Personalized approach in brain protection by hypothermia: Individual changes in non-pathological and ischemia-related glutamate transport in brain nerve terminals

Abstract

Background: Both deep and profound hypothermia are effectively applied in cardiac surgery of the aortic arch, when the reduction of cerebral circulation facilitates operations, and for the prevention of ischemic stroke consequences. Neurochemical discrimination of the effects of deep and profound hypothermia (27 and 17 °C, respectively) on non-pathological and pathological ischemia-related mechanisms of presynaptic glutamate transport with its potential contribution to predictive, preventive and personalized medicine (PPPM) was performed. Methods: Experiments were conducted using nerve terminals isolated from rat cortex (synaptosomes). Glutamate transport in synaptosomes was analyzed using radiolabel l-[14C]glutamate. Diameter of synaptosomes was assessed by dynamic light scattering. Results: Synaptosomal transporter-mediated uptake and tonic release of l-[14C]glutamate (oppositely directed processes, dynamic balance of which determines the physiological extracellular level of the neurotransmitter) decreased in a different range in deep/profound hypothermia. As a result, hypothermia-induced changes in extracellular l-[14C]glutamate are not evident (in one half of animals it increased, and in other it decreased). A progressive decrease from deep to profound hypothermia was shown for pathological mechanisms of presynaptic glutamate transport, that is, transporter-mediated l-[14C]glutamate release stimulated by depolarization of the plasma membrane and during dissipation of the proton gradient of synaptic vesicles by the protonophore FCCP. Conclusions: Therefore, the direction of hypothermia-induced changes in extracellular glutamate is unpredictable in healthy nerve terminals and depends on hypothermia sensitivity of uptake vs. tonic release. In affected nerve terminals (e.g., in brain regions suffering from a reduction of blood circulation during cardiac surgery, and core and penumbra zones of the insult), pathological transporter-mediated glutamate release from nerve terminals decreases with progressive significance from deep to profound hypothermia, thereby underlying its potent neuroprotective action. So, alterations in extracellular glutamate during hypothermia can be unique for each patient. An extent of a decrease in pathological glutamate transporter reversal depends on the size of damaged brain zone in each incident. Therefore, test parameters and clinical criteria of neuromonitoring for the evaluation of individual hypothermia-induced effects should be developed and delivered in practice in PPPM.