaptic SK2 channel activity after CA/CPR protects CA1 neurons. We will measure the time course and effects of ischemia on the SK2 and NMDAr contributions to glutamate transmission (EPSP), and NMDAr-mediated Ca2+ transients. 3. CA/CPR-induced ischemia causes PKA phosphorylation of spine SK2 channels, inducing channel endocytosis. Expression of PKA-immune SK2 channels will normalize the SK2 and NMDAr contributions to the EPSP, the NMDAr-dependent Ca2+ transient, and protect CA1 neurons from excitotoxic cell death. We will use control mice or mice expressing PKA-immune SK2 channels to determine: i) the sub-spine distribution of SK2 channels;ii) the SK2 and NMDAr contributions to the EPSP;iii) the spine Ca2+ transient;iv) CA1 viability. 4. The aberrantly sustained ischemia-induced loss of synaptic SK2 channels results in ischemic LTP (iLTP) that shifts ?m, the modification threshold, to higher stimulus frequencies and impairs further potentiation. Maintained expression of functional synaptic SK2 channels prevents iLTP and normalizes ?m. We will measure the long-term effects of CA/CPR-induced ischemia on synaptic plasticity.
Heart attack and the consequent cerebral ischemia is one of the leading causes of death and disability in the United States and, unfortunately, there are currently no drugs available that improve outcome following severe heart attack requiring cardio-pulmonary resuscitation. SK2 channels, one type of Ca2+- activated K+ channel, are anatomically and functionally poised to ameliorate brain damage following stroke. The proposed studies will demonstrate the neuroprotective role of SK2 channels and suggest novel interventional strategies to protect the brain following heart attack, improving survival, diminishing memory deficits, and improving quality of life.
